Influence of the non-ionic surfactant PEG-660-12-hydroxy stearate on the surface properties of phospholipid monolayers and their effect on lipid emulsion stability

The effect of the interaction between phospholipid monolayers and PEG-660-12-hydroxy stearate as a non-ionic surfactant on lipid emulsion stability in dynamic and static conditions was studied. The presence of PEG-660-12-hydroxy stearate molecules with phospholipid monolayers (static state) leads to a remarkable increase in the surface pressure (from 5 to 30 mN/m in the initial molecular area), whereas in the dynamic state, when the two emulsifiers are separated and each dissolved in one phase of the two emulsion phases, a sudden decrease in the surface pressures is observed. This indicates that PEG-660-12-hydroxy stearate molecules are intercalated between the phospholipid monolayers forming a molecular mixed film. At the same time, a part of the phospholipid monolayers interacts with the surfactant monomers to form a soluble or partially soluble association complex. This interpretation was also supported by interfacial tension measurements, where the interfacial tension in the dynamic state was lower than that in the static one. This indicates that in static conditions the phospholipids partially interact with PEG-660-12-hydroxy stearate resulting in a non-active association complex. Subsequently there is insufficient utilization of the available surfactants during the emulsification process. In contrast, in dynamic conditions both emulsifiers are available at the free surface from the beginning. This behaviour was substantiated by investigating the stability of emulsions which were prepared either by the static condition or the dynamic one during the autoclaving process. Received: 25 May 1998 Accepted in revised form: 18 September 1998