24 



SUBCELLULAR PARTICLES 



This conceptual scheme does not seem to have been explored extensively during 

 the past 20 years, although at present enormous effort is being expended on 

 examining the enzyme chemistry of cell surfaces and cellular particles. Let us 

 examine the basis of Danielli's assertion. 



It had been known for some time that the interfacial tension (I.T.) between 

 nonaqueous solutions of long-chain acids, or amines, and aqueous solutions, is 

 much lowered when the pH of the aqueous phase is such that the polar groups 

 of the solute at the interface are electrolytically dissociated. However, the range 

 of pH over which the I.T. change occurs with a fatty acid is more alkaline by 

 as much as 3 pH units than the titration curve of the acid (HA) in the bulk 

 aqueous phase. (The choice of buffer in these and related experiments is very 

 important, since buffer cations, such as acetate, may also accumulate in an inter- 

 face, particularly if an association with H^ ions is extant.) The change is more 

 marked with lower concentrations of cation. A simple assumption to account for 

 this pH shift phenomenon is that the pH at the interface is less than that of the 

 bulk aqueous phase because of the concentration of charge (minus) at the inter- 

 phase and a consequent concentration of H+ ions at the interface to form an ionic 

 double layer of thickness 8. 



Danielli derives an expression for the pH-shift by a consideration of a Donnan 

 equilibrium involving 'activities' of ions, which is 



where M^ is cation other than hydrogen, and the dissociation of the fatty acid at 

 the interface which is given by 



[H.M [A.-l = K4HA.l. (2) 



Surface phase, s 



Non-aqueous 

 phase with 

 source of minus 

 charges, A" 



Bulk phase, b 

 B' 



H* 



B- 



B' 



B- 



FiG. 1. Relations of pH ami I. T. change with fatty acitls. M* = cations other than H*, and 

 ' =z anions other than those from the fatty acid, A". 



