The Thermal Pressure and Energy–Volume Coefficients of Dimethyl Sulfoxide – Water Mixtures

Abstract

Thermal pressure and energy-volume coefficients are reported for several DMSO-water mixtures over the temperature range 13-55 "C. The energy-volume coefficient at 25 "C is found to pass through a maximum at 0.3-0.4 mole fraction DMSO and this observation is rationalized in terms of maximization of intercomponent interactions. The relationship between the energy-volume coefficient and the cohesive energy density of the binary liquid system is examined. The temperature dependence of the thermal pressure coefficient is discussed in terms of the effect of temperature on the susceptibility of the entropy of DMSO-water mixtures to isothermal expansion. Canadian Journal of Chemistry, 49, 611 (1971) IntroductionIn this paper we report the energy-volume Dimethylsulfoxide (DMSO) -water mixtures have been used extensively as solvent media for kinetic studies ( 1 4 ) including recent work reported by the authors concerning the effect of pressure on the solvolysis of benzyl chloride in several aqueous DMSO solvent systems (1). It was found in this study, and many others of the same type (5), that variation in solvent composition greatly influences the various psuedothermodynamic parameters of activation through differential effects on the properties of the initial and transition states. Since these properties are determined, at least in part, by the structural and molecular interaction properties of the solvent, it is necessary to possess detailed information on solvent structure and the extent of intercomponent interaction if the kinetic behavior is to be fully understood.It is generally accepted that deviation of liquid mixtures from ideality is a consequence of intercomponent interaction. Many studies, both thermodynamic and non-thermodynamic, have been performed in an effort to characterize the type and extent of these interactions in a great variety of liquid mixtures. Surprisingly, however, few studies appear to have been made on the energy-volume coefficient of binary liquid mixtures. Since both the energy-volume coefficient and the cohesive energy density are closely related to the extent and strength of intermolecular interaction, it seems that these two parameters would be of particular value in investigating the structural properties and intercomponent interactions in liquid mixtures.

Thermal pressure and energy–volume coefficients for the acetonitrile + water system

“…The constant volume cell and associated apparatus for the measurement of the thermal-pressure coefficients of liquid systems has been described (7,9). Pressure/temperature isochores were measured over the 0.1 to 40 MPa pressure range with data being collected in both the ascending and descending directions.…”

“…The temperature of the pressure vessel contents, including the constant volume cell, was measured with a calibrated platinum resistance thermometer encapsulated in a high temperature epoxy resin. The previously described (7,9) procedures were adopted to ensure thermal equilibration of the cell contents with the pressurising and thermostat fluids.…”

“…at this composition the internal energy and entropy of the system are most susceptible to isothermal increase in volume. Other aqueous-organic mixtures, including methanol + water (8), dimethyl sulfoxide + water (7), and tert-butanol + water (8) exhibit similar maxima in the highly aqueous region of composition. On the other hand, dimethyl sulfoxide + methanol system (9) exhibits a maximum in (aU/d V),at x(DMS0) = 0.5, as well as a temperature dependent minimum at a slightly higher DMSO mole fraction.…”

“…The flap, values were then corrected for the finite expansion and compression of glass in the usual manner (7,8,16).…”

“…Previous work in this series (6)(7)(8)(9) has demonstrated that P, and the derived energy-volume coefficient are sensitive to changes in the 'structure' of a binary system as the composition is changed. Accordingly, these parameters, together with the temperature derivative of fl, are well suited as probes for the study of bin& liquid mixtures.…”