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

• Sodium dioctylsulfosuccinate + NaCl system studied in aqueous ethylene glycol (EG).
• This surfactant exhibits special counterion binding behavior (SCB).
• The SCB exists if the medium has cohesive energy density between 2.3 and 2.75 J m−3.
• Change in the morphology of the surfactant aggregates is responsible for the SCB.
• An anomalous deviation from the Corrin–Harkins equation observed when EG is ⩾40%.

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.

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