Sum-frequency vibrational spectroscopy, with the help of surface pressure-area (π-A) isotherm, was used to study lipid Langmuir monolayers composed of molecules with positively and negatively charged headgroups as well as a 1:1 neutral mixture of the two. The spectral profiles of the CH(x) stretch vibrations are similar for all monolayers in the liquid-condensed (LC) phase. They suggest a monolayer structure of closely packed alkyl chains that are nearly all-trans and well oriented along the surface normal. In the liquid-expanded (LE) phase, the spectra of all monolayers appear characteristic of loosely packed chains with significant gauche defects. The OH stretch spectra of interfacial water for both positively and negatively charged monolayers are significantly enhanced in comparison with a neutral water interface, but the phase measurement of SFVS indicates that OH in the two cases points toward the bulk and the interface, respectively. The enhancement results mainly from surface-field-induced polar ordering of interfacial water molecules. For a charge-neutral monolayer composed of an equal number of positively and negatively charged lipid molecules, no such enhancement is observed. This mixed monolayer exhibits a wide range of LC/LE coexistence region extended to very low surface pressure and its CH(x) spectral profile in the coexistence region resembles that of the LC phase. This result suggests that in the LC/LE coexistence region, the mixed monolayer consists of coexisting LC and LE patches in which oppositely charged lipid molecules are homogeneously mixed and dispersed.

In situ imaging ellipsometry is employed to monitor the morphology of collapsed films of fatty acid Langmuir monolayers on pure water and on CaCl2 solution. The ellipsometry images reveal the existence of multilayer domains in the collapsed region, and analysis of the images yields the thicknesses of these domains. The multilayer films formed on water are mainly trilayers, while those on CaCl2 solution are mainly bilayers. The structure of the collapsed films also changes sensitively depending on the history of compression of the molecular layer.

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Hemicyanine dyes can be incorporated into silicalite-1 channels in uniform orientations. To find out the underlying mechanism, two different hemicyanine dyes having a long hydrophobic C18-alkyl chain on opposite ends of the dipolar nonlinear optical chromophore were adsorbed into the vertically oriented channels of silicalite-1 films supported on a glass plate. Second-harmonic phase measurement revealed that the above two molecules in the silicalite-1 channels were indeed oppositely oriented. A Langmuir monolayer of a hemicyanine was used as a reference sample to determine the absolute orientation of the dyes in the above samples, which allowed us to confirm that the hemicyanine dyes prefer to enter the zeolite channels with the hydrophobic tail part first.

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