On the temperature dependence of hydration thermodynamics for noble gases

Abstract

The hydration thermodynamics of five noble gases in the temperature range 0–60°C have been carefully determined. We analyse these data using the theoretical approach developed by Lee that emphasizes that there are different physical causes for the large and positive hydration Gibbs energy change and the large and positive hydration heat capacity change. The analysis confirms that the hydrophobicity of noble gases is caused by the excluded volume effect due to solute insertion and exaggerated by the small size of water molecules at any temperature. The reorganization of H-bonds in the hydration shell of noble gases is a compensating process that does not contribute to the Gibbs energy change, but it is the cause of the large and positive hydration heat capacity change and determines the temperature dependence of the hydration enthalpy and entropy changes. The modified Muller's model, despite its simplicity, proves able to satisfactorily describe the reorganization of H-bonds, indicating that the hydration shell does not resemble an iceberg. The H-bonds in the hydration shell are energetically slightly stronger but more broken than those in the bulk water.