Coacervate precipitates, surfactants

Many surfactants have been suggested as candidates for CO2 foam. However, at high salinity and temperature in the presence of oil, most surfactants foam poorly due to partitioning and emulsion formation and fail to control mobility during CO2 injection. This behavior is analogous to that observed in chemical (microemulsion) oil recovery (5-1). As the salinity, hardness and temperature increase, surfactants form water/oil emulsions, precipitate surfactant-rich coacervate phases, and partition into the oleic phase. CO2 decreases further the solubility of surfactant in the aqueous phase. 

This brief review has attempted to discuss some of the important phenomena in which surfactant mixtures can be involved. Mechanistic aspects of surfactant interactions and some mathematical models to describe the processes have been outlined. The application of these principles to practical problems has been considered. For example, enhancement of solubilization or surface tension depression using mixtures has been discussed. However, in many cases, the various processes in which surfactants interact generally cannot be considered by themselves, because they occur simultaneously. The surfactant technologist can use this to advantage to accomplish certain objectives. For example, the enhancement of mixed micelle formation can lead to a reduced tendency for surfactant precipitation, reduced adsorption, and a reduced tendency for coacervate formation. The solution to a particular practical problem involving surfactants is rarely obvious because often the surfactants are involved in multiple steps in a process and optimization of a number of simultaneous properties may be involved. An example of this is detergency, where adsorption, solubilization, foaming, emulsion formation, and other phenomena are all important. In enhanced oil recovery. 

Some hydrophobic cations, such as tetrabutyl ammonium, appear to have the opposite action on the CMC. OH acts as normal anion whereas practically has no effect up to ca 0.5 M and it rise the CMC at higher concentrations in view of the ether oxygen protonation of the ethylene oxide chain. As a result of electrolyte addition, strong specific binding of ions may make surfactants sometimes more soluble and, in contrast, it may cause coacervation. Surfactant precipitation or strong aggregation occurs in many cases. 

The oppositely charged polymers and surfactants may form a single phase without complex formation, or may form a soluble surfactant-polymer complex, or may result in phase separation (Piculell and Lindman 1992 Wang et al. 1999,2000). If the interaction leads to liquid-liquid phase separation, the process is called coacervation. If the interaction leads to a liquid-solid phase separation, then the process is named precipitation. Many factors affect the interaction of oppositely charged surfactants and polymers that consequently have an effect on the possible phase separation phenomena. Extensive studies have been done on both theoretical and practical aspects of phase separation. It is important to know the conditions under which coacervation occurs in applications dealing with the formulation of cosmetics and pharmaceuticals. 

Studies on the interaction between surfactants and styrene-ethylene oxide block co-polymers, however, indicate that the polymers exhibit, in the presence of surfactant, typical polyelectrolyte character. This, it has been suggested [264], is due to interaction repulsions between like charges of the NaDS ions adsorbed onto the polyoxyethylene blocks. Investigating the interaction of the same detergent with methylcellulose and poly(vinyl alcohol), Lewis and Robinson [265] also observed the polyelectrolyte character of the polymer-surfactant complexes. A complex between non-ionic surfactants and a polycarboxylic acid in water can solubilize oil-soluble dyes below the surfactant CMC [268]. The complex containing the solubilizate can be precipitated the solubilizate remains in the precipitated complex and is leached out only slowly on placing the precipitate in fresh solvent. This has potential pharmaceutical implications. Halothane uptake by coacervate systems of gelatin-benzalkonium [269] has... 

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