32 ACTIONS OF DRUGS ON SURFACES 



Straight line. Fig. 6 shows the action of heavy metals 

 on a number of biological systems. It is clear that the 

 relationship we have just obtained is approximately 

 true. 



Thus we have at present a plausible theory of the oli- 

 godynamic action of heavy metals, and of the differences 

 between their actions. It is an integral part of this theory 

 that the action involves the formation of an unionised 

 complex with ionogenic groups^ at a surface. The types 

 of ionogenic groups which we may particularly consi- 

 der as likely to be involved are phosphate, carboxyl and 

 SH. Much remains to be done before this theory can be 

 regarded as established. 



Dipole Interactions and Complex Formation 



When molecules are present side by side in a monolayer 

 the organisation of the monolayer is profoundly affected 

 by the forces operating between adjacent molecules. If 



^ It is of interest to consider mechanisms whereby a charge upon a 

 surface may arise. The mechanisms are four in number: i. The partition 

 of ions between the two phases; 2. The orientation of dipolar molecules 

 at the interface ; 3. The adsorption of ions; 4. The ionisation of ionogenic 

 groups. 



Mechanisms 2, 3, and 4 are obvious, but a word of explanation is 

 required for mechanism i. The different ions have different partition 

 coefficients between say oil and water. But in order to preserve electrical 

 neutrality the actual concentration of positive and negative charges 

 in both bulk phases must be equal. This equality is brought about 

 by the building up of an electrostatic potential at the interface which 

 effectively modifies the partition coeflftcients of the different ions. 



