Divalent–Anion Salt Effects in Polyelectrolyte Multilayer Depositions

Article pubs.acs.org/Langmuir

Divalent−Anion Salt Effects in Polyelectrolyte Multilayer Depositions Walter J. Dressick,*,† Kathryn J. Wahl,‡ Nabil D. Bassim,§ Rhonda M. Stroud,§ and Dmitri Y. Petrovykh*,∥,# U.S. Naval Research Laboratory †Divisions of Bio/Molecular Science & Engineering (Code 6910), ‡Chemistry (Code 6176), and § Materials Science and Technology (Code 6366), 4555 Overlook Avenue SW, Washington, DC 20375, United States ∥ Department of Physics, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *

ABSTRACT: We systematically investigate the effects of divalent anions on the assembly of polyelectrolyte multilayers by fabricating polystyrene sulfonate (PSS)/polyallylamine hydrochloride (PAH) multilayer films from aqueous solutions containing SO42−, HPO42−, or organic dicarboxylate dianions. The chosen concentrations of these anions (i.e., ≤0.05 M) allow us to isolate their effects on the assembly process from those of the polyelectrolyte solubility or solution ionic strength (maintained constant at μ = 1.00 M by added NaCl). Compared to a control film prepared from solutions containing only Cl− anions, stratified multilayers deposited in the presence of dianions exhibit increased UV absorbance, thickness, and roughness. From the dependence of film properties on the solution concentration of SO42− and number of polyelectrolyte layers deposited, we derive a generic model for the PSS/PAH multilayer formation that involves adsorption of PAH aggregates formed in solution via electrostatic interactions of PAH with bridging dianions. Experiments using HPO42− and organic dicarboxylate species of varying structure indicate that the separation, rigidity, and angle between the discrete negatively charged sites in the dianion govern the formation of the PAH aggregates, and therefore also the properties of the multilayer film. A universal linear relationship between film UV absorbance and thickness is observed among all dianion types or concentrations, consistent with the model.

1.0. INTRODUCTION Polyelectrolyte multilayers (PEMLs) are films fabricated by repetitive, alternating treatment of a substrate with separate polycation- and polyanion-containing solutions.1 The replacement of one or more polyelectrolyte layers by other charged species, such as nanoparticles2−7 or transition metal complexes,8,9 permits fabrication of heterocomposites having adjustable physicochemical properties useful for optical, electronic, and biomedical applications.4,10−16 Characteristics of the PEMLs, such as thickness, roughness, and structure, are determined by factors including the concentration, molecular weight, and chemical nature of polyelectrolytes, as well as the solvent, pH, ionic strength, and temperature during film deposition.10,12,17−23 PEML properties are also influenced by the polyelectrolyte deposition method (e.g., spray-coating,5,24−26 spin-coating,17,27 or dip-coating24,25), the rinsing protocol,7,28,29 and any cross-linking groups.14,30 The ionic strength of deposition solutions is critically important in determining the properties of PEMLs.4,10,12 Mobile cations and anions, whether univalent or multivalent, can ion-pair with oppositely charged sites on the polyelectrolyte chains to reduce intrachain electrostatic repulsions and allow otherwise weaker van der Waals and/or hydrogen bonding interactions to promote the formation of compact conformations (e.g., loops or coils).10,12,31−33 Adsorption of such coiled chains to an oppositely charged polyelectrolyte surface © 2012 American Chemical Society

produces a thick layer with an open structure amenable to interpenetration of polyelectrolyte chains from adjacent layers.34−37 For a univalent anion, the properties of PEMLs correlate well with that anion’s position in the Hofmeister series,38 as measured by the ion’s polarizability, hydration entropy, or Jones-Dole viscosity B parameter.23,34−36,39−41 The Hofmeister ordering of anions, for example, correlates with their effects on the thickness,17,23,28,34,41,42 degree of swelling,40 and extent of layer interpenetration36 for PEMLs formed by common polyelectrolytes: sodium polystyrene sulfonate (PSS) and polyallylamine hydrochloride (PAH) or polydiallyldimethylammonium chloride (PDDA). The Hofmeister ordering of cations similarly explains the salt dependence of PSS/PDDA17,23,41 and PSS/PAH20,23,35 film properties, though the effect of cations is muted by their smaller polarizability differences.23,41 The Hofmeister effect thus provides a general model for explaining the effects of univalent salts on PEML properties, with known exceptions traced to strong, specific interactions between ions and polyelectrolytes.20,43,44 In contrast to the case of univalent salts, however, a general model is lacking for the deposition of PEMLs in the presence of Received: August 15, 2012 Revised: September 20, 2012 Published: October 29, 2012 15831

dx.doi.org/10.1021/la3033176 | Langmuir 2012, 28, 15831−15843

Langmuir

Article

multivalent ions, which can promote polyelectrolyte intra- and intermolecular bridging. For example, treatment of aminebearing polyelectrolytes (e.g., PAH, PDDA) with SO42− promotes gelation in solution or collapse in surface-tethered chains.45−51 Bridging behavior similar to that of SO42− is reported for other multivalent anions, both inorganic (e.g., HPO42− or PO43−)45,51−54 and organic (e.g., citrate).46 Polycation-multivalent anion interactions have also been invoked to explain, for example, (1) a maximum film thickness for PSS/PDDA depositions at μ = 0.1 M,44 (2) linear growth of PSS/PDDA films from μ = 0.5 M solutions,36 and (3) decreased polyacrylate (PAA) adsorption rates and a critical ionic strength, above which adsorption halts, in the presence of phosphate.54,55 Likewise, strong interactions with multivalent cations are reported for anionic polyelectrolytes, such as PSS56 and PAA.57 We have recently observed the variation in film thickness and morphology of heterocomposite PEMLs (comprising PAH, PSS, and colloidal Au) as a function of the salt added to polyelectrolyte deposition solutions and of the rinsing protocol.6,7 Using SO42− as a model dianion, here we systematically investigate the effects of multivalent anions on PEML fabrication. From the dependence of the UV absorbance, thickness, roughness, and structure of deposited PSS/PAH multilayers on the solution SO42− concentration, [SO42−], we derive a generic anion-bridging model for the polyelectrolyte deposition mechanism. Additional experiments using HPO42− and organic dicarboxylates support our model and identify bridging dianion characteristics that control the PEML properties.

Chart 1. Structures and Abbreviations of the Ions Used in This Study

2.0. EXPERIMENTAL SECTION

deposition solution, followed by a 30−60 s rinse in 1.00 M NaCl (aq), a final ca. 3 min rinse in water, and drying in a N2 gas stream. The samples were then treated 20 min in the PAH solution, followed by the same rinsing and drying as after the PSS deposition. Alternate treatments with the PSS and PAH continued until films of 20 PSS/ PAH bilayers on Q-EDA (i.e., Q-EDA/(PSS/PAH)20) or up to 24 PSS/PAH bilayers on Si-EDA (i.e., Si-EDA/(PSS/PAH)24) had been deposited. To maintain >99% protonation of PAH amines (pKaμ=1M = 9.67)60 during PEML depositions, pH ≤ 7.67 (Table S-1) was ensured for all the PSS and PAH solutions (Supporting Information). In some cases, the salt rinse was replaced by a “water rinse” protocol whereby the substrates were triple-rinsed in H2O, rather than in salt solutions, after deposition of each polyelectrolyte layer (Table 1). For PEMLs containing the Na4Ru(BPS)3 complex of structure shown in Chart 1,61 trilayer structures of PSS/PAH/Ru (where Ru ≡ Ru(BPS)34−) were present in the films. Each Ru layer was deposited onto a PAH layer using an aqueous solution containing 2 mg Na4Ru(BPS)3 complex/mL water. Following a 30 min treatment, the sample was rinsed three times in water and dried using the filtered N2 gas stream. However, each PSS and PAH layer in the PSS/PAH/Ru trilayer multilayer was rinsed using the standard salt rinse protocol with 0.04 M Na2SO4/0.88 M NaCl (aq) solution, then 1.00 M NaCl (aq) solution, and finally water. Because deposition of PSS displaced some adsorbed Ru(BPS)34− from the film surface, fresh PSS solution was used for deposition of each PSS layer. For all PEML depositions, the pH values of the PAH, PSS, and initial salt rinse solutions were measured (Table S-1) immediately after solution preparation with a Corning Pinnacle 530 pH meter. All multilayer-coated substrates were stored in Fluoroware containers until needed for further study. 2.4. Spectroscopic Characterization. The XPS characterizations of the PEMLs on Si-EDA substrates were performed using the instrument and procedures described previously.7 FTIR spectra were acquired on a Nicolet FTIR spectrometer coupled to a single-bounce attenuated total reflectance (ATR) accessory (Specac Silvergate) with a 45° Ge prism and 7 mm sample window. Spectra were collected

The detailed list of the materials is presented in the Supporting Information. 2.1. Polyelectrolyte Solutions. The deposition solutions were prepared by addition of concentrated aqueous salt mixtures to a wellstirred solution of PAH (or PSS) in water. Each salt mixture contained NaCl and the ion to be studied at concentrations that produced 0.01− 0.05 M of the studied ion and μ = 1.00 M ionic strength af ter mixing with the PAH (or PSS) solution (note Table S-1). The concentrations of PAH and PSS after mixing were each 2 mg/mL. Before mixing, levels of the ion to be studied were typically ca. 0.070−0.075 M, limited for some of the dianions by solubility or irreversible gelation upon mixing with PAH. Even at these ion concentrations, gels were irreversibly formed if PAH solution was added to some salt mixtures. Adding the salt mixture to a stirred PAH solution sometimes initially generated a white precipitate, which quickly dissolved, however, to form a clear stable solution (the mixing procedure is described in detail in the Supporting Information). In contrast, no gelation was observed for PSS solutions. 2.2. Substrate Preparation. Silicon wafer and quartz substrates were cleaned with 1:1 v/v HCl/CH3OH and H2SO4, as described previously.58 Thereafter, a self-assembled monolayer (SAM) of N-(2aminoethyl)-3-aminopropyltrimethoxysilane (EDA) was immediately formed on each substrate, following a previously described protocol.59 The EDA-coated Si wafers (Si-EDA) and quartz (Q-EDA) were stored in Fluoroware containers and were sufficiently stable and clean for PEML deposition experiments for at least 6 months after preparation. 2.3. Polyelectrolyte Deposition. The PSS/PAH multilayer films were prepared on the lab bench under ambient conditions by dipcoating substrates in the PAH and PSS polyelectrolyte solutions containing the appropriate salts (Chart 1). For each PEML film, QEDA and Si-EDA substrates were initially treated with the PSS solution for at least 20 min in a Coplin jar. The treated substrates were then subjected to a “salt rinse” protocol involving initial rinsing for 30−60 s in a solution having the same salt composition as the 15832

dx.doi.org/10.1021/la3033176 | Langmuir 2012, 28, 15831−15843

Langmuir

Article

Table 1. Properties of Multilayer Films Deposited from Salt Solutions (Triple Water Rinse Protocol) thickness (nm) salta (substrate)b

absorbancec

NaClh (Q) NaClh (Si) NaH2PO4h (Q) NaH2PO4h (Si) MgCl2i (Q) MgCl2i (Si) Na2SO4i (Q) Na2SO4i (Si) MgSO4j (Q) Na2HPO4k (Si)

0.7558 − 0.7696 − 0.7466 − 2.8157 − 2.6300 −

total filmd − 97.5 80.4 100.5 77.4 93.5 347.6 411.6 382.0 304.9

± ± ± ± ± ± ± ± ±

2.2 1.6 0.7 7.2 4.0 12.5 13.6 50.0 5.5

roughness (nm) bilayere − 4.1 4.0 4.2 3.9 3.9 17.4 17.1 19.1 12.7

± ± ± ± ± ± ± ± ±

0.1 0.1 0.3 0.4 0.1 0.6 0.4 2.5 0.2

rqf − 0.83 0.95 0.84 1.27 0.99 15.90 − 35.55 2.91

± ± ± ± ± ±

Ng 0.37 0.12 0.09 0.16 0.50 4.80

± 5.88 ± 0.68

− 3 5 3 3 4 3 − 4 3

a

Identity of the univalent or divalent ion salt present in each polyelectrolyte solution during multilayer deposition. Each salt was present at 0.05 M concentration, except where noted, with sufficient NaCl added to fix the solution ionic strength at μ = 1.00 M. bIdentity of the substrate coated with the multilayer. Q = quartz and Si = silicon wafer. Substrates were coated with a chemisorbed EDA organosiloxane monolayer prior to multilayer deposition. Multilayer films have structure: Substrate-EDA/(PSS/PAH)x (x = 20 for quartz (Q) and x = 24 for silicon (Si) substrates). Films were triple-rinsed with water and dried in the filtered N2 gas stream after deposition of each PSS and PAH layer. cUV absorbance of the multilayer film at 225 nm on a quartz substrate. dThickness (±std. dev.) of the multilayer film as determined by AFM. eAverage thickness (±std. dev.) of a single PSS/ PAH bilayer within the multilayer film, calculated by division of the total film thickness by the number of bilayers deposited, x, with x = 20 (quartz) or x = 24 (Si). fRoot-mean-square roughness, rq, (±std. dev.) from AFM measurements. gNumber of measurements taken to determine roughness values. hContains 0.95 M NaCl added to adjust ionic strength. iContains 0.85 M NaCl added to adjust ionic strength. jContains 0.80 M NaCl added to adjust ionic strength. kPAH solubility issues limit the Na2HPO4 concentration to 0.011 M, with 0.957 M NaCl and with 0.010 M NaH2PO4 added for ionic strength and pH buffer control. using a MCT detector as 256 scans over the 400−4000 cm−1 wavelength region with 4 cm−1 resolution. UV−vis spectra of quartz slides bearing PEML films were acquired using a Cary 5000 UV−vis-NIR double-beam spectrophotometer equipped with Cary WinUV software. A baseline obtained from blank EDA-coated quartz slides was used to correct the UV−vis spectra. Components of our PEMLs produce distinct UV−vis absorbance features: PAH does not absorb above ca. 200 nm, whereas PSS exhibits an absorbance maximum at 225 nm and absorbs strongly below ca. 260 nm. The relative concentrations of available amine and sulfonate sites on terminal PSS and PAH layers were estimated from UV−vis spectra by using, respectively, the Ru(bpy)3(ClO4)2 and Na4Ru(BPS)3 complexes (Chart 1) as labels (Supporting Information).61−63 Although these Ru complexes absorb light at wavelengths 0.05 M, in agreement with previous observations.45−47 The neutralization of PAH chain charge by SO42− anions appears to be particularly effective, as the analogous interactions of the PSS −SO3− groups with Na+ (or Mg2+) cations do not induce gelation in any of our solutions. The constraints imposed by PAH aggregation on the process window for PEML fabrications are discussed in the Supporting Information. Interactions of PAH solutions with water provide further insight into the properties of PAH aggregates: addition of PAH solutions (μ = 1.00 M) containing SO42− or HPO42− dianions to water resulted in a faint precipitate exhibiting a bluish-white Tyndall effect, which suggests aggregates of ≥35−40 nm in size. No precipitate was observed upon addition of these PAH solutions to a mixed salt solution (containing the same SO42− 15834

dx.doi.org/10.1021/la3033176 | Langmuir 2012, 28, 15831−15843

Langmuir

Article

representative PAH-terminated films, no mobile cations (Mg2+ or Na+) were detected after rinsing, in agreement with previous observations.37,39,66 Retention of anions, on the other hand, depended upon the rinse protocol. Films rinsed using a solution having a salt composition identical to the final PAH treatment solution (followed by a water rinse) revealed the presence of the unique SO42− or H2PO4− anions, as appropriate, in addition to Cl− (XPS results not shown). In films that underwent a complete salt rinse protocol, i.e., received an additional rinse in 1.0 M NaCl (aq) solution prior to the final water rinse, all mobile anions were completely replaced by Cl−, as shown in Figure 1A.7 The charge balance, observed in agreement with previous reports,37,66 was confirmed within the ca. 10 nm XPS sampling depth by quantifying the S 2p doublet [BE(S 2p3/2) ≈ 168.1 eV] due to the SO3− groups of PSS,7,67 Cl 2p doublet [BE(Cl 2p3/2) ≈ 197.9 eV] due to chloride ions, and N 1s signal at BE ≈ 402 eV characteristic of protonated amines in PAH.7,68 Electroneutrality at the film surface is thus maintained by pairing protonated amines of PAH with anionic sulfonates of PSS and with mobile Cl− ions. The presence of 0.05 M univalent anions vs divalent SO42− anions produces the most dramatic effect on the film composition. In the presence of univalent anions, PAH and PSS adsorb in a nearly 1:1 ratio and few Cl− ions are required to neutralize the excess PAH component. In contrast, ca. 2−3 times more PAH than PSS is deposited from solutions containing SO42−. Therefore, those films contain large amounts of Cl− ions to charge-compensate the terminal PAH layers. Figure 1B corroborates this observation via relative measurements of the levels of accessible protonated cationic amine and deprotonated anionic sulfonate sites for PAH- and PSSterminated PEML films, respectively, using electrostatically bound Ru dyes (note Chart 1). Electrostatic binding of anionic Ru(BPS)34− dye to a Q-EDA/(PSS/PAH)2 PEML and cationic Ru(bpy)32+ dye to a Q-EDA/(PSS/PAH)2/PSS film readily occurs from aqueous dye solutions. Measurement of the absorbance of the bound Ru dyes provides an estimate (as described in the Supporting Information) of [−NH3+]/ [−SO3−] > 1.8 for PEMLs prepared using 0.037 M Na2SO4/ 0.889 M NaCl solutions. This estimate is in good agreement with the XPS results from Figure 1A for the PEML prepared with slightly higher [SO42−] (0.05 M Na2SO4/0.85 M NaCl) and suggests that deposition of excess PAH is facilitated by increasing [SO42−]. 3.5. Sulfate Solution Concentration. The unusual behavior of PEMLs prepared from solutions containing SO42−, as summarized in Figure 1 and Table 2, prompted us to examine these PSS/PAH multilayers further. Figure 2A (Table S-3) shows a plot of the absorbance at 225 nm due to the PSS film component vs the mole fraction of SO42− in polyelectrolyte solutions containing varying concentrations of Na2SO4 and NaCl at μ = 1.00 M total ionic strength. The mole fraction SO42− concentration, χ, in this case provides a relative measure of the competitive electrostatic binding of SO42− and Cl− ions at PAH protonated amine sites in solution. As Na2SO4 replaces a portion of the NaCl in the polyelectrolyte treatment solutions, the 225 nm absorbance of the Q-EDA/(PSS/PAH)20 PEML film increases from ca. 0.75, slowly at first and then more rapidly as solution SO42− levels rise. The increase in PEML absorbance due to the presence of SO42−, ΔA, can be estimated for each SO42− solution in Figure 2A as the difference between the absorbances of the respective PEML and a PEML deposited from 1.00 M NaCl (aq). The

by observing that PEMLs exhibit identical properties whether prepared using only univalent ions or in the presence of MgCl2 (cf., Tables 1 and 2), which indicates that PAH−Mg2+ interactions are unlikely contributors to PAH precipitation. 3.3. Effects of Anions on PEML Properties. Among the PEMLs prepared (via the salt rinse protocol) in the presence of various ions listed in Chart 1, Table 2 reveals the main systematic differences to be between films deposited in the presence of univalent vs divalent anions, while the identities of the cations appear to be unimportant. For each of the Na+ (or Mg2+) salts of the univalent Cl−, F−, H2PO4−, and H3CCOO− (acetate) anions, the UV absorbance at 225 nm (A225) and bilayer thickness (Tb) of the resulting PEMLs are nearly identical and do not significantly change when water rather than salt rinse protocol is used (cf., Tables 1 and 2). The A225 and Tb values increase dramatically, however, for films prepared using divalent anions, such as SO42−, HPO42−, or C2O42− (oxalate); the parameters of PEMLs prepared using dicarboxylate salts also vary with the identity of the dicarboxylates. 3.4. Film Compositions. To interpret the observations summarized in Table 2, we examined the composition of the PEMLs, with particular emphasis on films prepared in the presence of SO42− (Figure 1). In XPS analysis (Figure 1A) of

Figure 1. (A) Comparisons of the amounts of N, S, and Cl present in PAH-terminated PSS/PAH PEMLs prepared using μ = 1.00 M solutions containing SO42− dianion and representative univalent anions. Histograms representing the XPS Cl 2p signal due to Cl− anion (gray) and the N 1s signal due to PAH amine sites (black) are referenced to the S 2p signal due to the sulfonate sites of the PSS (white), which is arbitrarily set to unity to facilitate comparisons and estimate element ratios. (B) Visible absorbance spectra of the MLCT bands associated with Ru(BPS)34− adsorbed to a terminal PAH layer in a Q-EDA/(PSS/PAH)2 PEML (red, upper spectrum) and Ru(bpy)32+ adsorbed to a terminal PSS layer in a Q-EDA/(PSS/ PAH)2/PSS PEML (blue, lower spectrum) for estimation of the [−NH3+]/[−SO3−] ratio using eq. (S-4), Supporting Information. PEMLs were deposited from 0.037 M Na2SO4/0.889 M NaCl (aq) polyelectrolyte solutions using the salt rinse protocol. 15835

dx.doi.org/10.1021/la3033176 | Langmuir 2012, 28, 15831−15843

Langmuir

Article

tion from 1.00 M NaCl (aq) solutions. In addition, no correlation is observed between film roughness and the number of deposited PSS or PAH layers. These observations are indicative of linear, rather than exponential, PEML growth for films deposited from solutions having fixed [SO42−]. Linear film growth consistent with previous observations21,27,73 is confirmed for our PEMLs by linear correlations of both the film absorbance and total thickness with the number of deposited PSS/PAH bilayers (Figure 3A). 3.7. TEM Strata Imaging. To observe the linear growth regime by direct TEM imaging, we prepared PEMLs with PAH layers stained by Ru(BPS)34− shown in Figure 3B,C and SI Figures S-3, S-4. The protocol used to deposit PEMLs for Figure 3A (Table 2) was modified by treating each PAH layer with Ru(BPS)34− (aq) solution (as described in section 2.3) to electrostatically bind Ru(BPS)34− dye to the PAH protonated amine sites (Table S-4). Subsequent treatment of the film with fresh PSS solution displaced ca. 46 ± 5% of the bound dye from the PAH (Figure 3B, Table S-4). However, as expected, addition of PAH to begin the next deposition cycle did not displace the strongly bound PSS from the films (Figure S-4, Table S-4). Despite the partial displacement of Ru(BPS)34− dye during PSS deposition, the analysis (detailed in the Supporting Information) of absorbance data at 225 nm, 286 nm, and 460 nm in Figure 3C indicates robust and linear deposition of PEMLs comprising PSS/PAH/Ru trilayers, in which the Ru complex is incorporated without degradation (Figure S-3). The linearity of the A225 plot, which tracks both PSS and Ru(BPS)34− dye, indicates a linear growth similar to that of the corresponding PSS/PAH film in Figure 3A. Incorporation of the Ru dye does not significantly change the film roughness, but increases the thickness of the repeating unit (ca. 9.0 ± 0.2 nm for PSS/PAH/Ru trilayers vs 7.6 ± 0.1 nm for PSS/PAH bilayers). The STEM-HAADF image of the cross section of a Si-EDA/ (PSS/PAH/Ru)20 PEML is shown in Figure 4A. Uniform periodic strata comprising darker polyelectrolyte-rich (lower atomic number, Z) and lighter Ru-rich (higher Z) bands indicative of linear PEML growth are observed. The fwhm of the lighter bands in Figure 4A provides an estimated thickness of ca. 4.0 ± 0.5 nm for the Ru(BPS)34− dye-stained PAH regions, which corresponds to 44 ± 6%, i.e., a significant fraction, of each PSS/PAH/Ru trilayer. The validity of this estimate requires negligible diffusion of the Ru dye within the PEML stack, an assumption that we tested by fabricating a SiEDA/[(PSS/PAH)5Ru]2/(PSS/PAH)4 film in which every fifth PSS/PAH bilayer is stained with Ru(BPS)34− dye (Figure 4C). Discrete Ru-stained light bands of ca. 5 ± 1 nm thickness separated by the intervening PSS/PAH bilayer stacks are clearly observed in Figure 4C, supporting our assumption of the limited Ru(BPS)34− dye diffusion within the PEML. 3.8. Surface Morphology. Examination of the topography of the PEMLs described in Table 3 and Figure 3A reveals additional information concerning the structure of these films. Figure 5 illustrates AFM scans at intermediate stages during fabrication of PEMLs from 0.04 M Na2SO4/0.88 M NaCl solutions on a Si-EDA substrate. The PEML surface exhibits two distinct morphologies: for films having