pubs.acs.org/Langmuir © 2010 American Chemical Society
Beyond Cassie’s Law: A Theoretical and Experimental Study of Mixed Alkyl Monolayers David Polster, Harald Graaf,* Thomas Baumg€artel, and Christian von Borczyskowski Institute of Physics and nanoMA (Center for Nanostructured Materials and Analytics), Chemnitz University of Technology, D-09107 Chemnitz, Germany
Udo Benedikt and Alexander A. Auer Department of Chemistry, Chemnitz University of Technology, D-09111 Chemnitz, Germany Received December 14, 2009. Revised Manuscript Received March 30, 2010 Within this study, the influence of ester groups in mixed monolayers on the surface properties will be discussed. Detailed investigations on the macroscopic and microscopic characteristics on mixed monolayers with different content of ester groups in an alkyl surrounding are done by contact angle measurements and atomic force spectroscopy. Density functional theory (DFT) calculations show a statistical distribution and a directed orientation of the ester molecules. In the experiments an increasing amount of ester groups leads to a fast increasing polarity followed by a nearly constant polarity in the regime of 25% and 40% of ester in the monolayer and a further increase at higher amounts of ester groups, which clearly differ from the behavior expected by Cassie. By DFT calculations it can be shown that water molecules form ring-like structures around the ester group. These solvent shells increase the hydrophilic fraction on the surface explaining the disproportional growth in the polarity of the monolayer. This rise in polarity is maximal for single ester groups (monomers) or dimers of esters. The amount of these monomers and dimers is estimated by Monte Carlo simulation showing clearly that the linear regime at fractions between 0.25 and 0.4 are caused by the transition from mainly monomers to mainly dimers. Thus, we show for the first time that adsorbed water molecules forming a solvent shell around hydrophilic groups in hydrophobic surroundings influence the surface properties of mixed monolayers on a macroscopic and microscopic scale which therefore must be taken into account when preparing, investigating, using and understanding such monolayers.
Introduction In nearly all biological, technical, and everyday situations, surfaces and surface treatments are of vital importance. For example the essential breathing mechanism is impossible without special surfactants.1 To improve the design of surfaces it is necessary to create and study different procedures of surface treatment. Self-assembled organic monolayers are an often used method to create modified surfaces. Therefore, many studies have been performed on such systems,2 whereby some characteristics are only apparent on detailed investigations. This is one of the aims of the present paper. To study surface conditions of organic monolayers on silicon, contact angle and atomic force microscopy (AFM) measurements are a common approach.2-8 Many studies focus on static contact *Corresponding author. E-mail:
[email protected]. Telephone: þþ49-371-531-34807. Fax: þþ49-371-531-834807.
(1) Nogee, L. M.; Wert, S. E.; Proffit, S. A.; Hull, W. M.; Whitesett, J. A. Am. J. Respiratory Crit. Care Med. 2000, 161, 973. (2) Extrand, C. W. Langmuir 2004, 20, 4017. (3) Janssen, D.; De Palma, R.; Verlaak, S.; Heremans, P.; Dehaen, W. Thin Solid Films 2006, 515, 1433. (4) Liu, Y.-J.; Navasero, N. M.; Yu, H.-Z. Langmuir 2004, 20, 4039. (5) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudh€olter, E. J. R. Adv. Mater. 2000, 12, 1457. Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudh€olter, E. J. R. Adv. Mater. 1996, 118, 7225. (6) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudh€olter, E. J. R. Langmuir 1998, 14, 1759. (7) Sieval, A. B.; Vleeming, V.; Zuilhof, H.; Sudh€olter, E. J. R. Langmuir 1999, 15, 8288. (8) Sieval, A. B.; Linke, R.; Heij, G.; Meijer, G.; Zuilhof, H.; Sudh€olter, E. J. R. Langmuir. 2001, 17, 7554. (9) (a) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 1741. (b) Kaelble, D. H. J. Adhes. 1970, 2, 66. (c) Rabel, W. Phys. Bl€ atter. 1977, 33, 151.
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angles3,5,9a only, where static water contact angle measurements are the most common method4,5. Some AFM investigations have been carried out to estimate the adhesion between AFM tip and substrate using force spectroscopy.10,11 However, to the best of our knowledge a study that makes use of all of the abovementioned methods to characterize the surface of monolayers has not yet been reported. Monolayers are often prepared on silicon oxide.2,8,12,13 Organosilicon derivates with reactive groups such as chlorine or alkoxy groups are used for the preparation of the monolayer on silicon covered with silicon oxide (native or thermally grown). Polysiloxane intermediates interact with surface silanol groups forming Si-O-Si bonds. In contrast, alkenes and their derivates can not bind to surface related silanol groups to form stable monolayers.4,8 To prepare alkyl monolayers,the silicon oxide has to be chemically removed, resulting in hydrogen-terminated silicon. This hydrogen-terminated surface can react with the alkenes by formation of Si-C bonds.4,8,12,13 By using functionalized alkenes (e.g., carboxyl, hydroxyl, amino, or ester groups), the monolayer can be easily modified and in this manner the surface properties can be widely varied.4,14,15 (10) Alsteens, D.; Dague, E.; Rouxhet, P. G.; Baulard, A. R.; Dufr^ene, Y. F. Langmuir 2007, 23, 11977. (11) Shi, W.; Zhang, Y.; Liu, C.; Wang, Z.; Zhang, X. Langmuir 2008, 24, 1318. (12) Boukherroub, R.; Wayner, D. D. M. J. Am. Chem. Soc. 1999, 121, 11513. (13) Bansal, A.; Li, X.; Lauermann, I.; Lewis, N. S. J. Am. Chem. Soc. 1996, 118, 7225. (14) Strother, T.; Cai, W.; Zhao, X.; Hamers, R. J.; Smith, L. M. J. Am. Chem. Soc. 2000, 122, 1205. (15) Pike, A. R.; Lie, L. H.; Eagling, R. A.; Ryder, L. C.; Patole, S. N.; Connolly, B. A.; Horrocks, B. R.; Houlton, A. Angew. Chem., Int. Ed. 2002, 41, 615.
Published on Web 04/26/2010
DOI: 10.1021/la9046935
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Different methods have been employed in order to form monolayers of alkenes and their derivatives on hydrogenterminated silicon, including catalytic, electrochemical, photochemical, and thermal reactions.4,8,12,16 The last two methods are mostly used because of their mild reaction conditions, low costs and high quality of monolayers.4,8,12 The mechanism of the monolayer formation is a three step mechanism, where the homolytic cleavage of the hydrogen silicon bond induced by heat or irradiation with ultraviolet light is followed by a reaction of the formed silicon radical with the double bond of the alkene. Finally the radical at the alkyl chain leads to the formation of an additional silicon radical by interaction with hydrogen of a neighboring Si-H bond.17 This radical reaction mechanism can be used to prepare monolayers of solely one species of alkene molecules as well as mixed ones containing also ester-terminated alkenes.4,18,19 The ester groups do not react with the hydrogenterminated surface, which means that high quality monolayers with free functional groups are formed.4,6,18,19 In the present work a series of mixed 1-decene (DEC)/methyl10-undecenoate (MUD) monolayers on silicon were prepared and characterized. The aim of this study is to investigate the distribution of MUD within the DEC layer experimentally and theoretically. Of special interest are the contact angles and the related surface tension which appear between the liquid and the monolayer in comparison to the quantitative model reported by Cassie.20 A deviation from this model has been reported for some few mixed monolayers earlier, but the nature of this deviation was still unclear.4 Therefore, we studied the dependence of contact angle and surface tension by a series of different mixing ratios between DEC and MUD. Theoretical calculations allow us to understand the deviation especially for low molar ratios on a molecular level. As the contact angle method can only investigate large areas of a sample and will therefore integrate over this area, we used investigations of adhesion by force spectroscopy with an atomic force microscope (AFM) to determine the local properties of the film. Finally, we report a comparison of the microscopic and the macroscopic findings.
Experimental Section All chemicals were of reagent grade or the highest available commercial grade quality and purchased from Merck KGaA (Darmstadt, Germany). Deionized water (>18.2 MΩ 3 cm; 4 ppb TOC) was obtained from a Millipore Milli-Q Advantage water system (Millipore, Billerica, MA). Silicon (100) substrates (>3000 Ω 3 cm, n-type 10 10 mm2) were cleaned in acetone (at 40 °C), ethanol (40 °C), “piranha” (volume ratio H2SO4:H2O2 3:2, 80 °C, Caution! Piranha solution is highly corrosive and should be handled with care) (all liquids are from Merck in spectroscopic grade), and finally rinsed with ultrapure water. The cleaned silicon substrates were etched for about 2 min in hydrofluoric acid (aqueous solution, 3-4%) to remove native oxide and to form a hydrogen terminated silicon surface. Afterwards, the substrates were transferred directly into a flask containing ∼ 20 mL of 1-decene/methyl-10-undecenoate mixture (the mixtures were prepared in molar fractions, the chemical structures of the two molecules are shown in Figure 1) which was dissolved in mesitylene (volume ratio 1: 2) (all the reagents are from Merck in purest quality). This solution was deoxygenated prior to use for at least 30 min with argon. For the (16) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631. (17) Lopinski, A. B.; Wayner, D. D. M.; Wolkow, R. A. Nature 2000, 406, 48. (18) Aureau, D.; Ozanam, F.; Allongue, P.; Chazalviel, J.-N. Langmuir 2008, 24, 9440. (19) Condorelli, G. G.; Motta, A.; Bedoya, C.; Di Mauro, A.; Pellegrino, G.; Smecca, E. Inorg. Chim. Acta 2007, 360, 170. (20) Cassie, A. B. D. Discuss. Faraday Soc. 1948, 3, 11.
8302 DOI: 10.1021/la9046935
Figure 1. Molecular structures of 1-decene (a) and methyl10-undecenoate (b). reaction the solution with the silicon was boiled in the presence of bubbling argon for at least 5 h (the temperature was about 160-170 °C). Such a reaction time of a few hours is found to be suitable for the formation of monolayers with all-trans conformation of the alkyl chains.21 Afterwards, the modified substrate was rinsed with deionized water and cleaned in acetone and ethanol (all liquids are from Merck in spectroscopic grade). Contact angle measurements were carried out under ambient conditions with an OCA20 Data Physics (Data Physics Instruments, Filderstadt, Germany). To determine the contact angles the sessil-drop method was used: A liquid drop is growing on the syringe tip and picked up by the surface. After 10 s the drop is in equilibrium and the static contact angle is measured. AFM measurements were performed with a Level-AFM (Anfatec Instruments, Oelsnitz, Germany). For roughness measurements in the dynamic noncontact mode a Si-Cantilever (k = 5 N/m; υ = 160 kHz) and for force spectroscopy a Pt-coated cantilever (k = 3.5 N/m) were used. All measurements in one set of samples were performed with the same cantilever to avoid differences in the spring constant and slight geometric deviations. DFT calculations were carried out using the TURBOMOLE program package.22-24 For preoptimizations, the UFF module25 was used. All other calculations were carried out at the RI-DFT (BP-86/SVP) level of theory.26-29 In order to obtain a model for a monolayer on a Si-surface a slab was extracted from the X-ray crystal structure30 of the Si-(111) surface. It should be noted that while for the experiments the Si(100) surface was used, we have chosen to build the model for the theoretical investigations on a Si(111) surface as for this case previous investigations exist31 that contain detailed information about the molecular structure of the monolayer. Additionally, former experimental results show clearly that even compared to Si(111) the increased surface roughness of Si(100) which contains besides SiH also SiH2 and SiH3 groups,32 the monolayers formed on Si(100) surface are characterized by the same film thickness, electron density (and therefore density of alkyl chains) and physical properties as for similar SAMs on Si(111).6 It has been also shown by the same authors that the preparation mechanism— heat induced or light induced—do not change the properties of the (21) Ara, M.; Yamada, R.; Tada, H. Thin Solid Films 2006, 499, 8. (22) TURBOMOLE V5.8 2005, a development of University of Karlsruhe and Forschungszentrum Karlsruhe GmbH, 1989-2007, TURBOMOLE GmbH, since 2007; available from http://www.turbomole.com. (23) Ahlrichs, R.; B€ar, M.; H€aser, M.; Horn, H.; K€olmel, C. Chem. Phys. Lett. 1989, 162, 165. (24) Von Arnim, M.; Ahlrichs, R. J. Comput. Chem. 1998, 19, 1746. (25) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024. (26) Treutler, O.; Ahlrichs, R. J. Chem. Phys. 1995, 102, 346. € (27) Eichkorn, K.; Treutler, O.; Ohm, H.; H€aser, M.; Ahlrichs, R. Chem. Phys. Lett. 1995, 240, 283. € (28) Eichkorn, K.; Treutler, O.; Ohm, H.; H€aser, M.; Ahlrichs, R. Chem. Phys. Lett. 1995, 242, 652. (29) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem. Acc. 1997, 97, 119. (30) Becker, P.; Seyfreid, P.; Siegert, H. Z. Phys. B 1982, 48, 17. (31) Sieval, A. B.; van den Hout, B.; Zuilhof, H.; Sudh€olter, E. J. R. Langmuir 2001, 17, 2172. (32) Dumas, P.; Chabal, Y. J.; Jakob, P. Surf. Sci. 1992, 269/270, 867.
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Figure 2. (A) Substitution pattern of the Si(111) surface, hydrogen atoms are not shown. The centers labeled with “f” denote fixed chains and Alkyl chains indicated as 1 to 7, are allowed to relax during the optimization of the surface slab model. (B) View from side (alkyl chain 5 in the front) onto the DFT reduced slab model used in all quantum-chemical calculations for which all atoms in black and gray are fixed to minimize artifacts due to truncation of the slab model. obtained monolayer. Furthermore, for the simulations discussed in the following only a slice of the top of the monolayer is used. Because of this, the choice of an underlying Si(111) surface can be regarded as a technical issue and one can assume that the details of the underlying structure are of little importance for the properties of the surface of the SAM. The surface atoms of the Si(111) slab were saturated with hydrogen atoms and 50% of these surface hydrogen atoms were replaced with a C10H21 alkyl chain with a defined angle to the surface and all chains oriented in the same direction. The alkyl-chain substitution pattern was chosen as proposed by Sieval et al.31 as shown in Figure 2A. This structure was relaxed in a 300-step UFF optimization where groups at the edge of the slab (positions marked by an “f” in Figure 2A) were fixed in order to constrain the inner alkyl chains (at positions 1 to 7) to avoid effects of using the finite slab. This way, a model structure for the description of wetting processes and the influence of the substitutions on the surface of the SAM is obtained which does not only allow for detailed electronic structure calculations but should be transferable to similar SAMs on different substrates. To obtain a computationally feasible model of the surface that allows for several electronic structure calculations a reduced surface slab was constructed. For this purpose, the relaxed slab model was further reduced to a structure containing 241 atoms by cutting each of the alkyl chains to the topmost four atoms saturating of the open valences with hydrogen and thus neglecting the underlying Si-surface and parts of the alkyl chains. In order to avoid artificial geometry distortions due to the finite size of the reduced model slab the outermost molecules and also the CH3 groups at the bottom of the other molecules were fixed during all further calculations (atoms marked in black and gray in Figure 2B).
Results Pure as well as mixed monolayers with varying composition of MUD and DEC were prepared on hydrogen terminated silicon substrates by radical process as described previously,33,34 The samples were investigated on a macroscopic scale by static contact angle measurements and on the microscopic scale by AFM force-distance-curve measurements. Macroscopic Investigations. Static contact angles were obtained for mixed monolayers with different ratios of MUD (33) Buriak, J. M. Chem. Commun. 1999, 1051. (34) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudh€olter, J. R. Adv. Mater. 2000, 12, 1457.
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Figure 3. Static contact angle values of water (squares) and diiodomethane (stars) of mixed monolayers (1-decen/methyl-10-undecenoate-monolayers) as a function of mole fraction of methyl10-undecenoate in solution. The dotted lines are guides for the eye.
to DEC. Typical results are shown in Figure 3, where the contact angle of two liquids (water and diiodomethane) are displayed as a function of MUD content. Both liquids show generally the same behavior: For very small MUD contents (
