Solubilization of 1-hexanol in aqueous solutions of sodium dodecyl sulfate at pressures up to 140 MPa: partial molar volumes, compressibilities, and partition coefficients.

Abstract

Partial molar volumes and partial molar compressibilities of aqueous solutions of sodium dodecyl sulfate (SDS) and 1-hexanol at pressure up to 140 MPa have been determined. For aqueous SDS solutions the partial molar compressibility increases with pressure below the cmc and decreases with pressure above the cmc. The partial molar compressibility of aqueous 1-hexanol increases with pressure. The increased partial molar compressibility reflects that the structure of water is gradually broken down by increased pressure. Thus, the negative effect of electrostriction around the charged parts of SDS decreases as do the effects of hydrophobic hydration around the CH2 groups. In the micellar state the compressibility of the aggregate is the main factor, becoming less compressible as pressure increases. The cmc of SDS, as determined by speed of sound measurements, increases with pressure and goes through a shallow maximum at about 110 MPa. When 1-hexanol is added to SDS solutions, it will be partitioned between the aqueous and micellar (pseudo) phases. The partition coefficient has been determined from partial molar compressibilites, and it appears to decrease with pressure, reach a minimum around 80 MPa, and then increase, though the change with pressure is small.