Chapter II — 17 — Structure of Water 



at which point it equals the energy of solution of a gas. This explains the 

 decrease in solubility of gas in water with increasing temperature. Eley 

 relates this effect to the polarization of the water molecules around the 

 cavity. Since the energy required to make a cavity of molecular size equals 

 the internal energy of evaporation, for one mol at room temperature it 

 would be about 10 K cal. The energy of polarization for a molecule the 

 size of water is shown to be about 9 K cal. per mol. The difference, 1 K 

 cal. per mol, represents the net energy required to form cavities within the 

 liquid equivalent to one mole volume. 



Since the polarization of molecules around a cavity would disorganize 

 the open tetrahedral structure of water, the formation of cavities would 

 contribute to a close-packed structure. With increasing temperature, how- 

 ever, the energy supplied by polarization would decrease due to the in- 

 creasing rotational freedom from normal thermal effects. Consequently, 

 the net energy required to produce cavities would increase. It seems pos- 

 sible also that increased energy might be needed to form holes in the close 

 packed structure of liquid water. This would agree with the conclusion of 

 Morgan and Warren (1938) concerning the shift in structure between 

 1.5° C. and 83° C 



Eley postulates further that of all the cavities in water only certain ones 

 are available to neutral molecules, whereas ions and water molecules are 

 interchangeable in the lattice structure. Polarization of water about a cav- 

 ity occupied by an inert gas is limited to the immediate neighborhood ; about 

 an ion the force field extends through several molecular shells tending to 

 increase the regularity of molecular configuration. For compilation of the 

 physical and chemical properties of the water see the monograph by Dorsey 

 (1940). Detailed reviews of the structure of water as it relates to biologi- 

 cal problems are given by Barnes and Jahn (1934) and Blanchard 

 (1940). 



Summary of Water Structure : — This review of work on the structure of water 

 indicates that, far from being a simple mass of spherical molecules, randomly arranged, 

 and independently agitated by thermal energy, water is composed of polar molecules 

 coordinately arranged in some sort of lattice-like network and bound by a number of 

 intermolecular forces such as dipole attractions, London forces, and hydrogen bonds. 

 As ice, water apparently assumes a normal four-coordinated hexagonal structure with 

 12 HoO molecules per unit cell. Although variously assumed to have three-fold as- 

 sociation, hexagonal ring structure, and a four-coordinated hexagonal lattice (the lat- 

 ter seems best substantiated by modern methods of analysis), the important point is 

 that a regularity of structure compatible with geometrical arrangement of molecules 

 and intermolecular forces predominates in water in the solid form. 



As liquid, water apparently has abnormal coordination and several forms may 

 occur. These forms apparently coexist and the shift from one to another depends 

 largely upon temperature and the presence of solutes. Though liquid water forms a 

 continuum with sufficient order to give an x-ray diffraction pattern, the existence of 

 any one molecular configuration is only of statistical significance, the total structure 

 being in continual flux. 



Recent theories of liquid structure postulate points of abnormal coordination that 

 act as cavities or holes. Such holes are shared communally by all molecules of the 

 liquid, and viscous flow involves formation of double molecules that rotate into new 

 positions within the holes. With rise in temperature a shift in structure from the hexag- 

 onal ring lattice to a closer-packed quadrilateral structure apparently occurs but co- 

 ordination of the individual molecules decreases. 



The dipole character of water molecules and their tendency toward hydrogen bond- 

 ing are important in their role as solvent. In the formation of solutions the participa- 

 tion of both solute and solvent in the structure should be considered. 



As water passes into the vapor state, intermolecular distances increase immensely, 

 forces between molecules diminish, and, it seems agreed, coordinate structure gives way 

 to the chaos of the typical gaseous state. 



