Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2012
ABSTRACT Sodium dioctylsulfosuccinate (AOT) is a special surfactant and has a special counterion ... more ABSTRACT Sodium dioctylsulfosuccinate (AOT) is a special surfactant and has a special counterion binding (SCB) behavior as one of its special properties. The SCB refers to a sudden two-fold increase in the value of counterion binding constant when the concentration of added NaCl reaches about 0.015 mol kg−1. In order to examine the SCB of AOT when it is mixed with another surfactant which does not possess the SCB, aggregation and adsorption characteristics of mixtures of AOT and sodium dodecylsulfate (SDS) were studied in aqueous medium as a function of concentration of added NaCl by using surface tension method. The mixed systems with bulk mole fraction of AOT ≥ 0.245 have the SCB similar to that of neat AOT. Thus AOT controls the counterion binding behavior of AOT + SDS mixed surfactant system. It has been shown that the Corrin–Harkins equation used for determining the counterion binding constant of single ionic surfactants takes a modified form in the case of mixed surfactants. The compositions of the mixed micelle and adsorption layer were estimated. The added NaCl has a tendency to increase the attractive interaction between the two surfactants in both the micellar phase and the adsorption layer. Sodium ion has been found to have different specificities for sulfonate and sulfate headgroups.
The effect of hydrotropic organic coions on the physicochemical behavior of ionic surfactants has... more 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 pKh (Kh is the hydrolysis constant) values.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2012
ABSTRACT Sodium dioctylsulfosuccinate (AOT) is a special surfactant and has a special counterion ... more ABSTRACT Sodium dioctylsulfosuccinate (AOT) is a special surfactant and has a special counterion binding (SCB) behavior as one of its special properties. The SCB refers to a sudden two-fold increase in the value of counterion binding constant when the concentration of added NaCl reaches about 0.015 mol kg−1. In order to examine the SCB of AOT when it is mixed with another surfactant which does not possess the SCB, aggregation and adsorption characteristics of mixtures of AOT and sodium dodecylsulfate (SDS) were studied in aqueous medium as a function of concentration of added NaCl by using surface tension method. The mixed systems with bulk mole fraction of AOT ≥ 0.245 have the SCB similar to that of neat AOT. Thus AOT controls the counterion binding behavior of AOT + SDS mixed surfactant system. It has been shown that the Corrin–Harkins equation used for determining the counterion binding constant of single ionic surfactants takes a modified form in the case of mixed surfactants. The compositions of the mixed micelle and adsorption layer were estimated. The added NaCl has a tendency to increase the attractive interaction between the two surfactants in both the micellar phase and the adsorption layer. Sodium ion has been found to have different specificities for sulfonate and sulfate headgroups.
The effect of hydrotropic organic coions on the physicochemical behavior of ionic surfactants has... more 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 pKh (Kh is the hydrolysis constant) values.
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