Effect of Temperature and Added Salts on the Critical Micelle Concentration (CMC) and Some Thermodynamic Variables of Sodium DodecylBenzene Sulphonate Solutions

Abstract

Abstract

In this work the state-of-art Kruss K11 tensiometerwas employed to study the effect of temperature in the range of 283-333 K, and added salts on the critical micelle concentration (CMC) of sodium dodecyl benzene sulfonate (SDBS). The temperature range studied exceeded that of the temperature range used in previous studies. The salts were sodium chloride, calcium chloride and 1-Ethyl-3-Methylimidazolium Bromide (EMIB). To the best of my knowledge this is the first time that the effect of an organic salt (molten salt) on the CMC of surfactant has been investigated. It is found that the CMC decreases with increasing temperature up to 305 K and then increases up to the highest temperature studied (333 K). The addition of salt decreases the onset of CMC regardless of the identity of the salt and the effect of divalent salt is more pronounced than that of the monovalent salt. The effect of the molten salt is that the decrease of the surface tension is sharper than in the case of inorganic salt while the surface tension values are comparable to those of the monovalent salt but higher than those of the divalent salt. The surface excess concentration was calculated and shown to vary with both temperatures and concentrations of added salts. The onset of micellization is enhanced by increasing temperature and by the presence of the salt regardless of the identity of the salt. The Gibbs free energy of micellization is negative and becomes more negative with temperature and with the addition of salt. The enthalpy of micellization is also negative and exhibits a behavior similar to that of Gibbs free energy micellization. The calculated entropy of micellization is positive indicating that micellization is accompanied by an increase in the disorder of the system. The findings of this study are of benefit to the practical applications of ionic surfactants.