Deuterium NMR of a pentylcyanobiphenyl liquid crystal confined in a silica aerogel matrix

The temperature dependence of the deuterium NMR line shape was studied in a pentylcyanobiphenyl (5CB) nematic liquid crystal embedded into lecithin-treated and nontreated continuous-pore silica aerogel matrices. In the lecithin-treated matrix the temperature dependence of the NMR absorption spectra of 5CB exhibits a pronounced first-order nematic-isotropic transition, while in the nontreated matrix only a weak discontinuity was found. The observed almost continuous onset of nematic ordering in the latter case appears to be due to the spread of nematic-isotropic-transition temperatures ${\mathit{T}}_{\mathit{N}\mathit{I}}$ in different aerogel cavities. In the isotropic phase of 5CB a surface ordering effect of the silica aerogel matrix was observed and the surface order parameter was estimated to be ${\mathit{S}}_0$=0.02±0.01. The surface anchoring of the 5CB molecules is much stronger in the case of a nontreated matrix than in the lecithin-treated one.