Proton spin–lattice relaxation in a liquid crystal–Aerosil complex above the bulk isotropization temperature

• DOI: 10.1016/j.chemphys.2003.09.017
• Publication Date: 2003-11-15T12:00:19Z
• Authors: E. Anoardo; F. Grinberg; M. Vilfan; R. Kimmich
• Keywords: Fumed silica; Atmospheric temperature range; Lattice (music)

Abstract We present a study of the molecular dynamics in an octylcyanobiphenyl (8CB)–Aerosil complex above the bulk isotropization temperature. Using proton nuclear magnetic relaxation experiments in the laboratory frame ( T 1 −1 ) and in the rotating-frame ( T 1 ρ −1 ), we found a notable increase of the relaxation rates in the kHz frequency range as compared to the bulk 8CB liquid crystal at the same temperature. The field-cycling technique was used for the laboratory frame experiments while a conventional apparatus was used for the rotating frame method. The observed behavior is analyzed with the aid of Monte Carlo simulations on the basis of a two-phase fast-exchange model distinguishing surface-ordered and bulk phases. Two processes affecting the low frequency relaxation could be identified: reorientation mediated by translational displacements, accounting for molecular reorientations, and exchange losses of molecules from the surface to the bulk.