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Langmuir 1994,10,2064-2067
Electrochemical Detection of Anisotropic Probe Diffusion in the Liquid Crystalline Cesium PentadecafluorooctanoatelDZO System Timothy A. Postlethwaite, Edward T. Samulski,* and Royce W. Murray* Kenan and Venable Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received February 25, 1994. I n Final Form: May 16, 1994@ Anisotropic diffusion of electroactive probes incorporated into monodomain lamellar phases of 50 w t % cesium pentadecafluoroodanoate (CsPDFO)/D20 has been detected using cyclic voltammetry. The diffusional anisotropy ratios, Dl/Dll, were 2.0 for the anion [Fe(CN)6I4-and 19.3for the positively charged (ferrocenylmethy1)trimethylammoniumion (FcMTMA+). Differences in diffusional anisotropies for these two probes are attributed to hydrophobic and Coulombic interactions between the probe species and the ordered phase. Small time scale dependencies in the diffision coefficients of [Fe(CN)d4-in the lamellar phase were observed, possibly due to a perturbation of the bulk order by the electrode surface. This letter describes a n electrochemical measurement of anisotropic diffision of electroactive probes dissolved in the liquid crystalline system cesium pentadecafluorooctanoate (CF3(CF2)6CO2Cs)/&O(abbreviatedCsPDFO/ D2O) in its lamellar state. The electroactive probes are [Fe(CN)614- and the (ferrocenylmethy1)trimethylammonium ion ( C P F ~ C ~ C H ~ N ( C H abbreviated ~)~+, FcMTMA+) which on a charge basis should be repelled from and attracted to, respectively, the ordered, anionic CsPDFO lamellae. The detailed phase diagram of the CsPDFOD20 system determined by Boden and co-workers' shows that the system exists below its critical micelle concentration (cmc) as a simple aqueous amphiphile solution and above its cmc as disk-shaped micelles. The micellar solution exhibits two lyotropic liquid crystalline phases, the discotic nematic phase (at lower concentrations and higher temperatures) and the lamellar phase (at higher concentrations and lower temperatures). In the nematic phase, the positive diamagnetic susceptibility anisotropy of the discotic micelles aligns their minor axes along the applied magnetic fie1d;'s this magnetically induced order is retained as the nematic phase changes to the lamellar phase upon cooling. The exact nature of the lamellar phase remains a point of inquiry: Some evidence indicates that to form the lamellar phase the discotic micelles retain their identity and condense into planes of disks separated by layers ~ f w a t e r . ~Other - ~ studies suggest that the disks lose their identity and fuse into infinite bilayers separated by In this latter model, it is thought that the hydrophobic lamellae are perforated with defects, through which the water phase is continuous. Ionic condu~tivit+~ and N M R technique^'^-'^ have been applied to study transport in the isotropic and liquid
* Abstract published in Advance A C S Abstracts, June 15,1994.
(1)Boden, N.; Come, S. A.; Jolley, K W. J.Phys. Chem. 1987,91, 4092. (2)Boden, N.; McMullen, K J.;Holmes, M. C.; Tiddy, G. J. T. Springer Ser. Chem. Phys. 1980,1I,299. (3)(a)Boden, N.; Come, S. A.; Jolley, K. W. Chem. Phys. Lett. 1984, 105,99.(b)Boden, N.; Parker, D.; Jolley, K W. Mol. Cryst. Liq.Cryst. 1987,152,121. (4)Holmes, M.C.; Reynolds, D. J.;Boden, N. J.Phys. Chem. 1987, 91.5257. - - I
(5)Boden, N.; Come, S. A.; Holmes, M. C.; Jackson, P. H.; Parker, D.; Jolley, K. W. J.Phys. (Paris) 1986,47,2135. (6)Boden, N.; Jolley, K. W. Phys. Rev. A 1992,45, 8751. (7) Photinos, P.; Saupe, A. Phys. Rev. A 1990,41,954. (8) (a) haver, M. S.; Holmes, M. C. J . Phys. 11 1993,3, 105. (b) Holmes, M.C.; Smith, A. M.; h a v e r , M. S. J. Phys. 11 1993,3,1357. (9)(a) Photinos, P. J.; Saupe, A. J.Chem. Phys. 1986,84, 517. (b) Photinos, P. J.; Saupe, A. J . Chem. Phys. 1986,85, 7467.
crystalline phases of CsPDFOD20 and similar aqueous lamellar systems. Results show that transport in the lamellar phase is anisotropic, being more facile perpendicular to the macroscopic director (Le., parallel to the lamellae). There have been few reports of electrochemical measurements in liquid crystalline phases. Abruiia et al.,14 in studies of liquid crystal solvents for voltammetric measurements, encountered low ionic conductivities, but did find a small diffusional anisotropy of electrochemical probes in a parallel, surface-oriented, thermotropic nematic liquid crystal us the isotropic phase. Torres and FOX'^ noted small anisotropies in the conductivity of polypyrrolefilms electrodeposited from a surface-oriented thermotropic nematic liquid crystal. Morris and Osteryoung16 used pulse voltammetry to detect large changes in proton diffusion coefficients a t the order-to-disorder transitions in ordered polymer latex solutions. The present study sought to establish the feasibility of voltammetric measurements in lyotropic liquid crystalline media and thereby of detecting diffusional anisotropies in monodomain samples. The CsPDFO/D20 system was selected as a relatively well characterized system with high ionic conductivity3 and as being readily magnetically ordered into monodomain samples.
Experimental Section CsPDFO was prepared by neutralizing pentadecafluorooctanoicacid (Aldrich)with aqueous CsOH. The neutral solution was lyophilized, and the resulting off-white solid recrystallized three times from absolute ethanol to yield a glossy-white crystalline solid. (Ferrocenylmethy1)trimethylammoniumperchlorate [(FcMTMA)C104]was prepared by metathesizing (ferrocenylmethy1)trimethylammoniumiodide (Strem)with LiC104 in methanol followed by recrystallization from CHzClz upon the addition of diethyl ether (Warning: caution should be exercised in handling perchlorate salts as they are potentially explosive). D2O (Aldrich,99.9 atom % D),[Ru(NH&]C13 (Strem), and &Fe(CN)&HzO (Mallincrodt) were used as received. Liquid (10)Holmes, M. C.; Sotta, P.; Hendriks, Y.; Deloche, B. J. Phys. 11 1993. 3 . 1735. _.__ 7 - 7
~
(11)Chidichimo, G.;Coppola, L.; LaMesa, C.; Ranieri, G. A.; Saupe, A. Chem. Phys. Lett. 1988,145,85. (12)Tiddy, G. J. T. J . Chem. SOC., Faraday Trans. 1 1977,73,1731. (13)Ukleja, P.; Chidichimo, G.;Photinos, P.Liq. Cryst. 1991,9,359. ( 14)Serra, A.M.; Mariani, R. D.; Abrwia, H. D. J.Electrochem. SOC. 1986,133,2226. (15)Torres, W.; Fox, M. A. Chem. Mater. 1992,4,583. (16)Morris, S.E.; Osteryoung, J. G. InElectrochemistry in Colloids and Dispersions; Mackay, R. A., Texter, J.,Eds.; VCH: New York, 1992; Chapter 19.
0743-7463/94/2410-2064$04.50/00 1994 American Chemical Society
Langmuir, VoZ. 10, No. 7, 1994 2065
Letters
crystalline samplescreated by stirring the appropriate amounts of CsPDFO (50 w t %) and D2O in air-tight vials at cu. 60 "C (isotropicphase)weredegassedby stirringunder a D20-saturated nitrogen blanket at room temperature. Electrochemicalmeasurements were canied out with a Pine RDE4 potentiostat. Diffusional anisotropy was detected with an electrochemicalcell with twoAu workingelectrodespositioned at right angles to one another, vertically within a 1x 1cm cuvette cell. F't foil auxiliary and Ag wire pseudoreference electrodes were placed in the center of the cuvette. The Au electrodes were 150-nmAu films thermally evaporated onto oxidized Si wafers precoated with 15-nm Cr and subsequently cut into 2 x 0.8 cm rectangles. Wire contacts were made using Ag epoxyand the Au surface defined with a layer of insulating epoxy. The active areas were determined from the variation17of voltammetric peak current (z,) with potential scan rate ( u ) in solutions of [Ru(NH3)&13 in 0.1 M KCl(aq).l8 Above ca. 48 "C, the 50 w t % CsPDFOD20 solution is1 an isotropicmicellar solution (I),between 42 and 48 "C it is discotic nematic (ND),and below 42 "C it is lamellar (L). Monodomain lamellar liquid crystalline samples were obtained in the electrochemical cell by coolingthe sample, in a 1.1-Telectromagnet, through the I-ND-L transitions (generally from 60 to 30 "C at roughly 0.2 "C/min, holding for 10 min at 47 "C). The cell was placed in the magnet such that the director ofthe oriented sample was orthogonalto one workingelectrodeand parallel to the other. This configuration provides for an investigation of diffusion in two orthogonal directions in the oriented material as shown in the following measurement scheme:
DlI
400
I
I
I
200 n
i0i 2=t W
0 -200 -400 400
I
I
I
I
I
I
200 0
-200 -400 0.6
0.4
0.2
0.0
0.4
0.2
0.0
-0.2
volts vs. Ag wire Figure 1. Cyclicvoltammogramsof 2.5 mM&Fe(CN)s in 50% CsPDFO/D20 (A) at orthogonally positioned electrodes (A = 0.098 cm2)in the isotropic phase at 60 "C and (B) at electrodes positioned parallel and perpendicular to the director (see measurement scheme in the Experimental Section), respectively, in the oriented lamellar phase at 30 "C ( u = 50 mV/s). 800
800
600
600
% 3 400
400
200
200
W
ctor
1
lamellae Dll and DLare measured as noted. The cell temperature was controllableto 1"C. Voltammetry was generally performedwith the magnet off, but no differenceswere observedwith the magnet
on. Deuterium NMR spectra were recorded on a Bruker AMX300 spectrometer operating at 46.07 MHz.
Results and Discussion Deuterium NMR spectra of (1) 50%CsPDFO/D20, (2) 2.5 mM &Fe(CN)6 in 50% CsPDFO/D@, and (3) 2.5 mM (FcMTMA)C104in 50% CsPDFO/D20 confirmed that the lamellar phase was unperturbed by either of the two electrochemical probes. Each sample, heated in a 5-mm NMR tube to 60 "C (I phase) in the spectrometer, gave the expected singlet for deuterium in a n isotropic medium. Upon cooling to room temperature (L phase) in the magnetic field of the spectrometer, the singlet for each sample split into a well-resolved doublet, indicative of a monodomain ordered phase.lJg The splittings of (1)-(3) above, 795,790, and 791 Hz, respectively, are nearly the same and agree with that previously reported,' confirming that the electroactive probes do not alter the lamellar phase. (17) (a)i, = 0.4463nFA~(nF/RT)~u"20~, where n = 1,Fisthefaraday constant, A = electrode area, c = the probe concentration,and D = the diffusion coefficient. (b) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; p 218. (18) (a) For [Ru(NH&I3+in 0.1 M KC1, D = 7.1 x cm2/s. (b) Licht, S.; Cammarata, V.; Wrighton, M. S . J. Phys. Chem. 1990,94, 6133. (19) Boden, N.;Jolley, K. W.; Ukleja, P. Condens.Mutter News 1991, 1, 10.
e-
0 0.0 0.1 0.2 0.3 0.4 0.5
0 0.0 0.1 0.2 0.3 0.4 0.5
[scan rate (V/s)]'"
[scan rate (V/s)]'P
Figure 2. Potential scan rate dependence of oxidation peak currents for 2.5 mM [Fe(CN)d4- in 50% CsPDFOD20 in the (A)isotropic phase at 60 "C and (B)oriented lamellar phase at 30 "C. The currents are at electrodes parallel (0) and perpendicular (0) to the director (see measurement scheme in the Experimental Section). A = 0.098 cm2for both electrodes.
Figure 1A shows representative voltammograms of the [ F ~ ( C N ) G ] ~ couple - / ~ - in 50% CsPDFOlD20 at the two electrodes orthogonally positioned in the isotropic phase at 60 "C. As expected, the peak current densities a t the two electrodes and hence the diffision coefficient^^^ are the same in the isotropic medium. Upon cooling this sample to 30 "C (L phase) in a magnetic field, the voltammetric peak current densities at the electrodes parallel to and perpendicular to the magnetic field become different (Figure 1B). The differing peak current densities reflect a diffusional anisotropy of [Fe(CN)614-in lamellar CsPDFOlD20. These results are extended in Figure 2 with plots of i, against (potential scan rate)'". The identical slopes17in Figure 2A confirm that Dll and Dl are essentially the same for [Fe(CN)6I4-in the isotropic phase. From squares" of the slopes of the lines in Figure 2B, we find that the ratioD& = 2.0; Le., diffusion of [Fe(CN)6I4parallel to the lamellae (perpendicular to the director) is twice as rapid as that perpendicular to them. The true ratio may be higher than this (uide infra). The ratio Dl/Dllreturned to unity upon heating the sample back to 60 "C. Analogousvoltammetric results using FcMTMA+as the electroactive probe are given in Figure 3, where the Figure 3A data were taken in the isotropic phase where Dl/Dll= 0.97, and those in Figure 3B were collected in the magnetically oriented lamellar phase where the differing slopes show that Dl/Dll= 19.3. That is, diffusion of the
2066 Langmuir, Vol. 10,No.7,1994
Letters
Table 1. Comparison of "sport probe systema CS+ CsPDFO/DzO water CsPDFO/DzO NH&DF"/HzO
NH4+
Anisotropy Ratios Measured in Various Ordered Lamellar Phases
anisotropy ratio (I/ 11)"
methodb ionic conductivity
1.4-1.6 1.1-1.2 1.8-2.0 1.4-1.5 1.2-1.4 1.9-2.0 22.0 21.2-33.3 19.3
PFG NMR
ionic conductivity PFG NMR
ref 3 10 9 11 12 12
N&HDFN/H20 LiPDFOIH20d PFG NMR LiPDFO/H@ PFG NMR water CsPDFOD20 cyclic voltammetry [Fe(CN)d4this study PDFOCsPDFO/D20 PFG NMR 13 FcMTMA+ CsPDFO/D20 cyclic voltammetry this study NHJIDFN = ammonium heptadecafluorononoate,LiPDFO = lithium pentadecafluorooctanoate. * PFG NMR = pulsed field gradient NMR. Ranges given generally correspond to measurements over a range of temperatures. This system contained 9:l H20/D20. water Li+
300 h
70
I
I
240
8 .
4
180
b,
v
:E, 8
I
$ 3 1
120
O
O
O
0
ni 2 t
v
3
1L 2
0 0.0 0.1 0.2 0.3 0.4 0.5
0.0 0 . 1 0.2 0.3 0.4 0.5
[scan rate (V/s)]'P
[scan rate (V/s)]l/l
0
1
0.0 0.1 0.2 0.3 0.4 0.5
0.0 0 . 1 0.2 0 . 3 0.4 0.5
[scan rate (V/s)]1/2
[scan rate (V/s)]'/*
Figure 3. Potential scan rate dependenceof the oxidation peak current for 2.4 mM FcMTMA+in 50% CsPDFO/D20 in the (A) isotropic phase at 60 "C and (B) oriented lamellar phase at 30 "C. The currents are at electrodes parallel (0) and perpendicular (0)to the director (see measurement scheme in the Experimental Section). A = 0.095 and 0.10 cm2,respectively.
Figure 4. Potential scan rate dependence of diffusion coefficients" and their ratios, from data in Figure 2: (A) ratios in the isotropic phase ( 0 )at 60 "C and the lamellar phase (0) at 30 "C, (B) diffusion coefficients for electrodes perpendicular (0) and parallel (A) at 60 "C (I phase) and perpendicular (0) and parallel (0) to the director at 30 "C (L phase).
FcMTMA+ probe occurs ca. 20-fold faster parallel to the lamellae than perpendicular to them.20 We attribute the large anisotropy ratio for FcMTMA+ diffusion, relative to that for [Fe(CN)6I4-,to differences in interactions of the two probes with the CsPDFOD20 matrix. The transport anisotropy for [Fe(CN)614-is of the same order as observed for other probes in the same or analogous lamellar phases (the first six cases in Table 11, which are thought to reside mainly in the aqueous layers between the hydrophobic lamellae. Their small anisotropies are attributed to the approximately isotropic continuity of the water phase;3,6,9-12this continuity is caused by either spaces between the discotic micelles or defects in the otherwise continuous hydrophobic lamellae. We expect [Fe(CN),l4- to reside mainly in the aqueous phase, on the basis of its solubility in water and its electrostatic repulsion from the anionic head groups at the lamellae/water interface. This explanation is supported by the previous resultz1that [Fe(CN)6I4-has a small or zero partition coefficientbetween sodium dodecyl sulfate (SDS)micelles and the aqueous phase in micellar solutions of the anionic surfactant SDS. The large anisotropy ratio for FcMTMA+is reminiscent of that reported13for diffusion of the PDFO- amphiphiles themselves in the lamellar phase (Table 1). This result indicates that this species is strongly associated with the lamellar phase. Transport anisotropy of a species bound to a head group or residing in the hydrophobic part of the lamellar phase should be large a t least in part because transport perpendicular to the lamellae requires transiting the aqueous layers between the lamellae. FcMTMA+ is cationic and somewhat hydrophobic (its precursor, fer-
rocene, is insoluble in water). The combination of electrostatic association with the carboxylate interface (amphiphilic head groups) and a n affinity for the hydrophobic region may give FcMTMA+ a strong aninity for the hydrophobic lamellae. Other s t ~ d i e sin~ micellar ~,~~ SDS have found that the probes [Co(phen)312+,[Co(terpy)212+,and [Co(bpy)3I2+are highly partitioned into the micelles due to hydrophobic and Coulombic interactions. Ferrocene is also strongly bound to SDS micelles.% On-going studies are consistent with the above picture that FcMTMA+ is indeed more strongly associated with CsPDFO micelles than is [Fe(CN)614-. The degree of a probe's interaction with micelles can be qualitatively determined by the diminution of its diffusion coefficient upon addition of ~ u r f a c t a n t . ~ We ~ observe that the diffusion coefficient of FcMTMA+ decreases, in solutions whose compositions change from 0.1 M CsCl to 0.1 M CsPDFO, by a factor twice that observed for [Fe(CN)614-. Further studies are planned to determine probe/micelle or probdamellae binding and partitioning constants by the methods and theory outlined by R ~ s l i n gand ~ ~ ~ ~ ~ , ~ others,22,26and with other electrochemical probes of varying hydrophobicity and charge to shed light on the dependence of the anisotropyupon the probe environment. The i, us uV2 regression lines for [Fe(CN)614-in the lamellar phase of CsPDFO (in Figure 2B and in other experiments with this probe) persistently exhibit small nonzero intercepts. Such behavior is not seen in the
(20)The FCMTMA'+~+ voltammetry was slightlycomplicated by slow hydrolysis of the probe, resulting in the appearance of a second voltammetric wave at the formal potential of ferrocenemethanol, the expected hydrolysis product. This decomposition affects the accuracy of absolute diffusion coefficients for the probe, but not that of the ratio DdDii.
(21)Mandal, A.B.;Nair, B. U.; Ramaswamy, D. Langmuir 1988,4, 736.
(22)Davies, K;Hussam, A. Langmuir 1993,9,3270. (23)Kamau, G. N.; Leipert, T.; Shukla, S. S.; Rusling, J. F. J. ElectroanaZ. Chem. 1987,233,173. (24)(a) Dayalan, E.; Qutubuddin, S.; Texter, J. In Electrochemistry in ColloidsandDispersions;Mackay, R. A., Texter, J.,Eds.;V C H New York, 1992;Chapter 10. (b) Mandal, A.B. Langmuir 1993,9,1932. (c) Rusling, J. F.; Shi, C.-N.; Kumosinski, T. F. Anal. Chem. 1988,60, 1260. (25)Rusling, J. F. InEZectroanaZytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1994;Vol. 18. (26)Zana, R.; Mackay, R. A. Langmuir 1966,2,109.
Letters isotropic phase. The volume element probed in the voltammetric experimentchanges with potential scan rate; fast scan rates probe a thinner volume element next to the electrode/solutioninterface than do slow scan rates.27 The Figure 4A anisotropy ratios show that the ratio depends on the scan rate in the lamellar phase (0) but not in the isotropic phase (a),an effect persistently seen to a varying quantitative degree in experiments with this probe. Figure 4B shows that the apparent diffusion coefficients in the lamellar phase ( 0 , O ) at the two electrodes change in opposite directions with the scan rate. One possible explanation for the result in Figure 4B is that the electrode surface disrupts or lessens the degree of ordering of the lamellar phase. Such disruption would (27) (a) The diffusion time is related to the potential scan rate by t D = RT/Fu,and the diffusion length is related to this time by 1~ = (2DtIyZ.
(b) Bard, A. J.; Faulkner, L.R. Electrochemical Methods; Wiley: New York, 1980; p 129.
Langmuir, Vol. 10,No. 7, 1994 2067 yield a decreased transport anisotropy at fast potential scan rates, those scan rates probing the volume element closest to the electrode surface and thus most affected by disorder. This explanation suggests that the true anisotropy ratio (that for diffision far removed from the orderperturbing electrode surface)for [Fe(CN)6l4-may be larger than 2.0. Experiments t o investigate electrode-induced perturbations of the orientation of CsPDFO samples are underway.
Achnowledgments. This research is supported by grants from the Department of Energy and the National Science Foundation. T.A.P. acknowledges a fellowship from the Department of Education. The authors thank Julie Hutchison and Dr. C. D. Poon for help with the N M R experiments and Dr. Chris Velazquez for supplying the (FcMTMA)C104.
