Zeta Potential of Polyelectrolyte Multilayers Using the Spinning Disk

Jul 18, 2014 - Department of Chemistry and Biochemistry, The Florida State University, ... Universidad Politécnica de Valencia, 46021 Valencia, Spain...
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Zeta Potential of Polyelectrolyte Multilayers Using the Spinning Disk Method Maria Ferriz-Mañas†,‡ and Joseph B. Schlenoff*,† †

Department of Chemistry and Biochemistry, The Florida State University, Tallahassee, Florida 32306-4390, United States Escuela Técnica Superior de Ingenieros Industriales, Universidad Politécnica de Valencia, 46021 Valencia, Spain

‡

S Supporting Information *

ABSTRACT: Zeta potentials of surfaces bearing stable mono- or multilayers of polyelectrolyte were determined using the spinning disk method recently described by Sides et al. (Langmuir 2004, 20, 11493−11498). In this technique, the streaming potential difference between two electrodes, one at the disk surface, is quantitatively related to zeta potential. Variables such as rotation speed, electrolyte concentration, and electrode distance from the disk surface were explored and used to validate the recently-described theory, which emphasizes minimal contribution to net potential from surface conductivity. Layer-by-layer oscillations in sign and magnitude of the zeta potential were observed, in accord with prior work using electrophoretic mobility of multilayer-coated particles and other streaming potential measurements. The open geometry and the excellent mass transport of the spinning disk allowed in-situ observation of surface charge switching during the addition of a layer. As with all zeta potentials, especially those recorded at soft interfaces, translating results to quantitative densities of fixed surface charge is a challenge.

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INTRODUCTION Surface potential measurements under liquid flow are widely used to understand the behavior of particles and surfaces in solution.1 Classically, the surface, or zeta, potential of particles is measured by observing their velocity in response to an electric field. The zeta potential of larger samples is typically measured by either flowing electrolyte past them and measuring the voltage gradient developed (streaming potential) or by applying an electric field parallel to the surface and measuring the fluid velocity (electroosmotic flow).1 For flat samples, electrokinetic measurements are conveniently made by clamping the surface in proximity to another, forming a capillary channel, and pumping electrolyte through. The streaming potential is measured between electrodes at both ends of the channel. There are a few disadvantages to this technique.1 First, the flow channel material may be different from the test surface, requiring a correction. Second, a path for current flow through the excess ions collected at the surface can “short” the measured potential. This surface conductivity results in lower streaming potentials, requiring another correction for reliable zeta potentials. Finally, samples must be removed from solutions to which they were exposed then mounted in the capillary apparatus. It is thus difficult to watch any changes of surface potential during an adsorption experiment in real time. Sides and co-workers have recently described a promising technique for employing spinning disks to measure zeta potentials.2−5 In this method, a sensing electrode is placed in close proximity to the central axis of the disk and another is placed at some distance. The streaming current path goes outward along the disk surface and returns through the © XXXX American Chemical Society

solution, as shown in Figure 1. The electrodes record a streaming potential that, as these researchers have shown,

Figure 1. Sketch of the spinning disk for zeta potential measurements. Surface current flows radially outward along disk; the current returns through the bulk. The streaming potential generated can be read at any point on the disk, but it has a maximum value on the axis. Adapted from Figure 2 of ref 4.

provides the zeta potential. This spinning disk geometry does not suffer from contributions to measured potentials from cell materials. In addition, because the disk is spinning continuously, in-situ measurements of zeta potentials are possible with well-defined mass transport of reagents added to solution. Finally, in a recent theoretical analysis, Sides and Prieve6 have Received: April 23, 2014 Revised: June 30, 2014

A

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shown that surface conductivity effects at the spinning disk are negligible if the disk radius is larger than the distance from the disk surface to the sensing electrode. In the present work, we apply this new geometry, and the new theory that comes with it, to investigate zeta potentials at polyelectrolyte multilayers, PEMUs. Surface charges are believed to play a central role in the assembly of ultrathin films of polyelectrolytes or nanoparticles using the layer-bylayer method, where a substrate is alternately exposed to solutions of components.7 In general, it is understood that the surface charge alternates with the assembly process, switching back and forth between positive and negative as each component is added to the growing film. Particle or planar surfaces coated with multilayers show such charge alternation using electrophoresis,8−16 streaming potentials,17−19 or electroosmosis.20 The spinning disk method, despite its advantages, has thus far been used for a limited number of surface monolayer systems, such as silicon oxide on silicon (i.e., nascent silicon wafer),2,3 silicon carbide,21 and mica.4,5 We were interested in extending the method to polyelectrolyte multilayers. This work focuses on validating the spinning disk method for the multilayer system. Several predictions regarding rotation rate, electrolyte strength, and distance from the electrode are made by the theory. The multilayer system, however, has potentially very different properties compared to “hard” interfaces. For example, the surface charge is spread out over more than a monolayer and the film can be quite soft.22 With a thicker layer of charge, the system might be more susceptible to errors from surface conductivity. Our multilayer is a combination of poly(styrenesulfonate), PSS, and poly(diallyldimethylammonium), PDADMA. Much is known about PSS/PDADMA multilayers, including the fact that the surface of the film is much softer, almost gel-like, when terminated with PDADMA, whereas the PSS-capped film is glassy.22 Possible effects of high shear rates on surface potential and measurements thereof are unknown. Finally, we present an example, the first to our knowledge, of the real-time in-situ measurement of surface charge switching during the addition of a layer of polyelectrolyte during multilayering.

about 0.5 to about 5 mm. Another Ag electrode was located in the periphery of the cell shrouded in a glass tube. Three aluminum fins or baffles prevented excessive vortexing as the disk was spun. The cell was fitted with a water jacket and was maintained at 25 ± 0.1 oC with a Haake circulator. A DC power supply (BK Precision Multi Range 60 V/5A) was used to run the motor. The applied voltage was correlated to the rotation rate of the disk assembly in solution using a Meiko digital tachometer. The streaming potential between Ag electrodes was recorded with a Keithley 617 electrometer interfaced via an IEEE-488 bus to a computer running Labview data collection software. Two substrates were employed: PEEK or silicon wafer. The PEEK disk surface was polished first with progressively finer sandpaper, then with 5 μm alumina suspension, and finally with 0.3 μm alumina micropolish (both from Buehler Inc.). The silicon was 1 in. diameter polished Si[100] attached to the PEEK with Parafilm. The conductivity of the electrolyte, κ, was adjusted with NaCl and was measured with a Thermo Orion 3 Star conductivity meter fitted with a calibrated four-probe electrode. κ was measured before and after zeta potentials were recorded to verify that no contamination of solutions had occurred. Solutions were not buffered but were maintained at a pH in the range of 5.5−6.0 from ambient dissolved CO2. The following nomenclature for multilayers is used: substrate*priming layer-(A/B)x, where the substrate was either PEEK or Si wafer; the priming layer, used only for PEEK, was PEI; A and B are the polyelectrolytes used to build the multilayer; and x is the number of repeats for the A/B layer pairs. The terminating layer is on the right-hand side. For example, a PEEK substrate with PEI priming layer and five alternating PSS/PDADMA layer pairs terminated in PDADMA is PEEK*PEI-(PSS/PDADMA)5. If an additional PSS layer were added to terminate the multilayer, the system would be PEEK*PEI-(PSS/PDADMA)5PSS. The equation for converting the measured streaming potential ϕS to a zeta potential ζ is5

EXPERIMENTAL SECTION Anionic polyelectrolyte poly(4-styrenesulfonic acid) (molar mass ∼75 000, 18 wt % in water) and cationic polyelectrolyte poly(diallyldimethylammonium chloride) (molar mass 400 000−500 000 20 wt % in water) were purchased from Aldrich. PSS was neutralized with sodium hydroxide to a pH of 7 before use. Branched poly(ethylenimine), PEI, (molar mass ∼70 000, 30% aqueous solution) was purchased from Polyscience, Inc. Sodium chloride, hydrogen peroxide (30%), ammonium hydroxide (28−30% ammonia), and sodium hydroxide were used as received from Aldrich or Mallinckrodt. Sulfuric acid (17.8 M, 95−98%) was from EMD Chemicals. Deionized water (Barnstead, E-pure, Milli-Q) was used to prepare all aqueous solution. Its conductivity under ambient conditions (after dispending and standing in a beaker) was below 0.3 μS cm−1. The apparatus was based on systems described by Sides and Hoggard.2−4 A photograph of the cell is seen in the Supporting Information. A 1 in. diameter disk was machined out of polyetheretherketone (PEEK) rod and was press-fit on the end of a small DC motor (Portescap). A silver wire coated with AgCl was placed in a Luggin capillary. The tip of the capillary was located on the axis of the disk at a distance ranging from

(1)

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ζ=

1.96κν1/2 1 + z̅2 ϕS 3/2 εa Ω 1 − 2z ̅ 1 + z ̅ 2 + 2z ̅ 2

where z̅ ≡ z/a and where a, Ω, ν, and z represent the disk radius, disk rotation rate, kinematic viscosity, and axial distance from the disk, respectively. It is assumed that one reference electrode is at least one diameter away from the disk and that the other reference is centered on the axis. If z is less than 10% of the disk radius, the following simplification applies with less than 1% error:4 ζ=

1.96κν1/2 εaΩ3/2 ⎛ ⎜ 2⎜1 − ⎜ ⎝

1 z a

⎞ ⎟ 1 − 2 1/2 ⎟ ⎛z ⎞ ⎟ 2 2 +1 ⎝a ⎠ ⎠ ⎜

⎟

ϕS

(2)

The kinematic viscosity (m2 s−1) was calculated from the density (kg m−3) and dynamic viscosity (kg m−1 s−1) using v = η/ρ. Tabulated values of viscosity and density of salt solutions at 25 °C were obtained from the literature.23 The permittivity (ε) of water at 25 °C (6.94 × 10−10 F m−1 or 6.94 × 10−12 s Ohm−1 cm−1) was ε = εrε0, where εr is the relative permittivity (78.4 at 25 °C) and ε0 is the vacuum permittivity (8.85 × 10−12 B

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F m−1, all data from the CRC Handbook of Chemistry and Physics23). Multilayers were assembled using a robotic platform (StratoSequence V, nanoStrata Inc.) using 10 mM polyelectrolytes (concentration based on the repeat unit) in 0.10 M NaCl at room temperature. The dipping time in each polyelectrolyte was 10 min for PEEK substrates and 5 min for Si substrates. In between polymer layers, the films were rinsed three times in water for 1 min each. The PEI priming layer was adsorbed by manually dipping the PEEK into 10 mM PEI in 0.1 M NaCl for 60 min. PEI is often used as a priming layer for multilayer growth, especially on substrates that are not well characterized. In contrast, we have had extensive experience with silicon as a substrate, which needs no priming layer. Multilayer thicknesses on Si were measured with a Gaerner Scientific L116S Autogain ellipsometer using 632.8 nm radiation at 70° incident angle. A refractive index of 1.46 was employed. Thicknesses were recorded on 10 different regions of the wafer and were averaged. PEEK as a substrate was freshly polished before each polyelectrolyte deposition. Si wafers were first cleaned in piranha (80% vol H2SO4, 20% vol H2O2) for 10 min; were rinsed; then were cleaned in 50% vol NH4OH, 50% H2O2 for 10 more minutes; and finally, were rinsed with water and were dried with a stream of nitrogen. To measure the streaming potential, the motor was turned on until the streaming potential voltage stabilized (approximately 5 min), and then the motor was turned off and on for several equally spaced intervals of time, usually 20 s. This crude square wave modulation generated a quasi-square wave response in streaming potential. Zero streaming potential was taken to be at the base of these waves (no rotation), and the streaming potential was thus the height of the waves (peaks). Usually, four peaks of 20 seconds each were generated in sequence, and the streaming potential was the average of these. The parameters for the measurements were, unless otherwise specified, 1 mm electrode distance, 2580 rpm, and 10−4 M NaCl.

generated from motor switching, but these were ignored in measurements of streaming potential. An example of raw data is shown in Figure 2A, depicting sequential batches of four off−

Figure 2. Example of the raw response and stability of PEEK*PEI spinning disk. The test was run for 4 h in 5 × 10−4 M NaCl at 2850 rpm. Every 20 min, a series of four measurements was taken (the disk was switched off for 20 s). Note that the baseline fluctuates over time, while the peak heights (the streaming potential) remain approximately constant.

on motor cycles between longer on times (i.e., the disk spins continuously). A higher resolution graph of one of these batches is shown in Figure 2B. The streaming potential is the difference between on and off potentials. Streaming potentials for peaks in each batch were averaged. Figure 3 shows some results when streaming potentials are converted to zeta potentials with the aid of eq 2. This figure also illustrates the long-term (>2 h) stability of monolayer and multilayer coatings on PEEK under spinning. Following a slight change at short times (compare the first and second points), these systems, in 5 × 10−4 M NaCl, are quite stable with no evidence of detachment of the coating at the high shear rate

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RESULTS AND DISCUSSION The spinning disk proved to be a versatile and responsive method for measuring zeta potentials on substrates coated with monolayers and multilayers of polyelectrolyte. From the many possible combinations of polyelectrolyte and substrates, coatings on PEEK, a hard engineering plastic, and silicon wafer were explored. Polyelectrolytes PSS and PDADMA do not require solution buffering because they are fully charged over a wide range of operating pH. Branched PEI, bearing amines, was assumed to be fully charged over the pH range of the experiments (pH 5.5−6). All parameters could be accurately adjusted except for the position of the sensing electrode. For a distance of 1 mm, our positioning was, at best, about ±0.1 mm (±10%). For those experiments which did not involve reposition or replacement of the substrate, streaming potentials were highly repeatable. The PEEK itself showed negative zeta potentials after cleaning, but the magnitude of these potentials was not reproducible and the surface is considered undefined. Streaming potentials on the order of 1 mV were typically superimposed on a drifting background. Instead of sinusoidal modulation to distinguish streaming from background,5 a square wave was applied by turning the motor on and off (manually). Small spikes and overshoots were occasionally

Figure 3. Zeta potential of PEEK*PEI (□) and PEEK*PEI(PSS/ PDADMA)4PSS (◊) over time. For the latter, the difference between the first measurement and the last one is less than 15 mV. The disk spins continuously at 2850 rpm in 5 × 10−4 M NaCl. C

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employed. Zeta potentials are in the range of ±120 mV depending on the charge of the outside layer Similar results were obtained on silicon wafer. Figure 4 depicts zeta potentials of multilayers terminated in either PSS

The distance between the electrode and the disk surface was varied with the aid of a mechanical dial indicator (see picture in Supporting Information). Within the accuracy of this adjustable parameter, the streaming potential followed the expected dependence on distance, from 1 to 10 mm, from the rotating disk (Supporting Information). Streaming Potential versus NaCl Concentration. The theory for the spinning disk, eq 2, predicts an inverse relationship between streaming potential and electrolyte concentration for constant zeta potential. However, zeta potential is typically a function of electrolyte concentration, at least for hard interfaces.27 Figure 6 depicts streaming potentials at positive and negative multilayer surfaces as a function of electrolyte concentration.

Figure 4. Zeta potential of Si*(PDADMA/PSS)5 (◊), Si*(PDADMA/ PSS)5PDADMA (□), and silicon wafer (○) over time with the disk spinning continuously between measurements. Parameters: 10−4 M NaCl, electrode distance 1 mm, 2850 rpm.

or PDADMA, this time in 10−4 M NaCl. Again, a slight initial dropoff is seen in the zeta potentials at multilayer surfaces, after which the potentials are stable. Figure 4 also depicts the response of a clean silicon wafer. The zeta potential, about −100 mV in 10−4 M salt, is comparable to prior spinning disk measurements3 as well as to other literature values.24−26 The slight initial decrease in silicon zeta potential, possibly related to slow hydrolysis of the native oxide on the wafer, is beyond the scope of our discussion. The PEI monolayer showed the most stable zeta potential, whereas that for the multilayers decreased slightly at first, possibly evidence of initial surface or charge rearrangement. Streaming Potential versus Rotation Rate. From eq 2, streaming potential should be proportional to the [rotation rate]3/2. The rotation of the system used here could be adjusted to about 3500 rpm before solution vortexing became a serious issue. Figure 5 shows streaming potential as a function of Ω3/2 for two multilayers. The fit to the expected scaling is excellent. No deviations were seen for either the glassy PSS-terminated multilayer or the softer, more gel-like PDADMA-terminated film22 at higher rpm.

Figure 6. Streaming potential vs NaCl concentration for Si*(PDADMA/PSS)5 (◊) and Si*(PDADMA/PSS)5PDADMA (□). Parameters: electrode distance 1 mm and 2850 rpm. The crosses (X) represent dilution from about 10−3 NaCl to about 5 × 10−4 M and back to 10−3 M.

These streaming potentials are converted to zeta potentials (as a function of salt concentration) in Figure 7. The zeta potential remained fairly constant at the lowest salt concentrations, exhibiting a slight maximum at around 10−4 M, and then decreased slightly up to 10−3 M. The error increases toward higher [NaCl] since the streaming potential becomes much less than 1 mV (and signal-to-noise decreases),

Figure 7. Zeta potential vs NaCl concentration for Si*(PDADMA/ PSS)5 (◊) and Si*(PDADMA/PSS)5PDADMA (□). The crosses (X) represent the results when diluting back to 5 × 10−4 M and concentrating back to 10−3 M. Parameters: electrode distance 1 mm and 2850 rpm.

Figure 5. Streaming potential vs rotation rate3/2 for Si*(PDADMA/ PSS)5 (◊) and Si*(PDADMA/PSS)5PDADMA (□). The linear fit is expected from eq 2. Parameters: 10−4 M NaCl and electrode distance 1 mm. D

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although the values for the dilution and reconcentration steps were close to expected. The rapidly-diminishing streaming potential as a function of salt concentration points to an upper limit, in the millimolar range, for salt concentration during the measurement. Such a limit precludes measurements of zeta potentials for most conditions of multilayer assembly (typically in the range of 0.1−1 M salt).28 For hard interfaces such as silica, the following empirical relationship is observed29 ζ = a1 + a 2( −log ∑ ni) i

(3)

where a1 and a2 are constants (usually a1 ≈ 0) and where the concentrations of all counterions i are summed. Such behavior has been observed at silica using classical streaming potential techniques24−27 and the spinning disk method.3 The response of the multilayer electrokinetic data to salt concentration is quite different (Figure 7), that is, a1 is no longer negligible. Zeta Potential versus Number of Layers. Having validated eq 2 for typical variables, we turned to the measurement of zeta potential during multilayer buildup. The disk was dipped in 10 mM PDADMA in 0.10 M NaCl for 5 min at a rotation rate of 300 rpm and then was rinsed in three consecutive beakers of 10−4 M NaCl, 1 min each. The disk was transferred to the zeta potential cell containing 10−4 M NaCl and was spun at 2850 rpm. The disk was then removed, was coated with PSS (10 mM in 0.10 M NaCl), was rinsed with 10−4 M NaCl, and was reinserted into the zeta potential cell. The process was repeated for eight more layers. The layering process was repeated on pieces of silicon wafer, and the dry thickness was measured with an ellipsometer. The wet thickness of this multilayer was about 50% greater than the dry thickness.30 The zeta potential on silicon wafer decreased with time at first (in 2 h from −120 mV to −88 mV) as seen above. This drift was again ascribed to undefined silica chemistry. As with PEI on PEEK, a single polycation layer, PDADMA, was stable and reached the highest zeta potential (Figure 8). Thereafter, each layer reversed the surface charge, as seen previously with electrophoretic,9−16 capillary streaming,17−19 and electroosmotic20 measurements of multilayers. Within the error of our measurements, the zeta potential was independent of the number of layers. The magnitude of the zeta potential for positive and negative layers was similar: PEMUs terminated with PDADMA showed a zeta potential of 105 ± 18 mV and those capped with PSS were −94 ± 10 mV. The thickness of the multilayer depends on the number of layers (Figure 8) with slight upward curvature as seen previously for this system.31 The average thickness increment per layer is about 15 Å, but the PDADMA increments tend to be somewhat larger since it is the PDADMA that overcompensates the surface charge, whereas the PSS merely compensates the existing surface charge.31 The reproducibility of the data is lower than that for our previous electroosmotic measurements,20 slightly worse than the streaming potential measurements of Adamczyk et al.18 or Ladam et al.,17 but on par with electrophoretic measurements of multilayers on particles.11−16 Better reproducibility was obtained with a modified setup, built expressly for multilayering, that allowed removal of the disk assembly from the lid and reinsertion with greater position repeatability. An example of zeta potential alternation for multilayers built, using this assembly, in 0.2 and 0.3 M NaCl is shown in the Supporting

Figure 8. (A) Zeta potential (□) and dry thickness (◊) vs number of layers for a PDADMA/PSS multilayer built on Si wafer. Each layer was deposited from 10 mM polyelectrolyte in 0.1 M NaCl. Layer 0 corresponds to the Si wafer. Even layers are terminated with PSS, and odd layers are terminated with PDADMA. Parameters: 10−4 M NaCl, 1 mm electrode distance, and 2850 rpm. (B) Data from electroosmotic measurements in 0.02 M ionic strength buffer on PDADMA/PSS buildup from ref 20.

Information. Figure 8B shows data adapted from electroosmotic flow measurements of PDADMA/PSS multilayers coating silica capillaries.20 The electroosmotic mobility, μeo (m2 V−1 s−1), was converted to zeta potential using29 μ η ζ = eo εrε0 (4) where η is the viscosity (8.94 × 10−4 N s m−2 at 25 °C). The zeta potential values from μeo (Figure 8B) are consistently lower than those from the spinning disk (Figure 8A), but they were recorded in solutions of much higher ionic strength (0.02 M). Extrapolating the results from Figure 7 to this ionic strength suggests the methods are complementary. In Situ Layer Formation. The spinning disk geometry provides uniform access of solution species to the disk surface. The transport (flux) of materials to a spinning disk is welldefined. We took advantage of this by performing an in-situ experiment: adding dilute (10−6 M) PDADMA to a spinning multilayer capped with PSS. For the greatest time resolution, it was necessary to measure the streaming potential without modulation, in which case a stable background was required. Figure 9 shows an example of a spinning Si*(PDADMA/PSS)5 disk in 10−4 M NaCl. The disk potential was stable (no drift), even after switching off four times, giving a zeta potential of −82 mV. At the 6 min point, 10 μL of 10 mM PDADMA, making 10−6 M in the 100 mL cell, was added. The streaming potential steadily rose to a stable positive streaming potential (corresponding to +39 mV zeta potential), verified by turning the disk off four times. The switch from negative to positive surface took about 2 min. When the experiment was repeated with a Si*(PDADMA/PSS)5 film but using 10 times greater concentration of PDADMA in the cell (10−5 M), the switching E

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charge density profile, and hydrodynamic penetration depth. Because the charge density of the multilayer is much greater than that of the electrolyte, the decay length of the field in the latter, the Debye length, λD, is greater than in the former, λM. Estimates are made using the classical equation for λD

λD =

εrε0RT 2F 2C

(5)

where the concentration of electrolyte, C, is in mol m−3. The Debye length ranges from about 10 nm in 10−3 M NaCl to about 30 nm in 10−4 M NaCl. Dukhin et al.42 provide an estimate of the decay length inside a charged film

Figure 9. In-situ formation of a PDADMA layer on Si*(PDADMA/ PSS)5 by adding 10 μL of 10 mM PDADMA in 10−4 M NaCl (at 6 min). Four off cycles verify the streaming potential for the starting PSS surface. The noise at t = 6 min is from the addition of PDADMA. After the PDADMA adsorbs, reversing the potential, four off cycles reveal the streaming potential of the PDADMA layer.

λM =

εrε0RT F |ρf |

(6)

where ρf is the charge density (C m−3) of the fixed charges inside the film. In the case of polyelectrolyte complex, the concentration of charges is about 1 M or 9.6 × 107 C m−3, which yields a λM of about 0.1 nm (i.e., about a monolayer). This significant difference in decay lengths may explain the independence of zeta potentials on the thickness of each layer of charge added, which is, in turn, controlled by the salt concentration of the deposition solution. For PSS/PDADMA, the layer thickness is approximately proportional to deposition salt concentration,43 yet results here (see Figure S4 of the Supporting Information), and from electroosmotic flow experiments,20 reveal a zeta potential that is independent of layer thickness. Such a small value of λM implies that only the top monolayer of charge contributes to the measured zeta potential and that the charge underneath is screened. Additional detail on the surface charge of the multilayer is not directly evident from zeta potential measurements. Using radiolabeled counterions, we have measured the surface charge of PSS-terminated PSS/PDADMA multilayers.31 For deposition from 0.1 M NaCl, the surface charge is about 0.2 μmol m−2. In contrast, PDADMA-terminated multilayers have much more surface charge, leading to an asymmetric mechanism for multilayer buildup.31 The zeta potential measurements in Figure 8 do not reveal such asymmetry, although other streaming potential measurements support the conclusion that surface charge reversal is not a prerequisite for multilayer assembly.19 The inability of electrokinetic data to provide good estimates of the actual area density of ionic groups at a surface, even a glassy one, is due in part to the failure of Debye−Hückel approximations at high surface charge density. For example, in a recent analysis of zeta potentials of a fluorinated polyelectrolyte film (Nafion), Barbati and Kirby44 underpredicted the charge density of sulfonate groups by a factor of about 20. A (relatively low) charge density of −SO3− sites of about 0.2 μmol m−2 on our PSS-capped, glassy multilayer translates to 0.02 C m−2. Using the linearized Debye−Hückel approach, the effective surface charge density, q, is29

was observed but took only a few seconds. This time, the zeta potential switched from −92 mV to +68 mV. Further interpretation of results on surface charge switching or inversion in multilayering comes with at least three major caveats, a couple related to the fact that each layer is actually a thin zone of polyelectrolyte spread out over some finite thickness.7 First, the diffuseness of the surface−solution interface makes it difficult to locate the shear plane on which the theory for streaming potentials is based. For a hard interface such as silicon, the nominal position of zero fluid velocity is easier to define and is close to the interface. In contrast, hydrodynamic fluid flow is understood to penetrate a gel-like surface.32 Multilayers contain much water, and the water content oscillates depending on the terminating layer.33 For PSS/PDADMA, the surface of the multilayer is more hydrated and gel-like when capped with PDADMA.34,35 Nanoindentation measurements reveal an order of magnitude difference in effective surface modulus for PDADMA-capped, compared to PSS-capped, PEMUs.22 This strong alternation in water content, yielding significant differences in volume charge density, is not reflected in surface potential measurements (e.g., see Figure 8). Second, as with all measurements of surfaces immersed in aqueous solution, the zeta potential does not indicate the areal density of fixed charge groups on the surface. Typically, the effective surface charge calculated from the zeta potential is much lower than the number of ionized surface groups.27,29 For quantitative modeling of PEMU buildup, the number of counterion-balanced polyelectrolyte repeat units (the extrinsic charge) at the surface is needed.31 Finally, and again related to quantitative models for multilayers, the surface charge in PEMUs is known to be spread out up to several equivalent monolayers into the surface of the film.7,36 The zeta potential is effectively generated by the top layer of charge which screens contributions from buried charges. These buried charges play a critical role in how much polyelectrolyte is added with each layer.31 The challenge of modeling electrokinetic response from a soft, charged layer, where electrostatic and hydrodynamic boundaries may not coincide, has been taken up by several groups.32,37−40 Charges below the shear boundary behave like adsorbed, immobile ions and do not contribute to the measured potential. Recently, Duval et al.41 have considered streaming currents at polyelectrolyte multilayers. These theories require several parameters such as the Debye length, charge density,

q=

ζεrε0 λD

(7)

which yields 0.0023 C m−2 in 10−4 M NaCl. Even the nonlinear solution for q29 q= F

sinh(eζ /2kT ) λDe/2εrε0kT

(8)

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yields 0.0035 C m−2, also a severe underestimate of the density of −SO3−. One parameter not considered in the response of thin charged films is the effect of osmotic pressure on the charge density. Multilayers lose much water when they are subjected to external osmotic pressure,45 which would bring the charges closer. On the other hand, the salt concentrations over which osmotic dehydration of multilayers has been observed are in >0.1 M range30 (i.e., much higher than employed for streaming potentials).

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CONCLUSIONS The spinning disk method for zeta potentials works well with charged ultrathin films of polyelectrolyte or polyelectrolyte complex. The potentials yielded by this method are either comparable to, or of slightly greater magnitude than, those obtained by electrophoretic mobility (of particles) or classical streaming potential approaches, possibly because of negligible loss of potential to surface currents in the spinning disk. The open geometry of the spinning disk also allowed us to observe real time switching of surface potential on the addition of a layer during multilayering, although these experiments could not be conducted in the high salt concentrations typically employed for many layer-by-layer assemblies. Other limitations, which also apply to other methods of zeta potential measurement, included the difficulty of translating the zeta potential to a real surface charge density that would be useful in modeling the buildup of multilayers. The fact that surface charge is actually spread out into a thin film complicates interpretation of the zeta potential.

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ASSOCIATED CONTENT

S Supporting Information *

Photograph of the zeta potential cell and graphs of streaming potential and zeta potential. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants DMR-0939850 and DMR1207188 from the National Science Foundation. We thank Pokpong Rungthanaphatsophon and Ornsiree Junchaya for helping to develop the apparatus.

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REFERENCES

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