Program

Overview ▪ Monday ▪ Tuesday ▪ Wednesday ▪ Thursday ▪ Friday

Thursday June 25

Th3: Molecular films and 2D materials

Chair: D. Farías, Madrid, Spain

Contributed talk

Temperature induced conformational disorder in organic monolayers

J. Weber, A. Beier-Hannweg, T. Balgar, and E. Hasselbrink

Universität Duisburg-Essen, Fakultät für Chemie, Germany

The conformational order in different organic monolayers is investigated using vibrational sum-frequency generation spectroscopy (VSFS). The symmetric C-H stretching vibrations of both the terminal methyl and the methylene groups are utilized to monitor the chain conformation at various sample temperatures. At room temperature the film is typically well-ordered consisting almost entirely of all-trans configured chains. With increasing temperature a marked increase in the density of gauche-defects is observed before oxidative degeneration starts at Octadecylsiloxane [1] and Ca-arachidate [2] monolayers on glass substrates were studied. Films of various packing densities were prepared using the Langmuir-Blodget preparation technique. The conformational changes are entirely reversible, when heating to no temperature higher than 470 K. It is observed that the ratio of the methyl to methylene modes signal strength behaves like an equilibrium constant characterizing the system. From the temperature dependence of this ratio an apparent enthalpy change for the formation of gauche-defects is obtained. These were determined for systems of increasing packing density. The obtained values are an order of magnitude larger than those of similar gas phase molecules and increase markedly with packing density.

The effect of hydrocarbon chain length is presently studied.

[1] J. Weber et al., J. Phys. Chem. 139, 244902 (2013)

[2] J. Weber et al., J. Phys. Chem. 141, 044913 (2014)