Langmuir 2002, 18, 1919-1927
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Atomistic Modeling of the Adsorption of Benzophenone onto Cellulosic Surfaces Karim Mazeau*,† and Caroll Vergelati‡ CERMAV-CNRS, Universite´ J. Fourier, BP 53, 38041 Grenoble Cedex 9, France, and Rhodia recherches, centre de recherches de Lyon, 85 Av des fre` res Perret, BP 62, 69192 St Fons Cedex, France Received May 29, 2001. In Final Form: November 12, 2001 The interaction between cellulosic material and benzophenone was studied by molecular modeling. A model of the crystalline part of a native microfibril was built from previously published coordinates of the Iβ allomorph. This model presents three faces, namely (200), (110), and (11h 0), of about the same surface area. The energetical and geometrical characteristics of the benzophenone adsorption onto this microfibril were studied with a Monte Carlo protocol. It was shown that the interaction does occur on the three faces and was stabilized by both van der Waals and electrostatic forces. On the hydrophobic (200) face, a large number of interacting sites without specific geometry were sampled by the adsorbing molecule. The hydrophilic surfaces, (110) and (11 h 0), also have many interaction sites, but in contrast, the orientation of the adsorbed molecules is more strict. These two hydrophilic surfaces display equivalent behavior. Three surfaces (crystalline (11 h 0) and (200) and amorphous) subjected to periodic boundary conditions were also generated to study the process of the benzophenone monolayer formation. The calculated data showed that locally the amorphous surface displayed very favorable topology for benzophenone adsorption in which both van der Waals and electrostatic interactions were maximized. After fulfillment of these optimal sites, the amorphous surface behaves like the crystalline surfaces for which the adsorption sites are nonspecific. Finally, the interface between cellulose/benzophenone monolayer and water was studied by molecular dynamics. The density profiles showed that the benzophenone molecules penetrated the amorphous phase while they remained at the surface in the crystalline models.
1. Introduction Cellulose is of immense importance to mankind. It is the most abundant organic renewable resource, widely distributed in the plant kingdom, and possesses multifunctional properties.1 It exists as microfibrils of indefinite length. Many of the properties of cellulose are correlated to molecular interactions occurring at the surface of the microfibrils: adsorption and adhesion. Such interactions play a key role in a diversity of problems stemming from industry, technology, and biology. For example, direct dyes are used for textile applications, for histochemical observations of plant cell walls, and as additives in the pulp and paper industry. In the biosphere, cellulose interacts with hemicellulose, xyloglucan, and pectin molecules in the plant cell walls; it is believed that these interactions contribute to the cohesiveness, strength, and expansion properties of walls.2 Enzymes that have catalytic functions such as cellulases adsorb onto cellulose through the CBD (cellulose binding domain),3 a molecular recognition process that constitutes the first step of cellulose degradation. Man-made materials such as nanocomposites reenforced by natural microfibrils show an enhancement of certain physical properties as compared to pure matrix.4 Such materials must exhibit good adhesion between fiber and matrix. The exact supramolecular architecture of cellulose in the native state as well as in any of the processed states * Corresponding author. E-mail:
[email protected]. † Universite ´ J. Fourier. ‡ Centre de recherches de Lyon. (1) Schurz, J. Prog. Polym. Sci. 1999, 24, 481. (2) Cosgrove, D. J. Plant Phys. Biochem. 2000, 38, 109. (3) Valjamae, P.; Sild, V.; Pettersson, G.; Johansson, G. Eur. J. Biochem. 1998, 253, 469. (4) Bledzki, A. K.; Gassan, J. Prog. Polym. Sci. 1999, 24, 221.
remains an open question.5 Experimental evidence has shown that cellulose is composed of amorphous and crystalline domains. Spectacular progress in understanding the crystalline forms of cellulose came from electron diffraction6,7 and solid-state NMR.8 In nonregenerated celluloses (cellulose I) the crystalline regions consists of two allomorphs: a triclinic IR and a monoclinic Iβ phase. The conformation of the individual chains is the same in these two phases; because of the particular conformational properties of the glycosidic linkage, the glucose units alternate up and down in the chain forming 21 helical structures having a pitch of around 10.34 Å. In the two phases the chains are arranged parallel to each other, and the main difference between the two lattices appears to be a longitudinal shift of the polymer chains along the chain axis. IR observations7 suggest that these two allomorphs also differ in their hydrogen-bonding pattern. Unfortunately little is known about the structures of the amorphous phase. Experimental methods that can be used to characterize the molecular details of the organization of the cellulose molecules at the surface of a microfibril and on their interactions with a guest are difficult to achieve. Highresolution images from atomic force microscope (AFM) are so far limited to acid-treated microcrystalline cellulose;9-12 this chemical treatment, by suppressing the (5) Bayer, E. A.; Chanzy, H.; Lamed, R.; Shoham, Y. Curr. Opin. Struct. Biol. 1998, 8, 548. (6) Sugiyama, J.; Vuong, R.; Chanzy, H. Macromolecules 1991, 24, 2461. (7) Sugiyama, J.; Vuong, R.; Chanzy, H. Macromolecules 1991, 24, 4168. (8) Attala, R. H.; VanderHart, D. L. Science 1984, 223, 283. (9) Kuutti, L.; Peltonen, J.; Pere, J.; Teleman, O. J. Microsc. 1995, 178, 1. (10) Baker, A. A.; Helbert, W.; Sugiyama, J.; Miles, M. J. Appl. Phys. 1998, A66, S559.
10.1021/la010792q CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002
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modeling techniques are capable of providing atomic-level structural details with reasonable accuracy. The goal of this work is to use an atomistic molecular modeling protocol to study the details of the adsorption of benzophenone on model surfaces of cellulose microfibrils that has been recently reported. 2. Methods
Figure 1. Schematic representation of the cellobiose repeat unit showing the atom numbering and the torsion angles of interest.
amorphous chains, allows the observation of surface chains that are organized. AFM measurements gave images with periodicities along the chain axis of about 10.7 and 5.3 Å corresponding to cellobiose and glucose units repeat distances.9-12 The chains of the surface then adopt a conformation close to the expected 21 helical conformation as in the bulk crystal. Moreover, an intermolecular spacing of around 6 Å10-12 has been observed and corresponds approximately to the chain separation in the monoclinic phase. However, the exact organization of the surface chains is still a matter of controversy. The surface chains are organized like in the monoclinic phase for Valonia macrophysa cellulose9 or like the triclinic phase for Valonia ventricosa.10-12 However, solid-state NMR13 has shown on sugar beet pulp that the purification process as well as any acidic treatment affects the ultrastructural organization of cellulose chains within the microfibrils. Structural information can also be gained by chemical microstructural analysis.14,15 The reactivity of the different O2, O3, and O6 hydroxyl groups (see Figure 1 for the atom numbering) toward various reagents is correlated with the degree of organization of the cellulose. In the case of perfect order of the surface chains, the O3′ hydroxyl groups are engaged in an hydrogen bond with the ring oxygen O5 belonging to the adjacent glucose unit. Therefore, this hydroxyl is not accessible and displays no reactivity. On the other hand, it is expected that the three hydroxyl groups react equally in amorphous structures as a consequence of equivalent accessibility. Comparative studies shows that for highly crystalline valonia or bacterial celluloses the O3 is almost not accessible in contrast with the measured accessibility of the O3 in cotton fibers for which the structural order is far less perfect.16 Adsorption phenomena onto this very complex cellulosic material could be studied by spectroscopic methods. Diffuse reflectance infrared (DRIFT) spectroscopy17 was used to investigate the adsorption of benzophenone. It is possible to distinguish different environments for the probe molecule, depending on the organization level of the adsorbant. In particular, a distinction is observed between amorphous and crystalline domains of the cellulosic substrate. This technique thus gives key structural information on the interactions between both molecules. As a complement to these experimental studies, molecular (11) Hanley, S. J.; Giasson, J.; Revol, J. F.; Gray, D. G. Polymer 1992, 33, 4639. (12) Baker, AA.; Helbert, W.; Sugiyama, J.; Miles, M. J. J. Struct. Biol. 1997, 119, 129. (13) Heux, L.; Dinand, E.; Vignon, M. R. Carbohydr. Polym. 1999, 40, 115. (14) Rowland, S. P.; Roberts, E. J. J. Polym. Sci. A-1 1972, 10, 2447. (15) Rowland, S. P.; Roberts, E. J. J. Polym. Sci. A-1 1972, 10, 867. (16) Verlhac, C.; Dedier, J.; Chanzy, H. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 1171. (17) Ilharco, L. M.; Garcia, A. R.; Lopes da Silva, J.; Vieria ferreira, L. F. Langmuir 1997, 13, 4126.
2.1. Computational Details. Force Field. All calculations have been performed with the modeling package Cerius2 and Discover molecular modeling programs.18 Unless otherwise noted, we used the default setup of version 1.6. In all methods the consistent valence force field cvff(91)19-26 parameter set has been applied. This force field employs terms for the bond lengths, the bond angles, and the torsional potentials for the bonded terms of the potential energy function. It employs a van der Waals potential and an electrostatic potential for the nonbonded terms. A Morse potential is used for the bond lengths terms, a quadratic potential is used for the bond angle terms, and a single cosine form is used for the torsional term. A Lennard-Jones function is used for the van der Waals term, and a Coulombic form is used for the electrostatic term. Nonbonded terms are considered between cellulose molecules and between cellulose and the benzophenone. The charge equilibration approach27 was used to evaluate point charges on every atom. Minimization and Dynamics. All the minimizations were performed by using the all-atoms model and the conjugate gradient procedure with the root-mean-square of the atomic derivatives of 0.05 kcal/(mol·Å) as convergence criterion and the nonbonded and dielectric potentials cutoff distance between constituting groups set at 11 Å. The molecular dynamics were carried out in the (N,V,T) ensemble by imposing the minimum image convention in order not to duplicate nonbonded calculations. The system is coupled to a bath (T ) 323 K) and is allowed to equilibrate under fixed volumic conditions. The equations of motion were solved using the Verlet algorithm, with a time step of 1 fs. The length of the MD simulation was 1 ns (the first 0.1 ns being reserved for the equilibration of the system). To maintain the average temperature at 323 K, the velocities of the particles were rescaled. Random velocities are assigned to the atoms, corresponding to a Boltzmann distribution at 323 K. Several such simulations are performed to assess the range of accessible minimum energy structures and their relative energies. 2.2. Model Structures of Cellulose. Our study is limited to the Iβ crystallographic phase as it is reported to be more stable than the IR one.28 The cell dimensions were based on the experimental literature values:6,7 a ) 8.01 Å; b ) 8.17 Å; c ) 10.36 Å; γ ) 97°. A schematic representation of cellobiose, the building block of cellulose, is given in Figure 1. The initial chain conformation was derived from coordinates obtained in a previous study,29 and the Φ (O5-C1-O1-C4′) and Ψ (C1-O1-C4′-C5′) glycosidic torsion angles were initially set at -97.5 and -154.3°, respectively. Hydroxyl hydrogen atoms were generated in the trans position, and all the hydroxymethyl groups were in the tg conformation (ω ) 180°). (18) Accelrys Inc. (19) Hagler, A. T.; Huler, E.; Lifson, S. J. Am. Chem. Soc. 1974, 96, 5319. (20) Hagler, A. T.; Lifson, S. J. Am. Chem. Soc. 1974, 96, 5327. (21) Lifson, S.; Hagler, A. T.; Dauber, P. J. Am. Chem. Soc. 1979, 101, 5111. (22) Hagler, A. T.; Lifson, S.; Dauber, P. J. Am. Chem. Soc. 1979, 101, 5122. (23) Hagler, A. T.; Dauber, P.; Lifson, S. J. Am. Chem. Soc. 1979, 101, 5131. (24) Kitson, D. H.; Hagler, A. T. Biochemistry 1988, 27, 5246. (25) Kitson, D. H.; Hagler, A. T. Biochemistry 1988, 27, 7176. (26) Dauber-Osguthorpe, P.; Roberts, V. A.; Osguthorpe, D. J.; Wolff, J.; Genest, M.; Hagler. A. T. Proteins: Struct., Funct., Genet. 1988, 4, 31. (27) Rappe´, A. K.; Goddard, W. A. J. Phys. Chem. 1991, 95, 3358. (28) Yamamoto, H.; Horii, F.; Odani, H. Macromolecules 1989, 22, 4130. (29) Vietor, R. J.; Mazeau, K.; Lakin, M.; Perez, S. Biopolymers 2000, 54, 342.
Modeling of the Adsorption of Benzophenone
Figure 2. Schematic representation of a cross section of the Iβ crystal structure (a). The three crystal surfaces are also indicated. Connolly (11 h 0) (b) and (200) (c) surfaces are shown. CH groups are colored in light gray, while hydrophilic groups OH are in darker gray. Crucial for the correct description of the cellulose surfaces together with the correct determination of adsorption process by force field methods is an initial check of the force field to be used. To investigate the ability of the cvff force field19 to describe native crystalline cellulose, we performed molecular dynamics simulations of our model of the crystal structure. These calculations were carried out using periodic boundary conditions. The computational box consists of 3 × 3 × 2 unit cells. Symmetry and crystallographic translations are not fixed within the computational box. Refining both molecular and cell geometry of the initial model, by minimizing the energy, did not induce major changes in the packing arrangement. The average cell parameters are a ) 8.08 Å, b ) 8.51 Å, c ) 10.34 Å, and γ ) 98.2°. For the largest difference, the root-mean-square deviation of experimental and calculated cell geometry amounts to 4.1%. This minor deviation fits well within the expected error margin of force field methods together with the experimental uncertainty in the measures of the cell dimensions. The degree of conservation of the 2-fold screw axis and the crystallographic translations is an indication of the compatibility of the simulated model with experimental diffraction data and therefore of the suitability of the force field. The resulting coordinates were used to generate three model surfaces of the crystal together with a model of a microfibril in which three surfaces are present. These surfaces correspond to the (200), (110), and (11h 0) planes of the crystal (see Figure 2a). These molecular surfaces were placed in a computational box as follows. Two layers of cellulose chains are deposited parallel to the ac plane of the box according to the organization found in the Iβ allomorph. To do so, coordinates of cellulose chains along with periodic images are generated and positioned in a crystalline supercell subjected to periodic boundary conditions in all three directions. The edge dimensions of the supercell is exactly the sum of all the elementary cells that were used. Dimensions a and c correspond exactly to this sum while dimension b, perpendicular to the atomic surface, is enlarged to get a free volume above the cellulose chains. The dimension b is large enough to avoid interactions between the probe molecule and the side of the cellulosic surface that we are not interested in. Infinite surfaces are then modeled by pseudo-2D periodic boundary conditions. The amorphous phase of the cellulose is also described by a cube exhibiting periodic boundary conditions, the volume of which has been exactly defined from the experi-
Langmuir, Vol. 18, No. 5, 2002 1921 mental density of the system30 (1.490 g/cm-3). To get a uniform occupancy of the polymer into the cell, the propagation procedure of each representative chain follows the scanning method of Meirovitch, with a lookahead of 3.31,32 The characteristics of the different computational boxes are given in Table 1. On the other hand, a model of a microfibril was built; it consists of 10 chains of 12 residues each. In this model, a central chain is surrounded by each of the (110), (11 h 0), and (200) surfaces consisting of four chains each. For this model periodic boundary conditions were not applied. 2.3. Conformational Sampling. Conformation of the Benzophenone. The conformational space of benzophenone was explored by rotating both phenyl groups on a 10° grid over the 180° range for both torsion angles. At each point of the grid, a geometry optimization is performed by allowing the Cartesian coordinates of each atom to vary except those defining the two torsion angles. The results are presented in Figure 3 as a Ramachandran-like contour plot in which isoenergy values are plotted, relative to the lowest energy structure, as a function of the two torsion angles. Finally, the exact position of the minima is located by additional unconstrained minimization. Interaction of a Benzophenone Molecule with the Microfibril Model. Monte Carlo techniques were used to generate 106 different molecular orientations of benzophenone with respect to the cellulose. In this procedure which includes constraints arising from excluded volume, the coordinates of the atoms of the cellulose were kept fixed. At first, the geometric centers of both molecules are positioned at the origin of the Cartesian coordinate frame. Then a particular orientation of the benzophenone is determined by randomly choosing three Euler angles. A vector that points from the origin to the surface of a unit sphere is randomly chosen; the benzophenone is translated along this vector until the van der Waals surfaces of each molecule just touch each other. Finally, the interaction energy of this specific configuration is calculated, minimized, and stored. This procedure is repeated 106 times. Interaction of a Benzophenone Monomolecular Layer with Cellulosic Model Surfaces. Adding one benzophenone molecule at a time generates the first shell of adsorption of benzophenone on cellulose. A benzophenone molecule is placed at random on the cellulosic surface within the computational box (initial starting configurations have proved to be unimportant). Then a combination of molecular dynamics and energy minimization is used to locate the likely binding positions. Periods of dynamics are followed by a period during which the structure is minimized. The configuration in which the interaction energy is the lowest is selected and kept. In this procedure, all alcohol groups of the cellulose are allowed to move whereas all the remaining atoms are constrained to their initial position. In a final step, the empty space of the computational box was fulfilled by TIP3P33 water molecules. This allows study of the interaction of a preformed monomolecular layer of benzophenone preadsorbed on cellulosic model surfaces with water molecules.
3. Results and Discussion 3.1. Cellulose Surfaces. In our study, cellulosic microfibrils are modeled by two different systems: One originating from our coordinates of the 1β crystal phase model is ordered and represents the crystalline zones of the microfibril. The other system is not ordered and was obtained from packed coil conformations of cellulose molecules; it models amorphous zones of the microfibril. The model of the Iβ crystal structure29 is used to create different ordered atomic surfaces that are subjected to periodic boundary conditions together with the microfibril model. The three surfaces under investigation are the (200), (11h 0), and the (110) planes of the Iβ allomorph. The (11 h 0) and (110) faces have been experimentally observed,10 (30) Krassig, H. A. Cellulose: Structure, Accessibility and Reactivity; Polymer monographs, v. 11; Gordon and Breach Science Pub: Yverdon, Switzerland, 1993; p 123. (31) Meirovitch, H. J. Chem. Phys. 1983, 79, 502. (32) Meirovitch, H. Macromolecules 1985, 18, 569. (33) Jorgensen, W. J. Am. Chem. Soc. 1981, 103, 335.
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Table 1. Characteristics of the Periodic Model Systems surface cell params (Å) a b c molecular details tot. of glucosyl units amt of adsorbed benzophenone within the first shell amt of water molecules
(11 h 0)
(200)
amorphous
35.5 37.0 38.3 2 layers 6 chains/layer 7 units/chain 84 18 906
35.0 33.6 38.3 2 layers 4 chains/layer 7 units/chain 56 16 933
37.4 37.4 43.4 10 chains 10 units/chain 100 17 1287
Table 2. Summary and Nonbonded Contributions of the Average Interaction Energies (kcal/mol) between the Benzophenone Molecules and the Different Crystalline Surfaces of the Microfibril
Figure 3. Potential energy surface of the benzophenone molecule. Contours are shown in a 1 kcal/mol interval above the global minimum. Torsion angles T1 and T2 visited by the benzophenones when interacting with the three surfaces have been superimposed.
and there is indirect evidence that the (200) face does occur. Indeed it is the likely candidate for the adsorption of cellulases. The most striking 3D feature of the cellulosebinding domain of cellulases is a wedge shape with one of the two faces being very flat.3 This flat face is composed of three aligned aromatic residues (tyrosines) approximately 10.4 Å apart. This spacing equals the cellulose repeat unit; it suggests that the flat face would bind the (200) face of cellulose through the tyrosine residues. For this reason, this surface is important and should be studied. The (200) surface represent the faces that run through the b directions of the native crystal. The cellulosic chains exhibit C-H groups at the surface. This surface is flat and hydrophobic. Both (11 h 0) and (110) surfaces represent the faces that run through the diagonal of the ab plane of the native crystal. Cellulose chains are tilted by about 45° with respect to the ac plane. Grooves are extending parallel to the c axis; they are created by free spaces between chains. Hydroxyl groups point outward, emphasizing the hydrophilic character of these surfaces. Molecular drawings of surfaces (11h 0) and (200) are presented in Figure 2. 3.2. Conformational Analysis of Benzophenone. Because of obvious symmetry within the molecule, the T1, T2 Ramachandran-like potential energy surface is
cellulosic surf
E(tot.)
E(van der Waals)
E(electrostatic)
(110) (11 h 0) (200)
-16.6 -16.8 -15.0
-8.0 (48.5%) -7.0 (41.6%) -8.9 (59.3%)
-8.6 (51.5%) -9.8 (58.4%) -6.1 (40.7%)
also symmetrical (Figure 3). Therefore, only one energy minimum is predicted for this molecule at T1, T2 coordinates of -22.8° and -22.8°. In principle, conjugation effects would stabilize the conformation at values of 0° for both torsion angles. However, due to steric constraints torsion angles values deviate from 0°. As a consequence, the benzophenone molecule is a quasi-planar aromatic molecule in which the two planar rings make an angle of around 40° in the lowest energy conformation. 3.3. Adsorption of Benzophenone on the Microfibril Model. We used two methods to evaluate the adsorption energy of benzophenone. The first method follows the Monte Carlo procedure performed on a fibrillar model of cellulose in which the three crystallographic faces are present. A large number of adsorption sites have then been generated. Table 2 show, for each face, average values of the interaction energy together with their electrostatic and van der Waals contributions. The interaction energies are of the same order of magnitude, indicating that apparently there is no preferred face. However, in 81% of the cases, the adsorption takes place on the hydrophobic (200) surface. On this particular cellulose surface, benzophenone molecules do interact by maximizing stacking interactions between aromatic rings of the benzophenones and the apolar CH groups of cellulose. Therefore, benzophenone molecules tends to be oriented parallel to the cellulose surface. On this face, a large number of adsorption sites could be seen and, for each site, adsorption takes place without a specific geometry. This interesting feature is illustrated on Figure 4 in which several orientations of the benzophenone on the same adsorption site have been superimposed. In this figure the oxygen atom of the carbonyl group of benzophenone is precisely located. It is in interaction with a surface hydroxyl group O3H of a glucose unit through a hydrogen bond as suggested by the average oxygen to oxygen distance of about 2.9 Å. The remaining part of the molecule is able to freely rotate to 360° without loss in the quality of the interaction. Despite the obvious electrostatic character of this interaction due to the creation of a hydrogen bond, the dominant component of the interaction is the van der Waals term. The interaction is logically hydrophobic. Hydroxyl groups of the cellulose chains are involved in interstrand hydrogen bonds to maintain cohesion within the crystal structure.
Modeling of the Adsorption of Benzophenone
Figure 4. Adsorption of benzophenone onto a model of a cellulose microfibril. Several adsorbate structures are given simultaneously on the crystalline (200) surface.
Figure 5. Adsorption of benzophenone onto a a model of a cellulose microfibril. Adsorption is shown on the crystalline (11 h 0) surface.
Despite minor structural differences between the two hydrophilic (110) and (11h 0) surfaces, the benzophenone adsorption process is the same for the two surfaces. The calculated data show that electrostatic interactions are of greater importance in these interactions. As a consequence of the topological characteristics of those two faces, adsorption sites are specific. The probe molecules tend to orient their carbonyl group toward cellulosic surface hydroxyl groups that are located at the bottom of the grooves. Consequently, geometrical freedom of the interaction is restrained as compared with the adsorption behavior of the hydrophobic surface. The carbonyl group of benzophenone is always hydrogen-bonded with a hydroxyl group of the surface of the cellulose. Figure 5 shows a typical example of such interactions. The carbonyl group of benzophenone is in interaction with an O3H group, while the remaining part of the benzophenone molecule is almost not interacting with the cellulose.
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Figure 6. Interaction energy as a function of the adsorbed amount of benzophenone on three selected surfaces.
3.4. Adsorption of the First Layer of Benzophenone on 2D-PBC Surfaces. The second method to evaluate the adsorption energy of benzophenone on cellulose uses a combination of molecular mechanics and molecular dynamics procedures for travelling on the potential energy surfaces of each crystal and amorphous face that is subjected to periodic boundary conditions. Because of the strong similarity of the results concerning the two crystalline hydrophilic surfaces, the (110) face was rejected from the subsequent calculations. Calculations were then carried out on the ordered hydrophilic (11 h 0), ordered hydrophobic (200), and amorphous surfaces. Adsorption of the first layer of benzophenone on each of the faces was studied by following an iterative process to mimic the experimental conditions in which the probe molecule is primarily dissolved in a solvent. Benzophenone molecule number i is adsorbed onto a cellulose surface on which i - 1 probe molecules are already adsorbed. To obtain a monomolecular layer, between 16 and 18 benzophenone molecules are adsorbed onto each surface; the average covering level is about 93% of the total cellulosic surface (with respect to the mean value of the accessible area of the benzophenone). Figure 6 shows, for each surface, the evolution of the interaction energy between the probe molecule and the cellulose surface as a function of the number of benzophenone molecules adsorbed. The adsorption process takes place by following two different patterns: it is monotonic for crystal surfaces (within a narrow energetic range of roughly 4 kcal/mol), whereas it is a two-step process for the amorphous surface. On the highly ordered crystal surfaces, adsorption sites are qualitatively identical and they are repeated periodically. Figures 7 and 8 shows molecular drawings of the adsorbed first layer of benzophenone on the cellulosic (200) and (11 h 0) surfaces, respectively. The orientation of the benzophenone is described by the angles θ1 and θ2. The angle θ1 is the angle between a vector perpendicular to the average plane of the benzophenone molecule and the b axis, normal to the reference plane; θ1 ) 0° corresponds to molecule lying parallel to the cellulose surface. The θ2 angle describes the orientation of the carbonyl vector with respect to the fiber axis (c axis); a parallel and antiparallel alignment of the molecule would correspond to values of 0 and 180°, respectively, while for θ2 values close to 90° the molecule is then orientated perpendicular to the fiber axis. For the (200) surface displayed in Figure 7, the average value of θ1 is 2° for 15 molecules out of 16; the remaining benzophenone molecule have a θ1 value of 50°.
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Figure 7. Molecular drawing of the adsorbed first layer of benzophenone on the (200) cellulosic face. The cellulose is displayed by its corresponding Connolly surface as in Figure 1.
Figure 8. Molecular drawing of the adsorbed first layer of benzophenone on the (11 h 0) cellulosic face. The cellulose is displayed by its corresponding Connolly surface as in Figure 1.
Most of the benzophenone molecules do adsorb parallel to the surface plane to maximize stacking interactions, and only one molecule is tilted with respect to the surface. On the contrary, the θ2 values are extremely varied; as already described, there is no strict orientation of the benzophenone with respect to the helicoidal axes of the cellulose molecules, but multiple orientations are available without any selectivity. For the (11 h 0) surface displayed in Figure 8, values of the θ1 angle can be grouped in three families having average values of 6, 35, and 70°, respectively. In the first (7 molecules) and the last family (4 molecules), the benzophenones are oriented flat and perpendicular to
Mazeau and Vergelati
Figure 9. Molecular drawing of the adsorbed first layer of benzophenone on the amorphous face. The cellulose is displayed by its corresponding Connolly surface.
the surface, respectively. For the remaining family, benzophenones are slightly tilted; they coat the cellulose molecules which are oriented of about 45° with respect to the ac plane of the computational box. The θ2 values are either grouped around 90° (12 molecules) or close to 0 or 180° (6 molecules). The hydrophilic surface is more selective with respect to both the interaction site and the relative orientation of the probe with respect to cellulose molecules. Benzophenone molecules have a strong tendency to be tilted with respect to the average cellulose plane. Furthermore orientation is either parallel (for the most favorable cases) or perpendicular to the helicoidal axes of the cellulose molecules. For the amorphous surface of cellulose, displayed in Figure 9, the first molecules do adsorb on the most favorable sites with strong interaction energy. Then, when all these preferred sites are occupied, benzophenones do adsorb on sites that are energetically comparable to the crystalline sites. Geometrical anisotropy of the surface therefore creates two different adsorption sites. The θ1 and θ2 angles are randomly distributed underlying the amorphous character of the surface. Benzophenone molecules orient either parallel or perpendicular to the surface of cellulose, depending on the local geometry of the surface. Therefore, we can conclude that, with the exception of the first few adsorbed benzophenone molecules, the enthalpy of adsorption is comparable for the three studied faces. Conformational variations of the benzophenone are observed. While interacting, this molecule does not stay in the minimal energy conformation that was established in the isolated state. On the contrary the relative orientation of both conjugated benzene rings is adjusted to optimize intermolecular favorable contacts. The T1, T2 torsion angles are reported as dots on the potential energy surface in Figure 3. Internal cohesion of the monolayer is an interesting feature to evaluate to describe the stability of the interface. It is defined as the difference between the energy of cellulose and the energy of the whole system. Its values as a function of the different surfaces are reported in Table 3. It can be seen that the internal cohesion of the benzophenone layer is of the same order of magnitude as the interaction energy between the layer of benzophenone
Modeling of the Adsorption of Benzophenone
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Table 3. Cohesion Characteristics (kcal/mol) of the Relaxed Benzophenone Monolayer Adsorbed onto Different Cellulosic Surfaces cellulosic surf amorphous (11 h 0) (200)
internal cohesion of the adsorbed benzophenone monolayer E(tot.) E(intra) E(inter) 1108.9 1082.4 1078.7
1263.9 1259.5 1258.4
-155.0 -177.1 -179.7
interactn surf/monolayer -229.9 -166.3 -169.4
and the ordered cellulosic surfaces. This result suggests a great stability of the interface. On the other hand, for the amorphous surface, the cohesion energy of the layer is lower than the interaction energy between the layer and the surface; this indicates an interface that is not stable. In this case, the system may evolve. It can be expected that probe molecules do diffuse within the amorphous phase. Such an event may be promoted by the large computed affinity of the surface for the first adsorbed molecules. This process is comparable to the dying process: the dye layer tends to diffuse in the amorphous phase of the cotton fiber. 3.5. Comparison with Literature Data. Benzophenone adsorption on two different cellulosic samples of crystallinities of 73% and 40% has been studied by diffuse reflectance infrared (DRIFT) spectroscopy.17 Through the observed modifications of the carbonyl-stretching band, it was possible to distinguish three different environments for the benzophenone: entrapped between chains in crystalline domains, in amorphous domains, and as crystallites adsorbed at the cellulose surface. Unfortunately, the situation in which benzophenone molecules are entrapped within the cellulose (crystalline or amorphous) has not been considered in the present modeling investigation. However, the results of the two approaches show significant analogy. First, the observed data show that benzophenone adsorption does occur on both crystalline and amorphous domains. All the interaction energies between the probe and the cellulose are calculated to be favorable. Moreover, it was observed that, up to a certain benzophenone concentration, there were no visible changes in the spectra. This behavior is explained by a constant distribution of the adsorption sites. In agreement with these observations, our models shows a periodicity of those sites in the crystalline domains. Finally, for the entrapped benzophenone molecules the observed data do reveal that, for low concentration of probe molecules, the crystalline regions are the first ones to trap the benzophenone when solvent evaporates. If the concentration is high enough, dissolved benzophenone diffuses and also deposits in the amorphous regions or at the polymer surface. In contrast to those observations, our calculations do indicate that on disordered surfaces there are locally very favorable sites; when all those sites are occupied, the adsorption energies of benzophenone are comparable. We believe that this apparently divergent behavior simply reflects experimental differences in the accessibility of the amorphous and crystalline zones. It should be pointed out that the experimental results of Ilharco et al. are essentially based on the behavior of the carbonyl stretching band; the present molecular modeling study underlines the role of the van der Waals contribution on the stabilizing effect of the adsorption. This hydrophobic contribution arises from the phenyl groups of the benzophenone and the numerous CH groups of the cellulose. These van der Waals interactions are not seen in the experiments. Furthermore, the experimental data are averaged over many intermolecular arrangements. Finally, the technique that was used to prepare
Figure 10. Relative density profiles as a function of Z, normal to the cellulose plane, for the amorphous model before (top) and after (bottom) the molecular dynamics process. Phenyl carbons (dashed lines) and glycosidic carbons (continuous lines) are shown.
the samples involves, in a first step, a swelling of the cellulose induced by ethanol solvation. As previously mentioned, this treatment might induce a larger accessibility of the crystalline domains of cellulose. The wildly accepted conceptual view of the architecture of native cellulose microfibrils is that amorphous domains are surrounding the crystal phases. The present molecular modeling is carried out under three basic assumptions. In the first, the idealized model surfaces correctly describe the real surfaces of the microfibrils. Then the adsorption does occur at the cellulose/ vacuum interface which is, of course, an oversimplification of the reality particularly when compared to the experimental FTIR study of Ilharco et al. which uses a solvent. Finally, the thermodynamics of the adsorption process is assumed to be mainly governed by its enthalpic component. 3.6. Interface with Water. To test the hypothesis of the diffusion of benzophenone molecules within the amorphous phase, complementary computations were carried out. The model systems are the cellulosic surfaces on which the first layer of benzophenone is adsorbed. Water molecules fill in the empty space above the benzophenone molecules. Then 1 ns of molecular dynamics is performed at 323 K (integration step 1 fs). In this simulation, the coordinates of each atom were allowed to vary and all the previously defined constraints were removed. We are aware that the simulation time is not enough to draw definitive conclusions about the evolution of the system with time. Normalized density profiles of
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Mazeau and Vergelati
Figure 11. Same as Figure 10 for the (11 h 0) ordered face.
Figure 12. Same as Figure 10 for the (200) ordered face.
either cellulose or benzophenone along an axis perpendicular to the surface of cellulose are calculated. Comparisons are made between the begining and the end of the simulation time. Here again, singularities are observed between the different surfaces. Figure 12 shows initial and final density profiles for the hydrophobic (200) surface. Both profiles are comparable. The profile behavior of the ordered face (11 h 0) is displayed in Figure 11. At the end of the simulation, all the three peaks are enlarged at their base. This is probably due to a randomization of the orientation of the cellulose molecules. Indeed, at first, all the molecules have the same conformation and the same relative orientation. The initial sharp peak of the benzophenone shows that these molecules orient parallel to the cellulose. There is however a slight disorder as suggested by the peak shoulder. The final peak is enlarged. This illustrates the many different relative orientations of the benzophenone molecules. The interesting feature is that some interpenetration is able to occur between cellulose and benzophenone. In this case, benzophenone molecules tend to fill the voids that are created by some amorphization of the surface cellulose chains. Figure 10 shows the same density profiles for the amorphous system. The absence of order within the amorphous cellulose phase can be appreciated in this figure. Whereas in the two preceding plots a bilayer of cellulose is evident, such organization could not be seen in the present graph. Because the cellulose surface is not rigorously flat, benzophenone molecules occupy the holes, at the beginning of the simulation. Therefore, there is an overlap between the two density profiles. The final overlap is very large; there is a real penetration of benzophenone molecules within the cellulosic phase. Simultaneously, the cellulose
phase is swollen as compared with the initial state. The cellulose initial width is 1.75 nm; the final one is 2.25 nm. These results show that when in contact with a poor solvent for benzophenone, these molecules try to penetrate inside the cellulose phase. The most favorable sites are those of the amorphous phase of cellulose. 4. Conclusion Although cellulose is widely used as a substrate for adsorbing a variety of chemicals, characterization of the interactions between cellulose and its partner is far from complete. In this work, an approach to this problem has been made using molecular modeling. Native cellulose is a semicrystalline material in which crystal phases coexist with amorphous zones. Molecular models of an idealized crystalline microfibril were first generated and used as substrates to distinguish if adsorption occurs preferentially at specific surfaces. It was shown that, from an energetical point of view, adsorption could take place on all the surfaces. However, geometrical details of the adsorption are surface-dependent. For the hydrophobic flat (200) face a great variability is observed while, for the hydrophilic (110) and (11 h 0) faces, the geometry of adsorption is constrained. Molecular models of amorphous and crystalline faces, subjected to periodic boundary continuation, were used to investigate the formation of a monolayer of benzophenone up until full coverage of the surface. The calculated data have highlighted privileged sites of adsorption on the amorphous surface. These sites were characterized by very favorable adsorption energies. When these sites were filled, the energetic contribution of the adsorption process is the same for all surfaces. Most of the conclusions of this
Modeling of the Adsorption of Benzophenone
study showed general agreement with available experimental data. Analysis of the component energies of the benzophenone layer on one hand and the global interaction energy between the cellulose surface and the benzophenone shell on the other hand suggests that the monolayer would prefer interacting with the cellulose rather than retaining its integrity. This is not observed for the crystalline surfaces of cellulose. The interface between the calculated models and water, which does not solvate cellulose, has been studied
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by molecular dynamics. It shows that, for the amorphous surface, an evolutive process tends toward the diffusion of the probe molecules into the cellulose phase. It is remarkable to obtain such good agreement between the IR observation and the molecular modeling results. To our knowledge, this is the first time that this kind of study has been conducted. It gives an atomistic view of a very important process involving cellulosic materials. LA010792Q
