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In this work, we study experimentally the effect of an external osmotic pressure πext on the swelling of ionic mesoscopic gels in the weak screening limit, where the Debye screening length is larger than the mesh size of the gel. Variations in the osmotic pressure were induced by adding dextran to the solution. The results show that ionic microgels do not respond to πext below a given value of the normal stress; above this value the system de-swells with increasing osmotic pressure. The start of de-swelling is set by the gel charge density. The Flory thermodynamic theory for ionic gels captures the essential characteristics of the de-swelling behavior; in particular, it predicts with fairly good accuracy the value of πext at which de-swelling begins. Finally, due to the colloidal character of the gels, we observe that the system flocculates by a depletion interaction mechanism at high dextran concentrations.

The colloidal stability of polymer colloids with different functionality before and after adsorption of Triton X-100 has been studied. Experimental log W versus log [electrolyte] plots for bare particles were fitted using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, and values of diffuse potential (ψd) and the Hamaker constant (A) were obtained. In the same way, log W versus log [electrolyte] plots for some latex-surfactant complexes were fitted using the extended DLVO theory, which considered steric repulsive interaction. Kinetic information was employed in order to reduce the number of fitting parameters of the steric repulsive potentials. In this way, the thickness of the stabilizing layer (δ) was the fitting parameter. We have found that the extended DLVO theory explains directly the results of some of the latex-surfactant complexes. However, in other cases, additional mechanisms had to be considered to explain the results. All these discrepancies could indicate that electrostatic and steric repulsion energies are not completely independent.