The monolayer behavior and transfer characteristics of phospholipids at the air water interface

Abstract

The monolayer behavior of phospholipids at the air/water interface and their transfer characteristics on the solid substrates have been investigated with a constant-perimeter type Langmuir trough. From the surface pressure-area (II-A) isotherm, evaluated were the cross-sectional area of an oriented molecule, phase transition, and the miscibility of mixed monolayers. The monolayer state depends on the chain length and its mobility, and by adding proper salts, the monolayers were stabilized. The miscibility of mixed monolayers was also discussed with collapse pressure and excess area. The monolayers of all lipids were transferred into only one layer on hydrophobic substrates, and up to two layers on hydrophobic substrates;The multilayer formation of dipalmitoylphosphatidic acid (DPPA) was significantly affected by the subphase pH and the addition of multivalent salts. As a transfer promoter, DPPA or octadecylamine (ODA) was used as a component in a mixture with lipid materials, and their mixed monolayers resulted in good transfer characteristics. The transferred mass and film thickness of DPPA, determined by using a quartz crystal microbalance (QCM) and by an ellipsometry respectively, were proportional to the number of transfer. The lattice spacing of DPPA was 20.3 Angstrom per layer by ellipsometry, and 22.78 Angstrom per layer by X-ray diffraction. These indicated that the transferred multilayers had well-defined layered structures of Y-type.