Langmuir 1995,11, 4844-4847
4844
Location and Average Alignment of Alkylpyridinium Ions in Cationic Nematic Lyomesophases Boris E. Weiss-Lopez" and Consuelo Gamboa Departamento de Quimica, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile
Alan S . Tracey Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, V5A lS6, Canada Received March 6, 1995. I n Final Form: August 30, 1995@ Deuterium quadrupole splittings from the aromatic ring of a series of deuterium labeled n-alkylpyridinium ions with linear n-alkyl groups from C1 to c16 have been measured using 2H-NMR spectroscopy. These splittings were used to calculate the two order parameters describingthe average alignment of the individual pyridinium rings, and these values were used to determine the location and preferred orientation of the ions when contained in either of the lyotropic liquid crystals of this study. Two cationic mesophases, prepared from either hexadecylpyridinium chloride or tetradecyltrimethylammonium bromide, were used as the host systems. The smaller alkylpyridiniumions (c1-c5)were found predominantelyin the interstitial water. The intermediate size ions (CS-ClO), which were distributed more effectively into the micelle, have a disruptive effect in the mesophase, probably a result of modifications in the degree of alignment of the micelle, possibly because of modifications of its internal dynamics. Differences in the symmetry properties of the positively charged headgroups were found to impose restrictions on the motion of the dissolved alkylpyridinium ions.
Introduction When amphiphilic molecules are dissolved in a polar solvent such as water and their concentration is increased in a stepwise manner, a number of aggregates of varying shape are frequently formed. These aggregates and the phase changes associated with their formation have been characterized by a number of physical techniques.1-6 In the case of selected detergents, most frequently in conjuction with decanol and electrolyte, a nematic liquid crystalline material can be formed. According to the structure of the aggregates, the two most common nematic phases are the discotic mesophase of negative diamagnetic anisotropy, N,, closely affiliated with the lamellar structure, and the cylindrical mesophase of positive diamagnetic anisotropy, N;, that is closely allied to the hexagonal mesophase. A valuable property of these nematic phases is their ability to orient in an applied magnetic field with the director axis parallel or perpendicular to the applied field, as required by the diamagnetic anisotropy of the aggregate~.~-lO Evidence has also been presented for the formation of biaxial nematic mat e r i a l ~ . ~These ~ - ~ nematic ~ systems exhibit many of the properties of naturally occurring bilayer membranes, have Abstract published in Advance A C S Abstracts, November 15, 1995. (1)Jonsson, B.; Jokela, P.; Khan, A.; Lindman, B.; Sadaghiana, A. Langmuir 1991,7,889. (2)Auvray, X.;Abiyaala, M.; Duval, P.; Petipas, C.; Rico, I.; Lattes, A. Langmuir 1993,9,444. (3)Blackburn, J. C.; Kilpatrick, P. K. Langmuir 1992,8, 1679. (4)Alami, E.;Levy, H.; Zana, R.; Skoulius, A. Langmuir 1993,9, 940. (5)Bergenstahl, B. A.; Stenius, P. J . Phys. Chem. 1987,91,5944. (6)Warnheim, T.; Jonsson, A. J . Colloid Interface Sei. 1988,125, 627. (7)Radley, K.; Reeves, L. W.; Tracey, A. S. J . Phys. Chem. 1976,80, 174. ( 8 ) Yu,L. J.; Saupe, A. Phys. Reu. Lett. 1980,45,1000. (9)Hendrikx, Y.;Chavrolin, J.; Rawiso, M.; Liebert, L.; Holmes, M. C . J . Phys. Chem. 1983,87,3991. (10)Queiroz Do Amaral; Alvarenga Pimentel; Tavares, M. J . Chem. Phys. 1979,71,2940. @
0743-7463/95/2411-4844$09.00/0
the structures associated with other amphiphilic systems, and serve as good model systems for the study of such materials. Since the first observation of nematic amphiphilic liquid crystals15 a considerable number of NMR experiments have been undertaken in an attempt to understand the factors governing the localization and average alignment of solubilized molecular probes,16-ls the behavior of ions interacting with the micellar s u r f a ~ e , and ~ ~ -the ~ ~order profile along the hydrocarbon chains of the surfactant molecule^.^^-^^ The generation of cholesteric mesophases from their nematic precursors in the presence of chiral amphiphiles and the interactions between chiral surfaces and chiral molecules have all been topics of various studie~.~~-~~ Despite the observation that the internal dynamics of the micelle is strongly dependent on the nature of the interactions between the head-group and the solvent,22 little effort has been devoted to the investigation of the (11)Figuereido Neto, A.M.; Galerne, Y.; Levelut, A. M.; Liebert, L. J . Phys. Lett. 1986,46,499. (12)Figuereido Neto, A. M.; Levelut, A. M.; Liebert, L.; Galerne, Y. Mol. Cryst. Liq. Cryst. 1985,129,191. (13)Galerne,. Y.:. Fimereido Neto. A. M.: Liebert. L. J . Chem. Phvs. 1987,87,1851. (14)Bastos Do Santos, C. P.; Figuereido Neto, A. M. Langmuir 1991, 7,2626. (15)Lawson, K. D.; Flautt, T. J. J . Am. Chem. SOC.1967,89,5489. 177. (16)Radley, K.;Tracey,A. S. Mol. Cryst. Liq. Cryst. 1990,182B, (17)Fujiwara, F. Y ' Reeves, L. W. Can. J . Chem. 1980,58, 1550. Fujiwara, F. Y.;Reeves, L. W. Can. J . Chem. 1977, (18)Chen, D. M.; 55, 2404. (19)Tracey, A. S.; Boivin, T. L. J . Phys. Chem. 1984,88, 1017. (20)Tracey, A. S. Can. J . Chem. 1984,62,2161. (21)Iida, M.;Tracey, A. S. Langmuir 1991, 7,202. (22)Lee,Y.; Reeves, L. W.; Tracey, A. S. Can. J . Chem. 1980,58,110. (23)Fujiwara, F. Y.;Reeves, L. W. J . A m . Chem. SOC.1976,98,6790. (24)Reeves, L. W.; Tracey, A. S. J . A m . Chem. Sac. 1975,97,5729. (25)Tracey, A. S.; Zhang, X. J . Phys. Chem. 1992,96,3889. (26)Tracey, A. S.;Radley, K. Langmuir 1990,6,1221. (27)Tracey, A. S.; Radley, K. J . Phys. Chem. 1984,88,6044. (28)Khetrapal, C. L.; Kunwar, A. C.; Tracey, A. S.; Diehl, P. NMR Basic Principles and Progress; Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: Berlin, 1975;Vol. 9.
0 1995 American Chemical Society
Pyridinium Ions in Cationic Nematic Lyomesophases influence that the symmetry of the headgroup has on the orientation and mobility of solubilized guest molecules. Considering the methyl group as a unit, the trimethylammonium moiety belongs to the C3" molecular point group. In the alkyltrimethylammonium mesophases this head-group appears to have cylindrical symmetry due to rotation about the C1-N bond. For the corresponding alkylpyridinium surface such cylindrical symmetry is lost.29 Recent investigations have studied the role ofheadgroup/solvent interactions on the formation of liquid crystals from hexadecyltrimethylammonium bromide (HDTMABr) and hexadecylpyridinium bromide (HDPyBr). Different solvents, including water, were used in the It was found that formation of a particular mesophase was dependent on the cohesion energy of the solvent and on specific interactions between the solvent and the head-group. The orientation properties of a number of para-substituted pyridinium ions in anionic lyotropic mesophases have also been studied.31 In this case, the orientation was found to be highly dependent on the properties of the substituent group. In the present work, deuterium NMR spectroscopy has been used to study the location and orientation of a number of deuteriated n-alkylpyridinium ions in two different cationic nematic mesophases, both N, systems. The two mesophases were prepared either from hexadecylpyridinium chloride (HDPyC1) or from tetradecyltrimethylammonium bromide (TDTMABr). The alkylpyridinium ions were deuteriated in the aromatic ring, and the linear alkyl chain length varied from C1 to c16. These ions are particularly suitable for studies such as this because two order parameters are obtainable and these are sufficient to describe the average alignment of the aromatic ring of the ions.32
Experimental Section Synthesis of n-AlkylpyridiniumSalts. The n-alkylpyridinium salts, the C1 and C2 iodides, and the c3-c14 and Cle
bromides were prepared by mixing equimolar amounts ofpyridine (10% dg) with the desired alkyl halide and warming to 50 "C. This temperature was maintained until either a solid or an oil separated from the originalsolution. The oil was washed several times with anhydrous ether and then partially crystallized by cooling. The product was subjected to several crystallization/ wash cycles until a solid material was obtained. The solids were recrystallized several times from ethyl acetate/ethanoVether solutions. The short-chain products, CI-Clo, were very hygroscopic materials and were maintained over phosphorous pentoxide several weeks before use, to minimize the water content. Proton NMR spectra of all the salts dissolved in DzO were recorded and showed signals from the pyridinium moiety, from the a protons, and from the rest of the alkyl chain, centered at about 8.1,4.2, and 0.9 ppm from TMS, respectively. All reagents were from commercial suppliers and, except for the pyridine, were used as supplied. The pyridine was distilled,and the appropriate quantities were mixed with 99%deuteriated pyridine, comercially available, to give a 10% enriched product. Preparation of Mesophases. The TDTMABr and HDF'yC1 used for mesophase preparation were obtained from commercial sources and recrystallizedthree times from ethyl acetate/ethanol solutions before use. Reagent quality NaCl and NaBr were used without further purification. Decanol (DeOH)was purified by fractional crystallization and mixed with decanol-a-dz (20% v/v), previously synthesized by reduction ofdecanoic acid with LiAlD4. Distilled water containing 0.01%deuteriated water was used in all samples. (29)Reeves, L.W.; Tracey, A. S.; Tracey, M. M. Can. J. Chem. 1979, 57. 747. (30) T.: t. ,~~,Auvrav. Auvray, ~~.~ * , X.: X.; ~~,Perche. Perche, .~.~.., T.; _, P Petipas, -e. C.; Anthore, R.; Marti, M. J.; Rico, I.; Lattes, A. Langmuir 1992,8, 2671. (31)Spearman, S.A.; Goldstein, J. H. J.Mol. Struct. 1977,36,243. (32)Diehl, P.; Khetrapal, C. L. N M R Basic Principles a n d Progress; Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: Berlin, 1969; VOl. 1. ~~
~
Langmuir, Vol. 11, No. 12, 1995 4845
8000
LOO0
0
-LOW
-8000
nz
Figure 1. Deuterium NMR spectrum of the deuteriated
hexadecylpyridinium cation dissolved in HDPyCl mesophase. The center signal is from HOD, the three low-intensitydoublets from the pyridinium ion, and the high-intensity doublet from decanol. All splittings listed in Table 1can be measured directly from the spectrum. The composition of the phases, slightly different from previously reported mixture^,^^,^^ was 1.08 g HDPyCU2.11 mL H20/0.068 g NaCU270 pL DeOH and 0.84 g TDTMAEW2.00 mL H20/0.200 g NaBr/l5O pL DeOH
To 0.80 g of the desired mesophase was added 40 pmol of a deuteriated probe, and the solution was homogenized by repetitive stirring and warming in a 50 "C water bath. The samples were placed in a 5 mm NMR tube and allowed to spin for a few minutes before the NMR spectra were obtained. NMR Experiments. All NMR spectra were obtained at ambient temperature (21 "C) using a Bruker AMX-400 NMR spectrometer at S.F.U. and in a Bruker AMX-300 NMR spectrometer at the C.E.M., U. de Chile. Proton NMR spectra were obtained using a proton probe and the deuterium spectra obtained simply by using the lock channel of the same probe. A 30 kHz spectral window, a 15ps pulse, and 30 000 transients, obtained at the rate of 30 transient&, were used when measuring the deuterium spectra. Narrower spectral windows were utilized when high-qualityspectra ofthe low molecular weight pyridinium ion probes were desired.
Results and Discussion Figure 1 shows a deuterium NMR spectrum of the hexadecylpyridinium-d5 ion dissolved in the HDPyCl mesophase. First order quadrupole splittings from the ortho, meta, and para deuterium nuclei of the ion are easily obtainable as is the splitting from HOD in the water and from the a deuterons ofthe deuteriated decanol. Table 1summarizes the experimental results obtained for the various ions studied. The values observed for the quadrupole splittings of the monodeuteriated water in both cationic mesophases, of about 18 Hz for TDTMABr and 34 Hz for HDPyC1, are significantly smaller than those observed in anionic mesophases, where the splittings typically are on the order of 200-500 Hz.19921,24334~36 Since the value of the quadrupole splitting measures the mobility of the nucleus, this result suggests that the water molecules at these cationic interfaces interact weakly with the electrical bilayer, slightly decreasing its mobility. This view is supported by the fact that there is no observable effect on the dynamics of the water when the alkyl chain length is varied.36 However, there is a small decrease in the mobility of water on going from the ammonium, C3" local symmetry, to the pyridinium head-group, C2" local sym(33)Iida, M.;Tracey, A. S. J. Phys. Chem. 1990,94, 2590. (34)Diehl, P.; Tracey, A. S. Can. J. Chem. 1976,53,2755. (35)Hecker, L.;Reeves, L. W.; Tracey, A. S. Mol. Cryst. Liq. Cryst. 1979,53,71. (36)Frenot, M. P.; Nery, H.; Canet, D. J. Phys. Chem. 1984,88, 2884.
Weiss-Lopezet al.
4846 Langmuir, Vol. 11, No. 12, 1995 Table 1. Deuterium Quadrupole Splittings from the Ortho, Meta, and Para Positions of the Deuterium Labeled F'yridinium Ions,from DHO, and from Decanol-a&, in HDmCl and TDTMABr Mesophases. The Width of the 'H-NMR Signal from the Dipole-CoupledHydrocarbon Chain of the Detergents That Constitute the Mesophase, Measured at Half-Height, Is Listed in the Last Column. All Values Are in Hertz no. ofC's
o-
m-
p-
1 2 3 4 5 6 7 8 9 10 11 12 13 14 16
200 157 164 182 337 780 1710 2205 2590 3827 3852 5260 5386 6118 7073
162 110 140 154 387 1021 2357 2650 3537 5164 5284 7305 7468 8466 9934
HDPyCl 446 450 440 362 2 -810 -2448 -2980 -3664 -5259 -5690 -8024 -8510 -9655 -12187
1 2 3 4 5 6 7 8 9 10 11 12 13 14 16
184 190 192 204 452 882 1755 2300 2789 3021 4054 4798 5390 5830 6623
140 101 97 200 544 1052 2283 2950 3592 3991 5251 6218 7119 7768 8880
TDTMABr 376 414 359 238 153 -637 -1839 -2408 -2880 -3021 -4054 -4798 -6146 -6976 -8570
DeOH
DHO
Av
18973 17871 17433 17224 16862 15076 14310 13160 11784 14469 13435 16042 15247 15937 16686
28 34 34 34 36 38 34 32 33 32 34 33 36 35 38
7194 6854 6815 6850 6780 6032 6027 5743 5321 6031 5985 6795 6780 6703 7048
16536 17155 15384 14888 14021 10418 12817 12580 12073 11641 13158 13698 14333 14549 15068
19 16 17 18 17 16 16 15 17 16 18 19 18 19 19
6540 6460 6204 5603 5045 4105 3950 3843 4015 4650 5013 5020 5503 5507 6206
metry. A similar effect is also observed in the anionic phase^.^^,^^ This significantly bigger value of the quadrupole splitting of water in anionic mesophases has been attributed to strong interactions within the micelle interfa~e.~~,~~ The quadrupole splitting from the decanol a-deuterium atoms is a measurement ofthe overall alignment of decanol within the micelle. Circumstances that influence the alignment of the micelles or the behavior of the cationic amphiphile generating the mesophase can be expected to influence the decanol splitting. As can be seen from Table 1,the added pyridinium ions have a systematic influence on the decanol splittings. The splittingis not significantly influenced by the presence of the longer and shorter chain additives but drops substantially for the intermediate chain lengths, C,j-C10. This is observed in both mesophase systems. This suggests that the ions of intermediate chain length are having a disruptive effect on the integrity of the interface region. This phenomenon is not observed in the short-chain ions since, as discussed later, they are preferentially partitioned into the interstitial water and consequently have only minimal influence on the micellar structure. Considering the small amount of additive, the effect is rather large and most probably derives from modifications of the micelle structure, where small changes would be expected to lead to large effects. For instance, the individual micelles might be slightly smaller so that they tumble more isotropically. There may also be interface modifications so that the amphiphiles are less restrained in their motional properties. Support for this interpretation was obtained by measuring the overall line
Figure 2. Molecular fixed coordinate system and principal axis of the field gradient tensor (assumed to be along the C-D bond). Both coordinate systems can be related through the angles 81,82, and 83.
width of the dipolar coupled proton NMR spectrum of the hydrocarbon chains, a t half-height. As is evident from Table 1,the behavior of this parameter parallels that of the decanol deuterium splitting. The pronounced hygroscopic character of the intermediate-chain ions and their correspondingly small hydrophobic contribution to solubilization within the micelle could be responsible for such interface modifications. It seems likely that the balance between the hydrophobic and hydrophilic properties of the shorter chain ions is sufficiently different from that ofthe longer chain detergents that the pyridinium moiety of the intermediate-chain-length ions is located in a position or orientation that is different from the headgroup of the host amphiphile. These differences rapidly disappear for chains longer than Clo. More details of the interactions ofthese pyridinium ions with the mesophase can be obtained from the orientation matrix that describes the time-averaged alignment of a particular molecular axis with respect to the orientation of the magnetic field of the NMR spectrometer. These parameters can range from +1 for a completely parallel orientation to -0.5 for a completely perpendicular orientation and can be 0 for a randomly rotating axis or an axis aligned a t the magic angle. Although, for the general case, in a uniaxial liquid crystal five order parameters are required to completely specify the orientational probability function, this value is reduced to two for the present case of Czusymmetry, if the coordinate axes are fixed along the symmetry axes ofthe molecule, as depicted in Figure 2. The relation between the observed quadrupole splittings ( A v ) with the appropriate values for the deuterium quadrupole coupling constant (QD= 185 kHz) and the asymmetry parameter, q, for the various positions (7 = 0.051, is given by eq I.
Here the 8i are the angles between the principal axis of the electric field gradient tensor, assumed to be along the C-D bond axis, and the molecular coordinate axis of interest. The structural parameters employed in the calculations are = 34.5",82 = 28.0", and 0 3 = 90.0",as appears in Figure ZZ9 S,, is given by the traceless condition S, S,, S,, = 0. Figure 3 provides a graphical representation ofthe order parameters calculated for the various pyridinium ions in
+
+
Langmuir, Vol. 11, No. 12, 1995 4847
Pyridinium Ions in Cationic Nematic Lyomesophases 0.06 0.04 0.02
0
..., 1
2
3
4
0
I
5
6
"
1
'
9
-0.02
"
9
"
"
'
1011 1 2 1 3 1 4 1 5 1 6 A
.
4.04
'.
A
HydrocarbonChain Length
.
0.06
0%
b A s W
0.04
0
Sxx
0.02
0 0 0
0
*
1
.
.
2
.
3
I
E
4
5
6
I
8
...
~ 1 , 0 1 1 1 2 1 3 1 4 1 5 1 6
4.02
I
-0.04 HydrocarbonChain Length 4.06
Figure 3. Order parameters along theXand Ymolecular axis, S, (0)and S, (A), as a function ofthe hydrocarbon chain length of the solubilized guest, in (a) HDPyCl and (b) TDTMABr mesophases (see Figure 2). The variation of these parameters is consistent with a solubilization of the ions in a direction perpendicular to the magnetic field, as expected for a discotic nematic mesophase. the two mesophase systems. As observed from these figures the two sets ofvalues are similar for both systems. Not surprisingly, in keeping with the expectation that the smaller ions would be distributed efficiently into the aqueous region of the mesophase, the short-chain-length ions showed the smallest absolute values of the order parameters. However, somewhat unexpectedly, as seen in Figure 3, the C1-C5 ions all had values for the order parameters that did not differ significantly from each other. This finding strongly suggests that these shortchain ions spend most of their time in the aqueous interstitial region of the mesophases. Only for hydrocarbon chains of six carbons or more in length does the increase inquadrupole splittings suggest that the alkylpyridinium ions partition, to a significant amount, into
the micelle. This implies that, in these systems, the hydrophobic effect associated with at least a six carbon alkyl chain is required to compensate for the hydrophilicity of the ion and the repulsive electrostatic effects between the positively charged head-group and the similarly charged bilayer. If the quadrupole splittings at the two extremes are taken as the limiting values for the bound and free ions, then it can be estimated from a simple average that a Clo or C11 alkyl chain is required for the pyridinium ion to be partitioned, about 50% of the time, into the micellar structure of either the HDPyCl or TDTMABr mesophases. This result supports a recent study that concluded, from measurements of binding constants by fluorescence quenching, that the CIOpyridinium ion solubilized mainly in the aqueous region of a HDTMABr micellar solution.37 The behavior observed here for the pyridinium ions is quite different from that observed for alcohols in cationi~'~ amphiphilic ,~~ solution, where partition into the micelle is approximately linear with increase in chain length. Carboxylic acids in an anionic system similarly show a linearity with hydrocarbon chain length.18 The observations of alkyl carboxylates in a n anionic decyl sulfate mesophase18 reveals a n intermediate behavior between alkyl alcohols and the pyridinium ions of this study. There is a final point to be drawn from the order parameters. For the long-chain ions, the ratio of order parameters, S,,IS,, is about 0.3 for the hexadecylpyridinium system compared to about 0.5 for the tetradecyltrimethylammonium mesophase. This observation sugi gests that the pyridinium moiety rotates more freely about its N-C1 bond when in the TDTMABr mesophase than it does when in association with the pyridinium headgroups of the HDPyCl mesophase. This means that the head-groups have imposed motional restrictions on the solute that reflect the symmetry properties of the host amphiphiles. The values of the orientation parameters indicate that the incorporation of the pyridinium ions into the mesophase is with the hydrocarbon chain perpendicularly oriented with respect to the direction of the applied magnetic field.
Acknowledgment. The authors are pleased to acknowledge financial assistance from DTI, Universidad de Chile (Proyecto Q3545-9423), from Simon Fraser University (B.W.), and from NSERC Canada (A.S.T.). LA9501748 (37) Gamboa, C.; Olea, A. F. Langmuir 1993,9,2066. (38)Treiner, C. J. Colloid Interface Sci. 1983,93,33.
