Comparative study on the colloidal stability mechanisms of sulfonate latexes

In this work the colloid stability mechanisms of two sulfonated latexes prepared by different methods are studied. One of the latexes (JM1) was prepared by surfactant-free emulsion copolymerization of styrene (St) and sodiun styrene sulfonate (NaSS), while the second latex (SN9) was prepared by a two-stage "shot-growth" emulsion polymerization process using NaSS as ionic comonomer, which is included in a larger proportion in the second stage. The results are two latexes with nearly the same particle size but very different surface charge densities and, probably, one of them with a significant highly charged "hairy" surface. The anomalous electrokinetic behavior of the sulfonate polystyrene latexes was a consequence of their electric double-layer structure, and their high colloidal stability was not explained by the classical DLVO theory. This high colloidal stability could be a consequence of the hairy layer formed during the synthesis method (which includes the use of an ionic comonomer that can act as a polymerizable surfactant). By including several correction factors in the DLVO theory, such as the Stern layer thickness (ionic size) and the hydrodynamic interaction, a good correlation can be found between the theory and experimental results: the Hamaker constant value so obtained is equal to the theoretical one. If we try to explain the stability results by an electrosteric mechanism, it is possible to find a set of parameters which provide critical coagulation concentration (ccc) values which are in accordance with the experimental one; but, however, some controversies appear in relation to the values of some of the fitting parameters, because the Hamaker constant value is now lower than the theoretical one and the ions have to be dehydrated. Therefore, for the sulfonate latexes prepared by a "shot-growth" process, their high colloidal stability can be well explained by including some correction factors in the DLVO theory, and the use of a steric mechanism gives lower Hamaker constant values, which could mean that the hairy layer of these latexes does not seem to be large enough to produce a clear additional steric stabilization.