Chapter III —25— Solutions 



lattice of ice." Out of these definitions has grown the idea that in colloidal 

 systems water may exist in two different states, bound water and free water. 



Modern studies have changed this concept. Just as the idea of definite 

 water polymers has been displaced by the newer picture of coordinated 

 water molecules making up a continuum of quasi-lattice properties, so has 

 the concept of bound water been gradually broadened to the extent that all 

 inter- or intra-molecular forces affecting the activity of water molecules 

 are given consideration. For this broader concept the term hydration is 

 commonly used. From one viewpoint it seems that there is no such thing 

 as free water ; all water molecules in the liquid state have restricted mo- 

 tion because of their own intermolecular bonding. The restrictions and 

 the forces causing them may vary in intensity because of the variety of 

 possible types of hydration complexes ; the water molecule, however, does 

 not lose its chemical identity by reason of the forces with which it is held. 

 Although Bernal and Fowler postulate three types of water, they state 

 that they pass continuously into each other with change in temperature. 

 To deny the binding of water molecules would require a complete neglect 

 of those secondary valence forces that, through hydrogen bonding, account 

 for so many of the unusual properties of water. To adhere to the sharp 

 distinction between bound and free water, on the other hand, would neces- 

 sitate drawing a clear line between the behavior of such valence forces 

 toward crystalloidal, and colloidal substances, a line that is not indicated in 

 physico-chemical studies of solutions. That Gortner was fully aware of 

 the nature of these forces is evidenced by his statement, "We may con- 

 clude therefore that the forces which bind water on the surface of the 

 lyophilic colloids are of the same nature as the forces which cause the as- 

 sociation of water in bulk and which immobilize water molecules in the 

 ice crystal lattice. However, there is evidence that these forces on a sur- 

 face or at an interface may be of greater magnitude than the forces of asso- 

 ciation of water molecule for water molecule or the forces which tend to 

 arrange water molecules in the ordinary ice crystal lattice. Therefore at 

 least a part of the molecules of the bound water film may be expected to 

 have an activity which is less than the activity of the H2O molecule in the 

 ordinary ice lattice" (1938, p. 304). 



It should be pointed out here that these forces show no unique response 

 to colloids ; they are the forces that cause hydration of ions and molecules 

 as well as micelles and hence cause the major deviations from ideality of 

 aqueous solutions. 



It is often implied that bound water bears a numerical relationship to the 

 colloid {i.e., so many grams of water per gram of colloid) and that this 

 relationship is constant at different concentrations of the solution. If we 

 can believe the vapor pressure : water content curves this is not the case. 

 These curves show that as the water content of a solution decreases the 

 intensity of the binding force per water molecule increases. Therefore, if 

 equilibrium exists between the solute and the solvent, it seems evident that 

 the amount of water bonded to the colloid shifts continuously with water 

 content, there being much loosely held water at high water contents. As the 

 water content decreases, the remaining water is more tightly held. 



For example, in the hydration of cellulose it has been postulated that, 

 between the dry and saturated conditions, three different mechanisms are 

 responsible for holding water. At low water contents the water molecules 

 are pictured as being held as monomolecular layers, probably by hydrogen 

 bonds. At intermediate contents the layers become polymolecular with a 



