EFFECTS OF MOLECULAR DISSYMMETRY 271 



Table I gives a summary of these results to facilitate comparison. 



TABLE I 

 Surface Energy of a Hydrocarbon Molecule CwHsi Having a Volume 497. A^ 



Sphere 9.83 A diameter 

 304. A" surface 



Location of Molecule 

 In liquid hydrocarbon 



In water 



In vapor 



Adsorbed on water 

 (half immersed) 



or 



Surface energies used 

 carbon 59; hydrocarbon 

 Harkins et al. 



Ill calculations — water vs. vapor" 117, water vs. hydro- 

 vs. va|)or 50 ergs per cm. as taken from tables by 



The difference in energy between the spherical and cylindrical form 

 when the molecule is in the vapor phase or the water phase is so great that 

 the molecules will be nearly spherical. But with the molecule in the inter- 

 face the difference in energy (2 X lo"^'*) between the sphere and the 

 horizontal cylinder is so small that it will have little effect in determining 

 the shape, the exponential factor in Equation 5 being only 0.61. Thus the 

 configuration of the molecule will depend almost wholly on the a priori 

 probabilities. For reasons already given a nearly spherical form is im- 

 probable as compared to an extended or irregular form in which the larger 

 part of the molecule is exposed. These considerations lead us to conclude 

 that a hydrocarbon molecule adsorbed on a water surface will lie flat on the 

 .surface when not crowded by other absorbed molecules. 



This conclusion is the same as that reached by the writer from a study ^^ 

 of the surface tension of dilute solutions having non-saturated adsorbed 

 films. 



The very high value of 1 in the case of the vertical cylinder shows that 

 molecules can be oriented vertically only very rarely. The difference in 

 energy between the horizontal and vertical cylinder (127 X lO"^'*) gives, 

 when substituted in Equation (5), an exponential factor of 2 X lo"^"*. In 

 this case the a priori probabilities of the two orientations are the same so 

 that only one adsorbed molecule out of about 10^^ will be oriented in a 

 vertical direction. 



The energy 79 corresponding to the adsorbed molecule is very much less 



