40 



ACTIONS OF DRUGS ON SURFACES 



tension falls very rapidly, until the concentration is 

 reached at which micelle formation commences (Fig. 8). 

 At this point, the curv^e rapidly flattens out, so that the 

 surface tension becomes almost independent of concen- 

 tration. Once the curve has become flat, the concentra- 

 tion of single molecules does not increase with increase 



^■Concentration 



Fig. 8. The relationship between concentration, surface tension and 



+ 

 micelle formation for a substance such as CH3(CH2)9NMe3 



in concentration. What does increase is the concentration 

 of micelles. 



As a result of the possibility of micelle formation, the 

 following consequences may occur. 



I . If a substance can give rise to a micellar solution, 

 and if the physiological action of the substance is a 

 function of the concentration of single molecules in so- 

 lution, then over the range of existence of micelles the 

 action of a substance may be independent of its concen- 

 tration. This is because over the range of existence of 



