Abstract

The study of latex is characterized by the fact that it is always difficult to draw any definite conclusions. One is confronted with a substance which has been synthesized by a living organism, and whose extremely complex structure is very different from that of the sols currently studied by colloid chemistry. However, ultracentrifugation makes it possible to obtain reliable results as far as partition of the natural metallic components between the dispersing and dispersed phases is concerned. The existence of equilibria of adsorption as a function of the dilution of latex and comparison of the experimental curves with the isotherms of Freundlich and of Langmuir shows that, in the case of latex, adsorption is not a simple phenomenon. At least two different processes seem to be involved: (1) electrosorption of the metallic cations by the negative surface of the rubber particles, and (2) indirect adsorption, whereby the metal is bound in a slightly dissociable form to an organic anion which is itself strongly adsorbed. This would explain the much weaker adsorption of the alkali metals, which possess only a positive charge, and whose salts are in general more easily dissociated than those of the alkaline earth metals. The results obtained for metals added to latex in the form of soluble salts lead to the same conclusions. Measurement of the adsorption equilibria for samples of latex containing different concentrations of rubber show that, the smaller the proportion of dispersing phase to dispersed phase, the higher is the adsoprtion. This phenomenon, which is evident with alkali metals as well as with alkaline earth metals, can be explained by the increase of the charge on the particles when the latex is diluted. An examination of the critical adsorption values for the different metals studied confirms the fact that the behavior of latex with salts is that of a mixed sol with both lyophilic and lyophobic properties. These results would be more valuable if confirmed and determined more precisely by an electrokinetic study. The determination of the potential of the particles by measuring the cataphoretic velocity with respect to the adsorption of metal ions would surely permit a more accurate interpretation of the mechanism of this adsorption.