THE DISTRIBUTION OF MOLECULES 



81 



Some typical values of cp calculated from published experimental data 

 on vapor pressures of binary mixtures are given in Table I. It is seen that 

 the values of qp are of the order of magnitude of the surface tensions of 

 liquids, but are smaller than these in accord with the fact that in Equation 

 (17) (which gives qp) the coefficients are all less than unity and two of 

 them are negative. 



It can be shown from (17) that qp should approach zero if the A 

 molecules and the B molecules become alike in regard to both their values 

 of Y and their surface fractions a and b. We see from Table I that the 

 observed values of tp are very small for mixtures of closely related sub- 

 stances such as ethanol-methanol, benzene-toluene, and methyl acetate- 

 ethyl acetate. The series of benzene mixtures 9 to 15, which are arranged 

 in the order of the qp's, show how (p increases as the substance mixed with 

 the benzene becomes less like it, until with methanol cp becomes 10 ergs 

 per cm.^ 



TABLE I 

 "Mixture Energies" cp for Various Binary Mixtures 



In the case of mixture No. 6, hexane-ethyl iodide, there are only two 

 kinds of molecular surface (R and I) and therefore the 6 terms of 

 Equation (17) reduce to one, so that the value of y (R-I) can be calculated 

 from the observed value of qp. The three binary mixtures Nos. 3, 4 and 5 

 involve only 3 components. Because of the symmetry of carbon tetra- 



^ Data indicated by Z are those of Zavvidski (I.e.) ; S are from rough measure- 

 ments of the total pressures of binary mixtures, by G. C. Schmidt, Zeit. phys. Client., 

 99, 71 (1921) ; v.H. is from H. v. Halban, Zeit. Phys. Chem.. 84, 129 (1913). 



