Infrared reflection−absorption spectroscopic (IRRAS) measurements of octadecanoic acid monolayers on aqueous subphases containing 1 mM BaCl2, CuCl2, NiCl2, and ZnCl2, respectively, supplied insight into the molecular order and the coordination complexes of the system alkanoic acid/bivalent cation. The formation of highly ordered crystalline domains already at large areas per molecule is reflected by antisymmetric methylene stretching vibrations which are independent of the compression status, however, cation-dependent (Ba2+, Ni2+, ∼2917 cm-1; Cu2+, 2916.1 cm-1; Zn2+, unusually low, 2914.3 cm-1). The analysis of the antisymmetric and symmetric carboxylate stretching vibrations allowed the determination of coordination types of the respective alkanoic acid/cation system, while from the methylene scissoring vibration the subcell structure could be inferred in most cases. Ba2+ cations form an ionic alkanoate, the chain packing of which appears to be mainly orthorhombic. For Cu2+ cations both bridging bidentate and chelating bidentate complexes were found, and the chain packing is hexagonal. Ni2+ cations appear to show both monodentate and ionic complex formation, while for Zn2+ cations a single species was observed which is assigned to an unsymmetric chelating bidentate complex and a triclinic chain packing. Furthermore, the carboxylate and methylene scissoring bands allowed elucidation of the transformation mechanism that is encountered upon successive in situ acidification of the octadecanoic acid monolayer/Zn2+ system:  in addition to the unsymmetric chelating bidentate complex (triclinic) encountered at pH 6.7, an ionic interaction becomes increasingly evident between pH 6.1 and 5.55, and eventually at pH 5.0 the free alkanoic acid (orthorhombic?) is the dominant species.

The influence of the spreading solvents ethanol, hexane, and chloroform on the properties of monolayers of long-chain alcohols, hexadecanoic acid, hexadecanoic acid esters, and L-a-dipahnitoylphosphatidylcholine is investigated by surface pressure/area and surface potential/area isotherms, gas chromatography, surface viscosity measurements, spreading velocity measurements, and external infrared reflection-absorption spectroscopy. It is shown that the spreading solvent ethanol and mixtures containing ethanol cause film loss into the subphase, but on the other hand enforce a higher conformational order of the film-forming molecules of 1-hexadecanol monolayers and thus a higher surface viscosity. Ethanol solved in the subphase, however, does not influence the properties of the monolayer up to a concentration of about 1 mL/L. Different surface viscosity values are observed for monolayers spread from chloroform and hexane. The differences between the three spreading solvents are attributed to different spreading kinetics, which result in a different morphology of the monolayers. Infrared spectroscopic investigations do not supply any evidence of solvent molecules embedded in the monolayer.

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