EQUILIBRIUM STRUCTURAL MODEL OF LIQUID WATER - EVIDENCE FROM HEAT-CAPACITY, SPECTRA, DENSITY, AND OTHER PROPERTIES

Hydrogen bond strength depends on both temperature and pressure. The g radient for hydrogen bond strength with temperature, or pressure, depe nds upon the hydrogen bonded structure. These features create an intim ate connection between quantum mechanics and thermodynamics in the str ucture of liquid water. The equilibrium structural model of liquid wat er developed from analysis of the heat capacity at constant pressure i s complex. The model is based on the assumptions that: (i) the hydroge n bond length and molecular packing density of water both vary with te mperature; (ii) the number of different geometries for hydrogen bondin g is limited to a small set; (iii) water molecules that possess these hydrogen bonding geometries are in equilibrium with each other under s tatic conditions; (iv) significant changes in the slope of the heat ca pacity, Cp, and to a lesser extent other properties of the liquid, ref lect the onset of significant changes in the chemical structure of the liquid; (v) the partial molal enthalpies and entropies of the differe nt water arrays generated from these building blocks differ from each other in their dependence upon temperature; and (vi) the structure of the liquid is a random structural network of the structural components . The equilibrium structural model for liquid water uses four structur al components and the assumptions listed above. At the extrapolated-ho mogeneous nucleation temperature, 221 K a disordered hexagonal-diamond lattice (tetrahedrally hydrogen bonded water clusters) is the structu re of liquid water. At the homogeneous nucleation temperature, similar to 238 K: liquid water is a mixture of disordered tetrahedral water a rrays and pentagonal water arrays. The abundance of tetrahedral water structures at this temperature causes the system to self-nucleate. As the temperature increases to 266 K the proportion of disordered pentag onal water clusters in the equilibrium mixture increases. At 256 K, th e temperature of the previously unrecognized maximum in the heat of fu sion of water, ''planar''-hexagonal water arrays appear in the liquid. At 273 K the concentration of tetrahedral hydrogen bonded water appro aches zero. At the temperature of maximum density, 277 K, the liquid c onsists of a disordered dodecahedral-water lattice. The equivalence po int between pentagonal and ''plaar''-hexagonal water arrays occurs nea r 291 K, the approximate temperature of minimum solubility of large hy drocarbons in water. At temperatures above 307.6 K, the minimum in Cp, square water arrays first appear in significant concentrations. Penta gonal water arrays become insignificant in the liquid at the temperatu re of minimum isothermal compressibility, similar to 319 K. The equili brium point between ''planar''-hexagonal and square water arrays occur s near 337 K. As the temperature increases the liquid structure become s dominated by disordered cubic arrays of water molecules. Structures with fewer than four hydrogen bonds per water molecule appear in the l iquid near 433 K. ''Planar''-hexagonal clusters are no longer present in the liquid at the temperature of the maximum dissociation constant for water, 513 K. These views are certainly oversimplified. Simple mod els for density are introduced. A model for viscoscosity based on the variation of hydrogen bond strength with temperature is introduced. At tempts to model density, heat capacity, or other thermoodynamic proper ties of liquid water, using only two functions will, not capture the s ubtle complexity of the equilibrium process. The equilibrium structura l model of water has the potential to provide a basis for quantitative descriptions of the liquid's seeming anomalies. (C) 1998 American Ins titute of Physics. [S0021-9606(98)51841-7].