Chapter IV — 41 — Osmosis 



Causes for Non-Ideal Behavior of Solutions : — Two general ex- 

 planations may be given for departure of a solution from ideal behavior : 

 1 ) the two components may be so dissimilar in properties that they are in- 

 capable of forming an ideal system and so the escaping tendency of each 

 component is affected by the other; 2) the two components may have such 

 great attractive forces for each other that they form compounds {i.e., hy- 

 drates) or one component may associate or polymerize reducing the total 

 number of molecules present. 



The internal pressure (cohesion) of a liquid is a measure of the attrac- 

 tive forces between the molecules of that liquid, and differences in internal 

 pressure between solute and solvent determine, at least roughly, deviation 

 from ideal behavior of the solution. Deviations caused by differences in 

 internal pressure, and hence escaping tendencies greater than ideal, result 

 in positive departures from Raoult's law. If one component is highly polar 

 and the other less so, a positive deviation from ideality will result. 



On the other hand, attractive forces great enough to cause association 

 or compound formation alter the properties of solutions by reducing the 

 number of active molecules, and bring about negative deviations from 

 Raoult's law. Association of either solute or solvent leads to increase in 

 vapor pressure of the solution and positive deviations from Raoult's law 

 (Glasstone, 1942). Generally speaking, positive deviations from Raoult's 

 law result in osmotic pressures less than those calculated from concentra- 

 tion and negative deviations result in higher values. All of the deviations 

 mentioned are appreciable only as the concentration of the solute becomes 

 appreciable. 



Thermodynamically, it is possible to derive equations for non-ideal 

 solutions that are independent of the factors causing departures. Though 

 such treatment gives no hint as to the mechanism of the behavior of the 

 solution, it does give a convenient method for calculating values for vari- 

 ous functions of the solution. Because vapor pressure seems most ac- 

 curately to mirror the true escaping tendency of solvent molecules, be their 

 behavior ideal or non-ideal, vapor pressure measurements of the solvent 

 above a solution are made ; their departures from ideality are determined 

 by calculation from concentrations on the basis of Raoult's law ; the de- 

 parture times the mol fraction is termed the activity and the corrected vapor 

 pressure is named the fugacity. Because activity is measured in terms of 

 concentration, it may be expressed in units of mol fraction, concentration, 

 or molality. It is more accurate as an expression of the physical properties 

 of a solution than is osmotic pressure, in the same degree as vapor pressure 

 exceeds concentration in accuracy as an index of escaping tendency. 



Due to the untiring efforts of Berkeley and Hartley (1906), Morse 

 (1914), Frazer and Myrick (1916), Berkeley, Hartley, and Burton 

 (1919), and others, accurate measurements have been made on the osmotic 

 pressure, not alone of dilute solutions but, in the case of sucrose, of solu- 

 tions almost saturated. Values from these studies have been cited in tables 

 in this chapter. 



Physical chemists, realizing the difficulties in osmotic pressure measure- 

 ment, have been interested in the properties of solutions from a broader 

 viewpoint and have devised many methods for studying them. Because 

 the surface layer of a solution, in contact either with the vapor or the solid 

 solvent, acts like a semi-permeable membrane with respect to the escaping 

 tendency of the solvent molecules, measurements of vapor pressure, freez- 



