Role of hydrogen bonds in demixing phenomena in aqueous solutions

A microscopic mechanism for demixing in aqueous solutions of organic molecules is proposed. Analysis of the hydrogen bonding between water and organic molecules permits the identification of three main kinds of pair complexes: with two hydrogen bonds, with one, and without. The lower (upper) critical consolute point is connected with the temperature-induced appearance (disappearance) of a critical amount of pair complexes with one hydrogen bond between water and the organic molecules. The energetic distributions of pair complexes with different types of interactions with the solvent were calculated by the method of atom-atom potentials. The experimentally observed changes of the critical temperatures upon deuteration of water and addition of salt are well reproduced.