Chapter II — 7 — Structure of Water 



Studies on solution volume, viscosity, surface tension, compressibility, and other 

 properties of solutions have yielded a variety of evidence on the structure of water. 

 Richards and Palitzsch (1919) interpret their results as indicating that trihydrol is 

 bulky tending to dissociate under the influence of solutes. Pagliani (1920), Rabino- 

 vicH (1922) and Richards and Chadwell (1925) consider polymerization as one 

 factor in explaining viscosity and volume changes upon compression of solutions. 

 Bancroft and Gould (1934) studying the effect of the Hofmeister series on boiling 

 points, adsorption by gelatin, electromotive forces of cells, solubility of gases, and 

 heats of neutralization of acids, conclude that the two factors determining the order of 

 the series are 1) displacement of the water equilibrium and 2) selective adsorption. 

 With gelatin displacement of the water equilibrium is the more important ; with al- 

 bumin selective adsorption predominates. They conclude that, under ordinary condi- 

 tions, liquid water is a mixture of trihydrol, dihydrol, and monohydrol coexisting in 

 reversible equilibrium. 



According to this interpretation, chloride, bromide, thiocyanate, and iodide ions 

 tend to change dihydrol into monohydrol, and have no specific effect on trihydrol be- 

 yond that caused by the above mentioned shift in equilibrium. They concluded that 

 nitrate ion tends to convert trihydrol into monohydrol ; that the sulfate ion tends to 

 convert trihydrol and monohydrol into dihydrol ; that the electrolytic solution pressure 

 of hydrogen is greater in dihydrol than in monohydrol ; that of oxygen is less. Sulfur 

 dioxide tends to depolymerize water. Trihydrol tends to promote fluorescence, mono- 

 hydrol to check it. Changes in viscosity of water and salt solutions with changing 

 pressure fit the concept of displacement of the water equilibrium by pressure and by 

 salts. Since the Debye-Hiickel theory postulates interaction of the ions as the only 

 disturbing factor in the behavior of solutions, Bancroft and Gould consider that it 

 must break down at all concentrations at which displacement of the water equilibrium 

 is a disturbing factor. 



Rao (1934), from Raman spectrum studies concluded that water vapor consists of 

 single H2O molecules, water is predominately dihydrol and ice trihydrol. Though 

 ice contains no monohydrol, liquid water at 0° C. to 98° C. contains varying propor- 

 tions of all three molecular types. The shifting values that he presents fit well into 

 the picture of shifting coordination with temperature presented by more modern studies. 



"Normal" and "Polar" Liquids : — Starting with the properties of 

 pure water, and proceeding to studies on aqueous solutions, the investiga- 

 tions of physical chemists on the structure of water have thus led many 

 to the belief that the anomalous behavior of water and solutions can largely 

 be explained on the basis of association. On the other hand, studies involv- 

 ing many other Hquids and proceeding from a consideration of the prop- 

 erties of ideal solutions as described by the laws of van't Hoff, Raoult, 

 and Henry, give a different view. Though often attributing anomalous 

 behavior to association of liquids, this latter type of study emphasizes 

 intermolecular forces, stressing the properties of both solute and solvent 

 as they relate to the behavior of the solvent. As noted by Hildebrand 

 (1924, pp. 84-85) objections to assuming association as explaining devia- 

 tions from Raoult's law apply not so much to the concept as to the arbi- 

 trary choice of definite polymers. Chemists have long distinguished two 

 classes of liquids : one, the "normal liquids" having low dielectric constants, 

 surface tensions, heats of vaporization, etc. ; the other "polar Hquids" hav- 

 ing high values for these constants. Normal liquids are poor solvents for 

 electrolytes, polar liquids good solvents. The former include parafiins, 

 benzene, carbon tetrachloride, bromine, and carbon disulfide ; the latter 

 water, alcohol, ammonia, sulfuric acid, and acetone. Salts melt to form 

 polar liquids. 



The fundamental distinction between the normal and the polar or asso- 

 ciated liquids lies in the greater symmetry of the fields of force surround- 

 ing the molecules of the former, the field surrounding a polar molecule 

 being unsymmetrical. The polar molecules have great mutual attractions 

 resulting in greater cohesions, internal pressures, surface tensions, and 



