Neutron reflectivity studies of the free liquid surface of methylcyclohexane–perfluoromethylcyclohexane near the critical endpoint

Abstract

The surface structure of the liquid mixture methylcyclohexane (MCH)–perfluoromethylcyclohexane (PFMCH) has been investigated by specular neutron reflectometry from 42 K below to 10 K above the upper critical solution temperature T_UCS=46.13°C. The results have been interpreted in the light of the Abele′s layer model which indicates that the reflectivity throughout the experimental temperature range is satisfactorily represented by a model with two relatively thin layers interposed between the vapour phase and a well-characterised subphase. The twin-layer region is everywhere richer in PFMCH, the component of lower surface tension, than the immediate subphase and is thus regarded as an adsorbed layer. For T_UCS<T, in the region of one-liquid phase coexistence, the composition of the subphase is that of the critical mixture. The divergence of the composition, thickness and surface excess of the adsorbed layers in the very near vicinity of T_UCS is indicative of critical adsorption. In the region of two-liquid phase coexistence, i.e. T<T_UCS, the experimental data can be further divided into two separate categories depending on whether the temperature was rising or falling. For both categories the interposed adsorption region is again modelled by a minimum of two slab layers of similar thickness while the subphase composition varies with temperature and is well represented by the composition of the lower PFMCH-rich phase for temperatures just falling below T_UCS and by that of the upper MCH-rich liquid phase for much lower temperatures rising from around the measured wetting transition temperature T_W. The subphase scattering length density is calculated at the experimental temperature from the known liquid/liquid phase diagram. The vapour/liquid wetting phase is thus confirmed directly as the lower bulk phase, while the identification of T_W is indeterminate from the measurements due to hysteresis effects associated with the interface.