Effect of Poly(vinyl pyrrolidone) and Poly(vinyl alcohol) on Osmotic Coefficien and Sodium lon Activity in Aqueous Solution of Sodium Dodecyl Sulfate

Abstract

The osmotic coefficient, φ, of aqueous solution of sodium dodecyl sulfate(SDS) was measured with a vapour pressure osmometer(VPO) at 37° C, and the sodium ion activity, α Na+, was also measured with glass electrode (pNa meter). The degree of ionic dissociation of sodium ion from micelles determined withthese two methods agreed to be 0.213.
The osmotic coefficient, φ, of aqueous solutions of SDS decreased upon the addition of poly(vinyl pyrrolidone)(PVP) or poly(vinyl alcohol)(PVA), as shown in Fig.4, but the activity of sodiumion, α Na+, was little affected, as shown in Fig.5.
The activity, α Na+, of anion, DS-, was culculated from α Na+ and osmotic pressure, π. The anionic activity, α Ds-, of aqueous SDS solutions containing no polymer increased with SDS concentration and reached a maximum at 10 mmolii of SDS concentration, and then slightly decreased at higher concentrations. Thisbehavior of anionic activity of surfactant is the same as that of the anionic activity of sodium caprate shown by Ekwall et al. br
For the system containingpolymer, however, the maximum was not observed and the activity leveled off at higher concentration of SDS as shown in Fig.8. This means that, although DS anions form a complex with polymer, this complex formation does not affect the dissociation of sodium ion from SDS molecules. It wasconcluded that SDS anions bound to polymer form a micelle-like structure becausepolymer does not affect α Na+ value in spite of the complex formation.