Langmuir 1993,9, 3015-3025
3015
Kinetic Control in the Formation of Self-Assembled Mixed Monolayers on Planar Silica Substrates David A. Offord and John H. Griffin* Department of Chemistry, Stbnford University, Stanford, California 94305 Received May 6, 1993. In Final Form: July 22,1999 Self-assembled mixed monolayers (mixed SAMs) were formed by competitive adsorption of an n-alkyltrichlorosilaneand either n-butyl- or tert-butyltrichlorosilane on planar silica substrates. These studies were undertaken to provide information about the structures of silica-supportedmixed S A M s and the processes which control their formation. Three seta of mixed monolayer series were prepared. One set consisted of four series of mixed SAMsformed from combinationsof n-butyltrichlorosilane with n-octyl-, n-dodecyl-, n-hexadecyl-, and n-octadecyltrichlorosilaneusing total adsorbate concentrations of 1mM. Two additional seta consisted of five series each of mixed SAMs formed from tert-butyltrichlorosilanein combination with n-butyl-, n-octyl-, n-dodecyl-, n-hexadecyl-, and n-octadecyltrichlorosilane. These two seta differed with regard to the total solution concentration of elkyltrichlorosilane used (1mM versus 10 mM). Within each series, the solution concentration ratio of the competing adsorbates was varied from zero to infinity in order to produce surfaces with a range of compositions. Monolayers were characterized by contact angle goniometry, ellipsometry, and X-ray photoelectron spectroscopy (XPS). It was found that monolayer composition, a~ determined primarily by thickness measurements,varied with the solution concentration ratio of the shorter- and longer-chain adsorbate in a manner predicted by both simple kinetic and thermodynamicmodels for monolayer formation. Surface free energies for many of the mixed SAMs were found to be higher than those of single-component SAMs,which indicates that shorter- and longer-chainadsorbates are not macroscopicallyphasesegregated in these systems. n-Alkyltrichlorosilanes were found to adsorb with similar efficiency regardless of chain length and with considerably greater efficiency than tert-butyltrichlorosilane. In all, the results are consistent with a model in which monolayer composition is controlled by kinetic factors associated with the relative rates of surface adsorption of alkyltrichlorosilanes.
Introduction The self-assembly of organic monolayers on solid supports was first described by Zisman and co-workers.' The last decade has seen renewed and increasing interest in these materialss due to their ease of preparation, their usefulness in fundamental studies involving molecular structure control at solid/vapor and solid/liquid interfaces, and their potential for technical applications in electron transfer, adhesion, sensors, biocompatibility, catalysis, and other areas.6 Self-assembled monolayers (SAMs)have been produced using a variety of adsorbates and s u b s t r a t e ~ ~ 9 and ~ 3 ~are -~~ amenable to characterization by a number of surfacesensitive techniques.2238 The picture of SAMs formed from long-chain n-alkyl adsorbates that has emerged from
these studies is essentially the same as that predicted by Zisman et al.:' well-ordered, densely packed chains in extended conformations which are oriented nearly per(8)(a) Porter, M. D.; Bright, T. 3.; Allara, D. L.; Chideey, C. E. D. J. Am. Chem. SOC. 1987,109,3559-3568. (b) Bain, C. D.; Whitesides, G. M. J. Am. Chem. SOC. 1988,110,3665-3666. (c) Bain, C. D.; Whiteaides, G.M. J.Am. Chem. Soc. 1988,110,5897-5898. (d) Bain, C. D.; Whiteidea, G.M. Science 1988,240,62-63. (e) Bain, C. D.; Troughton, E.B.; Tao, 1989, Y.-T.; Evall, J.; Whitesides, G.M.; Nuzzo, R. G. J. Am. Chem. SOC. 111,321-335. (0Bain, C. D.; Whitesides, G.M. J.Phys. Chem. 1989,93, 1670-1673. (9) Bain, C. D.; Whitesides, G.M. J. Am. Chem. SOC. 1989, 111,7164-7175.
(h)Bain,C.D.;Evall,J.;Whitesides,G.M.J.Am.Chem.
Soc. 1989,111,7155-7164. (i) Prime, K. L.;Whiteaides, G.M.Science 1991,252,1164-1167. (i)Folkers, J. P.; Laibinii, P. E.; Whiteaides, G. M. Langmuir 1992, 8, 1330-1341. (k) Laibinis, P. E.;Nuzzo, R. G.; Whiteaides, G.M. J. Phys. Chem. 1992,96,5097-6105. (9) (a) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A; Porter, 1991,113,2370-2378. (b) Laibinie, P. E.; Fox, M. D. J. Am. Chem. SOC. M. A.; Folkers, J. P.; Whitesides, G.M. Langmuir 1991, 7,3167-3173.
(10) Laibinis,P.E.;Whiteaides,G.M.;Allara,D.L.;Tao,Y.-T.;Parikh,
* Author to whom correspondence should be addressed. *Abstract publishdin Advance ACSAbstracts, October 15,1993. (1) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. J. Colloid Sci. 1946, 1,513-538. (2) Sagiv, J. J. Am. Chem. SOC. 1980,102,92-98. (3) Nuzzo, R. G.;Allara,D. L. J.Am. Chem. SOC. 1983,105,4481-4483. (4) (a) Whitesides, G.M.; Ferguson, G.S. Chemtracts: Org. Chem. 1988,1,171-187. (b) Bain, C. D., Whitesides, G.M. Angew. Chem. Znt. Ed. Engl. 1989, 28, 506-512. (c) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990,6,87-96. (5) Ulman, A. An Introduction to Ultrathin Organic Films from
A. N.; Nuzzo, R. G.J. Am. Chem. SOC. 1991,113,7152-7167. (11) Sheen,C. W.; Shi,J. X.;Martensson, J.; Parikh, A. N.; Allara, D. L. J. Am. Chem. SOC. 1992,114, 1514-1515. (12) (a)Nuzzo, R. G.;Zegarski, B. R.; Duboii, L. H. J.Am. Chem. Soc. 1987,109,733-740. (b) Nuzzo, R. G.;Fusco, F. A.; Allara, D. L. J. Am. 1987, 109, 2358-2368. (c) Duboia, L. H.;Zegarski, B. R.; Chem. SOC. Nuzzo, R. G.R o c . Natl. Acad. Sci. U.S.A. 1987,84,4734.4742.
(13) Troughton, E. B.; Bain, C. D.; Whitesides, G.M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988,4, 365-385. (14) (a) Maoz, R.; Sagiv,J. J.Colloidlnterface Sci. 1984,100,485-496. (b) Pomerantz, M.; SegmWer, A.; Netzer, L.; Sagiv, J. Thin Solid Film 1985,132,153-162. (c) Gun,J.; Sagiv, J. J. Colbid Interface Sci. 1986, 112,457-472. (d) Maoz,R.;Sagiv, J. J. Colloid Interface Sei. 1986,112, Langmuir Blodgett to Self-Assembly; Academic Press: San Diego, CA, 457-472. (e) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 1991. (6) Swalen,J.D.;Allara,D.L.;Andrade,J.D.;Chandross,E.A.;Garoff,90,3054-3056. (0Maoz, R.; Sagiv, J. Langmuir 1987,3,1034-1044. (g) Maoz,R.;Sagiv,J. Langmuir 1987,3,1045-1051. (h) Tillman,N.; &an, S.; Isreaelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. SOC. 1988,110,6136J. F.; Wynne, K. J.; Yu, H.Langmuir 1987,3,932-960. 6144. (i) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Lungmuir (7) SAMs produced through the adsorption of alkanethiols on gold 1989,5,1074-1087. ti) Tillman, N.; Ulman,A.; Penner, T. L. Langmuir have received intense scrutiny.8 Ala0 studied have been SAMs formed 1989,5,101-111. (k) Silberzan, P.; LBger, L.; Auseerr6, D.; Benattar, J. by the adsorption of alkanethiols on silver: copper,1° and gallium J. Langmuir 1991, 7, 1647-1651. arsenide;" of dialkyl disulfides1*,dialkyl and alkylphosphinesah on gold; of alkyltrichlorosilanes on silicon oxide," gold,'6 mica,l8alumi(15) Finklea,H.O.;Robinson,L.R.;Blackburn,A.;Richter,B.;Allara, D.; Bright, T. Langmuir 1986,2,239-244. num>?and tin oxide;180falcoholdaminesl and iaocyanide~~~ on platinum; (16) Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532-538. and of carboxylic acids on aluminum oxidem and silver oxide.21
0743-7463/93/2409-3015$04.00/00 1993 American Chemical Society
Offord and Griffin
3016 Langmuir, Vol. 9,No. 11,1993
pendicular to the surface. Packing densities and tilt angles depend on the structures of the surface and adsorbate, and the nature of their interaction. Whitesides and coworkers have made extensive use of mixed SAMs formed by adsorption of two different n-alkyl mercaptans on gold surfaces as tools to probe the structures of SAMs and the processes which control their formation.4b*8byda-k Recently, they have demonstrated that the relationship between monolayer compositions and the compositions of solutions from which they are deposited deviatesfrom that predicted by a simple thermodynamicequilibriumtindependent sites (17)(a) Cave, N. G.; Kinloch, S. J.; Mugford, S.; Watts, J. F. Surf. Interface Anal. 1991,17,120-121. (b) Kallury, K. M. R.; Cheuug, M.; Ghaemmaghami, V.; Krull, U. J.; Thompson, M. Colloids Surf.1992,63, 1-9. (18)(a) Tabushi, I.; Kurihara, K.; Naka, K.; Yamamura, K.; Hatakeyama, H. Tetrahedron Lett. 1987, 28, 4299-4302. (b) Yamamura, K.; Hatakeyama, H.; Naka, K.; Tabushi, I.; Kurihara, K. J.Chem.SOC.,Chem. Commun. 1988,79-81. (19)Hick", J. J.; Zou, C.; Ofer, D.; Harvey, P. D.; Wrighton, M. S.; Laibinis. P. E.: Bain. C. D.: Whitesides, G. M. J. Am. Chem. SOC.1989, 111,7271-7272. . (20)(a) Allara,D. L.; Nuzzo,R. G. Langmuir 1986,1,45-52.(b) Allara, D. L.; Nuzzo, R. G. Langmuir 1985,I , 52-66. (c) Chen, S.H.; Frank, C. F. Langmuir 1989,5,978-987. (d) Allara, D.L.; Atre, S. V.; Elliger, C. A.; Snyder, R. G. J. Am. Chem. SOC.1991,113,1852-1854. (21)Schlotter, N. E.; Porter, M. D.; Bright, T. B.; Allara, D. L. Chem. Phys. Lett. 1986, 132,93-98. (22)SAMs are conveniently characterized by wetting contact angle measurements,= ellipsometry,- X-ray photoelectron s p e d r o s c o p y , ~ ~ They have ale0 been and Fourier transform infrared spectroscopy.&.12~2s probed using surface Raman scattering,% fluorescence,n electron- and X-ray diffraction,%electrochemistry,- secondary ion mass spectrometry,m laser desorption Fourier transform maes spectrometry?' scanning tunneling electron microscopy,= atomic force microscopy," interfacial force microscopy," surface acoustic wave analysis,36 X-ray specular reflectance,%high resolution electron energy loss spectroscopy,12.temperature-programmed desorption,'" electrochemical desorption?' and helium scattering.s8 (23)(a) Dimitrov, A. S.; Kralchevsky, P. A.; Nikolov, A. D.; Noshi, H.; Mataumoto, M. J. ColZoidInterfaceSci. 1991,145,279-282. (b)Ong, T. H.; Ward, R. N.: Davies, P. B.: Bain, C. D. J. Am. Chem. SOC. 1992.114, 6243-6245. (24)Laibinis, P. E.;Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991,95,7017-7021. (25)(a) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. SOC. 1990,112,558-569. (b) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. J. Chem.Phys. 1990,93,767-773. (c) Cheng, S.S.;Scherson,D. A.; Sukenik, C. N. J. Am. Chem. soc. 1992,114,5436-5437. (26)Bryant, M. A.; Pemberton, J. E. J. Am. Chem. SOC.1991,113, 3629-3637. (27)Chen, S.H.; Frank, C. W. Langmuir 1991,7,1719-1726. (28)(a) Strong, L.;Whitesides, G. M. Langmuir 1988,4,546-558. (b) Samant, M. G.; Brown, C. A.; Gordon, J. G. Langmuir 1991,7,437-439. (c) Tidswell, I. M.; Rabedeau, T. A.; Perahan, P. S.; Kosowsky, S. D.; Fokers, J. P.; Whitesides, G. M. J. Chem. Phys. 1991,95,2854-2861.(d) Samant, M. G.; Brown, C. A.; Gordon, J. G.Langmuir 1992,8,1615-1618. (29)(a) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A.M. J.Am. Chem. SOC.1990,112,4301-4306. (b) H i c k " , J. J.; Ofer, D.; Zou, C. F.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G . M. J. Am. Chem.Soc. 1991,113,1128-1132. (c) Collard,D.M.;Fox,M.A.Langmuir 1991,7,1192-1197. (d) Uosaki, K.; Sato, Y.; Kita, H. Longmuir 1991,7, 1510-1514. (e) Frisbie, C. D.;Fritachfaules, I.; Wollman,E. W.; Wrighton, M. S. Thin Solid F i l m 1992,210,341-347. (30)(a) Frisbie, C. D.; Martin, J. R.; Duff, R. R.; Wrighton, M. S. J. Am. Chem. SOC.1992,114,7142-7145. (b) Tarlov, M. J.; Newman, J. G. Langmuir 1992,8,1398-1405. (31)Li, Y.; H u n g , J.; McIver, R. T.; Hemminger, J. C. J.Am. Chem. SOC.1992,114,2428-2432. (32)(a) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. SOC. 1991,113,2805-2810. (b) Kim, Y. T.; Bard, A. J. Langmuir 1992,8, 1096-1102. (c) Yeo, Y. H.; Mcgonigal, G. C.; Yackoboski,K.; Guo, C. X.; Thomson, D. J. J. Phys. Chem. 1992,96,6110-6111. (33)Alves, C. A.; Smith,E. L.; Porter, M. D. J. Am. Chem. SOC.1992, 114,1222-1227. (34)Joyce, S. A.; Thomas, R. C.; Houston, J. E.; Michalske, T. A.; Crooks, R. M. Phys. Reo. Let. 1992,68,2790-2793. (35)Thomas, R. C.; Sun, L.; Crooks, R. M.; Ricco, A. J. Langmuir 1991,7,620-622. (36)Wasserman,S.R.; Whitesides,G. M.;Tidswell,I.M.; Ocko,B. M.; Pershan, P. S.;Axe, J. D. J. Am. Chem. SOC.1989,111,5852-5861. (37)Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. SOC. 1992,114,5860-5862. (38)Chidsey, C. E. D.; Liu, G.-Y.; Rowntree, P.; Scoles, G. J. Chem. Phys. 1989,91,4421-4424.
Y +5 SICI,
SICI,
$
SlCl,
$
SICI,
5
SICI,
$SICI:,
Figure 1. Alkyltrichlorosilane adsorbates used in the formation of self-assembled mixed monolayers on planar silica surfaces.
model.8j They have also provided evidence that the adsorbates in these mixed SAMs are neither fully mixed nor fully phase-segregated.8jpk We have undertaken a program of research directed at using the phenomenon of self-assembly to produce monolayers for potential applications in molecular recognition and catalysis. Our approach to monolayer receptors involves the use of combinations of a long- and a very short-chain adsorbate to form mixed SAMs which have structurally defined cavities with floors formed by the very short-chain adsorbate and walls formed by the long-chain adsorbate. Thisapproach is similar to that used to produce metal ion- and metal complex-selective, mixed-monolayercoated gold electrodes39 but differs from approaches involving partially or fully preformed receptorsav41or those which introduce structurally undefined defects by templating.18*42The success of our approach to thin-film receptors requires that stable mixed monolayers of longand very short-chain adsorbates can be formed and that the adsorbates are fully mixed (at least at low mole fractions of very short chains on the surface). In this paper, we report the synthesis and characterization of mixed SAMs formed by the coadsorption of n-butyltrichlorosilane or tert-butyltrichlorosilane and a series of longer-chain n-alkyltrichlorosilanes on silica surfaces (Figure 1). This work represents a systematic study of mixed SAMs on (39)(a) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988,332,426429.(b) Steinberg, S.;Rubinstein, I. Langmuir 1992,8,1183-1187. (c) Chailapakul, 0.; Crooks, R. M. Langmuir 1998, 9,884-888. (40)Arduengo, A. J., III; M o r a , J. R.; Rodriguez-Parada, J.; Ward, M. D. J. Am. Chem. SOC.1990,112,6153-6154. (41)Chung, C.; Cihal, C. A.; Smith, E.; Porter, M. D. Abstracts of Papers;197thNational Meetingof the American ChemicalSociety,Dallas, TX; American Chemical Society: Washington, DC, 1989; ANYL 41. (42)(a) Tao, Y.-T.; Ho, Y.-H. J. Chem. Soc., Chem. Commun. 1988, 417-419. (b) Kim, J.-H.; Cotton, T. M.; Uphaus, R. A. J. Phys. Chem. 1988,92,5575-5578. (c) Andersson, L. I.; Mandenius, C. F.; Mosbach, K. Tetrahedron Lett. 1988,29,5437-5440.(d) Yamamura, K.; Hatakeyama, H.; Tabushi, I. Chem. Lett. 1988,99-102. (e) Kallury, K. M. R.; Thompson, M.; Tripp, C. P.; Hair, M. L. Langmuir 1992,8,947-954.
Langmuir, Vol. 9, No.11,1993 3017
Structures of Mixed SAMs planar silica surface^'^^*^^ and provides a basis for comparison with mixed SAMs on gold. We find that mixed monolayers are readily formed from long- and very shortchain alkyltrichlorosilaneadsorbates. Further, we observe relationships between the structures and solution compositions of adsorbates and the surface compositions of mixed SAMs which indicate that monolayer formation is controlled by kinetic rather than thermodynamic factors in these systems.
Results Kinetic Model for Mixed Monolayer Formation. We anticipated that formation of alkylsiloxane monolayers might be dominated by kinetic effects associated with reactions of electrophilic chlorosilanes with nucleophilic silanols,l4‘resulting in the irreversibleformation of strong silicon-oxygen bonds (- 128 kcal/mol).44 In the simplest kinetic model for a two-component mixed SAM formed from a short-chain (SC) and a long-chain (LC) adsorbate, the rate of incorporation of each adsorbate into the monolayer depends upon an apparent rate constant characteristic of that adsorbate (k’) and the solution concentration of that adsorbate (eqs 1and 2). The overall rate of monolayer formation is the simple s u m of the rates of incorporation of the individual adsorbates (eq 3).
PSAM
-
= psc.sAM + (PLCSAM b A M ) x
R, = [SCl,~/[LClno~ (10) A plot of PSAM versus log R, (eq 11)generates a symmetric sigmoid in which the midpoint value of PSAMrepresents the point at which the SAM is composed of equal = 1.0) and concentrations of both adsorbates (eq 12,RSAM lies at a value of log R,h equal to log (k’wlk’sc),the relative rate of adsorptionfor the long- and short-chainadsorbates. 1 10’OB[‘k’sClk’LcLC’Rd”’ +1
=[SCls~/[LCls~ (12) This derivation follows closely that carried out by rate SCSm = d[SCl,,,/dt = k’,c[SCl,h (1) Folkers,Laibinis,and Whitesides for mixed SAMs in which rate LCsAM = d[LCls,,/dt = k’,,[LC],, (2) two components adsorb independently on a surface and are in thermodynamic equilibrium with adsorbates in rate total,,, = (d[SClsAM+ d[LCI,,,)/dt solution.*j They obtained a final relationship of the same = k’,,[SClnol, + k’,c[LCl,, (3) form as eq 11 in which the parameter k’sc/k’Lc (which depends upon the difference in the apparent free energy If an experimentallymeasurable property of a mixed SAM of activation for adsorption of one component versus (PSAM, such as wetting by probe liquids, thickness, or another) was replaced by A = exp(-a/RT), where a elemental composition) is linearly related to the molecular represents the difference in free energy of adsorption of composition of the SAM (i.e., eq 4 holds), the value of that one component versus another. Through a detailed property for a mixed S A M may be related to the limiting experimental analysis, they showed that the compositions values of the property observed with pure monolayers of of mixed SAMs formed upon adsorption of alkanethiola each adsorbate and the mole fraction of one component from ethanolicsolutionsonto gold substrates deviated from found on the surface of the mixed SAM (eq 5). those predicted by the simple independent sites/thermodynamic equilibrium model. This and additional results PSAM XSC,SA&’SC,SAM + XLC,SAMPLC,SAM (4) indicated that adsorption at adjacent surface sites is not PSAM = PSC,SAM + (PLC,SAM - PSC,SAM)XLC,SAM (5) independent and that surface compositions in alkanethioV gold systems are at least partly determined by kinetic In an irreversible,kineticallycontrolledprocess, the surface factors. Our analysis of a simple kinetic model predicts mole fraction of one adsorbate will be determined by the the same general relationship between RSAM and R,h as rate of adsorption of that adsorbate relative to the total is predicted by the simple thermodynamic model. Thus, rate of adsorption (eqs 6 and 7); thus, it is possible to finding that surface compositions of mixed S A M s vary in express PSAMsolely in terms of limiting property values, the expected fashion as a function of R,, would indicate concentrationsof adsorbates,and apparent adsorptionrate that monolayer formation is controlled by either simple constants for each species (eq 8). Equation 8 may be kinetic or simple thermodynamic factors, but it would not further simplified into a form with a single unknown distinguish between these two possibilities. To probe the parameter, the ratio of apparent rate constants for factors involved in the formation of self-assembled monoadsorption of the two components (eq 91, where R, is layers on planar silica surfaces, we have determined not the ratio of concentrations of short-chain and long-chain only how RSAMvaries with Reohbut also how competitive adsorbates in solution (eq 10). adsorption efficiencies vary with the structural characrate LC,,, teristics (length and branching) of adsorbates. XLCSAM = rate total,, (6) Self-Assembled Mixed Monolayers. SAMs were deposited from dry dicyclohexyl solutionsof the adsorbates onto polished Si/SiOz (111)wafers and glass slides.& Di(43) Mixturesof akyltrichlorosilnueshave been usedto produce mixed stationary phase8 for high-performanceliquid chromatography. See (a) cyclohexyl was chosen as the solvent for these studies in Abel, E. W.; Pollard, F. H.; Uden, R. C.; Nickless,G.J. Chromatogr. 1966, order tominimize the incorporation of solvent into cavities 22, 23-28. (b) Plueddemann, E. P. Silane Coupling Agents; Plenum that could be formed upon the self-assembly of mixed Prees: New York, 1982. (c) Jones, K.J. Chromatogr. 1987,392,l-10. (d) Jones, K.J. Chromatogr. 1987,392,ll-16. (e) Wirth, M. J.; Fatunmbi, monolayers.‘“ Three sets of mixed SAM series were H. 0. Anal. Chem. 1992,64,2783-2786. prepared from either n-butyltrichlorosilanane or tert-bu(44) This value was obtained from the bond dissociation energy of Mdi-OH, calculatedfrom a study of the gas-phasekinetics of the reaction of iodine with a seriesof silanes. The bond dissociationenergy is defined as the staudard enthalpy change for the procese A-B(g) k ( g ) + B’(g). Walsh, R. Acc. Chem. Res. 1981,14, 246-252.
-
(45) Only the data for mixed SAMEon silicon wafers are preaented. Advancing contact angles for mixed SAMs on glass slides were similar to those for mixed S A M s on silicon wafers.
3018 Langmuir, Vol. 9, No. 11,1993
Offord and Griffin -0.5 1
nic
1'120
2917.8
n
8
2850.3
I\
i
8o
2750 2800 2850 2900 2950 3000 3050
Wavenumbers
Figure 2. ATR-IR spectrum of a S A M formed from n-octadecyltrichlorosilane on a silicon (111)crystal. Positions, from left
toright,ofthe sy"etric-CHr (v,(CHd),antisymmetric-CHz (v,(CHz)),.and antisymmetric -CH3 (vn(CH3))stretching vibrations are indicated. tyltrichlorosilane and a series of n-alkyltrichlorosilanes (Figure 1). The first set consisted of four series of mixed SAMs formed from combinations of n-butyltrichlorosie with n-octyl-, n-dodecyl-, n-hexadecyl-, and n-octadecyltrichlorosilane (n-C$n-C,). These monolayers were deposited from solutions containing total adsorbate concentrations of 1 mM. The second set consisted of five series of mixed SAMs formed from combinations of tertbutyltrichlorosilane with n-butyl, n-octyl-, n-dodecyl-, n-hexadecyl-, and n-octadecyltrichlorosilane(t-Ch-C,). These monolayers were also deposited from solutions containing 1 mM total adsorbate. The third set consisted of four series of mixed SAMs formed from combinations of tert-butyltrichlorosilanewith n-octyl-, n-dodecyl-, nhexadecyl-, and n-octadecyltrichlorosilane, but these monolayers were deposited from solutions containing 10 mM total adsorbate. It was not possible to obtain the n-Cdn-C, series of mixed SAMs using 10 mM adsorbate concentrations, as n-butyltrichlorosilane formed macroscopic polymeric deposits on silica surfaces above a concentration of approximately 3 mM in dicyclohexyl. In each series, R,,h was varied in increments from zero to infinity in order to produce monolayers of varying surface composition and to determine the relationship between R,ln and RSAM.In addition, a S A M was prepared on a polished Si/SiOz (111)ATR crystal using a 1 mM solution of n-octadecyltrichlorosilanein order to probe the quality of this monolayer and our deposition protocol by attenuated total reflectance infrared spectroscopy (ATR-IR). Characterization. The S A M formed on the ATR crystal was characterized by contact angle goniometry to determine surface wettability and free energy, optical ellipsometry to determine film thickness, and FT-IRto determine surface order. For this surface, advancing contact angles for water and hexadecane probe liquids were 112O and 47O,respectively, and the thickness was 25 A. These values are in accord with those reported in the l i t e r a t ~ r e . ' ~In ~ *the ~ ATR-IR, the antisymmetric -CH3 stretch was observed at 2959.3 cm-'and the antisymmetric and symmetric -CH2 stretches were observed at 2917.8 and 2850.3 cm-l, respectively (Figure 2). In crystalline hydrocarbon-containingcompounds, antisymmetric- and symmetric - C H a vibrations occur at 2918 and 2851 cm-l, respectively." It has been established that absorptions due to -CH2- stretching vibrations broaden and shift to higher frequency with decreasingmonolayer cry stall in it^.^ (46) (a) Snyder, R. G.; Straws, H.L.; Elliger, C. A. J. Phys. Chem. 1982,86,5145-5150. (b) Snyder, R. G.; Maroncelli, M.; S t r a w , H.L.; Hallmark,V. M. J. Phys. Chem. 1986,90,5623-5630.
-
""1.h 1 .o
O L W - + - + W d
0.0
a
.s-1 .o -
v)
'3, 7
2 -2.0 C
-
Figure 3. Contact angle goniometry, ellipsometric thicknese, and XPS data as a function of log R,h for monolayers formed upon coadsorption of n-C1-SiC13 and n-C,-SiCl3 onto polished silicon wafers: top, cosine of advancing contact angles for water and hexadecane;center, ellipsometric thickness; bottom, natural logarithm of the ratio of carbon (1s) to silicon (2p) XPS peak intensities. Totalsilane concentration waa 1 mM, and depositions were carried out in dicyclohexyl solvent for 1day. Fitted curves are provided as guides to the eye. Most error bars lie within the plot characters and have been excluded for clarity.
The fact that we observed -CH2- stretching vibrations at energies equal to those reported in the literature for crystalline hydrocarbons and SAMs formed from n-octadecyl a d s ~ r b a t e s ~ ~indicates J ~ * ~ J that our monolayer is highly crystalline and that our deposition protocol may be used to form high-quality monolayers. SAMs on polished silicon wafers were characterized by contact angle goniometry, optical ellipsometry,and X-ray photoelectron spectroscopy (XPS,to determine surface atomic composition). Figures 3-5 present the cosines of the advancing contact angles for water and hexadecane (HD), the ellipsometric thickness, and the natural logarithms of the relative intensities of the XPS carbon (1s) and silicon (2p) peaks (ln(C1JSi2~))for each monolayer within each series as a function of log RaOh. Contact Angle Goniometry. Each surface in each series was probed with water, a polar liquid with a high surface tension (71"= 72 mN/m at 25 OC) and hexadecane, a nonpolar liquid with a low surface tension (71" = 27 mN/m at 25 "C). The advancing contact angles for water (Ba(H20))on surfaces formed from n-alkyltrichlorosilane
Structures of Mixed SAMs
Langmuir, Vol. 9, No. 11,1993 3019
-0.5 7
(1 20
-0.5 3
1120
-0.3
-1
O.'i 0.3
4
0.6
"V
I
I
O......
A
n n. ".V A
,
.cua-1 .o cn 's,
.
.......
€-6 A
-------- 4 ...........
I .
Figure 4. Contact angle goniometry, ellipsometric thickness, and XPS data as a function of log R,I, for monolayers formed upon coadsorption of t-Cr-SiC&and n-C,-SiCh onto polished silicon wafers. Total silane concentration was 1 mM. Deposition conditions and descriptions of the plota are as described in the caption to Figure 3.
adsorbates ranged from 102O with n-butyltrichlorosilane to 112O with octadecyltrichlorosilane. That lower values of B,(HzO) were obtained with shorter-chain adsorbates is consistent with their expected decreased order and ability to shield probe liquids from the polar silica surface.2The variation in B,(H20) on individual SAMs was less than 2 O . The variation in B,(HzO) on monolayers formed on different occasions from solutionsof n-alkyltrichlorosilanes having equivalent R,,h was also less than 2 O . In contrast, B,(HzO) for surfaces formed on different occasions from tert-butyltrichlorosilaneranged from 62 to 73O (from 1 mM adsorbate solution concentration) and 68to 76' (from 10 m M adsorbate solution concentration). The greater variability in Ba(H20) on surfaces formed from tertbutyltrichlorosilane, and the fact that higher concentrations of tert-butyltrichlorosilaneproduced monolayers of somewhat greater hydrophobicity and somewhat lesser variation suggests that the sterically hindered tertiary adsorbate adsorbs less readily and less reproducibly than n-akyltrichlorosilanes. This would not be surprising given that the estimated van der Waals diameter for a tertbutyl group, 5.6 A:' is greater than the 4.5-A distance (47) Charton, M. h o g . Phys. Org. Chem. 1971,8, 235-317.
Figure 5. Contact angle goniometry, ellipsometric thickness, and XPS data as a function of log R h for monolayers formed upon coadsorption of t-C&iCh and n-C,-Sic& onto polished silicon wafers. Totalsilane concentrationwaa 10mM. Deposition conditions and descriptions of the plota are as described in the caption to Figure 3.
between adjacent attachment sites on an idealized silicon oxide surface.48 A direct relationship between the cosine of the contact angle and the composition of surfaceswith fractional areas of different wettability is predicted by theoryqg and has been observed in some but not all mixed SAM cases examined.@ When plotted against log Rsoln,cos BJH20) for each series of mixed SAMsvaried in a smooth, sigmoidal fashion. To determine whether the observed relationships were well-described by the derived model, plots of corrected, normalized values of cos B,(H20) versus R,h were fit to eq 13. The experimental data were found to be very corrected, normalized PsAM =
)
(PSAM - PSC,SAM) (13) (PLCSAM - PSC,SAM)( ~ ' s d ' ~ ~ c ) %+ o l1n
-(
well described by eq 13. Transition midpoints (i.e., the values of logR,h at which cos B,(H20) lies midway between the values for the all-long chain and all-short chain SAMs) and correlation coefficients obtained from these analyses (48) Zhurnvlev, L. T. Langmuir 1987,3,316318. (49) (a) Caeeie, A. B. D. Discws. Faraday SOC.1948, 3, 11-16. (b) Israelachvili, J. N.; Gee, M. L. Langmuir 1988,5,288-289.
3020 Langmuir, Vol. 9, No. 11, 1993
Offord and Griffin
Table I. Transition Midpoints of Monolayer Properties as a Function of COB
series
e, H ~ O
thickness midpt R 1 mM
midpt
R
-0.48 -0.077 -0.16 -0.078 2.2 2.0 2.2 2.1 2.0
0.998 0.998 0.998 0.991 0.997 0.996 0.992 1.OOO 0.992
-0.29 -0.43 -0.25 -0.31 1.4 1.2 1.6 1.6 1.3
2.6 3.0 2.6 2.7
0.996 0.993 0.996 0.996
1.8 2.1 1.9 2.1
0.997 0.996 0.989 0.973 0.998 0.990
0.991 0.999 0.991 10 mM 0.995 0.996 0.997 0.983
midpt -0.05 (4.06) -0.26 (-0.33) 0.10 (0.06) 0.56 (0.12) 1.3 (1.3) 1.5 (1.5) 1.3 (1.2) 1.8 (1.9) 0.19 (0.11) 2.4 (2.5) 2.3 (2.5) 2.7 (2.7) 2.2 (1.8)
h ( C d Si%) R
n
0.991 (0.991) 0.983 (0.994) 0.899 (0.913) 0.703 (0.723) 0.916 (0.918) 0.951 (0.966) 0.916 (0.918) 0.899 (0.917) 0.834 (0.854)
0.85 0.62 0.60 7.0 1.3 0.59 0.80 0.58 0.61
0.933 (0.991) 0.955 (0.985) 0.940 (0.988) 0.980 (0.988)
0.41 0.49 0.46 6.3
a Midpoints (midpt) are expressed in units of log R,h = log [C4l/[n-C,],and theoretically represent the value of log R,h that produces RSAM= 1.0. Midpoints and correlation Coefficients (R2)were derived by fitting the corrected, normalized data shown in Figures 3-5 to eq 13. Values in parentheses indicate midpoints and correlation coefficients obtained from fits which included the cooperativity parameter n (eq 14).
-0.5 1
I
Figure 6. Plot of cos 8.(HzO) versus log R,h for the t-CJn-Cls
series and theoretical curves for noncooperative (n = 1.0,solid), negativelycooperative (n= 0.5,dotted),and positivelycooperative (n = 2.0, dashed) adsorption processes.
are presented in Table I. Addition of a parameter n which would account for cooperativity (i.e., greater or less than fiist-order dependence on R w d in the adsorption process (eq 14)@did not significantly increase the quality of the fits. This is illustrated in Figure 6, where it can be seen that the data for the n-Cr/n-Cle series is better fit by a noncooperative (n = 1.0) model versus negatively cooperative (n = 0.5) or positively cooperative (n = 2.0) models. corrected, normalized PsAM= 1 - PSC,SAM) (14) (PLC,SAM -PSC,SAM) (htsJkLc)(R80J' + 1 &AM
(
)
Within each set of monolayers, midpoints in PSAM varied little and nonsystematically with the length of the long chain component. For the n-C4/n-Cx set of SAMs, midpoints ranged from -0.48 to 0.077. For the two sets of t-C4/n-Cx mixed SAMs, midpoints ranged from 2.0 to 2.2 for monolayers formed from solutions containing 1 mM totaladsorbate concentration and 2.7 to 3.0 for SAMs formed from 10 mM solutions. The results indicate that n-alkyltrichlorosilanes adsorb with similar efficiency regardless of chain length, but with lW1000-fold greater efficiency than tert-butyltrichlorosilane,depending upon the total solution concentration of adsorbates employed. The advancing contact angles for hexadecane (B,(HD)) on these monolayers did not vary sigmoidally with log Rsoh. Rather, &(HD)decreased sharply near the 8,(Hz0) midpoints and eventually wet (B,(HD) < 10")at least one surface in each series, except in the n-Crln-C~,t-CJn-Cr, and t-C$n-Ce series. A t values of &,I,., above the midpoint,
8,(HD) increased and eventually reached values of 37O for n-butyl surfaces and 27 to 3 9 O (from 1 mM solution concentration) and 24 to 30° (from 10 mM solution concentration) for tert-butyl surfaces. A similar phenomenon has been previously noted in mixed S A M systems and is consistent with a model in which surfaces wet by hexadecaneconsist of isolated long chains which protrude from a mainly short-chain surface and expose disordered methylene groups that can engage in favorable van der Waals interactions with hexadecane.&*'J In the cases where hexadecane does not fully wet the surface, the difference in chain length between adsorbates is small and limits the amount of methylene character that may be displayed by these surfaces. Zisman Plots. The surface free energies for two series of mixed SAMs were analyzed in detail using advancing contact angle goniometry and 15 probe liquids covering The mixed SAMs a broad range of surface tensions (71~). studied were the n-Cdn-Claseries and the t-Cdn-Cuseries formed from solutions containing 1mM adsorbate. The probe liquids used and their respective surface tensions are listed in the Experimental Section. Zisman plots50 of cos 8, versus the probe liquid surface tension for each surface within each series are collected in Figure 7. From these plots it may be seen that a single relationship did not generally suffice to describe the data obtained with all probe liquids. Rather, separate relationships were observed for three different classes of probe liquids: (1) straight- and branched-chain hydrocarbons, (2) cyclic hydrocarbons,and (3) alcohols. To determine the critical surface tension (yc,equivalentto the solid/vapor interfacial tension, ysv)of each surface, the data from each class of probe liquids for each surface were fit to a quadratic equation.& yc was taken to be the value of ylv at which cos 8, was extrapolated to be 1.0, Le., the surface tension of a theoretical probe liquid that just wets the surface. Each class of probe liquids afforded somewhat different extrapolated values of yc,which are presented in Table 11. The data obtained with the alcohol probe liquids was not fit with high correlation and therefore are not considered to give reliable values of yC.& The data obtained with cyclic hydrocarbons provided highly correlated fits; however only three cyclic hydrocarbons were used. The data obtained with the largest class of probe liquids, the (50) (a) Fox, H.W.;Zisman,W.A.J. Colloid Sci. 1954,5,514-631.(b) Zisman,W.A. Adv. Chem. Ser. 1964,No. 43, 1-51.
Structures of Mixed SAMs
Langmuir, Vol. 9, No. 11, 1993 3021
A
15
19
23
y,
27
31
35
19
23
27
31
35
19
y&"/m)
"lm)
23
27
31
35
y&"/m)
1 .o
B
0.9 a'0.8
6 0.7 v)
0.6 0.5 1 .o 0.9 0'0.8
6 0.7 v)
0.6 0.5 1 .o 0.9 am 0.8 8 0.7 0.6
's
v)
b
0.5
15
19
23
y" "/m)
27 31
35
19
23
y,
27
31
35
19
23
27
31
35
Y&"/m)
"/m)
Figure 7. Zisman plots for two seriesof mixed monolayers adsorbed onto polished silicon wafers from dicyclohexyl solutionscontaining various proportions of Cd-Sic13 and n-Cl8-SiCls (1mM total silane concentration): A, n-Cdn-Clamixed SAMs;B, t-Cdn-Cl8 mixed SAMs; circles, straight/branchedhydrocarbon probe liquids;squares,cyclic hydrocarbon probe liquids;triangles,alcohol probe liquids. The limes are quadratic fits to the data and were used to obtain the critical surface tension, -ye, of each surface by extrapolation to COB
e.
= 1.0.
straight/branched hydrocarbons, was also fit with high correlation and are considered the most reliable for determining yc. The average value of ycfor n-Cls surfaces obtained from the straight/branched hydrocarbon data, 20.3 mN/m, agrees with the literature value for this system (20mN/ m)16 and is indicative of a surface consisting of closely packed methyl groups. The t-C4 monolayer had a higher surface tension (yc= 23.3 mN/m) than either of the other single component monolayers, n-C4 (21.5mN/m) and n-Cl8 (20.3 mN/m), consistent with a less ordered and less methyl-like surface. Within each mixed monolayer series, a subset of the mixed SAMs exhibited free energies that were higher than either of the single-component SAMs. The t-Cdn-Cl8 monolayer formed from logR,,b = 2.0 was wet or nearly wet by all probe liquids, similar to poly-
ethylene.61 We have assigned ycfor this surface to be a t least 27.6 mN/m, the highest surface tension among the straight/branched chain hydrocarbon probe liquids employed. The high free energy of this surface suggests a high degree of disorder, which would be consistent with a monolayer composed mainly of short chains with a significant fraction of randomly oriented ,disordered long ch&S.&Aj Ellipsometry. Provided that the appropriate index of refraction is used and that it does not vary with surface composition within a series of mixed SAMs, measured ellipsometric thicknesses will be directly related to monolayer compgsitions. By use of a refractive index of 1.45,96 the average ellipsometric thicknesses of single-component ~
(51) Ellison, A.
H.;Zisman, W.A. J. Phys. Chem. 1954,58,260-265.
Offord and Griffin
3022 Langmuir, Vol. 9, No. 11, 1993 Table 11. Critical Surface Tensions (re) of Mixed SAMs. ye straight/ yc straight/ branched yc cyclic branched log R,I. hydrocarbons hydrocarbons alcohols n-CJC1s --OD 20.3 21.2 18.0 -3.0 20.3 21.8 17.9 -2.0 20.3 21.3 18.1 -1.0 20.4 21.1 17.4 25.0 25.9 0.0 24.8 1.0 26.6 25.7 25.0 23.3 2.0 24.8 25.1 22.5 3.0 22.9 24.7 m 21.5 24.4 21.6 t-C4/C1e -a 20.3 22.0 14.5 0.0 20.3 21.4 15.9 1.0 20.3 22.6 18.1 1.7 24.2 24.3 24.5 2.0 127.61 24.9 127.61 3.0 23.8 25.2 24.7 4.0 24.3 25.3 24.6 5.0 23.5 25.2 23.4 m 23.3 25.2 22.6
-"'-
I
t
:-o'21 a* 0.0
t
90
$0
0.2
t
0.2t
t
Critical surface tensions were derived from the Zisman plots shown in Figure 6 by extrapolating quadratic fits of the data to cos 0, = 0. Values in brackets indicate the lowest possible value of ye for this surface given that it is wet by all the homologous liquids in this set.
monolayers formed from tert-butyl, n-butyl, n-octyl, n-dodecyl, n-hexadecyl, and n-octadecyltrichlorosilane were measured to be 4,9,12,18,24, and 26 A, respectively. These values compare well with those expected for closepacked monolayers having trans-extended alkyl groups ~,~ on tilted 1 5 O from the surface n ~ r m a l . ' ~Measurements individual surfaces and on corresponding surfaces formed on different occasions generally differed by f 2 A. Within each series, thicknesses of mixed SAMs varied in a smooth, sigmoidal fashion with log RBohand were well-described by eq 13 (Table I). Inclusion of the cooperativity parameter n (eq 14) did not significantly improve the fits. Values for the thickness transition midpoints varied little within each set of mixed SAMs, and ranged from -0.43 to -0.25 for the n-C4/n-C, set, 1.21.6 for the t-Cdn-C, set deposited from 1mM solutions, and 1.8-2.1 for the t-Cdn-C, set deposited from 10 mM solutions. The midpoints for the n-Cdn-C, set were near zero and similar to those obtained from the advancing water contact angle data. However, midpoint values for the t-C4/n-Cxseta obtained by ellipsometry are uniformly lower than those obtained from goniometry and indicate that n-akyltrichlorosilanes adsorb with 16-130-fold greater efficiency than tert-butyltrichlorosilane.This disparity suggests that data obtained for these seta by either one or both of these methods does not directly reflect monolayer composition. Figure 8 illustrates that ellipsometric thickness is linearly related to Ba(H20)for the n-C4/n-C18 series but not for the t-Cdn-Cl8 series formed from either 1or 10 mM solutions. Similar behavior was observed by Whitesidea and co-workersfor mixed S A M s on gold formed from 11-mercaptoundecanol and docosanethiol.8j XPS. Whitesides and co-workers have derived the expected relationship between surface composition in SAMs and X-ray photoelectron intensities and demonstrated that this relationship is followed with SAMs and mixed SAMs formed upon adsorption of n-alkyl mercap tans on silver and gold s u r f a ~ e s . ~By j ~monitoring ~~ the intensities of carbon (Is) and silver (3d) photoelectrons produced upon irradiation with a monochromatic A1 Ka source, the quantities l n ( C d A g d were found to vary linearly with expected surface thickness in monolayers containing adsorbates 11or more carbons in length. With
Thickness (A) Figure 8. Plots of cosine of advancing water contact anglesvem!us ellipsometric thickness for mixed SAMs: top, n-CdCle series formed from solutionscontaining1m M totalsilaneconcentration; center, t-CI/Cls series formed from solutions containing 1mM total silane concentration; bottom, t-CJClg series formed from solutions containing 10 m M total silane concentration.
adsorbates less than 11carbons in length, an exponential rather than linear drop in h(C~,/Agad)with thickness became apparent, in accord with expectations. Analogous studies of SAMs on planar silica surfaces have not been reported. Given the results of Whitesides et al., it was not expected that ln(C1$Si!$ would directly reflect surface composition in mixed SAMs formed using short-chain alkyltrichlorosilane adsorbates. Experimentally, it was found that ln(C1JSizp) varied in a sigmoidal fashion with log However, these data were not fit as well by eq 13 as were the cos Ba(H20) and thickness data. Transitions were in most cases more gradual than those predicted by eq 13 and inclusion of the parameter n (eq 14)afforded modestly to significantly better fits with values of n generally less than 1.0 (Table I). The inclusion of n in the curve-fitting process did not lead to large changes in the predicted transition midpoints, which were quite constant within each set of mixed SAM series with two notable exceptions: the n-C4/n-C8and t-Cr/n-C4 series. In these cases the spread in the data was small relative to the error and the fits were correspondingly poor. We believe that this indicates alimitation in the sensitivity of the method rather than of truly different surface compositions. Without considering the data for these series, midpoints obtained from the XPS data ranged from -0.33 to 0.12 for the n-C$ n-C, set, from 1.2 to 1.9 for the t-C4/n-Cz set deposited from 1mM solutions, and from 1.8-2.7 for the t-CJn-C, set deposited from 10mM solutions. These values agreed well with those obtained by goniometry and ellipsometry for the n-C4/n-Cxset, with those obtained by ellipsometry for the t-Cdn-C, set deposited from 1mM solutions, and fell in between those obtained by goniometry and ellipsometry for the t-Cdn-C, set deposited from 10 mM solutions.
Langmuir, Vol. 9, No.11,1993 3023
Structures of Mixed SAMs The XPS experiments also provided information about bonding in these monolayers. Photoelectrons from chlorine were not obtained from the SAMs, despite the large XPS cross section for this element. This suggests that, within the limits of XPS detection, alkyltrichlorosilane adsorbates react fully to form terminal Si-OH groups or Si-0-Si linkages with the surface and neighboring adsorbate chains.
Discussion Simple Kinetic and Thermodynamic Models Pre~~~~ for Mixed dict the Same R s A M : RRelationship SAMs. We have considered a simple model for kinetic control of mixed self-assembled monolayer formation and derived the expected relationship between experimentally determined properties that reflect monolayer composition (RSAM) and the compositions of solutions from which monolayers are deposited (RBOh). The derived relationship (eq 9) is of the same form as that derived previously for a simple thermodynamic equilibrium/independent sites model.8j This leads us to conclude that simple kinetic effects cannot be distinguished from simple thermodynamiceffects on the basis ofRs&Reoh relationshipsalone. We also conclude that mixed SAMs formed upon adsorption of alkanethiols onto gold or silver surfaces, in which R ~ A does M not vary with Rsohin the manner predicted by eq are formed by neither simple thermodynamic nor simple kinetic processes. Ellipsometric Thickness Measurements Indicate That Alkyltrichlorosilane/Silica Mixed SAMs Exhibit R s A M : R - ~Relationships Predicted by Simple Models. The variations in ellipsometric thickness with R,h for the alkyltrichlorosilane/silica mixed SAMs examined were well-described by eq 9. Providing that the measured thicknesses directly reflect monolayer composition, this indicates that formation of these monolayers under the described conditions is controlled by either simple kinetic or simple thermodynamic factors. We believe that it is valid to assume that ellipsometric thicknesses are directly related to surface composition in these systems, given that the precision of our thickness measurements (fl-2 A) is similar to the maximum error in thickness that would be expected for minor changes in refractive index such as those observed with n-octadecyltrimethoxysilane (1.44)versus tert-butyltrimethoxysilane (1.40). Variations in cos Ba(H20) as a function of R,h for the four series of n-C$n-C, mixed SAMs were also welldescribed by eq 9. Transition midpoints derived from the cos Ba(H20)data were similar to those derived from the thickness measurements, and cos Ba(HzO) and thickness were found to be linearly related in the n-Cdn-Cl8 series. While cos Ba(H20) varied as predicted with Reohfor the t-Cdn-C, mixed SAMs, transition midpoints derived from these data were uniformly higher than those derived from ellipsometry,and plots of cos Ba(H20)versus thickness for the t-Cdn-Cl8 series were curved, with the concave surface down. From this we conclude that cos Ba(H20) directly reflects surface composition for n-Crln-C, mixed SAMs but that it underestimates the amount of very short-chain adsorbate present in t-Cdn-C, mixed SAMs. It may be that n-alkylchains conceal tert-butyl groups from the very polar probe liquid water by folding over the hydrophobic cavity created by the tert-butyl adsorbate (Figure 9). The XPS data were in general not well described by the simplest models and were somewhat better described by models that suggest negative cooperativity in the selfassembly process. This stands in contrast to the results 9,@pkpSb
A
A
A
Figure S. Model for the concealment of short chain adsorbates by surrounding long chain adsorbates.
obtained by ellipsometry and goniometry as well as to the apparent positive cooperativity observed in the selfassembly of alkanethiols on gold and s i l ~ e r . However, ~j~~~ we have little confidence that ln(C1JSi2,) directly reflects the monolayer composition of the alkyltrichlorosilane/ silica SAMs studied given that theory and experiments with n-alkyl mercaptan/goldand n-alkylmercaptadsilver mixed SAM systems suggest that the linear relationship between ln(CldSi2,) and log Rsoh will break down with surfaces containing shorter-chain adsorbate^.^^ A better understanding of the relationship between XPS data and surface composition will require a more detailed XPS analysis of alkyltrichlorosilane/silicaSAMs. At the same time, we feel that the XPS data can be used to provide qualitative support for two important general results that emerged from the goniometry and ellipsometry studies: n-alkyltrichlorosilanesadsorb with significantly greater efficiency than tert-butyltrichlorosilaneand with similar efficiencies regardless of chain length. n-Alkyltrichlorosilanes Adsorb with Higher Efficiency than tert-Butyltrichlorosilane. In all cases examined, transition midpoints for t-Cdn-C, mixed monolayer series occurred at values of Rsoh>> 1(log R,h >> 0). By use of midpoints obtained from ellipsometry, the results indicate that the relative apparent rate for adsorption of n-alkyltrichlorosiconstants (k’~c/k’sc) lanes versus tert-butyltrichlorosilaneare 16-130. This finding does not distinguish between thermodynamic or kinetic models for adsorption. In a thermodynamic model, stabilizing van der Waals interactions between closepacked n-alkyl adsorbates would favor their presence on the surface relative to tert-butyl adsorbates. In a kinetic model, n-alkyltrichlorosilaneswould be expected to react more rapidly with silanol nucleophiles than would the stericallyhindered tert-butyltrichlorosilane. Quantitative rate data for nucleophilic substitution reactions of tertbutyltrichlorosilaneare not available, but it is known that tert-butyltrichlorosilaneis less easily hydrolyzed than are n-alkyltrichlorosilanes.62 n-Alkyltrichlorosilanes Adsorb with Similar Efficiency Regardless of Chain Length. For the n-C4/ n-C, set of mixed monolayer series, transition midpoints obtained by goniometry, ellipsometry, and XPS were similar and near zero. For the t-CJn-C, sets of mixed S A M series,transition midpointsderivedvia one particular analytical method (goniometry,ellipsometry,or XPS)were similar, even though midpoints obtained by the different methods did not agree in every case. Variationsin observed midpoints were generally minor and were not systematically correlated with increases in chain length of the n-C, component. The results indicate that n-alkyltrichlorosilanes adsorb onto silica surfaces with similar efficiency regardless of chain length. This finding is inconsistent with a thermodynamic model for monolayer formation, (52) (a) Tyler, L. J.; Sommer, L. H.; Whitmore, F. C. J. Am. Chem. SOC.1947,69,981. (b) Tyler, L. J.; Sommer, L. H.; Whitmore, F. C. J. Am. Chem. SOC.1948, 70, 2876-2878.
3024 Langmuir, Vol. 9, No. 11,1993
which would predict that adsorption efficiencies and midpoints would increase with increasing chain length of the n-C, component, and contrasts with the observed behavior of alkanethiol/gold systems in which midpoints increase with increasing differences between the lengths of shorter- and longer-chain components.&15~The results are consistent with the simple kinetic model, provided that n-alkyltrichlorosilanes with chains containing four or more carbon atoms react a t similar rates. This provision is not unreasonable given that the apparent activation energies for hydrolysis of ethyltrichlorosilane and n-pentyltrichlorosilane (10.4 and 11.9 kcal/mol, respectively) are similar but much larger than that for: methyltrichlorosilane (5.2 kcal/mol)." We conclude that formation of SAMs by adsorption of alkyltrichlorosilanes on silica is controlled by kinetic factors. Midpoints Increase with Increased Adsorbate Solution Concentration. Midpoints for the t-C4/n-Cx mixed SAM series deposited from 10 mM solutions of adsorbates were substantially higher than those for series deposited from 1mM solutions. No such concentration dependence would be expected if the ratio of apparent rate constants which controls the relationship between RSAMand R,,h in the kinetic model referred solely to the reaction of monomeric electrophilic alkyltrichlorosilanes with nucleophilic surface hydroxyl groups. However, soluble silicon electrophiles may also become incorporated into monolayers by reaction with oligomeric or polymeric silanols in solution which eventually adsorb onto the surface. The relative amounts of adsorption which occurs via these different channels (direct adsorption versus preassembly/adsorption) would be expected to depend upon solution adsorbate concentration, and we conclude that the observed changes in midpoints arise from different relative rates of reaction for direct adsorption versus oligomerization/adsorption. Subsets of Mixed SAMs Are of Higher Surface Energies. Implications for Surface Structure. We have shown that different classesof probe liquids (straight/ branched chain hydrocarbons, cyclic hydrocarbons, straight/branched chain alcohols) follow different quadratic correlations in Zisman plots of cos 8, versus ylvand lead to the extrapolation of different critical surface tensions. However,the three classesof probe liquids report the same two trends in ye for SAMs and mixed SAMs. First, surfaces formed from tert-butyltrichlorosilaneare of higher energy than those formed from n-butyltrichlorosilane, which are in turn of higher energy than those formed from n-octadecyltrichlorosilane.Second, a subset of mixed SAMs in each series are of higher energy than either of their respective C4 or CIS single-component monolayers. These trends are mirrored in cos 8,(HD) for these surfaces and underscore the noted utility of this probe liquid in less exhaustive contact angle analyses.& T h e finding of mixed SAMs with higher surface free energies than single-component SAMs has important implications for the surface structure of the mixed S A M S . ~ ~If *the ~ *long~ ~ and very short-chain components form a mixed SAM surface of macroscopically phasesegregated domains, the observed surface free energies would be expected to be a weighted average of, not higher
Offord and Griffin than, the free energies of the respective single-component SAMs. Thus, we conclude that macroscopic phase segregation is not the kinetically preferred pathway for formation of mixed monolayers by adsorption of alkyltrichlorosilanes on silica substrates. Our data do not allow us to distinguish whether adsorbates in these mixed SAMs are fully mixed or are microscopically phase-segregated (as has been inferred for n-alkyl mercaptan/gold mixed SAMs8j),but we believe that a kinetic preference for fully rather than partially mixed monolayers is most probable.
Experimental Section Adsorbates and Solvents. Octadecyl-, hexadecyl-, dodecyl, octyl-, n-butyl-, and tert-butyltrichlorosilanewere obtained from Htih America, Inc., and purified by bulb-to-bulb distillation at -0.075 Torr. Their purity was checked by 4OO-MHz 'H NMR in dry CDCb. All transfers involving akyltrichlorosilanes were conducted under dry Nz. Dicyclohexyl (Fluka) was purified and dried by passage through a column of Super I Basic Alumina (Fisher Scientific). Deionized/distilled water was used for all washings, and water purified by passage through a Milli-Qsystem was used for contact angle measurements. Straight/branched and cyclic alkanes used for the Zisman analysis were passed through Super I Basic Alumina (Fisher Scientific) 5 times the same day they were used. Each hydrocarbon passed the Zisman teat- just prior to use. The alcoholsused for the Zismananalysis were obtained anhydrous from Aldrich (except Gold Shield absolute ethanol) and were not further purified. Substrates. Cover glasses (18 X 18 mm) (Premium Glass, No. 1)were obtained fromFischer Scientific. Three-inch polished silicon (111)wafers (p doped) were obtained from Wacker and cut into 1x 3 cm pieces using a diamond-tipped stylus. For each series of monolayers formed on the polished silicon wafers, substrates were cut from the same wafer to ensure reproducibility. Polished wafers and cover glasses were cleaned just prior to monolayer formation by immersing them in 'piranha" solution (3730%aqueousHzO~:H&3O4)at9O0Cfor30min. (WARNING Piranha solution reacts violently, even explosively with organic materials.6' It shouldnotbe combined withsignificant quantities of organic materials and it should also not be stored for any length of time.) Clean substrates were rinsed thoroughly with deionized/distilledwater and blown dry with a stream of nitrogen. Overall, this treatment removes extraneous organic materials from the surfaces but does not completely dehydrate them --surface bound water molecules are needed to form siloxane cross-links between surface-bound adsorbates. Monolayer Formation. Mixed monolayers were spontaneously formed by immersing clean silicon oxide surfaces (cover glasses and polished Si wafers) into solutions of the silanes in dicyclohexyP (10 mL total volume, 1 or 10 mM total alkyltrichlorosilane concentration). Standard solutions of alkyltrichlorosilaues (1or 10 mM) in dicyclohexyl were prepared in a glovebox filled with dry Nz in glass volumetric flasks which had been preailaniid with octadecyltrichlorosilane. Individual depositionsolutionswere prepared by transferring the correct amount of each standard silane solution to presilanized 25-mL glass vials containing a Teflon-coated magnetic stir bar via a plastic-tipped automatic pipet. Each deposition solution was capped, removed from the glovebox, and stirred for 5 min. A cleaned cover glass and a polsihed silicon wafer substrate were then added to the solution. Dry Nz was blown into the vial, which was then capped and placed in a desiccator at room temperature (-20 O C ) for 1 day. Upon removal from the deposition solutions, the silanized substrates were immediately rinsed with approximately 10 mL of each of the following solvents in the following order: chloroform, 2-propanol, deionized/distilled water, 2-propanol, chlo~~
(53) We have studied nualogoue system of tert-butyl mercaptan, n-butyl mercaptan, and n-alkyl mercaptan adsorbates on gold surfaces (Linford, M. R;Offord, D. A.; Griffii, J. H., unpublished), and have observed that transition midpoints within seta of mixed SAMs increase with increases in the length of the n-alkyl adsorbate employed and that tert-butyl mercaptan adsorb less efficiently than n-alkyl mercaptans. (54) Shaffer, L. H.; Flanigen, E. M. J. Phys. Chem. 1957,61, 15951600.
(55) (a) Zisman, W. A. J. Chem. Phys. 1941,9,534-551. W. A. J. Chem. Phys. 1941,9,72%741.
~
(b) Ziman,
(66) In this test, a drop of the liquid is placed on the surface of 0.1 N HCl or NaOH and ita spreading observed for 1h. If it doee not spread noticeably, it is considered to be sufficiently pure of polar contaminants. Liquids used in the present studies did not spread for several hours. (57) (a) Dobba, D. A.; Bergman, R. G.;Theopold, K. H. Chem. Eng. News 1990,68 (17),2. (b) Wnuk, T. Chem. Eng. News 1990,68 (26), 2. (c) Matlow, S. L. Chem. Eng.News 1990, 68 (30), 2.
Structures of Mixed SAMs roform. Finally, the substrates were Soxhlet extracted with hot chloroform for at least 30 min prior to characterization. Each deposition was repeated at least twice. Contact Angle Goniometry. Contact angleswere determined on a h e - H a r t Model 100 Goniometer at room temperature (-20 OC) and ambient relative humidity for water and hexadecane probe liquids. Advancing contact angles (83 were measured by lowering a 1-pL drop from a blunt-ended needle attached to a 2-mL syringe onto the surface and expanding the drop until it began to spread across the surface. The needle was then raised to detach it from the drop and the angle between the drop and the surface was measured within 1min. Both sides of three separate drops per surface were measured (six data points total)and averaged to obtain 8, for a particular surface. Receding contact angles for water were also measured (data not shown). It was found that the advancing and receding contact angles differed by as little as 2O for single-componentmonolayers formed from dodecyl-, hexadecyl-, and octadecyltrichlorosilane,and by as much as 12O for mixed monolayers or for single-component monolayers formed from n-octyl-, n-butyl-, and tert-butyltrichlorosilane, and that increased hysteresis was correlated with increased interfacial free energy. Zisman Plots. Advancing contact angles of various probe liquids on the t-CrlCl8 and n-C&8 monolayer series (both from 1 mM silane deposition solutions) were measured using the method described above. An environmental chamber saturated with the probe liquid was used for the more volatile probe liquids. The contact angle of the drops did not decrease noticeably for several minutes under these conditions. The following liquids were used with each liquid’s surface tension (mN/m)68in parentheses: straight/branched hydrocarbons, 2,3-dimethylpentane (20.11, 2,2,4-trimethylpentane (20.61, n-octane (21.7), nnonane (22.9),n-undecane (24.8),and n-hexadecane (27.6); cyclic hydrocarbons, cyclohexane (25.4), cyclooctane (29.9), and dicyclohexyl (32.8); alcohols, 2-propanol (21.6), ethanol (22.5), 1-propanol (23.8), 1-butanol (25.5),l-pentanol (25.9), and l-octan01 (27.6). Ellipsometry. Ellipsometric measurements were made with a Gaertner variable angle ellipsometer L116A using a heliumneon laser of wavelength 6328 A and an incident angle of 70°. The Gaertner GC5A+SubCA+SCGA+SC7A automatic ellipsometry program for an IBM PC was used to calculate film thicknesses. Six measurements of the bare substrate’s optical constants N , and K,were made at different positions on each polished silicon wafer substrate immediately after cleaning and averaged. Immediately following monolayer deposition and cleaning, six thickness measurements were taken at different positions and averaged for each surface. A refractive index of 1.45 for the hydrocarbon monolayers was assumed.3B XPS. X-ray photoelectron spectra were obtained on a VG source ESCALAB Mark I1 spectrometer equipped with a Mg KCY (58) Jasper, J. J. J . Phys. Chem. Ref. Data 1972, 1, 841-1009,
Langmuir, Vol. 9, No. 11,1993 3025 (1253.6 eV), quartz monochromator, concentric hemispherical analyzeroperating a t a constant analyzermode, and a channeltron detector. The pressure in the analytical chamber during analysis mbar. A take-off angle of -30° from was approximately 1X the surface was employed. Survey spectra (&lo00 eV binding energy) were recorded with a 50-eV pass energy, 1.00-eV step size, W m s dwell time, 1-mm spot size, and 7 scans (350 a). High resolution spectra of the C1. region (274-294 eV binding energy, 15scans) and the Si%region (both the elemental and oxide peaks, 94-114 eV binding energy, 5 scans) were recorded with a 20-eV pass energy, 0.25-eV step size, 250-ma dwell time, and 1-mmspot size. The areas under the unsmoothed C1n and Siz, peaks were measured and the quantity In(CIJSi,) was used to compare different surfaces in a given series. ATR-IR. Attenuated total reflectance infrared spectra of an n-Cl8 monolayer formed on a 45O incidence, 50 X 10 X 1mm ATR crystal (Harrick Scientific) were obtained using p-polarized light on a Model RS loo00 FTIR (Mattson Instruments) spectrometer. The spectrum was run in a dry air (COZand H20 removed by a Balston 74-5041 pure air generator) filled sample compartment. One thousand scans were recorded at a resolution of 2 cm-l. A narrow-band MCT detector, cooled with liquid nitrogen, was used to detect the reflected light. The moving mirror speed was 2.5 cm/s (40 kHz) in the forward direction and 6.3 cm/s (100 kHz)in the reverse direction. Data were collectedon the forward scan. The monolayer spectrum was referenced to a background spectrum for the uncoated crystal obtained under the same conditions. Data Analysis. Curve-fitting was performed using the program KaleidaGraph from Synergy Software. As guides to the eye, the reference curves provided in Figures 3-5 were generated by fitting the contact angle, thickness, and X P S data using the weighted curve fit function. The general curve fit function was employed to determine the correlation between the corrected, normalized experimental data and the derived eq 13. Values of R,b corresponding to PSM midpoints were converted to logarithm form for ease of comparison with the data presented in Figures 3-5. The second-order polynomial curve fit function was used to derive values of yc from the Zisman plots.
Acknowledgment. The work was supported by Stanford University, a Camille and Henry Dreyfus Foundation New Faculty Award in Chemistry, a Shell Foundation Faculty Career Initiation Award, and the NSF-MRL Program through the Center for Materials Research at Stanford University. We are grateful to Dr. Michael Hochella for his assistance with the X-ray photoelectron spectrometer and to Professor C. E. D. Chidsey and Mr. M. R. Linford for access to and assistance with the FT-IR spectrometer.
