Adsorption and micellar behavior of aqueous ionic surfactant systems

Abstract

In the present work, the thermodynamic behavior of surface adsorption and micellization of cetyltrimethylammonium bromide (CTAB) and Tetradecyltrimethylammonium bromide (TTAB) in pure water and aqueous NaCl solution was investigated by surface tentiometric and conductometric methods. The CMC values of the surfactants in pure water gradually increased with increasing temperature. On the other hand, the CMC values of CTAB and TTAB in presence of NaCl increased and then decreased with increasing temperature and the values were found to be much lower than the corresponding values in pure water. Thus the CMC-Temperature data can be shown by a Λ −shaped curve. The micellar dissociation (fraction of counter-ion binding) and energetic parameters (free energy, enthalpy and entropy) of both adsorption and micellization were calculated. The processes were found to be both enthalpy and entropy controlled and appeared to be more and more enthalpy driven with an increase in temperature. An enthalpy-entropy compensation rule was found to hold for both adsorption and micellization processes. The krafft temperature (Tk) of the surfactant was found to decrease significantly with an increase in concentration of NaCl, which is a sharp contrast to the usual behavior of the effect of electrolytes on the Tk values of classical ionic surfactants. The surface excess concentration (Γ) of both surfactants in pure water was found to decrease gradually with increasing temperature. However, the values of Γ were much higher in the presence of NaCl compared to the corresponding values in pure water. The aggregation number (Nagg) of the surfactants in presence and in absence of NaCl has been measured. The aggregation numbers (Nagg) of the surfactants in the presence of NaCl were found to be higher than those in pure water. The solubilization behavior of a water insoluble dye, sudan red B (SRB) in the micellar system was studied by UV-visible spectrophotometric technique. The solubilization ratio in the presence of NaCl was found to be about three times higher than in pure water, indicating that the solubilization of SRB in the CTAB and TTAB micelles significantly increased with an increase in NaCl concentration.