Effect of size of tetraalkylammonium counterions on the temperature dependent micellization of AOT in aqueous medium

Abstract

Different tetraalkylammonium, viz. N⁺(CH₃)₄, N⁺(C₂H₅)₄, N⁺(C₃H₇)₄, N⁺(C₄H₉)₄ along with simple ammonium salts of bis (2-ethylhexyl) sulfosuccinic acid have been prepared by ion-exchange technique. The critical micelle concentration of surfactants with varied counterions have been determined by measuring surface tension and conductivity within the temperature range 283–313 K. Counterion ionization constant, α, and thermodynamic parameters for micellization process viz., $\Delta G_m^{\text{0}}$, $\Delta H_m^{\text{0}}$, and $\Delta S_m^{\text{0}}$ and also the surface parameters, Γ_max and A_min, in aqueous solution have been determined. Large negative $\Delta G_m^{\text{0}}$of micellization for all the above counterions supports the spontaneity of micellization. The value of standard free energy, $\Delta G_m^{\text{0}}$, for different counterions followed the order ${\text{N}}^{\text{ + }} \left( {{\text{CH}}_{\text{3}} } \right)_4 >{\text{NH}}_{\text{4}}^{\text{ + }} >{\text{Na}}^{\text{ + }} >{\text{N}}^{\text{ + }} \left( {{\text{C}}_{\text{2}} {\text{H}}_5 } \right)_{\text{4}} {\text{ $>$ N}}^{\text{ + }} \left( {{\text{C}}_{\text{3}} {\text{H}}_{\text{7}} } \right)_4 >{\text{N}}^{\text{ + }} \left( {{\text{C}}_{\text{4}} {\text{H}}_{\text{9}} } \right)_4$, at a given temperature. This result can be well explained in terms of bulkiness and nature of hydration of the counterion together with hydrophobic and electrostatic interactions.