Alignment of small organic solutes in a nematic solvent: the effect of electrostatic interactions

The origin of the alignment with respect to the director observed for solutes in a nematic host remains unclear, and various mechanisms ranging from steric repulsions to dispersive or electrostatic interactions have been invoked. Here we present atomistic molecular dynamics (MD) computer simulations of rigid solutes of small dimensions dissolved in a nematic liquid crystal solvent, 4-n-pentyl-4'cyanobiphenyl (5CB), that aim to quantitatively predict the orientational order. We have validated the results comparing the dipolar couplings obtained by atomistic simulation with their experimental NMR counterparts. To help assess the separate effect of the various types of anisotropic interactions on the orientational order of solutes, we have modeled solute molecules with their partial atomic charges present or absent (switching them to zero), finding that, at least for the cases studied, the alignment mechanism is largely dominated by steric and van der Waals dispersive forces rather than Coulomb ones. We have compared the anisotropic aligning potential with the predictions of the Maier-Saupe and surface tensor models and discussed their performance.