Langmuir-Blodgett monolayers of zinc arachidate in a solid-solid contact have been studied by total internal reflection (TIR) Raman scattering in the pressure range of 3-6 kbar.  The spectra show the retention of a high degree of chain order over this pressure range.  The intensity of spectra acquired in the C-H stretching region with s-polarised light decreases with increased pressure while p-polarised spectra in the same region increase in intensity with increased pressure, indicating a progressive increase in chain tilt with pressure.  A quantitative model is presented for the interpretation of Raman intensities from thin films at interfaces based on integration of the electric field over the microscope objective, calculated using the Lorentz Reciprocity Theorem. This model is applied to the calculation of the tilt of the Zn arachidate chains under pressure, by analysis of the intensities of the symmetry and antisymmetric C-H stretches. The pressure is determined from the Newton rings and Hertz's theory of deformation.  Hence the compressibility of the Zn arachidate monolayers under normal load can be calculated. The limitations of the model used to interpret the Raman spectra are discussed.

Quantitative analysis of Raman spectra of fatty acid lubricants under pressure

253rd National Meeting (2017)

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