The chief methods employed in the production of concentrated latex have been confined to the controlled evaporation of water from the latex by the application of heat, concentration by centrifuging, and separation of the latex into layers of different concentrations by the action of creaming agents. Recently, another process has been used commercially, which produces concentrates similar to those obtained by the centrifuging and creaming methods in that a substantial part of the serum constituents is removed during the concentration. The new method is based on phenomena observed by Pauli and his co-workers on the behavior of colloidal sols when subjected to electrodialysis. It was noted that in the purification of sols, such as silicic acid and serum albumen, by subjecting them to electrodialysis between vertical semipermeable membranes, separation was frequently obtained into superimposed layers containing different concentrations of the sol. The more concentrated layers were uppermost, or vice-versa, according to the relative densities of the sol and dispersing medium. This stratification is probably the result of the combined influence of the electrical potential and gravitational force under suitably chosen conditions. The colloidal particles migrate to one of the membranes under the influence of the electric field, with consequent concentration at the membrane surface. If the field strength is not too great, the particles become more under the influence of gravitational force than of the electrical force, tending to cause deposition on the membrane. The concentrated layer of particles tends, therefore, to move upwards or downwards along the surface of the membrane in accordance with its relative density with respect to that of the surrounding liquid. Conversely, there is a corresponding reduction in the concentration of the sol at the other membrane surface, which promotes a flow of more dilute dispersion in the opposite direction to that of the stream on the other membrane. If this action is allowed to continue, the dispersions of different concentrations spread out through the cell liquid in the form of horizontal layers which may be decanted.
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