Macroscopic viscosity and the dielectric relaxation of some substituted benzenes

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1 Macroscopic viscosity and the dielectric relaxation of some substituted benzenes S. C. SR~VASTAVA AND J. CROSSLEY~ Deparrment of Chemistry, Lakehead Utliuersity, Thunder Bay, Ontario P7BSEl Received December 24, 1975 S. C. SRIVASTAVA and J. CROSSLEY. Can. J. Chem. 54, 1418 (1976). Microwave dielectric constants and losses, and viscosities (v) have been obtained for acetophenone, anisole, bromobenzene, and p-dimethoxybenzene in 11-heptane, 11-hexadecane. paraffin oil, and three 11-heptane + paraffin oil mixtures between 15 and 60 "C. The dielectric results have been analyzed for mean relaxation times (so). Plots of In T ~T against In 7 for bromobenzene are compared with those for the other solutes in an attempt to determine the effect of contributions from intramolecular relaxation processes. The results suggest that measurements employing several solvents at one temperature provide a better insight into mechanisms of dipole reorientation than those with one solvent at several temperatures. S. C. SRIVASTAVA et J. CROSSLEY. Can. J. Chem. 54, 1418 (1976). On a obtenu les constantes et les pertes dielectriques de microonde ainsi que les viscosites (v) de I'acetophCnone de l'anisole, du bromobenzene et du p-dimkthoxy-benzene dans le n-heptane. le 11-hexadecane, l'huile de paraffine et trois melanges de n-heptane et d'huile de paraffine entre 15 et 60 "C. On a analysi les resultats dielectriques afin d'itablir les temps de relaxation moyen ro. On a compare la courbe de In rot en fonction de In v pour le bromobenzene avec celles des autres solutes dans un effort pour determiner l'effet des contributions des processus de relaxations intramol6culaires. Les risultats suggkrent que les mesures faisant appel i plusieurs solvants ii une temperature fournissent une meilleure vue des micanismes de reorientation de dipole que ceux dans un solvant ii plusieurs tempkatures. [Traduit par le journal] Introduction The effect of increasing macroscopic viscosity upon the dielectric relaxation of some n-alkylbromides and alkanones was examined in an earlier paper (1). It was evident that additional, more systematic, studies were required before any conclusions, regarding the possibility of employing viscous solvents to provide some insight into mechanisms of dipole reorientation, could be reached. The Debye (2) equation culated from eq. 1 are considerably longer than the experimental values. This is especially true for small spherical solute molecules and for viscous solvents. Several modifications of the Debye equation have been put forward (4), but it is seldom possible to predict, with any certainty, the relaxation time of a polar molecule from molecular dimensions and the viscosity of the medium. Kalman and Smyth (5) found that the empirical relationship [ 11 T = 4rga3/kT = 3g V/kl' [21 T = constant X gz/t in which a is the radius of a spherical nlolecule of volume V in a homogeneous fluid of viscosity g, k is the Boltzmann constant and T is the absolute temperature, gives relaxation times, T: in reasonable agreement with experiment only when the volume of the solute is about three times greater than that of the solvent (3). In the majority of cases, however, relaxation times cal- 'To whom correspondence to be addressed. Present address: Edward Davies Chemical Laboratories, University College of Wales, Aberystwyth, Dyfed, U.K. SY23 1NE. satisfactorily represented their data for polar solutes in the viscous Nujol solvent over a small temperature range. It can be shown (6) that, for a particular solute-solvent system, the ratio of the activation energy for dielectric relaxation to that for viscous flow n(ah,*/ah,*) should equal x, provided the entropies of activation for the two processes are the same. Higasi (7) finds that x is close to zero for small spherical solute molecules and approaches unity for large nonspherical solute molecules. Furthermore, the x value for benzophenone, which relaxes by whole

2 SRIVASTAVA AND CROSSLEY 1419 molecule rotation, is almost twelve times that for diphenylether, a molecule of similar shape and size which relaxes primarily by a fast intrainolecular process. Thus the parameter x should provide some insight into the mechanisms of dipole reorientation. The majority of relaxation time - viscosity studies have been confined to the relatively small viscosity changes possible with the conventional nonpolar solvents, or, by small temperature variations. Grubb and Smyth (5) employed Nujol (paraffin oil) to separate the two relaxation processes, overall illolecular rotation and intramolecular rotation, for some aromatic ethers. A more systematic approach has been made by Balogun and Cumper (9) who used benzene + liquid paraffin mixtures to obtain a wide range of solvent viscosities. They made their measurements at several temperatures and were thus able to obtain AH,*, AH,*, and x values. Unfortunately, they did not include nonrigid aromatic compounds amongst their solutes, and benzene nlay not have been a particularly good solvent for diluting the paraffin oil, since it interacts quite significantly with many polar molecules (10). In addition, paraffin oil is a mixture of aliphatic hydrocarbons which are of course molecularly quite different from benzene. This paper reports dielectric relaxation time and viscosity data for both rigid and nonrigid substituted benzenes in n-heptane, n-hexadecane, paraffin oil, and paraffin oil + 11-heptane inix- tures between 15 and 60 C. Kalman and Smyth x values (eq. 2) are obtained from (i) temperature variations for a given solution and (ii) viscosity variations, produced by using different solvents, at fixed temperatures. Experimental The apparatus and procedure used to determine the dielectric constant (E') and the dielectric loss (e") at 35, 24, 16, 9.3, 2.5, 1.9, 1.5, and 0.98 GHz have been described previously (11, 12). Low frequency dielectric constants (eo) were measured at 2 MHz using a WTW Dipolmeter. The solution viscosities (v) were measured using calibrated Ubbelohde type viscometers and density bottles. All the chemicals were dried prior to their use. Each solute was dissolved (about 0.2 to 0.4 moll-') in pure tr-heptane, pure 11-hexadecane, pure paraffin oil (p.o.), and paraffln oil + 11-heptane mixtures made up in the following percentages by volume: (i) 75'j;; paraffin oil + 25% 11-heptane (), (ii) 859; paraffin oil + 15% 11-heptane (p.o.85), and (iii) 95% paraffin oil + 5%?I-heptane () I FIG. 1. Cole-Cole plots for (a) acetophenone and (6) anisole in paraffin oil at 25 'C. The frequencies are: 35.11, 23.98, 16.20, 9.31, 2.5, 1.9, 1.5 (0.98 for acetophenone only) and GHz. c' Results Cole-Cole plots (13) for all the acetophenone. bromobenzene, 11-dimethoxybenzene, and the majority of the anisole solutions were quite syminetrical (Fig. la). The anisole in pure paraffin oil solutions showed some indication of separation into two dispersion regions (Fig. lb). Mean relaxation times (T~), Cole-Cole distribution parameters (a) and high frequency dielectric constants (em) were determined for all the solutions, by a computer fit of the experimental E', E", and 0 data to the Cole-Cole equations as described previously (14). The errors involved in this type of analysis have been discussed in detail (14). Previous work has shown that the dielectric absorptions of the anisole and p- dimethoxybenzene solutions contain contributions from two relaxation processes (8, 15, 16). In view of the distribution of relaxation times observed for bromobenzene, which has only one relaxation process, however, we have refrained from analyzing the data for the other systems in terms of two discrete relaxation times. The mean relaxation times, Cole-Cole distribution parameters, and viscosities are listed in Table 1.

3 1420 CAN. J. CHEM. VOL. 54, 1976 Enthalpies of activation for dielectric relax- In 7 against 1/T plots respectively. The values are ation (AH?) and viscous flow (AH,*) were presented in Table 2. obtained from In T~T against 1/T (Fig. 2) and Values of x in the Kalman and Srnyth equation TABLE I. Relaxation times, distribution parameters, and solution viscosities for polar solutes in several nonpolar solvents T ' 7'' rokt 102~ x - Solvent ("C) (ps) a (cp) (PSI (PSI 39 p.o.85 p.o.85 Bromobenzene Anisole

4 SRIVASTAVA AND CROSSLEY T 70 'I 7' 7" rokt 10'4 X - Solvent ("C) (ps) a (cp) (PS) (PSI 3'1 Acetophenone p-dirnethoxybenzene : were obtained from plots of In rot against In a given solution at the four tenlperatures using tl (Fig. 3) by two methods: the relationship (a) Linear regression analysis of the points for [3] In rot = In A + x' In tl

5 CAN. I. CHEM. VOL TABLE 2. The effect of temperature on the dielectric relaxation and viscosity for polar solute - nonpolar solvent solutions AH,,': AH," (kcal (kcal ah,; A k 1014 x - Solvent mol-1) mol-1) AH,," x' 10SXA p.o (b) Linear regression analysis of the points for a solute in each of the solvents, at a given temperature, using the relationship The constants A and Bin the above equations are dependent upon the solute and solvent, and replace 3V/k in the Debye equation. Values of A and B have been determined from the intercepts of the In TOT against plots by methods (a) and (b), respectively. In order to assess the linearity of these plots relaxation tiines T' and T" calculated from x' and A, and, x" and B respec tively are compared with the experiinental TO values in Table 1. Discussion Dielectric relaxation times for rigid polar molecules of the same shape measured in the same solvent at the same temperature are found Bromobenzene Anisole Acetophenone p-dimethoxybenzene to be proportional to their nlolecular volumes (17). For acetophenone, anisole, bromobenzene. and p-dinlethoxybenzene the calculated molecular volumes (18) are 121, 108, 107, and 132 X cc, respectively. The TO values (Table 1) for anisole and p-dimethoxybenzene, especially in the inore viscous solvents, are significantly shorter than those for the rigid bromobenzene. Consequently, the dielectric absorption of the two ethers must, in accord with earlier measurements (8, 15, 16), contain contributions from an intramolecular relaxation process, i.e. methoxy group rotation. It has been suggested (19) that acetyl group rotation makes a significant contribution to the dielectric absorption of acetophenone. However, the mean relaxation times for acetophenone and bromobenzene are very nearly in the same ratio as their molecular volumes. Table 1 gives the molecular volumes ( ~okt/3~) for the polar solutes calculated from the experi-

6 SRIVASTAVA AND CROSSLEY 1423 I- with corresponding data for bromobenzene reveals that they are not sensitive indicators of contributions from intramolecular relaxation processes. Volumes (AK/3 in Table 2) calculated from the intercepts of the In TOT against In 7 plots are, of course independent of temperature. However, they are considerably smaller than the true values and they decrease with increased solvent viscosity. Obviously some additional parameter(s) must be contained in the A values k-? - in order to obtain better agreement. The A, and thus AK/3, values in pure paraffin oil are the only ones in Table 2 which reflect the contribution that intramolecular rotation makes to the dielectric absorption of the two ethers. In addition, the A value for acetophenone in paraffin oil is larger than that for bromobenzene which suggests that acetophenone behaves es- sentially as a rigid molecule. It is clear, however, 30 U to1 that n, x', and A do not provide any appreciable 4 FIG. 2. Plots of In T~T against 1/T for brornobenzene in heptane A; 0; p.o.85 0; + ; and 0. mental TO and solution viscosity values. These calculated volumes are much smaller than the true values, and they decrease markedly with increased viscosity, which illustrates the inadequacy of the Debye equation and the use of a macroscopic viscosity for these systems. The AH,* values (Table 2) increase with increased viscosity and are similar to those reported for benzene + paraffin oil solutions by Balogun and Cumper (9). In contrast, the AH,* values do not increase markedly with increased viscosity for the heptane + paraffin oil solutions. Plots of In TOT against In 7 for a given solution at the different temperatures are linear (Fig. 3) and the slopes x' are, as predicted (6), very similar to the respective n values. The relaxation times, T', calculated from the slopes and intercepts of these plots are in excellent agreement with the experimental TO values. For all the solutes, n and x' (Table 2) tend to decrease with increased solvent viscosity and follow the same trend as those obtained by Balogun and Cumper (9). The values in paraffin oil are quite similar to those reported, for 1-chloronaphthalene (0.38) and isoquinoline (0.43) in Nujol, by Kalman and Smyth (5). A comparison of the n and x' data in Table 2 for anisole and p-dimethoxybenzene In 7 FIG. 3. Plot of In against In 7 for acetophenone in several solvents at 15 "C, v; 25 "C, 0; 40 "C, 0; and 60 "C, 0. The solid lines are drawn through the points, for each of the six solvents, at a fixed temperature (slope x"). The broken lines are drawn through the points for the n-heptane and solutions at four ternperatures (slope x').

7 1424 CAN. J. CHEM. VOL. 54, 1976 TABLE 3. The effect of solvent on the dielectric relaxation for polar solute - nonpolar solvent solutions Bk Solute T("C) s" 10SXB 1024XX3 Brornobenzene Anisole Acetophenone p-dirnethoxy benzene insight into the mechanisms of dielectric absorption in these systems. Table 3 lists the slopes (x"), intercepts (B), and volumes (Bk/3) obtained from In TOT against In?I plots for each of the solutes at fixed temperatures, i.e. both TO and are increased by increasing the solvent viscosity. Again the calculated relaxation times, T", are in good agreement with those determined by experiment. The x" values for a particular solute are, with the exception of p-dimethoxybenzene, relatively independent of temperature and, of course, the solvent. Higasi (7) obtained x" values of 0.20, 0.23, and 0.31 for the rigid molecules o-dichlorobenzene, isoquinoline, and 1-chloronaphthalene respectively, although he did not indicate the viscosity range. Crossley (I) obtained x" values of about 0.27 for a number of alkylbromides and alkanones in paraffin oil + heptane mixtures at 25 "C. The x" and B values for the nonrigid anisole and p-dimethoxybenzene are clearly smaller than those for the rigid bromobenzene. The ratio of the B values for acetophenone and bromobenzene is about 1.25 compared with 1.14 for their molecular volumes, which further indicates that acetyl group rotation does not significantly contribute to the dielectric absorption of acetophenone. The results in Tables 2 and 3 suggest that dielectric measurements of a polar solute in a series of different viscosity solvents may provide a better insight into the mechanisms of dipole reorientation than illeasurements with one solvent at several temperatures. It will be interesting to note whether these conclusions are substantiated by measurements on polar solutes of a different shape and size. Acknowledgement We wish to thank the National Research Council of Canada for providing a grant which supported this work. 1. J. CROSSLEY. J. Chern. Phys. 61, 866 (1974). 2. P. DEBYE. Trans. Faraday Soc. 30, 679 (1934). 3. R. J. MEAKINS. Trans. Faraday Soc. 54, 1160 (1958). 4. K. H. ILLINGER. Progress in dielectrics. Edited by J. B. Birks and J. Hart. Heywood, London, Vol F. KALMAN and C. P. SMYTH. J. Am. Chern. Soc. 82, 783 (1960). 6. J. P. SHUKLA, D. D. SHUKLA, and M. C. SAXENA. J. Phys. Chern. 73, 2187 (1969). 7. K. HIGASI. Dielectric relaxation and molecular structure. Research Institute of Applied.. Electricity, Sapporo E. L. GRUBB and C. P. SMYTH. J. Am. Chern. Soc (1962). 9. G. A. BALOGUN and C. W. N. CUMPER. J. Chern. Soc. Faraday 11, 1172 (1973). 10. J. CROSSLEY and C. P. SMYTH. J. Am. Chern. Soc. 91, 2482 (1969). 11. W. F. HASSELL, M. D. MAGEE, S. W. TUCKER, and S. WALKER. Tetrahedron, 20, 2137 (1964). 12. S. E. KEEFE and E. H. GRANT. Rev. Sci. Instrurn. 39, 1800 (1965). 13. K. S. COLE and R. H. COLE. J. Chern. Phys. 9, 341 (1941). 14. J. CROSSLEY, S. P. TAY, and S. WALKER. Adv. Mol. Relax. Processes. 6, 69 (1974). 15. D. B. FARMER and S. WALKER. Can. J. Chern. 47, 4645 (1969). 16. W. E. VAUGHAN, S. B. W. ROEDER, and T. PROVDER. J. Chern. Phys. 39, 701 (1963). 17. W. F. HASSELL and S. WALKER. Trans. Faraday Soc. 62, 2695 (1966). 18. J. T. EDWARD. Chern. Ind. (London). 774 (1956). 19. D. B. FARMER, P. F. MOUNTAIN, and S. WALKER. J. Phys. Chern. 77, 714 (1973).

D ielectric relaxation of some polar molecules and their binary m ixtures in non polar solvents

In d ia n J. P h y s. 70B (2), 119-125 (1996) IJP B an international journal D ielectric relaxation of some polar molecules and their binary m ixtures in non polar solvents S L Abd-El-Messieh Department