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Langmuir 1996, 12, 4324-4328
Articles Solubilization of N-Alkylpyridinium Ions in Anionic Nematic Lyomesophases Boris E. Weiss-Lo´pez,* Jaime Vicencio-Gonza´lez, and Consuelo Gamboa Departamento de Quı´mica, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile Received April 4, 1996. In Final Form: June 12, 1996X Deuterium quadrupole splittings from the aromatic ring of a series of linear N-alkylpyridinium-d5 ions, with alkyl chains from 1 to 16 carbon atoms, were measured using 2H-NMR spectroscopy. The pyridinium ions, 10% deuteriated in the aromatic ring, were dissolved in nematic anionic lyomesophases prepared from sodium decyl sulfate (SDS) and cesium decyl sulfate (CsDS). With these splittings, the two order parameters that completely describe the average orientation of the aromatic ring with respect to the magnetic field were calculated. The added pyridinium ions have a dramatic effect on the mobility of the CsDS mesophase components. The smaller N-methyl and N-ethylpyridinium ions, C1 and C2, have a disruptive effect on the integrity of the mesophase, C3 and C4 do not have an appreciable effect, and the larger ones, C5 to C12, show the opposite effect, increasing the order of the system with the length of the alkyl chain. This phenomenon, not observed in the SDS mesophase, may be attributed to differences in charge distribution between both surfaces. This interpretation is supported by estimation of the degrees of dissociation and first cmc of SDS and CsDS using conductometric measurements. The results could also be explained if the CsDS mesophase was near to a phase transition and stabilized by the added ions.
Introduction The importance of interactions and distribution of ions and molecules, including polymers, in micellar solutions and bilayers of amphiphilic molecules has been recognized for many years and continues to be the subject of a significant number of theoretical and experimental investigations. The different NMR spectroscopic techniques, as well as other methodologies, have been widely used for this purpose.1-15 The use of nuclear quadrupole splittings in the determination of order parameters of specifically labeled molecules dissolved in anisotropic solutions remains a very active field of research.16-18 As early as 1976 it was observed that the concentrations of Li+, Na+, K+, Rb+, and Cs+ counterions and additives such as sodium sulfate and ethylene carbonate, constitute an important factor in the type of nematic phase obtained X
Abstract published in Advance ACS Abstracts, August 1, 1996.
(1) Forsten, K. E.; Kozack, R. E.; Lauffenburger, D. A.; Subramaniam, S. J. Phys. Chem. 1994, 98, 5580. (2) Abrahmsen-Alami, S.; Stilbs, P. J. Phys. Chem. 1994, 98, 6359. (3) Regev, O.; Kong, C.; Khan, A. J. Phys. Chem. 1994, 98, 6619. (4) Bocker, J.; Brickman, J.; Bopp, P. J. Phys. Chem., 1994, 98, 712. (5) Iliopoulos, I.; Olsson, U. J. Phys. Chem. 1994, 98, 1500. (6) Radley, K.; McLay, N. J. Phys. Chem. 1994, 98, 3071. (7) Baber, J.; Ellena, J. F.; Cafiso. D. S. Biochemistry 1995, 34, 6533. (8) Mandal, A. B.; Wang, L.; Brown, K.; Verrall, R. J. Colloid Interface Sci. 1993, 161, 292. (9) Maltesh, C.; Somasundaran, P. J. Colloid Interface Sci. 1993, 157, 14. (10) Raudino, A. J. Phys. Chem. 1995, 99, 15298. (11) Li, X.; Lin, Z.; Cai, J.; Scribven, L. E.; Davis, H. T. J. Phys. Chem. 1995, 99, 10865. (12) Kwon, K. O.; Kim, M. J.; Abe, M.; Ishinoimori, T.; Ogino, K. Langmuir 1994, 10, 1415. (13) John, A. C.; Rakshit, A. K. Langmuir 1994, 10, 2084. (14) Wang, G. J.; Engberts, J. B. F. N. Langmuir 1994, 10, 2583. (15) Brasher, L. L.; Herrington, K. L.; Kaler, E. W. Langmuir 1995, 11, 4267. (16) Huo, S.; Vold, R. R. J. Phys. Chem. 1995, 99, 12391. (17) Tansho, M.; Ikeda, S.; Ohki, H.; Ikeda, R. J. Phys. Chem. 1995, 99, 4335. (18) Sheikh-Ali, B. M.; Khetrapal, C. L.; Weiss, R. G. J. Phys. Chem. 1994, 98, 1213.
S0743-7463(96)00320-4 CCC: $12.00
from decyl sulfate/decanol/D2O solutions.19 In 1984, it was reported that the type of counterion-headgroup interactions depends on the properties of both interacting entities. For instance, in mesophases prepared with decylpyridinium, Cl- is bonded to two headgroups and Br- to three, whereas in alkyltrimethylammonium mesophases, Cl- is bonded to three headgroups and Br- to two. Also, detailed information was obtained about the binding of Li+, Na+, K+, Rb+, and Cs+ ions to the carboxylate headgroup and of Cl- and Br- ions to the ammonium headgroup in mesophases prepared from mixtures of potassium dodecanoate and alkyltrimethylammonium bromide.20,21 Using methyl phosphonate ion as a probe, modifications in the pH and reverse of the diamagnetic anisotropy sign by addition of aromatic counterions were observed in nematic mesophases containing cesium decyl sulfate (CsDS) and sodium decyl sulfate (SDS) mixed with trimethylanilinium decylsulfate. The results were interpreted in terms of ion binding of the different protonated methyl phosphonate species that can exists at different pH values.22 More recently,23 the incorporation of three porphyrin derivatives into bilayers of dioctadecyldimethylammonium chloride was studied using UV-visible, fluorescence, and EPR spectroscopies. For the two molecules where conclusive results were obtained, the more hydrophobic derivative is located in the interior of the bilayer, whereas the more hydrophilic derivative is found near the aqueous interface. In both cases the long axes of the porphyrin molecules are found to be parallel to the bilayer surface. It has also been recently observed that addition of methylsalicylic acid and hydroxybenzoic acid to micellar solutions of cetyltrimethylammonium bromide causes spherical micelles to undergo (19) Radley, K.; Reeves, L.; Tracey, A. S. J. Phys. Chem. 1976, 80, 174. (20) Tracey, A. S. Can. J. Chem. 1984, 62, 2161. (21) Tracey, A. S.; Boivin, T. L. J. Phys. Chem. 1984, 88, 1017. (22) Radley, K.; Tracey, A. S. Liq. Cryst. 1989, 6, 75. (23) van Esch, J. H.; Feiters, M. C.; Peters, A. M.; Nolte, R. J. M. J. Phys. Chem. 1994, 98, 5541.
© 1996 American Chemical Society
Solubilization of N-Alkylpyridinium Ions
a phase transition to wormlike micelles.24 In a previous study, we have observed that intermediate chain length N-alkylpyridinium ions (C5 to C10) have a disruptive effect on the integrity of the interface of lyotropic liquid crystals made from hexadecylpyridinium chloride and tetradecyltrimethylammonium bromide.25 Thus, for many years it has been known that the interactions between different counterions, or added guest molecules, and the surface of micelles or bilayers of amphiphilic molecules, may introduce important modifications to the properties of the phases involved. In this work we present a 2H-NMR study about the location and average alignment of a series of N-alkylpyridinium-d5 ions in anionic nematic liquid crystals. The pyridinium ions, with linear alkyl chain lengths from 1 to 16 carbon atoms and 10% deuteriated in the aromatic ring, were dissolved in type II mesophases19 prepared from SDS and CsDS. The 2 order parameters that completely describe the alignment of the pyridinium moiety were calculated from the observed quadrupole splittings. The experimental observations can be rationalized in terms of the balance between the hydrophilic and hydrophobic properties of the pyridinium ions and the differences in surface charge distribution between both detergents. Conductivity measurements26 to estimate the degrees of dissociation and first critical micellar concentration (cmc) of SDS and CsDS support this interpretation. Other possible origins cannot be discarded. Experimental Section Synthesis of N-Alkylpyridinium Salts. The synthesis of the pyridinium salts, their purification, and characterization have been previously described.25 All these compounds are permanently stored under vacuum over phosphorus pentoxide, due to their high hygroscopicity, particularly the short chain derivatives (C1 to C10). Preparation of Mesophases. In order to prepare the mesophases it was necessary to synthesize both detergents. SDS and CsDS were synthesized by allowing the reaction between decanol and sulfuric acid (1:1.5 molar ratio) to proceed at 5 °C for 7 days and later neutralizing with concentrated solutions of either NaOH or CsOH to produce mainly SDS or CsDS, respectively.27 After precipitation of the inorganic sulfate by adding ethanol and evaporation of the solvent, the solids were recrystallized four times from anhydrous ethanol and the crystals vacuum dried for several days before using. The final products are white crystalline solids. The compositions of the SDS mesophase were initially obtained from previous work28 and optimized for the present work. The composition of the CsDS mesophase was obtained by reproduction of the SDS composition, on a molar basis, and later optimized. The compositions of all samples are listed in Table 1. NMR Experiments. All NMR spectra were recorded at 300 K on a Bruker AMX-300 NMR spectrometer at the Centro de Equipamiento Mayor (CEM), Facultad de Ciencias, Universidad de Chile. Deuterium NMR spectra were obtained with a broad band inverse probe tuned to the deuterium frequency, and proton spectra were obtained using the proton channel of the same probe. A pulse length of 10 µs, which corresponds to a 90° magnetization flip angle, and a spectral window of 35 kHz were used for the deuterium spectra. Between 10 000 and 50 000 transients were acquired at a rate of 30 transients/s; smaller ions showed narrower lines and required fewer acquisitions to achieve the same signal/ noise ratio.
Results and Discussion Figure 1 shows the 2H-NMR spectrum of pentylpyridinium-d5 ion dissolved in the CsDS mesophase. First(24) Lin, Z.; Cai, J. J.; Scriven, L. E.; Davis, H. T. J. Phys. Chem. 1994, 98, 5984. (25) Weiss-Lopez, B. E.; Gamboa, C.; Tracey, A. S. Langmuir 1995, 11, 4844. (26) Evans, H. C. J. Chem. Soc. 1956, part 1, 579. (27) Reeves, L.; Tracey, A. S. J. Am. Chem. Soc. 1975, 97, 5729. (28) Fujiwara, F. Y.; Reeves, L. W. Can. J. Chem. 1980, 58, 1550.
Langmuir, Vol. 12, No. 18, 1996 4325
Figure 1. Deuterium NMR spectra of pentylpyridinium-d5 dissolved in type II CsDS mesophase. The aggregate orients with the director axis perpendicular to the magnetic field. All the splittings listed in Table 1 can be measured directly from the spectra. The central pair of signals arise from HDO, the three low-intensity doublets from the pyridinium ring, and the outer doublet from decanol-R-d2. Table 1. Compositions of the SDS and CsDS Mesophases Used for the Different Samples SDS Mesophase no. of carbons
SDS (mg)
Na2SO4 (mg)
decanol (µL)
HDO (µL)
pyridinium (mg)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 16
481.7 480.4 479.6 478.7 477.5 480.3 479.7 475.8 471.7 471.8 471.6 471.6 472.0 471.5 471.0
41.3 41.4 41.6 41.7 41.5 41.3 41.1 40.9 41.4 41.8 41.5 41.3 41.0 41.9 41.6
97 97 97 97 97 97 97 97 97 97 97 97 97 97 97
600 600 600 600 600 600 600 600 600 600 600 600 600 600 600
14 17 17 18 24 15 19 14 17 15 20 18 17 17 16
CsDS Mesophase no. of carbons
CsDS (mg)
K2SO4 (mg)
decanol (µL)
HDO (µL)
pyridinium (mg)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 16
338.1 337.0 335.6 334.5 333.8 332.9 321.2 330.0 330.7 330.5 330.6 330.6 330.6 330.9 330.6
20.2 20.3 19.8 20.0 20.1 20.0 19.8 19.9 20.3 20.1 19.8 19.6 20.1 20.0 20.4
40 40 40 40 40 40 40 40 40 40 40 40 40 40 40
400 400 400 400 400 400 400 400 400 400 400 400 400 400 400
14 16 16 19 20 10 20 16 15 16 21 21 21 20 21
order quadrupole splittings from decanol-R-d2, from HDO, and from the ortho, meta, and para positions of the pyridinium-d5 ions can be measured from the spectra. All the experimental results obtained in this work appear in Table 2. As may be seen from Table 2, the value of the quadrupole splitting of HDO in the SDS mesophase is about 400 Hz and remains practically constant for all the samples. The same splittings measured in CsDS are smaller, but more surprisingly, there is a strong dependence of this value on the length of the alkyl chain of the added pyridinium
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Table 2. Deuterium Quadrupole Splittings from the Ortho, Meta, and Para Positions of the Ring-Labeled Pyridinium-d5 Ions, from Decanol-r-d2 and from HDO, in SDS and CsDS Mesophasesa no. of carbons
DeOH-R-d2
HDO
ortho
meta
para
4321 7069 9944 11394 12535 11713 14078 15311 16821 18330 20635 19726 18540 19950 18830
-1228 -6059 -8880 -9721 -9835 -9846 -10685 -11192 -11192 -10623 -10904 -10519 -10730 -10737 -10920
2309 3943 7187 8817 12036 13789 17872 17713 20055 20527 23028 24142 24554 24205 24109
-258 -2599 -5195 -5440 -6159 -7504 -6678 -7525 -7434 -7534 -7142 -6686 -6657 -6779 -7247
1 2 3 4 5 6 7 8 9 10 11 12 13 14 16
14500 15346 15933 16381 16470 14312 15730 15414 15532 15691 16791 15631 14850 15533 14830
SDS Mesophase 389 3905 417 6852 436 9944 449 11394 453 12258 416 11713 463 13815 464 15058 455 16430 418 17394 444 19600 420 18686 423 17650 419 19284 420 17750
1 2 3 4 5 6 7 8 9 10 11 12 13 14 16
10251 10185 12768 13076 15644 16315 18278 16207 17655 17885 19112 19573 19568 19271 18872
CsDS Mesophase 166 1981 166 3730 210 6914 217 8385 256 11300 294 13039 282 16456 274 15502 295 18449 284 18985 296 21116 290 21999 273 22238 283 22001 289 21952
a All values are in Hz and the error in the measured splittings is ( 3 Hz.
ions, ranging from around 166 Hz for the smaller ions to about 300 Hz for the larger ones. This result suggests that the interaction of the water molecules with the surface of the SDS aggregate is always stronger than the interaction with the surface of the CsDS mesophase,29,30 but in CsDS it is also a function of the alkyl chain length of the added pyridinium ions. This important difference on the mobility of water indicates that there may be significant differences on the properties of both surfaces. A closer inspection of Table 2 reveals that the behavior of the quadrupole splitting from decanol-R-d2 is similar to that observed for HDO. It is constant for the SDS mesophase, about 15 kHz, but in CsDS it is a strong function of the length of the alkyl chain of the added pyridinium ions. The value of this splitting is a parameter indicative of the order of the aggregate near the interface and has a value of 13 kHz in the absence of additives; however, in CsDS it ranges from 10 to 19 kHz. These results show that the small ions, N-methylpyridinium and N-ethylpyridinium ions, C1 and C2, have a disruptive effect on the integrity of the CsDS mesophase similar to previous observations in cationic micelles.25 C3 and C4 pyridinium ions do not have a significant effect and, as the chain length of the pyridinium ions increases, there is a progressive decrease in the mobility of this region, becoming significantly more rigid than in the original mesophase. The later observation suggests the existence of strong interactions between the pyridinium ions and the surface, affecting the mobility of the molecules at the interface in the CsDS aggregate. (29) Lee, Y.; Reeves, L. W.; Tracey, A. S. Can. J. Chem. 1980, 58, 110. (30) Hecker, L.; Reeves, L. W.; Tracey, A. S. Mol. Cryst. Liq. Cryst. 1979, 53, 77.
Considering the small amounts of additive used, the magnitude of the observed effect is large and most probably arises from strong electrostatic interactions between the surface and the added ions. The possible origins of the observations mentioned above are discussed later. We do not expect modifications in the diamagnetic anisotropy or pH effects in these systems, as previously observed.22 We have measured the pH of the samples, and it is 6.0 for SDS and 6.2 for CsDS. To obtain more detailed information about the location and average alignment of the alkylpyridinium ions, and the way in which they interact with the surface of these anionic liquid crystals, we have calculated the order parameters necessary to describe completely the average orientation of the pyridinium moiety with respect to the magnetic field. Five order parameters are needed to describe the average orientation of a rigid nonsymmetric body in the magnetic field; however, this number is reduced to two for the present C2v symmetry, if the molecular coordinate system is coincident with the axis of symmetry. As is known, these parameters can have values ranging from -0.5, for an axis perpendicular to the magnetic field, to +1.0, for an axis parallel to the field, while a value of 0 indicates a randomly rotating axis or an axis oriented at the magic angle. The relation between the observed quadrupole splitting of position i, ∆νi (i ) ortho, meta, and para), and the order parameters Sxx and Syy of the ring, is obtained from eq I31
∆νi ) (3/2) QD [Syy - cos2 θi(Syy - Sxx) + (η/3)(2Sxx + Syy + cos2 θi (Syy - Sxx))] (I) Here θi are the angles between the principal axis of the local electric field gradient tensor, considered to be along the C-D bond, and the molecular axis of the ring; QD is the quadrupole coupling constant and η the asymmetry parameter of the electric field gradient. The values of the structural and spectroscopic parameters involved in this equation, as well as the definition of the axis for the pyridinium ring, have been previously reported,25 and the same orientation and values of the constants were used in the present work. The Y axis is coincident with the C2 symmetry axis of the ring and the X axis, oriented perpendicularly, is contained in the ring plane. The order parameter Sxx represents motional averaging around the C2 symmetry axis of the ring and a smaller value implies an easier rotation of the ring around the torsion defined by the CR-N bond. The order parameter Syy represents motional averaging that can be separated in terms of contributions of tumbling around the X and/or Z molecular axes. These latter motions of the ring involve motions of the complete molecule, including the alkyl chain. The calculated order parameters Sxx and Syy of the pyridinium ions dissolved in SDS and CsDS mesophases are listed in Table 3. Figure 2 shows a plot of these values as a function of the alkyl chain length of the pyridinium ions. This figure also shows, for comparison, previously reported values of the same order parameters, measured in a hexadecylpyridinium chloride (HDPyCl) mesophase.25 In this figure it can be seen from the magnitude of the deuterium quadrupole splittings that the pyridinium ions spend more time attached to the anionic surface than to the cationic surface, in contrast with previous observations in cationic systems, where at least six carbon atoms were required in the alkyl chain of the pyridinium ion for it to start to partition to a significant extent into the micelle.25 (31) Reeves, L.; Tracey, A. S.; Tracey, M. M. Can. J. Chem. 1979, 57, 747.
Solubilization of N-Alkylpyridinium Ions
Langmuir, Vol. 12, No. 18, 1996 4327
Figure 2. Order parameters of the pyridinium ions along the X and Y molecular axis, Sxx and Syy, as a function of the hydrocarbon chain length of the pyridinium ions, measured in SDS and CsDS mesophases. Table 3. Order Parameters, Sxx and Syy, of the Pyridinium Ions Dissolved in CsDS and SDS Mesophases CsDS mesophase no. of carbons
Sxx
1 2 3 4 5 6 7 8 9 10 11 12 13 14 16
0.0108 0.0242 0.0454 0.0536 0.0701 0.0816 0.0980 0.0945 0.1090 0.1130 0.1230 0.1270 0.1280 0.1270 0.1280
SDS mesophase Syy
no. of carbons
Sxx
Syy
-0.0013 -0.0100 -0.0198 -0.0210 -0.0241 -0.0293 -0.0269 -0.0298 -0.0299 -0.0304 -0.0294 -0.0279 -0.0278 -0.0282 -0.0299
1 2 3 4 5 6 7 8 9 10 11 12 13 14 16
0.0227 0.0467 0.0656 0.0740 0.0817 0.0789 0.0914 0.0988 0.1050 0.1080 0.1130 0.1170 0.1080 0.1130 0.1124
-0.0051 -0.0227 -0.0329 -0.0358 -0.0375 -0.0375 -0.0409 -0.0429 -0.0412 -0.0400 -0.0418 -0.0411 -0.0408 -0.0391 -0.0420
In the present anionic mesophases, even the short chain pyridinium ions are partially incorporated into the micelle, despite the strong hygroscopic character of these ions. This is probably due to the attractive Coulombic interaction between the cationic pyridinium ions and the anionic surface. Not surprisingly, in this case the incorporation of the ions also occurs with the Y axis perpendicular to the surface. Figure 2 shows that the mobility of the Y axis of the ring, represented by the value of Syy, is always greater in CsDS than in SDS. Furthermore, it may be seen that for alkyl chain lengths of six or more carbon atoms, Syy remains constant in both host systems. This observation reflects the fact that with six or seven carbon atoms, the hydrophobic effect of the alkyl chain and the interaction between the positive ions and the negative surface are sufficient to incorporate the alkyl chain completely into the superstructure of the aggregate, in both nematic systems. This imposes maximum restrictions to the tumbling around the X or Z axis of the ring. The difference in the mobility of the Y axis between both nematic systems is approximately constant for all the samples. As shown in Figure 2, the behavior of Sxx is completely different from that of Syy. For the short chain pyridinium ions, C1 to C6, the values of Sxx in the SDS mesophase are greater than those observed in the CsDS micelle. As the alkyl chain length increases to about seven carbon atoms, both Sxx curves cross. For the long chain pyridinium ions, Sxx measured in the CsDS mesophase becomes larger than the value found for the SDS mesophase. A possible explanation for these observations arises from the particular hydrophilic/hydrophobic properties of the pyridinium ions and from differences in the
characteristics of the counterions of both detergents. Cesium has an ionic radius of about 1.67 Å whereas that for sodium it is only 0.97 Å.32 On account of this difference, sodium generates a significantly stronger electrostatic potential, which interacts more effectively with water molecules, making the sodium ion a considerably more strongly solvated cation than cesium. This solvation effect presumably leads to the SDS surface becoming more negatively charged than the CsDS surface. This phenomenon could explain the observation that the short chain ions, C1 to C6, are less mobile in both directions, X and Y, in the SDS surface than in the CsDS surface, since for these relatively hydrophilic cations the attractive Coulombic interaction with the negatively charged surface is more important than the hydrophobic effect. The crossing of the Sxx curves can also be understood in terms of the differences in surface charge. Sxx represents the mobility of the ring around the torsion axis defined by the CR-N bond and, due to the increasing hydrophobic effect of the longer alkyl chain ions, the headgroup of the pyridinium ions becomes progressively more incorporated into the interface region, imposing more restrictions to this motion. There is a position at which the positive pyridinium ring starts to interact significantly with the positively charged counterions. Since the CsDS surface has more counterions attached to it, this interaction is more important and, we postulate, strong enough to modify the location of the pyridinium head, pushing the pyridinium moiety more to the interior of the bilayer. This allows an extra stabilization of the aggregate by increasing the effectiveness of the interaction between the pyridinium headgroups and the negative charges of the sulfate heads. It is possible that counterion displacement occurs simultaneously with this process. This hypothesis could also explain the observed dependence of the mobility of water upon the characteristics of the added pyridinium ions, as a progressively more rigid interface will interact more efficiently with the water molecules. This is not observed in the SDS mesophase, probably because the SDS surface has a significantly smaller number of counterions attached to it. To support this hypothesis, the ratio between the degrees of dissociation, R, and the cmc of spherical micelles of SDS and CsDS, was estimated by the conductometric method of Evans.26 The measured cmc’s of SDS and CsDS were 3.3 × 10-2 and 2.3 × 10-2 M, respectively. The ratio between the dissociation degrees, RSDS/RCsDS, was estimated from the ratios between the slopes after and before the cmc, and the obtained value is 1.48. Both results show that the SDS micelle is significantly more dissociated than the CsDS micelle. Alternative explanations for the particular behavior of the CsDS mesophase can be suggested. It is possible that due to the mixing of cationic and anionic surfactant molecules the micellar size increases, decreasing the mobility of the complete system. The amounts of pyridinium ions added to the mesophase are always less than 10% (w/w) of the total detergent, and since the observed effect is rather large, this possibility does not seems very likely. Another possible explanation is that the CsDS mesophase may be near a phase transition and being stabilized by modifications of the ionic strength of the medium or surfactant concentrations. This possibility does not seem likely since the smaller ions, C1 and C2, present a disruptive effect on the integrity of the interface, opposite to what is expected for an ionic strength stabilization of the aggregate. The minimum amount of K2SO4 necessary to obtain a homogeneous anisotropic phase was 12 mg per sample; we used 20 mg. Also a (32) CRC Handbook of Chemistry and Physics, 69th ed.; CRC Press: Boca Raton, FL, 1988-1989.
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Table 4. Normalized Order Parameters, Sxx and Syy, of the Pyridinium Ions Dissolved in CsDS and SDS Mesophases CsDS mesophase
SDS mesophase
no. of carbons
Sxx
Syy
no. of carbons
Sxx
Syy
1 2 3 4 5 6 7 8 9 10 11 12 13 14 16
0.0105 0.0237 0.0356 0.0410 0.0448 0.0500 0.0536 0.0583 0.0619 0.0629 0.0642 0.0647 0.0657 0.0659 0.0675
-0.0012 -0.0098 -0.0155 -0.0161 -0.0154 -0.0179 -0.0147 -0.0184 -0.0169 -0.0170 -0.0154 -0.0142 -0.0142 -0.0146 -0.0158
1 2 3 4 5 6 7 8 9 10 11 12 13 14 16
0.0150 0.0308 0.0419 0.0463 0.0496 0.0551 0.0581 0.0637 0.0676 0.0707 0.0718 0.0743 0.0784 0.0765 0.0758
-0.0034 -0.0150 -0.0210 -0.0224 -0.0229 -0.0262 -0.0260 -0.0276 -0.0265 -0.0264 -0.0264 -0.0263 -0.0295 -0.0266 -0.0286
series of experiments to optimize the composition of the other mesophase components were performed. Starting from the original composition, we introduced variations in the concentration of detergent and decanol. The used concentration was the middle point between the two extremes that showed a constant value for the decanolR-d2 quadrupole splitting. If the observed effect could be attributed to some instability of the CsDS mesophase, it should affect equally the X and Y axis of the ring. This interpretation of the results cannot be completely ruled out at this time. Finally, to obtain information about the interaction of the alkylpyridinium ions and the surfaces of SDS and CsDS, in the absence of specific influences of the added ions on the CsDS mesophase, the quadrupole splittings from the aromatic ring were normalized by dividing them by the splitting of decanol-R-d2, for both host systems, multiplied by 10 000, and the order parameters recalculated. Table 4 lists the values obtained for the normalized parameters and Figure 3 shows a plot of these numbers
Figure 3. Normalized order parameters, Sxx and Syy, as a function of the hydrocarbon chain length of the pyridinium ions, measured in SDS and CsDS mesophases.
versus the alkyl chain length of the pyridinium ions. It may be seen from this figure that the Sxx curves no longer cross. This figure represents the mobility of the pyridinium ions as influenced only by the different surface charge, independent of the specific effect observed in CsDS. It can be seen that, in the absence of specific interactions, the SDS surface imposes more restrictions on the motions of the pyridinium ions than the CsDS surface, in both axes, mainly due to differences in the surface charge of the micelles. Alternatively, in the absence of specific effects, the pyridinium ions should spend more time attached to the SDS micelle than to the CsDS micelle. Acknowledgment. The authors are pleased to acknowledge financial assistance from Universidad de Chile, Grant DTI Q3545-9633. We are also very pleased to acknowledge Dr. Alan S. Tracey for invaluable suggestions and discussions. LA960320P
