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Abstract
One synergism of using surfactant mixtures is the reduction in the equilibrium extent of and rate of precipitation. The overall time required for calcium-induced precipitation of mixed sodium dodecyl sulfate (SDS) and sodium octylbenzene sulfonate (SOBS) over a particular range of ratios has been found to increase dramatically when compared to either SDS or SOBS alone. In this study, light transmission and isoperibol calorimetry were used to measure the delay in the precipitation reaction, while scanning electron and optical micrographs of crystals formed give insight into the precipitation mechanism. The smaller the difference in the supersaturation ratio of the two precipitating surfactants, the longer the induction time is. The delay in the extent of precipitation is due to the interruption of crystal formation from dissimilar precipitating surfactants.
Keywords Surfactant precipitation * Kinetics * Calcium * Anionic surfactants * Surfactant crystallization
Introduction
Precipitation of anionic surfactants can be detrimental in several applications including surfactant-based separation processes, enhanced oil recovery, and detergency. Anionic surfactants tend to precipitate with cations such as Na+, Ca^sup 2+^, and Mg^sup 2+^ and other positively charged molecules such as cationic surfactants and polymers. Single anionic surfactants are generally found to precipitate with calcium within minutes [1-3]. The surface activity of the surfactants is reduced when precipitation of the surfactant occurs and the surfactant is removed from solution, often reducing their useful properties. Builders are often used in formulating detergents in order to avoid precipitation of surfactants with cations [4]. There are extensive studies on equilibrium precipitation with single anionic surfactants precipitating with monovalent and divalent cations and with cationic surfactants [5-27]. It is well known that use of mixtures of surfactants rather than isomerically pure surfactants (anionic or anionic/cationic surfactant mixtures) inhibit their tendency to precipitate at equilibrium as characterized by decreasing Krafft temperatures, increasing hardness tolerance, and shrinking precipitation phase boundary concentration regimes as reviewed in detail by Scamehorn and Harwell [28].
Kinetic studies of precipitation in surfactant systems have received little attention in the scientific literature. This could be due to the difficulty in quantifying the rate of precipitation, especially in systems with complex surfactant mixtures. Clarke et al. first studied the effect of micelle formation related to the kinetics of precipitation of calcium dodecyl sulfate by stopped-flow...