Size and shape effects on the orientation of rigid molecules in nematic liquid crystals

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

Section: Resultsmentioning

confidence: 99%

“…[2]; where S I is the solvent order parameter and A12 describes the strength of the solute-solvent interaction. In dilute solution the solute-solute interaction can be ignored and A 12 is assumed to contain the two contributions due to short and long-range interactions.…”

Section: Resultsmentioning

confidence: 99%

Orientation of some disk-shaped solutes in nematic liquid crystals

Yim¹,

Gilson²

1988

“…This difference was attributed to a change in sign of the electric field gradient in the two solvents and the resulting effect upon the interaction between the molecular quadrupole moment of the solute and the average electric field gradient from the solvent. The importance of this interaction in determining the solute orientation was first demonstrated in studies of D2 and HD dissolved in nematic solvents by Burnell and co-workers (3)(4)(5)(6), who also showed that a mixture of the two solvents in which the field gradient is zero could be used to eliminate the long-range interaction and limit the potential to a short-range interaction. This latter contribution was interpreted using a Hooke's law force constant for the displacement of the solvent molecules with the molecular circumference of the solute defining this displacement (6).…”

Section: Introductionmentioning

confidence: 99%

“…In eqs. [2]- [6], it is assumed that AL is constant and that the electric field gradient is invariant over all the local environments that a solute molecule might prefer, so that the potential can be factorized into solvent-dependent and solute-dependent terms. This type of factorization is usually considered valid for dispersion forces (9, lo), but has also been suggested for repulsion forces (1 1).…”

Section: Introductionmentioning

confidence: 99%

The orientational ordering of solute molecules in nematic solvents: cyanobenzenes, bromobenzenes, and p-benzoquinone in EBBA and 1132

Yim¹,

Gllson²

1990

. Can. J. Chem. 68, 875 (1990). The orientational order parameters of benzonitrile and o-, m-, and p-dicyanobenzene, monobromo-and p-dibromobenzene, and p-benzoquinone, dissolved in two nematic solvents, 1132 and EBBA, have been measured as functions of temperature, and used to determine the potential energy parameters for each solute-solvent pair. These parameters have been correlated with a short-range interaction based upon a shape and size function of the solute molecule and a long-range contribution due to the interaction between the solute molecular quadrupole moment and the average electric field gradient. Introduction When solutes are dissolved in nematic liquid crystal solvents, the interaction between solute and solvent molecules results in an orientational ordering of the solute. An understanding of solvent-solute interactions in these anisotropic systems is desirable in view of their importance, for instance, as analogues of biological membranes and in various technological applications. Recently, the orientational behaviour of mono-and dichlorobenzenes in two nematic solvents, EBBA (a Schiff-base) and 11 32 (a mixed phenylcyclohexylcyanide solvent), was studied as a function of temperature and of concentration (1,2). The results showed that chlorobenzene molecules were more highly oriented in 1132 than in EBBA. This difference was attributed to a change in sign of the electric field gradient in the two solvents and the resulting effect upon the interaction between the molecular quadrupole moment of the solute and the average electric field gradient from the solvent. The importance of this interaction in determining the solute orientation was first demonstrated in studies of D2 and HD dissolved in nematic solvents by Burnell and co-workers (3-6), who also showed that a mixture of the two solvents in which the field gradient is zero could be used to eliminate the long-range interaction and limit the potential to a short-range interaction. This latter contribution was interpreted using a Hooke's law force constant for the displacement of the solvent molecules with the molecular circumference of the solute defining this displacement (6). Diehl and co-workers (7) have studied a series of eleven chlorobenzenes and interpreted the orientation using a bond interaction model. They concluded that a molecular shape function must be included and adopted the force constantmolecular circumference model. In our earlier studies on

“…Burnell and co-workers (19)(20)(21)(22)(23) have proposed that the interaction between the solute molecular quadmpole moment and the solvent electric field gradient makes an important contribution to the ordering potential, particularly for small molecules. In recent studies of the orientation behaviour of monochlorobenzene (17), dissolved in two nematic liquid crystal solvents of different properties, EBBA and 1132, it was shown that the ring moiety played an important role in determining the difference in the solute orientational order in these two solvents.…”

Section: Introductionmentioning

confidence: 99%

The orientational behaviour of dichlorobenzenes in nematic liquid crystal solvents

Yim¹,

Gilson²

1989

. Can. J. Chem. 67, 54 (1989). The orientational order parameters of ortho-, meta-, and para-dichlorobenzene, dissolved in the nematic solvents EBBA and 1132, have been measured as functions of temperature and concentration, and used to determine the values of the potential energy parameters for each solute-solvent pair. These potentials have been interpreted in terms of a short-range contribution, which depends upon the shape and size of the solute molecule, plus a long-range term due to the interaction between the average electric field gradient from the solvent and the molecular quadmpole moment of the solute.