Effect of ethylene glycol on the special counterion binding and microstructures of sodium dioctylsulfosuccinate micelles

Sodium dioctylsulfosuccinate (AOT) micelle has a special counterion binding behavior (SCB), which refers to the abrupt twofold increase in the counterion binding constant (β) at a critical concentration (c*) of added NaCl (in water c* ≈ 0.015 mol kg(-1)). In this paper, the SCB of AOT has been studied in a mixture of water and ethylene glycol (EG) by applying surface tension, fluorescence, and small angle neutron scattering (SANS) methods. The SCB exists in water + 10% (w/w) EG as well, but disappears when the EG% is ≥20. It has been found out that the SCB of AOT occurs in media having cohesive energy density values in the range of 2.3-2.75 J m(-3). SANS data indicate co-existence of vesicles and cylindrical micelles of AOT in water + 10% EG when the added NaCl concentration is greater than c* thereby revealing that change in the morphology of aggregated species is the probable cause for the SCB of AOT. From this study it has become clear that the Corrin-Harkins (CH) equation, commonly used for determining β, can be applied only above a limiting concentration (ce(#)) of added electrolyte. In aqueous organic or pure organic polar solvents below ce(#) sharp deviation from the CH equation occurs with reversal of slope rendering this equation inapplicable for the determination of β.