J. Phys. Chem. 1992, 96, 8176-8180
8176
will be different. This would be consistent with the phase behavior of the alkali halides for which the hdvier alkali halides behave as their light and analogues under pressure.
Acknowledgment. We thank Dr.S. Hull (RAL)for help with the neutron diffraction experiments. This work was supported by SERC (U.K.). J.H.acknowledges the award of a fellowship by NSERC (Canada). Registry No. LiOD, 12159-20-5.
References pnd Notes (1) Bastow, T. J.; Amm, D.T.; Segel; S. W.; Heyding, R. D. Z . Narurforsch. 1986, 41a, 283. (2) Bastow, T. J.; Elcombe, M. M.; Howard, C. J . Solid Srare Commun. 1986, 57, 339. (3) Bastow, T. J.; Elcombe, M. M.; Howard, C. J. Solid Stare Commun. 1986, 59, 257. (4) Elschner, S.; Bastow, T. J. Solid Srare Commun. 1986, 60, 75. ( 5 ) White, M.A.; Moore, S. A. J. Chem. Phys. 1986, 85, 4629. (6) Bastow, T. J.; Elcombe, M.M.; Howard, C. J. Solid Srare Commun. 1987, 62, 149.
(7) Jacobs, H.; Mach, B.; Lutz, H.-D.; Henning, J. 2.Anorg. AIIg. Chem. 19877
5441
28.
( 8 ) Jacobs, H.; Mach, B.; Harbrecht, B.; Lutz, H.-D.; Henning, J. Z. Anorg. Allg. Chem. 1987, 544, 55.
(9) Mach, B.; Jacobs, H.; Schgfer, W. Z . Anorg. Allg. Chem. 1987,553, 187. (10) Bastow, T. J.; Elcombe, M. M.; Howard, C. J. Ferroelectrics 1988, 79* 269. (1 1) Bastow, T. J.; Segel, S. L.; Jeffrey, K. R. Solid Srare Commun. 1991, 78, 565. (12) Ernst, T. Z. Phys. Chem. 1933, B20, 65. (13) Dachs. H. Z. Krisrallow. 1959. 112. 60. (14j Adams, D. M.; HainesYJ. J. Phys. Chem. 1991, 95, 7064. (15) Barnett, J. D.; Block, S.; Piermarini, G. J. Rev. Sci. Insrrum. 1973, 44, 1. (16) Klug, D. D.; Whalley, E. Rev. Sci. Insrrum. 1983, 54, 1205. (17) Minceva-Sukarova, B.; Sherman, W. F.; Wilkinson, G. R. J. Phys. C Solid Srare Phys. 1984, 17, 5833. (18) Kruger, M. B.; Williams, Q.;Jeanloz, R. J. Chem. Phys. 1989, 91, 5910. (19) Pistorius, C. W. F. T. Z . Phys. Chem. 1969, 65, 51. (20) Hiiller, A. Faraday Discuss. Chem. Soc. 1980, 69, 66.
(21) Khitrov, V. A.; Khitrova, N. N.; Khmel'kov, V. F. Zh. Obshchei Khim. 1953, 23, 1630.
NMR Study of the Effects of Electric Dipole Interactions on the Ordering of Chaln Solutes in the Nematic Phase D. J. Photinoqt C.-D. Peon,? E.T.Samulski,**tand H.ToriumiS Department of Chemistry, Venable and Kenan Laboratories, University of North Carolina, Chapel Hill, North Carolina 27599-3290; Department of Physics, University of Patras, Patras 261 10, Greece; and Department of Chemistry, University of Tokyo, Komaba, Meguro, Tokyo 153, Japan (Received: March I O , 1992)
The existence of residual electric dipole interactions in nematic liquid crystals is demonstrated by comparatively analyzing deuterium NMR spectra of a,w-dibromoalkanesand n-alkanes dissolved in nematic solvents. It is shown that these. interactions generate an additional orientational bias that has sizable and readily measurable effects on the order parameter profiles of the dibromoalkane solute. The residual electric dipole interaction modifies both the local and global ordering of the electric dipole carrying chain solutes. The study of the dependence of these changes in solute ordering on the dipole moment constitution of the solvent offers new insights into the interplay of the different types of intermolecular interactions operative in the liquid crystal's average intermolecular potential.
I. Introduction With very few exceptions, molecules forming thermotropic nematic or smectic liquid crystal phases possess strong electric dipoles. Aside from their obvious role in the formation of polar mesophases (e.g., ferroelectric smectic phases), electric dipoles are often assumed to be a determining factor of thermodynamic stability and/or self-organization in apolar mesophases as well (pair association' giving reentrant phases? biaxial smectics? etc.). While it is generally agreed that the dominant mechanism accounting for the molecular orientational ordering in apolar phases originates from packing considerations (excluded vol~me),"~ here we show experimental NMR data for dibromoalkane solutes in different nematic solvents which clearly imply the existence of residual solute-solvent electric dipole interactions. In apolar mesophases electric dipole interactions are effective only over a short range of intermolecular distances, and therefore their contribution to the ordering depends strongly on the shape of the molecule and the location of the dipole moments on the molecule. An accurate evaluation of the orientational bias experienced by a solute molecule as a result of dipoldipole interactions with the nematic solvent molecules is a major task even for molecules having simple geometry. Nevertheless, crude es-
* Corresponding author.
University of North Carolina. 'University of Patras. 'University of Tokyo.
0022-3654/92/2096-8 176$03.00/0
TABLE I Molecular Primary Structures
50CB
Idealized Shapes & Dipoles
N-c-@@o-(cH,),-~H,
5CB
DBr-C& c10
CHJ-(CH2)g-CHJ
&%w
timates of the anisotropy of this bias for molecular dimensions and dipole moment strength usually encountered in liquid crystals give values as high as 1 kcal/mol and, therefore, is comparable to the typical order of magnitude of the total anisotropy of the nematic potential of mean torque. Accordingly, the dipole-generated bias could play an important role in the alignment Here we initiate an inquiry into this subject by studying the orientational ordering differen- exhibited by related solutes with 0 1992 American Chemical Society
Dipole Interactions in Nematics
Figure 1. Schematic representation of electric dipolar interactions between solvent dipoles ( p ) and solute dipoles (p’) located respectively at s and s’ relative to each molecule’s inversion center. R (R’)represents the intercenter distance in configuration a (b); the distance between dipoles r remains unchanged on inverting the dipole directions. The shaded region in (d) indicates an excluded-volume violation.
and without electric dipole moments dissolved in nematic solvents having different electric dipole distributions. Our present study consists of a comparative analysis of the orientational ordering (as measured by *HNMR) of a,o-dibromononane (DBr-C9), a,o-dibromodecane (DBr-ClO), and n-decane (C10) in three nematic solvents differing with respect to their dipole moment composition and arrangement (see Table I): Phase V, a mixture of four mesogens with alkyl tail chains of N = 1,3 and the azoxy oxygen located at either nitrogen; 4,4’-(penty1oxy)cyanobiphenyl (50CB); and 4,4’-pentylcyanobiphenyl(5CB). The bromine atoms at the termini of the solute dibromoalkanechain place two centers of strong electric dipole interactions on the solute without seriously perturbing its geometrical structure or its conformer statistical weights relative to the n-alkane. The fact that in a positionally uniform and apolar medium it is possible to have an effective orientational energy originating from a purely polar intermolecular interaction may appear paradoxica19 within the mean field approximation. This effect is, however, quite analogous to the well-known generation of an anisotropicorientational potential by averaging a purely isotropic attractive intermolecularpotential in a manner that properly takes into account the excluded volume of the two molecules (in short, by averaging an isotropic interaction over a nonspherically symmetric region).4q5 In order to illustrate this point, we focus on the dipole-dipole interaction of a probe solute molecule with a molecule of the nematic solvent. (Although this interaction is more accurately described in terms of local charges, it is simpler to think in terms of dipoles.) For simplicity, we consider symmetric molecules with dipole moments positioned at distances s,s‘ from the centers of symmetry of the molecules and directed along the major molecular axes (Figure 1). Thus, the orientations of the two molecules are represented by the direction of their respective dipole moments p and p’. As a result of the apolarity of the nematic phase, the orientations p and -p are statisticallyequivalent (and similarly for p and -p’) since they are obtained by inverting the molecule about its center of symmetry. For sufficiently large separations of the two molecules this inversion symmetry, combined with the positional translation symmetry of the nematic phase, results in a complete cancellation of the dipole-dipole interaction. As shown in Figure 1 for any configuration of the two molecules (Figure la), there is a statistically equivalent configuration (Figure lb) in which the relative orientation of the dipoles is inverted. (The two configurations in Figure la,b differ at most in the intermolecular vector R (R’)that joins the centers of the two molecules; the distance r between dipoles is unchanged.) The situation is different at short intermolecular distances where excluded-volumeconstraints restrict simultaneous translational
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 8177 freedom and inversion symmetry. Thus, if the dipoles are not positioned on the centers of the molecules,1° the configuration in Figure ICdoes not have a corresponding configuration in which the dipole p’ is inverted while maintaining the same position relative to p, because the latter configuration is forbidden by molecular overlapping (Figure Id). En this case there cannot be complete cancellation of the dipole-dipole interaction. A similar effect is induced by excluded volume restrictions in the averaging of transverse dipole-dipole interactions. In this case, due to the 2-fold rotational symmetry about the longitudinal axis of the nematic solute molecule (transverse apolarity), there is complete cancellation of the interaction except at short distances where excluded-volume constraints restrict simultaneous translational freedom and 2-fold rotational freedom. A net effective interaction will thus be obtained upon averaging over the solute molecule’s positions and orientations if the transverse dipole is located off the 2-fold symmetry axis. We stress that for both cases, longitudinal and transverse electric dipoles, there is macroscopic apolarity in the mesophase. The residual orientational potential experienced by the dipolar solute molecule is a result of the restrictions imposed by the short-range interactions (excluded-volume effect) on the solvent positional averaging in conjunction with the inherent anisotropy of the nematic phase. This orientational potential is symmetric with respect to inversions of the solute molecule orientation ( p -p) and is, of course, additional to the (dominant) orientational bias resulting from shape anisotropy of the solute molecule (i.e., the pure excluded-volume interaction between solute and nematic solvent in the absence of any dipole interactions). The preceding remarks show how the dipole-dipole interaction would be manifested on the average intermolecular potential-the basic quantity in the mean field approximation-for the case where all but excluded volume correlations are ignored. In the case of sufficiently large dipole moments, not uncommon in nematogen molecules, dipole-dipole correlations1° may also contribute substantially7v8to the effective bias that governs the single molecule orientational distribution. In either case, the additional orientational bias originating from dipole interactions depends on the sizes and shapes of the two molecules and also on the location (9,s’) and orientation of the dipoles within each molecule. The use offlexible solutes having both dipole carrier and noncarrier sites offers the possibility of distinguishing between the action of the bias on different parts of the molecule. On the other hand, the dipolar bias acting on a rigid solute molecule will modify only the overall orientational order of the latter in a way that depends inextricably on all of the above geometrical characteristics. Among our findings is an interesting aspect of the dipole bias, namely, its dual effect on the local (segmental) and global ordering of the flexible solute molecule. Indeed, although the dipoles are localized on specific parts of the molecules, the bias is obtained as the net result of averaging their interactions over a region determined by excluded volume, i.e., by the global shapes of the molecules. The deuterium NMR technique has the requisite specificity to observe both the local and global aspects of the dipole bias.
-
II. NMR Segmental Orientational Order Profiles of Chain Solutes in Nematic Solvents 2H NMR spectra of fully deuterated a,o-dibromononane (DBr-C9) and a,w-dibromodecane (DBr-C 10) dissolved in the nematic solvent Phase V were measured within the temperature range 300-335 K in steps of 5 deg. Spectra of the same dibromoalkanes were also measured in the nematic phases of 5CB and 50CB at temperatures of 291 and 325 K, respectively, for the purpose of investigating the effects of the solvent on the orientational ordering of the flexible solutes at the same reduced temperature. In order to obtain information on the solute ordering in the absence of dipolar interactions with the solvent, spectra of perdeuterated n-decane (C 10) were recorded in these three solvents. Representative *H NMR spectra of the chain solutes in Phase V are shown in Figure 2. The resonance assignment is straightforward and corresponds to increasing quadrupolar
Photinos et al.
8178 The Journal of Physical Chemistry, Vol. 96, No. 20, 1992
I
CIO
I
cl m SCB
' lill
1
DBr-CIO
I
I
1
DBr-C9
-40
-30
.20
lil
L 1
0
10
3
4
5
6
j
Figure 3. Plots of the quadrupolar splittings (Mz)exhibited by DBr-ClO (circles), DBr-C9 (triangles), and C10 (squares connected by dotted line) versus carbon atom position j in (a) Phase V, Td = 0.950; (b) 50CB, Trd= 0.953; and (c) 5CB, Trd= 0.950. In C10, j = 1 corresponds to the methyl group splitting which exhibits an intrinsic reduction due to the local C, symmetry.
I
-10
2
20
30
:
40
CBO-C10
kHz
Figure 2. Representative deuterium NMR spectra of the partially oriented solutes n-decane-d22(ClO), l,lO-dibromodecane-dm(DBr-ClO), and 1,9-dibromodecane-d,,(DBr-C9) in the nematic solvent 50CB (325
K).
splittings on moving toward the middle of the chain. The segmental orientational order profiles of these solutes in the three solvents (Phase V, SOCB, and 5CB) are given in the plots of the measured quadrupolar splittings Av in Figure 3. (The carbon atoms are numbered 1, 2, ... from the chain end.) The spectra of DBr-C10 and DBr-C9 are nearly identical, except for a small difference in the outermost splitting. This small difference is associated with the central methylenes of DBr-C9 and DBr-C10 and are slightly more visible in 50CB and 5CB (Figure 3b,c); it reflects the usual variations in the ordering of the central part of the chain molecule with chain parity. The shape and the relative magnitudes of the outer four peaks (j = 2-5) in the DBr-CN spectra are also quite similar to those of the corresponding peaks (j = 2-5) in the C10 spectrum. (The innermost doublet of C10 (j = 1) is from the terminal methyl groups and is not directry comparable as the direction of its (averaged) electric field gradient is different, and it is further reduced because of the local C, symmetry.) There are important, readily perceived, qualitative features in Figure 3 associated with the electric dipolar bias present in the DBr-CN solutes: (i) The magnitudes of the DBr-CN splittings in Phase V are increased in the centers of the chains by more than 20% relative to the corresponding C10 splittings (Figure 3a). The situation is analogous in the SOCB, where the DBr-CN splittings are increased to a smaller extent (roughly 10%) compared to the C10 splittings (Figure 3b), and the differences are even less in 5CB (Figure 3c). (ii) The overall magnitudes of the quadrupolar splittings of C10 (and DBr-CN) change from the Phase V solvent to 50CB and SCB at nearly the same reduced temperature (Td 0.95). This is a amsequence of intrinsic structural and conformational (shape) differences between the two classes of solvent molecules. These intrinsic differences should be subtracted out when considering the net effect of dipole interactions on changing solvent. This is conveniently done by comparing ratios of DBr-CN quadrupolar splittings relative to those of decane in the different solvents, since the decane molecule experiences only the shape-dependent part of the total ordering bias and is roughly of the same length and conformer structure (at least in the central part of the chain) as the DBr-CN molecules. Thus,if we evaluate ratios of the splittings for the central methylenes of the DBr-CN chains (j= 5) relative
-
0
1
2
3
4
5
6
J
Figure 4. Plots of the quadrupolar splittings (kHz)exhibited by the solutes 1,1O-bis(4,4'-cyanobiphenyloxy)decane (CBO-ClO), a dimer mesogen (triangles); DBr-ClO (circles), and C10 (squares) versus carbon atom position j in the nematic solvent 50CB (Trd= 0.953).
-
to the corresponding splitting of C10 in the same solvent (Td 0.95), we find that Av5(DBr-CN)/Avv,(C10)has a higher value in Phase V, 1.26 [1.23], than in SOCB, 1.19 [1.12] or in SCB, 1.06 [0.96], for DBr-C10 [DBr-C9]. Hence, the dipolar bias appears to have a larger effect on the ordering of the central methylenes of the DBr-CN solutes in Phase V than in 50CB and SCB. (iii) Next we contrast the ordering at the chain ends with the ordering at the chain center within the DBr-CN solutes. We evaluate the Av,/Av5 ratio in the two classes of solvents and find that it is lower in Phase V (AvI/Av5 = 0.583 [0.605] for DBr-C10 [DBr-C9]) than in 50CB (Avl/Av5 = 0.645 [0.680]) or in 5CB (0.625 [0.660]). In other words, the Avl value shows relatively small enhancement in Phase V compared to the other two solvents (compare the differences between Avl and Avz in the three solvents) despite the fact that the enhancement of the overall orientation of DBr-CN is higher in Phase V. This finding would be surprising if one regarded the enhanced DBr-CN ordering found in Phase V (see (ii) above) to be exclusively driven by strong alignment of the terminal, polar C-Br bonds. Apparently, this is not the dominant mechanism operative (if it is operative at all) because if it were, the magnitudes of the Avl/Av5 ratios would have been the other way around in Phase V and the other two CB solvents. A further clear indication that the ordering of the chain does not just follow passively the excessive ordering of the DBr-CN terminal segments is provided by comparing, in the same nematic solvent, the qualitative trend in the variation of Av, along the DBr-CN chain with the trend observed for, say, a mesogenic
Dipole Interactions in Nematics dimer solute CBO-C1O-a substituted alkyl chain with orientable terminal segment comprised of cyanobiphenyl ether units." It is clearly seen in Figure 4 that there is clearly a strong terminal segment ordering influence in the case of the CBO-C10 dimer and the Avj values decrease toward the middle of the dimer spacer chain, quite contrary to the increasing 'alkane-like" Avj trend observed for the DBr-C10 and C10. In summary, the NMR data shows that (i) the central part of the DBr-CN chains is more ordered than the central part of the decane chain, (ii) the relative ordering (Le., with respect to decane ordering in the same solvent) in the central part of the DBr-CN c h a i i is higher in the Phase V solvent than in the alkyl-substituted cyanobiphenyl (CB) solvents, and (iii) at the same time, however, the enhancement of the terminal segment ordering in the DBr-CN solutes is smaller in Phase V than in the CB solvents.
m. Discussion The interpretation of the NMR data of the preceding section rules out the simple mechanism of terminal-segment-drivenenhancement of the ordering in the central part of the chain. In order to understand what kind of mechanism produces the observed segmental ordering profiles of DBr-CN chains in the different solvents, two questions must be addressed: (1) Does the dipolar interaction increase or decrease the ordering at the termini of the dibromoalkane chain? In other words, is the ordering observed in the outer parts of the chain higher or lower than what one would observe if one hypothetically turns off just the solvent-solute dipole interaction? (2) Since there is no direct dipole interaction between the central part of the dibromoalkane chain and the solvent molecules, why is the ordering of that part of the DBr-CN solute chain higher (in both Phase V and the CB solvents) than the ordering in the central part of decane, and moreover, why is it so in a way that the trend in the relative ordering of the central part in the two classes of solvents (higher relative ordering in Phase V than in the CB mesogens) is opposite to the trend in the relative ordering at the chain termini (lower in Phase V than in the CB mesogens)? Regarding question (l), there is no direct way of determining from the present NMR data whether the ordering at the termini is increased or decreased by the dipolar bias. A quantitative comparison with the ordering of the terminal segments (C-CH, bonds) of decane (or an n-alkane in general) is complicated because of differences in the geometry and the conformation statistics caused by the bromine atoms. However, there are certain strong indications that the terminal ordering is substantially reduced by the dipolar bias. First, the dipole moments in the dibromoalkane molecule (directed essentially along the C-Br bonds) are at an angle with respect to the local hydrocarbonchain excluded-volume contour (-35O; see Table I). Hence, energetically favored associations of the C-Br dipole with the principle solvent dipole conflicts with excluded-volume alignment of the chain contour parallel to the major axis of the mesogenic core. In this sense, the resulting ordering of the dibromoalkane (in either class of solvents) is clearly a product of competition, rather than of cooperation, between dipolar and excluded-volume interactions. Furthermore, the substantial transverse component of the principal dipole moment of Phase V mesogens favors the alignment of the C-Br bond perpendicular to the director and thus directly opposes the ordering imposed on the terminal segments by their own local excluded-volume interactions. Finally, model calculations1*J3 reproducing very accurately the observed orientational ordering profiles of the alkanes and the dibromoalkanes indicate that the pure excluded-volume interactions of the latter with the solvent molecules would produce substantially higher ordering at the termini than what is actually ob~erved.'~ The question raised in (2) can now be treated by just adding the following to the above considerations: While there is direct conflict between the alignment of the terminal dipoles and the alignment of the chain contour, the dipole interactions modify the positional distribution of the solvent molecules around the solute and thus could indirectly cause an enhancement of chain ordering. Indeed, the principal dipole moment in the Phase V solvent
The Journal of Physical Chemistry, Vol. 96, No. 20, 1992 8179 molecules is situated close to the center of the aromatic mesogenic core. Thus, the dipolar interaction with the terminal segments of the DBr-CN solutes necessarily brings the entire solute chain in proximity with the mesogen's core. This in turn enhances the excluded-volume-based ordering of the central part of the chain (while opposing the alignment of its ends). On the other hand, the principal dipole moment in the CB mesogens is located at the m N group (the dipole moment assoCiated with the ether oxygen in 50CB is much smaller than the C=N moment); it is a longitudinal moment and is located on one end of the mesogenic core (see Table I). This allows more direct interaction with the dipoles at the termini of the DBr-CN and thus modifes to a smaller extent the positional distribution of the central part of the solute chain relative to the solvent mesogenic core. Furthermore, there is considerable dipole association among the cyanobiphenyl cores themselvesI5 which causes a reduction of the effective dipole moment of the solvent, hence weakening the solvent-lute dipolar interaction. Accordingly, the dipolar bias in the CB solvents will produce moderate ordering of the central part of the chain, in comparison with Phase V, and also a weaker disorientation of the terminal segments, thus making the overall solute ordering appear relatively stronger in the CB solvents than in Phase V. It is interesting to note in this connection that competing effects from this interplay between dipole interactions and excluded volume would have been confounded in a rigid solute. In the absence of molecular flexibility it is not possible to observe segmental contributions to the ordering; the competition between the alignment of polar segments and the packing-enforced alignment would be manifested rather indirectly through moderate modifications of the ordering tensor of the entire rigid molecule (due to mutual cancellations resulting from the competing character of these effects). Thus, while the molecular flexibility of the DBr-CN solutes introduces additional complicationsin the analysis and modeling of NMR data (e.g., averages over solute conformations), the unique probe characteristics of these alkyl chainbased solutes more than compensate for this complexity.
IV. Conclusions The basic conclusion reached in this study is that electric dipoledipole interactions have quite measurable effects on the orientational ordering of chain solutes in nematic liquid crystals. The residual dipolar bias affects solute ordering in two ways: via local interactions (ordering of the dipolecarryins sites of the solute molecule) and through global interactions (via general changes in the average proximity of solutesolvent sites, i.e., solute sites not directly interacting with the solvent dipole moment). Furthermore, there is significant diversity in these effects depending on the dipole moment structure of the solvent molecules: dipole moments positioned toward the inner part of the mesogenic core (Phase V) produce more enhancement of the ordering of the central part of the dibromoalkane chain than dipole moments positioned at the ends of the mesogenic core (50CB, 5CB). The qualitative solventdependentdifferences between normal alkanes and substituted alkanes reported here in conjunction with a reliable mode112J3of excluded-volume ordering of such solutes provide an opportunity for quantitative characterization of the dipolar bias. In a forthcoming publicati~n'~ the dipolar bias is placed on a rigorous formal foundation (in terms of the solute-solvent pair distribution), and a quantitative separation of the excluded-volume ordering from the dipolar bias is presented. The dipoltdipole interactions survive in spite of the apolar symmetry of the nematic phase and even in the absence of substantial intermolecular dipoltdipole correlations. In the absence of such correlations the interactions survive as a result of incomplete averaging caused by excluded-volume restrictions in conjunction with the nematic phase anisotropy and off-center positioning of the dipole moments on the solute and solvent molecules. In all instances of solvents and DBr-CN solutes studied here, the dipole moments are situated at a distance from the inversion center of the molecules (or the solvent 2-fold rotational symmetry axis), and their direction points away from that center. This arrangement was found to enhance to some degree the ordering of the
J. Phys. Chem. 1992, 96, 8180-8183
8180
References and Notes
central part of the chain and reduce ordering at the termini. However, as simple inspection of Figure 1 shows, the surviving dipoledipole ordering bias changes sign upon reversing the polarity of either the solvent or the solute dipoles (in the absence of substantial dipoltdipole correlations). It is thus expected that if, for example, a solute with the dipoles pointing away from the molecular center were to interact with a solvent in which the dipole moment points toward the molecular center, these trends would be reversed. Accordingly, a judicious mixture of solvents each with different dipole arrangements could produce a null effect for the dipolar bias experienced by a particular solute with a specific dipole moment composition (i.e., the null effect will be solute dependent). In summary, we feel that the significance of dipole generated ordering in the nematic phase has been overlooked in NMR studies of molecular ordering in nematics (perhaps due to some vague notion about the apdarity of the nematic phase and a simultaneous neglect of excluded-volume/short-distancecontributions to the averaging of directional interactions), Clearly, the implications of dipoltdipole (and d i p o l e i n d u d dipole) interactions should be studied more extensively before positing a role for higher rank terms of the multipole expansion (e&, quadrupoles9J6) in explanations of nematic solvent effects.
(1) de Jeu, W. H. Phil. Trans. R. Soc. London 1983, A309, 217. (2) Cladis, P. E. Phys. R N . Lett. 1975, 35, 48. (3) McMillan, W. L. Phys. Reu. 1973, A8, 1921. (4) Gelbart, W. M. J. Phys. Chem. 1982, 86. 4298. (5) Cotter, M. A. Phil. Trans. R. Soc. London 1983, A309, 127. (6) Romano, S. Liq. Cryst. 1988, 3, 323. (7) Perera, A.; Patey, G. N. J . Chem. Phys. 1989, 93, 3045. (8) Zarragoicoechea, G. J.; Levcsque, D.; Weis, J. J. Mol. Phys. 1992, 75, 989. (9) Emsley, J. W.; Palke, W. E.; Shilstone, G. N. Liq. Crysr. 1991,9,643, 649. (10) Strictly speaking, the pair distribution function is not invariant with
respect to independent inversions of the directions of the two molecules even if their shape is symmetric. When the dipole moment composition of the molecules is not symmetric, the dipole interaction will break the full inversion symmetry of the pair distribution wen in a nonpolar medium. (The single molecule distribution is of course inversion symmetric.) Accordingly, there will be an additional source of incomplete averaging of the dipole interaction at short distances as a result of dipoltdipole correlations. This additional source produces a dipole contribution to the effective orientational potential for molecules carrying a single dipole moment wen when the dipole is positioned at the center of the molecule (Le., 8,s‘ = 0). See: Dunmur, D. A.; Palffy-Muhoray, P. Mol. Phys. 1992, 76, 1015 and references therein. (1 1) Photinos, D. J.; Samulski, E. T.; Toriumi, H. J. Chem. Soc., Furaday Trans. 1992, 88, 1875. (12) (a) Photinos, D. J.; Samulski, E. T.; Toriumi, H. J. Phys. Chem. 1990, 94,4688. (b) Photinos, D. J.; Samulski, E. T.; Toriumi, H. J . Phys. Chem. 1990, 94,4694. (13) Photinos, D. J.; Samulski, E. T.; Toriumi, H. Mol. Cryst. Liq. Crysr. 1991, 204, 161. (14) Manuscript in preparation. (15) Dunmur, D. A,; Toriyama, K.Liq. Crysr. 1986, 1, 169. (16) Est, A. J. van der; Kok, M. Y.; Burnell, E. E. Mol. Phys. 1987,60, 397.
Acknowledgment. This work was supported by a subcontract from the University of Pennsylvania (DARPA/ONR Grant NOOO14-90-J- 1559). Registry No. DBr-C9, 4549-33-1; DBr-C10, 4101-68-2; 5CB, 40817-08-1; SOCB, 52364-71-3; C10, 16416-29-8; Phase V, 37268-47-6.
Continuous Deionization of Latex Suspensions T. Palberg,*>tW. Hart1,t U. Wittig,g H. Versmold,* M. W M 4 t and E. Simnacbert Department of Physics, University of Konstanz. Konstanz, FRG. Institute of Physical Chemistry II, RWTH Aachen, Aachen, FRG, and Rcithel GmbH & Co., Bochum. FRG (Received: March 13, 1992; In Final Form: June 15, 1992)
A new, fast, and reproducible method of preparing ordered colloidal suspensions at very low salt concentrations is reported. Preparation times are reduced by 2 orders of magnitude compared to those for the usual methods. Particle concentration and salt contents can be monitored and controlled by optical and conductivity measurements. Transient effects appearing during continuous deionization of the sample are reported, and qualitative explanations of these effects are suggested.
Introduction Monodisperse, charged submicron spheres interacting via screened Coulomb potentials may form systems of fluidlike, crystalline, or amorphous interparticle structure, if the salt concentration in the suspension is low enough. For these systems, the terms “colloidal fluid”, “colloidalcrystal”, or “colloidal glass” are used, indicating that ordered colloids provide a model system for atomic substances. In contrast to the latter, typical length scales are in the order of a few hundred nanometers; thus, the structure and dynamics can be investigated by light-scattering methods. Also time scales are altered significantly. In recent publications, a number of effects have been reported and measured quantitatively, which are not always easily investigated in atomic substances, either because of unaccessible time scales or because experimental procedures demand very high expenditure. Examples include nucleation and crystal growth processes from an undercooled melt,’J nonequilibrium phase t r a n s i t i ~ ntransversal6 ,~~ and longitudinal’ lattice vibrations, structural relaxation processes, ‘University of Konstanz. *RWTH Aachen. IR6thel GmbH & Co.
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and two-dimensional phenomena.* Experiments are supported by extensive theoretical work.+’* Quantitative experiments are to be performed under well-defined and reproducible conditions concerning particle and salt concentrations. A number of sample preparation methods are reported, which usually include the more or less complete removal of salt ions and the subsequent addition of a known amount of salt. Alternatively, Myers and Saville” presented a sedimentation-decantation method, where the desired salt concentration is reached directly. We will, however, deal mainly with the former technique and focus on the deionization problem. Diutzis in 1952 passed protein solutions through a column of both anionic and cationic ion-exchange resinsI4 (IEX). This procedure can also be applied to suspensions, but care has to be taken at high volume fractions, because colloidal particles may coagulate on the IEX;’* suspended material is lost, and aggregates may contaminate the suspension. The amount of coagulation may be reduced significantly, if particla are predialyzed and the IEX is conditioned by special methods.16J7 Further methods of latex purification are dialy~is~*.~~ or agitation with mixed-bed IEX.M The choice of the preparation method has no influence on the chemical properties of the latices, as determined 0 1992 American Chemical Society
