Influence of Some Hofmeister Anions on the Krafft Temperature and Micelle Formation of Cetylpyridinium Bromide in Aqueous Solution

Abstract In this work, the effect of some Hofmeister anions on the Krafft temperature (TK) and micelle formation of cetylpyridinium bromide (CPB) have been studied. The results show that more chaotropic anions increase, while the less chaotropic ones lower the TK of the surfactant. More chaotropic I− and SCN− form contact ion pairs with the cetylpyridinium ion and reduce the electrostatic repulsion between the CPB molecules. As a result, these ions show salting‐out behavior, with a consequent increase in the TK. In contrast, less chaotropic Cl− and NO3− increase the activity of free water molecules and enhance hydration of CPB molecules, showing a decrease in the TK. A rather unusual behavior was observed in the case of SO42− and F−. These strong kosmotropes shift from their usual position in the Hofmeister series and behave like moderate chaotropes, lowering the TK of the surfactant. Because of the high charge density and the strong tendency for hydration these ions preferentially remain in the bulk. Rather than forming contact ion pairs, these ions stay away from the CPB molecules, decreasing the TK of the surfactant. In term of decreasing the TK, the ions follow the order NO3− > SO42− > Cl− > F− > Br− > SCN− > I−. The critical micelle concentration (CMC) of the surfactant decreases significantly in the presence of these ions due to the screening of the micelle surface charge by the excess counterions. The decreasing trend of the CMC in the presence of the salts follows the order SCN− > I− > SO42− > NO3− > Br− > Cl− > F−.