l62 



ATOMS, IONS, SALTS, AND SURFACES 



of atoms increases the work of attraction for water by i lo per cent, which is suffi- 

 cient evidence that the oxygen of the alcohol must be oriented toward the surface of the 

 water. 



If the alcohol surface is pulled from the water surface at the interface between the 

 two the interface disappears and a water {A ) surface and an alcohol {B) surface ap- 

 pear. The work done is aided by the free 

 energy of the surface which disappears, and 

 hindered by those which appear, so 



WA=yA-]-yB—yAB . 



Values of the interfacial tension (t^b) for 

 a number of liquids are plotted in Figure 15, 

 and for the work of adhesion in Figure 16. 



THE SPREADING OF ONE LIQUm ON THE 

 SURFACE OF ANOTHER 



Most organic liquids will spread on water, 

 but water spreads on almost no organic 

 liquids. It is easy to show that spreading or 

 non-spreading is determined by the work of 

 adhesion {Wa) between the liquids and the 

 work of cohesion (Wc) for the upper liquid 



S=Wa-Wc(B). 



If S is positive, the liquid B will spread on the 

 surface of A;ilS is negative, it will not spread. 

 The presence of polar groups in the organic 

 liquid is not essential for spreading, since 

 hexane and octane, as well as benzene, spread 

 on water. Not only organic liquids, but water 

 as well, spread on a clean surface of mercury: 



Fig. 14. — Octyl alcohol over water. Illus- 

 trates the oreintation of the alcohol mole- 

 cules at the interface. 



S = yA-(y -{-y ) . 



2. Evidence for orientation of molecules in surfaces of pure liquids; comparison of 

 energy of surf ace formation with heat of vaporization. — If a liquid consists of molecules 

 with one end polar and the other end non-polar, the energy required to lift the non- 

 polar end (the "light" end from the standpoint of electrical forces) into the surface is 

 much less than for the polar end, so the orientation theory predicts that in the outer 

 layer of molecules the non-polar groups will be at the surface. However, if such a 

 molecule passes into the vapor state, the polar end of the molecule must be separated 

 from the liquid, and thistwill require a relatively large amount of energy. Therefore, 

 according to the theory, the energy per molecule of surface formation (/?) should be 

 small as compared with the energy of vaporization (X). If the molecule is symmetri- 



