An analysis of the different behavior displayed by OMCTS and 6CB molecules confined between mica surfaces

COE Laboratory, Tohoku University* IMRAM, Tohoku University**
â—‹Fabio Pichierri* Masashi Mizukami** Kenichi Kusakabe** Kazue Kurihara**

Recent experiments performed with the shear force resonance method have shown that the octamethyl-cyclotetrasiloxane (OMCTS) and 4-cyano-4'-hexylbiphenil (6CB) molecules behave differently when confined between mica surfaces [1]. Whereas the resonance peak of 6CB decreased under compression that of OMCTS did not change in the range of applied pressures. Furthermore, upon further compression OMCTS molecules were completely removed from the gap whereas this did not happen with the 6CB molecules under the same pressure conditions. These results were interpreted by taking into account the differences in the interaction between these two molecules and mica surface as well as the differences in the intermolecular interactions that operate within these two liquids. Here we present an analysis of the molecule-surface and molecule-molecule interactions with the aid of electronic structure calculations. The latter indicate that 6CB is characterized by a large dipole moment (~4 Debye) which is likely to affect both the interaction with mica and the structuring process via the formation of dimers. Also, plausible models of these two liquids confined in nanometer-scale spaces are constructed by using the available crystallographic data.
[1] M. Mizukami, K. Kusakabe, K. Kurihara, Progr. Colloid Polym. Sci. 128 (2004) 105.