Atomistic Modeling of the Adsorption of Benzophenone onto Cellulosic Surfaces

Atomistic Modeling of the Adsorption of Benzophenoneonto Cellulosic Surfaces

Karim Mazeau*,† and Caroll Vergelati‡

CERMAV-CNRS, Universite J. Fourier, BP 53, 38041 Grenoble Cedex 9, France, andRhodia recherches, centre de recherches de Lyon, 85 Av des freres 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. Amodel of the crystalline part of a native microfibril was built from previously published coordinates ofthe Iâ allomorph. This model presents three faces, namely (200), (110), and (11h0), of about the same surfacearea. The energetical and geometrical characteristics of the benzophenone adsorption onto this microfibrilwere studied with a Monte Carlo protocol. It was shown that the interaction does occur on the three facesand was stabilized by both van der Waals and electrostatic forces. On the hydrophobic (200) face, a largenumber of interacting sites without specific geometry were sampled by the adsorbing molecule. Thehydrophilic surfaces, (110) and (11h0), also have many interaction sites, but in contrast, the orientation ofthe adsorbed molecules is more strict. These two hydrophilic surfaces display equivalent behavior. Threesurfaces (crystalline (11h0) and (200) and amorphous) subjected to periodic boundary conditions were alsogenerated to study the process of the benzophenone monolayer formation. The calculated data showed thatlocally the amorphous surface displayed very favorable topology for benzophenone adsorption in whichboth 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 moleculardynamics. The density profiles showed that the benzophenone molecules penetrated the amorphous phasewhile they remained at the surface in the crystalline models.

1. Introduction

Cellulose is of immense importance to mankind. It isthe most abundant organic renewable resource, widelydistributed in the plant kingdom, and possesses multi-functional properties.1 It exists as microfibrils of indefinitelength. Many of the properties of cellulose are correlatedto molecular interactions occurring at the surface of themicrofibrils: adsorption and adhesion. Such interactionsplay a key role in a diversity of problems stemming fromindustry, technology, and biology. For example, direct dyesare used for textile applications, for histochemical ob-servations of plant cell walls, and as additives in the pulpand paper industry. In the biosphere, cellulose interactswith hemicellulose, xyloglucan, and pectin molecules inthe plant cell walls; it is believed that these interactionscontribute to the cohesiveness, strength, and expansionproperties of walls.2 Enzymes that have catalytic functionssuch as cellulases adsorb onto cellulose through the CBD(cellulose binding domain),3 a molecular recognitionprocess that constitutes the first step of cellulose degra-dation. Man-made materials such as nanocompositesreenforced by natural microfibrils show an enhancementof certain physical properties as compared to pure matrix.4Such materials must exhibit good adhesion between fiberand matrix.

The exact supramolecular architecture of cellulose inthe native state as well as in any of the processed states

remains an open question.5 Experimental evidence hasshown that cellulose is composed of amorphous andcrystalline domains. Spectacular progress in understand-ing the crystalline forms of cellulose came from electrondiffraction6,7 and solid-state NMR.8 In nonregeneratedcelluloses (cellulose I) the crystalline regions consists oftwo allomorphs: a triclinic IR and a monoclinic Iâ phase.The conformation of the individual chains is the same inthese two phases; because of the particular conformationalproperties of the glycosidic linkage, the glucose unitsalternate up and down in the chain forming 21 helicalstructures having a pitch of around 10.34 Å. In the twophases the chains are arranged parallel to each other,and the main difference between the two lattices appearsto be a longitudinal shift of the polymer chains along thechain axis. IR observations7 suggest that these twoallomorphs also differ in their hydrogen-bonding pattern.Unfortunately little is known about the structures of theamorphous phase.

Experimental methods that can be used to characterizethe molecular details of the organization of the cellulosemolecules at the surface of a microfibril and on theirinteractions with a guest are difficult to achieve. High-resolution images from atomic force microscope (AFM)are so far limited to acid-treated microcrystalline cellu-lose;9-12 this chemical treatment, by suppressing the

* 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.

(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.

1919Langmuir 2002, 18, 1919-1927

10.1021/la010792q CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 02/08/2002

amorphous chains, allows the observation of surface chainsthat are organized. AFM measurements gave images withperiodicities along the chain axis of about 10.7 and 5.3 Åcorresponding to cellobiose and glucose units repeatdistances.9-12 The chains of the surface then adopt aconformation close to the expected 21 helical conformationas in the bulk crystal. Moreover, an intermolecular spacingof around 6 Å10-12 has been observed and correspondsapproximately to the chain separation in the monoclinicphase. However, the exact organization of the surfacechains is still a matter of controversy. The surface chainsare organized like in the monoclinic phase for Valoniamacrophysa cellulose9 or like the triclinic phase for Valoniaventricosa.10-12 However, solid-state NMR13 has shownon sugar beet pulp that the purification process as wellas any acidic treatment affects the ultrastructural orga-nization of cellulose chains within the microfibrils.Structural information can also be gained by chemicalmicrostructural analysis.14,15 The reactivity of the differentO2, O3, and O6 hydroxyl groups (see Figure 1 for theatom numbering) toward various reagents is correlatedwith the degree of organization of the cellulose. In thecase of perfect order of the surface chains, the O3′ hydroxylgroups are engaged in an hydrogen bond with the ringoxygen O5 belonging to the adjacent glucose unit. There-fore, this hydroxyl is not accessible and displays noreactivity. On the other hand, it is expected that the threehydroxyl groups react equally in amorphous structuresas a consequence of equivalent accessibility. Comparativestudies shows that for highly crystalline valonia orbacterial celluloses the O3 is almost not accessible incontrast with the measured accessibility of the O3 in cottonfibers for which the structural order is far less perfect.16

Adsorption phenomena onto this very complex cellulosicmaterial could be studied by spectroscopic methods.Diffuse reflectance infrared (DRIFT) spectroscopy17 wasused to investigate the adsorption of benzophenone. It ispossible todistinguishdifferentenvironments for theprobemolecule, depending on the organization level of theadsorbant. In particular, a distinction is observed betweenamorphous and crystalline domains of the cellulosicsubstrate. This technique thus gives key structuralinformation on the interactions between both molecules.As a complement to these experimental studies, molecular

modeling techniques are capable of providing atomic-levelstructural details with reasonable accuracy. The goal ofthis work is to use an atomistic molecular modelingprotocol to study the details of the adsorption of ben-zophenone on model surfaces of cellulose microfibrils thathas been recently reported.

2. Methods

2.1. Computational Details. Force Field. All calculationshave been performed with the modeling package Cerius2 andDiscover molecular modeling programs.18 Unless otherwise noted,we used the default setup of version 1.6. In all methods theconsistent valence force field cvff(91)19-26 parameter set has beenapplied. This force field employs terms for the bond lengths, thebond angles, and the torsional potentials for the bonded termsof the potential energy function. It employs a van der Waalspotential and an electrostatic potential for the nonbonded terms.A Morse potential is used for the bond lengths terms, a quadraticpotential is used for the bond angle terms, and a single cosineform is used for the torsional term. A Lennard-Jones functionis used for the van der Waals term, and a Coulombic form is usedfor the electrostatic term. Nonbonded terms are consideredbetween cellulose molecules and between cellulose and thebenzophenone. The charge equilibration approach27 was used toevaluate point charges on every atom.

Minimization and Dynamics. All the minimizations wereperformed by using the all-atoms model and the conjugategradient procedure with the root-mean-square of the atomicderivatives of 0.05 kcal/(mol·Å) as convergence criterion and thenonbonded and dielectric potentials cutoff distance betweenconstituting groups set at 11 Å.

The molecular dynamics were carried out in the (N,V,T)ensemble by imposing the minimum image convention in ordernot to duplicate nonbonded calculations. The system is coupledto a bath (T ) 323 K) and is allowed to equilibrate under fixedvolumic conditions. The equations of motion were solved usingthe Verlet algorithm, with a time step of 1 fs. The length of theMD simulation was 1 ns (the first 0.1 ns being reserved for theequilibrationof thesystem).Tomaintain theaverage temperatureat 323 K, the velocities of the particles were rescaled. Randomvelocities are assigned to the atoms, corresponding to a Boltzmanndistribution at 323 K. Several such simulations are performedto assess the range of accessible minimum energy structuresand their relative energies.

2.2. Model Structures of Cellulose. Our study is limited tothe Iâ crystallographic phase as it is reported to be more stablethan the IR one.28 The cell dimensions were based on theexperimental literature values:6,7 a ) 8.01 Å; b ) 8.17 Å; c )10.36 Å; γ ) 97°. A schematic representation of cellobiose, thebuilding block of cellulose, is given in Figure 1. The initial chainconformation was derived from coordinates obtained in a previousstudy,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 thetrans position, and all the hydroxymethyl groups were in the tgconformation (ω ) 180°).

(11) Hanley, S. J.; Giasson, J.; Revol, J. F.; Gray, D. G. Polymer1992, 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.

(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.

Figure 1. Schematic representation of the cellobiose repeatunit showing the atom numbering and the torsion angles ofinterest.

1920 Langmuir, Vol. 18, No. 5, 2002 Mazeau and Vergelati

Crucial for the correct description of the cellulose surfacestogether with the correct determination of adsorption process byforce field methods is an initial check of the force field to be used.To investigate the ability of the cvff force field19 to describe nativecrystalline cellulose, we performed molecular dynamics simula-tions of our model of the crystal structure. These calculationswere carried out using periodic boundary conditions. Thecomputational box consists of 3 × 3 × 2 unit cells. Symmetry andcrystallographic translations are not fixed within the compu-tational box. Refining both molecular and cell geometry of theinitial model, by minimizing the energy, did not induce majorchanges in the packing arrangement. The average cell parametersare a ) 8.08 Å, b ) 8.51 Å, c ) 10.34 Å, and γ ) 98.2°. For thelargest difference, the root-mean-square deviation of experi-mental and calculated cell geometry amounts to 4.1%. This minordeviation fits well within the expected error margin of force fieldmethods together with the experimental uncertainty in themeasures of the cell dimensions. The degree of conservation ofthe 2-fold screw axis and the crystallographic translations is anindication of the compatibility of the simulated model withexperimental diffraction data and therefore of the suitability ofthe force field.

The resulting coordinates were used to generate three modelsurfaces of the crystal together with a model of a microfibril inwhich three surfaces are present. These surfaces correspond tothe (200), (110), and (11h0) planes of the crystal (see Figure 2a).These molecular surfaces were placed in a computational box asfollows. Two layers of cellulose chains are deposited parallel tothe ac plane of the box according to the organization found in theIâ allomorph. To do so, coordinates of cellulose chains along withperiodic images are generated and positioned in a crystallinesupercell subjected to periodic boundary conditions in all threedirections. The edge dimensions of the supercell is exactly thesum of all the elementary cells that were used. Dimensions aand c correspond exactly to this sum while dimension b,perpendicular to the atomic surface, is enlarged to get a freevolume above the cellulose chains. The dimension b is largeenough to avoid interactions between the probe molecule andthe side of the cellulosic surface that we are not interested in.Infinite surfaces are then modeled by pseudo-2D periodicboundary conditions. The amorphous phase of the cellulose isalso described by a cube exhibiting periodic boundary conditions,the volume of which has been exactly defined from the experi-

mental density of the system30 (1.490 g/cm-3). To get a uniformoccupancy of the polymer into the cell, the propagation procedureof each representative chain follows the scanning method ofMeirovitch, with a lookahead of 3.31,32 The characteristics of thedifferent computational boxes are given in Table 1.

On the other hand, a model of a microfibril was built; it consistsof 10 chains of 12 residues each. In this model, a central chainis surrounded by each of the (110), (11h0), and (200) surfacesconsisting of four chains each. For this model periodic boundaryconditions were not applied.

2.3. Conformational Sampling. Conformation of theBenzophenone. The conformational space of benzophenone wasexplored by rotating both phenyl groups on a 10° grid over the180° range for both torsion angles. At each point of the grid, ageometry optimization is performed by allowing the Cartesiancoordinates of each atom to vary except those defining the twotorsion angles. The results are presented in Figure 3 as aRamachandran-like contour plot in which isoenergy values areplotted, relative to the lowest energy structure, as a function ofthe two torsion angles. Finally, the exact position of the minimais located by additional unconstrained minimization.

Interaction of a Benzophenone Molecule with theMicrofibril Model. Monte Carlo techniques were used togenerate 106 different molecular orientations of benzophenonewith respect to the cellulose. In this procedure which includesconstraints arising from excluded volume, the coordinates of theatoms of the cellulose were kept fixed.

At first, the geometric centers of both molecules are positionedat the origin of the Cartesian coordinate frame. Then a particularorientation of the benzophenone is determined by randomlychoosing three Euler angles. A vector that points from the originto the surface of a unit sphere is randomly chosen; thebenzophenone is translated along this vector until the van derWaals 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 Layerwith Cellulosic Model Surfaces. Adding one benzophenonemolecule at a time generates the first shell of adsorption ofbenzophenone on cellulose. A benzophenone molecule is placedat random on the cellulosic surface within the computationalbox (initial starting configurations have proved to be unimpor-tant). Then a combination of molecular dynamics and energyminimization is used to locate the likely binding positions. Periodsof dynamics are followed by a period during which the structureis minimized. The configuration in which the interaction energyis the lowest is selected and kept. In this procedure, all alcoholgroups of the cellulose are allowed to move whereas all theremaining atoms are constrained to their initial position. In afinal step, the empty space of the computational box was fulfilledby TIP3P33 water molecules. This allows study of the interactionofapreformedmonomolecular layerofbenzophenonepreadsorbedon cellulosic model surfaces with water molecules.

3. Results and Discussion

3.1. Cellulose Surfaces. In our study, cellulosicmicrofibrils are modeled by two different systems: Oneoriginating from our coordinates of the 1â crystal phasemodel is ordered and represents the crystalline zones ofthe microfibril. The other system is not ordered and wasobtained from packed coil conformations of cellulosemolecules; it models amorphous zones of the microfibril.

The model of the Iâ crystal structure29 is used to createdifferent ordered atomic surfaces that are subjected toperiodic boundary conditions together with the microfibrilmodel. The three surfaces under investigation are the(200), (11h0), and the (110) planes of the Iâ allomorph. The(11h0) 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.

Figure 2. Schematic representation of a cross section of theIâ crystal structure (a). The three crystal surfaces are alsoindicated. Connolly (11h0) (b) and (200) (c) surfaces are shown.CH groups are colored in light gray, while hydrophilic groupsOH are in darker gray.

Modeling of the Adsorption of Benzophenone Langmuir, Vol. 18, No. 5, 2002 1921

and there is indirect evidence that the (200) face doesoccur. Indeed it is the likely candidate for the adsorptionof cellulases. The most striking 3D feature of the cellulose-binding domain of cellulases is a wedge shape with oneof the two faces being very flat.3 This flat face is composedof three aligned aromatic residues (tyrosines) approxi-mately 10.4 Å apart. This spacing equals the celluloserepeat unit; it suggests that the flat face would bind the(200) face of cellulose through the tyrosine residues. Forthis reason, this surface is important and should bestudied.

The (200) surface represent the faces that run throughthe b directions of the native crystal. The cellulosic chainsexhibit C-H groups at the surface. This surface is flatand hydrophobic. Both (11h0) and (110) surfaces representthe faces that run through the diagonal of the ab planeof the native crystal. Cellulose chains are tilted by about45° with respect to the ac plane. Grooves are extendingparallel to the c axis; they are created by free spacesbetween chains. Hydroxyl groups point outward, empha-sizing the hydrophilic character of these surfaces. Mo-lecular drawings of surfaces (11h0) and (200) are presentedin Figure 2.

3.2. Conformational Analysis of Benzophenone.Because of obvious symmetry within the molecule, theT1, T2 Ramachandran-like potential energy surface is

also symmetrical (Figure 3). Therefore, only one energyminimum is predicted for this molecule at T1, T2coordinates of -22.8° and -22.8°. In principle, conjugationeffects would stabilize the conformation at values of 0° forboth torsion angles. However, due to steric constraintstorsion angles values deviate from 0°. As a consequence,the benzophenone molecule is a quasi-planar aromaticmolecule in which the two planar rings make an angle ofaround 40° in the lowest energy conformation.

3.3. Adsorption of Benzophenone on the Mi-crofibril Model. We used two methods to evaluate theadsorption energy of benzophenone. The first methodfollows the Monte Carlo procedure performed on a fibrillarmodel of cellulose in which the three crystallographic facesare present. A large number of adsorption sites have thenbeen generated. Table 2 show, for each face, average valuesof the interaction energy together with their electrostaticand van der Waals contributions.

The interaction energies are of the same order ofmagnitude, indicating that apparently there is no pre-ferred face. However, in 81% of the cases, the adsorptiontakes place on the hydrophobic (200) surface. On thisparticular cellulose surface, benzophenone molecules dointeract by maximizing stacking interactions betweenaromatic rings of the benzophenones and the apolar CHgroups of cellulose. Therefore, benzophenone moleculestends to be oriented parallel to the cellulose surface. Onthis face, a large number of adsorption sites could be seenand, for each site, adsorption takes place without a specificgeometry. This interesting feature is illustrated on Figure4 in which several orientations of the benzophenone onthe same adsorption site have been superimposed. In thisfigure the oxygen atom of the carbonyl group of ben-zophenone is precisely located. It is in interaction with asurface hydroxyl group O3H of a glucose unit through ahydrogen bond as suggested by the average oxygen tooxygen distance of about 2.9 Å. The remaining part of themolecule is able to freely rotate to 360° without loss in thequality of the interaction. Despite the obvious electrostaticcharacter of this interaction due to the creation of ahydrogen bond, the dominant component of the interactionis the van der Waals term. The interaction is logicallyhydrophobic. Hydroxyl groups of the cellulose chains areinvolved in interstrand hydrogen bonds to maintaincohesion within the crystal structure.

Table 1. Characteristics of the Periodic Model Systems

surface

(11h0) (200) amorphous

cell params (Å)a 35.5 35.0 37.4b 37.0 33.6 37.4c 38.3 38.3 43.4

molecular details 2 layers 2 layers 10 chains6 chains/layer 4 chains/layer 10 units/chain7 units/chain 7 units/chain

tot. of glucosyl units 84 56 100amt of adsorbed benzophenone within the first shell 18 16 17amt of water molecules 906 933 1287

Figure 3. Potential energy surface of the benzophenonemolecule. Contours are shown in a 1 kcal/mol interval abovethe global minimum. Torsion angles T1 and T2 visited by thebenzophenones when interacting with the three surfaces havebeen superimposed.

Table 2. Summary and Nonbonded Contributions of theAverage Interaction Energies (kcal/mol) between the

Benzophenone Molecules and the Different CrystallineSurfaces of the Microfibril

cellulosic surf E(tot.) E(van der Waals) E(electrostatic)

(110) -16.6 -8.0 (48.5%) -8.6 (51.5%)(11h0) -16.8 -7.0 (41.6%) -9.8 (58.4%)(200) -15.0 -8.9 (59.3%) -6.1 (40.7%)


Despite minor structural differences between the twohydrophilic (110) and (11h0) surfaces, the benzophenoneadsorption process is the same for the two surfaces. Thecalculated data show that electrostatic interactions are ofgreater importance in these interactions. As a consequenceof the topological characteristics of those two faces,adsorption sites are specific. The probe molecules tend toorient their carbonyl group toward cellulosic surfacehydroxyl groups that are located at the bottom of thegrooves. Consequently, geometrical freedom of the inter-action is restrained as compared with the adsorptionbehavior of the hydrophobic surface. The carbonyl groupof benzophenone is always hydrogen-bonded with ahydroxyl group of the surface of the cellulose. Figure 5shows a typical example of such interactions. The carbonylgroup of benzophenone is in interaction with an O3Hgroup, while the remaining part of the benzophenonemolecule is almost not interacting with the cellulose.

3.4. Adsorption of the First Layer of Benzophe-none on 2D-PBC Surfaces. The second method toevaluate the adsorption energy of benzophenone oncellulose uses a combination of molecular mechanics andmolecular dynamics procedures for travelling on thepotential energy surfaces of each crystal and amorphousface that is subjected to periodic boundary conditions.Because of the strong similarity of the results concerningthe two crystalline hydrophilic surfaces, the (110) facewas rejected from the subsequent calculations. Calcula-tions were then carried out on the ordered hydrophilic(11h0), ordered hydrophobic (200), and amorphous surfaces.Adsorption of the first layer of benzophenone on each ofthe faces was studied by following an iterative process tomimic the experimental conditions in which the probemolecule is primarily dissolved in a solvent. Benzophenonemolecule number i is adsorbed onto a cellulose surface onwhich i - 1 probe molecules are already adsorbed. To obtaina monomolecular layer, between 16 and 18 benzophenonemolecules are adsorbed onto each surface; the averagecovering level is about 93% of the total cellulosic surface(with respect to the mean value of the accessible area ofthe benzophenone). Figure 6 shows, for each surface, theevolution of the interaction energy between the probemolecule and the cellulose surface as a function of thenumber of benzophenone molecules adsorbed. The ad-sorption process takes place by following two differentpatterns: it is monotonic for crystal surfaces (within anarrow energetic range of roughly 4 kcal/mol), whereasit is a two-step process for the amorphous surface. On thehighly ordered crystal surfaces, adsorption sites arequalitatively identical and they are repeated periodically.Figures 7 and 8 shows molecular drawings of the adsorbedfirst layer of benzophenone on the cellulosic (200) and(11h0) surfaces, respectively. The orientation of the ben-zophenone is described by the angles θ1 and θ2. The angleθ1 is the angle between a vector perpendicular to theaverage plane of the benzophenone molecule and the baxis, normal to the reference plane; θ1 ) 0° correspondsto molecule lying parallel to the cellulose surface. The θ2angle describes the orientation of the carbonyl vector withrespect to the fiber axis (c axis); a parallel and antiparallelalignment of the molecule would correspond to values of0 and 180°, respectively, while for θ2 values close to 90°the molecule is then orientated perpendicular to the fiberaxis. For the (200) surface displayed in Figure 7, theaverage value of θ1 is 2° for 15 molecules out of 16; theremaining benzophenone molecule have a θ1 value of 50°.

Figure 4. Adsorption of benzophenone onto a model of acellulose microfibril. Several adsorbate structures are givensimultaneously on the crystalline (200) surface.

Figure 5. Adsorption of benzophenone onto a a model of acellulose microfibril. Adsorption is shown on the crystalline(11h0) surface.

Figure 6. Interaction energy as a function of the adsorbedamount of benzophenone on three selected surfaces.


Most of the benzophenone molecules do adsorb parallel tothe surface plane to maximize stacking interactions, andonly one molecule is tilted with respect to the surface. Onthe contrary, the θ2 values are extremely varied; as alreadydescribed, there is no strict orientation of the benzophe-none with respect to the helicoidal axes of the cellulosemolecules, but multiple orientations are available withoutany selectivity. For the (11h0) surface displayed in Figure8, values of the θ1 angle can be grouped in three familieshaving average values of 6, 35, and 70°, respectively. Inthe first (7 molecules) and the last family (4 molecules),the benzophenones are oriented flat and perpendicular to

the surface, respectively. For the remaining family,benzophenones are slightly tilted; they coat the cellulosemolecules which are oriented of about 45° with respect tothe ac plane of the computational box. The θ2 values areeither grouped around 90° (12 molecules) or close to 0 or180° (6 molecules). The hydrophilic surface is moreselective with respect to both the interaction site and therelative orientation of the probe with respect to cellulosemolecules. Benzophenone molecules have a strong ten-dency to be tilted with respect to the average celluloseplane. Furthermore orientation is either parallel (for themost favorable cases) or perpendicular to the helicoidalaxes of the cellulose molecules.

For the amorphous surface of cellulose, displayed inFigure 9, the first molecules do adsorb on the mostfavorable sites with strong interaction energy. Then, whenall these preferred sites are occupied, benzophenones doadsorb on sites that are energetically comparable to thecrystalline sites. Geometrical anisotropy of the surfacetherefore creates two different adsorption sites. The θ1and θ2 angles are randomly distributed underlying theamorphous character of the surface. Benzophenone mol-ecules orient either parallel or perpendicular to the surfaceof cellulose, depending on the local geometry of the surface.Therefore, we can conclude that, with the exception of thefirst few adsorbed benzophenone molecules, the enthalpyof adsorption is comparable for the three studied faces.

Conformational variations of the benzophenone areobserved. While interacting, this molecule does not stayin the minimal energy conformation that was establishedin the isolated state. On the contrary the relativeorientation of both conjugated benzene rings is adjustedto optimize intermolecular favorable contacts. The T1, T2torsion angles are reported as dots on the potential energysurface in Figure 3.

Internal cohesion of the monolayer is an interestingfeature to evaluate to describe the stability of the interface.It is defined as the difference between the energy ofcellulose and the energy of the whole system. Its valuesas a function of the different surfaces are reported in Table3. It can be seen that the internal cohesion of thebenzophenone layer is of the same order of magnitude asthe interaction energy between the layer of benzophenone

Figure 7. Molecular drawing of the adsorbed first layer ofbenzophenone on the (200) cellulosic face. The cellulose isdisplayed by its corresponding Connolly surface as in Figure1.

Figure 8. Molecular drawing of the adsorbed first layer ofbenzophenone on the (11h0) cellulosic face. The cellulose isdisplayed by its corresponding Connolly surface as in Figure1.

Figure 9. Molecular drawing of the adsorbed first layer ofbenzophenone on the amorphous face. The cellulose is displayedby its corresponding Connolly surface.


and the ordered cellulosic surfaces. This result suggestsa great stability of the interface. On the other hand, forthe amorphous surface, the cohesion energy of the layeris lower than the interaction energy between the layerand the surface; this indicates an interface that is notstable. In this case, the system may evolve. It can beexpected that probe molecules do diffuse within theamorphous phase. Such an event may be promoted by thelarge computed affinity of the surface for the first adsorbedmolecules. This process is comparable to the dyingprocess: the dye layer tends to diffuse in the amorphousphase of the cotton fiber.

3.5. Comparison with Literature Data. Benzophe-none adsorption on two different cellulosic samples ofcrystallinities of 73% and 40% has been studied by diffusereflectance infrared (DRIFT) spectroscopy.17 Through theobserved modifications of the carbonyl-stretching band,it was possible to distinguish three different environmentsfor the benzophenone: entrapped between chains incrystalline domains, in amorphous domains, and ascrystallites adsorbed at the cellulose surface. Unfortu-nately, the situation in which benzophenone moleculesare entrapped within the cellulose (crystalline or amor-phous) has not been considered in the present modelinginvestigation. However, the results of the two approachesshow significant analogy. First, the observed data showthat benzophenone adsorption does occur on both crystal-line and amorphous domains. All the interaction energiesbetween the probe and the cellulose are calculated to befavorable. Moreover, it was observed that, up to a certainbenzophenoneconcentration, therewerenovisible changesin the spectra. This behavior is explained by a constantdistribution of the adsorption sites. In agreement withthese observations, our models shows a periodicity of thosesites in the crystalline domains.

Finally, for the entrapped benzophenone molecules theobserved data do reveal that, for low concentration of probemolecules, the crystalline regions are the first ones totrap the benzophenone when solvent evaporates. If theconcentration is high enough, dissolved benzophenonediffuses and also deposits in the amorphous regions or atthe polymer surface. In contrast to those observations,our calculations do indicate that on disordered surfacesthere are locally very favorable sites; when all those sitesare occupied, the adsorption energies of benzophenoneare comparable. We believe that this apparently divergentbehavior simply reflects experimental differences in theaccessibility of the amorphous and crystalline zones.

It should be pointed out that the experimental resultsof Ilharco et al. are essentially based on the behavior ofthe carbonyl stretching band; the present molecularmodeling study underlines the role of the van der Waalscontribution on the stabilizing effect of the adsorption.This hydrophobic contribution arises from the phenylgroups of the benzophenone and the numerous CH groupsof the cellulose. These van der Waals interactions are notseen in the experiments. Furthermore, the experimentaldata are averaged over many intermolecular arrange-ments. Finally, the technique that was used to prepare

the samples involves, in a first step, a swelling of thecellulose induced by ethanol solvation. As previouslymentioned, this treatment might induce a larger acces-sibility of the crystalline domains of cellulose. The wildlyaccepted conceptual view of the architecture of nativecellulose microfibrils is that amorphous domains aresurrounding the crystal phases.

The present molecular modeling is carried out underthree basic assumptions. In the first, the idealized modelsurfaces correctly describe the real surfaces of the mi-crofibrils. Then the adsorption does occur at the cellulose/vacuum interface which is, of course, an oversimplificationof the reality particularly when compared to the experi-mental FTIR study of Ilharco et al. which uses a solvent.Finally, the thermodynamics of the adsorption process isassumed to be mainly governed by its enthalpic compo-nent.

3.6. Interface with Water. To test the hypothesis ofthe diffusion of benzophenone molecules within theamorphous phase, complementary computations werecarried out. The model systems are the cellulosic surfaceson which the first layer of benzophenone is adsorbed.Water molecules fill in the empty space above thebenzophenone molecules. Then 1 ns of molecular dynamicsis performed at 323 K (integration step 1 fs). In thissimulation, the coordinates of each atom were allowed tovary and all the previously defined constraints wereremoved. We are aware that the simulation time is notenough to draw definitive conclusions about the evolutionof the system with time. Normalized density profiles of

Table 3. Cohesion Characteristics (kcal/mol) of theRelaxed Benzophenone Monolayer Adsorbed onto

Different Cellulosic Surfaces

internal cohesion of the adsorbedbenzophenone monolayercellulosic

surf E(tot.) E(intra) E(inter)interactn

surf/monolayer

amorphous 1108.9 1263.9 -155.0 -229.9(11h0) 1082.4 1259.5 -177.1 -166.3(200) 1078.7 1258.4 -179.7 -169.4

Figure 10. Relative density profiles as a function of Z, normalto the cellulose plane, for the amorphous model before (top)and after (bottom) the molecular dynamics process. Phenylcarbons (dashed lines) and glycosidic carbons (continuous lines)are shown.


either cellulose or benzophenone along an axis perpen-dicular to the surface of cellulose are calculated. Com-parisons are made between the begining and the end ofthe simulation time. Here again, singularities are observedbetween the different surfaces. Figure 12 shows initialand final density profiles for the hydrophobic (200) surface.Both profiles are comparable. The profile behavior of theordered face (11h0) is displayed in Figure 11. At the endof the simulation, all the three peaks are enlarged at theirbase. This is probably due to a randomization of theorientation of the cellulose molecules. Indeed, at first, allthe molecules have the same conformation and the samerelative orientation. The initial sharp peak of the ben-zophenone shows that these molecules orient parallel tothe cellulose. There is however a slight disorder assuggested by the peak shoulder. The final peak is enlarged.This illustrates the many different relative orientationsof the benzophenone molecules. The interesting featureis that some interpenetration is able to occur betweencellulose and benzophenone. In this case, benzophenonemolecules tend to fill the voids that are created by someamorphization of the surface cellulose chains. Figure 10shows the same density profiles for the amorphous system.The absence of order within the amorphous cellulose phasecan be appreciated in this figure. Whereas in the twopreceding plots a bilayer of cellulose is evident, suchorganization could not be seen in the present graph.Because the cellulose surface is not rigorously flat,benzophenone molecules occupy the holes, at the beginningof the simulation. Therefore, there is an overlap betweenthe two density profiles. The final overlap is very large;there is a real penetration of benzophenone moleculeswithin the cellulosic phase. Simultaneously, the cellulose

phase is swollen as compared with the initial state. Thecellulose initial width is 1.75 nm; the final one is 2.25 nm.These results show that when in contact with a poorsolvent for benzophenone, these molecules try to penetrateinside the cellulose phase. The most favorable sites arethose of the amorphous phase of cellulose.

4. Conclusion

Although cellulose is widely used as a substrate foradsorbing a variety of chemicals, characterization of theinteractions between cellulose and its partner is far fromcomplete. In this work, an approach to this problem hasbeen made using molecular modeling.

Native cellulose is a semicrystalline material in whichcrystal phases coexist with amorphous zones. Molecularmodels of an idealized crystalline microfibril were firstgenerated and used as substrates to distinguish ifadsorption occurs preferentially at specific surfaces. Itwas shown that, from an energetical point of view,adsorption could take place on all the surfaces. However,geometrical details of the adsorption are surface-depend-ent. For the hydrophobic flat (200) face a great variabilityis observed while, for the hydrophilic (110) and (11h0) faces,the geometry of adsorption is constrained. Molecularmodels of amorphous and crystalline faces, subjected toperiodic boundary continuation, were used to investigatethe formation of a monolayer of benzophenone up untilfull coverage of the surface. The calculated data havehighlighted privileged sites of adsorption on the amor-phous surface. These sites were characterized by veryfavorable adsorption energies. When these sites werefilled, the energetic contribution of the adsorption processis the same for all surfaces. Most of the conclusions of this

Figure 11. Same as Figure 10 for the (11h0) ordered face. Figure 12. Same as Figure 10 for the (200) ordered face.


study showed general agreement with available experi-mental data. Analysis of the component energies of thebenzophenone layer on one hand and the global interactionenergy between the cellulose surface and the benzophe-none shell on the other hand suggests that the monolayerwould prefer interacting with the cellulose rather thanretaining its integrity. This is not observed for thecrystalline surfaces of cellulose.

The interface between the calculated models andwater, which does not solvate cellulose, has been studied

by molecular dynamics. It shows that, for the amorphoussurface, an evolutive process tends toward the diffusionof the probe molecules into the cellulose phase.

It is remarkable to obtain such good agreement betweenthe IR observation and the molecular modeling results.To our knowledge, this is the first time that this kind ofstudy has been conducted. It gives an atomistic view ofa very important process involving cellulosic materials.

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Atomistic Modeling of the Adsorption of Benzophenone onto Cellulosic Surfaces