Substrate-induced order in the isotropic phase of a smectogenic liquid crystal: A deuteron NMR study

Using deuteron nuclear magnetic resonance, we study the pretransitional wetting behavior of the smectogenic and nematogenic liquid crystal 8CB in the isotropic phase confined to 200 nm diameter channels of Anopore membranes. The channels were treated with aliphatic acid to impose homeotropic alignment. Just above the nematic-isotropic transition we observe an enhanced line splitting compared to non-smectogenic homologs such as 5CB; moreover, the line splitting is characterized by a discontinuity of its slope at approximately 5 K above the transition temperature. This deviation from a purely nematic wetting scenario is attributed to the substrate-stabilized smectic order within the wetting film, which appears to be stimulated by the proximity of the smectic-A phase: in 8CB, the smectic-A-nematic transition occurs approximately 7 K below the nematic-isotropic transition. We interpret the data in terms of a semimicroscopic Landau--de Gennes type model, which allows us to estimate the thickness of the presmectic film---which consists of 2 bilayers---as well as the parameters of the surface interaction and the effective smectic-nematic coupling.