Chapter III — 29 — Solutions 



lar in that they bring about a reduction in the total volume of the system 

 (solute, or imbibant, plus solvent). This indicates an ordering of mole- 

 cules and an evolution of energy and invariably results in a reduction in 

 diffusion pressure. Thus water of imbibition includes "bound water" as 

 described by Gortner in the quotation on page 24. Though hydration 

 has been defined to include both osmotic and imbibitional phenomena, it 

 may be very convenient to make a qualitative distinction between them. 



According to modern views on the structure of water, as explained in 

 Chapter II, forces of coordination resulting from hydrogen bonds tend to 

 hold water in a quasi-crystalline lattice and solutes capable of satisfying 

 these forces may enter into this structure. Numerical relations of coordi- 

 nation are of statistical significance only as the molecules are in a con- 

 tinuous state of flux, coordination becoming less and less definite with in- 

 creasing temperature. The firmness of binding is a matter of the nature 

 and intensity of the bonds involved. Hydrogen bonds are relatively weak, 

 covalent bonds stronger, and ionic bonds of great strength. When water 

 of hydration or imbibition near the saturation point is removed, little energy 

 is required; as more and more water is removed greater force is necessary. 

 For most colloids, however, water has little or no stoichiometric relation 

 to the hydrated or imbibing compounds. Probably both hydrogen and 

 covalent bonds account for its binding. With water of crystallization, 

 definite numerical relations exist and much energy is required to remove 

 such water from the crystal ; it is largely held by covalent bonds. Undoubt- 

 edly all possible types of bonds are involved in the hydration of cell walls, 

 protoplasm, and vacuole. 



The eflfects of force fields upon the packing of water molecules have 

 been discussed by Bernal and Fowler (1933),, Morgan and Warren 

 (1938), and others. Though the arrangement of molecules may differ 

 with the resulting variation shown by the properties of liquid water, water 

 of crystallization, and ice, the state of aggregation does not change exces- 

 sively as shown by the limited variation in coordination throughout the 

 range from water at 100° C. to ice. Eley (1944) points out that the molal 

 heat capacity of hydrate water approximates the value -9 cal. deg'^ mol"^, 

 the value for ice. For further consideration of the effect of binding forces 

 upon the density of water as shown by adsorption on cellulose see Babbitt 

 (1942). Heuser (1944) devotes a complete chapter to the reactions of 

 cellulose with water. 



In contrast to the solvent, the state of aggregation of the solute mole- 

 cules may vary widely and the definition of hydration involves solutes 

 that form ions, that dissolve as undissociated molecules, or that form 

 molecular aggregates of great magnitude. Though the forces causing 

 hydration obey a hyperbolic law and hence at certain proportions of solute 

 to solvent may exhibit a high rate of change, this region of high curvature 

 of the free energy : water content curve does not represent a break. Figure 

 9 shows two such curves for soils (Veihmeyer and Edlefsen, 1936). 

 Others are presented by Briggs (1932, Figures 1 and 2) and Edlefsen 

 and Anderson (1943, Figure 1). Greenberg (in Schmidt, 1938, page 

 472) presents a similar curve for swelling pressure. 



There is considerable literature to indicate that a water content of 0.3 

 to 0.5 grams per gram of colloid may be a critical region for gelatin, casein, 

 acacia, fibrin, agar, and many other biocolloids (Gortner, 1938). Pos- 

 sibly this water content may fall within the region of maximum curvature 



