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Wetting-Induced Reconstruction in Molecular Surfaces N. Kacker,† S. K. Kumar,*,† and D. L. Allara*,†,‡ Department of Materials Science and Engineering and Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802 Received August 14, 1997X Attempts to understand and control wetting phenomena at organic surfaces have been hampered by a lack of molecular understanding of the interfaces, in particular, the extent of interpenetration and reconstruction of the adjoining phases. Through molecular dynamics simulations we show that subtle details of interfacial reconstruction can dramatically affect the stability of interfaces between a wetting liquid and a self-assembled monolayer (SAM). These results strikingly rationalize experiments which show a sharp transition in contact angles with small variations in chain length of the SAM. This finding provides an initial basis for the quantitative description of wetting phenomena, e.g., contact angle measurements, and offers significant progress toward a general molecular-based understanding of more complex interfacial phenomena observed with macro- and biomolecular systems.
The surface behavior of organic materials is of critical importance in many common contexts, e.g., biological processes, chromatography, and polymer adhesion. While a predictive understanding of these phenomena would unquestionably be of immense value, no quantitative pictures have yet emerged to describe such suspected critical aspects as the extent of solvent interpenetration into and consequent reconstruction of the adjoining phase.1-4 Recent experiments with self-assembled monolayers (SAMs)5-8 have shown intriguing effects of SAM chain length and composition on contact angles.3,9-12 Particularly interesting are a set of systematic experiments on controlled density, loosely packed SAMs which show a rather sharp transition of hexadecane wetting as a function of the SAM chain length.13 We have applied molecular dynamics (MD) simulations to model these experiments and observe that this transition appears to be strongly correlated with subtle details of interfacial reconstruction. This finding provides an initial basis for the quantitative description of wetting phenomena, e.g., contact angle measurements, and offers significant progress toward a general molecular-based understanding of more complex interfacial phenomena observed with macro- and biomolecular systems.1,14 †
Department of Materials Science and Engineering. Department of Chemistry. X Abstract published in Advance ACS Abstracts, November 1, 1997. ‡
(1) Andrade, J. D. Surface and Interfacial Aspects of Biomedical Polymers; Plenum Press: New York, 1985. (2) Timmons, C. O.; Zisman, W. A. J. Colloid Interface Sci. 1966, 22, 165. (3) Atre, S.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882. (4) Laibinis, P. E.; Bain, C. D.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1995, 99, 7663. (5) Ulman, A. An Introduction to Ultrathin Organic Films, from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (6) Whitesides, G. M. Crit. Rev. Surf. Chem. 1993, 3, 49. (7) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (8) Maoz, R.; Sagiv, J. Langmuir 1987, 3, 1034, 1045. (9) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097-5105. (10) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330-1341. Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. J. Adhes. Sci. Technol. 1992, 6, 1397-1410. (11) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370-1378. (12) Ulman, A.; Evans, S. D.; Shnidman, Y.; Sharma, R.; Eilers, J. E.; Chang, J. C. J. Am. Chem. Soc. 1991, 113, 1499-1506. (13) Judge, E.; Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1994, 100, 761. (14) Good, R. J.; Kotsidas, E. D. J. Adhes. 1979, 10, 17; J. Colloid Interface Sci. 1978, 66, 360.
S0743-7463(97)00917-7 CCC: $14.00
The wetting response of a solid surface is typically defined by the liquid drop contact angle, θ, which can be related to the relevant interfacial Gibbs free energies by Young’s equation:
cos θ ) (γSV - γSL)/γLV where γSV, γLV, and γSL are the solid-vapor, liquid-vapor, and solid-liquid tensions, respectively.15,16 The wetting behavior of mixed compositions surfaces is generally described by semiempirical composition-wetting angle correlations,17,18 for example, the Cassie equation:17 N
〈cos θ〉 )
fj cos θj ∑ j)1
where for N species at the wetting surface, fj is the fractional composition of species j, and θj is the corresponding wetting angle for the pure j surface. Recent reports3,4,19 of strong deviations from this simple type of correlation implicate surface reorganization as a controlling factor. In the present report we focus on a recent systematic set of n-alkane wetting experiments utilizing controlled, ∼50% coverage, n-alkyl chain SAMs, CnH2n+1 with n ) 6-24, formed on amorphous SiO2 substrates,13 a very general type of hydrophobic, partial-coverage sructure. While octane wets all SAMs examined, in contrast, the hexadecane (HD) θ changes sharply from complete wetting to high nonwetting values (∼39°) with increasing SAM length. Since infrared spectroscopy indicated no sharp structural variations with chain length in the “dry” monolayer structures, these results point to the underlying influence of solvent-induced reconstruction. However, while this conjecture forms a reasonable basis for understanding these types of anomalous wetting phenomena, no direct connections between the interfacial tensions and SAM structure have been established to date. Hints of such connections are given by previous computer simulations in densely-packed SAMs20,21 where the prediction of slight solvent-induced surface group restructur(15) Young, T. Philos. Trans. R. Soc. London 1805, 95, 65. (16) de Gennes, P. G. Rev. Mod. Phys. 1985, 57, 827. (17) Cassie, A. B. D. Discuss. Faraday Soc. 1952, 75, 5041. (18) Israelachvili, J. N.; Gee, M. L. Langmuir 1989, 5, 1. (19) Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156. (20) Hautman, J.; Klein, M. L. Phys. Rev. Lett. 1991, 67, 1763. (21) Hautman, J.; Klein, M. L. J. Chem. Phys. 1989, 91, 4994.
© 1997 American Chemical Society
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ing can be taken as an indication that extensive reorganization might occur in more loosely-packed structures. In order to resolve this issue we conducted molecular dynamics (MD) simulations. The wetting structure model carefully followed the reported experiments13 and consisted of (1) a planar SiO2 substrate with ∼2.7 randomly distributed SiOH grafting sites/nm2, equivalent to ∼50% of the maximum packing allowed for all-trans hydrocarbon chains, (2) covalent -C-O-Sit SAM-substrate linkages, where the C-O bond22 was forced to be near perpendicular to the surface with the aid of a potential, (3) chain moieties modeled as united atoms23 (beads) connected by rigid bonds24,25 with incorporation of realistic bond and torsion angle potentials,20,21 and (4) beads further than four bonds apart on the same chain and all beads on different chains interacting with Lennard-Jones potentials20,21 while all bead-substrate interactions follow a z-dependent potential.26 The initial structures consisted of 40 all-trans chains oriented approximately perpendicular to the substrate for the SAM and 48 HD molecules oriented either perpendicular or parallel for the solvent. A standard MD method was utilized27 with (1) the SHAKE algorithm to constrain the bond lengths at the prespecified value,23 (2) periodic boundary conditions to increase the system size, and (3) a 5 fs time step. After an initial equilibration of 150-300 ps, which employed velocity rescaling27 to attain a temperature of 300 K, we conducted simulations in the microcanonical ensemble. Effectively no changes in properties were found in all cases after the equilibration. We averaged properties at least over a further 500 ps and often as long as 2-3 ns. We first consider the “dry” SAMs at the vacuum interface. Since the grafting density is independent of SAM length, chain crowding will result with increasing n. Figure 1 shows this effect manifested in a monotonic increase in the mean trans populations with n, in good agreement with experimental infrared spectroscopy data.13 The simulation data can also be used to calculate the average surface composition of CH2 and CH3 moieties. Although the precise definition of the ambient surface is ambiguous, a reasonable definition is the Gibbs plane,28 i.e., the “halfway” point in the smoothly decreasing density tail. This represents the outer ∼5 Å of the surface, recognized as the active region in wetting.3,4 If there were no reconstruction, one might expect that the wetting behavior of hexadecane on these SAMs could be predicted by the Cassie equation. The known values of θ ∼ 0 and 45° for hexadecane on pure CH2 and CH3 surfaces, respectively,3,4 together with these derived surface composition data, were utilized to predict 〈cos θ〉. However, as Figure 2 (top) clearly shows, this wetting prediction based on the “dry” structures is in strong disagreement with experiment. This discrepancy is strikingly resolved by examining the solvent-induced reorganization of the SAM. Simula(22) For convenience, we have modeled the -O- moiety in our simulations by employing the parameters for -S- (using model II) from ref 21. (23) Ryckaert, J.-P. Bellemans, A. Discuss. Faraday Soc. 1978, 66, 95. (24) Jorgensen, W. L.; Madura, J. D.; Swenson, C. J. J. Am. Chem. Soc. 1984, 106, 6638. (25) Jorgensen, W. L. J. Phys. Chem. 1986, 90, 1284. (26) Klatte, S. J.; Beck, T. L. J. Phys. Chem. 1993, 97, 5727. While we have employed the same size parameters as Beck to characterize the wall-united atom potential, we have chosen the energy parameter w to be 61K in order to properly reproduce adsorption data for alkanes on SiO2. (27) Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Clarendon Press: Oxford, 1987. (28) Rowlinson, J.; Widom, B. Molecular Theory of Capillarity; Clarendon Press: Oxford, 1989.
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Figure 1. SAM alkyl chain trans populations plotted as a function of n, the number of carbon groups in the chain. Filled symbols refer to the dry SAM, while the open symbols are from the wet SAMs. Lines are a guide to the eye.
Figure 2. (top) Plots of the cosine of the liquid contact angle on the SAM as a function of n. Filled squares are the previously reported experimental data of Parikh et al. (ref 13) obtained by contacting hexadecane with the SAM. Filled circles are results obtained from application of the Cassie equation on the dry SAM as described in the text. (bottom) Plot of the simulation derived value of the spreading parameter (defined in the text), S, for hexadecane on the SAM as a function of n. The vertical dashed line is given as a guide to the location of the n value at the onset of the wetting change.
tions were carried out by placing an equilibrated dry SAM in contact with a HD or an octane overlayer. Generally 48 solvent molecules were utilized, but in a few runs with HD the number was doubled to 96 with no observation of finite size effects. After initial equilibration, we found that the average energy and average z position of the center
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Figure 3. Plots of the z-position of the solvent density profile Gibbs plane as a function of time in the MD simulations of hexadecane covered SAMs of varying chain lengths. The results show that the Gibbs plane assumes a stable position at very early times in the simulations and demonstrates the overall stability of the SAM-solvent interface.
of mass of the solvent film were independent of the starting orientation of the solvent chains. Figure 3, a plot of the z position of the solvent Gibbs plane, clearly illustrates the fact that a stable solvent profile at the interface is established over relatively short times. Subsequently, data were collected for up to 2-3 ns, and no systematic changes occurred in any property during this time. We now focus on the structural changes induced in the SAM by the presence of the HD phase. Figure 1 shows for small n that the mean trans population of the SAM chains is significantly increased, relative to the dry state, while with increasing n the conformational populations in the dry and wet states merge. For octane, the trans
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fraction is ∼74% for 5 < n < 24. The presence of the solvent clearly alters the conformations of the SAM chains, at least for the short chains in the case of HD and for all the chain lengths examined for octane. A more incisive view of the HD/SAM restructuring is observed in Figure 4, where the segment (bead) density z-profiles of the SAM and HD are plotted vs n. The left hand plot of the SAM density shows the expected sharp modulations near the substrate due to the layering of the beads. As the distance increases the wall-bead correlations begin to diminish and collapse into fairly smooth density tails for all values of n giving no clues of the observed sharp wetting change at n ) 15. In contrast, the HD density (right hand plot) for n e 15 shows a frontal peak in F at ∼4 Å from the SiO2 substrate while for n ) 16 the peak vanishes to give an ∼3 Å retraction of the wetting liquid front. Note that at this chain length the SAM has changed from a wetting to a nonwetting behavior experimentally (Figure 2, top). This strong correlation implies that the wetting is remarkably sensitive to the solvent-SAM mixing profile. Since the frontal HD peak coincides spatially with the minimum in the wall-segment potential, we conclude that the reorganization of the SAM is driven by a strong attraction of the HD molecules to the SiO2 substrate coupled with the diminishing spatial segment density of the SAM at low z. Intuitively, this behavior follows from a drive for the system to maximize the net surface-molecule and intermolecular van der Waals interactions. In contrast, for octane a peak is observed at ∼4 Å at all values of n. This evidence strongly implies that the fundamental basis of the transition in wetting behavior for HD with n is the change in the interfacial penetration by the solvent. If solvent-induced reorganization were key to the structure-wetting correlation, then these effects also would be reflected in the system thermodynamics. Since our simulations reveal that the internal energy balance, while favorable, is effectively independent of n, it is clear
Figure 4. Three-dimensional representations of the CH3 (bead) density (g‚cm-3) as a function of the distance from the substrate for a series of SAM/hexadecane (HD) structures with different chain-length SAMs. The total bead density has been separated into the SAM and liquid contributions and the same axis scales are maintained for each plot except for the direction of the SAM chain-length axes which were given opposite directions for viewing clarity. In the right-hand plot of the isolated HD profiles, arrows are used to mark the sharp change in the penetration depth of the HD into the SAM between n ) 15 and 16. This change corresponds closely to the position of the sharp onset of change in the wetting response in Figure 2. Note that such behavior is not present in the left-hand plot of the SAM density profile. For reference, the position of the solvent Gibbs plane is indicated for the C22 SAM by a vertical line marked with a G.
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that the free energies, as expressed in the γ values in Young’s equation,29 must be determined. However, since past work30 suggests that the direct MD evaluation of γLV values can have as much as 100% error, we turned to the approximation of square gradient theory.31 In this method the free energy of an inhomogeneous system is expanded in a Taylor series about the homogeneous value up to second order in gradients. An expression for the surface tension of the system then results:
∫0∞κ(dF dz)
γ)2
2
dz
where κ is a constant that embodies the interactions in the system28 and F is the density at that z value. We have utilized this method, with κ ) 0.5 from van der Waals theory, on slabs of HD exposed to vacuum at T ) 300 K and find γLV ∼ 20 dyn/cm. The reasonable agreement of this value with the experimental number of ∼25 dyn/cm32 suggests the approximate validity of this method. The extension of these ideas to the calculation of γSV and γLS requires incorporation of the strong molecule-substrate potential. This was achieved by a method proposed recently.33 These data, when combined into the spreading parameter,16 defined as S ≡ γSV - γSL - γLV, conveniently capture the spontaneity of the wetting process, i.e., θ ) 0 when S g 0 and θ > 0 when S < 0. The results of these calculations are given in Figure 2 (bottom) where we plot the n-dependence of S. In striking agreement with experiment, the simulations predict that S of HD changes sign for n ) 15-17 while S of octane is always positive. More careful examination shows that γLV changes sign at n ) 16, consistent with the notion derived from Figure 3 that the sharp wetting change in this case is driven primarily by changes in the solvent (29) In past work, Klein (see ref 20) had suggested that solvents which had high contact angles would spontaneously self-assemble into drops on the solid surface. The contact angle of the drop of liquid with the substrate could then be determined through standard geometry. In our simulations we do observe the solvent self-assembling into a drop in all cases where non-zero contact angles are predicted. However, the drop shapes show fluctuations too large to allow accurate estimates of contact angles. Consequently, this methodology (ref 20) does not appear to be appropriate in the context where one deals with relatively small contact angles. (30) Harris, J. G. J. Phys. Chem. 1992, 96, 5077. Chen, L.-J. J. Chem. Phys. 1995, 103, 10214. (31) Cahn, J. W.; Hilliard, J. E. J. Chem. Phys. 1955, 28, 258. We have utilized a variety of κ values ranging from 0.4 to 0.6 and found that the n value where S for HD changed sign was relatively unaffected. Consequently, our results are not significantly sensitive to the precise value of this parameter. (32) CRC Handbook of Physics and Chemistry; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1989. (33) Chen, Z. Y.; Noolandi, J.; Izzo, D. Phys. Rev. Lett. 1991, 66, 727.
penetration into the SAM. It is important to stress that the simulation-generated S values are not in quantitative agreement with the experiment, and the predicted contact angle values are different from experiment by a factor of 2. We speculate that this discrepancy could have been caused by factors such as the approximate character of the square gradient theory as well as approximations associated with relating microscopic ideas to a macroscopic experiment. In this regard though we also point out that these calculations represent, to our knowledge, the first direct theoretical prediction of spreading parameter values. A critical issue concerns the generality of the importance of solvent-induced reconstruction in wetting measurements. In the case of densely-packed SAMs where intrusion of solvent is sterically blocked, the results of Hautman and Klein20,21 suggest that solvents, such as water, show a tendency to perturb the conformational states of the outermost surface groups. On the other hand, for loosely-packed systems, e.g., polymers, solvent reorganization effects are speculated to occur widely.1 Our findings for contact angle solvents on the broad system of relatively loosely-packed, disordered SAMs show strong effects. Thus we conclude, supporting recent conjectures in a variety of experimental systems,4,14,19,34 that solventinduced effects play a ubiquitous role in contact angle measurements. In conclusion, we have demonstrated from computer simulations that solvent-induced reconstruction of molecular surfaces can be a key factor in controlling wetting behavior. This guiding principle puts on firm grounds previous notions that contact angle measurements may frequently be intrusive probes of molecular structures. Further, we suggest that empirical rules, e.g., Cassie’s law, which are based on data derived from pure, tightly packed surface structures, are limited to applications involving similarly dense surfaces which cannot undergo surface reconstruction. In contrast, in the limit of the more typical cases of highly flexible and open structures such as found in biological or polymeric systems, these reconstruction effects will be of profound significance in determining the contact responses of the associated surfaces.1,14 Acknowledgment. The financial support of the National Science Foundation is gratefully acknowledged. The authors thank J. Banavar, B. Garrison, and J. D. Weinhold for many useful discussions. LA970917K (34) Mansky, P.; Russell, T. P. Science 1997, 275, 1458.
