CHAPTER IX 



THE ORIGIN OF THE ELECTRICAL CHARGES OF MICEL- 

 LAE, AND OF LIVING CELLS AND TISSUES 



1. STABILITY or SUSPENSIONS, ELECTRICAL CHARGES OF 

 MICELLAE, AND DONNAN EQUILIBRIUM 



The stability of suspensions is, perhaps, the chief problem of a 

 theory of colloidal behavior. Hardy 1 has shown that this prob- 

 lem is linked with the problem of the origin of the electrical 

 charges of the particles in suspension, since such particles are 

 forced by mutual electrostatic repulsion to remain in suspension. 

 By his experiments on the migration of suspended particles of 

 coagulated white of egg in an electrical field he proved that they 

 have a positive charge in the presence of acid, a negative charge 

 in the presence of alkali, and no charge at an intermediate point 

 which he termed the isoelectric point of the particles. He was 

 able to demonstrate that the stability of colloidal suspensions is 

 a minimum at the isoelectric point. 



He and others found, moreover, that low concentrations of 

 neutral salts diminish the stability of colloidal suspensions in 

 the presence of acids or alkalies and that the efficient ion of the 

 salt has the opposite sign of charge to that of the colloidal 

 particle, since the precipitating efficiency of a salt increases 

 rapidly with the valency of that ion of the salt which has the 

 opposite sign of charge to that of the colloidal particles. It 

 seemed natural to infer that the precipitation of colloidal sus- 

 pensions by low concentrations of a salt was caused by an 

 annihilation of the charge of the colloidal particle. The problem 

 of the stability of the colloidal suspension then developed into 

 the problem of accounting for this peculiar behavior of the 

 electrical charges of colloidal particles. 



Hardy's original idea was that the H ions of the acid or OH 



1 HARDY, W. B., Proc. Roy. Soc., vol. 66, p. 110, 1900. 



150 



