Micellization behaviors of surfactants and the study of the location of solubilized species within the micellar system by complexation with metalss

Abstract

Knowledge of Micellar behavior of a surfactant is essential for its wide range of applications. In the present work a comprehensive study of micellization, adsorption and thermodynamic behavior of TTAB, CTAB and OTAB (in pure water and also in presence of NaCl) has been conducted by conductivity and surface tension methods. From the CMC studies it is observed that increase of temperature causes increase of CMC of TTAB and CTAB which is due to thermal solubility of the surfactant monomers. Then slight decreases of the CMC were observed with further increase of temperature (above 313 K) which is due to dehydration of the head groups. These CMC studies have been utilized to evaluate the Stauff‐ Klevens empirical relation of log CMC= A – Bn, where n is the number of carbon atom in the alkyl chain of the surfactant and A is constant for a particular ionic head at a given temperature. This empirical relation was found to become insignificant in presence of NaCl. The surface excess concentration (Γm) and the counter ion binding (β) values of the surfactants were found to be greater in presence of NaCl than in pure water. From the thermodynamic calculations it was found that both the adsorption and micellization processes are enthalpy and entropy controlled. Δ om G and Δ o m H values were found to be negative whereas Δ o m S values were positive. For each of the surfactants Δ om G and Δ o m H values were found to increase, whereas Δ o m S values decrease, with increase of temperature. Greater Δ om G and Δ o m S values were obtained for the surfactant with lager alkyl chain. The CMC measurement and the thermodynamic studies were not possible, for OTAB below temperature of 313 K since the Krafft temperature of OTAB is higher than this temperature (the TK is 309.5 K). Experimental results show that TK of TTAB, CTAB and OTAB are 12.4oC, 24.7 oC and 36.5 oC respectively. The Krafft temperature (TK) the surfactants were found to decrease in presence of NaCl, NaF, Na2SO4 where as the TK values increase in presence of NaBr and NaI . NMR studies and the metal (zinc, cadmium and mercury) – ligand complexation studies were carried out to identify solubilization site the ligand (DiphenylCarbazide). The experimental results show that (a) The solubilization site for DPC in TTAB is in between the hydrophilic groups and the first few carbon atoms of the hydrophobic groups that comprise of outer core of the micelle interior. (b) The solubilization site for DPC in CTAB is more deeply seated in the palisade layer of the micelle. (c) The solubilization site for DPC in OTAB is even more deeply seated than the palisade layer, almost near about the inner core of the micelle.