415 



So ill this water layer we meet witli a solution in which the 

 hexane occurs in "gas concentration". If we now question what 

 osmotic pressure would occur when we bring this hexane-water 

 layer (in three-phase equilibrium) into contact with pure water by 

 means of a membrane only permeable to water, we can therefore 

 easily calculate this pressure by the aid of Van 't Hoff's laws 

 on the osmotic pressure. In the three-phase layer rich in water 

 less than 0.06 gram-molecule of hexane is found to 100 gram- 

 molecules of water. A hundred gram-molecules of pure water 

 have a v^olume of 1,8 X 1,195 liters under their own vapour pres- 

 sure at 220°, as the specific volume of water at this temperature 

 amounts to 1,195. Now the pressure on this water layer at 220° is 

 not the saturated vapour pressure of water (22,9 atm.), but the 

 three-phase pressure (50,2 atm.). So we should have to take the 

 compressibility of water into account at the said temperature. As, 

 however, the water is here still xery far from the critical tempe- 

 rature, the compressibility is probably slight here, and roughly 

 we may estimate the volume at 1.8X1-195 liters := 2.15 liters. 

 A gram-molecule of iiexane, therefore, is found in more than 



2.15 



liters = 36 liters. 



0.06 



At 0° C. and one atmosphere one gram-molecule of gas occupies 



273 + 220 



a volume of 22,41 liters, so at 220° C. one of ~ 22.41!.= 



273 



40.5 1. Hence the osmotic pressure of the hexane is smaller than 



40,5 



— 7- or about 1.1 atmospheres. 

 36 



So the three-phase mixture with a pressure of 50.2 atm. would 

 be osmotically in equilibrium with water of 50.2 — 1.1 = 49.1 atm. 



If, therefore, the pressure of the pure water is lower than 49.1 

 atm., water from the three-phase mixture will pass through the 

 membrane, and so when we bring tiie three-phase mixture into 

 contact with water under its saturated vapour pressure (22.9 atm.), 

 water passes through the membrane. Hence we should get here 

 unmixing, so splitting up of the hexane-water mixture in the components. 



One might surmise a contradiction here to the second law of 

 thermodynamics. For if one considers that hexane and water both 

 under their own vapour pressure, spontaneously mix under formation 

 of the three-phase mixture, and that the three-phase mixture on the 

 other hand cedes waicr to water under its own vapour pressure 

 through a semi-permeable membrane, one might conclude to a con- 

 flict with the second law. But of course this is only seemingly so. 



