Interactions between modified mica surfaces in triglyceride media

Results obtained from surface force measurements using modified nonpolar mica surfaces immersed in triolein are presented. The force vs distance curves were determined for different water activities in the interaction medium. Two oscillations with a periodicity of 20 Angstrom were observed in the force curve measured across anhydrous triolein. The force barriers appear at separations of 45-40 and 20-30 Angstrom. It is suggested that triolein has no clear preferential orientation of the oleic acid chains outside a nonpolar surface. This is different from outside a polar mica surface where triolein adopts conformations with the three oleic acid residues directed toward the bulk. At high water contents the triolein molecules outside nonpolar surfaces suddenly change their orientation when a high compressive force is applied. The forces acting between mica surfaces were measured in triolein solutions containing phospholipids at different; water activities. It was shown that the phospholipid self-assembled onto the mica surfaces and rendered them nonpolar. The forces between such surfaces in anhydrous triolein are similar to these observed between mica hydrophobized using the Langmuir-Blodgett technique. In addition, at high water activities a weak long-range repulsive force was observed. This force was interpreted as being due to weakly adsorbed phospholipid aggregates. We discuss the implications of the results for the stability and physical properties of colloidal particle dispersions in nonpolar media. Adsorption isotherms for the phospholipid from refined vegetable oil at a low water activity on mica and sucrose crystals are presented. They show that the phospholipid adsorbs in a monolayer on mica. On sucrose more than monolayer coverage is observed, which we interpret in terms of a phase separation of phospholipid into crevices and cracks.