Salt-Induced Modulation of the Krafft Temperature and Critical Micelle Concentration of Benzyldimethylhexadecylammonium Chloride

In this work, the effect of some sodium salts on the Krafft temperature (T (K)) and critical micelle concentration (CMC) of benzyldimethylhexadecylammonium chloride (C16Cl) in aqueous solution has been studied. It was observed that the T (K) can be modulated to lower and higher values and the CMC can be depressed significantly upon the addition of the electrolytes. More chaotropic Br- and I- raise the T (K) with an increase of the concentration of the ions. On the other hand, less chaotropic NO3 (-) initially lowers and then raises the T (K). Kosmotropic F-, SO4 (2-) and CO3 (2-) gradually lower the T (K) with increasing concentration of the electrolytes. The more chaotropic ions form contact ion pairs with the surfactant and decrease the solubility with a consequent increase in the T (K). On the other hand, kosmotropic ions, being extensively hydrated in the bulk, remain separated from the surfactant by hydrated layers of water molecules. As a result, a significant electrostatic repulsion exists between the charged headgroups of the surfactant, resulting in a decrease in the T (K). The CMC of the surfactant decreases significantly in the presence of these ions. The surface tension at the CMC (gamma(CMC)) also decreases in the presence of all the salts except for F-. The electrostatic repulsion between the charged headgroups is significantly reduced because of screening of the surface charge of both micelles and adsorbed monolayers by the associated counterions, resulting in a decrease in both the CMC and gamma(CMC).