Hydrophilic and Hydrophobic Colloids and the Influence of Electrolytes on Membrane Potentials and Cataphoretic

I t has been recognized for some time that it is necessary to subdivide colloids into two groups, the hydrop~lic and the hydrophobic colloids, according to the influence of electrolytes on their precipitation. To the hydrophilic colloids belong among others proteins and soaps, and to the hydrophobic colloids, collodion, mastic, graphite, metal oxides, etc. The hydrophobic colloids (such as mastic) are precipitated from aqueous suspensions b y comparatively low concentrations of salts and the precipitating effect of a salt increases rapidly with increasing valency of that ion of the salt which has the opposite sign of charge to that of the particle. Hydrophilic colloids, such as genuine crystalline egg albumin or gelatin, are only precipitated in very high concentrations of electrolytes and the sign of charge of the precipitating ion has no relation to the sign of charge of the protein. All attempts to account for this difference in the action of electrolytes on the two kinds of colloids by a common theory have failed for the reason that the forces which prevent the particles from coalescing in an aqueous solution are entirely different in the two cases. Hydrophilic colloids, such as genuine proteins, form true solutions in water, i.e. the forces by which the protein molecules or protein ions are kept in solution are the strong forces of chemical affinity between molecules of water and protein;t while the forces