i6o 



ATOMS, IONS, SALTS, AND SURFACES 



molecule may dissolve in the oil and the polar group in the water. Thus, each end of the 

 molecule is highly soluble in the liquid toward which it turns. If this is true: (a) butyric 

 acid should be very much more soluble in the interface than in either the water or the 

 benzene; {b) the butyric acid molecules should be oriented with their polar ends toward 

 the water and their non-polar ends toward the hexane. (Fig. ii). 



_ — Water Phase —-^ 



__^. 



GENERAL STATEMENT OF THE THEORY OF ORIENTATION OF MOLECULES IN SURFACES 



The following statement, written in 1916, gives in concise form the general funda- 

 mental principles of the orientation theory: 



I. The molecules in the surfaces of liquids 

 seem to be oriented, and in such a way that the 

 least active or least polar groups are oriented 

 toward the vapor phase. The general law for 

 surfaces seems to be as follows : // we suppose 

 the structure of the stirface of a liquid to be at first 

 the same as that of the interior of the liquid, then 

 the actual surface is always formed by the orienta- 

 tion of the least active portion of the molecule 

 toward the vapor phase, and at any surface or 



INTERFACE THE CHANGE WHICH OCCURS IS SUCH 

 AS TO MAKE THE TRANSITION TO THE ADJACENT 



PHASE LESS ABRUPT. This last Statement ex- 

 presses a general law, of which the adsorption 

 law is only a special case. If the molecules are 

 monatomic, and s^onmetrical, then the orienta- 

 tion will consist in a displacement of the 

 electromagnetic fields of the atom. This molec- 

 ular orientation sets up what is commonly 

 called a ''double electrical layer" at the sur- 

 faces of liquids and also of solids. 



This law, if applied to special cases, indi- 

 cates for a few pure liquids the following orientation: In water the hydrogen atoms 

 turn toward the vapor phase and the oxygen atoms toward the liquid. With organic 

 paraffin derivatives the CH3 groups turn outward, and the more active groups, such 

 as NO. CN, COOH, COOM, COOR, NH„ NHCH3, NCS, COR, CHO, I, OH, or 

 groups which contain N, S, O, I, or double bonds, turn toward the interior of the 

 liquid. 



If any of these organic compounds are dissolved in water, their orientation in the 

 water surface is the same as that just given, with the active groups inward. 



At interfaces between two pure liquids the molecules turn so that their like parts 

 come together in conformity with the general law. With solutions, the solute mole- 

 cules orient so that the ends of the molecules toward the liquid A are as much like A 

 as possible, and the ends toward B are as much like B as possible. So at interfaces be- 

 tween organic liquids and water, for example, the organic radical sets toward the or- 

 ganic liquid, and the polar group toward the water. 



Fig. II. — A two-phase system of water 

 and benzol which contains butyric acid. 

 The greatest concentration of the acid is at 

 the interface. The acid is more or less as- 

 sociated in the benzol. 



