JOHN H. NORTHROP 



799 



NATURE OF THE COHESIVE FORCE 



The theory of the stabiHty of colloidal suspensions outlined above predicts that 

 coagulation will occur whenever the cohesive force is greater than the repulsive force. 

 There is, in addition, very good evidence that the velocity of migration in an electric 

 field is a measure of the repulsive force. There is every reason to believe that this rate 

 of migration is, in turn, proportional to the potential difference between the surface 

 of the particle and the surrounding liquid, and that the repulsive force is due to the 

 mutual repulsion of this electric charge. The nature of the attractive force, however, 

 is much less certain. The majority 

 of writers on the subject state 

 that it is a "surface tension" or 

 "capillary" effect, and Billitzer' 

 assumed that the change in the 

 potential was simply a measure 

 of the change in the surface ten- 

 sion, the latter approaching a 

 maximum as the p.d. approached 

 a minimum. The same view is 

 expressed by Michaelis.- 



Green and Halvorson^ have 

 shown that, hypothetically, 

 changes in surface energy might 

 serve as the cohesive force and 

 could be responsible for the ag- 

 glutination of suspensions. 



Mellon's^ experiments are of 

 interest in this connection. He 

 found that Sodium oleate pre- 

 vented the agglutination while caprylic and nonylic alcohol did not, although the 

 latter have as great an effect on the air-solution inte'rfacial tension as does sodium 

 oleate. The experiment shows, therefore, as might be expected, that measurement at 

 the air-solution interface has no relation with the force at the particle-solution 

 interface. 



Von Smoluchowskis also assumes that the attractive force is increased as the po- 

 tential decreases. Freundlich^ assumes the attractive force to be constant. If the 

 writer's measurements of this force are significant, there is no doubt that it does not 

 vary in any way with the potential. There is no valency effect, and the effects of salt 

 are all in concentrations so high that the potential is very low or absent. It must be 



' Billitzer, J.: Ztschr. phys. Chemie, 51, 128. 1905. 



^Michaelis, L.: Pie Wasserstoffionenkonzentration (ist ed.), p. 49. 1914. 



3 Green, R. G., and Halvorson, H. O.: J. Inject. Dis., 35, 5. 1924. 



4 Mellon, R. R., et al.: op. cit., p. 365. 1924. 



s von Smoluchowski, M.: Physik. Ztschr., 17, 557, 583. 1916. 

 ^Freundlich, H.: Kolloid-Ztschr., 23, 163. 1918. 



Molar.concentration.of.buffer.mixture 



Fig. 10. — Effect of glycocoU-phosphate-acetate buffer, 

 pH 7.0, in varying molar concentration, upon potential of 

 pneumococcus type I, (a) alone, {b) treated with 1/20 

 normal and (c) with 1-20 specific serum, and {d) of globu- 

 lin particles obtained from the specific serum. 



