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J. Phys. Chem. C 2007, 111, 4294-4304
Structure of Mixed Carboxylic Acid Terminated Self-Assembled Monolayers: Experimental and Theoretical Investigation M. L. Carot,† V. A. Macagno,† P. Paredes-Olivera,‡ and E. M. Patrito*,† Departamento de Fisicoquı´mica and Unidad de Matematica y Fı´sica, Facultad de Ciencias Quı´micas, UniVersidad Nacional de Co´ rdoba, Instituto de InVestigaciones en Fisicoquı´mica de Co´ rdoba (INFIQC), Ciudad UniVersitaria, 5000 Co´ rdoba, Argentina ReceiVed: October 3, 2006; In Final Form: January 16, 2007
The structure and stability of mixed self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) and 11-mercaptoundecanoic acid (MUA) prepared by immersion in ethanolic solutions were studied by cyclic voltammetry, electrochemical impedance spectroscopy, ellipsometry, and STM as a function of the thiol composition of the forming solution. The presence of a single reductive desorption peak in the voltammograms of the mixed SAMs and the lack of phase segregation observed by STM support the formation of homogeneous SAMs despite the large chain length difference between MPA and MUA. To explain the driving force leading to the formation of a homogeneous mixture, intermolecular interactions within the SAM were investigated using density functional theory. The carboxyl groups of adjacent MPA and MUA molecules in a compact monolayer can form a stable head to head cyclic dimer with a hydrogen bond strength of 16.2 kcal/mol. The flexibility of the alkyl chain of MUA allows the carboxyl groups of adjacent MPA and MUA molecules to be located on the same plane. However, the carboxyls of adjacent MPA-MPA and MUA-MUA pairs form much weaker hydrogen bonds because steric constraints avoid the formation of the stable cyclic dimer. Therefore, the prevalence of MPA-MUA interactions over MPA-MPA and MUA-MUA interactions explains the homogeneous mixing of MPA and MUA. The potential chemical switchability properties of mixed monolayers of mercaptoalcanoic acids of different chain lengths as a function of the solution pH are discussed.
Introduction Surface modification by using ω-functionalized alkanethiols is the simplest way to create chemically or biologically functionalized surfaces. One of the most interesting functions in this context is a carboxylic acid group, since -COOHterminated surfaces are of interest for a wide range of applications in surface science,1-7 electrochemistry,8,9 biomineralization,10,11 surface engineering,12-15 sensor development,16 nanoparticles,17,18 and biology.19-23 Often this type of SAM is used as a starting point for fabricating biologically active surfaces to immobilize proteins or other biological agents for further analysis.24,25 Reversible switching surfaces have been designed with carboxylate-terminated SAMs.26 The molecules were deposited in dilute fashion on a surface, and the carboxylate end group was electrochemically pushed toward or away from the electrode with the associated conformational and hydrophilic-to-hydrophobic surface-property changes. In case of the alkanethiols (CH3 termination), the adsorption process leads to the formation of a highly ordered, crystalline adlayer.27 It is not certain a priori whether replacing the terminal -CH3 group by other functions is compatible with this high degree of order or whether possible interactions between the functionalities added to the chain termini lead to a different ordering pattern or even to disorder. Chain dynamics in selfassembled monolayers28,29 of alkanethiolates on gold results in * To whom correspondence should be addressed. Phone: 54-3514334169/54-351-4334180. Fax: 54-351-4334188. E-mail: martin@ fcq.unc.edu.ar. † Departamento de Fisicoquı´mica. ‡ Unidad de Matematica y Fı´sica.
surface reconstruction due to trans-gauche conformational changes in chain termina. This process is especially noticeable in hydroxy-terminated SAMs.30 Driven by the formation of correlated hydrogen bonds, and to minimize surface free energy, terminal hydroxy groups get buried in the surface, resulting in an increase of the measured water contact angle from ∼20 to ∼60°, and in hydroxyl groups less available for chemical reactions, such as esterification.30 Unlike CH3-terminated SAMs, the quality of COOH- and NH2-terminated SAMs is harder to control. For COOH-SAMs, a wide range of contact angles have been reported, such as 70°.33 Therefore, improved methods for the preparation of carboxylic acid34,35 and amine34 terminated SAMs have been proposed. The formation of better SAMs can be attributed to the disruption of interplane hydrogen bonds.34 A rather strong dependence on preparation conditions is observed for carboxyl-terminated SAMs. One of the first comprehensive studies36 of the structure and formation of mercaptohexadecanoic (MHA) acid adsorbed on Au(111) observed well-packed SAMs through the sharpness of Fourier transform infrared (FTIR) CH2 stretching peaks; this study determined that the films exhibited a high degree of molecular orientation and that there was only a small number of carboxylic acid groups linked together by hydrogen bonds. A later IR study37 also found a highly oriented structure of a MHA SAM. These authors considered the presence of slightly distorted headto-head bound -COOH‚‚‚HOOC- dimers consistent with the presence of a highly oriented SAM. Subsequent, near-edge X-ray absorption fine structure (NEXAFS) measurements showed a high degree of disorder in these films.38,39 It was proposed that the disorder is linked to the high flexibility of
10.1021/jp066513v CCC: $37.00 © 2007 American Chemical Society Published on Web 02/23/2007
Mixed Carboxylic Acid Terminated SAMs the alkyl chain anchoring the carboxylic acid to the substrate or to hydrogen-bond formation between neighboring carboxylic groups. However, subsequent FTIR studies, with a strict protocol for SAM preparation, confirmed the ordering in these SAMs that is highly dependent on the state of the carboxyl group.40 The flexibility in surface design is greatly enhanced by the use of mixed SAMs. The structure of this type of surfaces results from the competition between thermodynamic and kinetic controls. This was recognized in the early studies by Whitesides and co-workers.41,42 Kinetic factors favor the formation of a random mixture of adsorbates, while thermodynamics favors the formation of phase-separated domains of different adsorbates. When coadsorbed from a two-component solution, the monolayers exhibit increasing tendency toward phase separation on the mesoscopic scale with increasing difference in chain lengths, regardless of their terminating groups.41-45 This is because the van der Waals interactions among the hydrocarbon chains overcompensate the entropy of mixing the more the components differ in their number of methylene units. Molecular dynamics simulations report on a threshold value of the chain length difference of 3 methylene units.46 If the hydrocarbon backbones are of equal chain length, most studies including reductive desorption,47,48 wetting,44 or friction force microscopy49 suggest rather perfect mixing despite differences in the polarity of the terminating groups. However, high-resolution STM investigations50,51 have found nanoscale domains for certain systems (mixed SAMs with -CH3 and -OCH3 endgroups, C16 backbone). The terminal functionalities of compact single-component monolayers often show steric packing which limits their accessibility for further reactions. In this context, mixed SAMs with different chain lengths may provide an alternative for embedding the longer alkanethiol with the reactive terminal group in a matrix of a shorter alkanethiol. For example, the amount of immobilized DNA reaches a maximum at a surface composition of 5% acridine derivative.52 The association constant of carbonic anhydrase reaches its maximum when the surface composition of benzenesulfonamide group is about ∼10%.53 The immobilization of catalase decreases in the order of mixed SAMs > 11-mercaptoundecanoic acid > 3-mercaptopropanoic acid due to steric effects.25 However, the formation of homogeneous mixtures of alkanethiols of different chain lengths is difficult to achieve due to phase segregation.54-56 One approach to avoid phase segregation in mixed SAMs of different chain lengths is by coadsorption at high temperatures.57 The formation mechanism under these conditions is close to a kinetically controlled process, thus leading to a homogeneous mixture. In this work we show that homogeneous mixed SAMs of carboxylic acid terminated SAMs can be obtained at room temperature from ethanolic solutions. The structure of mixed SAMs of MUA + MPA was investigated from an experimental and a theoretical point of view. Despite the large chain length difference, the mixture is homogeneous, indicating that interactions involving the -COOH group prevail over the van der Waals interactions among the alkyl chains. DFT calculations show that the flexibility of the MUA alkyl chain allows the formation of stable head-to-head cyclic dimers between the carboxylic groups of adjacent MUA and MPA. The lack of phase segregation in these mixed SAMs is ascribed to the prevalence of MPA-MUA interactions over the interactions of the like molecules: MPA-MPA and MUA-MUA.
J. Phys. Chem. C, Vol. 111, No. 11, 2007 4295 Experimental Section Au films (500 nm thick) evaporated on heat-resistive glass were employed as substrates. These substrates were annealed in a hydrogen flame for 2 min and cooled to room temperature in a stream of nitrogen. The voltammograms were obtained with an electrochemical workstation (IME6, Zahner, Germany) and a conventional electrochemical cell with separate compartments for reference (Ag/AgCl/Cl-) and counter electrode (Pt wire). Impedance spectra were recorded in the frequency range 0.1 Hz-10 kHz. The signal amplitude to perturb the system was 10 mV. 3-Mercaptopropionic and 11-mercaptoundecanoic acids were purchased from Sigma-Aldrich. Self-assembled monolayers were prepared by immersing the previously annealed gold substrates in millimolar ethanolic solutions of the thiols for 12 h. All chemicals were used as received without further purification. Electrolytic solutions were prepared with Milli-Q water (Millipore Corp., Billerica, MA). Merck suprapure chemicals were employed to prepare 0.1 M KOH and 0.025 M phosphate buffer. The electrolytes were thoroughly deaereated by bubbling with nitrogen prior to each experiment. The STM measurements were performed with a Molecular Imaging microscope (Phoenix, AZ), using tungsten tips electrochemically etched from a 0.25 mm diameter wire in aqueous 2.5 M NaOH. To minimize faradaic currents at the tipelectrolyte interface, the tips were coated with Apiezon wax. A platinum wire was used in the STM cell as counter electrode. A Rudolph Research rotating-analyzer automatic ellipsometer with a 75 W tungsten halogen light source was used. All measurements were made at an angle of incidence of 70.00°. Measurements were performed at three wave lengths: 632.8, 546.1, and 405.0 nm. Theoretical Methods and Surface Modeling The MUA-MPA, MPA-MPA, and MUA-MUA interactions were first investigated in vacuum using localized basis sets at the PW91/6-31+G(d,p) level of theory. The Gaussian03 program was used in these calculations.58 The interaction between MUA and MPA on the Au(111) surface was modeled using the periodic supercell approach with a (2x3 × x3)R30° unit mesh having one MPA and one MUA molecule. This structure corresponds to a total thiol surface coverage of 0.33 with a 1 to 1 relation between MPA and MUA. The surface was represented by a slab with 4 layers of metal atoms. The positions of all the atoms in the unit cell were relaxed with the exception of the metal atoms in the fourth layer. The convergence criterion for geometry optimizations was a rms force of 0.01 eV/Å. A vacuum thickness of 8 Å was introduced between the thiolated slabs to avoid spurious interactions between neighboring replicas. Only one side of the metal slab was covered by thiols. The first principle atomistic calculations were performed using state of the art plane wave periodic DFT as implemented in the PWSCF code.59 Ultrasoft pseudopotentials60 were used for the atomic species. The electron wavefunctions were expanded in a plane-wave basis set up to a kinetic energy cutoff of 22 Ry (180 Ry for the density). Brillouin zone integration was performed using a (6 × 6 × 1) MonkorstPack mesh.61 Other details of the calculations are given in our previous works on alkanethiol adsorption on metal surfaces.62,63 For both the Gaussian and plane-wave basis sets, gradient corrections were included in the exchange correlation functional using the PW91 formulation.64 In molecular calculations, hybrid functionals like B3LYP may provide a better description for hydrogen bonding65 than the nonhybrid functionals like PW91.
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Figure 1. (a) Reductive desorption of pure (labeled MUA and MPA) and mixed SAMs. The labels for the mixed SAMs indicate the MPA: MUA composition of the forming solution. Electrolyte: 0.1 M KOH. Scan rate: 20 mV/s. (b) Variation of the reductive desorption potential as a function of the MPA mole fraction of the forming solution.
However, for consistency between the molecular and periodic calculations, we employed the same functional. For the gasphase calculations, both functionals gave similar results. For example, we obtained a MPA-MPA interaction energy of 16.1 kcal/mol using B3LYP whereas we obtained 16.8 kcal/mol using PW91. Hydrogen bond lengths obtained with PW91 were on average 0.1 Å shorter than those obtained with B3LYP. Results and Discussion Structure and Stability. The structure and stability of the mixed SAMs were investigated by means of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and ellipsometry. Figure 1a shows the linear sweep voltammograms for the reductive desorption of the mixed MPA-MUA monolayers in 0.1 M KOH at a scan rate of 20 mV/s, together with the corresponding votammograms for the single-component monolayers. The reductive desorption peak potentials of pure MPA and MUA are observed at -0.78 and -0.94 V, respectively. As expected, the desorption potential decreases with the chain length.66 For the mixed monolayers we also obtained voltammograms similar in shape to those of single-component SAMs. The peak width of the mixed monolayers was broader than the peak width of the single component SAMs. This indicates that intermolecular interactions are different for the mixed and single component SAMs. This issue will be addressed in the theory section. A striking feature of the voltammograms of the mixed
Carot et al.
Figure 2. (a) Impedance spectra of an MPA SAM (2), an MUA SAM (b), and a mixed SAM (0) formed from an ethanolic solution with an MPA:MUA composition of 80:20%. (b) Equivalent circuit used to fit the impedance spectra. (c) Variation of the monolayer capacitance and the power n of the CPE element as a function of the MPA mole fraction of the forming solution.
SAMs is the presence of a single reductive desorption peak. The presence of two current peaks, which is a common feature for mixed monolayers consisting of a mixture of singlecomponent domains,54-56 was not observed for any of the compositions investigated. It has been shown that domains whose size exceeds 15 nm2 produce two reductive desorption current peaks.56 Therefore, the single desorption peak observed for the mixed SAMs indicates that MPA and MUA are mixed homogeneously. Homogeneous mixtures of alkanethiols of different chain lengths have only been achieved using careful preparation protocols. Homogeneous monolayers of MPA and n-alkanethiols of various chain lengths have been constructed on Au by controlling the surface structure of the substrate with underpotentially deposited lead atoms.67 A homogeneous SAM of MHA and n-butanethiol was obtained from an initial low-density SAM of MHA obtained after the cleavage of a bulky tail group of a MHA derivative26 and subsequent immersion in a n-butanethiol solution. For both the MPA + n-alkanethiol and the MHA + n-butanethiol homogeneous SAMs, a single desorption peak was detected in the electrochemical reductive desorption experiments. However, preparation of the mixed SAMs from immersion in a solution of both thiols produced two desorption peaks corresponding to the coexistence of two distinct phases.26,67
Mixed Carboxylic Acid Terminated SAMs
Figure 3. Ellipsometric thickness of the pure and mixed monolayers as a function of the MPA mole fraction of the forming solution.
Figure 4. MPA surface composition as a function of the MPA composition of the forming solution. The surface composition was calculated using the reductive desorption peak potential (b), the monolayer capacitance (2), and the ellipsometric thickness (9).
Therefore, our finding of a single reductive desorption peak for the mixed MPA + MUA monolayer indicates that a homogeneously mixed SAM has been obtained using the simple protocol of immersion in an ethanolic mixture of both thiols at room temperature. We note that homogeneous mixed SAMs of n-alkanethiols having chain length differences of 4-10 carbon atoms have only been obtained by the immersion method by increasing the temperature of the forming solution to 50 °C.57 Figure 1b shows that the potential of the reductive desorption peak for the mixed monolayers follows a continuous trend with the composition of the forming solution. For the reductive desorption charge we observed no variation with the composition of the forming solution. We obtained an average value of 83 ( 9 µC cm-2 for all the monolayers investigated which is indicative of a compact structure with a surface coverage of 0.33.68 Figure 2a shows typical impedance spectra for the pure and mixed SAMs. The spectra were recorded at a potential of 0 V, inside the potential window of the SAM stability. The spectra are dominated by a capacitive behavior corresponding to a linear log |Z| vs log(f) relationship and phase angles close to 90 deg. The solid lines in Figure 2 correspond to the fit according to the circuit in Figure 2b. A constant phase element (CPE) with impedance ZCPE ) 1/[(jω)nCm] was used to describe the dielectric behavior of the monolayer. Cm is the monolayer capacitance, n is the CPE powerm and j ) x-1. A value of n approaching unity indicates ideal capacitive behavior. The variation of the monolayer capacitance and the CPE power n
J. Phys. Chem. C, Vol. 111, No. 11, 2007 4297 as a function of the MPA mole fraction in solution is shown in Figure 2c. The mixed monolayers have effective capacitance values which are intermediate between those of the pure components. A different trend is observed for the CPE powers. The mixed monolayers have CPE powers which are lower than those of the pure MPA and MUA monolayers. The lower CPE powers of the mixed monolayers are indicative of structural disorder within the SAMs. Pinhole defects and collapsed sites at domain boundaries and structural defects are the main factors causing structural disorder in pure SAMs. In the case of single component SAMs, it has been found that when 0.95 e n e 0.97, the monolayers still have a small number of collapsed sites and pinhole defects whereas for n > 0.97 the monolayers are essentially free of pinholes defects and contain only collapsed sites.69 The thickness of the single component and mixed monolayers was determined by ellipsometry. Immediately after the flameannealing process, the refractive index of each bare gold sample was measured and then it was immersed directly in the appropriate thiol solution for 12 h. After removal from the thiol solution, the samples were again measured. The film thickness calculation was based on a three-phase ambient/SAM/gold model in which the film was assumed to be isotropic and assigned a refractive index value of 1.45 + 0.0i. Figure 3 shows the variation of the thickness of the SAM as a function of the mole fraction of MPA in solution. It can be observed that the thicknesses of the mixed monolayers are intermediate between those of the pure components. For the single component MPA and MUA monolayers, we obtained average thicknesses of 5.5 and 14.2 Å, respectively. The ellipsometric thicknesses of the MPA and MUA monolayers are shorter than the corresponding molecular lengths of 6.0 and 16.0 Å, respectively. From the ellipsometric thicknesses and the molecular lengths we obtained tilt angles of 23° (MPA) and 27° (MUA) in agreement with the values reported for n-alkanethiols SAMs which are around 30°.27 Surface Composition. To determine the surface composition χ, we assume that a given property of the mixed monolayer (Pmix) is a simple weighted average of the corresponding property of SAMs derived from the individual components (PMPA and PMUA):70
Pmix ) χMPAPMPA + χMUAPMUA
(1)
χMPA + χMUA ) 1
(2)
From eqs 1 and 2, the surface composition of MPA can be calculated as
χMPA )
Pmix - PMUA PMPA - PMUA
(3)
Equation 3 assumes that the packing density of alkanethiolates does not change with surface composition. This was confirmed by the calculation of reductive desorption charges, which remained constant for all the compositions investigated. We considered three independent properties: the potential of the reductive desorption peak; the monolayer thickness determined from ellipsometry; the monolayer capacity determined from EIS. The MPA surface compositions obtained from these properties are plotted in Figure 4 as a function of the MPA composition of the forming solution. It can be observed that all the properties give the same trend and, as expected, the surface tends to be enriched by the thiol with the longest alkyl chain. STM Measurements. The surface topography of the mixed monolayers was investigated in-situ by STM in a 0.025 M
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Figure 6. Equilibrium configurations for the MPA-MPA and MPAMUA cyclic dimers.
Figure 5. (a) STM image of MPA/Au(111) in 0.025 M phosphate buffer (pH ) 7.2) showing different rotational domains. (b) STM image of a mixed SAM on Au(111) formed in a solution with an MPA:MUA composition of 80:20%.
phosphate buffer solution of pH ) 7.2. At this pH the carboxylic end-groups remain protonated.71 In the case of a pure MPA monolayer, medium-resolution STM images readily show monoatomically deep vacancy islands connecting differently ordered rotational domains (Figure 5a). The MPA domains are characterized by stripe patterns with missing rows in agreement with previous studies.72-74 For the mixed monolayers, we observed the characteristic features of the SAM/Au(111) interface such as monatomic step edges and vacancy islands. However, we could not observe differentiated domains. Figure 6b shows a typical image for a mixed SAM prepared from a solution with an 80:20% MPA:MUA composition. Similar images were obtained for mixed SAMs prepared from other solution compositions. Zooming into the flat terraces of Figure 5b did not reveal any evidence of phase segregation. We conclude that the lack of order in the mixed SAMs effectively reflects the homogeneous mixture between MPA and MUA. It is surprising to obtain homogeneous mixtures of MPA and MUA considering that their alkyl chains differ by 8 carbon atoms. The miscibility of two components in a SAM (as well as the miscibility of liquids) is mainly determined by the difference in the interaction energy between the adsorbed species. The experimental results therefore indicate that MUAMPA interactions prevail over MUA-MUA and MPA-MPA interactions. Since the mixed monolayers in this study were prepared in ethanol, the phase properties could also be influ-
enced by solute-solvent interactions on Au surfaces. It is known that the dielectric constant of the solvent as well as intermolecular interactions in the SAM play a crucial role in the final composition of the mixed SAMs.75 Therefore, to obtain a deeper insight into the nature of intermolecular interactions in the mixed SAM, we investigated from a quantum mechanical point of view the interactions in a mixed monolayer with a 1 to 1 composition of MPA and MUA. Quantum Mechanical Calculations. The flexibility of the alkyl chains of long mercaptoalcanoic acids allows the formation of hydrogen bonds between adjacent carboxyl groups leading to a significant amount of disorder in -COOH-terminated SAMs.40 Under low surface coverage conditions, the carboxyl may even bind to the gold substrate when applying positive potentials.26 However, the interactions between carboxyl groups within a SAM have not been investigated yet from a theoretical point of view. Theoretical studies of alkanethiols on metal surfaces have been mainly focused on the interaction between the sulfur headgroup and the metal surface (see refs 62 and 63 and references therein). In this section we first investigate intermolecular interactions in vacuum and then on the Au(111) surface. (a) MPA-MPA, MUA-MPA, and MUA-MUA Interactions in Vacuum. The most stable structure between carboxylic groups is the head to head cyclic dimer as shown in Figure 6 for the MUA-MPA and MPA-MPA dimers. The interaction energy between the molecules in the dimer is 16.8 kcal/mol yielding a hydrogen bond strength of 8.4 kcal/mol. The hydrogen bond length is shown in the figure. These values for the hydrogen bond strength and length correspond to a strong interaction. As a reference, the corresponding values for the water dimer at the same level of theory are 6.1 kcal/mol and 1.90 Å, respectively.
Mixed Carboxylic Acid Terminated SAMs
J. Phys. Chem. C, Vol. 111, No. 11, 2007 4299
Figure 8. Equilibrium structure of the MUA-MPA dimer obtained constraining the S-S distance to 5 Å.
Figure 7. (a) Hydrogen bond strength and (b) hydrogen bond length for the MPA-MPA interaction calculated as a function of the S-S distance.
The interaction energy of 16.8 kcal/mol obtained for the MPA dimer is in agreement with the values reported recently for substituted formic acid dimers76 calculated using high levels of theory. For example, the B3LYP/6-311++G(2d,2p) and MP2/ 6-311++G(2d,2p) methods yield interaction energies of 18.53 and 16.66 kcal/mol for the CH3COOH dimer.76 When the interaction energy is extrapolated to an infinite basis set, the MP2 method yields a value of 17.57 kcal/mol. This implies that the cyclic dimers of carboxylic acids have strong hydrogen bonds of around 8 kcal/mol. The hydrogen bond length of 1.60 Å (Figure 6) obtained for the MPA-MPA dimer is in agreement with the hydrogen bond length of 1.66 Å calculated at the B3LYP/6-311++G(2d,2p) level for the CH3COOH dimer.76 The slightly shorter value obtained for the MPA dimer is due to the standard 6-31+G(d,p) basis set that we employed, whereas the basis sets used in ref 76 have more diffuse and polarization functions. For the free molecules in vacuum, the opposite -COOH groups lie in the same plane (Figure 6). However, in a compact monolayer with a surface coverage of 0.33, the sulfur atoms are separated by a distance of 5 Å on the Au(111) surface.27 Therefore, we computed the MPA-MPA interaction as the sulfur atoms approach the limiting value of 5 Å to evaluate the influence of the packing density on the strength of the hydrogen bond between adjacent molecules. A series of geometry optimizations were performed constraining the S-S distance to different fixed values. The interaction energies and some representative structures are shown in Figure 7. As the S-S distance decreases, the facing carboxyl groups do not lie on the same plane and the hydrogen bond interaction is considerably weakened. At an S-S interatomic distance of 5 Å, the
MPA-MPA interaction is only 1.2 kcal/mol whereas, in the equilibrium configuration of Figure 6, the interaction energy is 16.8 kcal/mol. The picture is quite different when the S-S distance is decreased for MUA-MPA starting from the equilibrium structure shown in Figure 6. In this case, the flexibility of the alkyl chain of MUA avoids the rupture of the hydrogen bond interactions between the facing -COOH groups. Figure 8 shows the lowest energy configuration obtained when the distance between the sulfur atoms is 5 Å. The interaction energy for this structure (taking as a reference the isolated molecules in their linear configurations) is 14.4 kcal/mol, which is only 2.4 kcal/mol less stable than the interaction between the linear chains (16.8 kcal/mol, Figure 6). Therefore, the bending of the alkyl chain of MUA has a small effect on the strength of the MUA-MPA hydrogen bond. This contrasts the behavior of the MPA-MPA interaction which is only 1.2 kcal/mol when the sulfur separation is 5.0 Å (Figure 7a). In this case, the hydrogen bonds are partially broken because the short alkyl chain of MPA does not have the flexibility to keep the COOH groups in the same plane as the distance between the sulfur atoms decreases. Figure 9a shows the equilibrium structure for a geometry optimization started with two adjacent MUA molecules in the all-trans configuration. The hydrogen bond interaction between the carboxyls induces the bending of the alkyl chains leading to a maximum separation between the carbon atoms of 6.7 Å. The presence of gauche defects within the alkyl chains readily allows the formation of the cyclic dimer. Figure 9b is an example of the many possible structures in which the carboxyls lie on the same plane and form strong hydrogen bonds. The structure in Figure 9b is 5.0 kcal/mol more stable than that of Figure 9a. Structures such as those in Figure 9a,b will not give rise to compact SAMs. We conclude that the hydrogen bond interaction between the carboxyls and the van der Waals interactions between the alkyl chains are opposite forces. Van der Waals interactions are maximized when the packing of the alkyl chains is compact, whereas the hydrogen bond interaction between the carboxyl groups induces a disordering (bending and gauche defects) of the alkyl chains. This explains the difficulty found experimentally to form compact -COOHterminated SAMs with reproducible properties as discussed in the introduction section. It is clear that the state of the carboxyl (protonated or deprotonated) will have a profound effect on the ordering of the SAM as observed experimentally.40 When the sulfur atoms of the MUA molecules are separated by a distance of 10 Å (corresponding to 2x3 times the cell parameter of the Au(111) surface), the cyclic dimer between
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Figure 9. (a, b) Equilibrium structures of MUA-MUA dimers optimized with a constrained S-S distance of 5 Å. (c) Equilibrium structure of a MUA-MUA dimer optimized with a constrained S-S distance of 10 Å. (d) MUA-MPA-MUA structure optimized by keeping a fixed S-S distance of 5 Å. The S atoms are located along the same line.
the carboxyls readily forms due to the bending of the topmost part of the alkyl chain as shown in Figure 9c. An MPA molecule can be intercalated between the MUA molecules without interfering in the hydrogen bonding between the carboxyls of MUA as shown in Figure 9d. Figures 8 and 9d anticipate that, in a compact mixed SAM, a MUA molecule can form strong hydrogen bonds with an adjacent MPA molecule 5 Å apart or with a MUA molecule 10 Å apart. Either structure favors the mixing of MUA and MPA. (b) MUA-MPA Interaction on Au(111). The bent structure of Figure 8 (without the hydrogens bound to the sulfur atoms) was taken as the starting geometry on the Au(111) surface. The whole structure as well as the first three metal layers was optimized using a (2x3 × x3)R30° unit cell. Figure 10 shows side and top views of the equilibrium structure, and Figure 11 shows the main interatomic distances. The sulfur atoms of adjacent MPA-MUA dimers are separated by 6.34 Å, whereas the S-S distance within the dimer is 4.0 Å. The driving force for the decrease of the S-S distance of the dimer from the initial value of 5.0 Å is the minimization of the repulsion between the OH group of MPA and the adjacent alkyl chain of MUA (around 3.7 Å in the equilibrium structure; Figure 11). During the decrease of the S-S distance from 5.0 to 4.0 Å, the first layer of gold atoms suffered a considerable relaxation. Figure 10c shows that the S atom of MUA is bicoordinated to the Au surface with S-Au distances of 2.50 and 2.44 Å, whereas the S atom of MPA is tricoordinated with the following S-Au
distances: 2.45, 2.47, and 2.52 Å. The shortest C-C distance of 4.54 Å is compatible with the nearest neighbor distance of 4.5 Å, which is observed in the herringbone packing of hydrocarbon chains.77 The magnitude of the adsorbate-induced metal relaxation depends on the surface coverage of the adsorbate, the structure of the adsorbate layer, and the nature of the adsorbate-metal bond. Among the coinage metals, the sulfur-metal bond for alkanethiol monolayers has the highest directionality on gold due to the important covalent contribution to the surface bonding.62 The directionality of the thiol-Au bond implies that adsorbate reorganization will influence the metal structure. In the case of the compact (x3xx3)R30° structure of alkanethiols, the adsorbate induced metal relaxation is small because the alkyl chains are in the all-trans configuration, they have the same tilt with respect to the surface normal, and the sulfur head groups are equally spaced by 5 Å. However, the minimum energy structure of the mixed MPA-MUA monolayer has sulfur head groups with two different separations (Figure 11) whereas the alkyl chains of MPA and MUA have different tilt angles with respect to the surface normal (Figures 10a and 11). Therefore, the structure of the MPA-MUA monolayer together with the highly directional surface bonding explains the considerable surface relaxation of the first layer of metal atoms. The influence of the adsorbate structure on the surface relaxation is evident from the comparison of Figures 11 and 12. The surface relaxation of the latter is smaller because the alkyl chains are
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J. Phys. Chem. C, Vol. 111, No. 11, 2007 4301
Figure 11. Side view of the MPA-MUA equilibrium structure on Au(111) showing the main interatomic distances in Å.
Figure 10. (a) Side view of the equilibrium structure of the MUAMPA dimer on Au(111) calculated using a (2x3 × x3)R30° unit cell. In the image the unit cell was replicated 3 times in each direction. (b) Top view. (c) Top view only showing the first layer of metal atoms and the C and O atoms of the MPA and MUA molecules.
arranged in the all-trans configuration and they have the same tilt with respect to the surface normal. Despite the gold surface and alkyl chain relaxation observed upon adsorption of the MUA-MPA complex, the geometry of the facing carboxyl groups remained virtually unaltered, indicating that the hydrogen bonds were not weakened. The short hydrogen bonds in Figure 11 correspond to a strong interaction. The hydrogen-bonded MUA + MPA structure in Figure 11 is 7.8 kcal/mol more stable than the structure in which the MPA and MUA molecules are in the all-trans configuration (Figure 12). This shows that the trans-gauche conformational changes
Figure 12. Optimized geometry of the MUA and MPA molecules in the all-trans configuration in a (2x3 × x3)R30° unit cell.
of MUA driven by the formation of hydrogen bonds within the mixed SAM are energetically favorable. The energy difference of 7.8 kcal/mol between the all-trans structure of Figure 12 and the hydrogen-bonded structure of Figure 11 has contributions from different processes: the substrate relaxation; the bending of the alkyl chain of MUA; the formation of the hydrogen bonds between the facing carboxyl groups. Therefore, the value of 7.8 kcal/mol cannot be taken as a measure of the hydrogen bond strength of the molecules within the SAM. To evaluate the
4302 J. Phys. Chem. C, Vol. 111, No. 11, 2007
Carot et al.
Figure 13. Structures employed for the calculation of the intralayer hydrogen bonding within the SAM: (a) equilibrium structure; (b) breakage of the carboxyl dimer performed by rotating the dihedral angle defined by the last 4 C atoms. After the rotation, the carboxyl of MUA points away from the surface and the H atoms of the last two CH2 groups are in a trans configuration.
strength of the hydrogen bonds between the carboxyl groups, we rotated the COOH group of MUA away from the surface as shown in Figure 13. The rotation only involved the change of the dihedral angle defined by the last four carbon atoms of MUA. After this rotation, only the carboxyl groups of MUA and MPA were optimized. For both structures of Figure 13, the surface relaxation, the sulfur adsorption sites, and the position of the methylene groups closer to the surface remained identical. Therefore, the energy difference is a measure of the hydrogen bond strength. We found that the hydrogen-bonded structure in Figure 13a is 16.2 kcal/mol more stable than that of Figure 13b. Thus, the hydrogen bonding within the SAM has nearly the same strength as for the free molecules in vacuum. The structure in Figure 8 corresponds to an MPA:MUA composition of 50:50% on the Au(111) surface, whereas the structure in Figure 9d corresponds to an MPA:MUA composition of 33:66% on Au(111). As the surface composition depends on the composition of the forming solution, both structures could occur depending on the forming conditions. The structure of Figure 8 fits into a (2x3 × x3)R30° unit cell on the Au(111) surface, whereas the structure of Figure 9d fits into a (3x3 × x3)R30° unit cell. In the structure of Figure 9d there are 50% less hydrogen bonds/unit area than in the structure of Figure 8. However, the structure of Figure 9d is expected to have stronger van der Waals interactions due to a more compact packing. Therefore, the energetics for the formation of both structures are expected to be comparable. Another factor influencing the structure of mixed SAMss besides intermolecular interactionssis the nature of the solvent. As the mixed monolayers were prepared from ethanolic solutions, we investigated the interaction of methanol with the carboxylic end-group. Figure 14 shows the equilibrium structure between MPA and methanol molecules. The strength of the hydrogen bond interaction is 11.3 kcal/mol. This value is lower than the interaction between facing carboxyl groups, which is 16.8 kcal/mol (Figure 6). This implies that, during the formation of a monolayer in an ethanolic solution containing carboxylic acids, the terminal COOH groups of the SAM will form intralayer hydrogen bonds with adjacent molecules or interplane
Figure 14. Equilibrium structure of the MPA-ethanol dimer showing the corresponding hydrogen bond lengths.
hydrogen bonds with free alkanethiols in the bulk solution. The second mechanism can lead to a partial second layer of alkanethiols on the top of the SAM. This has motivated the development of improved methods for the formation of compact -COOH-terminated SAMs on the basis of the disruption of interplane hydrogen bonds by small molecules such as CF3COOH34 or CH3COOH35 capable of forming hydrogen bonds in the ethanolic solution of alkanethiols. With the picture of intramolecular interactions derived in this section, we can explain the reason that the mixed monolayers of MPA + MUA are homogeneous, despite the large chain length difference of 8 carbon atoms. The driving force for the formation of the homogeneous mixture is the prevalence of MPA-MUA interactions over the interactions of the like molecules: MPA-MPA and MUA-MUA. The hydrogen bonding between two adjacent MPA molecules whose sulfur atoms are separated by 5 Å is only 1.2 kcal/mol (Figure 7) because the facing carboxyl groups do lie on the same plane. The same occurs in the case of adjacent MUA molecules in the all-trans configuration (Figure 9a). However, for adjacent MPA-MUA molecules, the flexibility of the alkyl chain of MUA allows the facing carboxyl groups to be located on the same plane (Figure 11), giving rise to the formation of a strong hydrogen bond interaction of 16.2 kcal/mol. In the case of adjacent MUA molecules with the all-trans configuration, van der Waals interactions among the long alkyl chains are important and could compensate the lack of hydrogen bond interactions. In the condensed phases of hydrocarbon chains, the attractive interaction/CH2 group is between -1.5 and -1.8 kcal/mol77
Mixed Carboxylic Acid Terminated SAMs depending on the chain packing. This implies that, for MUA, with 10 methylene units, van der Waals interactions are between 15 and 18 kcal/mol in the case of a compact monolayer. If the monolayer is not compact due to the presence of gauche defects, van der Waals interactions will be lower and the MPAMUA hydrogen bonding will prevail over MUA-MUA van der Waals interactions. The formation of compact all-trans MUA SAMs from an ethanolic mixture of MPA + MUA is unlikely as the presence of MUA-MPA dimers from the initial stages of the SAM formation will readily induce gauche defects in the MUA alkyl chain. The formation of dimers between the carboxylic groups of the thiols is favored in ethanolic solutions because the carboxyl-alcohol interaction (Figure 13) is weaker than the carboxyl-carboxyl interaction as discussed in previous paragraphs. The hydrogen-bonded MPA-MUA structure of Figure 10a exposes the methylene groups of MUA. Therefore, this monolayer is expected to show a hydrophobic behavior, in contrast to the hydrophilic behavior expected for the all-trans mixed monolayer shown in Figure 12 which exposes the carboxylic groups. It has been observed that pure SAMs of short- and longchain CH2OH-terminated thiols are hydrophilic, whereas the mixed SAMs are substantially more hydrophobic.43 These changes of wetting behavior have been ascribed to the disorder of the mixed SAM.43 Under the light of our results, we propose that the disorder is mainly caused by intralayer hydrogen bonding. Intermolecular hydrogen bonding has been observed using IR spectroscopy for mixed SAMs of CH2OH- and CH3terminated alkanethiols.44 For the mixed monolayers with longchain CH2OH and short-chain CH3-terminated alkanethiols, a random mixing of both components was observed together with a significant degree of intermolecular hydrogen bonding.44 A structure like in Figure 9d explains the cause of the random mixing: the protruding CH2OH-terminated chains are involved in hydrogen-bonding interactions. On the contrary, for mixed SAMs of long-chain CH3-terminated alkanethiols and shortchain CH2OH-terminated alkanethiols, phase segregation was observed.44 We attribute this behavior to the prevalence of van der Waals interactions among the long-chain CH3-terminated alkanethiols. Besides, in a random mixture of long-chain CH3terminated alkanethiols and short-chain CH2OH-terminated alkanethiols, steric effects are expected to inhibit hydrogen bonding. The structure of mixed monolayers of mercaptoalcanoic acids of different chain lengths is expected to be sensitive to the solution pH. In acid solutions, the protonated carboxylic endgroups will be involved in intralayer hydrogen bonding inducing the formation of gauche defects in the longer alkyl chain. This increases the hydrophobic nature of the SAM due to the exposure of methylene groups (see Figure 10), whereas, for alkaline solutions, the molecules are expected to be in the alltrans configuration due to the repulsion between the carboxylate groups and the solvent hydration of these groups. Therefore, an important chemical switchability is predicted for mixed SAMs of mercaptoalcanoic acids of different chain lengths as a function of pH, giving rise to a hydrophilic behavior in alkaline solutions and to a more hydrophobic behavior in acid solutions. Even in the case of single-component COOH-terminated SAMs, conformational changes have been reported with pH. Mercaptohexadecanoic SAMs undergo a stark reorientation upon deprotonation of the end group by rinsing in a KOH solution,35 which leaves the carboxylate ion end groups in an upright configuration.
J. Phys. Chem. C, Vol. 111, No. 11, 2007 4303 Conclusions The structure of mixed SAMs of MPA + MUA prepared from ethanolic solutions was studied by cyclic voltammetry, electrochemical impedance spectroscopy, ellipsometry, and STM. The reductive desorption of the mixed SAMs is characterized by the presence of a single peak with a peak potential which is intermediate between the potentials of the peaks of the pure component SAMs. The capacitance and the thickness of the mixed SAMs also showed a continuous variation between the values corresponding to the pure SAMs. The mixed monolayers had a compact structure, as deduced from the reductive desorption charge of 83 ( 9 µC cm-2. However, the intermolecular interactions were different for the mixed- and single-component SAMs because the peak width of the mixed monolayers was broader than the peak width of the singlecomponent SAMs. For all the mixed monolayers, we observed lower CPE powers than for the pure SAMs, indicating a higher degree of structural disorder within the monolayers. The presence of a single reductive desorption peak together with the different nature of the intermolecular interactions, the structural disorder revealed by EIS, and the lack of phase segregation observed by STM, all support the formation of a homogeneous mixture of MPA + MUA. The driving force for the formation of the homogeneous mixture is the strong hydrogen bond between adjacent MPA and MUA molecules. The hydrogen bonding between adjacent MPA-MPA and MUA-MUA molecules is considerably weaker because for these pairs the carboxyl groups do not lie on the same plane. However, the flexibility of the long alkyl chain of MUA allows its carboxyl group to form a head to head cyclic dimer with the carboxyl group of an adjacent MPA molecule, with an interaction energy of 16.2 kcal/mol. The theoretical calculations show that the intralayer hydrogen bonding not only affects the structure of the bulk of the monolayer itself but also the structure of the metal substrate. In the hydrogen-bonded MPA-MUA configuration, the bending of the MUA molecule exposes the methylene groups, leading to the formation of a more hydrophobic SAM. The control of the surface hydrophobic-hydrophilic properties by the formation or disruption of intralayer hydrogen bonds with the solution pH outlines the potential switchability properties of mixed SAMs of mercaptoalcanoic acids of different chain lengths. Acknowledgment. Financial support from the FONCyT (Grant 06-03195), CONICET, Agencia Co´rdoba Ciencia, and SECYT-UNC is gratefully acknowledged. M.L.C. thanks CONICET for the fellowship granted. References and Notes (1) Yang, H. C.; Dermody, D. L.; Xu, C. J.; Ricco, A. J.; Crooks, R. M. Langmuir 1996, 12, 726. (2) Smith, D. A.; Wallwork, M. L.; Zhang, J.; Kirkham, J.; Robinson, C.; Marsh, A.; Wong, M. J. Phys. Chem. B 2000, 104, 8862. (3) Kokkoli, E.; Zukoski, C. F. J. Colloid Interface Sci. 2000, 230, 176. (4) Kokkoli, E.; Zukoski, C. F. Langmuir 2001, 17, 369. (5) Ashby, P. D.; Chen, L. W.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 9467. (6) Fisher, G. L.; Hooper, A. E.; Opila, R. L.; Allara, D. L.; Winograd, N. J. Phys. Chem. B 2000, 104, 3267. (7) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 6704. (8) Boubour, E.; Lennox, R. B. Langmuir 2000, 16, 4222. (9) Sugihara, K.; Shimazu, K.; Uosaki, K. Langmuir 2000, 16, 7101. (10) Kuther, J.; Seshadri, R.; Knoll, W.; Tremel, W. J. Mater. Chem. 1998, 8, 641. (11) Kuther, J.; Tremel, W. Thin Solid Films 1998, 329, 554.
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