Langmuir 1993,9, 591-596
591
Surfactant Monolayers on Electrode Surfaces: Self-Assembly of a Viologen Derivative Having a Cholesteryl Hydrophobic Residue Jing Li and Angel E. Kaifer' Chemistry Department, University of Miami, Coral Gables, Florida 33124 Received September 14, 1992. In Final Form: November 24, 1992
The Self-assembly properties of N-[(cholesteryloxy)carbonyll-N'-ethylviologen (12+) and N-ethyl-N'octadecylviologen (22+)at the electrode surface were compared on Au in order to assess the effecta of the two different hydrophobic tails. Voltammetric data indicated that both compounds self-assemble from aqueous solution forming compact monolayers with the following saturation surface coverages: 3.7 X 10-10 mol/cm2 for 12+ and 4.2 X 10-10 mol/cm2 for 22+. The formal potential for viologen reduction ia more positive in 12+ monolayers (-0.345 V vs SSCE)than in 22+ monolayers (-0.468 V vs SSCE). The voltammetric reduction wave for interfacial 12+is much broader than that for reduction of 22+. However,the monolayers of the cholesterylderivativeseem to be less organized than those made from the octadecyl analog, asjudged from the blocking properties of the two monolayers for the reduction of the water-soluble complex Ru(NH3)63+. These results suggest that the cholesteryl-based monolayers are less organized due to the difficulties associated with the packing of the rigid and sterically bulky cholesteryl subunits. Introduction One of the most important properties of amphiphilic molecules is their tendency to accumulate spontaneously at interfaces, giving rise to organized monolayer or multilayer assemblies. This property has been extensively utilized in electrochemistry,especially during the last few years, to control the molecular architecture of the electrode-solution interface. Among many different approaches, a particularly simple one consists of exposing a clean and smooth electrode surface to a dilute aqueous solution of an amphiphile to drive its self-assembly at the electrode-solution If the amphiphilecontains covalently attached electroactive residues, the formation and structure of the interfacial assembly can be assessed using electrochemicaltechniques. The advantages derived from the inherent simplicity of this approach, which does not require sulfur or silicon functionalization of the amphiphiles and is relatively insensitive to the electrode material utilized, are at least partially offset by the dynamic nature of the self-assembled structures, which results in their dissolution if the amphiphilicmonomer is not present in the contacting solution. The literature contains reports on the self-assembly of amphiphiles containing viologen,' ferrocene? and other electroactive Thus far, all the amphiphiles investigated had long alkyl tails as hydrophobicresidues. (1) (a) Gomez, M.; Li, J.; Kaifer, A. E. Langmuir 1991, 7, 1797. (b) Bee, I. T.; Huang, H.; Yeager, E. B.; Scherson, D. A. Langmuir 1991,7, 1558. (c) Cotton, T.M.; Kim, J. H.; Uphaus, R. A. Microchem. J. 1990, 42,44. (d) Widrig, C. A.; Majda, M. Langmuir 1989,5,689. (e) Dim, A.; Kaifer, A. E. J. Electroanal. Chem. 1988,249, 333. (0Lee, K. A. B.; M o m , R.; Mclennan, G.; Finklea,H. 0.J. Electroanal. Chem. 1988,246, 217. (g) Finklea, H. 0.;Fedyk, J.; Schwab, J. InElectrochemica[Surjace Science; Soriaga, M.,Ed.; ACS SympoeiumSeriee378;AmericanChemical Society: Washington, DC, 1988; p 431. (h) Miller, C. J.; Majda, M. J. Am. Chem. SOC.1986,108,3118. (i) Lee, C.-W.;Bard,A. J. J.Electroanal. Chem. 1988,239,441. (2) (a) Charych, D. H.; Landau, E. M.; Majda, M. J. Am. Chem. SOC. 1991,113,3340. (b)DeLmg,H.C.;Donohue,J.J.;Buttry,D.A.Langmuir 1991,7,2196. (c) Nordyke, L.L.; Buttry, D. A. Langmuir 1991,7,380. (d) Donohue, J. J.; Buttry, D. A. Langmuir 1989,5,671. (e) Facci, J. S. Langmuir 1987, 3, 525. (3) (a) Van Galen, D. A.; Majda, M. Anal. Chem. 1988,60, 1549. (b) Gosa, C. A.; Miller, C. J.; Majda, M. J. Phys. Chem. 1988,92,1937. 14) Jain. M.K. Introduction t o Biological Membranes, 2nd ed.: John Wiley & Sons: New York, 1988. (5) Stryer,L.Biochemistry,3rded.;Freeman:NewYork, 1988;Chapter 12.
Chart I. Monolayer Packing and Balance of Cross-Sectional Areas: (A) A Monolayer with Lipophilic Tails Having Smaller Cross-sectional Area Than the Electroactive Groups, (B) A Monolayer with Good Balance between the Cross-sectional Areas of the Lipophilic Tails and the Electroactive Groups
E'* hpaphtllcIlr
/
As a result of this, there is a mismatch between the crosssectionalarea of the hydrophobic residue (a full extended alkyl tailhas a cross-sectional area of about 20 2, and the bulkier electroactive subunit (typically, 40-60 A2)in the monolayer-forming amphiphile. This disparity in the cross-sectional areas leads to loose packing of the hydrophobic taila in the monolayer assemblies and correspondingly poor barrier properties toward hydrophilic species. For instance, Majda et al.ld and our group1*have noted that the degree of packing of self-assembled monolayers formed by asymmetric alkylviologens is determined by the packing of the electroactive viologen groups as represented pictorially in Chart IA. The fundamental importance of this issue in understanding monolayer packing and electrochemical properties led ua to explore the self-assembly of electroactive amphiphilee with hydrophobicgroups having cross-sectional areas larger than that of a straight alkyl chain. Our first choice as a bulky hydrophobic residue was cholesterol. Several reasons can be advanced to support this selection. First, cholesterolhas a cross-sectional area approximately equal to 45 A294 a value similar to that found for the viologen subunit, which we selected as the electroactive residue. Second, cholesterol plays an important role in the regulation of cellular membrane fluidity in mammals.5 Third, the synthetic accessibility of cholesterylviologenderivatives seemed reasonable. Thua, we prepared the cholesterylviologen 12+ and compared ita
(0 1993 American Chemical Society 0743-7463/93/2409-0591~04.~/0
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592 Langmuir, Vol. 9, No. 2, 1993
interfacial self-assembly and electrochemical properties with those of the asymmetric alkylviologenZZ+.We report the results of this study here.
,
-0:70
000 POTENTIAL,V vs SSCE
22+
Experimental Section Materials. l-(PF&was prepared in three steps. First, the monoquatemized precursor l-ethyl-4-(4'-pyridyl)pyridinium bromide was synthesized by reacting 1.0 g of 4,4'-bipyridine (Fluka) with an excess (5 mL) of bromoethane in 10 mL of refluxing benzene. After a reaction time of 15 h the precipitated product was collectedby filtration and washed in a large volume of toluene to remove unreacted starting materials. The product was finally isolated by filtration and recrystallized from hot acetonitrile. Second,themonoquaternized product was reacted with equimolar amounts of NaI and cholesteryl chloroacetate (Aldrich) in refluxing chloroform for 24 h. The orange-brown precipitate was collected by filtration and washed several times with chloroform. This solid was identified as a 12+salt containing a mixture of C1-, Br-, and I- counterions. The pure hexafluorophosphate salt was isolated by counterion exchange with NH4PFs in water. l.(PFs)2 was dried under vacuum and characterized 1H NMR (400 MHz solvent DMSO-ds) 6 9.40 (d, 2 H), 9.30 (d, 2 H), 8.85 (d, 2 H), 8.80 (d, 2 H), 5.75 (s,2 H), 5.37 (8, 1 H), 4.70 (t,2 H), 4.60 (p, 1H), 2.40-0.80 (m, 46 H); FAB-MS (matrix = 3-nitrobenzyl alcohol, mle) M+ = 612, M2+PF6-= 757. Anal. Calcd for C41H~N202P2F12: C, 54.54; H, 6.70. Found: C, 54.44; H, 6.73. The asymmetric alkylviologen 2.(PFs)2 was prepared as previouslydescribed.'. Ru(NH3)&13 was purchased from Strem Chemicals (Newburyport, MA), and 8-cyclodextrin was a gift from the American Maize-Products Co. All solutions were freshly prepared with distilled water which was further purified with a Barnstead Nanopure four-cartridge system. Equipment. The electrochemical instrumentation has been described elsewhere." NMR spectra were recorded in a Varian VXR-400-S spectrometer. Mass spectra were obtained in a VG Trio-2 spectrometer. Procedures. Experimental protocols for the self-assembly of amphiphilicviologenmonolayerson gold surfaceswere identical to those already published.la Flame-annealed, gold beads were used as working electrodes in the electrochemical experiments and as substrates for the self-assembly of viologen amphiphiles. Roughness factors in the range 1.1-1.2 were typical for these gold surfaces as measured from the anodic stripping of a monolayerof chemisorbediodinea6Usual values for the geometric area of the gold bead electrodes were in the range 0.064.15 cm2. The concentration of the viologen derivative was adjusted by adding small aliquota of 5.0 mM viologen stock solution to the supporting electrolyte solution (10.0 mL of 0.5 M Na2S04)in the electrochemical cell. Acetonitrile and DMSO were used as the solvents to prepare the stock solutions of l.(PFd2and 2.(PF&, respectively. Control experiments indicated that the small amounts of these organicsolventsadded to the aqueous electrolyte solution do not affect the observed electrochemical properties.
Results To the best of our knowledge, lz+is the first amphiphilic viologen derivative in which lipophilicity is introduced by (6) Rodriguez,J. F.;Mebrahtu,T.;Soriaga,M.P.J.Electroanal. Chem.
1988,249,333.
-0.70
0.00 POTENTIAL, V vs SSCE
Figure 1. Voltammetric response at 0.5 VIS of a gold bead electrode immersed in 0.5 M NazSO4 also containing (A) 35 pM 12+and (B)15 pM 22+. Table 1. Voltammetric Parameters of Self-Assembled Monolayers of 12+ and 22+ on Gold at 25 O C 22'
12+ ~
E"' (V vs SSCE) hE, (mV, measured
at 500 mV/s) Efwh (mV, measured at 500 mV/s) rma(mol/cm2)
~~~~
-0.345
15
-0.458 33
210
125
(3.7& 0.2) x 10-10
(4.2i 0.1) X 10-lo
using a cholesteryl subunit.' Figure 1A shows the cyclic voltammogram (CV)recorded with a clean gold bead electrode immersed in a 35 p M solution of la+ also containing 0.5 M NaZS04 as supporting electrolyte. The set of voltammetric waves observed corresponds to the reversible reduction of the viologen subunit, e.g., the 12+/+ redox couple. Due to the very low concentration of 12+ in the solution, the small peak-to-peak potential difference, and the linearity of cathodic peak current VB scan rate plots, we conclude that the voltammogram of Figure 1A results from the adsorption of 12+on the electrodesurface. We have observed similar adsorption or interfacial selfassembly behavior with asymmetric alkylviologens.l* The voltammetric behavior observed with 12+ is thus quite significant because it indicates that the interfacial selfassembly of viologen derivatives can be fostered by functionalization with cholesteryl moieties as well as by derivatization with long alkyl chains. However, only general similarities can be drawn from the data because the voltammetric response of 12+ is different from that recorded with the octadecyl analog 22+ (see Figure 1B).A (7) Buttry and co-workershave prepared thiol-functionaliiedviologen derivatives containing choleabryl subunita as lipophilic taila: 179th Meeting of the Electrochemical Society, 1991; Abstracta 663 and 694.
Surfactant Monolayers on Electrode Surfaces
Langmuir, Vol. 9, No. 2, 1993 593
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Figure2. Electroactive surfacecoverage on gold bead electrodes as a function of the 12+ concentration in the contacting 0.5 M NazSOl solution.
comparison of relevant voltammetric parameters is presented in Table I. The reduction potential of la+is clearly less negative than that for 22+. Furthermore, the voltammetric waves are much broader for the cholesterylviologen than they are for the octadecyl analog. This is reflected by the full width potential values at half-height (Ebb)* given in the table. The shape of the la+reduction wave is clearly distorted by the presence of a "shoulder" slightly shifted to more negative potentials. This wave shape was found to be independent of the scan rate in the range 0.1-50 V/s and, thus, does not seem to be caused by kinetic effects. Since the purity of the cholesteryl derivative was high enough to pass elemental analysis, we safely discard the possibility that the shoulder could result from viologencontaining impurities in the sample. Two possible explanations for the shape of the voltammetric waves observed with la+ are (i) the presence of at least two populations of viologensites residing in different interfacial microenvironments, and (ii) the predominance of strong viologen-viologen interactions in the monolayer (perhaps related to dimerization of the cation radical forms)." The reduction potential is significantly more positive for interfacial la+than for 22+. This effect is substantial (113mV)andmaybecausedbydifferencesintheelectronic effecta exerted by the substituents on the viologen nucleus. The total charge involved in the reduction of the surfaceconfined la+(for both populations of viologen groups) can be readily determined by the integration of the reduction current wave. This charge value can be easily expressed as surface coverage (moVcmZ),takingthe geometricsurface area of the electrode as a good approximation for its real surface area, an assumption which is justified by the low roughnew factors of the gold bead electrodes used in this work. Figure 2 shows the dependence of the surface coverageof 1*+on its concentration in the adjacent solution. The graph exhibits the typical shape of an adsorption isotherm. A saturation surface coverage of (3.7 0.2) X 10-lO mol of 12+/cm2is reached at solution concentrations in the range 2630 pM. This value is slightly lower than that previously reported for 22+ [(4.2f 0.1) X 10-lomol/ cm21.la The data in Figure 2 can be treated, according to
*
(8) Murray, R. W.In Electroanalytical Chemiatry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, p 191.
l0 o
, 1 0.1
0.2
7
0.3
3
c x I O , mol/cm Figure 3. C/I'vs C Langmuir plots for (A) 1*+and (B)2*+.
the simple Langmuir adsorption model, using the following equation:
where C is the bulk concentration of viologen, I' is the experimental surface coverage, rmuis the maximum surface coverage, and K is the equilibrium constant for the adsorption process. Plots of C/r vs C are linear (see Figure 3)and yield calculated values for rmar and K from the regression lines. From the slopes we determined rmax values of (3.8f 0.1) X 10-lomol/cm2for 12+ and (4.3f 0.1) X 10-lo mol/cm2 for 22+. The calculated K values were (7.2 0.9) X 106 and (1.5 X 0.6) X 108 M-l, respectively. These values correspond to the following free energy changes for adsorption: -33.4 f 0.3 and -35.2 f 0.2 kJ/ mol for la+ and 22+, respectively. The good fit of the experimental data to eq 1 should not be taken as confirmation of the validity of the Langmuir adsorption model. This has been noted before by others.'d In fact, the adsorption of these viologen amphiphiles is not expected to adhere strictly to a Langmuir isotherm since attractive van der Waalsinteractions between neighboring lipophilic groups and repulsive electrostatic interactions between adjacent viologen subunits are likely to take place. All these results indicate that la+self-assembles at the electrde-solution interface forming a structure which seems to be a monolayer on the basis of the coverage data. Experiments with 8-cyclodextrin demonstrate that the interfacialself-assemblyof la+is driven by the lipophilicity of the cholesteryl subunits. 8-Cyclodextrin is a natural receptor known to form a stable inclusion complex with
*
Li and Kaifer
594 Langmuir, Vol. 9, No. 2, 1993
cholesterol in aqueous media.9 When millimolar concentrations of 6-cyclodextrin were added to a solution containing 35 pM 12+, the voltammetric waves correspondingto the reduction of the surface-confined viologen groups decreased substantially, indicating that It+ is removed from the electrode surface. The addition of ~-cyclodextrinat the 4 mM level causes the total disappearance of the surface-confined waves. Under these conditions, only small waves were observed at more negative potentiale than the wavea for adsorbed 12+. These waves have the characteristics of a diffusion-controlled electrochemical process and are thus assigned to the reduction of the 8-cyclodextrin complex of 12+. We conclude that cyclodextrin complexation of the cholesterylviologen derivative increases its solubility in water due to the replacement of the hydrophobic periphery of the cholesteryl moiety by the more hydrophilic periphery of the cyclodextrin receptor. Therefore, the increased aqueoussolubility is responsible for the disruption of the interfacial monolayer assembly. Similar observationswere recorded before with asymmetricalkylviologen derivatives, such as 22+, in the presence of the receptor a-cyclodextrin.la How well organized are the monolayers of 12+? This question can be addressed by investigating the monolayer blocking properties on the electrochemistryof hydrophilic species. We have shown already that Ru(NH3)s3+ is particularly appropriate for this purpose.1a The fast Ru(NH3)s3+l2+ redox couple exhibits a half-wave potential positive from that required for viologen reduction. Therefore, when the viologen monolayer is well organized and packed, the direct reduction of the Ru(II1) complex by the electrode surface is so hindered that electron transfer mediated by the monolayer viologen sites takes place, according to the following equation: Ru(NH,)r
+ V+
-
Ru(NH,)?
+ V2+
(2)
--
-0.70
-0.70
POTENTIAL, V vs SSCE
POTENTIAL, V va SSCE
I
30
0.00
On scan reversal, no clearly defined oxidation peaks are detected because the reduced viologen sites have been depleted in the reduction of the Ru(II1) species and the reduced Ru(I1) complex cannot undergo fast oxidation at the monolayer-modified electrodesurface. This is exactly what happens in the case of 22+ monolayers, as we have reported previously.1a For comparison purposesa typical voltammogram is given in Figure 4B. Note that the only clearly defined peak in the potential range surveyed is cathodic and correspondsto the process given in eq 2. The cathodic peak potential is -0.39 V w SSCE at a scan rate of 100 mV/s. The situation is different with monolayers of 12+ (see Figure 4A). The voltammogram shows only one cathodic peak, but an anodic peak is also observed. At a scan rate of 100 mV/s the cathodic and anodic peak potentiale are -0.34 and -0.20 V vs SSCE, respectively. Since the formal potential for the reversible reduction of the viologen sites is more negative for 22+ monolayers thanfor 12+ monolayers,the thermodynamicdriving force for the mediation process of eq 2 is smaller in the case of the choleateryl derivative. The onset of mediation requires that the direct electrode reduction of the Ru(1II) complex be hindered to some extent by the presence of the monolayer. Therefore, it is Wicult to extract conclusions about the relative levels of molecular organization of the two monolayers from the cathodic segments of the voltammograms in Figure 4. However, the observation of an anodic peak in the voltammogram of Figure 4A (correspondingto the monolayer-coveredelectrode) clearly
indicates that the choiesteryl monolayer is not very effective at keeping the Ru(I1) complex away from the electrode surface. Although some level of anodic current is observed on the corresponding voltammogramof Figure 4B,the absence of clearly defined peaks indicates that the 22+ monolayer is more effective at preventing the reoxidation of the Ru(I1) complex. This suggests that the interfacial monolayers formed by compound 22+ exhibit a higher degree of organization than the corresponding interfacial monolayers of 12+. The monolayers formed by the spontaneous interfacial self-assembly of these amphiphilic viologen derivatives are dynamic entities. Due to the lack of covalent attachment to the electrode surface, the monomers in the monolayer undergo fast exchange with those diseolved in the contacting solution, as it is the case, for example, in micellar solutions.1° Therefore, when monolayer-covered electrodes are transferred to solutions of pure supporting electrolyte, the electroactive surface coverage decreases as a function of time while the monolayer assembly redissolves in the contacting solution. The rate of m o m layer redissolution is dependent on the structure of the
(9) (a) Taneva, 5.; Ariga, K.; Okahata, Y.; Tagaki, W. Langmuir 1989, 5, 111. (b) Kempfle, M.; Mueller, R.; Winkler, H. Eiochim. Eiophys. Acta 1987,923,83. (c) Lach, J.; Pauli, W. J. Phorm. Sci. 1966,66, 32.
(10) Fendler, J. H. Membrane Mimetic Chemistry; John Wiley & Sons: New York, 1982; Chapter 2.
Figure4. (A) Cyclic voltammetric response (at 0.5 V/s) of agold bead electrode immersed in a 1.0mM RU(NH&~+ solution in 0.5 M NazSOl in the absence (dotted line) and in the presence (continuous line) of 35 pM l*+.(B)Similarcyclic voltammograms in the absence (dottedl i e ) and in the presence (continuousline) of 15 pM 2'+.
Surfactant Monolayers on Electrode Surfaces
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Langmuir, Vol. 9, No. 2,1993 595 a
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Figure 5. Time dependence of the fractional surface coverage measured after transferring the gold bead electrode from a solution containing (A) 35 pM 12+ + 0.5 M Na2SO4 and (B)15 pM 2*+ + 0.5 M Na2SO4 into pure 0.5 M Na2S04. The plotted fractional coverage is defined as the ratioof the coverage measured a certaintime after the transfer vs the coverage before the transfer.
amphiphileas shownby the data in Figure 5. Cholesterylbased monolayers undergo dissolution much more slowly than octadecyl-based monolayers in pure supporting electrolyte solution. The plot in Figure 5A shows that, in the case of 12+,about 90%of the initial electroactivesurface coverage remains on the electrode surface after 70 min of exposure to amphiphile-free solution. In contrast, only 16% of the initial surface coverage of 22+remains after 70 min of exposure to pure supporting electrolyte solution (see Figure 5B). Another interestingfeature of the interfacialmonolayers formed by 12+ is their high sensitivity to the presence of 0 2 in the solution. During our experiments with monolayers of 12+ we observed that particularly long and thorough protocols to remove dissolved 0 2 were needed. In the presence of even small concentrations of dissolved 0 2 the cathodic wave showed enhanced currents while the anodic currents were clearly decreased. In contrast, the monolayersof 22+ were not as extremelysensitiveto minute concentrations of dissolved 0 2 . This difference in sensitivityto 0 2 might be related to the greater hydrophobicity of the cholesteryl-based monolayers compared to those formed by the octadecyl counterparts, since a more hydrophobic environment would increase the affinity of molecular oxygen for the monolayer assembly. Discussion The voltammetric results obtained with gold bead electrodes immersed in aqueous solutions containing micromolar concentrations of either 12+ or 22+ clearly indicate that both amphiphilic viologen derivatives ag-
Figure 6. Proposed mode of docking of two 12+ ions showing a-8 face contact. The conformationof the 12+ion was minimized using the M M X field (PCMODEL software).
gregate at the electrode-solution interface, forming assemblies which, on the basis of the surface coverage data, seem to be monolayers. In these monolayer assemblies, the viologen derivatives align themselves with their main molecular axis perpendicular to the electrode surface. In principle, our voltammetric data show that cholesteryl subunits are similar to long aliphatic chains in the sense that both confer interfacial self-assembly properties to amphiphilic viologen ions. Additional experimentation with amphiphiles possessing other head groups will be necessary to confirm the generality of this conclusion. The comparisonof the voltammetricdata for monolayers of 12+ and 22+ in the presence of Ru(NH3)s3+is quite interesting because it reveals that monolayers of the cholesteryl derivative have poorer blocking properties. In principle, the cholesterylviologen derivative is expected to produce a more hydrophobic monolayer assembly than the correspondingalkylviologenderivativedue to the larger cross-sectional area of the respective lipophilic subunits. Majda et al. proposed a model for the interfacial selfassembly of N-methyl-N'-octadecylviologenon goldld in which the amphiphilic viologen ions aggregate with their main molecular axisperpendicular to the electrodesurface. The degree of packing of the resulting assembly is controlled by the packing of the relatively bulkier viologen head groups. Our group reached similar conclusionsin a systematic study of the interfacial self-assemblyof a series of asymmetric alkylviologen ions.la However, according to this model, the lipophilic region of the interfacial assembly is loosely packed (the situation is pictorially represented in Chart IA) and does not lead to the effective rejectionof water moleculesfrom the monolayer. A bulkier lipophilic subunit (Chart IB) is expected to improve the degree of packing of the lipophilic region of the assembly, thus decreasing the penetration of water molecules in the monolayer and improving its overall hydrophobic character. These ideas are further supported by the maximum surface coverage that can be reached in a monolayer of 12+ (3.7 X 10-lo mol/cm2) which is clearly lower than the corresponding value for a monolayer of the alkyl analog 22+(4.2 X 10-lomol/cm2). This suggests that the degree of molecular packing in a monolayer of 12+ is no longer controlled by the viologenhead groups. Otherwise,similar maximum surface coverage values would be obtained in both cases. In proposing a model for the interfacial self-assembly of 12+ ions, it is important to realize that cholesteryl
596 Langmuir, VoZ. 9, No. 2, 1993
B
Figure 7. PCMODEL rendition of two possible conformations for the 12+ ion obtained by rotation around the ester C-O bond.
subunits do not possess cylindrical symmetry and show specific preferences for certain docking modes. The cholesterol molecule has two well-defined faces that are usually referred to as a! and @.ll Van der Waals contacts are maximized when two cholesterol molecules approach each other in such a way that the @ face of one molecule docks on the a!face of the second molecule. If this is applied to two 12+ions, onereachesa situation likethat represented in Figure 6. Note that the docking of cholesteryl derivatives cannot be treated like the docking of cylindrical objects, an important difference with alkyl-based amphiphiles. Furthermore, note that in the structure of 12+ the angle between the viologen head group and the cholesteryl subunit i s substantially different from 180'. (11) Hauser, H.; Poupart,G. In The Structure ofBiofogicafMembranes; Yeagle, P., Ed.; CRC Press, Inc.: Boca Raton, FL, 1991; Chapter 1.
Li and Kaifer
This is also the case in 22+, but the lipophilic group is indeed much more rigid in the cholesterylderivative. Then, one can visualize conformations for 12+ in which the rigid cholesteryl subunit may hinder monolayer organization, leading to defective sites. Figure 7 presents two possible conformationsof 12+. The first one (Figure 7A) is identical to that shown in Figure 6. The second one (Figure 7B) was obtained simply by rotation around the ester C-O bond and shows clearly how relatively minor conformational changesmay alter substantially the overall molecular shape of this amphiphilic viologen ion, resulting in monolayer defects. Thus, face-to-facestacking of 12+ions, as proposed in Figure 6, is probably the main molecular assembly mode in the monolayer. We hypothesize that the lack of cylindrical symmetry of the cholesteryl group may foster one-dimensional molecular aggregation patterns in which chains of 12+ions are formed by continuous a!-@
[email protected]. These long lines of stacked viologens ions, however, cannot completely cover the two-dimensional electrode surface, and inevitably, areas connecting different chains of stacked 12+ ions may arise. In these borderline areas, which we can effectively consider monolayer defects, the viologen units are more exposed to water molecules and, thus, exhibit a more negative reduction potential than the viologen groups embedded in the wellorganized, hydrophobic chains of stacked ions. These arguments are consistent with the observed shoulder on the negative side of the reduction wave of interfacial 12+. In conclusion, we have shown that cholesteryl residues can be used as lipophilic subunits to foster the interfacial self-assembly of amphiphilic viologen ions. While the bulkier and more rigid structure of cholesterol compared to that of an aliphatic chain gives rise to more hydrophobic interfacial structures, paradoxically, the same rigidity of the cholesteryl subunit, as well as its tendency to lateral or face-to-face aggregation, produces monolayers containing a higher density of defective sites than the corresponding interfacial assemblies formed with amphiphiles based on less bulky, but more flexible, aliphatic chains. Acknowledgment. The authors are grateful to the National Science Foundation for the support of this research (Grant CHE-9000531).
