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 (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.

Slowing Down of the Mutual Diffusion Process Approaching the Liquid/Liquid Coexistence Curve of a Nonionic Surfactant/Deuterium Oxide System: Study of the System C12E5/D2O

“…For “simple” systems the regular solution model (upper critical point) has been used to calculate the temperature and composition dependence of (∂μ i /∂ x i ) P , T . The calculated temperature and composition dependence of the diffusion coefficient is in qualitative agreement with experimental data. ,,, The observed slowing down of the diffusion process in a wide temperature−composition region along the coexistence curve indicates that from a physical point of view the system is “close” to its binodal curve.…”

“…It is well established that the slowing down of the mutual diffusion process approaching the liquid/liquid coexistence curve of systems with a miscibility gap is a system independent phenomenon. [1][2][3][4][5] The data shown in Figure 4 are replotted in the form of isotherms in the upper part of Figure 5 to bring out similarities to results reported by Kato et al for the same system 30 The critical temperature of the system C 12 E 5 /water is about 20 K lower than that of the system C 12 E 6 /water. In the system C 12 E 6 /water Strey et al have carried out temperature jump experiments to study the dynamics of the system.…”

“…C 4 E 1 /H 2 O), polymer solutions (e.g. polystyrene/cyclohexane), and polymer melts (polystyrene- h /polystyrene- d ). − …”

“…The slowing down of the diffusion process also shows up in the kinetics of the liquid/liquid phase separation studied along the binodal curve after crossing the coexistence curve by fast temperature or pressure jumps. These are system independent properties. − …”

Study of the Viscosity of Nonionic Surfactant/Deuterium Oxide Mixtures and of the Self-Diffusion Coefficient of the Surfactant Approaching the Liquid/Liquid Coexistence Curve

“…It is well established that the mutual diffusion coefficient D at constant composition of binary liquid mixtures with a miscibility gap shows that D decreases approaching the liquid−liquid coexistence curve. This is a material independent property. − For a system with a lower critical point the mutual diffusion coefficient decreases with increasing temperature in the vicinity of the binodal curve. Dynamic light scattering experiments with C 12 E 5 /D 2 O mixtures in the vicinity of the lower critical point have shown that this occurs in the “large aggregate” region of the phase diagram in a region close to the T η ,max ( y ) curve shown in Figure .…”