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LV. Molecular Thermodynarnics. III. By Bernard 

 A. M. Cavanagh, B.A., Balliol College, Oxford*. 



Solvation of Solutes. 



THE partial solvation of a solute — that is, the com- 

 bination of some fraction of it with the substance 

 which, in the free state, constitutes the solvent — to form 

 a "solvate" or "solvates" is a phenomenon probably of 

 the very widest prevalence in solutions, particularly in our 

 more common and valuable solvents. It has unfortunately 

 been too readily ignored, because in dilute solutions its 

 effects are not of the first order of magnitude. They are 

 nevertheless considerable, in many cases much more con- 

 siderable than has generally been allowed for ; and, in any 

 attempt to pass beyond the region of the most dilute 



solutions (say, ^), the question of solvation of the solutes 



must receive very serious consideration. 



In the present treatment, it is to be understood, the 

 4i solvates " considered are true chemical compounds, at 

 any rate in the one sense required by molecular thermo- 

 dynamics t, and the conclusions reached are not to be 

 expected to apply in any degree to any other "" associations " 

 or " complexes " to which the name " solvate " might be 

 given. 



The following are the general circumstances considered 

 here : — 



Any number of " solutes " dissolved in a complex J 

 solvent are indefinitely solvated, without dissociation or 

 association — that is to say, each forms one unsolvated 

 molecular species and any number of solvate-species by 

 combination of one unsolvated molecule with different 

 amounts of solvent. 



The residual solvent not so " absorbed " by combination 

 with the solutes may be called the " free solvent "" §. 



The separable or experimental solutes have been called 

 the " unsolvated " solutes, and it would appear a little 

 paradoxical to say that these may be solvates ; but, in fact, 



* Communicated by Dr. J. W. Nicholson, F.R.S. 



t See second paper, Phil. Mag. xliv. p. 229 (1922), 1st section. 



X See second paper, loc. cit. The case where the solvent is simple is 

 included as a limiting case. 



§ In the previous paper the term " solvent " was used to signify 

 "free solvent," since the combined or " hound " solvent was not there 

 under consideration. 



