Stability and structure of polyelectrolyte multilayers deposited from salt free solutionsBasel Abu-Sharkh Citation: The Journal of Chemical Physics 123, 114907 (2005); doi: 10.1063/1.2008254 View online: http://dx.doi.org/10.1063/1.2008254 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/123/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Dynamics and stability of dispersions of polyelectrolyte-filled multilayer microcapsules J. Chem. Phys. 126, 244901 (2007); 10.1063/1.2743432 Utilization of water/alcohol-soluble polyelectrolyte as an electron injection layer for fabrication of high-efficiencymultilayer saturated red-phosphorescence polymer light-emitting diodes by solution processing Appl. Phys. Lett. 89, 151115 (2006); 10.1063/1.2358942 Linear polyelectrolytes in tetravalent salt solutions J. Chem. Phys. 124, 044904 (2006); 10.1063/1.2155484 Elasticity of polyelectrolyte multilayer microcapsules J. Chem. Phys. 120, 3822 (2004); 10.1063/1.1644104 Structure of polyelectrolytes in 3:1 salt solutions J. Chem. Phys. 119, 12621 (2003); 10.1063/1.1625367

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Stability and structure of polyelectrolyte multilayers depositedfrom salt free solutions

Basel Abu-Sharkha�

Department of Chemical Engineering, King Fahd University of Petroleum and Minerals (KFUPM),Dhahran 31261, Saudi Arabia

�Received 6 June 2005; accepted 1 July 2005; published online 22 September 2005�

Molecular-dynamics �MD� simulation results show that polyelectrolyte multilayers deposited fromsalt free solutions on charged planar surfaces are thermodynamically stable structures that formspontaneously regardless of the method of deposition. The simulation also shows that thepolyelectrolyte multilayers are “fuzzy” in nature and molecules in one layer interpenetrate otherlayers. The influence of chain length, surface charge, and polymer charge is also investigated. Layerthickness was found to be independent of chain length. The ratio of surface to chain charge wasfound to influence the thickness of the first layer and the amount of polymer absorbed in the first fewlayers. The thickness of the subsequent layers was found to be independent of the charge ratio.© 2005 American Institute of Physics. �DOI: 10.1063/1.2008254�

I. INTRODUCTION

Thin layers of polyelectrolytes �PEs� may be depositedon a variety of surfaces by alternate exposure to solutions ofpolyanions and polycations.1–5 Over the past ten years, therehas been increasing interest in the application of these layersas well as the fundamental understanding of their nature. Thetechnique of polyelectrolyte multilayering has been appliedto numerous classes of materials including proteins, syn-thetic polyelectrolytes, clay minerals, dendrimers, metal col-loids, and other inorganic particles.6–13 Applications devel-oped using this technology include conducting layers,sensors, light-emitting films, selective membranes, area pat-terning, catalysis, corrosion protection, encapsulation, andgene therapy among many others.14–22

It is generally believed that the driving force for forma-tion of a polyelectrolyte multilayer is the electrostatic attrac-tion between the surface and the polyelectrolyte. Experi-ments have shown that charge overcompensation isnecessary for the formation of multilayers.23,24 Many factorsinfluence the structure of the multilayers including degree ofcharge of the polymer, ionic strength of solution, salt type,deposition duration, and polymer concentration.

Analytical models have been developed to describe theformation of polyelectrolyte multilayers on chargedsurfaces.25–27 Recently, very few Monte Carlo �MC� simula-tion studies were devoted to investigating multilayer forma-tion on spherical, planar, and cylindrical surfaces.24,28–30

Those studies focused on the effect of nonelectrostatic inter-actions on multilayer formation and the mechanism of chargeovercompensation.

A recent molecular-dynamics simulation study of the ki-netics of multilayer deposition of short charged chains on aspherical surface concluded that formation of multilayers is anonequilibrium process that appeared only in short simula-tion runs. When the multilayer system was equilibrated for a

long period of time, the multilayered structure was reducedto a bilayer.31 It was subsequently concluded that polyelec-trolyte multilayers are kinetically trapped, nonequilibriumstructures that, if given sufficient time, will collapse to abilayer structure. However, the nature of the curved surfaceused in this study raises questions concerning the generalityof conclusions. Recent simulation studies by Messina ofmultilayer formation on small-diameter nanotubes have indi-cated that multilayer formation is prohibited by the high cur-vature of the surface because of the high entropy penalty.28

The objective of this paper is to investigate the thermo-dynamic equilibrium structure of PE multilayers depositedfrom salt free solutions on flat surfaces. We also investigatethe molecular structure of the formed layers and the influ-ence of the surface charge and PE charge on assembly of themolecules and compare simulation with experimental results.To the best of our knowledge, such investigation has notbeen conducted in the past and will contribute to answeringsome of the questions related to the stability and structure ofPE multilayers. In addition, this paper provides the firstsimulation data of PE multilayer deposition on a structuredflat surface using realistic chains.

II. SIMULATION DETAILS

We conduct coarse-grained molecular-dynamics �MD�simulation of multilayer assembly from solutions of poly-electrolyte chains consisting of Np=16, 64, and 128 mono-mer beads. The absolute value of the charge on each chargedmonomer bead is equal to 1. A coarse-grained system wasused to reduce the overall number of particles in the systemand to subsequently reduce the number of computations pertime step. In addition, the overall dynamics of the system isaccelerated because the free-energy profile in the system isless bumpy.32–34 Charge densities of f =1, 0.75, and 0.5 wereused corresponding to a charge on each bead, an unchargedbead every three beads, and alternating charged and un-charged beads. Multilayers were deposited on a metal sur-a�Electronic mail: sharkh@kfupm.edu.sa

THE JOURNAL OF CHEMICAL PHYSICS 123, 114907 �2005�

0021-9606/2005/123�11�/114907/6/$22.50 © 2005 American Institute of Physics123, 114907-1

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face composed of 144 iron spherical particles that were con-strained in place. The dimensions of the surface are 31.27�31.27 Å2 and is located at Z=0. Total surface charges of−144, −72, and −36 were used in the simulation correspond-ing to a negative charge on each bead, a negative charge onevery other bead, and a negative charge in four beads. Thevarious surface charge densities were used to investigate theinfluence of surface charge on multilayer formation. In addi-tion, realistic surfaces usually do not dissociate beyond acertain charge density. In another variation, a surface com-posed of beads identical to the chain beads was also used.This surface was also composed of 144 beads and has dimen-

sions of 36.68�36.68 Å2. In general, changing the surfacecharacteristics was not found to change any of the conclu-sions of this study.

The polyelectrolyte chains are modeled as realisticunited atom zigzag bead spring chains composed of Np

monomers. Each bead has a mass of 12 amu. Increasing themass of each bead to 72 was not found to influence theequilibrium configuration of the system. The force field usedto model the chains is a simplified form of the polymer-consistent force field �PCFF� and is described by an equationof the following form:35,36

�1�

The force field employs a quartic polynomial for bondstretching �Eq. �1�� and angle bending �Eq. �1�� and a three-term Fourier expansion for torsions �Eq. �1��. Eq. �1� is theCoulombic interaction between the atomic charges and Eq.

�1� represents the van der Waals interactions. The force fieldparameters of the chain are given in Table I.35,36 Electrostaticinteractions between charged beads are calculated using theEwald sum method.37 A relative dielectric constant ��r� of 80

TABLE I. Force field parameters used in the simulation. The units are in kcal/mol �energy�, Å �distance�, anddegrees �angle�.

T 350 KMassC 12.011Fe 55.847Bond potential b0 K2 K3 K4

1.53 299.67 −501.77 679.81Angle potential �0 H2 H3 H4

112.67 39.516 −7.443 −9.5583Torsion potential V1 �1

0 V2 �20 V3 �3

0

0 0 0.0514 0 −0.143 0van der Waals interactions � �

Chain beads 4.0100 0.0540Surface beads 2.6595 13.889

114907-2 B. Abu-Sharkh J. Chem. Phys. 123, 114907 �2005�

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was used to account for screening of charges by an implicitsolvent �water�.35 van der Waals cross interaction parametersare calculated using32,33

�ij = 2��i� j

��i3� j

3���i

6 + � j6�

, �2�

�ij =��i

6 + � j6�1/6

2. �3�

Nonbonded van der Waals interactions were calculatedwith a cutoff distance of 2.5�, where � is van der Waaldiameter of a chain bead. Standard long-range correctionswere applied.39 Simulations are carried out in the NVT en-semble with periodic boundary conditions. A constant tem-perature was accomplished by linking the system to athermostat.40 A simulation time step of 3 fs was used.

Simulations were performed following a modified formof the method of Panchagnula et al.31 The charged iron sur-face was constructed and counterions were dispersedthroughout the simulation box. A neutral soft repulsive wallwas placed at the top of the simulation box to avoid theescape of counterions and chains to the lower side of thecharged surface. The upper wall is identical to the lowersurface with the exception that it interacts with other par-ticles by a force field that corresponds to the repulsive termof the van der Waals potential described in Eq. �1�. An 80-Å-thick layer of vacuum was placed on top of the neutralwall to eliminate periodic electrostatic interactions betweenthe charged wall and particles inside the simulation box. Anumber of positively charged polyelectrolyte molecules werethen inserted in the box along with their counterions. Theconcentration of chains was kept at 0.03�−3. The simulationbox was subsequently equilibrated for 20 ns during whichequilibration was confirmed by monitoring the total energy,radial distribution functions, and concentration profiles of thevarious species in the system. Subsequently, counterionstrapped within the layer were removed. These counterionswere removed to simulate realistic PE multilayers depositedfrom salt free solutions.41 Various experimental studies haveshown that surface and PE counterions are displaced by ad-sorbing polymer segments and that PE multilayers containno counterions.41 The system was subsequently equilibratedfor 20 ns. Unabsorbed polyelectrolyte molecules were thenremoved along with their counterions, representing a rinsingstep. An equal amount of the oppositely charged polyelectro-lyte was subsequently added to the box along with oppositelycharged counterions. The system was again equilibrated for20 ns. The trapped counterions were subsequently removedfrom the multilayer followed by further equilibration for 20ns. Subsequently, the unabsorbed polyelectrolyte was thenremoved along with its counterions and the system was fur-ther equilibrated for 20 ns. Overall charge neutrality wasalways maintained in the system. The two depositions repre-sent a complete dipping cycle. This process was repeated forsix dipping cycles. After completing the depositions, the sys-tem was annealed further for a total of 50 ns. Equilibrationwas confirmed again by monitoring the energy and concen-tration profiles in the system.

III. STRUCTURE OF THE MULTILAYERS

Figure 1 shows the structure of the multilayered systemcontaining chains composed of 128 beads after annealing.The oppositely charged polyelectrolytes are shaded in whiteand gray. The multilayers can be clearly observed. In the firsttwo layers, the molecules are parallel to the charged surface.In addition, the first two layers are thinner and more orderedthan subsequent layers. Figure 2 shows the concentrationprofiles of the cationic and anionic layers in the Z direction�surface is located at Z=0�. Concentration profiles of poly-electrolyte beads as a function of distance from the plate �Z�are calculated by taking volumetric slices of 0.2-Å thicknessin the Z dimension. A histogram of Z positions was thenconstructed as a trajectory averaged quantity. Number densi-

FIG. 1. Configuration of the multilayer system with Np=128.

FIG. 2. Concentration profiles of the cationic and anionic PEs; Np=128 andsurface charge=−72.

114907-3 Stability and structure of polyelectrolyte multilayers J. Chem. Phys. 123, 114907 �2005�

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ties are therefore in units of beads/Å3. Fairly sharp peaks areobserved especially in the first three layers. This result coin-cides with experimental observations which show that thefirst few layers are thinner and have a different structurefrom subsequent layers.5 The peaks corresponding to the toplayers are wider and indicate a lower level of order. Theaverage thickness of a layer is 4.5 Å corresponding to amonomolecular layer. This result corresponds with experi-mental observations which indicate that the first few layersdeposited from salt free solutions are monomolecular with athickness of around 3 Å.42 The difference in layer thicknessbetween the experimentally observed and the simulation val-ues is a result of the large van der Waals diameter of thesimulation beads. The concentration profiles indicate thatthere is a substantial level of interpenetration between thetwo oppositely charged polyelectrolytes especially beyondthe fourth layer. This concentration profile corresponds to the“fuzzy layer” structure suggested by Decher43 and confirmedby experimental measurements.44–46 Figure 3 shows asample conformation of a single chain from the first layer.The train, loop, and tail conformations observed in Fig. 3correspond with experimentally observed conformations.47

Figure 4 shows a comparison between the concentration pro-files of the anionic PE observed for short �Np=16� and longchains �Np=128� in the Z direction. The peaks correspondingto the long chains are sharper, indicating a higher level oforder and less penetration of the cationic layers. The higherlevel of order observed for the long chains is a result of theirlower entropy caused by their higher connectivity. In addi-tion, short chains contain more free end segments, hence

more tail segments which participate in the charge overcom-pensation mechanism. However, layer thickness was not in-fluenced by molecular weight. Again, this result correspondswith experimental findings which indicate that the thicknessof thin layers is independent of the molecular weight of thechain.48

IV. THERMODYNAMIC STABILITY

In order to investigate thermodynamic stability of themultilayers, we study their spontaneous deposition from awell-mixed polyelectrolyte solution. A system was preparedby mixing an equal number of anionic PE and cationic PEchains with Np=64 and f =1 along with counterions at aconcentration of 0.03�−3.49 The mixture was placed on top ofthe iron surface that is completely charged. Subsequently,three simulations were conducted. The first simulation wasconducted on a charged surface with a charge of 1 per bead.The procedure of the first simulation is as follows: The simu-lation box was equilibrated for 20 ns. Subsequently, counte-rions trapped within the absorbed polymers were removedand the size of the simulation box was adjusted to maintainthe concentration of unadsorbed PE chains �in solution� at aconstant level. Electroneutrality was always maintained. Thisprocedure was repeated for several cycles until four layerswere deposited. The second simulation was conducted bytaking the equilibrium structure resulting from the first simu-lation and further equilibrating it after removing chargesfrom the surface. Finally, the equilibrium structure resultingfrom the second simulation was simulated again after re-charging the surface. The duration of the second and thirdsimulations was 50 ns. Figure 5 displays the equilibriumconcentration profiles of the cationic PE in the Z directionresulting from the first and second simulations. The systemwith the charged surface formed well-structured multilayers�first simulation�. Removing charges caused the layers tolose their multilayered structure �second simulation�. Re-charging the surface caused the layers to reassemble and theoriginal multilayer structure reassembled. The same multi-layered structure could also be obtained by simulating a mix-

FIG. 3. Structure of a chain adsorbed on the charged surface.

FIG. 4. Concentration profiles of the anionic polymer in the Z direction forNp=16 and Np=128.

FIG. 5. Concentration profiles of anionic chains with Np=64 on a chargeand a neutral surface.

114907-4 B. Abu-Sharkh J. Chem. Phys. 123, 114907 �2005�

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ture of oppositely charged PEs without their counterions.These simulations clearly show that multilayer formation is aspontaneous equilibrium process that does not depend on thestarting configuration or method of deposition. This conclu-sion is valid for PE multilayers deposited from salt free so-lutions. In addition, at this stage this conclusion is limited tothe first few layers. This result has a very useful practicalimplication as PE multilayers do not have to be depositedsequentially by the tedious and time consuming process ofalternate dipping of a substrate in solutions of cationic andanionic PEs. Alternatively, a thin layer can be initially depos-ited on the substrate by, for example, high-speed spinningfollowed by annealing in an appropriate solvent and at anappropriate temperature.

Figure 6 shows that reducing the charge of the polymerchain to f =0.75 did not change the layer thickness, however,it influenced surface coverage and concentration of the cat-ionic PE especially in the first three layers. Subsequent lay-ers appeared to have similar characteristics. The higher con-centration of the cationic PE is a result of the need of morecationic beads to neutralize and overcompensate the surfacecharge. This effect propagates to the third layer beyondwhich polymer charge does not seem to influence polymerconcentration as well as layer thickness.

Figure 7 shows the influence of surface charge onmultilayer structure for chains with Np=64 and f =0.5. It canbe seen that a higher surface charge leads to the formation ofa thicker first layer that is composed of two monomolecularlayers. Multiple layers are needed to neutralize and overcom-pensate the surface charge. Layers located further away fromthe surface have the same thicknesses that are independent ofsurface charge.

V. CONCLUDING REMARKS

In conclusion, our MD simulations show that multilayerformation on flat surfaces from salt free solutions is a ther-modynamic equilibrium process that does not depend on themethod of deposition. This conclusion apparently contradictsthat of Panchagnula et al.31 who conducted simulations on a

spherical substrate. This apparent contradiction can be attrib-uted to three reasons. First, multilayer formation on curvedsurfaces is prohibited because of the high entropy penaltystemming from the low dimensionality of the substrate atstrong curvature.30 Second, chains used in our simulationsare more realistic compared to the freely jointed necklacemodels used by other investigations because they are re-strained by the angle and dihedral potentials. Subsequently,our chains have lower entropy and their entropy penalty re-sulting from multilayering is lower than the freely jointednecklace chains. Finally, the multilayers formed on thespherical substrate contained counterions. Failure to displacecounterions from the multilayers leads to a physical situationthat resembles deposition from solutions of high ionicstrength in which the chemical potential of the salt ions ishigh and PE/PE ion pairs are replaced by PE/salt ion pairs.42

Many of the experimental observations, for example, themonomolecular structure of the first layer and the fuzzy na-ture and interpenetration of the layers were reproduced bythe simulation. Detailed investigation of the influence of sur-face to PE charge ratio on layer structure as well as detailedinvestigation of molecular structure of all layers will be in-vestigated in a future publication.

The author acknowledges support provided by KFUPM.

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