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.
