Ultrasonic investigation of the behaviour of sodium dodecyl sulphate in aqueous poly(N-vinyl- 2-pyrrolidone)

Abstract

The interaction of sodium dodecylsulphate (SDS) and aqueous poly(N-vinyl-2-pyrrolidone) (PVP) has been investigated over a large range of SDS concentrations and at [PVP]=0.4 and 0.2% (w/v) at 313.15 K using ultrasound absorption and velocity measurements. The results suggest that SDS in dilute solutions ([SDS] < 7 m<em>M</em>) forms premicellar "associates" some of which undergo either hydrophobic binding with PVP or get adsorbed on to the PVP coil surface. Na⁺ ions from the bulk solution also get bound to the O^δ- ends of PVP monomer units. The latter process triggers uncoiling of PVP coils followed by the simultaneous binding of the negatively charged premicellar SDS "associates" with the N^δ+ end of the PVP monomer units in the range 7 < [SDS] < 11 m<em>M, </em>beyond which electrostatic repulsion between the negatively charged, bound SDS "associates" on the PVP coils causes further stretching of the PVP coils and consequent greater binding of SDS "associates". The binding process gets completed at [SDS] ≈ 18 m<em>M, </em>beyond which pure SDS micelles, that contain some PVP- SDS "associates" entities, are formed.