Current Opinion in Colloid & Interface Science 19 (2014) 25–31

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Current Opinion in Colloid & Interface Science

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Competing mechanisms in polyelectrolyte multilayer formation andswelling: Polycation–polyanion pairing vs. polyelectrolyte–ion pairing

Dmitry Volodkin a, Regine von Klitzing b,⁎a Fraunhofer-Institut für Biomedizintechnik (IBMT), Am Mühlenberg 13, D-14476 Potsdam–Golm, Germanyb Stranski-Laboratorium für Physikalische und Theoretische Chemie, Technische Universität Berlin, Strasse des 17. Juni 124, D-10623 Berlin, Germany

⁎ Corresponding author.E-mail address: klitzing@mailbox.tu-berlin.de (R. von

1359-0294/$ – see front matter © 2014 Elsevier Ltd. All rihttp://dx.doi.org/10.1016/j.cocis.2014.01.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 November 2013Received in revised form 12 December 2013Accepted 1 January 2014Available online 11 January 2014

Keywords:Polyelectrolyte multilayersPolyanion–polycation complexesIon exchangeHofmeister seriesHydrophilic/hydrophobic balance

The competition of interactions between charged groups of polyanions and polycations and their interactionwithsmall counterions strongly affect the formation and stability of polyelectrolyte multilayers. This has conse-quences for the properties of polyelectrolytemultilayers likemechanics, polymermobility and swelling inwater.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Polyelectrolyte multilayers (PEMs) of alternating polyanions andpolycations [1] are of strong scientific and technological interest dueto their ease of preparation bydipping, spinning or spraying. In addition,there are almost no geometrical restrictionswith respect to the shape ofsubstrate or template. Mostly, they are used as coatings of planar sub-strates or of particles and as wall material of hollow capsules. An out-standing feature, which distinguishes them from most of the otherpolymer films, is their pronounced ability to be tailored on a molecularlevel. The choice of polyelectrolytes and salt with respect to concentra-tion and type determines the structure and dynamics of PEMs and theirresponse to outer stimuli like swelling in water. On a molecular levelthe important parameters are the number density and strength ofpolyanion/polycation complexation sites. They can be tuned eitherduring or after preparation by varying e.g. pH and temperature or byadding different types of salt. The review addresses the competition be-tween forming and dissolving these polyanion/polycation complexesand focuses on the electrostatic Layer-by-Layer (LbL) method.

The free enthalpy (Gibbs energy) of complexation of oppositelycharged functional groups of polyelectrolytes consists of several contri-butions like the formation of the complex itself (enthalpic contribution),the release of small counterions and the partial removal of thehydrationshell around ionized groups (entropic contributions). Short-ranged vanderWaals interactions and the contribution of hydrophobic parts of the

Klitzing).

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polyelectrolytes in terms of liberation of structured water moleculesaround them play a minor role.

Among enthalpic contributions Coulombic interactions between op-positely charged groups play themajor role. They depend on the chargedensity of the polyelectrolytes and of the substrate and the screening ofcharges by counterions. Other less important interactions are hydrogenbonding, and hydrophobic and van der Waals interactions.

The review focusesmainly on the competition between theCoulombinteractions and the ion specific effects on Coulomb interactions. Strongion specific interactions between counter ions of added salt andpolyelectrolytes avoid the complexation between oppositely chargedpolyelectrolyte groups and lead to extrinsic charge compensation. Incontrast, weak polyelectrolyte/counter ion interactions cause intrinsiccharge compensation related to a high density of polyanion/polycationcomplexation sites. The density of complexation sites is strongly relatedto the type of growth, and affects mechanical properties, chain mobilityand the swelling behavior in water.

2. Formation of multilayers at low salt concentrations

2.1. Linear vs. exponential growth

Although a number of comprehensive reviews have been published[2–5] themechanism of the PEMgrowth is still not fully understood andattracts a significant attention in the scientific community focusing onpolymer self-assembly. There are twogrowth regimes identified in liter-ature for the PEM: i) linear and ii) supralinear or exponential. Lineargrowth takes place if the increment in thickness per deposited polyelec-trolyte bilayer is constant. For exponentially growing multilayers the

26 D. Volodkin, R. von Klitzing / Current Opinion in Colloid & Interface Science 19 (2014) 25–31

increment increases with the number of deposited layers. As examples,in presence of low salt concentration, pairs of synthetic polyelectrolyteslike polystyrene sulfonate (PSS)/polyallylamine hydrochloride (PAH)and PSS/polydiallyldimethylammonium chloride (PDADMAC) showlinear growth behavior while combinations of most biopolymerslike hyaluronic acid (HA)/poly-L-lysine (PLL) or poly-L-glutamic acid(PGA)/PLL grow exponentially.

In general, the growth of any LbL assembled multilayer can be pre-sented by a graph as shown in Fig. 1A. The growth profile consists oftwo parts, exponential and linear. The switch point presents the transi-tion between these two parts. The difference in the multilayer growthregime will be defined by the ratio between both parts. For linearlygrowing films the exponential part is short (switch is at a few layersor less) and sometimes cannot be well identified. For exponentiallygrowing films the exponential part is larger and can be easy identified.For instance, the switch is at 18 bilayers in Fig. 1A. The crucial funda-mental question is why the growth profile has a switch from the expo-nential to a linear part. Two models have been proposed in order todescribe the nature of the switch, namely “roughness” and “diffusion”models. Schemes in Fig. 1B and C represent these models showing theposition of the switch point with respect to the growth profile ingraph A.

According to the “roughness” model (Fig. 1B), isolated islands areformed at the beginning. The growth rate is exponential because theheight and radius of the islands and therefore the surface increase. Fi-nally, the islands coagulate resulting in aflatmultilayer growing linearlydue to a constant surface area. An alternative explanation related to theroughness increase is based on the formation of branch-like structuresduring the exponential growth [7]. The transition point corresponds tosterical limitations for polymer molecules resulting in a constantnumber of binding sites. The roughness model seems to be suitableto explain deviations from a linear growth for the first few layers,

Fig. 1. A— Growth profile of HA/PLL multilayer showing a switch from exponential to lin-ear growth regimes. B — Scheme of “roughness”model representing a coalescence of ini-tially formed islands at the switch point during the multilayer build-up. C — Scheme of“diffusion”model representing a formation of restructuring zone (polymer diffusion is re-stricted) underneath the diffusion zone at the switch point. Adapted from Ref. [6].

which are of the thickness of molecular dimensions. Therefore itcan be used to explain the so-called “substrate effects” in lineargrowing systems. In contrast, exponentially growing multilayers(HA/PLL) are of hundreds of nanometers or even micrometers thick[8,9,6]. This is far away from dimensions of a single polymer mole-cule. Therefore, increasing roughness cannot be the dominating fac-tor for HA/PLL film exponential growth up to 12 bilayers, and anothermodel (“diffusion” model) has been proposed.

The “diffusion”model (Fig. 1B) is based on the polymer diffusion INand OUT of themultilayer [10,8,9,11]. According to this model, the typeof growth is linear if polymers are not able to diffuse into themultilayer.If at least one of both polyelectrolytes is able to diffuse into the entiremultilayer, the multilayer grows exponentially. During adsorption ofoppositely charged polyelectrolyte the excess polyelectrolyte withinthemultilayer will move to the interface of PEM and polyelectrolyte so-lution and will form complexes with the freshly adsorbing oppositelycharged polyelectrolyte.With increasing thickness of the PEM its capac-ity for polyelectrolyte uptake increases. Hence, the thickness incrementper bilayer increaseswith increasing number of layers. The thickness in-crement is usually much smaller for linearly growing multilayers thanfor exponentially growing ones in their linear growth regime (typi-cally a few nanometers versus hundreds of nanometers). One expla-nation for the switch from exponential to linear growth might bethat during contact time tcontact with the polyelectrolyte solutiononly the polyelectrolytes from a depth Δx∝

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

D � tcontactp

(D = diffu-sion coefficient) could reach the surface and form complexes. Fromamultilayer thickness Δx on the depth fromwhich the polyelectrolytescould diffuse to the surface remains constant (=Δx), and therefore thegrowth behavior becomes linear [10]. If this would be the case, thenumber of layers where the switch from exponential to linear growthtakes place should increase with contact time of the polyelectrolyte so-lution andwith decreasingmolecular weight of the polyelectrolytes. Onthe contrary, it was found that the switch takes place always at 12 bilay-ers [8] irrespective of contact time [8] or molecular weight [9]. In orderto solve this contradiction, another model was developed which as-sumes a “restructuring” inner compartment with restricted polymerdiffusion (so-called “forbidden zone”) and a higher density [8](Fig. 1C). Thus, upon formation of this zone the amount of free polyelec-trolytes in themultilayer is constant irrespective of the total PEM thick-ness. This results in the switch from exponential to linear regime(Fig. 1C). Indeed, the restricted polymer diffusion and the related tran-sition from exponential to linear growth are observed for polyelectro-lytes of high molecular weight like 360 kDa PLL. In contrast, short20 kDa PLL can diffuse into the whole multilayer [9]. This should keepthe growth exponential, since the amount of free polyelectrolyteswithin the PEM increases with increasing PEM thickness. Obviously,this not the case: Also short chains show a transition from exponentialto linear growth. Here is still a lack of understanding. One idea is thatthe short PLL can diffuse out of the whole multilayer and form com-plexes with the oppositely charged HA outside the multilayer [9]. An-other explanation is that polyelectrolyte chains are still mobile withinthe multilayer system, but they cannot reach the surface due to anelectrostatic barrier formed at the surface [7]. That would lead toovercompensation for one species. Indeed, recently, it was found thatafter a dozen number of layers an excess of polycation is present inthe multilayer [12].

In order to prove the consistency of the describedmodels one needsto utilize methods of direct monitoring of the multilayer compositionwith high precision in space and time in a wet state. Such an approachhas been elaborated, studying the internal multilayer structure byusing optical methods like confocal laser scanning fluorescence micros-copy [9]. Recently, this approach has been even improved [13]: Themultilayer has been constructed onto a glassmicrofiber allowing gettingside view of the multilayer internal structure. Molecular distributionand transport can be analyzed by combining it with fluorescence recov-ery after photobleaching (FRAP). Utilization of microfluidics for fine

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control over the deposition conditions including deposition time andpolymermass transport has shown to be a perspective for precise analy-sis of growth profiles [6,14]. Such approaches or combination of themcould be useful in the nearest future to better understand themultilayergrowth mechanism.

2.2. Polyanion–polycation complexes

The general assumption is that linearly growing multilayers arecomposed of intrinsically charge compensated polyelectrolyte com-plexes and exponentially growing multilayers of both intrinsically andextrinsically charge compensated ones. This gives a higher density ofcomplexation sites in case of linearly growing multilayers than forexponentially growing ones. According to this it is assumed that the“diffusion” compartment in Fig. 1C is more loosely packed (intrinsicand extrinsic charge compensation) and the “restructuring” compart-ment is of higher density with rather insoluble polyelectrolyte

Fig. 2. A— Schematic presentation of the internal structure of polyelectrolyte multilayers.Upper part of the multilayer shows extrinsically compensated polyelectrolyte complexwith many free charges surrounded by the counterions. The inner part of the multilayerpresents intrinsically compensated polyelectrolyte complex when the complexation isachieved through ion pairs formed between the polyelectrolyte backbones. B — Growthprofile of PSS/PDADMAC multilayers assembled at different salt (NaCl) concentrations.An increase of the salt concentration results in a change of the growth regime from linearto exponential. Adapted from Ref. [15].

complexes (intrinsic charge compensation). The difference in chainconformation of both compartments is shown in Fig. 2A.

In order to get a deeper insight into the formation of polycation/polyanion complexes in bulk solution, isothermal titration calorimetry(ITC) measurements in water were carried out. Firstly, the formationof polyelectrolyte complexes (PEC) in bulk solution is strongly dominat-ed by a gain in entropy. The reasons against enthalpy, i.e. electrostaticinteractions, as driving force are at least threefold: 1) A charge reversioncosts a lot of energy, and probably the adsorption would stop at zerosurface charge. 2) In addition, electrostatically, it makes no differenceif polyelectrolyte charges are compensated by small counterions or byoppositely charged polyelectrolytes. 3) The formation also takes placeeven at high ionic strength, where the electrostatic interaction is moreor less screened [16]. Nevertheless, important differences in enthalpycould be identified for different types of growth: The complexation isstrongly exothermic for linearly growing multilayers (ΔH more nega-tive than −1000 J/mol) [17,18] and it is endothermic (positive ΔH)for exponentially growing multilayers. Weakly exponential growthwas observed for ΔH less negative than −500 J/mol [18]. According tothe equation ΔG = ΔH − TΔS in case of exothermic processes both(negative) change in enthalpy and (positive) change in entropy favorcomplexation and the complexes are expected to be strong and lineargrowth occurs. In contrast, for endothermic processes both the changein enthalpy and entropy are positive and counter active. Therefore the ef-fect of temperature on the PEM formation should be much more pro-nounced in case of exponential growth than for linear growing PEMs.Indeed, for linear growing systems no or a relatively small increasein multilayer thickness with increasing temperature is observed[19–21,18]. For exponentially growingmultilayers a pronounced increasein thickness is detectedwith increasing preparation temperature [18,21].

Salomäki et al. showed that in case of PSS/PDADMACmultilayers thetype of growth can be switched from linear to exponential by increasingthe temperature. The rate of exponential buildup Γk + 1 = Γ1 ∗ expβk isdetermined by the growth exponent β (k: number of deposited bilay-ers). The exponent shows Arrhenius-like behavior [21]. In general, thisis an indication for an increasing probability to overcome activation bar-riers. In case of multilayers it means a weakening of polyanion/polycation complexes and therefore less hindrance in polyelectrolytetransport to the surface.

Although some thermodynamical aspectswere discussed above, it isworth tomention that polyelectrolyte multilayers are not in thermody-namic equilibrium, but kinetically hindered. This has been proven byheating the system above the preparation temperature after the prepa-ration is finished which leads to annealing effects like e.g. smoothening[22]. Further, it is worth to be noted that the thermodynamic parame-ters measured for PECs in bulk solution have not to be the same as forcomplexes in PEMs. Nevertheless, similarities of the polyanion/polycation complexes were found in both geometries, e.g. in Infrared-Spectra [23] and by theoretical considerations [24,25].

2.3. Influence of charge density of polyelectrolytes and of their complexes

ITC can be also used to determine the stoichiometry of polycation/polyanion complexes [17]. It is calculated from the point of charge satu-ration, i.e. the concentration of titrating polyelectrolyte beyond whichno further complexes are formed. Interestingly, for most preparationconditions no 1:1 stoichiometry, assumed by other groups before [25],could be found. Obviously, the stoichiometric coefficient for higher mo-lecular weight is higher than for shorter chains due to worse accessibil-ity of the charges because of stronger coiling. A comparison withturbidity studies and PEM formation shows that complexes close tothe charge compensation point are not water soluble and give ratherthickmultilayers,where highly charged complexes (high stoichiometriccoefficient) are water soluble and unsuitable for PEM formation [17].The relationship between solubility of complexes and the ability toform multilayers becomes very comprehensible in Ref. [26] and was

Fig. 3.A) Ellipsometry data for PDADMAC/PSSmultilayers containing 75% charged or 100%charged PDADMAC adsorbed fromNaBr solutions of two different concentrations: 0.1 and0.25 M. For comparison data of multilayers containing 75% charged PDADMAC preparedfrom 0.1 NaCl solution are also shown. Adapted from Ref. [33]. B) Total amount of waterof (PSS/PDADMAC)6 multilayers in dependence of salt concentration and type of ion ofthe preparation solutions. Adapted from Ref. [34].

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generalized to other types of systems in Ref. [3]. The multilayer andouter solution are considered as two phases of a system with an uppercritical solution temperature (UCST). The temperature is replaced bythe salt concentration and the UCST by a critical salt concentration be-yond which no stable PEM can be formed anymore. The miscibilitygap presents the regime of unsoluble complexes, i.e. stable multilayers.The solubility of complexes can be varied by several parameters like theaddition of salt (see chapter 3), temperature, solvent, pH and polyelec-trolyte charge density.

A minimum charge density is required for the formation of multi-layers [27,16,28,29]. Below this charge threshold the number of com-plexation sites may not be high enough to form stable complexes.Often the thickestmultilayers are obtained for a charge density betweenthis threshold and the nominal 100% charge density level. The reason isan increase in solubility of the polyelectrolyteswith increasing degree ofcharges, which counteracts the ability for adsorption. Not only the aver-age charge density, but also the distribution of the charges along thechains plays an important role in building up multilayers. The adsorp-tion of block-copolymers showed that a short strongly charged block(10% of the total number of monomer units per chain) is sufficient forthe formation of multilayers, even if the average charge density isbelow the charge threshold that is required for multilayer formation[30].

The diffusion coefficient is mainly affected by the charge density ofthe polymers [31]. The charge distance of polycations is in the order ofPAH b PDADAMC (100% charged) b PDADMAC (75% charged) and incase of polyanions PSS b HA. It was clearly shown by FRAP that thevalues of lateral diffusion coefficient follow the same order.

During PEM formation one would expect an alternating overcom-pensation of charges by polycations and polyanions. In case of PEC inbulk solutions this would correspond to an overcharging of the PEC byan excess of polycations andpolyanions, respectively. Onewould expecta symmetric reduction in concentration of free counterions at both sidesof the point of equal charge due to increasing (Manning) condensationwith increasing charge. While overcharging with polyanions followsmore or less this assumption, overcharging with polycations showsmore or less no effect on the concentration of free counterions [32].This is explained by a rather homogeneous distribution of the excesspolyanion within the PEC, but by microphase separation into domainsof neutralized polyanion/polycation complexes and excess polycations.Since the charge distancewithin thepolycationmicrodomains is shorterthan the Bjerrum length the all over all counterion condensation withinthe PEC with polycation excess is higher, and the concentration of freecounterions is lower than for the PEC with polyanion excess. Thismight explain also the excess of counterions for PSS/PDADMAC multi-layers with PDADMAC as outermost layer, observed by Ghostine et al.[12]. PSS terminated PEM does not show any excess of counterions.

3. Effect of counterions

3.1. Influence of salt concentration

The complexation between polyanions and polycations can bestrongly affected by additional counterions added during preparation.Counterions screen the charges along the polyelectrolyte chains andlead to stronger coiling related to a reduced solubility in water. This inturn results in thicker PEM. The interaction between the functionalgroups of the polyelectrolyte with the counterions causes a weakeningof the complexation between polyanions and polycations.With increas-ing salt concentration the polyelectrolyte complexes become less stableand their number density decreases, which makes the polyelectrolytemultilayers less stable. From a certain salt concentration on nostable PEM can be formed anymore. This threshold salt concentration(stable–unstable) depends strongly on the used salt (see below) andthe charge density of the polyelectrolytes. For instance PSS/PDADMACmultilayers prepared with fully charged PDADMAC show an increase

in thickness and a switch from linear to exponential growth with in-creasing NaBr concentration from 0.1 M to 0.25 M during preparation(Fig. 3A). In contrast, PEM prepared with 75% charged PDADMACshows already a strongly unstable behavior, when they are preparedwith 0.25 M NaBr. The adsorbing polyelectrolyte catches the formerlyadsorbed oppositely charged polyelectrolyte and the polyanion/polycation complex is desorbed. A kind of “zigzag curve” appears inthe diagram of the thickness vs. number of deposited layers and noPEM is formed (see Fig. 3A).

The added salt ions screen the charges of the polyelectrolytechains and reduce the number density of polyion pairs within themultilayer. This facilitates the diffusion of polyelectrolyte chains [31].Therefore an exponential growth becomes more probable. Indeed, forPSS/PDADMAC [33,15] and PGA/PAH [18] a change from linear to expo-nential growth regime is reported with increasing salt concentration(see Fig. 2B). The ratio between the thicknesses of the diffusion com-partment and the restructuring compartment is assumed to increasewith increasing salt concentration. As far asweknow, there is no clear ex-perimental evidence for that to date. The results about the effect of saltconcentration indicate that exponential growth represents an intermedi-ate state in terms of stability and chain mobility between linear growingmultilayers and unstable or non-growing (“zig-zag”) multilayers.

Obviously, the addition of salt has qualitatively the same effect likeincreasing temperature during preparation as long as the solvent qual-ity is reduced. This is indicated with the arrow in Fig. 2B. Treatmentafter the preparation is finished leads to annealing for both parameters,but the effect on the multilayer thickness is different. While the PEM

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becomes thicker with increasing salt concentration [35,36] its thicknessis reduced with increasing temperature [22].

3.2. Effect of type of salt (Hofmeister series)

Discussion of ion specific effects often refers to the “Hofmeister se-ries”. Leontidis has summarized the properties of the “Hofmeisterions” in a well comprehensible way in Ref. [37]. Small ions like F− andLi+ have a relatively small polarizability, high electric fields at short dis-tances and prefer to keep their water of hydration. They have a well or-dered large hydration shell (cosmotropic ions, water structure makers).Chaotropic ions like I− and Cs+ are largewith a significant polarizability,a weak electric field and their hydration water can be easily removed(water structure breakers). Therefore chaotropic ions can interactmore strongly with the oppositely charged polyelectrolyte thancosmotropic ions, leading to a stronger screening of polyelectrolytecharges related to stronger coiling, thicker multilayer thickness andlarger roughness [38,39,33]. The thickness and roughness ofmultilayersincrease in the order: F− b Cl− b Br− for anions and Li+ b Na+ b K+.For instance an increase in attraction of larger anions by polycationswas proven by conductivity measurements in solution. This in turn re-sults in a stronger chain coiling due to a reduction in intrachain repul-sion, confirmed by reduction in solution viscosity [40]. The density ofcomplexation sites will be reducedwhich leads to a higher chainmobil-ity [31]. Ion specific effects in PEM are detected at ionic strengths above0.1 mol/l [33]. Up to 0.1 mol/l electrostatic effects seem to be dominant.Ion specific effects occur from about 3 bilayers, but not close to the sub-strate [33]. Even a transition from linear to exponential growth [38,33]or a system instability can be induced with increasing ion size [33](see also Fig. 3A). With this background one can understand the factthat K+ ions give thicker PSS/PAH multilayers than Na+ and Cs+ ions[19]. The interaction between Cs+ ions and PSS is already so strongthat the multilayers become less stable and the thickness decreases.

From this perspective the elastic modulus is expected to decreasewith increasing ion size, which was not confirmed by quartz crystal mi-crobalance measurements [41]. The reason for this unexpected resulthas not been clarified, so far.

Especially, in the presence of macromolecules it is important not tostudy the ion alone. Models are being developed that depend upon di-rect ion-macromolecule interactions as well as interactions with water

Fig. 4. Summary of different parameters affecting the water content of polyelectrolyte mcharged polyelectrolytes (left hand side) and PEM with strong polyion–ion interaction. Top: D(F− or Br−)–polyion interactions and polyanion–polycation complexation. Consequences of co

molecules in the first hydration shell of the macromolecule [42–45].Often the problem of interactions between ions and macromoleculesor aggregates is broken down to the adsorption of ions at a nonpolarsurface. Recent simulations show that polarizable and nonpolarizableforce fields give very similar results. This might indicate that the ion hy-drophobicity rather than the polarizability is the dominant parameter[46]. To our knowledge, experimentally, the effects of ion size, polariz-ability, and hydrophobicity cannot be separated since all of these pa-rameters are highly correlated.

The solvent affects the interactions between counterions and oppo-sitely charged polyelectrolytes. For instance,methanol and ethanol havea poorer solvating effect on the ions than water, which leads to a stron-ger ion–polyelectrolyte association, i.e. a stronger coiling of thepolyelectrolyte chains. This is the reason for increasingmultilayer thick-ness with increasing ethanol concentration [47] and a decreasing con-ductivity of polyelectrolyte solutions (only free ions contribute to theconductivity) [40].

3.3. Influence on swelling in water

The amount of water in PSS/PAHmultilayers is more or less insensi-tive to the change in preparative salt concentration [48]. In contrast,PSS/PDADMAC multilayers show a pronounced swelling when the saltconcentration is increased during preparation [34].

Fig. 3B shows that the water uptake of PSS/PDADMAC multilayersincreases with increasing preparative salt concentration [34]. This is ex-plained by a reduction in complexation sites related to a transition fromintrinsic to more extrinsic charge compensation. It causes higher chainmobility [31] and a stronger ability for the system to swell. It can becompared to a sponge, where larger mesh sizes between the complexa-tion sites give a larger flexibility to the system. In addition, Fig. 3Bindicates that swelling in water is affected by the type of saltadded during preparation. The total water content increases in theorder of F− b Cl− b Br−. The order in water content is explained againby an increase in extrinsic charge compensation and reduced densityin complexation sites.

Dodoo et al. separated the total amount ofwater into two types of in-corporated water: “void water” and “swelling water” [34]. Usually, theamount of water is measured by the swelling ratio between dry andswollen conditions by e.g. AFM [35], ellipsometry [49] or neutron

ultilayers. Comparison of PEM obtained via strong interactions between oppositelyifferent preparation parameters. Middle: Molecular scheme of competition between ionmpeting interactions for PEM (and PEC) properties.

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reflectometry. Analyzing the amount of water by neutron reflectometrya mismatch between the amount of water determined by the swellingratio and by the change in scattering length density (SLD) wasfound. This difference is explained by the fact that under dry conditions(vacuum) voids are formed which are filled with vacuum. In otherwords the SLD of the PEM is reduced with respect to the one of thepure polyelectrolytes. After exposure to water certain amount of the in-coming water will fill these voids without changing the PEM thicknessbut the SLD. This fraction of the total amount of water is called the“void water”. The much larger part of the water leads to an increase inthickness, and is called “swelling water”. The two types of water showopposite behavior with respect to the dependence on salt concentrationand type of salt. The “voidwater” decreaseswith increasing salt concen-tration and in the order of F− N Cl− N Br−. This means that the densityof the dry multilayer is the highest for Br− and high salt concentration.A stronger coiling of the chains due to either stronger anion/PDADMACinteraction or increasing ionic strength leads to a more compactPEM structure due to easier adaptation of the polyelectrolytechains. On the other hand the reduction of complexation sites al-lows the system a stronger swelling in water, i.e. a higher amountof swelling water. The “swelling water” dominates the total amountof water.

Guzman et al. [15] calculated the water content of PSS/PDADMACmultilayers via the volume change and found values close to the datafor the “swelling water” presented above and in Ref. [34].

In order to avoid the problem of the “hidden” void water x-ray mi-croscopy is a useful method [50]. It was carried out at capsules withPEM walls.

4. Summary, conclusion and outlook

A complex interplay between different interactions determines theformation of PEM: Polyanion–polycation, ion-oppositely chargedpolyion and ion–solvent interactions, the solubility of the polyanion–polycation complexes vs. the interactions between the complexeswith the surface. The review addressesmainly the competition betweenthe formation of complexes of oppositely charged polyions on one handand polyion–ion interactions on the other hand. As summarized in Fig. 4large ions of high polarizability and a small hydration shell can easier actwith the oppositely charged polyelectrolytes than small ions with alarge hydration shell.

The consequence is a higher extrinsic charge compensation by coun-ter ions and a lower density of complexation sites. This in turn leads to amoremobile polymermatrix related to an exponential growth with en-dothermic formation of complexes. In case of pronounced extrinsiccharge compensation the PEM is denser with fewer voids in the drystate due to stronger screening of the polyelectrolyte charges. Thisgives a higher flexibility of the polymer chains and an easier adjustmentof the chains to the environment. In contrast, these PEMs swell morestrongly in water than the ones built up in presence of small ions. Dueto the lower density of complexation sites the “mesh sizes” are largerand can take up more water.

PEM built-up in presence of larger ions is less stable. In the case ofstrong decrease in number of complexation sites due to strongpolyion–ion interactions and/or high ionic strength the PEM formationmight even become impossible. As mentioned in Section 2.3 the regionof stability of the PEM corresponds to a miscibility gap in a phase dia-gram (salt concentration vs charge of [PEM + solution]). The descrip-tion may be generalized to other parameters and mapped in onesingle diagram: Instead of the salt concentration [26,3] one should con-sider the solvent quality in terms of Flory Huggins parameter as the or-dinate of the phase diagram. The Flory Huggins parameter includestemperature-, polyelectrolyte charge, salt- and solvent effects and thesecond viral coefficient.

One of the open questions concerns the explanation of the expo-nential growth. None of the proposed models explains the fact that

the growth switches to linear after 12 bilayers irrespective of molec-ular weight of the polymers and contact time to the polyelectrolytesolution. There is for instance no experimental proof for the “forbid-den zone’, where no polyelectrolyte diffusion takes place within the“restructuring” compartment. A further question is why charge com-pensation is observed only for the polycation and never for thepolyanion. A strong argument against the “roughness” model isthat at the point of transition at 12 bilayers the thickness of thePEM is already in the μm range, i.e. much thicker than the molecularlength scales.

Another question addresses the interaction between polyelectro-lytes and ions. Ions with a large polarizability show strong interactionswith polyelectrolytes. So far, there is no hint that “like seeks like”,whichwould include that small ionswith a large hydration shell shouldstrongly interact with polyelectrolyte with highly hydrated functionalgroups.

List of abbreviationsPEM polyelectrolyte multilayerLbL Layer-by-LayerPSS polystyrene sulfonatePAH polyallylamin hydrochloridePDADMAC polydiallyldimethylammonium chlorideHA hyaluronic acidPLL poly-L-lysinePGA poly-L-glutamic acidFRAP fluorescence recovery after photobleachingITC isothermal titration calorimetryPEC polyelectrolyte complex in bulk solutionUCST upper critical solubilisation temperatureSLD scattering length density

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