Colloidal dispersion stability of unilamellar DPPC vesicles in aqueous electrolyte solutions and comparisons to predictions of the DLVO theory

The colloidal dispersion stability at 25 degrees C of aqueous dispersions of sonicated DPPC (dipalmitoylphosphatidylcholine) vesicles was quantitatively evaluated from the Fuchs-Smoluchowski stability ratio W. Data of average particle size vs. time were obtained with dynamic light scattering measurements. Dispersions in water, 1, 10, or 150 mM aqueous sodium chloride, and phosphate buffer saline (PBS) solutions were tested. The W-values ranged from about 8 x 10(6) to 6 x 10(8). The dispersed particles had small but significant zeta-potentials zeta, implying that the vesicles had some significant charge due to preferential adsorption of negative hydroxyl ions over hydronium ions in water, and chloride ions over sodium ions in the electrolyte solutions. Hence, there was some contribution of the double-layer electrostatic forces to the dispersion stability. The initial values of zeta ranged from -30 to 8 mV. The vesicle non-retarded Hamaker constants were estimated from the published values of the Hamaker constants of DPPC and water in vacuum, and they varied with the vesicle size. A new dimensionless model of the DLVO theory for spherical particles was formulated, focusing on the conditions for the existence of a positive interaction potential energy maximum, phi(max), which is linked to W. The dimensionless number N, which is defined as the ratio of the electric double layer energy to the Hamaker constant A, was found to be the key determinant of phi(max) and W. DLVO calculations of phi(max), and W with error analysis show that the charged DPPC vesicles tested are quite more stable than predicted. This discrepancy highlights some significant weaknesses in the premises and approximations of the DLVO theory, and the need for improved theories, possibly considering other repulsive forces. (C) 2009 Elsevier Inc. All rights reserved.