In Situ IRRAS Studies of NH Stretching Bands and Molecular Structures of the Monolayers of Amphiphiles Containing Amide and Amine Units at the Air-Water Interface

The monolayers of the amphiphiles containing amide and amine units at the air-water interface on water surface have been studied using infrared reflection absorption spectroscopy (IRRAS). Sharp NH stretching bands are observed in the monolayers of N-octadecanoyl-L-alanine (ALA) (at pH 3.0) and N-octadecyloxalamide (OXA), while no NH stretching band is clearly detected in the monolayers of octadecan-1-amine (OA) and N-hexadecylethane-1,2-diamine (EDA) (at pH 9.3). It is considered that the presence and absence of the NH stretching bands are related to properties of amides and amines, roles of N-H bonds in hydrogen bond donor/acceptor, and orientation of transition moment directions of NH stretching vibrations. The intermolecular hydrogen-bonding interactions between the adjacent amides of ALA and OXA induce the parallel packing of the CCC planes of corresponding alkyl chains, while the chains are uniaxially oriented in the monolayers of OA and EDA. In the presence Of CuCl2 in the subphase, Cu2+ ions coordinate to the headgroups of OA and EDA in the monolayers; moreover a new peak at 1220 cm(-1) is observed for the EDA monolayer and assigned to the CH, twisting and wagging modes relevant to the ethylenediamine headgroups. The presence of the peak is related to the dehydration of the headgroups induced by the twisted tetrahedral coordination, and the coordinated headgroups undergo an orientation change with surface pressure.