CHAPTER II 



Possible Actions of Drugs on Surfaces 



Ionic Interactions 



Ions may react with the charged groups of surfaces. The 

 molecules composing a surface may have charges upon 

 them which can be represented either as an electrostatic 

 dipole, a fully ionised group, or as a combination of 

 dipoles and fully ionised groups. When an interfacial 

 layer of molecules is composed mainly of dipolar mol- 

 ecules, the electrostatic potential difference between the 

 two phases is commonly of the order of 500 mV. Ions 

 may affect the packing of these molecules if they come 

 close to or are adsorbed upon the interface. The effect 

 of the ions is, of course, in such a case mainly mediated 

 by the interactions between the ionic charge and the 

 dipoles of the molecules. Where fully ionised groups 

 are present in the interface, such as carboxyl, phosphate 

 and amino groups, the action of ions is mainly mediated 

 by the interaction between the ion and the ionised group 

 of the interface. The effect of ions upon an ionised sur- 

 face is commonly greater than the effect of ions upon a 

 surface composed of dipolar molecules. As a result of 

 the interaction with ions the structure of a surface and 



