The hydrogen bond properties of water from 273 K to 573 K; equations for the prediction of gas-water partition coefficients
An equation due to Plyasunov and Shock for the calculation of gas to water partition coefficients, as log Kw, at high temperature requires knowledge of experimental values of log K298w, ΔH298w the standard enthalpy of hydration at 298 K, ΔCp298w the heat capacity of hydration at 298 K and b′ that is a constant that refers to the variation of ΔCpw 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 Kw, ΔH298w, ΔCp298w and b′ can be estimated. Known values of the four input quantities, where available, together with estimated values enabled values of log Kw 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 Kw values for the 555 solutes. These LFERs can then be used to estimate further values of log Kw between 273 and 573 K for any solute with already determined descriptors, so that knowledge of experimental values of log K298w, ΔH298w, ΔCp298w 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 Kw. 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.