Cationic-anionic surfactant systems

The synergisms of mixtures of anionic-cationic surfactant systems can be used to form middle-phase micro emulsions without adding short-chain alcohols [109, 110]. The surfactants studied were sodium dihexyl sulphosuccinate and benzethonium chloride. The amount of sodium chloride required for the middle-phase microemulsion decreased dramatically as an equimolar anionic-cationic surfactant mixture was approached. Under optimum middle-phase microemulsion conditions, mixed anionic-cationic surfactant systems solubilised more oil than the anionic surfactant alone. Upadhyaya et al. [109] proposed a model for the interaction of branched-tail surfactants (Fig. 8.16). According to this model the anionic-cationic pair allows oil to penetrate between surfactant tails and increases the oil solubilisation capacity of the surfactant aggregate. Detergency studies were conducted to test the capacity of these mixed surfactant systems to remove oil from... 

Robinson, B.H. Bucak. S. Fontana, A. On the concept of driving force applied to micelle and vesicle self-assembly. Langmuir 2000, 16. 8231-8237, and references therein. Shioi. A. Hatton, T.A. Model for formation and growth of vesicles in mixed anionic/cationic surfactant systems. Langmuir 2002. 18, 7341-7348. 

Frese et al. [38] studied the adsorption kinetics of mixed anionic/cationic surfactant systems, finding that net charge-neutral mixed micelles were formed. They found that the dynamic surface tension was higher than expected for small-surface ages, possibly due to an activation barrier associated with the mixed-micelle complexes. 

Figure 3 Precipitation phase diagram for the anionic-cationic surfactant system sodium decyl sulfate (SDS)-DPC at two different temperatures.

It is important to note that complexes, other than those containing oxyethylene moieties can also exhibit cloud point phenomena. Figure 12 shows the phase diagram of the anionic-cationic surfactant system sodium- -lauroyl-iV-methyl-p-alanine-stearyltrimeth-ylammonium chloride [38]. 

Favaro, Y.L., Reinsborough, V.C. Micellar catalysis in mixed anionic/cationic surfactant systems. Can. J. Chem. 1994, 72(12), 2443-2446. 

The surfactants used in the emulsion polymerization of acryhc monomers are classified as anionic, cationic, or nonionic. Anionic surfactants, such as salts of alkyl sulfates and alkylarene sulfates and phosphates, or nonionic surfactants, such as alkyl or aryl polyoxyethylenes, are most common (87,98—101). Mixed anionic—nonionic surfactant systems are also widely utilized (102—105). 

The phase behavior of anionic-cationic surfactant mixture/alcohol/oil/ water systems exhibit a similar effect. First of all, it should be mentioned that because of the low solubility of the catanionic compound, it tends to precipitate in absence of co-surfactant, such as a short alcohol. When a small amount of cationic surfactant is added to a SOW system containing an anionic surfactant and alcohol (A), three-phase behavior is exhibited at the proper formulation, and the effect of the added cationic surfactant may be deduced from the variation of the optimum salinity (S ) for three-phase behavior as in Figs. 5-6 plots. Figure 16 (left) shows that when some cationic surfactant is added to a SOWA system containing mostly an anionic surfactant, the value of In S decreases strongly, which is an indication of a reduction in hydrophilicity of the surfactant mixture. The same happens when a small amount of anionic surfactant is added to a SOWA system containing mostly a cationic surfactant. As seen in Fig. 16 (left), the values of In S at which the parent anionic and cationic surfactant systems exhibit three-phase behavior are quite high, which means that both base surfactants, e.g., dodecyl sulfate... 

Figure 5. Simplified extraction mechanism for the alkylammonlum cationic surfactant system In organic solvents. CA refers to a solubilized co-extractant, X Is the counterion of the surfactant, and X and W refer to the species (anions or anionic metal complexes) to be extracted from the aqueous phase.

Doan, T., Acosta, E., Scamehorn, J.F. and Sabatini, D.A. (2003) Formulating middle-phase microemulsions using mixed anionic and cationic surfactant systems. /. Surfactants Detergents, 6,215-224. 

Li, X., and Kunieda, H. (2000) Solubilization of micellar cubic phases and their structural relationships in the systems anionic-cationic surfactant-dodecane-water. Langmuir, 16, 10092-10100. 

Further, just as in micelle formation, the addition of counterions can reduce the repulsion between the head groups of the anionic surfactants by compressing the electrical double layer between them. This compression acts to increase the adsorption. This increase in adsorption was not observed for addition of divalent cations to the cationic surfactants systems, however. 

Reference has already been made to the interesting finding by Laurent and Scott (65) that precipitation of various polyanion/cationic surfactant systems can be totally inhibited by the addition of a sufficient amount of simple salt. This work allowed the definition of a critical electrolyte concentration (c.e.c.), which was found to vary from system to system. Clearly, electrostatic screening effects are again involved. This phenomenon has been confirmed and examined in some detail by Lindman and co-workers (see next section). Less work has been carried out in this respect on polycation/anionic surfactant systems and, at least in some systems involving cationic cellulosic polymer/SDS combinations, resolubilization by salt addition was found not to be facile (59,103). 

Much more work of this type, involving polyion/oppositely charged surfactant mixtures, has been carried out on polyanion/cationic surfactant systems (72,73), mainly because it was found easier to home-make cationic surfactant electrodes Now that anionic-surfactant-specific electrodes are available commercially (e.g., Orion Research Model 93-42), this situation is likely to change, especially in the cosmetics field with its high interest in polycationic conditioning agents. In the case of ideal behavior, the electrode allows determination of the equivalent weight of the polycation. 

If it is possible to create mesostructured phases from an anionic metal oxide/cationic surfactant system, as in the case of MCM-41, it should also be feasible using the reversed system the combination of metal oxide polycations with anionic surfactants should lead to similar materials. In both systems the ionic interactions of the metal oxide and the surfactant should be comparable and the mechanism in the reversed system should be analogous to the synthesis of MCM-41. 

FIGURE 15.23 Oil recovery percentages obtained with some surfactant systems. The results were determined as a function of injected porous volume for surfactant systems with various concentrations above the CMC. Graphs (a), (b), (c), and (d) refer to different nonionics graphs (e) and (f) correspond to the anionics graph (g) shows the behavior of the cationic surfactant system graph (h) is the result of a nonionic and anionic surfactant mixture. 

The metal ion can conveniently be recovered from the acid micellar solution by ultrafiltration. Ismael and Tondre demonstrated the use of ultrafiltration in the back extraction of metal ions from micellar solutions containing Ni + and Cu + using the extractants 8-hydroxyquinoline (HQ) and an alkylated derivative (Cn-HQ) in the presence of cationic micelles of cetyltrimethylammonium bromide and 1-butanol. Decreasing the pH to pH = 3 led to effective extraction of Ni " in the presence of Cu + but, for Cu + extraction, it was necessary to reduce the pH further to less than 1. It is interesting to note that for these systems cationic surfactant systems were used rather than the anionic systems described previously. 

The interactions between cationic and nonionic surfactants and proteins has received substantially less attention than the anionic case. Nonylphenol-POE nonionic surfactants undergo limited binding with proteins, although there is fittle evidence for sufficient interaction to induce the conformational changes found in the case of anionic materials. The limited number of results published on protein-cationic surfactant systems indicate that little cooperative association occiu s in those systems, even if the native protein charge is negative. 

---