Diffusivity in nitrobenzene-isooctane mixtures of non-critical composition approaching the liquid–liquid coexistence curve

Abstract

Dynamic light-scattering experiments with mixtures of non-critical composition of the nitrobenzene–isooctane system are carried out to obtain information about the temperature and composition dependence of the mutual diffusion coefficient, D. The system has a mixing gap with an upper critical point. It is shown that D decreases in a characteristic way along two pathways in the phase diagram of the system: approaching the temperature of phase separation in a mixture of constant composition or approaching the critical point along the two branches of the liquid–liquid coexistence curve. From a thermodynamic point of view this behaviour is interpreted as being caused by the temperature and composition dependence of the thermodynamic factor (∂µ_i/∂x_i)_{T, p}. An equivalent interpretation is based on the assumption that it is caused by the temperature and composition dependence of the correlation length of the local composition fluctuations. Evidence for the validity of the two interpretations is given. The described temperature and composition dependence of D appears to be a system independent property of binary liquid mixtures with a miscibility gap.