Critical adsorption at the surface of a polymer solution. Analysis of ellipsometric data on the depletion layer near the critical solution point

The structure of the liquid-gas interface of a polymer solution, polystyrene (MW=110.000) in cyclohexane, has been studied at temperatures and concentrations near the critical solution point (Tc=294.75 K, wc=10.2wt%) using ellipsometry. In the one-phase region of the liquid (T >Tc) the ellipticity ρ measured at fixed bulk concentrations increases on approaching the phase separation temperature Tb but exhibits a finite maximum 0.2 to 0.5K above Tb. This behaviour is analysed in terms of model profiles of the optical dielectric constant ϵ(z), based on the conjectured scaling form of the composition profile of semi-critical interfaces, which predicts a divergence of the profile width proportional to the correlation length ζ on approaching criticality. The pronounced increase of ḡ9 with the bulk concentration of the polymer at temperatures well above Tc can be explained by the persistence of a depletion layer of considerable thickness (typically 10 nm) away from the near-critical region.