42 PHENOMENA, ATOMS, AND MOLECULES 



into the interior of the liquid. Thermal agitation tends to counteract this 

 tendency. According to the Boltzmann equation the relative distribution 

 of molecules between two regions of differing energy is measured by exp 

 (W/kT), where W is the difference in the energy of the molecule in the 

 two states. Now, judging from a comparison of the values of X for octane 

 and octyl alcohol, the energy necessary to bring an OH group from a hydro- 

 carbon environment out into free space is at least 26 X io~^^ erg. The 

 energy corresponding to kT at room temperature is about 4 X io~^^ erg. 

 Since W is thus more than 6.5 times as great as this, we may conclude that 

 nearly all of the molecules [all but about exp (—6.5) = io~^] in the 

 surface of octane will be oriented in such a way that the hydroxyl graups 

 do not reach the surface. 



We may now understand why the surface energy of octyl alcohol is the 

 same as that of octane. We have previously seen that Yo can be determined 

 by the energy needed to separate a prism into two parts. We may conceive 

 of this separation as taking place in two steps. The molecules on the op- 

 posite sides of an imaginary dividing plane may first be oriented so that 

 the hydroxyl groups are turned away from this plane, leaving only the 

 hydrocarbon parts of the molecule in contact with the plane surface. The 

 two bodies of liquids are then separated from each other along this plane, 

 which would require only the same energy as for pure octane. Because of 

 the orientation of the molecules, Yo thus differs from that of octane only 

 by the amount of energy needed to turn the molecules around in the interior 

 of the liquid, which is probably negligible. 



Referring again to Table I we see that a comparison of X/S with Yo 

 affords a measure of the effect of orientation. In general, for molecules with 

 uniform fields of force, "k/S is from 0.5 to 0.55 of the value of Yo- 



When, however, a part of the molecular surface has a much weaker 

 field of force than that of other parts, the surface of the liquid consists 

 principally or wholly of the least active parts, so that Yo is lower than 

 normal. Thus for octyl alcohol X/S is 0.82 of Yo while for water it is 1.18, 

 indicating that the water molecule is very unsymmetrical and is strongly 

 oriented at the surface. 



At present these illustrations will suffice. In a paper published in 191 6 

 I showed that this theory was applicable to the surface tension of organic 

 liquids in general, including cases of substituted benzene derivatives where 

 the values of Yo depend to a marked degree on the relative positions of the 

 substituted groups. Somewhat later W. D. Harkins and his co-workers 

 investigated large numbers of organic substances in this way and com- 

 pletely demonstrated the importance of orientation of molecules in the 

 surface layer of liquids consisting of unsymmetrical molecules. 



Oil Films on Water (18) (21), 



