IONIC INTERACTIONS 27 



owing the changes in viscosity of the surface film. 



As a result of the forces operating between a charged 

 surface and the ions present in an underlying medium, 

 the ionic composition of the interfacial region may be 

 very different from that of the surrounding bulk phase. 

 To give an example: if a monolayer of palmitic acid is 

 spread upon a solution containing 2,000 sodium ions to 

 I calcium ion, in the interfacial region the ratio is of the 

 order of 0.3 sodium ions to one calcium ion (Danielli 

 and Webb). This large difference in the ionic ratio is 

 partly caused by the fact that the charge on the surface 

 attracts multivalent ions much more strongly than it 

 attracts univalent ions. But this is only part of the cause. 

 In the case of the sodium palmitate monolayer just men- 

 tioned, if the electrostatic effect were entirely responsible 

 for the difference in ratio between the surface and bulk 

 phases, the ratio in the surface would be 100 sodium 

 ions to I calcium ion : the difference between this figure 

 and the actual figure of 0.3 sodium ions to i calcium ion 

 is due to a second factor. The second factor is the for- 

 mation of unionised complexes between certain ions and 

 groups in the surface. 



In the case of egg albumin molecules in solution it has 

 been possible to carry this analysis to a quantitative 

 conclusion (Danielli). The closed circles of Fig. 5, 

 show experimental values obtained for sodium : cal- 

 cium ratios in ovalbumin solutions. Ultrafiltrates were 

 prepared from the solutions, and the difference between 



