Association colloids anionic

One characteristic property of surfactants is that they spontaneously aggregate in water and form well-defined structures such as spherical micelles, cylinders, bilayers, etc. (review Ref. [524]). These structures are sometimes called association colloids. The simplest and best understood of these is the micelle. To illustrate this we take one example, sodium dode-cylsulfate (SDS), and see what happens when more and more SDS is added to water. At low concentration the anionic dodecylsulfate molecules are dissolved as individual ions. Due to their hydrocarbon chains they tend to adsorb at the air-water interface, with their hydrocarbon chains oriented towards the vapor phase. The surface tension decreases strongly with increasing concentration (Fig. 3.7). At a certain concentration, the critical micelle concentration or... 

In ternary systems, too, where the anion- and cation-active association colloids are replaced by a non-ion-active colloid of the polyoxyethylene type, similar conditions were found, with different mesophases separated by two-phase and three-phase regions. For instance, in the alkylpoly-oxyethylene-oleic acid-water system all five of known mesophases E, D, F, C, and B, are found as well as two extremely stiff, completely transparent gels, which are optically isotropic (19, 21). Their x-ray patterns, which are similar but differ from the other phases on essential points, have so far resisted satisfactory interpretation. These mesophases, Ii and I2, occur in different parts of the system it is obvious that one structure is the reverse of the other—thus parallel to E and F. The structure would possibly be one of spheres with the densest cubical packing. 

Kinetic data obtained under these conditions have been fitted by pseudophase models in terms of k by solving the PBE or in terms of k / Vm by using the PIE model [10], However, because these treatments contain reasonable but unproven assumptions [64] and because values of parameters such as //, V, and are only approximate, values of k may not be unique [123], Extensive evidence shows that k kw for many reactions of anionic nucleophiles, but this generalization does not hold for anionic electrophiles [83,124,125], Therefore, although a great many kinetic data are consistent with the assumption that counterions concentrate at surfaces of ionic association colloids, it is difficult to obtain quantitative estimates of interfacial ion concentrations from measured rate constants. 

FIGURE 3.11 Representations of hydrophilic, hydrophobic, and association colloidal particles the hydrophilic colloids are bonnd to water as indicated by the dashed lines. Positively charged counter ions surround negatively charged hydrophobic and association colloidal particles. The micelle of the association colloid may be visnalized as a clnster of organophilic hydrocarbon chains inside a ball with anionic heads attracted to water on the snrface of the ball. 

Ion bridging is a specific type of Coulombic interaction involving the simultaneous binding of polyvalent cations (e.g., Ca, Fe, Cu ) to two different anionic functional groups on biopolymer molecules. This type of ionic interaction is commonly involved in associative self-assembly of biopolymers. As a consequence it is also an important contributory factor in the flocculation (via bridging or depletion) of colloidal particles or emulsion droplets in aqueous media containing adsorbed or non-adsorbed biopolymers (Dickinson and McClements, 1995). 

The above discussion clearly shows that the increase of anionic copolymer concentration or the strength of chain interaction can lead to a transition from intrachain contraction to interchain association. It can result in a mesoglobular phase in which a limited number of copolymer chains are associated together to form polymeric colloidal particles stable between microscopic single-chain globular phase and macroscopic phase separation (precipitation). It is not a surprise to see the formation of such stable mesoglob-... 

The present article was stimulated by the recent experimental data on protein-covered latex colloidal systems immersed in various electrolyte solutions NaCl, NaNC>3, NaSCN and Ca(NOg)2, which showed strong specific anionic effects on the restabilization curves.1 In the opinion of Lopez-Leon et al.,1 the above polarization model for double layer/hydration forces could explain only some of their experiments, but not all of them. However, they assumed that at pH = 10 the adsorption of anions was negligible hence specific anion effects could not be predicted by their association with the positive sites of the surface. Furthermore, at pH = 4 they assumed the... 

It is clear that a perfect agreement with experiment cannot be provided by a theory which ignores the additional interactions between ions, and ions and surfaces, not included in the mean field potential (such as image forces,14 excluded volume effects,15 and ion-dispersion16 or ion-hydration forces17). However, it will be shown that the experimental results reported by Lopez-Leon et al.1 can be more than qualitatively reproduced for uniunivalent electrolytes by the present polarization model for hydration/double layer forces, if one accounts for the association equilibria with the surface sites for all the ions present in the electrolyte (H+, OH , anions, and cations).11 Some additional reasons for the quantitative disagreements, involving the structural modifications of the adsorbed protein layer and the nonuniformity of the colloidal particles, will be also noted. 

Surfactants are employed in emulsion polymerizations to facilitate emulsification and impart electrostatic and steric stabilization to the polymer particles. Sicric stabilization was described earlier in connection with nonaqueous dispersion polymerization the same mechanism applies in aqueous emulsion systems. Electrostatic stabilizers are usually anionic surfactants, i.e., salts of organic acids, which provide colloidal stability by electrostatic repulsion of charges on the particle surfaces and their associated double layers. (Cationic surfactants are not commonly used in emulsion polymerizations.)... 

When referring to Ti02-based photocatalytic systems it is important to note that, in most cases, the semiconducting oxide is associated there with a noble metal or/and a noble metal oxide catalyst. While the role played by these catalysts in (partial) cathodic reactions seems relatively well understood it remains less clear with regard to the photoanodic reactions. In particular, the exact function of the extensively used ruthenium dioxide catalyst has been questioned The role of Ru02 as a hole-transfer catalyst has, for example, been established through laser-photolysis kinetic studies in the case of photo-oxidation of halide (Br and CP) ions in colloidal titanium dioxide dispersions. In fact, the yields of Brf and ClJ radical anions, photogenerated in the course of these reactions. 

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