Benzenes rotational viscosity

Low rotational viscosities, which are necessary for outdoor or video applications of liquid crystal displays, are obtained in mixtures with unpolar substances having short side chains such as, for example, dialkylcy-clohexylphenyls or the corresponding alkenyl compounds [83, 84]. Lateral substitUT ents lead to a viscosity increase as well as the substitution of hydrogen atoms at the benzene ring by halogens. The effect is small for fluorine atoms and increases in the sequence Flateral cyano group is more pronounced. 

Figure 19. Rotational viscosity coefficient y at 25 °C as a function of the free volume coefficient Vfg = 1 - fcp, where is the molecular packing coefficient (1) al-kyloxycyanobiphenyls (2) alkylbicyclooctylcyano-benzenes (3) alkylpyridylcyanobenzenes (4) alkylcyanobiphenyls (5) alkylcyclohexylcyanobenzenes. The crosses show the rotational viscosity for some members of the homologous series and the lengths of the vertical lines give the accuracy of measurement.

Fluorescence is measured in dilute solution of model compounds for polymers of 2,6-naphthalene dicarboxylic acid and eight different glycols. The ratio of excimer to monomer emission depends on the glycol used. Studies as functions of temperature and solvent show that, in contrast with the analogous polyesters in which the naphthalene moiety is replaced with a benzene ring, there can be a substantial dynamic component to the excimer emission. Extrapolation to media of infinite viscosity shows that in the absence of rotational isomerism during the lifetime of the singlet excited state, there is an odd-even effect In the series in which the flexible spacers differ in the number of methylene units, but not in the series in which the flexible spacers differ in the number of oxyethylene units. 

The rotational correlation times of a benzene solution of a nitroxide imbibed in polystyrene of varied crosslinking density and the analogous compound covalently attached to the chains of the crosslinked polystyrene were measured. In the latter case the modified polystyrene matrix was also equilibrated with benzene. An estimate of the influence of crosslinking density on the internal viscosity can be obtained... 

Recently, we have observed a similar inapplicability of the SED law to a much larger molecule (benzene) in water in a high pressure NMR relaxation measurement The initial compression does not decrease but increases the rotational correlation time for a benzene molecule in water at 30 C. In other words, the viscosity exponent defined by Eq. (3) is negative at lower pressures. The exponent is turned over to a large positive at higher pressures. 

The vibrational modes of the Raman lines used in this experiment belong to the totally symmetric species, so that the rotational diffusion constant obtained from their Raman linewidths is for the rotational motion (tumbling motion) of the solute molecule around the x axis perpendicular to the molecular z axis. Figs. 1-3 show the rotational diffusion constants obtained for benzene, cyclohexane and chloroform-d in aqueous solution and reference solution plotted against the reciprocal viscosity of the solvent. 

For the three solutes, D s obtained for the solutions other than aqueous solution fall on the straight lines, but in aqueous solution deviates from the lines below the lines for benzene and cyclohexane, while above the line for chloroform. This indicates that the rotational motion of benzene or cyclohexane molecules are hindered more than would be predicted from the viscosity effect and that the motion of chloroform is enhanced in aqueous solution. It is clear that other factors such as the dielectric constant cannot explain the deviation of D s in aqueous solution all the solvents used are similar to each other in the solvent effect on spectroscopic properties. These facts suggest that the deviation of aqueous solution could be attributed to the iceberg... 

Figure 3.10 Rotational relaxation times from light-scattering. Plots of against viscosity of solution. Solutes benzene, toluene, p-xylene. Temperature 23.6 C. Solvents CCI4, i-pentane, t-BuOH, mixtures. From Ref. [15].

Figure 3.12 Rotational relaxation times for benzene in various solvents. The relaxation times about the two axes, Tj and T, are plotted against the viscosity of the solution, o solutions pure benzene. See text. From Ref. [1, p. 334].

In addition, segmental motion and solvent dynamics have the common property that r for rotational diffusion need not extrapolate to zero as r/ 0. Figures 6.3 and 6.4 show r for segmental motion has a nonzero low-viscosity limit, i.e., r 0 when r 0. Similarly, Bauer, et al. studied rotational diffusion by simple aromatics including benzene and mesitylene(34), finding that r is linear in r but r > 0 in the limit t] 0. 

In another type of application a low molecular fluorescent probe is added to a system containing macromolecules. As would be expected, the rotation of a small species is insensitive to the molecular weight of high polymers, but depends on the "microscopic viscosity" which is a function of free volume. For instance, Nishijima has shown that the microscopic viscosity of liquid paraffin hydrocarbons levels off for molecular weights above 1000 and that the microscopic viscosity of polystyrene containing 10 volume"/ benzene is only 200 times as high as that of benzene (15). Nishijima also showed that the emission anisotropy is a useful index of molecular orientation. Since both the excitation and the emission are anisotropic, the method yields the fourth moment of the distribution function of orientations, while other optical properties (dichroism, birefringence) depend on the second moment (15). 

The longer r,( Hg) of HgMe2 when the solvent is benzene rather than cyclohexane (0.96 vs. 0.56 s) has been rationalized in terms of the effect of complex formation on spin-rotation and shielding anisotropy relaxation, but this ignores the difference in viscosities. 

---