Microstructure of dispersions of lamellar droplets carrying anchoring hydrophobically endcapped poly(sodium acrylate)s as novel steric stabilizers

We have studied the influence of anchoring hydrophobically single-endcapped poly(sodium acrylate)s on the microstructure and colloidal stabilization o f self-assembled lamellar droplets formed from a mixture of anionic and non ionic surfactants in concentrated aqueous electrolyte solutions. A fluoresc ently labeled hydrophobically endcapped poly(sodium acrylate) has been synt hesized and characterized using time-resolved fluorescence spectroscopic te chniques; it appears that the fluorophore has considerable freedom of inter nal rotation. Using this labeled poly(sodium acrylate), the presence of an adsorbed polymer layer bound to the surface of the droplets was imaged by c onfocal scanning laser microscopy, providing visual evidence that the dropl ets are sterically stabilized. Laser diffraction and refractive index measu rements were employed to determine average particle sizes of the colloidal particles, and it was established that increasing the molecular weight of t he hydrophilic (pendant) backbone at a constant (hydrophobic) anchor densit y, or increasing the concentration of polymer in the dispersion at constant molecular weight, results in a decrease of the average droplet size. This is in agreement with theoretical predictions that an increased lateral pres sure in the adsorbed layer, due to a higher polymer segment density near th e surface, is relieved by increasing the curvature of the lamellar droplets . Finally, the adsorption of hydrophobically endcapped polymers to lamellar droplets has been described in terms of a Freundlich isotherm, reflecting the degressive increase of the amount of polymer adsorbed onto the surface of the droplets with increasing polymer concentration. Again, an increase o f lateral pressure with surface coverage is held responsible fdr this effec t.