Chapter II 



— 15 — 



Structure of Water 



whose molecules, though tetrahedral in structure, behave as spheres with 

 directed intermolecular forces. The low (fourfold) coordination results 

 from hydrogen bonds that affect both viscosity and thermal properties. 



HiLDEBRAND ( 1937) Stated that the term "association" under which all 

 departures from normal behavior of liquids have been lumped must now 

 be subdivided into association arising from the interaction of dipoles, and 

 that due to the formation of definite chemical bonds. To hydrogen bonds 

 he assigned great importance. Substances containing hydroxyl, carboxyl, 

 or amino groups show a type of association markedly different from that of 

 most other dipoles. These bonds are of particular interest to biochemists 

 because of their existence in proteins. 



Current Research: — Morgan and Warren (1938) by carefully con- 

 ducted x-ray diffraction analysis of water showed that, whereas water in the 

 neighborhood of freezing temperature has open tetrahedral structure as in 



ice, at higher temperatures the tetrahedral 

 bonding becomes less sharply defined. In 

 ice the intermolecular separation of neighbor- 

 ing molecules is 2.76A ; of next nearest 

 neighbors it is 4.5A. In liquid water this 

 distance for nearest neighbors is 2.90A ; for 

 next nearest neighbors it is 4.5A or slightly 

 more. Because the density of water at 0° C. 

 is greater than that of ice, Morgan and 

 Warren believe that the increased density 

 is due, not to a crowding in of next nearest 

 neighbors, but to a filling in between the first 

 and second neighbors, in other words a loss 

 of the highly regular pattern of the ice lat- 

 tice and substitution of a closer packed struc- 

 ture. They picture this as a shift from the 

 hexagonal lattice of ice to a quadrangular 

 pattern having a density 27 per cent greater 

 as indicated in Figure 8. The line indicat- 

 ing the next nearest neighbor distance of 

 4.5A is strong in their diffraction picture at 1.5° C. ; it is less distinct at 

 13° C, and still less at 30° C, and disappears at 62° C. This adds weight 

 to their conclusion that the structure of water changes with rise in tempera- 

 ture through this range. Although the average number of molecules per 

 unit space (density) is greater, the coordination may decrease; from an 

 average value of 4 at 0° C. the coordination number decreases approaching 

 a value of zero in the vapor. 



The concept expressed by Bernal (1937) that a liquid possesses points 

 of abnormal coordination that act as "holes" into which neighboring mole- 

 cules may "flow" has been elaborated by later workers on liquids (Altar 

 1937). In their theory of liquid structure Hirschfelder, Stevenson, and 

 Eyring (1937) picture a mechanism of viscous flow involving the forma- 

 tion of double molecules that rotate into new positions and then separate. 

 The ability of such double molecules to rotate depends upon empty spaces 

 or holes into which the molecules pass during rotation and the activation 

 energy of viscous flow they consider to be needed for the formation of 

 these holes. Since the activation energy of normal liquids is around y^ 

 to 14 the heat of vaporization they assume the required hole to be ^ to 



Fig. 8. — Diagrammatic illustra- 

 tion of the shift from the hexagonal 

 lattice of ice to a closer packed 

 structure for water. Redrawn from 

 Morgan and Warren (1938). 



