Orientation of some disk-shaped solutes in nematic liquid crystals

Abstract

The solute orientation parameters for benzene, trichloro-, and tribromobenzene have been measured, as functions of temperature, in the nematic solvents EBBA and ZLI-1132. The potential functions for orientation were derived for the model based on short and long-range contributions to the potential, with the former dependent on either a function of molecular shape or of molecular volume. There is reasonable agreement between calculated and experimental solute order parameters in both cases, and the difference in solute order parameter in the two solvents arises mainly from the difference in solvent electric field gradient.Utilisant les solvants nematiques EBBA et ZLI-1132, on a mesure les paramktres d'orientation de solute du benzkne et des trichloro-et tribromo-benzknes, en fonction de la temperature. On a obtenu les fonctions de potentiel du modkle en se basant sur les contributions au potentiel 2i courte ainsi qu'i longue distance; les premikres dependent soit de la forrne soit du volume molCculaire. Dans chacun de ces cas, on observe une bonne concordance entre les parametres calculCs et les paramktres experimentaux pour l'ordre des solutes; dans ces deux solvants, la difference dans le paramktre d'ordre du solute provient principalement de la difference dans le gradient du champ Clectrique.[Traduit par la revue] Introduction There have been numerous attempts to extend the original Maier-Saupe theory of nematic liquid crystals (1) to account for the orientation of a solute molecule dissolved in a nematic solvent, and, in particular, to relate this orientation to some molecular property of the solute. Recently, Bumell and coworkers (2-4) have proposed two approaches to this problem. The first is that the intermolecular forces responsible for the orientation of the solute arise from both short:range and longrange interactions, which depend upon the solute molecular size and shape and on the interaction of the solute molecular quadmpole moment with the electric field gradient in the solvent, respectively. The second theory (4) requires that a plot of the

The orientational behaviour of dichlorobenzenes in nematic liquid crystal solvents

“…For example, if w is assumed to be independent of the solute, then w can be obtained in those cases where the molecular quadrupole moments are known. From the results obtained in earlier studies (18) on benzene, trichloro-, and tribromobenzene, the values of w are 0.90, 1.24, and 1.24, respectively. However, these ratios are subject to the rather large uncertainties involved in the experimental molecular quadru-…”

“…The derivation of eq. [3] requires that B should be independent of solvent and of temperature, but, as was observed in the case of chlorobenzene (17) and for a number of other aromatic solutes (12,15,18), there is a strong solvent dependence of the parameter B (or its equivalent). Figure 2 also shows that there is a smaller variation with temperature and concentration, where the experimental value of B increases with increasing temperature in EBBA, but decreases in 1132.…”

“…[4] with the appropriate molecular dimensions and van der Waals radii. The earlier results on benzene, trichlorobenzene, and tribromobenzene (18) are included. A reasonable correlation exists and the behaviour resembles that shown in Fig.…”

“…Early attempts by Saupe and Nehring (1,2) to derive such expressions used a bond additive model for various fluoro-and chloro-benzene derivatives, which gave approximate agreement with the experimental results, but, due to the lack of experimental data at that time, this approach did not examine the effects of temperature, concentration, and solvent. Subsequent studies (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18) have shown that the behaviour is more complicated, and the mean field approach, with a dependence on dispersion forces alone, is probably an oversimplification.…”

A temperature dependence study of small solutes in zero electric field gradient nematic liquid crystal mixtures

“…A few studies have used quantities such as the reduced temperature T r [27,28]and the shifted temperature [29,30] in comparing the orientational nature of liquid crystals. In our study a common probe solute, TCB, is dissolved in all the sample tubes, and the dipolar coupling D HH (TCB) of TCB was measured from the same spectrum as the solute.…”