Co-ordination of water molecules in isotopic mixtures

► OH stretching vibrations in isotopic mixtures prove the organisation of liquid water. ► The isotopic exchange in H 2O/D 2O depends on the correlation between the molecules. ► Liquid water is a mixture of two fractions having different degree of correlation. ► The correlated fraction does not participate in the mixing process. ► Water molecules at the boundary of the two fractions are the most reactive. The essential question of whether the interactions within liquid water may best be described by either the continuum or the micro domain model has for some time been a matter of some debate within the scientific community. We studied isotopic exchange in H 2O/D 2O mixtures by the inspection of IR spectra, and found that it depends strongly on the correlation between an individual molecule and its environment. It is further found that in such mixtures a certain proportion of the water molecules is not involved in the mixing process at all. The physical properties of these molecules determine the behaviour of interfacial water that is often described in the biological literature.