PROPERTIES OF LIQUID WATER - ORIGIN OF THE DENSITY ANOMALIES

Strong support for a ''mixture'' model of liquid water can be found fr om an analysis of the accurate experimental density data (H2O) over th e range t similar to -30 degrees C in the supercooled regime to t simi lar to +70 degrees C. Published density data can be fit to this mixtur e model with six- to seven-decimal-point precision. Remarkably, the ou tput parameters from these fits indicate the presence of capacious int ermolecular bonding with a density extremely close to that of ordinary ice-Ih, intermixed with compactly bonded regions having a density nea r that of the common dense forms of ice, in particular ice-II. Densiti es at higher temperatures could also be fit to good precision with suc h a model, though the model must clearly become less valid as the temp erature rises and more varied bonding forms contribute. The fitting pr ocedure also shows that both the capacious and dense components have p ositive thermal expansion coefficients that are similar in magnitude t o those of their respective ice forms. As T approaches the vicinity of 225 K in the deep supercooled regime, the structure of the liquid app roaches disordered ice-I-type bonding, with no contribution from the d ensely bonded component. Combined with the differential X-ray scatteri ng data of Bosio, Chen, and Teixeira on liquid water, and structural d ata on the ice polymorphs from Kamb's work, it can be concluded that t he bonding differences between the dense and capacious structures are not at the nearest-neighbor level but occur instead in the outlying no n-hydrogen-bonded next-nearest-neighbor O...O structure. Because of th e long-range structural implications of this conclusion, uncertainties arise in molecular dynamics modeling of the liquid and on the usefuln ess of attempts to learn about the liquid from the study of small gas- phase clusters.