D.Phil. Student, University of Oxford, 1998–2002
In Situ Vibrational Spectroscopy of Thin Organic Films Confined at the Solid–Solid Interface
RAMAN SPECTROSCOPY was used to study thin films of hexadecane, octamethyltetrasiloxane (OMCTS), 1-undecanol and Langmuir–Blodgett (LB) monolayers consisting of zinc stearate, zinc arachidate and zinc behenate, all at the solid–solid interface. This thesis contains the first unenhanced Raman spectrum of an organic monolayer confined in the contact between two solid surfaces. The LB monolayers were also investigated with SUM-FREQUENCY SPECTROSCOPY in order that comparisons could be made between results from the two techniques. Thin films were confined between an optical prism and an optical lens at pressures ranging from 30 MPa to 200 MPa.
I have shown that the deposited LB monolayers were conformationally ordered and that this high degree of order was retained at applied pressures of up to 200 MPa. However, the application of pressure caused the hydrocarbon chains to tilt from the surface normal. The changes observed in the overall intensity of the Raman spectra on formation of the solid–solid contact can be explained by changes in electric field strengths, but this interpretation cannot be made in the case of the SF spectra. The SF signal arising from the monolayer confined between the two solid surfaces was often much lower than predicted, and this is discussed in terms of structural changes and transfer of monolayer material from one surface to the other.
Liquid lubricants were squeezed almost completely out of the solid-solid contact at pressures of 40 MPa. However, the use of a total internal reflection (TIR) excitation geometry in the Raman experiments increased the sensitivity of this technique sufficiently for spectra to be obtained from sub-monolayer amounts of material that had collected in small surface defects, with acquisition times of minutes.
D. A. Beattie, R. Fraenkel, S. Haydock, A. Petersen and C. D. Bain "Sum-Frequency Spectroscopy of an Organic Monolayer at the Solid–Solid Interface" submitted for publication in Journal of Physical Chemistry B.
D. A. Beattie, S. Haydock and C. D. Bain "A Comparative Study of Confined Organic Monolayers by Raman Scattering and Sum-Frequency Spectroscopy" Vibrational Spectroscopy 2000, 24, 109–123 (DOI).