﻿414 Size of Colloid Particles and Adsorption of Electrolytes. 



Discussion of Residis. 



It will be seen that, under the conditions imposed and 

 within the limits of the experiments, a comparison of the 

 data obtained, upon the basis of the total interfacial surface 

 in unit volume, leads to uniformity in the curves. Such 

 uniformity is not to be observed when the comparison is 

 based upon the mass of the disperse phase, or the number 

 of particles, in unit volume. It appears that adsorption is 

 very largely conditioned by the amount of interfacial surface 

 exposed. It is to be noticed, however, that the minimal 

 concentration of electrolyte is higher throughout for the 

 fraction containing small particles than for that containing 

 large particles. This may be brought about in two ways. 

 The smaller particles may bear a higher charge per unit 

 area of their surface, or the critical value to which their 

 charge must be reduced before coagulation begins may be 

 lower than in the case of the larger particles. The latter 

 explanation is more probably correct, since it is known that 

 the surface tension of large particles is greater than that of 

 small ones. It seems probable, if the existence of a critical 

 potential difference for coagulation between a particle and 

 the dispersion medium be admitted, that this should be 

 lower in the case of small particles which have less tendency 

 to adhere, and should thus permit of a greater freedom of 

 approach between the particles. If the former explanation 

 were correct, we should expect a separation of the particles 

 according to size in an electric field ; but this is contrary to 

 experience, the particles move at the same rate independently 

 of their size. According to the Helmholtz theory of the 

 electrical double la}^er, this effect is due to equal density of 

 *the charge upon unit area of the surface. It appears 

 probable, therefore, that the smaller particles have a lower 

 critical potential difference for coagulation. The behaviour 

 of the solution containing mixed particles of different size is 

 in some respects curious. With both A1 2 (S0 4 ).3 and NaCl 

 the curve representing the coagulation of this solution 

 is more flattened relatively than the other two curves. A 

 lack of uniformity in ths size of the particles appears to 

 render the solution less sensitive to change in concentration, 

 in the case of coagulation by univalent and trivalent ions. 



