Influence of Kosmotropes and Chaotropes on the Krafft Temperature and Critical Micelle Concentration of Tetradecyltrimethylammonium Bromide in Aqueous Solution

Abstract

The study aims to show the effect of some Hofmeister series anions on the Krafft temperature (TK) and the critical micelle concentration (CMC) of tetradecyltrimethylammonium bromide (TTAB) in aqueous solution. The TK of TTAB decreases in the presence of strong kosmotropes and moderate chaotropes but increases in the presence of strong chaotropes. Strong kosmotropes and moderate chaotropes remain hydrated in the bulk phase of the aqueous solution and promote dispersion of the surfactant. On the other hand, strong chaotropes form contact ion pairs with the cationic part of TTAB and exhibit salting out effects showing an increase in TK. The CMC of TTAB decreases in the presence of all ions irrespective of their chaotropic and kosmotropic nature. This can be ascribed to screening of the micelle surface charge by the added counterions. The standard Gibbs energy ( $$ \Delta G_{\text{m}}^{\text{o}} $$ ) of micellization was found to be negative, indicating spontaneity of the process. Solubilization of a water-insoluble dye, Sudan II in TTAB micelles, was quantified by the molar solubilization ratio (MSR). The MSR value was found to be higher in the presence of NaCl compared to that in pure water indicating that the added NaCl facilitated solubilization of the dye.