Selective Adsorption of DMSO from an Aqueous Solution at the

May 22, 2003 - The dynamics of wetting a hydrocarbon self-assembled monolayer by water/DMSO (dimethyl sulfoxide) mixtures and the resulting structure ...
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Langmuir 2003, 19, 5383-5388

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Selective Adsorption of DMSO from an Aqueous Solution at the Surface of Self-Assembled Monolayers John Vieceli and Ilan Benjamin* Department of Chemistry, University of California, Santa Cruz, California 95064 Received February 24, 2003. In Final Form: April 22, 2003 The dynamics of wetting a hydrocarbon self-assembled monolayer by water/DMSO (dimethyl sulfoxide) mixtures and the resulting structure are studied by molecular dynamics computer simulations. A mixture of the two liquids in contact with a methyl terminated C18H37 self-assembled monolayer shows preferential wetting of the surface by the DMSO molecules over a wide range of concentrations. The dynamics of the water desorption and DMSO adsorption starting from a nonequilibrium state are followed. The process is shown to involve a continuous replacement of approximately four water molecules for every single DMSO molecule. In the final equilibrium state, the DMSO methyl groups are oriented toward the surface while the SO bonds form hydrogen bonds with the water. This gives rise to an overlayer of water adjunct to the DMSO surface layer.

I. Introduction Elucidating the nature of the interactions between liquids and organic surfaces has been receiving growing attention in recent years. In atmospheric chemistry, organic aerosols can affect cloud properties through their interactions with water and provide a medium for reactions with significantly different kinetics and thermodynamics than those in bulk solution.1-3 In analytical chemistry, some chemical sensors utilize self-assembled monolayers (SAMs) that selectively adsorb trace organic molecules from solution.4,5 In engineering, phenomena such as lubrication, coating, and wetting6 are controlled by the nature of interactions at the liquid/solid interface. At the more fundamental level, the nature of wetting transitions of binary liquid mixtures at solid boundaries is of major current interest.7,8 In this case, one of the fluid components, which is energetically more favorable at the interface, will phase-separate and form a film separating it from the other liquid phase. While the above phenomena (and, in particular, the wetting transition) are beginning to be studied in detail using X-ray reflectometry,8 atomic force and scanning tunneling microscopy,9 and other techniques,10-12 very little has been done to understand solid-liquid interactions at the molecular level. In particular, most of our current understanding of wetting * To whom correspondence should be addressed. E-mail: [email protected]. (1) Andreae, M. O.; Crutzen, P. J. Science 1997, 276, 1052. (2) Kaufman, Y. J.; Fraser, R. S. Science 1997, 277, 1636. (3) Elisson, G. B.; Tuck, A. F.; Vaida, V. J. J. Geophys. Res. 1999, 104 (D9), 11633. (4) Scheller, F. W., Schubert, F., Ferrowitz, J., Eds. Frontiers in biosensorics; Birkhauser: Berlin, 1996. (5) Collins, G. E.; Gibbs, C. G.; Gutsche, C. D. Transducers ′97 1997, 1327. (6) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley: New York, 1990. (7) Dietrich, S. In Phase Transitions and Critical Phenomena; Domb, C., Lebowitz, J. L., Eds.; Academic Press: London, 1988; Vol. 12, p 1. (8) Plech, A.; Klemradt, U.; Huber, M.; Peisl, J. Europhys. Lett. 2000, 49, 583. (9) Giancarlo, L. C.; Flynn, G. W. Annu. Rev. Phys. Chem. 1998, 49, 297. (10) Croxton, C. A., Ed. Fluid Interfacial Phenomena; Wiley: New York, 1986. (11) Olbris, D. J.; Ulman, A.; Shnidman, Y. J. Chem. Phys. 1995, 102, 6865. (12) Eisert, F.; Dannenberger, O.; Buck, M. Phys. Rev. B. 1998, 58, 10860.

transitions in liquid mixtures on solid surfaces is based on phenomenological theories (such as Landau theory) and mean field approaches.13 One system of particular interest concerns the interfacial properties of water/dimethyl sulfoxide ((CH3)2SO, DMSO) mixtures, both their free (liquid/vapor) surface and their interaction with SAMs. DMSO is an intermediate in the process which leads to the formation of sulfuric acid in atmospheric aerosols.14 The adsorption of DMSO on metal surfaces has been extensively studied15 because of its role in double layer phenomena in electrochemistry.16 Selective adsorption of DMSO from its mixture with water at the SAM surface is also relevant to protein immobilization studies at interfaces.17 The adsorption of DMSO at water surfaces has been the subject of several recent experimental studies employing surface tension and surface potential measurements,18 resonance-enhanced surface second harmonic generation spectroscopy,19 and surface sum frequency generation spectroscopy.20 These studies indicate that the surface region is rich in DMSO relative to the bulk. Such surface partitioning effects have been found in other fully miscible water/polar liquid mixtures.21,22 A mean field theoretical model23 gave information about the density profiles and hydrogen bonding as a function of the distance from the interface and was in reasonable agreement with experimental data on surface excess properties over the entire concentration range. (13) Sullivan, D. E.; Gama, M. M. T. d. In Fluid Interfacial Phenomena; Croxton, C. A., Ed.; Wiley: New York, 1986; p 45. (14) DeBruyn, W. J.; Shorter, J. A.; Davidovits, P.; Worsnop, D. R.; Zahniser, M. S.; Kolb, C. E. J. Geophys. Res. 1994, 99, 16927. (15) Jarzabek, G.; Borkowska, Z. J. Electroanal. Chem. 1988, 248, 399. (16) Damaskin, B. B.; Tyurin, V. Y.; Dyatkina, S. L. Elektrokhimiya 1991, 27, 1358. (17) Hodneland, C. D.; Lee, Y.-S.; Min, D.-H.; Mrksich, M. Proc. Natl. Acad. Sci. 2002, 99, 5048. (18) Dabkowski, J.; Zago´rska, I.; Dabkowska, M.; Koczorowski, Z.; Trasatti, S. J. Chem. Soc., Faraday Trans. 1996, 92, 3873. (19) Karpovich, D. S.; Ray, D. J. Phys. Chem. B 1998, 102, 649. (20) Allen, H. C.; Gragson, D. E.; Richmond, G. L. J. Phys. Chem. B 1999, 103, 660. (21) Zhang, D.; Gutow, J. H.; Eisenthal, K. B.; Heinz, T. F. J. Chem. Phys. 1993, 98, 5099. (22) Wolfrum, K.; Graener, H.; Laubereau, A. Chem. Phys. Lett. 1993, 213, 41. (23) Luzar, A. J. Chem. Phys. 1989, 91, 3603.

10.1021/la034320i CCC: $25.00 © 2003 American Chemical Society Published on Web 05/22/2003

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We have previously used molecular dynamics simulations to study the structure, thermodynamics, and dynamics of the free surface of water/DMSO mixtures with various concentrations.24 Despite the complete miscibility at all concentrations, DMSO tends to aggregate at the surface, with the CH3 group pointing away from the bulk, in agreement with experiments.18-20 By starting some of the calculations with all the DMSO molecules in the bulk water, we were able to demonstrate that the approach to equilibrium is in good agreement with a simple diffusion model. In the present paper, we use the same water/DMSO model potential energy functions and potential energy functions we recently developed for investigating the wetting behavior of water on self-assembled monolayers to study the structure and the energetics involved in the wetting of the SAM by water/DMSO mixtures. By starting the simulation with all the “adsorption sites” on the surface occupied by water molecules and all the DMSO molecules in the bulk phase, we can also follow the dynamics of desorption of water molecules and the simultaneous adsorption by DMSO molecules. The rest of the paper is organized as follows: In section II, we briefly describe the molecular systems, the potential energy functions, and methods used in the simulations. In section III, we discuss the results. In section IV, we present the conclusions. II. Systems, Potentials Energy Functions, and Methods A. Systems and Potential Energy Functions. We study four different mixtures of water and DMSO in contact with a self-assembled monolayer made of 100 C18H37 molecules covalently attached to a 43 Å × 43 Å silica surface. Periodic boundary conditions are used in the (xy) plane parallel to the surface but not in the direction normal to the interface. This creates a liquid/vapor interface on one end. A reflecting wall is placed in the vapor phase at a distance of approximately 30 Å from the liquid/vapor interface to prevent the escape of gas-phase water molecules and maintain a fixed vapor pressure. More details about the self-assembled monolayer system studied here can be found elsewhere.25-27 Each hydrocarbon molecule consists of a flexible chain of CH2 groups modeled as united atoms of mass 14, terminated by a CH3 group which is modeled as a united atom of mass 15. The intramolecular potential includes harmonic stretching and bending terms, a three-term Fourier series for the torsional energy,28 and nonbonded interactions between two atomic centers separated by three or more bonds. The harmonic force constants, the torsional parameters, and the equilibrium bond lengths and bond angles are taken from the Amber force field29 and Jorgensen’s TIPS parameters.30 These parameters are given in Table 1. The intramolecular nonbonded interactions are modeled using the Lennard-Jones potential with parameters σ ) 4.0 Å and  ) 0.1 kcal/mol. This interaction is scaled down by a factor of 2 for the 1,4 carbon atoms in each chain.28 The intermolecular interactions between different hydrocarbon molecules are modeled as a sum of Lennard(24) Benjamin, I. J. Chem. Phys. 1999, 110, 8070. (25) Rudich, Y.; Benjamin, I.; Naaman, R.; Thomas, E.; Trakhtenberg, S.; Ussyshkin, R. J. Phys. Chem. A 2000, 104, 5238. (26) Squitieri, E.; Benjamin, I. J. Phys. Chem. B 2001, 105, 6412. (27) Vieceli, J.; Benjamin, I. J. Phys. Chem. B 2002, 106, 7898. (28) Burkert, U.; Allinger, N. L. Molecular Mechanics; American Chemical Society: Washington, DC, 1982. (29) Weiner, S. J.; Kollman, P. A.; Nguyen, D. T.; Case, D. A. J. Comput. Chem. 1986, 7, 230. (30) Jorgensen, W. L. J. Am. Chem. Soc. 1981, 103, 335.

Vieceli and Benjamin Table 1. Intramolecular Harmonic Force Field Parameters for DMSO and Hydrocarbon Molecules DMSO kS-O rS-O kS-C rS-C kC-S-C θC-S-C kC-S-O θC-S-O

SAM

2510 kJ/mol‚Å-2 1.53 Å 1674 kJ/mol‚Å-2 1.80 Å 837 kJ/mol‚rad-2 97.4° 837 kJ/mol‚rad-2 106.75°

2176 kJ/mol‚Å-2 1.53 Å 527 kJ/mol‚rad-2 112°

kC-C rC-C kC-C-C θC-C-C

Table 2. Lennard-Jones Parameters and Partial Charges for Water and DMSO atom

σ (Å)

 (kJ/mol)

Q (au)

S O(DMSO) CH3(DMSO) O(water) H(water) CH2(in C18H37) CH3(in C18H37)

3.4 2.8 3.8 3.165 0.0 3.905 3.905

1.00 0.30 1.23 0.65 0.00 0.50 0.73

0.139 -0.459 0.160 -0.82 0.41 0.00 0.00

Jones potentials between every pair of united atoms,

[( ) ( ) ]

uij(r) ) 4ij

σij r

12

-

σij r

6

(1)

The Lennard-Jones parameters σij and ij for the interaction between united atoms of types i and j are computed from the standard parameters of the CH2 and CH3 groups, using the relation31

ij ) xij, σij ) (σi + σj)/2

(2)

Each DMSO molecule is represented by four atomic sites by replacing each methyl group with a united atom of mass 15. The intermolecular interactions between DMSO molecules are represented by a sum of pairwise additive Coulomb and 6-12 Lennard-Jones interactions with parameters taken from the work of Luzar and Chandler.32 This model gives a structure in good agreement with neutron diffraction experiments on bulk water/ DMSO mixtures over a wide concentration range.33 Our DMSO model is fully flexible: The intramolecular vibrational potential is represented by harmonic bond stretching and angle bending terms, with parameters given in Table 1. The water model is the flexible SPC model used previously to study the properties of bulk and interfacial water. The intermolecular part is also represented by a sum of pairwise additive Coulomb and 6-12 LennardJones interactions. The water intramolecular potential is a fit to the spectroscopic data of gas-phase water.34 The water-DMSO, water-SAM, and DMSO-SAM interactions are all modeled using Coulomb and 6-12 Lennard-Jones interactions and the mixing rule (eq 2). All the Lennard-Jones parameters and charges for the DMSO, water, and chain molecules are given in Table 2. B. Methods. We consider four water/DMSO mixtures A, B, C, and D corresponding to the DMSO mole fractions 0.15, 0.2, 0.25, and 0.30, respectively. The mixtures are prepared by starting with a system in which 968 water molecules are in equilibrium with the SAM at 300 K. Then, a number of water molecules are removed at random from (31) Hansen, J.-P.; McDonald, I. R. Theory of Simple Liquids, 2nd ed.; Academic: London, 1986. (32) Luzar, A.; Chandler, D. J. Chem. Phys. 1993, 98, 8160. (33) Soper, A. K.; Luzar, A. J. Phys. Chem. 1996, 100, 1357. (34) Kuchitsu, K.; Morino, Y. Bull. Chem. Soc. Jpn. 1965, 38, 814.

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Table 3. Summary of System Sizes and Other Properties syst

Nwatera

NDMSOb

xDMSOc

xDMSO(bulk)d

A B C D

823 774 726 678

145 194 242 290

0.15 0.20 0.25 0.30

0.13 0.19 0.25 0.30

a Number of water molecules. b Number of DMSO molecules. Total DMSO mole fraction. dBulk DMSO mole fraction. Calculated from the density profiles of Figure 7.

c

groups of the SAM. The water profile is determined from a 0.5 ns simulation with 968 molecules, and the DMSO profile is determined from a 0.5 ns simulation with 290 DMSO molecules. The oscillations in the density profiles are typical of the structure of a liquid in contact with a solid.35 The oscillations in the water density decay to the bulk density value FB ) 0.0334 Å-3 when the distance from the surface is greater than 12 Å, while the oscillations in the DMSO profile persist farther out (FB ) 0.008 49 Å-3). For water, the ratio of peak density to bulk density, Fmax/FB ) 1.46, is smaller than that of DMSO, for which Fmax/FB ) 2.07. This indicates a more favorable interaction between the DMSO and the SAM than between water and the SAM. Indeed, the average interaction energy between an adsorbed water molecule and the SAM is -0.7 kcal/mol compared with -3.4 kcal/mol for the interaction between DMSO and the surface. The water results are consistent with experimental and theoretical studies25 of poor wetting of a smooth hydrocarbon SAM surface by water. A quantitative discussion of the adsorption of liquids on solid surfaces typically utilizes the concept of surface excess. For the case of a planar interface, denoting the density profile of any liquid in contact with the solid by F(z), where z is the coordinate normal to the interface, the surface excess is defined as

Γ)

Figure 1. Density profile of neat water (top panel) and of neat DMSO (bottom panel) in contact with a self-asssembled monolayer made of C18H37 hydrocarbon molecules chemisorbed on the surface of silica at 300 K. The vertical line at 21 Å indicates the average position of the hydrocarbon terminus.

a 7 Å thick lamella centered in the bulk region and replaced by the same number of DMSO molecules, as indicated in Table 3. We run a brief equilibration period until the temperature is stable at 300 K. Of course, the system is still out of equilibrium because DMSO molecules may diffuse and adsorb at the SAM interface, replacing adsorbed water molecules. The dynamics of this adsorption process at the interface, for each one of the four systems, are monitored for the next 2 ns using a constant temperature trajectory with an integration time step of 1 fs. The number of adsorbed water and DMSO molecules at the SAM interface and also their interaction energy with the SAM surface are followed as a function of time by performing averages over 10 ps intervals. This choice of the time interval is a compromise between the need for a relatively short time for an accurate description of the dynamic evolution and the need for enough data to get reasonable statistical averages. Final equilibrium averages are computed over the last 0.5 ns time interval in which no further change is observed as a function of time. This procedure gives information about the time evolution of any property and its final equilibrium value. III. Results and Discussion A. Neat and Initial Density Profiles. We begin with a brief discussion of the interaction of each neat liquid with the SAM surface. Figure 1 shows the density profiles of pure water (top panel) and of pure DMSO (bottom panel) in equilibrium with the SAM. Note that z ) 0 corresponds to the location of the silica surface onto which the hydrocarbon chains are attached and the vertical line at z ) 21 Å indicates the average location of the top methyl

∫zz (F(z) - FB) dz B

A

(3)

where zA is a reference plane and zB is a point in the liquid phase far enough from the surface such that the density is equal to the bulk density FB. If A is the surface area, then ΓA is the excess number of molecules in the surface relative to the number expected if the density remains fixed at the bulk value up to the reference plane zA. For water, zB is any point in the region 33 Å < z < 37 Å where the water attained its bulk density FB ) 0.0334 Å-3. When discussing the adsorption of two or more liquids at a solid boundary, it is useful to choose the reference plane to make the surface excess of one of the components be zero. For water, this surface is located at zA ) 23.2 Å, which is 2.2 Å away from the methyl terminated headgroups (toward the water), suggesting poor wetting of the surface by water.25 While the surface excess is useful for a thermodynamic treatment,36,37 it requires the use of a standard reference density, which may not be well defined. Another choice, which will be used henceforth, is molecular in nature and involves actual counting of interfacial molecules. Thus, it requires a definition of the surface region. This quantity is related to the surface coverage used in statistical models of adsorption, and it is particularly useful here as a quantitative measure for following the dynamics of adsorption. We define the surface density of the water (σw) and the DMSO (σd) by counting the corresponding number of molecules in the surface region z < zs and dividing by the surface area. For water, zs is taken to be the location of the first minimum in the neat water density profile (Figure 1). A similar quantity is determined for DMSO. We have for water zs ) 26.8 Å and for DMSO zs ) 27.2 Å. The water surface density in the neat water/SAM system is σ°w ) 0.111 Å-2, corresponding to approximately two water molecules per CH3 (35) Henderson, D., Ed. Fundamentals of Inhomogeneous Fluids; Marcel Dekker: New York, 1992. (36) Nicholson, D.; Parsonage, N. G. Computer Simulation and the Statistical Mechanics of Adsorption; Academic Press: New York, 1982. (37) Rowlinson, J. S.; Widom, B. Molecular Theory of Capillarity; Clarendon: Oxford, 1982.

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Figure 2. Initial density profiles of the four systems studied. In each panel the solid thick line is the DMSO density profile, the thin line is the water density profile, and the thick vertical line is the average position of the outer CH3 united atom of the SAM. In panel A, we also show (dotted line) the density profile of the original water-only density profile in contact with the SAM. The DMSO overall molar fractions are 0.15 (A), 0.2 (B), 0.25 (C), and 0.3 (D).

group (their density is 0.054 Å-2). The DMSO surface density in the neat DMSO/SAM system is σ°d ) 0.0367 Å-2 , which is three times smaller. This is due to the larger volume occupied by the DMSO molecule. The effective volume of a DMSO molecule in the bulk is about FB(water)/ FB(DMSO) ) 3.9 times larger than that of water. The initial density profiles (following a short equilibration period, as discussed above) along the surface normal of water and DMSO in each mixture in contact with the SAM are shown in Figure 2. In panel A, we again show (dotted line) the density profile of the neat water/SAM system of Figure 1. Comparing this neat profile with the density profiles of the four panels shows that, in the short equilibration period following the insertion of the DMSO molecules, the systems have significantly expanded in the liquid/vapor interface region. In contrast, the two “layers” of water molecules near the SAM (approximately corresponding to the two peaks in the water density profiles) experience almost no change, and no DMSO is near the SAM (within the region corresponding to the first water peak) yet. B. Time-Dependent Adsorption. Figure 3 shows the time-dependent surface densities, starting from the initial configuration of Figure 2, over the entire 2 ns simulation. Each data point is obtained by averaging the surface densities from a 10 ps interval. Note that the initial water densities in the four systems are slightly larger than the σ°w ) 0.111 Å-2 value. This is due to statistical uncertainty (10 ps interval compared with the 500 ps used to calculate σ°w) and possibly due to an initial temporary increase in the water density following the insertion of the DMSO in the center, which increases the initial normal pressure. The DMSO molecules diffuse to the SAM surface and replace water molecules in a process that is completed in less than 2 ns. Clearly, this time scale depends on the initial distance of the DMSO from the surface and its initial bulk concentration. However, by calculating the normalized correlation function

C(t) )

σ(t) - σ(∞) σ(0) - σ(∞)

(4)

Figure 3. Time-dependent surface concentration (see text for exact definition) of water and of DMSO in each of the four systems. The thick solid line, dashed line, solid thin line, and dotted line correspond to systems A, B, C, and D, respectively. The inset shows the normalized correlation function C(t) (eq 4).

Figure 4. Time-dependent relative change in the number of adsorbed water and DMSO molecules. See text for an exact definition. The lines for each system are the same as those in Figure 3.

which is shown in the inset of each panel, one can show that the approach to equilibrium of these systems takes place on a similar time scale of 0.4 ns (from a fit to a single-exponential decay) for both liquids. This is similar to what is obtained by a solution of a diffusion equation for the change in concentration as a function of time, suggesting a barrierless adsorption and desorption process.24,38 This will be confirmed below when we discuss the adsorption energies of water and of DMSO. The final water surface density varies from about 0.050 Å-2 for system A to 0.039 Å-2 for systems B and C and 0.031 Å-2 for system D. The final DMSO density is less sensitive to the bulk concentration and is given by 0.019, 0.022, 0.023, and 0.025 Å-2, for systems A-D, respectively. The dynamic fluctuations around these average values reflect the continuous desorption/adsorption process that each liquid experiences even after equilibrium is reached. Figure 4 shows the ratio [σw(t) - σw(0)]/[σd(t) - σd(0)] as a function of time. Except for a short transient period that lasts 0.2 ns (during which σd(t) - σd(0) is a small number, resulting in a large statistical error), this is nearly constant (38) MacRitchie, F. Chemistry at Interfaces; Academic Press: San Diego, CA, 1990.

Adsorption of DMSO on SAMs

Figure 5. Time-dependent total DMSO-SAM (thick lines) and water-SAM (thin line) interaction energy.

Figure 6. Time-dependent total DMSO-SAM (thick lines) and water-SAM (thin lines) interaction energy, normalized by the number of surface molecules.

throughout the process and is equal to about -3.7. Thus, each DMSO molecule replaces about four water molecules during the process, and this ratio is independent of the bulk concentration (at least in the range of systems A-D) and of time. This suggests that the DMSO adsorption/ water desorption process is a concerted process. The tendency of the DMSO to replace water at the SAM surface is in part due to the larger (more negative) adsorption energy of this molecule. Figure 5 shows the time-dependent SAM-water (thin lines) and SAMDMSO (thick lines) interaction energies for the four systems. The time scale for the approach to equilibrium of these energies seems to be very similar to the change in surface densities shown in Figure 3. Indeed, the normalized interaction energy per molecule shown in Figure 6 is remarkably independent of time (except for a transient period for DMSO that lasts less than 0.2 ns) and of the system. It is equal to -0.7 kcal/mol for water and -3.5 kcal/mol for DMSO, which is very close to the average interaction energy between each neat liquid and the surface. Thus, every four water molecules leaving the surface increases the interaction by +2.8 kcal/mol and this is more than made up for by the adsorption of one DMSO molecule.

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Figure 7. Final equilibrium density profiles of water (thin lines) and DMSO (thick line) calculated from the last 0.5 ns of the simulation.

C. Final Equilibrium Characterization. Figure 7 shows the final density profile of the two liquids calculated from the last 0.5 ns of each system. We clearly see that the water experienced a dewetting transition: The first peak is much smaller than the second, in contrast with the initial density profile shown in Figure 1. The DMSO is clearly wetting the surface. The large second peak of the water immediately to the right of the DMSO peak reflects the strong solvent-solvent hydrogen bonding (one end of the DMSO molecule is a strong proton acceptor and can hydrogen bond with water). This is also consistent with recent molecular dynamics simulations of bulk aqueous solutions of DMSO and neutron scattering experiments, which show the enhancement of waterDMSO (and water-water) hydrogen bonding.32,33,39 The density profiles of Figure 7 give a reasonably constant density (especially for DMSO) in the “bulk” region of the system. This allows us to compute the bulk concentration of each liquid, which can be used to test if the adsorption at the SAM surface follows a simple adsorption isotherm. The mole fraction of DMSO in the bulk of the four systems at equilibrium is given in Table 3. It is interesting to note that it matches the overall mole fraction in the system. (Of course, the initial mole fraction before the onset of adsorption was much larger.) This suggests that the system is still far from saturation (in that increasing the DMSO concentration in the bulk will lead to additional adsorption at the surface). Indeed, a plot of the DMSO surface concentration (data given in the paragraph after eq 4) as a function of its bulk mole fraction gives a linear plot (not shown), expected for the region far from saturation in the Langmuir adsorption isotherm.6 Similar results are obtained by calculating the surface excess (using as the reference plane that which gives zero surface water excess in each of the four systems). In particular, the surface excess of DMSO in each of these systems is near zero, which matches the mole fraction data given in Table 3. This is consistent with the qualitative conclusion one may draw from Figure 7: While the DMSO wets the surface, there is a significant peak of the water density in the layer just next to the DMSO (on the bulk side) which, over the concentration range of this (39) Vaisman, I. I.; Berkowitz, M. L. J. Am. Chem. Soc. 1992, 114, 7889.

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Figure 8. Final equilibrium probability distributions for several molecular orientations with respect to the surface normal at the SAM/liquid interface (left panels) compared with the liquid/vapor interface (right panels). Panels A and B show the water dipole orientation. Panels C and D show the orientation of the OH bond. Panels E and F show the orientation of the SO bond. In all cases, θ ) 0° corresponds to the oxygen (of water and of DMSO) pointing away from the bulk. The lines for each system are the same as those in Figure 3. In addition, in panels A-D the dashed-dotted lines correspond to the orientations of water molecules in the neat water/SAM system, and in panels E and F the dashed-dotted lines correspond to the orientations of the SO bond of DMSO in the neat DMSO/ SAM system.

paper, gives for the total surface excess (an integral over the whole region) a value of zero. Finally, Figure 8 shows the molecular orientations of the adsorbed water and DMSO molecules (left panels) in comparison with the orientations at the liquid/vapor interface of the mixtures (right panels). The orientations are almost independent of the concentration in the range

Vieceli and Benjamin

examined here. Panels A and C show that the water dipole is nearly parallel to the SAM surface with a slight tilt of the water oxygen toward the SAM surface. One OH bond is parallel to the surface, and the other hydrogen is pointing toward the bulk water. In contrast, the significant population near θ ) 180° in panel D suggests some free OH bonds at the liquid/vapor interface40,41 but much fewer at the SAM. The DMSO is oriented such that the methyl groups are more likely to point toward the surface of the SAM than the oxygen, similar to the results at the liquid/ vapor interface.20,24 It is likely that this reflects the DMSO oxygen atoms’ participation in hydrogen bonding with the water molecules located in the surface region (corresponding to the broad peak near 90°) and with water molecules in the second layer (corresponding to the significant population around 180°). Each panel also includes (dashed-dotted lines) the orientational probability distributions of the neat liquid in contact with the SAM and in contact with its own vapor. We note that while some differences between the distributions in the mixtures and in the neat liquids are clearly observed, the general behavior described above is unchanged. IV. Conclusions Water and DMSO are fully miscible liquids over the entire concentration range. Yet, a mixture of these two liquids on a self-assembled monolayer exhibits a wettinglike transition in which the DMSO preferentially adsorbs on the surface due to its more negative adsorption energy. The DMSO is oriented with the methyl group pointing toward the organic surface, thus maximizing hydrogen bonding with the water in the second adsorbed layer. Acknowledgment. This work has been supported by a grant from the National Science Foundation (CHE9981847). Discussions with Prof. Gang-yu Liu are greatly appreciated. LA034320I (40) Du, Q.; Superfine, R.; Freysz, E.; Shen, Y. R. Phys. Rev. Lett. 1993, 70, 2313. (41) Du, Q.; Freysz, E.; Shen, Y. R. Science 1994, 264, 826.