76 PHENOMENA, ATOMS, AND MOLECULES 



In Fig. 2 the molecule AC is shown first by itself and secondly, sur- 

 rounded by others in such a way that the relative areas of the two kinds 

 of surface in contact with each part of the given molecule are in the ratio 

 of a to c. 



The area of that part of the molecule which has an A-surface is Sa 

 and of this the fraction c is in contact with the C-surface of surrounding 

 molecules. The corresponding surface energy is Sacyac- Where the 

 A-surface of one molecule is in contact with the A-surface of another 

 there is of course no surface energy. 



Similarly the area of contact of the C-surface of the given molecule 

 with the A-surfaces of neighboring molecules is Sea and the correspond- 

 ing energy is Sacyac- Thus the total surface energy of a given molecule in 

 contact with others is 2Sacyac- 



Let us now remove the molecule AC to a vapor phase so that it is not 

 in contact with others. Then at the surface of the molecule there is the 

 energy vS'(aYo + <^"Yc) and there is an equal surface energy at the surface 

 of the cavity in the liquid. This latter disappears when the cavity is allowed 

 to collapse but at the same time the new energy Sacyac appears because of 

 the new interfacial contacts. This energy Sacyac is half as great as the 

 original energy of the molecule in the liquid, the factor ^ being due to 

 the fact it takes two opposing surfaces to make an interface. Thus when 

 the surface 6* collapses the total interfacial area formed (counting surfaces 

 A A and CC) is only ^6'. 



The increase in surface energy involved in transferring a molecule from 

 the liquid to the vapor phase is thus 



l = S (aya — acyac+ eye) (?) 



However if the molecule is a large one it may be so flexible that sur- 

 face forces tend to make it assume a spherical form when it is in the vapor 

 phase although it may be chain-hke in the liquid phase. Also, active groups 

 such as hydroxyl will tend to be drawn into the interior of a large mole- 

 cule of vapor, just as they are drawn below the surface of the liquid phase. 

 We may neglect the change in S in passing from the liquid to the vapor 

 phase, but may profitably consider that the surface fractions a and c are 

 different in the two cases. 



If ayS and c^S are the areas of A-surface and C-surface in the mole- 

 cule of vapor, then {a-av)S is the A-surface which becomes buried within 

 the molecule of vapor. Thus we find that the energy for the transfer of a 

 molecule from the liquid to the vapor is 



l = S [avya + (a — av — ac) yac + Cvyc] (8) 



An analysis of the available data on the heats of evaporation and 



