THE ELECTROKINETIC PROPERTIES OF PHOSPHOLIPID-STABILIZED FAT EMULSIONS - .3. INTERDROPLET POTENTIALS AND STABILITY RATIOS IN MONOVALENT ELECTROLYTES

The interdroplet potential in a typical parenteral fat emulsion in the presence of sodium ions has been estimated from electrolyte stability data in a conventional manner as a sum of electrostatic, Van der Waals, and hydration forces. The hydration forces are important at all electrolyte concentrations, and result in the abolition of the primary potential minimum. At high (> 100 mM) electrolyte concentrations a secondary minimum is observed into which the droplets can flocculate reversibly. The stability ratios of the emulsion in monovalent electrolyte solutions were calculated from the interdroplet potentials. Good agreement was found between the calculated values and those measured by turbidimetric means. Optimization of the model yielded a value for the Hamaker constant (A11) of soya oil of 4.75 × 10-20J (A121 = 6.5 × 10-22J in water). The importance of the hydration forces in determining the interdroplet potential suggests that these should receive at least as much attention as the electrostatic forces in the consideration of emulsion stability. It is suggested that amino acids exert their stabilizing effect in parenteral feeding mixtures by modifying the hydration forces, possibly allowing them to be more efficiently propagated through the solvent medium. The results offer hope that it may ultimately be possible to predict the stability of TPN mixtures from their composition alone. © 1990.