Counter-ion effects on the krafft temperature and micelle formation of ionic surfactants in aqueous solution

Abstract

In this work the effect of some sodium and chloride salts on the Krafft temperature (TK) and critical micelle concentration (CMC) of two classical ionic surfactants, Octadecyltrimethylammonium bromide (OTAB) and Sodium dodecyl sulfate (SDS) have been investigated by conductometric and tensiometric method. Sodium salts of different monovalent and divalent anions belonging to the Hofmeister series found to decrease or increase the TK of OTAB. In terms of decreasing the TK the propensity follows the order: C7H5O3− > C7H5O2− > C6H5SO3− > SO42− > Cl− > NO3− > F− > Br− > SCN− > I−. The results show that hydrotropic and kosmotropic counter-ions decrease while chaotropic counter-ions increase the TK of the surfactant. Chloride salts of monovalent cation such as Li+, Na+, Cs+, K+ affect the solubility of SDS and hence TK of the surfactant. Some salts increase while some decrease the TK of the system. In terms of deceasing the TK the ions follows the trend: Li+ > Na+ > Cs+ > K+. Added counter-ions screen the charge of the micelle head group and facilitate closer packing of the surfactant. Thus added salts always decrease the CMC of the surfactant. Different salts interact differently with surfactant and thus decrease the CMC differently. For SDS the effectiveness in lowering the CMC the ions follows the order: Cs+ > K+ > Na+ > Li+. On the other hand, in terms of OTAB the ions follow the following trends in decreasing the CMC: C6H5SO3− > C7H5O2− > C7H5O3− > SO42− > NO3− > Br− > Cl− > F−. Thermodynamic parameters (Gibbs free energy, enthalpy and entropy changes) of micellization and adsorption were calculated from the specific conductance and surface tension data. The negative value of free energy change indicates the process to be spontaneous. The enthalpy and entropy terms are found to compensate each other for both micellization and adsorption. For most of the cases surface excess concentration (Г) was found to be higher in presence of salts than pure surfactant showing lower equilibrium surface tension of the system. The solubilization behavior of a water insoluble dye, Sudan Red B (SRB), in the micellar system was studied by the UV–visible spectrophotometric technique. The solubilization of SRB in OTAB in the presence of Na2SO4 was found to be about 1.33 times higher than that in pure water. In the case of SDS the value was found to be 1.07 times in the presence of NaCl. This indicates that the solubilization of SRB in the surfactant micelles significantly increases in the presence of added salts.