174 SURFACES AND MEMBRANES 



sess as high a value as 465 ergs/cm 2 . The surface energy possessed by 

 some of the more common physiological fluids is shown in Table V-2, 

 each of which has a surface energy lower than water. 



TABLE V-2 



Liquid-Liquid Interface Surface Energy 

 Interfacial energy E in ergs per square centimeter, 20° C 



T . . . Liquid- Water Liquid- Air 



Liquid E M E 



Liquid-Liquid Interface 



Interface energy also exists at the surface between two immiscible 

 liquids. It has been found that the interfacial energy between two 

 liquids is less than the energy of the liquid possessing the higher surface 

 energy when in contact with air. In this case the reduction in energy 

 is a measure of the reduction of the surface energy of the boundary due 

 to the presence of the superimposed liquid. 



Here one liquid surface is in close contact with a second liquid surface. 

 The molecules of one liquid surface attract the molecules lying in the 

 opposite face of the adjoining liquid across the contact boundary. This 

 attraction diminishes the effective pull exerted by each of the liquids on 

 its own surface molecules. The condition for complete miscibility of 

 two liquids is that this interfacial energy shall be zero. Table V-2 

 shows how much the interfacial energy of water in contact with an 

 organic liquid has been reduced below that of water in contact with air. 



Effects of Substances in Solution on Surface Energy 



Substances dissolved in a liquid may either lower or raise the surface 

 energy at the interface between liquid and air. When many inorganic 



