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Advances in Colloid and Interface Science (v.151, #1-2)
Rheology of emulsions by Svetlana R. Derkach (pp. 1-23).
The review is devoted to the historical and modern understanding of rheological properties of emulsions in a broad range of concentration. In the limiting case of dilute emulsions, the discussion is based on the analogy and differences in properties of suspensions and emulsions. For concentrated emulsions, the main peculiarities of their rheological behaviour are considered. Different approaches to understand the concentration dependencies of viscosity are presented and compared. The effects of non-Newtonian flow curves and the apparent transition to yielding with increasing concentration of the dispersed phase are discussed. The problem of droplet deformation in shear fields is touched. The highly concentrated emulsions (beyond the limit of closest packing of spherical particles) are treated as visco-plastic media, and the principle features of their rheology (elasticity, yielding, concentration and droplet size dependencies) are considered. A special attention is paid to the problem of shear stability of drops of an internal phase starting from the theory of the single drop behaviour, including approaches for the estimation of drops' stability in concentrated emulsions. Polymer blends are also treated as emulsions, though taking into account their peculiarities due to the coexistence of two interpenetrated phases. Different theoretical models of deformation of polymer drops were discussed bearing in mind the central goal of predictions of the visco-elastic properties of emulsions as functions of the properties of individual components and the interfacial layer. The role of surfactants is discussed from the point of view of stability of emulsions in time and their special influence on the rheology of emulsions.
Keywords: Rheology; Dilute emulsions; Highly concentrated emulsions; Interfacial layers; Surfactants; Polymer mixtures
A review of polymeric dispersant stabilisation of titania pigment by Saeed Farrokhpay (pp. 24-32).
A review of past and present published works examining the interaction of polymeric dispersants with titania pigment particles is presented. Titania is the most important white pigments currently used in the world and its suspension properties are very important for consumer industries such as paints, papermaking and plastics; if aggregates are present, the end-use properties including gloss, opacity and storage stability will be highly affected. As polymeric dispersants are generally used to disperse titania pigment particles, it is very important to understand the interactions between the pigment particles and polymeric dispersants of varying functionality. Although, in principle, the adsorption of polymers onto titania pigment and influences on pigment dispersion and stabilisation are fairly known, it is nevertheless hardly possible to forecast the behaviour of a given polymeric dispersant in advance, unless to have a broad knowledge of the interaction occurring between pigment and dispersants and effect of dispersant structure upon adsorption. While only titania pigment is discussed, the issues raised may also apply to other mineral oxides such as alumina or zirconia.
Keywords: Titania pigment; Polymeric dispersant; Stabilisation; Polymer adsorption; Electrosteric stabilisation; Suspension
Bioinspired control of colloidal silica in vitro by dual polymeric assemblies of zwitterionic phosphomethylated chitosan and polycations or polyanions by Konstantinos D. Demadis; Konstantinos Pachis; Antonia Ketsetzi; Aggeliki Stathoulopoulou (pp. 33-48).
This paper focuses on the effects of biological and synthetic polymers on the formation of amorphous silica. A concise review of relevant literature related to biosilicification is presented. The importance of synergies between polyelectrolytes on the inhibition of silicic acid condensation is discussed. A specific example of a zwitterionic polymer phosphonomethylated chitosan (PCH) is further analyzed for its inhibitory activity. Specifically, the ability of PCH to retard silicic acid condensation at circumneutral pH in aqueous supersaturated solutions is explored. It was discovered that in short-term studies (0–8 h) the inhibitory activity is PCH dosage-independent, but for longer condensation times (>24 h) there is a clear increase in inhibition upon PCH dosage increase. Soluble silicic acid levels reach 300 ppm after 24 h in the presence of 160 ppm PCH. Furthermore, the effects of either purely cationic (polyethyleneimine, PEI) or purely anionic (carboxymethylinulin, CMI) polyelectrolytes on the inhibitory activity of PCH is systematically studied. It was found that the action of inhibitor blends is not cumulative. PCH/PEI blends stabilize the same level of silicic acid as PCH alone in both short-term (8 h) and long-term (72 h) experiments. PCH/CMI combinations on the other hand can only achieve short-term inhibition of silicic acid polymerization, but fail to extend this over the first 8 h. PCH and its combinations with PEI or CMI affect silica particle morphology, studied by SEM. Spherical particles and their aggregates, irregularly shaped particles and porous structures are obtained depending on additive or additive blend. It was demonstrated by FT-IR that PCH is trapped in the colloidal silica matrix.
Keywords: Silicification; Chitosan; Inulin; Silica; Inhibitors; Zwitterionic polymers; Assembly
Surface properties of the derivatives of lysosomotropic substances against other quaternary ammonium salts by Katarzyna Dopierala; Jacek Luczynski; Krystyna Prochaska (pp. 49-56).
Equilibrium adsorption at the air/water interface of cationic surfactants belonging in the group of quaternary ammonium bromides was studied. Quaternary ammonium salts, derivatives of lysosomotropic substances with different alkyl chain numbers and hydrophobicities were investigated. Surface properties of considered compounds, were examined and presented against other quaternary ammonium bromides of different chemical structure and with different number of alkyl chains in the molecule. The Frumkin equation and reorientation model were used for a quantitative description of the process of their adsorption. It was found that considered biologically active derivatives reveal strong surface activity. Chemical structure–surface activity relationship was analyzed and referenced to biological activity. Symmetry of the molecules resulting from the number of alkyl chains is the factor determining the behavior of the surfactants at the air/water interface as well as in the bulk. Surfactants with asymmetrical structure show a strong tendency to reorientation, in opposite to tetraalkylammonium bromides and double chain surfactants. Furthermore, the role of ester bond was emphasized.
Keywords: Quaternary ammonium salts; Lysosomotropic substances; Chemical structure–surface activity relationship; Reorientation model