The Energy of Interaction between Water Monolayer and Carbon Surface and the Determination of H-Bond Energy from the Isotherms of Water Desorption by Organic Molecules into an Aqueous Solution

It is proven that, when passing from a liquid into an adsorption phase on a carbon surface, the maximal number of H-bonds between water molecules decreases from four to three because of the screening of one unpaired electron of the oxygen atom of an adsorbed water molecule by aromatic rings of the carbon surface. An energy gain equal to the energy of one H-bond arises upon water desorption by organic molecules adsorbed from an aqueous solution. The ratio between the number of H-bonds of a group of water molecules, which is displaced into a solution by one organic molecule, in the solution and in the adsorption phase is independent of the number of molecules in this group and is, on the average, equal to 2.038 for all possible structures of H-bonds in both phases. The allowance for this ratio in the isotherm of water desorption into a solution and the introduction of a coefficient, which depends on the relative water content () in the adsorption phase, in the form of into the equation of the desorption isotherm make it possible to determine the balance of the change in the Gibbs energy at the desorption equilibrium and the standard Gibbs energy = –1.76 kJ/mol of water desorption into a solution from a carbon surface. This value determined by an independent method is = –1.79 kJ/mol; i.e., both values are close to each other. The RTlnf energy of the additional H-bond, which is formed between water molecules upon passing from the adsorption phase into the solution, is found by the extrapolation of the isotherms of water desorption by molecules of several benzene derivatives. This energy ranges from 9.13 to 9.24 kJ/mol, thus corresponding to the energy of one H-bond, as measured by IR spectroscopy and NMR.