The pressure and temperature dependence of the orientational order of a biaxial solute dissolved in a nematic solvent

Abstract

We have determined the principal components of the Saupe ordering matrix for anthracene-dlo dissolved in the nematogen trans-4-n-heptyl(cyanophenylcyclohexane) as a function of both temperature and pressure, using deuterium NMR spectroscopy. The availability of an equation of state for the solvent has allowed us to extract the temperature variation of the solute ordering matrix at constant volume as well as the usual constant pressure. This provides the first real test of molecular field theories of the ordering of biaxial particles in a liquid crystal solvent, since such theories assume that the system is at constant volume. As we had expected, the predictions of theory are found to be in better accord with the results at constant volume than at constant pressure, although the agreement is not perfect even at constant volume. We have measured the solute equivalents of the thermodynamic parameter F~l){---(d In T/d In V)C2.o } suggested by Alben for pure nematogens to gauge implicitly the relative importance of attractive and repulsive forces in stabilising nematic order. The relationship of the solute to the solvent F parameters is discussed together with the significance of their values.

Nematic Potential and Order Parameter Determined from Dielectric Measurements

“…Apparently, the relation q ~ S is not fulfilled along the isochoric line as was reported in [7]. The absolute S-values for 7 PCH, however, are smaller than derived from Emsley et al [5,7], On the other hand, for the points belonging to the isobaric lines, q ~ S is roughly valid. This can be understood, when we take into account the following experimental findings.…”

“…Results have been published e.g. for some nCBs [2][3][4] and 7PCH [5], but the authors point out that many corrections are necessary, partially due to experimental flaws [3,4]. In spite of these shortcomings we have used the S data for the analysis of the high-pressure behaviour of the nematic potential q (calculated from the retardation factors) under various conditions [1,6,7], In particular, we examined the proportionality between q and S, which is an important assumption in the Maier-Saupe theory [8].…”

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

“…The temperature dependences of S yy -S xx and S zz have been analysed in detail in previous studies [29,30,46,47]. However, these studies utilized liquid crystals for which the mean efg was almost certainly not zero.…”

“…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.…”

Orientational order in a liquid crystalline mixture studied by molecular dynamics simulation and NMR

“…Another problem is that the NMR experiments are performed at constant pressure, while the Maier-Saupe theory refers to a constant volume situation. This issue has been addressed by several authors 31,32 and it is well established that molecular ordering is both temperature and volume dependent. The extent and precise form of the volume dependence is however not yet fully understood.…”