Thermodynamics of aggregation of Tweens in the presence of diclofenac sodium

The effect of the addition of diclofenac sodium (DS) to the aqueous solution of non-ionic surfactants, polyoxyethylene sorbitan monolaurate, Tween-20 (T-20) and polyoxyethylene sorbitan monooleate, Tween-80 (T-80) has been investigated by means of conductivity and thermodynamic (density and speed of sound) studies at 25 °C. Studies have been carried out with variation in surfactant concentration at fixed [DS] = 5, 7, 9, 12 mM. The formulations have been characterized through the UV–vis spectroscopy. The spectra show a shift in λmax in the presence of Tweens. Critical micellization concentration, cmc, has been obtained from the conductivity–concentration plots. Conductivity results show an apparent change in curvature on addition of DS. Standard free energy of micellization, Δ G m ° , has been estimated based upon the mass–action model. Apparent molar properties, viz. Vϕ,s and Kϕ,s, have been computed from the density and ultrasonic velocity measurements. Limiting parameters of ϕ v ° and ϕ k ° have been used to explain the solvation of surfactant monomers in association with DS.