The diffusion process in solutions of polystyrene (Mw = 2.74 x 10s) in cyclohexane of noncritical composition slows down upon approaching the liquid/liquid coexistence curve from the homogeneous single fluid phase region of the phase diagram at constant temperatures below the critical temperature and along the liquid/liquid coexistence curve approaching Tc. This effect reflects the influence of the thermodynamic factor (9µ/9 )xy (µ, chemical potential of polystyrene; , volume fraction of polystyrene) on the diffusion coefficient D. The experimental data are discussed in terms of the Flory-Huggins theory of polymer solutions. The composition and temperature dependence of the thermodynamic factor calculated from this model leads to ( , ) curves which qualitatively reflect all features of the experimental data. These findings support the assumption that a slowing down of the diffusion process upon approaching the liquid/liquid coexistence curve of binary mixtures with a miscibility gap is a system independent phenomenon.

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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 (8pj/8xJT, , , . 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 systemindependent property of binary liquid mixtures with a miscibility gap.

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A quick response of laser beam attenuation is found when a temperature change is imposed on near-critical SF, at reduced gravity in space ("piston effect"). The time to reach thermodynamic equilibrium is not correspondingly short. Density equilibration is a long lasting slow process. The times of density equilibration suggest a scaling with the temperature difference (7'-T,).