Anisotropic solute-solvents interaction

Liquid crystals are usually excellent solvents for other organic compounds. Non-mesomorphic solute molecules may be incorporated into liquid-crystalline solvents without destruction of the order prevailing in the liquid-crystalline matrix. The anisotropic solute-solvent interaction leads to an appreciable orientation of the guest molecules with respect to the axis of preferred solvent alignment. The consequences may be useful as shown by the use of liquid crystals as anisotropic solvents for spectroscopic investigations of anisotropic molecular properties [166]. Ordered solvent phases such as liquid crystals have also been used as reaction media, particularly for photochemical reactions cf. for example [111, 155, 163] and Section 5.5.9. 

A very interesting property of liquid crystals is their ability to orientate molecules of solute. The anisotropic solute-solvent interactions depend critically on the geometry of the guest molecule and the applications reported are based on this property. 

The anisotropic solute—solvent interaction depends critically on the geometry of the guest molecules and therefore provides a very sensitive physical parameter to distinguish between two geometrical isomers of a molecule. For this reason liquid crystals may be used most successfully as substrates in gas-liquid chromatography. This type of application is described in Section 5. 

The additive contribution of the individual bonds to the anisotropic potential draws attention to the rule of the additivity of bond polarizabilities [122]. This strongly suggests that the average orientation of the substituted benzenes is directly related to their principal polarizabilities and that the anisotropic solute-solvent interaction is determined by London dispersion forces. Considering dispersion forces (in dipolar approximation) one obtains the following expression for the... 

The first sum accounts for the anisotropic solute—solvent interaction, while the second term characterizes the anisotropic contribution to the solute—solute interactions. Sg and denote the solute and the solvent order, respectively. The coefficients Ag and Agg are negative quantities and are proportional to the coefficient A/V (cf Equ. (24)). An analogous expression is valid for the solvent molecules. Accordingly, the total average orientational energy of the mixed solute-solvent system is given by [129]... 

Figure 19. Temperature dependencies of the retention times of para- and meta-xylene in 4,4 -di-hexyloxyazoxybenzene. At the isotropic-to-nematic transition (at about 125°C) the retention times decrease abruptly in a temperature range of about 5°C. A good separation of the isomers is only possible in the liquid crystalline states of the substrate, where the activity coefficient (and thus the retention time) is determined primarily by the anisotropic solute-solvent interaction (from Ref. 127).

The thermodynamic parameters, measured at various temperatures for the isotropic, nematic, or smectic phase of a liquid crystalline stationary phase, are related to changes in the ability of a solute to interact with the solvent. Several theoretical treatments, including the refined infinite dilution solution model [453, 480, 481] and the lattice model [455,456], have been postulated for the general interpretation of the thermodynamic parameters in terms of the retention and selectivity parameters in gas chromatographic separations. However, attempts to correlate the thermodynamic quantities with specific solute-solvent interactions remain qualitative. Nevertheless, a few general observations can be noted. Upon cooling from an isotropic phase into a liquid crystalline phase, the values of decrease and those of 7 increase. Plots of Vg or 7 versus temperature show discontinuities at phase transition temperatures. The activity coefficient of any solute in an anisotropic environment (Ya) is found to be more positive (and typically, 7r>l) than in its isotropic phase (7i°°). This trend in is a reflection of vari-... 

The anisotropic potential acting on a probe is a solute-solvent property and therefore the study of probes gives valuable insight into the mechanism governing solute-solvent interactions. It has been found that very often the probe and the solvent can interact more or less specifically and in some cases an exchange between two or more sites has been inferred from large effects on the apparent geometry of the dissolved molecule. 

Many ceUulosic derivatives form anisotropic, ie, Hquid crystalline, solutions, and cellulose acetate and triacetate are no exception. Various cellulose acetate anisotropic solutions have been made using a variety of solvents (56,57). The nature of the polymer—solvent interaction determines the concentration at which hquid crystalline behavior is initiated. The better the interaction, the lower the concentration needed to form the anisotropic, birefringent polymer solution. Strong organic acids, eg, trifluoroacetic acid are most effective and can produce an anisotropic phase with concentrations as low as 28% (58). Trifluoroacetic acid has been studied with cellulose triacetate alone or in combination with other solvents (59—64) concentrations of 30—42% (wt vol) triacetate were common. 

Both the degree of order in liquid crystals and the average orientation of guest molecules in liquid crystals are closely related to the anisotropy of the intermolecular forces. The measurements of the solute or the solvent order are therefore most important in order to test theoretical models of the forces acting between non-spherical molecules. The use of nematic phases as model systems for the investigation of anisotropic intermole.cular interaction potentials is another important scientific application of liquid crystals. 

The relationship between electrolytic and hydrodynamic solution properties is still under intensive study and is not treated in this paper. Many instances of specific electrolyte-low-dielectric solvent interaction need to be investigated fully. The equivalent problems in anisotropic solvents are not completely understood. This review is presented with that thought in mind. 

Lyotropic LCPs are polymers whose solutions exhibit liquid crystallinity, that is, anisotropic domains in a fluid system, over a characteristic range of concentrations. In more concentrated solutions the system may be multiphasic and contain crystalline particles, amorphous gel particles and anisotropic solution coexisting with one another. Upon dilution, the anisotropic liquid crystalline solution turns biphasic, where anisotropic and isotropic solutions of the same polymer in the same solvent coexist. Upon further dilution, the solution becomes fully isotropic. Polymers that exhibit lyotropic mesomorp-hicity are either stiff-backbone polymers with strong interchain interaction in the absence of solvent or polymers whose backbones are so extended and rigid that, upon breakup of their crystalline order by the addition of some solvent, the stiff polymer chains retain substantial measure of parallel alignment to remain in mobile anisotropic domains. 

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