EFFECTS OF MOLECULAR DISSYMMETRY 267 



To produce isothemially a new surface of unit area we must expend the 

 work Y, but the total energy increases by more than this, the difference 



— T-T^being derived from heat energy, and thus causing a cooling effect. 

 a J. 



dy 

 In general the temperature coefficient -^ is practically constant, and 



therefore by Eq. (4) yo is constant and equal to the value of y at T = O. 

 From the linear change of y with temperature it is probable that the varia- 

 tion of y is a direct result of the thermal agitation of the molecules, and is 

 not due to any change in the structure of the surface. In studying the effects 

 of molecular orientation we must, therefore, consider the total energy yo 

 rather than the free energy. 



In the early work on the orientation of the molecules in the surfaces of 

 organic liquids, the writer pointed out that the different portions of the 

 molecules produced their characteristic effects independently of the pres- 

 ence of the other portions of the molecular surface. Thus it was stated : ^^ 



The surface energy of a liquid is not a property of the molecules, 

 but depends only on the least active portions of the molecules and on 

 the manner in which these are able to arrange themselves in the surface 

 layer. In liquid hydrocarbons of the paraffin series the molecules arrange 

 themselves so that the methyl groups at the ends of the hydrocarbon 

 chains form the surface layer. The surface layer is thus the same no 

 matter how long the chain may be. As a matter o£ fact the surface 

 energies of the hydrocarbons from hexane to molten paraffin, have 

 substantially the same surface energy (46-48 ergs per cm.-) although 

 the molecular weights differ very greatly. If we consider the alcohols 

 such as CH3OH, C2H5OH, etc., we find that their surface energies are 

 practically identical with those of the hydrocarbons. The reason for this 

 is that the surface layer in both cases consists of CH3 groups. 



We see that this conception of surface forces is in agreement with the 

 conclusions that we have reached by our more general considerations of 

 intermolecular forces. We may, however, use this idea of separate surface 

 energies for different parts of the molecular surface, to explain the orienta- 

 tion of the molecules and many other phenomena observed in liquids. 



Let us apply the idea of surface energy to the surface of a molecule in 

 the same manner as we apply it to a large surface of a liquid. As an example 

 we will consider a molecule of palmitic acid, C15H31COOH and calculate 

 the difference in surface energy when the molecule is in various environ- 

 ments. From the density 0.85 and the molecular weight 256 the volume per 

 gram molecule is 301 cm^ and from the Avogadro number we find the 

 volume per molecule to be 497 X lo'^^cm^. Because of surface tension 



