PHYSICOCHEMICAL STUDIES OF AQUEOUS SODIUM DODECYL-SULFATE SOLUTIONS IN PYRIDINE AND ISOMERIC PICOLINES

Specific conductance and surface tension values have been reported for aqueous solutions of sodium dodecyl sulphate (SDS) with or without co solvents, namely, pyridine, alpha-, beta- and gamma-picolines at 298.1 5, 308.15 and 318.15 K. The observed specific conductance data have be en utilised to evaluate values of the counter-ion association constant (alpha) and the limiting equivalent conductance (Lambda(0)) obtained from Onsager's relation. Surface properties, namely surface pressure a t the CMC surface excess concentration and minimum area per molecule o f surfactant at the air-liquid interface have been obtained from surfa ce tension data. Thermodynamic quantities of micellisation, adsorption at the liquid-air interface and transfer of surfactant molecules from aqueous solution to mixed solvents, evaluated from the CMC data, are reported. While the process of micellisation as well as adsorption at the air-liquid interface are exothermic, transfer of SDS from water to mixed solvent is accompanied by an increase in enthalpy. The observed positive values of Gibb's energy for the transfer of SDS from water t o mixed solvent suggests the non-feasibility of such a transfer. The o bserved data have been interpreted in terms of intermolecular interact ions operating between the component molecules in the solution.