pubs.acs.org/Langmuir © 2009 American Chemical Society
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Stability of Binary SAMs Formed by ω-Acid and Alcohol Functionalized Thiol Mixtures Frederik Tielens,*,†,‡ Vincent Humblot,†,‡ Claire-Marie Pradier,†,‡ Monica Calatayud,§, and Francesc Illas^ UPMC Univ Paris 06 and ‡CNRS and UMR 7197, Laboratoire de R eactivit e de Surface, Tour 54-55, 2 eme etage - Casier 178, 4, Place Jussieu, F-75005 Paris, France, §UPMC Univ Paris 06 and CNRS, UMR 7616, Laboratoire de Chimie Th eorique, Casier 137, 4, Place Jussieu, F-75005 Paris, France, and ^Departament de Quı´mica Fı´sica & Institut de Quı´mica Te orica i Computacional (IQTCUB), Universitat de Barcelona, Martı´ i Franqu es 1, E-08028 Barcelona, Spain )
†
Received March 31, 2009. Revised Manuscript Received June 29, 2009 A simple model is presented to describe the mixing process of acid- and alcohol-terminated thiol self-assembled monolayers (SAMs) on Au(111). It was found that the low-concentration acid-terminated SAMs are enriched in acid thiol compared with the original solution from which the SAM is made. Due to the relatively strong interaction between acid and alcohol head groups, homogeneously mixed SAMs showing alcohol/acid pairs are preferred when the acidterminated thiol fraction in solution is below or equal to 50%. This particular behavior affecting the mixing process is explained using atomistic first-principle thermodynamics. The calculated phase diagram has been discussed and interpreted with the help of XPS.
Introduction Self assembled monolayers (SAMs) of n-alkanethiols on metals have attracted scientific interest and become technologically relevant targets, because they provide an easy and controlled way to create surfaces with specific chemical functionalities (nonpolar, polar, electroactive, biologically active).1,2 Under appropriate reaction conditions, terminal functional groups exposed at the surface of the SAM, immersed in a solution of ligands, can react directly with the molecules present in solution. Many direct immobilization techniques have been adapted from methods for DNA, polypeptides, and protein recognition reactions on glass or metal surfaces. Key parameters for the binding and further reactivity of these macromolecules are a good dispersion and a controlled density of the SAM terminal functions. Many of those SAMs consist of more than one component. Those so-called mixed SAMs can be prepared using different methods. One of the experimentally simplest and most broadly applied methods, developed for activating SAMs in a controlled way, is the formation of amide linkages via an interchain anhydride intermediate.3 In this method, the SAM is terminated with carboxylic groups dehydrated with trifluoroacetic anhydride to yield an interchain anhydride. This activated surface exposed to amines generates amide bonds on one out of two chains. This socalled “anhydride method” produces a SAM with a 1:1 mixture of functional groups on the surface (-COOH and -CONHR). *Corresponding author.
[email protected]. (1) Briand, E.; Salmain, M.; Compere, C.; Pradier, C. M. Colloids Surf., B: Biointerfaces 2006, 53, 215–224. (2) Briand, E.; Salmain, M.; Herry, J. M.; Perrot, H.; Compere, C.; Pradier, C. M. Biosens. Bioelectron. 2006, 22, 440–448. (3) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 6704–6712. (4) Takami, T.; Delamarche, E.; Michel, B.; Gerber, C.; Wolf, H.; Ringsdorf, H. Langmuir 1989, 11, 3876–3881. (5) Smith, R. K.; Reed, S. M.; Lewis, P. A.; Monell, J. D.; Clegg, R. S.; Kelly, K. F.; Bumm, L. A.; Hutchison, J. E.; Weiss, P. S. J. Phys. Chem. B 2001, 105, 1119–1122. (6) Li, L.; Cheng, S.; Jiang, S. Langmuir 2003, 19, 3266–3271. (7) Shevade, A. V.; Zhou, J.; Zin, M. T.; Jiang, S. Langmuir 2001, 17, 7566–7572.
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Another method is the adsorption of asymmetric disulfides.4-9 By this method, different mixed SAMs have been prepared, such as SAMs with thiols having different tail groups4,8 and chain lengths10 and others.9 A simpler way to produce mixed SAMs is the direct immersion of the support in a solution of a thiol mixture. Various mixed SAMs have been realized using this approach: alkanethiols with different chain lengths,11-13 and different terminal groups,4,6 or decanthiol and amide-containing alkanethiol of similar length.5 However, some drawbacks are found for the direct immersion method such as the formation of microscopic subdomains and the fact that the composition of the SAM may significantly differ from that in solution.12,14,15 Our group characterized experimentally and theoretically16-19 SAMs prepared by this method.1 Rather uniform SAMs (characterized via AFM1) were obtained, without requiring any chemical modification of the head groups. However, we were unable to estimate the quality of the acidalcohol-terminated thiol mixture. In the present work, SAMs constituted of thiols terminated by either an acid or an alcohol (8) Chen, S.; Li, L.; Boozer, C. L.; Jiang, S. J. Phys. Chem. B 2001, 105, 2975– 2980. (9) Sch€onherr, H.; Ringsdorf, H.; Jaschke, M.; Butt, H.-J.; Bamberg, E.; Allinson, H.; Evans, S. D. Langmuir 1996, 12, 3898–3304. (10) Heister, K.; Allara, D. L.; Bahnck, K.; Frey, S.; Zharnikov, M.; Grunze, M. Langmuir 1999, 15, 5440–5443. (11) L€ussem, B.; M€uller-Meskamp, L.; Karth€auser, S.; Waser, R.; Homberger, M.; Simon, U. Langmuir 2006, 22, 3021–3027. (12) Bain, C. D.; T., E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321–335. (13) Chen, S.; Li, L.; Boozer, C. L.; Jiang, S. Langmuir 2000, 16, 9287–9293. (14) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330– 1341. (15) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. J. Phys. Chem. 1994, 98, 563–571. (16) Tielens, F.; Humblot, V.; Pradier, C.-M. Int. J. Quantum Chem. 2008, 108, 1792–1795. (17) Tielens, F.; Costa, D.; Humblot, V.; Pradier, C.-M. J. Phys. Chem. C 2008, 112, 182–190. (18) Doneux, T.; Tielens, F.; Geerlings, P.; Buess-Herman, C. J. Phys. Chem. A 2006, 110, 11346–11352. (19) Tielens, F.; Humblot, V.; Pradier, C.-M. Surf. Sci. 2008, 602, 1032–1039.
Published on Web 07/24/2009
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group were prepared experimentally, and characterized by photoelectron spectroscopy (XPS). In parallel, the obtained mixed SAM layers were also modeled by first-principle techniques. The SAMs consisted of ω-substituted undecanethiol chains for the experiments and ω-substituted propanethiol for the theoretical calculations.1,2 At present, a representation of the composition and the structure of mixed SAMs is getting clear.20 However, the effect of the concentration of a two-thiol-mixture in solution on the finally obtained SAM is not perfectly understood. Only very few theoretical studies have been treated the mixing of thiols in the SAM.11 The present work aims to supply unbiased answers to the mechanisms of the formation of mixed SAMs by means of accurate density functional calculations carried out using appropriate periodic models. From the density functional energies, an analysis of the thermodynamical stability of the SAMs as a function of the thiol concentration is presented. The theoretical predictions are supported by XPS experiments. We provide evidence that the composition of SAM on the Au(111) surface is not equal to the composition of a binary mixture of 11-mercaptoundecanoic acid (MUA) and 11-mercaptoundecanol (MUOH) in solution throughout the whole range of composition; the mixing behavior and the structural arrangement on the surface are discussed, with respect to the formation of perfect mixed SAMs.
Experimental Details In the experiments, MUA and MUOH are used and purchased from Sigma Aldrich (St.-Quentin Fallavier, France). All solvents are reagent-grade. Reagents are used without any further purification. Experiments are carried out at room temperature if not specified otherwise. The surfaces are constituted of glass substrates (11 mm11 mm) coated successively with a 50 A˚ thick layer of chromium and a 200 nm thick layer of gold purchased from Arrandee (Werther, Germany). The gold coated substrates are annealed in a butane flame to ensure a good crystallinity of the topmost layers and rinsed in a bath of absolute ethanol 15 min before adsorption. The substrates are immersed in binary mixtures at 0.01 M (MUA100, 10 mM of MUA; MUA75, 7.5 mM of MUA and 2.5 mM of MUOH; MUA50, 5 mM of MUA and 5 mM of MUOH; MUA25, 2.5 mM of MUA and 7.5 mM of MUOH; MUA0, 10 mM of MUOH) in 10 mL of absolute ethanol for 3 h, in order to ensure an optimal crystallinity of the adlayer,2 and thorough rinsed in ethanol and Milli-Q water and dried under a flow of dry nitrogen. All XPS analyses have been performed on a PHOIBOS 100 X-ray photoelectron spectrometer from SPECS GmbH (Berlin, Germany) with the Mg KR X-ray source (hν=1253.6 eV) operating at 10-10 Torr or less. Spectra were recorded with a 20 eV pass energy for the survey scan and 10 eV pass energy for the C1s, O1s, S2p, and Au4f regions in a “fixed analyzer transmission” analysis mode, with a 7 20 mm2 entrance slit, leading to a resolution of 0.1 eV for the spectrometer. A takeoff angle of 90 from the surface was employed for each sample, and binding energies were calibrated against the Au4f7/2 binding energy at 84.0 eV, which leads to the CC, CH C1s contribution at 284.9 eV. Element peak intensities were corrected by Scofield factors,21 the spectra were fitted using the Casa XPS v.2.3.13 Software (Casa software Ltd. UK) and applying a Gaussian/Lorentzian ratio, G/L equal to 70/30. (20) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (21) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129–137.
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Computational Details The RPBE functional has been used to solve the periodic Kohn-Sham equations as implemented in the VASP code.22,23 PAW pseudopotentials are combined with a plane-wave basis (cutoff 500 eV). More details on the computational techniques and models are given elsewhere.16,17,19 a. Model. The SAMs are modeled using a repeated slab model for Au(111) consisting of five atomic layers , the two bottom layers fixed to bulk positions, and four thiol chains (See Figure 1). On the surface, S-C2H4-COOH (MPA, used as a model for MUA) and/or 3 S-C2H4-CH2OH (MPOH, used as a model for MUOH) is adsorbed, to optimize the prize/quality ratio of the calculation. This choice is justified because the interchain interactions are dominated in this case by the terminal groups, and the flexibility of the long alkyl chains is compensated by the rigidity of the SAM structure (crystallinity of long alkyls chains SAMs).2,24,25 It should be noted that beside the interaction with the Au(111) surface, the thiol chain interacts with its neighbors via hydrogen bonds between the tail groups; this is probably important in the self-organization of the SAM.17 Long-range C-H 3 3 3 C-H dispersion forces are also present, but are expected to be negligible compared with the strength of the H-bond interactions in the present case. √ √ The 2 32 3R30 unit cell used to build the different mixing configurations contains four thiol chains and about 110 atoms (from which 60 are Au atoms), depending on the thiol used (see Table 1). The schematic representation of the unit cell is represented by a square containing the letters A or/and B, with A representing an alcohol-terminated thiol and B an acid-terminated thiol. As was already observed in a previous study,17 the thiol chains tail groups interact with each other forming weak dimer complexes or pairs. With this taken into account, the models contain only relevant combinations of thiol pairing mixtures. The pure alcohol thiol SAM is modeled with a unit cell containing four alcohol thiol chains, the 25% acid thiol SAM is modeled by a unit cell containing one acid thiol and three alcohol thiol chains, and the 50% acid thiol has two alcohol chains and two acid thiol chains; three configurations were considered (see Table 1), AB/AB, AB/BA, and AA/BB, from which only two are shown. The AB/BA configuration has a similar energy to the AB/ AB one. The 75% acid SAM contains three acid thiol chains and one alcohol acid chain, and finally, the 100% acid SAM contains four acid thiol chains. A vacuum of 15 A˚ is used to separate the repeated slabs in the z-direction. The SAM systems are geometrically optimized without constraints, starting from different possible geometries. b. Energetics. It is usually assumed that stable SAMs involve thiolate radicals after RS-H bond cleavage through the reaction:26 RS-HþAuð111Þ f RS-Auð111Þþ1=2H2
ð1Þ
We define the adsorption energy of the thiol molecules on the Au(111) surface with respect to the RS 3 radical and not to the RSH thiol. Even in the simplest scenario of UHV conditions and well-characterized surfaces, many fundamental aspects of the initial adsorption of self-assembly process are still a matter of debate.26 So, since it is not the aim of this paper to discuss the adsorption process, but only to describe the final thiol SAM system, the adsorption energy was calculated as follows: (22) Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49, 14251–14269. (23) Kresse, G.; Furthm€uller, J. Comput. Mater. Sci. 1996, 6, 15–50. (24) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87–96. (25) Nuzzo, R. G.; D., L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558– 569. (26) Vericat, C.; Vela, M. E.; Benitez, G. A.; Matin Gago, J. A.; Torrelles, X.; Salvarezza, R. C. J. Phys.: Condens. Matter 2006, 18, R867–R900.
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Figure 1. (top) Selected side view of the model used in this work for the most stable mixed 50% MPA/MPOH thiol SAM on Au(111). (bottom) Top view of the same model showing the stabilizing H-bonds and the unit cell (surface gold atoms not shown). ΔEads ¼ EðSAMÞ - EðAuð111ÞÞ - nEðMPAÞ - ð4 -nÞEðMPOHÞÞ
ð2Þ
with n the number of acid thiols per unit cell. Negative numbers indicate exothermic adsorption processes with respect to the clean surface and the RS 3 radical. The larger the ΔEads, the larger the stability of the resulting layer. In order to include the effect of the relative concentrations in the adsorption process of a thiol mixture, the stability of the SAMs has to be investigated by considering the surface free energy, which is obtained through the chemical potential μ and the effective chemical potential Δμ defined as μ - μ (0 K). This computational modeling enables the inclusion of realistic thermodynamic conditions and the comparison of systems represented by different unit cells in a meaningful way.27 The stability of the (27) Torres, D.; Carro, P.; Salvarezza, R. C.; Illas, F. Phys. Rev. Lett. 2006, 97, 226103.
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various mixtures of acid- and alcohol-terminated thiols on the Au(111) surface was studied, using ab initio atomistic thermodynamics, a formalism already applied with success on several systems28-30 including SAMs.27 To represent realistic thermodynamic conditions, we consider the surface in contact with an acid thiol (MPA) and alcohol thiol (MPOH) atmosphere at a given pressure and temperature. The thiol molecules in the gas phase act as a reservoir interchanging molecules with the surface. One can define the surface free energy of the Au(111) surface with respect to the adsorbates (RS 3 radicals): ΔγðΔμÞ ¼
1 ½ΔEads -nMPA ΔμMUA -ð4 -nMPA ÞΔμMUOH -γclean A ð3Þ
(28) Sun, Q.; Reuter, K.; Scheffler, M. Phys. Rev. B 2003, 67, 205424. (29) Meyer, B. Phys. Rev. B 2004, 69, 045416. (30) Tielens, F.; Calatayud, M.; Franco, R.; Recio, J. M.; Minot, C.; Perez-Ramirez, J. Solid State Ionics 2009, 180, 1011-1016.
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Table 1. Schematic Representation of the Different Thiol Configurations in the Considered Unit Cell of MPA/MPOH Mixed SAMs with Their Corresponding Relative Concentrations and Adsorption Energies in eV (see text for definition)
where A is the surface area, n the number of acid thiol chains on the surface unit cell, ΔEads the adsorption energy defined in eq 2. Since the slab model exhibits two surfaces, a relaxed one covered with thiol molecules, and an unrelaxed clean one, γclean, the surface free energy of the unreconstructed clean one has to be subtracted because it does not play a role in the studied system.
Results and Discussion A first observation interesting to mention is the behavior of the gold surface layers upon thiol adsorption. In our previous study, we already mentioned the high mobility of the surface gold atoms. In the present calculations, we confirm this trend, and one can even speculate that reconstructions of the (111) surface are more than probable. Recently, the adsorption of thiols on surface gold adatoms was proposed to be the most stable adsorption geometry.31-33 Geometry optimization of the perfect (111) surface after thiol adsorption goes into that direction. Gold atoms are pulled out of the surface as can be seen in Figure 1. Pure alkyl thiol chain adsorption does not affect the Au(111) surface as much as ω-terminated alkyl thiols do, due to their tail-tail interactions (results not shown here). All adsorption energies calculated using eq 2 and schemes representing the thiol organization are shown in Table 1. Two general remarks should be given to the adsorption energies. As can be seen, the energy differences between the different phases are relatively small, so that entropy effects may also be important in the stabilization of the SAM. RPBE energies, which have shown their validity for adsorption processes, might, however, be at the limit of accuracy. This stresses the importance of experiments (see XPS data) supporting the calculations. From these adsorption energies, one can conclude that MPA is more strongly adsorbed to the Au(111) surface than MPOH, with a difference of 0.36 eV per thiol chain. A question arising from this finding is: In the presence of a solution containing the two thiols, will the surface be covered by a homogeneous MPA (31) Fonticelli, M. H.; Benitez, G.; Carro, P.; Azzaroni, O.; Salvarezza, R. C.; Gonzalez, S.; Torres, D.; Illas, F. J. Phys. Chem. C 2008, 112, 4557. (32) Wang, J. G.; Selloni, A. J. Phys. Chem. C 2007, 111, 12149. (33) Gr€onbeck, H.; H€akkinen, H.; Whetten, R. L. J. Phys. Chem. C 2008, 112, 4557.
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Figure 2. XPS high-resolution spectra of C1s for SAMs with different MUA/MUOH concentrations.
(acid-terminated thiol) SAM, or at least by a strongly MPAenriched layer? The first answer to that question was brought up by performing XPS characterization of gold surfaces after their immersion in MUA/MUOH thiol mixtures. Figure 2 shows the high-resolution XPS spectra in the region of the C1s peak corresponding to different SAM mixtures. On the pure MUA layer (100% MUA), the C1s peak could be decomposed into three contributions, DOI: 10.1021/la901127g
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at 284.9, 286.0, and 289.3 eV, respectively. The first one at 284.9 eV is obviously due to the carbon atoms of the aliphatic chain (C-C, C-H); the second one at 286.0 eV may be ascribed to the carbon in R position of the COOH terminal group, while the last one at 289.3 eV is clearly attributed to the carbon in COOH or COO- groups. Turning now to the pure MUOH layer (0% MUA), only one contribution, in addition to the one of the aliphatic carbons at 284.9 eV, was necessary to fit the C1s peak, recorded at 286.6 eV and was assigned to the carbon atoms in the CH2-OH tail groups. Eventually, on the mixed MUOH/MUA layer, the C1s peak was best fit with four contributions consistent with the fact that both MUA and MUOH are present on the surface. One can notice the decrease of the CdO and C;COOH contributions at 289.3 and 286.0 eV, alongside with the expected increase of the C;OH contributions at 286.6 eV, as a function of decreasing percentage of MUA in solution, while the C-C, C-H contribution at 284.9 eV remains almost constant.
Figure 3. Comparison between calculated atomic % (star) and experimental atomic % (square) of the acidic carbon contribution in solution vs at the surface. The atomic % is defined as CdO(OH) contribution/C total contribution.
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Comparing the areas of the C 1s contribution at 289.2 eV, enables to monitor the evolution of the CdO group as a function of the MUA% in solution, and thus of the SAM composition. In Figure 3, the dashed line represents the fraction of the COOH/ COO-, C 1s contribution corresponding to the surface concentration equal to that in solution, while the plain line is a polynomial fit of the experimental XPS values. One can notice that, for MUA% in solution between 0 and 50%, the fraction of adsorbed MUA is higher than that in solution, while for the higher fraction of MUA in solution, between 50% and 100%, the percentage of adsorbed MUA is very similar to the one expected. In other words, the MUA low-concentration SAMs are enriched in MUA compared with the original solutions. This result is supported by the calculated adsorption energies of the thiols. The thiols adsorb on the Au(111) surface as weak dimers linked together via their tail groups. A H-bond is formed between the different alcohol and acid groups, thus forming thiol pairs. The adsorption energy increases with increasing acid thiol concentration. In other words: A perfect homogeneous layer of mixed acid-alcohol thiol pairs (Table 1, c) is more favored than acid-alcohol pairs surrounded by alcohol thiol pairs (Table 1, b), but less favored than acid-alcohol pairs surrounded by acid thiol pairs (Table 1, e). This behavior shows that it is easier to obtain homogeneous mixtures in MPOH than in MPA excess conditions, since the A-B pairs are more stabilized surrounded by MPOH (A) than by MPA (B) thiols. The results shown in Figure 3, concerning the compositions of the SAM in comparison with that of the solution, can also be explained by the calculated phase diagram (Figure 4). This phase diagram was obtained using eq 3 and shows the effect of the relative concentration of acid/alcohol-terminated thiol on the SAM structure. It should be noted that, on the basis of the computed results, on a very limited number of highly ordered structural models with simple MPA-MPOH stoichiometries (0, 1/ 4, 1/2, 3/4, 1), one cannot rule out the existence of ordered or disordered homogeneous phases for a general stoichiometry. Looking for more combinations is unfortunately out of our present computational possibilities. Nevertheless, the proposed models are in line with the experimental XPS results, and based on
Figure 4. Calculated surface Gibbs free energy diagram as a function of the acid thiol (MPA) and the alcohol thiol (MPOH) chemical potential, showing the domains of the different stable SAM mixtures on Au(111). 9984 DOI: 10.1021/la901127g
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Figure 5. Schematic representation of the SAM composition on Au(111) as a function of the MPA concentration.
limited but reliable experimental and theoretical data, a possible scenario for the process of increasing MPA concentration in a MPOH SAM is schematized in Figure 5 and explained as follows. The low MPA concentration SAM (25%, e.g.) is not present in the phase diagram. Adding some MPA to a 100% pure MPOH SAM thus favors the formation of perfectly 50% MPA/MPOH mixed SAMs in a “sea” of MPOH rather than isolated MPA chains well dispersed in the MPOH SAMs (Figure 5b,c). This behavior can be explained as a consequence of the higher stability of an equally mixed (50%) SAM compared with the 25% MPA SAM (-4.78 vs -4.38 eV). Increasing further the MPA concentration favors the perfectly mixed SAM, and the mixed SAM domains become more important than the pure MPOH SAM domains (Figure 5d), until the whole surface is covered with a homogeneous mixed MPOHMPA SAM (Figure 5e). The 50% MPA SAM prefers to be homogeneously mixed, forming MPA-MPOH pairs (Table 1, Figure 5c,e). MPOH-MPOH and MPA-MPA pairing configuration of the thiol chains (Table 1, d) is less stable by 0.25 eV. If the MPA concentration increases beyond 50%, some MPA chains will not be able to pair with a MPOH chain, and the homogeneously mixed SAM starts to show domains with isolated MPOH chains in a “sea” of a 50% mixed SAM (Table 1 e and Figure 5f). At 75% MPA, the surface is completely covered by a SAM constituted of MPOH chains surrounded by MPA chains (Table 1, Figure 2 and Figure 5g). Between 75% and 100% of MPA, domains of pure acid thiol layers surrounded by the 75% phase are formed (Figure 5h), until no MPOH chains are present on the surface, forming a 100% MPA SAM. In summary, the SAMs containing proportionally more alcohol thiol are enriched in acid thiol because the formation of
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isolated acid thiol chains is not favorable, as shown in the phase diagram (Figure 4). The corresponding 25% phase is not present in the diagram. In contrast, the symmetric “opposite” phase, i.e., 75% of MPA, is stable, and the 50% phase does not dominate at this concentration as for 25% of MPA where the formation of domains with different compositions (segregation) is favored.
Conclusions The combination of theoretical quantum calculations and experimental XPS spectra was successfully used to describe the mixing behavior of acid thiol/alcohol thiol binary SAMs. The gold surface is found to be more affected after adsorption of terminated thiols than after the adsorption of non-interacting pure alkyl thiols. From XPS experiments, it was concluded that immersing a gold surface in a MUA-MUOH solution results in a SAM enriched in acid thiol at low acid thiol solution concentrations. This result is in line with the first-principle results, which predict an unstable 25% acid thiol phase, in contrast with the symmetric opposite phase, the 75% acid thiol phase, which is stable, and the 50% phase that does not dominate at this concentration as for the 25% acid thiol phase where the formation of domains with different compositions is favored (50% phase and pure alcohol thiol phase). This finding may be very important when using mixed SAMs to attach receptor molecules, for instance, to the acid terminal functions of the thiol. Acknowledgment. The computation facilities are provided by IDRIS, CINES and by CCRE (Universite Pierre et Marie Curie). A significant part of the computing time was provided by the Barcelona Supercomputing Centre (BSC).
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