Response of Adsorbed Polyelectrolyte Monolayers to Changes in

Nov 21, 2012 - *E-mail: [email protected]; phone: +41 22 379 6405. .... Katarzyna Kubiak , Zbigniew Adamczyk , Michał Cieśla. Colloids and ...
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Response of Adsorbed Polyelectrolyte Monolayers to Changes in Solution Composition Maria Porus,† Plinio Maroni, and Michal Borkovec* Department of Inorganic and Analytical Chemistry, University of Geneva, 30, Quai Ernest-Ansermet, 1205 Geneva, Switzerland ABSTRACT: Reflectometry and quartz crystal microbalance are used to study the response of adsorbed polyelectrolyte monolayers to solutions of variable composition. These techniques respectively yield the dry and wet masses of the adsorbed layer, and by combing these results, one obtains the water content and the thickness of the polyelectrolyte films. The systems investigated are films of adsorbed poly(allyl amine) (PAH) and poly-L-lysine (PLL) on silica and films of poly(styrene sulfonate) (PSS) on amino-functionalized silica. When such films are adsorbed from concentrated polyelectrolyte solutions containing high levels of salt, they are found to swell reversibly up to a factor of 2 when incubated in solutions of low salt. This swelling is attributed to the strengthening of repulsive electrostatic interactions between the adsorbed polyelectrolyte chains. PAH films may also swell upon decrease of pH, and collapse upon a pH increase. This transition shows a marked hysteresis and can be rationalized by the competition of electrostatic repulsions between the chains and their attraction to the surface. The presently observed swelling phenomena are caused by a collective process driven by the electrostatic repulsion between the densely adsorbed polyelectrolyte chains. Such responsive layers are only obtained by adsorption from high polyelectrolyte and salt concentrations. Layers absorbed at low polyelectrolyte and salt concentrations show only minor swelling effects, since the adsorbed polyelectrolytes layers are dilute and the adsorbed polyelectrolyte chains interact only weakly.

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INTRODUCTION Dissolved polyelectrolytes are known to respond to variations in solution composition with changes in their charge,1,2 conformation,3,4 or both.5−7 These features are being exploited by numerous researchers to create surfaces with switchable properties.8−19 The most frequently investigated surface architectures involve polyelectrolyte brushes or polyelectrolytes multilayers. The envisioned applications include, for example, controllable wetting properties, nanoscale actuation, or catalysis. Polyelectrolyte brushes may swell or collapse depending upon solution composition.12−19 For strong polyelectrolytes, the brush thickness increases with increasing grafting density, but is independent of the salt concentration at low salt levels. At higher salt levels, the brush thickness decreases with increasing salt levels. The transition between these two regimes was demonstrated with X-ray reflectivity, ellipsometry, or direct force measurements with brushes on planar substrates consisting of poly(styrene sulfonate) (PSS), poly(acrylic acid) (PAA), and poly(vinyl pyridine).16−19 The decrease of brush thickness with increasing salt level was also demonstrated by means of dynamic light scattering with fully ionized brushes grafted of PAA or with PSS brushes made from adsorbing block copolymers on colloidal particles.12−15 Brushes containing weak polyelectrolytes capable of binding protons or polyelectrolytes with ligands complexing other types of ions normally swell with increasing linear charge density, but may also show more complex behavior.18−21 The extent of swelling or shrinking of © 2012 American Chemical Society

polyelectrolyte brushes can be rather impressive, as their thickness may vary by more than 1 order of magnitude. Many of these features are also well understood from the theoretical point of view.9,22−25 Polyelectrolytes multilayers obtained by adsorption of oppositely charged polyelectrolytes by the layer-by-layer technique can be influenced through variations of the salt level as well as the type of the fabrication method (i.e., dipping, spraying, spin-coating).26−30 Such multilayers may also respond to changes in solution composition by swelling or collapsing. Planar multilayers consisting of poly(allyl amine) (PAH) and PSS were shown to swell substantially upon variations in pH31,32 or salt.33 Swelling of polyelectrolyte multilayers could be equally induced by rinsing with pure water.34,35 Similar response of multilayers made of poly-L-lysine (PLL) and hyaluronic acid was observed upon changes of pH.36 Such multilayers may swell substantially, whereby an increase in thickness up a factor of 4 has been reported.31,32 Variations in the permeability of polyelectrolyte multilayers were shown through release of dyes from capsules11 or by following electrochemical reactions on electrodes.37 Given the responsiveness of polyelectrolyte brushes or polyelectrolyte multilayers, one expects that adsorbed polyelectrolyte monolayers should also respond to changes in Received: October 3, 2012 Revised: November 21, 2012 Published: November 21, 2012 17506

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The solutions were introduced through the circular flow-though cell, whereby the solutions are forced through a thin layer cell through two eccentric holes with a peristaltic pump. The adsorbed wet mass per unit area Γwet is detected through a shift Δf of the resonance frequency of the quartz crystal according to50

solution composition or other stimuli. However, the information available on the responsiveness of polyelectrolyte monolayers in the literature is scarce and conflicting. From direct force measurements it was concluded that adsorbed polyelectrolyte monolayers swell substantially upon a decrease in the salt concentration.38−40 Studies of adsorbed carboxymethyl cellulose films by quartz crystal microbalance (QCM) lead to conflicting results. While swelling of such films with decreasing the salt level was reported,41 the opposite effect was observed as well.42 The aim of the present article is to systematically study the response of adsorbed polyelectrolyte monolayers to changes of salt concentration and pH. Our approach is to use reflectivity to obtain the dry mass, which yields unambiguous information concerning the adsorbed amount. From QCM the wet mass is obtained, which is sensitive to swelling or collapse of these layers. By comparing the dry and wet mass, the water content and the layer thickness can be deduced as well. On the basis of this approach, we find that polyelectrolyte monolayers may swell in thickness up to a factor of 2 upon decrease of the salt concentration.

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Δf = −

m Γwet C

(1)

where m is the overtone number and C = 0.177 mg Hz−1 m−2 is the mass sensitivity constant. The fact that this equation yields an accurate estimate of the adsorbed wet mass can be verified in two different ways. First, the wet mass determined from different overtones must remain constant.50−52 In the present case, the measured adsorbed wet mass is identical for the overtones m = 5, 7, and 9 within 5%. The frequency shifts were well reproducible for different crystals as well as on the same crystals cleaned several times. Since the resonance frequency depends on the solution density and viscosity, the baseline must be determined for each electrolyte separately when different electrolytes are used within the same experimental run.51 The shift in the dissipation signal ΔD was investigated as well. The dissipation is given by D=

EXPERIMENTAL SECTION

E′ 2πE

(2)

where E′ is the energy loss during one oscillation and E is total energy of the oscillator. Changes in dissipation were around 10−6 or less and they were proportional to the adsorbed wet mass whereby the ratio was −ΔD/Δf < 5 × 10−7 s. This condition also indicates that eq 1 is applicable.50,52,53 The adsorbed wet mass was normally determined from the fifth overtone (m = 5), which is characterized by the lowest noise, and leads to a detection limit of about 20 μg/m2. The shift in dissipation was substantially less reproducible than the shift in frequency. This behavior of the dissipation signal might be related to larger shifts originating from different solution compositions, influence of roughness, or variability among different crystals. Due to the small changes in the dissipation signal and its poor reproducibility, its changes could not be interpreted with confidence. The QCM response was also analyzed with the viscoelastic model.50,51 The different overtones were fitted with a least-squares algorithm, and they yielded similar adsorbed masses but inconsistent results for the elastic modulus and viscosities of the films. Interesting results can be obtained from this model for substantially thicker layers than the ones investigated here.27,50 In the present case, the unfavorable signal-to-noise ratio prevents the extraction of any reliable information from this model. Reflectivity. The dry mass of the adsorbed polyelectrolytes was measured with a home-build fixed-angle reflectometer with two rotating arms, which are equipped with a light source and a detector. The light source is polarized green diode laser with a wavelength of 533 nm, which can be modulated at a frequency of 229 Hz. The light beam is focused with an incident angle of 60° to the stagnation point. The actual angle of incidence is 71° due to light refraction in the prism. The reflected beam is detected with a polarizing beam splitter and two photodiodes. The beam splitter separates the beam in its perpendicular and parallel components, and their intensities are measured with lock-in amplification. The same crystal as used for the QCM measurements is mounted in a stagnation point flow cell and covered by a capped prism. The stagnation point is centered in the rotation axis of the goniometer. The solutions are pumped with a peristaltic pump through the bore hole in the prism. Further details on the reflectivity setup can be found elsewhere.54 These intensities measured by the two photodiodes are proportional to the reflectances RP and Rs that are related to the measured reflectivity signal R as

Materials. Poly(allylamine hydrochloride) (PAH) with a molecular mass in the range of 120−200 kg/mol (Alfa Aeser, Germany) and PLL hydrobromide of about 300 kg/mol (Sigma Aldrich, Switzerland) were used as cationic polyelectrolytes. The cationic polyelectrolytes were investigated at pH 4 and 10. The solution characteristics of these polyelectrolytes were studied earlier.43−45 Both polyelectrolytes are fully ionized at pH 4, and their degree of ionization at pH 10 is about 30% and 80% for PAH and PLL, respectively. The hydrodynamic radii of the two polyelectrolytes in 1 M monovalent salt solution are around 20 and 40 nm, respectively. The sodium salt of the anionic polyelectrolyte PSS with molecular masses of 70 and 300 g/mol (Alfa Aeser, Germany) and 5640 kg/mol (Polymer Standards, Germany) was used as well. PSS obtained from these two suppliers gave consistent results. The hydrodynamic radii in 1 M monovalent salt solution increase from around 5 nm for the lowest molecular mass to 50 nm for the highest one.47,48 Polyelectrolytes were dissolved in Milli-Q water containing NaCl, and solution pH was adjusted by adding HCl or NaOH. Fresh solutions were prepared weekly. QCM sensor crystals were purchased from Q-Sense (QSX301, Gotheborg, Sweden). They are coated with a gold layer of about 100 nm in thickness. A titanium layer of about 30 nm in thickness is sputtered on the gold layer, and on the top of the latter one a silica layer of about 300 nm was deposited. The crystals were cleaned with Milli-Q water, dried with nitrogen gas, and treated in an UV-ozone cleaner for 20 min. The cationic polyelectrolytes were adsorbed to these crystals directly. Anionic polyelectrolytes were adsorbed to positively charged amino-functionalized crystals, which were prepared as follows. The cleaned silica-sputtered crystals were placed overnight in a Petri dish aside of a sessile drop of about 40 μL of 3(ethoxydimethylsilyl)propylamine (Sigma Aldrich, Switzerland) in an evacuated desiccator. The silanized crystals were rinsed with ethanol and Milli-Q water, and finally dried with nitrogen. The crystals were cleaned after use as described above, and they could be reused several times yielding reproducible results. The root-mean-square roughness determined by atomic force microscopy (AFM) imaging in noncontact mode is 5.4 nm for the bare crystal and 10.3 nm for the functionalized one. This roughness prevents obtaining AFM images of the adsorbed polyelectrolytes. The precise thicknesses of the titanium and silica layers were determined by scanning angle null-ellipsometry, and these values agreed rather well with the ones given by the manufacturer. More details on the characterization of these sensor crystals can be found elsewhere.49 Quartz Crystal Microbalance. The wet mass of the adsorbed polyelectrolytes was measured by Q-sense E4 (Gotherborg, Sweden).

R=B

Rp Rs

(3)

where B is an unknown instrumental constant. This constant is eliminated by normalizing the reflectivity signal R(t) to its initial value 17507

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Article

R(t ) − R(0) R(0)

Surface roughness may influence these estimates, as one assumes homogeneous films for the analysis of reflectivity and QCM.62 We only refer to differences in the frequency shifts, and these shifts will only be influenced by changes in the roughness of the surface, and not by the actual roughness of the sample. However, the roughness is not expected to change much given the small thickness of the adsorbed polyelectrolyte layers. We suspect that such roughness effects could lead to minor frequency shifts well below 1 Hz. Roughness effects in the optical reflectivity are expected to be negligible, as they start to become important at much larger length scales.63 A similar approach has been used to determine the water content and layer thickness of polyelectrolyte multilayers.64 Note that the present polyelectrolyte films are too thin for their thickness to be determined by ellipsometry reliably.

(4)

From the reflectometry signal S(t) one obtains the adsorbed dry mass per unit area Γdry according to55

S(t ) = A Γdry

(5)

where A is the sensitivity constant. This constant A is calculated based on a homogeneous four-slab model. Thereby, the fourth topmost slab corresponds to the adsorbed polyelectrolyte layer whose refractive index is estimated from the ideal mixing law n = ns +

Γdry dn · L dc

■

(6)

RESULTS AND DISCUSSION Polyelectrolyte layers adsorbed from concentrated polyelectrolyte solutions containing high levels of salt may swell substantially upon changes in solution composition. This phenomenon will be evidenced by combining reflectivity and QCM experiments on planar silica substrates. Stimulus by Changes in Salt Concentration. The experiment shown in Figure 1 illustrates the swelling of a

where ns is the refractive index of water, L the thickness of the top layer, and dn/dc is the refractive index increment of the polyelectrolyte. These calculations require the refractive indices of silica, titanium, and gold at 533 nm, which are 1.457, 1.846 + 2.535i, and 0.458 + 2.426i as published earlier.56,57 The layer thicknesses of the silica and titanium layers needed in the calculation are taken from ellipsometric measurements. For the polyelectrolyte salts, the following known values of dn/dc were employed, namely 0.18 g/L for PLL,58 0.225 g/L for PAH,59 and 0.17 g/L for PSS.59 Given the thin layers investigated here, the results are basically independent of the thickness L. The typical value for sensitivity factor A for silica crystals was about 0.02 m2/mg. This value leads to a detection limit of about 3 μg/m2. One should note that reflectivity may achieve substantially better detection limits an oxidized silicon wafer due to the larger sensitivity factor.54 When different electrolytes are studied in the same experimental run, the respective baselines for each electrolyte must be determined separately due to the dependence of the reflectivity signal on the refractive index of the electrolyte solution. Water Content and Layer Thickness. The swelling of adsorbed polyelectrolyte films will be normally expressed as the relative change in the wet mass ΔΓwet Γ(0) wet

=

Γwet − Γ(0) wet Γ(0) wet

(7)

where Γwet is the wet mass of the film in the presence of the stimulating solution, and Γ(0) wet is the wet mass in an appropriate reference state. On the other hand, combining the dry mass per unit area Γdry obtained by reflectivity and the wet mass per unit area Γwet obtained by QCM, the mass fraction x of trapped water in the film can be estimated from the relation x=1−

Γdry Γwet

(8) Figure 1. Swelling of PAH layers adsorbed on silica at pH 10 induced by decreasing NaCl concentration probed by reflectivity (left) and QCM (right). The experimental trace is shown on top, while the adsorbed amount at the bottom. The response of the bare substrate is investigated by repeated flushing 1 M and 0.1 mM NaCl solutions of pH 10 (solution I and II). The layer is grown by adsorption from a PAH solution of a concentration 1 g/L in 1 M NaCl and pH 10 (solution III). Swelling of the polyelectrolyte layer is evidenced by QCM by repeated flushing with 1 M and 0.1 mM NaCl solutions of pH 10 (solution I and II). Reflectivity indicates that very little desorption takes place.

On the basis of this relation, one can show that the resulting change in the water content is Δx = x − x(0) = (1 − x(0))

ΔΓwet Γ(0) wet

(9)

(0)

where x is the water mass fraction of polyelectrolyte in the reference state. When the water content is known, the density ρ of the adsorbed film can be estimated from the ideal mixing relation

1 1−x x = + ρ ρp ρs

(10)

where ρs is the density of the NaCl solution, and ρp is the density of polyelectrolyte. The densities of NaCl solutions are tabulated, and the densities of PSS, PAH, and PLL are 1.2 g/mL, 1.1 g/mL, and 1.1 g/ mL, respectively.60,61 Once the density of the film is known, the average layer thickness of the polyelectrolyte can be obtained from

L=

Γwet ρ

PAH layer upon a decrease in the salt concentration at pH 10. The reflectivity experiments are shown in the left column of Figure 1. The bare silica substrate is first equilibrated with a 1 M NaCl solution (solution I). Then the substrate is flushed with 0.1 mM NaCl solution (solution II) and rinsed with 1 M NaCl solution again (solution I). The substrate responds in the same fashion when this process is repeated for a second time.

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Figure 2 shows the same experiment at a much lower PAH concentration of 5 mg/L. The adsorbed dry mass of the original

The reflectivity signal changes substantially due to the difference in the refractive indices of the respective solutions even though there is basically no change in the adsorbed amount. When the substrate is flushed with a solution of PAH in 1 M NaCl at a polyelectrolyte concentration of 1 g/L (solution III), one observes an increase in the reflectivity signal, which corresponds to an increase of 0.92 mg/m2 in the adsorbed dry mass. The dry mass measured by reflectivity yields only the mass of the adsorbed polymer and does not include any contributions from the eventually trapped water. The substrate is then flushed with a polyelectrolyte-free 1 M NaCl solution (solution I). A small amount of the adsorbed polyelectrolyte desorbs, but subsequent rinsing with the salt solutions does not lead to any further desorption. The corresponding QCM experiment demonstrating the swelling is shown on the right column in Figure 1. The bare silica substrate is again flushed with a sequence of 1 M and 0.1 mM NaCl solutions (solutions I and II). While a substantial frequency shift between the two solutions can be observed, this shift is related to differences in the density and viscosity of the solution, and does not reflect changes in the adsorbed amount. When the substrate is flushed with the PAH solution (solution III), one observes an increase in the adsorbed wet mass of about 1.9 mg/m2. The wet mass obtained by QCM includes the mass of the adsorbed polymer as well as the mass of the trapped water. The fact that the wet mass is about twice as large as the dry mass suggests that the film consists of comparable quantities of water and polyelectrolyte. The precise numbers are given in Table 1. When the film is flushed with a

Figure 2. Swelling of PAH polyelectrolyte layers adsorbed on silica at pH 10 induced by decreasing NaCl concentration probed by reflectivity (left) and QCM (right). The experimental trace is shown on top, while the adsorbed amount is at the bottom. The response of the bare substrate is investigated by repeated flushing 1 M and 0.1 mM NaCl solutions of pH 10 (solution I and II). The layer is grown by adsorption from a PAH solution of a concentration 5 mg/L in 1 M NaCl and pH 10 (solution III). Swelling of the polyelectrolyte layer is evidenced by QCM by repeated flushing with 1 M and 0.1 mM NaCl solutions of pH 10 (solution I and II). Reflectivity indicates that basically no desorption takes place. Swelling is much less pronounced than in the situation shown in Figure 1 since here the polyelectrolyte film is adsorbed from a solution of much lower salt concentration.

Table 1. Properties of Polyelectrolyte Layers Obtained Adsorbed to Silica from Solutions Containing 1 M NaCl and 1 g/L of the Polyelectrolyte PAH

dry mass (mg/m2)a wet mass (mg/m2)b water mass fractionc thickness (nm)c

PLL

PSS

pH 4

pH 10

pH 4

pH 10

70 300 kg/mol kg/mol

5640 kg/mol

0.53

0.73

0.40

0.68

1.4

1.6

1.7

1.4

1.9

1.4

1.5

1.7

2.8

3.2

0.63

0.61

0.71

0.55

0.18

0.43

0.47

1.3

1.7

1.3

1.4

1.5

2.5

2.9

film is 0.72 mg/m2, and the wet mass is 1.5 mg/m2. The swelling of such films is less pronounced than for the case discussed above, and the relative increase in wet mass is only 13%. Adsorption from solutions of lower polyelectrolyte concentrations leads to thinner and more heterogeneous films, and such films have a smaller capacity to swell. Figure 3 shows an analogous experiment at a high polyelectrolyte concentration but at low salt level. The adsorbed dry and wet masses are 0.22 mg/m2 and 0.74 mg/ m2. No swelling is observed in this situation. Similarly, no swelling has been observed when the film is adsorbed at low polyelectrolyte concentration and low salt concentration. The data for the latter experiment will not be discussed in detail as they are similar to the one shown in Figure 3. In both cases, one deals with thin and heterogeneous films, where the individual polyelectrolyte chains are well separated on the surface. The swelling of such films cannot be detected. The fact that high salt and high polyelectrolyte concentrations lead to the formation of thicker and more homogeneous films can be explained by the finite relaxation time of polyelectrolyte chains.49 At low polyelectrolyte concentrations, the individual adsorption events involving neighboring chains are infrequent. Therefore, an adsorbed chain will have time to relax, and will adopt a flatter and a more

a

Measured by reflectivity. bMeasured by QCM. cEvaluated by combining reflectivity and QCM results. The errors of the reported quantities are in the range of 10−20%.

polyelectrolyte-free 1 M NaCl solution, one observes that minor desorption takes place, as already suggested by the reflectivity experiment. The original layer after rinsing is always used as the reference state for the subsequent swelling studies. Stimulating the original film with a 0.1 mM NaCl solution leads to a substantial increase in the wet mass to about 3.6 mg/ m2 even though the dry mass remains constant. This increase indicates that the mass of water trapped in the adsorbed polyelectrolyte film increases when the salt level is lowered. In other words, the polyelectrolyte film swells upon a decrease in salt concentration. The resulting increase in the wet mass is about 95%. Repeated flushing of the film with low and high salt solutions indicates that swelling and collapse of the film are fully reversible. 17509

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Figure 4. Swelling of PAH polyelectrolyte layers adsorbed on silica at polyelectrolyte concentrations of 1 g/L and in 1 M NaCl solution stimulated with NaCl solutions of lower concentrations at pH 4 and 10. (a) Adsorbed wet mass obtained by QCM, and (b) the relative change in the wet mass relative to 1 M NaCl solution. (c) Layer thickness and (d) water mass fraction in the film obtained by combining QCM and reflectivity experiments.

Figure 3. Swelling of PAH polyelectrolyte layers adsorbed on silica at pH 10 induced by decreasing NaCl concentration probed by reflectivity (left) and QCM (right). The experimental trace is shown on top, while the adsorbed amount is at the bottom. The response of the bare substrate is investigated by repeated flushing 0.1 mM and 1 M NaCl solutions of pH 10 (solution I and II). The layer is grown by adsorption from a PAH solution of a concentration 1 mg/L in 0.1 mM NaCl and pH 10 (solution III). Swelling of the polyelectrolyte layer is evidenced by QCM by repeated flushing with 0.1 mM and 1 M NaCl solutions of pH 10 (solution I and II). Reflectivity indicates that no desorption takes place during these experiments. Swelling is much less pronounced than in the situation shown in Figure 1 since the polyelectrolyte film is adsorbed from a solution of much lower polyelectrolyte concentration.

same. The layer thickness increases from 1.7 to 3.3 nm at pH 10, while at pH 4 it changes from 1.3 to 2.5 nm, which corresponds to an increase of about 90%. The water mass fraction increases in both cases from about 0.6 to 0.8. The polyelectrolytes are strongly flattened since the layer thickness is substantially smaller than the hydrodynamic radius in solution.48 Figure 5 shows similar data for the swelling of adsorbed PLL films on silica at pH 4 and 10. Their original properties are summarized in Table 1. One observes that the PLL films swell upon decreasing the NaCl concentration in a very similar fashion as PAH films, reaching a relative increase in the wet mass of 70%. The main differences between the PLL and PAH system are that the adsorbed wet mass is basically independent of pH for PLL, while for PAH it increases with pH. The layer thickness is independent of pH for PLL, while for PAH the thickness increases with pH. On the other hand, the water mass fraction is independent of pH for PAH, while for PLL it increases with pH. We interpret this different behavior between PAH and PLL films as follows. PLL is known to undergo in solution a conformational transition between pH 9 and 10 from a random coil to an α-helix.65 This transition is accompanied by a collapse of the chain.46,66 Therefore, the adsorbed PLL at pH 10 is more compact, which leads to a smaller water content and less pronounced swelling. On the other hand, the adsorbed dry mass is larger at higher pH (see Table 1). These opposing effects result in comparable layer thicknesses at pH 4 and 10. PAH undergoes no conformational transition, and therefore the layer has similar water content at these two pH values. The adsorbed amount again increases with increasing pH and therefore leads to a larger layer thickness. However, the relative swelling behaviors of the PAH and PLL films remain similar.

compact conformation on the surface. At high polyelectrolyte concentrations, the adsorbing chains do not have sufficient time to relax due to simultaneous arrivals of other adsorbing chains into their neighborhood, and therefore they pack on the surface more densely, resulting in a thicker and a more homogeneous film (see graphical abstract). The adsorbed amount increases with increasing salt level due to the screening of electrostatic repulsion between the adsorbing chains. With increasing salt levels, the finite relaxation time of the adsorbing chains becomes more important. At low salt concentrations, the adsorbing polyelectrolytes repel more strongly, and individual adsorbed polyelectrolyte chains remain further separated on the surface, leading to thinner and laterally more heterogeneous films. At high salt levels, on the other hand, the resulting layers are thicker, denser, and laterally more homogeneous. Figure 4 shows the swelling of PAH layers adsorbed at high salt and high polyelectrolyte concentrations when stimulated with solutions of different salt concentrations. The situations at pH 4 and 10 are compared. The properties of the original adsorbed layers are summarized in Table 1. Figure 4a shows the increase of the wet mass for layers adsorbed in 1 M NaCl, while Figure 4b displays the corresponding relative increase by taking the original layer formed in 1 M NaCl as reference. One observes that the wet mass continuously increases with decreasing salt concentration of the stimulating solution. While the adsorbed amount is higher at pH 10 than at pH 4, the relative increase is in both solutions approximately the 17510

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quantities are also subject to experimental errors that are mainly caused by the variability between different crystals. Adsorbed layers of high molecular mass PSS swell to 80% when the NaCl concentration is decreased. However, the capacity of swelling decreases substantially with decreasing molecular mass. PSS of the lowest molecular mass investigated does not swell at all. This trend can be again rationalized by the finite relaxation time of the polyelectrolyte. This relaxation time increases strongly with increasing molecular mass, and therefore films made of high molecular mass polyelectrolyte are thicker and less compact. Therefore, such films swell more extensively. Based on the different systems investigated, the following generic trends can be deduced. Polyelectrolyte films adsorbed from solutions of high polyelectrolyte concentrations and high salt levels are stable in polyelectrolyte-free salt solutions, but they swell substantially when the salt level is decreased. The stability of the films can be explained with the irreversible attachment of the charged polyelectrolyte segments to the oppositely charged surface. The swelling of the layer originates from the progressively increasing repulsion between the different adsorbed polyelectrolyte chains with decreasing salt level. The swelling originates from a collective crowding phenomenon that involves interactions between adsorbed polyelectrolyte chains. Therefore, it can be only observed in dense and homogeneous films, which form in solutions of high polyelectrolyte concentrations and high salt levels. Films formed in solutions of low polyelectrolyte concentrations and low salt levels do not swell, since these films are dilute and the adsorbed polyelectrolytes interact only weakly. Similar swelling of polyelectrolyte monolayers with decreasing salt level was also observed with QCM with carboxymethyl cellulose films adsorbed on hydrophobic surfaces.41 On the other hand, carboxymethyl cellulose films adsorbed on regenerated cellulose were reported to shrink when flushed with deionized water.42 While the different substrate might be responsible for this opposing behavior, but it is also conceivable that the frequency shifts induced by changes in electrolyte solutions were not properly taken into account in the latter study. Another comparison can be made with direct force measurements between adsorbed polyelectrolyte monolayers.38−40,67 Polyelectrolytes layers were adsorbed to silica substrates from concentrated polyelectrolyte solutions in 1 M NaCl. Repulsive forces between such layers were measured at different salt concentrations, and the layer thickness of the adsorbed film was estimated from the range of these forces. The authors have concluded that such adsorbed layers progressively swell with decreasing salt concentration,38 and that their thickness increases with increasing molecular mass of the polyelectrolyte.39 While these findings are in full qualitative agreement with the present observations, the thicknesses estimated are in substantial disagreement. From the direct force measurements, values in the range of 20−200 nm were obtained, while the present reflectometry and QCM techniques yield thicknesses only between 2−5 nm. However, the present analysis is based on the simplifying assumption of a box segment profile, and the actual profile may strongly deviate from this idealization. These conflicting results could be rationalized with a two-layer profile. A compact and thin sublayer situated close to the interface would be mainly composed of polyelectrolyte trains. A dilute and more diffuse sublayer extending further away from the surface could resemble a brush consisting mainly of loops and tails. The

Figure 5. Swelling of PLL polyelectrolyte layers adsorbed on silica at polyelectrolyte concentrations of 1 g/L and in 1 M NaCl solution stimulated with NaCl solutions of lower concentrations at pH 4 and 10. (a) Adsorbed wet mass obtained by QCM, and (b) the relative change in the wet mass relative to 1 M NaCl solution. (c) Layer thickness and (d) water mass fraction in the film obtained by combining QCM and reflectivity experiments.

Figure 6 shows the dependence of the swelling of PSS films adsorbed to amino-functionalized silica on the molecular mass at pH 5.6. Their original properties are given in Table 1. The layer thickness as well as the water content of the original film increase with increasing molecular mass, and these quantities seems to saturate at high molecular mass. However, these

Figure 6. Swelling of polyelectrolyte layers of PSS with different molecular masses adsorbed on silica at polyelectrolyte concentrations of 1 g/L and in 1 M NaCl solution stimulated with NaCl solutions of lower concentrations at pH 5.6. (a) Adsorbed wet mass obtained by QCM, and (b) the relative change in the wet mass relative to 1 M NaCl solution. (c) Layer thickness and (d) water mass fraction in the film obtained by combining QCM and reflectivity experiments. 17511

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presently used surface sensitive techniques are likely to weigh more strongly the compact sublayer close to the interface, while the force measurements will be more sensitive to the diffuse sublayer situated further away. An analogous two-layer model was proposed recently to describe the internal structure of adsorbed layers of poly(ethylene imine) on silica.68 Similar differences between layer thicknesses obtained direct force measurements and ellipsometry were also reported for dense grafted brushes of poly(vinyl pyridine) and poly(acylic acid).17 However, the differences between the different techniques were a mere factor of 2, which is substantially smaller than the discrepancies reported above. Another reason for this discrepancy could be related to the lateral heterogeneity of these films. This heterogeneity increases with decreasing salt level and seems to be present up to micrometer length scales.40,67 The presently observed swelling of adsorbed polyelectrolyte layers with decreasing salt concentration qualitatively resembles swelling polyelectrolyte brushes in the so-called salted brush regime.9,12−19 In this case, the thickness of the polyelectrolyte brush scales as c−3/2 where c is concentration of monovalent salt. However, changes in the thickness of polyelectrolyte brushes can be substantial, sometimes even approaching 2 orders of magnitude. This characteristic feature of brushes contrasts the present findings for polyelectrolyte monolayers, where the salt dependence is substantially weaker, and the observed changes do not exceed a factor of 2. Stimulus by Changes in pH. Another possibility to trigger swelling of adsorbed polyelectrolyte films is by changes in solution pH. In the previous section we have shown that pH may influence the extent of swelling, albeit such effects were relatively minor. In the following, we will demonstrate that changes in solution pH may also trigger swelling and collapse of adsorbed polyelectrolyte films at constant salt level. Figure 7 shows that an adsorbed PAH film swells upon a decrease in pH. The film was deposited at high polyelectrolyte and 1 M NaCl solution at pH 10. The film was first rinsed with the polyelectrolyte-free 1 M NaCl solution of pH 10, and then with 0.1 mM NaCl solution of pH 10. Since the salt level is decreased, the film swells as discussed above. The film in this swollen state is now taken as a reference. When the film is flushed with a 0.1 mM NaCl solution of pH 4, one observes swelling of the film by additional 30%. Repeated rinsing of the film with these two solutions shows that the polyelectrolyte swells reversibly, even though some desorption can be observed too. Note that there is no substantial shift in the baseline upon change in pH, but a substantial one upon changing the salt concentration. Figure 8 shows a similar experiment, except that the film was adsorbed at pH 4. This film is finally incubated in 0.1 mM NaCl solution of pH 4 and then in the same solution of pH 10. Again one observes that the film swells upon decreasing the pH by about 30%. The swelling of the film upon decreasing pH can be understood through the change of the line charge density of the PAH. The polyelectrolyte is strongly charged at pH 4, and but only weakly at pH 10. We explain the swelling of the layer by electrostatic repulsions between the adsorbed polyelectrolytes at the surface. These chains are highly charged at low pH, and they stand up due to the crowding resulting from the electrostatic repulsion acting between the chains. These chains are weakly charged at high pH, and they collapse due to the weaker electrostatic repulsion. This mechanism is very similar to swelling of weak polyelectrolyte brushes, which swell when

Figure 7. Swelling of PAH polyelectrolyte layers adsorbed on silica at pH 10 induced by decreasing the pH probed by reflectivity (left) and QCM (right). The experimental trace is shown on top while the adsorbed amount in the bottom. The response of the bare substrate is investigated by flushing of 0.1 mM NaCl solutions of pH 10 and 4 (solution I and II) and by 1 M NaCl solution of pH 10 (solution III). The layer is grown by adsorption from a PAH solution of a concentration 1 g/L in 1 M NaCl and pH 10 (solution IV) and flushed again with the pure electrolyte (solution III). Swelling of the polyelectrolyte layer is evidenced by QCM by repeated flushing with 0.1 mM NaCl solutions pH 10 and 4 (solution I and II). Reflectivity indicates that only little desorption takes place during these experiments.

the polyelectrolytes are highly charged, but collapse when they are close to neutral.12,13,21 Figure 9 compares results of two sets of more extensive experiments where PAH films were incubated in solutions of different pH. In the first set, PAH was adsorbed at pH 10 at high salt solution and then incubated in 0.1 mM NaCl of pH 10 as discussed above. This film was then subsequently exposed to 0.1 mM NaCl solutions of decreasing pH, and the relative change in the wet mass is recorded with the QCM. The film remains compact down to about pH 6.2 and then swells abruptly almost to 30% within a narrow pH range. In the second set of experiments, the PAH film was deposited at pH 4.0 and incubated in 0.1 mM NaCl of pH 4 as above. This film was exposed to 0.1 mM NaCl solutions of increasing pH. In this case, the film remains swollen all the way to about pH 7−8 and then suddenly collapses for pH above 8. While such a discontinuous swelling-collapse hysteresis may seem surprising, a very similar hysteresis was reported in the swelling of PAH/PSS multilayer films.31 This study reported that the thickness of films deposited at certain pH would swell appreciably in a very narrow pH range. This range would be situated at low pH when the pH was decreased, and at high pH when the pH was increased, leading to a substantial hysteresis of the swelling process, analogous to the one observed here. Our interpretation of the hysteresis in the swelling process is similar to the one proposed earlier.31 Repulsive electrostatic interactions between the polyelectrolyte chains compete with attractive electrostatic or hydrophobic interactions between the 17512

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variations of pH and salt.69,70 Similarly to PAH monolayers, the adsorbed dendrimers swell with deceasing pH. Even though this behavior is qualitatively similar to the one described here, we suspect that the mechanism is different. The swelling of the dendrimers appears to be governed by interactions acting between a single adsorbed dendrimer and the substrate. When the strength of these attractive forces between the dendrimer and the substrate decreases, the dendrimers swell. For polyelectrolyte monolayers, however, the swelling appears to be a collective phenomenon within the dense adsorbed polyelectrolyte layer. Thereby, repulsive interactions between the adsorbed polyelectrolyte chains compete with attractive interactions between the polyelectrolytes and the substrate. The following three observations illustrate this difference in the swelling mechanisms. First, the salt dependence is opposite in both sytsems. When the salt concentration is decreased, adsorbed dendrimers collapse, while polyelectrolyte monolayers swell. Second, dendrimers swell even when they are adsorbed at low salt conditions, where the individual dendrimers are widely separated on the surface and the monolayer is very dilute. Polyelectrolyte monolayers do not swell under such conditions. Finally, the pH dependence is continuous for the dendrimers, while it shows a hysteresis for the polyelectrolyte monolayer. The existence of a hysteresis strongly suggests that this transition is driven by a collective effect. Comparison with Adsorption of Polyelectrolyte Monolayers. We would also like stress that properties of polyelectrolyte films adsorbed on oppositely charged substrates depend very differently on the composition of the polyelectrolyte-containing solution from which this layer is adsorbed, and on the composition of the polyelectrolyte-free solution used to stimulate a previously adsorbed film. How properties of adsorbed polyelectrolyte films vary with the polyelectrolytecontaining solution from which the adsorption takes place was studied frequently in the past.71−76 These properties were also studied for the same systems as the ones discussed here.49 Adsorbed amount and thickness of adsorbed polyelectrolyte layers normally increase with increasing salt concentration of the polyelectrolyte-containing solution. Thereby, the water content of the film remains approximately constant. This effect originates from the weakening of attractive interactions between the adsorbed polyelectrolyte and the oppositely charged substrate. On the other hand, an adsorbed polyelectrolyte film exposed to polyelectrolyte-free solutions of decreasing salt concentrations swells, and the water content of the film increases. This behavior originates from the repulsive interactions between the adsorbed polyelectrolyte chains, and only occurs in dense layers. The adsorbed amount of weak cationic polyelectrolytes on silica normally increases with pH and so does the film thickness. This phenomenon can be explained by weakening of the attractive interactions by the presence of the increasing charged substrate.72,73,77 On the other hand, such polyelectrolyte films swell when they are incubated in solutions of decreasing pH. In this case, the adsorbed amount remains constant, but the thickness increases due to swelling. This swelling is induced by repulsive interactions between the adsorbed chains.

Figure 8. Swelling of PAH polyelectrolyte layers adsorbed on silica at pH 4 induced by increasing the pH probed by reflectivity (left) and QCM (right). The experimental trace is shown on top while the adsorbed amount in the bottom. The response of the bare substrate is investigated by repeated flushing of 0.1 mM solutions of pH 4 and 10 (solution I and II). The layer is grown by adsorption from a PAH solution of a concentration 1 g/L in 1 M NaCl and pH 4 (solution III). Swelling of the polyelectrolyte layer is evidenced by QCM by repeated flushing with 0.1 mM NaCl solutions pH 4 and 10 (solution I and II). Reflectivity indicates that only little desorption takes place during these experiments.

Figure 9. Comparison of the swelling of adsorbed layers of PAH on silica with pH. The polyelectrolyte layer is adsorbed in 1 M NaCl electrolyte pH 4 or pH 10, and then flushed with 0.1 NaCl of same pH. The relative change in the wet mass is measured with QCM when the layer is exposed to 0.1 mM NaCl solutions of decreasing pH. The wet mass of the layer at pH 4 is taken as reference.

polyelectrolyte and the substrate. At low pH, the repulsive chain−chain interactions dominate, while at high pH the attractive chain-substrate interactions become more important. Due to the presence of attractive interactions, this conformational change may proceed by a mechanism that resembles a first-order phase transition. Such transitions are often accompanied by a hysteresis, and they have also been proposed to be operational for conformational transitions in solution of hydrophobic polyelectrolytes.4,5,7 Comparison with Swelling of Adsorbed Dendrimers. Poly(amido amine) (PAMAM) dendrimers adsorbed to silica substrates were shown to undergo substantial swelling upon

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CONCLUSIONS Polyelectrolyte monolayers may become responsive when adsorbed from solutions of high polyelectrolyte concentrations and containing high concentrations of salt. Films involving 17513

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(7) Uyaver, S.; Seidel, C. First-order conformational transition of annealed polyelectrolytes in a poor solvent. Europhys. Lett. 2003, 64, 536−542. (8) Cohen Stuart, M. A.; Huck, W. T. S.; Genzer, J.; Muller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Emerging applications of stimuli-responsive polymer materials. Nat. Mater. 2010, 9, 101−113. (9) Ballauff, M.; Borisov, O. Polyelectrolyte brushes. Curr. Opin. Colloid Interface Sci. 2006, 11, 316−323. (10) Zhou, F.; Huck, W. T. S. Surface grafted polymer brushes as ideal building blocks for “smart” surfaces. Phys. Chem. Chem. Phys. 2006, 8, 3815−3823. (11) Kozlovskaya, V.; Kharlampieva, E.; Mansfield, M. L.; Sukhishvili, S. A. Poly(methacrylic acid) hydrogel films and capsules: Response to pH and ionic, strength and encapsulation of macromolecules. Chem. Mater. 2006, 18, 328−336. (12) Guo, X.; Ballauff, M. Spatial dimensions of colloidal polyelectrolyte brushes as determined by dynamic light scattering. Langmuir 2000, 16, 8719−8726. (13) Guo, X.; Ballauff, M. Spherical polyelectrolyte brushes: Comparison between annealed and quenched brushes. Phys. Rev. E 2001, 6405, 051406. (14) Hariharan, R.; Biver, C.; Mays, J.; Russel, W. B. Ionic strength and curvature effects in flat and highly curved polyelectrolyte brushes. Macromolecules 1998, 31, 7506−7513. (15) Hariharan, R.; Biver, C.; Russel, W. B. Ionic strength effects in polyelectrolyte brushes: The counterion correction. Macromolecules 1998, 31, 7514−7518. (16) Ahrens, H.; Forster, S.; Helm, C. A. Charged polymer brushes: Counterion incorporation and scaling relations. Phys. Rev. Lett. 1998, 81, 4172−4175. (17) Drechsler, A.; Synytska, A.; Uhlmann, P.; Elmahdy, M. M.; Stamm, M.; Kremer, F. Interaction forces between microsized silica particles and weak polyelectrolyte brushes at varying pH and salt concentration. Langmuir 2010, 26, 6400−6410. (18) Biesalski, M.; Ruhe, J. Scaling laws for the swelling of neutral and charged polymer brushes in good solvents. Macromolecules 2002, 35, 499−507. (19) Biesalski, M.; Johannsmann, D.; Ruhe, J. Electrolyte-induced collapse of a polyelectrolyte brush. J. Chem. Phys. 2004, 120, 8807− 8814. (20) Schneider, C.; Jusufi, A.; Farina, R.; Pincus, P.; Tirrell, M.; Ballauff, M. Stability behavior of anionic spherical polyelectrolyte brushes in the presence of La(III) counterions. Phys. Rev. E 2010, 82, 011401. (21) Wang, Y. L.; Chang, Y. C. Synthesis and conformational transition of surface-tethered polypeptide: Poly(L-lysine). Macromolecules 2003, 36, 6511−6518. (22) Netz, R. R.; Andelman, D. Neutral and charged polymers at interfaces. Phys. Rep. 2003, 380, 1−95. (23) Lyatskaya, Y. V.; Leermakers, F. A. M.; Fleer, G. J.; Zhulina, E. B.; Birshtein, T. M. Analytical self-consistent-field model of weak polyacid brushes. Macromolecules 1995, 28, 3562−3569. (24) Zhulina, E. B.; Borisov, O. V. Poisson−Boltzmann theory of pHsensitive (annealing) polyelectrolyte brush. Langmuir 2011, 27, 10615−10633. (25) Biesheuvel, P. M. Ionizable polyelectrolyte brushes: Brush height and electrosteric interaction. J. Colloid Interface Sci. 2004, 275, 97−106. (26) Dodoo, S.; Steitz, R.; Laschewsky, A.; von Klitzing, R. Effect of ionic strength and type of ions on the structure of water swollen polyelectrolyte multilayers. Phys. Chem. Chem. Phys. 2011, 13, 10318− 10325. (27) Guzman, E.; Ritacco, H.; Rubio, J. E. F.; Rubio, R. G.; Ortega, F. Salt-induced changes in the growth of polyelectrolyte layers of poly(diallyl-dimethylammonium chloride) and poly(4-styrene sulfonate of sodium). Soft Matter 2009, 5, 2130−2142.

weak polyelectrolytes may also swell upon pH variations due to variations in their line charge density. This swelling behavior of adsorbed polyelectrolyte monolayers qualitatively resembles polyelectrolyte multilayers as well as grafted polyelectrolyte bushes, as the latter two also swell upon lowering the salt level or upon increasing the line charge density. However, two main differences remain. Swelling of adsorbed polyelectrolytes films is smaller than for polyelectrolyte multilayers and quite modest when compared to polyelectrolyte brushes. We have observed that adsorbed polyelectrolyte monolayers may swell in thickness up a factor of 2, while polyelectrolyte multilayers up to a factor of 4,31,32 and polyelectrolyte brushes by more than an order of magnitude.12,16,17 We have reported hysteresis effects in the swelling-collapse transition of adsorbed PAH films, which is similar to the one observed ion polyelectrolyte multilayers,31,32 while in brushes such transitions seem to always be reversible.12,13 The response to stimulus of polyelectrolyte monolayers is a poorly investigated subject, but the present study clearly demonstrates that such systems respond systematically to stimuli induced by variations of salt levels and pH. The unique feature of adsorbed polyelectrolyte monolayers is that they are ultrathin, typically a few nanometers, which is substantially smaller than the thickness of polyelectrolyte multilayers or brushes. Given their responsive behavior described here, they may represent interesting alternatives in material science applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; phone: +41 22 379 6405. Present Address †

ABB Schweiz AG, Baden-Dättwil, Switzerland.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Christiane Helm for useful comments on the manuscript. This research was supported by the Swiss National Science Foundation and the University of Geneva.

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REFERENCES

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