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, ΔGm°, 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 ϕv° and ϕk° have been used to explain the solvation of surfactant monomers in association with DS.