Spectroscopic Study of Water-Hydrocarbon Mixtures at High Temperatures and Pressures.

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We have elucidated the following properties of mixed fluids at high temperatures and pressures.(1) Water concentration in a hydrocarbon-rich phase initially increases dramatically as temperature rises at constant pressure, and reach maximum at a temperature close to one-phase critical curve, and then steeply decreases at higher temperatures.(2) Water molecules in benzene and alkylbenzene at temperatures below 523K exist as monomers or dimers, and the relative ratio of the dimers increases with increasing temperature.(3) π-hydrogen bonding interaction between water and a benzene ring exists even at high temperatures under pressure.(4) An anomalously large volume expansion occurs on mixing of water and hydrocarbons in a certain temperature-pressure range. This finding could be observed for the first time by the in situ infrared measurements of the fluid mixtures. For water-benzene mixtures, the volume expansion is particularly large in the region enclosed by (a) the gas-liquid equilibrium curve of water, (b) an extended line of the three-phase equilibrium curve, and (c) the one-phase critical curve of the mixtures.(5) Solubility of benzene in water is an order of magnitude smaller than that of water in benzene, and the solubility of benzene in water increases with increasing pressure at constant temperature, while that of water in benzene decreases. This asymmetry of the mutual solubilities can be understood by differences in molecular size and in isothermal compressibility between water and benzene.(6) For high-temperature-pressure water, it is found that (i) in the compressed gas or gas-like condition a monomer-dimer equilibrium exists and the dimerization enthalpy is estimated to be (-15±3)kJ/mol, (ii) an appreciable proportion of water molecules rotate quite freely even at 673K at 400 bar, and (iii) ratio of hydrogen bond formation is dependent on molar density irrespective of temperature.