Static light scattering experiments with aqueous solutions of the nonionic tenside C12E5 with critical and non‐critical composition

Abstract

AbstractConcentration fluctuations with long range correlations as well as non‐critical background contributions determine the light scattering properties of C12E5/H2O mixtures of critical composition near the lower critical point (critical mass fraction yc = 0.0123; critical temperature Tc = 31.8°C). If the influence of the non‐critical contributions is neglected, an evaluation of the light scattering data in terms of the power laws ξ = ξo · ϵ‐ν and χT = χT,o · ϵ−γ (ξ being the correlation length of local concentration fluctuations and χT the generalized susceptibility; ϵ = (Tc−T)/Tc is the reduced temperature) leads to values of the critical exponents ν and γ which are closer to the mean field values (ν = 0.5; γ = 1) than to the values theoretically expected for binary liquid mixtures from the Landau‐Ginzburg‐Wilson model (ν = 0.63, γ = 1.24). If the background contributions are taken into account empirically by adding temperature independent background terms ξb and χT,b respectively to the simple power laws, the analysis of the light scattering data leads to values of ν ≈︁ 0.65 and γ ≈︁ 1.2, which are close to the values given by the Landau‐Ginzburg‐Wilson model. The background term ξb has a value of about ξb ≈︁ 6 nm and the critical amplitude ξo a value of about ξo ≈︁ 1.2 nm. This value for ξo is one order of magnitude larger than that found for simple binary liquid mixtures (O(ξo) ≈︁ 0.1 nm). — In mixtures of non‐critical composition (−0.79 < (y−yc)/yc < 0.63) the scattered intensity shows an angular dependence which is also a function of temperature corresponding to correlation lengths 20 nm < ξ < 206 nm for 0.5 K < (TP−T) < 7.0 K. They are assumed to reflect contributions of micelles which also contribute to the scattering of the mixture of critical composition away from the critical temperature.