SURFACE CHEMISTRY 43 



When a pure saturated liquid hydrocarbon is placed upon the water, it 

 remains on the surface as a drop or lens which has no effect on the surface 

 tension of the surrounding water. If, however, an insoluble fatty or oily 

 substance, such as the common vegetable and animal oils, is placed upon 

 clean water, it spreads out almost instantly as a thin film over the surface. 

 If the motions of the surface are made visible by dusting the surface with 

 powdered talc, it may be seen that, with a limited amount of oil, the film 

 only spreads out sufficiently to cover a definite area, or at least if the area 

 exceeds a rather definite value, the oil has no effect on the surface tension 

 of the water. A comparison of various insoluble organic substances has 

 proved that the spreading tendency depends upon the presence of certain 

 active groups or radicals in the organic molecule, these being the groups 

 which tend to increase the solubility of organic substances in water. For 

 example, pentane, C5H12, is practically insoluble in water but amyl alcohol, 

 C5H11OH, is relatively soluble. Thus the OH groups in organic molecules 

 exert strong attractive forces on the OH groups in the water molecules 

 and these manifest themselves by an increase in solubility. Similarly the 

 carboxyl group COOH tends to make the lower fatty acids much more 

 soluble in water than the corresponding hydrocarbons. 



Hydrocarbons with high molecular weight such as CisHas are ex- 

 tremely insoluble in water. If the carboxyl group replaces the CH3 group at 

 the end of the chain CigHss, one end of the molecule only tends to dissolve 

 in water, whereas the rest of the molecule still retains the insolubility of 

 the hydrocarbons. By spreading over the surface of the water, molecules 

 of this kind can bring their carboxyl group in contact with the water 

 without separating from one another. 



An oil film formed in this way must consist of a single layer of mole- 

 cules packed closely on the surface layer of the water. If there is a surplus 

 of the fatty acid, as compared with the limited area over which it can 

 spread, the endeavor of the carboxyl groups to come in contact with 

 water causes the molecules to become so crowded at the surface that they 

 stand nearly erect, side by side on the surface. The area occupied by each 

 molecule is thus determined by the cross section of the hydrocarbon chain 

 or by that of the head in contact with water, if this happens to be larger 

 than the cross section of the chain. The thickness of the film is then deter- 

 mined by the length of the hydrocarbon chain. 



By dissolving known weights of liquid or solid, oily or fatty sub- 

 stances in a volatile solvent such as hexane, and placing known amounts 

 of these dilute solutions on the water surface, a definite number of mole- 

 cules can be transferred to the water surface. The oil film can be confined 

 to a given portion of the surface of water in a long tray by a floating strip 

 of paper reaching across the tray. By measuring the forces exerted on the 



