The Influence of Charged Lipids on the Flocculation and Coalescence of Oil-in-water Emulsions. I: Kinetic Assessment of Emulsion Stability

The influence of various negatively charged lipids on the stability of phospholipid stabilized oil-in-water emulsions in the presence of varying concentrations of calcium chloride was examined by a measurement of the changes in the turbidity of the emulsion over time. The data were described well by the following equation: (Ai - A) = (Ai - A0) exp(-kt) where Ai is the maximum spectrophotometric absorbance achieved in the system, A is the absorbance at time t, A0 is the absorbance at the start of the experiment and k is the flocculation or coalescence rate constant. Plots of k and Ai/A0 vs. concentration of Ca++ provided an indication of the rate and extent of emulsion flocculation/coalescence, respectively. The sodium salts of phosphatidic acid (SPA) and oleic acid (SO) increased the critical flocculation concentration of calcium while sodium phosphatidylinositol (SPI) and sodium phosphatidylserine (SPS) had no effect on the critical flocculation concentration compared to the control emulsion. The addition of all lipid salts increased the rate of flocculation compared to the control emulsion, however, emulsions containing SPI demonstrated the highest values of k. In contrast, emulsions containing SO coalesced to the largest extent, as indicated by large values of Ai/A0. Systems containing charged phosphatides regained some stability in higher concentrations of Ca++ while those containing oleate were comparatively more unstable up to 20 mM Ca++. Based upon the results of the present studies, it appears that phosphatidic acid is the most important fraction of the anionic phosphatides in stabilizing an emulsion in the presence of calcium ion.(ABSTRACT TRUNCATED AT 250 WORDS)