The effect of solute structure on the nematic–isotropic transition in binary mixtures

Abstract

Systematic thermodynamic and statistical-mechanical studies of nematic–isotropic (NI) phase equilibria in binary mixtures have provided significant information on the effects of molecular size, shape and flexibility on the orientational order and stability of nematic mesophases [1–3]. The addition of solute to a nematogenic solvent either depresses or elevates the NI transition temperature (T NI) of the pure solvent and gives a two-phase region. The phase diagram in the T −x 2 plane T* = T/T NI; x 2 = solute mole fraction) yields coexistence curves that are virtually linear for x 2 < ∼0.10. Of interest is the negative of the slope of the lower phase-boundary line (nematic/nematic + isotropic), β N = −(d T/d x 2) N, which is measure of the order-destroying (positive β N) or order-enhancing (negative β N) ability of a solute. Experimental β N results are presented for mixtures of quasispherical (tetra-n-alkyl tins), chain-like (n-alkanes) and rodlike ( p-polyphenyls) solutes dissolved in nematogenic solvents (MBBA and 5CB). These results are compared with the predictions of lattice models, which stress the predominant role of repulsive interactions.