Orientational analysis of pelargonic acid at liquid–solid interfaces, drops, and ultrathin liquid films, by polarized Raman spectroscopy

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Abstract
Polarized Raman spectra of pelargonic acid (PA), CH3–(CH2)7–CO2H, at liquid–solid interfaces were recorded, using the excitation of the surface plasmon of silver. In this device, the exciting electric field decreases exponentially (or quasi-exponentially) into the sample. Therefore, the molecules in the vicinity of the solid surface are preferentially illuminated. This study was performed on two types of systems: 1°—One interface: a drop of PA is deposited on a solid surface [silver, silica, cetyltrimethylammonium bromide (CTAB), or Langmuir–Blodgett (LB) layers of barium stearate]; 2°—two interfaces: liquid ultrathin films of PA are placed between silica or barium stearate LB layers; the thickness of the film (250 to 5 nm) being measured by an interferential method. The shape of the 2800–3000 cm−1 band, which is very sensitive to the chain conformation, was studied. Since in the convenient polarization, the spectra of the drops were different from the spectrum of the isotropic liquid (classical cell), we can assume that the PA molecules have an anisotropic distribution in the vicinity of a solid surface and that the molecules are, therefore, more or less uniaxially oriented. The orientation is dependent on the nature and the smoothness of the solid surface, as is the distance over which the orientation spreads. (1°) If the distance of orientation is short relative to the penetration depth (≂100 nm) of the exciting wave, most of the illuminated molecules belong to an isotropic distribution: there is, therefore, little difference between the spectra of the drop and of the isotropic liquid, but, as in the ultrathin films only the oriented molecules are illuminated, spectra of films and liquid are very different. (2°) If the orientation distance is long (PA on barium stearate LB layers), the greater part of the illuminated molecules are oriented: the spectrum of the drop is therefore very different from the spectrum of the bulk (isotropic liquid); and the spectra of ultrathin films are similar to the spectrum of the drop. The differences observed in the band shapes are due to an increasing intensity of the peaks assigned to the symmetric vibrations of the CH2 and CH3 groups. These modifications can be explained by the comparison of the calculated scattering activities of uniaxially oriented molecules and random ones. The causes of the orientation and conformation change of the hydrocarbon chain have been discussed.