Liquid isotropic solvents

Benzene is an isotropic solvent its viscosity is the same in every direction. However, a liquid crystal solvent is an anisotropic solvent its viscosity is smaller in the direction parallel to the long axis of the molecule than the perpendicular direction. Methylhenzene is a small, spherical molecule, so its interactions with either solvent are similar in all directions. 

Equilibrium electrostatic interactions between a solute and a solvent are always nonpositive - tliey are zero if the solute is characterized by no electrical moments (e.g., a noble gas atom) and negative otherwise, i.e., attractive. It is easiest to visualize the electrostatic interactions as developing in a stepwise fashion. Consider a solute A characterized by electrical moments for simplicity, consider only die dipole moment. When A passes from the gas phase into a solvent, the solvent molecules, if diey have permanent moments of their own, reorient so that, averaged over thermal fluctuations, their own dipole moments oppose that of the solute. In an isotropic liquid with solvent molecules undergoing random thermal motion, the average electric field at any point will be zero however, the net orientation induced by the solute changes this, and the lield induced by introduction of the solute is sometimes called the reaction field . 

When the sample is allowed to equilibrate in a strong static magnetic field, the direction of B0 defines the z axis, and all the anisotropic interactions discussed earlier in this chapter are displayed. Many commercially important materials are liquid crystals, as are biological membranes, and NMR can be used to investigate their structures. In addition, small molecules that would tumble randomly in an isotropic solvent are impeded in a liquid crystal solvent to the extent that they, too, display anisotropic interactions. However, the order parameter for such a small molecule is usually much less than the maximum values of + 1 or — y2. Typically, values of S 0.1 are observed for molecules dissolved in... 

Liquid crystals possess physical properties which lie somewhere between those of solids and liquids cf. Section 3.1 and [725]. The rigidity which is present in a solid matrix is absent in liquid crystals, thus permitting molecular motion as well as conformational flexibility of the dissolved solute molecules. At the same time, due to the order in the liquid-crystalline phase, the randomness in motion and conformational flexibility of the dissolved solute molecules is to some extent restricted. If the structures of the solute and solvent molecules are compatible, then solute molecules can be incorporated into the liquid-crystalline phase without disrupting its order. Thus, the reactivity of substrate molecules incorporated into liquid crystals without destroying their order should be different from that in isotropic solvents. Apart from the first report on the influence of liquid crystals on chemical reactions by Svedberg in 1916 [726], the use of liquid crystals as... 

The rates and activation parameters for the thermal cis tram isomerization of A,A -distearoylindigo have been determined in both isotropic and liquid-crystalline solvents [727]. 

As a consequence of the alignment of solute molecules in liquid-crystalline solvents, the ratio of products formed in competitive reaction pathways can be different from that observed in isotropic liquids. This is illustrated by the Norrish type II photolysis of alkyl phenyl ketones with varying alkyl chain length in the isotropic, smectic, and... 

Similar results have been observed in the photodimerization reaction of acenaphthylene [732, 733] cf. Eq. (5-161) in Section 5.5.8. A considerable increase in the production of the traro-adduct was reported in cholesteric liquid-crystalline media compared to the isotropic solvent benzene, in which the cu-adduct is the dominant product... 

Surprisingly, some Diels-Alder cycloaddition reactions show no variation in endojexo product ratio with changes in solvent phase. Ordered liquid-crystalline solvents are not able to differentiate between endo- and exo-activated complexes in the Diels-Alder reaction of 2,5-dimethyl-3,4-diphenylcyclopentadienone with dienophiles of varying size (cyclopentene, cycloheptene, indene, and acenaphthylene), when it is carried out in isotropic (benzene), cholesteric (cholesteryl propionate), and smectic liquid-crystalline solvents at 105 °C [734]. 

Other rate processes studied include isopropyl group rotation, (529) restricted rotations in phosphine derivatives of cyclopentadienyl complexes of iron and nickel, (530) and ring and nitrogen inversion in (en) complexes of praseodymium. (531) Molecular geometries of molecules in solution can be accurately determined from NMR spectra of the molecules oriented in a nematic phase of a liquid crystalline solvent. The effect of Eu(dpm), on the nematic phase spectrum of pyridine has been examined. The pyridine geometry is unaffected by the LSR. The observed LIS values can be separated into isotropic and anisotropic components. (532)... 

Like ebulliometry, the cryoscopic method is also limited to relatively low-molecular-weight polymers with up to 50,000. A newer variation of this method involves the solute-induced depression of the nematic-isotropic transition temperature for a liquid crystal solvent such as p-azoxyanisole [2]. This technique appears to be particularly convenient for high-molecular-weight polymers and may be applicable for M values as high as 10 . 

Reaction quantum yield determinations using conventional chemical actinometers in isotropic solution are problematic in studies employing liquid crystalline solvents, because liquid crystals (particularly smectic phases) tend to scatter and/or reflect light, and impurity quenching... 

Table 3. Relative quantum yields for the Norrish II fragmentation of 11 and 13 in isotropic and liquid crystalline solvents at 30°C. ...

There are three combinations of the two components of nematic network gels (1) an isotropic network swollen by a nematic solvent, reminiscent of the polymer dispersed liquid crystal systems (PDLC). This case (1) was discussed by Brochard (1979) and Ballauff (1991) (2) a nematic network swollen by an isotropic solvent was actually studied experimentally by Carudo et al. (1992) and theoretically by Warner and Wang (1992a) (3) both components can order at a temperature above the glass transition. Actually the first two systems are special cases of the last one which has been experimentally investigated (Zentel, 1986 Barnes et al., 1989 Kishi et al., 1994) and theoretically studied (Wang Warner, 1997). 

The results reported in this paper demonstrate that colloidal dispersions in thermotropic liquid crystals are challenging systems for discovering and studying new physical effects and structures. The anisotropy of the continuous phase leads to the observation of phenomena that are markedly different from that known in isotropic solvents. We have shown that a thermally induced phase sep-... 

The integral equation formulation PCM (lEF-PCM) method is a generalization of the PCM method that allows one to deal with anisotropic solvents such as liquid crystals, as well as with isotropic solvents [E. Canc s, B. Mennucci, and J. Tomasi, J. Chem. PhysL, 107, 3032 (1997)]. lEF-PCM has given good results when applied to aqueous ionic solutions [M. Cossi et al., Chem. Phys. Lett., 286,253 (1998)]. 

The birefiingence of the polymer network was estimated by measuring the birefringence after replacing the liquid crystal solvent with an isotropic solvent such as octane. The birefiingence of the liquid crystal composite was modeled as follows. The polymer network was assumed to be a square array of parallel fibrils of radius R and order parameter Sp. The order parameter of the liquid crystal solvent was assumed to be equal to that of the polymer network at the interface, and continuously decreasing away from this interface. 

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