IRRAS studies on chain orientation in the monolayers of amino acid amphiphiles at the air-water interface depending on metal complex and hydrogen bond formation with the headgroups

Monolayers of N-octadecanoyl-L-alanine at the air-water interface on pure water and metal ion containing subphases have been studied using polarized infrared reflection-absorption spectroscopy (IRRAS). The metal complex and hydrogen bond formation with the headgroups give rise to a change in chain order depending on metal ion in the subphase. On pure water and Ag+-/Pb2+-containing subphase, the antisymmetric CH2 stretching band intensity [nu(a)(CH2)] undergoes a slower increase than the symmetric one [v,(CH2)] below the Brewster angle, so the intensity ratios Of nu(a)(CH2)/nu(s)(CH2) are less than 1 in the cases of Ag+ and Ph2+. Beyond the Brewster angle, the nu(a)(CH2) band intensities are substantially reduced in comparison with the nu(s)(CH2) ones in the cases of pure water and Ag+, but the nu(a)(CH2) bands still remain negative-oriented in the presence of Ph2+. These unusual spectral features indicate that the alkyl chains take a preferential orientation with their C-C-C planes parallel to the water surface. The parallel packing of the alkyl chains results from the intermolecular hydrogen bonds C=O center dot center dot center dot H-N between the neighboring amide groups, strengthened by the metal complex of covalent interaction. On the Ca2+-/Cu2+-containing subphase, the corresponding polarized spectra display a usual behavior. The alkyl chains are roughly estimated to be inclined around 35-40 degrees from the surface normal on the assumption of chain segment orientation for the monolayers in the liquid-expanded phase. The chain conformation and tilt are closely related to the formation of intramolecular hydrogen bonds and the ionic interaction of the metal complex in the cases of Ca2+ and Cu2+.