Langmuir 2009, 25, 2689-2695
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Molecular Dynamics Simulation of Oxygen Transport through ω-Alkoxy-n-alkanethiolate Self-Assembled Monolayers on Gold and Copper Piyush Srivastava,† Walter G. Chapman,*,† and Paul E. Laibinis*,†,‡ Department of Chemical and Biomolecular Engineering, Rice UniVersity, Houston, Texas 77005-1827, and Department of Chemical and Biomolecular Engineering, Vanderbilt UniVersity, NashVille, Tennessee 37235-1604 ReceiVed October 15, 2008. ReVised Manuscript ReceiVed December 11, 2008 We have used molecular dynamics (MD) simulations to investigate the influences of the position of the ethereal oxygen on the ability of ω-alkoxy-n-alkanethiolate self-assembled monolayers (SAMs) to act as barrier films against through-film oxygen transport as relevant to the uses of these films in corrosion inhibition. Our MD simulations reveal that when the ether linkage is too close to the metal surface or to the chain ends, the free-energy barrier of SAMs toward oxygen diffusion was ∼5 kJ/mol less than for a non-ether-containing n-alkanethiolate SAM having the same chain length. MD simulations show that SAMs having an ether linkage near a chain end contain a highly disordered terminal region. As a result, the SAMs allow a more rapid transport of oxygen across these monolayers than through n-alkanethiolate SAMs of similar length lacking the ether unit. Additionally, SAMs with the ether linkage close to the metal surface undergo a structural transition to an alternating flipped structure that is less crystalline compared to that of an n-alkanethiolate SAM. Together, these factors diminish the barrier properties of the ω-alkoxy-n-alkanethiolate SAMs below those for their unsubstituted analogues.
Introduction Self-assembled monolayers (SAMs) formed by the adsorption of n-alkanethiols (CnH2n+1SH) onto gold and copper surfaces1-5 provide barriers against the transport of species such as O2,6-9 H2O,10 and various ions11-14 to the underlying metal substrate. Their ability to act as barriers and protect the underlying metal surfaces is relevant to the use of these films for corrosion inhibition6-10,14-17 and molecular electronics.5,18 The alkanethiols in these films are densely packed, chemically bonded to the metal surface, and contain chains in a quasi-crystalline state.1-5,19 These attributes, coupled with their ease of preparation and ability * Corresponding authors.
[email protected]. † Rice University. ‡ Vanderbilt University.
E-mail:
[email protected].
E-mail:
(1) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321–335. (2) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152–7167. (3) Ulman, A. Chem. ReV 1996, 96, 1533–1554, and references therein. (4) Laibinis, P. E.; Palmer, B. J.; Lee, S.-W.; Jennings, G. K. In Thin Films: Self-Assembled Monolayers of Thiols; Ulman, A., Ed.; Academic Press: New York, 1998; Vol. 24, pp 1-41. (5) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV 2005, 105, 1103–1169, and references therein. (6) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022– 9028. (7) Ishibashi, M.; Itoh, M.; Nishihara, H.; Aramaki, K. Electrochem. Acta 1996, 41, 241–248. (8) Jennings, G. K.; Munro, J. C.; Yong, T.-W.; Laibinis, P. E. Langmuir 1998, 14, 6130–6139. (9) Jennings, G. K.; Munro, J. C.; Laibinis, P. E. AdV. Mater. 1999, 11, 1000– 1003. (10) Jennings, G. K.; Laibinis, P. E. Colloids Surf. 1996, 116, 105–114. (11) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559–3568. (12) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682–691 Groat, K. A.; Creager, S. E. Langmuir 1993, 9, 3668–3675. Finklea, H. O. Electroanal Chem. 1996, 19, 109–335. Zamborini, F. P.; Crooks, R. M. Langmuir 1997, 13, 122–126. Protsailo, L. V.; Fawcett, W. R. Langmuir 2002, 18, 8933– 8941. (13) Zamborini, F. P.; Campbell, J. K.; Crooks, R. M. Langmuir 1998, 14, 640–647. (14) Zamborini, F. P.; Crooks, R. M. Langmuir 1998, 14, 3279–3286.
to include a wide range of chemical functionalities,1-5 contribute to the continued interest in these systems. The barrier properties of n-alkanethiolate SAMs on both gold and copper surfaces have been shown by various experimental techniques to depend strongly on the chain length, n, of the n-alkanethiol.6-11,14,15,20-22 For example, the coating resistances of n-alkanethiolate SAMs on copper, as measured by electrochemical impedance spectroscopy (EIS), show dramatic increases with chain length for n g 16 while displaying negligible resistance, in comparison, for n e 12.8 Furthermore, the rate of oxygen transport through n-alkanethiolate SAMs on copper from XPS measurements of surface oxidation rates was found to diminish exponentially with the chain length, n, of the monolayer.6 A similar exponential decrease with chain length was observed for the permeability of oxygen through n-alkanethiolate SAMs on copper from electrochemical measurements.7 On gold, cyclic voltammetry (CV) experiments have shown that n-alkanethiolate (15) Yamamoto, Y.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1993, 140, 436–443. Li, G.; Ma, H.; Jiao, Y.; Chen, S. J. Serb. Chem. 2004, 69, 791– 805. (16) Jennings, G. K.; Yong, T.-H.; Munro, J. C.; Laibinis, P. E. J. Am. Chem. Soc. 2003, 125, 2950–2957. (17) Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1994, 141, 2018–2023. Itoh, M.; Nishihara, H.; Aramaki, K. J. Electrochem. Soc. 1995, 142, 3696–3704. Ma, H. Y.; Yang, C.; Yin, B. S.; Li, G. Y.; Chen, S. H.; Luo, J. L. Appl. Surf. Sci. 2003, 218, 144–154. (18) Xia, Y. N.; Zhao, X. M.; Whitesides, G. M. Microelectron. Eng. 1996, 32, 255–268. Huang, Z. Y.; Wang, P. C.; MacDiarmid, A. G.; Xia, Y. N.; Whitesides, G. M. Langmuir 1997, 13, 6480–6484. Wang, W.; Lee, T.; Reed, M. A. Phys. ReV. B 2003, 68, 035416. Lee, T.; Wang, W.; Reed, M. A. Ann. N.Y. Acad. Sci. 2003, 1006, 21–35. Akkerman, H. B.; Blom, P. W. M.; De Leeuw, D. M.; De Boer, B. Nature 2006, 441, 69–72. (19) Cossaro, A.; Mazzarello, R.; Rousseau, R.; Casalis, L.; Verdini, A.; Kohlmeyer, A.; Floreano, L.; Scandolo, S.; Morgante, A.; Klein, M. L.; Scoles, G. Science 2008, 321, 943–946. (20) Miller, C.; Cuendet, P.; Graetzel, M. J. Phys. Chem. 1991, 95, 877–886. Oncins, G.; Vericat, C.; Sanz, F. J. Chem. Phys. 2008, 128, 044701. (21) Qian, Z.; Shi, X.; Zhuang, J.; Kong, J.; Deng, J. Bioelectrochem. Bioenerg. 1998, 46, 193–198. Sur, U. K.; Lakshminarayanan, V. J. Electroanal. Chem. 2001, 516, 31–38. (22) Mendes, R. K.; Freire, R. S.; Fonseca, C. P.; Neves, S.; Kubota, L. T. J. Braz. Chem. Soc. 2004, 15, 849–855.
10.1021/la803423a CCC: $40.75 2009 American Chemical Society Published on Web 02/03/2009
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SAMs exhibit a strong chain-length dependence in their barrier properties.14,21 The above trends suggest that the n-alkanethiolate SAMs having longer chain lengths on gold and copper exhibit substantially superior barrier properties as compared to those having shorter chain lengths. The improved barrier properties of these films with increasing chain length have been attributed to the increased thickness of the films as well as to the increased crystallinity within these monolayers as observed by reflectance infrared spectroscopy.2,8,9 To establish further connections between the chain length of the n-alkanethiol and the structure of the SAM and its barrier properties, we recently performed molecular dynamics (MD) simulations of these films on gold and copper.23 Our MD simulations revealed the existence of a middle region within these SAMs for n g 12 that adopted a more crystalline-like structure with a substantially lower gauche population than in other parts of the monolayer. For these SAMs having chain lengths of n g 12, the thickness of this middle region increased with chain length (n) and yielded substantial enhancements in the barrier properties of the SAMs with increasing n. These experimental results and MD simulation studies indicate that in order to achieve the best barrier properties from an n-alkanethiolate SAM on gold or copper its chain length should be as high as possible. Unfortunately, n-alkanethiols with chain lengths of n > 22 require custom syntheses, often involving chain-coupling reactions24 that are challenged by the limited solubilities of reaction products and intermediates and difficulties in separating reaction intermediates from starting materials and byproducts. A comparatively easier way of synthesizing long-chain molecules is to use an ether unit to link hydrocarbon chains. These molecules have better solubility than their purely hydrocarbon analogs because of the polarity provided by the ethereal oxygen. Previous studies have formed SAMs that are almost 60 Å thick in a single step by the adsorption of ω-alkoxy-n-alkanethiolate onto gold25 and copper16 surfaces. These SAMs contain a densely packed hydrocarbon backbone and are structurally similar to unsubstituted n-alkanethiolate SAMs on these substrates.16,25 As such, they provide a possible strategy for generating thick monolayer films with enhanced barrier properties. Our motivation for this work comes from EIS experiments that showed that the coating resistances of ω-alkoxy-n-alkanethiolate SAMs on copper depend on the position of the ether linkage in the monolayer.16 The coating resistances were highest and comparable to that of a corresponding n-alkanethiolate SAM with the same overall chain length when the oxygen atom was positioned sufficiently far away from the metal surface and the chain ends. The resistances were markedly inferior to that of the corresponding unsubstituted n-alkanethiolate SAM when the ether linkage was close to the metal surface or a chain end. This influence on the barrier properties of the ω-alkoxy-n-alkanethiolate SAMs is intriguing because it suggests that the ability of these monolayers to generate crystalline domains similar to those in the corresponding n-alkanethiolate SAMs depends on the position of the ether linkage. The introduction of an ether linkage into an isolated alkyl chain can result in gauche conformations being favored over trans conformations by ∼0.03 kJ/mol.26 Here, the ether linkage might also introduce a permanent dipole within the monolayer that could affect chain ordering. IR studies (23) Srivastava, P.; Chapman, W. G.; Laibinis, P. E. J. Phys. Chem. B 2009, 113, 456–464. (24) Ca´rdenas, D. J. Angew. Chem., Int. Ed. 2003, 42, 384–387, and references therein. (25) Laibinis, P. E.; Bain, C. D.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 7663–7676. (26) Miwa, Y.; Machida, K. J. Am. Chem. Soc. 1989, 111, 7733–7739.
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Figure 1. Schematic illustrations of CH3(CH2)20 - pO(CH2)pS SAMs on gold and copper showing their different averaged canted structures.
performed on ω-alkoxy-n-alkanethiolate SAMs on gold indicate that ether linkages can introduce regions of local disorder within a monolayer.25 It has been suggested16 that this local disorder caused by an ether linkage, when near chain ends having a high gauche population,23,27,28 produces an extended region of disorder within a SAM that is responsible for the poor barrier properties of these monolayers observed experimentally. Present experimental data are limited and cannot directly establish connections between the position of an ether in a SAM and its influence on the barrier property of the SAM or whether the extent of local disorder caused by an ether linkage is the most important factor. To investigate these issues, we performed MD simulations to study the transport of oxygen through ω-alkoxy-n-alkanethiolate SAMs on gold and copper, having a fixed overall chain length and differing in the position of an ethereal oxygen within the chain. MD simulations have been used previously to model the structure of n-alkanethiolate SAMs27-30 and have provided structural connections to their macroscopic wetting,28,31 frictional,32,33 adsorptive,33,34 and barrier properties.23 MD simulations have also been employed to study the transport of small molecules through systems including lipid bilayers,35-38 polymers,39 and monolayers.40 Our MD simulations focused on oxygen transport through CH3(CH2)20 - pO(CH2)pS SAMs on gold and copper, having a fixed overall chain length of n ) 22 and differing in the position of the ethereal oxygen on gold and copper surfaces (Figure 1). The difference on these surfaces is that the average chain tilt for n-alkanethiolate SAMs is ∼30° from the surface normal on gold whereas the chains are oriented (27) Hautman, J.; Klein, M. L. J. Chem. Phys. 1989, 91, 4994–5001. (28) Srivastava, P.; Chapman, W. G.; Laibinis, P. E. Langmuir 2005, 21, 12171–12178. (29) Bareman, J. P.; Cardini, G.; Klein, M. L. Phys. ReV. Lett. 1988, 60, 2152–2155. Hautman, J.; Klein, M. L. J. Chem. Phys. 1990, 93, 7483–7492. Bareman, J. P.; Klein, M. L. J. Phys. Chem. 1990, 94, 5202–5205. Mar, W.; Klein, M. L. Langmuir 1994, 10, 188–196. Shevade, A. V.; Zhou, J.; Zin, M. T.; Jiang, S. Langmuir 2001, 17, 7566–7572. Rai, B.; Sathish, P.; Malhotra, C. P.; Pradip; Ayappa, K. G. Langmuir 2004, 20, 3138–3144. Ghorai, P. K.; Glotzer, S. C. J. Phys. Chem. C 2007, 111, 15857–15862. (30) Hautman, J.; Bareman, J. P.; Mar, W.; Klein, M. L. J. Chem. Soc., Faraday Trans. 1991, 87, 2031–2037. (31) Hautman, J.; Klein, M. L. Phys. ReV. Lett. 1991, 67, 1763–1766. (32) Tupper, K. J.; Brenner, D. W. Thin Solid Films 1994, 253, 185–189. Park, B.; Chandross, M.; Stevens, M. J.; Grest, G. S. Langmuir 2003, 19, 9239–9245. (33) Leng, Y.; Jiang, S. J. Am. Chem. Soc. 2002, 124, 11764–11770. (34) Zheng, J.; Li, L.; Chen, S.; Jiang, S. Langmuir 2004, 20, 8931–8938. Zheng, J.; Li, L.; Tsao, H.-K.; Sheng, Y.-J.; Chen, S.; Jiang, S. Biophys. J. 2005, 89, 158–166. (35) Xiang, T.-x. Biophys. J. 1993, 65, 1108–1120. Xiang, T.-x.; Anderson, B. D. Biophys. J. 1994, 66, 561–573. Jedlovszky, P.; Mezei, M. J. Am. Chem. Soc. 2000, 122, 5125–5131. Bemporad, D.; Luttmann, C.; Essex, J. W. Biophys. J. 2004, 87, 1–13. (36) Marrink, S.-J.; Berendsen, H. J. C. J. Phys. Chem. 1994, 98, 4155–4168. (37) Marrink, S. J.; Sok, R. M.; Berendsen, H. J. C. J. Chem. Phys. 1996, 104, 9090–9099. (38) Marrink, S. J.; Berendsen, H. J. C. J. Phys. Chem. 1996, 100, 16729– 16738. (39) Neogi, P., Ed. Diffusion in Polymers; Marcel-Dekker: New York, 1996; Vol. 32. Indrakanti, A.; Ramesh, N.; Duda, J. L.; Kumar, S. K. J. Chem. Phys. 2004, 121, 546–553. (40) McKinnon, S. J.; Whitterburg, S. L.; Brooks, B. J. Phys. Chem. 1992, 96, 10497–10506. Vieceli, J.; Ma, O. L.; Tobias, D. J. J. Phys. Chem. A 2004, 108, 5806–5814.
Oxygen Transport through Self-Assembled Monolayers
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more closely along the normal on copper (Figure 1), resulting in different packing densities for the two SAMs.2 We selected an overall chain length of n ) 22 that was long enough to yield densely packed, quasi-crystalline n-alkanethiolate SAMs on copper and gold without being computationally prohibitive. With these systems, the MD simulations provide insight into the effects of the ether position on both the structure and the barrier properties of these SAMs and allow comparisons with experiments.
ω-Alkoxy-n-alkanethiolate SAMs on Gold and Copper. We followed the approach adopted by Hautman and Klein27 for the MD simulations of n-alkanethiolate SAMs and used model parameters available for n-alkyl ethers.41 Our molecular model for the ω-alkoxy-n-alkanethiolate [CH3(CH2)20 - pO(CH2)pS] SAMs on gold and copper had 23 united atomss1 neutral sulfur atom, 20 methylene groups, 1 oxygen atom, and 1 methyl tailgroupsthat were connected by rigid bond-length constraints. The united atoms were each represented by a single interaction site. The united atoms interacted through a Lennard-Jones potential
VL-J(rij) )
{
[( ) ( ) ]
4εij 0
12
σij rij
6
rij e rc
(1)
rij > rc
where rij is the interatomic distance between a pair of united atoms i and j, εij and σij are Lennard-Jones interaction parameters, and rc is the cutoff distance. An intramolecular Lennard-Jones interaction was also used between atoms in a chain that were at least three united atoms apart. The united atoms representing the oxygen atom and the adjacent methylene or methyl groups each contained one charge site. Long-range electrostatic interactions operated between charge sites on different molecules and were described by
VQ(rij) )
qiqj rij
(2)
where rij is the interatomic distance between a pair of interacting sites i and j and q is the charge on a site. The bond-bending potential for the interactions experienced by the united atoms was described by the expression
1 Vb(θ) ) Kθ(θ - θo)2 2
(3)
where Kθ is a force constant and θo is an equilibrium bond angle. The metal-S-C bond angle was left unrestrained. The torsional potential for the system was described by a series expansion in the cosine of the dihedral angle φ:
Vt(φ) ) a0 + a1[1 + cos(φ)] + a2[1 - cos(2φ)] + a3[1 + cos(3φ)] (4) United atoms interacted with the underlying gold substrate through a 12-3 potential
V(z) )
C12 (z - zo)
12
-
C3 (z - zo)3
SAMsa
O2e
site
σ (nm)
ε (K)
q (e)
CH3 CH2 O Sc S-Sd O2
0.375 0.395 0.280 0.355 0.425 0.336
98.0 46.0 55.0 126.0 200.0 120.0
0.25b 0.25b -0.5 0.0 0.0 0.0
a
Simulation Details
σij rij
Table 1. Lennard-Jones Parameters and Charges for SAMs and Oxygen
(5)
where C3 and C12 are 12-3 interaction parameters, z is the distance from the metal surface, and zo is a constant. Values for the interaction parameters in eq 1-5 are listed in Tables 1-3. All simulations were performed in the NVE ensemble. The equations of motion were integrated numerically for 200 ps by utilizing the velocity Verlet algorithm with a time step of 0.005
Reference 41. b For sites adjacent to the ether oxygen. c For the interactions between S and other atoms, except for S-S interactions. Adapted from ref 27. d For S-S interactions only. Adapted from ref 27. e Reference 43.
Table 2. Valence Parameters for SAMsa,b bonds
S-CH2
C-C
C-O
rij (nm)
0.1812
0.154
0.141
angle
S-C-C
C-C-C
C-O-C
Kθ(×103 K/rad2) θe
62.5 114.4°
62.5 109.5°
112°
dihedral angle
C-C-C-C
C-C-O-C
C-C-C-O
a0/kB a1/kB a2/kB a3/kB
0.0 K 335.03 K -68.19 K 791.32 K
0.0 K 725.35 K -163.75 K 558.20 K
0.0 K 176.62 K -53.34 K 769.93 K
a Adapted from refs 27 and 41. b Parameters for the dihedral angle S-X-X-X were assumed to be the same as for C-X-X-X.
Table 3. Parameters for the 12-3 Potential Described by Equation 4a site
C12 (10-5 K nm12)
C3 (K nm3)
zo (nm)
CH3 CH2 Ob S O2b
3.41 2.81 2.81 4.089 2.81
20.8 17.1 17.1 180.6 17.1
0.0860 0.0860 0.0860 0.0269 0.0860
a Adapted from ref 27. b Parameters for the oxygen atom and the oxygen molecule are assumed to be the same as for the methylene group.
ps. An equilibration period of 300 ps was used in each simulation prior to the 200 ps of integration. During the equilibration period, the velocities were scaled to a temperature of 298 K. The bond lengths in the n-alkanethiol molecules were constrained using the RATTLE algorithm.42 The cutoff distance used for the LJ potentials was 4.5σ, where σ is the segment diameter of the methylene group. No cutoff was used for the electrostatic interactions. The packing arrangements for the ω-alkoxy-n-alkanethiolate SAMs appear to be similar to those for n-alkanethiolate SAMs on gold and copper.16,25 As a result, our MD cell consisted of chains arranged in a triangular lattice with nearest-neighbor spacings of 0.497 nm for ω-alkoxy-n-alkanethiolate SAMs on gold and 0.462 nm for those on copper. These lattice spacings are the same as those used in MD simulations of unsubstituted n-alkanethiolates on gold30 and copper23 that were successful in reproducing experimentally determined structural quantities2 such as the chain tilt and the film thickness for SAMs on these supports. The initial configurations for the chains were all trans-zigzagextended and normal to the surface. Periodic boundary conditions were used in the x and y directions for the SAM but not in (41) Stubbs, J. M.; Potoff, J. J.; Siepmann, J. I. J. Phys. Chem. B 2004, 108, 17596–17605. (42) Andersen, H. C. J. Comp. Phys. 1983, 52, 24–34. (43) Murad, S.; Gupta, S. Fluid Phase Equilib. 2001, 187, 29–37.
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the z direction. The simulation cell consisted of 90 CH3(CH2)20 - pO(CH2)pS molecules and a hard wall placed above the SAM. The size of the simulation box was 10.89σ × 11.32σ × 10.89σ (length (L) × width (W) × height (H)) for n-alkanethiolate SAMs on gold and 10.12σ × 10.54σ × 10.24σ for that on copper. A conformer was considered to be in a gauche state when its dihedral angle, φ, satisfied the expression cos φ g 0.5. Oxygen Transport through ω-Alkoxy-n-alkanethiolate SAMs on Gold and Copper. We used the so-called z-constraint algorithm36-38 to compute the free-energy barriers provided by the SAMs against through-film oxygen transport. We recently used this approach to examine the effect of chain length on the barrier properties of unsubstituted n-alkanethiolate monolayers on gold and copper.23 The CH3(CH2)20 - pO(CH2)pS SAMs on gold and copper were equilibrated for 300 ps, and the resulting configurations were used as the starting point for further simulations. An oxygen molecule was then inserted and constrained at a particular z position in the monolayer and was allowed to move only in the xy plane for 200 ps using a constraining force F(z, t). This process was repeated for various z positions chosen to sample the whole monolayer in equidistant steps. The oxygen molecule was represented by a single LJ site,43 with interaction parameters listed in Table 1. The time average of the constraining force, 〈F(z)〉t, was used to compute the local free-energy barrier, ∆G(z), at a particular z position as
∆G(z) ) -
z 〈F(z′)〉t dz′ ∫outside
(6)
The resulting free-energy barrier profiles, ∆G(z) versus z, for CH3(CH2)20-pO(CH2)pS SAMs on both gold and copper allowed a comparison with those for n-C22H45S SAMs on these substrates when determining the effect of positioning an ether unit at particular locations along the chain on the barrier properties of the film. We estimate the uncertainties in the ∆G values to be (1 kJ/mol.
Figure 2. Free-energy barrier profiles for oxygen transport through CH3(CH2)20 - pO(CH2)pS SAMs on gold for different values of p as computed by MD simulations.
Results and Discussion MD Simulations of Oxygen Transport through ω-Alkoxyn-alkanethiolate SAMs on Gold and Copper. The transport of a species through a monolayer film will depend on variations in the structure and free volume present within the SAM and how these features are affected locally by the presence of a diffusing species. The idealized static structures in Figure 1 for the SAMs on gold and copper reveal differences in the average tilt and packing density for the chains on these supports but do not capture variations that exist in-plane or along a chain that may contribute to transport rates through the film. Our molecular dynamics simulations provided energy information about the response of the CH3(CH2)20 - pO(CH2)pS monolayers on gold and copper to the presence of a oxygen as it diffused through each of the films. In these simulations, the z constraint method36-38 fixed the position of an oxygen molecule within each monolayer at selected distances from the metal surface and allowed its in-plane position as well as the structure of the SAM to vary. The results from these MD simulations provided quantitative information about the free-energy barriers for oxygen transport through these coatings as a function of the position of the ether unitsexpressed here by the number of methylene groups, p, between the sulfur atom and the ethereal oxygensin the monolayer. Figures 2 and 3 show the free-energy barrier profiles for oxygen transport through CH3(CH2)20 - pO(CH2)pS SAMs on gold and copper, respectively, for different values of p as computed by MD simulations using eq 6. These free-energy profiles for SAMs contain an ether linkage that shares attributes with those observed
Figure 3. Free-energy barrier profiles for oxygen transport through CH3(CH2)20 - pO(CH2)pS SAMs on copper for different values of p as computed by MD simulations.
for unsubstituted SAMs23 in that the outer region of the monolayer provides a slope similar to the barrier profiles and the middle region is responsible for the maximum free-energy barrier for
Oxygen Transport through Self-Assembled Monolayers
Figure 4. Maximum free-energy barriers against oxygen transport within CH3(CH2)20 - pO(CH2)pS SAMs on gold and copper as a function of the number of methylene groups between the sulfur atom and the ethereal oxygen, p. The dashed lines show the maximum free-energy barriers for unsubstituted CH3(CH2)21S SAMs on the two substrates for comparison.
the monolayer. In Figures 2 and 3, the free-energy barriers for a SAM on copper were higher than for that SAM on gold, owing to the denser chain packing (or lesser free volume) in the former system. Similar differences were observed between unsubstituted n-alkanethiolate SAMs on copper and gold.23 In Figures 2 and 3, the free-energy barrier profiles for oxygen transport through the films display an appreciable drop of ∼5 kJ/mol in many cases that occurs near the position of the ethereal oxygen. Local disordering events that can occur in the region of an alkyl chain near an ether linkage26 may be responsible for the observed dips in the free-energy barriers for the SAMs. The effects of ether on the free-energy barrier profiles appeared to be larger on the copper surfaces, where the unsubstituted n-alkanethiolate SAMs are able to provide superior barrier properties on copper26 as a result of their more dense packing on this support.2 The dip in the free-energy barrier profiles in Figures 2 and 3 at the location of the ethereal oxygen in the ether-containing SAMs also had a big impact on the maximum free-energy barrier attained by these monolayers against through-film oxygen transport. Figure 4 shows the maximum free-energy barriers attained by each of the CH3(CH2)20 - pO(CH2)pS SAMs on the two substrates against oxygen transport as a function of the number of methylene groups, p, separating the adhering sulfur atom of the SAM and the internal ethereal oxygen. The maximum freeenergy barriers for oxygen transport through unsubstituted CH3(CH2)21S SAMs on the two substrates (i.e., SAMs where the ethereal unit is replaced by a methylene group) are shown by the dashed lines in Figure 4 for comparison. For SAMs with an ether linkage on gold, the maximum freeenergy barriers against oxygen transport for p ) 10, 11, 14, and 17 were comparable to that determined for the corresponding unsubstituted C22 SAM on gold, whereas free-energy barriers through the CH3(CH2)20 - pO(CH2)pS SAMs were J2 kJ/mol lower for p ) 2, 5, 6, 8, 9, and 20, with those for p ) 5, 6, 8, and 9 having the lowest free-energy barriers in the series. On copper, the maximum free-energy barriers for these monolayers when p ) 2, 4, 8, 9, 10, 11, 14, and 17 were similar to those calculated for a C22 SAM on copper and were found to be J3 kJ/mol lower for p ) 5, 6, 7, and 20. In Figure 4, the maximum free-energy barriers for the CH3(CH2)20 - pO(CH2)pS monolayers, as calculated by MD simulations, suggest that having the ether substitution in the middle part of a C22 monolayer limits its impact on the barrier properties of the film. In contrast, SAMs having an ether linkage
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close to the metal surface or at the outer chain end provide less of a barrier for through-film oxygen transport as compared to unsubstituted C22 SAMs on these supports. The barrier property trends from MD simulations showed similarities to experimental work where EIS measurements on ω-alkoxy-n-alkanethiolate SAMs on copper16 yielded barrier properties for these SAMs that were comparable to those for their corresponding unsubstituted n-alkanethiolate analogs when the ether substitution was sufficiently far away from the metal surface and the chain ends. An unexpected feature in Figure 4 is that the maximum freeenergy barriers for oxygen transport through SAMs on copper for p ) 2 and 4, where the ether linkage is close to the metal surface, are similar to that for an unsubstituted C22 SAM and greater than for those having the ether linkage a little further away from the metal surface. This variation, along with a detailed structural analysis, is discussed in the next subsection. Equilibrium Structures of ω-Alkoxy-n-alkanethiolate SAMs on Gold and Copper from MD Simulations. Although the MD simulations of oxygen transport described above captured the trend that the barrier properties of CH3(CH2)20 - pO(CH2)pS SAMs on gold and copper depend on the location of the ether unit, the results are unable to provide structural details regarding the origin of these observations. To gain insight into the structural features within these films that may be responsible for the trends in barrier properties shown here by simulations and elsewhere16 in experiments, we performed MD simulations of CH3(CH2)20 - pO(CH2)pS SAMs on gold and copper to investigate the differences in their equilibrium structures for various values of p. In general, our MD simulations of these SAMs found that gauche densities in the vicinity of the ethereal oxygen were higher than those calculated along methylene segments within unsubstituted C22 SAMs.23 For the ether-containing SAMs, their higher gauche densities appear because of a local disordering of the chain caused by the ether linkage.26 The effects were more concentrated for the SAMs on copper because bonds in the region of two below to two above the position of the ether in these SAMs exhibited enhanced levels of gauche density, reflecting disorder in the chain. Regions further away from the ether exhibited gauche densities similar to those in unsubstituted SAMs. On both metals, the highest levels of gauche density occurred in the chains for adsorbates having roughly five methylenes separating the ether unit from the sulfur headgroup on both metals. The presence of disorder around the location of the ethereal oxygen and its effects on the nearby regions of the alkyl chain underlie the observed dips of 2-5 kJ/mol in the free-energy barrier profiles for these SAMs noted earlier (Figures 2 and 3). When regions away from the ether linkage contain gauche densities comparable to those in unsubstituted n-alkanethiolate SAMs, the free-energy barriers for oxygen transport are similar to those for their unsubstituted analogs (Figure 4). Experimentally, the barrier properties of such systems are similar to those of n-alkanethiolate SAMs, at least on copper, when the ether is positioned sufficiently far from the metal and outer surface. For example, impedance results for ether-containing SAMs CH3(CH2)17,21O(CH2)19S/Cu and CH3(CH2)21O(CH2)22S/Cu are comparable to those of their unsubstituted analogs.16 For SAMs having ether linkages at their outer chain ends (p ) 20), the conformer percentages in this terminal region in a gauche configuration were 12 and 4% for SAMs on gold and copper, respectively. These values were roughly 5 times higher than those calculated for C22 SAMs on gold and copper,23 revealing a much less organized outer surface. Experimental studies involving reflectance infrared spectroscopy and wetting
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measurements performed on SAMs having an ether linkage at its terminus indicate that the terminal region of these methoxyterminated monolayers are very disordered.25 From MD simulations of the barrier profiles for SAMs,23 the organization of the outer region of a monolayer is responsible for the initial slope of the free-energy profiles at the SAM/air interface and is an important factor in determining the maximum free-energy barrier for the SAM. The very disordered outer region that occurs for the methoxy-terminated SAMs (i.e., p ) 20) allows an oxygen molecule to penetrate these systems more easily and appears to be responsible for the lower free-energy barriers obtained for these monolayers compared to those for unsubstituted C22 SAMs. Our MD simulations of CH3(CH2)20 - pO(CH2)pS SAMs on gold and copper showed some unexpected deviations from the linear rodlike structures illustrated in Figure 1. These differences depended on the position of the ether unit within the SAM. Figures 5 and 6 show the equilibrium configurations at the end of MD simulation runs for CH3(CH2)18O(CH2)2S (p ) 2), CH3(CH2)13O(CH2)7S (p ) 7), and CH3CH2O(CH2)19S (p ) 19) SAMs on gold and copper, respectively. As readily seen, the equilibrium structures for SAMs on both gold and copper for p ) 7 were remarkably different from those for p ) 2 and 19. For SAMs with p ) 7, a large fraction of the chains in these monolayers adopted a structure where the chain regions above and below the ether unit tilted in different directions from one other. In contrast, SAMs where p ) 2 and 19 contained chains that tilted uniformly in one direction occurred for unsubstituted n-alkanethiolate SAMs.23,27,28 The kinked structures shown in Figures 5 and 6 for p ) 7 were also observed for SAMs on gold for p ) 5, 6, 8, and 9 and for SAMs on copper for p ) 5 and 6, albeit to varying degrees. Figure 7 depicts the fraction of CH3(CH2)20 - pO(CH2)pS molecules within SAMs on gold and on copper that adopt a kinked structure where the chains above and below the ether unit orient in different directions as a function of p from the MD simulations. Over half of the chains in these SAMs adopted the kinked configuration for values of p ) 5, 6, and 7 and p ) 5 and 7 for SAMs on gold and copper, respectively. In contrast, fewer than 20% of the chains adopted the kinked configuration for p e 4 and p g 10 on both substrates and for values of p ) 8 and 9 as well on copper surface. In general, chains within the SAMs on copper were less likely to adopt the kinked configuration than on gold, possibly because of the higher chain density and the greater tendency to align chains along the surface normal on copper. From Figure 7, positioning the ether in a region closer to the metal surface results in a greater fraction of chains with the kinked structure than in the middle of the SAM or closer to the outer surface. The observation that these kinked structures occur more frequently within these SAMs when the ether linkage is near the metal surface is intriguing and requires further analysis. MD simulations have shown that the orientation of chains in n-alkanethiolate SAMs on gold is extremely sensitive to the configurations of the methylene segments linked to the sulfur headgroup that is anchored to the metal surface.27 In contrast to polymethylene chains where a transzigzag-extended structure is preferred, the dihedral angle of a carbon-carbon bond adjacent to an ethereal oxygen has been shown to favor a gauche conformation over trans.26 This proclivity of the dihedral angle of a -CH2CH2CH2O- sequence to favor a gauche conformation may be instrumental in forcing these SAMs to adopt a flipped structure when the ethereal oxygen is close to the metal surface and is among the first few segments of the chains. Moreover, the permanent dipole in the monolayer due to the ethereal oxygen may also cause chain reordering and direct the chain to adopt a
SriVastaVa et al.
Figure 5. Side views of the equilibrium configurations at the end of the MD simulation runs for CH3(CH2)18O(CH2)2S (p ) 2), CH3(CH2)13O(CH2)7S (p ) 7), and CH3CH2O(CH2)19S (p ) 19) SAMs on gold.
kinked structure. These departures from fully trans-extended structures for SAMs having an ether linkage close to the metal surface adversely affect the organization and packing of the monolayer. These changes appear to be responsible for the lesser barrier properties of these monolayers in MD simulations and experiments as compared to those of unsubstituted SAMs having the same overall chain length. The strong anchoring provided by the sulfur headgroup to the metal surface appears to be responsible for the observation that the SAMs do not adopt a kinked structure when the ether linkage in these monolayers is close to the sulfur headgroup (i.e., for p ) 2-4). This preference may explain why the barrier properties calculated by MD simulations for these SAMs were comparable to those for an unsubstituted C22 SAM. As the distance between the ether linkage and the metal surface increases (p ) 5-7), the preference of the ether unit to introduce gauche
Oxygen Transport through Self-Assembled Monolayers
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Figure 7. Fraction of chains in CH3(CH2)20 - pO(CH2)pS SAMs on gold and copper adopting a kinked structure as a function of p, the number of methylene units between the ether unit and the sulfur headgroup.
explanation of the barrier property trends observed experimentally and by MD simulations.
Conclusions
Figure 6. Side views of the equilibrium configurations at the end of the MD simulation runs for CH3(CH2)18O(CH2)2S (p ) 2), CH3(CH2)13O(CH2)7S (p ) 7), and CH3CH2O(CH2)19S (p ) 19) SAMs on copper.
conformers may overcome preferences exerted by the sulfur-metal interaction on the chain structure. For longer separations between the ether and the metal surface (p g 8 for copper and p g 10 for gold), preferences by the polymethylene chain to adopt a transextended configuration appear to dominate the SAM structure. For these systems, they adopt relatedly tilted structures and exhibit similar barrier properties as for unsubstituted SAMs. The MD simulations reveal local energy and structural information for establishing links between the position of the ethereal oxygen in a monolayer and its influence on the barrier properties of a SAM. The existence of a disordered outer region in these monolayers when the ether linkage is at the chain ends and the observed kinked structures for SAMs having the ether linkage close to the metal surface provided the structural
MD simulations were performed to obtain insight into the effect of the position of a constituent ethereal oxygen on the barrier properties of various ω-alkoxy-n-alkanethiolate SAMs on gold and copper. The MD simulations allowed a systematic investigation of the complete range of ether linkage positions within a system that would be a challenge to access experimentally. The MD simulations showed that when the ethereal oxygen is placed close to the metal surface or to the outer chain end the free-energy barriers against the transport of oxygen offered by these SAMs were roughly 5 kJ/mol lower than for a corresponding unsubstituted n-alkanethiolate SAM of the same equivalent chain length. In general, the free-energy barrier trends revealed by MD simulations were in good agreement with observations from EIS measurements that had been performed previously on a subset of these systems and had suggested that the presence of an ether group located in specific regions within a SAM can lead to the formation of lesser organized films. The MD simulations found that the placement of the ether linkage at the chain end causes the terminal region of the SAM to become populated with a higher fraction of gauche conformers than do unsubstituted n-alkanethiolate SAMs with the same overall chain length. The higher gauche density is likely responsible for these coatings being less effective as barriers to through-film oxygen transport. In the MD simulations, whereas most of the ether-containing SAMs formed canted trans-extended structures, placement of the ether linkage at some positions along the chain resulted in some fraction of the SAM containing chains in a kinked configuration. These kinked structures, likely the result of the preference of bonds near an ether unit to adopt a gauche configuration, were observed to occur at the highest levels for SAMs having ether linkages spaced roughly five to seven methylene units from the coordinating sulfur headgroups. SAMs containing these kinked structures had lesser barrier properties than did those that adopted a predominately trans-extended structure as occurs for unsubstituted n-alkanethiolate SAMs on the substrates. Acknowledgment. We gratefully acknowledge the financial support of the NSF Center for Biological and Environmental Nanotechnology at Rice University (CBEN, EEC-0118007) and the Robert A. Welch Foundation (C-1241). This work was supported in part by the Rice Terascale Cluster funded by NSF under grant EIA-0216467, Intel, and Hewlett-Packard. LA803423A
