Interactions between cationic surfactant and poly ethylene glycol: Effect of the polymer concentration and alkyl chain length of surfactant

The interaction between a homologous series of n-alkyltrimethylammonium bromides ($C_n{H}_{2n+1}\stackrel{+}{N}{\left(C{H}_3\right)}_{3}B{r}^{-}$; n = 12 (DOTAB), 14 (TTAB), and 16 (CTAB)) and polyethylene glycol 4000 (PEG) were investigated in aqueous solution using electrical conductivity method and zeta potential (ζ-potential) technique at 298.15 K. The variation of the specific conductivity vs. surfactant concentration shows one breakpoints, which is called the critical micelle concentrations (CMC). This point (CMC) in the presence of aqueous PEG solutions (0.1, 0.5, 1, 2.5, 4 and 6 %w/v) were determined for different surfactants by using the Carpena method. Some physicochemical parameters have been calculated such as pre- and post-micellar slopes (S₁ and S₂), degree of counter-ion binding (α), standard Gibbs free energy of micellization ($\Delta G_{mic}^0$) and the Gibbs free energy of transfer ($\Delta G_{\mathrm{trans}}^0$). It can be seen that the CMC value increases almost linearly with increasing polymer concentration in each system (except CTAB) and the slopes depends on alkyl chain length of surfactant. The calculated values of $\Delta G_{mic}^0$ are negative in all studied systems indicating the spontaneity and stability of the micellization. In the second section of manuscript, a comprehensive study of the ζ-potential of PEG (at two different percentages) in contact with aqueous solutions containing DOTAB, TTAB, and CTAB surfactants is studied. From the obtained results, it can be suggested that PEG molecules can give solvophobic effect which increases the CMC of cationic surfactants compared to that in pure water.