Molecular interactions between phospholipids and electrolytes in a monolayer of parenteral lipid emulsion

The particle size of parenteral lipid emulsions containing phosphatidylcholine decreased in solutions of electrolytes. The sizes of lipid particles in solutions of KCl or CaCl2 decreased to 90% compared with those in water. To determine the reasons for this, the effects of electrolytes on the size of lipid particles were investigated in terms of the molecular area (limiting area). The molecular areas at the air/water interface were obtained from the surface pressures of monolayers by the Wilhelmy method. The molecular area of phospholipids which formed the interfacial membrane of a lipid emulsion decreased when electrolyte was added to the water phase. The molecular areas for monolayers of all phospholipids decreased to approximately 90% with 10 mM KCl solution. These findings suggest that the molecules of phospholipids pack closely since the repulsion between molecules weakens with increase in ionic strength. On the other hand, in the presence of CaCl2, the areas decreased to 80% due to electrical adsorption of Ca2+ onto the phospholipid membrane surface. Furthermore, we studied the mixed monolayers dynamically. The excess free energy of a mixed monolayer of a phosphatidylcholine and an oleic acid became negative with the addition of Ca2+, suggesting that the mixed monolayer formed a condensed membrane. These results show that the addition of electrolytes induces membrane condensation and decreases the size of lipid emulsion. (C) 1997 Elsevier Science B.V.