Interfacial properties of poly(N-vinyl-2pyrrolidone) at the air/water interface

Spread films of poly(N-vinyl-2-pyrrolidone) (PVP) were studied at the air/water interface by using the Langmuir technique. Influence of the subphases of different thermodynamic conditions were analyzed. Surface pressure-area (π-A) isotherms change sensitively with the temperature. Adsorption layers (Gibbs surface excess layer) of aqueous solutions of PVP were also investigated using the interfacial tension measurements. The influence of the PVP concentration and temperature on the adsorption behavior of PVP solutions was determined. The behavior of spread films is contrasted to the behavior of surface excess layers for this polymer. The driving force for adsorption of PVP at the air/aqueous interface seems to arise from an entropie contribution. It was observed that when the polymer's hydrodynamic volume decreases (0.55 M Na₂ θ-solvent), a more favorable entropy adsorption was obtained. Comparison of the spread film and surface excess layer data seems to indicate that the interfacial segment density is lower in the surface excess layer than in the spread film.