(190v) Precipitation of Calcium Dodecyl Sulfate Under the Influence of Sodium Decyl Sulfate In the Binary Anionic Surfactant Systems

Precipitation of anionic surfactants by counterions contained in hard water has been a problem, especially in the detergent and cleaning industries. Precipitation is known by the consumer as scum formation, and this scum formation is an extremely undesirable result. The use of mixed surfactant systems can prolong the induction time of surfactant precipitation compared to a single anionic surfactant systems, however, it is currently not known whether this inhibition is due to a change in the thermodynamics of the system, inhibition of nuclei formation, or inhibition of crystal growth. In this study, the induction time for calcium dodecyl sulfate has been investigated with several supersaturation ratios via visual observation both in single and dual surfactant systems: sodium dodecyl sulfate is mixed with sodium decyl sulfate with added calcium chloride. For the pure sodium dodecyl sulfate, the induction time is far smaller for solutions with a surfactant concentration above the critical micelle concentration (cmc) than for solutions below the cmc. This observation implies that micelles are involved in nucleation of the precipitate. The crystal growth rates, after the induction time is past, are essentially identical in the single and dual systems, indicating that crystal growth kinetics do not play a significant role in the reduction of scum formation. Knowing that micelles act as a template in the single surfactant system and that the growth rates are the same in the single and dual system suggests that the mechanism of inhibition is related to the mixed micelle that forms in the dual system.