Molecular Dynamics Simulations of Adsorption of Catechol and

Article pubs.acs.org/JPCC

Molecular Dynamics Simulations of Adsorption of Catechol and Related Phenolic Compounds to Alumina Surfaces In-Chul Yeh, Joseph L. Lenhart, and B. Christopher Rinderspacher* Macromolecular Science & Technology Branch, Materials & Manufacturing Science Division, U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, United States

ABSTRACT: We performed atomistically detailed molecular dynamics simulations to study adsorption behaviors of catechol, which is a key functional group in marine bioadhesives, to two different alumina surfaces in both anhydrous and aqueous conditions. In anhydrous conditions, without competing interactions from water molecules, catechol adsorbed to both hydroxylated and nonhydroxylated alumina surfaces. In aqueous conditions, catechol and several analogous phenolic compounds displaced water molecules and were strongly attracted to the nonhydroxylated alumina surface, which is more hydrophobic. When comparing the phenolic moieties near the hydroxylated alumina surface in aqueous conditions, the catechol molecules displayed the strongest adsorptions mainly through cooperative hydrogen bonding interactions of two neighboring hydroxyl groups with the surface hydroxyl groups of alumina as evidenced by the longer hydrogen bonding lifetimes and the larger number of adsorbed molecules near the surface. Insights gained from this study can be used in design of novel bioadhesives or antifouling surface coatings. coatings and surfaces.20−22 For example, Saiz-Poseu et al.23,24 investigated adsorption behaviors of an alkyl catechol and a catechol-based macrocycle at graphite/nonanoic acid and Au(111)/n-tetradecane solid/liquid interfaces, respectively, in combined experimental and molecular dynamics simulation studies. Adsorptions of catechol on surfaces of TiO2(110) and germanium in vacuum were also investigated by STM and surface spectroscopy experiments and theoretical methods.25,26 Adsorptions of catechol to other types of metal oxide substrates in aqueous conditions have not been studied yet with molecular dynamics simulations. Alumina or aluminum oxide (Al2O3) is widely used in many technological applications due to its high melting temperature and very high hardness.27 Interactions that control adsorption of organic molecules to the alumina surface is of importance in both practical and fundamental perspectives. In this study, we performed atomistically detailed molecular dynamics simulations of catechol near alumina surfaces to understand the structural details and underlying driving forces behind the adsorption behavior of catechol with alumina. Even though chemisorptions of catechol and related phenolic compounds on aluminum oxides such as gibbsite, boehmite,

1. INTRODUCTION While synthetic adhesives are susceptible to extreme humidity, aquatic organisms such as mussels and barnacles possess a remarkable ability to adhere to various surfaces in aqueous environments.1−4 The 3,4-dihydroxy-L-phenylalanine (DOPA) containing catechol moiety found usually flanked by cationic amino acid residues in the adhesive proteins mfp-35 and mfp-56 of marine mussels is considered to be one of the key functional groups for the adhesive properties.4,7,8 The catechol functional group contains two adjacent hydroxyl groups and a benzene ring, which can contribute to adhesion with hydrophilic and hydrophobic interactions, respectively,9,10 in addition to chemisorptions.11,12 Many synthetic bioadhesives containing the catechol functional group have been designed and studied experimentally.13−19 However, despite recent advances in experimental techniques such as scanning tunneling microscopy (STM), it remains a challenge to characterize fully the atomistic details of the adsorption behaviors of solvated phenolic compounds with solid substrates, which may arise from many complex competing interactions among the phenolic compounds, the substrate, and water molecules. Atomistically detailed molecular dynamics simulations can be a useful tool to complement the experimental efforts to understand the structural and dynamical details of such interactions and design synthetic adhesives in extreme humidity and antifouling This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society

Received: December 22, 2014 Revised: March 13, 2015

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DOI: 10.1021/jp512780s J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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and noncrystalline alumina were observed and can contribute to the adsorption behavior,11,12 we focused on nonreactive physisorptions of phenolic compounds to alumina surfaces, which can be described by classical force fields. To investigate the effect of the hydrophobicity of the surface on the adsorption behavior of catechol, two different types of alumina slabs were prepared: one with hydroxylated and the other with nonhydroxylated surfaces. First, adsorptions of catechol molecules to the alumina surfaces in vacuum without solvent molecules were investigated. Next, distributions of water molecules near the two different alumina surfaces without added organic molecules were investigated to estimate the intrinsic hydrophilicity of the alumina surfaces. Then, interactions of catechol near the alumina surfaces in aqueous conditions were investigated and compared with those obtained in vacuum. We also investigated interactions of several related phenolic compounds, resorcinol, hydroquinone, and phenol, with the alumina surfaces to determine the importance of the number and proximity of hydroxyl groups to the adsorption behavior of catechol with the solid substrate. Finally, the free energy profiles of catechol and related phenolic compounds near the alumina surfaces were obtained with extensive biased simulations and compared with those obtained with unbiased simulations.

2. METHODS 2.1. Description of Simulated Systems. As described earlier, to investigate the effect of the hydrophilicity of the alumina surface on the adsorption behavior, we prepared two different types of alumina slabs with hydroxylated and nonhydroxylated surfaces. For alumina slabs with hydroxylated and nonhydroxylated surfaces, thin slabs of alumina with thicknesses of 12 and 13 Å, respectively, were prepared by replicating the unit cell of the crystalline α-Al2O3 10 times in a and b directions and cutting along the oxygen-terminated (0001) surface. The slightly different thicknesses of alumina slabs were chosen to satisfy the charge neutrality constraint for each alumina surface. The resulting lengths of the simulation cell in x and y directions were 47.59 and 41.21 Å, respectively. For the hydroxylated surface, hydrogen atoms were attached to the oxygen atoms at the surface. For the nonhydroxylated alumina surface, the oxygen atoms on the surface remained exposed. The positions of the atoms in the alumina slab with the nonhydroxylated surface were fixed during energy minimization and subsequent dynamic simulations to preserve the configuration with exposed oxygen atoms, which may be unstable.28 Typical snapshots of simulations with hydroxylated surfaces are shown in Figure 1. It also illustrates two types of interfacial systems used in our simulations. Figure 1(a) illustrates a system where the solvated phenolic compounds form interfaces with the alumina surface on one side and a vacuum on the other side. Figure 1(b) shows a system where the solvated phenolic compounds are enclosed between two alumina surfaces without forming interfaces with vacuum. Figure 1(c) shows schematics of phenolic compounds used in our study. Table 1 summarizes the simulations performed in this study. First, alumina/catechol interfaces without any water molecules were prepared with both hydroxylated and nonhydroxylated alumina surfaces to study the interactions of catechol with alumina in anhydrous conditions (simulations 1 and 2 in Table 1). Next, alumina/water interfaces without phenolic compounds were prepared to understand the interactions of water

Figure 1. Illustrations of the simulation setup and the phenolic compounds used in this study. (a) An aqueous solution of catechol was in contact with the hydroxylated alumina surface on one side and the vacuum on the other side. (b) The solvated catechol was enclosed between two hydroxylated alumina surfaces. A significant vacuum gap spanning across the interface present in both (a) and (b) in conjunction with an electrostatic correction term for the slab geometry implemented effectively a two-dimensionally periodic boundary condtion. (c) The schematics of the phenolic compounds used in our study.

with the two different types of alumina surfaces (simulations 3 and 4). Effects of different concentrations of catechol involving one or two hydroxylated alumina surfaces were studied in simulations 5−9 as illustrated in Figures 1(a) and (b), respectively. Interactions of three other phenolic compoundsresorcinol, hydroquinone, and phenolwith hydroxylated alumina surfaces were also studied in simulations 10−12 in order to assess the effect of hydroxyl placement. Interactions of phenolic compounds with nonhydroxylated surfaces in aqueous condition were investigated in simulations 13−16. In simulations 17−24, the interactions of phenolic compounds with alumina surfaces in aqueous condition were probed by estimating the free energy or potential-of-mean force (PMF)29−31 profiles of phenolic compounds across the interfaces with umbrella sampling.32 2.2. Force Fields. As mentioned earlier, in this study, nonreactive interactions of phenolic compounds with alumina surfaces, which can be described by classical force fields, are the focus. The mixed interactions of organic and inorganic systems as in catechol and alumina pose challenges in conventional applications of force fields, which are typically designed for a particular class of system, either organic or inorganic.33−36 Recently, Heinz et al.37 developed force fields that can describe the inorganic−organic interface, but the parameter set for alumina is not readily available yet. The CLAYFF force field,34 which treats most interatomic interactions in crystalline B



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Table 1. Summary of Simulations Performed in This Studya simulation

alumina

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

1 1 1 1 1 1 1 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1

(hydroxylated) (nonhydroxylated) (hydroxylated) (nonhydroxylated) (hydroxylated) (hydroxylated) (hydroxylated) (hydroxylated) (hydroxylated) (hydroxylated) (hydroxylated) (hydroxylated) (nonhydroxylated) (nonhydroxylated) (nonhydroxylated) (nonhydroxylated) (hydroxylated) (hydroxylated) (hydroxylated) (hydroxylated) (nonhydroxylated) (nonhydroxylated) (nonhydroxylated) (nonhydroxylated)

water

phenolic compounds

none none 3000 3000 1000 1000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000 3000

45 (catechol) 45 (catechol) none none 100 (catechol) 50 (catechol) 50 (catechol) 150 (catechol) 45 (catechol) 45 (resorcinol) 45 (hydroquinone) 45 (phenol) 45 (catechol) 45 (resorcinol) 45 (hydroquinone) 45 (phenol) 1 (catechol) 1 (resorcinol) 1 (hydroquinone) 1 (phenol) 1 (catechol) 1 (resorcinol) 1 (hydroquinone) 1 (phenol)

mixing procedure described below. For van der Waals interactions in PCFF, a 9-6 LJ potential energy function was used ⎡ ⎛ ⎞9 ⎛ σij′ ⎞6 ⎤ σij′ U (rij) = ϵ′ij⎢⎢2⎜⎜ ⎟⎟ − 3⎜⎜ ⎟⎟ ⎥⎥ r ⎝ rij ⎠ ⎦ ⎣ ⎝ ij ⎠

The prime symbol signifies van der Waals parameters that can be used for the 9-6 LJ function. The parameters σ′ij and ϵ′ij were set by applying the following sixth power mixing rules36 suitable for the PCFF force field ⎤1/6 ⎡1 σij′ = ⎢ ((σi′)6 + (σj′)6 )⎥ ⎦ ⎣2

ϵ′ij =

(σi′)6 + (σj′)6

σi′,CLAYFF = 21/6σi ,CLAYFF

(6)

(7)

A similar mixing scheme was used and optimized by Heinz et al.43 in simulations of surfaces and interfaces of face-centered cubic metals. A test simulation with a 12-6 LJ function and the Lorentz−Berthelot mixing rule with a modified parameter σi,PCFF = 2−1/6σi,PCFF ′ of PCFF atom i was also performed. However, the 9-6 LJ function with the adjusted parameter in eq 7 for the mixed interactions between CLAYFF and PCFF atoms reproduced the experimental water solubility and adsorption properties of phenolic compounds better and were used consistently in all of our simulations. 2.3. Simulation Details. As shown in Figure 1, each system was prepared in a simulation cell of a rectangular prism with the alumina surface normal to the Z-axis and included the large vacuum gap along the z direction to remove the interactions between periodic images in the z direction. The molecular dynamics simulations were performed using the LAMMPS program44 with a time step of 1 fs. The cutoff distance of 12 Å was used for LJ interactions as in our recent simulation study.45 The electrostatic interactions were calculated with the particle− particle−particle mesh (PPPM) method46 using a real space cutoff distance of 12 Å. The three-dimensional periodic boundary condition was applied with the Ewald correction term for the slab geometry47,48 implemented in LAMMPS. The large vacuum gap in conjunction with the Ewald correction term for the slab geometry essentially represents twodimensionally periodic systems. The simulations were performed under constant volume at room temperature of 300 K (NVT ensemble). However, the simulated system adjusted to its optimum density because of the vacuum gap along the z direction. Number density distributions of water

(1)

where the rij is the distance between the two atoms designated by i and j, and the parameter σij and the potential well depth ϵij are given by applying the Lorentz−Berthelot rule42 1 σij = (σi + σj) (2) 2 and ϵiϵj

2 ϵ′i ϵ′j (σi′)3 (σj′)3

where σ′i and ϵ′i are PCFF van der Waals parameters for atom i. It is to be noted that the minimum potential energy −ϵij′ is obtained at rij = σij′ in the 9-6 LJ function described by eq 4. For the mixed interaction between atoms i and j described by CLAYFF and PCFF force fields, respectively, the 9-6 LJ function and the sixth power mixing rules in eqs 4−6 were used, but the van der Waals parameter σi,CLAYFF of CLAYFF atom i was modified by the following relation so that the potential energy minimum with the PCFF 9-6 LJ function occurs at the same distance as with the CLAYFF force field.

materials as nonbonded, was successfully used to study interfacial water and ethanol at the alumina surface by Striolo et al.38−40 Therefore, we used the CLAYFF force field and the simple point charge (SPC) model of water41 to describe alumina and water. With a future extension to polymeric systems in mind, the polymer consistent force field (PCFF) force field35 was chosen to describe phenolic compounds, and the partial charges on atomic sites were assigned with those corresponding to the condensed-phase optimized molecular potentials for atomic simulation studies (COMPASS) force field.36 The nonbonded van der Waals interactions between the atoms subjected to the CLAYFF force field were described by a 12-6 Lennard-Jones (LJ) function

ϵij =

(5)

and

a The 1st column shows simulation identification numbers. The 2nd column shows numbers and type (in parentheses) of alumina surfaces in the simulation. The 3rd column lists number of water molecules in the simulated system. The 4th column lists numbers and type (in parentheses) of phenolic compounds in each simulation.

⎡⎛ ⎞12 ⎛ ⎞6 ⎤ σij σij U (rij) = 4ϵij⎢⎢⎜⎜ ⎟⎟ − ⎜⎜ ⎟⎟ ⎥⎥ r ⎝ rij ⎠ ⎦ ⎣⎝ ij ⎠

(4)

(3)

to CLAYFF van der Waals parameters σi and ϵi for atom i. The minimum potential energy in the 12-6 LJ function in eq 1 is −ϵij obtained at rij = 21/6σij, which will be used in the adjustment of van der Waals parameter σi in the force field C



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and phenolic compounds across the interface were monitored to ensure that the proper equilibration had been reached. It was determined that most of the simulation systems were equilibrated after 1 ns. However, rather long simulations were necessary to sample properly distributions at low concentrations. Therefore, each simulation was performed with an additional 19 ns after equilibration. 2.4. Density Distributions and Potentials of Mean Force. To understand the structures and adsorption preferences of the phenolic compounds near the alumina surfaces, we calculated the number density n(z) of atoms of interest along the z direction from the simulation trajectory. The average z position of oxygen atoms at the alumina surface was taken as a reference z position (z = 0 Å). The potential of mean force (PMF) along the z direction w(z) was estimated by w(z) = − RT log n(z) + C

(8)

where R, T, and C represent the gas constant, temperature, and an undetermined constant, respectively. 2.5. Potential of Mean Force Calculation with Umbrella Sampling. To estimate the free energy profiles of the phenolic compounds across the alumina/water/vacuum interface more directly, we performed a series of simulations where the center of mass z positions of the phenolic compounds were biased at various points across the interface by a harmonic potential with a force constant of 1 kcal/mol. The phenolic compounds were initially placed at 78 Å away from the alumina surface (defined as the center-of-mass z position of oxygen atoms on the alumina surface), corresponding to the vacuum region of the last configuration of the aluminina/water/vacuum interface simulation. The biased reference z positions were selected at 1 Å intervals down to the point near the alumina surface. The simulations were performed for the four phenolic compounds described above with both hydroxylated and nonhydroxylated alumina surfaces. Each simulation at the biased z reference position lasted 2 ns, of which the last 1 ns was used for analysis. The free energy or PMF profiles29 were obtained with the weighted histogram analysis method (WHAM)32 and were compared with the distributions calculated from corresponding MD simulations performed without biased sampling.

Figure 2. Final snapshots, number density distributions, and orientations of catechol molecules near hydroxylated and nonhydroxylated alumina surfaces in anhydrous conditions. The side and top views indicate the snapshots viewed parallel and perpendicular to the surface plane, respectively. The catechol molecules were initially sparsely distributed across the interface. (a) The number density distributions of catechol molecules as a function of the z distance of the center of the catechol benzene ring near hydroxylated and nonhydroxylated alumina surfaces represented by solid and dashed lines, respectively. The z location of the alumina surfaces was defined as the average z position of the oxygen atoms on the surface in this and following figures. (b) The average of cos θdipole,Z, where θdipole,Z is the angle between the catechol bisector and the Z-axis, as a function of the z distance from the surface. The catechol bisector was defined by a vector from the center of the benzene ring to the midpoint of the two carbon atoms bonded to the hydroxyl groups, which is closely aligned with the catechol dipole. (c) The probability distributions of cos θdipole,Z for catechol molecules corresponding to the first peak near the surface. The statistically insignificant distributions beyond z = 6 Å were omitted. (d) The probability distributions of cos θdipole,Z for catechol molecules corresponding to the second nearest peak to the surface.

3. RESULTS AND DISCUSSIONS 3.1. Adsorption of Catechol in Anhydrous Conditions. To probe the intrinsic interaction between catechol molecules and two different alumina surfaces without competing interaction with water molecules, we analyzed distributions of catechol molecules in vacuum without water molecules near hydroxylated and nonhydroxylated alumina surfaces. Forty-five catechol molecules were initially placed at various points away from the alumina surface. Representative snapshots shown in Figure 2 indicate that most of the catechol molecules are concentrated near the surface for both types of surfaces. The density distribution of catechol molecules in Figure 2(a) displays two distinct peaks near the surface for both hydroxylated and nonhydroxylated surfaces even though the positions of the second peaks differ slightly. Figure 2(b) shows profiles of the average value of cos θdipole,Z, where θdipole,Z is the angle between the catechol bisector and the Z-axis, as a function of the z distance from the surface. The catechol bisector was defined by a vector from the center of the benzene ring to the midpoint of the two carbon atoms bonded to the hydroxyl groups, which closely aligned with the catechol dipole.

The average value of cos θdipole,Z in Figure 2(b) is close to 0 for the first peaks at z = 3.2 Å for both nonhydroxylated and hydroxylated alumina surfaces, indicating benzene rings of catechol molecules lying nearly parallel to the surface with van der Waals interactions. At the second peaks located at z = 4.7 and 5.2 Å, respectively, for hydroxylated and nonhydroxylated alumina surfaces, the average values of cos θdipole,Z are close to −1, which indicates that the catechol dipole was nominally oriented perpendicular to the alumina surface with catechol hydroxyl groups pointing to the surface in the absence of water. The distributions of cos θdipole,Z values at the first and second peaks shown in Figures 2(c) and (d), respectively, confirm the orientational preferences described above. However, slightly negative cos θdipole,Z values observed with the catechol molecules in the first peak near the hydroxylated surface shown in Figure 2(c) indicate that specific hydrogen bonding interactions exist between catechol and the hydroxylated D



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between water oxygen and the hydroxylated alumina surface shown in Figure 3(a) is replaced by an indistinct shoulder with nonhydroxylated surface in Figure 3(b). This indicates weaker hydrogen bonding interactions between water and the nonhydroxylated alumina surface. Figures 3(c) and (d) compare the number density and PMF distributions, respectively, of water oxygen atoms near hydroxylated and nonhydroxylated surfaces. With both hydroxylated and nonhydroxylated surfaces, the number density of water oxygen near the bulk-like region (z > 12 Å) is about 0.0333 Å −3 corresponding to the water mass density of 0.996 g/cm3, which is close to the mass density of bulk water. More pronounced density peaks and lower minimum PMF values were observed near the hydroxylated alumina surface. This indicates that the hydroxylated alumina surface is significantly more hydrophilic than the nonhydroxylated alumina surface. 3.3. Catechol/Alumina Adsorption in Aqueous Conditions. Next, we studied adsorption of catechol to the hydroxylated alumina surface under aqueous conditions. Figure 4 shows atomic number density distributions of oxygen atoms

alumina surface in addition to van der Waals interactions. In contrast, more symmetric cosine distributions around zero observed with catechol molecules near the nonhydroxylated alumina surface indicate weaker interactions of the hydroxyl groups in catechol molecules with the nonhydroxylated alumina surface. 3.2. Alumina/Water Interface. We investigated distributions of water molecules near the two different alumina surfaces without added organic molecules to estimate the intrinsic hydrophilicity of the alumina surfaces. Figure 3(a) shows the

Figure 3. Distributions of water molecules near hydroxylated and nonhydroxylated alumina surfaces. (a) Distributions of the number density of oxygen (a solid line with circles) and hydrogen (a dashed line) atoms of water molecules near the hydroxylated alumina surface and the hydrogen atoms of the hydroxyl groups on the alumina surface (a dot-dashed line). (b) Distributions of the number density of oxygen (a solid line with squares) and hydrogen (a dashed line) atoms of water molecules near the nonhydroxylated alumina surface. (c) Comparison of oxygen atoms of water molecules near hydroxylated and nonhydroxylated alumina surfaces shown as lines with circles and squares, respectively. (d) The potential of mean force (PMF) or free energy profiles of the water molecules near hydroxylated and nonhydroxylated alumina surfaces.

Figure 4. Distributions of oxygen atoms of catechol molecules near the hydroxylated alumina surface at different concentrations and numbers of alumina surfaces in contact with the solvated catechol. The distributions in (a), (b), and (c) were from simulations 5, 6, and 7 summarized in Table 1, respectively, where aqueous solutions of catechol were in contact with an alumina surface and vacuum as illustrated in Figure 1(a). The distributions in (d) and (e) were from simulations 8 and 9 summarized in Table 1, respectively, where aqueous solutions of catechol were in contact with two alumina surfaces as illustrated in Figure 1(b).

distributions of oxygen and hydrogen atoms of water molecules as well as hydrogen atoms of hydroxyl groups on the hydroxylated alumina surface. Similar distributions of oxygen and hydrogen atoms of water molecules were obtained previously by Argyris et al.38 using the CLAYFF force field with SPC/E water model.49 A well-defined water layer characterized by pronounced oxygen and hydrogen peaks was observed near the hydroxylated alumina surface. Figure 3(b) shows distributions of oxygen and hydrogen atoms of water molecules near the nonhydroxylated surface. Broader distributions of oxygen and hydrogen atoms of water molecules were observed near the nonhydroxylated alumina surface. The distinct peak in the water hydrogen density distribution

from the hydroxyl groups of catechol at several different concentrations as well as with different numbers of alumina surfaces in contact with catechol solutions. In the top three plots (Figures 4(a)−(c)), the aqueous catechol solutions have interfaces with an alumina slab on one side and vacuum on the other side as illustrated in Figure 1(a), corresponding to simulations 5, 6, and 7 in Table 1, respectively. Distinct peaks E



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using eq 8. The undetermined constant C in eq 8 was set to 0 in Figure 5(a) but was adjusted to have the minimum PMF values in Figure 5(b) coincide. Even though the density distributions at different concentrations of catechol were different as shown in Figure 4, the PMF profiles near the surfaces were quite similar as shown in Figure 5(b) when they were adjusted to have the minimum PMF energies coincide, indicating that the catechol−alumina adsorption behavior is consistent across the various concentrations studied here. We chose the lowest concentration of catechol (3:200 catechol:water ratio) between two alumina surfaces shown in Figure 4(e) as our main simulation system and used it in the subsequent analysis. We adopted the same setup to simulations involving other phenolic compounds and nonhydroxylated alumina surfaces described below. 3.5. Comparison of Catechol, Resorcinol, Hydroquinone, and Phenol. In addition to catechol, we studied adsorptions of several related phenolic compounds to alumina to understand the nature of adsorption between catechol and alumina. Figure 6(a) compares number density distributions of

of catechol oxygens near the alumina surface were observed in addition to broader peaks near the water/vacuum interfaces. In the bottom two plots (Figures 4(d) and (e)), the catechol/ water solutions are sandwiched between two alumina slabs as illustrated in Figure 1(b), corresponding to simulations 8 and 9 in Table 1, respectively. This type of sandwiched configuration with two alumina surfaces has the advantage that we can focus on catechol/alumina adsorption in the absence of water/ vacuum interfaces. The distributions of number density of atoms enclosed between two alumina slabs were calculated with respect to the average z position of the surface oxygen atoms of one of the two alumina slabs in Figures 4(d) and (e). As a result, the distribution of atoms near the other alumina slab farther away from the z reference point was broader. Similar distributions were obtained when calculated with respect to the alumina slab on the other side (data not shown). It is possible that accumulations of catechol molecules near the alumina surface are driven by the high initial concentration of catechol in the aqueous phase rather than intrinsic adsorption preferences. However, even at very low concentrations of catechol, there were significant populations of catechol near the alumina surface, which indicates that the preferential distributions of catechol near interfaces were not due to oversaturation but caused by specific adsorption interactions. Near the alumina surface, three distinct peaks (one at z = 2.6 Å and two minor peaks at z = 5.3 and 9.5 Å) were observed in all distributions. As was the case in the atomic density distributions of water at the alumina/water interface without the phenolic compounds shown in Figure 3, after approximately 12 Å away from the alumina surface, the distributions of the catechol molecules lacked structure and became bulk-like. Figure 5 compares the PMF or free energy profiles of potential obtained from catechol oxygen density distributions shown in Figure 4

Figure 6. Number density distributions of phenolic compounds near the hydroxylated alumina surface as a function of the z distance from the surface. (a) Distributions of all hydroxyl oxygen atoms of phenolic compounds. (b) Distributions of oxygen atoms of the hydroxyl group closer to the surface. (c) Distributions of the z center of the benzene ring in phenolic compounds. The solid line represents the results for catechol. Lines with circles, squares, and diamonds represent the results for resorcinol, hydroquinone, and phenol, respectively.

the hydroxyl oxygen of catechol, resorcinol, hydroquinone, and phenol enclosed between two hydroxylated alumina surfaces. In all cases, 45 phenolic compounds and 3000 water molecules were placed between the two hydroxylated surfaces of alumina as described earlier and summarized in Table 1 as simulations 10−12. The highest number density of oxygen near the alumina surface was observed with catechol. The number density of oxygen from resorcinol with two hydroxyl groups located in the

Figure 5. Free energy profiles of oxygen atoms of catechols near the hydroxylated alumina surface obtained from the oxygen density distributions in Figure 4. (a) Distributions without adjusting undetermined constants. (b) Distributions with adjustment of undetermined constants to have the same minimum free energy value. F



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meta-position on the benzene ring was smaller than for catechol but larger than for hydroquinone with its two hydroxyl groups in the para-position. Phenol with a single hydroxyl group has the smallest oxygen number density. The distributions with respect to the alumina surface on the other side are similar to those shown in Figure 6(a) (data not shown). The higher number densities of oxygen atoms in dihydroxybenzene compounds compared with phenol can be partly attributed to twice the number of oxygens present in the dihydroxybenzene compounds in addition to potential cooperative hydrogen bonding interactions of two hydroxyl groups instead of one. To resolve this issue, we calculated the number density distribution only for the hydroxyl oxygen closest to the surface (see Figure 6(b)). As expected, the oxygen number densities of the dihydroxybenzene compounds are reduced and have similar number densities to phenol in the bulk-like region (z > 12 Å). However, the relative rankings of phenolic compounds for the oxygen number density near the alumina surface remain the same as in Figure 6(a). Figure 6(c) compares the distributions of the center of the benzene ring. In contrast to a single major peak near the alumina surface in oxygen density distributions shown in Figures 6(a) and (b), two distinct peaks are observed near the alumina surface in the distributions of the benzene ring center, as shown in Figure 6(c). The small peaks near z = 3.3 Å correspond to benzene rings adsorbed parallel to the alumina surface as observed in catechol orientation in vacuum, shown in Figure 2. More pronounced peaks near z = 4.6 Å indicate benzene rings oriented perpendicular to the surface. The relative ranking of populations of phenolic compounds near the surface in oxygen density distributions is preserved in the distributions of the z center of the benzene ring. 3.6. Orientational Properties. To investigate the structural properties of the phenolic compounds near the alumina surface in more detail, we analyzed orientations of the bisector of the benzene ring with respect to the surface normal (Z-axis). The direction of the bisector was defined by a vector originating from the center of the benzene ring to the ring carbon atom bonded to the hydroxyl group closest to the surface. For catechol, another bisector, which closely aligned with the direction of its dipole moment, was used as in the analysis of catechol orientation near alumina surfaces in vacuum, shown in Figure 2(b), (c), and (d). Figure 7(a) compares distributions of cos θbisect,Z, where θbisect,Z is the angle between the ring bisector and the Z-axis, for phenolic compounds whose hydroxyl groups are near the hydroxylated alumina surface, corresponding to the first peak near the surface in Figure 6(b). The distributions in Figure 7(a) confirm two distinct orientations, one parallel to the surface characterized by the smaller peak near cos θbisect,Z = 0 and the other one pointing to the surface as indicated by the larger peak near cos θbisect,Z = −1. Figure 7(b) shows the coplanarity of the two hydroxyl groups in the dihydroxybenzene compounds by distributions of the difference of z positions of two hydroxyl groups near the surface. It is shown that the hydroxyl groups in catechol are located coplanarly near the surface, while hydroxyl groups of the other dihydroxybenzene compounds are separated in the z direction, which indicates cooperative hydrogen bonding of catechol with the alumina surface. 3.7. Hydrogen Bonding. To investigate the role of hydrogen bonding in adsorption of phenolic compounds to the alumina surface, we calculated the average number of phenolic compounds that made hydrogen bond contacts with

Figure 7. Orientation of the phenolic compounds near the surface. (a) The distributions of cos θbisect,Z, where θbisect,Z is the angle between the ring bisector (defined by a vector from the center of the benzene ring to the carbon atom bonded to the hydroxyl group closer to the surface) and the Z-axis, for phenolic compounds corresponding to the first peak near the surface in Figure 6(b). A distribution with another definition of a bisector closely aligning with the catechol dipole used in Figure 2 is shown with a dashed line. (b) Distributions of the difference of the z positions of hydroxyl oxygens of dihydroxybenzene compounds near the surface selected in (a). This estimates the coplanarity of the two hydroxyl groups. The zero Δz means that two hydroxyl groups are coplanar.

the alumina surface and their average lifetimes. A hydrogen bond contact was considered to be established when the nonbonded hydrogen−oxygen distance is smaller than 3 Å and the angle ∠O−H···O formed by the oxygen, hydrogen, and oxygen atoms from alumina and phenolic compounds is larger than 130°. A similar definition of hydrogen bond has been used in recent simulation studies.45,50 The majority of observed hydrogen bond contacts (68% on the average) was between the hydrogen atoms of phenolic compounds and the hydroxyl groups from the alumina surface. Table 2 summarizes our Table 2. Analysis of Hydrogen Bonding Interactionsa phenolic compound catechol resorcinol hydroquinone phenol

nHB (nm−2) 0.218 0.108 0.086 0.035

± ± ± ±

0.002 0.001 0.001 0.001

τHB (ps) 6.92 5.49 4.92 4.93

± ± ± ±

0.57 0.18 0.29 0.17

nHB and τHB signify average number of molecules hydrogen bonded to the hydroxylated alumina surface per unit area (nm2) and the average lifetime of each hydrogen-bonded molecule, respectively. The data were obtained from simulations 9−12 summarized in Table 1. a

results. Catechol had the most molecules making hydrogen bond contacts with the alumina surface per unit area, 0.218 nm−2, and the longest lifetime of the hydrogen-bonded interaction, 6.92 ± 0.57 ps. Resorcinol with the hydroxyl groups in meta-position had the next highest number and duration of hydrogen-bonded molecules. Hydroquinone with the hydroxyl groups in the para-position had a smaller number of molecules hydrogen bonded to the alumina surface than catechol or resorcinol but a larger number than phenol with a G



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single hydroxyl group. However, hydroquinone and phenol had very similar lifetimes of hydrogen-bonded interaction, indicating that little cooperative hydrogen bonding interactions with the alumina surface exist with hydroquinone, consistent with the distant hydroxyl groups in the para-position. Indeed, only 2.1 and 4.9% of hydrogen-bonded hydroquinone and resorcinol molecules, respectively, made simultaneous hydrogen bond contacts with the alumina surface with their two phenolic hydroxyl groups, which can be achieved only by the benzene rings oriented parallel to the surface for hydroquinone. In contrast, 24.6% of hydrogen-bonded catechol molecules made simultaneous hydrogen bond contacts with the alumina surface. It was observed that the lifetime of the hydrogen bond contact of catechol was also extended by switching of the hydroxyl groups making a hydrogen bond contact with the alumina surface. Catechol with neighboring hydroxyl groups can form cooperative hydrogen bonds with the hydroxyl groups on the alumina surface, resulting in more frequent and longer lasting hydrogen-bonded interactions. The case for the longer lifetime of hydrogen bond contact of catechol compared to other phenolic compounds was also made previously on the basis of experimental measurements of adhesion by DOPA-containing proteins to mica.8 3.8. Interactions with the Nonhydroxylated Alumina Surface. We also investigated the adsorption behavior of catechol and other phenolic compounds with a nonhydroxylated alumina surface in aqueous conditions. As illustrated in the snapshot in Figure 8 and the density distribution of the benzene ring center in Figure 8(a), with a nonhydroxylated alumina surface, catechol molecules are predominantly distributed near the surface instead of in the bulk aqueous phase. This is similar to the corresponding distribution in a vacuum shown in Figure 2(a) but is in contrast to the corresponding distribution with the hydroxylated surface in aqueous solution also compared in Figure 8(a), which shows a significant population in the bulk aqueous phase in addition to the distinct peaks near the surface. The order of relative magnitudes of the first and second major peaks near the hydroxylated alumina surface is reversed near the nonhydroxylated alumina surface. The first peak at z = 3.2 Å dominates near the nonhydroxylated alumina surface, while the second peak is more populated near the hydroxylated alumina surface. Figure 8(b), (c), and (d) shows orientations of the catechol dipole in aqueous solution as a function of the zdistance from the alumina surface as in Figure 2(b), (c), and (d) for catechol in vacuum. Figure 8(b) and (c) shows that, at the peak nearest the nonhydroxylated alumina surface, the average cos θdipole,Z value is close to zero with its distribution symmetrical with respect to zero, indicating that the benzene rings of the catechol molecules lie parallel to the alumina surface, as observed under anhydrous conditions, shown in Figure 2(c). However, the distribution of cos θ dipole,Z corresponding to the second shoulder peak near z = 4.4 Å away from the nonhydroxylated surface, shown in Figure 8(d), does not show a peak near −1 as observed with the hydroxylated surface or also in the corresponding distributions under anhydrous conditions, shown in Figure 2(d), but instead shows a larger population at the positive values of cos θdipole,Z, indicating an orientational preference to point away from the surface toward water molecules in the bulk phase, which provides stronger hydrogen-bonding potential for the hydroxyl groups in catechol molecules than the nonhydroxylated alumina surface. Similar orientational preferences near the nonhydroxy-

Figure 8. Representative snapshot and profiles of number density and orientations of catechol molecules near the nonhydroxylated surface in aqueous condition shown as dashed lines. The corresponding profiles with the hydroxylated surface are also shown as solid lines. Figure 2 shows similar plots in anhydrous conditions. (a) Distributions of the center of the benzene ring. (b) The average cosine of the angle between the ring bisector and the Z-axis as a function of z distance from the surface. (c) The cosine distributions of the catechol contributing to the first peak near the surface. (d) The cosine distributions for catechol molecules for the second peak.

lated surface were observed for other phenolic compounds used in this study (data not shown). Figures 9(a) and (b) compare the number density distributions of the benzene ring center in the phenolic compounds and the corresponding PMF distributions, respectively, near nonhydroxylated alumina surfaces. As observed with catechol in Figure 8, all phenolic compounds are strongly attracted to the nonhydroxylated surface, which may have acted as a more hydrophobic surface compared to the hydroxylated alumina surface, displacing polar water molecules and attracting the phenolic compounds mainly through hydrophobic van der Waals interactions. We calculated the number of phenolic compounds adsorbed to the surface defined by the z distance of 4.8 Å, which corresponds to the first minimum in the distribution of water molecules near the nonhydroxylated alumina surface shown in Figure 3(b). The areal densities of adsorbed catechol, resorcinol, hydroquinone, and phenol were 15.02, 13.42, 12.63, and 10.82 nm−2, respectively. The corresponding decreases of areal densities of adsorbed water molecules due to displacement by catechol, resorcinol, hydroquinone, and phenol were 70.37, 59.62, 55.96, and 50.94 nm−2, respectively, which were estimated by comparing distributions of water with and without phenolic H



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profiles in Figure 9(b) obtained with unbiased simulations with multiple phenolic compounds display shallower minima than those obtained with biased umbrella sampling of a single phenolic compound. These may be due to the increased competition to be near the surface among multiple phenolic compounds in unbiased simulations. Figure 10 compares the PMF profiles of phenolic compounds obtained with biased simulations obtained with

Figure 9. Number density distributions of phenolic compounds near the nonhydroxylated alumina surface as a function of the z distance of the center of the benzene ring from the surface. (a) Distributions of the center of benzene ring. (b) The free energy profiles of the phenolic compounds near the surface calculated with the density distributions in (a). (c) The free energy profiles as a function of the z center-ofmass position obtained with biased umbrella sampling.

compounds. The average number of adsorbed water molecules displaced by each catechol, resorcinol, hydroquinone, and phenol was estimated to be 4.69, 4.44, 4.43, and 4.71, respectively. This indicates that catechol molecules are most strongly attracted to the nonhydroxylated alumina surface as well as to the hydroxylated surface with stronger hydrogen bonding in addition to the hydrophobic interaction. The PMF distributions shown in Figure 9(b) also show that phenolic compounds are strongly attracted to the surface. However, it shows that phenols have the lowest PMF value despite having the smallest peak in density distributions in Figure 9(a). This discrepancy results from the uncertainty in the determination of the reference points of PMF distributions beyond the adsorbed layer due to the strong attraction of phenolic compounds to the surface at the low concentration. 3.9. PMF Calculations with Biased Simulations. To understand the intrinsic adsorption interactions between phenolic compounds and alumina in the low-concentration limit, we also obtained the PMF profiles with biased simulations of single phenolic molecules. Figure 9(c) shows the PMF profiles of phenolic compounds with the nonhydroxylated alumina surface obtained with biased simulations. The PMF profiles with biased simulations confirm strong attractions of the phenolic compounds to the nonhydroxylated surface, as observed in the PMF profiles obtained via unbiased simulations shown in Figure 9(b). Even though the exact comparison of the relative magnitudes of PMF minimum values of different phenolic compounds is difficult due to the statistical error, the PMF minimum for phenol is the highest for phenol in contrast to the results shown in Figure 9(b) but in agreement with the density distribution in Figure 9(a). In addition, the PMF

Figure 10. (a) Free energy profiles of phenolic compounds across the water/vacuum interface. (b) The free energy profiles obtained from simulations with biased umbrella samplings for phenolic compounds near the hydroxylated alumina surface as a function of the z center-ofmass position. (c) The free energy profiles of phenolic compounds near the hydroxylated alumina surface obtained with distributions of the z center-of-mass positions calculated from unbiased simulations.

the hydroxylated alumina surface. Figure 10(a) shows the PMF profiles of the phenolic compounds across the water/vacuum interface region. Solvation free energies of phenolic compounds can be estimated by the difference between free energy values in vacuum (z > 60 Å) and bulk aqueous regions (z ≈ 30 Å) in the PMF profiles.51 The estimated solvation free energies of catechol, resorcinol, hydroquinone, and phenol were −8.8, −11.5, −10.9, and −6.6 kcal/mol, respectively. Catechol is expected to be less available for solvation than resorcinol and hydroquinone because of the restricted space created by the adjacent hydroxyl groups. In agreement with expectation, the magnitude of the solvation free energy of catechol is smaller than resorcinol and hydroquinone but larger than phenol. In addition, the solvation free energy of phenol is in good agreement with the experimental value of −6.62 kcal/mol,52 confirming the validity of our interaction parameters between phenolic compounds and water molecules. Overall, the PMF profiles of phenolic compounds near the hydroxylated alumina surfaces in Figure 10(b) show small populations of molecules adsorbed parallel to the surface near z = 3.3 Å and larger populations of molecules oriented perpendicular to the surface at z between 4 and 5 Å, in qualitative agreement with the I



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density distributions obtained with unbiased MD simulations, shown in Figure 6(c). The free energy of phenol in the adsorbed state was higher than those of the dihydroxybenzene compounds, indicating the weakest adsorption of phenol with the hydroxylated alumina surface. The free energy barrier of catechol escaping from the second well was the highest, which is consistent with the stronger adsorption of catechol molecules, as observed in unbiased MD simulations. Figure 10(c) shows PMF distributions of the center of mass of phenolic compounds obtained with unbiased simulations with multiple (45) phenolic compounds. All the main features in the PMF profiles with biased simulations of a single phenolic compound in Figure 10(b) are present in Figure 10(c). The fluctuations along the PMF profile are reduced because of the improved statistical sampling due to the larger number of phenolic compounds. In addition, with a smaller number of phenolic compounds near the hydroxylated surface, a better match is expected between the PMF profiles of phenolic compounds obtained from biased and unbiased simulations near the hydroxylated surface than near the nonhydroxylated surface. Catechol displayed the lowest free energy in the adsorbed state compared with the PMF value in the solvated state near z = 7 Å in the PMF profile in Figure 10(c). Phenol displayed the highest free energy in the adsorbed state, indicating the weakest interaction with the hydroxylated surface. Hydroquinone and resorcinol displayed intermediate free energy values at the minima in the adsorbed state. These are consistent with the trends observed in the density distributions and the hydrogen bonding analysis.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +1 410 306 2811. Fax: +1 410 306 0676. E-mail: berend. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by an appointment to the Postgraduate Research Participation Program at the U.S. Army Research Laboratory (ARL) administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and ARL. The DoD HPC Modernization Office supported this project by supplying supercomputer time under the Computing Challenge Project C5M. We thank Drs. Joshua Orlicki and Daniel Knorr for helpful discussions and comments on our work.

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4. SUMMARY AND CONCLUSIONS We have performed atomistic molecular dynamics simulations to study adsorption behaviors of catechol, which is a key functional group in marine bioadhesives, and several analogous phenolic compounds near two different types of alumina surfaces under both anhydrous and aqueous conditions. In anhydrous conditions, without competing interactions from water molecules, catechol adsorbs to both hydroxylated and nonhydroxylated alumina surfaces with the benzene rings in the first layer oriented parallel to the surface and the hydroxyl groups in the second layer pointing to the surface. In aqueous conditions, near the nonhydroxylated alumina surface, which is more hydrophobic, the phenolic compounds displace water molecules and are strongly attracted to the surface, through van der Waals hydrophobic interaction between the benzene ring and the surface. Near the hydroxylated alumina surface, the catechol molecules display the strongest adsorptions mainly through cooperative hydrogen bonding interactions of the two neighboring hydroxyl groups with the surface hydroxyl groups of alumina as evidenced by the longer hydrogen bonding lifetime and the deeper minimum in the free energy profile compared with other phenolic compounds. These dual abilities of catechol to adhere to surfaces with different hydrophilicities through both hydrophilic and hydrophobic interactions may be critical to the unique properties L-DOPA to adhere to many different types of surfaces and can be utilized in the future development of bioadhesive polymer materials and antifouling coatings. We plan to extend our study to investigate adhesive properties of more realistic polymeric systems containing the catechol moiety with atomistic molecular dynamics simulations. J



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Molecular Dynamics Simulations of Adsorption of Catechol and

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