Chapter III — 31 — Solutions 



is not the purpose of the present publication to go further into this subject. Interested 

 readers are referred to the books of Remick (1943) and Hober (1945) which have 

 been drawn on freely for the contents of this chapter. Further source material on the 

 dielectric properties of matter appears in Chemical Reviews of 1936. Articles by Debye, 

 SiDGwiCK, Wyman, Cohn, Kirkwood, and Scatchard in this review should all prove 

 of interest to biologists. A symposium on structure and molecular forces in pure liquids 

 and solutions held in 1936 in Edinburgh is reported in volume 33 of the Transactions 

 of the Faraday Society. Further detailed work on solutions is reviewed by Kincaid, 

 Eyring, and Stearn (1941), and by Scatchard and Epstein (1942). The thermo- 

 dynamics of electrolytic solutions have received excellent treatment in a recent mono- 

 graph by Harneo and Owen (1943). 



Summary: — Two viewpoints are expressed in explaining the properties of aqueous 

 solutions. One emphasizes the importance of the molecular constitution of water re- 

 lating the discrepancies from ideal behavior to polymerization, hydration, and complex 

 formation involving water molecules. The other stresses the effects of intermolecular 

 forces between solute and solvent and their efifect upon colligative properties. 



Molecular interactions involve electrical forces resulting from the charges on elec- 

 trons and atomic nuclei. They are comprised of ion to ion attractions, ion : dipole 

 forces, and interaction between various types of polar molecules. They result in the 

 numerous types of valence forces known to exist between molecules; ionic bonds, 

 covalent bonds, and hydrogen bonds. The latter are of great interest in biology as 

 they account for the coordination of water, the hydration of crystalloids and colloids, 

 and the formation of bridges responsible for the three dimensional archiecture of bio- 

 colloids. 



Studies on the hydration of biological materials has given rise to the concept of 

 "bound water." Well deserved criticism has been directed toward certain of the meth- 

 ods used to determine this water. While some would deny its existence entirely, a more 

 plausible view is that a small amount of water can be bonded to colloidal surfaces by 

 intense forces. Such bound water must differ from free vrater in some of its physical 

 and chemical properties ; its molecular constitution, however, remains unchanged. No 

 sharp line can be drawn between bound and free water, there being a smooth deviation 

 in the average intensity of the binding forces as water is added to or withdrawn from 

 the system. 



Hydration is defined as the resultant of any interaction between solute and water 

 tending to reduce the activity of the latter. Water does not vary greatly in closeness 

 of packing but the state of aggregation of solutes may vary from ions to colloidal mi- 

 celles. The forces causing hydration obey a hyperbolic law and the curve relating 

 free energy to water content is smooth and continuous. Breaks in such curves involve 

 changes of state or very low water contents. 



The vapor pressure method should be best for measuring hydration because it inte- 

 grates all factors causing deviations from ideality. It avoids supercooling and the use 

 of reference solutes. 



High ion mobilities of H+ and OH- are explained in the basis of a proton jump 

 along coordinated chains of water molecules when favorable configurations occur. 



