Influence of pH on colloidal properties and surface activity of polyglycerol fatty acid ester vesicles

Abstract

Certain polyglycerol esters of fatty acids (PGE) form dispersions of uni- or multilamellar vesicles in dilute aqueous solution. These self-assembled aggregates reduce the surface-activity of PGE monomers such that interfacial films may take several hours to form. This is undesirable for processes, which rely on rapid surfactant adsorption, for example foaming. In the present work, we study the effect of pH on the colloidal (size distribution, morphology, surface charge) and interfacial (adsorption kinetics) properties of a commercial, non-purified PGE. Using dynamic light scattering, zeta-potential measurements and cryo-SEM, we show that changing the pH of the dispersion media can cause agglomeration and eventually osmotic rupture of PGE vesicles. The change in dispersion state also impacts the adsorption behavior at the water surface. Direct evidence that destabilized vesicle dispersion are more surface-active is provided by comparing the dynamic surface tension of solutions of different pH. The faster adsorption kinetics at low pH correlate with a remarkably increased foaming power. We suggest that an osmotic shock induced by changes in pH causes vesicles to deform and partially open, so that their hydrocarbon core is exposed to the dispersion media. This energetically unfavorable condition promotes the hydrophobically driven adsorption of surfactant monomers at surfaces and hence stimulates the foaming ability.