J . Phys. Chem. 1992, 96, 5224-5228
5224
thanks the Deutsche Forschungsgemeinschaft for support under Grant No. Sch. 428/1-1.
References and Notes
Figure 3. Variation of the vertical tip displacement across the 4 3 X 22 Au(ll1) surface (a) before and (b) after immersion in a DMF solution containing -0.5 mM I (x = 5 ) . (c) Schematicdrawing of the proposed monolayer structure on the d 3 X 22 A u ( l l 1 ) surface.
Au surface (Figure 1) and those observed in images of films of I on the same surfaces (Figure 2a,c), we propose that the chemical reactivity of the reconstructed Au( 111) toward adsorption of I is spatially modulated by periodic electronic and/or topographic features of the substrate. The patterns observed in Figure 2a,c can thus be rationalized by assuming that I adsorbs preferentially on sites in the transition region between hexagonal close-packed (hcp) and fcc stacking regions (i.e., the corrugation lines, vide supra), Figure 3. In such a model, row pairs of molecules should be observed in the images with a pair-to-pair separation of -65 A and separation of individual rows (within a pair) of -30 A, in excellent agreement with our experimental observations. A full account of our study will be reported in the near future. Acknowledgment. This work was supported by the Office of Naval Research and the National Science Foundation. J.H.S.
(1) The synthesis and characterization of R~(bpy)~bpy(CH~),bpy~+ is described in: Schmehl, R. H.; Auerbach, R. A.; Wacholtz, W. F.; Elliott, C. M.; Freitag, F. A.; Merkert, J. W. Inorg. Chem. 1986, 25, 2440. (2) Hallmark, V. M.; Chiang, S.;Brown, J. K.; Woll, C. S.Phys. Rev. Len. 1991, 66, 48. (3) Widrig, C. A.; Alves, C. A.; Porter, M.D. J . Am. Chem. SOC.1991, 113, 2805. (4) Yau, S. L.; Vitus, C. M.; Schardt, B. J . Am. Chem. SOC.1990,112, 3677. (5) Hallmark, V. M.; Chiang, S.;Rabolt, J. F.; Swalen, J. D.; Wilson, R. T. Phys. Rev. Lett. 1987, 59, 2879. (6) Haiss, W.; Lackey, D.; Sass, J. K.; Besocke, K. H. J . Chem. Phys. 1991, 95, 2193. (7) Van Hove, M. A.; Koestner, R. J.; Stair, P. C.; Biberian, J. P.; Kesmodel, L.; Bartes, I.; Somorjai, G. Surf Sci. 1981, 103, 189. (8) Harten, U.; Laher, A. M.; Toennies, J. P.; WBII, C. S. Phys. Rev. Lett. 1985, 54, 2619. (9) Huang, K. G.; Gibbs, D.; Zehner, D. M.; Sandy, A. R.; Mochrie, S. G.J. Phys. Rev. Lett. 1991, 65, 3313. (IO) Barth, T. V.; Brune, H.; Ertl, G.; Behm, R. T. Phys. Rev. B. 1990, 42, 9307. ( 1 1 ) Chambliss, D. D.; Wilson, R. T.; Chiang, S.J . Yac. Sci. Technol. 1991, 89, 2933. (12) Hossick Schott, J.; White, H. S . Lungmuir, in press. (1 3) A Nanoscope I1 scanning tunneling microscope was used with mechanically cut Pt/Ir (70/30) tips. All images were recorded in the constant current mode at a scan rate of 8.6 Hz. (14) Au surfaces were prepared according to the procedure of Hsu and Cowley:" a 2-cm length of 99.999% 0.5-"diameter Au wire was flame cut; Au spheres (1-2" diameter) were formed by heating one end of the wire in a H2/02flame until molten. Upon cooling in Ar or air, highly reflective, optically flat facets appear on the surface. Spheres that were not further annealed typically displayed large atomically flat unreconstructed areas on the facet in STM images. Spheres annealed in a cooler H2/02flame typically exhibited large reconstructed areas. (15) Hsu, T.; Cowley, T. M. Ultramicroscopy 1983, 11, 125. (16) Gao, X.;Hamelin, A.; Weaver, M. J. J. Chem. Phys. 1991,956993, (17) Acvedo, D.; Abruila, H. D. J . Phys. Chem. 1991, 95,9590.
Self-Assembled Thiol Monolayers with Carboxylic Acid Functionality: Measuring pH-Dependent Phase Transitions with the Quartz Crystal Microbalance Juan Wang, Lynn M. Frostman, and Michael D. Ward* Department of Chemical Engineering and Materials Science, University of Minnesota, Amundson Hall, 421 Washington Ave. SE, and the Center for Interfacial Engineering, Minneapolis, Minnesota 55455 (Received: March 25, 1992; In Final Form: May 6,1992)
The resonant frequency of piezoelectric 5-MHz quartz crystal microbalances (QCMs), coated with self-assembledmonolayers prepared from HS(CH2),SCOOH,exhibited unusually large sigmoidal increases (e1200 Hz for AT-cut quartz) when the pH of aqueous solutions in contact with the monolayer was increased. The Af-pH curves for the monolayers indicated pK, values (K,= the ionization constant) and transition region widths that were significantly greater than the values for carboxylic acids in solution. The frequency shift observed during titration was essentially independent of counterion (M"+ = Na+, K+, Cs+, Ca2+)and could not be explained on the basis of simple mass changes. Double-resonator experiments with AT- and BT-cut quartz resonators suggested that an increase in tensile stress with increasing pH contributed partially to the QCM response. The major contribution to the frequency response is attributed to changes in the viscoelastic properties of the hydrodynamic layer in contact with the QCM.
Introduction Self-assembled monolayers on solid substrates provide a convenient route to the modification of surface properties, a capability that will likely have significant impact on numerous areas of fundamental and technological importance, including corrosion, lubrication, electronics, specific binding, and sensors.Iv2 We describe herein preliminary investigations with the piezoelectric quartz crystal microbalance (QCM) of the chemical and rheo-
* To whom correspondence should be addressed. 0022-365419212096-5224$03.00/0
logical properties of self-assembled monolayers with carboxylic acid functionality at the monolayer-water interface. Specifically, pH-dependent phase transitions are manifested in unusually large changes in QCM resonant frequency, and the ionization constants of the immobilized monolayers can be determined conveniently. The QCM response is attributed to stress effects in the monolayer and significant changes in the viscoelasticity of the hydrodynamic layer in contact with the quartz resonator. The results demonstrate that large frequency changes can accompany interfacial processes on the QCM even though the actual mass changes are negligible. 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5225
Let ters
1600 1200
1n t
I
1600 I
1
/
800
E.
’
400
3
0
-400 2
4
6
8
10
12
14
PH Figure 1. Dependence of resonant frequency on pH for 5-MHz AT-cut (m) and 5-MHz BT-cut (0)QCMs modified with monolayers prepared from HS(CH2)&OOH, a bare AT-cut QCM (A),an AT-cut QCM modified with monolayers prepared from HSCH2CH2COOH(0),and an AT-cut QCM modified with monolayers prepared from HS(CH2)15CH, (A). The titration curve was collected by incrementally increasing the pH from pH = 3 (established by addition of sodium phosphate buffer; final [Na+] = 0.7 M) with 0.1 M NaOH. Identical behavior is observed in the opposite direction.
Results and Discussion The QCM comprises a thin piezoelectric quartz crystal sandwiched between two gold electrodes that provide an alternating electric field which induces shear vibration of the quartz crystal at very high freq~ency.~ If rigid layer behavior and no slip at the resonator-fluid boundary are assumed, the series resonant frequency (r,) can be used to measure mass changes on the surface of the QCM according to eq 1: where Af is the measured frequency shift, f o the initial (resonant) frequency of the quartz crystal, Am the mass change, A the piezoelectrically active area defined by two gold excitation electrodes, pq the density of quartz (2.648 g cm-I), and pq the shear modulus (2.947 X 10” dyn cm-2 for AT-cut quartz). Interfacial slip, film modulus, and viscosity also play significant roles in measurements made with QCMs and other piezoelectric transducer^.^ The QCM resonant frequency can be determined by inserting the crystal into the feedback loop of a broad-band R F amplifier or by connecting the crystal to an impedance analyzer.6 Impedance analysis conveniently provides measurement of the frequency of maximum conductance, fGmanr which corresponds closely to fr, while also providing insight into energy dissipation and coupling of the resonator to the fluid in which it is submerged.
Self-assembled monolayers were prepared on the gold electrodes of the QCM by immersing the QCM in ethanol solutions of HS(CH2)15COOH.7One side of the QCM was then immersed in 1 mL of an aqueous solution adjusted to pH = 3 with sodium phosphate buffer (final [Na+] = 0.7 M). When the pH of the solution in contact with the monolayer on an AT-cut 5-MHz QCM was increased incrementally with 0.1 M NaOH, a sigmoidal increase in f, exceeding 1200 Hz accompanied titration of the monolayer (Figure 1). The behavior was reversible, and no hysteresis was evident on the time scale of our measurement (>2 min) if the ionic strength was constant. The onset of the frequency increase was evident near pH = 6,and the width of the transition region was approximately 4 pH units. The ionization constant, pKa, of the monolayer estimated from the inflection point of Af-pH curve exceeded 8, significantly greater than values typically observed for carboxylic acids in solution (pKa = 4). The width of the transition region also was larger than typical values for carboxylic acids in solution (=2 pH units). The pKa determined from Af-pH curves varied slightly among different samples under identical conditions (h0.2 pH units), but the magnitude of the frequency change over the entire titration
-400 2
8 10 12 14 PH Figure 2. Dependence of resonant frequency on pH at different NaCl concentrations for 5-MHz AT-cut QCMs modified with monolayers prepared from HS(CH2)&OOH: [NaCI] = 0.15 M (0),0.50 M (0), and 1.1 M (m). The pH was established at pH = 3 by addition of a minimal amount of H3P04. 4
6
region was identical for all samples to within several percent. The pK, values and the width of the transition region decreased with increasing [Na+] in the aqueous phase, with a shift of approximately 1 pH unit for a decade change in [Na+] (Figure 2). Indeed, for very low [Na+] pKa > 9. (In order to perform titrations a t low [Na+], the pH was adjusted to pH = 3 with a minimal amount of H,PO,.) The QCM frequency did not change when identical experiments were performed with bare gold electrodes or electrodes coated with monolayers prepared from HS(CH2)&H3, establishing the participation of the carboxylic acid functionality in the observed behavior. However, identical experiments performed with monolayers prepared from HSCH2CH2COOH gave only a slight decrease in the QCM resonant frequency. These experiments establish that changes in the density and viscosity of the solution due to added electrolyte during titration were not responsible for the observed behavior. The features of the Af-pH curves for the HS(CH2)&OOH monolayer are reminiscent of phase transition behavior reported for fatty acid monolayers spread on aqueous subphases, which exhibit sigmoidal dependences of surface potential and area on the pH of the subphase.6 Contact angle titrations of carboxylic acid functionalized polyethylene and thiol monolayers reveal related b e h a ~ i o r .The ~ high pKa values and large transition width can be attributed to the low dielectric constant of the monolayer compared to the bulk aqueous phase,I0 hydrogen-bonding stabilization of the acid form of the monolayer,” nearest-neighbor electrostatic interactions between emerging carboxylate ions, and surface potential terms.I2 The [Na+] dependence implicates the contribution from the increasing surface potential to both the pKa and transition width. However, the onset of the frequency increase at pH = 6 was essentially independent of ionic strength, indicating that the onset is not influenced by surface potential field effects. The rather large frequency change accompanying titration of the HS(CH2)&OOH monolayer was particularly surprising. The formation of carboxylate anions and subsequent metal ion complexation a t the film/water interface during excursions to high pH should result in a net decrease in frequency, rather than the experimentally observed increase. The expected Af for Na+ binding at the monolayer is only -1 Hz, based on an area of 25 A2 per thiol molecule. Titration with KOH or CsOH gave identical Af-pH curves, although titrations with CaOH gave slightly smaller frequency shifts (1 100 Hz) and slightly lower pKa values. Simple mass changes at the interface therefore do not account for the observed frequency changes. This suggests that changes in the viscoelasticity of the hydrodynamic layer (which includes the monolayer and associated hydration layers) in contact with the resonator are responsible for the observed frequency shifts. These changes can be expressed in terms of the loss tangent (tan A) given by eq 2, where 27rf7 is the surface loss modulus, 9 the tan A = 27rf7/t (2) surface longitudinal or shear viscosity, and t the surface longitudinal elasticity or storage modulus. The frequency increase upon
5226 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992
Letters
SCHEME I
bulk water
bulk water
layer
I &OH-
!!At
w
H+ I
I
AS = (KAT- KBT)-l[tqATAfAT/fiAT - tqBTAfBT/fiBT] (3) AT-cut and BT-cut quartz crystals, respectively, gave AS = +5.5 X lo5dyn The positive sign indicates increasing tensile stress in the film with increasing pH or increasing compressive stress with decreasing pH. We also have observed very small increases ( 10, which would suggest a decrease in t, but this contribution does not appear to parallel the data in Figure 1. The stress change appearing in the film during titration is consistent with the behavior observed for fatty acid monolayers on aqueous subphases, which undergo film expansion (Le., increases in molecular area) upon deprotonation due to electrostatic repulsion between neighboring carboxylate anions and loss of hydrogen bonding between acid groups a t the interface. Stress in the self-assembled monolayers, however, cannot be relieved by expansion owing to immobilization of the chains on the surface via the strong goldsulfur bonds. The 550-Hz increase in frequency of the BT-cut resonator with increasing pH indicated that the stress change determined in this manner was only a minor component of the frequency response, contributing only 240 Hz to the total frequency change observed on the AT-cut resonator. After accounting for stress effects and the negligible mass changes, a 960-Hz contribution to the total frequency change remains. Impedance analysis indicated a small decrease in the equivalent inductance (L)with increasing pH, consistent with a decrease in the coupling of the QCM vibrations with the bulk fluid (Figure 3).15 Notably, a significant decrease in the equivalent resistance ( R ) from nearly 4500 0 at low pH to 1000 at high pH was observed, corresponding to decreasing energy dissipation with increasing pH and suggesting a decrease in the viscosity of the hydrodynamic layer. The R values in water of a bare QCM (700 a) and a QCM coated with HS(CH2)&H3 (700a) were constant over the entire pH range. It is evident that R for the HS(CH2)15COOHmonolayer at low pH is exceptionally large compared to typical values, suggesting that viscosity effects dominate the observed behavior.
I
I
quartz
quartz excursions to high pH is consistent with either an increase in t (resulting in a higher acoustic velocity and a corresponding higher frequency) or a decrease in 7. A recent report describing stearic acid monolayers suggested decreasing values of 2~f7,and increasing values o f t , with increasing pH.13 Changes in t were implicated by experiments performed with 5-MHz BT-cut quartz resonators, which gave the same results qualitatively but with a total Af of only 550 Hz. The stress coefficients for the AT and BT orientations are nearly identical in magnitude but opposite in sign (KAT= 2.75 X cm2 dyn-I; KBT= -2.65 X 10-l2cm2 dyn-I). The change in stress, AS, was evaluated using a "double-resonator" technique in which the frequency shifts for the same interfacial process on both resonators are compared; in the absence of other factors, stress changes will cause changes infi of identical magnitudes but opposite sign for the two orientation^.'^ Calculation of AS upon titration from pH = 3 to 12 with eq 3, where tqAT and tqBTare the thickness of the
k
-
I
5000
1(
- 4000 a
0.056
3000
- \/
1
t 1000
\\ "
2
4
4
'
'
6
'
8
"
10
12
0.054
7
0.052
I 0.048 14
PH
Figure 3. Dependence of the equivalent inductance ( L ) and resistance (R) on pH for AT-cut QCMs modified with monolayers prepared from HS(CH2)&OOH monolayers. The inductance reflects the inertial mass coupled to the resonator and the resistance reflects energy dissipation of the acoustic shear wave. The viscosity of the hydrodynamic layer depends on several factors, including the bulk viscosity, interfacial slip associated with interaction of the monolayer with the solvent and water drag by hydrated molecules at the interface, hydrophobic interactions between the alkyl chains in the monolayer, the degree of hydrogen bonding in the monolayer, and the order of the hydrodynamic In a viscous fluid, the amplitude of the shear wave parallel to the surface of the QCM decreases with distance from the resonator as an exponentially damped cosine function with the decay length (6) depending upon the frequency of the resonator and the density ( p L ) and viscosity (qL) of the liquid according to eq 4.18 The decay length defines the hydrodynamic layer, whose 6 = (~L/T~oPL)~"
(4)
thickness in water is approximately 2500 A for& = 5 MHz. Equation 5 describes the frequency change expected for changes
in the effective viscosity of the hydrodynamic layer under the assumption of no slip at the resonator-liquid boundary, where pLo and qLo are the initial density and viscosity, respectively. For example, when one side of a 5-MHz resonator is transferred from air (where pLoqLo is negligible) to water (qL = 1.0 X lo-* g cm-l S-I), a frequency decrease of 714 Hz is observed. The changes in the viscosity and density of the solution during titration are negligible, as evident from the control experiments with bare gold electrodes and electrodes covered with monolayers prepared from HS(CH2)&H3. Therefore, changes in the viscosity and density of the bulk solution are not responsible for this behavior. Contact angle titrations indicated lower contact angles at high pH, suggesting an increase in the viscosity at the monolayepliquid interface and a corresponding decrease in slipI9 with increasing pH. These effects would give a frequency decrease
The Journal of Physical Chemistry, Vol, 96, No. 13, 1992 5227
Letters
by reactive spreading and contact angle hysteresis that can inwith increasing pH, contrary to observation. fluence contact angle measurements. We are currently examining An alternative explanation for the 960-Hz contribution involves the effects of temperature, resonant frequency, monolayer comphase transitions that are conceptually similar to crystallineglassy position, and chain length on the frequency response in order to transitions, in which the viscosity of the hydrodynamic layer at gain insight into the contributions of longitudinal viscosity and low pH is significantly greater than at high pH. For purposes storage modulus. The unusual behavior described here demonof illustration, one may consider a low-viscosity interfacial region strates that the resonant frequency of the QCM can be affected at high pH in which complete slip occurs, either within the significantly by factors other than mass changes at the interface. monolayer or at a slip plane above the monolayel-liquid interface. It therefore is evident that caution should be exercised when Based on eq 5 , in this case the observed frequency change could interpreting frequency changes for thin films, particularly during be explained by a no-slip condition at low pH with an effective processes that may involve ionization at the resonator/fluid inviscosity of qL =2.0 X g cm-' s-', approximately twice the terface or stress changes in the films. value for water. The hydrodynamic layer defined by the decay length of the shear wave for this viscosity (3700 A) would require Acknowledgment. We gratefully acknowledge financial support substantial long-range ordering of the aqueous phase at low pH. from the National Science Foundation (NSF/CTS-9111000 and It can be shown that the frequency change corresponding to this NSF/DMR-9107179). L.M.F. acknowledges support from a effective qL value also can be described in terms of formation of National Science Foundation Graduate Fellowship. a 500-A-thick rigid transition layer between the resonator and bulk water (Scheme I), in which the shear wave propagates References and Notes without loss (6 = m). (1) (a) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; The equivalent R value at high pH for the HS(CH2)15COOH Garoff, S.; Israelachvilli, J.; McCarthy, T. J.; Murray, R. W.; Pease, R. F.; monolayer was similar to that of a bare QCM in water. In Rabolt, J. F.; Wynne, K. J.; Yu, H. Lungmuir 1987, 3, 932. (b) Bregman, J. I. Corrosion Inhibitors; MacMillan: New York, 1983; Chapter 5. (c) addition, upon immersion of the QCM treated with HS(CBowden, F. P.; Tabor, D. The Friction and Lubricotion of Solids; Oxford H2)15COOHin an aqueous solution of pH = 12 the frequency University Press: London, 1968; Part 11, Chapter 19. (d) Zisman, W. A. decreased by -865 Hz, indicating viscous coupling at high pH that Friction and Wear; Davis, R., Ed.; Elsevier: New York, 1959. is slightly greater (apparent 1) 1.4 X g cm-l s-') than that (2) (a) Blackman, L. C. F.; Dewar, M. J. S. J. Chem. Soc. 1957, 162. (b) Whitesides, G. M.; Laibinis, P. E. Lungmuir 1990, 6, 87-96. (c) Chidsey, observed for a bare QCM and arguing against the presence of C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691. (d) Laibinis, P. E.; complete slip discussed in the preceding paragraph. If this degree Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. of viscous coupling at high pH is assumed and the interfacial slip J. Am. Chem. SOC.1991, 113, 7152. (e) Porter, M. D.; Bright, T. B.; Allara, does not change during titration, the 960-Hz frequency change D. L.; Chidsey, C. E. D. J . Am. Chem. SOC.1987,109,3559. (f) Arduengo, A. J. 111; Moran, J. R.; Rodriguez-Parada, J.; Ward, M. D. J. Am. Chem. represents an effective viscosity of the hydrodynamic layer at low SOC.1990. 112. 6153-6154. le) Rubinstein. I.: Steinbere. S.: Tor. Y.: pH of vL = 6.2 X g cm-I s-l (6 6000 A). Under these Shanzer, A,;Sa&, J. Nature 1 9 6 , 332,426-429.' (h) Duevel, R.' V.; Corn; conditions, the frequency change also can be described in terms R. M. Anal. Chem. 1992,64, 337. of a 1900-A-thick rigid transition layer between the resonator and (3) Ward, M. D.; Buttry, D. A. Science 1990, 249, 1000. (4) Sauerbrey, G. Z . Phys. (Munich) 1959, 155, 206. bulk water. We note that the frequency shifts were identical for (5) (a) Grate, J. W.; Wenzel, S.W.; White, R. M. Anal. Chem. 1992, 64, all monovalent cations and only slightly smaller frequency shifts 413. (b) Martin, S.J.; Frye, G. C. Appl. Phys. Lett. 1990, 57, 1867. (c) were observed for Ca2+,although larger viscosities for Langmuir Wohltjen, H. Sens. Actuators 1984, 5, 307. (d) Duncan-Hewitt, W. C.; monolayers with Ca2+in the subphase have been r e p ~ r t e d . ' ~ * ' ' . ~ ~ Thompson, M. Anal. Chem. 1992, 64, 94. (d) Grate, J. W.; Klusty, M.; McGill, R. A,; Abraham, M. H.; Whiting, G.; Andonian-Haftvan, J. Anal. This further suggests that the observed behavior was dominated Chem. 1992, 64, 610. (e) Ricco, A. J.; Martin, S. J. Appl. Phys. Lett. 1987, by the surface viscosity at low pH, where metal ions may be 50, 1474. (f) Hager, H. E. Chem. Eng. Commun. 1986,43,25. (g) Reed, expected to play a less important role in the interfacial structure. C. E.; Kanazawa, K. K.; Kaufman, J. H. J. Appl. Phys. 1990, 68, 1993. (6) For a description of impedance analysis in QCM applications, see: (a) These apparent viscosity changes and the corresponding tranButtry, D. A.; Ward, M. D. Chem. Rev., in press. (b) Muramatsu, H.; sition layer thicknesses suggest ordering of the interfacial region Tamiya, E.; Karube, I. Anal. Chem. 1988,60, 2142. (c) Beck, R.; Pitterman, over length scales that are unexpectedly large. The monolayer U.; Weil, K. G. Ber. Bunsen-Ges. Phys. Chem. 1988.92, 1363. (d) Kipling, may be more ordered at low pH due to the presence of the hyA. L.; Thompson, M. Anal. Chem. 1990,62, 1514-1519. Tiean, Z.; Liehua, N.; Shouzhou, Y. J. Electroanal. Chem. Interfacial Electrochem. 1990, 293, drogen-bonding network, but its thickness is only -20 A. The 1. (e) Martin, S. J.; Granstaff, V. E.; Frye, G. C. Anal. Chem. 1991,63, 2272. interactions of interfaces with water have been suggested to result (7) (a) The QCM comprised plano- lano 1-in.-diameter quartz crystal in ordered layers of water between the interface and the bulk fluid; with 2000-A-thick Au electrodes on 200-Lthick Ti underlayers (for adhesion). in our case ordering may arise from unique hydrogen bonding of An asymmetric electrode format was used in which the electrode (0.35 cm2) facing solution was larger than the electrode (0.20 cm2) on the opposite side the aqueous transition layer to the monolayer at low PH.~'It was of the crystal. The electrodes only overlapped in the center of the resonator. reported recently that the longitudinal viscosity increased with For details see: Ward, M. D. J. Phys. Chem. 1988, 92, 2049. (b) Self-ascoverage of water molecules, based on the slip time deduced from sembled monolayers on the gold electrodes of the QCM were prepared by impedance analysis of a QCM.Z2 Estimates of the thicknesses of immersing the QCM in a 1 X lo-' M ethanol solution of HS(CH,)15COOH (1) for 24 h. After formation of the monolayers the QCM was rinsed setransition regions have ranged from a few molecular layers to quentially with ethanol and deionized water. The resulting surfaces were micron^,^^-^^ but results obtained recently with a surface forces hydrophilic by contact angle measurements, consistent with expectations for apparatus suggest that only films less than 10 molecular layers carboxylic acid monolayers. FT-IR reflectance spectra of 16-MHDA on the thick exhibit unusually high viscosities.26 gold electrode of the QCM exhibit vCd = 1723 cm-I compared to uCd = 1700 cm-' for the neat material. This has been previously attributed to Although the molecular details are not yet understood, our "sideways" hydrogen bonding in the two-dimensional plane in other thiol results clearly indicate significant changes in the viscoelasticity, monolayers containing carboxylic acid functionality: Nuzzo, R. G.; DuBois, and therefore the structure, of the hydrodynamic layer that L. H.; Allara, D. L. J. Am. Chem. SOC.1990, 112, 558. Ihs, A,; Liedberg, contains the monolayer and the aqueous solution associated with B. J. Colloid Interface Sci. 1991, 144, 282-292. (8) (a) Schulman, J. H.; Hughes, A. H. Proc. R. Soc. London 1932, AZ38, the monolayer. The absence of pH-driven frequency changes for 430-450. (b) Schulman, J. H.; Rideal, E. K. Proc. R. SOC.London 1930, the short-chain monolayer of HSCH2CH2COOHsuggests that A130.284-294. (c) Peters, R. A. Proc. R. Soc. London 1931, A133, 140-154. the behavior is sensitive to the thickness, order, and structural (d) Caspers, J.; Goormaghtigh, E.; Ferreira, J.; Brasseur, R.; Vandenbranden, features of the monolayer. Phase transitions in the monolayer M.; Ruvsschaert, J.-M. J. Colloid Interface Sci. 1983, 91, 546-551. (e) Patil, G. S.;Matthews, R. H.; Cornwell, D. G. Ado. Chem. Ser. 1975, No. 144, are not unexpected, having been noted previously for thiol-based 44-66. molecular monolayers and fatty acids on aqueous sub phase^.^' (9) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370-1378. Indeed, temperature-dependent phase transitions in multiple-layer (10) Whitesides, G. M.; Biebuyck, H. A.; Folkers, J. P.; Prime, K. L. J. LB films have been reported to dramatically affect the QCM Adhesion Sci. Technol. 1991, 5. 57. ( 1 1 ) Mille, M. J. Colloid Interface Sci. 1981, 81, 169. resonant frequency.28 (12) (a) Bagg, J.; Haber, A. H. Proc. R. SOC.London 1932, A138, 430. Based on these preliminary investigations, the quartz crystal (b) Betts, J. J.; Pethica, B. A. Trons. Faraday SOC.1956, 52, 1581. microbalance is a convenient device for determination of the pK, ( 1 3 ) Yazdanian, M.; Yu, H.; Zografi, G.; Kim, M. W. Lungmuir 1992, values of certain monolayers. This approach is not compromised 8, 630.
-
-
5228
J . Phys. Chem. 1992, 96, 5228-5231
(14) (a) EerNisse, E. P. J . Appl. Phys. 1973, 44, 4482-4485. (b) EerNisse, E. P. J . Appl. Phys. 1972,43, 1330-1337. (c) Cheek, G. T.; OGrady, W. E. J . Elecrroanal. Chem. 1990, 277, 341. (15) Impedance analysis was performed with a Hewlett-Packard 4194A impedancc/gain-phase analyzer capable of performing measurements over a frequency range of 100 Hz-40 MHz in the impedance mode. Data collection was accomplished via an HPIB interface with a Macintosh personal computer. (16) Colacicco, G.;Buckelew, A. R., Jr.; Scarpelli, E. M. J . Colloid Interface Sci. 1974, 46, 147. (17) Colacicco, G.; Basu, M. K.; Littman, J.; Scarpelli, E. M. Adu. Chem. Ser. 1975, No. 144, 239. (18) (a) Kanazawa, K. K.; Gordon, J. G.,I1 Anal. Chem. 1985, 57, 1770-1771. (b) Kanazawa, K. K.; Gordon, J. G., I1 Anal. Chim. Acta 1985, 175, 99-105. (19) (a) Thompson, M.; Arthur, C. L.; Dhaliwal, G.K. Anal. Chem. 1986, 58, 1206-1209. (b) Rajakovic, L. V.; Cavic-Vlasak, B. A.; Ghaemmaghami, V.; Kallury, K. M. R.; Kipling, A. L.; Thompson, M. Anal. Chem. 1991.63, 615-621. (c) Kipling, A. L.; Thompson, M. Anal. Chem. 1990, 62, 1514-1519. (d) Khurana, A. Phys. Today 1988,41, :7. (e) Krim, J.; Widom, A. Phys. Rev. 1988, 838, 12184.
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Steady-State and Time-Resolved Direct Detection EPR Spectra of Fullerene Triplets in Liquid Solution and Glassy Matrices. Evidence for a Dynamic Jahn-Teller Effect in Triplet Ceot Gerhard L. Gloss,*** Pennathur Gautam,*+sDaisy Zhang,* Department of Chemistry, The University of Chicago, Chicago, Illinois 60637
Paul J. Krusic,* Steven A. Hill, and Edel Wasserman* Central Research and Development, E . I. du Pont de Nemours & Co., Wilmington, Delaware 19 90-0328 (Received: April 7, 1992; In Final Form: May 4, 1992) UV irradiation of methylcyclohexane solutions of C, produces a very narrow, transient EPR absorption which is assigned to the first excited triplet state of Cm The line width of only 0.14 G, uncommon for motionally narrowed triplet EPR spectra, is attributed to a very rapid interchange of the magnetic axes by pseudorotation converting the degenerate Jahn-Teller states into each other. Time-resolved, direct-absorption EPR measurements with a time resolution of 0.5 ps support this conclusion. They indicate that in solution the triplet EPR absorption decays at rates comparable with those obtained by optical methods for )Cb0. A relaxation time TIof 8 ps was obtained from the oscillations observed in the early stages of the decay curve and s, too following laser excitation. This T , , and the line width in solution, require correlation times between short for rotation. Polarized, partially averaged powder triplet spectra were also observed in methylcyclohexane glasses at low temperatures. The pseudorotation proposal is supported by the distinctly different behavior of C70.
Among the many spectroscopic measurements made recently on CU and other fullerenes,l there has been the detection of the EPR spectra of the lowest triplet states of C , (3C,) and c70 (3C70) in rigid matrices at 5 K.2 The spectrum of 3C, had nonvanishing zero-field splitting parameters indicating the loss of spherical symmetry in the triplet state as is expected from the Jahn-Teller distortions in the excited states of C,.2c In this communication we wish to report the CW and time-resolved EPR spectra of the triplet states of CU and CT0in liquid solution and in glasses at different temperatures. When a degassed and saturated solution of C60in methylcyclohexane is irradiated inside the cavity of an EPR spectrometer with a xenon arc lamp a t temperatures between 300 and 180 K, a very sharp (0.14 G) line is observed at g = 2.001 35 (Figure 1A). As shown in the inset, the line width does not change appreciably from room temperature to 200 K. Below 180 K the line begins to broaden and can no longer be detected in a conventional EPR experiment below 145 K. This signal decays rapidly
'du Pont Contribution No. 6222. 'Deceased, May 24, 1992. f Present address: Center for Fast Kinetics Research, University of Texas at Austin, Austin, TX 78712.
when the light is extinguished and can be observed repeatedly without loss of intensity, indicating the absence of efficient photochemical changes. A possible candidate for the carrier of the spectrum is the lowest triplet state of Cm. Optical studies reported by Foote and collaborators3 and corroborated by others4 have determined lifetimes of 40 ps and longer for the triplet state. To obtain evidence that the EPR signal originates from the triplet state, time-resolved EPR experiments were carried out using the direct detection method with a time resolution of 0.5 ps.5 In these experiments the carrier of the EPR signal is generated by pulses from an excimer laser with a wavelength of 308 nm and width of 12 ns fwhm. The laser repetition rate is set at 80 Hz, and the magnetic field is swept a t 5 G/min. Using a boxcar integrator with a 100-ns gate width and a 5-ps delay between the laser pulse and the sampling gate, an absorption spectrum is obtained and is shown in Figure 1B. The line width and its gvalue are the same as in the steady-state experiment, assuring that the carrier is the same in the two different experiments. By changing the delay between the laser pulse and the sampling gate, it is possible to obtain the decay kinetics of the signal. They are displayed in Figure 2 and show complex behavior a t short times and an exponential decay after 20 ps. The time evolution of the EPR signal can be simulated reasonably well by solving the Bloch equations to which a damping term has been added to account for the slower
0022-3654/92/2096-5228%03.00/0 0 1992 American Chemical Society I
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