No abstract

Excess partial molar enthalpies of 1-propanol, H 1P E, in 1-propanol−NaCl−H2O were measured directly, accurately, and in small increments in mole fraction of 1-propanol, x 1P, at 25 °C in the range x NaCl < 0.04. x NaCl is the mole fraction of NaCl. The enthalpic interaction function, H 1P - 1P E, between 1-propanol molecules was then evaluated. H 1P - 1P E is a convenient, model-free measure for the intermolecular interaction in terms of enthalpy. The behavior of these thermodynamic quantities was compared with that of the binary 1-propanol−H2O. Based on the knowledge accumulated in our laboratory on the binary aqueous 1-propanol, the effect of NaCl on H2O became apparent. Our tentative conclusions are that (1) a NaCl molecule “binds” to seven or eight molecules of H2O on dissolving into H2O, and (2) the reminder of bulk H2O away from solute NaCl is not affected and stays almost the same as pure H2O.

The solubilities of benzene, toluene, ethylbenzene, and «-propylbenzene in water were measured at 25 °C at pressures to 400 MPa (100 MPa for benzene). The maxima in the solubilities are observed at 200, 150, and 140 MPa for toluene, ethylbenzene, and n-propylbenzene, respectively. The volume changes, 0, accompanying the solution are estimated at high pressures from the pressure dependences of the solubilities. The compressions of ethylbenzene, «-propylbenzene, and m-xylene were measured at 25 °C up to 350 MPa with a mercury-trapped piezometer in order to estimate the molar volumes, Vo, of the solutes. The partial molar volumes, °, of benzene and alkylbenzenes in water at high pressures up to 400 MPa were estimated from the °and V°, and the isothermal partial molar compressibilities, ß °[=l/F°(dV°/dP)T], were also determined. The ß °of ethylbenzene and n-propylbenzene at atmospheric pressure were -1.6 X 10"1 234 and -2.1 X 10"4 MPa"1, respectively; these quantities became positive at ca. 50 MPa. The ß °for the other solutes were positive and decreased with increasing pressure. The compressibilities, ft,, of the hydration shells around the solutes were estimated by a model of the hydration state. The results suggest that the hydration shell is destroyed by compression and disappears above 100 MPa.

The solubility of toluene, ethylbenzene, and propylbenzene in water was measured over conditions of 0.10-400 MPa and 273.2-323.2 K. Solubility was found to initially increase with increasing pressure and then decrease from a maximum at around 100 or 200 MPa. On the other hand, the solubility-temperature curve at 0.10 MPa exhibited a minimum at around 290 K. The solubility minimum of toluene disappeared at high pressures. The overall solubility-pressure-temperature surface resembles a portion of a distorted hyperboloid. The partial molar volume of these solutes in water was estimated from the pressure coefficient of solubility using the compression of the solutes in the pure liquid state, a property that was also measured in the present study. Expansion coefficient and isothermal compressibility of partial molar volume of hydrophobic groups in water at atmospheric pressure are estimated from literature data. The expansion coefficient of partial molar volume was found to be significantly larger at atmospheric pressure than at high pressure. Partial molar volume in the low-temperature region was found to increase with increasing pressure and then to decrease at pressures over ca. 100 MPa, resulting in a maximum. However, the maximum was found to disappear in the high-temperature region, where the volume decreases regularly. This behavior can be ascribed to a unique property of pure water: the compressibility of water increases with decreasing temperature, or the expansion coefficient increases with increasing pressure in the range of pressure and temperature in the present study. Such a unique property seems to weaken or disappear in the water under hydrophobic hydration.