Counteranion-Specific Hydration States of Cationic Polyelectrolyte

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Materials and Interfaces

Counter Anion-Specific Hydration States of Cationic Polyelectrolyte Brushes Yuji Higaki, Yoshihiro Inutsuka, Hitomi Ono, Norifumi L Yamada, Yuka Ikemoto, and Atsushi Takahara Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00210 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 31, 2018

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Industrial & Engineering Chemistry Research

Counter Anion-Specific Hydration States of Cationic Polyelectrolyte Brushes Yuji Higaki,†,‡,§,∥ Yoshihiro Inutsuka, ‡ Hitomi Ono, ‡ Norifumi L. Yamada,⊥ Yuka Ikemoto, # Atsushi Takahara†,‡,§,∥* †

Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku,

Fukuoka 819-0395, Japan ‡

Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-

0395, Japan §

International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu

University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ∥

Japan Science and Technology Agency (JST), ERATO, Takahara Soft Interfaces Project, 744

Motooka, Nishi-ku, Fukuoka 819-0395, Japan ⊥

Neutron Science Laboratory, High Energy Accelerator Research Organization, Ibaraki 319-

1106, Japan #

Japan Synchrotron Radiation Research Institute/SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun,

Hyogo 679-5198, Japan

ACS Paragon Plus Environment

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ABSTRACT

While polyelectrolyte brushes have received extensive attention due to their particular surface properties, the ion-specific hydration states remain largely unknown. Here, we report the counter anion-specific hydration states of cationic poly[2-(methacryloyloxy)ethyltrimethylammonium] (PMTA) brushes in salt-free water. The water droplet contact angle on the PMTA brushes depends on the counter anion species, and the order is consistent with the Hofmeister series. Weakly hydrated chaotropic counter anions are strongly bound to weakly hydrated quaternary ammonium (QA+) cations in the PMTA brush chains, which induces a reduction in the ζpotential, dehydration and collapse of the PMTA brushes. The PMTA brushes with strongly hydrated chloride counter anions produce a more diffuse tail and less swollen bound layer under salt-free deuterium oxide than brushes with weakly hydrated thiocyanate counter anions. Ion pairing disturbs the ordering of hydrated water in the PMTA brushes. Our work enhances the understanding of the ion specificity in the hydration states of polyelectrolyte brushes and encourages the rational design of charged polymer materials.


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INTRODUCTION Polymer brushes are thin films of polymer chains covalently bound to surfaces, which allow for surface modification, where the surface properties can be tailored by designing the chemical structure of the surface-tethered polymer chains.1-3 Charged polymer brushes, including polyelectrolyte (polymers with net charge) brushes and net-neutral zwitterionic polymer brushes, show various properties such as lubrication and anti-fouling under wet conditions.4-8 Those performances arise from the hydration of the charged polymer chains.9 The surface properties of polyelectrolyte brushes depend on the fraction of dissociated ionic groups, the dielectric constant of the solution, and the salt concentration, as the hydration state is modulated by these factors.1018

As counterions are bound to the polyelectrolyte chains by electrostatic interactions, the charge

fraction is dependent on how much the counterions interact with pendant charges; therefore, the hydration state is rationally assumed to be counterion-specific. Tunable wettability of polyelectrolyte brushes by exchanging the counterions was reported by Huck et al.19-20 Ion specificity is generally described by the well-known Hofmeister series.2127

Hofmeister studied the relative efficiency of ions in precipitating suspensions of egg white

lecithin, and proposed the classic Hofmeister series. The order was later modified by taking effect of counterions into account to give the standard Hofmeister sequences: SO42– > OH– > F– > Cl– > Br– > NO3– > I– > SCN– > ClO4– for anions at fixed cation, and NH4+ > K+ > Na+ > Cs+ > Li+ > Rb+ > Mg2+ > Ca2+ > Ba2+ for cations at fixed anion.22 The anions on the left side are called kosmotropes, and those on the right side are called chaotropes. Because the stability of colloids or proteins depends on the solution conditions, like


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pH or buffer type besides salt concentration, the minor variation and reverse ordering exist. Kosmotrope anions exhibit strong interactions with water, whereas chaotrope anions exhibit weak interactions with water. Similar to the remarkable ion specificities in proteins, artificial polyelectrolytes also exhibit Hofmeister effects.14-15, 25, 28-30 Interactions of ions with polymer brushes have been investigated by quartz crystal microbalance with dissipation (QCM-D).15-16, 29 Liu et al. reported a systematic QCM-D study for the counter anion-specific hydration state of poly[2-(methacryloyloxy)ethyltrimethylammonium]

(PMTA)

brushes

with

a

series

of

Hofmeister anions including SO42−, HPO42−, CH3COO−, Cl−, Br−, NO3−, I−, and SCN−.15 The mass and stiffness variation triggered by counter anion exchange and subsequent hydration was verified by means of QCM-D. The ion-specific interactions between quaternary ammonium (QA+) cations in the PMTA brush chains and the anions have significant effects on hydration. The more chaotropic anions induced dehydration and collapse of the PMTA brush chains due to the strong binding of highly polarized large anions. Meanwhile the ion-specific interactions between the PMTA brushes and kosmotropic anions are dominated by the competition of QA+ cations and kosmotropic anions for interaction with water. However, QCM-D suffers from a lack of aspect on the polymer chain density profile in the swollen PMTA brushes. Neutron reflectivity (NR) provides quantitative determination of polymer density profile in polymer films normal to the substrate; thereby, it has been employed to the investigation of hydration state of polyelectrolyte brushes.17, 30-35 Because of the large contrast between 1H and 2

H which allows selective labelling by deuteration, NR provides detail insights for the swollen

chain density profile of the charged polymer brushes in aqueous solutions from the scattering length density (SLD) profile. Dunlop et al. studied swollen PMTA-Cl brush structure by NR.17 The PMTA-Cl brush adopted two-region structure consisting of a dense surface region and a


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diffusive swollen layer extending from the surface. The collapse of the PMTA-Cl brushes under electrolytes and oppositely charged surfactants was also verified. Specific anion effect and pHresponse of weak polyelectrolyte brushes has been extensively studied by NR.32-34 However, the counter anion-specific swollen structure modulation of the strong poly(quarternary ammonium cation) brushes has never been investigated by NR, and remains largely unknown. Here, we present the first known experimental study of the counter anion-specific hydration state of a cationic polyelectrolyte PMTA brushes by means of NR and infrared (IR) spectroscopy. Our work provides a clear picture of the strong cationic polyelectrolyte brushes and their relation to liquid wetting and/or anti-fouling behaviors, which will encourage materials scientists to rationally design “smart” surfaces exhibiting ion-sensitive interfacial properties. EXPERIMENTAL SECTION Materials. Thin plate silicon wafers (single side polished, 40 mm × 10 mm × 0.5 mmt) and thick silicon blocks (single side polished, 50 mm × 20 mm × 8 mmt) were used as substrates. Silica nanoparticles (SiNPs) with an average radius of 100 nm dispersed in water (40 wt%) were kindly

supplied

by

Nissan

Chemical

Industries,

Tokyo,

Japan.

2-

(Methacryloyloxy)ethyltrimethylammonium chloride (MTAC) solution (Aldrich, 80 wt% in H2O) was purified by repeated precipitation in THF. Milli-Q water (Millipore Inc., Billerica, MA) with a resistance above 18 MΩ·cm was used. Deuterium oxide (D2O, Merck, 99.96%), sodium chloride (NaCl, Kanto Chemical, molecular biology grade, >99%), potassium thiocyanate (KSCN, Wako Pure Chemical Industries, 98.5%), sodium acetate (NaAc, Wako Pure Chemical Industries, 98.5%), and lithium perchlorate (LiClO4, Wako Pure Chemical Industries, 98%) were used without further purification.


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Preparation of the Polymer Brushes. Poly(MTAC) (PMTA-Cl) brushes were prepared by surface-initiated atom transfer radical polymerization (ATRP).19 The detail preparation procedure is given in the Supporting Information. The characteristics of the PMTA-Cl brushes used in this paper are summarized in Table 1. Contact angle measurement and FTIR spectroscopy were carried out using a thick polymer brush (PMTA-Cl_HM), while NR was performed using a thin polymer brush (PMTA-Cl_LM). Thick polymer brushes are suitable to investigate hydrogen bonding network structure of hydrated water in PMTA brushes by FTIR because the FTIR signals of uptake water in the PMTA brushes were hardly observed in thin PMTA brushes. The NR profile exhibits clear Kiessig fringes when the polymer brush thickness is in the range of 20-40 nm, which is appropriate for capturing the slight change in hydration states. Counter anion exchange was conducted by soaking the PMTA-Cl brushes in 100 mM aqueous solutions of KSCN, NaAc, and LiClO4 for over 24 hours to obtain PMTA brushes with acetate (PMTA-Ac), thiocyanate (PMTA-SCN), and perchlorate (PMTA-ClO4) counter anions, respectively. The chemical structures of the polymer brushes are shown in Figure 1.

Figure 1. Chemical structure of the PMTA brushes


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Table 1. Characteristics of the PMTA-Cl brushes Mna

Mn/Mwa

Thicknessb

graft densityc

(nm)

(chains/nm2)

PMTA-Cl_LMd

107,000

1.23

12.8

0.09

PMTA-Cl_HMe

―

―

230

―

a

Mn and Mw/Mn were determined by SEC equipped with a refractive index detector. The molecular weights were calibrated against the absolute molecular weight of PMTA-Cl obtained by SEC-MALS (multi-angle light scattering) measurement. bThickness estimated by ellipsometry under ambient conditions (relative humidity: ca. 40%) at 298 K. The refractive index of PMTACl, 1.49, was applied to the calculation. cGraft density, σ, was calculated from Mn and the thickness (L) using the equation σ = ρLNA/Mn, where ρ is the bulk density of the free (unbound) polymer and NA is Avogadro’s constant. The bulk density of PMTA-Cl, 1.31 g mol-1, was employed to the calculation. The bulk density of PMTA-Cl brush was calculated from the SLD value (0.874) obtained by NR. dSamples for NR measurement. eSamples for contact angle measurement and FT-IR spectroscopy. Sacrificial free initiator was not used to obtain extremely thick polymer brushes.

SiNPs were modified with a silane coupling reagent containing a ATRP initiating group according to previously reported protocol.36 PMTA-Cl brush growth from the SiNP surface was achieved by surface-initiated ATRP following identical polymerization conditions for the flat silicon wafer. Counter anion exchange was carried out by dispersing the PMTA-Cl brushmodified SiNPs in 100 mM aqueous solutions of KSCN, NaAc, and LiClO4 for over 24 hours, respectively. The PMTA brush-modified SiNPs were separated by centrifugation and washed by repeating dispersion in Milli-Q water and collection by centrifugation. The PMTA brushmodified SiNPs were used in the ζ-potential measurements. Measurements. Detailed measurement conditions for size exclusion chromatography (SEC), X-ray photoelectron spectroscopy (XPS) with Ar gas cluster ion etching apparatus, and ellipsometry are provided in the Supporting Information.


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Contact Angle Measurement. Static contact angles of water droplets were recorded using a droplet placing and droplet shape analysis system equipped with a video camera (Attension Theta, Biolin Scientific AB, Stockholm, Sweden). A 1 µL water droplet was placed on the polymer brush surface using a micro-syringe, and the contact angle was recorded. The relative humidity and ambient temperature were approximately 40% and 298 K, respectively. Over five measurements were conducted with a polymer brush sample, and the averaged contact angles were recorded. Contact angle data were acquired by the use of a single PMTA brush sample by repeating the counter ion exchange. FT-IR Spectroscopy. Humidity-controlled Fourier transform infrared (FT-IR) spectroscopy was conducted at the BL43IR beamline of SPring-8 (Hyogo, Japan) with a FT-IR microscope system (VERTEX 70 and HYPERION 2000, Bruker) equipped with an MCT detector. The IR beam size was adjusted to 20 µm × 20 µm by apertures. A polymer brush sample was installed into a homemade humid vapor flow cell with a BaF2 window.37 The gap through which the humid vapor passed was adjusted to 2 mm by a silicone rubber sheet spacer. The relative humidity of the flow gas was controlled by a humidity-control apparatus HUM-1 (RIGAKU, Tokyo, Japan), and the temperature and humidity were monitored at the gas outlet. IR spectra were recorded in transmission mode with a resolution of 4 cm−1 and integrated over 512 scans within 100 seconds under a successive humidity increase of 1.0% min-1. The humid vapor temperature was kept at 27.5 ± 0.2 °C. The water vapor absorptions were subtracted from the raw data by using the IR spectrum of a pristine Si wafer under humid vapor. ζ-potential Measurement. The ζ-potential of the PMTA brush-modified SiNPs was determined using an electrophoretic light-scattering spectrophotometer (ELS Z, Otsuka Electronics Co., Ltd., Osaka, Japan) equipped with a semiconductor laser and a photomultiplier


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tube detector. The ζ-potential was estimated from the electrophoretic mobility, u, using the Helmholtz-Smoluchowski equation: ߞ=ఌ

ఎ బ ఌೝ

‫ݑ‬

(1)

where ε0, εr and η are the dielectric constant of vacuum, the dielectric constant of the medium, and the viscosity of the medium, respectively. All measurements were repeated at least five times to obtain reliable averaged values. ζ-potential data were acquired by the use of a single PMTA brush modified particle sample by repeating the counter ion exchange. Neutron Reflectivity. NR was conducted using a reflectometer (BL16 SOFIA, Materials and Life Science Facility (MLF), Japan Proton Accelerator Research Complex (J-PARC), Tokai, Japan) providing 25 Hz pulsed neutron radiation.38 The wavelength (λ) of the incident neutrons was selected to be 0.20–0.88 nm using a disk chopper. The reflected neutrons were recorded by a two-dimensional position-sensitive scintillation detector. The neutron momentum transfer vector is defined as qz = (4π/λ)sinθ, where θ is the specular reflection angle with respect to the polymer brush interface. A silicon block with a polymer brush (50 mm × 20 mm × 8 mmt) was covered with a quartz trough filled with salt-free D2O. A Viton rubber frame was sandwiched between the trough and silicon block, and the couple was fixed with aluminum plates. The sample was installed in a temperature-controlled chamber at 298 K. A 10 mm width and 30 mm transverse footprint on the sample surface was adjusted by slits. The NR of the air/polymer brush interface was acquired under dry nitrogen gas flow at 298 K. The MOTOFIT program was employed to fit the reflectivity profiles to the model SLD profiles, which were composed of the thickness, SLD, and Gaussian roughness.39 The fitting curve with the lowest χ2 error was employed. The SLDs of


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Si, SiO2, and D2O were set to 2.07 × 10−4 nm−2, 3.47 × 10−4 nm−2, and 6.2 × 10−4 nm−2, respectively. The polymer volume fraction φ(z) profiles were simply calculated by the following equation, ߩே (‫ߩ)ݖ(߶ = )ݖ‬ே,௉ௌ஻ + ሾ1 − ߶(‫)ݖ‬ሿߩே,஽ଶை

(2)

where the ρN(z), ρN,PSB, and ρN,D2O are the SLD at a position(z), the SLD of the PMTA brush, and the SLD of D2O, respectively. The average brush thickness, Lbrush, was determined by the following equation, ಮ

‫ܮ‬௕௥௨௦௛ =

ଶ ‫׬‬బ ௭థ(௭)ௗ௭

(3)

ఋ೏ೝ೤ ஶ

ߜௗ௥௬ = ‫׬‬଴ ߶(‫ݖ݀)ݖ‬

(4)

where δdry is the effective dry thickness of the polymer brushes.30 The curve fitting was repeated under the constraint of the area under the polymer volume fraction profiles (δdry) to ensure mass balance of the polymer chains. However, the fitting curve was inconsistent with reflectivity data at all; thereby we relaxed the mass balance constraint (mass balance: PMTA-Cl/PMTA-SCN = 1.0/0.9) to establish consistency between the curve fitting and mass balance. The disagreement would be derived from experimental error. RESULTS AND DISCUSSION Preparation of PMTA Brushes with a Variety of Counter Anions. The chloride anions in the PMTA-Cl brushes were replaced by soaking the PMTA-Cl brush in aqueous solutions of NaAc, KSCN, or LiClO4. The ion exchange was verified by XPS measurement (Figure S1). The chloride signal completely disappeared, and signals assignable to the exchanged counter anions


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appeared. The XPS signals are derived from the outermost surface region because of the limited inelastic mean free path of a photoelectron; therefore, ion exchange at the bottom of the polymer brushes was not verified. The internal chemical composition was verified by successive XPS measurement during etching by Ar gas cluster ion beam. The Ar gas cluster ion beam consists of several hundred to several thousand Ar atoms and has been reported as a soft etching source with high sputtering yield and low penetration or damaging effects to organic materials.40 The depth profile of the elements in the PMTA-SCN brushes showed no chloride signals even when signals from the silicon wafer substrate were detected, indicating complete ion exchange in the PMTA brush chains (Figure S2). Complete ion exchange was also ensured for the other counter anions. Wetting and ζ-potentials. Side-view images of a water droplet on the PMTA brushes are shown in Figure 2. The wetting properties drastically changed after replacing the counter anions. ClO4– and SCN– are typical chaotropic anions, while Ac– and Cl– are classified as at the boundary between kosmotropes and chaotropes. PMTA brushes with strongly hydrated counter anions (Ac–, Cl–) showed contact angles below 20°, whereas those with weakly hydrated chaotropic counter anions (SCN–, ClO4–) showed much higher contact angles. The water droplet contact angles implied that strongly hydrated counter anions promote water droplet wetting, whereas weakly hydrated counter anions made the PMTA brushes slightly hydrophobic. In accordance with the Hofmeister series (order of ionic hydration strength), the water droplet contact angle follows the trend Ac– < Cl– < SCN– < ClO4–. The wetting order is consistent with previous report by Huck et al.


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Figure 2. Side view of a water droplet on (a) PMTA-Ac, (b) PMTA-Cl, (c) PMTA-SCN, and (d) PMTA-ClO4. The averaged static contact angles are depicted in the images.

Most of the polymer films without any positively charged groups show a negative ζpotential. Meanwhile, cationic polyelectrolyte brushes exhibit a positive ζ-potential, and the potential depends on the charge fraction of the polyelectrolyte chains. SiNPs modified with PMTA brushes with strongly hydrated counter anions show a more positive ζ-potential than those with weakly hydrated counter anions (Figure 3). The large charge fraction of the QA+ cations in the PMTA-Ac and PMTA-Cl brushes results from the dissociation of the hydrophilic kosmotropic counter anions.

Figure 3. ζ-potential of PMTA brush-modified SiNPs containing different counter anions.


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The counterion-dependent wettability of the PMTA brushes has already been reported, and our results are consistent with the previous report.20 The variation in the surface tension is associated with the charge fraction of the PMTA brush chains modulated by the inter-ionic interaction between the QA+ cations and the counter anions. The chaotropic anions significantly bind with QA+ cations to shield electrostatic interactions, leading to reductions in the absolute ζpotential and surface tension. Manning’s counterion binding theory states that a portion of counterions are bound to the pendant charges in polyelectrolyte chains, producing temporary neutral ion pairs.41 An empirical rule for ion-specific pairing between cations and anions was proposed by Collins.42 According to the theory, kosmotropic (chaotropic) cations and kosmotropic (chaotropic) anions tend to form strong ion pairs, whereas kosmotropic (chaotropic) cations and chaotropic (kosmotropic) anions are likely to dissociate in water. This tendency is explained in terms of the ion-water interactions, the energetic cost for ion dehydration, and the energetic benefit for the production of water-water pairs. Because the QA+ cation is a weakly hydrated cation, the interaction force between QA+ cations and anions follows the series Ac– < Cl– < SCN– < ClO4– according to Collins’ rule. The more chaotropic anions effectively bind to QA+ cations, leading to charge neutralization, as seen in the reduced ζ-potential. Meanwhile nonelectrostatic ion-specific dispersion interaction forces play an important role in Hofmeister series.43 The larger and polarizable anions substantially interact with QA+ cations through van der Waals interactions. Counterion binding causes the dehydration of QA+ groups because the close approach of ions induces the mutual perturbation of hydration layers. The ion-paired PMTA brushes lose their hydrophilicity due to the weak


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surface tension of PMTA brushes and large interfacial tension at the PMTA brushes/water interface. Hydrogen-Bonding Network Structure of Hydrated Waters. The FT-IR spectra of the PMTA-Cl and PMTA-SCN brushes exposed to humidity-controlled gas flow are shown in Figure 4. The absorption is assigned to the O-H stretching vibration of water introduced into the PMTA brushes, as the signal was hardly observed in the spectrum of the bare silicon wafer. The absorptions were decomposed to three Gaussian peaks at approximately 3550 cm-1 (A1), 3400 cm-1 (A2), and 3250 cm-1 (A3), which can be assigned to free water, weakly interacting water and strongly interacting water, respectively (Figure S3).44-45 The double-peak absorption in the vibrational spectrum of water is also assigned to the symmetric stretching vibrations split by Fermi resonance with an overtone of the water bending mode.46 The hydrogen-bonding network structure of the hydrated water was determined by integration of the components.


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Figure 4. IR spectra in the O-H stretching vibration region for (a) PMTA-Cl and (b) PMTASCN brushes swollen under water vapor at various humidity.

PMTA-Cl brushes exhibit stronger O-H stretching absorptions than PMTA-SCN brushes, indicating that water uptake is preferable in the PMTA-Cl brushes. The PMTA-Cl brushes exhibited strong A3 absorption even at low humidity. The PMTA-SCN brushes showed hardly any A3 absorption at low humidity, while A3 absorption was weakly observed at high humidity. Each absorption peak for the PMTA-Cl brushes showed a redshift from that of the corresponding PMTA-SCN brushes. The redshift and significant A3 absorption by the PMTA-Cl brushes


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suggest the growth of a hydrogen-bonding network in strongly hydrated counter anions. Strongly hydrated anions promote the dissociation of the ionic pairs to produce charged QA+ cations and anions. The dissociated charged groups encourage water uptake and ordering of water. On the other hand, chaotropic anions are well associated with the QA+ cations, reducing the water uptake capability. Ion pairs of QA+ cations and chaotropic anions disturb the hydrogen-bonding network, probably due to the disordered hydrophobic hydration around the charge-coupled neutral ion pairs. Neutron Reflectivity. The polymer volume fraction profiles of the hydrated PMTA brushes in salt-free D2O were investigated by NR. The NR curves and SLD profiles of the PMTA-Cl and PMTA-SCN brushes under dry nitrogen gas flow are given in Figure S4. The NR profiles showed clear Kiessig fringes and were successfully fitted with a single slab model (Table S1, S2). The reflectivity profiles were almost identical; therefore, counter anion-specificity was not observed in the dry state. The thickness was determined to be 11.3 nm, which is comparable to the thickness obtained from ellipsometry (see Table 1). The PMTA brushes exhibit a less diffusive collapsed conformation with low interfacial roughness in which the chains are leveled off. The reflectivity curves and polymer volume fraction profiles of the PMTA-Cl and PMTA-SCN brushes under salt-free D2O are shown in Figure 5. The dual slab model was employed for fitting of the reflectivity (Table S3, S4). The PMTA brushes consist of a bound layer, a dense (less swollen) layer, and a diffuse outer tail extending toward the liquid phase, which is approximately consistent with previous reports.17, 35 The bound layer with relatively low SLD was required to fit the reflectivity precisely. The Kiessig fringes were substantially damped, indicating that the PMTA brushes were well-swollen in D2O, producing a diffuse interface. The


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broad polymer density gradient in the tail corresponds to a rough interface, which smears out the high-frequency oscillations associated with the thickness from the reflectivity. The bound layer is associated with low-frequency oscillations in the reflectivity, as seen in the broad peak at approximately 1.5 nm−1. The charged PMTA brush chains are well hydrated, and the charged chains are extended because of a combination of short-range excluded volume interactions, longrange electrostatic repulsive interactions and translational entropy of counterions. Previous NR studies of polyelectrolyte brushes required a dense layer in the substrate side prior to a diffuse tail to fit the reflectivity data adequately.17, 35 The dense layer is the bottom part of the polymer brush chains where ionic dissociation is unfavorable on account of charge crowding due to the high chain density.41, 47 The ultra-thin bound layer would be produced by the strong binding of the segments to SiO2 by electrostatic interactions.


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Figure 5. (A) NR curves of (a) the PMTA-Cl brush (red circles) and (b) the PMTA-SCN brush (blue circles) in salt-free D2O at 298 K. The black solid lines are the fitting curves. (B) Polymer volume fraction profiles for (a) the PMTA-Cl brush in D2O (red line) and (b) the PMTA-SCN brush in D2O (blue line). Polymer volume fraction profiles for PMTA-Cl and PMTA-SCN in dry state are shown as dotted lines.

Although the PMTA brushes showed similar response in the swollen structure under saltfree D2O, ion specificity was observed in the polymer volume fraction profiles. Weak oscillations were observed in the reflectivity profile of the PMTA-SCN brushes, whereas the


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oscillations were hardly observed in the profile of the PMTA-Cl brushes. This result suggests that the PMTA-Cl brushes produced a more diffusive tail than the PMTA-SCN brushes. Indeed, the PMTA-SCN brushes exhibit lower SLD and interfacial roughness than the PMTA-Cl brushes. The fitting parameter values in MOTOFIT are shown in Table S3 and Table S4 in Supporting Information. Strongly hydrated chloride anions induce the dissociation of the ionic pairs; therefore, hydration of the QA+ cations is promoted in PMTA-Cl. The electrostatic repulsive force and excluded volume effect of well-charged PMTA chains decorated with water result in the well-swollen diffuse tail. The PMTA-Cl brushes showed a lower polymer volume fraction in the less swollen inner layer than the PMTA-SCN brushes. The average thickness of the PMTA brushes calculated by eq. 3 changes from 11.3 nm under dry nitrogen to 38.7 nm in salt-free D2O with Cl– counter anions and 27.1 nm in salt-free D2O with SCN– counter anions. Meanwhile, the PMTA-Cl brushes exhibited a higher polymer volume fraction in the bound layer than the PMTA-SCN brushes. The highly-charged chains favorably interact with negatively charged SiO2 through electrostatic attractive interactions. The charge-coupled bound layer becomes less hydrophilic; therefore, the bound layer at the SiO2-substrate interface is less swollen in high charge fraction PMTA-Cl brushes. The PMTA-Cl brushes took up much more water than the PMTA-SCN as shown in the IR absorption intensity, whereas the NR results indicated that both PMTA-Cl and PMTA-SCN brushes were well-swollen and introduced almost identical volume of water in contact with saltfree liquid D2O. Namely, the contrast between the PMTA-Cl brushes and PMTA-SCN brushes was not drastic in NR in comparison with IR. The inconsistency is attributed to the difference between water uptake from humidity controlled vapor in IR and liquid water in NR. While the PMTA brushes were soaked in salt-free D2O in NR, the PMTA brushes were exposed to


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humidity controlled H2O vapor in the IR measurements. Swollen structure of polymer brushes in vapor of the good solvents was investigated by NR measurement.31, 48 The apparent enrichment of solvent vapor at polymer brush/air interface was verified. The affinity of water with PMTA brushes would be different between liquid water and vapor to cause significant variation in the volume of uptake water. The counter anion-specific hydration state of PMTA brushes is schematically illustrated in Figure 6. It should be noted that the schematic representation is emphasized to support readers’ understanding. The schematics of anion-specific and ionic strength dependent PMTA brush conformation were previously proposed on the basis of QCM-D data which provide mass and stiffness variation of the films.15 The schematics is not consistent with our pictures that is depicted on the basis of NR data which unravel the SLD profile of the swollen films. We verified the detail density profiles of the PMTA brushes in salt-free D2O, and elucidated that chaotropic anions induce dehydration of PMTA chains whereas kosmotropic anions encourage hydration and producing significantly diffusive interface.


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Figure 6. Schematic representation of counter anion-specific hydration states in PMTA brushes.

CONCLUSIONS The counter anion-specific hydration states of a strong polyelectrolyte PMTA brushes in water and the relation to water droplet wetting were unraveled. Strongly hydrated Cl– anions induce the dissociation of ion pairs composed of the QA+ cations in the PMTA brushes and the counter anions, while weakly hydrated SCN– anions strongly interact with the weakly hydrated QA+ cations. The ion binding state is associated with the ζ-potential. The PMTA brushes consist of a bound layer, a dense layer, and a diffusive tail. The PMTA-SCN brushes produce a less-swollen tail and a more-swollen bound layer than the PMTA-Cl brushes because of the preferable ion pairing interaction between the SCN– anions and QA+ cations. The hydrogen-bonding network structure of water in the PMTA brushes is promoted by PMTA-Cl but not by PMTA-SCN. The Hofmeister effect on water droplet wetting on PMTA brushes is supported by the insight into the counterion-specific hydration state of the PMTA brushes. We believe that this research provides an important concept for the development of polyelectrolyte-based materials employed in crowded environments.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI://////.


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(1) Preparation procedure for the PMTA-Cl brushes; (2) conditions for size exclusion chromatography, X-ray photoelectron spectroscopy with Ar gas cluster ion etching apparatus, and ellipsometry; (3) XPS spectra of the PMTA brushes; (4) depth profile of elements in the PMTA-SCN brushes; (5) a typical decomposition of the O-H stretching IR absorption peak; (6) NR curves and SLD profiles of the PMTA-Cl and PMTA-SCN brushes under dry nitrogen gas flow; and (7) fitting parameter values in MOTOFIT for a swollen PMTA brushes in salt-free D2O from neutron reflectivity are available.

AUTHOR INFORMATION Corresponding Author * E-mail [email protected]; Tel +81-92-802-2517; Fax +81-92-802-2518 (A.T.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Photon and Quantum Basic Research Coordinated Development Program of the Ministry of Education, Culture, Sports, Science and Technology, Japan. This work was funded by ImPACT Program of Council for Science, Technology and


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Innovation (Cabinet Office, Government of Japan). This work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”. This work was supported by the MEXT Project of “Integrated Research Consortium on Chemical Sciences”. FT-IR measurements were performed at BL43IR (2015B1313, 2016A1329, 2016B1703, 2017A1753) in SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI). NR measurements were performed on BL-16 in the Materials and Life Science Facility (MLF), J-PARC, Japan (program no. 2009S08 and 2014S08). We gratefully acknowledge the kind support of Y. Harada, K. Yamazoe, and Y. Cui (Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo) with the humidity-controlled IR measurements in BL43IR.

ABBREVIATIONS QA+, quaternary ammonium; PMTA, poly[2-(Methacryloyloxy)ethyltrimethylammonium]; SLD, scattering length density; SiNP, silica nanoparticle.

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Counteranion-Specific Hydration States of Cationic Polyelectrolyte

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