Surface effects and dipolar correlations of confined and constrained liquids investigated by NMR relaxation experiments and computer simulations Farida Grinberg*, Rainer Kimmich Sektion Kernresonanzspektroskopie, Universita ¨t Ulm, 89069 Ulm, Germany Abstract Local order and molecular dynamics of liquids near surfaces strongly deviate from the behavior in the bulk. This in particular refers to liquid crystals above the bulk isotropization temperature. Transverse relaxation data of 5CB examined in porous glasses with different pore sizes are reported. A strong pore size effect was found. For the interpretation, a simple diffusion-adsorption computer simulation was carried out. Molecules can diffuse from the isotropic bulk part of the pore fluid to the ordered surface layer and vice versa. The residual dipolar correlation function is characterized by a slowly decaying tail owing to repeated returns of molecules to the surface. At each return the molecular orientation correlation is recovered as far as the surface sites visited have orientations correlated to the initial site. That is, molecular orientation is controlled by the “reorientation mediated by translational displacement” process considered in previous papers. © 2001 Elsevier Science Inc. All rights reserved. Keywords: Liquid crystals; Porous glasses; Dipolar correlations; NMR relaxation; Computer simulations 1. Introduction This study predominantly refers to liquid crystals con- fined in the void space of porous materials. There are im- portant applications of such systems in new electro-optic technologies [1]. Confinements by pores and surface con- straints strongly influence order and dynamics both below and above the bulk clearing point. Surface interactions give rise to orientational order even above the nematic-isotropic transition [2]. In this work, we examine this effect based on transverse relaxation experiments and computer simula- tions. Surface constraints are strongest in the near vicinity of the pore walls [2,3]. We therefore focused on materials with pore sizes of less than a few nanometers, i.e. systems that have attracted relatively little attention so far. The angular part of the dipole-dipole interaction of sys- tems consisting of two spins 1 /2 is described by time-depen- dent spherical harmonics. Nuclear magnetic relaxation is induced by stochastic modulations of these functions as a consequence of molecular reorientations. The analytical treatment of such surface effects is restricted to the simplest geometries such as planar surfaces. More realistic void shapes and surface textures can numerically be treated with a suitable computer simulation program [4]. In the follow- ing, we will describe a procedure for the elucidation of the reduced dipolar correlation function. 2. Materials and instruments 4'-n-pentyl-4-cyanobiphenyl (5CB) was confined in po- rous silica glasses Bioran (mean pore dimension R = 5 nm), Vycor (Vyc, R = 2 nm), and controlled porous glasses (CPG-1.5, R = 1.5 nm; CPG-4, R = 4 nm). The bulk isotropization temperature ( T NI ) of 5CB is 309.6 K. Atten- uation curves of the transverse proton magnetisation were recorded at 90 and 400 MHz using high-power Bruker instruments. The transverse proton relaxation rates, T 2 -1 , were measured with the standard Hahn echo pulse sequence [5]. 3. Computer simulations The displacement and orientation of a molecule, hence- forth called the “random walker”, was considered at or near * Corresponding author. E-mail address: farida.grinberg@physik.uni-ulm.de (F. Grinberg). Magnetic Resonance Imaging 19 (2001) 401– 404 0730-725X/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S0730-725X(01)00256-9