Physicochemical characterization of parenteral lipid emulsion: determination of Hamaker constants and activation energy of coalescence.

The particle size of lipid emulsion (LE) is changed by flocculation and coalescence. This change in particle size was studied using values obtained for maximum total interaction energy (Vtmax) for flocculation and activation energy for coalescence (E). Vtmax was calculated using DLVO theory, and E was calculated from the rate of increase in particle size in LE. Two LEs (PC99LE and PC70LE) were prepared from lecithins containing 99% and 70% phosphatidylcholine, respectively. The Hamaker constants for PC99LE and PC70LE were found to be 1.4 x 10(-22) J and 3.1 x 10(-21) J, respectively. Vtmax for PC99LE was 4.7 kT at 121 degrees C, and E was 1.5 x 10(-19) J, while Vtmax for PC70LE at 121 degrees C was 151 kT and E was 3.2 x 10(-19) J. These findings suggest that PC99LE readily underwent flocculation and coalescence with increase in particle size, but that the particle size of PC70LE changed little. The degrees of flocculation and coalescence of LE were determined separately using values of Vtmax and E. These parameters are thus quite useful in predicting the stability of LE.