Nematic-Isotropic Phase Equilibria
The Journal of Physical Chemistry, Vol, 82, No. 15, 1978
support of the National Research Council of Canada and of the Ministere de 1'Education de la Province de Qu6bec. References and Notes (1) A. C. de Kock. Z. fhvs. Chem.. 48. 129 (1904). (2j B. Kronberg, D.'F. R. &on, and D. Patterson, C k m . Soc.,Faraday Trans. 2 , 72, 1673 (1976). (3) . . D. E. Martire, G. A. Oweimreem, G. I. Agren, S. G. Ryan, and H. T. Peterson, J . Chem. Phys., 64, 1456 (7976). (4) B. Kronberg and D. Patterson, J . Chem. SOC., Faraday Trans. 2 , 72, 1686 (1976). (5) B. Kronberg, I. C. Bassignana, and D. Patterson, J. fhys. Chem., following paper in this issue. (6) G. W. Smith, 2 . G. Garlund, and R. J. Curtis, Mol. Cryst. Liq. Cryst., 19. 327 11973): H. Kelker and B. Scheurle. Anaew. Chem.. Int. Ed. E&/., 8,884 (1969); M. Sorai, T. Nakamura, an; S. Seki, Bull. Chem. SOC.Jpn., 47, 2192 (1974).
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(7) P. J. Flory, Discuss. Faraday SOC.,49, 7 (1970). (8) H. Hocker, G. J. Blake, and P. J. Flory, Trans. Faraday SOC.,67, 2258 (1971); B. Bahadur and S. Chandra, J . fhys. C , Solid State fhys., 9, 5 (1976); A. K. Rastogi, Ph.D. Thesis, McGill University, Montreal, 1969. (9) H. T. Peterson, D. E. Martire, and M. A. Cotter, J. Chem. Phys., 61, 3547 (1974); G. I. Agren and D. E. Martire, J. phys. (Paris),36,C1-141 (1975); G. 1. Agren, Phys. Rev. A, 11, 1040 (1975); R. L. Humphries and G. R. Luckhurst, Roc. R. SOC. London, Ser. A , 352, 41 (1976). (10) E. W. Fischer, fhys. Non-Clyst. Solids, f r o c . Int. Conf., 4th, 34 (1977). (11) G. Delmas, J . Appl. folym. Sci., 12, 839 (1968). (12) H. G. Elias and H. Lys, Macromol. Chem., 92, 1 (1966). (13) P. G. Assarson, P. S. Leung, and G. J. Stafford, folym. Prepr., Am. Chem. SOC.,Div, folym. Chem., 10, 1241 (1969); J. L. Koenig and A. C. Angood, J. Polym. Sci., A-2, 8, 1787 (1970); L. W. Kessler, W. D. O'Brien, and F. J. Dunn, J . Phys. Chem., 74, 4096 (1970); S. H. Maron and F. E. Filisko, J. Macromoi. Sci.-phys., B6, 79 (1972).
Effect of Solute Size and Shape on Nematic-Isotropic Phase Equilibria in EBBA 4Aromatic Hydrocarbon Systems Bengt Kronberg, Isabella Bassignana, and Donald Patterson* DeparfmeRt of Chemistty, Otto Maass Chemistry Building, McGill University, Montreal H3A 2K6, Canada (Recelved December 20, 1977; Revised Manuscript Received March 7, 1978) Publication costs assisted by the National Research Council of Canada
Phase diagrams have been obtained for p-ethoxybenzylidene-p-n-butylaniline (EBBA) containing aromatic hydrocarbon solutes. Slopes PN and p' were obtained for the ( T , z ~boundary )~ where the isotropic phase first appears on heating and the (T,x2)'boundary where the nematic phase occurs on cooling. They have been corrected to infinite dilution and used to give the difference of solute activity coefficients in the nematic and isotropic liquid crystal, Le., (yzN"- yZ'")/y;", and the free energy of transfer of solute from the isotropic to the nematic phase. These quantities together with PN and P' are indicators of order destruction by the solute. They increase monotonically with solute molecular weight for solutes composed of aromatic rings which are not fused together and where the molecular flexibility is high, indicating a low or zero degree of orientational correlation between the EBBA and solute molecules. For the poly-p-phenyl series, considered to be stiff rods, the order-destruction parameters pass through a maximum with molecular weight and decrease, becoming negative for p-quaterphenyl which increases the EBBA order. This behavior is predicted by the lattice-model theory developed by Peterson, Martire, and Cotter for rodlike solutes. Data are also given for a number of platelike solutes consisting of fused benzene rings. The order in the EBBA-plate system is very sensitive to the shape of the plate being increased by any anisotropy, which apparently promotes a correlation of the EBBA and plate orientations.
Introduction Figure 1 shows the phase diagram for the nematicisotropic transition in the system p-ethoxybenzylidenep-n-butylaniline (EBBA) + 1,2-diphenylethaneS The straight-line phase boundaries are typical for systems where the solute molecule is no larger than the liquid crystal molecule. Recent work1 with p-methoxybenzylidene-p-n-butylaniline (MBBA) has shown that solute size and shape affects the depression of the nematic-isotropic transition and also the free energy of solute transfer from the isotropic to the nematic phase. The solutes used were: (a) highly branched alkanes or other globular molecules with an alkyl-group surface, or (b) normal alkanes considered to be flexible rods. The globular solutes have relatively large effects on the nematic order which increased with solute size. The normal alkanes, presumably because of their ability to correlate their molecular. orientations with those of the nematogen, have a smaller effect which is independent of alkane chain length. As first found by Chen and Luckhurst,2 there is a functional relationship between the depression of the nematic-isotropic 0022-365417812082-1719$01.0010
transition temperature and the lowering, by the solute, of the order parameter S of the liquid crystal. Thus, with the globular and n-alkane solutes, analogous effects of solute size and shape were found when studying S or the depression of the transition temperature. In the present work we give only phase data for aromatic hydrocarbon solutes in EBBA, which has a higher transition temperature than MBBA and hence is a better solvent for a solid solute. Because of their wide variety of molecular structures, most of which are seen in Figure 2, the aromatic hydrocarbons are well suited to a study of effects of solute size and shape, extending the work of ref 1. Experimental Section Materials. The liquid crystal, EBBA, has been described in the accompanying paper,3 designated as I. The solutes are listed in Table I, and the structures of most of them are given in Figure 2. Except for MBBA, they were obtained from Aldrich Chemical Co., Milwaukee, Wisc., with purities ranging from 95 to 99%. All compounds with purities less than 99% were purified by sublimation. The
0 1978 American
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B. Kronberg, I. Bassignana, and D. Patterson
The Journal of Physical Chemistry, Vol. 82, No. 15, 1978
TABLE I : Phase Diagram Data for EBBA t Aromatic Solute Systems solute
ON
benzene ethylbenzene diphenylmethane 1,2-diphenylethane o-terphenyl polystyrene( 600) biphenyl p-terphenyl p-quaterphenyl MBBA' EBBA
o1
(ON")
(r*N"/
(PI")
M,
4.31 4.3 5.0 6.2 8.0
4.98 5.9 8.8 11.4 9.3
4.3 4.3 4.3
9.3 13.6 17.8 17.3 18.4
78.1 106.2 168.2 182.3 230.3 600 154.2 230.3 306.4 267.4 281.4
0.117 0.090 0.067 0.047 0.123
4.98 5.15 4.98 5.23 6.19
6.71 8.97 9.11 11-09 6.71
128.2 166.2 178.2 216.3 152.2
0.329 0.420 0.300 0.333 0.335 1.265
0.105 0.111 0.123 0.061 0.071 0.044
5.51 6.71 6.71 6.71 6.28 6.28
9.15 9.15 9.29 11.24 11.25 12.68
178.2 202.3 252.3 252.3 228.3 278.4
0.281 0.422
0.145 0.092
7.91 7.91
9.15 9.15
228.3 276.3
nonfused rings 0.454 (0.475) 0.520 (0.554) 0.640 (0.702) 0.618 (0.681) 0.780 (0.887) 1.431 (1.701) 0.517 (0.566) 0.211 (0.237) - 0.319 (- 0.254) 0.108 (0.111) 0 0
0.352 0.395 0.421 0.436 0.436 0.346 0.469 1.079 2.434 0.617
0.093 0.110 0.144 0.139 0.188 0.436 0.112 0.044 - 0.043 0.020
0.426 0.534 0.421 0.369 0.435
(0.531) (0.560) (0.613) (0.324) (0.370) (0.235) (0.710) (0.474)
0.541 0.654 0.875 0.847 1.182 2.830 0.683 0.274 - 0.180 0.116 0
(0.518) (0.615) (0.803) (0.775) (1.055) (2.443) (0.629) (0.2 47) (- 0.243) (0.11 3)
naphthalene fluorene anthracene 2,3-benzofluorene acenaphthylene
0.704 0.544 0.392 0.271 0.748
(0.655) (0.501) (0.376) (0.265) (0.691)
fused rings " 0.542 (0.586) (0.460) 0.421 0.337 (0.352) (0.253) 0.247 (0.615) 0.564
phenanthrene pyrene perylene 3,4-benzopyrene l,2-benzanthracene 1,2;5,6-dibenzanthracene triphenylene 1,12-benzoperylene
0.611 0.666 0.715 0.352 0.407 0.277 0.845 0.548
(0.586) (0.623) (0.689) (0.344) (0.396) (0.245) (0.814) (0.517)
0.509 0.521 0.589 0.315 0.359 0.205 0.683 0.445
0
1,A
r21")-1 w,A
K
acenaphlhylene
benzene
e9
260
8ot
i
ethylbenzene
w
305
@%
-
1,2 diphenylelhone
biphenyl
200 pyrene 307
0 - terphenyl 503 p - quoterphenyl
@g
@--@@-@
I 0.10
I
0.05
I
x2
+
Figure 1. Typical nematic-isotropic phase diagram (for EBBA 1,2-diphenylethane) showing the ( T , x , ) line ~ where the isotropic phase first appears on heating and the (T,x,)' line where the nematic phase appears on cooling.
lriphenylene 395
-
1,2,5,6 dibenzanlhracene 126
@
3,4- benzopyrene
MBBA
5 4
@@@
134
310
-128
@@
291
p - terphenyl
12 6
NEMATIC
189
2,3-benzafluarene
@-w
380
anthracene
phenanthrene
diphenylmethone 393
339
58
173
polystyrene (6001
perylene
1056
339
naphthalene
I, 12 - benzaperylene
323
m
257
fluorene
251
MBBA, used here as a solute, was obtained from Eastman Kodak Chemicals, Rochester, N.Y. Methods. The experimental methods are essentially as outlined in ref 3. Due to the relatively small size of the solute molecules, it was always possible to observe the ( 7 ' ~line, ~ ) obviating ~ recourse to the lever rule. The experimental error of the slopes of the (T,x2INand (T,x2)' boundaries (the P parameters) was estimated to be less than *l%. The phase diagrams have been determined in the region 0.01 < x2 < 0.10 except for solutes with a molecular weight higher than 250 where, due to the low solubility of these solutes in the EBBA, they were determined in the region 0.001 < x2 c 0.01.
Flgure 2. Structures of the aromatic hydrocarbons with their molar free energies of transfer, in J/mol, from isotropic to nematic EBBA.
Results and Discussion General Characteristics of the Phase Diagrams. Table I lists the systems for which phase diagrams were obtained, Figure 1for EBBA 1,2-diphenylmethane being typical. )~ Table I also lists the reduced slopes of the ( T , x ~and (T,x2)Iphase boundaries, Le., PN and /3I as defined by eq 7 of I. In addition, the table gives the experimental width of the two phase regions, K , as defined by eq 8 of I. According to theory," the value of K at infinite dilution is given by K~ = AhQf RT@where Aho is the heat of the nematicisotropic transition at the transition temperature, T@. A
+
The Journal of Physical Chemistry, Vol. 82, No.
Nematic-Isotropic Phase Equilibria
zP
2I-
J'"
1
'0
, e
~
A\
-
0-
1
I
I
The various parameters ON", PI", (yzNm - yz'")/y2", and AG(trans) are measures of the effectiveness of the solute in disturbing the nematic order. Discussion of Solute Size and Shape Effects. The systems in Table I are divided into those with nonfused ring and fused ring solutes. Nonfused Ring Solutes. The groups includes both isotropic, globular molecules of different sizes and more anisotropic rodlike molecules. Also included are ethylbenzene and polystyrene with a molecular weight of 600 g mol-l, abbreviated PS(600), the data being taken from
I. Because of the lack of molar volume data for the solutes, the molecular weight is used as a measured of the solute size. Figure 3 shows values of the PN parameter against molecular weight for the nonfused ring solutes. A similar diagram is found using the AG(trans) values listed in Figure 2. In Figure 3, two series of solutes may be distinguished, each with benzene as the first member. In one series, composed of benzene, ethylbenzene, diphenylmethane, 1,2-diphenylethane,o-terphenyl, and PS(600), ON is a monotonically increasing function of the molecular weight. This is the same behavior as was found for branched alkane and globular solutes in MBBA and indicating that there is little or no correlation of the orientation of the EBBA molecules with these solute molecules, or in the case of the PS with segments of the solute. The series of rodlike solutes in the p-n-phenyl group, where 1 5 n 5 4, show a quite different behavior. Here the PN or AG(trans) parameters increase with solute size,
75, 7978 1721
from benzene to biphenyl then decrease with p-terphenyl and p-quaterphenyl. Thus, as the solute molecule increases in length, increasing orientational correlation lowers PN and AG. It is of interest that the solute MBBA in EBBA solvent lies on the curve established hy the ppolyphenyls as seen in Figure 3, as does EBBA considered as a solute in itself where the parameters have zero values. Orientational correlation between the solute and the liquid crystal molecule also occurred for n-alkanes in MBBA.l However, there PN and AG were merely independent of n-alkane chain length and did not actually decrease toward negative values. Presumably the polyphenyls, due to the interaction between the hydrogens on phenyl rings, are less flexible than the n-alkanes and hence are better able to correlate their orientations with the liquid crystal molecules. The negative values of the /3 and AG parameters for p-quaterphenyl deserve special attention. Negative /3 parameters mean that the slopes of the (T,x2)boundaries are positive, i.e., the transition temperature is increased by the solute. p-Quaterphenyl thus increases the nematic order of the solvent. This is unusual for a solute that does not exhibit a liquid crystalline phase in its pure state. A mesomorphic phase may, however, be hidden below the rather high melting point of the solute. If one assumes that the mean of the ( T , x J Nand (T,x2)Ilines varies linearly throughout the concentration range, one finds an estimation of the solute nematic-isotropic transition temperature to be 167 "C. This is clearly too far below the melting point (320 " C ) to be observed through a supercooling of the liquid. However, the next member of the series, p-quinquephenyl, melts at 401 "C, becoming a nematic with a transition temperature 44 "C higher.6 The negative value of AG indicates that the solute is more "stable" in the nematic phase, where it can correlate its molecular orientations with those of the EBBA, than in the isotropic phase where it cannot do so. p-Quaterphenyl has thus a better correlating ability than EBBA itself, which gives a zero value of the P and AG parameters, and also better than MBBA for which solute the parameters are positive. The order p-quaterphenyl > EBBA > MBBA correlates with the order of their nematic-isotropic transition temperatures, 167 > 80 > 45 "C. The importance of solute shape is emphasized in comparing the two ortho and para isomers of terphenyl. Roughly a factor of 4 separates the values of the P and AG parameters for the two isomers, where the correlating p-terphenyl has the lower value. Again biphenyl, diphenylmethane, and 1,2-diphenylethaneall contain two phenyl groups. However, in biphenyl, the phenyl groups are close to coplanar, but this is not so for diphenylmethane leading to a considerably more globular shape which is consistent with the higher AG value. In the case of 1,2-diphenylethane, there is a possibility of coplanarity of the phenyl groups but apparently the high flexibility of the molecule again leads to a high value of AG. The prediction of the lattice model theory for a mixture of hard rods7 is given by curve a in Figure 3. The theory is correct in predicting that ON should pass through a maximum and then decrease as the solute size is increased. The length to width ratio of the solvent molecule used in the theory is equal to five, while that of the EBBA is approximately equal to 3.2. Decreasing the length to width ratio of the solvent in the theory will, however, increase the effect of the ~ o l u t ei.e., , ~ the ON parameter will be larger than shown in Figure 3. Thus, the discrepancy between the lattice theory and experiment will increase. Nevertheless, the qualitative agreement between the theoretical
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The Journal of Physical Chemistry, Vol. 82, No. 15, 1978 I
4
a
B. Kronberg, I. Bassignana, and D. Patterson
,
* /
1\-O
1
-100
5
10
15
2:
Flgure 4. The molar free energy of transfer of the fused aromatic ring solutes against the solute molecular length in A. For comparison also the poly-p-phenyl (M) from benzene to quaterphenyl: (0)MBBA and EBBA; (A)solutes of width 5.0-5.2 A in Table I; (A) solutes of width 5.5-6.7 A; ( 0 )noncorrelating aromatics, diphenylmethane, 1,2-diphenylethane, and o-terphenyl.
prediction and experiment is satisfying, and the introduction of an attractive potential would probably increase the predicted order and hence lower the effect of the solute, decreasing PN. Fused Ring Solutes. Molecules in this group have roughly the same thickness, i.e., that of benzene ring while the other dimensions of the “plate” vary greatly. The molecular size varies from benzene to 1,12-benzoperylene, with six benzene rings, and the shape from round “plates” such as benzene to more rodlike “plates” such as anthracene. The data in Table I and Figure 2 show striking effects of solute shape. For instance, phenanthrene has a considerably larger AG value than its more anisotropic isomer anthracene. Similar comparisons may be made between triphenylene (4rings) and its more anisotropic isomer 1,Zbenzanthracene or between perylene (5 rings) and 1,2;5,6-dibenzanthracene.It is also interesting that molecule size does not have a pronounced effect, e.g., benzehe and 1,12-benzoperylene (6 rings) have the same AG and /3 values. We find that the results are amenable to an interpretation similar to that apparent for the nonfused solutes. The molecular geometry of the platelike molecules will be characterized by the two quantities 1 and w; 1 is the length of the longest axis in the molecule and w is the largest width perpendicular to this axis. The quantities 1 and w have been calculated with the interatomic distances rc-c (arom.) = 1.39 A and rc-H = 1.10 A (the van der Waals radius of the hydrogen atoms was not taken into account). The interpretation of the MBBA + n-alkane systems was based on the assumption of an alignment of the MBBA and n-alkane molecules. The important factor in determining the destruction of order was the two-dimensional density or “cross-sectional area fraction” of aligned n-alkanes in a plane perpendicular to the long axis of the n-alkane and MBBA molecules. If one makes the reasonable assumption that the platelike solute molecules line up with their long axes parallel to the long axes of the EBBA molecules, one can adopt the width (w) of the molecule, perpendicular to its long axis, as a measure of the cross-sectional area, since all the solute molecules in this group have roughly the same thickness. In Table I we see that the solute molecules can be divided into groups, where the molecules in each group have approximately the
same values of the width, 10, and hence differ only in their length, or anisotropy. Each group should thus be analogous to the p-n-phenyl series in that only the length, or anisotropy, is varied. Within each group we thus expect to see a similar behavior of the /3 and AG parameters as for the p-n-phenyl series. In Figure 4 the AG parameter is shown as a function of solute molecular length for two different groups of similar w values. Each of these groups describes approximately the same behavior as the p-n-phenyl series, i.e., the AG parameter decreases with the anisotropy of the solute molecules. Note that the group with the large w gives larger values of AG and both groups run approximately parallel to the p-n-phenyl series. The stiff platelike rods thus show an increasing ability to correlate their molecular orientations with those of the EBBA as the anisotropy increases. In the aliphatic hydrocarbon + MBBA systems, we found ON to be proportional to the cross-sectional area of the solute. Since the thickness is the same for all the fused-ring solutes, the quantity ON/w should be independent of the cross-sectional area, or width, of the solute molecule. This turns out to be the case, and a single curve of ON/ w against molecular length is found for all fused-ring solutes, indicating a simple relation between ON and the cross-sectional area of the solute molecule. We thus conclude that the nematic order is affected by both the cross-sectional area and the length, or anisotropy, of the solute molecule. The contributions affect the order in opposite manner; a larger cross-sectional area, or width in this case, decreases the order while a longer molecule increases the order. It is interesting to compare benzene, triphenylene, and 1,12-benzoperylene. These platelike solute molecules are symmetrical in the sense that 1 w. It is easily seen from Figure 2 that 1,12-benzoperylene is a larger plate than triphenylene although the two molecules have identical values of both 1 and w. Values of /3 and AG increases from benzene to triphenylene but then decrease to 1,12benzoperylene. This molecule has dimensions large compared with those of the EBBA with a high degree of correlation of orientations between it and the EBBA. Such a behavior has been predicted by Albent who used a lattice model of hard rods and plates to calculate nematic-isotropic phase diagrams. The results indicate that the /3 parameters decrease with the size or molecular weight of the platelike solute. The theory also predicts that for even larger platelike solutes the nematic-isotropic transition temperature should increase, Le., the fl parameters should be negative. This prediction has not yet been confirmed experimentally but the present results certainly point in the same direction as the theory. Acknowledgment. We gratefully acknowledge the support of the National Research Council of Canada and of the Ministere de YEducation de la Province de QuBbec. References and Notes B. Kronberg, D. F. R. Gilson, and D. Patterson, J. Cbem. Soc., Faraday Trans. 2, 72, 1673 (1976). D. H. Chen and G. R. Luckhurst, Trans. Faraday Soc., 65,656 (1969). B. Kronberg, I. C. Bassignana, and D. Patterson, J. Phys. Chem., preceding paper In this issue. B. Kronberg and D. Patterson, J . Chem. Soc., Faraday Trans. 2, 72, 1686 (1976). G. W. Smith and 2 . G. Garlund, J. Chem. Pbys., 59, 3214 (1973). D. Vorlander, Z. Phys. Cbem., A126, 449 (1927). H. T. Peterson, D.E. Martire, and M. A, Cotter, J. Cbem. Phys., 61, 3547 (1974). R. Alben, J . Cbem. Phys., 59, 4299 (1973); and in “Liquid Crystals and Ordered Fluas” Vol. 2, J. F. Johnson and R. S.Porter, Ed., Plenum Press, New York, N.Y., 1974.
