Advances in Colloid and Interface Science 207 (2014) 325–331

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Advances in Colloid and Interface Science

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Historical perspective

About different types of water in swollen polyelectrolyte multilayers

Ralf Koehler a, Roland Steitz a, Regine von Klitzing b,⁎a Helmholtz–Zentrum, Lise-Meitner Campus, Hahn-Meitner-Platz 1, 14109 Berlin, Germanyb Stranski-Laboratorium für Physikalische und Theoretische Chemie, Tech-nische Universität Berlin, Strasse des 17. Juni 124, D-10623 Berlin, Germany

⁎ Corresponding author. Tel.: +49 30 31423476; fax: +E-mail address: klitzing@chem.tu-berlin.de (R. von Kli

0001-8686/$ – see front matter © 2014 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.cis.2013.12.015

a b s t r a c t

a r t i c l e i n f o

Available online 30 January 2014

Keywords:Polyeletrolyte multilayersWater contentNeutron reflectivityNeutron reflectometryVoid waterIon specific effects

The review addresses swelling of polyelectrolyte multilayers in water. Different models for the determination ofthe water content are compared. It is clearly shown that voids under dry conditions present cavities for waterwhich contribute to the water content of the multilayer in the swollen state. This so-called “void water” doesnot lead to any changes in thickness but in scattering length density during swelling. The “swelling water”leads to both changes in scattering length density and in thickness. Depending on the preparation conditionslike the type polymers, polymer charge density, ionic strength and type of salt the ratio of “void water” differsbetween 1 and 15 vol.% while the amount of “swelling water” is of several ten's of vol.%.

© 2014 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3252. Determination of water content: models and techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3263. Non-linear swelling with increasing relative humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3274. Odd–even effect due to swelling in water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3275. Water distribution across a polyelectrolyte multilayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3286. Effect of type of polyelectrolyte and polyelectrolyte charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3287. Effect of ionic strength and type of salt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3298. Conclusions and future directions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330

1. Introduction

Due to miniaturization of devices ultrathin coatings become moreand more important. One challenge of the design of thin coating filmsis that they serve to modify surface properties in an easy way. Suitablefilms can be prepared by the layer-by-layer (LbL) method wherepolyanions andpolycations are alternately adsorbed fromaqueous solu-tions [1,2]. The main features are that the thickness can be easily tunedwith nanometer precision by the numbers of deposited layers or theionic strength and the macroscopic properties can be controlled by thetypes of polyelectrolyte used for preparation. During the alternatingadsorption process complexes between oppositely charged groups ofthe polyelectrolytes are formed [3] driven by entropy due to the releaseof small counterions [4]. Themutual charge compensation of oppositelycharged polyelectrolytes is called intrinsic charge compensation, whileextrinsic charge compensation refers to charge compensation of

49 30 31426602.tzing).

ghts reserved.

polyelectrolytes by counterions. In general, a strong extrinsic chargecompensation is related to a lower density of complexation sites thanthe intrinsic one. The density of complexation sites can be easily con-trolled by preparation conditions like the type polymers, polymercharge density, ionic strength and type of salt.

Polyelectrolyte multilayer assemblies (e.g., planar films or walls ofhollow capsules) are well-known to be sensitive to external parameterssuch as ionic strength and pH [5–8] temperature [9] or humidity [10].These features make them particularly attractive for technical applica-tions like sensors and containers for drugs.

The review addresses especially the response of polyelectrolytemul-tilayer (PEM) to the exposure of different relative humidities and liquidwater. Differentmethods to measure and to calculate thewater contentare compared.

The water mobility within the multilayers was studied by NMR[11,12] and the swelling behavior mainly by scanning force microscopy(SFM) [13] and ellipsometry [5,10,14]. The disadvantage of SFM andellipsometry is that the swelling ratio can be only determined by the dif-ferences in thickness, since the multilayers are too thin for getting

326 R. Koehler et al. / Advances in Colloid and Interface Science 207 (2014) 325–331

information about e.g. the refractive index. In order to get more infor-mation about the internal structure and the water distribution neutronreflectivity is used [15–19]. Often not all information of the neutron re-flectivity data is used, and the water content is calculated only by thechange in thickness. The review shows the importance and the powerof the full analysis of neutron reflectivity data using a combination ofchange in thickness and scattering length density.

Another promising technique for measuring the total amount of in-corporated water within PEM is transmission X-ray microscopy (TXM)[20]. TXM allows studying the water content of PEM in microcapsules,which is compared to the one of PEM at planar surfaces.

2. Determination of water content: models and techniques

Polyelectrolyte multilayers are very sensitive to the exposure towater vapor with respect to changes in their structure. A common prob-lem is to compare the water content between different studies, since forlong time and even nowadays the relative humidity of the environmentis often not properly controlled. This problem is discussed in details in[21]. In the following the suffix “dry” refers to a relative humidity closeto 0%. Usually 1% r.h. can be reached. The suffix “swollen” addresses ex-periments in a certain relative humidity or against liquid water.

In order to calculate the water content within the multilayers threedifferent models are applied in literature:

(1) The simplestmodel (thicknessmodel) calculates the amount of in-corporated water ϕ due to changes in thickness before (ddry) andafter swelling in water or different relative humidities (dswollen):

ϕswell ¼dswollen−ddry

dswollen: ð1Þ

This model assumes that the whole water penetrating into thePEM leads to swelling of the polyelectrolyte multilayer. It is themost common model, since it can be applied to data obtained bymany simple lab methods like ellipsometry and SFM. In contrast,the two models described in the following, can be only appliedto neutron reflectometry (NR). In 2000, first NR measurementsat PEMs against liquid water were performed [16]. Generally, aflow cell is used, where the neutron beam is irradiated throughthe Silicon wafer side, which presents both substrate of the poly-electrolyte multilayer and the top of the flow cell.

(2) The density model takes into account that the change in scatteringlength density of the PEMNbswollen−Nbdry originates from the in-corporation of water [16,22]:

ϕ′swell ¼

Nbswollen−NbdryNbwater−Nbdry

: ð2Þ

Nbwater is the scattering length density of water which is known(D2O: 6.37 · 10−6 Å−2, H2O:−0.56 · 10−6 Å−2). If only two com-pounds would have been taken into account, i.e. water and

Fig. 1. Scheme explaining model 3: a) dry state with vacuum voids; b) “void water”, which cowhich contributes to changes both in thickness and in scattering length density.

polymer, the water content determined via the change in thick-ness or density due to exposure to water (or vapor) should bethe same. This is not the case. For instance, a polystyrene sulfonate(PSS)/polyallylamin hydrochloride (PAH) multilayer exposed toliquid water gives a ϕswell of 0.29, but a ϕswell′ of 0.46 [23].Hence, a more complex model is needed solving this paradox.

(3) The third model assumes that there are vacuum voids in the dryPEM (see Fig. 1a). During exposure to water, the volume fractionof voids is filled with water, which contributes to the change inscattering length but not to the change in PEM thickness (seeFig. 1b). This water fraction is called void water ϕvoid. The largeramount of water contributes to both swelling and change in scat-tering length density and is called swelling water,ϕswell, and corre-sponds to the relative change in thickness in Eq. (1) (see Fig. 1c).

The total amount of water ϕtotal is the sum of both water fractions[14,23]:

ϕtotal ¼ 1−xð Þ 1−ϕswellð Þ þ ϕswell ¼ ϕvoid þ ϕswell ð3Þ

where x the fraction of polymer is given by

x ¼ NbdryNbwater

−Nbswollen−ϕswellNbwater

1−ϕswellð ÞNbwaterþ 1: ð4Þ

For instance for PSS/PAH multilayers, ϕvoid is about 0.12 and there-fore the total amount of water (ϕtotal) is equal to 0.41 with ϕswell equalto 0.29 as mentioned above [23]. The two latter values are in goodagreement with measurements in saturated water vapor (100% r.h.)[15]. The total amount of water (0.41) is similar but not equal to thewater content, calculated by the change in density (0.46). There is asmall difference:

ϕtotal−ϕ′swell ¼ ϕvoid 1− Nbwater

Nbwater−Nbdry

!: ð5Þ

That means ϕtotal b ϕswell′ for exposure to D2O and ϕtotal N ϕswell′ forexposure to H2O [23]. In other words ϕswell′ depends on the isotopeand ϕtotal do not.

Taking void water into account for analysis of experimental data,gives similar amounts of incorporated water for heavy and light water.Obviously, there is no isotopic effect irrespective of the ionic strengthand type of salt used for preparation [14]. Differences in swelling againstD2O and H2O, reported in other studies [24,25] are minor.

To summarize, the total amount of water should be determined bythe third model via Eqs. (3) and (4). Beside the determination of twotypes of water and of the multilayer density in the swollen and drystate, the presented method allows also an independent calculation ofthe scattering length density of the pure polyelectrolytes.

An overview about the swelling behavior of specific PEM systemscalculated by different models is given in [21].

ntributes to changes in scattering length density but not in thickness; c) “swelling water”

Fig. 3. Ellipsometry: thickness and refractive index in dependence of the outer relativehumidity for a multilayer consisting of 9 PSS/PAH bilayers and an additional PAH layeras outermost layer. Taken from Ref. [10].

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Another technique to get access to thewater content of PEM is trans-mission X-raymicroscopy (TXM) [20]. This technique is mainly applica-ble to volume systems, i.e. hollow microcapsules, where the wallsconsist of PEM. TXM allows studying soft matter in aqueous environ-ment with a high resolution of 20 nm. In the so-called “water window”

(E= 284–543 eV) a density contrast is obtained. Temperature sensitivemicrocapsules with walls consisting of (PAH/PSS)7/PAH, irreversiblyshrink upon heating with a simultaneous increase of the shell thickness[20] (Fig. 2, top row). After imaging capsules heated at different temper-atures (20, 80 and 99 °C), transmission profiles were extracted alongthe diameters of the shells (Fig. 2, bottom row). Before heating, thePEM in the microcapsule wall is highly hydrated (70% water). It isworth to note that thewater content of the capsulewalls ismuch higherthan of the same material prepared on planar interfaces (42–56%water). This indicates a more loosely packed structure in the capsulewall than at the planar substrate and a strong effect of the solid sub-strate on the PEM structure. Another difference between PEM inmicro-capsule walls and at planar surfaces is the response to heating. Whilethe PEM in a microcapsule wall looses about 40% of the water, the lossof water of PEM at planar substrates is in the order of a few % [9].

3. Non-linear swelling with increasing relative humidity

In general PSS/PAH multilayers show a nonlinear increase in swell-ing behavior with increasing water vapor pressure [24,10,22]. Thestrongest increase in swelling occurs between 70 and 100% r.h., whichleads to a pronounced decrease in refractive index. The effect of relativehumidity on both thickness and refractive index is shown in Fig. 3.

The non-linear increase in swelling might be explained by an expo-nential increase in ion mobility found by conductivity measurements[26].

From the humidity dependence of swelling parameters the Flory–Huggins parameter χ, was determined. With χ = 0.91 for PSS/PAHmultilayers [24] and χ = 0.85 for PSS/polydiallyldimethylammoniumchloride (PDADMAC) multilayers [22], water is a bad solvent for bothmultilayer types (remember: χ = 0.5 for θ solvent). That is a hint for

Fig. 2. TXM micrographs of (PAH/PSS)7/PAH capsules (a) at 20 °C before any heat treatment atransmittance profiles are shown below in the micrographs (d–f). Taken from [20].

strong intrinsic charge compensation leading to the formation of ratherhydrophobic complexes.

4. Odd–even effect due to swelling in water

Since more than 15 years the so-called “odd–even effect” isdescribed in literature [27]. Itmeans that thewater content of a PEMde-pends on the composition of the outermost layer. It has nothing to dowith the loss in material which is claimed for some unstable systems[28,29].

At high r.h. (98% r.h.) PSS/PAH multilayers with PSS as outermostlayer are thicker than the ones with PAH in the terminated layer [10].This leads to a kind of zig–zag increase in thickness with increasingnumber of deposited layers. In contrast, the respective dry PEMs(3% r.h.) show a monotoneous increase in thickness, which meansthat no material is lost during the adsorption of PAH. The resulting

nd after treatment at (b) 80 °C and (c) 99 °C for 20 min. The corresponding fitted radial

328 R. Koehler et al. / Advances in Colloid and Interface Science 207 (2014) 325–331

oscillation in water content between PSS and PAH terminated PEM isvisible in Fig. 4a.

Schwarz and Schönhoff found an increased mobility and/or watercontent in the outer shell of PSS terminated PEM deposited onto silicaparticles by using nuclear magnetic resonance (NMR) [30]. Later thisissue was addressed by neutron reflectometry [31–33]. Recently, thehigher amount of water in PSS terminated PEM was supported by neu-tron reflectometrymeasurements (28% for PSS and 21% for PAH). Underpressure water was pressed out but PSS terminated PEM still preservedmore water than PAH terminated ones (16% for PSS and 10% for PAH)[34].

An explanation for the odd–even effect could be that in case of PSS asoutermost layer a potential with an exponential decay towards theinner part of the multilayer could be monitored and not for PAH termi-nated PEM [35]. Klitzing and Möhwald considered the PSS surface asseparating interface between two dielectrica: the aqueous solutionand the PEM. Both dielectrica have different dielectric permittivitiesand different ionic strengths. This leads to different decay lengths ofthe PSS surface potential into the aqueous solution and into the PEM.The potential within the multilayer was scanned via a pH sensitivedye. The negative potential (and higher counter ion concentration)could bring more water into the film. The decay in potential wouldalso explain, why the odd–even effect decreases with increasing num-ber of layers [35,36]. So far, it could not be clarified, why the potentialin case of PAH terminatedmultilayers remains constant within themul-tilayer [35].

Also the contact angle shows an odd–even effect where thepolyanion terminated multilayers show a lower contact angle than thepolycation terminated ones as observed for PSS/PAH, PSS/PDADMACand polyacrylic acid (PAA)/PAH multilayers [10,37,38]. This wouldlead to a stronger attraction of water and stronger swelling for thepolyanion terminated multilayer as observed e.g. for PSS/PAH multi-layers. On the other hand PDADMAC as outermost layer gives a muchhigher contact angle than PAH and the oscillation in contact angle be-tween PSS and PDADMAC terminated layer is much more pronouncedthan for a PSS/PAH system. However no odd–even effect is observedin swelling for the PSS/PDADMAC layers.

According to our knowledge the phenomenon of the odd/even effectin swelling is not fully understood.

5. Water distribution across a polyelectrolyte multilayer

In the first papers dealing with the water content of PEM, an almosthomogeneous water distribution was claimed [39,40,15]. In the mean-while an inhomogeneous distribution of water across the PEM is statedin many papers.

Fig. 4.Water content of PSS/PAHmultilayers; Left: Amount of swellingwater (calculated by Eq. (thickness ddry is measured at 3% r.h. and dswollen at 98% r.h.. Right: Scheme of the polyelectrolyt

A hint for inhomogeneous swelling of PEM vertical to the surface isgiven by neutron reflectometry measurements using a super latticestructure of deuterated and non-deuterated layers, whichwas inventedby Lösche and coworkers [39]. For a homogeneous swollen PEM onewould expect that the total thickness d of a PEM is given by the distancet between two deuterated layers (lattice unit) times the repetitionnumber n of the lattice unit:

d ¼ n � t ¼ 2πΔq

¼ n � 2πΔQ

: ð6Þ

Δq corresponds to the distance between two Kiessig fringes, comingfrom the superposition of reflected beams at the outer PEM interfaces(solid/PEM and PEM/water). ΔQ refers to the position of the Bragg re-flexes due to internal reflections at the deuterated layers. In contrast,it is claimed that the ratio between the total thickness and the latticeunit is incommensurable [21].

Just by measuring the change in PEM thickness due to swellingshows a strong dependency of number of deposited layers. For 3, 6,and 13 PSS/PAH bilayers the water content ϕswell is reduced from 50%over 30% to 23%, respectively [10] as shown in Fig. 4a. This gives a stronghint for a higher polyelectrolyte density close to the substrate decreas-ing towards the PEM/water interface as shown in Fig. 4b. A thin PEMconsists more or less completely of loosely packed polyelectrolytechains and becomes denser by adsorption of more polyelectrolytelayers, where the outer layers remain loosely packed. With increasingnumber of adsorbed layers the ratio of the loosely packed compartmentbecomes less and less important with respect to the whole PEM thick-ness leading to a decrease of the all-over-all water content. A structurebased,multi-zone-model of swelling comes into play (Fig. 4b). Inhomo-geneous water distribution in PEM was also shown by Tanchak et al.[17]. For PAA/PAH multilayers, they found a linear decay of waterfrom the outer PEM/water interface to the solid/PEM interface: Theouter layers contain 54% of water, the water content in the core of thePEM lays between 30 and 45%, but the inner, interfacial zone close tothe substrate stores ≈ 18% water, only.

Recently Prescott and coworkers observed a kind of odd–even effectfor the distribution of water within the PEM [34]. In case of PAH termi-nated PEM, the amount of water close to the silicon substrate wasreducedwhile in case of PEMwith PSS as outermost layer thewater dis-tribution was more or less uniform.

6. Effect of type of polyelectrolyte and polyelectrolyte charge

First let's startwith themost commonpolyanion/polycation combina-tions used for polyelectrolyte multilayers: PSS/PAH and PSS/PDADMAC

1)) in dependence of the number of PSS and PAH layers deposited onto a Siliconwafer. Thee multilayer swelling inhomogeneously in water. Taken from Ref. [10].

329R. Koehler et al. / Advances in Colloid and Interface Science 207 (2014) 325–331

multilayers. The studies of the multilayers containing strong polyelectro-lytes PSS and PDADMAC against vacuum show no effect of the former ex-posure to D2O or H2O on the scattering curve. This leads to the conclusionthat after preparation in H2O no hydration water is exchanged [23]. Theother explanation for coincident neutron reflectivity curves is that nohydrationwater is kept after evacuation. Obviously, the amount of hydra-tion water, i.e. water which does not leave the PEM even at 0% r.h., de-pends on different preparation conditions. For instance in [18] 0% r.h. isobtained by exposure to N2 and hydration water is claimed to remainin the PEM. In contrast, in [23] 0% r.h. is received by creating a vacuum(10−6–10−5 bar) and no hydration water could be identified as men-tioned above.

If PDADMAC is replaced by aweak polyelectrolyte like PAH, the scat-tering curves against vacuum after former exposure to D2O or H2O arenot identical anymore. This is explained by an average exchange ofabout one proton per functional group at the nitrogen of PAH [23]. Theexchange takes place against a saturated vapor atmosphere or water,but not against ambient lab conditions (about 44% r.h.) [23].

According to the model including void water the dry PSS/PDADMACmultilayer is denser than the PSS/PAHmultilayer. ϕvoid is about 0.12 forPSS/PAH but only 0.05 for PSS/PDADMAC. The total water content isabout 0.4–0.45 for both types of multilayers but PSS/PAH multilayerswells less than the PSS/PDADMAC multilayer.

The difference in swelling between PSS/PAH and PSS/PDADMAClayers can be explained by different chain mobility within the PEM.Nazaran et al. showed a clear relationship between the chain mobilitymeasured by Fluorescence Recovery after Photobleaching (FRAP) andthe charge density along the polyelectrolyte chains [41]. PAHhas a nom-inal charge distance of 2.5 Å while PDADMAC has a charge distance ofabout 3.6 Å leading to a lower density of complexation sites with PSS.It is worth to note that not all polyelectrolyte charges are accessibledue to counterion condensation. According to the concept of Manningcondensation onewould expect no effect of polyelectrolyte charge den-sity on the PEM structure below a charge distance of the Bjerrum length(about 7.1 Å in water at room temperature) due to counterion conden-sation. On the other hand, counterion condensation in PEM is not asstrong as predicted via Manning condensation: For PSS/PDADMACmultilayers changing the PDADMAC charge from 50% (charge distance(CD) = 7.2 Å) via 75% (CD = 4.8 Å) to 100% (CD = 3.6 Å) gives a pro-nounced effect on PEM thickness [42]. A reason for the strong deviationfromManning concept might be the high local electric (Coulomb) fieldcreated by the charged groups of the polyelectrolytes, which actsisotropically in all 3 dimensions and cannot be completely screened bycounterions on a nanometer length scale. In addition, gain in entropydue to the release of counterions is a strong driving force for the forma-tion of complexes between the ionic groups of oppositely charged poly-electrolytes [4].

Fig. 5. Effect of ionic strength and type of salt on the water content: themodel of “void and swetweenwaterwhich does not contribute to swelling (ϕvoid) (lower part of (b)) andwater whichsolutions of varying ionic strength and type of salt. Adapted from Ref. [14].

A reduction in density of complexation sites in PEM e.g. due to re-duction of the polyelectrolyte charge density leads to higher mobility/flexibility of PSS/PDADMAC multilayers which causes less/smallervoids in the dried state. The PEM structure can be described as a poly-mer network where the complexation sites between polyanions andpolycations present the cross-linkers. A reduced density of complexescauses larger mesh sizes in the polymer matrix and a stronger swellingin water can occur. This leads for instance to an increase in water con-tent of P(DADMAC–stat–NMVA) multilayers with decreasing chargedensity [43,21].

PEMs containing hyaluronic acid (HA) are known to swell strongly(e.g. [44]). The nominal charge distance of HA is larger (10 Å) than ofPDADMAC (3.6 Å). In addition the strong ability of HA for hydrogenbinding might give a further explanation for strong swelling.

Steitz et al. showed that water content and volume change is not amaterial constant but depends on preparation conditions as salt con-tent, and sample history. The dried and re-swollen PSS/PAH multilayercontained about 42 vol.% of water and shrinks by about 10% upon dry-ing. A PSS/PAH multilayer which was adsorbed in-situ and has neverbeen dried after preparation contains 56% water and shrinks by 32%upon drying. It had been shown earlier that drying has an enormous im-pact on the structure of PEM [45].

7. Effect of ionic strength and type of salt

The amount of water in PSS/PAHmultilayers is more or less insensi-tive to the change in preparative salt concentration. In contrast, PSS/PDADMACmultilayers show a pronounced swelling when the salt con-centration is increased during preparation.

Fig. 5a shows that thewater uptake of PSS/PDADMACmultilayers in-creases with increasing preparative salt concentration [14]. This is ex-plained by a reduction in complexation sites related to a transitionfrom intrinsic to more extrinsic charge compensation. It causes higherchain mobility [41].

Guzman et al. investigated also PSS/PDADMAC multilayers. Thewater content calculated via the volume change (model 1, without cor-rection for the void water) was found to be dependent of the salt con-centration upon preparation: ϕswell = 0.3 for 0.1 M NaCl and 0.6 for0.5 M NaCl [46]. These values are close to the data for the swellingwater ϕswell presented by Doodo et al. [14].

In addition, Fig. 5a indicates an effect of the type of salt addedduring preparation. The total water content ϕtotal increases in theorder F− b Cl− b Br−. These ions present a part of the Hofmeister series.The order inwater content is explained again by an increase in extrinsiccharge compensation leading to thicker and rougher multilayers [29]and a higher mobility [41]. It is assumed that the larger anions with

lling water” delivers (a) the total amount of incorporated water (ϕtotal) and (b) differs be-does (ϕswell) (upper part of (b)). PEM of 6 bilayers PSS/PDADMAC prepared in different salt

Fig. 6. Summary of different parameters affecting thewater content of polyelectrolytemultilayers. The− and+ in themolecular images are polyelectrolyte charges and the circles with−and + are the counterions coming either from polyelectrolytes or from added salt.

330 R. Koehler et al. / Advances in Colloid and Interface Science 207 (2014) 325–331

smaller hydration shells can better interact with the polycations thansmaller anions with a larger hydration shell do.

Dodoo et al. quantified the two types of incorporated water: “voidwater” and “swelling water” in dependence of ionic strength and typeof salt. The results are summarized in Fig. 5b. The void water decreaseswith increasing salt concentration and in the order of F− N Cl− N Br−

(lower part of Fig. 5b). This means that the density of the drymultilayerincreases in the order of F− b Cl− b Br− andwith increasing salt concen-tration. A stronger coiling of the chains due to either stronger anion/PDADMAC interaction or increasing ionic strength leads to a more com-pact structure. The decreasing density of complex sites due to extrinsiccharge compensation makes it easier for the system to rearrange and toadapt, leading to higher density. On the other hand the reduction ofcomplex sites allows the system a stronger swelling in water, i.e. ahigher amount of swelling water (upper part of Fig. 5b).

8. Conclusions and future directions

The review addresses different strategies for calculating the watercontent. It is stated clearly that the general method of measuring theswelling behavior neglect a part of water, the “void water” which doesnot contribute to the swelling but to a change in scattering length den-sity. Beside the fact that adding “void water” delivers the total amountof water of a multilayer, considering “void water” gives deeper insightinto the structure of the polyelectrolyte multilayers.

Asmentioned in a former review [47] changing different preparationconditions can lead to similar effects on the polyelectrolyte multilayerstructure.

Fig. 6 gives an overview about some preparation parameters affectingthe water content. Decreasing the degree of polymer charge, increasingthe ionic strength and using salt ions with a high polarizability duringmultilayer preparation leads to a low density of complexation sites relat-ed to an extrinsic charge compensation and a high mobility of polyelec-trolyte chains [47]. The present review shows that this in turn leads to adenser packing, i.e. low amount of voids, in the dry state and a strongability for swelling in water like a sponge (high ϕswell). In general thetotal amount of water in these liquified PEM is higher than in the moreglassy multilayers (left hand side in Fig. 6) prepared with low ionicstrength and with salt ions with a low polarizability.

The review shows clearly how important it is to consider both con-tributions: “swelling water” and “void water”. Beyond polyelectrolytemultilayers this route of determination of the water content presents

a high impact for all hydrophilic coatings exposed to water. This mightbe also important for other polyelectrolyte architectures like brushesand hydrogels deposited at surfaces.

In terms of applications the amount of water and especially the loca-tion of water is important for manymetal/polymer connections as theyoccur in hybrid systems like in ultralight materials for cars or for theproduction of compact electronics. Often thewater induces some corro-sion which should be avoided, of course. Therefore, with respect toquestions of both basic research and technology the determination ofthe location of water is still of interest and challenging.

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