SURFACE-ANALYSIS OF LIPID LAYERS AT AIR WATER INTERFACES

In order for the surface layer composition of lipid mixtures to be probed, Langmuir-Blodgett (L-B) films were deposited from surface layers of spread films or of aqueous dispersions of lung surfactant components. The composition and density of the deposited L-B films were determined by thin-layer-chromatography/optical densitometry (TLC/OD) or attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR). In FTIR, L-α-dipalmitoylphosphatidylcholine (DPPC) with perdeu-terated chains (d-DPPC) was also used for the hydrocarbon stretching bands in mixtures to be better separated. TLC/OD and IR results indicated that DPPC and PG (phosphatidylglycerol) were the main components of the sheep lung surfactant used. Results from monitoring changes in the surface layer composition during surface layer compression with the use of the above two analysis methods were consistent. L-B films from DPPC-PA (palmitic acid) and d-DPPC-PA showed no enrichment of DPPC in the surface layer following compression from a surface pressure of II = 10 to II = 50 dyn/cm. However, depositions from spread films of DPPC-PG and d-DPPC-PG on water at 25 °C showed a significant enrichment of DPPC, from initially 50 to over 80 mol % as the surface pressure increased from 20 to 60 dyn/cm. The difference in the behavior of DPPC-PA and DPPC-PG films may be due to the difference in the solution nonidealities of the two mixtures. This hypothesis is supported by the surface pressure-area (II–A) isotherms of the mixtures. The II-A isotherm of a sheep lung surfactant dispersion resembled that of the DPPC-PG mixture, in that two plateaus (at II = 50 and 70 dyn/cm) were observed. As suggested from the isotherm data, enrichment of DPPC in the surface layer of the lung surfactant dispersion was measured. L-B depositions from spontaneously adsorbed and subsequently compressed surface layers of sheep lung surfactant dispersions showed a significant enrichment of DPPC, from ~55 to ~80 wt %, as the layer was compressed from II = 40 (uncompressed surface layer) to 70 dyn/cm. The results indicate the central role of DPPC in controlling the extraordinarily low surface tensions of aqueous mixtures of lung surfactants and supports the previously proposed “squeeze-out” hypothesis for surface layers of mixed lipids. © 1990, American Chemical Society. All rights reserved.