Journal of Colloid And Interface Science (v.296, #1)
Double-layer phenomena in electrochemistry: Controversial views on some fundamental notions related to electrified interfaces
by G. Horányi; G.G. Láng (pp. 1-8).
The problem of the distinction between the so-called free charge and the thermodynamic (total) charge of electrodes was discussed in the light of the relevant IUPAC definitions calling attention to the strict relationships existing between charge and mass balances during the formation of a double layer at electrodes or/and particles. It is demonstrated that the origins of controversial views concerning partial charge transfer and electrosorption valency, notions widely used nowadays in the electrochemical literature, could be ascribed to confusion of the free and thermodynamic charges. Although there are, even if sporadic, evidences and theoretical considerations in the literature proving that electrosorption valency as usually defined is an extrathermodynamic and self-contradictory concept, it is widely used by many authors for the interpretation of electrosorption phenomena. One of the most important aims of the present work was to demonstrate that independent of the thermodynamic considerations, the concept of electrosorption valency cannot be reconciled with elementary laws of electrochemistry. On the basis of analysis of real processes occurring in the interfacial layer of electrodes it was urged to avoid the treatment of some double-layer phenomena in terms of electrosorption valency in order to eliminate far-reaching misinterpretations leading to serious contradictions.
Keywords: Thermodynamic charge; Free charge; Double layer; Adsorption; Electrosorption valency
Interfacial characteristics of heterogeneous layers of linear polyvinylamine and branched polyethyleneimine or polyethyleneimine grafted with C12â€“C22 alkyl chains
by Otman Oulanti; Emile Pefferkorn; Simon Champ; Helmut Auweter (pp. 9-15).
We investigated the characteristics of heterogeneous layers composed of linear hydrolyzed polyvinylamine and branched polyethyleneimine adsorbed at silica/water interfaces. The studies also included heterogeneous layers where branched polyethyleneimine was replaced by polyethyleneimine modified by grafting with C12â€“C22 alkyl chains. Surface area exclusion chromatography was used to determine the interfacial relaxation and surface affinity of the polymer molecules within homogeneous layers. The relaxation of bare and grafted polyethyleneimine was found to be small and of equal extent but to develop at different rates. Comparatively, the relaxation of hydrolyzed polyvinylamine was faster and of greater extent. Within heterogeneous layers composed of polyvinylamine and bare or grafted polyethyleneimine, the relaxation of the different molecules was strongly increased as compared to that prevailing in homogeneous layers. The chromatographic method was then used to determine the mode of layer establishment. The polymer coating profiles on successive glass fiber filters were found to depend on the sequence of injection of the two polymers, due to the interfacial stability or instability of the initially established layer. It was shown that a previously established extremely thin layer of bare or grafted polyethyleneimine molecules strongly modified the adsorption profile of subsequently adsorbed polyvinylamine molecules.Surface area exclusion chromatography that provides the adsorption figures on the stationary phase resulting from elution of the column is applied to various polymer mixtures.
Keywords: Polyethyleneimine adsorption on silica; Polymer interfacial reconformation; Polymer layer relaxation; Surface area exclusion chromatography; Relaxation in homogeneous polymer layers; Relaxation in heterogeneous polymer layers; Selectivity in polymer adsorption
Characterization and adsorption properties of tetrabutylammonium montmorillonite (TBAM) clay: Thermodynamic and kinetic calculations
by Mehmet Akçay (pp. 16-21).
The montmorillonite has been subjected to modification through ion-exchange reaction by tetrabutylammonium bromide (TBAB). The modified sample was studied by X-ray diffraction (XRD) technique, Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA) methods. The basal spacing of modified montmorillonite was determined as 14.40 Ã…. The IR spectra of modified montmorillonite showed CH vibrations. The characterization of tetrabutylammonium montmorillonite (TBAM) and the adsorption of p-chlorophenol ( p-CP) on organomontmorillonite was studied as a function of the solution concentration and temperature. The observed adsorption rates were found to fit to the pseudo-second-order kinetics. The rate constants were calculated for temperatures ranging between 25.0â€“35.0â€‰Â°C at constant concentration. The adsorption energy, E, and adsorption capacity, ( qm), for phenolic compounds adsorbing on organomontmorillonite were estimated using the Dubininâ€“Radushkevich (Dâ€“R) equation. Thermodynamic parameters (Î”ga=âˆ’11.063 andâˆ’11.802kJ/mol,Î”ha=âˆ’30.032 andâˆ’30.789kJ/mol,Î”sa=âˆ’0.0636 andâˆ’0.0637kJ/molâ€‰K for 298 and 308 K, respectively) were calculated by a new approximation from the adsorption isotherms of p-CP on organomontmorillonite. These isotherms were modeled according to Freundlich and Dubininâ€“Radushkevich adsorption isotherms, through which the first-order and second-order coefficients (K1ads=0.0152 and 0.0127 Î¼mol/gâ€‰min,K2ads=0.0130 and 0.0108 L/minâ€‰Î¼mol, respectively) were obtained at 298 and 308 K.
Keywords: Characterization; Tetraalkylammonium; Adsorption; Thermodynamics and kinetics; Organoclays
Coadsorption of Cd(II) and oxalate ions at the TiO2/electrolyte solution interface
by W. Janusz; M. Matysek (pp. 22-29).
The study of the adsorptions of cadmium and oxalate ions at the titania/electrolyte interface and the changes of the electrical double layer (edl) structure in this system are presented. The adsorption of cadmium or oxalate ions was calculated from an uptake of their concentration from the solution. The concentration of Cd(II) or oxalate ions in the solution was determined by radiotracer method. For labeling the solution14C and115Cd isotopes were used. Coadsorption of Cd(II) and oxalic ions was determined simultaneously. Besides, the main properties of the edl, i.e., surface charge density and zeta potential were determined by potentiometer titration and electrophoresis measurements, respectively. The adsorption of cadmium ions increases with pH increase and shifts with an increase of the initial concentration of Cd(II) ions towards higher pH values. The adsorption process causes an increase of negatively charged sites on anatase and a decrease of the Î¶ potential with an increase of initial concentration of these ions. The adsorption of oxalate anions at the titania/electrolyte interface proceeds through the exchange with hydroxyl groups. A decrease of pH produces an increase of adsorption of oxalate ions. The processes of anion adsorption lead to increase the number of the positively charged sites at the titania surface. However, specific adsorption of bidenate ligand as oxalate on one surface hydroxyl group may form inner sphere complexes on the metal oxide surface and may overcharge the compact part of the edl. The presence of oxalate ions in the system affects the adsorption of Cd(II) ions on TiO2, increasing the adsorption at low pH range and decreasing the adsorption at high pH range. Using adsorption as a function of pH data, some characteristic parameters of adsorption envelope were calculated.
Keywords: Coadsorption; Titanium dioxide; Electrical double layer; Zeta potential; Cadmium ions; Oxalate ions
The influence of temperature on the adsorption of mellitic acid onto goethite
by Michael J. Angove; John D. Wells; Bruce B. Johnson (pp. 30-40).
The adsorption of mellitic acid (benzene-1,2,3,4,5,6-hexacarboxylic acid) onto goethite was investigated at five temperatures between 10 and 70â€‰Â°C. Mellitic acid adsorption increased with increasing temperature below pH 7.5, but at higher pH the effect of increasing temperature was to reduce the amount adsorbed. Potentiometric titrations were conducted and adsorption isotherms were measured over the same temperature range, and the data obtained were used in conjunction with adsorption edge data to develop an Extended Constant Capacitance Surface Complexation Model of mellitic acid adsorption. A single set of reactions was used to model the adsorption for the three different experiment types at the five temperatures studied. The adsorption reactions proposed for mellitate ion (L6âˆ’) adsorption at the goethite surface (SOH) involved the formation of two outer-sphere complexes:SOH+L6âˆ’+3H+â‡Œ[(SOH2)+(LH2)4âˆ’]3âˆ’,2SOH+L6âˆ’+2H+â‡Œ[(SOH2)22+(L)6âˆ’]4âˆ’. This mechanism is consistent with recent ATR-FTIR spectroscopic measurements of the mellitate-goethite system. Thermodynamic parameters calculated from the temperature dependence of the equilibrium constants for these reactions indicate that the adsorption of mellitic acid onto goethite is accompanied by a large entropy increase.Adsorption of mellitic acid onto goethite at temperatures between 10 and 70â€‰Â°C. Below pH 7 adsorption is greater at lower temperature; above pH 7 uptake is greater at higher temperature.
Keywords: Sorption; Surface complexation; Carboxylic acid
Adsorption energies for a nanoporous carbon from gasâ€“solid chromatography and molecular mechanics
by Thomas R. Rybolt; Katherine A. Ziegler; Howard E. Thomas; Jennifer L. Boyd; Mark E. Ridgeway (pp. 41-50).
Gasâ€“solid chromatography was used to obtain second gasâ€“solid virial coefficients,B2s, in the temperature range 342â€“613 K for methane, ethane, propane, butane, 2-methylpropane, chloromethane, chlorodifluoromethane, dichloromethane, and dichlorodifluoromethane. The adsorbent used was Carbosieve S-III (Supelco), a carbon powder with fairly uniform, predominately 0.55 nm slit width pores and a N2 BET surface area of 995 m2/g. The temperature dependence ofB2s was used to determine experimental values of the gasâ€“solid interaction energy,Eâˆ—, for each of these molecular adsorbates. MM2 and MM3 molecular mechanics calculations were used to determine the gasâ€“solid interaction energy,Ecalâˆ—, for each of the molecules on various flat and nanoporous model surfaces. The flat model consisted of three parallel graphene layers with each graphene layer containing 127 interconnected benzene rings. The nanoporous model consisted of two sets of three parallel graphene layers adjacent to one another but separated to represent the pore diameter. A variety of calculated adsorption energies,Ecalâˆ—, were compared and correlated to the experimentalEâˆ— values. It was determined that simple molecular mechanics could be used to calculate an attraction energy parameter between an adsorbed molecule and the carbon surface. The best correlation between theEcalâˆ— andEâˆ— values was provided by a 0.50 nm nanoporous model using MM2 parameters.
Keywords: Adsorption; Henry's law; Adsorption; gasâ€“solid chromatography; Adsorption; carbon; on; B; 2; s; correlations of; Virial coefficients; gasâ€“solid; Molecular mechanics surface energy; Adsorption energy
Competitive adsorption of organic matter with phosphate on aluminum hydroxide
by Xiao-Hong Guan; Chii Shang; Guang-Hao Chen (pp. 51-58).
The effects of orthophosphate on the adsorption of natural organic matter (NOM) on aluminum hydroxide were investigated using three organic compounds as surrogates, including humic acid (HA), phthalic acid, and 2,3-dihydroxybenzoic acid (2,3-DHBA). The adsorption of phthalic acid and 2,3-DHBA was very limited compared to that of HA, whereas their adsorption was reduced much more significantly than that of HA by phosphate. The efficiency of phosphate in reducing HA adsorption increased with increasing phosphate concentration. Phosphate adsorption was slightly reduced by phthalic acid and 2,3-DHBA but moderately suppressed by HA. The adjacent carboxylic groups mainly contributed to the adsorption of humic acid at low pH, while the adjacent phenol groups were responsible for the adsorption of humic acid at high pH. HPLCâ€“SEC and SUVA analysis revealed that HA molecules with high molecular weight were adsorbed preferentially but were easily displaced by the specifically adsorbed phosphate. TMâ€“AFM images revealed that the aggregation of HA molecules and the protonation of carboxylic groups at low pH facilitated the adsorption under acidic conditions. The presence of phosphate increases the coagulant dosage for NOM removal as some sites on the coagulant precipitates become utilized by phosphate.The SEC elution curves. Competitive adsorption between phosphate and organic acids on aluminum hydroxide was investigated. Humic acid molecules weight were adsorbed preferentially but were easily displaced by the specifically adsorbed phosphate.
Keywords: Aggregation; Carboxylic group; Hydrophobic effect; Natural organic matter; Phenolic group
Biosorption of copper(II) from aqueous solutions by Spirogyra species
by V.K. Gupta; Arshi Rastogi; V.K. Saini; Neeraj Jain (pp. 59-63).
Batch studies were conducted to investigate the kinetics and isotherms of Cu(II) biosorption on the biomass of green alga Spirogyra species. It is observed that the biosorption capacity of the biomass strongly depends on pH and algal dose. The maximum biosorption capacity of 133.3 mg Cu(II)/g of dry weight of biomass was observed at an optimum pH of 5 in 120 min with an algal dose of 20 g/L. Desorption studies were conducted with 133.3 mg/g of Cu(II) loaded biomass using different desorption agents including HCl, EDTA, H2SO4, NaCl, and H2O. The maximum desorption of 95.3% was obtained with HCl in 15 min. The results indicate that with the advantages of high metal biosorption capacity and satisfactory recovery of Cu(II), Spirogyra can be used as an efficient and economic biosorbent material for the removal and recovery of toxic heavy metals from polluted water.Biosorption and desorption studies were conducted to investigate the potential of green alga Spirogyra species for removal of Cu(II) from aqueous solutions. The maximum biosorption capacity of 133.3 mg of Cu(II)/g of dry weight of biomass was observed at an optimum pH of 5 in 120 min with an algal dose of 20 g/L. The maximum desorption of 95.3% was obtained with HCl in 15 min.
Keywords: Adsorption; Biosorption; Spirogyra; Biomass; Copper; Desorption
Adsorption of poly(styrene sulfonate) of different molecular weights on Î±-alumina: Effect of added sodium dodecyl sulfate
by Anne Marit Blokhus; Ketil Djurhuus (pp. 64-70).
The adsorption of poly(styrene sulfonate), PSS, of different molecular weights (70,000, 500,000, and 1,000,000 mol/kg), from aqueous solutions on Î±-alumina has been investigated. PSS of the lower molecular weight adsorbs less than the others whose adsorption isotherms overlap. The adsorption is found to increase with increasing ionic strength of the solutions indicating that both electrostatic and non-electrostatic contributions are involved in the adsorption process. Upon addition of the anionic surfactant, sodium dodecyl sulfate, SDS, PSS is found to adsorb less the more SDS added. SDS is found to be preferentially adsorbed as shown both from the simultaneous adsorption of the components and also from the sequential adsorption process where SDS in all cases displaces preadsorbed PSS from the solid surface. The displacement of preadsorbed polyelectrolyte by surfactant is a very slow process and the displacement is less pronounced as the molecular mass of the polyelectrolyte increases indicating the fewer number of contact points to the surface. This is further underlined by the effect on the displacement of PSS by SDS upon increasing the ionic strength of the solutions.The adsorption of poly(styrene sulfonate), PSS, of different molecular weights from aqueous solutions on Î±-alumina has been investigated. The effect of added anionic surfactant and the influence of ionic strength on both adsorption and desorption of PSS has also been monitored.
Keywords: Competitive adsorption; Desorption; Polyelectrolyte; Surfactant; Oxide surface
IR and quantum-chemical studies of carboxylic acid and glycine adsorption on rutile TiO2 nanoparticles
by Lars Ojamäe; Christian Aulin; Henrik Pedersen; Per-Olov Käll (pp. 71-78).
Nanocrystalline TiO2 powders of the rutile polymorph, synthesized by a solâ€“gel method, were treated with water solutions containing, respectively, formic, acetic, and citric acid and glycine in order to study the adsorption properties of these organic species. The samples were characterized by FTIR, Raman, powder XRD, and TEM. It was found that HCOOH, CH3COOH and HOC(COOH)(CH2COOH)2â€”but not NH2CH2COOHâ€”adsorbed onto TiO2. The adsorption of HCOOH, CH3COOH and NH2CH2COOH onto the (110) surface of rutile was also studied by quantum-chemical periodic density functional theory (DFT) calculations. The organic molecules were from the computations found to adsorb strongly to the surfaces in a bridge-coordinating mode, where the two oxygens of the deprotonated carboxylic acid bind to two surface titanium ions. Surface relaxation is found to influence adsorption geometries and energies significantly. The results from DFT calculations and ab initio molecular-dynamics simulations of formic acid adsorption onto TiO2 are compared and match well with the experimental IR measurements, supporting the bridge-binding geometry of carboxylic-acid adsorption on the TiO2 nanoparticles.
Keywords: Titanium oxide; Nanoparticle; Adsorption; Carboxylic acid; Glycine; FTIR; Raman; Quantum-chemical calculations
Phenobarbital interactions with derivatized activated carbon surfaces
by Aktham Aburub; Dale Eric Wurster (pp. 79-85).
The interactions between phenobarbital and activated carbon surfaces were studied in detail in this work. This was accomplished by utilizing different reagents to manipulate the surface polar functional group compositions of different activated carbons, and determining how those modifications changed phenobarbital adsorption. Oxidation of an activated carbon surface caused a systematic decrease in the basal carbon surface, resulting in a concurrent systematic decrease in the non-specific adsorption of phenobarbital. Even more interesting, it was shown for the first time that chemical reduction of some of the carbonyl-containing functional groups on the activated carbon surface caused a significant increase in the specific adsorption of phenobarbital without any significant effect on the non-specific adsorption. These results support the notion that the OH groups on activated carbon surfaces are the specific adsorption sites for phenobarbital from aqueous solutions, and that the basal carbon surface is the region where non-specific adsorption takes place.
Keywords: Specific adsorption; Non-specific adsorption; Modified Langmuir-like equation; Heat of displacement; Phenobarbital; Hydrophobic bonding; Activated carbon
Film drainage and interfacial instabilities in polymeric systems with diffuse interfaces
by A.N. Zdravkov; G.W.M. Peters; H.E.H. Meijer (pp. 86-94).
We report an experimental investigation on the effect of mutual diffusion in polymeric systems on film drainage between two captive drops. The main objective is to study the influence of diffuse interfaces on film drainage. This is done by using material combinations with different interfacial properties and interferometric visualization of the film between two interacting drops. For highly diffusive systems film drainage is observed to be, in contrast to immiscible systems, non-axisymmetric and unstable immediately after the film formation (at a few micrometers film thickness). Depending on whether the total thickness of the diffusion layers in the film is smaller or larger than the thickness of the film, Marangoni convection is found to enhance or delay film drainage. Enhanced film drainage is determined to be in order of 100 times faster than predicted by the current models, while delayed film drainage is observed after a drainage period where experimental and predicted results (assuming, a partially mobile interface) are in close agreement.
Keywords: Film drainage; Film drainage instability; Polymerâ€“polymer inter-diffusion; Diffuse interface layers; Drop coalescence; Marangoni convection
Synthesis and characterization of nanoheterostructures based on oligothiophene functionalized Ru nanoparticles
by Tarik Matrab; Abderrahim Yassar; Guillaume Viau; Nassira Chakroune; Fernand Fievet; Pierre Camille Lacaze (pp. 95-101).
Monodisperse ruthenium nanoparticles functionalized by electroactive oligothiophenes have been prepared and characterized. Using TEM, UVâ€“visible and FTIR we established that the organization of these nanoparticles into nanospheres can be directly controlled via modulation of theÏ€â€“Ï€ interaction between the organic components adsorbed on the surface. This finding also shows that the self-assembled nanoheterostructures may be switched from monodisperse nanoparticles to ordered nanospheres by tuning the pH.
Thermo-responsive behavior and microenvironments of poly( N-isopropylacrylamide) microgel particles as studied by fluorescent label method
by Yuriko Matsumura; Kaoru Iwai (pp. 102-109).
Poly( N-isopropylacrylamide) (PNIPAM) microgel particles labeled with a fluorescent monomer 4- N-(2-acryloyloxyethyl)- N-methylamino-7-N,N-dimethylaminosulfonyl-2,1,3-benzoxadiazole (DBD-AE) were prepared by emulsion polymerization under various crosslinker concentrations. The thermo-responsive behavior and the microenvironment of the microgel particles were studied in water by turbidimetric and fluorescence analyses. For the microgel particles prepared under the crosslinker concentration of 1 mM, the turbidity began to increase at ca. 32.5â€‰Â°C, but the relative fluorescence intensity dramatically increased and the wavelength at the maximum fluorescence intensity(Î»max) was dramatically blue-shifted both at ca. 31.5â€‰Â°C with increasing the temperature, suggesting the hydrophobicity around the DBD-AE unit was dramatically increased and the subsequent shrinking of the microgel particles occurred. As the crosslinker concentration increased from 0.5 to 20 mM, the transition temperature determined by turbidimetric analysis was constant upto 2 mM, rose between 2 and 10 mM, leveled off above 10 mM, and was ca. 34â€‰Â°C at 20 mM. The temperature-induced microenvironmental change inside the microgel particles was also reduced at high crosslinker concentrations. The results obtained from the fluorescence of the DBD-AE unit and another fluorescent monomer unit 3-(2-propenyl)-9-(4-N,N-dimethylaminophenyl)phenanthrene (VDP) suggested that the heterogeneity inside the microgel particles prepared under the crosslinker concentration of 20 mM became high.Poly( N-isopropylacrylamide) (PNIPAM) microgel particles labeled with a polarity-sensitive fluorescent monomer DBD-AE were synthesized under various crosslinker concentrations. The thermo-responsive behavior and the microenvironments of these microgel particles in water were studied by turbidimetric and fluorescence analyses.
Keywords: Poly(; N; -isopropylacrylamide) microgel particle; Crosslinker concentration; Fluorescent labeled hydrogel
Fabrication and characterizations of oligopyrrole doped with dodecylbenzenesulfonic acid in reverse microemulsion
by Mingjuan Han; Ying Chu; Dongxue Han; Yang Liu (pp. 110-117).
The pyrroleâ€“oligomer nanoparticles doped dodecylbenzenesulfonic acid (DBSA) have been fabricated in waterâ€“DBSAâ€“isopropyl alcoholâ€“isooctane reverse microemulsion, since DBSA could play the roles of both surfactant and dopant. We have found the optimum synthetic conditions by the analysis of factors affecting the yield and conductivity of oligopyrrole. The effect of solubility on the improved properties of DBSAâ€“oligopyrrole in some organic solvents was studied employing UVâ€“visâ€“NIR spectroscopy. This indicates that doping and pyrrole-oligomers may improve the poor processibility of PPy. Its environmentally stable properties were characterized by thermogravimetric and differential thermal analysis (TG-DTA). The maximum room temperature conductivity of products is 8.01 S/cm using the four-probe technique. The pyrrole-oligomers have been determined by the analysis of molecular weight measurement, and X-ray photoelectron spectroscopy (XPS) and elemental analysis were employed to investigate the formation of DBSAâ€“oligopyrrole complexes. Correspondingly, the structural properties of DBSAâ€“oligopyrrole were studied by infrared spectroscopy (IR), X-ray diffraction (XRD), transmission electron micrographs (TEM), and scanning electron microscopy (SEM).The pyrroleâ€“oligomer nanoparticles doped dodecylbenzenesulfonic acid (DBSA) have been fabricated in waterâ€“DBSAâ€“isopropyl alcoholâ€“isooctane reverse microemulsion, since DBSA could play the roles of both surfactant and dopant. We have found the optimum synthetic conditions by the analysis of factors affecting the yield and conductivity of oligopyrrole. The effect of solubility on the improved properties of DBSAâ€“oligopyrrole in some organic solvents was studied employing UVâ€“visâ€“NIR spectroscopy. Infrared spectroscopy (IR), transmission electron micrographs (TEM), scanning electron microscopy (SEM), differential thermal analysis (TG-DTA), and elemental analysis, etc. were employed to investigate DBSAâ€“oligopyrrole particles.
Keywords: Oligopyrrole; DBSA; Doped; Nanoparticles; Reverse microemulsion polymerization
Preparation, characterization, resistance to protein adsorption, and specific avidinâ€“biotin binding of poly(amidoamine) dendrimers functionalized with oligo(ethylene glycol) on gold
by Chi Ming Yam; Maxence Deluge; David Tang; Amit Kumar; Chengzhi Cai (pp. 118-130).
Protein-resistant films derived from the fifth-generation poly(amidoamine) dendrimers (PAMAM G5) functionalized with oligo(ethylene glycol) (OEG) derivatives consisting of various ethylene glycol units (EG n,n=3, 4, and 6) were prepared on the self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA) on gold substrates. The resulting films were characterized by ellipsometry, contact angle goniometry, and X-ray photoelectron spectroscopy (XPS). About 35% of the peripheral amines of the dendrimers were reacted with Nâˆ’hydroxysuccinimide-terminated EG n derivatives (NHS-EG n). The dendrimer films showed improved stability over octadecanethiolate SAMs on gold in hot solvents, attributed to the formation of multiple amide bonds per PAMAM unit with underlying NHS-activated MUA monolayer. The EG n-attached PAMAM surfaces withn=3 reduced the adsorption of fibrinogen to âˆ¼20% monolayer, whereas 2â€“3% forn=4 or 6. The dendrimer films with various densities of EG n molecules on PAMAM surfaces were prepared by immersion of the NHS-terminated MUA-functionalized gold substrates in ethanolic solutions containing PAMAM and NHS-EG n of various mole ratios. The density ( r) of the EG n molecules on the PAMAM surfaces is consistent with the mole ratio (râ€²) of NHS-EG n/free amine of PAMAM in solutions. The resistance to protein adsorption of the resulting surfaces is correlated with the surface density and the length of the EG chains. At their respective r, the EG n-modified dendrimer films resisted âˆ¼95% adsorption of fibrinogen on gold surfaces. Finally, the specific binding of avidin to the âˆ¼5% and âˆ¼40% biotinylated EG3 dendrimers (surface density of biotin with respect to the total number of terminal amino groups on PAMAM G5) gave rise to about 50% and 100% surface coverage by avidin, respectively.
Keywords: Oligo(ethylene glycol); Poly(amidoamine) dendrimers; Protein resistance; Gold substrates; Avidinâ€“biotin interaction
Preparation of montmorillonite/titania nanocomposite and enhanced electrorheological activity
by Liqin Xiang; Xiaopeng Zhao (pp. 131-140).
We prepared a new type of electrorheological particle composed of TiO2 nanocrystallites-coated montmorillonite (MMT/TiO2) nanocomposite by the solâ€“gel technique. The characterizations including TGA, XRD, TEM, SEM, EDS, and FTIR showed that TiO2 was deposited on the surface of the MMT flakes with anatase nanocrystallite. An obviously enhanced ER effect was found in the MMT/TiO2 nanocomposites based ER fluids compared with pure MMT and TiO2. Furthermore, the temperature and sedimentation stabilities of the MMT/TiO2 ER fluids had also been improved greatly. Interestingly, the content of TiO2 was demonstrated to have an important influence on the ER effect. When the content of TiO2 was about 20 wt%, the ER effect of MMT/TiO2 ER fluid reached its maximum, which was about 5 times that of pure MMT ER fluid and 27 times that of pure TiO2 ER fluid. Based on dielectric analysis, the significant ER enhancement by formation nanocrystallites-coated montmorillonite was attributed to the enhanced interfacial polarization in this nanocomposite particle due to the effective limitation of the long-range drift of active ions in montmorillonite particles.MMT/TiO2 nanocomposites was prepared by the solâ€“gel technique, which showed the enhanced electrorheological effect, better temperature and sedimentation stabilities compared with pure MMT and TiO2.
Keywords: Electrorheology; Nanocomposite; Montmorillonite; TiO; 2; Interfacial polarization
Preparation and some properties of organically modified layered alkali titanates with alkylmethoxysilanes
by Yusuke Ide; Makoto Ogawa (pp. 141-149).
The silylation of K2Ti4O9â‹… nH2O with organosilanes (methyl, n-butyl, n-octyl, n-dodecyl, n-octadecyltrimethoxysilanes and n-octadecyldimethylmethoxysilane) was conducted using the octylammonium-exchanged form as the intermediate. The surface coverage of the octadecylsilylated derivative was controlled by changing the employing amounts of octadecyltrimethoxysilane. The swelling behaviors of the octyl, dodecyl, and octadecylsilylated derivatives in organic solvents were investigated to show that the degree of the swelling varies depending on the kind of solvents, the alkyl chain length of the attached alkylsilyl groups, and the surface coverage. The octadecylsilylated derivative with the largest surface coverage was converted to film with a thickness of ca. 500 nm by casting the chloroform suspension on a substrate. The octadecylsilylated derivative showed a reversible thermoresponsive change of the basal spacing by ca. 0.5 nm in the temperature range between 15 and 60â€‰Â°C.
Keywords: Layered alkali titanate; Silylation; Swelling; Film; Phase transition
Electrical double layer interactions between dissimilar oxide surfaces with charge regulation and Sternâ€“Grahame layers
by Derek Y.C. Chan; Thomas W. Healy; Tharatorn Supasiti; S. Usui (pp. 150-158).
Models of surfaces with intrinsic ionisable amphoteric surface sites governed by the dissociation of acidâ€“base potential determining ion species together with the capacity for the adsorption of anion and cations of the supporting electrolyte are required to describe both the results of electrokinetic and titration measurements of inorganic oxides. The Gouyâ€“Chapmanâ€“Sternâ€“Grahame (CGSG) model is one such model that has been widely used in the literature. The electrical double layer interaction between two dissimilar CGSG surfaces has been studied by Usui recently [S. Usui, J. Colloid Interface Sci. 280 (2004) 113] where erroneous discontinuities in the slope of the pressureâ€“separation relation were observed. We revisit this calculation and provide a simple general methodology to analyse the electrical double layer interaction between dissimilar ionisable surfaces with ion adsorption.
Keywords: Double layer interaction; Dissimilar double layer; Charge regulation; Gouyâ€“Chapmanâ€“Sternâ€“Grahame model; Oxideâ€“water interfaces; Site-binding model; Surface force
High-salt stabilization of Laponite clay particles
by Alvin Y. Huang; John C. Berg (pp. 159-164).
Colloidal radioactive transuranic wastes are currently buried in large tanks in the form of dense colloids in high salt, high pH aqueous media. These facilities are beginning to fail, so it is necessary to transport and â€œpackageâ€? them for more permanent disposal, processes requiring understanding of the microstructure that develops under such conditions. Laponite RD clay is believed to be a good simulant for the colloids in the waste tanks, and the present study addresses their behavior under high salt conditions, where previous studies have frequently observed the phenomenon of â€œrestabilization,â€? i.e., the attainment of aggregation stability at high electrolyte conditions. Specifically, the aggregation kinetics and the resulting cluster structure (fractal dimension) of Laponite RD clay colloids at high concentrations of BaCl2 (an electrolyte previously shown to lead to restabilization) are investigated. At low-to-intermediate electrolyte concentrations, the clay is found to behave in accord with DLVO theory, i.e., low salt conditions yield slow aggregation into densely-packed aggregates, whereas intermediate salt concentrations, sufficient to cause double layer collapse, produce rapid aggregation into open aggregates. High salt concentrations, however, show slow rates of aggregation. The aggregate structure under these conditions is found to mimic that found for very low electrolyte concentrations, i.e., high fractal dimension. Further experiments show that a sudden increase in salt concentration in a system containing young open aggregates produced under intermediate salt concentrations causes them to reform into more compact structures.
Keywords: High-salt stabilization; Non-DLVO forces; Hydration forces; Aggregate structure; Fractal dimension; Aggregation kinetics
Synthesis and characterization of an electroactive surface that releases Î³-aminobutyric acid (GABA)
by Chun Yan; Wakana Matsuda; David R. Pepperberg; Steven C. Zimmerman; Deborah E. Leckband (pp. 165-177).
We report the synthesis and characterization of a new electroactive surface capable of releasing the neurotransmitter Î³-aminobutyric acid (GABA) upon reduction. The GABA was anchored to an alkanethiol via electrochemically active quinone (abbreviation, TM-GABA). The quinone unit, upon reduction to the hydroquinone, cyclizes to release GABA into solution. The half-life is 99 s. The self-assembled monolayer (SAM) of TM-GABA on gold was prepared and characterized with several surface sensitive techniques. X-ray photoelectron spectroscopy (XPS) explored the SAM formation of TM-GABA on Au surfaces. Cyclic voltammograms showed the ability to electrochemically control the quinone unit at the distal end of the chain. GABA was selectively released upon electrochemical reduction at a potential of âˆ’700 mV. The functional GABA terminal group was detected by surface plasmon resonance measurements of anti-GABA antibody binding.
Keywords: Self-assembled monolayers (SAMs); XPS; SPR; Cyclic voltammogram; Electrochemical reduction; GABA
Oscillating adhesive contacts between micron-scale tips and compliant polymers
by K.J. Wahl; S.A.S. Asif; J.A. Greenwood; K.L. Johnson (pp. 178-188).
Adhesion of micron-scale probes with model poly(dimethylsiloxane), PDMS, elastomers was studied with a depth-sensing nanoindenter under oscillatory loading conditions. For contacts between diamond indenters (radiusR=5 or 10 Î¼m) and PDMS, forceâ€“displacement curves were highly reversible and consistent with Johnsonâ€“Kendallâ€“Roberts (JKR) behavior. However, our experiments have revealed striking differences between the experimental measurements of tipâ€“sample interaction stiffness and the theoretical JKR stiffness. The measured stiffness was always greater than zero, even in the reflex portion of the curve (between the maximum adhesive force and release), where the JKR stiffness is negative. This apparent paradox can be resolved by considering the effects of viscoelasticity of PDMS on an oscillating crack tip in a JKR contact. Under well described conditions determined by oscillation frequency, sample viscoelastic properties, and the Tabor parameter (with variables R, reduced elastic modulus,Eâˆ—, and interfacial energy, Î” Î³), an oscillating crack tip will neither advance nor recede. In that case, the contact size is fixed (like that of a flat punch) at any given point on the loadâ€“displacement cycle, and the experimentally measured stiffness is equal to the equivalent punch stiffness. For a fixed oscillation frequency, a transition between JKR and punch stiffness can be brought about by an increase in radius of the probe or a decrease in PDMS modulus. Additionally, varying the oscillation frequency for a fixedEâˆ—, R, and Î” Î³ also resulted in transition between JKR and punch stiffness in a predictable manner. Comparisons of experiments and theory for an oscillating viscoelastic JKR contact are presented. The storage modulus and surface energy from nanoscale JKR stiffness measurements were compared to calculated values and those measured with conventional nanoindentation and JKR forceâ€“displacement analyses.
Keywords: Adhesion; Viscoelastic; Elastomer; PDMS; JKR; Nanomechanics; Force curve; AFM; Thin films
Kinetic evidences of the existence of positively charged species at the quartz-aqueous solution interface
by O.S. Pokrovsky; S.V. Golubev; J.A. Mielczarski (pp. 189-194).
To probe the surface speciation of quartz in strong acidic solutions (pH 0â€“3), where surface titration and electrophoresis are extremely difficult to perform, dissolution rates of this mineral were measured at 25â€‰Â°C and constant ionic strength (1.0 M) using mixed-flow and batch reactors. Dissolution rates increase with activity of protons at0â©½pHâ©½3, which suggests the adsorption of H+ on the mineral surface, leading to polarization of SiO bonds and detachment of the silicon atom from the structure. This scheme is consistent with the presence of a non-negligible amount (i.e., up to 30â€“50% at pH close to 0) of protonated >SiOH+2 species on the surface, as was recently demonstrated using X-ray photoelectron spectroscopy (XPS) analysis of exactly the same quartz sample [Y. Duval, J. Mielczarski, O.S. Pokrovsky, E. Mielczarski, J.J. Ehrhardt, J. Phys. Chem. B 106 (2002) 2937â€“2945]. A 2-p K electrical double layer (EDL) constant capacitance surface speciation model has been used to model the obtained kinetic data. A set of surface stability constants consistent with previous spectroscopic XPS measurements (pK1=âˆ’1.0 and pK2=4.0) and EDL capacitance of 1.5 F/m2 provide adequate description of the dissolution rate with reaction order with respect to [>SiOH+2] close to 1. Although the CCM model used in this study presents some limitations on surface charge versus pH dependences, the developed kinetic approach opens new possibilities of probing the surface speciation at the SiO2â€“aqueous solution interface under extreme solution conditions.
Keywords: Quartz; Vitreous silica; Surface; Complexation; Dissolution; Kinetics
Self-assembled monolayers (SAMs) of alkoxycyanobiphenyl thiols on goldâ€”A study of electron transfer reaction using cyclic voltammetry and electrochemical impedance spectroscopy
by V. Ganesh; Santanu Kumar Pal; Sandeep Kumar; V. Lakshminarayanan (pp. 195-203).
Self-assembled monolayers (SAMs) of liquid crystalline thiol-terminated alkoxycyanobiphenyl molecules with different alkyl chain lengths on Au surface have been studied for the first time using electrochemical techniques such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The barrier property of the SAM-modified surfaces was evaluated using two different redox probes, namely potassium ferro/ferri cyanide and hexaammineruthenium(III) chloride. It was found that for short-length alkyl chain thiol (C5) the electron transfer reaction of hexaammineruthenium(III) chloride takes place through tunneling mechanism. In contrast, redox reaction of potassium ferro/ferri cyanide is almost completely blocked by the SAM-modified Au surface. From the impedance data, a surface coverage value of >99.9% was calculated for all the thiol molecules.Self-assembled monolayer formed by liquid crystalline molecule of thiol-terminated alkoxycyanobiphenyl mediates electron transfer between Au electrode and [Ru(NH3)6]2+/3+ when the alkyl chain length is short and retards the process when the chain length is longer.
Keywords: Self-assembled monolayer (SAM); Alkoxycyanobiphenyl thiol; Nematic liquid crystal; Cyclic voltammetry; Electrochemical impedance spectroscopy; Redox reaction; Electron transfer; Tunneling
Electroactive films of heme protein-coated multiwalled carbon nanotubes
by Liyun Zhao; Hongyun Liu; Naifei Hu (pp. 204-211).
A novel method for fabricating proteinâ€“MWNT films on pyrolytic graphite (PG) electrodes was described. Positively charged hemoglobin (Hb) or myoglobin (Mb) in buffers at pH 5.5 or 5.0 was first adsorbed on the surface of acid-pretreated, negatively charged multiwalled carbon nanotubes (MWNTs) mainly by electrostatic interaction, forming a coreâ€“shell structure. The aqueous dispersion of protein-coated MWNTs was then cast on PG electrodes, forming proteinâ€“MWNT films after evaporation of solvent. The proteinâ€“MWNT films exhibited a pair of well-defined, quasi-reversible cyclic voltammetric peaks, characteristic of heme Fe(III)/Fe(II) redox couples. The protein films were characterized by voltammetry, UVâ€“vis spectroscopy, and scanning electron microscopy (SEM). This approach for assembly of proteinâ€“MWNT films showed higher surface concentration of electroactive proteins than the simple cast method, and the amount of proteins in the films could be controlled more precisely compared with the dipping method. Furthermore, the film assembly using this method was more stable than that using simple cast method. The proteins in MWNT films retained their near-native structure, and electrochemically catalyzed reduction of oxygen and hydrogen peroxide, suggesting the potential applicability of the films as the new type of biosensors or bioreactors based on direct electrochemistry of enzymes.Hemoglobinâ€“MWNT films cast on PG electrodes with (a) untreated and (b) pretreated MWNTs, and (c) myoglobinâ€“MWNT films with pretreated MWNTs at 0.2 Vâ€‰sâˆ’1 in pH 7.0 buffers.
Keywords: Hemoglobin; Myoglobin; Multiwalled carbon nanotubes; Direct electrochemistry; Electrocatalysis
Effect of packing on orientation and cisâ€“ trans isomerization of azobenzene chromophore in Langmuirâ€“Blodgett film
by Masashi Takahashi; Takashi Okuhara; Tomohiro Yokohari; Koichi Kobayashi (pp. 212-219).
Photoactive film material of long-chain azobenzene derivative, p-( Ï‰-trimethylammoniodecyloxy)-pâ€²-octyloxyazobenzene bromide (TAOAB), was fabricated into a Langmuirâ€“Blodgett (LB) film by a polyion-complex technique using poly(sodium 4-styrenesulfonate) as a polyanion. To investigate the effect of the packing state of the azobenzene chromophore on its orientation and cisâ€“ trans isomerization, TAOAB was mixed with methyl stearate in the LB film matrix at various mole fractions(XTAOAB), and structural characterizations were subsequently carried out by means of Fourier transform infrared and ultravioletâ€“visible spectroscopies, X-ray diffraction analysis, and atomic force microscopy. The results obtained show that as the degree of packing increases, both the azobenzene chromophores and the hydrocarbon chains orient more perpendicularly to the surface of the LB film. In addition, reversible cisâ€“ trans photoisomerization of TAOAB took place upon alternate irradiation with UV and visible light even in a mixed LB film with the chromophores in a dense lateral packing state. In the process of thermal cis-to- trans isomerization, we found that the reaction rate is strongly affected by the packing state of TAOAB at 20â€‰Â°C, reflecting the differences in steric hindrance among LB films of variousXTAOAB. In addition, higher activation energy was obtained for thermal cis-to- trans isomerization when the free volume around the chromophores became smaller.
Keywords: Langmuirâ€“Blodgett films; Monolayers; Polyion-complex technique; Azobenzene derivative; Reversible; cis; â€“; trans; photoisomerization; Isomerization kinetics
Spinodal instability and pattern formation in thin liquid films confined between two plates
by Ruhi Verma; Ashutosh Sharma; Indrani Banerjee; Kajari Kargupta (pp. 220-232).
The instability, morphology and pattern formation engendered by the van der Waals force in a thin liquid film of thickness h confined between two closely placed solid surfaces (at distanced>h) are investigated based on nonlinear 3D simulations. The initial and the final stages of dewetting and pattern formation are found to be crucially dependent on the volumetric (thickness) ratio of air and liquid and its deviation from the location of the maximum of the spinodal parameter versus volumetric ratio curve. On a low energy surface, relatively thinner films and wider air gaps favor initial dewetting of the lower plate by the formation of holes, whereas thicker films with thinner air gaps initially evolve by the formation of columns/bridges that join the upper plate. In the later stage of evolution, the initial holes in thinner films evolve into columns/drops, while a rapid coalescence of columns in the thicker films eventually causes formation of holes. Thus, a phase inversion, either from liquid-in-air to air-in-liquid dispersion or vice versa, occurs during the final stages of evolution. A thin film confined between two high-energy solid surfaces forms columns (bridges) only when its mean thickness,h0, is greater than a critical thickness(hc) or the air gap is smaller than a critical distance. The patterns can be aligned by using a topographically patterned confining surface. Conditions on pattern periodicity, amplitude, and the volumetric ratio of air and liquid in the gap are explored for the formation of various types of ordered patterns including annular rings of columns, concentric ripples, parallel channels and a rectangular array of complex features. The results are of significance in soft lithographies such as LISA, soft stamping and capillary force lithography.Self-organized patterns in thin liquid films confined between two plates with implications for soft lithographies such as LISA and capillary force lithography.
Keywords: Confined thin film; Dewetting; Soft lithography; Templating; Patterning
Monolayer behavior of silica particles at air/water interface: A comparison between chemical and physical modifications of surface
by Yuh-Lang Lee; Zhung-Ching Du; Wei-Xin Lin; Yu-Min Yang (pp. 233-241).
Silica particles are hydrophobized either by chemical graft of alkyl chains or by physical adsorption of cationic surfactants, alkyltrimethylammonium bromide. The effects of the two modification methods on the monolayer behavior of silica particles at the air/water interface are studied, as well as the packing structure of the particulate films. The results show that the hydrophobicity of particles chemically modified by octanol (SiO2-C8) and dodecanol (SiO2-C12) are similar and higher than that modified by butanol (SiO2-C4). The monolayer composed of particles with higher hydrophobicity shows a large lift-off area, higher compressibility, and significant hysteresis due to the higher particleâ€“particle interaction. As a result, the particulate films exhibit 2-dimensional (2D) aggregative domains of closely-packed structure, but with particle free regions presenting among the domains. The monolayer prepared by SiO2-C4 shows a contrary behavior resulted from the higher particleâ€“water interaction. The particles modified by adsorption of cationic surfactants have an amphiphilic property at the air/water interface. Such monolayer exhibits lower compressibility and hysteresis, higher re-spreading characteristic, and a lower collapse pressure compared with those of the chemically modified particles. A particulate film with high uniformity and closely-packed structure can be obtained by using the octyltrimethylammonium bromide (OTAB) modified particles. When the alkyl chain of surfactant increases, the packing of the particles becomes looser. Such phenomenon is probably caused from the higher probability for the long-chain surfactants to stay at the air/water interface which obstructs the intimate contact of particles.The surface of silica particles are chemically grafted by alkyl chain (left), or modified by adsorption of surfactant (right). The behaviors of such particles at air/water interface were studied.
Keywords: Monolayer; Silica particle; Surface modification; Langmuirâ€“Blodgett deposition
Effect of calcium and carbonate concentrations on anionic membrane fouling during electrodialysis
by Monica Araya-Farias; Laurent Bazinet (pp. 242-247).
A previous study on electrodialysis of calcium and carbonate high concentration solutions demonstrated that calcium migrated through the cation-exchange membrane (CEM) was blocked by the anion-exchange membrane (AEM) where it formed another fouling. The aim of the present work was to complete the identification of the deposit formed on AEM during electrodialysis and to characterize its physical structure at the interface of the membrane. No fouling was found on the anionic membranes treated without calcium chloride in presence of sodium carbonate, while membranes used during ED process of solutions containing calcium chloride and sodium carbonate were slightly fouled. A thin layer of precipitates was observed on the anionic membrane surface. The appearance of precipitates was typical of a crystalline substance. The size and form of crystal increased in proportion to the concentration of calcium chloride in solution. Large and cubic crystals were the best defined on the membrane treated at 1600 mg/L of CaCl2. The precipitate was identified as calcium hydroxide. However, this fouling was not found to affect significantly the electrical conductivity and the thickness of the membranes. Furthermore, the fouling formed was reversible.
Keywords: Electrodialysis; Fouling; Anionic membranes; Sodium carbonate; Calcium chloride
Diffusion of ionic species in bentonite
by P. Leroy; A. Revil; D. Coelho (pp. 248-255).
We present a macroscopic model of ionic diffusion in bentonites including the effect of the hydraulic-electrical-chemical couplings expected in such charged porous medium. The anomalous concentrations of the ions in the pore water of the bentonite are modeled with a modified Donnan model in which we account for the partition of the counterions between the Stern and Gouyâ€“Chapman layers. This is accomplished using an electric triple layer (TLM) model combined with an explicit complexation model at the mineral/water interface. The porosity entering into the determination of the formation factor of the medium is an effective porosity obtained by removing the fraction of hydration water covering the surface of the clay minerals. We investigate two different cases of diffusion. In the first case, we consider a salinity gradient between two reservoirs in contact with a cylindrical sample of bentonite. The model predicts an increase of the diffusivity of the salt with the salinity of the solution in contact with the bentonite in agreement with experimental data. In the second case, we analyze a self-diffusion experiment of an ionic tracer. The model predicts an increase of the diffusivity of anions with the effective porosity and with the ionic strength. This is also in good agreement with experimental data.
Keywords: Clay; Bentonite; Diffusion; Porous media
In situ small-angle neutron scattering study of nitrogen adsorption and condensation in mesoporous silica glass CPG-10-75
by E. Hoinkis; B. Röhl-Kuhn (pp. 256-262).
Film formation and capillary condensation of nitrogen at 78 K on the mesoporous controlled pore glass CPG-10-75 have been studied at certain relative pressures by in situ small-angle neutron scattering. On desorption ramified clusters of vapor filled voids have been observed, but not on adsorption. The kinetics of adsorption and desorption have been followed. The experimental results are discussed with respect to recent theoretical studies of fluids in complex pore systems.
Keywords: In situ SANS; Controlled pore glass; Vitreous silica; Adsorption; Capillary condensation; Percolation; Cavitation; Self-similar growth
Two-dimensional facets in Langmuir monolayers of 1- O-hexadecyl- rac-glycerol at the airâ€“water interface
by Md.N. Md. Nazrul Islam; Teiji Kato (pp. 263-268).
We study the surface phase behavior in Langmuir monolayers of 1- O-hexadecyl- rac-glycerol (C16G) by film balance and Brewster angle microscopy over a wide range of temperatures. A cusp point followed by a pronounced plateau region in the pressureâ€“area ( Ï€â€“ A) isotherm indicates a first-order phase transition between a lower density liquid expanded (LE) phase and a higher density liquid condensed (LC) phase at the airâ€“water interface. A wide variety of condensed domains are found to form just after the appearance of the cusp point. The observed surface morphology was compared with that of ethylene glycol mono- n-hexadecyl ether (C16E1) that bears an ethylene oxide (EO) unit in the head-group. As usually observed, the domains of C16E1 are found to be circular at lower temperatures and fractal at higher temperatures. Contrary to this usual behavior, the domains of C16G are found to be strip-like structures at lower temperatures, which attain increasingly compact shape as the temperature increases and finally attain faceted structures atâ©¾25Â°C. It is concluded that a higher degree of dehydration around the head-group region of C16G appreciably reduces the hydration-induced repulsive interactions between the head-groups and imparts to the molecules an increase in hydrophobicity, thereby a closer molecular packing. As a result, the molecules form increasingly compact domains as the temperature increases. Since the head-group of C16E1 is much smaller than that of C16G, dehydration effect cannot appreciably increase its hydrophobic character. Rather, increases in subphase temperature result in a decrease in the line tension of the interface giving fractal structures at higher temperatures. In addition, the changes in enthalpy(Î”H) and entropy(Î”S) values were also calculated to understand the thermodynamic nature of condensation of the molecules in the LEâ€“LC transition region.
Keywords: Langmuir monolayers; Film balance; Brewster angle microscopy; Phase transition; Condensed-phase domains
Viscoelastic properties of insoluble amphiphiles at the air/water interface
by F. Miano; C.P. Winlove; D. Lambusta; G. Marletta (pp. 269-275).
The effects of the presence of a molecular monolayer on the dilatational properties of the air/water interface have been investigated. Two water insoluble amphiphiles, dipalmitoyl phosphatidyl choline and quercetin 3- O-palmitate, were spread onto a pendant drop and the dynamic surface pressure was measured by means of drop shape analysis. The surface dilatational elasticity and viscosity of the spread monolayers were also determined by the oscillating drop technique. Constraints on the range of measuring conditions were investigated and we demonstrated that the pressureâ€“area isotherms derived from oscillatory dynamic measurements display phase behaviour similar to that found in equilibrium measurements, albeit at reduced resolution. Both the amphiphiles formed purely elastic films that were characterised by a dilatational modulus that depended on the surface concentration and obeyed a power scaling law. The exponent of the relationship could be related to the thermodynamic conditions prevailing at the interface. The phospholipid monolayer scaling exponent was 2.8 in a temperature range of 20â€“26â€‰Â°C indicates a favourable solvency of molecules in the bidimensional matrix. A very high scaling exponent (11.8 at 7â€‰Â°C) for quercetin palmitate was interpreted assuming that molecules self-organise in fibre-like structures. This interface structure and the phase behaviour was found consistent with observations of the surface film obtained by Brewster angle microscopy. The structured quercetin 3- O-palmitate monolayers are disrupted by temperature increase or by adding a 0.2 molar fraction of the immiscible dipalmitoyl phosphatidyl choline.
Electrowetting-induced capillary flow in a parallel-plate channel
by Jiann H. Chen; Wen H. Hsieh (pp. 276-283).
This paper investigated, theoretically and experimentally, the electrowetting-induced capillary rise in a parallel-plate channel. The measured equilibrium height of the meniscus was proportional to the square of the applied potential. A model, based on the kinetic equation of capillary flow with the consideration of an electrowetting dynamic contact angle, was established to simulate the capillary rise. The effects of the electrostatic charge and the contact-line friction were linearly added to describe the electrowetting dynamic contact angle. The model was found to be able to adequately describe the experimental data under different initial heights and applied voltages. The non-Poseuille flow effect had little influence in the meniscus rising phenomenon studied in this work.
Keywords: Electrowetting; Dynamic contact angle; Capillary flow
Oscillatory loading of a viscoelastic adhesive contact
by J.A. Greenwood; K.L. Johnson (pp. 284-291).
The cycle of loading and unloading of a spherically-tipped probe against an adhesive, viscoelastic plane specimen is studied by numerical integration of the relations between crack speed and apparent surface energy previously found for a linear 3-element viscoelastic solid with a Maugisâ€“Dugdale law of force across the crack. It is found that even when the rate of loading is so slow that the loading and unloading curves almost coincide, suggesting purely elastic behaviour, the pull-off force can be appreciably greater than the elastic (JKR) value. When the normal force is modulated with a small amplitude sinusoidal variation during unloadingâ€”in order to find the contact stiffnessâ€”the contact radius barely changes, and the stiffness is close to that for a rigid flat punch instead of having the expected JKR value.
Keywords: JKR; Viscoelastic; Contact stiffness; Loading cycle; Surface energy
Molecular connectivity methods for the characterization of surface energetics of liquids and polymers
by M. Bortolotti; M. Brugnara; C. Della Volpe; D. Maniglio; S. Siboni (pp. 292-308).
In the topological approach to structureâ€“property relationships, the molecular structure is described in terms of appropriate weighted graphs to which suitable topological parameters, usually known as molecular connectivity indices, can be associated. Molecular connectivity indices are here applied to the prediction of surface free energy and Goodâ€“van Ossâ€“Chaudhury acidâ€“base components of organic compounds. To this aim, some quantitative structureâ€“property relationships (QSPRs) are determined, involving both topological indices and group indicator variables of the customary functional group theory. The semiempirical models obtained to appear satisfactory and show significant advantages with respect to the models based on the purely functional group approach.An example of the optimal prevision of the surface tension values for a common class of liquids, the branched hydro carbons, as obtained by the methods devised in the paper.
Keywords: QSPR; Topology; Molecular connectivity
Influence of temperature on the mixed micelles of Pluronic F127 and P103 with dimethylene- bis-(dodecyldimethylammonium bromide)
by Mandeep Singh Bakshi; Shweta Sachar (pp. 309-315).
The steady state fluorescence measurements have been performed on Pluronic F127 and P103 with dimethylene- bis-(dodecyldimethylammonium bromide) (12-2-12) mixtures at 21â€“40â€‰Â°C. From the pyrene fluorescence, the critical micelle concentration (cmc), micelle mole fraction ( x), micropolarity, and aggregation number have been computed for both mixtures over the whole mixing range at different temperatures. These micelle parameters indicate that the mixed micelle formation between the unlike components of both mixtures takes place due to the synergistic interactions and which increase with an increase in temperature.The steady state fluorescence measurements have been performed on Pluronic F127 and P103 with dimethylene- bis-(dodecyldimethylammonium bromide) (12-2-12) mixtures at 21â€“40â€‰Â°C. From the pyrene fluorescence, the critical micelle concentration (cmc), micelle mole fraction ( x), micropolarity, and aggregation number have been computed for both mixtures over the whole mixing range at different temperatures. These micelle parameters indicate that the mixed micelle formation between the unlike components of both mixtures takes place due to the synergistic interactions and which increase with an increase in temperature.
Keywords: Triblock polymers; Cationic Gemini surfactant; Mixed micelles; Temperature effect; Steady state fluorescence
Synthesis and characterization of new cationic quaternary ammonium polymerizable surfactants
by A. Samakande; P.C. Hartmann; R.D. Sanderson (pp. 316-323).
Two new polymerizable surfactants (surfmers), (11-acryloyloxyundecyl)dimethylethylammonium bromide (ethyl surfmer) and (11-acryloyloxyundecyl)dimethyl(2-hydroxyethyl)ammonium bromide (hydroxyethyl surfmer), were synthesized and characterized. The binary phase diagrams of both surfmer/water systems are described. Both surfmers can form isotropic solutions and lamellar lyotropic liquid crystalline phases. The hydroxyethyl surfmer/water system forms a lamellar phase for weight concentrations of surfmer between 70 and 90% relative to between 75 and 85% for the ethyl surfmer/water system. The differences in the self assembly of these surfmers were attributed to the ability of hydroxyethyl surfmer to form hydrogen bonds (between two head groups and with water), whereas no such interactions can occur with the ethyl surfmer system.
Keywords: Reactive surfactant; Surfmer; Phase transitions; Lyotropic liquid crystals
A fluorimetric and circular dichroism study of hemoglobinâ€”Effect of pH and anionic amphiphiles
by Swati De; Agnishwar Girigoswami (pp. 324-331).
In this work, bovine hemoglobin (Hb) has been studied mainly by the fluorescence method. pH has been found to exert a profound effect on Hb structure. This has been confirmed by fluorescence and circular dichroism (CD) studies. The pH-induced change in quaternary structure of Hb indirectly affects its secondary structure. This in turn affects ligand binding to Hb at various pH. The binding of two amphiphiles, a bile salt and a surfactant, have been investigated. The pH-induced structural modification of Hb has been confirmed by studies with the well-known denaturant urea and the polarity probe ANS, which has been used as an extrinsic fluorophore.
Microstructure of Î²-lactoglobulin-stabilized emulsions containing non-ionic surfactant and excess free protein: Influence of heating
by Sven Kerstens; Brent S. Murray; Eric Dickinson (pp. 332-341).
The influence of the non-ionic surfactant Tween 20 on the microstructure of Î²-lactoglobulin-stabilized emulsions with substantial excess free protein present was investigated via confocal microscopy. The separate distributions of oil droplets and protein were determined using two different fluorescent dyes. In the emulsion at ambient temperature the excess protein and protein-coated oil droplets were associated together in a reversibly flocculated state. The pore-size distribution of the initial flocculated emulsion was found to depend on the surfactant/protein ratio R, and at higher values of R the system became more inhomogeneous due to areas of local phase separation. Evidence for competitive displacement of protein from the oilâ€“water interface by surfactant was obtained only on heating (from 25 to 85â€‰Â°C) during the process of formation of a heat-set emulsion gel. By measuring fluorescence intensities of the protein dye inside and outside of the oil-droplet-rich areas, we have been able to quantify the evolving protein distribution during the thermal processing. The results are discussed in relation to previous work on the competitive adsorption of proteins and surfactants in emulsions and the effect of emulsion droplets on the rheology of heat-set protein gels.Confocal microscopy demonstrates emulsion reorganization and competitive displacement of protein by non-ionic surfactant on heating a Î²-lactoglobulin-stabilized emulsion containing substantial excess protein.
Keywords: Confocal laser scanning microscopy; Competitive displacement; Emulsion gel; Phase separation; Î²; -Lactoglobulin; Non-ionic surfactant; Whey protein; Tween 20
Comparison of morphology and mechanical properties of surfactant aggregates at waterâ€“silica and waterâ€“graphite interfaces from molecular dynamics simulations
by Kunal Shah; Patrick Chiu; Susan B. Sinnott (pp. 342-349).
Cationic surfactants are important for a wide range of applications, including controlled drug delivery systems, emulsifiers, and chemical mechanical polishing. It is therefore important to better understand surfactant structure and properties at the solidâ€“liquid interface. Here, classical molecular dynamics simulations with empirical potentials are used to compare the structures and mechanical properties of cationic surfactant micelles at hydrophobic (graphite) and hydrophilic (silica) surfaceâ€“water interfaces. In particular, the morphology of monolayers and bilayers of C12TAB ( n-dodecyltrimethylammoniumbromide) at these interfaces, and their responses to atomic force microscopy indentation, are examined. The simulations predict that surfactant monolayers and bilayers on silica evolve into a spherical micelle structure, in agreement with theoretical models of surfactant morphology. In contrast, surfactant monolayers on graphite evolve into a hemi-cylindrical structure, in agreement with experimental findings. In the simulated indentation of the micelle/silica system, the spherical micelle breaks apart and forms a surfactant monolayer. The indentation force curve has a maximum value of 2.25 nN. On the other hand, the simulated indentation of the micelle/graphite system causes the hemi-cylindrical micelle structure to break apart and the surfactant tails to wrap around the graphite indenter. The indentation force curve has a maximum value of 13 nN.
Keywords: Molecular dynamics simulations; Cationic surfactants (C; 12; TAB); Structure and shape of micelles; Mechanical properties of micelles; Hydrophobic (graphite) surface; Hydrophilic (silica) surface; AFM
Phase behaviors of AOT/longer chain n-alkanes reverse micelles in the presence of compressed ethylene
by Dong Shen; Yu Dong; Buxing Han; Jiawei Chen; Jianling Zhang (pp. 350-355).
It was found that, in a suitable pressure range, ethylene could increase the amount of solubilized water in reverse micelles of sodium bis-2-ethylhexylsulfosuccinate (AOT) in longer chain n-alkanes considerably, where the phase separation was induced by a micelleâ€“micelle interaction mechanism. The microenvironments in the ethylene-stabilized reverse micelles were investigated by the UVâ€“vis adsorption spectra using methyl orange (MO) as a probe. The maximum absorption of MO decreased with the increase of ethylene pressure at constantW0 value. Conductivity measurements demonstrated that the percolation temperature of the reverse micellar system increased considerably after compressed ethylene was added. The results of this work confirm that some small-molecule gases have the function of cosurfactants to stabilize reverse micelles.
Keywords: AOT; Reverse micelle; Ethylene
The use of acridine orange base (AOB) as molecular probe to characterize nonaqueous AOT reverse micelles
by R. Darío Falcone; N. Mariano Correa; M. Alicia Biasutti; Juana J. Silber (pp. 356-364).
The behavior of acridine orange base (AOB) in nonaqueous reverse micelles composed of n-heptane/AOT/polar solvent has been performed. Ethylene glycol (EG), propylene glycol (PG), glycerol (GY), formamide (FA), dimethylformamide (DMF), and dimethylacetamide (DMA) were employed as water substitutes. The studies were performed by static and time-resolved emission spectroscopy. Thus, the distribution of AOB between the two pseudophases of the aggregates was quantified by measuring the partition constants from emission spectra at different surfactant concentration. Similar values to those obtained by means of absorption spectroscopy were obtained. This match is indicating that AOB is not experiencing partition during the lifetime of the excited state. Partitioning to the micelles is strongly favored in micelles containing hydrogen-bond donor (HBD) solvents rather than non-HBD solvents. Variations of fluorescence lifetimes with AOT concentration confirm these results. By the solvatochromic behavior of AOB in the different systems it is shown that the microenvironment at the interface is distinct from that of the bulk polar solvent, indicating that the probe senses no â€œfreeâ€? solvent. The steady state anisotropy (âŒ©râŒª) was measured for EG/AOT/ n-heptane and DMF/AOT/ n-heptane systems as representatives for HBD and non-HBD polar solvents, respectively. The value ofâŒ©râŒª is higher in the micelles containing EG than that obtained with DMF, and increases with AOT concentration. This is explained as due to highly structured polar solvents in the inner core. EG is interacting with the polar heads of AOT through hydrogen-bond interaction, while DMF can only interact with the Na+ counterions. This is confirmed by the time-resolved emission spectra (TRES) of the probe in the micellar systems, in comparison with the bulk solvents.AOB is a very powerful micropolarity and microviscosity probe to investigate the interface of nonaqueous reverse micelles.
Effect of a neutral water-soluble polymer on the lamellar phase of a zwitterionic surfactant system
by Amir Maldonado; Ricardo López-Esparza; Raymond Ober; Thaddée Gulik-Krzywicki; Wladimir Urbach; Claudine E. Williams (pp. 365-369).
We have studied the effect of adding a water-soluble polymers (PEG) to the lamellar phases of the ternary system tetradecyldimethylaminoxide (C14DMAO)â€“hexanolâ€“water. The results of Freeze-Fracture Electron Microscopy (FFEM) and Small Angle X-ray Scattering (SAXS) experiments show that the addition of the polymer induces the spontaneous formation of highly monodisperse multilayered vesicles above a threshold polymer concentration.
Keywords: Surfactant; Polymer; Zwitterionic; Lamellar; Vesicle
Effect of temperature on the unfavorable mixing between tetraethylene glycol dodecyl ether and Pluronic P103
by Mandeep Singh Bakshi; Aman Kaura; Gurpreet Kaur (pp. 370-373).
The fluorescence measurements of tetraethylene glycol dodecyl ether (C12E4) and triblock polymer (Pluronic P103), poly(ethylene oxide)â€“poly(propylene oxide)â€“poly(ethylene oxide), (EO)17(PO)60(EO)17, binary mixtures have been performed over the whole mixing range in the temperature range of 20â€“40â€‰Â°C. The results have been evaluated by computing various micellar parameters and excimer formation. It has been concluded that mixed micelle formation takes place due to unfavorable mixing at lower temperature range, and the magnitude of which decreases with the increase in temperature up to 40â€‰Â°C. The reduction in the unfavorable mixing has been attributed to the dehydration of P103 micelles with the increase in temperature.
Keywords: Temperature effect; Triblock polymers; Non-ionic surfactants; Fluorescence measurements; Micellar parameters
Anisotropic Brownian diffusion near a nanostructured surface
by Michael R. King (pp. 374-376).
The Brownian diffusion of sub-micrometer-sized particles (diameter 0.52 Î¼m) in the vicinity of a nanostructured surface was experimentally characterized. The surface consisted of a repeating pattern of rectangular grooves with depth 35 nm and pitch 400 nm. It was found that the one-dimensional particle diffusivity parallel to the nanogrooves was significantly higher than the diffusivity perpendicular to the grooves (1.518Â±0.274 (S.E.) Î¼m2/s compared to0.704Â±0.090Â Î¼m2/s). No such anisotropy in the one-dimensional Brownian diffusivity was found for particles near a flat surface.
Keywords: Brownian diffusion; Nanostructured surface; Anisotropic diffusion
Comparative thermogravimetric and adsorption study of highly ordered mesoporous materials
by Lucildes P. Mercuri; Jivaldo R. Matos; Zuojiang Li; Mietek Jaroniec (pp. 377-380).
A comparative study was carried out for highly ordered mesoporous materials using high resolution thermogravimetry (HR-TG) and adsorption techniques. These materials were synthesized with mixed surfactants of various alkyl chain lengths. For thermogravimetry measurements n-butanol was used to probe the adsorbent surface and high-resolution TG curves as well as their 1st and 2nd derivatives were obtained for this probe molecule. It is shown that the values of the mesopore volume and specific surface area evaluated from TG data are in a good agreement with those evaluated from low-temperature nitrogen adsorption isotherms. This comparative study was performed to confirm the usefulness of the HR-TG technique as an alternative method for evaluation of the mesopore volume and specific surface area of ordered mesoporous materials.
Keywords: Thermogravimetry; BET surface area; MCM-41; Nitrogen adsorption; Butanol thermodesorption; Mesoporous materials
by Arthur Hubbard (pp. 381-381).
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