Abstract
Aqueous solutions of 2-propanol and ethanol are experimentally studied in order to obtain the location of the maximum of density versus temperature at high pressure. To this aim, a new technique based on measuring the pressure against temperature for several quasi-isochores was developed. These data are combined with isothermal compressibility, experimentally obtained from high pressure density data, to determine the isobaric thermal expansivity in the quasi-isochores. The temperature of maximum density is obtained by detecting the temperature for which this magnitude becomes zero. These results are compared with those obtained using highly precise densitometry at atmospheric pressure and also with literature data. The differences between the temperature of maximum density for solutions and that of water are positive for very diluted solutions, but they become negative as concentration is raised, which constitutes a quite unusual behavior as compared with other systems. As pressure is raised, this anomalous behavior becomes much more pronounced with positive deviations that exceed 1.5 K at the highest pressures. The underlying microscopic picture that could explain the observed behavior is suggested.
Highlights
The maximum of density at 277.13 K and atmospheric pressure is perhaps the most known of the plethora of physicochemical anomalies that water shows [1,2]
Pressure moves the temperature of maximum density (TMD) towards lower temperatures; for instance, at 10 MPa the TMD is decreased more than 2 K for pure water
TMD is determined from this magnitude, by calculating the pressure and temperature where it becomes zero
Summary
The maximum of density at 277.13 K and atmospheric pressure is perhaps the most known of the plethora of physicochemical anomalies that water shows [1,2]. This structure is broken, and, as a result, water becomes more disordered and denser, which explain the increase in density and the eventual advent of the maximum. There are only one scientific work that has dealt with this issue: G Cotter, in his Ph. D. thesis [19] measured the TMD up to 7.5 MPa for some salts, alcohols, and sugars, he did not publish these results. A new experimental methodology for determining the temperature of maximum density under high pressure is presented and applied to 2-propanol and ethanol aqueous solutions, which previous works have revealed as two of the substances that show the strongest anomaly in the TMD behavior. The microscopic phenomena that could explain the observed trends are proposed
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