WILLIAM D. HARKINS 



i6i 



2, If at an interface the transition from a liquid A to the liquid B is made by a 

 saturated film of solute molecules which we may call A-B, i.e., they have one end like 

 A and the other like B, then the free surface energy is greatly reduced. 

 For example, with water and benzene with sodium oleate as the solute, 

 the free energy falls as low as 2 ergs per square centimeter. 



3. If the solvent is polar, such as water, then solutes will in general 

 be positively adsorbed in the surface if they are less polar than water (or 

 if a part of the molecule of the substance is less polar than water), and 

 the least polar end of the molecule will be turned outward. Solutes more 

 polar than water are negatively adsorbed. 



EVIDENCE FOR ORIENTATION OF MOLECULES IN SURFACES 

 AND INTERFACES 



I. Evidence from the energy of rupture. — The orientation theory indi- 

 cates that if a bar of liquid octyl alcohol (Fig. 12) were to be pulled 

 apart, one of the steps in the process would be for the molecules to 

 orient where the break is to occur (Fig. 13) in such a way that this will 

 become the weakest part of the bar. Evidently this means that the final 

 break will occur between the non-polar ends of the molecules. If octane 

 (CsHis) is ruptured, the work done for a bar of i sq.cm. cross-section (Wc) 



is 43.5 ergs. When octyl alcohol 



— - - :Alcphol-_ - z-i 



Fig. 12. — 

 Bar of liquid 

 of unit cross- 

 section. 



is pulled apart, additional energy 

 must be utilized in orienting the 

 molecules, so it is not surprising that the work 

 of rupture (Wc) is slightly higher (55.0). 



If, however, the octyl alcohol is to be 

 pulled away from water, and the molecules of 

 the alcohol are oriented in the interface, then 

 t/ie final break must come between the polar 

 hydroxyl ( — OH) groups of the alcohol and the 

 polar molecides of water (Fig. 14): therefore 

 (the orientation theory predicts that in this 

 case) the work of rupture (work of adhesion, 

 Wa) should be high. The experimental results 

 show that this prediction of the theory is 

 justified, since the work of adhesion is found 

 to be 92 ergs, or 60 per cent higher than the 

 work required to rupture the alcohol. Even 

 more remarkable is the fact that this is 1 11 per 

 cent higher than the work required to rupture 

 octane, and 1 10 per cent higher than the amount of work necessary to separate octane 

 from water. The extremely remarkable nature of these results is evident when it is 

 considered that the molecule of octane contains 26 atoms, while that of octyl alcohol 

 contains these same 26 atoms and only one more, an atom of oxygen which gives the 

 polar nature to the molecule. Thus an increase of less than 4 per cent in the number 



D 



Alcohol 



Fig. 13. — Represents the orientation 

 of the molecules which occurs if a bar of 

 alcohol is pulled apart. 



