An Undergraduate Laboratory Experiment for the Direct Measurement of Monolayer-Formation Kinetics D. S. Karpovich and G. J. ~ l a n c h a r d '
Michigan State University, East Lansing, MI 48824 The study of surfaces and interfaces has become increasingly important in chemical research. The reason for this widespread interest in interfacial systems lies in their range of applicability, such a s separations, catalysis, chemical sensing, and trihology Unfortunately, the traditional undergraduate chemistry curriculum leaves students with only a brief introduction to surface and interface chemistry. We present a n undergraduate physical chemistry experiment that introduces the student to a class of chemically modified surfaces that are the focus of a great deal of interface and materials science research. We report here the use of a mass sensor to study the adsorption and desorption kinetics of self-assemhling alkanethiol monolayers on gold. The use of this simple, low cost technology and the direct information it provides on monolayer formation makes this experiment attractive for undergraduate physical chemistry labs. Alkanethiol self-assemhling monolayers are the focus of much chemistry research because they are interesting from a fundamental standpoint and are potentially useful for surface modification, chemical sensor, and molecular electronics applications (1-27). The utility of these monolayers lies in their highly ordered structure, where the predominantly all-trans alkyl chains are tilted a t about 30" with respect to the surface normal (4, 15, 18,231.The snrface properties of the monolayer can he altered significantly (3, 8, 9, 11, 12, 17, 27) by incorporating thiols with different chain lengths or different functionalities into a single monolayer. However, to tailor the synthesis of such monolayers, we must first understand the mechanism of adsorption in detail. There are a variety of means available for the examination of interface adsorption and desorption kinetics. Timeresolved vibrational spectroscopy is a potentially useful technique, hut the cost of a research-grade FTIR makes its widespread use in a n undergraduate laboratory impractical. We use, a s a n alternative, a simple mass sensor, capa'~uthorto whom correspondence should be addressed.
466
Journal of Chemical Education
hle of detecting nanogram mass changes, as the substrate for the experiment. The quartz crystal microbalance (QCM) is sensitive, in principle, to mass changes as small as several nanograms (22, 2831). The electrodes of the QCM are gold that is vapor-deposited onto a chromium underlayer. The basis of the QCM is a piezoelectric quartz crystal that oscillates a t a resonance frequency determined by several factors, including the mass of the crystal. The resonant frequency of the crystal depends on mass loading according to the Sauerhrey equation (22, 2832). This relationship holds well for gas-phase measurements, but in situ solutionphase measurements using the QCM are in many cases only semiquantitative due to capacitive and colligative effects due to the solvent (30, 31, 3337). We use the QCM here to perform in situ solution-phase kinetic measurements. Because we are concerned only with rates of mass change in this experiment, we do not need to determine the absolute mass change associated with the formation of the monolayer. We discuss below how the rate of monolayer growth can he measured and used to obtain fundamental kinetic and equilibrium properties. I n this discussion, we detail how these measurements are well-suited to an undergraduate physical chemistry laboratory. Overview of Surface Kinetics The net chemical reaction we detect is Au + RSH Z?
Au-SR
+ %Hz
This chemisorption reaction, which self-terminates a t one monolayer under the conditions used here, proceeds a t a gold surface where the thiols react with the gold to form a gold thiolate monolayer (Au-SR) (4, 7, 11,26). The Langmuir adsorption isotherm can he used to describe this adsorution reaction. The L a n m u i r isotherm is based on the assumptions that adsorption is limited to one monolayer, all surface sites are eauivalent. and adsorption to one site is independent of the bccupandy condition of the adjacent
-
This result can be simplified by substituting hobs for k,C + kd and IC for
giving
Fitting the Data
l o l ' ' ' ' l ' l ' l ' l 0
10
20
30
40
50
time (s) Figure 1. Raw data from the experimental setup. The frequency range is given in Hz, and does not show the about 6-MHz offset.The exact oscillation freauencvvaries amona QCM crvstals. The solution concentration used ior this scan was 1.60 x 104M. sites (2,38,39). Although there are defects on the gold surface (25),the resultant modulation of surface-site energies is apparently small enough that the Langmuir approximation holds. The alkyl chains do interact with each other, but this interaction is only significant for nearly complete surface coverage due to the inherently short-range nature of dispersive forces (about r-12).Thus, adsorption to one site can be considered independent of whether adjacent sites are filled for fractional coverage less than unity. The Langmuir isotherm was derived initially for physisorption, not chemisorption, but the bond strength of the alkanethiolate-gold bond is small enough that this condition is not violated seriously (vide infra). We do not know the rouehness of the gold surface a t the atomic scale, but this is not critical to our experiment. The coverage of the surface is e x ~ r e s s e da s a unitless quantity, 0, thelfraction of available sites t h a t have reGted, or equivalently, the fraction of a monolayer. Using 0, we can derive the Langmuir isotherm (2, 38, 39). The Langmuir isotherm dictates that the rate of the surface reaction is given by
where 0 is the fraction of surface covered; (1 - 0) is the fraction of surface exposed; C i s the alkanethiol concentration; and k, and kdare the association and dissociation constants, repsectively. Integration of eq 1 yields the time course of the monolayer formation.
We can use the QCM to monitor the adsorption of alkanethiols to gold. With a QCM suspended i n solution, we introduce a n alkanethiol aliquot and monitor the QCM resonant frequency change subsequent to alkanethiol introduction. The data are of the form frequency vs. time. The raw data can be fit to eq 3. For fitting the data, kOa,is independent o f K . This is an important point because absolute frequency changes vary from one run to the next due to the sensitive impedancematching conditions required by the electronics and the variation of the gold surface morphology from crystal to crystal (25, 35). Because 8 is unitless, the absolute mass chanee is not i m ~ o r t a nto t the kinetics determination. As IIIJSP rl~ange ment~onedabove, measurement.< of al~iolutc~ in liauids are comolicr~trdhv dielcctiic and VISCOUS elrects. The QCM oscillation freque"ncy in solution before thiol introduction represents a baseline, and plotting the (negative) QCM frequency change vs. time after thiol introduction yields d a t a of t h e form z W i vs.t w h e r e z i s a n arbitrary scaling factor. Evaluation of the Equilibrium Constant Fitting the experimental data to eq 3 gives k,~,,.Because hob,= h,C + kd, a plot of k,h, vs. C for a series of measurements over a range of thiol concentrations gives a line with a slope of k. and an intercept of kd, allowing evaluation of the equilibrium constant, K,,,for the monolayer formation. k
K =B eq
k,,
(4)
We obtain two important physical quantities for this system from K,,. The steady state fractional coverage of the gold surface, 0(-1, is given by
and the free energy of formation of the monolayer i s found from
Mixing Effect The example curve in Figure 1shows that the maximum initial rate of monolayer formation is not reached immediately upon injection. Several seconds may be needed to achieve the maximum adsorption rate, depending on the volume and concentration of thiol aliquot injected, due to limitations inherent in mixing of the solvent and the thiol aliquot. The aliquot-mixing time is substantially shorter than the monolayer-formation tir-e by measurement of the mixing time using a colored dye solution. The smaller the amount injected, the more time is required for thorough mixing. The "roll-on" of the maximum initial rate i s caused by the concentration gradient introduced by mixing. We can account for this mixing effect by modifying the variable t in eq 3 to t - to. Volume 72 Number 5 May 1995
467
BNC connector
stock chid solution added
frequency counter oscillator +5 M C
temperature eont,ol (293 K)
Figure 3. Detailed side view of the spring clip mount for the QCM crystal. The contacts for the QCM electrodes are electronically isolated from the spring clip. The BNC connection couples directly to the oscillator circuit.
quartz rnicmbalance with
evaporated gold electrodes (AT cuf 6 MHz)
stirrer
Fioure 2. Exoerimental setuo, iacketed and , . indicatino the OCM. the , lehperal.r&onlro ea reacl on veise , OCM moLnt, ana c r. cdltry oala processing eqL pment.
Fitting the portion of the euwe from the onset of the maximum rate through a significant portion of equilibrium yields the variable t,, which is the predicted time of injection if mixing were instantaneous. We have observed deviations from the Langmuir isotherm a t both hieh .. and low concentrations. and the limiting concentrations depend on the solven~and alkanethiol chain lenmh. These deviation are ~mderstood,nno we discuss them in detail elsewhere (40). Experimental QCM Setup
A schematic of the experimental setup is shown in Figure 2. The QCM must be suspended in solution firmly with isolated electrical connections for both gold electrodes. This can be done by modifying a common electrical spring clip (see Fig. 3). Due to the relatively strong spring pressure needea to ensure reliable electrical contact, we k e d QCM's (McCoy Electronics, part no. 78-18-4) that were snfficiently robust to avoid breakage. These crystals are AT cut with a resonance frequency of about 6 MHz. The QCM is connected via the modified snrine c l i to ~ a &MHz oscillator circuit (Maxtek part no. 154200-4; The oscillator circuit requires 5-V dc input. A frequency counter i s connected i n parallel with t h e oscillator and QCM. The freauencv counter is c a ~ a b l eof 1-Hz frequency resolution using a 0.28-s gate time. The frequencycounter requires a n interface to allow direct connection to a computer or a chart recorder. All connections, especially those associated directly with the oscillator circuit, should be made using shielded cables. Insufficient shielding can cause a baseline frequency instability due to stray RF pickup. Cables should be routed away from any possible source of ac electrical interference, 468
microbalance (viewed from side)
Journal of Chemical Education
such as the magnetic stirrer motor, temperature-controller motor, or fluorescent lights. These measurements must be carried out in a jacketed beaker connected to a temperature controller: The oscillation frequency of the QCM i s strongly temperature-dependent, and variations in ambient conditions can introduce drift to the data. The results we report were taken a t 20.0 "C. We have found that 150-mljacketed beakers work well with 100-mL total solution volume because the solutions are deep enough to allow space for a stir bar and the QCM. Stirring is accomplished with a magnetic stirrer and poly(tetrafluoroethy1ene)-coated s t i r bar. The stirring speed is set to be as fast as possible without introducing fluctuations to the QCM baseline frequency. Alkanethiol Solutions We have developed this experiment usine n-hexane as the solvent and 1:octadecane'thiol as the azsorbate. This svstem exhibits Lanemuir behavior in the concentration range 0.003 mM to 0.3 mM 1-octadecanethiol in n-hexane. Other solvents with comparably low dielectric constants (e.g., cyclohexane) also work (40), but solvents with higher zero-frequency dielectric constants, eo (e.g., ethanol) produce excessive noise (&?, probably due to solvent-mediated capacitive leakage between t h e QCM electrodes. Alkanethiols other than l-octadecanethiol may also be used (40). However, different thiols or solvents will exhibit Langmuir behavior over different concentration ranges, and these ranges - must be determined for each system (40). Frequency measurements should begin with pure solvent in the temperature-controlled beaker to establish a baseline. An aliquot of stock thiol solution is introduced hy svrinee . -iniection. " Asolution of 0.01 M 1-octadecanethiol in n-hexane is a satisfactory stock solution. The stock solution should be kept a t the same temperature as the solvent in the reaction beaker to avoid thermal fluctuations upon injection. Dilutions from this solution allow a wide dynamic range to be used with convenient aliquot sizes. For example. to make a measurement a t 0.1 mM thiol concentration, 99 mL of n-hexane i s used in the jacketed beaker. After temperature equilibration and with the stirrer on, collect data for 10 s to establish a baseline. Injection of 1 mL of stock solution yields a 0.1 mM final concentration.
We report i n Table 1the concentrations and corresponding volumes of solvent and stock solution used i n developing and evaluating this experiment (40). Other volumes may be used, so long a s they are known accurately. Volumetric glassware can be used. Glassware Cleaning
Cleaning the glassware properly is crucial in determining the success of this experiment. Residual alkanethiols in the reaction beaker, on the stir bar, or on the QCM holder from a previous run can contaminate a clean QCM before thiol injection, resulting in suppressed adsorption or, in severe cases,.total loss of frequency change on injection. The glassware, stir bars, QCM crystals, and QCM holders must be cleaned thoroughly before each run. Clean the glassware and stir bars by immersion in chromic acid cleaning solution for a t least 5 min. then rinse them i n distilled o r deionized water, and allow to dry. These components can also be rinsed with tetrahvdrofuran (THF) to remove the water, then rinsed in hexaie to remove the THF, and finally dried i n a clean environment. The use of solvent rinses serves to minimize the drying time and thereby minimizes exposure to potential contaminants. The cleaned components should be used shortly after cleaning to minimize adventitious organic vapor-phase and particulate contamination. The gold-coated QCM crystals can be cleaned by a number of methods. Cleaning by immersion in fresh piranha solution (3:l cone. sulfuric acid:30% hydrogen peroxide) for 5 min, followed by rinsing i n distilled or deionized water is satisfactory. We find that chromic acid attacks the chromium underlayer of the QCM electrodes, rendering them unusable. Water can be removed from the cleaned QCM by washing i n high purity THF to displace the water and then i n high purity n-hexane to displace the THF. The clean crystals should be used immediately after cleaning. The crystal holder can be cleaned adequately by immersion in clean n-hexane. Safety Precautions Caution: All students should take proper safety precautions
when handling chemicals in this experiment. Gaggles and acid-resistantgloves are necessary when using chromic acid and piranha cleaning solutions because they are extremely corrosiue. These cleaning solutions should be used in a fume hood to minimize possible exposure to their corrosive vapors. Tetrahydrofuran (THF)and n-hexane are highly flammable, and care should be taken to keep them away from ignition sources. THF can irritate the eyes, skin, and mucous membranes and is a narcotic at high vapor concentrations. Hexane can cause irritation and can also he a narcotic at high vapor concentrations. Exposure to THF and n-hexane should be minimized by wearing gaggles and gloves and by using them in a fume hood. -Alkanethiols have an unpleasant odor and exposure can cause irritation. Exposure should be limited by wearing gloves and goggles and by working in a fume hood.
.
beaker. I'repare s QCM, as desrribed above. Clamp the QCM in the holder as shown in Fi~rure3, and louw the crystal into the sol\.ent until it is cnmpletcly immcrsc>d. Stirrmg should be a s fast a s poss~blewithout afkcting the stnhilitv of the QCM bnselinr frrauencv. Allow a few minutes fog the QCM to equilibrate to the solvent. When ready to inject, fill the syringe with the required amount of stock thiol solution (see Table 1). Initiate the data acauisition hardware (strio chart recorder. comuuter. etc.). ~ i i 10 t s, and then inject.inject the stock~olut'iona s r a ~ i d l vas ~ossible.beina careful not to iniect directlv onto but rather toward the base of the vessei. The thk Q ~ M equivalent time for a n instantaneous injection can be determined during t h e curve-fitting procedure. The frequency should change quickly after injection and then reach a n equilibrium value a t a frequency lower than that of the clean QCM. Data should be collected for a period long enough to ensure t h a t the adsorption process has reached equilibrium. Figure 1shows raw data of a typical run (40). If a slight frequency drift is present, i t can be accounted for before curve fitting by using the slope of the initial 10-s baseline, or if necessary, a longer baseline. A strong, positive. monotonic freauencv drift mav indicate that the eold electrode is lifting away &om the sibstrate due to damage incurred during use or cleaning, and the crystal should be discarded. Repeat the above experiment for a minimum data set of three re~licationseach for a series of a t least three concentrations.
.
d
Data Processing
Normalize the initial baseline frequency of each curve to zero. Plot -Afvs. time ( 4 , and fit the data to eq 7. The fits to the data will yield the exponential growth constant hob, and to, the equivalent time of a n instantaneous injection. Average the kOb,values for each concentration, and plot hob, vs. alkanetbiol formal concentration. These data should yield a line k,b. = k.C + ha. From this data, where h, is the slope and hd is the y-intercept, find the equilibrium constant K,, (eq 4). The steady state fractional coverage for each concentration can be calculated using eq 5, and AGads for this system can be determined using eq 6. Table 1. Solution Compositions Used in the Determination of AGads
Formal alkanethiol concn (M)
Volume of 0.01 M stock solution (mL)
Volume of solvent in beaker (mL)
3.00x lo4
3.00
97.00
1.00~10~
1.00
99.00
3.00x'.01
0.30
99.70
1.00~10"
0.10
99.90
0.03
99.97
3.00x
lo"
Data Reduction and Analysis
After collection, the data are fit to eq 7 using a nonlinear least-squares curve fitting routine. w e have i s e d Synergy KaleidaGraph for Macintosh systems and MicroCal Origin for Window; systems.
Table 2. Measured Monolayer Growth Rate Constants as a Function of Thiol Concentration
Alkanethiol concentration (M)
lbbh (5.') 95% C.1,
Suggested Experimental Procedure If the stock solution and syringes are ready and all labware is cleaned thoroughly, this experiment can be com~ l e t e dwithin one 3-h lab period. Initial meparation and n adding iabware cleaning should take about 2 h. ~ e & by the appropriate amount of n-hexane and a stir bar to the
Volume 72 Number 5 May 1995
469
Table 3. Calculated Fractional Coverage as a Function of Alkanethiol Formal Concentration
Alkanethiol concentration (M) 3.00~ lo4 1.00~10~ 3.00 x 1.OO x 1 3.00 x 1od
o-~
Fractional surfacecoverage (8) 0.82 +0.05, -0.12 0.60 +0.09, -0.16 0.31 +0.09, -0.12 0 1 3 +0.06. -0.06 0.04 +0.02, -0.02
I
0
10
20
30
40
50
time (s) Figure 4. Example of processed data with fitted curve. The data were fitted to eq 3, with kabs = 0.44 i0.14. The data are not fit through - Af = 0 due tothe early time mixing effects described in the text. Example Calculations
Table 2 shows the averages of kOb,values we have determined for various alkanethiol concentrations. These data were obtained by fitting raw -Afvs. time responses to eq 7 (Fig. 4). Plotting k,k vs. thiol concentration gives a line with a slope of 2440 f 1074 and a y-intercept of 0.16 f 0.03 (Fig. 5). Using these values in eq 4 gives a n equilibrium constant of 15,250 f 7300. Equation 5 can then be used to find the steady state fractional coverage for each concentration, and these data are shown in Table 3. Equation 6 yields AGa* = -5.6 + 0.4, -0.2 kcallmol for l-octadecanethiol in n-hexane. Conclusion We have reported a n experiment for studying adsorption and desorption kinetics of self-assembling alkanethiol monolavers on eold usine -a QCM. . This ex~erimentis wellsuited to a n undergraduate physical chemistry laboratory because it introduces the student to surface and interface science as well as to chemical mass sensors a t a low cost. From this ex~eriment.students will learn how a simde mass sensor Ean be used to extract fundamental phys~cal information about a chemical system.
-
-
Acknowledament We are grateful for support ol'th~sresearch through grant C111.: 92-11237 from the National Sc~enct? Found;~tion. Literature Cited 1.Camillone Ill, N.:Chidsex C.E. D.;Liu, G.: Seoles, G. J Cham. Phys. 1993,98,42344245. 2. Chen. S. H.:Frank. C. WLangmuir 1989.5.978-967. 3. Chidsey.C.E, D.:Loiamno, D.N.Langmuir1990,6,682691. 4. Porter, M. D.: Bright, T. B.: Allara,D, L.: Chidsex C, E. D. J Am. Chem, Soc. 1987, 109.35593568. 5.Hahner, G.: Woll. Ch.: Buck, M.: Gmnze, M. Longmuir 1993,9,1965-1958. 6.K m , Y; McCarioy, R. L.; Bard, A. J.Langmuir 1983,9,1941-1944. 7.Nuzm. R. G.; Zegarski, B. R.: Dubois. L. H. J A m Chem Sac. 1987,109,733-740. 8.Whitesides, G. M.; Laibinis, P. E. Langmuir 1590.6,87-96. 9.Bain, C. D.: Whitesides, G. M. Science 1988,240.62-63.
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Journal of Chemical Education
n-C18H3+3Hconcentration (M) Figure 5. Plot of kobEVS.alkanethiol concentration,shown with anendant best-fit linear regression. 10.Nuno. R.G.: Korenir. E. M.; 0ubois.L. H , J Chem. Phys. 1890.93.767-773. ll.Bain,C.D.;Troughton.E B.;Yao,Y; Euall, J.:Whitesides,G.M.;Nuzzo,R.G.J A m . Chem. Sac. 1989.111.321335. 12.Bain, C.D.;Evsll, J.:Whiterider,G. M. J A m Chrm Soc. 1989,111,715&7164. 13.Thomas. R.C.; Sun,L.; Cmoks, R. M. Longmuir 1991,7,620-622. 14.Kim.Y.:McCsrlev. R. L.: 9ard.A. . . . J.J~". Phvr C b m . 1992.96.7418-7121. . . 15.Strong, L.: Whitesides, G. M. Langmuir 1988,4,546-558. 16.Widrig. C.A.;Alvea, C.A.;Porter,M. O J A m Chrm. Sac. 1891,113,2805-26iO. 17.Dubois, L. H.;Zegarski. B. R.;Nuzzo, R. G. J A m Chem Soc. 1990.112.570579. 18.Nuzzo, R.G.; Dubois, L. H.;Allsra, 0. L. J Am. Chem Sac. 1990 112.558-569. 19.Chidsex C. E.D.: Liu, G.: Rowntree, P: Sealer. G. J. Chsm. Phys 1989,91,44214423. 20.Chidsex C. E.D.: Liu, 0 . : Scoles, G.: Wang, J.Langmuir 1990.6.16W1806. 21. Camillone 111,N.: Chidrey C. E. D.: Liu. G.: Putnnrki, T M.: Scoles. G. J. Chern. Ph.ys. 1991.94 8493.6502. 22. MeCarlex R. L.: Kim,Y.:Bard,A. J. J Phw. Chem. 1993,97,211-215. D. L. J. Chem. Phvs 1992.96.927445. 23. P a r i k h. A N.:Allara. . 24. Chidsex C. E. D.; Bcrtozzi, C. R.; Putvin8ki.T. M.; Mujsce. A.M. J Am. Cham. Soe. 1990. 112.43014306. 25.Creager, S. E.:Hoekett, L. A,: Rowe. G. K Longmuir 1992,8,854-861. 26.Widrig, C.A.;Chung.C.; Porter.M. D. J. Eleciroonal. Chem. 1991,310.335359. 27.bin. C. B.: Whiteside%G. M. J . Am. Chem S o c 1989. 111.7164-7175. 28.Hlsvay J.:Guilbault, G. G A n d Cham. 1977,49,1890.1898. 29.Sauerbny, G . Z . Phyr. 1959,165.206-222. 30.Ward. M.D.: Butfry D. A. Sciene 1990.249.1000-1007. 31.Deakin, M. R.: Buttry D. AAnol. Chrm 1989.61.1147A-115U. 32.Lu. C J Voc. Sei. Tkhnni. 1915.IZ.578-583. 33.Nomura. T;Okuhnra, M. Ann1 Chim. Acln 1982,142,281-284. 34.Bruckenstein. S.: S h q M. Eklrachim. Ado. 1985.30,1295-1300. 35.Ymg, M.: Thompson, M. Langmuir 1993,9,199&1994. 36.Varimau, P. T.;Buttry,D.A. J Phys Chem 1987.91. 1292-1295. 37. Kanazawa, K. K.:Gordon 1I.J.G.Anal. Chim.Ado 1985.175.99-105. 38. Laidler, K. J. Chemical Kindics, 3rd. ed.; Harper Calms: New York, 1987;231-232. 39.Atkins, P WPhysimi Chemidry, 4th ed.; Freeman: New York, 1990:86it887. 40.Karpovich. D. S.:B1anehard.G. J. Langmuir 1994,10 831-322. ~
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