Structure of Adsorbed Polyelectrolyte Monolayers Investigated by

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Structure of Adsorbed Polyelectrolyte Monolayers Investigated by Combining Optical Reflectometry and Piezoelectric Techniques Maria Porus, Plinio Maroni, and Michal Borkovec* Departement of Inorganic, Analytical and Applied Chemistry, University of Geneva, 30, Quai Ernest-Ansermet, 1205 Geneva, Switzerland ABSTRACT: Polyelectrolyte monolayers on solid substrates are studied with optical reflectivity and the quartz crystal microbalance (QCM). In particular, we investigate the adsorption of anionic poly(styrene sulfonate) (PSS) on amino-functionalized silica as well as cationic poly(allylamine hydrochloride) (PAH) and poly-Llysine (PLL) on bare silica. By comparing the dry and wet masses measured on identical substrates with these two techniques, we obtain information on the layer thickness and water content of these layers. Monolayers typically feature an adsorbed dry mass of about 0.1−2 mg/m2, a layer thickness of 0.5−2 nm, and a water content of 20−50%. One finds that the layer thickness increases with increasing concentrations of monovalent salts and polyelectrolytes.

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chains.28,29,32,33 Less highly charged strong polyelectrolytes or weak polyelectrolytes at low degrees of ionization show more complex behavior, including maxima in the adsorbed amount.30,34,35 The role of the substrate was also investigated, and it was found that the adsorbed amount increases with increasing charge of the substrate. 28,29 The fact that polyelectrolyte adsorption is an irreversible process was shown by electrophoresis experiments with colloidal particles13,36 and for planar substrates by rinsing the adsorbed layer with pure electrolyte solutions29,37,38 as well as investigating the desorption kinetics upon variations in the molecular mass.38 Although the adsorbed mass is the most relevant characteristic of an adsorbed polyelectrolyte film, different structures of a film with a given mass per unit area can be realized. These different structures principally differ through their lateral heterogeneity and their thickness. An adsorbed polyelectrolyte film can be laterally homogeneous or heterogeneous. The lateral heterogeneity of adsorbed polyelectrolyte films has been clearly demonstrated with poly(amido amine) (PAMAM) dendrimers by atomic force microscopy (AFM).28,39 Such globular polyelectrolytes adsorb on silica and mica in dilute fluidlike monolayers. In the case of linear polyelectrolytes, the situation is less clear, but numerous experimental40,41 and theoretical32 studies suggest that such adsorbed films are frequently laterally heterogeneous as well. An adsorbed polyelectrolyte film can be thin and compact or thick and swollen. Currently, only a few techniques are available to measure the thickness or equivalently the water content of an adsorbed polyelectrolyte monolayer. The principal difficulty

INTRODUCTION The adsorption of polyelectrolytes on water−solid interfaces is important in numerous applied disciplines, such as water purification, ceramics processing, and biomedicine.1−11 In water purification, polyelectrolytes are added to neutralize the charge of suspended particles, leading to the formation of aggregates that can be separated by sedimentation.1 In ceramics processing, polyelectrolytes are employed to form a stabilizing layer on particle surfaces in order to control the rheological properties of particle slurries.2,3 In biomedicine, adsorbed polyelectrolytes provide protective or functional coatings, for example, by exploiting comblike architectures4,5 or polyelectrolyte multilayers fabricated by layer-by-layer or spraying techniques.7−10 Because of the relevance of these applications, experimental and theoretical research is being pursued to elucidate the properties of adsorbed polyelectrolyte layers on solid substrates. Experimental activities have developed in two different directions, namely, studies of polyelectrolytes adsorbed on colloidal particles or on macroscopic planar substrates. The adsorbed amount on colloidal particles has been studied for a long time by classical batch depletion techniques.12,13 Measurements of the adsorbed amount on planar substrates became possible only recently, after the development of sufficiently sensitive optical or piezoelectric surface techniques. The optical techniques include mainly ellipsometry,14 reflectivity,15−17 surface plasmon resonance,18 optical waveguide light-mode spectroscopy,19 and dual-polarization interferometry.20,21 The piezoelectric techniques mainly refer to different variants of the quartz crystal microbalance (QCM).22−25 With these techniques, it was shown that the number of highly charged adsorbed polyelectrolytes generally increases with increasing salt levels.26−31 This trend has been explained by the progressive screening between the adsorbing polyelectrolyte © 2012 American Chemical Society

Received: December 9, 2011 Revised: January 28, 2012 Published: January 29, 2012 5642

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Gothenburg, Sweden). Prior to use, they were cleaned with Milli-Q water, dried in a flow of nitrogen, and treated in a UV-ozone cleaner for 20 min. These crystals were used directly for the adsorption of the cationic polyelectrolytes. For the adsorption of anionic polyelectrolytes, the crystals were functionalized with amino groups by silanization in vacuum to render them positively charged in water. The cleaned crystals were placed in a Petri dish beside a sessile drop of about 40 μL of 3-(ethoxydimethylsilyl)propylamine (Sigma-Aldrich, Switzerland) in an evacuated excicator overnight. Finally, the crystals were rinsed with ethanol and then with Milli-Q water, dried in a flow of nitrogen, and used immediately thereafter. The roughness of the bare and functionalized crystals was determined by AFM imaging (Nanoscope II, Veeco, USA) in noncontact mode. One finds a rootmean-square roughness of 5.4 nm for the bare crystal and 10.3 nm for the functionalized one. After use, the crystals were cleaned as described above. They could be reused many times. The precise thickness of the silica layer was determined by scanning angle null ellipsometry (Multiskop, Optrel, Berlin, Germany) with a laser of 633 nm wavelength. This technique measures the change in the polarization state of light upon reflection from a surface, which is described by the ellipsometric amplitude angle Ψ and phase angle Δ. These angles are related to Fresnel reflectivity coefficients rp and rs referring to parallel and normally polarized light as57

is that such films are very thin, typically a few nanometers. Elucidating the structure of adsorbed polyelectrolyte monolayers on colloidal particles is experimentally challenging, but progress could be made by improvements to the sensitivity in dynamic light scattering (DLS)42,43 or small angle neutron scattering (SANS).44 These techniques show that such layers have a thickness of a few nanometers and that they contain polyelectrolyte and water in about comparable quantities. DLS measurements have revealed that the thickness increases with the salt level and molecular mass.42,43 These observations are in semiquantitative agreement with theoretical predictions.32,33,45,46 The water content of adsorbed polyelectrolyte films has been obtained on planar substrates with QCM through heavy water exchange47,48 or by comparisons between optical techniques and QCM.21,30,31,49−51 Optical techniques measure the dry mass, which corresponds to the actual mass of the adsorbed polymer, and QCM measures the wet mass, which also includes the mass of the solvent trapped within the layer. The thickness and water content of the adsorbed polyelectrolyte layers have been obtained by combining reflectivity and QCM measurements.30,31,49,50 These techniques confirm that these films contain polyelectrolytes and water in about comparable quantities, but studies on how these quantities vary with the salt level or surface properties are lacking. Related information is available for polyelectrolyte multilayers assembled by the layer-by-layer technique. Because of their substantially larger adsorbed mass, reliable measurements of the layer thickness and of the water content could be performed with colloidal particles with DLS and SANS.52 Analogous information was obtained with planar substrates with ellipsometry53 and neutron reflectivity.18,54−56 These studies reveal that polyelectrolyte multilayers contain about comparable quantities of polymer and water and that the thickness of one bilayer of oppositely charged polyelectrolytes is 5−20 nm. These numbers suggest that the thickness of a polyelectrolyte monolayer film is a few nanometers. The thickness of the polyelectrolyte multilayers was also found to increase with increasing salt levels.56 Although such information is clearly relevant to polyelectrolyte multilayers, it may not be transferable to single adsorbed polyelectrolyte monolayers. This article provides the corresponding information for adsorbed polyelectrolyte monolayers with a comprehensive set of measurements of adsorbed mass, water content, and layer thickness. Optical reflectivity and QCM techniques are used to measure the dry and wet adsorbed masses on identical silica substrates. A comparison of these two experimental quantities yields the layer thickness and its water content.

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rp rs

= tan Ψ exp(− iΔ)

(1)

Both ellipsometric angles were measured as a function of the angle of incidence and fitted to a three-slab model within the Abeles matrix formalism.57 The topmost layer corresponds to silica (SiO2), the middle layer corresponds to the titanium (Ti) metal adhesion layer, and the bottom layer corresponds to gold (Au) metal. The data were fit with the refractive indices set to the respective values at 633 nm of 1.461 for silica, 2.153 + 2.925i for titanium, and 0.197 + 3.090i for gold published earlier.58,59 A least-squares fit of the ellipsometry data does not yield a good description of the data. For this reason, the imaginary part of Ti was adjusted as well, leading to a value of 2.65. The resulting

Figure 1. Characterization of the QCM quartz crystal coated with a topmost silica (SiO2) layer, a thin titanium (Ti) adhesion layer, and a thick gold (Au) layer (inset). (a) Results of scanning angle ellipsometry with the best fit of a three-layer homogeneous slab model from which thicknesses of 37 nm for the titanium layer and 306 nm for the topmost silica layer are obtained. (b) Reflectivity sensitivity factor as a function of the thickness of the topmost silica layer for the QCM crystal shown in the inset.

EXPERIMENTAL SECTION

Materials. As cationic polyelectrolytes, poly-L-lysine (PLL) hydrobromide with a molecular mass of about 300 kg/mol (Sigma-Aldrich, Switzerland) and poly(allylamine hydrochloride) (PAH) with a molecular mass of 120−200 kg/mol (Alfa Aeser, Germany) were used. The sodium salt of the anionic polyelectrolyte poly(styrene sulfonate) (PSS) with molecular masses of 70 and 300 g/mol (Alfa Aeser, Germany) and 666 and 5640 kg/mol (Polymer Standards, Germany) was studied. PSS obtained from these two suppliers gave consistent results. The polyelectrolytes were dissolved in Milli-Q water containing NaCl, and the pH was adjusted by additions of HCl or NaOH. Solutions were stored at room temperature for not more than a week. QCM quartz sensor crystals coated with a gold layer of about 100 nm thickness and sputter coated on its top with a silica layer of about 300 nm in thickness were purchased from Q-Sense (QSX301,

fit was satisfactory and is shown in Figure 1a. The obtained mean thicknesses of the silica and titanium layers were 308 and 34 nm, respectively, which agreed reasonably well with the values of 300 and 30 nm given by the manufacturer. The overall thickness varies about 2% from crystal to crystal. With scanning angle ellipsometry, no differences could be established between the new and used crystals or between silanized and nonsilanized crystals, even after many cleaning and adsorption cycles. Reflectivity. A home-built fixed-angle reflectometer was used to measure the dry mass per unit area of the adsorbed polyelectrolytes. 5643

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We refer to the mass determined by this optical technique as the dry mass because it reflects the mass of the adsorbed polyelectrolyte but does not include the mass of the trapped water. The crystal is installed in a stagnation-point flow cell and covered with a capped prism separated from the crystal by a spacer. The cell is mounted on the rotational axis of a goniometer with two arms, which are equipped with a light source and a detector. The solutions are pumped with a peristaltic pump through the bore hole in the prism. The polarized green diode laser with a wavelength of 533 nm, which can be electronically modulated at a frequency of 229 Hz, is projected onto the stagnation point at the crystal surface with an angle of incidence of 60°. As a result of light refraction in the prism, the actual angle of incidence on the surface is 71°. Upon reflection, the beam is separated into its perpendicular and normal components by a polarizing beam splitter, and their intensities are measured by photodiodes with a lockin amplification detection scheme. Further details on the reflectivity setup are given elsewhere.60 These intensities are proportional to reflectances Rp and Rs, respectively, which are related to the measured reflectivity signal R as

R=B

reproduced within 10% on different crystals and on the same crystals after cleaning. Quartz Crystal Microbalance (QCM). Q-sense E4 (Gothenburg, Sweden) with a flow-through cell was used to measure the wet mass per unit area of the adsorbed polyelectrolytes. We refer to the mass determined by QCM as the wet mass because it reflects the mass of the adsorbed polyelectrolyte as well as the mass of the trapped solvent. A peristaltic pump was used to pump solutions through the circular thin-layer cell, which features two eccentric holes as the inlet and outlet. With QCM we explore the piezolelectricity of quartz, whereby a resonance oscillation of the crystal is excited by applying an alternating electric voltage. The adsorbed wet mass per unit area Γwet is detected through a decrease in the resonance frequency Δf according to25

Δf = −

(2)

R(t ) − R(0) R(0)

(3)

This reflectometry signal S(t) is proportional to the adsorbed dry mass per unit area Γdry according to15 S(t ) = A Γdry

D=

Γdry dn L dc

E′ 2πE

(7)

where E′ is the energy loss during one oscillation and E is the total energy stored in the quartz oscillator. Typical changes in dissipation of about 10−6 were observed, and they increased with the increasing mass adsorbed. The QCM results were well reproduced for different crystals and on the same crystals after cleaning. We have also attempted to analyze the QCM response with the viscoelastic model.25,63 Least-squares fits of the overtones gave similar adsorbed masses but inconsistent results in the elastic modulus and viscosity of the film. This model may yield interesting results for substantially thicker layers than the ones investigated here,25 but in the present case, one cannot extract reliable information because of the less favorable signal-to-noise ratio. Determination of Layer Thickness and Water Content. By combining the dry mass per unit area Γdry obtained by reflectivity and the wet mass per unit area Γwet from QCM, one can obtain the layer thickness and water content of the polyelectrolyte layer.25,64 The mass fraction of the trapped water x can be obtained from the ratio of the dry and wet masses as

(4)

where A is the sensitivity constant. The sensitivity constant A is calculated from a homogeneous four-slab model, where the fourth topmost slab corresponds to the adsorbed polyelectrolyte layer. Its refractive index is obtained from the mixing law

n = ns +

−2

where n is the overtone number and C = 0.177 mg Hz m is the mass sensitivity constant. This equation yields an accurate estimate of the adsorbed mass for rigid layers.25 In this case, the overtones with n = 5, 7, and 9 lead to the same adsorbed mass per unit area within 5%. The frequency shift of the fifth overtone (n = 5) was used for the determination of the adsorbed wet mass. The frequency shift of this overtone can be measured to an accuracy of about 0.2 Hz−1, and the shifts of other overtones can be measured with a lower accuracy. The limit of detection of this QCM setup is about 20 μg/m2. Moreover, the shift in the dissipation signal ΔD was investigated. The dissipation is defined as

where B is an unknown instrumental constant. To eliminate this constant, the reflectivity signal R(t) is normalized to its initial value

S(t ) =

(6) −1

Rp Rs

n Γwet C

(5)

where ns is the refractive index of water, L is the thickness of the top layer, and dn/dc is the refractive index increment of the polyelectrolyte. The refractive index at 533 nm of silica is 1.457, that of titanium is 1.846 + 2.535i, and that of gold is 0.458 + 2.426i as published earlier.58,59 The layer thicknesses of the silica and titanium layers were taken from the ellipsometric measurements. For the various polyelectrolyte salts, the following values of dn/dc were used, namely, 0.18 g/L for PLL,61 0.225 g/L for PAH,62 and 0.17 g/L for PSS.62 For the thin layers investigated here, the results are practically independent of the thickness L. A typical value of the sensitivity factor A for silica crystals was about 0.02 m2/mg, which leads to a detection limit in the adsorbed amount of about 3 μg/m2. Note that the reflectivity technique can achieve a substantially lower detection limit on an oxidized silicon wafer because of the larger sensitivity factor.60 The sensitivity is essential to the studies presented here, and it cannot be achieved easily with a He−Ne laser operating at 633 nm because of the noise originating from the mechanical chopper needed. For this reason, we prefer to work with the electronically modulated laser operating at 533 nm and to correct for the wavelength dispersion in the refractive indices of the substrates. Test measurements with the He−Ne laser were performed on adsorbed layers with the highest adsorbed mass, and the adsorbed amounts agreed to better than 10% at the two wavelengths used. Figure 1b demonstrates the dependence of the sensitivity factor on the thickness of the silica layer on the QCM crystal. One observes that this thickness is critical to achieving a good sensitivity in the adsorbed mass with reflectivity, and the crystal to be used must be carefully chosen. At a wavelength of 533 nm, the sensitivity is sufficient for the commercially available crystals with a thickness of about 300 nm. For this reason, such crystals were used. The reflectivity results were well

x=1−

Γdry Γwet

(8)

The volume fraction of water in the film ϕ can be deduced from the ideal mixing law ϕ=

ρpx ρpx + ρs(1 − x)

(9)

where ρs is the density of the NaCl solution and ρp is the density of the polyelectrolyte. In the present case, for PSS, PAH, and PLL the densities were taken as 1.2, 1.1, and 1.1 g/mL as reported elsewhere.51,65 Once the water volume fraction is known, the layer thickness can be obtained from the relation Γ L = wet ρ

(10)

where the density of the wet film is given by

ρ = ρp(1 − ϕ) + ρsϕ 5644

(11)

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All of these relations are approximate because they assume ideal mixing and a homogeneous film. The determination of the water content was also attempted with QCM by exchanging normal water with heavy water.48 Although this method was applied to layers with high adsorbed masses with success, in the present system the noise in the frequency and dissipation shifts was too substantial to extract the water content accurately even for the layers with the largest adsorbed mass investigated.

figure, the crystal is rinsed with the polyelectrolyte-free electrolyte solution at a later time. The optical reflectivity signal S is shown in Figure 2a, and the shifts in the frequency Δf and the dissipation factor ΔD observed by QCM are shown in Figure 2b. The resulting dry mass obtained from the reflectivity is shown in Figure 2c, and the wet mass from the QCM experiment is displayed in Figure 2d. Both techniques indicate that the adsorbed mass increases linearly with time at first. After a certain time period, the adsorbed mass attains a plateau, which indicates that the surface is saturated with the polyelectrolyte. Although the initial rise depends only weakly on the salt concentration, the plateau value increases with the increasing salt concentration. One also observes that the layer remains stable when the cell is flushed with pure electrolyte solution. This stability indicates that the adsorption process is irreversible. Similar results are found for the other polyelectrolyte systems investigated. The molecular mass dependence of the properties of the adsorbed layer will be illustrated for PSS and aminofunctionalized silica. Figure 3 shows the dependence of the

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RESULTS AND DISCUSSION A comprehensive set of data on the adsorbed mass per unit area, layer thickness, and water content of adsorbed polyelectrolyte monolayers on silica surfaces is presented. The thickness and water content of these films is obtained by measuring the adsorbed dry mass with reflectivity and the wet mass with QCM on identical substrates. The combination of these two techniques permits us to obtain new information concerning the structure of adsorbed polyelectrolytes layers. Three linear polyelectrolytes were investigated, namely, anionic poly(styrene sulfonate) (PSS) on amino-functionalized silica as well as cationic poly(allylamine hydrochloride) (PAH) and poly-L-lysine (PLL) on bare silica. Layer Thickness and Water Content. Typical experimental results for the adsorption of PSS with a molecular mass of 300 kg/mol on amino-functionalized silica at different concentrations of monovalent salt NaCl are shown in Figure 2.

Figure 3. Adsorption data derived from reflectivity and QCM for saturated layers of adsorbed PSS of different molecular masses on amino-functionalized silica vs the NaCl concentration at pH 5.6. (a) Dry mass obtained from reflectivity and (b) wet mass obtained from QCM. By combining the data, we obtain (c) the layer thickness and (d) the water content of the film. The legend given in a applies to all graphs. The structure of PSS is given in b. Figure 2. Reflectivity and QCM data for PSS with a molecular mass of 300 kg/mol adsorbing on amino-functionalized silica at different NaCl concentrations at pH 5.6. The surface is initially flushed with the electrolyte solution (solution I). At time zero, a PSS solution of 5 mg/ L is introduced (solution II), and later the surface is flushed again with electrolyte solution (solution I). Time dependence of the (a) reflectivity and (b) QCM signal. Corresponding (c) dry and (d) wet adsorbed masses per unit area. The sequence of the solutions is indicated by the bar above. The legend given in d applies to all graphs.

dry and wet adsorbed masses per unit area of adsorbed PSS films on the salt concentration in NaCl solutions at pH 5.6 and a polyelectrolyte concentration of 5 mg/L for molecular masses in the range from 70 to 5640 kg/mol. The dependence was fit to the empirical relation Γ = Kc α

(12)

where c is the electrolyte concentration, α is an exponent, and K is an empirical constant. The exponent found to be α = 0.2 ± 0.1. Although the main purpose of this fit is to obtain a simple interpolation of the experimental data, we note that the exponent lies between the values of α = 0 and 1/2, as suggested theoretically.46,66 The layer thickness and water volume fraction were calculated with eqs 9 and 10 from the experimental data,

The cell is first flushed with the pure electrolyte solution of pH 5.6 to achieve a stable baseline, which serves as a reference for the subsequent measurements. At time zero, a solution with a polyelectrolyte concentration of 5 mg/L in the same electrolyte and the same pH is injected. As indicated in the top bar of the 5645

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molecular mass as observed here. This agreement indicates that the observed dependence is mainly dictated by single adsorbed chains. Let us now discuss the effects of pH on similar adsorption data for weak polyamines on silica surfaces. Figure 4 shows the

but more reliable estimates could be obtained from the fits to eq 12. One observes that the dry adsorbed mass is in the range of 0.3−1.5 mg/m2 and the wet mass is about 2 to 3 times as large. From these numbers, one finds that the water content is about 20−40% and that the layers thickness is about 0.5−2.5 nm. For the PSS with the highest molecular mass of 5640 kg/mol investigated, the radius of gyration is between 30−100 nm under comparable conditions in solution.67,68 Therefore, the polyelectrolyte chains flatten substantially during the adsorption, namely, by a factor of about 50. The adsorbed amount (i.e., dry mass) and the layer thickness increase with increasing salt level. The water content of about 20−40% increases slowly with the salt concentration. The adsorbed mass increases with the molecular mass very weakly or not at all, and the layer thickness increases with the molecular mass more strongly, especially at high salt concentrations. The water content increases with the molecular mass as well, but only weakly. The adsorbed amount is in good agreement with the adsorbed amounts for PSS on alumina69 and on amidine latex particles.43 For alumina, the adsorbed amount shows a weak increase with the molecular mass as reported here, and a weak decrease was found for latex particles. However, all studies agree that the molecular mass dependence of the adsorbed amount of PSS is extremely weak. The hydrodynamic layer thickness of PSS adsorbed on amidine latex as measured by DLS was shown to increase strongly with the salt concentration, with little dependence on the molecular mass at low salt concentrations but with the thickness increasing more rapidly at high salt concentrations.43 These trends are very similar to the ones presented here. However, the hydrodynamic layer thickness measured by DLS is systematically larger by a factor of 2 to 3 than the thicknesses reported here. This difference can be explained by the extended concentration profile of the layer and the existence of a few dangling tails extending into the solution. The reported values for the layer thickness are comparable to the results obtained with SANS on poly(diallyldimethyl ammonium chloride) adsorbed on latex particles at low salt concentrations.44 Such layers were found to have a thickness of about 0.8 nm and a water content of about 74% when expressed as a mass fraction.44 Similar values for the water content were found by QCM for adsorbed modified dextrins on alkanethiol monolayers48 and for cationic linear copolymers on silica by combing reflectivity and QCM, albeit not exactly for the same substrates.30,31,49 The presently reported values for the water content of the PSS layers are lower than the values reported in these studies. The increase in the adsorbed amount with increasing salt level can be explained by electrostatic screening between the adsorbing chains.28,29,32,34,66 At low salt levels, each polyelectrolyte chain is surrounded by an extensive diffuse layer. As the adsorption to the surface progresses, the repulsion between these diffuse layers leads to a saturation of the adsorption process at low coverage and results in low adsorbed amounts. At higher salt levels, the diffuse layer is less extended, leading to higher surface coverage and higher adsorbed amounts. The molecular mass dependence of the layer structure of adsorbed polyelectrolytes is poorly understood. Computer simulations of single polyelectrolyte chains adsorbed on oppositely charged substrates45 suggest a similar dependence of the layer thickness on the salt concentration and on the

Figure 4. Adsorption data derived from reflectivity and QCM for saturated layers of adsorbed PAH on silica vs NaCl concentration for different pH values. (a) Dry mass obtained from reflectivity and (b) wet mass obtained from QCM. By combining the data, the (c) layer thickness and (d) water content of the film are obtained. The legend given in a applies to all graphs. The structure of PAH is given in b.

dry and wet adsorbed masses for PAH adsorbed on silica as a function of the NaCl concentration for different pH values. The adsorbed amount and the layer thickness again increase with increasing salt concentration and increasing pH. The water content remains approximately constant. The observed dry mass is in good agreement with earlier reflectivity measurements of adsorbing PAH on silicon wafers.62 The presently observed values for the adsorbed amount, layer thickness, and water content as well as their dependence on the salt concentrations are similar as found for PSS. The increase in the adsorbed amount with pH can be rationalized by the increased magnitude of the silica surface charge. The surface charge of silica is controlled by the silanol groups, which are protonated and uncharged under acidic conditions but deprotonated and negatively charged under basic conditions.60,70,71 The surface charge is neutralized by counterions in the diffuse layer; therefore, a higher surface charge leads to a higher local salt concentration close to the surface. This local increase in the salt concentration leads to more effective screening of the repulsive forces between the polyelectrolyte chains close to the surface and therefore to greater adsorbed amounts. The validity of this mechanism was demonstrated for adsorbed cationic dendrimers on silica quantitatively.28 Another possible explanation is the weakening of the repulsive interactions between the polyelectrolyte chains due to the decrease in the degree of ionization of PAH with increasing pH because PAH is fully ionized at pH 4 but only half ionized at pH 10.72,73 However, we suspect that the effect due to PAH ionization is secondary because of the substantial screening of the polyelectrolyte charge by condensed counter5646

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Low versus High Polyelectrolyte Concentrations. Typical experimental results on the dependence on the polyelectrolyte concentration are shown in Figure 6 for the

ions within the chain. The fact that ionization effects are unimportant follows from similar trends with pH observed with strong polyelectrolyte poly(diallyldimethyl ammonium chloride).29 Similar patterns in the adsorption behavior of PLL are summarized in Figure 5. The dry and wet adsorbed masses and

Figure 5. Adsorption data derived from reflectivity and QCM for saturated layers of adsorbed PLL on silica vs NaCl concentration for different pH values. No fit is shown for data at pH 11. (a) Dry mass obtained from reflectivity and (b) wet mass obtained from QCM. By combining the data, the (c) layer thickness and (d) water content of the film are obtained. The legend given in a applies to all graphs. The structure of PLL is given in b.

Figure 6. Reflectivity and QCM data for PAH adsorbing on a silica substrate at different polyelectrolyte concentrations in 1 M NaCl solution and pH 10. The surface is initially flushed with the electrolyte solution (solution I); at time zero the PAH solution is introduced (solution II), and the surface is flushed again with pure electrolyte solution (solution I). Time dependence of the (a) reflectivity and (b) QCM signal. Corresponding (c) dry and (d) wet adsorbed masses per unit area. The sequence of the solutions is indicated in the bar above. The legend given in d applies to all graphs.

the layer thickness increase with the salt concentration and solution pH. The experimental data were again fitted with eq 12 to facilitate the prediction of a common trend for the layer thickness and the water content of the film. The main difference between these two systems is that the adsorbed mass and layer thickness are substantially lower for PLL than for PAH. Moreover, at pH 11 one observes a maximum in these quantities. Although a similar maximum can also be recognized in the PAH data shown in Figure 4, it is more developed for PLL than for PAH. Moreover, the water content of the PLL film decreases substantially with increasing pH, whereas this quantity remains approximately constant for PAH. The dependence of the adsorbed amount on the pH and salt concentration is similar to earlier results on PLL adsorption on silicon wafers in the NaBr electrolyte.34 The adsorption of weak cationic polyelectrolyte poly(dimethylamino ethyl methacrylate) on silica also shows a maximum at high pH,35 which is reminiscent of the maximum observed for other weakly charged polyelectrolytes.30,74,75 This maximum is probably related to the competition between cations originating from the background electrolyte and the weakly charged polyelectrolyte, and it was also predicted on theoretical grounds.32,76,77 The other reason for the differences between PLL and PAH is that PLL undergoes a conformational transition from a random coil to an α helix, which happens around pH 10.78,79 The denser structure of the PLL layer at high pH could be related to the more compact nature of the prevailing α helix. However, PAH does not show any conformational transition at high pH.80

adsorption of PAH in 1 M NaCl solution and pH 10. Experimental traces are shown on the top, and the adsorbed mass per unit area is shown at the bottom. The left column refers to the reflectivity experiment, and the right column refers to the QCM data. The substrate is first equilibrated with pure salt solution, the polyelectrolyte solution in the same medium is injected at time zero, and the film is finally flushed with the pure electrolyte solution again. One observes that the adsorption plateau is reached much more rapidly for high polyelectrolyte concentrations than for lower ones, which reflects the fact that the kinetics of the adsorption process is first order in polymer concentration.28,29,81 The initial increase in the adsorbed amount is similar for the dry and wet masses, but a precise comparison cannot be made because the flow profiles in the reflectivity and QCM cells are different. Reflectivity reveals that the adsorption plateau depends on the polyelectrolyte concentration only weakly whereas a strong dependence of the frequency shift is observed with the QCM. After saturation, the cell is rinsed with pure salt solution. The fact that the reflectometry and QCM signals remain constant during this rinsing process indicates that the polyelectrolyte layer is stable in pure salt solution even when deposited at high polyelectrolyte concentration and that no desorption takes place. Only the QCM dissipation signal indicates a small decrease upon rinsing, which is probably related to the different density and viscosity of the polyelectrolyte solution and eventually to minor rearrangements within the film. The 5647

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other polyelectrolyte systems investigated were found to behave analogously. Figure 7 shows how the adsorption of PSS depends on the polyelectrolyte concentration for two different molecular

Figure 8. Dependence of the adsorption data derived from reflectivity and QCM for saturated layers on the polyelectrolyte (PE) concentration for PAH. The conditions in 10−3 and 1 M NaCl and pH 4 and 10 are compared. (a) Dry mass obtained from reflectivity and (b) wet mass obtained from QCM. By combining the data, we obtained the (c) layer thickness and (d) water content of the film. The legend given in a applies to all graphs.

Figure 7. Dependence of the adsorption data derived from reflectivity and QCM for saturated layers on the polyelectrolyte (PE) concentration for PSS of different molecular masses at pH 5.6. The conditions of 10−3 and 1 M NaCl are compared. (a) Dry mass obtained from reflectivity and (b) wet mass obtained from QCM. By combining the data, we obtain the (c) layer thickness and (d) water content of the film. The legend given in a applies to all graphs.

masses and two different salt levels. The top row demonstrates that the dry mass increases only slightly whereas the wet mass increases much more strongly. The bottom row shows the corresponding layer thickness and volume fraction of water within the film obtained from eqs 9 and 10. The data have been fitted with eq 12, whereby c has now been interpreted as the polyelectrolyte concentration. The analogous exponent was found to have a value of α = 0.07 ± 0.03. One observes that the layer thickness and the water content increase with increasing polyelectrolyte concentration as well. The effect is most pronounced at high molecular mass and at high salt concentration. Figures 8 and 9 show the corresponding data for PAH and PLL at pH 4 and 10 in 1 M NaCl solution and at pH 10 in 10−3 M NaCl solution. No data in 10−3 M NaCl at pH 4 are reported because they are close to the QCM detection limit and the estimates of the water content are unreliable. The largest increase in the adsorbed mass with increasing polyelectrolyte concentration is found at a salt level of 1 M and pH 10. Thereby, the wet mass increases much more rapidly than does the dry mass. This increase is also accompanied by a slight increase in the water content. Therefore, the layer formed at high polyelectrolyte concentrations is thicker and contains more water. Figure 8 shows that the behavior at pH 4 is similar. In this case, the dry mass remains roughly constant and the wet mass increases substantially. As a consequence, the layer thickness and the water content increase with the polyelectrolyte concentration. At pH 4, however, the structure of the adsorbed layer remains constant.

Figure 9. Dependence of the adsorption data derived from reflectivity and QCM for saturated layers on the polyelectrolyte (PE) concentration for PLL. The conditions in 10−3 and 1 M NaCl and pH 4 and 10 are compared. (a) Dry mass obtained from reflectivity and (b) wet mass obtained from QCM. By combining the data, we obtained the (c) layer thickness and (d) water content of the film. The legend given in a applies to all graphs.

These observations can be rationalized by the finite relaxation time of the adsorbing polyelectrolyte coils as suggested previously for polyelectrolytes29 and proteins.82−84 At low polyelectrolyte concentration, an individual adsorbing polyelectrolyte chain has time to adopt a flat conformation on the surface because the neighboring chains will adsorb only after it has relaxed. Such an adsorbed layer will saturate at lower coverage, and the resulting layer will be thin and compact. At 5648

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(5) Zhen, G.; Eggli, V.; Voros, J.; Zammaretti, P.; Textor, M.; Glockshuber, R.; Kuennemann, E. Immobilization of the enzymelactamase on biotin-derivatized poly(L-lysine)-g-poly(ethylene glycol)coated sensor chips: a study on oriented attachment and surface activity by enzyme kinetics and in situ optical sensing. Langmuir 2004, 20, 10464−10473. (6) Decher, G. Fuzzy nanoassemblies: toward layered polymeric multicomposites. Science 1997, 277, 1232−1237. (7) Decher, G., Schlenoff, J. B., Eds.; Multilayer Thin Films; WileyVCH: Weinheim, Germany, 2003. (8) Porcel, C.; Lavalle, P.; Ball, V.; Decher, G.; Senger, B.; Voegel, J. C.; Schaaf, P. From exponential to linear growth in polyelectrolyte multilayers. Langmuir 2006, 22, 4376−4383. (9) Izquierdo, A.; Ono, S. S.; Voegel, J. C.; Schaaf, P.; Decher, G. Dipping versus spraying: exploring the deposition conditions for speeding up layer-by-layer assembly. Langmuir 2005, 21, 7558−7567. (10) Caruso, F.; Caruso, R. A.; Möhwald, H. Nanoengineering of inorganic and hybrid hollow spheres by colloidal templating. Science 1998, 282, 1111−1114. (11) Fery, A.; Dubreuil, F.; Möhwald, H. Mechanics of artificial microcapsules. New J. Phys. 2004, 6, 18. (12) Bonekamp, B. C.; Lyklema, J. Conductometric and potentiometric monitoring of polyelectrolyte adsorption on charged surfaces. J. Colloid Interface Sci. 1986, 113, 67−75. (13) Kleimann, J.; Gehin-Delval, C.; Auweter, H.; Borkovec, M. Super-stoichiometric charge neutralization in particle-polyelectrolyte systems. Langmuir 2005, 21, 3688−3698. (14) Odberg, L.; Sandberg, S.; Welinklintstrom, S.; Arwin, H. Thickness of adsorbed layers of high-molecular-weight polyelectrolytes studied by ellipsometry. Langmuir 1995, 11, 2621−2625. (15) Dijt, J. C.; Cohen Stuart, M. A.; Fleer, G. J. Reflectometry as a tool for adsorption studies. Adv. Colloid Interface Sci. 1994, 50, 79− 101. (16) Heinrich, L.; Mann, E. K.; Voegel, J. C.; Koper, G. J. M.; Schaaf, P. Scanning angle reflectometry study of the structure of antigenantibody layers adsorbed on silica surfaces. Langmuir 1996, 12, 4857− 4865. (17) Kleimann, J.; Lecoultre, G.; Papastavrou, G.; Jeanneret, S.; Galletto, P.; Koper, G. J. M.; Borkovec, M. Deposition of nanosized latex particles onto silica and cellulose surfaces studied by optical reflectometry. J. Colloid Interface Sci. 2006, 303, 460−471. (18) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Ultrathin multilayer polyelectrolyte films on gold: construction and thickness determination. Langmuir 1997, 13, 3422−3426. (19) Voros, J.; Ramsden, J. J.; Csucs, G.; Szendro, I.; de Paul, S. M.; Textor, M.; Spencer, N. D. Optical grating coupler biosensors. Biomaterials 2002, 23, 3699−3710. (20) Cross, G. H.; Reeves, A.; Brand, S.; Swann, M. J.; Peel, L. L.; Freeman, N. J.; Lu, J. R. The metrics of surface adsorbed small molecules on the Young’s fringe dual-slab waveguide interferometer. J. Phys. D 2004, 37, 74−80. (21) Aulin, C.; Varga, I.; Claesson, P. M.; Wagberg, L.; Lindstrom, T. Buildup of polyelectrolyte multilayers of polyethyleneimine and microfibrillated cellulose studied by in situ dual-polarization interferometry and quartz crystal microbalance with dissipation. Langmuir 2008, 24, 2509−2518. (22) Kanazawa, K. K.; Gordon, J. G. Frequency of a quartz microbalance in contact with liquid. Anal. Chem. 1985, 57, 1770− 1771. (23) Rodahl, M.; Hook, F.; Krozer, A.; Brzezinski, P.; Kasemo, B. Quartz crystal microbalance setup for frequency and Q-factor measurements in gaseous and liquid environments. Rev. Sci. Instrum. 1995, 66, 3924−3930. (24) Edvardsson, M.; Rodahl, M.; Kasemo, B.; Hook, F. A dualfrequency QCM-D setup operating at elevated oscillation amplitudes. Anal. Chem. 2005, 77, 4918−4926. (25) Hook, F.; Kasemo, B.; Nylander, T.; Fant, C.; Sott, K.; Elwing, H. Variations in coupled water, viscoelastic properties, and film thickness of a Mefp-1 protein film during adsorption and cross-linking:

high polyelectrolyte concentration, an individual chain has less time to rearrange its solution conformation because the adsorbing polyelectrolyte chains fill up the surface rapidly. These layers will feature a larger adsorbed mass, and they will be thicker and more porous. The PSS data support this interpretation because the relaxation time of a polymer chain is known to increase strongly with the molecular mass.85 From the present data, one can further conclude that polyelectrolyte chains relax more slowly at high salt concentration and at lower line charge densities.

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CONCLUSIONS This article studies the adsorption of linear polyelectrolytes on water−solid interfaces, namely, anionic poly(styrene sulfonate) (PSS) on amino-functionalized silica as well as cationic poly(allylamine hydrochloride) (PAH) and poly-L-lysine (PLL) on bare silica. By comparing the adsorbed mass measured with optical reflectivity and QCM on the same substrates, the thickness and water content of the adsorbed layer can be extracted. By systematically studying these two quantities, novel information concerning the structure of adsorbed polyelectrolytes is obtained. In particular, one observes that the thickness of the polyelectrolyte monolayers increases with increasing salt concentration and polyelectrolyte concentration. These findings can be rationalized by conformational changes in single adsorbed chains. The dependence on the salt concentration suggests that at low salt concentrations the chain adsorbs in a flat conformation whereas at higher salt concentrations the conformation is more globular. The dependence on the polyelectrolyte concentration reflects the finite relaxation time of the adsorbing chains. At higher polyelectrolyte concentrations, the surface is filled rapidly and an adsorbing polyelectrolyte retains its globular conformation. At lower concentrations, an adsorbing polyelectrolyte has more time to relax and adopts a flatter conformation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +41 22 379 6405. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Swiss National Science Foundation and the University of Geneva. We thank Raffaele Mezzenga for a helpful discussion and Olivier Vassalli for expert laboratory help.

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REFERENCES

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