The hydrogen bond properties of water from 273 K to 573 K; equations for the prediction of gas-water partition coefficients

Abstract

An equation due to Plyasunov and Shock for the calculation of gas to water partition coefficients, as log K_w, at high temperature requires knowledge of experimental values of log K²⁹⁸_w, ΔH²⁹⁸_w the standard enthalpy of hydration at 298 K, ΔCp²⁹⁸_w the heat capacity of hydration at 298 K and b′ that is a constant that refers to the variation of ΔCp_w with temperature. Linear free energy relationships based on Abraham descriptors for solutes have been constructed for all of these four input quantities, so that log K_w, ΔH²⁹⁸_w, ΔCp²⁹⁸_w and b′ can be estimated. Known values of the four input quantities, where available, together with estimated values enabled values of log K_w from 273 to 573 K to be calculated for 555 solutes. At various fixed temperatures LFERs again based on Abraham descriptors were constructed from the obtained log K_w values for the 555 solutes. These LFERs can then be used to estimate further values of log K_w between 273 and 573 K for any solute with already determined descriptors, so that knowledge of experimental values of log K²⁹⁸_w, ΔH²⁹⁸_w, ΔCp²⁹⁸_w and b′ is no longer necessary. The obtained LFERs show how the solvation properties of water vary with temperature. Both water hydrogen bond acidity and hydrogen bond basicity towards solutes decrease substantially with increase of temperature until at 573 K the hydrogen bond properties of water resemble those for solvents such as nitrobenzene and acetonitrile at 298 K. A characteristic property of water at ambient temperatures is that water will not solvate non-polar solutes, thus leading to large Henry's law constants and small values of K_w. Although this property also decreases with increase of temperature, our results indicate that it still remains to some extent even at 573 K, so that the general solvation properties of water at 573 K do not resemble the solvation properties of organic solvents at 298 K.