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Journal of Colloid And Interface Science (v.327, #1)

Editorial Board (pp. ofc).

Adsorption of ethoxylated styrene oxide and polyacrylic acid and mixtures there of on organic pigment by W.K. Wijting; J. Laven; R.A.T.M. van Benthem; G. de With (pp. 1-8).
The adsorption of two polymeric surfactants on an organic pigment was investigated. As surfactants the anionic polyacrylic acid sodium salt (PANa,MW=15,000) and a non-ionic block copolymer surfactant based on styrene oxide (SO) and ethylene oxide (EO) (MW=1500) were used. The adsorption behavior was analyzed by size exclusion chromatography of the supernatant after centrifugation of the pigment dispersions. It was found that PANa has no affinity to the pigment, whereas SO–EO has a strong affinity to the pigment surface. Competitive adsorption of PANa and SO–EO was not observed. Addition of SO–EO yields stable dispersions.In order to disperse pigment particles in paint systems, the adsorption of surfactants on such a pigment is investigated. Furthermore rheology of pigment dispersions is investigated.

Keywords: Adsorption; Colloids; Diblock-copolymer; Organic pigment; Polymer; Poly-electrolyte; Surface tension; Styrene oxide; Surfactant; Dispersant; Zeta-potential

Adsorption behaviors of three polymeric adsorbents with amide groups for phenol in aqueous solution by Man-Cai Xu; Yun Zhou; Jian-Han Huang (pp. 9-14).
Efficient removal of phenol and its derivatives from wastewater is of significant environmental concern. In the present study, the adsorption behaviors of three polymeric adsorbents with amide groups, PMVBA, PMVBC, and PMVBU, for phenol were investigated in aqueous solution. The adsorption in relation to the solution pH value was discussed and the neutral pH was optimum. The adsorption dynamic cure was determined and it was shown that the pseudo-second-order rate equation characterized the adsorption better and the intraparticle diffusion was the rate-limiting step. The adsorption isotherms were measured and correlated to Freundlich isotherms, and it was shown that PMVBU held the largest adsorption capacity. The adsorption thermodynamic parameters were calculated and it was seen that the adsorption enthalpy of phenol onto the three adsorbents follows the order PMVBU>PMVBA>PMVBC. Analysis of the adsorption mechanism suggested that hydrogen bonding played a predominant role and multiple hydrogen bonding was involved for phenol and PMVBU.Strong hydrogen bonding is formed between phenol and PMVBA (N,N-dimethylacetamide), PMVBC, and PMVBU ( N-methylurea andN,N′-dimethylurea), and multiple hydrogen bonding is involved for phenol and PMVBU.

Keywords: Polymeric adsorbent; Phenol; Adsorption; Hydrogen bonding

Sorption of uranyl ions on titanium oxide studied by ATR-IR spectroscopy by G. Lefèvre; J. Kneppers; M. Fédoroff (pp. 15-20).
ATR-IR spectroscopy was used to study the sorption of uranyl ions (10−4 M) onto titanium oxide (mixture of rutile and anatase). A circulation setup, filled with a solution in D2O, allowed recording of the evolution of the antisymmetric OUO stretching of uranyl species onto titanium oxide particles deposited on the ATR crystal. The band centered at 915 cm−1 has been decomposed in two Gaussian peaks at 920 and 905 cm−1. From these values, and the observation that the ratio of the areas of the two peaks vs pH was constant, we have proposed that uranyl sorption on titanium oxide in the pH range 4–7 leads to the formation of one surface complex where uranium atoms have two different chemical environments. A trimer surface complex linked by two uranium atoms to the titanium oxide surface would be consistent with this interpretation.

Keywords: ATR-IR; Uranium; Titanium oxide; Surface complex; Spectroscopy; D; 2; O

Concentration-resolved 2D correlation gel permeation chromatography study of aggregate–aggregate interactions in the polymerization products of triethoxysilyl-terminated polystyrene silane-coupling agent by Keita Suzuki; Jun-Ichi Oku; Ken-Ichi Izawa; Hiro-Fumi Okabayashi; Isao Noda; Charmian J. O'Connor (pp. 21-30).
Concentration-resolved 2D correlation gel permeation chromatography (GPC) has been used to examine the intricate details of the HCl-catalyzed polymerization of a polymeric silane-coupling agent (SCA), triethoxysilyl-terminated polystyrene (TESiPS). The concentration-resolved 2D correlation GPC maps directly reflect the marked difference in the aggregate–aggregate interactions of dilute and concentrated monomeric units, which govern the differences in the polymerized products. There is an optimum concentration of SCA for the enhancement of interfacial strength and subsequent polymerization. Thus, the concentration-resolved 2D correlation GPC technique can be used as a powerful tool for elucidation of aggregate–aggregate interactions and reaction mechanisms in a surface- or interface-enhanced reaction system. It has been shown that the yield value of polymerization products can be improved to a marked extent by choosing a high initial monomer concentration, due to the increase in the production of oligomers. Multiple reaction processes are promoted by the self-assembly of the monomeric and oligomeric components.In region I (lower C i), it is mainly the monomeric species and dimeric and trimeric components that coexist. As C i increases (region II″), the correlation of the oligomers strengthens.

Keywords: Concentration-resolved; 2D correlation; Gel permeation chromatography; Triethoxysilyl-terminated polystyrene; Aggregate–aggregate interactions; Polymerization; Oligomer; Yield; Mechanisms

Langmuir–Blodgett films of diazobenzene molecules by Josmary R. Silva; Nara C. de Souza; Vânia C. Fernandes; Paula Homem-de-Mello; Osvaldo N. Oliveira Jr. (pp. 31-35).
Langmuir–Blodgett (LB) films from diazobenzene Sudan III have been investigated using surface potential measurements as a function of number of layers and deposition pressures, with the surface potential data being related to molecular dipole moments obtained from theoretical electronic structure calculations. The surface potential increased with the number of layers for SIII LB films, and then tended to saturate. Results from density functional theory (DFT) and UV–vis spectroscopy indicated that the increase is due to addition of layers with oriented molecular dipoles, with the saturation tendency being attributed to a decrease in the amount of material deposited in each layer. The surface potential increased with the surface pressure used for deposition, probably owing to a higher contribution from the vertical component of the dipole moment as a closer molecular packing, which is associated with decreasing conformational entropy, was reached.Energy diagram of HOMO and LUMO for the diazobenzene Sudan III molecule (energies obtained with DFT calculations).

Keywords: Diazobenzene molecules; Langmuir–Blodgett films; Surface potential; Density functional theory

Kinetics of metal ions adsorption at heterogeneous solid/solution interfaces: A theoretical treatment based on statistical rate theory by Władysław Rudzinski; Wojciech Plazinski (pp. 36-43).
The statistical rate theory combined with a two-component competitive adsorption model is applied to describe the effect of pH on the kinetics of metal ions adsorption at energetically heterogeneous solid/solution interfaces. The surface heterogeneity has been represented by both Gaussian-like and rectangular functions of the adsorption energy distribution. A concept of effective heterogeneity parameters is found to represent very well the combined effects of surface energetic heterogeneity and of the electrostatic lateral interactions in the adsorbed phase, described by using the mean field approximation. The applicability of our approach is demonstrated by a quantitative analysis of two sets of experimental data reported in literature. Our theoretical expressions have been able to successfully correlate kinetic and equilibrium data in both these cases.The role of lateral interactions between adsorbed species can be expressed by introducing a so-called “effective surface heterogeneity” parameter.

Keywords: Sorption; Kinetics; Surface heterogeneity; Lateral interactions; pH

Controlled deposition of calcium carbonate particles on porous membranes by using alternating current system by Junji Watanabe; Mitsuru Akashi (pp. 44-50).
The deposition of calcium carbonate particles in the absence of additives was performed through the application of an alternating current. Solutions of calcium chloride and sodium carbonate were filled in glass cells, and a porous polymer membrane was interposed between the glass cells. An alternating current with a sine waveform (frequency: 10 Hz) was applied by using a platinum electrode. In this process, the reciprocal migration of calcium and carbonate ions toward the interface of the pores on the membrane takes place. Moreover, ion mixing occurs to a small extent at the interface of the pores. An alternating current was applied for 30 min during which the ion mixing continued. When observed using a scanning electron microscope, a majority of the formed calcium carbonate particles was found to exhibit a spheroidal vaterite crystal structure. This crystal structure was confirmed to be vaterite after characterization by X-ray diffraction. In the absence of an alternating current, only calcite was formed on the porous polymer membrane. Taking this result into account, it was concluded that the alternating current could induce reciprocal ion migration through the pores of the membrane, and vaterite deposition can be achieved. This is one of the methods employed for the calcium carbonate formation on the porous membrane by using an alternating current system.Vaterite and calcite were formed on both sides of porous membrane under alternating current.

Keywords: Calcium carbonate; Vaterite; Calcite; Polymorph; Alternating current

Influence of the functionalization pattern of ethyl cellulose on the interactions with polystyrene latex particles in aqueous mixtures by Alexandra Wallström Wennerstrand; Martin Olsson; Lars Järnström; Andreas Koschella; Dominik Fenn; Thomas Heinze (pp. 51-57).
The interactions between polystyrene latex particles and ethyl cellulose (EC) with different functionalization pattern have been investigated. 3-Mono- O-EC and EC with statistical functionalization pattern in the anhydroglucose units were studied in aqueous solutions and dispersions. EC belongs to a group of polymers that phase separate upon heating. The two types of EC showed large differences in phase separation temperature, which was explained as an effect of different interactions with water due to different functionalization pattern. Both types of EC did adsorb on polystyrene particles, which indicated a favorable interaction between EC and polystyrene latex particles, however, in a different manner depending on the structure of EC. The conventionally synthesized ethyl cellulose with statistical functionalization pattern formed much stronger networks with polystyrene latex particles than 3-mono- O-EC did. The lower phase separation temperature and the slightly higher molecular weight of the conventional ethyl cellulose gave it higher preference for interacting with polystyrene latex particles to form network. Throughout the study, comparison is made with other cellulose derivatives like ethyl(hydroxyethyl) cellulose (EHEC) and carboxymethyl cellulose (CMC).The functionalization pattern of ethyl cellulose was shown to have a substantial influence on loss modulus and other rheological properties of polystyrene suspensions to which the cellulose ether was added.

Keywords: Cellulose ethers; Polystyrene; Adsorption; Cloud point; Coating; Functionalization pattern; Pair correlation function; Rheology; Suspensions

Preparation of highly monodisperse fluorescent polymer particles by miniemulsion polymerization of styrene with a polymerizable surfactant by Tatsuo Taniguchi; Naoki Takeuchi; Shotaro Kobaru; Takayuki Nakahira (pp. 58-62).
Miniemulsion polymerization of styrene (St) in the presence of a hydrophobe (hexadecane:HD) using a cationic polymerizable surfactant (N,N-dimethyl-N-n-dodecyl- N-2-methacryloyloxyethylammonium bromide:C12Br) and a cationic initiator (2,2′-azobis(2-amidinopropane) dihydrochloride:V50), called St/C12Br/V50 hereafter, proceeded efficiently compared with that using sodium dodecyl sulfate (SDS) and potassium persulfate (KPS), i.e., St/SDS/KPS, providing monodisperse polystyrene latex particles with a narrower particle size distribution. In St/C12Br/AIBN, where an oil-soluble initiator, i.e., 2,2′-azobisisobutyronitrile (AIBN), was used in place of V50, little changes in polymerization kinetics or in particle size distribution were observed, while a significant drop in polymerization rate and a broad particle size distribution were observed with St/SDS/AIBN. A polymerizable pyrene derivative (1-pyrenylmethyl methacrylate: PyMMA) was quantitatively incorporated into monodisperse latex particles in St/PyMMA/C12Br/V50 compared to pyrene (Py) in St/Py/C12Br/V50. Contrary to our expectation, however, increased excimer emission was observed with St/PyMMA/C12Br/V50 particles, indicating less evenly distributed pyrene chromophores in the particles. The fluorescence lifetime of pyrene chromophores in St/Py/C12Br/V50 particles was determined to be 286 ns, which was 17 times longer than that of pyrene in THF solution.Highly monodisperse fluorescent polymer particles containing pyrene fluorophores were prepared by miniemulsion polymerization of styrene with a polymerizable surfactant.

Keywords: Miniemulsion polymerization; Polymerizable surfactant; Pyrene; Fluorescence lifetime

Preparation and characterization of asymmetric planar supported bilayers composed of poly(bis-sorbylphosphatidylcholine) on n-octadecyltrichlorosilane SAMs by Saliya N. Ratnayaka; Ronald J. Wysocki Jr.; S. Scott Saavedra (pp. 63-74).
Planar supported lipid bilayers (PSLBs) have been widely studied as biomembrane models and biosensor scaffolds. For technological applications, a major limitation of PSLBs composed of fluid lipids is that the bilayer structure is readily disrupted when exposed to chemical, mechanical, and thermal stresses. A number of asymmetric supported bilayer structures, such as the hybrid bilayer membrane (HBM) and the tethered bilayer lipid membrane (tBLM), have been created as an alternative to symmetric PSLBs. In both HBMs and tBLMs, the inner monolayer is covalently attached to the substrate while the outer monolayer is typically composed of a fluid lipid. Here we address if cross-linking polymerization of the lipids in the outer monolayer of an asymmetric supported bilayer can achieve the high degree of stability observed previously for symmetric PSLBs in which both monolayers are cross-linked [E.E. Ross, L.J. Rozanski, T. Spratt, S.C. Liu, D.F. O'Brien, S.S. Saavedra, Langmuir 19 (2003) 1752]. To explore this issue, HBMs composed of an outer monolayer of a cross-linkable lipid, bis-sorbylphosphatidylcholine (bis-SorbPC), and an inner SAM were prepared and characterized. Several experimental conditions were varied: vesicle fusion time, polymerization method, and polymerization time and temperature. Under most conditions, bis-SorbPC cross-linking stabilized the HBM such that its bilayer structure was largely preserved after drying; however these films invariably contained sub-micron scale defects that exposed the hydrophobic core of the HBM. The defects appear to be caused by desorption of low molecular weight oligomers when the film is removed from water, rinsed, and dried. In contrast, poly(bis-SorbPC) PSLBs prepared under similar conditions by Ross et al. were nearly defect free. This comparison shows that formation of a cross-linked network in the outer leaflet of an asymmetric supported bilayer is insufficient to prevent lipid desorption; inter-leaflet covalent linking appears to be necessary to create supported poly(lipid) assemblies that are impervious to repeated drying and rehydration. The difference in stability is attributed to inter-leaflet cross-linking between monolayers which can form in symmetric bis-SorbPC PSLBs.Figure: Asymmetric bilayer composed of poly(lipid) on SAM.

Keywords: Poly(lipid); Lipo-polymer; Asymmetric supported bilayer; Hybrid bilayer membrane; Lipid polymerization; Bis-SorbPC; Self-assembled monolayer; SAM

The antimicrobial reagent role on the degradation of model cellulose film by D. Jausovec; D. Angelescu; B. Voncina; T. Nylander; B. Lindman (pp. 75-83).
The effect of the antimicrobial agent TMPAC (3-(trimethoxysilyl)-propyldimethyloctadecyl ammonium chloride) on the cellulase activity on model cellulose substrate was investigated by in situ-null ellipsometry. The cellulases used were extracted from Trichoderma viride and Aspergillus niger, and the model cellulose film was prepared by spin-coating silicon oxide wafers with cellulose solubilized in N-methylmorpholine- N-oxide/dimethyl sulfoxide solution. Upon enzyme addition to the previously equilibrated cellulose film, the initial enzyme adsorption on the substrate was followed by an overall decrease in film mass owing to enzymatic digestion of the cellulose. The loss of cellulose film mass was associated with a non-monotonously behavior of the cellulose film thickness. The activities of the two enzymes were different, a much higher degradation rate being observed for the Trichoderma viride cellulase. The degradation rate with this cellulase decreased significantly when the cellulose film was treated with the antimicrobial agent. The antimicrobial agent did not affect the cellulose degradation catalyzed by the Aspergillus niger cellulase. It was, hence, demonstrated for the first time that, depending on the cellulase type, the antimicrobial agent can inhibit enzymatic activity at the solid–liquid interface.The effect of the antimicrobial agent TMPAC (3-(trimethoxysilyl)-propyldimethyloctadecyl ammonium chloride) on the cellulase activity on model cellulose substrate was investigated by in situ-null ellipsometry.

Keywords: Antimicrobial agent; Model cellulose film; Enzymatic degradation; Ellipsometry

Effect of clay aggregation on water diffusivity using low field NMR by Xavier Guichet; Marc Fleury; Eric Kohler (pp. 84-93).
Water diffusivity D measured by using NMR techniques in Na-smectite suspensions decreases with increasing smectite fraction (up to 50 wt%), but increases with increasing salinity (NaCl or CaCl2 aqueous solutions) at a fixed clay fraction. The increase, larger for CaCl2 solutions, is explained by aggregation of clay particles when high salinities are reached. Macroscopic organisation of dense mixtures of clay and aqueous solutions can be inferred byT2 transverse NMR relaxation times which are sensitive to the volume to surface ratio. Dispersed suspensions exhibit mono-modalT2 distributions, whereas bimodalT2 distributions are observed for flocculated systems. The bimodalT2 distributions are interpreted as a measurement of the spacing between clay particles within aggregates and between aggregates. Finally, the diffusion data can be gathered in an unique curve using the Debye length and the measured spacing between particles. When the thickness of the electro-diffuse layer (Debye length) is of the same order as the spacing between clay particles, the water diffusivity decreases. Otherwise it is constant at about2.22±0.25×10−9 m2/s. This last result illustrates clearly the effect of electro-chemical properties of smectite on water diffusivity.Normalised water diffusivity versus the ratioRl of the two characteristic lengths of the suspension, i.e. the apparent spacing between clay particles divided by the Debye length.

Keywords: Smectite; Clay particles; Colloid materials; Transport properties; Diffusion; NMR; Electro-chemical properties; Microstructure; Porous materials

Novel dimeric cholesteryl-based A(LS)2 low-molecular-mass gelators with a benzene ring in the linker by Min Xue; Kaiqiang Liu; Junxia Peng; Qiuhong Zhang; Yu Fang (pp. 94-101).
Three novel dimeric cholesteryl-based A(LS)2 low-molecular-mass organic gelators (LMOGs) with phthaloyl, isophthaloyl, or terephthaloyl moieties in the linkers were designed and prepared. According to the linker structures, the compounds are denoted as1 ( o-),2 ( m-), and3 ( p-), respectively. Gelation tests revealed that the difference of relative positions of two cholesterol moieties in the benzene ring can produce a dramatic change in the gelation behaviors of the compounds. Importantly,2 and3 are more efficient gelators than1, and their self-assembly behaviors are also very different from each other as revealed by scanning electron microscopy (SEM) measurements. Very interestingly,2 gels xylene spontaneously at room temperature, and the sol–gel phase transition of the system is mechanically controllable. FTIR and1H NMR spectroscopy studies revealed that hydrogen bonding andπ–π interactions between the molecules of the gelators play an important role in the formation and maintenance of the gels. The X-ray diffraction (XRD) analysis revealed that in the gel of2/benzene,2 aggregated into a layered structure with an interlayer distance of 3.54 nm, which is just the length of2.Difference of the relative positions of two cholesterol moieties in a benzene ring can produce a dramatic change in gelation behavior. More interestingly, a distinct thixotropic gel system was discovered.

Keywords: Supramolecular gels; Gelator; Cholesterol; A(LS); 2

Template assisted synthesis of silica-coated molecular crystal nanorods: From hydrophobic to hydrophilic nanorods by Rabih O. Al-Kaysi; Robert J. Dillon; Lingyan Zhu; Christopher J. Bardeen (pp. 102-107).
We report a method for the preparation of silica-coated molecular crystal nanorods. A sol–gel method was used to make silica nanotubes inside anodized alumina templates. The nanotubes were then loaded with 9-anthracene carboxylic acid (9-AC) and solvent annealed to produce silica-coated organic nanorods. The core–shell structure was confirmed using electron microscopy, and the highly crystalline organic core was characterized using powder X-ray diffraction and transmission electron microscopy. The silica-coated 9-AC rods had much improved dispersal properties in aqueous solution, and were also able to undergo reversible bending under UV illumination, as observed previously for uncoated 9-AC rods. This work demonstrates that it is possible to make surface-coated molecular crystal nanorods that retain their useful functionalities.

Keywords: Templated materials; AAO; Silica nanotubes; Photoresponsive nanorods; Hydrophobic and hydrophilic

Effects of raw material texture and activation manner on surface area of porous carbons derived from biomass resources by Feng Zhang; Guo-Dong Li; Jie-Sheng Chen (pp. 108-114).
Porous carbons have been prepared from biomass resources, such as cornstalks, rice straws, pine needles and pinecone hulls, through a simple carbonization and KOH solution activation process. The pore sizes of the obtained porous carbons are mainly distributed in the range of 1–2 nm, whereas the surface areas of the materials vary from 1000 to more than 3000 m2 g−1 depending on the raw materials and preparation conditions. It is found that the biomass texture and the activation manner play key roles in determination of surface areas of the porous carbons. In addition, the amount of activation agent, the activation temperature and the activation time also affect the surface area of the porous carbons but to a less extent. The obtained porous carbons with high surface areas show good performance when used as Pt-catalyst supports for cinanamaldehyde hydrogenation.Porous carbons, with main pore sizes of 1–2 nm and surface areas of 1000 m2 g−1 to more than 3000 m2 g−1, have been successfully prepared from biomass resources.

Keywords: Biomass; Texture; Porous carbon; Activation manner; Catalyst support

Improved capacitance characteristics of electrospun ACFs by pore size control and vanadium catalyst by Ji Sun Im; Sang-Wook Woo; Min-Jung Jung; Young-Seak Lee (pp. 115-119).
Nano-sized carbon fibers were prepared by using electrospinning, and their electrochemical properties were investigated as a possible electrode material for use as an electric double-layer capacitor (EDLC). To improve the electrode capacitance of EDLC, we implemented a three-step optimization. First, metal catalyst was introduced into the carbon fibers due to the excellent conductivity of metal. Vanadium pentoxide was used because it could be converted to vanadium for improved conductivity as the pore structure develops during the carbonization step. Vanadium catalyst was well dispersed in the carbon fibers, improving the capacitance of the electrode. Second, pore-size development was manipulated to obtain small mesopore sizes ranging from 2 to 5 nm. Through chemical activation, carbon fibers with controlled pore sizes were prepared with a high specific surface and pore volume, and their pore structure was investigated by using a BET apparatus. Finally, polyacrylonitrile was used as a carbon precursor to enrich for nitrogen content in the final product because nitrogen is known to improve electrode capacitance. Ultimately, the electrospun activated carbon fibers containing vanadium show improved functionality in charge/discharge, cyclic voltammetry, and specific capacitance compared with other samples because of an optimal combination of vanadium, nitrogen, and fixed pore structures.Specific capacitances of samples at different current density. It is clear that the development of micro- and mesoporosity by chemical activation leads to a fivefold increase in specific capacitance.

Keywords: EDLC; Electrode; Carbon; Electrospinning; Vanadium pentoxide

Studies on wettability of polypropylene/methyl-silicone composite film and polypropylene monolithic material by W.X. Weixin Hou; Bo Mu; Q.H. Qihua Wang (pp. 120-124).
A polypropylene/methyl-silicone superhydrophobic surface was prepared using a simple casting method. Varying the ratio of polypropylene and methyl-silicone results in different surface microstructure. The wetting behavior of the as-prepared surface was investigated. A polypropylene monolithic material was also prepared. Its superhydrophobicity still retains when the material was cut or abraded. The as-prepared material can also be used to separate some organic solvents from water.

Keywords: Superhydrophobic; Monolithic material; Separation

The transition from inertial to viscous flow in capillary rise by N. Fries; M. Dreyer (pp. 125-128).
We investigate the initial moments of capillary rise of liquids in a tube. In this period both inertia and viscous flow losses balance the pressure generated by the meniscus curvature (capillary pressure). It is known that the very first stage is purely dominated by inertial forces, where subsequently the influence of viscosity increases (visco-inertial flow). Finally the effect of inertia vanishes and the flow becomes purely viscous. In this study we derive the times and meniscus heights at which the transition between the time periods occur. This is done in an attempt to provide a method to determine a priori which terms of the momentum balance are relevant for a given problem. Analytic solutions known from previous literature are discussed and the time intervals of their validity compared. The predicted transition times and the calculated heights show good agreement with experimental results from literature. The results are also discussed in dimensionless form and the limitations of the calculations are pointed out.The times and heights of transition from inertial to viscous flow are examined.

Keywords: Capillary tube; Analytic solution; Capillary rise; Lucas–Washburn equation; Washburn equation; Imbibition; Liquid penetration; Flow regimes

Thermo-responsiveness of poly(N,N-diethylacrylamide) polymers at the air–water interface: The effect of a hydrophobic block by Amélia M.P.S. Gonçalves da Silva; Sónia I.C. Lopes; Pedro Brogueira; Telmo J.V. Prazeres; Mariana Beija; José M.G. Martinho (pp. 129-137).
The poly(N,N-diethylacrylamide) (h-PDEA) homopolymer and the poly( N-decylacrylamide)- b-PDEA (PDcA11- b-PDEA231) diblock copolymer were studied in the range of 10 to 40 °C, at the air–water interface. Theπ–A isotherms of h-PDEA appear nearly invariant with temperature while theπ–A isotherms of PDcA11- b-PDEA231 deviate significantly to lower areas with the temperature increase evidencing the thermo-responsiveness of this diblock copolymer at the interface. For the copolymer, the limiting area per segment versus temperature shows a break point around 29 °C, slightly lower than the lower critical solution temperature (LCST) of h-PDEA in water (31–33 °C). AFM images of LB monolayers transferred at 40 °C revealed for both polymers the presence of hydrophobic aggregates due to the conformational changes (collapse) of chains that occur at the LCST. Differences in the morphology of these aggregates, flat irregular structures for h-PDEA and round-shaped domains for PDcA11- b-PDEA231, were related with the condensing effect of the hydrophobic block. The PDcA11 block, anchoring the polymer to the interface, ensures a better stability and cohesion to the film and preserves the thermo-sensitivity of the h-PDEA at the interface.Aggregates of variable size were observed at the air–water interface above the LCST of the PDcA11- b-PDEA231 thermo-responsive copolymer.

Keywords: Thermo-responsive polymers; Copolymers; LB films; Interfaces; Poly(; N; ,; N; -diethylacrylamide); Lower critical solution temperature (LCST)

The effect of particle hydrophobicity, separation distance and packing patterns on the stability of a thin film by G. Morris; M.R. Pursell; S.J. Neethling; J.J. Cilliers (pp. 138-144).
Hydrophobic particles attached to bubble films in foams can increase the capillary pressure required to cause coalescence or bursting. Previous studies have considered the effects of changing particle spacing and contact angle in 2 dimensions (2D), but there are limitations to this approach; in 2D when the separation distance is zero and the particles are touching, the critical capillary pressure tends to infinity as there is no exposed film. In 3 dimensions (3D) spherical packing ensures that there are always exposed sections of film between particles even when they are close packed. Using Surface Evolver, the effects of contact angle and particle separation on the stability of a particle laden film were investigated in 2D and 3D. The 2D model was compared and validated with an analytical approach developed by Ali et al. [Ind. Eng. Chem. Res. 39 (2000) 2742–2745] and a 3D model was used to investigate the critical capillary pressures of square and hexagonal packing of monodisperse particles. It was found that when the stability of the film was compared with the area of film per particle both packing patterns have the same stability.The effect of particle separation distance on the capillary pressure required to rupture a thin film was modelled in 3D. The film surface formed a complex shape.

Keywords: Particles; Films; Hydrophobicity; Stabilisation; Flotation; Capillary pressure; Surface Evolver

Capillary rise of a meniscus with phase change by Guy Ramon; Alexander Oron (pp. 145-151).
The Lucas–Washburn equation, describing the motion of a liquid body in a capillary tube, is extended to account for the effect of phase change – evaporation or condensation. The system is found to always possess a stable equilibrium state when the temperature jump across the interface is confined to a certain range. We show that phase change affects the equilibrium height of the meniscus, the transition threshold from monotonic to oscillatory dynamics, and the frequency of oscillations, when present. At higher mass transfer rates and/or large capillary radii, vapor recoil is found to be the dominant factor. Evaporation lowers the equilibrium height, increases the oscillation frequency and lowers the transition threshold to oscillations. For condensation, two regimes are identified: at high mass transfer rates similar trends to those of evaporation are observed, whereas the opposite is found for low mass transfer rates, resulting in an increased equilibrium height, lower oscillation frequencies and a shift of the transition threshold toward monotonic dynamics.

Keywords: Capillary rise; Evaporation; Condensation; Vapor recoil; Lucas–Washburn equation

Effect of counterion substitution on the viscosity anomaly in AOT microemulsions by Xiaoming Pan; Surita R. Bhatia (pp. 152-156).
AOT-based water-in-oil microemulsions display an anomalous maximum in the viscosity with X, the water to surfactant ratio. Several explanations for this phenomenon have been offered. In this work, we investigate viscosity and droplet interactions in Ca(AOT)2/water/ n-decane and KAOT/water/ n-decane microemulsions and compare our results with the commonly studied NaAOT/water/oil system. The Ca(AOT)2 system demonstrates a maximum in relative viscosity and droplet attraction nearX=15, similar to the NaAOT system, although the maximum occurs at a higher value of X in the Ca(AOT)2 system. By contrast, the viscosity and interparticle interactions in the KAOT system do not strongly depend on the amount of water in the system. We attribute the differences in behavior between the two systems to different hydration characteristics of the counterion, and we believe that our results are consistent with a previously proposed model that attributes interdroplet attractions to charge fluctuations and surfactant exchange. Our findings support the connection between the viscosity anomaly and interparticle interactions.

Keywords: AOT; Microemulsion; Counterion; Hydration; Calcium; Potassium; Viscosity

Formation of micelles of Pluronic block copolymers in PEG 200 by Chaoxing Zhang; Jianling Zhang; Wei Li; Xiaoying Feng; Minqiang Hou; Buxing Han (pp. 157-161).
The formation of micelles of Pluronic block copolymers in poly(ethylene glycol) (PEG) was studied using fluorescence, solubilization measurements, and frozen fracture electron microscopy (FFEM) methods at 40 °C. It was discovered that surfactants L44 (EO10PO23EO10), P85 (EO26PO40EO26), and P105 (EO37PO56EO37) can form micelles in PEG 200 (PEG with a nominal molecular weight of 200), and the critical micellization concentration (CMC) decreases with increasing molecular weight of the surfactants. The size of the micelles formed by these Pluronic block copolymers is in the range of 6–35 nm. The CMC values in PEG 200 are higher than those in aqueous solutions.PEO–PPO–PEO block copolymers can form micelles in poly(ethylene glycol) with an average molecular weight of 200 g/mol.

Keywords: Micellization; PEG 200; CMC; Pluronics; L44; P85; P105

Mixed monolayers of Gemini surfactants and stearic acid at the air/water interface by Rong Li; Q.B. Qibin Chen; D.H. Dazhi Zhang; H.L. Honglai Liu; Ying Hu (pp. 162-168).
The properties of mixed monolayers composed of the cationic Gemini surfactant ([C18H37(CH3)2N+(CH2)3N+(CH3)2C18H37],2Br+, abbreviated as 18-3-18,2Br−1) and stearic acid (SA) at the air/water interface were investigated by using a Langmuir film balance. The excess areas at the different mixed monolayer compositions were obtained and used to evaluate the miscibility and nonideality of mixing. Due to the electrostatic attractive interactions between 18-3-18,2Br−1 and SA, the excess areas indicated negative deviations from ideal mixing. Moreover, 18-3-18,2Br−1 and SA were miscible at the air/water interface, as was confirmed by atomic force microscopy (AFM) images of the LB films transferred onto mica substrates. The attenuated total reflectance (ATR) infrared spectra showed that SA in the mixed monolayers was ionized completely at a compositionXSA=0.67 and formed a “cationic–anionic surfactant,” i.e., the carboxylate, with 18-3-18,2Br−1 owing to the electrostatic interaction between the head groups.Schematic structure of the “cationic–anionic surfactant” formed by stearic acid and a cationic Gemini surfactant.

Keywords: Gemini surfactant; Mixed monolayer; Stearic acid; LB film

Method for analysis of the composition of acid soaps by electrolytic conductivity measurements by Peter A. Kralchevsky; Mariana P. Boneva; Krassimir D. Danov; Kavssery P. Ananthapadmanabhan; Alex Lips (pp. 169-179).
Here, we propose a method for determining the stoichiometry of acid-soap crystallites. The method is based on dissolving the crystallites in water at an appropriate working temperature, followed by measurement of the electrolytic conductivity of the obtained solutions. The working temperature is chosen in such a way that the only precipitate in the solutions is that of carboxylic acid, whereas the carboxylate salt is dissociated, and its content in the dissolved crystals determines the solution's conductivity. In the theoretical model for data interpretation, we took into account the dependence of the molar conductance on the ionic strength. The method was applied for determining the stoichiometry of acid-soap crystals collected from solutions of potassium myristate (tetradecanoate) at 25 °C. The crystals were dissolved in water at working temperature of 40 °C, at which the conductivity was measured. The stoichiometry of all samples determined in the present study coincides with that independently obtained by another method that is based on in situ pH measurements.The acid-soap crystallites that are spontaneously formed in carboxylate soap solutions represent complexes of j molecules carboxylic acid and n molecules neutral soap. A method is proposed for determining their stoichiometry,j:n, by electrolytic conductivity measurements.

Keywords: Carboxylate soap solutions; Acid-soap crystallites; stoichiometry; Potassium myristate; Electrolytic conductivity; Molar ionic conductances

Wormlike micelles in poly(oxyethylene) surfactant solution: Growth control through hydrophilic-group size variation by Toufiq Ahmed; Kenji Aramaki (pp. 180-185).
Viscoelastic micellar solutions are formed in poly(oxyethylene) cholesteryl ether (ChEO m,m=15, 30) aqueous solutions on addition of tri(ethyleneglycol) mono n-dodecyl ether (C12EO3). The steady-shear and dynamic rheological behavior of the systems is characteristic of wormlike micellar solution. In either system, the plateau modulus (G0) and relaxation time ( τ) are found to increase with increasing cosurfactant mixing fractions. The plateau modulus of the ChEO30–C12EO3 system at the maximum viscosity region is found to be higher than that in the ChEO15–C12EO3 system at the maximum viscosity region, whereas for the relaxation time the opposite relation is found. The maximum viscosities obtained in the two systems are of the same order of magnitude. In the ChEO30–C12EO3 system, the maximum viscosity is obtained at a higher cosurfactant mixing fraction than that in the ChEO15–C12EO3 system. It is concluded that decreasing the head-group size of the hydrophilic surfactant favors micellar growth. Monolaurin, another hydrophobic surfactant known to induce growth in some systems, is found to cause phase separation before significant micellar growth occurs in ChEO m solutions, although the effect of head-group size of ChEO m is found to be similar to the ChEO m–C12EO3 systems.The one-dimensional micellar growth occurring on addition of hydrophobic cosurfactant to solutions of hydrophilic surfactant can be controlled by changing the head-group sizes of the hydrophilic surfactant over a large range.

Keywords: Poly(oxyethylene) cholesteryl ether; Mixed surfactant system; Nonionic surfactants; Wormlike micelles; Rheology; Maxwell model; Viscoelasticity

Thermodynamics of mixed micelles: Determination of the aggregate composition by Pierre Letellier; Alain Mayaffre; Mireille Turmine (pp. 186-190).
In most studies concerning surfactant mixtures, the determination of the composition of mixed micelles is often tricky. This composition can be obtained by different ways. One of them, undoubtedly the most used, supposes, a priori, that the surfactant in the micelle follows the regular solution model. This poses a problem on the thermodynamic point of view because in these studies, a model of behavior is first admitted for deducing a composition. In a correct thermodynamic approach, a composition should first be determined and then, a model of behavior which accounts for the observed physicochemical properties can be found. This approach is all the more questionable since the application of the Gibbs–Duhem relationship to the pseudo-phase allows aggregate composition to be determined simply, without using a solution model, because the composition of the bulk at the threshold of aggregation is known. In this article, we describe and validate a simple procedure, which supplements that proposed by Rodenas et al. [E. Rodenas, M. Valiente, M.D. Villafruela, J. Phys. Chem. B 103 (1999) 4549], and which allows determination of the activities of the components of the micellar aggregate and its composition. The results are compared to those obtained with other approaches such as molecular–thermodynamic model.The composition of a mixed micelle made of a surfactant 2 (DTABr in this example) and a surfactant 3 (Brij 35 here) can be simply calculated by applying the Gibbs–Duhem relationshipdlna3=−1−x3x3dlna2.

Keywords: Aggregates; Surfactants; Mixed micelles; Composition

Thermodynamics and bending energetics of toruslike micelles by L. Magnus Bergström (pp. 191-197).
The self-assembly of surfactants forming toruslike or toroidal micelles has been investigated from a theoretical point of view, in particular the structural behaviour and stability of tori in terms of the three bending elasticity constants spontaneous curvature (H0), bending rigidity (kc) and saddle-splay constant (k¯c). It is demonstrated that the size of toruslike micelles increases with an increasing bending rigidity, but is independent of both spontaneous curvature and saddle-splay constant. Similar to conventional micelles, toruslike micelles are found to be stable over bilayers as the spontaneous curvature times the surfactant layer thickness exceeds 1/4. Moreover, it is shown that toruslike micelles, in general, are favoured at the expense of long spherocylindrical micelles as a result of elimination of the unfavourable end-caps. However, conventional micelles that are able to grow with respect to both width and length (tablets) may be stable over tori as well as spheres in much wider regimes of different bending elasticity constants. As a result, toruslike micelles are predicted to be stable over conventional micelles, including rods, at large values of the effective bending constantkeff≡2kc+k¯c, i.e. in the same region where infinite cylinders are expected to be observed. This result is consistent with the fact that toruslike micelles have usually been observed to coexist with large networks of branched cylinders.Predominance diagram showing different geometrical shapes of micelles in terms of the bending elasticity constants. Toruslike micelles are predicted to be stable over conventional micelles at large values of the effective bending constant.

Keywords: Surfactants; Self-assembly; Bending elasticity; Toroidal micelles

Aggregation and adsorption properties of sodium dodecyl sulfate in water–acetamide mixtures by D. Das; K. Ismail (pp. 198-203).
The critical micelle concentration (cmc) of sodium dodecyl sulfate was determined in water + acetamide media from 0 to 70 wt% of acetamide and at temperatures in the range from 20 to 40 °C by using conductance, surface tension, and fluorescence methods. The cmc increases with increase in acetamide concentration and the reported [M.S. Akhter, Colloids Surf. A 121 (1997) 103] decrease in cmc was not observed. The limiting surface tension at the cmc does not have any dependence on the amount of acetamide added. The cmc data as a function of temperature were used to estimate the free energy, enthalpy, and entropy terms for micellization. Enthalpy–entropy compensation takes place during micellization. Counterion binding constant, surface excess, and aggregation number of SDS decrease with increasing acetamide concentration and become almost constant for weight percentages of acetamide greater or equal to 30. Pyrene appears to move from the interior of the SDS micelle to the micellar interface at about 30 wt% acetamide. The empirical relations reported by Aguiar et al. [J. Aguiar, P. Carpena, J.A. Molina-Bolivar, C. Carnero Ruiz, J. Colloid Interface Sci. 258 (2003) 116] between the parameters of a sigmoid-type expression for the ratio of fluorescence emission intensities of pyrene and surfactant properties are found to be applicable to SDS in water + acetamide medium below 20 wt% acetamide only. Standard free energy of micellization has linear correlations with reciprocal of dielectric constant and Gordon parameter of the solvent. The water + acetamide medium behaves similar to mixed solvents containing water and any polar liquid nonaqueous solvent and this study highlights the significance of solvophobicity.The critical micelle concentration of SDS increases on increasing the level of acetamide in an aqueous medium. On the other hand, the surface excess decreases and becomes constant above 30 wt% acetamide. Solvophobicity controls the adsorption and micellization in these mixed solvents.

Keywords: Sodium dodecyl sulfate; Water–acetamide mixture; Critical micelle concentration; Polarity; Aggregation number; Surface excess

Stability and rheological behavior of concentrated monodisperse food emulsifier coated microbubble suspensions by Yuyi Shen; Marjorie L. Longo; Robert L. Powell (pp. 204-210).
Nearly monodispersed populations of microbubbles were produced using flow focusing with a food grade emulsifier. The microbubbles produced by this technique have diameters in the range of 120–200 μm. The flow focusing device uses metered streams of air and liquid to produce a jet that periodically pinches to make individual microbubbles. The size of the microbubbles can be controlled by changing the relative flow rates of the gas and the liquid. The emulsifier consists of a mixture of monoglycerides, diglycerides and sodium stearoyl lactylate in combination with polyethylene glycol (PEG)-40 stearate. The emulsifier forms a thin shell that stabilizes the microbubbles. The microbubbles are stable over time with their sizes remaining roughly constant over 2 h. Such stability allows suspensions of microbubbles to be formed and their rheological properties tested. The sizes of the microbubbles are also monitored off-line while testing, examining the effect of shearing on the bubble sizes, as well as their stability over time. These results show that the microbubble suspensions are viscoelastic and exhibit power law behavior. The relationship between the air fraction of the suspension and fluid viscosity is determined.

Keywords: Microbubble; Rheology; Monodisperse; Viscosity; Viscoelasticity

Two strategies for the self-assembly of gold nanoparticles: Photoreaction and radical reaction by Sung Huh; Boknam Chae; Seung Bin Kim (pp. 211-215).
Gold nanoparticles modified with a newly synthesized cinnamate moiety (CI-AuNPs) were prepared using the phase method. The cinnamate moiety, which is well known for its photoreactive properties, could be reacted by means of radical attack and UV irradiation. Crosslinking of this moiety by the two reactions causes aggregation of the CI-AuNPs, which demonstrates the feasibility of fabricating self-assembled spherical structures by means of chemical crosslinking of gold nanoparticles through two distinct mechanisms, namely, photocycloaddition and radical reaction upon addition of an initiator. Both methods yielded monodispersed spherical CI-AuNPs assemblies, and especially the radical-induced system showed the ability to construct linear-chain structures. Here, we suggest the preparation of gold nanoparticles modified with a simply synthesized photoreactive group and their self-assembly via two selective strategies (photoreaction and radical reaction). We expect that this study will contribute to the strategic fabrication of self-assembled nanostructures.Gold nanoparticles modified with cinnamate group (CI-AuNPs) build up the suprastructures upon UV irradiation and radical reaction.

Keywords: Gold nanoparticles; Cinnamate moiety; Photoreaction; Radical reaction; Assembled gold nanoparticles

The effect of shear on colloidal aggregation and gelation studied using small-angle light scattering by Tahereh Mokhtari; A. Chakrabarti; Christopher M. Sorensen; Chung-yin Cheng; Dennis Vigil (pp. 216-223).
In situ light scattering measurements were performed to investigate the effect of low shear rates (0.13–3.56 s−1) on an aggregating colloidal system made of 20 nm polystyrene particles. The aggregating system was subjected to a shear for a short period (ca. 33 s) and only once at various times after the onset of aggregation. The effect of shear (aggregation kinetics and morphology) was studied both in a cluster dilute and in a cluster dense regime (see introduction). Our results have shown that shear can enhance the aggregation and gelation. Shear induced growth can yield hybrid superaggregates when the system is dense.In this paper we show that shear can enhance the aggregation and gelation. Shear induced growth can yield hybrid superaggregates when the aggregating system is dense.

Keywords: Light scattering; Shear; Gelation; Superaggregates

Thermal hysteresis of some important physical properties of nanoparticles by Tapan Sarkar; Shibsekhar Roy; Jaydeep Bhattacharya; Dhananjay Bhattacharya; Chanchal K. Mitra; A.Kr. Anjan Kr. Dasgupta (pp. 224-232).
Gold nanoparticles show thermal hysteresis with properties such as surface plasmon absorption, conductivity, and zeta potential. The direction of the incremental change in plasmon peak position and its extinction depend on the nature of surface conjugation. The thermal profile of a surface plasmon resonance spectrum for nanoparticles may serve as a signature for the associated small molecule or macromolecule on which it is seeded. The thermal responses of zeta potential and conductivity profile are found to be independent of the surface conjugation with the later being subjected to a phase transition phenomenon as revealed by a temperature criticality.The hysteresis of conductivity and wavelength of plasmon maximum shown on the background of the TEM image of GNP.

Keywords: Gold nanoparticle; Surface plasmon; Hysteresis; Phase transition

Supramolecular gels based on organic diacid monoamides of cholesteryl glycinate by Kaiqiang Liu; Ni Yan; Junxia Peng; Jing Liu; Qiuhong Zhang; Yu Fang (pp. 233-242).
Three diacid monoamides of cholesteryl glycinate (1,2, and3) were designed and synthesized. The gelation behaviors of these compounds and their corresponding ammonium salts (1′,2′, and3′) were tested in 28 solvents. It was found that1,2, and3 were weak gelators, but their neutralization with ammonia enhanced their gelling ability significantly. In particular,1′ behaves as an “ambidextrous gelator.” It gelatinizes both apolar solvents and water. More interestingly, some of the alkyl alcohols and water can be gelatinized at room temperature by simply bubbling ammonia into the system, which contained a suitable amount of1. Several techniques, such as polarizing optical microscopy and scanning electron microscopy (SEM), revealed that1′ aggregates into spherulite microparticles in 1-pentanol, at the dilution or the gel state. Moreover, SEM monitoring studies revealed that1′ first aggregated into microparticles and then these particles aggregated into fiber-like structures and finally into a three-dimensional (3D) network structure. Attenuated total reflection Fourier infrared transform spectroscopy and salt effect studies demonstrated respectively that hydrogen bonding formation between NH and CO and electrostatic interaction are two of the main driving forces for the formation of the gels.Neutralization with ammonia turns succinic acid or glutaric acid monoamide of cholesteryl glycinate into an “ambidextrous gelator.” More interestingly, some gels form spontaneously at room temperature.

Keywords: Supramolecular gels; Ambidextrous gelator; Cholesteryl glycinate; Chemo-stimulus

Comparative studies on adsorption behavior of thionine on gold nanoparticles with different sizes by Yuanhua Ding; Zhuqing Chen; Ju Xie; Rong Guo (pp. 243-250).
The adsorption behavior of thionine on gold nanoparticles of two different mean diameters, 18 and 5 nm, was compared by using UV–vis spectroscopy, fluorescence spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), dynamic light scattering (DLS), and quantum chemical calculations. It is found that the addition of small particles makes the monomer peak of thionine finally disappear, and the corresponding dimer peak is significantly increased. Small gold nanoparticles make the equilibrium between the monomer and H-type dimer forms of thionine move largely toward the dimer forms. Due to the stronger binding between thionine and small gold nanoparticles, the fluorescence quenching of thionine by small particles is enhanced compared to large particles, and the quenching is both static and dynamic. TEM images indicate that the addition of thionine results in a heavy clustering for small particles, and the resulting thionine–gold nanoclusters of about 45 nm were obtained. Quantum chemical calculations, which were based on the density functional theory (DFT) at the B3LYP level, and infrared spectroscopic studies show that the nitrogen atoms of the NH2 moieties of thionine bind to the gold nanoparticle surfaces. For 18 and 5 nm particles, the surface-to-volume atomic ratios are about 0.0597 and 0.2148, respectively. The higher surface-to-volume atomic ratio and the higher surface free energy result in stronger binding of thionine on small particle surfaces, which can be used to modulate the arrangement of dye molecules on particle surfaces, and thus control the properties of organic–inorganic nanocomposite materials.The size effect of gold nanoparticles on the adsorption characteristics of thionine was investigated, and analyzed according to the binding model for thionine–Au complex obtained from quantum chemical calculations.

Keywords: Gold nanoparticles; Thionine; Size effect; Adsorption behavior; Quantum chemical calculation; Surface structure parameter

Adhesion between a charged particle in an electrolyte solution and a charged substrate: Electrostatic and van der Waals interactions by Oxana Malysheva; Tian Tang; Peter Schiavone (pp. 251-260).
The equilibrium separation between a charged particle in an electrolyte solution and a substrate with an initially uniform surface charge density is obtained using the classical Derjaguin–Landau–Verwey–Overbeek theory. The electrostatic free energy is obtained by coupling the electric response of the substrate with the electric potential obtained from the solution of the Debye–Hückel equation. The van der Waals free energy is calculated by integrating the 6–12 Lennard-Jones potential. Metallic, dielectric, and semiconducting substrates are considered in turn. At low ionic strength, our results demonstrate a distinct response to the charged particle in each case. For example, in the case of a metallic substrate, the attached state (corresponding to equilibrium separation at short range) is always close to the van der Waals energy minimum. In addition, the application of a surface charge of sign opposite to that of the particle facilitates the transition from the detached state (corresponding to large separation at which the interaction between the particle and the substrate is negligible) to the attached state but scarcely changes the equilibrium separation. In the case of a dielectric substrate, the attached state is located at a distance of around two orders of magnitude larger than that for a metallic substrate and this equilibrium separation decreases as the (opposing) surface charge increases. A semiconducting substrate can behave either like a metal or like a dielectric, depending on the ratio of its Debye length to that of the electrolyte solution.

Keywords: Debye–Hückel equation; DLVO theory; Electrostatic interaction; Van der Waals interactions; Charged particle; Charged substrate; Equilibrium separation

Field induced phase segregation in ferrofluids by E.S. Kooij; A.C. Gâlcă; B. Poelsema (pp. 261-265).
We study the phase segregation in magnetite ferrofluids under the influence of an external magnetic field. A phase with lower nanoparticle density and corresponding higher optical transmission is formed in the bottom of a glass cell in the presence of only a very modest magnetic field gradient (smaller than25T/m). The flux density in our magnetic configuration is simulated using finite element methods. Upon switching off the external magnetic field, the low-density phase develops into a ‘bubble’-like feature. The kinetics of this ‘bubble’ in the absence and presence of a magnetic field are described and analyzed in terms of a simple model, which takes into account buoyancy and drag forces.Field-induced phase segregation in magnetite ferrofluid, as observed by video imaging, showing the formation of a low density ‘bubble’. The ‘bubble’ height increases continuously with time due to buoyancy.

Keywords: Magnetic field; Magnetite; Ferrofluid; Interaction

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