Small Angle Neutron Scattering Experiments with Aqueous Solutions of the Nonionic Surfactant C12E5 with Critical and Noncritical Composition

Abstract

AbstractResults of small angle neutron scattering experiments with four solutions of the nonionic surfactant C12E5 in D2O with critical and noncritical compositions in the vicinity of the lower critical point are reported (1.11·10−4 ≤x ≤ 2.042·10−3; x, mole fraction of C12E5; xc = 5.74·10−4, critical mole fraction). They were obtained at temperatures away from the liquid/liquid coexistence curve. The observed angular dependence of scattered neutron intensity is assumed to be caused by micelles formed by surfactant molecules. Two characteristic lengths Ξ1 and Ξ2 are calculated from the scattering profiles. Ξ1 has a value of the order of 10 nm. It is attributed to the extension of noncritical concentration fluctuations. Ξ2 has a value of the order of 1 nm and is attributed to the core of micelles. At constant temperature Ξ1 decreases with increasing surfactant concentration, whereas at constant surfactant concentration it increases with increasing temperature weakly (compared with the divergence of the correlation length of local composition fluctuations approaching Tc). ‐ A combination of light scattering and small angle neutron scattering data obtained with the system C12E5/water gives support to the assumption that in the vicinity of the lower critical point a continuous change will take place from a comparatively weak temperature dependence of the length characterizing the structure of the liquid to a strong temperature dependence of the correlation length of the critical concentration fluctuations.