Advances in Colloid and Interface Science (v.97, #1-3)

Electrorheological suspensions by Tian Hao (1-35).
The objective of this article is to give a review of electrorheological (ER) suspensions whose rheological properties can abruptly change under an external electric field. Attention is given to the physical backgrounds behind ER phenomena reported recently. The criteria on how to design a high performance ER fluid and mechanisms explaining how an ER suspension displays the ER effect are focused upon. We begin with a brief historic introduction, ER materials, followed by positive ER effect, negative ER effect and photo-ER effect discussions. The physical parameters that can substantially affect the ER effect are discussed thereafter, and physical processes occurring in ER suspensions under an electric field are reviewed. The mechanisms of the ER effect proposed before are summarized. A future outlook on the ER material development and ER fluid applications is given.
Keywords: Colloidal suspension; Electrorheological effect; Electrorheological materials; Dielectric; Conductivity;

Hematite was prepared using Fe(NO3)3·9H2O. Bentonite was first saturated with sodium and then was transformed to the hydrogen form. The bentonite–hematite (b–h) system was prepared by the mixing of hematite and bentonite at pH 6.0, and a hematite-coated bentonite surface was created. The physical and chemical properties of the above solids were studied from the information given by X-ray, IR and NMR spectra and by measurement of specific surface and point of zero charge. The results of the above study show that the bentonite–hematite system is not a physical mixture of hematite and bentonite. Hematite moved into bentonite's interlayer space, coating these planes throughout. Consequently, Al-substitution and structural H2O/OH displacement of hematite took place. The system has a specific surface, less than bentonite and greater than hematite, while its p.z.c. is higher than bentonite and lower than hematite. Adsorption experiments of phosphates at different concentrations and pH were carried out and the constant capacitance model was used to describe phosphate adsorption by hematite, bentonite and the bentonite–hematite system. The model is characterized by a ligand exchange mechanism based on the assumption that the charge is attributed both on adsorbate and adsorbent, and is proven to successfully describe the adsorption of phosphate on these materials along with the effect of varying pH values. Ma-Za 1 and Ma-Za 2 (two computer programs) were used for this purpose. The model successfully gave a description (both quantitative and qualitative) of the adsorption of phosphate anions across the pH range (3.8–9.0). Furthermore, it can be successfully applied in systems as the bentonite–hematite system by utilizing the surface protonation–dissociation constant of hematite. The model, when applied in such systems, procreates the exact shape of the adsorption isotherms for the entire pH range of 3.8–9.0. The constant capacitance model was able to describe phosphorous adsorption on hematite and the bentonite–hematite system. The accurate fit of the model to phosphorous adsorption on hematite and the bentonite–hematite system suggests that inner sphere surface complexation is the appropriate adsorption mechanism for these materials.
Keywords: Adsorption; Phosphate; Hematite; Bentonite; Bentonite–hematite system; Constant capacitance model;

Theoretical prediction of emulsion color by David Julian McClements (63-89).
The perceived quality of many commercial products that are based on emulsions is determined by their color. In this article, a theory is presented to relate the color of emulsions to their composition and microstructure. First, the scattering characteristics (Q s and g) of individual droplets are calculated using Mie Theory. Second, the scattering (S) and absorption (K) coefficients of a concentrated emulsion are calculated using radiative transfer theory. Third, the reflectance spectrum (R) of the emulsion is calculated using Kubelka–Munk Theory. Finally, the tristimulus coordinates (XYZ, or L*a*b*) of the emulsion are calculated using color theory. There is excellent agreement between theoretical predictions and experimental measurements of the influence of droplet and chromophore characteristics on the tristimulus coordinates of concentrated oil-in-water emulsions.
Keywords: Emulsions; Optical properties; Color; Light scattering;

This article presents studies on the photophysical and photochemical behavior of probes within micellar systems: organized emulsifier/polymer aggregates; the intra- and interpolymer association of amphiphilic polymers; monomer-swollen micelles (microdroplets); and the interfacial layer. Pyrene (Py) as a probe is particularly attractive because of its ability to measure the polarity of its microenvironment. Dipyme yields information on the microviscosity of micellar systems. Probes such as laurdan and prodan can be used to explore the surface characteristics of micelles or microdroplets. The dansyl group has a special photophysical property that gives information about the local polarity and mobility (viscosity) of the microenvironment. The organized association of amphiphilic polymer and emulsifier introduces a heterogeneity in the local concentration of the reactants. This heterogeneity also results from the attractive interaction between hydrophilic monomer and emulsifier in the case when the monomer carries a positive charge and the counterpart a negative one, and vice versa. Some emulsifiers can bind to the amphiphilic copolymers by simple partitioning between the aqueous phase and the polymer — non-cooperative association. The interaction between micelles (microdroplets) and charged polymers leads to the formation of mixed micelles. Binding emulsifiers to these polymers was detected at emulsifier concentrations much below the critical micellar concentration (CMC). Emulsifiers often interact cooperatively with polymers at the critical aggregation concentration (CAC) below the CMC, forming micelle-like aggregates within the polymer. The CAC can be taken as a measure of interaction between the emulsifier and polymer. A decrease in the monomer fluorescence intensity of probe-labeled polymer results from increased excimer formation, or higher aggregates within the unimolecular polymeric micelles. An increase in the monomer fluorescence intensity of probe-labeled polymer within the micellar system can be ascribed to shielding of the probe chromophores by emulsifier micelles. The quenching of probe emission by (un)charged hydrophilic monomer depends on partitioning of the monomer between the aqueous phase and the micelles. Penetration of reactants into the interfacial layer determines the quenching of the hydrophobic probe by hydrophilic quencher, or vice versa. Quenching depends on the thickness, density and charge of the interfacial layer. Compartmentalization prevents the carbonyl compound and unsaturated monomer from coming into sufficiently close contact to allow singlet or triplet–monomer interaction. All negatively charged carbonyl probe molecules are quenched with significantly lower rates than the parent neutral hydrophobic benzophenone molecules, which were located further inside the aggregates. This results from the different conformation and allocation of reactants within the micellar system. In the reverse micelles, quenching depends on the amount of water in the interfacial layer and the total area of the water/oil interface.
Keywords: Organized association of amphiphilic polymer; Polymer–emulsifier interactions; Fluorescence measurements; Probe/quencher behavior;

Coagulation efficiency of colloidal particles in shear flow by Marco Vanni; Giancarlo Baldi (151-177).
The mechanism of shear-induced coagulation of colloidal particles is reviewed, in order to define a method to evaluate the collision efficiency according to the present knowledge of the phenomenon. Therefore, a detailed description of the procedure for trajectory analysis and identification of collision is presented, as well as of the role and estimation of colloidal forces. Recent analytical expressions have been adopted for the Van der Waals interaction, that are based on the Lifshitz theory of dispersion forces and that are capable of considering ionic screening. The obtained results have been compared with those of older expressions for the dispersion force. The two formulations agree fairly well for the case of strongly destabilised systems; on the contrary, they may lead to significantly different results for slow coagulation. The proposed method results in a fully predictive procedure for estimating collision efficiency. The results have been compared favourably with experimental data concerning the system polystyrene–water for the regime of fast coagulation. Finally, the transition between primary and secondary coagulation has been analysed in detail and it has been shown that, in the fast coagulation regime, the type of coagulation depends only on surface potential and ionic strength, but not on particle size and shear rate.
Keywords: Coagulation; Aggregation; Shear flow; Colloids; Suspensions;

Hydrophobic mechanochemical treatment of metallic surfaces by V Roucoules; F Gaillard; T.G Mathia; P Lanteri (179-203).
Hydrophobicity, lubrication and anticorrosion properties of steel substrates have been obtained by a deposition of thin film (i.e. by mechanochemical treatment) at room conditions. Stearic acid and paraffin were chosen as reactive molecules. Different abrasive powders were selected to generate active sites on the treated surfaces for adsorption of the reactive molecules and then, the results were compared. The surfaces were analyzed by reflection–absorption infrared spectroscopy (RAIRS). The results emphasize that, a thick layer of mixed stearic acid/paraffin was deposited onto the metallic surface after the treatment. After hexane rinsing we could only detect a very thin layer of oriented stearic acid molecules chemically adsorbed onto the metallic surface and which engages strong interactions with it. Whereas, RAIRS only provides molecular analysis, the XPS technique was complementary for discriminating the different surfaces. It was possible to show differences in thickness as well as in coverage according to the size and shape of abrasive particles. Furthermore, we could conclude that deposit layer is not uniform. Defects were always present and were dependent on abrasive powders used. Then wettability was assessed as a way to test the homogeneity of thin films generated by the mechanochemical treatment. In agreement with theoretical data, receding contact angle was very dependent on the defects in the deposited film. If holes or aggregates were increased in the deposit layer, the receding contact angle was decreased while advancing contact angles and equilibrium contact angles remained constant. A very important point for technological applications was that the homogeneity of the deposited film was governed by abrasive powder involved in mechanochemical treatment and contact angle values were a direct measurement of the homogeneity of surfaces generated by mechanochemical treatment.
Keywords: Organic thin films; Mechanochemical treatment; Abrasive particles; RAIRS; XPS; Contact angles;

Dimeric and oligomeric surfactants are novel surfactants that are presently attracting considerable interest in the academic and industrial communities working on surfactants. This paper first presents a number of chemical structures that have been reported for ionic, amphoteric and nonionic dimeric and oligomeric surfactants. The following aspects of these surfactants are then successively reviewed the state of dimeric and oligomeric surfactants in aqueous solutions at concentration below the critical micellization concentration (cmc); their behavior at the air/solution and solid/solution interfaces; their solubility in water, cmc and thermodynamics of micellization; the properties of the aqueous micelles of dimeric and oligomeric surfactants (ionization degree, size, shape, micropolarity and microviscosity, solution microstructure, solution rheology, micelle dynamics, micellar solubilization, interaction between dimeric surfactants and water-soluble polymers); the mixed micellization of dimeric surfactants with various conventional surfactants; the phase behavior of dimeric surfactants and the applications of these novel surfactants.
Keywords: Dimeric surfactants; Oligomeric surfactants; Gemini surfactants;

Thermophoresis is an important mechanism of micro-particle transport due to a temperature gradient in the surrounding medium and has found numerous applications, especially in the field of aerosol technology. Extensive studies, both theoretical and experimental, have been done to understand the nature of this phenomenon. However, it is clear that a lot more of work needs to be done before we can predict thermophoresis accurately for any given gas-particle system as well as particle shape and orientation in any flow regime. This paper reviews the existing theories and data in two major categories, for spherical particles and for non-spherical particles, as well as the various techniques in making thermophoresis measurements. The current state of development for thermophoresis studies is that for spheres the theories and experimental data agree with each other fairly well but for non-spherical particles in the transition regime the theories are yet to be developed and experimental data showing the effect of particle shape are much needed in all Knudsen number range. The best techniques of thermophoretic force measurements involve the use of electrodynamic balances to work on single micro-particles and the use of microgravity to minimize the effect of convection. A combination of the above two has not been attempted and should provide the most accurate data.
Keywords: Thermophoresis; Thermophoretic force; Non-spherical particle;

Electrodeposition of ceramic materials can be performed by electrophoretic (EPD) or electrolytic (ELD) deposition. Electrophoretic deposition is achieved via motion of charged particles towards an electrode under an applied electric field. Electrolytic deposition produces colloidal particles in cathodic reactions for subsequent deposition. Various electrochemical strategies and deposition mechanisms have been developed for electrodeposition of ceramic and organoceramic films, and are discussed in the present article. Electrodeposition of ceramic and organoceramic materials includes mass transport, accumulation of particles near the electrode and their coagulation to form a cathodic deposit. Various types of interparticle forces that govern colloidal stability in the absence and presence of processing additives are discussed. Novel theoretical contributions towards an interpretation of particle coagulation near the electrode surface are reviewed. Background information is given on the methods of particle charging, stabilization of colloids in aqueous and non-aqueous media, electrophoretic mobility of ceramic particles and polyelectrolytes, and electrode reactions. This review also covers recent developments in the electrodeposition of ceramic and organoceramic materials.
Keywords: Electrophoretic deposition; Electrolytic deposition; Ceramic; Polyelectrolyte; Polymer;