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

Abstract

Abstract 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 NO 3 − initially lowers and then raises the T K . Kosmotropic F − , SO 4 2− and CO 3 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 (γ 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 γ CMC .