Infrared reflection absorption spectroscopy (IRRAS) was used to analyze the adsorption of the cationic surfactants n-dodecyl-, n-tetradecyl-, and n-cetyltrimethylammonium bromide (DTAB, TTAB and CTAB) at the air-water interface and to study their interaction with 1,2-dimyristoyl-d(54)-sn-glycero-3-phosphocholine (DMPC-d(54)) and 1,2-dimyristoyl-d(54)-sn-glycero-3-phosphoglycerol (DMPG-d(54)) Langmuir monolayers. From the Gibbs adsorption isotherms of the pure surfactants on 0.1M NaCl as subphase the value of surface excess concentration Gamma and the standard free energy of adsorption DeltaG(0) was determined as a function of the interface coverage Theta. The conformational and orientational changes of the surfactant alkyl chains were followed by analyzing the frequencies of the antisymmetric and symmetric methylene stretching vibrational bands nu(CH(2)) during adsorption at the air-water interface. The interaction of the surfactants with lipid monolayers was also followed by IRRAS. Deuterated lipids were used to distinguish between hydrocarbon chains of the surfactants and the phospholipids. Thus, information on the conformational changes of the lipid molecules during incorporation of surfactant molecules into the lipid monolayer could be obtained. The absorbance values of the antisymmetric and symmetric methylene stretching vibrational bands (nu(CD(2)) and nu(CH(2))) were used for the direct calculation of the surface partition coefficients P of the surfactants. The influence of the surfactant headgroup charge and its alkyl chain length on the incorporation into lipid monolayers is discussed.
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