Microstructure of Dispersions of Lamellar Droplets Carrying Anchoring Hydrophobically Endcapped Poly(sodium acrylate)s as Novel Steric Stabilizers

• Published in: Langmuir Vol. 15; no. 15; pp. 4989 - 5001
• Main Authors: Kevelam, Jan, Martinucci, Silvia, Engberts, Jan B. F. N, Blokzijl, Wilfried, van de Pas, John, Blonk, Han, Versluis, Peter, Visser, Antonie J. W. G
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 20.07.1999
• Subjects: Biochemie; Biochemistry; Chemistry; Colloidal gels. Colloidal sols; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry; Ethylene oxide polymer; Anionic surfactant; Stabilization; Non ionic surfactant; Coated particle; Sodium acrylate polymer; Sodium compound; Aqueous solution; Colloidal dispersion
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la981636v

We have studied the influence of anchoring hydrophobically single-endcapped poly(sodium acrylate)s on the microstructure and colloidal stabilization of self-assembled lamellar droplets formed from a mixture of anionic and nonionic surfactants in concentrated aqueous electrolyte solutions. A fluorescently labeled hydrophobically endcapped poly(sodium acrylate) has been synthesized and characterized using time-resolved fluorescence spectroscopic techniques; it appears that the fluorophore has considerable freedom of internal rotation. Using this labeled poly(sodium acrylate), the presence of an adsorbed polymer layer bound to the surface of the droplets was imaged by confocal scanning laser microscopy, providing visual evidence that the droplets are sterically stabilized. Laser diffraction and refractive index measurements were employed to determine average particle sizes of the colloidal particles, and it was established that increasing the molecular weight of the hydrophilic (pendant) backbone at a constant (hydrophobic) anchor density, 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 pressure in the adsorbed layer, due to a higher polymer segment density near the 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 of lateral pressure with surface coverage is held responsible for this effect.