Mechanisms of solute orientational order in nematic liquid crystals

Conformational problem of alkanes in liquid crystals by NMR spectroscopy: a mini-review

Recent discoveries of the role of alkane flexibility in determining liquid-crystal behaviour are surveyed. With the impetus for understanding the alkane conformational problem established, recent model dependent (1)H NMR work on the topic will be reviewed where progress is made but the need to circumvent models eventually becomes evident. A closer look at the rigid basic units of alkanes will provide the way forward where it is shown that the orientational ordering and anisotropic potentials of these molecules dissolved in liquid crystals scale with each other. Once this relationship is established, a series of works using anisotropic and isotropic (1)H NMR spectroscopy to study alkane conformational statistics will be covered, wherein the influence of the gas, isotropic condensed and anisotropic condensed phases will be described.

NMR of solutes in nematic and smectic A liquid crystals: the anisotropic intermolecular potential

Orientational order parameters determined from (1)H NMR spectroscopy of solutes in liquid crystals that form both nematic and smectic A phases are used to determine the solute smectic A order parameters and the smectic-nematic coupling term. For the analysis, it is necessary to know the nematic part of the potential in the smectic A phase: various ways of extrapolating parameters from the nematic phase to the smectic phase are explored.

NMR of short-chain hydrocarbons in nematic and smectic a liquid crystals

The proton NMR spectra of ethane, propane, and n-butane codissolved and orientationally ordered in two liquid-crystal solvents that exhibit both nematic (N) and smectic A (SmA) phases (one of which also has a reentrant nematic (RN) phase) are analyzed using CMA-ES (covariance-matrix adaptation evolution strategy). To deal with problems arising from the broad liquid-crystal background signal, a smoothed experimental spectrum is fitted to a smoothed calculated spectrum. The ethane and propane dipolar couplings and anisotropic energy parameters scale with each other, and the n-butane couplings are assumed to behave likewise. This restriction on the relative values of the n-butane energy parameters facilitates a fit to the temperature dependence of the n-butane dipolar couplings, in which the six n-butane energy parameters (three for trans and three for gauche), the isotropic trans-gauche energy difference E(tg), and its temperature coefficient E(tg)', and the methyl CCH angle decrease are obtained. Unlike earlier studies, the fit does not employ a model for the anisotropic intermolecular potential. The nematic potential in the SmA phase of the liquid-crystal mixture 6OCB/8OCB is estimated by interpolated values obtained from the ethane spectra for the N and RN regions. Analysis of results for the SmA phase involves adding a nematic-smectic coupling prefactor and a smectic prefactor. Spectra obtained in the liquid-crystal 8OCB are calculated with no adjustable parameters, scaling the potentials using the ethane values, and the agreement between experiment and calculation is outstanding. Conformer populations are affected by the environment, and the isotropic solute-solvent interactions result in increased gauche populations, whereas anisotropic interactions increase the trans probability. The trans probability in the SmA phase is slightly lower than expected from the nematic results.

Separation of nematic and smectic-A potential effects on solute ordering in a series of binary 6OCB–8OCB mixtures

1H NMR and optical microscopy have been used to study four solutes dissolved in samples of a binary mixture of 4-n-hexyloxy-4′-cyanobiphenyl (6OCB) and 4-n-octyloxy-4′-cyanobiphenyl (8OCB) that are close to the region of the phase diagram where the smectic-A (SmA) and re-entrant nematic (RN) phases exist. Optical microscopy clearly indicates that one of the four studied samples shows the phase sequence of isotropic–nematic–SmA–RN. The derived solute order parameters were interpreted by means of two Maier–Saupe mechanisms in conjunction with the Kobayashi–McMillan theory in the case of the SmA phase. The novel feature of this study is that the nematic potential is extrapolated from the nematic to the SmA phase based on concentrations of the 6OCB–8OCB mixtures. The derived smectic order parameters for each of the studied solutes clearly show a maximum absolute magnitude near the centre of the SmA temperature range. The different partitioning of these solutes in the binary mixture is also discussed.

Orientational mechanisms in liquid crystalline systems. 1. A reaction field analytical description of the interaction between the electric quadrupole moment of a probe solute and the electric field gradient of a nematic solvent

In the present paper, the fundamental problem of calculating the electric field gradient (EFG) experienced by a highly idealized solute, represented by a general point quadrupole immersed in an anisotropic uniaxial medium, has been tackled. Following a generalized reaction field approach (based upon the original ideas and the "mean-field philosophy" due to Kirkwood and Onsager) in the linear response approximation, a closed analytical expression of the EFG has been derived (to the best of our knowledge, for the first time). The obtained expression is particularly simple and elegant, also thanks to the oversimplifying approximation that the virtual cavity containing the solute is assumed to be perfectly spherical. This compact and manageable formula, obtained by a rigorous mathematical derivation (unlike other mean-field phenomenological models previously suggested in literature) can be useful to investigate and better understand a likely orientational mechanism, partly responsible for the ordering of small solutes dissolved in nematic mesophases, based on the interaction between the electric quadrupole of the solute and the electric field gradient of the anisotropic uniaxial medium (in the next paper of this issue, the formulation obtained in this work is widely tested on a variety of uniaxial and biaxial solutes dissolved in different nematic solvents).

Orientational mechanisms in liquid crystalline systems. 2. The contribution to solute ordering from the reaction field interaction between the solute electric quadrupole moment and the solvent electric field gradient

In the previous paper of this issue, [Celebre, G.; Ionescu, A. J. Phys. Chem. B doi: 10.1021/jp907310g], following a generalized reaction field approach in the linear response approximation, we were successful in obtaining an analytical compact expression for the mean-field anisotropic orientational potential U(Q-EFG) theoretically experienced by a highly idealized nonionic and apolar solute, considered as a point quadrupole immersed in a uniaxial polarizable continuum medium (model of a nematic solvent comprised of dipolar mesogenic molecules). The term U(Q-EFG) describes the electrostatic interaction between the electric quadrupole of the solute and the electric field gradient induced at the solute by the surrounding medium polarized by the distribution of electric charges representing the quadrupolar solute itself. In the present paper, the obtained potential has been considered as an additional orientational interaction contributing to the solute ordering, besides the well-recognized and very effective "short-range" (size-and-shape-dictated) mechanisms. Since in our theory the solvent is characterized by its dielectric tensor, the model has been widely tested by taking as references the experimental order parameters of several uniaxial and biaxial different small rigid probe molecules (H(2), N(2), acetylene, allene, propyne, benzene, hexafluorobenzene, 1,4-difluorobenzene, and norbornadiene) dissolved in the nematic solvents ZLI1132 (Deltaepsilon >> 0) and EBBA (Deltaepsilon < 0); moreover, the order parameters of the same solutes in the so-called nematic "magic mixture" (45 wt % EBBA + 55 wt % ZLI1132), where the short-range orientational effects are commonly believed to be very dominant, have been conventionally assumed as reference of the absence of electrostatic orientational effects. The experimental order parameters of the treated solutes, obtained in the past by liquid crystal NMR and available from literature, have been then compared with those theoretically predicted by our theoretical approach in order to obtain useful hints about two basic points, (a) the real physical nature of the interactions (other than the "size-and-shape") involved in the orientational mechanisms and (b) the conceptual effectiveness of the suggested mean-field approach in describing this kind of phenomena. Successes and failures of the approach in the predictions are discussed at length, along with their possible reasons and implications.

The smectic effect on solute order parameters rationalized by double Maier-Saupe Kobayashi-McMillan theory

From the dipolar couplings obtained by NMR spectroscopy we have calculated the order parameters of a wide variety of solutes in the nematic and smectic A phases of the liquid crystals 8CB and 8OCB. These measurements are then rationalized with the previously tested two Maier-Saupe Kobayashi-McMillan interaction potential from which smectic order parameters are calculated.