Physicochemical studies on effect of additives on clouding behavior and thermodynamics of polyoxyethylene (20) sorbitan monooleate

Abstract

Cloud point (CP) of nonionic surfactant, namely polyoxyethylene (20) sorbitan monooleate (Tween-80) in aqueous environment and in the presence of different additives such as salts (sodium chloride, NaCl; sodium sulfate, Na₂SO₄; sodium nitrate, NaNO₃) and polymers (polyvinylpyrrolidone, PVP; polyvinyl alcohol, PVA; polyethylene glycol, PEG) have been studied. The CP values of Tween-80 in aqueous medium are found to decrease with increase of surfactant concentration. The CP values of Tween-80 are lower in magnitude in the presence of sodium salts as compared to those in aqueous medium; the effect of sodium salts (between 0.1 and 10 mmol L^-1) in reducing CP values were found to be in the order: Na₂SO₄ ˃ NaCl ˃ NaNO₃. The CP values of Tween-80 solutions also decrease with increasing concentrations of polymers used. The values of ∆G⁰_c are positive in all the cases, indicating the non-spontaneous nature of clouding. ∆H⁰_c and ∆S⁰_c values were found to be almost negative in the presence of salts and polymers, except in the case of the PEG-water system. The negative values of ∆H⁰_c and ∆S⁰_c decrease with increasing concentrations of salts and polymers.