Influence of hydrotropic coions on the shape transitions of sodium dioctylsulfosuccinate aggregates in an aqueous medium

Abstract

The effect of hydrotropic organic coions on the physicochemical behavior of ionic surfactants has not yet been understood clearly, although as counterions they are known to have a remarkable effect on the microstructures of ionic surfactants. In this paper therefore the aggregation behavior of sodium dioctylsulfosuccinate (AOT) has been studied in aqueous media in the presence of sodium benzoate (NaB)/sodium salicylate (NaSa)/sodium meta-hydroxybenzoate (Na-mHB)/sodium para-hydroxybenzoate (Na-pHB). Surface tension and small angle neutron scattering (SANS) techniques are mainly used. AOT is known to have a special counterion binding behavior (SCB) with respect to sodium counterion, viz., at about 0.015 mol kg⁻¹ of added NaCl the counterion binding constant of AOT increases suddenly by two fold which is attributed to the shape change of the AOT micelle. The SCB of AOT is found to exist in the presence of benzoate, m-hydroxybenzoate and p-hydroxybenzoate coions, but not in the presence of o-hydroxybenzoate (salicylate) coions. The SANS data, on the other hand, showed that shape transition of the AOT aggregate takes place in the presence of all the four hydrotropic coions; the concentration of NaSa required to induce the shape transition was however higher. A sharp increase in the value of the hydrodynamic diameter determined by the dynamic light scattering (DLS) experiments also indicates the shape transition of AOT aggregates in the presence of the salts. Chemical shift values of benzoate and hydroxybenzoates confirmed the binding of these coions to an AOT micelle. The binding of the salicylate coion is shown to be stronger due to its low hydration energy and high pK_h (K_h is the hydrolysis constant) values.