Interfacial effects and emulsion stabilization by in situ surfactant generation through the saponification of esters

Abstract

The stabilization of oil-in-water emulsions is critical for a wide variety of fields. Here, we explore the reaction dynamics of and emulsion stabilization by in situ surfactant generation through the saponification of esters that are commonly used in cosmetics and drug delivery. We combine measurements of dynamic interfacial tension and surface elasticity with emulsion stability to determine the dynamics of the interfacial reaction and its effect on interfacial properties. The rate of surfactant production through the in situ reaction depends on the surface area and thus the dynamics measured in a pendant drop may not reflect the dynamics in emulsification. To test this, we also measure the dynamics of surfactant production after emulsification. Further, we explore the effect of surfactant chain length and solution pH on the interfacial tension dynamics and the emulsion stability. The results show that the interfacial tension transients in a pendant drop correlate with the in situ reaction dynamics but that there are others factors to consider. The steady state interfacial tension represents a pseudo-steady state between rate of surfactant produced and that desorbing into the aqueous phase. The interfacial tension dynamics and emulsion stability also depend strongly on the charge. The emulsions retain stability after aqueous pH neutralization, suggesting that a portion of the adsorbed surfactant molecules remain ionized at neutral pH. Emulsion stability increases with increasing surfactant chain length, which is the expected trend because a longer chain length reduces aqueous solubility thereby increasing the surfactant surface coverage. Therefore, the approach of emulsion stabilization by in situ surfactant production is more useful for surfactants with very low aqueous solubility. Finally, the transients of surfactant concentration in the emulsions show a reduction in the rate of the in situ reaction with increasing coverage of the surface by the adsorbed anionic surfactant most likely due to the buildup of surface charge that reduces the concentration of the hydroxyl ions at the surface.