Characterizing the Hydrophobicity of Surfaces Using the Dynamics of

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Characterizing the Hydrophobicity of Surfaces Using the Dynamics of Interfacial Water Molecules Selemon Bekele, and Mesfin Tsige J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01353 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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The Journal of Physical Chemistry

Characterizing the Hydrophobicity of Surfaces Using the Dynamics of Interfacial Water Molecules Selemon Bekele and Mesfin Tsige∗ Department of Polymer Science, The University of Akron, Akron, Ohio 44325

E-mail: [email protected] Phone: 330-972-5631 Abstract As most interfacial processes of practical interest occur in aqueous media where the presence of water may have an impact on desired functional properties, it is important to understand the structural and dynamical properties of interfacial water. Using molecular dynamics simulations, we investigated the properties of interfacial water molecules in contact with model atactic polystyrene surfaces of varying polarity. We find that interfacial water molecules which do not make hydrogen bonds with the substrate have a faster dynamics and appear to have a universal water-water hydrogen bond relaxation time of about 5 ps. The diffusion coefficients and the relaxation times of the water molecules involved in hydrogen bonding with the surface, on the other hand, have strong dependence on surface polarity and reveal a hydrophobic to hydrophilic transition regime with contact angle in the range 40 − 50o . The results presented will be of broad interest to researchers working in the area of surface science, biotechnology, nanotechnology, and in all forms of coating applications.

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Introduction In spite of its simple molecular structure, water is a complex liquid with many specific properties largely attributed to hydrogen bonding at the molecular level. 1,2 The formation and breaking of hydrogen bonds occurring in the subpico-second time scale in pure water is generally accepted to be the fundamental dynamical process responsible for water’s peculiar behavior. The interaction of water with surfaces plays a critical role in a large number of chemical, physical, biological and environmental processes. 3,4 Adsorption of proteins on a surface and its interaction are major concerns in a number of fields, such as biology, medicine, biotechnology, and food processing. 5 The adsorption behaviors of model proteins at various solid-liquid interfaces were investigated by Arai et al. 6,7 where they concluded that hydrophobicity/hydrophilicity, charge density variations, and electrostatic interactions play critical roles in protein adsorption on polystyrene latex surfaces. In particular, bio-medical applications involving liquid/bioploymer interfaces are extremely important since the human body is essentially a water-polymer interface system. As a result, polymeric implants into the human body (e.g. heart valves, hip and knee-cap replacements, contact lenses) find themselves in aqueous environments and their biocompatibility is highly influenced by interactions with water and biological molecules in the aqueous media. 8–10 Interfacial properties such as adhesion, wettability, lubrication, friction etc. largely determine the performance of polymeric materials such as the most commonly studied polystyrene (PS), poly(methyl methacrylate) (PMMA) and many others. 11–18 Wettability gradient surfaces were used to investigate the interactions of model proteins and cells with polymeric materials of varying hydrophilic/hydrophobic nature. 19 Curtis et al. 20 had suggested long ago that hydroxylation of a polystyrene surface makes it very adhesive for cells and a small increase in the surface oxygen concentration of polymers can lead to marked improvements in adhesion and in biocompatibility. A huge amount of computational effort has been directed over the past several decades 2


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or so towards understanding the structure of water near hydrophobic and hydrophilic surfaces. 21–26 In general, calculations of density profile relative to an average planar surface of amorphous materials result in smoothly varying sigmoidal density profiles, 27 a behavior largely attributed to the intrinsic roughness of the surface and the presence of thermal fluctuations of the surface called capillary waves. As we have shown in our recent work 28 on the properties of polymer/vacuum and polymer/solid interfaces, not taking into account the surface fluctuations will hide information on the nature of the density profiles around the interfacial region. 29–33 In this article, we present results from a molecular dynamics simulation study on the structure and dynamics of water molecules and hydrogen bonds relative to the instantaneous surface of atactic polystyrene (aPS) surfaces of varying polarity. To the best of our knowledge, the dynamics of interfacial water molecules has not been previously investigated relative to the instantaneous solid polymer surface to characterize hydrophobicity.

Computational Details The LAMMPS 34 simulation package was employed for all simulations. The atactic polystyrene (aPS) system consisted of 160 chains, each consisting of 40 repeat units (40-mers), was generated by randomly placing the chains in a box. The system was initially equilibrated at 600 K for 5 ns in an NPT ensemble followed by another 5 ns in an NVT ensemble at 600 K to readjust, using LAMMPS fix-deform, the dimensions of the box in order to match the equilibrated density of the system at the end of the initial NPT run. Periodic boundary conditions in all directions were used for this preparatory part. The last configuration of the NVT run was used to generate a film with interfaces in the xy-plane that was then cooled down to 300 K. The resulting film had a thickness of around 60 Å. Numerous studies have used spectroscopic methods such as X-ray photoelectron spectroscopy or contact angle measurements to characterize modified polymer surfaces. 35–37 Ex-

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perimentally, one technique used to chemically modify organic surfaces is UV irradiation which in the case of polystyrene introduces oxygen containing functional groups to the ortho and meta positions on the phenyl rings near the surface. We mimic the surface treatment of polystyrene by randomly replacing the hydrogen atoms at the ortho and metha positions of the phenyl rings by oxygen. The modified phenyl rings are those within 1 nm of the surface and are also randomly selected. The polarity of the polystyrene surface is varied by increasing the surface oxygen concentration. The oxidized polystyrene at each concentration was then equilibrated for 1 ns at 300 K. Other surface properties such as surface roughness and chemistry are kept constant allowing for investigation of changes in the structure and dynamics of the interfacial water molecules as a function of only one variable. To model the interactions within the polystyrene system, the optimized potentials for liquid simulation all-atom (OPLS-AA) 38 force field was used since it was shown by Tatek and Tsige 39 to provide structural and dynamics properties of aPS that compare well with experimental results. A water slab of dimensions 146 Å × 146 Å × 66 Å cut out from an already equilibrated bulk water system, was placed on top of the aPS with an initial 3 Å separation between them. The rigid extended simple point charge (SPC/E) 40 model was employed to describe the interaction between the water molecules. The SHAKE algorithm 41 was used to keep the OH bond distances and the HOH bond angles fixed. The Velocity-Verlet algorithm was used for integrating the equations of motion. An integration time step of 1 fs was used consistently for all cases. The cutoff radius for the Lennard-Jones term was set to 12 Å. The Coulomb interactions were calculated with a particle-particle/particle-mesh Ewald (PPPM) algorithm. After a 3 ns equilibration of the combined water/polystyrene system, data were recorded every 10 fs for an additional 30 ps for each oxygen concentration. The ITIM technique 32,33 was used to calculate the aPS surface in contact with water using a spherical probe with radius of 2.0 Å. This value is set based on a study of how the number of interfacial atoms varies with the size of the probe as shown in Figure S1a. The

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probe was started outside the region occupied by the polystyrene and was moved vertically 2

down along test lines centered at 1 × 1 Å (x,y) grids covering the entire XY surface of the box until it came within the van der Waals radius of the topmost carbon atom in each grid. The surface thus determined is taken to be the z−position of the carbon atom plus the z-component of the vector joining the centers of the carbon atom and the probe. The instantaneous polystyrene surface obtained is shown in Figure S1b.

Results and Discussion The instantaneous surface shown in Figure S1b is calculated for each frame or snapshot of the simulation. Particle number and hydrogen bond distributions are then computed relative to the instantaneous surface every 10 fs and averaged over 30 ps. 2

0.0 O / Å 2 0.03 O / Å 2 0.06 O / Å 2 0.09 O / Å 2 0.12 O / Å

bulk

2

ρ(z)/ρ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

0

-4

-2

0

2 4 Z (Å )

6

8

10

Figure 1: Normalized water density relative to the polystyrene instantaneous surface.

Water Density Profile Figure 1 shows the normalized density of water as a function of distance relative to the water/polystyrene instantaneous interface. The density profiles show a layering structure in the interfacial region independent of polarity. The use of the instantaneous surface has effectively removed the smearing effect of the capillary fluctuations and inherent surface roughness. The vertical dashed line indicates the position of maximum density which is not 5



changing with the polarity of the substrate. While the maximum in the density varies from 2.09 to 2.38 as the polarity increases, any changes in the width of the density profile (≈ 0.5Å) are smaller than the size of a water molecule. The implication is that there is hardly any depletion of water molecules from the interfacial region, consistent with observations made by Godawat et al. 42 However, as shown in Figure S2, the total number of interfacial water molecules clearly shows a strong dependence on the polarity of the surface. Our results indicate that the use of the instantaneous surface may be very important and calls for further investigation if water density near interfaces is also a good way of characterizing the hydrophobicity of surfaces as implied from Figure S2 and the magnitude of the peaks of Figure 1.

Hydrogen Bond Structure To investigate any structural and dynamical heterogeneity that may be present in the interfacial region, the water molecules within the first minimum ( z < 3.75Å ) in Figure 1 were considered as interfacial water molecules. Hydrogen bonds were then defined using a set of geometrical cuts based on a two dimensional map of angle θOHO (aPS oxygen - water hydrogen - water oxygen) versus distance rOH (aPS oxygen - water hydrogen). 27 The geometric cuts used in this analysis are rOH < 2.5Å, θOHO > 2.1 rad (120o ) and rOO < 3.2Å (the position of the first minimum in the oxygen-oxygen and oxygen-hydrogen radial distribution function). The water molecules were divided into two classes based on whether a given water molecule makes hydrogen bonding with substrate atoms. For each class of water molecules, we calculated the number of hydrogen bonds per water molecule weighted by the appropriate number of water molecules contributing in order to find out whether there is considerable enhancement in the ratio or not. The corresponding number distributions of water molecules and hydrogen bonds are given in Figures S3 and S4. Figure 2a shows the probability distribution for the average number of water-water hydrogen bonds per water molecule. It can 6


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be seen that there is a greater likelihood for a water molecule to participate in 3 hydrogen bonds. The probabilities for making 1, 2 and 4 hydrogen bonds are similar but smaller by almost 15%. The data also indicate that it is possible for water molecules to be involved in 5 hydrogen bonds with very low probability. Overall, only slight changes with polarity are observed.

HB

P(N /H2O)

0.4

2

0.0 O / Å 2 0.03 O / Å 2 0.06 O / Å 2 0.09 O / Å 2 0.12 O / Å

a

0.3 0.2 0.1

0

1

2

3 4 NHB/H2O

5

6

0 0

1

2

3 4 NHB/H2O

5

6

0.4

HB

P(N /H2O)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


b

0.3 0.2 0.1

Figure 2: Probability distribution of the number of hydrogen bonds per water molecule in the interfacial region (a) for the water molecules that have not made hydrogen bonding with the substrate, (b) for the water molecules that are involved in hydrogen bonding with the substrate. The largest error bar is 0.003 which is smaller than the symbol size.

For the water molecules that are involved in hydrogen bonding with the substrate, as Figure 2b depicts, the probability distribution for the number of hydrogen bonds per water molecule exhibits noticeable variation with changes in polarity. Relative to the data in Figure 2a, the probabilities for 1 and 2 hydrogen bonds per water molecule are substantially reduced whereas 3 and 4 hydrogen bonds per water molecule show an enhanced probability 7



of occurrence. The number of hydrogen bonds per water molecule as a function of position relative to the instantaneous surface are shown in Figure S5. There appears to be enhancement in the number of hydrogen bonds per water molecule on the lower end of the interfacial region with increasing polarity but, we emphasize that a substantially small number of water molecules contribute to the ratio.

Water Diffusion Since the water molecules are constantly in motion, the underlying molecular structure of water is quite dynamic exhibiting large fluctuations and reorganizations on time scales from femtoseconds to picoseconds. These motions allow water to play a key role in a number of chemical and biological processes that occur at the water/biomolecule interface. 43 Moreover, the dynamics of interfacial water molecules determine a variety of processes related to corrosion, the ability of small molecules to diffuse to and near interfaces, the performance of coatings, and many more. H2O-H2O

2

D( x 105cm2/s)

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H2O- H2O -Sub

1.8 1.6 1.4 1.2 1 0.8

0

0.02

0.04

0.06

NO / Å

0.08

0.1

0.12

2

Figure 3: Diffusion coefficient of interfacial water molecules as a function of hydrophilicity. The lines are linear fits to indicate the existence of a transition region. Figure 3 shows the diffusion coefficients, extracted from mean-squared displacement (MSD) curves shown in Figure S6, as a function of hydrophilicity for interfacial water molecules not in hydrogen bonding with the substrate (circles) and those which are involved in hydrogen bonding with the substrate (squares). The values obtained for interfacial

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water molecules not in hydrogen bonding with the substrate are well below the diffusion coefficient of 2.9 × 10−5 cm2 /s calculated for the water molecules within the bulk region of the water slab. The diffusion coefficient decreases with increasing oxygen concentration that saturates at high concentrations. The water molecules in hydrogen bonding with the substrate exhibit a similar diffusion behavior as a function of polarity except that they diffuse much slower than those which are not in hydrogen bonding with the substrate. In both cases, 2

the data exhibit inflection points around oxygen concentration values 0.06-0.08 oxygens/Å . The saturation behavior may be explained in terms of the total number of interfacial water molecules as a function of surface polarity. As shown in Figure S2 of the supporting information, it is evident that the total number of interfacial water molecules initially increases 2

with polarity but appears to saturate beyond an oxygen concentration of 0.06 O/Å . This 2

observation suggests that, for oxygen concentrations more than 0.06 O/Å , once a water molecule breaks a hydrogen bond and is free to move, it diffuses through an environment which is characterized by similar number of water molecules irrespective of the polarity of the surface. As a result, the diffusion constant does not change beyond the transition region. Using MD data on the wetting of the same oxidized polystyrene studied here, 27 this range of oxygen concentration values corresponds to the range of contact angles of 40-50o which may be considered to be a hydrophobic to hydrophilic transition regime. Recently, Zhang et al. 44 have measured contact angles of water droplets on substrates of varying surface energies and indicated the existence of a hydrophilic-hydrophobic boundary with a contact angle in the range 34-55o which is consistent with the results obtained in this work. This conclusion is also consistent with the contact angle value of ∼ 45o as a hydrophobic to hydrophilic transition point obtained by Godawat et al. 42 through calculations of excess chemical potentials in their simulation of interfacial properties of water on self assembled monolayer surfaces of varying polarity.

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Hydrogen Bond Dynamics The dynamic behavior of hydrogen bonds is studied in terms of the history independent (or intermittent) autocrrelation function which is given by CHI (t) =

hh(0)h(t)i , hh(0)i

where h(t) = 1 if

the pair of particles considered is bonded at time t and h(t) = 0 if they are not bonded at time t. Being insensitive to interim disruption of hydrogen bonds, the relaxation time associated with CHI (t) is called the structural relaxation time of the hydrogen bond network. 45,46 H2O-H2O

40

H2O-H2O -Sub

〈τ〉 (ps)

H2O- H2O-Sub

30 20 10 0

0

0.02

0.04

0.06

NO / Å

2

0.08

0.1

0.12

Figure 4: Relaxation times of hydrogen bonds as a function of hydrophilicity extracted from time autocorrelation functions (TACF). The lines are linear fits to indicate the existence of a transition region.

0 H2O-H2O

E [kcal/mol]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-5 H2O- H2O-Sub

-10 -15 -20

H2O- H2O-SubO

0

0.02

0.04

0.06 0.08 2 NO/Å

0.1

0.12

Figure 5: Interaction energy as a function of hydrophilicity of interfacial water molecules with substrate. The red squares and green triangles are for water molecules bonded to another water molecule and substrate oxygens. The red squares show the interaction energy with the whole substrate, while the green triangles show the interaction energy with the substrate oxygens only. The lines are linear fits to indicate the existence of a transition region.

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Figure S7 shows the intermittent correlation functions of hydrogen bonds near the water/PS interface. We note that the hydrogen bond dynamics near the water/PS interface is clearly heterogeneous. Hydrogen bonds in the interfacial region are generally found to display non-exponential relaxation characteristics. Figure 4 shows the dependence of the mean relaxation time hτ i on the polarity of the substrate. It is interesting to note that, for the hydrogen bonds between water molecules at the interface but not making hydrogen bonds with the substrate (HB1, circles), the relaxation is not affected by changes in polarity. Intrigued by this observation, we further investigated this case on other surfaces (self assembled monolayers (SAMs), PMMA, hydroxylated quartz and hydroxylated sapphire surfaces) and, overall, the data suggest a universal hydrogen bond relaxation time of about 5 ps for water molecules which are not in hydrogen bonding with the substrate. However, it is quite interesting that their diffusion (see Figure 3) is responsive to the changing environment as the oxygen density on the surface is varied. For the water molecules which are involved in hydrogen bonding with the substrate, we further differentiate between two types of hydrogen bonds a given water molecule makes. The first type is between a water molecule and a substrate atom (HB2, triangles) and the second is between water molecules at least one of which has also made a hydrogen bond with the substrate (HB3, squares). The results in Figure 4 clearly indicate that HB2 relax much more slowly than HB3 which in turn relax more slowly than HB1. In addition, there seems to be an inflection point in the data for the water-substrate hydrogen bonds in a similar range of concentrations as for the diffusion data. Straight line fits to this data indicate changes in the relaxation behavior occurring 2

around oxygen concentrations of 0.06-0.08 oxygens/Å which provides further support to the prediction that a transition from hydrophobic to hydrophilic behavior takes place at the corresponding contact angle values in the range 40-50o . One may naturally expect that the stability of hydrogen bonds is correlated with the hydrogen bond length and probably anticipate a decrease in length with increasing relaxation time. However, analysis of the distribution of distances between a water hydrogen and

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a substrate oxygen shows similar mean hydrogen bond lengths for all substrate polarities and types of water molecules considered. Then, the question arises as to what could be responsible for the monotonic increase in the relaxation time of the water-substrate hydrogen bonds and the slow dynamics of the associated water molecules with increase in surface oxidation. In order to understand these effects, we computed the energy of interaction of the interfacial water molecules with the substrate. Figure 5 shows the total interaction energy of interfacial water molecules with the substrate for the two classes of water molecules. The water molecules which have not made hydrogen bonds with the substrate (circles) have an energy of around 1-2 kCal/mol per water molecule which is comparable to kT and thus making the hydrogen bonds between water molecules susceptible to breaking under ambient conditions resulting in the small relaxation times observed. On the other hand, the water molecules involved in hydrogen bonding with the substrate (squares) have larger interaction energies of 5-9 kCal/mol per water molecule giving rise to bigger relaxation times. Despite the fact that the strength of the total potential field a water molecule feels shows only a slight increase as the polarity changes, the increase in relaxation time for these class of water molecules may be explained in terms of the number density of substrate oxygens around each water molecule which generates interaction energies between a water hydrogen and substrate oxygens (triangles) that grow substantially with increasing oxygen concentration. The lines shown in Figure 5 are linear fits to the green triangles which lend additional support to the existence of a transition region and it seems the hydrogen bonds to the substrate oxygens are the ones that play a key role here.

Conclusions In summary, interfacial water molecules at the polymer/water interface are characterized by structural and dynamical heterogeneity. The diffusion behaviors of water molecules in the two classes are markedly different from each other as are the relaxations of the water-water

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and water-substrate hydrogen bonds. The interfacial water-water hydrogen bonds have a universal mean relaxation time of 5 ps. The total interaction energy per water molecule offers clues to the different behaviors in hydrogen bond dynamics in the interfacial region. A close look at the energy systematics for this class of water molecules reveals that the total energy of interaction between a water hydrogen and substrate oxygens increases with the polarity of the surface suggesting that it may be the local charge distribution, i.e., the increasing densities of surface oxygen atoms, that plays a role in the stability of the water-substrate hydrogen bonds. The data presented suggest a hydrophobic to hydrophilic transition regime in the range of contact angles 40-50o consistent with experimental and simulation results in the literature.

Supporting Information Available Surface construction using the ITIM method, total number of interfacial water molecules as a function of surface polarity, water molecule distributions relative to the polystyrene instantaneous surface, hydrogen bond distributions in the interfacial region, number of hydrogen bonds per water molecule, mean-squared displacement, and time autocorrelation functions of hydrogen bonds.

This material is available free of charge via the Internet at

http://pubs.acs.org/.

Acknowledgement This work was supported by the National Science Foundation (DMR-1410290). The authors would like to thank A. Dhinojwala for helpful discussions.

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