The adsorption and longitudinal diffusion behaviors of a series of hemicyanine dyes to phospholipid vesicle membranes were studied by second-harmonic generation (SHG) and fluorescence spectroscopies. It was observed that the longitudinal diffusion of cationic hemicyanine dyes takes place immediately after the initial adsorption of these dyes to the outer surface of the vesicle membrane. In contrast, hardly any amount of a zwitterionic hemicyanine dye with a sulfonate group diffused across the vesicle membrane within the measurement time (<2000 s). Based on the difference in the time-course responses of SHG and fluorescence spectroscopies for all of the hemicyanine dyes tested, we propose that hydration of the sulfonate group is mainly responsible for the low diffusivity of the zwitterionic hemicyanine dye.

Alkali metal ion recognition with [2-hydroxy-5-(4-nitrophenylazo)phenyl]-methyl-15-crown-5 (azoprobe 1) at the heptane/water interface was investigated by in situ second harmonic generation (SHG) spectroscopy. Upon addition of alkali metal ions, the second harmonic (SH) intensity of azoprobe 1 at the heptane/water interface was found to increase selectively. The observed selectivity of K+ > Na+ > Li+ > TMA+ was essentially the same order as that of the extractability for alkali metal ions in the 1,2-dichloroethane/water extraction. Although the Li+ extractability by azoprobe 1 was very low in the 1,2-dichloroethane/water system, the observed increase of SH intensity suggested that the Li+ complex could distribute to some extent at the heptane/water interface. The red shift of the SHG spectra for azoprobe 1 revealed that the heptane/water interface had an intermediate polarity between those of the two solvents. The light polarization analysis of the SH intensity exhibited a clear orientation change of azoprobe 1 at the heptane/water interface on forming the alkali metal complexes. It was experimentally clarified that the Na+ and K+ complexes were flatter while the Li+ complex exhibited a lift-up orientation as compared with the free anionic azoprobe 1 at the heptane/water interface.

The chemical processes occurring at the interface between two immiscible liquids are recognized to play important roles in many fields, including separation chemistry 1 and molecule recognition chemistry. 2 Consequently, various analytical techniques have been applied to reveal the chemical and physical characteristics of liquid/liquid interfaces. In particular, time-resolved total internal reflection (TIR) fluorescence spectroscopy 3 and second harmonic generation (SHG) spectroscopy [4][5][6][7][8][9][10][11] have been used to obtain molecular level information on these interfaces. Of these two spectroscopies, SHG is more advantageous because of its inherent sensitivity toward molecules at liquid/liquid interfaces. 12 SHG is based on second-order nonlinear optical process through which fundamental light with a frequency of ω undergoes conversion into light with a frequency of 2ω. 12,13 Since the second-order nonlinear process is forbidden for a medium with inversion symmetry under the electric dipole approximation, adsorbed molecules at liquid/liquid interfaces can be analyzed by SHG spectroscopy without interferences from molecules in a bulk medium. On the basis of this advantage, SHG spectroscopy has been applied for molecules adsorbed at liquid/liquid interfaces to elucidate the orientation, 4,5 chemical equilibria, 6 molecular ordering, 7 and molecular association. 8,9Recently, solvatochromic dyes have been used as probe molecules to examine the physical properties of liquid/liquid interfaces by time-resolved TIR fluorescence 14 and SHG 10,11 spectroscopies. The probe molecules often consist of an anionic head group and a chromophore that shows solvatochromism. 11,14 The distance between the anionic head group and the chromophore is adjusted by an alkyl spacer with a suitable length, and the probe molecule serves as a surfactant. It has been reported that the solvations of a solvatochromic dye at the liquid/liquid interface depend on the distance between the head group and the solvatochromic chromophore. 11,14 The interfacial polarity at liquid/liquid interfaces has also been discussed from SHG spectroscopic measurements utilizing solvatochromic dyes as probe molecules. 10,11 Thus, the characterization of surfactant molecules adsorbed at interfaces can offer deeper understanding of probe molecules adsorbed at liquid/liquid interfaces. In particular, the conformation of surfactant molecules at liquid/liquid interfaces can be clarified to illustrate the structure of the probe molecules located there.In the present study, lauric acid (LA) is used as a surfactant and its adsorption behavior at the heptane/water interface is discussed based on the results obtained by interfacial tensiometry and SHG spectroscopy. The dynamic adsorption process of LA and the adsorption equilibrium are obtained by interfacial tensiometry. The SHG responses from LA adsorbed at the interface are analyzed by considering the interfacial concentration, molecular orientation, and surface electric field generated by the anionic LA lay...

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Development of analytical methods sensitive to interfaces is an essential requirement to elucidate specific behaviors of molecules at liquid-solid and liquid-liquid interfaces. A resonant second harmonic generation spectroscopy is newly developed and applied to analysis of molecules at interfaces. Analysis by electrochemical STM and time-resolved total internal reflection fluorescence spectroscopy is also described. Complexation reactoins and molecular recognitions at liquid-liquid interfaces are analyzed by measurements of surface tension and ion transfer polarogram, and unusual selectiveity is achieved using suitable host molecules.