Structure and Mechanism of the Deposition of Multilayers ofPolyelectrolytes and Nanoparticles

Basel Abu-Sharkh*

Department of Chemical Engineering, KFUPM, Dhahran 31261, Saudi Arabia

ReceiVed July 23, 2005. In Final Form: February 2, 2006

MD simulation of the layer-by-layer assembly of polyelectrolytes (PEs) and nanoparticles (NPs) revealed that theassembly process is electrostatically driven with alternating charge reversal and an overcompensation mechanism.Layers were observed to grow in the lateral direction as well as in a direction normal to the surface. Weakly adsorbedPE molecules were observed to desorb from the flat and NP surfaces. Those molecules are attracted by suspendedNPs in solution. PE molecules do not only pull NPs toward the surface but bridge NPs both in solution and on thesurface, forming agglomerates and islands. The first double layer differs in structure from the second double layeras a result of strong adsorption of the PE molecules to the rigid surface.

1. Introduction

Layer-by-layer assembly (LBL) is one of the most importantmethods of thin-film deposition.1 This technique has beensuccessfully used for the deposition of oppositely chargedpolyelectrolytes (PEs), nanoparticles (NPs), and various othermaterials. The method uses very simple equipment and produceshigh-quality films with thickness that can be controlled on thenanometer scale. Unlike other techniques of NP-polymercomposite formation, LBL produces highly homogeneouscomposite films of PEs and NPs.

Experimental studies of the multilayers formed by PEs andNPs show that the thickness of the layers follows a linear trendand that the thickness of each monolayer corresponds to thediameter of the nanoparticle or the thickness of a monolayer ofthe PE chain.2-4 In some cases, two to three monolayers areformed in each deposition cycle, indicating the looping/entanglement of charged polymer chains with charged nano-particles.5-7 In the case of large NPs, for example, yittrium irongarnet and other large hydrophilic NPs made from oxides, thethickness obtained was noted to be significantly lower than theaverage value expected for densely packed layers of the samediameter, and very low surface coverage was observed. However,stable growth of the NPs with poly(diallyl dimethylammonium)chloride (PDDA) was observed for 50 depositions. A study ofthe effect of NP size on the structure of the multilayer filmindicated that the density and thickness of the films as well asthe adsorption kinetics appear to be strongly dependent on thesize of the particles, with smaller particles favoring the formationof smooth, dense films with a higher content of NPs.8 The growthof YIG films prepared by layer-by-layer assembly was found to

occur via two deposition modes: sequential adsorption of denselypacked layers (normal growth mode) and in-plane growth ofisolated particle domains (lateral expansion mode). Microscopyresults indicate that lateral growth is based on the interplay ofparticle/particle and particle/polyelectrolyte interactions ratherthan on a substrate effect. The lateral expansion mode is a generalattribute of layer-by-layer deposition and can be observed forvariousaqueouscolloids.Theswitch from lateral tonormalgrowthmode was found to be effected by grafting charged organichydrophobic groups to YIG nanoparticles. Hydrophobic interac-tions between the hydrocarbon groups of the modified YIG andpolyelectrolyte significantly increase the attractive componentof the particle/polyelectrolyte and particle/particle interactions.

The effect of ionic strength on the adsorption behavior andstructure formation of PE/NP films was also investigated.9 Lowionic strength solutions gave stable adsorbed films with areproducible stratified multilayer structure. The films formed inhigh ionic strength solutions were initially much thicker but alsoless stable. A significant desorption was observed to take placein conjunction with the second exposure to NPs.10

Despite numerous experimental investigations of the structureof PE/NP multilayer assemblies, the theoretical models ofelectrostatic self-assembly are still very limited. All of thetheoretical studies considered only oppositely charged PEmultilayer assembly.11-14 None of these models describes theassembly of PE/NP multilayers.

Computer simulation is a valuable tool that can give insightinto the molecular phenomena and mechanism of PE/NPmultilayer formation. It canbeused toelucidate factors influencingthe self-assembly, explain some experimental observations, andverify theoretical models. Very few MC and MD simulationstudies have been devoted to investigating multilayer formation

* E-mail: sharkh@kfupm.edu.sa.(1) Kotov, N. A. InMultilayer Thin Films; Decher, G., Schlenoff, J. B., Eds.;

Wiley-VCH: Weinheim, Germany, 2003.(2) Lvov, Y. M.; Rusling, J. F.; Thomsen, D. L.; Papadimitrakopoulos, F.;

Kawkami, T.; Kunitake, T.Chem. Commun.1998, 1229-1230.(3) Teldeshi, C.; Mohwald, H.; Kirstein, S. J. Am. Chem. Soc.2001, 123,

954-960.(4) Fang, M.; Kim, C. H.; Saupe, G. B.; Kim, H. N.; Waraksa, C. C.; Miwa,

T.; Fujishima, A.; Mallouk, T. E.Chem. Mater. 1999, 11, 1526-1532.(5) Rogach, A. L.; Koktysh, D. S.; Harrison, M.; Kotov, N. A.Chem. Mater.

2000, 12, 1526-1528.(6) Serizawa, T.; Takeshita, H.; Ahashi, M.Langmuir1998, 14, 4088-4094.(7) Hicks, J. F.; Shon, Y. S.; Murray, R. W.Langmuir2002, 18, 2288-2294.

(8) Bogdanovic, G.; Sennerfors, T.; Zhmud, B.; Tiberg, F.J. Colloid InterfaceSci.2002, 255, 44-51.

(9) Sennerfors, T.; Bogdanovic, G.; Tiberg, F.Langmuir 2002, 18, 6410-6415.

(10) Ostrander, J. W.; Mamedov, A A.; Kotov, N. A. J. Am. Chem. Soc.2001,123, 1101-1110.

(11) Netz, R. R., Joanny, J. F.Macromolecules1999, 32, 9013-9025.(12) Park, S. Y.; Rubner, M. F.; Mayes, A. M.Langmuir2002, 18, 9600-

9604.(13) Castlenov, M.; Joanny, J. F.Langmuir2000, 16, 7524-7532,(14) Solis, F. J.; del la Cruz M. O.J. Chem. Phys. 1999, 110, 11517-11522.

3028 Langmuir2006,22, 3028-3034

10.1021/la052004t CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 03/01/2006

on spherical, planar, and cylindrical surfaces.15-20 However, nomolecular simulation studies have been reported for the multilayerassembly of PEs and NPs.

The objective of this article is to investigate the mechanismof formation of PE/NP multilayers and visualize the interactionbetween the NP and PE molecules during the deposition process.A second objective of this study is to investigate the structureof the multilayers deposited from salt-free solutions on a flatsurface.

2. Simulation Details

We conducted a coarse-grained MD simulation of the LBLassembly of polyelectrolyte molecules and nanoparticles fromdilute solutions. The polyelectrolyte chains consist ofNp ) 64monomer beads. The absolute value of the charge on each chargedmonomer bead is equal to 1. A coarse-grained system was usedto reduce the overall number of particles in the system andsubsequently to reduce the number of computations per timestep. In addition, the overall dynamics of the system is acceleratedbecause the free-energy profile in the system is less bumpy.21-23

The diameter of a beadσ in the chain is 4 Å, which correspondsto 1.6 monomers of NaPSS with a monomer size of 2.5 Å.

The nanoparticle was modeled using a C60 fullerene sphere.Two surface charge densities were used for nanoparticles: ahigh charge of-30 corresponding to a single-charge monovalentcharge on every second carbon and a low charge of-6 corres-ponding to six monovalent charged beads of C60 distributeduniformly over the surface. All charges in the system aremonovalent, discrete, and fixed on the beads. The diameter ofthe particle is 7.114 Å. Each bead of the C60NP or PE has a massof 12 atomic mass units. Increasing the mass of each bead to 72was not found to influence the equilibrium configuration of thesystem.

Multilayers were deposited from dilute solutions with a volumefraction of 0.05 on a hexagonal packed surface composed of 289spherical particles that are constrained in place. The dimensionsof the surface are 42.8 Å× 42.8 Å. Each bead has a mass of12 atomic mass units. The surface is located atZ ) 0. The totalsurface charge is-144, corresponding to a negative monovalentunit charge on every other bead. A neutral soft repulsive wallwas placed at the top of the simulation box to avoid the escapeof counterions and chains to the lower side of the charged surface.The upper wall is identical to the lower surface with the exceptionthat it interacts with other particles by a force field that correspondsto the repulsive term of eq 5. A 100-Å-thick layer of vacuumwas placed on top of the neutral wall to eliminate interactionbetween the lower side of the charged wall and particles insidethe simulation box. The force field used to model the chains isa simplified form of the polymer-consistent force field (PCFF)described by an equation of the form24,25

The force field employs a quartic polynomial for bond stretching(term 1) and angle bending (term 2) and a three-term Fourierexpansion for torsions (term 3). Term 4 is the Coulombicinteraction between the atomic charges, and the attractive partof term 5 represents the van der Waals interactions. The forcefield parameters of the chain are given in Table 1.24,25Electrostaticinteractions between charged beads are calculated using the Ewaldsum method.23 A relative dielectric constantεr of 80 was usedto account for the screening of charges by an implicit solvent(water).24 van der Waals cross-interaction parameters arecalculated using24,25

Nonbonded van der Waals interactions were calculated with acutoff distance of 2.5σ, whereσ is the diameter of a chain bead.Standard long-range corrections were applied.25Simulations werecarried out in theNVTensemble with periodic boundary conditionsin three dimensions. A constant temperature was accomplishedby linking the system to the Andersen thermostat.26A simulationtime step of 3 fs was used.

Simulations were performed using a procedure that is similarto the procedures used for the simulation of polyelectrolytemultilayers.15-20 The procedure resembles the experimentaldeposition of multilayers that proceeds by the immersion of acharged substrate into a dilute polyelectrolyte solution followedby a rinsing step to remove excess, unadsorbed molecules andfinally immersion in a dilute suspension containing the nano-particles followed by a second rinsing step. The charged surfacewas constructed, and its counterions were dispersed throughoutthe simulation box. Ten positively charged polyelectrolytemolecules were then inserted into the box along with theircounterions. The concentration of chains was kept at a volumefraction of 0.05. The simulation box was subsequently equilibratedfor 20 ns, during which time equilibration was confirmed bymonitoring the total energy and concentration profiles of thevarious species in the system. Unadsorbed polyelectrolytemolecules were then removed along with their counterions,representing a rinsing step. These molecules were deleted fromthe simulation box. Equilibrating the system for 200 ps followingdeletion of the unadsorbed molecules was not found to cause anydesorption or major reorganization of adsorbed molecules. Twenty

(15) Messina, R.Macromolecules2004, 37, 621.(16) Messina, R.; Holm, C.; Kremer, K.Langmuir2003, 19, 4473.(17) Messina, R.J. Chem. Phys.2003, 119, 8133.(18) Panchagnula, V.; Jeon J.; Dobrynin, A. V.Phys. ReV. Lett. 2004, 93,

037801.(19) Panchagnula, V.; Jeon, J.; Rusling, J. F.; Dobrynin, A. V.Langmuir2005,

21, 1118-1125.(20) Abu-Sharkh, B.J. Chem. Phys. 2005, 123, 114907/1-114907/6.(21) Marrink, S. J.; Mark, A. E.J. Am. Chem. Soc.2003, 125, 15233-15242.(22) Stevens, M.; Hoh, J. H.; Woolf, T. B.Phys. ReV. Lett.2003, 91, 188102-

188104.(23) Aksimentiev, A.; Schulten, K.Proc. Natl. Acad. Sci. U.S.A. 2004, 101,

4337-4338.(24) Sun, H.; Mumby, S. J.; Maple, J. R.; Hagler, A. T.J. Am. Chem. Soc.

1994, 116, 2978.(25) Sun, H.J. Comput. Chem.1994, 15, 752. (26) Karasawa, N.; Goddard, W. A.Macromolecules, 1992, 25, 7268.

term 1: ∑i

K2(bi - b0)2 + K3(bi - b0)

3 + K4(bi - b0)4 +

term 2: ∑i

H2(θi - θ0)2 + H3(θi - θ0)

3 + H4(θi - θ0)4+

term 3: ∑i

V1[1 - cos(æi - φ10)] +

V2[1 - cos(2φi - φ20)] + V3[1 - cos(3φi - φ3

0)] +

term 4: ∑i>j

qiqj

εrrij

+

term 5: ∑i>j

εij[2(σij

rij)9

- 3(σij

rij)6] (1)

εij ) 2xεiεj

(σi3σj

3)

(σi6 + σj

6)

σij )(σi

6 + σj6)1/6

2

Polyelectrolyte/Nanoparticle Multilayer Deposition Langmuir, Vol. 22, No. 7, 20063029

(high charge) or 80 (low charge) negatively charged nanoparticleswere subsequently added to the box along with their oppositelycharged counterions at a volume fraction of 0.05. The systemwas again equilibrated for 20 ns. Unadsorbed NPs were thenremoved along with their counterions, representing a rinsingstep. After a molecule or a nanoparticle was adsorbed on thesurface, the volume of the simulation cell was adjusted so thatthe concentration of molecules in the solution phase remainedconstant. In addition, it was observed that the molecules that areclose to the surface interact with the surface whereas moleculesthat are far away do not feel the presence of the surface. As aresult, after adjusting the volume of the simulation cell, themolecules and nanoparticles in the solution phase were redis-tributed uniformly throughout the liquid phase to simulate a realsolution in which molecules are uniformly distributed. Thisprocess was repeated three or four times until no more adsorptionwas observed, indicating equilibrium between the surface andthe solution. Another layer of the PE and nanoparticles wassubsequently added using the same procedure described above.Overall charge neutrality was always maintained in the systemusing counterions. The four depositions represent two completedipping cycles. After completing the depositions, the systemwas further annealed for a total of 20 ns. Equilibration wasconfirmed again by monitoring the energy and concentrationprofiles in the system.

Deciding which molecules are adsorbed was based on thefollowing criterion: If a bead or more of a PE molecule or anNP is less than a distance of 1.1σ from a surface particle or analready adsorbed PE molecules, then that molecule is consideredto be adsorbed on the surface. Otherwise, the molecule or NPis considered not to be adsorbed and is removed in the rinsingstep.

3. Results and Discussion

The first deposition step of the cationic PE on the negativelycharged substrate resulted in the formation of a layer of adsorbedmolecules that assume three different conformations: train, loop,and tail as shown in Figure 1. The tails and loops extend intothe solution phase in the absence of added salt because of strongintramolecular electrostatic repulsion. However, the train sectionsof the chains adhere to the surface. These structures have beenexperimentally observed and reported in the literature for PEmolecules adsorbed on charged surfaces.27

During the initial stages of the highly charged NP depositionstep, three processes were observed to occur (Figure 2). Initially,the extended tail segments reach for an NP and start adsorbingand folding at its surface (Figure 2a). Once this adsorption takesplace, the NP either pulls the PE from the surface or the PE pullsthe NP toward the surface (Figure 2b and c). The mechanism thatdominates depends on the fraction of the PE molecule that is indirect contact with the surface in the train conformation. Moleculeswith a small train fraction were easily pulled away from thesurface. It is expected that this process depends on many factors,for example, charges on the surface, PE, and NP in addition tothe sizes of the PE molecule and NP. Molecules that are stronglyanchored to the surface start folding around the NP, bringing itgradually closer to the surface. It was also observed that PEmolecules pulled from the surface bridge NPs and bring themtogether in small agglomerates composed of two to three NPs.Those globules are removed from the simulation box during therinsing step (Figure 2d). The desorption of PE molecules or NPsfrom previously deposited layers has been experimentallyobserved.1 Upon adsorption of the nanoparticles, the net chargeof the system changes from+112 to-132, corresponding to theadsorption of six NPs and the desorption of one cationic PEmolecule. Deposition of the particles with low charge proceededfollowing a similar mechanism; however, much less desorptionof PE chains by NPs was observed. Deposition mostly proceededby the adsorption of a PE end on an NP surface, followed byfolding and gradual pulling of the NP toward the surface. Thesecond cationic PE layer was deposited using a procedure similarto the one described earlier. The mechanism of deposition in thisstep proceeded by the initial adsorption of one or both ends ofthe PE chain on the surface of a nanoparticle(s) or an uncoveredsurface, followed by gradual folding and adsorption of the chainon the surface of the NP. In this case, the NP appeared to be verystable on the surface, and no desorbtion of NPs was observed.Similar to the first PE layer, some tail segments were also extendedto the solution phase. A second nanoparticle layer was subse-quently deposited using the same procedure described earlier.The three processes that were observed during the first depositioncycle are also observed during the second layer deposition. Inaddition, some NPs were deposited in the uncovered areas of thesurface. Furthermore, the NPs were stacked not only parallel but

(27) Klitzing, R. V.; Steitz, R. InHandbook of Polyelectrolytes and TheirApplications; Tripathy, S. K., Kumar, J., Nalwa, H. S., Eds.; American ScientificPublishers: Stevenson Ranch, CA, 2002; Vol. 1.

Table 1. Force Field Parameters

T ) 300 KmassC 12.011Fe 55.847

bond potential b0 K2 K3 K4

C-C 1.53 299.67 -501.77 679.81

angle potential æ0 H2 H3 H4

C-C-C 112.67 39.516 -7.443 -9.5583

torsion potential V1 φ10 V2 φ2

0 V3 φ30

C-C-C-C 0 0 0.0514 0 -0.143 0

nonbond interactions σ εFe 2.6595 13.889C 4.0100 0.0540

εij ) 2xεiεj

(σi3σj

3)

(σi6 + σj

6)

σij )(σi

6 + σj6)1/6

2

3030 Langmuir, Vol. 22, No. 7, 2006 Abu-Sharkh

also perpendicular to the surface. Because many nanoparticlesare bridged together by the polyelectrolyte, both in solution andon the surface, the topolgy of the deposited group depends onthe topology of the bridged particles. In general, the particlesassemble together in the form of islands that are depositedhorizontally or vertically on the surface. Islands containing upto three NPs bridged by one or more chains were observed. Asimilar formation of islands has been observed experimentally.15

After a very long equilibration time of 100 ns, the NP and PEsystem forms a highly ordered system in which particles areassembled in well-defined layers. It is also observed that onechain from the first polymer layer can extend to encapsulatenanoparticles from the first and second layers. The system withlow-charge NPs provided higher surface coverage, and manymore NP were deposited in the first cycle compared to the numberof strongly charged NPs. Figure 3 shows the conformation of thesimulation box after two deposition cycles for the strongly chargedNP system (a) and after four cycles for the system with low NPcharge (b). The different chain colors in Figure 3b indicate PEchains deposited at different cycles. It can be observed that morethan one layer can be deposited in one deposition cycle. A similarphenomenon was observed during the deposition of HgTe- andcitrate-stabilized gold NPs in addition to large latex colloids

deposited from solutions in the presence of salt.5,6 It is alsoobserved that well-defined layers are formed in both cases.

We also observed that the strongly charged NPs do notcompletely cover the substrate in the first deposition and that theNPs are not densely packed as shown in Figure 4a. A similarphenomenon was observed for 32 nm yittrium iron garnet (YIG)NPs as well as for some other large hydrophilic NPs made ofoxides. However, the density of the NPs was high enough toreverse the surface charge as shown in Figure 5. It is also observedin Figure 4 that the NPs form island. A similar phenomenon wasobserved using scanning electron microscopy of PDDA/bariumferrate NPs and PDDA/latex films. Our simulations show thatsuch islands probably start to form in the solvent phase beforethe NPs are deposited onto the substrate. However, some of thesurface area that was not covered in the first deposition cyclewas covered by PE and NP after the second deposition cycle(Figure 4b). Figure 5 shows the net charge of the system withhighly charged NPs (excluding counterions) after each depositionstep. It can be observed that the level of charge reversal is nearlyuniform for the PE and NP steps. The charge reversal effectedby the NP layer is slightly higher than that caused by the PElayer.

The concentration profiles in theZ direction of the NP andPE beads are shown in Figure 6, part a for the system with highlycharged NPs and in part b for the system with low-charge NPs.Concentration profiles of beads as a function of the distancefrom the plate (Z) are calculated by taking volumetric slices ofthickness 0.2 Å in theZ dimension. A histogram ofZ positionswas then constructed as a trajectory-averaged quantity. Numberdensities are therefore in units of beads/Å3. The sharp peaks ofthe first PE and NP layers result from the adsorption of PE tailsegments on the rigid surface and the subsequent adsorption of

Figure 1. Structure of the first cationic PE layer.

Figure 2. Mechanism of deposition of the first layer of nanoparticles.

Figure 3. Structure of the PE/NP system with (a) high-charge NPsand (b) low-charge NPs.

Polyelectrolyte/Nanoparticle Multilayer Deposition Langmuir, Vol. 22, No. 7, 20063031

NPs onto those PE segments. The subsequent layers are moreflat and have a more uniform distribution of concentration. Inaddition, significant overlap and penetration of the two com-ponents are observed. The lateral expansion of the NP layer inthe system containing high-charge NPs is illustrated in Figure

7, which shows the concentration profiles of the PE moleculesand NPs after the depositions of the first and second layers. Itcan be clearly seen that the concentration of both PE moleculesand NPs in the first layer (first peaks in Figure 7a and b) increasesafter the deposition of the second layer, indicating that more PEmolecules and NPs were deposited in the first layer during thesecond cycle of depositions.

Figure 8 shows the intermolecular radial distribution functionof the PE segments and NP beads. Comparison between thelow-charge and high-charge NP systems indicates that the NPsare more strongly correlated in the low-charge system. NPs aremore closely packed and can approach one another more easilythan in the high-charge NP system because of less-repulsiveinteractions. Furthermore, the PE molecules are more stronglycorrelated at short distances in the high-charge NP system becausemany of them need to assemble on the highly charged NP surfacein order to neutralize it, resulting in close proximity of the PEmolecules. In addition, the need for more PE segments to

Figure 4. Top view of (a) the first double layer and (b) two doublelayers of the system with high-charge NPs.

Figure 5. Net charge after the first, second, and third depositionsin the system with high-charge NPs.

Figure 6. Concentration profiles of the cationic PE and NP normalto the surface for the system with (a) high-charge NPs and (b) low-charge NPs.

3032 Langmuir, Vol. 22, No. 7, 2006 Abu-Sharkh

neutralize the highly charged NP causes the NP-PE correlationto be stronger in the high-charge NP system.

Figure 9 shows the normalized distribution of the radius ofgyration of PE molecules in the system with low charge. Abimodal curve can be observed. The second peak correspondsto the molecules that are adsorbed or are near the surface. Thesemolecules are more expanded as a result of the adsorption of alarge part of them on the flat surface in a stretched conformation,resulting in a large radius of gyration. The first peak correspondsto molecules in the second layer and subsequent layers. Moleculesin these layers are less expanded because they fold around theNPs, resulting in a smaller radius of gyration.

The local orientation of chains relative to the surface may bemeasured using the orientation correlation function. To this end,we define unit vectors between adjacent monomers:

The scalar product between two such unit vectors describes theangle between the chain tangent vector and the surface vector:

The distancer denotes the distance between the centers of massof the chain segment and the surface vector. The orientationcorrelation is defined by using the second Lengendre polynomial:

Figure 10 shows the orientation correlation function of the threefirst PE layers in the low-charge NP system. For the first layer,it can be observed that a near-perfect parallel orientation isobserved at short distances (P2 ≈ 1). This results from the strongadsorption of PE segments on the surface. As the distance betweenthe surface and chain segments increases, the parallel orientationis maintained to a lower extent as indicated by the positive values

Figure 7. Concentration profiles of (a) the PE and (b) the NPs afterdeposition of one layer and two layers.

u )ri - ri - 1

|ri - ri - 1|(2)

Figure 8. Intermolecular radial distribution function of the systemwith (a) high-charge NPs and (b) low-charge NPs.

cosR(r) ) uchain‚usurface (3)

P2(r) ) 12[3 cos2 R(r) - 1] (4)

Polyelectrolyte/Nanoparticle Multilayer Deposition Langmuir, Vol. 22, No. 7, 20063033

of P2 even as far as 20 Å from the surface. The second layer alsoshows a high level of orientation (P2 ≈ 0.8) with the surface ata short distance of 11 Å, but a perpendicular orientation is observedat a distance of 17 Å. This parallel orientation is associated withsegments that are adsorbed on the sides of the NPs. The thirdlayer also shows a high level of parallel orientation (P2 ≈ 0.8)with the surface at short distances (21 Å).

Figure 11 shows the orientation correlation function betweenPE molecules deposited in subsequent layers. The negativeP2

at closest contact indicates near-parallel orientation. This parallelorientation is the only possible orientation of segments at closestcontact. The value ofP2 increases to 0.6 for the first- to second-layer correlation and to 0.4 for the second- to third- and third-to fourth-layers correlations. The surface effect is apparent in thefirst- to second-layer orientation correlation.

4. Conclusions

In conclusion, multilyers of PE and NPs were found to beelectrostatically driven with alternating charge reversal and anovercompensation mechanism. Layers were observed to grow inthe lateral direction as well as normal to the surface. Lateralgrowth is facilitated by the adsorption of more PE molecules onthe uncovered areas of the surface. The newly adsorbed PEmolecules attract more oppositely charged NP particles to thesurface, resulting in lateral growth. Growth normal to the surfaceis caused by PE chains adsorbed onto NP surfaces. Weaklyadsorbed PE molecules were observed to desorb form the flatand NP surfaces. Those molecules are attracted by suspendedNPs in solution. PE molecules do not only pull NPs toward thesurface but also bridge NPs both in solution and on the surface,forming agglomerates and island. The first double layer differsin structure from the second double layer as a result of a strongsurface effect.

The charge of the NP has a strong influence on the structureof the multilayers. High charge leads to the formation of islandand incomplete coverage of the surface whereas the system withlow NP charge tends to provide more surface coverage. Thismight be a result of strong repulsive interactions between differentNPs and NPs with similarly charged surfaces. The lateral repulsionof NPs prevents the packing of NPs on the surface, thus islandsare formed. This strong repulsion is not sufficiently neutralizedby the PE, resulting in the adsorption of only a small numberof NPs in each step. This problem is not encountered in thelow-NP-charge system, and as a result, more surface coverageis feasible. There are similarities between the NP-PE and PE-PE multilayer systems. For example, the mechanism of depositionis very similar, in which a particle or part of the PE chain isinitially adsorbed, followed by reorientation and packing on thesurface. Rearrangement and desorption of weakly adsorbed chainsis also observed in both cases, although NPs demonstrate astronger ability to desorbing previously adsorbed chains thanoppositely charged PEs. In both cases, the first two layers havea structure that is different from that of subsequent layers. Thefirst layers are well structured, and their structure is stronglyinfluenced by the highly ordered wall structure.

LA052004T

Figure 9. Normalized distribution of the radius of gyration of PEmolecules in the system with low-charge NPs. The first peakcorresponds to molecules in the second layer and subsequent layers.The second peak corresponds to molecules in the first layer.

Figure 10. Orientation correlation function of the first, second, andthird layers with the surface in the system with low-charge NPs.

Figure 11. Orientation correlation function of the first with second,second with third, and third with fourth layers in the system withlow-charge NPs.

3034 Langmuir, Vol. 22, No. 7, 2006 Abu-Sharkh