The Influence of Secondary Interactions during the Formation of

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J. Phys. Chem. B 2007, 111, 8426-8434

The Influence of Secondary Interactions during the Formation of Polyelectrolyte Multilayers: Layer Thickness, Bound Water and Layer Interpenetration† Manesh Gopinadhan,‡ Oxana Ivanova,‡ Heiko Ahrens,‡ Jens-Uwe Gu1 nther,‡ Roland Steitz,§ and Christiane A. Helm*,‡ Institut fu¨r Physik, Ernst-Moritz-Arndt UniVersita¨t, Felix-Hausdorff-Straβe 6, 17487 Greifswald, Germany, and Hahn-Meitner Institut, Glienicker Straβe 100, D-14109 Berlin, Germany ReceiVed: NoVember 9, 2006; In Final Form: February 23, 2007

With X-ray and neutron reflectivity, the structure and composition of polyelectrolyte multilayers from poly(allyl amine) (PAH) and poly(styrene sulfonate) (PSS) are studied as function of preparation conditions (salt concentration and solution temperature, T). The onset of a temperature effect occurs at 0.05 M NaCl (Debye length ∼ 1 nm). At 1 M salt, the film thickness increases by a factor of 3 on heating the deposition solution from 5 to 60 °C. The PAH/PSS bilayer thickness is independent of the kind of salt (NaCl or KCl), yet its composition is different (more bound water for NaCl). At low T, the internal roughness is 33% of the bilayer thickness; it increases to 60% at high T. The roughening is accompanied by a total loss of bound water. At which temperature the roughening starts is a function of the kind of salt (50 °C for NaCl and 35 °C for KCl). The strong temperature dependence and the eventual loss of bound water molecules may be attributed to the hydrophobic force; however, there is an isotope effect, since the loss of bound water is less pronounced in the deuterated layers.

Introduction In the past decade, the field of nanostructured material formation has progressed significantly. Polyelectrolyte multilayers1 formed by sequential adsorption of alternating charged polyelectrolytes have been instrumental in this progress. During each dip of the substrate, a small amount of oppositely charged polyelectrolyte is adsorbed; the surface charge is reversed; and thus, the adsorption of an oppositely charged polyion is possible again.2,3 Obviously, electrostatic force dominates the adsorption process. Therefore, polyelectrolyte multilayers form twodimensionally stratified layers that are growing step-by-step into the third dimension. With this technique, layered polymeric multicomposites with nanometer control can be built, even on rough or bent surfaces.4 The temperature effect, however, shows that nonelectrostatic interactions are also important for the formation of polyelectrolyte multilayers. If the films are built from 1 M salt solution, the film thickness increases by a factor of 2-10 if the temperature of the deposition solution is raised from 5 to 50 °C.10-12 How large the effect is, depends on the chemical nature of polymers and salt; however, if the polyelectrolyte multilayers are built from pure water, no temperature dependence of the film thickness is found.11 A necessary condition for the acting of temperature-dependent secondary forces seems to be that range and amplitude of the electrostatic force are shielded by many ions, which are provided by a high salt concentration in the deposition solution.13,14 † Part of the special issue “International Symposium on Polyelectrolytes (2006)”. * To whom correspondence should be addressed. Phone: 0049 (0)3834 864710. Fax: 0049 (0)3834 864712. E-mail: helm@ physik.uni-greifswald.de. ‡ Ernst-Moritz-Arndt Universita ¨ t. § Hahn-Meitner Institut.

SCHEME 1: Structure Formulas of Poly(styrene sulfonate), PSS, and Poly(allyamine hydrochloride), PAH

For polyelectrolyte multilayers, the effect of salt concentration (NaCl) in the preparation solution has been explored systematically, especially for the polyanion/polycation pair poly(styrene sulfonate)/poly(allylamine hydrochloride) (PSS/PAH, cf Scheme 1),3,19 yet only at room temperature. For that system, the multilayer thickness increases linearly with the number of adsorption cycles. It is possible to increase dBL, the thickness per deposited polyelectrolyte bilayer, by one order of magnitude, from 6.5 Å (salt free) to 66 Å (3 M NaCl); however, the layer interpenetration as evidenced by neutron reflectivity remains constant. The interfacial roughness, σ, between adjacent polyelectrolyte layers increases almost linearly with the thickness, dBL, of a polyelectrolyte bilayer pair, σ ≈ 0.4dBL. This partial interpenetration shows that the adsorbing polyelectrolyte does not mix with the multilayer film,12,20 but simply adsorbs. With PSS/PAH multilayers from 1 M KCl solution prepared at different temperatures, one finds the same internal roughness, σ ≈ 0.4dBL, at low temperatures, yet when the PSS precipitation temperature in solution (55 °C) is approached, the internal roughness increases dramatically.21 It is not clear if this feature is correlated with the precipitation temperature or if it is a general temperature effect. Furthermore, little is known about the respective influence of temperature and salt concentration in the deposition solution on the formation of polyelectrolyte multilayers. Both parameters need to be varied to find out at which salt concentration the

10.1021/jp067402z CCC: $37.00 © 2007 American Chemical Society Published on Web 04/27/2007

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J. Phys. Chem. B, Vol. 111, No. 29, 2007 8427

SCHEME 2: Schematic of Architecture and Resulting Scattering Length Density Profile of the Polyelectrolyte Multilayer PEI{(PSS/PAH)3PSSd/PAH}4(PSS/PAH)3a

a The scattering length density profile is shown assuming zero internal roughness (left) and including roughness (right), the latter together with the parameter assignment according to the kinematic model of the polyelectrolyte multilayer.

temperature effect starts. For these first experiments, the film thickness is determined with X-ray reflectivity. In a second series of experiments, the preparation temperature and the nature of salt are varied. To quantify layer interpenetration and water content, neutron reflectivity measurements are performed at 0% r.h. With selectively deuterated polyelectrolyte layers, a superstructure is formed (cf. Scheme 2), leading to Bragg peaks in the reflectivity measurements. In the past, we prepared films showing one Bragg peak only.21 In that case, four independent parameters can be extracted reliably from reflectivity data: the scattering length density (i) of the deuterated PSSd-layer (from the height of the superstructure peak) and (ii) of the protonated layers (from the amplitude of the Kiessig oscillations); (iii) the thickness, dBL, of a polyelectrolyte bilayer pair within a repeat unit (from the position of the superstructure peak); and (iv) the total film thickness (from the periodicity of the Kiessig oscillations). With only one Bragg peak, the internal roughness, σ, between adjacent layers is not measured directly, but calculated from the scattering length density profile. This approach was found to be valid when the salt concentration in the solution is varied.7,22 With two Bragg peaks as obtained in the experiments described below, it is possible to determine the internal roughness directly from the ratio between the peak heights. The data are good enough to make consistency checks of the scattering length density profile that describes the various protonated and deuterated layers that form the thin film. For instance, if a layer swells due to incorporation of water, its thickness increase and its change in scattering length density should lead to the same volume fraction of incorporated water. Otherwise, the model needs an overhaul. The paper is organized as follows: After describing the experimental setup and the theoretical background of intermolecular interactions and the neutron reflectivity, the results and the discussion will follow. The details of the integration and the tables showing all fitted parameters (cf. Scheme 2) can be found in the Supporting Information, which is available free of charge via the Internet. Materials and Methods The polyelectrolytes used are poly(ethylene imine), PEI; poly(allylamine hydrochloride), PAH; and poly(styrene sulfonate),

PSS. Most silicon wafers are a generous gift from Wacker, Burghausen; some are bought from Matthias Schmehl, Rostock, Germany. The wafers are made hydrophilic by RCA cleaning. The branched polycation PEI (75 kDa, Aldrich) has proved to be an efficient first layer for charge reversal. The polycation PAH (70 kDa) is also from Aldrich, whereas the polyanion PSS (65 kDa) and deuterated PSSd (83.7 kDa) are from Polymer Standard Service, Mainz, Germany. Ultrapure water is from Millipore (Milli-Q); NaCl and KCl, (99.9%) from Merck (Darmstadt, Germany). P2O5 (Merck, Darmstadt) is used to obtain 0% relative humidity in the sample chamber for the neutron measurements. For each adsorption step during polyelectrolyte multilayer build-up, the substrate is immersed in a 0.003 mol/L (monomer) polyelectrolyte and salt solution for 30 min and then washed in three beakers of pure water (soaking time, 1 min each). All beakers are kept at the same temperature, which is adjusted externally by a thermostat (Haake, Germany). The films are prepared by a robot (Riegler & Kirstein, Berlin, Germany). Small-angle X-ray reflectometry experiments are performed with a Seifert XRD 3003 TT diffractometer (Seifert, Germany) using Cu KR radiation (wavelength λ ) 1.54 Å). The neutron setup is instrument V6 at Hahn-Meitner Institut (Berlin, Germany) (λ ) 4.66 Å). For neutron reflectivity measurements, polyelectrolyte multilayers with selectively deuterated layers are prepared. The film architecture is PEI(PSS/PAH)3{PSSd/(PAH/ PSS)3PAH}3PSSd. There are always four deuterated polyelectrolyte bilayers, PAH/PSSd. Mostly, three additional nondeuterated polyelectrolyte bilayers are added at the film/air interface, to obtain PEI{(PSS/PAH)3PSSd/PAH}4(PSS/PAH)3. The films were characterized during three different beam times at HahnMeitner Institut. The films were investigated with neutrons 1-3 weeks after preparation. In the reflectivity experiments, the deviation δ ) 1 - n of the refractive index, n, from 1 depends linearly on a material constant, which is directly related to the constituting molecules. For X-rays, this material constant is the electron density, Fe (δ ) λ2Fero/2π, with Thomson radius ro). For neutron optics, the scattering length density, F, is the relevant parameter (δ ) λ2F/2π). In both cases, n deviates only by ≈10-5 from 1. Therefore, approximations are possible, and the measured reflectivity, R,

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may be described as the Fresnel reflectivity, RF, of an infinitely sharp interface modulated by interference effects from a thin surface layer.23 Above about two critical angles of total external reflection (Rc ) x2δ) the reflectivity is given by the kinematic approximation

|

R 1 ) RF Fsub

∫F′(z)eiQ z dz| z

2

(1)

Fsub is the electron density (or scattering length density, respectively) of the substrate, F′(z) is the gradient of the electron density (or scattering length density) along the surface normal, and Qz ) 4π/λ sin R is the wave vector transfer normal to the surface (R is the incident angle that is equal to the exit angle in a specular scattering geometry). To quantify the molecular parameters, the exact, optical matrix formalism (dynamical approach) is used.24 The surface layer is parametrized as consisting of different slabs (each with a density and a thickness, as well as a roughness parameter). In all cases, the simulated reflectivity is convoluted with the angular divergence of the respective spectrometer (X-ray, 0.012°; neutrons, 0.017°). Theoretical Background: Intermolecular Interactions The importance of the electrostatic force on polyelectrolyte multilayer formation is obvious. The range of the electrostatic interaction is measured by the Debye length, 1/κ, and is a function of the ionic strength, 1/κ ) 0.3 nm/xI (I is the concentration of monovalent salt given in M).5 The Debye length at 1 M salt is 0.3 nm, a value that corresponds to the diameter of a water molecule, suggesting that at high ion concentrations, it may be necessary to revisit some assumption of electrostatic theory (i.e. “point charges” immersed in a “polarizable isotropic continuum”). In addition to the range, the amplitude of the electrostatic interaction also decreases with increasing salt concentration. Pure electrostatics of a stiff polyelectrolyte predicts a flat adsorption layer,6 yet experimentally increasing layer thickness is found on increasing the salt concentration.7 The parameters determining adsorption are very different for neutral and charged polymers. For neutral polymers, the important parameters are polymer length, monomer/interface adsorption energy, and the “polymer interaction parameter χ” according to the language of polymer theory.8,9 χ is calculated from the (negative) inter- and intramolecular contact energies, , between solvent molecules (s) and monomeric segments (p) of the polymer chain, χ ) sp - 1/2(ss + pp). All the interactions within solutions of neutral polymers are presumed to be short-ranged, that is, on the order of a molecular diameter. For χ < 0.5 (good solvent conditions), entropy dominates, and the radius of gyration, RG, is proportional to N0.6. For large χ (i.e., χ > 0.5), polymer-solvent contact is unfavorable; the chain contracts; and on further increase of χ, it precipitates. From neutral polymers, it is known that in the case of an attractive force between surface and monomeric segments, a higher polymer surface coverage is achieved on increase of χ.6 A likely short-ranged interaction force between polyelectrolytes would be the hydrophobic force acting between the apolar backbones of the chains, since the signature of hydrophobicity is its temperature dependence.15,16 In cold water, the water molecules surrounding an apolar solute form good hydrogen bonds (low enthalpy) in structured cages (low entropy) that avoid “wasting” hydrogen bonds. In hot water, the shell “melts”, and most first-shell water conformations are accessible (higher entropy and higher enthalpy). Therefore, for small nonpolar

molecules dissolved in water, the molar free energy, ∆µ°, is expressed in terms of enthalpic and entropic components, ∆µ° ) ∆h° - T∆s°, where ∆h° is the molar enthalpy and ∆s° is the molar entropy. Of course, this equation is always valid, but for most solute/solvent systems, neither the molar enthalpy nor the entropy change on an increase in temperature; therefore, the equation is not particularly helpful. With small nonpolar molecules dissolved in water, the increase of molar enthalpy and entropy occur simultaneously; thus, the molar free energy, ∆µ°, is fairly constant on temperature variation, even though dramatic changes occur on a molecular level. When the FloryHuggins parameter χ ) sp - 1/2(ss + pp) is derived to calculate the inner energy of mixing, it is neglected that each p monomer is not fully accessible for making noncovalent contacts, because it is covalently connected to two other p monomers in the chain. However, these covalent contacts will affect the shape and energy of the first-shell water cages, with their strong orientation-dependent hydrogen bonds. There are other secondary interactions that may contribute to the increased polyelectrolyte coverage at elevated temperatures. In addition, apolar compounds dissolve in salt solutions to different degrees, depending on the type of salt. In 1888, F. Hofmeister discovered that different salts have different propensities to precipitate or dissolve proteins in solution.17 Additionally, van der Waals attractions need to be considered; furthermore, the deuteration may affect the hydrogen bonds, causing an isotope effect.18 Theoretical Background: Bragg Peaks in Neutron Reflectometry Although the dynamic optical matrix formalism is exact, the results are not easy to understand. The main advantage of the kinematic model given in eq 1 is it simplicity. With analytical equations, it is possible to find correlations between a property of the measured reflectivity (peak height, periodicity of Kiessig oscillations, ...) and a multilayer property (water content, layer thickness, ...), as derived in the Supporting Information. The main idea is to describe the deuterated layer not as a slab of width 2∆ and two roughness parameters σ, but as a Gaussian with width ξ, (an approximation that is justified if ∆ e σ. Actually, in the case ∆ , σ, one obtains ξ ) σ, as shown in the Supporting Information). All films consist of a base layer (thickness lbase) and four repeat units (thickness ld which corresponds to the separation between two deuterated layers; cf. Scheme 2). Some films have additional bilayers on top of the repeat units, at the film/air interface (thickness ltop). Therefore, the total thickness lh of the films investigated in this paper is given by

lh ) lbase + (N - 1)ld + ltop

(2)

with N ) 4 the number of repeat units in the films of this work. Due to the pronounced layer interdigitation, without the deuterated layers, the film would be homogeneous, with the average scattering length density (for the contrast due to deuteration, see Table 1).

Fh )

bPSS + bPAH + nbH2O VPSS + VPAH + nVH2O

(3)

Fh is derived from the volume, V, and the scattering length, b, of the water molecules (H2O) and the monomeric segments (PAH and PSS, respectively), with n as the number of water

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J. Phys. Chem. B, Vol. 111, No. 29, 2007 8429

TABLE 1: Contrast in the Thin Film Due to Deuteration scattering electron scattering length no. of density density length electrons Fe (Å-3) bN (10-5 Å) (10-6 Å-2)

molecule

vol (Å3)

H2O D2O PSS PSSd PAH Si PAH + PSS PAH + PSSd

29.98 29.98 200 200 97 20.016 297 297

10 10 95 95 33 14 128 128

-1.675 19.145 47.203 120.07 -0.614 4.149 119.46 46.589

0.334 0.334 0.475 0.475 0.34 0.699 0.431 0.431

-0.559 6.386 2.36 6.004 -0.063 2.073 4.022 1.569

molecules per PAH/PSS monomer pair. Therefore, the scattering length density of the multilayer film can be described as

F(z) ) Fh(z) + Fd(z)

(4)

Fh(z) describes a protonated, homogeneous layer, and Fd(z), the additional scattering length density due to the deuterated layers. Every fourth layer in the repeat unit is a deuterated PSSd/ PAH layer. This layer is described by a Gaussian with intensity Fd and width ξ. Its thickness is 1/4 of the thickness of the repeat unit, ld, assuming that each polyanion/polycation monomer is hydrated by the same number of water molecules (independent of degree of deuteration). The additional scattering length density of the deuterated layer is

Fd )

bPSSd - bPSS V 4x2πξ PSS + VPAH + nVH2O ld

RBragg(Qz) 1 ) | F′d(z)eiQzz dz|2 ) RF(Qz) Fsub2

∫

2πe-Qz ξ

2 2

Fd2

Fsub

2 2 2Qz ξ

sin2(NQzld/2) 2

Qzmf2πm/ld

98

sin (Qzld/2) 8π3e-4π ξ /ld m 2 2

Fd2

ξ2

Fsub

ld2

2 2

2 2m

N2 (8)

Because of the definition of the wavevector, Qz ) 4π/λ sin R, the maxima occur at mλ ) 2ld sin R. This relationship resembles the Bragg condition; therefore, the maxima caused by the superstructure are often referred to as Bragg peaks. From the difference between the positions of the first and second peak, ld can be calculated,

∆Qz ) 2π/ld

(9)

the periodicity of the superstructure peaks. Since in our films, ld is less than 25% of lh, the superstructure peaks exhibit a larger periodicity than the Kiessig fringes in reciprocal space. Note that the intensity increases with the square of the number of repeat units (i.e., deuterated layers). Indeed, in our measurements, the normalized intensity of the first Bragg peak is between 7 and 25 (cf. Figures 3, 4). Since the superstructure peaks have so much more intensity than the Kiessig fringes caused by the homogeneous layer (cf. eq 8), the normalized reflectivity is plotted on a logarithmic scale (cf. Figure 3). On a linear scale, the intensity of the superstructure peaks is readily

(5)

bPSSd is the scattering length of a deuterated PSS monomer, and ξ is related to the internal roughness σ by ξ = σ (derivation in the Supporting Information). If the film is homogeneous (i.e., consists only of protonated slabs), it can be described by the thickness, lh; the roughness parameters, σair and σsub for the film/air and film/substrate interfaces, respectively; and the corresponding scattering length densities. Then the reflected intensity is given by

RKiessig(Qz)

)

RF(Qz) Fh2

1

∫

| F′h(z)eiQzz dz|2 ) 2

Fsub

(Fh - Fsub)2 -Qz2σsub2 e + Fsub2

(Fh - Fsub)Fh -Qz2(σsub2+σair2)/2 e cos(Qzlh) (6) Fsub2

e-Qz σair - 2 2

Fsub

2

2

The last term of this equation is important; it describes the socalled Kiessig oscillations. The film thickness can be deduced from the separation ∆Qz of two neighboring minima of the interference fringes according to

∆Qz ) 2π/lh

(7)

the periodicity of the Kiessig fringes. In contrast, if only the contribution of the deuterated bilayers is considered (i.e., Fh(z) ) 0) and its scattering length density is given by Fd(z), the reflectivity is a multiple slit diffraction pattern (making it similar to a diffraction grating that consists of a large number of equally spaced parallel slits).

Figure 1. Top: X-ray reflectivity curves normalized relative to the Fresnel reflectivity of polyelectrolyte multilayers prepared at the salt concentrations and temperatures indicated. For clarity, the curves are shifted vertically. Bottom: Thickness dBL per polyelectrolyte bilayer pair as a function of the salt concentration in solution for the temperatures indicated. The architecture of all films is PEI(PSS/ PAH)5PSS.

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Figure 2. X-ray reflectivity curves normalized relative to the Fresnel reflectivity of polyelectrolyte multilayers with selectively deuterated layers. The films are prepared from 1 M NaCl and 1 M KCl solution, respectively, at the temperatures indicated. The architecture of the films from NaCl and KCl solutions at temperatures between 15 and 50 °C is PEI{(PSS/PAH)3PSSd/PAH}4(PSS/PAH)3; otherwise (5 and 55 °C), it is PEI(PSS/PAH)3{PSSd/(PAH/PSS)3PAH}3PSSd.

seen, and the high-frequency oscillations are almost invisible (cf. Figure 4). From the intensity ratio of the first to second Bragg peak, the internal roughness can be determined (cf eq 8):

(

)

RBragg(Qz1) RF(Qz2) ξ2 ≈ ln 4 /12π2 2 R (Q ) R (Q ) ld F z1 Bragg z2

(10)

It is useful that the height of the Bragg peaks and their decay is independent of (i) the thickness of polyelectrolyte bilayers above and below the repeat units (lbase and ltop) and (ii) the respective roughnesses of the substrate/film and film/air interfaces (σsub and σair). There are also mixed terms,

2

|∫

Fsub2

() ()

|

F′h z F′d z eiQzz dz 2

(11)

which lead to two oscillations with opposite sign and periodicities very similar to the Kiessig fringes. Therefore, at high Qz, the Kiessig fringes are shifted. In addition, one finds some contributions with extremely low periodicities and very low amplitudes (see the Supporting Information).

Figure 3. Top: Neutron reflectivity curves normalized relative to the Fresnel reflectity of polyelectrolyte multilayers with selectively deuterated layers. The films are prepared from 1 M NaCl solution at the temperatures indicated. Always, the relative humidity is 0%. Bottom: The deduced scattering length density profiles. The architecture of the films is PEI{(PSS/PAH)3PSSd/PAH}4(PSS/PAH)3, except for those prepared at 5, 55, and 60 °C: there, it is PEI(PSS/PAH)3{PSSd/(PAH/ PSS)3PAH}3PSSd.

Results X-ray reflectivity curves from PEI(PSS/PAH)5PSS films prepared at 15 °C and at different NaCl concentrations are shown in Figure 1 (top). Clearly, with increasing salt concentration in the preparation solution, one observes a narrowing of the oscillations, indicating film thickening (cf. eq 7). If the thickness per polyelectrolyte bilayer pair is plotted as function of the inverse Debye length (Figure 1, bottom), one observes a roughly linear increase of the thickness (for I < 1 M), as published before.3,25,26 If both the salt concentration and the solution temperature are varied, the temperature causes an additional thickening if the salt concentration exceeds 0.05 M. The Debye length corresponding to this salt concentration is 13 Å, a value that is similar to the diameter of a PSS chain (12 Å), suggesting that the secondary interactions are shortranged and of the same order of magnitude as the molecular dimensions. Below 25 °C, the temperature effect is rather weak, then it gets stronger. Actually, it is possible to produce apparently identical films by two different sets of preparation parameters. An example is shown in Figure 1 (top): either 15 °C and 0.5 M or 35 °C and

Polyelectrolyte Multilayers

Figure 4. Normalized neutron reflectivity curves with selectively deuterated layers shown on a linear scale. The films are built from 1 M NaCl and 1 M KCl solution, respectively, at temperatures indicated. For most films from NaCl and KCl solutions, the architecture is PEI{(PSS/PAH)3PSSd/PAH}4(PSS/PAH)3, except for those from 5, 55, and 60 °C: there, it is PEI(PSS/PAH)3{PSSd/(PAH/PSS)3PAH}3PSSd. All measurements are performed at 0% r.h.

0.1 M. The periodicity of the X-ray reflectivity curves is the same; therefore, both films have the same thickness, yet the amplitudes of the oscillations differ slightly, suggesting different water contents. Another example would be room temperature and 3 M or 50 °C and 1 M. Both films show 66-Å-thick polyelectrolyte bilayer pairs.7 To quantify the bound water, neutron reflectivity measurements are performed (cf. Figures 3, 4) at 0% r.h. The films are organized in a superlattice of seven protonated layers and one deuterated layer in four repeat units. To get thick films and additionally a strong contribution from the secondary interactions, all films are prepared from 1 M salt solution, and the highest preparation temperature achieved is 60 °C from NaCl and 45 °C from KCl solutions. Higher preparation temperatures, T, lead to large-scale inhomogeneties of the films (optical interference of thin films suggests various coexisting thicknesses). Already, the films prepared 10 °C below the highest possible T show substantially increased film/air roughness in X-ray reflectivity measurements (cf. Figure 2), as compared to those films prepared at lower T. Oscillations of modified Kiessig fringes as well as two Bragg peaks due to an internal superstructure caused by the deuterated layer can be

J. Phys. Chem. B, Vol. 111, No. 29, 2007 8431 discerned (cf. Figure 3). On increase of the preparation T, the superstructure peaks shift to lower Qz and eventually decrease in intensity. To get a first idea about the internal roughness, the normalized reflectivity plots are shown on a linear scale (cf. Figure 4). For the films prepared from 1 M NaCl solution, two things are obvious: (i) If films are prepared at 50 °C and higher temperatures, the second Bragg peak has much less intensity than the first one. The second Bragg peak of the 60 °C film is almost invisible on a linear scale (cf. Figure 4, top), and only clearly visible on a logarithmic scale (cf. Figure 3, top). This observation suggests that heating the preparation solution to 50 °C and higher leads to a continuous rise in the internal roughness (cf. eq 10). (ii) The film prepared at 40 °C exhibits the most intense first Bragg peak, and the films prepared at lower temperatures still provide fairly high first Bragg peaks. The damping factor for the Bragg peak intensity in eq 8, exp(-4π2ξ2/ld2) ≈ 0.57, is constant (with the factor ξ/ld ≈ 0.12 from the decay between second and first peak), and its height is proportional to Fd2 ∝ ((bPSSd - bPSS)/(VPSS + VPAH + nVH2O))2. The only term that can vary is the water content, symbolized by n in the denominator. The bright first Bragg peaks suggest that there is less bound water in the film prepared at 40 °C than in films from lower temperature. At higher preparation temperatures, the first Bragg peaks decrease in intensity, too. Actually, the peak intensity is so low that the simple kinematic model suggests in addition to an increase of the internal roughness, an increase in the water content. Similar qualitative features are also observed if the films are prepared from KCl instead from NaCl solutions: (i) The Bragg peaks are very intense for preparation temperatures of 35 °C and less, suggesting constant but low water content, even lower than that of the films from NaCl solutions. (ii) For higher preparation temperatures (40 and 45 °C), the second Bragg peak shows increasingly less intensity, suggesting a pronounced rise in the internal roughness. To quantify these qualitative observations, least-squares fits according to the matrix formalism are performed (cf. Supporting Information Tables 1 and 2). We use a simple model to describe the neutron reflectivity measurements. In a repetition unit consisting of four PSS and four PAH layers, the thickness of the deuterated PSSd/PAH-layer amounts to 1/4 of the length of the repetition unit, the thickness of the nondeuterated layers to 3/4 of the repetition unit. Two additional slabs are necessary to describe the outer layers: the base layer adjacent to the Si wafer (thickness lbase) and the air-adjacent top layer (thickness ltop), respectively. For the least-squares fits of the neutron data, the roughness of the film/air and the film/substrate interfaces is taken from the fits to the X-ray reflectivity measurements (Figure 2, note that the film/air roughness increases for those films in which the neutron reflectivity measurements suggest increased internal roughness), since the larger Qz range of the X-ray measurements allows quantification of the decay of the oscillation amplitudes more reliably. Then the fits have only five free parameters: (1) the scattering length density of the PAH/PSSd-layer, (2) the scattering length density of the nondeuterated spacer layers, (3) the thickness of a repeat unit, (4) the interfacial roughness, and (5) the thickness of the base layer. In the case of a top layer above the last deuterated bilayer, a sixth parameter is necessary. Figure 5 shows that a rise in the preparation temperature from 5 to 60 °C causes an increase in the thickness per polyelectrolyte bilayer, dBL, from ∼32 to 92 Å; no effect of the salt in the deposition solution (NaCl or KCl) can be discerned.

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Figure 5. Thickness dBL per polyelectrolyte bilayer pair as function of deposition temperature (top left), and ratio of the internal roughness σ and dBL (right), together with the water content in the protonated (PSS/PAH) layers. Also shown (bottom left) is the water content in the deuterated (PSSd/PAH) and protonated spacer (PSS/PAH) layers. All parameters are determined from neutron reflectivity measurements shown in Figures 3 and 4.

Analyzing the composition of the films shows an influence of the salt used: the amount of tightly bound water is fairly constant at low preparation temperatures (e35 °C, ∼3.6 or 2.3 water molecules per PSS/PAH monomer pair for films from NaCl or KCl solution, respectively). Then the amount of bound water almost disappears; it is 0.2 ( 0.3. Interestingly, the internal roughness starts to increase as soon as the water content is less than two water molecules per PAH/PSS monomer pair. This is true for films from both 1 M NaCl and 1 M KCl salt solutions, even though the temperature characteristic for the onset of the roughening is ∼15 °C higher for NaCl solutions. The internal roughness amounts to 30-35% of the thickness of a PAH/PSS bilayer pair dBL up to 45 °C (or 30 °C) for films from NaCl (or KCl) solutions, which is almost the same value as found when varying the ion concentration.7 On heating the preparation solution, the ratio σ/dBL increases until it is ∼0.6. Then, at least with the dipping method, film preparation is no longer possible. On closer inspection, there is another remarkable feature connected with the bound water (cf. Figure 5). In the kinematic model, the underlying assumption is that the same amount of water is contained in the deuterated and the spacer bilayers. As shown in Figure 5, this assumption is reasonable for preparation temperatures less than or equal to 40 or 35 °C for films from NaCl or KCl solutions, respectively. At higher preparation temperatures, the water content in the protonated spacer layers decreases to about nothing (0.2 ( 0.3 per PSS/PAH monomer pair); however, in the deuterated layers, it amounts to 3 ( 0.9 per PSSd/PAH monomer pair. We tried to fit the reflectivity data with the same amount of water in both the deuterated and perprotonated parts of the films (using the same assumption as in the kinematic model). Although this gives not good, but reasonable, fits for temperatures e35 or e40 °C for films from 1 M KCl or 1 M NaCl, respectively, it gives remarkably bad fits for films from higher preparation temperatures. The first Bragg peak is described well (having the highest intensity; naturally, the least-square methods try to fit it first), yet neither the second Bragg peak nor the

high-frequency oscillations are described. Furthermore, the water content in the spacer layers is remarkably high to accommodate for the low intensity of the first Bragg peak. Discussion The main findings can be summarized as follows: (i) An increase in the preparation temperature increases the thickness per polyelectrolyte bilayer pair, if and only if the salt concentration in the deposition solution exceeds 0.05 M (Debye length in solution should be e13 Å). (ii) The thickness per PSS/PAH bilayer increases by a factor of 3 if the preparation temperature is raised from 5 to 60 °C. (iii) Although polyelectrolyte bilayers deposited at different preparation conditions may have the same thickness, the amount of bound water depends on the temperature, the concentration, and the kind of salt in the deposition solution. (iv) If the water content decreases beneath two water molecules per PAH/PSS monomer pair in a protonated bilayer due to a heated preparation solution (with 1 M salt), the internal roughness starts to increase dramatically. At which temperature this effect occurs depends on the kind of salt. (v) Even if the protonated layers contain very little, if any, bound water, the deuterated layers still have 2-3 water molecules per PAH/PSSd monomer pair. For a qualitative explanation of these phenomena, the hydrophobic effect is important. The strong temperature dependence is definitely there. It seems that the range of the attractive force is ∼10 Å, the size of a PSS diameter, since only for short Debye lengths do the secondary interactions contribute.28 In cold water, the water molecules surrounding the apolar solute form good hydrogen bonds in “structured cages”. In hot water, more first-shell water conformations are accessible, but some of them have weaker or unformed hydrogen bonds, van der Waals interactions, or both. Melting the first-shell water allows a closer approach of the polymer groups, making the short-ranged secondary forces such as van der Waals attraction5

Polyelectrolyte Multilayers more effective. However, eventually, the first-shell water is unstable, and no water is bound any more within the polyelectrolyte multilayer. Then it seems that the polyelectrolytes not so much adsorb but collapse in a random coil conformation onto the oppositely charged substratesin this case, the polyelectrolyte multilayer. With the melting of the first-shell water, the chains probably lose much of their ability to burrow into the underlying multilayer. The increase in the air/film roughness accompanying the increase of the internal roughness is consistent with such a model. Also consistent with a loss of interpenetration is the declining stability of polyelectrolyte multilayers prepared at high T when dipped in an aqueous solution. In addition to the strong temperature effect, other secondary forces manifest themselves. K+ appears to bind more tightly onto PSS than Na+: at 55 °C, it causes precipitation of PSS in a 1 M KCl solution, whereas PSS in 1 M NaCl is stable at all temperatures. But it does not lead to thicker polyelectrolyte bilayers. At low T, the stronger binding of K+ to PSS in solution leads to fewer bound water molecules in the polyelectrolyte multilayers, as compared to films from NaCl solutions. At these temperatures, the internal roughness is comparable, 0.33 and 0.36 dBL, respectively. We should comment on the fact that we give for the multilayers an internal roughness of 0.33 dBL, whereas for PAH/ PSS multilayers built from different salt concentrations3,25,26 or different temperatures,21 σint ≈ 0.4dBL is published. This apparent difference is due to the selection of the slab model, to be exact, to the assignment of different slabs to different molecular groups. We chose as the deuterated slab the PAH/ PSSd layer and assumed that its thickness is 1/4 of the repeat unit. Another option would have been to assume that the PSSd layer provides the contrast all alone. Then its thickness would be 1/6 of the repeat unit, because the PSS monomer volume is VPSS ) 200 Å3, and VPAH ≈ 100 Å3. This gives for the total volume of the monomers in the repeat unit (one monomer from each adsorption layer): 4VPSS + 4VPAH ) (4‚200 + 4‚100) Å3 ) 1200 Å3. Therefore, the deuterated PSS layer contributes 1/6 to the volume and, thus, 1/6 to the thickness of the repeat unit. The thickness of the protonated slab would be 5/6 of the repeat unit. This 1:5 model is only justified if the same amount of water per monomer is found within the deuterated and the protanated layers. The amount of water within a slab can be calculated from the corresponding scattering length density. In the low-temperature region characterized by low internal roughness, with the 1:5 model, we get fits to the reflectivity profiles that are identical to those shown in Figure 3. The internal roughness is the same as published before, σint ≈ 0.4dBL. However, we find for the KCl samples about three water molecules per PSSd monomer in the deuterated layer, and about 0.8 water molecules per monomer in each protonated layer. From this, the relationship between the volumes is calculated: (3VH2O + VPSS)/(7‚0.8VH2O + 3VPSS + 4VPAH) ) 1:4 (with VH2O ) 30 Å3). The different swelling of the slabs leads to inconsistencies in the model, which suggests that the thickness of the deuterated layer is larger than the 1:5 model assumed. Mathematically, the higher roughness of the deuterated layer in the 1:5 model is reasonable. As discussed in the Supporting Information Section, the scattering length density profile of the deuterated layer is smeared at both interfaces; since the deuterated layer is so thin, it can be described by a Gaussian. If the deuterated layer is made even thinner, one needs a larger roughness to obtain the same profile (i.e., the same Gaussian). Since, with the 1:3 model, we get in the low-temperature region the same amount of bound water for the protonated and

J. Phys. Chem. B, Vol. 111, No. 29, 2007 8433 the deuterated layers, these numbers are reliable. However, in the high-T region, things are more complicated. The volume ratio for the KCl samples is (3.3VH2O + VPAH + VPSS)/(3‚ 0.14VH2O + 3VPSS + 3VPAH) ) 1:2.3. Apparently, the deuterated slab, or rather, the strongly hydrated slab, is even broader than we assumed. It seems that a bit more bound water is stabilized in the vicinity to the deuterated layer, and our numbers are on the high side. The strong attractive interaction between the deuterated PSS and water molecules leads to more bound water molecules in the deuterated layer when there is almost no water in the protonated layers. This is a typical isotope effect,27 and detailed knowledge of the structure would be necessary to understand it quantitatively. In terms of polymer language, the decreased water binding within the polyelectrolyte films prepared from hot salt solutions is evidence of an increased polymer/solvent interaction parameter, χ. Clearly, the attractive interaction between polymer segments and water molecules decreases. The qualitative differences, i.e., (i) melting of the first-shell water and (ii) its eventual disappearance, go beyond simple polymer theory. We studied polyelectrolyte multilayer films from PAH/PSS, which is known to form one of the most stratified polyelectrolyte multilayers.1 However, we think that the temperature-dependent secondary interactions are important for any polyelectrolyte with hydrophobic backbones (i.e., not polylysine) or aromatic groups such as the benzene ring of PSS adsorbing onto an oppositely charged substrate. The fact that electrostatic forces need to be screened in order to allow secondary interactions to determine the polyelectrolyte surface coverage is probably a very general result. Obviously, the binding constant between polyelectrolyte and water varies from system to system, yet it will decrease on heating (that is the hydrophobic force). Our results suggest that in the case when polyelectrolyte multilayers are used as a membrane, with the aim to control the transport of selected molecules, it may be important to watch the temperature because it will affect the water’s binding and, thus, the water’s mobility within the multilayer. Finally, we would like to remark that any physiological solution has a monovalent salt concentration of 0.15 M, and the body temperature of most animals is above 30 °C. This allows nature to control any polyelectrolyte adsorption not only with electrostatics but also with secondary interactions, especially hydrophobic forces. Conclusion The growth of polyelectrolyte multilayers from PAH and PSS is studied at different salt concentrations and temperatures. At ion concentrations exceeding 0.05 M NaCl, a rise in the preparation temperature increases the thickness per deposited PSS/PAH bilayer pair, indicating that attenuation of the electrostatics (range e1 nm) is necessary to allow secondary forces to contribute. The thickness of PAH/PSS bilayers from 1 M NaCl or KCl solution increases on an increase in the temperature (5 to 60 °C) by a factor of 3. The polyelectrolyte bilayers have the same thickness, independent of the kind of salt, yet different compositions. At low temperatures, it is 3.6 or 2.3 water molecules per PAH/PSS monomer pair, respectively. On heating the preparation solution, eventually the amount of bound water decreases to almost nothing, 0.2 ( 0.3. If fewer than two water molecules are bound to a PAH/PSS monomer pair, the interpenetration (i.e., the internal roughness normalized by the bilayer thickness, σ/dBL) as well as the film/ air roughness start to increase. At which temperature this roughening occurs depends on the kind of salt (50 or 35 °C for

8434 J. Phys. Chem. B, Vol. 111, No. 29, 2007 films from 1 M NaCl or KCl, respectively). Even when there is no water in the PAH/PSS monomer pairs, the deuterated layers are still hydrated with two to three water molecules per PAH/ PSSd pair. We suggest that for strongly screened electrostatics, on an increase in the temperature, the hydrophobic force dominates the changes in polyelectrolyte multilayer growth and composition. Although weak first-shell water surrounding the hydrophobic groups allows short-ranged segment/segment attractions to manifest themselves, the total loss of bound water molecules is adverse to the stability of the polyelectrolyte multilayer (higher film/air roughness, larger internal roughness). At which temperature the water loss in the polyelectrolyte multilayers occurs depends on other secondary forces, and the kind of salt used in the preparation solution has an effect. Furthermore, there is a pronounced isotope effect. Acknowledgment. The financial support of the DFG (He 1616 9-4), the TR24, and the State of Mecklenburg-Vorpommern is appreciated. Supporting Information Available: The Supporting Information contains a detailed derivation of the analytical equations necessary to describe the neutron reflectivity showing both Kiessig fringes and at least two superstructure peaks. Additionally, two tables are shown with all parameters necessary to fit the data given in Figures 3 and 4. These parameters were then used to calculate the scattering length density profile shown in Figure 3. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Scho¨nhoff, M. Curr. Opin. Colloid Interface Sci. 2003, 32, 8695. (2) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831-835. (3) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893-8906.

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