Langmuir 1990,6, 1221-1224 Table 111. Stability of a Dispersion Containing 0.17 g/L of Dye I and Sodium Chloride % dye extracted at N ~ C concn I of treatment before extraction, after addinn 0 M 0.001 M 0.01 M 0.10 M " NaCl extracted immediately 1.9 3.0 8.7 30 allowed to stand for 15 h 2.0 2.5 8.1 28 agitated for 15 min on the 2.0 3.0 8.6 28 tumbler used for extraction agitated in a blender for 5 min 2.9 4.9 31 59
time period elapsed between salt addition and extraction. These results indicate that the rate of flocculation is much slower than the rate at which the dye is extracted by a solvent from the dispersion. The extraction rate shows the effect electrolytes have on shear stability of a dispersion. In the absence of a solvent, mechanical action of the extraction procedure had
1221
no effect on the extraction rate. However, vigorous agitation of the dispersion prior to extraction increased the extraction rate considerably. The destabilizing effect of the electrolyte is clearly evident (Table 111). When electrolytes flocculate a dispersion, the aggregates formed are more accessible to the solvent and are extracted more rapidly than smaller particles. However, flocculation of the dispersion is not a prerequisite for the extraction to occur. A dispersion may be apparently stable, but extraction with a solvent can reveal differences in stability and detect destabilization by electrolytes, heat, or agitation. Hence, the extraction technique can indicate the stability of a "live" dispersion,without the need to cause flocculation and measure the flocculation rate in a "dead" dispersion. Registry No. I, 34446-26-9; NaC1, 7641-14-5; MgC12, 718630-3; BaC12, 10361-37-2;NaZS04, 7757-82-6; CaC12, 10043-52-4; sodium lignosulfonate, 8061-51-6.
Amphiphilic Cholesteric Lyotropic Liquid Crystals Prepared from Potassium N-Dodecanoyl-L-threoninate Alan S. Tracey" and Keith Radleyt Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada, V5A 1S6 Received September 1, 1989. In Final Form: January 25, 1990 Cholesteric lyotropic liquid crystalline solutions prepared from potassium N-dodecanoyl-Lthreoninate have been investigated by NMR spectroscopy, optical microscopy, and laser diffraction techniques. The results showed that the pitch of the helical axis generated by the two chiral centers of the amphiphile was less for this system than for the analogous alaninate system where the amphiphile has only one center of optical activity. In contrast to this, the difference in the partial alignment of the D- and L-alanines is about 50% greater in the threoninate as compared to the alaninate amphiphilic system. This difference is almost independent of the amphiphile concentration but increases both with temperature and with decanol content. The behavior of the quadrupolesplittings from 2H,7Li, and 23Na was investigated as a function of amphiphile and decanol concentration and of temperature. Replacement of the L amphiphile with its D antipode in a stepwise manner decreased and then reversed the pitch of the helical axis and the relative orientation of the D- and L-alanines but had no significant effect on the water or lithium and sodium ion quadrupole splittings.
Introduction The influence that chiral centers have on the behavior of nematic lyotropic liquid crystals has been an area of developing interest over the past few Much of this work has concentrated on chiral detergents which have only one optical center and has provided detailed information concerning the mechanism which leads to cholesteric behavior in amphiphilic systems. t Present address: 12 New Street, Skelmanthorpe, Huddersfield, W. Yorkshire, United Kingdom, HD8 9BL. (1) Tracey, A. S.; Diehl, P. FEBS Lett. 1975,59, 131-132. (2) Covello, P. S.; Forrest, B. J.; Marcondes Helene, M. E.; Reeves, L. W.; Vist, M. J . Phys. Chem. 1983,87,176-181. (3) Yu, L. J.; Saupe, A. J . Am. Chem. SOC.1980, 102, 4879-4883. (4) Forrest, B. J.; Mattai, J. Chem. Phys. Lipids 1984,35, 1-10. (5) Tracey, A. S.; Radley, K. J. Phys. Chem. 1984,88,6044-6048. (6) Tracey, A. S.; Radley, K. Mol. Cryst. Lip. Cryst. 1985,122,77-87. (7) Melnik, G.; Saupe, A. Mol. Cryst. Liq. Cryst. 1987, 145, 95-110.
0743-7463/90/2406-1221$02.50/0
In achiral systems, the formation of nematic liquid crystals is determined by the occurrence of long-range interactions between micelles. In cholesteric liquid crystals, these interactions must be asymmetric. Cholesteric states thus are nematic states where a chiral center induces a spontaneous twisted structure via an asymmetric interaction. In amphiphilic cholesteric mesophases, unlike their thermotropic analogues, the interaction leading to cholesteric behavior may proceed via two possible mechanisms. The first involves pairwise long-range interactions between chiral molecules of adjacent micellesSs The second model involves specific short-range interactions between the optical centers of the chiral molecules within the individual micelles. Because of the asymmetry in such interactions, the micelles themselves will be twisted into a chiral shape. The available evidence suggests that it is 0 1990 American Chemical Society
Tracey and Radley
1222 Langmuir, Vol. 6, No. 7, 1990 the long-range interactions between the chiral micelles which provides t h e major contribution to the occurrence of cholesteric b e h a v i ~ r . ~ * ~ , ~ t ~ An interesting question about the generation of cholesteric behavior concerns t h e effect that a second chiral center in t h e amphiphile will have on t h e cholesteric behavior of t h e mesophase. It is now well-known that alkanamides prepared from amino acids readily form cholesteric mesophases2 even for rather complex amino acids such a s aspartic acid and proline.6J0 Although a number of cholesteric lyotropic systems have now been investigated, a system containing two chiral centers has not been reported. Such a system offers t h e possibility of either cooperative or compensating effects. In this report, the results of investigations into the N-dodecanoylthreonine system have been discussed. T h e cholesteric mesophase prepared from this material readily f o r m s a n d is s t a b l e o v e r a r a t h e r w i d e r a n g e of compositions. One aspect of lyotropic cholesteric materials is the length of the repeating unit along the helical axis. This distance, the pitch, gives rise to a selective color reflectance which for lyotropic cholesteric materials lies in infrared light b u t is in the visible region for many thermotropic materials. T h e lyotropic cholesteric systems which have provided the shortest pitch have been prepared from N-d~decanoylalaninate,~,~~ but here also n o selective reflectance of visible light was obtained. In order to induce a sufficiently highly twisted helical axis, it may be necessary to have more than one chiral center in the amphiphile, and this study with threonine represents some preliminary work in this regard. For this study, only the L (-) and D (+) threonines were utilized for the formation of the potassium N-dodecanoylthreoninates. L-Threonine ((2S,3R)-2amino3-hydroxylbutanoic acid) is t h e n a t u r a l l y occurring material, a n d D-threonine is its enantiomer. Neither of the diastereoisomers (2R3R or 2S,3S) were investigated in this study, and these systems should provide additional information concerning the requirements for compensation or enhancement of cholesteric behavior.
Experimental Section The potassium salts of D- or L-N-dodecanoylthreoninate (KDDT) were prepared by the acylation of threonine using dodecanoyl chloride as previously described.llJ2 Elemental analysis of the products gave L-KDDTof 56.48% carbon, 8.89% hydrogen, and 3.96% nitrogen and D-KDDTof 56.18% carbon, 8.97 71 hydrogen, and 3.93% nitrogen compared to the calculated composition based on the formula Cl~H3004NKof 56.64% carbon, 8.88% hydrogen, and 4.13% nitrogen. The melting points were determined: L-KDDT, 155 "C; D-KDDT, 154 "C. The optical rotations were determined: L-KDDT,G O D = 3.24 (HzO); D-KDDT,t y 2 0 ~= 2.98 (HzO). Samples were prepared by using procedures previously described.8 The compositions are presented in Tables 1-111. The standard sample contained 0.20 g of L-KDDT,0.04 g of decanol, and 0.50 g of DzO stock solution. The stock solution was made up with 30 g of DzO containing 0.15 g of L-alanine, 0.15 g of D,L-alanine, 1.0 g of NaBr, and 0.5 g of LiCl. Doubly distilled D2O and specially purified decanol were used throughout. Various samples were investigated by use of polarizing microscopy. The samples were contained in unsealed CAMLAB (8) Radley, K.; Saupe, A. Mol. Phys. 1978,35, 1405-1412. (9) Saupe, A. II Nuouo Cimento 1984, 30, 16-28.
(10)Alcantara, M. R.; Correia de Melo, M. U. M.; Paoli, V. R.; Vanin, J. A. Mol. Cryst. Li9. Cryst. 1983, 90,335-347. (11) Radley, K.; Tracey, A. S. Can. J. Chem. 1984, 63,95-99. (12) Jungermann, E.; Gerecht, J. F.; Krems, I. J. J . Am. Chem. SOC. 1956, 78, 172-174.
Table I. Various NMR Parameters and Pitch Measurements Obtained as a Function of the Concentration of Potassium N-Dodecanoyl-L-threoninate.
L-KDDT,~ mg AS, 70 13.0 150 160 13.0 17.2 170 15.6 180 14.9 190 14.7 200 14.3 210 14.4 220 16.8 230
1/P, cm-'
680 935 1210 1315 1755 1940 2200 2445 2485
quadrupole splittinas 2H AUQ, 'Li AWQ, 23Na PWQ, Hz Hz kHz 244.6 4.77 387.0 364.0 241.7 4.41 325.4 218.9 3.88 323.9 227.8 3.86 3.14 267.6 197.2 242.7 2.78 188.3 2.13 189.2 148.7 146.9 120.6 1.61 2.31 223.0 200.6
"Measurements were made at 303 K. Weight of L-KDDT (mg) in 0.50 g of D20 stock solution and 40 mg of decanol (see Experimental Section). Table 11. Various NMR Parameters and Pitch Measurements Obtained as a Function of the Substitution of L-KDDTfor D-KDDTin the Standard Sample.
quadrupole splittings
+ D),
L/(L
1/P,
% ' L-KDDT
AS, 76
cm-l
100
14.7 9.4 2.9 -3.0 -9.5 -15.8
1938 1226 368 355 1147 1960
80 60
40 20 0 a
2H AUQ, 'Li AUQ, Hz Hz 185.3 179.7 197.0 189.9 194.8 186.5
242.6 241.7 270.7 261.0 265.6 247.1
23Na
Pug,
kHz 2.78 2.68 2.96 2.82 2.93 2.78
Measurements were made at 303 K.
Table 111. Various NMR Parameters and Pitch Measurements Obtained as a Function of the Decanol Concentration in the Standard Sample.
quadrupole splittings decanol, mg
37 40 43 46 49
AS, (r, 14.6 14.7 15.0 15.7 15.2
1/p, cm-l
1805 1938 1630 1184 915
'H AUQ, Hz 290.0 185.3 227.0 263.9 285.0
7Li AWQ, 23Na AUQ, Hz Hz 327.0 3.29 242.7 2.78 300.3 3.47 355.0 4.18 389.0 4.67
Measurements were made at 303 K. microslides or between a microscope slide and a cover slip. Conoscopic observations were made with a Bertrand lens and a full-wave retardation plate at 294 K in order to determine the sign of the birefringence.7 All NMR measurements were made at 303 K except variabletemperature measurements, which were made on the standard sample. 'H, ZH, and alkali metal ion NMR spectra for this study were obtained by using a Bruker 400-MHz NMR spectrometer operating in the FT mode. The pitches of the helical axis of the amphiphilic cholesteric liquid crystal samples were measured by use of laser diffraction. The wavelength of the light from the laser was 6.328 X 10-6 cm. Very sharp diffraction bands were obtained after the sample contained in a NMR tube had been aligned in the spectrometer. The temperature of the samples was controlled to within 0.1 "C by placing the samples in a brass block suitably drilled for water flow, sample placement, and optical path. The temperature was controlled by circulating water from a thermostated water bath. The temperature of the brass block was determined by using a calibrated thermocouple thermometer. Except for the variable-temperature studies, all measurements were made at 303 K.
Results and Discussion T h e amphiphilic cholesteric liquid crystals samples used in t h e study, when viewed under a polarizing microscope,
Langmuir, Vol. 6, No. 7, 1990 1223
Amphiphilic Cholesteric Lyotropic Liquid Crystals gave rise to fingerprint textures with negative birefringence. Compensated cholesteric-phase samples in the study gave rise to pseudoisotropic textures with positive birefringence, under the same conditions. This indicated that the compensated cholesteric has a disk-shaped micelle structure. T h e reversal of the birefringence corresponding to the nematic-cholesteric transition has been observed before, both in thermotropic and amphiphilic cholesteric liquid ~rysta1s.l~In the cholesteric phase, the optical axis is parallel to the helix axis whereas in the nematic phase the optical axis is parallel to the director axis of t h e micelle. T h i s l a t t e r axis is perpendicular to helix axis in the cholesteric phase. 2HNMR spectra were observed for all the samples as a function of time. The diamagnetic anisotropies of the cholesteric phase samples were found to be positive and in the compensated cholesteric phases negative. Similar results were observed previously for the cholesteric potassium N-dodecanoyl-L-alaninate(L-KDDA)mesophase. The reason is that the unique axis, which in the nematic phase corresponds to the micelle axis, in the cholesteric phase corresponds to the helical axis. There is an asymmetry in the interactions between the two optically active solute molecules, D- and L-alanine in the aqueous region of the mesophase, and the chiral amphiphiles of the micelles. The result of this asymmetry, in general, will mean that the two solutes are aligned differently in the mesophase, with respect to the micellar surface. This difference in alignment is reflected in a difference in the observed dipolar couplings between the hydrogens of the alaninate m ~ i e t i e s .A ~S is then given by the difference in dipolar couplings for the two isomers divided by the average value and is reported as a percentage. For the purpose of this paper, AS refers only to the alanine methyl groups. AS, the twist (inverse pitch), and the quadrupole split7Li,and 23Nawere determined as a function tings from 2H, of the L-KDDTconcentration (Table I), of the proportion of L-KDDT in mixtures of L and D amphiphile (Table 11), and of the decanol content (Table 111). The results revealed a close parallel in behavior between the threoninate system and the alaninate system which has previously been ~ t u d i e d . ~ J l From Table I, it is seen that the alkali metal quadrupole splitting decreases by more than a factor of 2 as the concentration of the threoninate amphiphile was increased by 50%. This reverse dependence is similar to that observed for the alaninate system5 and occurs because the alignment of the headgroup of the amphiphile decreases with increasing concentration. The quadrupole splittings from the alkali metals and the water derive principally from binding interactions with the functional groups of the amphiphile. Although not quantitatively measured, it could be seen from observation of the lH spectra that the headgroup alignment follows the metal ion quadrupole splittings as has been studied much more rigorously in the alaninate ~ y s t e m . ~ J l In contrast to the above behavior, replacement of the L-threoninate amphiphile by its optical isomer in a stepwise manner (Table 11) does not have a significant effect on either the 2H, 7Li, or 23Na quadrupole splittings. An increase in the decanol content (Table 111)does, however, cause an increase in the ion splittings. Figure 1 shows the twist (l/P)and the A S (%) for Dand L-alanine as a function of the stepwise substitution of the detergent D-KDDTfor the detergent L-KDDT. No ~
~
~~~
(13)Gibson, H.W.In Liquid Crystals-The Fouth State of Matter; Saeva, F. D., Ed.; Marcel Dekker: New York, 1979;p 113.
I
I
I
80
20 L I ( L + D I Yo
Figure 1. Magnitude of the twist (1/P)(0) induced in the amphiphilic cholesteric liquid crystal as the amphiphileis changed in a stepwise manner from the D-threoninate derivative to the L derivative and corresponding changes in the average difference in alignment ( 0 )of the D- and L-alanine incorporated into the sample. Measurements at 303 K.
1
,
.
0.17
,
,
[KDDT I
,
,
,
1
0.21
Figure 2. Behavior of the twist (1/P)(0) and the difference in the alignment of the D- and L-alanines ( 0 )as a function of the detergent concentration in the standard sample. Measuremenb at 303 K. attempt was made to determine the sign of the twist, although it was assumed the sign of the twist was reversed for the D-KDDT to L-KDDT mesophases. For the AS during the predominantly D to predominantly L amphiphile transition, the sign change was easily observed. The D- and L-alanine concentrations were made unequal in the DzO solution when making up the samples, and the relative positions of the NMR transitions were seen to change places. During the D-KDDTto L-KDDTtransition, the AS was seen to move through zero for the racemic amphiphile and then change sign. The AS and 1/Poriginate by two quite separate mechanisms; however, other things being equal, both the twist and the AS are dependent on the number of chiral centers present. Figure 2 shows the behavior of twist (1/P)and the A S as a function of the amphiphile concentration in the L-KDDTcholesteric mesophase. If the present results are compared with those of the alaninate (L-KDDA)cholesteric phase," the magnitude of AS is 50% greater in this system than in the alaninate system, although the gradients of the plots are similar. The magnitude of the twist is about 25% smaller in L-KDDTphases than in the L-KDDA phases, but again the gradients of the plots are similar. An increase in the surfactant concentration induces a change in the headgroup orientation with a concurrent increase in the twisting power of the amphiphilic cholesteric liquid crystal.5 The increase of twisting power with increasing amounts of the amphiphiles must result from the changes in the orientation of the headgroup, and the twist is therefore a measure of the distortion induced in the micelle by the chiral center. The effect of changes in the decanol concentration on the twist and the A S is illustrated in Figure 3. With increasing decanol concentration, A S slightly increases,
1224 Langmuir, Vol. 6, No. 7, 1990
Tracey and Radley 15
2000
s v,
I
'P
t
4 5
1000
'Oo0
307 TEMPERATURE Figure 4. Effect of the temperatureon the twist (1/P)(0) and on the difference in the alignment of the D- and L-alanines is depicted ( 0 )for the standard sample. Units of temperature,K. 295
40 46 [Decanol I Figure 3. Behavior of the twist (1 P ) (0) and the difference in the alignment of the D- and L-a anines ( 0 )as the decanol concentration is increased in the standard sample. Measurements at 303 K .
i
which contrasts with the alaninate system, where a slight decrease was observed. The twist decreases with increased decanol content to about one-half the value at low decanol concentrations. The decanol dilutes the headgroups in the micelle surface, and increasing the decanol content has the opposite effect of increasing the detergent concentration. When the decanol is incorporated into the micelles, its hydroxyl group becomes an integral part of the micelle surface. The decanol serves to shield the interacting chiral centers from each other, so it might be expected that with an increase in the decanol content in the cholesteric system there would be a corresponding decrease in the twist. The cholesteric/isotropic transition temperature for the standard sample was about 41 "C. Below this temperature, the temperature dependence of the AS and the twist was observed, and the results are presented in Figure 4. A S increases about 15% for a 20 "C increase in temperature, and this probably occurs because the alaninate partitions more effectively into the micelle at higher temperature and is in closer contact with the chiral centers of the amphiphiles. The twist showed only a moderate temperature dependence in this L-KDDTsystem, which contrasts with the alaninate system where a dramatic increase in twist with increased in temperature was observed. For cholesteric thermotropic liquid crystals, two mechanisms have been proposed to explain the dependence on the twist on temperat~re.'~ Both involve the increase in the thermal motion, which leads to an increase in the molar volume. The variation in the molar volume affects the pitch of the helix by either (i) varying the intermolecular distance along the helix axis or (ii) varying the average displacement angle of the director of one molecule (or micelle) with respect to the adjacent cholestogen. These two processes have opposite effects on the pitch length, and if both are important, they may effectively cancel each other. Mechanism ii will operate at two levels in lyotropic cholesteric systems. At the most fundamental level, it applies to the interactions between the adjacent optically active amphiphiles as in thermotropic mesophases. This is the interaction which leads to the generation of the chiral micelle. At a higher level, the mechanism will apply to the interactions between the adjacent micelles. These two effects need not reinforce each other with changes in temperature, and an insensitivity to temperature changes might be observed. Studies of the potassium N-dodecanoyl-L-alaninate system have suggested t h a t mechanism i is t h e
predominant source of the change in pitch length with temperature in that system. Since both the alaninate and the threoninate systems are micellar systems, it is difficult to see why t h e mechanism for generation of the macroscopically observed twist should change on going from one system to the other. However, the threoninate amphiphile has two chiral centers, and it may be that with changes in temperature the population distribution of the rotational isomers is changed. In this event, the distortion induced in the micelles might have a significant temperature dependence. If the change in rotamer population leads to a more distorted micelle and mechanism i is also operative, then the pitch could well be insensitive to temperature as observed. Conclusions Investigation of the alignment of D- and L-alanine in the cholesteric N-dodecanoyl-L-threoninate mesophase system has shown that the two alanine isomers give rise to dipole couplings that are about 50% larger than previously determined for these compounds under similar conditions system. The pitch of the in the N-dodecanoyl-L-alaninate induced helical axis in the threoninate system is, however, only about 75% of that in the alaninate system. Both the difference in the alignment of the two alanine isomers and the generation of the helical axis of the mesophase itself depend on specific interactions between chiral centers. The results indicate that alanine is aligned more effectively by the two chiral centers of the threoninate than by the one center of the alaninate. It appears that the interactions of the two alanine isomers with the individual chiral centers of the threoninate amphiphile is, to an extent, additive, in the sense that difference in alignment is larger and not smaller than for the alaninate amphiphile system, which has only one chiral center. I t seems that this additivity does not extend to the generation of the helical axis of the mesophase itself. The generation of the helical axis has its origins in the asymmetry of the interactions between adjacent chiral amphiphiles. The longer pitch length in the threoninate system as compared to the alaninate system suggests that the two chiral centers, 2s and 3R, of the threoninate amphiphiles are partially compensating each other. If this is true then it may be that the chiral centers of the other isomer of thronine, 2s and 3S, (or their optical antipode) will reinforce each other so as to lead to more highly twisted micelles. Interactions between the resultant chiral micelles should lead to a shorter pitch and perhaps a selective reflection of visible light. Acknowledgment. Thanks are gratefully extended by A.S.T.to NSERC Canada for its financial support of this work. Registry No. L-KDDT,127232-60-4; D-KDDT,127232-615; decanol, 112-30-1;D-alanine, 338-69-2;L-alanine, 56-41-7.
