Abstract:
In the present work, the influence of some counterions on the Krafft temperature (TK) and critical micelle concentration (CMC) of tetradecyltrimethylammonium bromide (TTAB) has been investigated. The thermodynamics of adsorption and bulk micellization as well as the solubilization of a water insoluble dye, sudan in surfactant solution has also been investigated. It was found that the TK of TTAB decreases in the presence of less chaotropic NO3 and Cl as well as kosmotropic SO42 and to some extent F and increases in presence of more chaotropic SCN , I and to some extent Br. The order of ions in terms of decreasing the TK is NO3 SO42 Cl F Br SCN I. More chaotropic SCN and I form contact ion pairs with the surfactant cation, which results in the salting-out behavior with a consequent increase in the TK. The activity of free water molecules increases in presence of less chaotropic NO3 and Cl and thereby hydration of TTAB molecules increases, showing a decrease in the TK. Kosmotropic SO42 and F having high charge density remain strongly hydrated in the bulk rather than forming contact ion pairs and thereby decrease the TK. It was found that the CMC of TTAB decreases in presence of all the salts due to the screening of the micelle surface charge by the associated counterions with SO42 being most effective in lowering the CMC and the propensity follows the order: SO42 NO3 Br Cl F. The CMC of TTAB in pure water and in the presence of NaCl was found to increase gradually with increasing temperature. The thermodynamic parameters (changes in Gibbs free energy, enthalpy and entropy) for both adsorption and bulk micellization have also been calculated. The Δ𝐺𝑚0 and Δ𝐺𝑎𝑑0 values are found to be negative which indicates that both the processes are spontaneous. For both adsorption and micellization the enthalpy and entropy terms are found to compensate with each other. The surface excess concentration () was found to decrease gradually with increasing temperature and the values are much higher in the presence of NaCl than the corresponding values in pure water. From the solubilization study it has been found that the CMC of TTAB decreases in presence of sudan . It has also been found that the solubilization capacity in presence of NaCl is higher than that of pure TTAB.
