ORIENTATION AND ORGANIZATION IN FUNCTIONALIZED PHOSPHOLIPID MONOLAYERS AT GOLD SURFACES AS MEASURED BY POLARIZATION MODULATION FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

Polarization modulation Fourier transform infrared reflectance (PM-FTIR) spectroscopy has been employed to study the pressure-dependent molecular structure of functionalized phospholipid monolayers at the gold-air interface. "Head-down" monolayers of dipalmitoylphosphatidylcholine (DPPC) and the anthraquinone- and anthracene-functionalized analogues (DPPC-AQ and DPPC-AN) have been prepared on vapor-deposited polycrystalline gold surfaces using Langmuir-Blodgett techniques. PM-FTIR spectra of monolayers deposited with varying molecular densities reveal differing degrees of order within the aliphatic chains and slight changes in orientation of the aromatic rings. The molecular order within the alkyl chains can be estimated from relative peak frequencies and intensities in the CH stretching region and follows the trend DPPC > DPPC-AQ > DPPC-AN. More expanded films of all three amphiphiles are found to possess more disordered alkyl chains. The relative intensities of the carbonyl and ring stretch bands of DPPC-AQ have been employed to estimate the molecular orientation of the anthraquinone moieties within the monolayer. The PM-FTIR spectroscopic results are used to refine the conclusions of previous film balance, electrochemical, and ellipsometric studies of the functionalized phospholipids within the free and supported monolayers.