Anion-specific effects on the behavior of pH-sensitive polybasic brushes

Anion-Speci

fic Effects on the Behavior of pH-Sensitive Polybasic

Brushes

Joshua D. Willott,

Timothy J. Murdoch,

Ben A. Humphreys,

Steve Edmondson,

Erica J. Wanless,

and Grant B. Webber*

†_{Priority Research Centre for Advanced Particle Processing and Transport, University of Newcastle, Callaghan, NSW 2308, Australia} ‡_{School of Materials, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom}

*

ABSTRACT: The anion-specific solvation and conformational behavior of weakly basic poly(2-dimethylamino)ethyl methacrylate (poly(DMA)), poly(2-diethylamino)ethyl methacrylate (DEA)), and poly(2-diisopropylamino)ethyl methacrylate (poly-(DPA)) brushes, with correspondingly increasing inherent hydro-phobicity, have been investigated using in situ ellipsometric and quartz crystal microbalance with dissipation (QCM-D) measure-ments. In the osmotic brush regime, as the initial low concentration of salt is increased, the brushes osmotically swell by the uptake of solvent as they become charged and the attractive hydrophobic inter- and intrachain interactions are overcome. With increased ionic strength, the brushes move into the salted brush regime where they desolvate and collapse as their electrostatic charge is

screened. Here, as the brushes collapse, they transition to more uniform and rigid conformations, which dissipate less energy, than similarly solvated brushes at lower ionic strength. Significantly, in these distinct regimes brush behavior is not only ionic strength dependent but is also influenced by the nature of the added salt based on its position in the well-known Hofmeister or lyotropic series, with potassium acetate, nitrate, and thiocyanate investigated. The strongly kosmotropic acetate anions display low affinity for the hydrophobic polymers, and largely unscreened electrosteric repulsions allow the brushes to remain highly solvated at higher acetate concentrations. The mildly chaotropic nitrate and strongly chaotropic thiocyanate anions exhibit a polymer hydrophobicity-dependent affinity for the brushes. Increasing thiocyanate concentration causes the brushes to collapse at lower ionic strength than for the other two anions. This study of weak polybasic brushes demonstrates the importance of all ion, solvent, and polymer interactions.

■

Many physicochemical phenomena exhibit specific ion effects which cannot be wholly explained by classical theories based on hydration, water dipole, internal pressure, and electrostatic or van der Waals interactions.1 These effects were first identified through work on the ability of ions to increase or decrease the stability of egg white proteins, leading to the creation of the Hofmeister series of ions.2 Since then, extended variations of this lyotropic series have been shown to correlate with the stability of proteins1 and colloidal suspensions,3 solution properties such as osmotic pressure,1 and interfacial phenom-ena such as ion partitioning at the air−water interface.4 No theory has comprehensively explained these effects, with minor variations in the order of ions and even complete series reversal observed.1,5 Traditionally, these effects have been rationalized in terms of ion ability to order (kosmotropic) or break (chaotropic) the structure of water. However, recent evidence suggesting that ions have little effect on the overall structure of water beyond thefirst hydration sheath which together with the lack of consideration of the nature of the solute challenges this explanation.6−9 The techniques used to probe Hofmeister

effects often cannot directly interrogate the interactions that occur between ion, solute, and solvent. This has led many studies to correlate the physicochemical response of solutes such as macromolecules, to properties such as polarizability,9 the Gibbs free energy of hydration,10 and surface charge density.11 Interactions may be better understood through simulations such as molecular dynamics which can probe equilibrium interactions.12−14 However, there is no guarantee that these modeled interactions apply broadly to specific ion effects. Recently, it has been suggested that nonelectrostatic ion-specific dispersion forces between solute and ion play a vital role in understanding Hofmeister effects.15−20 _{At salt}

concentrations ≥100 mM, it has been shown that counterion condensation in polyelectrolytes is strongly influenced by the ion-specific dispersion potential, which becomes significant at these concentrations where the electrostatics are highly screened.17 Received: July 24, 2014 Revised: March 5, 2015 Published: March 13, 2015 Article pubs.acs.org/Langmuir

Published 2015 by the American Chemical

copying and redistribution of the article or any adaptations for non-commercial purposes.

Downloaded via UNIV TWENTE on April 18, 2019 at 11:53:14 (UTC).

During the past two decades the chemistry of polyelec-trolytes localized at solid−liquid interfaces has attracted frequent theoretical, computational, and experimental attention. Polymer brushes are surface coatings of densely packedflexible polymer chains anchored by one end to an interface.21They are typically formed via a controlled polymerization technique, such as atom transfer radical polymerization (ATRP)22 from surface-bound initiator sites and can be tailored by suitable monomer selection to respond to various environmental stimuli. One major class of responsive brushes are those formed from weakly acidic or basic polyelectrolytes which respond to changes in solution pH, ionic strength, and in certain cases temperature.23Their behavior is in contrast to that of strong polyelectrolytes in which the number and position of the charged monomers are fixed. Here we present the first known experimental study of the ion-specific behavior of three tertiary amine methacrylate polybasic brushes in response to three different Hofmeister series anions.

Weak polybasic polymer brushes such as those formed from tertiary amine methacrylates including poly(2-(diethylamino)-ethyl methacrylate) (poly(DEA)),24−30 the less hydrophobic dimethylamino analogue (poly(DMA)),30−34 and the more hydrophobic diisopropylamino analogue (poly(DPA))29,30,35 have been reported to exhibit physicochemical responses to variations in solution temperature, pH, and ionic strength. Such brushes are pH-responsive due to the presence of ionizable basic tertiary amine moieties located along the length of the polymer chains that readily associate with protons in aqueous solution dependent on the apparent brush pK_a. This functionality affects the electrostatic and hence osmotic balance inside the brush by controlling the charge on the brush. This consequently controls the associated uptake or release of solvent molecules and ionic species by the brush. At pH values below the brush pK_aa polybasic brush becomes charged as the basic groups associate with protons in solution and swell via the uptake of solvent, while at pH values above the brush pK_a protons are dissociated and the brush becomes uncharged and collapses by expelling solvent.

The response of a weak polybasic brush to dissolved electrolyte is more complex than Derjaguin−Landau− Verwey−Overbeek (DLVO) electrostatic screening. This is because the degree of brush charge is also governed by ionic strength not just pH. Consequently, these brushes display behavior with increasing solution ionic strength due to the competing effects of electrostatic screening and ionization.36−39

Raising the ionic strength decreases the range and strength of the repulsive electrostatic interactions, while simultaneously increasing the number of charged monomer groups on the polymer. Therefore, along with pH, varying ionic strength represents a second, and less-studied, mechanism that can be exploited to manipulate the structure and properties of surfaces modified with weak polyelectrolyte brushes.

For a weak polybasic brush, at a hypothetical zero added ionic strength and below the apparent brush pKa, each

monomer group is associated with a hydroxide ion in order to maintain brush electroneutrality.36,40,41 The corresponding locally high pH within the brush shifts the acid−base equilibrium to favor a neutral state driven by the decrease in free energy relative to the charged state.41Here the polymer− polymer interactions dominate and the brush adopts a collapsed conformation, dependent on polymer hydrophobic-ity. As the ionic strength is increased, salt anions, which cannot participate to any appreciable extent in the acid−base

equilibrium, replace the hydroxide counterions from within the brush to maintain brush electroneutrality. This lowers the local pH toward the bulk pH, thus driving the acid−base equilibrium toward the products resulting in more charges along the polymer chain. This increase in charge causes more water and counterions to be absorbed by the brush, and the osmotic pressure rise swells the brush. This is known as the “osmotic brush” regime, and brush thickness increases with ionic strength. In this regime polymer hydrophobicity plays a vital role in determining brush behavior.28 With further increasing ionic strength a point is reached at which the concentration of bulk electrolyte approaches that of the ions within the brush and the fraction of charged monomers can no longer increase: the“salted brush” regime. With any additional rise in ionic strength, the screening of the electrostatic repulsion between monomer groups becomes significant, and the brush contracts. In this salt-dominated regime the weak polybasic brush behaves like a strong polybase where the degree of charged monomer is constant.39

Since the seminal scaling theory work of Zhulina and co-workers36 which provided asymptotic dependencies for brush thickness with added inert salt for ionizable brushes, more complete understanding of polyelectrolyte brushes has been realized by accounting for short-range excluded volume interactions and finite chain extension which had previously been ignored or lacked appropriate treatment.42 Self-consistent-field theory,38,43−45 _{Poisson−Boltzmann theory,}39,46

Monte Carlo simulations,47dissipative particle dynamics simulations,48 and molecular dynamic simulations49,50have all been employed to study the behavior of polyelectrolyte brushes. Although the majority of these studies deal with strong polyelectrolyte brushes, they are relevant to weak polyelectrolyte brushes in the salted brush regime as above.39 Recently, molecular dynamic simulations have shown that in the salted brush regime, for strong polyelectrolytes, brush thickness decreases with a weaker dependency with increasing ionic strength, contrary to scaling theory.42,48−50 The authors rationalize this difference by arguing that a brush cannot continue to collapse without restriction, and brush thickness should level off at higher salt concentrations due to excluded volume interactions. Indeed, Attili et al.51 have shown experimentally that a weakly acidic hyaluronic brush (pKa∼ 3) at high ionic strength (>150 mM

NaCl) exhibits a positive excluded volume effect which causes the brush to collapse with a weaker dependency on salt concentration than predicted by scaling theory.

Theoretical analysis accounting for excluded volume effects in weak polyelectrolyte brushes is limited to the work of Israëls et al.52They found that in the osmotic brush regime (low salt) the onset of nonelectrostatic excluded volume interactions caused the brush thickness to be higher than predicted by scaling theory. Several publications have experimentally investigated the effects of salt on a weak polyelectrolyte brush in order to compare with theoretical scaling laws.28,53−57 Significantly, three of these studies report weaker scaling exponents for brush thickness with increasing ionic strength for the salted brush regime.28,55,57Furthermore, our previous work on a poly(2-diethylamino)ethyl methacrylate brush showed that in the osmotic brush regime the brush transitions from a collapsed to swollen conformation over a much narrower concentration range than predicted theoretically.28 This behavior was attributed to the hydrophobicity of the polymer which is not accounted for in the theory. It is clear from these above-mentioned studies that the true behavior of

polyelec-trolyte brushes is influenced by excluded volume interactions as well as polymer hydrophobicity. Moreover, these theories assume that the added salt is inert. However, salt-specific effects have been seen in many polyelectrolyte systems ranging from microgels11,58 to polymer brushes55,59−62 which introduce additional complexity to brush behavior.

No systematic studies varying the nature and concentration of anions have been reported for any weak polybasic brushes. In this study we have chosen to work with the potassium salts of three diverse Hofmeister series anions: acetate, nitrate, and thiocyanate. Anions exhibit stronger ion-specific effects than cations due to their greater range in polarizability.1 In most classifications of Hofmeister series salts, the acetate anion is characterized as strongly kosmotropic, thiocyanate as strongly chaotropic, and nitrate as being mildly chaotropic. Herein in situ ellipsometry and quartz crystal microbalance with dissipation (QCM-D) measurements have been used to study the behavior of three polybasic tertiary amine methacrylate brushes of varying hydrophobicity, poly(DMA), poly(DEA), and poly(DPA), in response to variations in the nature and concentration of added salt. As such, the major aim of this work is to elucidate the roles that polymer hydrophobicity, ionic strength, and salt identity play in determining the behavior of weak polyelectrolyte brushes.

■

Figure 1 shows the chemical structures of the three polymer brushes. From poly(DMA) up to poly(DPA) the brushes display increasing

hydrophobicity due to the increased pendant alkyl group length and substitution of the tertiary amine group.30,63,64Also, the steric bulk increases with the same trend as hydrophobicity.

Materials and Chemicals. Native oxide silicon wafers were purchased from Silicon Valley Microelectronics, Santa Clara, CA. Quartz crystal microbalance sensors with a 50 nm silica coating (QSX 303,∼4.95 MHz fundamental resonant frequency) were purchased from Q-Sense, Sweden. Initiator functionalization reagents including (3-aminopropyl)triethoxysilane, tetrahydrofuran, triethylamine, and 2-bromoisobutyryl bromide were purchased from Sigma-Aldrich and used as received. The tetrahydrofuran and triethylamine were dried over 4 Å molecular sieves (ACROS Organics) before use (for at least 1 day). The inhibitor was removed immediately before polymerization from the (dimethylamino)ethyl methacrylate (DMA), 2-(diethylamino)ethyl methacrylate (DEA), and 2-(diisopropylamino)-ethyl methacrylate (DPA) monomers, all purchased from

Sigma-Aldrich, by gravity feeding through a 10 cm length and 2 cm diameter alumina column (activated, basic). Polymerization reagents copper(II) bromide (99.999%), 2,2′-bipyridine (≥99%),L-ascorbic acid (≥99%),

and (+)-sodium L-ascorbate (≥98%) were purchased from

Aldrich and used as received. Methanol (99.8%, anhydrous, Sigma-Aldrich) and isopropyl alcohol (99.7%, Chem-Supply Pty Ltd.) were used as solvents in the polymerizations. Measurements were performed in the presence of potassium nitrate (Asia Pacific Specialty Chemicals Ltd., >99%), potassium acetate (Alfa Aesar, >99%), or potassium thiocyanate (Alfa Aesar, >98%) electrolyte solutions in the concentration range of 0.05−500 mM. Solution pH values were accurate to±0.1 pH unit. For the nitrate and thiocyanate solutions pH adjustments were made by adding a minimum amount of 0.01 M nitric acid (RCI Labscan Ltd.) or potassium hydroxide (AR grade, Chem-Supply Pty Ltd.). For the acetate solutions acetic acid was used for the pH adjustments. Feed reservoir of pH-controlled salt solution was flowed through the sample chambers using air-impermeable silicone tubing and a peristaltic pump. Milli-Q water (18.2 MΩ·cm at 25 °C, Millipore) was used to prepare all solutions.

Surface Initiator-Functionalization and Brush Polymeriza-tion. Cleaned wafers and QCM sensors (see Supporting Information) were amine-functionalized by exposure to (3-aminopropyl)-triethoxysilane (APTES) vapor at <5 mbar for 30 min at room temperature before being annealed in air at 110°C.28−30 Bromine-functionalization of the APTES layer was performed by immersing the surfaces in a solution of 2-bromoisobutyryl bromide (0.26 mL) and anhydrous triethylamine (0.3 mL) in anhydrous tetrahydrofuran (10 mL), under a nitrogen atmosphere, for 60 min.28,30The poly(DMA), poly(DEA), and poly(DPA) brushes were synthesized from the bromine initiator moieties using activators continuously regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) methodology. A typical brush polymerization was performed in either an isopropyl alcohol/water mixture (9:1 v/v, for DMA and DPA) or pure methanol (for DEA) using an ARGET ATRP recipe consisting of monomer/catalyst/ligand/reducing agent in molar ratio of 2500/1/10/10. Copper(II) bromide was used as the catalyst, with either 2,2′-bipyridine (for DEA and DPA) or 1,1,4,7,10,10-hexamethyltriethylenetetramine (for DMA) as the ligand and either sodium ascorbate (for DEA) or ascorbic acid (for DMA and DPA) as the reducing agent. The ratio of monomer to solvent was 50:50 v/v for all syntheses. For a more detailed synthetic procedure please see the Supporting Information.

Our previous work has shown that the ideal dry thickness for the brushes to be investigated by ellipsometry is ∼20 nm.26,28,30 Polymerization times were selected accordingly, and measured brush thickness values are given in Table 1. For the QCM-D measurements

similar dry thickness brushes were used for poly(DEA) and poly(DPA). For poly(DMA), a brush of comparable dry thickness was initially studied but displayed changes in resonant frequency (reversal of sign) and dissipation that indicated a loss of coupling between the brush and solvent. This was attributed to the high degree of solvation of the poly(DMA) brush which was subsequently confirmed by ellipsometry. This type of behavior has previously been observed by other groups.65,66Therefore, a poly(DMA) brush with a lower dry brush thickness was synthesized and studied using Figure 1. Chemical structures of poly(2-dimethylamino)ethyl

methacrylate (poly(DMA)), poly(2-diethylamino)ethyl methacrylate (poly(DEA)), and poly(2-diisopropylamino)ethyl methacrylate (poly-(DPA)).

Table 1. Dry Brush Thickness Values for the Poly(DMA), Poly(DEA), and Poly(DPA) Brushes Studied

ellipsometric brush thicknessa(nm) ellipsometry studies QCM-D studiesb poly(DMA) 23.4± 0.7 13.9± 0.2 poly(DEA) 21.2± 0.5 21.2± 0.5 poly(DPA) 21.1± 0.3 22.8± 0.3

a_{Error bars are the standard deviation from multiple measurement}

areas on a single brush surface.bMeasured for brushes grown on sister wafers (see Supporting Information).

QCM-D, as listed in Table 1. To ensure the results obtained were as comparable as possible, a single brush was used for each of the in situ ellipsometry and QCM-D experiments.

Ellipsometry Studies. Measurements were performed using a Nanofilm EP3 single wavelength (532 nm green laser) imaging ellipsometer operated using EP3View software. The ellipsometric parameters (Ψ and Δ) were modeled using WVASE32 software; for details please see the Supporting Information. A Nanofilm SL fluid cell with optical glass windows and laser beam positioned at a 60° angle of incidence was used to house the brush-modified wafers. Only a single angle of incidence was available since the beam must intersect the cell windows perpendicularly. The cell geometry was trapezoidal with an internal volume of 0.70 mL and exposed sample area of 10 mm× 18 mm. Data were collected every 15 s. Subsequently, eachΨ and Δ pair wasfitted to determine the solvated (wet) brush thickness using a multilayer-slab model (see Supporting Information). The maximum error of the solvated brush thickness measurement was approximately ±3 nm based on the noise in the Ψ and Δ data. A measured Δ offset of−1.5°, which is due to the glass windows of the fluid cell interfering with the measurement ofΔ, was accounted for in the optical model.67 QCM-D Studies. Measurements were performed using a KSV Z500 QCM-D (KSV, Finland) with the sensor housed in a parallel flow chamber sealed with an O-ring and glass window. The cell geometry was cylindrical with a cell volume of 0.25 mL and exposed sample area of 14 mm in diameter. QCM-D works by recording the changes in frequency (Δf) and energy dissipation (ΔD) of the oscillating piezoelectric quartz sensor as the mass coupled to the sensor vibration changes. Here the driving voltage applied to the QCM sensor is turned off and on intermittently, and the sensor’s oscillation, which is left to decay freely, is measured. QCM-D is sensitive to changes in the solvent retained within or hydrodynamically coupled to polymer brushes.68 Oscillations of the brush-coated sensors at the fundamental resonant frequency (∼5 MHz) and higher overtones (∼15, ∼25, ∼35, and ∼45 MHz) were measured with frequency (Δfn/

n) and dissipation (ΔDn) data for the third (n = 3) overtone (∼15

MHz) presented herein. The maximum errors associated with the recorded frequency and dissipation values were ±3 Hz and ±1.5 × 10−6, respectively.

The major challenge facing the use of QCM-D when studying solvated polymer brushes is differentiating the frequency response from changes in mass and viscoelasticity associated with the brush. For sufficiently rigid films that behave elastically, generally when the absolute value ofΔDn/(Δfn/n) is less than 0.4× 10−6 Hz−1,69 the

Sauerbrey model can be applied to calculate the areal mass density of the coating.70This also includes the mass of solvent entrained within and coupled with the coating. In this study, however, as is the case for most soft dissipativefilms, the above condition is not met and the use of the Sauerbrey model is not applicable. Therefore, we report the recorded changes in resonant frequency as a measure of brush solvation instead of determining changes in mass associated with brush response.

Equilibrium Brush Measurements. Equilibrium measurements, both in situ ellipsometry and QCM-D were performed under a constant flow rate of the desired salt solution, ∼4.3 mL min−1, at controlled pH and controlled temperature of 21 ± 0.2 °C. This relatively highflow rate was not found to have any impact on the frequency or dissipation response of the sensors due to the parallel flow geometry of the QCM cell. These conditions were maintained over time scales of at least 25 min until a steady state in response was achieved. For overnight storage the respectiveflow chambers were left filled with Milli-Q water at ambient pH (∼5.5) with the flow stopped. Each brush remained solvated over the entire length of the individual experiments.

■

Since the degree of charge and hence behavior of the weak polybasic brushes is influenced by pH,30,40,71all measurements were performed at controlled pH. For the poly(DMA) and poly(DEA) brushes pH 5.5 was selected while for the

poly(DPA) brush the measurements were performed at pH 4.5, with the following reasoning. Figure S1 (Supporting Information) shows the response of the three brushes in 10 mM potassium nitrate electrolyte as the pH was incrementally decreased from pH 9 to an acidic regime and then increased back to pH 9. At high pH above the apparent brush pKa, all

three brushes collapse since neutralization of the ionizable tertiary amine groups causes the brushes to discharge, allowing the inherent polymer hydrophobicity to dominate the electro-static repulsion. Conversely, at low pH below their apparent pKa, the brushes osmotically swell via the uptake of solvent and

counterions as they become charged. In this work, a pH value ∼1.5 pH units below the apparent brush pKa was selected

where at intermediate potassium nitrate concentration (10 mM) the brushes are fully swollen (see Figure S1).30 This design feature enabled the investigation of the influence of a wide range of salt concentrations from 0.05 to 500 mM of different Hofmeister series potassium salts (nitrate, acetate, and thiocyanate) on the swelling behavior of these three polybasic tertiary amine methacrylate brushes.

Ionic-Strength-Dependent Behavior As Monitored by Ellipsometry. Figure 2 shows the equilibrium ellipsometric response of the three polybasic brushes as a function of the ionic strength (0.05−500 mM) of the three anions studied, at a fixed pH below the apparent brush pKa. The experiments were

performed by increasing the ionic strength, starting from 0.05 mM for each salt. Ellipsometry measures the overall ellipsometric thickness of the brush layer and can be regarded as an ensemble average over the entire measurement area. Furthermore, since it is necessary to use a slab model for the polymer density profile in this work (i.e., constant polymer density throughout the fitted thickness; see Supporting Information) the ellipsometry measurements will have low sensitivity to any “tail” of polymer density extending out into solution.34 The data shown were recorded when no further change in the real-time monitoring of the brushes was observable; i.e., the brush is assumed to be at equilibrium at each data point. Measurements as the salt concentration was decreased (data not shown) closely overlay the increasing salt data, confirming the attainment of equilibrium. Swelling ratio (ratio of wet to dry brush thickness) is used here as it is a measure of the amount of solvent within the brush and removes any dependence on dry brush thickness. This facilitates comparison between brushes.

Our previous work showed that a poly(DEA) brush in the presence of potassium nitrate at pH 5.5 swelled over a narrow (low) concentration range;28this is confirmed by the data in Figure 2b. Here the change from 0.1 to 1 mM potassium nitrate equates to an increase in the brush thickness from a collapsed state to a significantly swollen state. In the osmotic brush regime (low salt) for the three brushes to swell salt anions and associated water molecules must penetrate the polymer−liquid interface between the collapsed polymer chains to replace the hydroxide ions that are present to preserve brush electro-neutrality.40This process effectively charges the brush since the salt anions do not appreciably participate in the acid−base ionization equilibrium of the tertiary amine groups of the polymers. At a certain increasing concentration of salt anions within the brush the hydrophobic polymer−polymer inter-actions are overcome and the brush swells due to the increased osmotic pressure imparted by the counterions and associated water molecules.

In Figure 2, the behavior of the poly(DMA) and poly(DPA) brushes in the presence of potassium nitrate in the osmotic brush regime (low salt) are markedly different from each other, and both are dissimilar to the poly(DEA) brush. This behavior is attributed to the hydrophobicity difference between the three brushes. Recent contact angle measurements30have shown that poly(DMA) brushes are the least hydrophobic, poly(DEA) more hydrophobic, and poly(DPA) the most hydropho-bic.30,63,64 The poly(DMA) brush is highly solvated in the osmotic brush regime with its swelling ratio increasing only slightly in the range 0.05−10 mM for the three different salts. Being the most hydrophilic of the three polymers, the brush charge density required for the poly(DMA) brush to overcome the attractive poly(DMA)−poly(DMA) interactions and swell is the lowest, and as such the brush is highly solvated in this regime. Recently, we have shown that even when uncharged, the poly(DMA) brush remains well swollen with a swelling

ratio of ∼6.30 This is in stark contrast to the other more hydrophobic poly(DEA) and poly(DPA) brushes that collapse to a swelling ratio of ∼2 at low ionic strengths. Poly(DPA), being the most hydrophobic, requires a higher potassium nitrate concentration (between 1 and 10 mM) to swell the brush than poly(DEA). This may be rationalized by considering that a higher degree of overall brush charge density must be reached before the hydrophobic poly(DPA)−poly(DPA) interactions are overcome. Unsurprisingly, poly(DEA) exhibits intermediate behavior in terms of the osmotic regime collapse to swelling transition in accord with its hydrophobicity.63,64

At intermediate potassium nitrate concentrations, for poly-(DMA) and poly(DEA) between 1 and 10 mM and for poly(DPA) between 10 and 50 mM, brush thickness is effectively independent of ionic strength. Here the competing effects of increased charge and increased screening of the charge are largely balanced. Looking at Figure 2b for poly(DEA), at higher ionic strength the brush enters the salted brush regime; for the nitrate anion the brush begins to collapse between 100 and 500 mM. The poly(DMA) and poly(DPA) brushes also undergo collapse at increased concentrations of potassium nitrate. In this regime, for potassium nitrate all protonated amine groups are associated with a nitrate counterion; thus, any additional salt anions that enter the brush have to interact, at least partially, with the hydrophobic brush surface. Here as more anions enter the brush, the electrostatic inter- and intrachain interactions are screened and the inherent polymer hydrophobicity drives chain attraction, expelling solvent and collapsing the brush.

Anion-Specific Brush Behavior As Monitored by Ellipsometry. From Figure 2, at intermediate ionic strength, there is a clear anion-specific dependence of brush thickness (shown by swelling ratio) evident for the poly(DEA) and poly(DPA) brushes, with the brushes being most swollen in acetate, then nitrate, and least swollen in the presence of the thiocyanate anion. These differences cannot be solely attributed to hydrated ion size, as the hydrated diameters are actually in the reverse order: acetate = 0.434 nm, nitrate = 0.446 nm, and thiocyanate = 0.484 nm.72One striking feature of Figure 2c is that the poly(DPA) brush does not undergo any considerable swelling in thiocyanate from the initial state in the osmotic brush regime which indicates a direct polymer−ion interaction. This is attributed to the strong chaotropic nature of the thiocyanate anion. It has been shown that thiocyanate strongly associates with hydrophobic surfaces not only via electrostatic interactions but also by attractive van der Waals forces.11,20,73 With poly(DPA) being the most hydrophobic of the three polymers it is reasonable to suggest that the concentration of thiocyanate within the brush is therefore the highest here, and as such the locally high ionic strength prevents the brush from swelling. Furthermore, thiocyanate is a weakly hydrated anion,74 thus allowing the brush to more easily expel water molecules and remain collapsed. The anion-specific swelling at intermediate salt concentrations may also be affected by the effective charge of the alkylammonium group of the polymers. It has been shown that the chaotropic iodide is more efficient at screening charge for low charge density ammonium moieties compared to the kosmotropic fluoride, with the reverse observed for high charge density ammonium moieties.75

In Figure 2, the impact that the different anions have on brush behavior is most apparent above 50 mM. This is due to the fact that in this salted brush regime the brush charges from the protonated tertiary amine groups are completely

Figure 2. Ellipsometric swelling ratio of the (a) poly(DMA), (b) poly(DEA), and (c) poly(DPA) brushes as a function of ionic strength for potassium salts of three different Hofmeister series anions: acetate (■), nitrate (●), and thiocyanate (▲). The experiments were performed from low to high ionic strength. The pH wasfixed at 5.5 ± 0.1 for poly(DMA) and poly(DEA) and at pH 4.5 ± 0.1 for poly(DPA).

compensated by salt anions, and any additional salt anions must enter the brush through attraction with the polymer itself, i.e., via van der Waals and hydrophobic interactions, rather than predominately electrostatic interactions. The chaotropic thiocyanate anion has the most obvious impact, with the three brushes being considerably collapsed. Poly(DMA) collapses in the range 50500 mM potassium thiocyanate. The more hydrophobic poly(DEA) brush begins to collapse at lower thiocyanate concentrations, between 10 and 50 mM, while at the same concentrations of potassium nitrate and acetate the poly(DEA) brush remains highly swollen. As discussed above, the poly(DPA) brush remains collapsed over the entire concentration range for potassium thiocyanate.

For all three brushes in the presence of the nitrate anions (less chaotropic than thiocyanate) a higher salt concentration is required to drive the excess ions into the brush and the degree of brush collapse is considerably less. Significantly, in the case of the kosmotropic acetate anion, in the salted brush regime, the influence of increasing salt concentration is minimal with the brushes remaining highly solvated. This is true for all three brushes, which suggests that the acetate anions do not readily associate with the hydrophobic polymers via van der Waals forces, but rather via the electrostatic forces which are largely present at lower ionic strengths. We propose that the affinity of the acetate anion for the hydrophobic brushes is sufficiently low and that, in the concentration range studied, no appreciable ingress of ions into the brush occurs above those electrostati-cally associated with the brush. This is unsurprising since kosmotropic ions, like acetate, are known to strongly favor interactions with water5 as well as be excluded from the solvation layers of hydrophobic macromolecules in solution.1

Moreover, these data are consistent with the prior work of Swann et al. on poly(DEA)-based microgels, where the microgel was most swollen in kosmotropic anions and least swollen when immersed in chaotropic anions.11They proposed that larger, weakly hydrated ions such as thiocyanate favor binding to the microgel, thus reducing their availability to contribute to osmotic swelling. Note that the proposed higher concentration of thiocyanate anions in the brush may result from interactions of the ion with both the polymer backbone and ammonium groups, with both predicted by Paterová et al.13 Ionic-Strength-Dependent Behavior As Monitored by QCM-D. QCM-D measurements were also performed to provide complementary insight into the solvation of the brushes and the energy transfer between the oscillating brush-coated sensor and the solvent (i.e., brush viscoelasticity). Figure 3 presents the resonant frequency and dissipation values collected by QCM-D measurements for the poly(DMA), poly(DEA), and poly(DPA) brushes as a function of the ionic strength of the three anions studied. For polymer brushes, variations in Δf can be used to provide an indication of the degree of solvation of the grafted chains.76Here a decrease in Δf equates to an increase in coupled mass on the sensor, and since the mass of polymer brush is invariant, this signals an increase in the amount of solvent coupled to the motion of the brush.ΔD is a measure of how rapidly the motion of the sensor is damped by the sensor coating and is typically used as an indicator of chain extension and also the comparative softness (elasticity) or rigidity of the brush layer as it swells or collapses.76 Energy dissipation by polymer brushes is significantly influenced by the behavior of the peripheral polymer chains of a swollen brush.77,78 A higher dissipation

Figure 3.Equilibrium QCM-D frequency response,Δf3/3, of (a) poly(DMA), (b) poly(DEA), and (c) poly(DPA) brush and the corresponding

dissipation changes,ΔD3, of the three brushes (d), (e), and (f), respectively, as a function of the solution concentration of potassium acetate (■),

nitrate (●), and thiocyanate (▲). The values presented are for the third resonant overtone. ZeroΔf3/3 andΔD3values correspond to the brush

response when equilibrated in 0.05 mM of the respective salt solution. Thefilled and open symbols correspond to the brushes swelling in the osmotic brush regime and collapsing in the salted brush regime, respectively, as the ionic strength of each salt solution was incrementally increased. Brush response was the same, within error, when the brushes were exposed to decreasing ionic strength. The pH was controlled at 5.5± 0.1 for poly(DMA) and poly(DEA) and at pH 4.5± 0.1 for poly(DPA).

value means a more elastic brush as it is able to more readily dissipate energy through the motion of the brush to the solvent. The opposite is true for a lower dissipation value, which indicates a more rigidfilm that interacts less with the solvent. A classical QCM-D experiment is designed so that zero frequency and dissipation represent the response of the bare sensor, and an adsorbed amount can subsequently be determined. However, this is not possible here since the polymer brush is, by necessity, synthesized external to the instrument environment. Instead, zero Δf3/3 and ΔD3

correspond to the equilibrium brush response at 0.05 mM of each respective salt solution, and so the behavior of the brushes is interpreted in comparison to this state. Here, the degree of solvation of the brush is unknown which makes quantitative comparison between the three brushes impossible. More importantly, however, the impact that the different anions have on the brushes themselves can be investigated. In Figure 3, for all three brushes, there are two distinct regimes which correspond to the brushes swelling in the osmotic brush regime at low salt concentrations (filled symbols) and collapsing in the salted brush regime (open symbols), as seen in Figure 2 for the ellipsometry data. Starting in the osmotic brush regime, in all cases Δf (Figure 3a−c) decreases and ΔD (Figure 3d−f) simultaneously increases as the ionic strength is incrementally raised from the baseline of 0.05 mM. This corresponds to the brushes adopting more extended conformations (ΔD), with more solvent entrained within the brush and/or coupled to the oscillatory motion of the brushes (Δf). It is immediately evident for poly(DEA) and poly(DPA) that the salt concentration required for the brushes to swell as measured by QCM-D is lower than measured by ellipsometry. This difference is attributed to the measurement technique. Ellipsometry measures the ensemble average across the entire measurement area. Conversely, the QCM-D measurement would be greatly influenced by the polymer chains that extend further into solution than the average. These chains would impede the motion of solvent over the brush during the sensor oscillation, and so the frequency response and hence coupled mass would effectively increase. Here, ΔD is also sensitive to this low brush density at the brush periphery.77,78 These fundamental differences in the two techniques make QCM-D, through analysis of both the frequency and dissipation response, more sensitive to subtle changes in brush conformation. In the salted brush regime,Δf increases while simultaneouslyΔD decreases as the brushes collapse, expelling solvent, with increasing ionic strength.

Anion-Specific Brush Response Measured by QCM-D. In Figure 3 as ionic strength is increased, the magnitude ofΔf and ΔD is the lowest for all three brushes immersed in the most chaotropic thiocyanate anion. As we move to nitrate, a weaker chaotrope, and then acetate, a strong kosmotropic anion, the magnitude of the Δf and ΔD response increases, consistent with less solvent being taken up by the brushes when immersed in solutions of the more chaotropic anions. This is supported by the decreasing hydration number from acetate to thiocyanate,72 and the decreasing strength with which water molecules are bound to the increasingly more chaotropic anions,79as well as being concordant with the ellipsometry data in Figure 2. In the salted brush regime for each brush and each potassium salt (open symbols), Δf increases as the brushes expel solvent while at the same timeΔD decreases, indicating the brushes collapse, with the polymer chains interacting less with the solvent and more among themselves, i.e., more rigid

behavior. In the case of potassium thiocyanate, as polymer hydrophobicity increases, the brushes desolvate and collapse at lower salt concentrations, having the greatest impact on the most hydrophobic poly(DPA) brush. As discussed earlier, thiocyanate has been shown to strongly associate with hydrophobic surfaces; essentially it acts like a hydrophobe.20 Thus, as polymer hydrophobicity increases, the thiocyanate anions are attracted more to the polymer; specifically this occurs at lower concentrations. Furthermore, since thiocyanate anions are weakly hydrated,74the brushes can more easily expel the associated water molecules and hence collapse to a greater degree compared to the brushes in the nitrate and acetate solutions.

Additional insight concerning the conformational changes occurring within the three brushes as the nature and concentration of the potassium salt is varied can be garnered by plotting ΔD as a function of Δf over the entire ionic strength range for each salt. Here the relationship between the two quantities, a ΔD−Δf plot, can be used to describe the viscoelastic behavior of each brush78 as the nature and concentration of the salt are varied. These plots are presented in Figure 4 for the poly(DMA), poly(DEA), and poly(DPA) brushes. Figure 4 is composed of the same data as Figure 3 with the dotted lines tracking the brush response as ionic strength is increased from zeroΔf and ΔD; recall this has been set as the data for 0.05 mM of each respective salt. What is immediately obvious from these three plots are the relative differences in magnitude for the frequency and dissipation responses. However, as mentioned above, it is impossible to compare between brushes in terms of these absolute numbers because the degree of solvation of each of the brushes is unknown. Instead, and more importantly, the influence of the three potassium salts on the behavior of an individual brush can be discussed.

In Figure 4, there are two distinct regions present in the ΔD−Δf relationship for the brushes for each anion which coincide with the osmotic (filled symbols) and salted brush (open symbols) regimes. These three plots suggest the brushes transition to different conformations at low and high ionic strength dependent on the nature of the salt. In thefirst region, ΔD increases with decreasing Δf, indicating the solvation of the polymer and swelling of the brushes as they dissipate more energy to the solvent upon increasing ionic strength, as noted previously from Figure 3. In the second region, the decrease in ΔD with increasing Δf is indicative of the desolvation of the polymer accompanied by the collapse of the brushes at further increasing ionic strength, again as per Figure 3. However, Figure 4 highlights that in the salted brush regime theΔD−Δf relationship traces a different path. Here the value of ΔD is much lower for the same frequency response than in the osmotic brush regime; this is not immediately apparent from Figure 3. This shows that for a given degree of brush solvation, less energy is dissipated by the brushes undergoing the salt induced collapse at high ionic strength than when the brushes are swelling upon the addition of salt. This behavior arises from the differences in brush conformation that occur as a function of ionic strength.

Focusing on the behavior of the brushes in potassium nitrate, at low ionic strength they are in a collapsed conformation, with the exception of poly(DMA) which is highly solvated over the entire ionic strength range studied (from Figure 2). As ionic strength increases, nitrate counterions penetrate the periphery of the brush, and the brushes become increasingly charged and

swell by the uptake of solvent: the osmotic brush regime. Here, the surface of the brush has a lower polymer density than deeper in the brush, giving rise to an increasing value ofΔD, as the polymer brush dissipates more energy to the solvent. This behavior is depicted schematically in Figure 5. In contrast, as the concentration of nitrate ions within the brushes increases, they collapse as the polymer charge is screened by the greater number of ions within the brush. This collapsed brush conformation has a lower value of ΔD for the same degree of solvation (Δf) than for the same brush in the osmotic brush

regime. Here, at high salt concentrations the brushes adopt more rigid yet similarly solvated conformations to those adopted at low added salt, illustrated in Figure 5b. As indicated by the lower ΔD value, the interactions between the brushes and the nitrate anions prevent individual polymer chains from extending into solution. For example, all three brushes exhibit a similarΔD value, i.e., similar rigidity, when in the presence of both 0.05 and 500 mM potassium nitrate. However, at 500 mM the brushes are significantly more solvated, i.e., higher Δf value, than in 0.05 mM. This ionic-strength-dependent conforma-tional behavior is also influenced by the nature of the potassium salt.

From Figure 4, it is clear that the brushes are solvated to a lesser extent in thiocyanate over the entire concentration compared to the nitrate and acetate anions, as expected from Figures 2 and 3. Here, the poly(DMA) brush is more desolvated and more collapsed at high thiocyanate concen-trations compared to its 0.05 mM state. Like the nitrate data, the more hydrophobic poly(DEA) and poly(DPA) brushes display lowerΔD values for similar degrees of brush hydration at higher thiocyanate concentrations; however, unlike the nitrate data, they behave even more rigidly than the collapsed brushes at low salt (0.05 mM). This signals a stronger association of the brushes with the more chaotropic anion. Again, not unexpectedly, the behavior of the three brushes in potassium acetate solution is significantly different than the more chaotropic nitrate and thiocyanate anions. Here at high ionic strength the same trends as the other salts are visible; however, the magnitude of the drop in ΔD is less. So even though the brushes do behave more rigidly at high acetate concentrations for similar degrees of solvation to the brushes in the osmotic brush regime, the extent of this behavior is much less compared to that exhibited in the presence of the more chaotropic anions, as the affinity of the acetate anions for the polymer is much lower than for the more chaotropic anions as expected.

■

We have demonstrated that the specific anion effects on three polybasic brushes are dependent on polymer hydrophobicity. In the osmotic brush regime (low salt) chaotropic anions more readily partition within the brush, more so as polymer hydrophobicity increases, so the brushes are able to swell at lower salt concentrations than for more kosmotropic anions. In the salted brush regime, in the presence of high concentrations of the kosmotropic acetate anion the brushes remain highly solvated, while for the mildly chaotropic nitrate the brushes

Figure 4.Change in dissipation (ΔD3) versus change in frequency

(Δf3/3) for the three brushes (a) poly(DMA), (b) poly(DEA), and

(c) poly(DPA) as a function of the solution concentration of potassium acetate (■), nitrate (●), and thiocyanate (▲) starting from 0.05 mM. Both dissipation and frequency are relative to the starting 0.05 mM value for each respective salt solution for each brush. The filled and open symbols are the same data from Figure 3 for the brushes in the osmotic and salted brush regimes, respectively. The inset graph in (b) shows the same data as in the main graph but with magnified ΔD and Δf axes.

Figure 5. Schematic illustration of the ionic- strength-dependent conformational brush behavior in the (a) osmotic and (b) salted brush regimes. The brush in (a) has a higher dissipation as measured by QCM-D than the brush in (b) due to the tails of polymer at the periphery of the brush. The ionic strength range as well as the degree of brush solvation is anion-specific.

collapse and for the strong chaotropic anion, thiocyanate, the brushes collapse even further. QCM-D measurements confirm that the viscoelastic behavior of the brushes is different at low and high ionic strength with the nature of the anion having a major influence. Strong interactions between the highly chaotropic thiocyanate and the hydrophobic sections of the brushes result in brushes with uniform, rigid conformations. These data demonstrate conclusively that when interpreting weak polyelectrolyte brush behavior all interactions between ions, solvent, and the polymers themselves warrant consid-eration.

■

*

Details of synthetic protocols, modeling of ellipsometric data, and Figures of the pH response of the brushes measured by ellipsometry and QCM-D. This material is available free of charge via the Internet at http://pubs.acs.org.

■

Corresponding Author

*E-mail: grant.webber@newcastle.edu.au (G.B.W.).

Notes

The authors declare no competingfinancial interest.

■

This research is supported by the Australian Research Council (DP110100041).

■

(1) Lo Nostro, P.; Ninham, B. W. Hofmeister Phenomena: An Update on Ion Specificity in Biology. Chem. Rev. 2012, 112, 2286− 2322.

(2) Hofmeister, F. On the Understanding of the Effects of Salts. Arch. Exp. Pathol. Pharmakol. 1888, 24, 247−260.

(3) López-León, T.; Santander-Ortega, M. J.; Ortega-Vinuesa, J. L.; Bastos-González, D. Hofmeister Effects in Colloidal Systems: Influence of the Surface Nature. J. Phys. Chem. C 2008, 112 (41), 16060−16069.

(4) Pegram, L. M.; Record, M. T. Hofmeister Salt Effect on Surface Tension Arise from Partitioning of Anions and Cations between Bulk Water and the Air-Water Interface. J. Phys. Chem. B 2007, 111, 5411− 5417.

(5) Marcus, Y. Effect of Ions on the Structure of Water: Structure Making and Breaking. Chem. Rev. 2009, 109 (3), 1346−1370.

(6) Funkner, S.; Niehues, G.; Schmidt, D. A.; Heyden, M.; Schwaab, G.; Callahan, K. M.; Tobias, D. J.; Havenith, M. Watching the Low-Frequency Motions in Aqueous Salt Solutions: The Terahertz Vibrational Signatures of Hydrated Ions. J. Am. Chem. Soc. 2012, 134, 1030−1035.

(7) Jungwirth, P.; Cremer, P. S. Beyond Hofmeister. Nat. Chem. 2014, 6, 261−263.

(8) Omta, A. W.; Kropman, M. F.; Woutersen, S.; Bakker, H. J. Negligible Effects of Ions on the Hydrogen-Bond Structure in Liquid Water. Science 2003, 301, 347.

(9) Zhang, Y.; Cremer, P. S. The Inverse and Direct Hofmeister Series for Lysozyme. Proc. Natl. Acad. Sci. U. S. A. 2009, 106 (36), 15249−15253.

(10) Thormann, E. On Understanding of the Hofmeister Effect: How Addition of Salt Alters the Stability of Temperature Responsive Polymers in Aqueous Solutions. RSC Adv. 2012, 2, 8297−8305.

(11) Swann, J. M. G.; Bras, W.; Topham, P. D.; Howse, J. R.; Ryan, A. Effect of the Hofmeister Anions upon the Swelling of a Self-Assembled pH-Responsive Hydrogel. Langmuir 2010, 26 (12), 10191−10197.

(12) Heyda, J.; Hrobarik, T.; Jungwirth, P. Ion-Specific Interactions between Halides and Basic Amino Acids in Water. J. Phys. Chem. A 2009, 113, 1969−1975.

(13) Paterova, J.; Rembert, K. B.; Heyda, J.; Kurra, Y.; Okur, H. I.; Liu, W. R. L.; Hilty, C.; Cremer, P. S.; Jungwirth, P. Reversal of the Hofmeister Series: Specific Ion Effects on Peptides. J. Phys. Chem. B 2013, 117, 8150−8158.

(14) Rodríguez-Ropero, F.; van der Vegt, N. F. A. Ionic Specific Effects on the Structure, Mechanics and Interfacial Softness of a Polyelectrolyte Brush. Faraday Discuss. 2013, 160, 297−309.

(15) Boström, M.; Williams, D. R. M.; Ninham, B. W. Specific Ion Effects: Why DLVO Theory Fails for Biology and Colloid Systems. Phys. Rev. Lett. 2001, 87 (16), 168103−168103.

(16) Boström, M.; Williams, D. R. M.; Ninham, B. W. Ion Specificity of Micelles Explained by Ionic Dispersion Forces. Langmuir 2002, 18 (16), 6010−6014.

(17) Boström, M.; Williams, D. R. M.; Ninham, B. W. The Influence of Ionic Dispersion Potentials on Counterion Condensation on Polyelectrolytes. J. Phys. Chem. B 2002, 106 (32), 7908−7912.

(18) Boström, M.; Kunz, W.; Ninham, B. W. Hofmeister Effects in Surface Tension of Aqueous Electrolyte Solution. Langmuir 2005, 21 (6), 2619−2623.

(19) Parsons, D. F.; Boström, M.; Maceina, T. J.; Salis, A.; Ninham, B. W. Why Direct or Reversed Hofmeister Series? Interplay of Hydration, Non-Electrostatic Potentials, and Ion Size. Langmuir 2009, 26 (5), 3323−3328.

(20) Boström, M.; Williams, D. R. M.; Ninham, B. W. Specific Ion Effects: Why the Properties of Lysozyme in Salt Solutions Follow a Hofmeister Series. Biophys. J. 2003, 85 (2), 686−694.

(21) Barbey, R.; Lavanant, L.; Paripovic, D.; Schüwer, N.; Sugnaux, C.; Tugulu, S.; Klok, H. A. Polymer Brushes via Surface-Initiated Controlled Radical Polymerization: Synthesis, Characterization, Properties, and Applications. Chem. Rev. 2009, 109 (11), 5437−5527. (22) Matyjaszewski, K. Atom Transfer Radical Polymerization (ATRP): Current Status and Future Perspectives. Macromolecules 2012, 45 (10), 4015−4039.

(23) Ballauff, M.; Borisov, O. Polyelectrolyte Brushes. Curr. Opin. Colloid Interface Sci. 2006, 11 (6), 316−323.

(24) Wang, X.; Liu, G.; Zhang, G. Conformational Behavior of Grafted Weak Polyelectrolyte Chains: Effects of Counterion Condensation and Nonelectrostatic Anion Adsorption. Langmuir 2011, 27 (16), 9895−9901.

(25) Cheesman, B. T.; Willott, J. D.; Webber, G. B.; Edmondson, S.; Wanless, E. J. pH-Responsive Brush-Modified Silica Hybrids Synthesized by Surface-Initiated ARGET ATRP. ACS Macro Lett. 2012, 1 (10), 1161−1165.

(26) Cheesman, B. T.; Smith, E. G.; Murdoch, T. J.; Guibert, C.; Webber, G. B.; Edmondson, S.; Wanless, E. J. Polyelectrolyte Brush pH-Response at the Silica-Aqueous Solution Interface: A Kinetic and Equilibrium Investigation. Phys. Chem. Chem. Phys. 2013, 15 (34), 14502−14510.

(27) Topham, P. D.; Glidle, A.; Toolan, D. T. W.; Weir, M. P.; Skoda, M. W. A.; Barker, R.; Howse, J. R. The Relationship between Charge Density and Polyelectrolyte Brush Profile Using Simultaneous Neutron Reflectivity and In Situ Attenuated Total Internal Reflection FTIR. Langmuir 2013, 29 (20), 6068−6076.

(28) Willott, J. D.; Murdoch, T. J.; Humphreys, B. A.; Edmondson, S.; Webber, G. B.; Wanless, E. J. Critical Salt Effects in the Swelling Behavior of a Weak Polybasic Brush. Langmuir 2014, 30 (7), 1827− 1836.

(29) Fielding, L. A.; Edmondson, S.; Armes, S. P. Synthesis of pH-Responsive Tertiary Amine Methacrylate Polymer Brushes and Their Response to Acidic Vapour. J. Mater. Chem. 2011, 21 (32), 11773− 11780.

(30) Willott, J. D.; Humphreys, B. A.; Murdoch, T. J.; Edmondson, S.; Webber, G. B.; Wanless, E. J. Hydrophobic Effects within the Dynamic pH Response of Polybasic Tertiary Amine Methacrylate Brushes. Phys. Chem. Chem. Phys. 2015, 17 (5), 3880−3890.

(31) Jia, H.; Wildes, A.; Titmuss, S. Structure of pH-Responsive Polymer Brushes Grown at the Gold−Water Interface: Dependence on Grafting Density and Temperature. Macromolecules 2012, 45 (1), 305−312.

(32) Sanjuan, S.; Perrin, P.; Pantoustier, N.; Tran, Y. Synthesis and Swelling Behavior of pH-Responsive Polybase Brushes. Langmuir 2007, 23 (10), 5769−5778.

(33) Nordgren, N.; Rutland, M. W. Tunable Nanolubrication between Dual-Responsive Polyionic Grafts. Nano Lett. 2009, 9 (8), 2984−2990.

(34) Moglianetti, M.; Webster, J. R. P.; Edmondson, S.; Armes, S. P.; Titmuss, S. Neutron Reflectivity Study of the Structure of pH-Responsive Polymer Brushes Grown from a Macroinitiator at the Sapphire−Water Interface. Langmuir 2010, 26 (15), 12684−12689.

(35) Sun, W.; Zhou, S.; You, B.; Wu, L. Polymer Brush-Functionalized Surfaces with Reversible, Precisely Controllable Two-Way Responsive Wettability. Macromolecules 2013, 46 (17), 7018− 7026.

(36) Zhulina, E. B.; Birshtein, T. M.; Borisov, O. V. Theory of Ionizable Polymer Brushes. Macromolecules 1995, 28 (5), 1491−1499. (37) 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 (10), 3562−3569.

(38) Zhulina, E. B.; Klein Wolterink, J.; Borisov, O. V. Screening Effects in a Polyelectrolyte Brush: Self-Consistent-Field Theory. Macromolecules 2000, 33 (13), 4945−4953.

(39) Zhulina, E. B.; Borisov, O. V. Poisson−Boltzmann Theory of pH-Sensitive (Annealing) Polyelectrolyte Brush. Langmuir 2011, 27 (17), 10615−10633.

(40) Witte, K. N.; Kim, S.; Won, Y.-Y. Self-Consistent Field Theory Study of the Effect of Grafting Density on the Height of a Weak Polyelectrolyte Brush. J. Phys. Chem. B 2009, 113 (32), 11076−11084. (41) Nap, R.; Tagliazucchi, M.; Szleifer, I. Born Energy, Acid-Base Equilibrium, Structure and Interactions of End-Grafted Weak Polyelectrolyte Layers. J. Chem. Phys. 2014, 140 (2), 024910.

(42) Guptha, V. S.; Hsiao, P. Y. Polyelectrolyte Brushes in Monovalent and Multivalent Salt Solutions. Polymer 2014, 55 (12), 2900−2912.

(43) Pryamitsyn, V.; Leermakers, F.; Fleer, G.; Zhulina, E. Theory of the Collapse of the Polyelectrolyte Brush. Macromolecules 1996, 29 (25), 8260−8270.

(44) He, S. Z.; Merlitz, H.; Chen, L.; Sommer, J. U.; Wu, C. X. Polyelectrolyte Brushes: MD Simulation and SCF Theory. Macro-molecules 2010, 43 (18), 7845−7851.

(45) Biesheuvel, P. M.; de Vos, W. M.; Amoskov, V. M. Semianalytical Continuum Model for Nondilute Neutral and Charged Brushes Including Finite Stretching. Macromolecules 2008, 41 (16), 6254−6259.

(46) Zhulina, E. B.; Borisov, O. V. Structure and Interaction of Weakly Charged Polyelectrolyte Brushes: Self-Consistent Field Theory. J. Chem. Phys. 1997, 107 (15), 5952−5967.

(47) Hehmeyer, O. J.; Arya, G.; Panagiotopoulos, A. Z.; Szleifer, I. Monte Carlo Simulation and Molecular Theory of Tethered Polyelectrolytes. J. Chem. Phys. 2007, 126 (24), 244902.

(48) Ibergay, C.; Malfreyt, P.; Tildesley, D. J. Mesoscale Modeling of Polyelectrolyte Brushes with Salt. J. Phys. Chem. B 2010, 114 (21), 7274−7285.

(49) Kumar, N. A.; Seidel, C. Polyelectrolyte Brushes with Added Salt. Macromolecules 2005, 38 (22), 9341−9350.

(50) Chen, L.; Merlitz, H.; He, S. Z.; Wu, C. X.; Sommer, J. U. Polyelectrolyte Brushes: Debye Approximation and Mean-Field Theory. Macromolecules 2011, 44 (8), 3109−3116.

(51) Attili, S.; Borisov, O. V.; Richter, R. P. Films of End-Grafted Hyaluronan are a Prototype of a Brush of a Strongly Charged, Semiflexible Polyelectrolyte with Intrinsic Excluded Volume. Bio-macromolecules 2012, 13 (5), 1466−1477.

(52) Israëls, R.; Leermakers, F. A. M.; Fleer, G. J. On the Theory of Grafted Weak Polyacids. Macromolecules 1994, 27 (11), 3087−3093.

(53) Currie, E. P. K.; Sieval, A. B.; Fleer, G. J.; Cohen Stuart, M. A. Polyacrylic Acid Brushes: Surface Pressure and Salt-Induced Swelling. Langmuir 2000, 16 (22), 8324−8333.

(54) Biesalski, M.; Johannsmann, D.; Rühe, J. Synthesis and Swelling Behavior of a Weak Polyacid Brush. J. Chem. Phys. 2002, 117 (10), 4988.

(55) Zhang, H.; Rühe, J. Swelling of Poly(methacrylic acid) Brushes: Influence of Monovalent Salts in the Environment. Macromolecules 2005, 38 (11), 4855−4860.

(56) Wu, T.; Gong, P.; Szleifer, I.; Vlček, P.; Šubr, V.; Genzer, J. Behavior of Surface-Anchored Poly(acrylic acid) Brushes with Grafting Density Gradients on Solid Substrates: 1. Experiment. Macromolecules 2007, 40 (24), 8756−8764.

(57) Lego, B.; Skene, W. G.; Giasson, S. Swelling Study of Responsive Polyelectrolyte Brushes Grafted from Mica Substrates: Effect of pH, Salt, and Grafting Density. Macromolecules 2010, 43 (9), 4384−4393.

(58) Firestone, B. A.; Siegel, R. A. pH, Salt, and Buffer Dependent Swelling in Ionizable Copolymer Gels: Tests of the Ideal Donnan Equilibrium Theory. J. Biomater. Sci., Polym. Ed. 1994, 5 (5), 433−450. (59) Farhan, T.; Azzaroni, O.; Huck, W. T. S. AFM Study of Cationically Charged Polymer Brushes: Switching Between Soft and Hard Matter. Soft Matter 2005, 1 (1), 66−68.

(60) Azzaroni, O.; Brown, A. A.; Huck, W. T. S. Tunable Wettability by Clicking Counterions Into Polyelectrolyte Brushes. Adv. Mater. 2007, 19 (1), 151−154.

(61) Wei, X.; Ngai, T. Ion-Induced Hydrophobic Collapse of Surface-Confined Polyelectrolyte Brushes Measured by Total Internal Reflection Microscopy. Polym. Chem. 2012, 3 (8), 2121−2128.

(62) Matsuoka, H.; Nakayama, S.; Yamada, T. X-ray Reflectivity Study of the Effect of Ion Species on Nanostructure and its Transition of Poly(styrenesulfonate) Brush at the Air/Water Interface. Chem. Lett. 2012, 41 (10), 1060−1062.

(63) Bütün, V.; Armes, S. P.; Billingham, N. C. Synthesis and Aqueous Solution Properties of Near-Monodisperse Tertiary Amine Methacrylate Homopolymers and Diblock Copolymers. Polymer 2001, 42 (14), 5993−6008.

(64) Thavanesan, T.; Herbert, C.; Plamper, F. A. Insight in the Phase Separation Peculiarities of Poly(dialkylaminoethyl methacrylate)s. Langmuir 2014, 30 (19), 5609−5619.

(65) Ishida, N.; Biggs, S. Salt-Induced Structural Behavior for Poly(N-isopropylacrylamide) Grafted onto Solid Surface Observed Directly by AFM and QCM-D. Macromolecules 2007, 40 (25), 9045− 9052.

(66) Dunér, G.; Thormann, E.; Dėdinaitė, A. Quartz Crystal Microbalance with Dissipation (QCM-D) Studies of the Viscoelastic Response from a Continuously Growing Grafted Polyelectrolyte Layer. J. Colloid Interface Sci. 2013, 408 (0), 229−234.

(67) Rodenhausen, K. B.; Kasputis, T.; Pannier, A. K.; Gerasimov, J. Y.; Lai, R. Y.; Solinsky, M.; Tiwald, T. E.; Wang, H.; Sarkar, A.; Hofmann, T.; Ianno, N.; Schubert, M. Combined Optical and Acoustical Method for Determination of Thickness and Porosity of Transparent Organic Layers Below the Ultra-Thin Film Limit. Rev. Sci. Instrum. 2011, 82 (10), 103111.

(68) Moya, S. E.; Irigoyen, J. Recent Advances in the Use of the Quartz Crystal Microbalance with Dissipation for the Study of the Collapse of Polyelectrolyte Brushes. J. Polym. Sci., Part B: Polym. Phys. 2013, 51 (14), 1068−1072.

(69) Reviakine, I.; Johannsmann, D.; Richter, R. P. Hearing What You Cannot See and Visualizing What You Hear: Interpreting Quartz Crystal Microbalance Data from Solvated Interfaces. Anal. Chem. 2011, 83 (23), 8838−8848.

(70) Sauerbrey, G. Use of Quartz Vibration for Weighing Thin Films on a Microbalance. J. Phys. 1959, 155, 206−212.

(71) Nap, R.; Gong, P.; Szleifer, I. Weak Polyelectrolytes Tethered to Surfaces: Effect of Geometry, Acid−Base Equilibrium and Electrical Permittivity. J. Polym. Sci., Part B: Polym. Phys. 2006, 44 (18), 2638− 2662.

(72) Marcus, Y. Thermodynamics of Solvation of Ions. Part 5. -Gibbs Free Energy of Hydration at 298.15 K. J. Chem. Soc., Faraday Trans. 1991, 87 (18), 2995−2999.

(73) Rembert, K. B.; Paterová, J.; Heyda, J.; Hilty, C.; Jungwirth, P.; Cremer, P. S. Molecular Mechanisms of Ion-Specific Effects on Proteins. J. Am. Chem. Soc. 2012, 134 (24), 10039−10046.

(74) Mason, P. E.; Neilson, G. W.; Dempsey, C. E.; Barnes, A. C.; Cruickshank, J. M. The Hydration Structure of Guanidinium and Thiocyanate Ions: Implications for Protein Stability in Aqueous Solution. Proc. Natl. Acad. Sci. U. S. A. 2003, 100 (8), 4557−4561.

(75) Mason, P. E.; Heyda, J.; Fischer, H. E.; Jungwirth, P. Specific Interactions of Ammonium Functionalities in Amino Acids with Aqueous Fluoride and Iodide. J. Phys. Chem. B 2010, 114 (43), 13853−13860.

(76) Liu, G.; Zhang, G. QCM-D Studies on Polymer Behavior at Interfaces; Springer: New York, 2013.

(77) Liu, X.; Dedinaite, A.; Rutland, M.; Thormann, E.; Visnevskij, C.; Makuska, R.; Claesson, P. M. Electrostatically Anchored Branched Brush Layers. Langmuir 2012, 28 (44), 15537−15547.

(78) Wang, T.; Long, Y.; Liu, L.; Wang, X.; Craig, V. S. J.; Zhang, G.; Liu, G. Cation-Specific Conformational Behavior of Polyelectrolyte Brushes: From Aqueous to Nonaqueous Solvent. Langmuir 2014, 30 (43), 12850−12859.

(79) Robinson, J. B.; Strottmann, J. M.; Stellwagen, E. Prediction of Neutral Salt Elution Profiles for Affinity Chromatography. Proc. Natl. Acad. Sci. U. S. A. 1981, 78 (4), 2287−2291.