Counterion Effect on Krafft Temperature and Related Properties of Octadecyltrimethylammonium Bromide

Abstract

AbstractIn the present work, we have investigated the effect of some counterions on the Krafft temperature (TK) and the micelle formation of octadecyltrimethylammonium bromide (OTAB) in aqueous solution. The results showed that the ions with more chaotropic nature increase the TK while those with a kosmotropic, hydrotropic and less chaotropic nature lower the TK of the surfactant. More chaotropic SCN− and I−, being weakly hydrated, form contact ion pairs with the octadecyltrimethylammonium ion and reduce the electrostatic repulsion between the surfactant molecules. As a result, these ions exhibit salting out behavior and raise the TK of the surfactant. On the other hand, less chaotropic Cl− and NO3−, kosmotropic SO42− and F− and hydrotropic benzoate and salicylate ions increase the solubility of the surfactant, with a consequent decrease in the TK. SO42−, F−, benzoate and salicylate cannot form contact ion pairs with the weakly hydrated cationic part of OTAB. Rather, being extensively hydrated and kosmotropic in nature, these ions do not show any tendency to shed their hydrated water molecules to form contact ion pairs with the weakly hydrated octadecyltrimethylammonium ion and therefore, stay apart. As a result, the TK of the surfactant decreases significantly in the presence of these ions. The critical micelle concentration (CMC) of the surfactant decreases significantly in the presence of these ions due to screening of the micelle surface charge by the added counterions. Consequently, the surfactant molecules attain better packing because of substantial reduction in the electrostatic repulsion between the charged head‐groups, showing a significant decrease in the CMC.