Association colloids cationic

All these results are consistent with the hypothesis that aryl cations react in aqueous media at diffusion-controlled rates with all nucleophiles that are available in the immediate neighbourhood of the diazonium ion. On this basis Romsted and coworkers (Chaudhuri et al., 1993a, 1993b) used dediazoniation reactions as probes of the interfacial composition of association colloids. These authors determined product yields from dediazoniation of two arenediazonium tetrafluoroborates containing ft-hexadecyl residues (8.15 and 8.16) and the corresponding diazonium salts with methyl groups instead of Ci6H33 chains. ... 

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. 

Menger, F.M., Williams, D.Y, Underwood, A.L., Anacker, E.W. Effect of counterion geometry on cationic micelles. /. Colloid Interface Sci. 1982,90(2), 546-548. Loughlin, J.A., Romsted, L.S. A new method for estimating counter-ion selectivity of cationic association colloid trapping of interfacial chloride and bromide counterions by reaction with micellar bound aryldiazonium salts. Colloids Surf. 1990, 48(1-3), 123-137. 

Cationic micelles are aggregates of cationic surfactant monomers (Fig. 1). Surfactants are schizophreiuc molecules typically composed of linear hydrocarbon chains afflxed to water-soluble head groups. In addition to cationic surfactants (with anioiuc coimterions), micelles may be composed of surfactants with aiuoiuc (with cationic counterions), zwitterionic, or nonionic head groups. Micelles, like microemulsions, are association colloids and... 

A delicate balance of forces controls the stability, size, and shape of cationic micelles and association colloids in general, and their properties are... 

Scheme 3 shows the reactions of a number of different weakly basic nucleophiles with z-AtN2 that have been studied in association colloids (published work is indicated by reference numbers in brackets and unpublished work by a bold asterisk in Scheme 3). In addition to the work described here, chemical trapping with Ifi-ArNj has already been used to determine the ion exchange constant between Cl and Br counterions in cationic micelles [26], Cl concentrations at the surfaces of zwitterionic micelles [27] and phospholipid micelles and vesicles [28], hydration numbers and terminal OH...

Surfactant molecules can be considered as building blocks for certain forms of geometry in colloidal chemistry. Various forms of association molecules can be obtained as the concentration of surfactant in water is increased and/or physicochemical conditions are changed (e.g. CMC, Craft-point, etc.). Figure 2 schematically shows the most likely structural configurations and assemblages of surfactants association in an aqueous system (26). Upon addition of oil and a short-chain alcohol, for example, one can convert the oil-in-water micelles into water-in-oil microemulsions. It is therefore possible to induce a transition from one structure to another by changing the physicochemical conditions such as temperature, pH and addition of mono or di-valent cations to the surfactant solution. It should be also noted that the sur-... 

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). 

No attempt was made to reduce the ash content of the humic acid samples, since drastic purification methods can cause abnormal changes in humate characteristics, and it was believed that most organic acids in their natural environment would be in salt form and associated with other colloidal matter. Though competition from displaced cations may have contributed to the smaller uptake by humic acid (HA II) in the adsorption stage (as shown in Table 2.1), any residual counterions should have had little effect on the metal-ion extraction step. 

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. 

Because of the high solubility of these bases, over-application can raise the pH above 9, which is undesirable. Owing to the monovalent nature of the associated cations (Na+ or K+), such bases act as colloid dispersants (see Chapter 9). Thus, they increase suspended solids. Additional acid ameliorates are shown in Table 12.8. These phosphate compounds are highly effective in precipitating heavy metals as well as manganese and iron. However, the use of such compounds may increase suspended solids by increasing the surface electrical potential of clays or metal-oxides (see Chapters 3 and 9). 

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.)... 

Not only are accurate data for trace metals in rivers sparse, there are complications that exist at the river-sea interface. The increase in salinity occurring at the river-sea water interface, with its concomitant increase in the concentrations of the major seawater cations, can lead to flocculation and sedimentation of trace metals such as iron (Boyle et al., 1978 Sholkovitz and Copeland, 1983) or to desorption from suspended riverine particles of trace metals such as barium (Edmond et al., 1978). In organic-rich rivers a major fraction of dissolved trace metals can exist in physiochemical association with colloidal humic acids. Sholkovitz and Copeland (1983) used product-mode mixing experiments on filtered Scottish river water, and observed that iron removal was almost complete due to the flocculation of strongly associated iron-humic acid colloids in the presence of the increased... 

Experimental studies. Sorption of radionuclides by colloids is affected by the same solution composition parameters discussed in the previous section on sorption processes. The important parameters include pH, redox conditions, the concentrations of competing cations such as Mg " " and K, and the concentrations of organic ligands and carbonate. The high surface area of colloids leads to relatively high uptake of radionuclides compared to the rock matrix. This means that a substantial fraction of mobile radionuclides could be associated with carrier colloids in some systems. The association of radionuclides with naturally occurring colloids and studies of radionuclide uptake by colloids in laboratory systems give some indication of the potential importance of colloid-facilitated radionuclide transport in the environment as discussed below. 

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