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Wetting the (110) and (100) Surfaces of Iβ Cellulose Studied by Molecular Dynamics Karim Mazeau* and Alain Rivet Centre de Recherches sur les Macromolécules Végétales (CERMAV-CNRS), ICMG FR 2607, BP 53, 38041 Grenoble Cedex 9, France, Affiliated with the Joseph Fourier University Received December 14, 2007 Revised Manuscript Received February 4, 2008 Accepted February 7, 2008
Introduction When observing the molecular details of the surfaces of cellulose microfibrils, one is struck by their heterogeneity, indicating that there are areas that are purely hydrophilic, whereas others have the hydrophobic character. Nature benefits from this ambivalence in many ways. For instance in plant cell walls, cellulose is able to interact not only with hydrophilic adducts such as hemicelluloses but also with the hydrophobic aromatic moieties of lignin.1 Other hydrophobic interactions occur when the aromatic part of a cellulase strongly binds to cellulose with the help of the so-called “cellulose-binding module”.2,3 In the case of the Iβ cellulosesthe most abundant allomorph in higher plantssthe external morphology of the crystalline microfibrils corresponds mostly to the (110) and (11j0) surfaces, which are highly decorated by protruding hydroxyls. As a consequence, cellulose possesses a dominant hydrophilic character, which makes it incompatible with most synthetic polymers that are hydrophobic.4 In addition to these dominant hydrophilic surfaces, the (100) surface is believed to be much less represented. Since this surface exposes exclusively hydrophobic C-H moieties to the surrounding medium, cellulose also possesses the additional property to capture nonpolar solvents such as chloroform5 or cyclohexane.6 It is also this (100) surface that is responsible for the cellulases initial adsorption and further attack.7 The measurement of the contact angle of water drops deposited on a surface is the traditional method to determine the hydrophilic/hydophobic character of this surface.8 The test consists in measuring at the contact point the angle between the tangent drawn at the drop surface and that of the tangent of the supporting surface. In addition to the significance of this angle, the surface energy may also be derived from these measurements. Given the importance of the wettability of cellulosics, contact angles and their associated surface energies have been extensively measured for native and regenerated cellulose.9–13 The contact angle is a macroscopic parameter; its value gives an average of the surface characteristic but does not account for the chemical heterogeneity of the surface. The hysteresis effect often observed in dynamical wetting experiments partly accounts for this heterogeneity, but this phenomenon is difficult to interpret as hysteresis depends on many other factors.14 In an attempt to better characterize the surface anisotropy of cellulose, Yamane and his collaborators have recently demonstrated that the wetting ability of regenerated cellulose toward water could be lowered to some extent when the samples had * Correspondence to Karim Mazeau: phone, +33(0)4 76 03 76 39; fax, +33(0)4 76 54 72 03; e-mail,
[email protected].
been dipped into liquids of low dielectric constant. According to these authors, the ensuing increase in contact angle was likely due to a rearrangement of the hydrophilic surfaces, which consequently became more hydrophobic. The molecular mechanism of such rearrangement, if it can be proven, involves the rotation of some of the cellulose surface chains (and probably some of those in the crystal core as well) around their molecular axis. With the goal of ultimately verifying this debatable hypothesis, we have undertaken a preliminary study devised to mimic the wetting of cellulose surfaces by water, using atomistic molecular dynamics simulations. The question of the theoretical wetting limit of cellulose is addressed by considering separately the surfaces (110) and the (100) of the Iβ allomorph.
Materials and Methods The model of the crystalline bulk of cellulose was taken from our previous study.15 This model was derived from duplicates of the experimental two-chain monoclinic unit cell of the Iβ allomorph,16 further equilibrated by a molecular dynamics run of 1 ns in the isothermal–isobaric NPT ensemble (constant number of particles, pressure, and temperature) at 1 atm and 300 K, and then optimized to reach its equilibrium geometry. The resulting superlattice possessed the (100) and (010) surfaces parallel to the faces of the periodic cube. The cell parameter normal to the (100) surface was then enlarged to 100 Å to provide a sufficient volume above the cellulose surfaces to insert the water drop. Another superlattice having the (110) and (110j) faces parallel to the faces of the periodic cube was also defined. The cell parameter normal to the (110) surface was enlarged similarly. The resulting models were then duplicated in the directions parallel to the surfaces. The final (110) surface contains two layers of 20 cellulose chains each having 20 residues each, its dimensions are 106 Å × 104 Å (chain axis), while the (100) surface contains four layers of 12 cellulose chains each having 20 residues, its dimensions are 98 Å × 104 Å (chain axis). Both models have a thickness of about 15 Å. The water nanodroplet considered in this study contains 460 water molecules. The simple point charge model (SPC) was used for the water molecules. The initial configuration of the water droplet is generated in two steps. First, a MD simulation with periodic boundary conditions is performed to obtain correct bulk structure. Then, an additional MD simulation without periodic boundary conditions is run to obtain an equilibrated isolated droplet. We used the Cerius2 molecular modeling package.17 Energies were calculated with the second generation all atom force-field PCFF18–22 that we have already used to model cellulose.23–25 The charge equilibration method was used to calculate charges for each atom.26 For the Coulombic term, the dielectric constant value was set to 8. The minimization uses the conjugate gradient procedure, with a convergence criterion stating that the root-mean-square of the derivative of energy change due to modification in the atomic positions, should be less than 0.05 kcal mol-1 Å-1. Molecular dynamic calculations were based on the canonical NVT ensemble (constant number of particles, volume, and temperature). The equations of motion were solved using the standard Verlet algorithm,27 with a time step of 1 fs. The system is coupled to a bath at T ) 300 K using Nose’s algorithm.28 All calculations were performed at the Centre d’Expérimentation et de Calcul Intensif, CECIC, Grenoble.
Results and Discussion A cluster of 460 water molecules equilibrated by molecular dynamics spontaneously takes the spherical shape of a droplet. Such droplet was placed on top of the two cellulose surfaces.
10.1021/bm7013872 CCC: $40.75 2008 American Chemical Society Published on Web 03/07/2008
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Figure 1. Wetting of the (110) surface (left) and the (100) surface (right) of the Iβ allomorph of cellulose. Snapshots are taken at 0 (initial configuration, top), 5, 10, and 15 ns (bottom).
The initial height of a droplet on a cellulose surface was determined in such a way that no severe overlaps between the droplet and cellulose were permitted. Molecular dynamics simulations were then performed at 300 K for the droplets while the model surfaces were kept fixed to save computational efforts. Instantaneous configurations selected at different simulation times are shown in Figure 1. The two drops spread at the surface during dynamics. The water molecules were attracted by cellulose and adsorbed at the surface. A spontaneous evaporation took place occasionally, but only few water molecules escaped and those, which did so, generally went back to the droplet. The instantaneous contact angle of irregular microscopic droplet was calculated from the average height of the mass center of the water droplet with respect to the planar surface (defined as the average height of the surface atoms).12 Figure 2 gives the evolutions with time of the predicted contact angles for the two cellulose surfaces. An obvious result was that the equilibrium regime was not reached even after a simulation time of 15 ns. Only the advancing dynamics of the water drop was observed and, unfortunately, not the thermodynamic equilibrium state. Equilibrium contact angles of the water droplets on the two surfaces could however be estimated by extrapolation (see
Supporting Information). Equilibrium contact angle of the water droplet on the (110) surface was estimated at 43°. This value is close to 46°, measured for bacterial cellulose.11 Because this surface is the most represented in the external morphology of the native fibers, its contribution is expected to be statistically dominant. Thus, cellulose has, on average, a highly hydrophilic character, which is characteristic of a surface of high surface energy. In contrast, the equilibrium contact angle is predicted at 95° for the (100) surface. As its wetting is lower than that of the (110) surface, the (100) surface therefore has the characteristic of a surface of moderate surface energy. Contact angles deduced from molecular modeling are difficult to compare with measured ones for the following reasons. On the one hand, the models of the surfaces used in this study are idealized: they are infinite (because of the periodic boundary conditions and because of the covalent bonds created across the periodic cell), perfect from a structural point of view (roughness is of the order of angstroms), and free from contamination. On the other hand, real cellulose surfaces are finite, rough, and quasi-systematically contaminated. This apparent drawback offers a theoretical opportunity to reveal to which extent the cellulose surfaces might differ from one another. There is a strong difference between the modeled
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long simulation times. This information is available free of charge via the Internet at http://pubs.acs.org.
References and Notes
Figure 2. Time evolution of the two contact angles of the (110) (squares in red) and the (100) (circles in blue) surfaces of the Iβ allomorph of cellulose.
contact angle, which is nanoscopic, and the measured one, which is macroscopic. The modeled water drop is necessary very small compared to a real one and line tension effects are not negligible at the nanometer scale.8 The contact angle reported by modeling is a theoretical or ideal angle; in addition the results are obviously dependent on both the force field and the simulation conditions. Even if in this work we did not calculate the surfaces energies, they can be estimated following the same approach by calculating contact angles of sessile drops of different polarities. The approach, which has been developed here, also opens the opportunity to characterize the different exposed surfaces of other cellulose allomorphs, either native or regenerated. The effect of chemical modifications at the cellulose surface could also be addressed as well. Acknowledgment. We thank H. Chanzy for valuable discussions. Supporting Information Available. An extrapolation procedure was performed to estimate the contact angles at very
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