The partitioning of PGE1 in a lipid emulsion has been shown to be consistent with a three-phase model which assumes that solute may reside in the bulk aqueous and oil phases and at the oil-water interface. Ultrafiltration and dialysis techniques were used to estimate the relative percentages of PGE1 in each phase at various pHs. The amount of PGE1 present in the bulk oil phase of the emulsion was concluded to be insignificant. At emulsion pH values of less than five, PGE1 resides preferentially at the interface. With increasing pH, the percentage of PGE1 in the aqueous phase increases. A model which assumes that both the non-ionized and ionized PGE1 species may be present at the interface, depending on pH, was shown to be consistent with the data. Estimates were made of the distribution coefficients of the ionized and non-ionized PGE1 between the interface and the aqueous phase and their concentration dependence. The distribution coefficients were used to generate a distribution profile of the various PGE1 species as a function of pH. The overall dehydration kinetics of PGE1 in a lipid emulsion were found to fit a model whereby the k(apparent) measured at each pH is simply the sum of the product of the fraction of the PGE1 at the interface, f(i), and the rate constant at the interface, k(i), plus the product of the fraction of the PGE1 in the aqueous phase, f(aq), and the rate constant in the aqueous phase, k(aq). The k(aq) and k(apparent) were experimentally determined as a function of pH. The k(i) was indirectly determined from the stability data in the emulsion. Microscopic rate constants for dehydration of PGE1 in the aqueous phase and interface were estimated from the experimental data. The dehydration kinetics were also shown to be affected by the addition of charged surface active agents.
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