The effect of alcohols on the change in partial molal volumes and compressibilities at micelle formation of sodium decanoate at 25°C

Abstract

The effect of methanol, ethanol, 1-propanol, and 2-propanol on the change in partial molal volume, ΔV m , and compressibility, Δκ s m , during micelle formation of sodium decanoate has been determined at 25°C. Partial molal volumes were determined from density measurements, and partial molal compressibilities were determined from ultrasound measurements. It was found that both ΔV m and Δκ s m had a maximum in the concentration range 1–3 m alcohol, and then decreased rapidly on further addition of alcohol. These results have been interpreted from changes in the separate values of the partial molal quantities of the surfactant in the singly dispersed state and in the micellar state, respectively. The effect of the alcohols on the CMC at 25°C has been determined by conductivity measurements. It was found that the CMC decreases with increasing concentration and chain length of the alcohol. The fraction of associated counterions, β, has been determined by EMF measurements, and it was found that ,β decreases upon alcohol addition. This result has been interpreted from the reduced surface charge of the micelles caused by the alcohols.