Ion-Specific Hydration States of Zwitterionic Poly(sulfobetaine

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Ion-Specific Hydration States of Zwitterionic Poly(sulfobetaine methacrylate) Brushes in Aqueous Solutions Tatsunori Sakamaki, Yoshihiro Inutsuka, Kosuke igata, Keiko Higaki, Norifumi L Yamada, Yuji Higaki, and Atsushi Takahara Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03104 • Publication Date (Web): 15 Nov 2018 Downloaded from http://pubs.acs.org on November 22, 2018

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Ion-Specific Hydration States of Zwitterionic Poly(sulfobetaine methacrylate) Brushes in Aqueous Solutions Tatsunori Sakamaki,‡ Yoshihiro Inutsuka,‡ Kosuke Igata,‡ Keiko Higaki, Norifumi L. Yamada,

‡

⊥

Yuji Higaki,‡,§,#,†

*

and Atsushi Takahara‡,§,#

#

*

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

Fukuoka 819-0395, Japan

§

Institute for Materials Chemistry and 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

⊥

Neutron Science Laboratory, High Energy Accelerator Research Organization,

Ibaraki 319-1106, Japan

ACS Paragon Plus Environment

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ABSTRACT

The ion-specific hydration states of zwitterionic poly(3-(N-2-methacryloyloxyethyl-

N,N-dimethyl)ammonatopropane sulfonate) (PMAPS) brushes in various aqueous solutions were investigated by neutron reflectivity (NR) and atomic force microscopy (AFM). The asymmetric hydration state of the PMAPS brushes was verified from the NR scattering-length density profiles, while the variation in their swollen thickness was complementary determined from AFM topographic images. PMAPS brushes got thicker in any salt solutions, while the extent of swelling and the dimensions of swollen chain structure were dependent on the ion species and salt concentration in the solutions. Anion specificity was clearly observed, whereas cations exhibited weaker modulation in ion-specific hydration states. The anion specificity could be ascribed to ion-specific interactions between the quaternary ammonium cation in sulfobetaine and the anions. The weak cation specificity was attributed to the intrinsically weak cohesive interactions between the weakly hydrated sulfonate anion in sulfobetaine and the strongly hydrated


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cations. The ion-specific hydration of PMAPS brushes was largely consistent with the ion-specific aggregation state of the PMAPS chains in aqueous solutions.

INTRODUCTION

Zwitterionic polymers exhibit distinctive properties from those of nonionic polymers and polyelectrolytes.1 Zwitterions contain positively and negatively charged groups; hence, the formal charge on a zwitterionic polymer is neutral, however

they

are

completely

different

from

nonionic

polymers

such

as

poly(ethylene glycol). The charged groups are hydrophilic, and become strongly hydrated through charge–dipole interactions while the net charge of the polymer chain remains neutral.2-3 The opposite charges in the zwitterions produce large local

dipole

moments,

leading

to

strong

charge–dipole

and

dipole–dipole

interactions. Thus, the hydration states of zwitterionic polymers are modulated by interactions with other ions present in solution, furthermore depend on the coexisting ion species and salt concentration of the solution.4-7


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Poly(sulfobetaine)s (PSBs) are zwitterionic polymers composed of sulfobetaine (SB) pendant groups that include a quaternary ammonium cation and a sulfonate anion with an alkyl chain spacer. PSB chains typically aggregate in salt-free water due to inter/intra chain dipole–dipole cohesive interactions between SB groups, whereas the SB pairs dissociate in aqueous electrolyte solutions because of the attractive

dipolar

interaction

screening

caused

by

ion

binding.

The

salt-

concentration dependent solubility of PSBs, in contrast to that of polyelectrolytes, is known as “anti-polyelectrolyte effect.” Kikuchi et al. studied the chain dimensions of

a

PSB,

poly(3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropane

sulfonate) (PMAPS), using small angle X-ray scattering (SAXS) and light scattering.8 Their comprehensive study determined that the long-range excludedvolume strength is dominant in the chain dimension of PMAPS rather than the short-range interactions of the chain stiffness parameters, and that 74 mM NaCl aqueous solution is a -solvent. Schlenoff et al. studied the ion-specific hydrodynamic radius of gyration (Rh) of poly(sulfobetaine acrylamide)s in aqueous solutions of various sodium salts.9 It was determined that the dependence of Rh


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on [NaX] was significantly influenced by the anion species present in solution. Less hydrated anions promote dissolution of the PSB while encouraging collapse of the PSB chains to produce compact coils by ion exclusion.

Zwitterionic polymer brushes are a class of thin films comprising dense surfacebound zwitterionic polymer chains. Their outstanding resistance to protein adsorption and anti-fouling capabilities have been extensively studied, and these unique surface properties are often attributed to their hydration states and neutral net charge.10-12 The PSB brushes are typically well-hydrated due to the high osmotic pressure, but the engagement of SB groups through dipole interactions produces a network structure that prevents massive hydration in the absence of salts.13-14 Ions in aqueous solution can penetrate the hydrated PSB brush to dissociate the SB couples through charge screening via ion binding. The SB disengagement induction order is largely consistent with the well-known Hofmeister series. Liu et al. investigated the ion-specific hydration state modulation of PSB brushes using a quartz crystal microbalance with dissipation (QCM-D) technique.1518

The resonance frequency shift coupled with the dissipation factor shift as a


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function of salt concentration indicated that the PSB brushes swelled with increasing salt concentration. The PSB brushes were softened by breaking the network structure, facilitating subsequent hydration. The ion-specific mass and stiffness variations of the PSB brushes were also systematically studied. However, the ion-specific hydration states of PSB brushes remain controversial with respect to the swollen chain dimensions. As the PSB brushes are tethered to the substrate at the chain end, their hydration state is certainly asymmetric at the substrate and liquid interfaces. Although the QCM-D studies have provided indications of mass and stiffness variation of the polymer brushes, the hydrated chain dimension and vertical heterogeneous density distribution of the swollen PSB brushes in response to various ions remain unknown.

Neutron reflectivity (NR) can provide quantitative data with regard to the density profile in the hydrated polymer thin films normal to the substrate. This technique was adapted for the investigation of charged polymer brushes.13-14, 19-24 NR can easily distinguish deuterated organic components because of the large difference between the scattering-length densities (SLD) of protons and deuterium. The


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degree of hydration in hydrogenated PSB brushes can be determined using deuterium oxide as a solvent instead of water. Specific ion effects on the structure of weak polyelectrolyte brushes23 and poly(N-isopropylacrylamide)22 brushes as well as their pH and temperature dependence were investigated using NR.

In this study, we addressed to the ion-specific hydration states of PMAPS brushes by NR and atomic force microscopy (AFM). The aggregation state of PMAPS chains was also studied through temperature-dependent transmittance of the aqueous solutions. Sodium chloride (NaCl), sodium thiocyanate (NaSCN) and tetramethylammonium chloride (TMAC) were used as electrolytes to evaluate ion specificity.

EXPERIMENTAL SECTION

Materials

Silicon blocks (single side polished, 50 mm × 20 mm × 8 mmt, Matsuzaki Seisakusyo Co., Ltd., Japan) and silicon wafers (single side polished, 0.5 mm


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thickness, SUMCO Corporation, Japan) were used after sonication in ethanol followed by irradiation with vacuum-ultraviolet rays ( = 172 nm) for 10 min under reduced pressure. Milli-Q water (Millipore Inc., Billerica, MA) with a resistance of >18 MΩ·cm was used. Sodium chloride (NaCl, Nacalai Tesque Inc., 99.5%), sodium thiocyanate (NaSCN, FUJIFILM Wako Pure Chemical Corp., 99.0%), tetramethylammonium chloride (TMAC, Tokyo Chemical Industry Co., Ltd., 98.0%) and deuterium oxide (D2O, Kanto Chemical Co., Inc., 99.8%) were used as received.

Preparation of PMAPS Brushes and Unbound PMAPS

The PMAPS brushes were prepared on silicon substrates by surface-initiated atom transfer radical polymerization.25 The sample list is provided in Table S1, and detailed synthetic procedures are given in the Supplementary Information (SI). The thickness of PMAPS brushes for NR measurement was adjusted to approximately 20 nm in the dry state (PMAPS brush_01 and PMAPS brush_02 in Table S1) because the Kiessig fringes can be clearly observed in this thickness


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range, and minor changes in hydration state are reflected in the NR profiles. The degree of polymerization was controlled to approximately 140 to achieve the thickness. The NR profile of PMAPS brushes under dry nitrogen gas flow showed clear Kiessig fringes (see Figures S2 and S3). The PMAPS brushes collapsed to produce uniform smooth surfaces, as indicated by the sharp decline at the surface in the SLD profile. The SLD of dry PMAPS brushes was determined to be 0.98 × 10-4 nm-2, which was largely consistent with the calculated SLD value of free PMAPS (0.92 × 10-4 nm-2; component: C11H22NO5S, density: 1.34 g / cm3), and the SLD value was involved in the polymer volume fraction calculation. The thicknesses determined by NR were almost consistent with those determined by ellipsometer. A thick PMAPS brush (PMAPS brush_03 in Table S1) was involved in the AFM experiment because the thick polymer brushes are preferable to figure out the ion specific response due to the large thickness variation in the topographic images. Unbound PMAPS was used for the turbidity test. High molecular weight PMAPS (PMAPS_04 in Table S1) was used in the turbidity test because the temperature dependent aggregation of PMAPS also depends on the


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molecular weight, and the ion-specific response in the solution turbidity is significant for high molecular weight PMAPS.

Measurements

Detailed conditions for size exclusion chromatography and ellipsometry are provided in SI.

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) with a 25 Hz pulsed neutron beam.26-27 The settings used were similar to those in our previous report.14 The sample was installed in a temperature controlled chamber at 25°C. The neutron momentum transfer vector is defined as qz = (4/ )sin, where 2 is the specular reflection angle with respect to the polymer brush interface and  is the wavelength of the incident neutrons. The SLDs of 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 MOTOFIT program was used to fit the NR


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profiles to obtain SLD profiles.28 The volume fraction profiles were calculated from the SLD profiles according to the procedure in previous report.14

AFM Observation

AFM topographic images were observed using scanning force microscopy installed with environmental control scanner (Cypher ES; Asylum Research, Santa Barbara, CA). Topographic images were obtained by contact mode at 25°C with a back-side Au-coated silicon nitride triangular cantilever (OMCL-TR800PSA, Olympus Corporation, Tokyo, Japan; factory-specified spring constant: 0.57 Nm-1, tip radius: less than 15 nm) under ambient (25°C, approximately 40%RH) or aqueous solutions. The ambient and swollen thicknesses were obtained from height gap (H) at a scratch track on the polymer brush. The scratch track was given by scratching with a sharp needle. The normal force was calibrated by the deflection sensitivity and spring constant of the cantilever, and was adjusted to approximately 6 nN in all imaging. At least 10 topographic images were taken at multiple locations for multiple scratches, then 5 line profiles were obtained from


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each image to give 50 height gap data. The swollen PMAPS brush thickness was provided by averaging the height gap data.

Turbidity Test

The turbidity tests of the PMAPS aqueous solutions were conducted using an ultraviolet-visible-near

infrared

(UV-VIS-NIR)

spectrophotometer

(UV-3600,

Shimadzu Corp., Kyoto, Japan) equipped with a thermostat cell holder. Quartz cells (optical pass: 10 mm) were placed in the holder, and the temperature was controlled by circulating temperature-controlled water. The temperature was directly monitored using a thermocouple arranged in the reference cell solution. The PMAPS concentration was adjusted to 0.1 wt% or 1.0 wt%. Transmittances of the PMAPS solutions at 550 nm were recorded at various temperatures.

RESULTS AND DISCUSSION

Anion Specificity in Hydration States of PMAPS Brushes


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The NR profiles of a PMAPS brush (PMAPS brush_01 in Table S1) in saltfree D2O, NaCl, and NaSCN aqueous solutions with fitting curves based on model SLD profiles, SLD profiles determined by curve fitting of the NR profiles, and polymer volume fraction profiles calculated from the SLD profiles are shown in Figure 1. The NR profiles of the swollen PMAPS brushes in all aqueous solutions were accurately reproduced by employing a three-layered model shown in Figure 2. The PMAPS brushes exhibited ideal NR profiles with clear Kiessig fringes in salt-free D2O and 10 mM NaCl solutions over a wide q range, indicating that the hydration states of the PMAPS brushes were nearly identical in both the solutions, and showed clear interfaces without significant interfacial roughness (Figure 1a and b). The swollen thickness of the PMAPS brushes was approximately twice that of the dry state thickness. A thin high density bottom layer was required at the substrate interface to fit the slope in the high q region. Above the bottom layer, the hydrated PMAPS brushes exhibited a less hydrated middle layer, followed by a well-hydrated top layer. The interface thicknesses were thin enough to yield a stepped layered structure. In other words, the PMAPS brushes adopted


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an asymmetric swollen structure with comparatively clear boundaries after swelling.

Figure 1. (I) NR profiles of a PMAPS brush (PMAPS brush_01) in (a) salt-free D2O, (b) 10 mM NaCl, (c) 100 mM NaCl, (d) 10 mM NaSCN and (e) 100 mM NaSCN D2O solutions. Intensities are offset by arbitrary factors. (II) SLD profiles obtained by curve fitting of NR profiles. (III) Polymer volume fraction profiles calculated from the SLD profiles.


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Figure 2. Schematic representation of the layered structure model applied to the curve fitting of NR profiles.

The inter/intra PMAPS chain binding through dipolar SB pairing inhibits hydration in the salt-free or low salt concentration aqueous solutions. The SB pairing is significant in the middle layer likely because of the high lateral chain density resulting in reduced hydration, while the top layer is relatively hydrated due to the reduction of coupled SB moieties. The low lateral chain density in the outer region due to the molecular weight distribution in the grafting chains would induce the stepped layer structure after swelling. The inter/intra chain binding through dipolar SB pairing prevents the extension of the hydrated diffusive interfacial layer while the charged groups are locally well-hydrated.


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The Kiessig fringes were largely dampened in the 100 mM NaCl solution (Figure 1c), while the weak oscillation remained. The three-layered model was also useful to reproduce the less characteristic reflectivity, while the SLD and interfacial roughness significantly increased (Table S3). As shown in the large SLD, the volume fraction of the PMAPS brush decreased, and the liquid interface became unclear, producing a diffusive interfacial tail. Chloride anions and sodium cations penetrate the PMAPS brush then bind to the SB groups, screening the SB binding interaction. The charge screening induces disengagement of the inter/intra-bound PMAPS chains, releasing them from the network followed by extension and water uptake to produce a well-hydrated diffusive tail in high salinity conditions. The boundary between the middle and top layers became unclear, and the SLD smoothly changed from the bottom layer to solution phase. The ions can access to the middle layer by disrupting to the osmotic pressure and subsequently inducing the dissociation of SB groups.

The Kiessig fringes in the NR profiles were drastically dampened in the NaSCN solutions (Figure 1d and e). Significant damping was observed even in the 10


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mM NaSCN solution (Figure 1d), while periodic oscillations were not observed after the initial drop in reflectivity in the 100 mM NaSCN solution (Figure 1e). As shown in the polymer volume fraction profiles, the PMAPS brushes were wellswollen to produce substantially diffusive tail, and layered structure no longer remained in the 100 mM NaSCN solution. This anion-specific hydration is consistent with the results of the QCM-D mass variation studies.15 It is plausible that the striking hydration in the NaSCN solutions is induced by strong interactions between the thiocyanate anions and the quaternary ammonium cations in the SB groups. Collins proposed an empirical rule, in which oppositely charged ions with similar affinity toward water produce strong ion pairs.29-30 Consequently, oppositely charged ions with markedly different water affinities separate in water and dissociate in dipolar media. This universal rule is valid for the ion-specific interactions between ions and charged groups in SB. The quaternary ammonium cation in the SB is extremely polarizable and exhibits weak hydration,31 while the thiocyanate anion is also polarizable with a weak hydration shell.32-33 The weakly hydrated thiocyanate anions efficiently interact with the quaternary ammonium


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cations owing to the similarity in their affinity to water. This favorable interaction results in the anion-specific weakening in dipole–dipole attraction between SB groups, promoting the dissociation of SB pairs and resulting in the significant hydration of the PMAPS brushes even at low salt concentrations. In addition, the asymmetric ion adsorption to the quaternary ammonium cation and sulfonate anion in the SB units induces net charge augmentation of the zwitterionic PMAPS chains, leading to an enhanced solubility.6

The anion-specific hydration state modulation of PMAPS brushes was also verified through swollen thickness determination by AFM. The swollen thickness of the PMAPS brushes in aqueous solutions normalized by the thickness in saltfree milli-Q water is shown in Figure 3. Typical AFM topographic images of the PMAPS brushes in aqueous solutions showed smooth PMAPS brush/water interface geometry (Figure S4). The swollen thicknesses were determined by the average height gap in the line profiles. The swollen thickness increased in all salt solutions, and it increased with an increase in salt concentration. NaSCN promoted the swelling of PMAPS brushes significantly more than NaCl. When


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compared to the thickness of PMAPS brushes in milli-Q water, the averaged swollen thickness of the PMAPS brushes in 100 mM NaSCN was 1.7 times greater, while it was 1.3 times greater in 100 mM NaCl solution. The AFM results are consistent with the results of the NR study, and draw identical trends where the weakly hydrated thiocyanate anions more significantly promoted the hydration of PMAPS brushes compared to the strongly hydrated chloride anions. However, the difference in swollen thickness (1.3 or 1.7 times increase in 100 mM solutions) is greater than that obtained in the NR results (Figure 1B). Because the PMAPS brush sample (PMAPS brush_03) has a relatively large molecular weight distribution, a thicker diffusive layer with low polymer chain density is produced in the outermost region. The topographic image of the soft matter in the contact mode AFM depends on the deflection set-point of the cantilever.34 At the minimum set-point, the thickness determined from the topographic gap is overestimated. Meanwhile, the cantilever tip penetrates the soft swollen polymer brush at large set-point leading to underestimation. In addition, the tip penetration also depends on the stiffness of the diffusive tail. In this study, the deflection set-point was


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adjusted to the minimum point required for clear imaging. Therefore, the thickness determined by AFM topographic gap is somewhat inaccurate, although the ionspecific trend is reliable.

Figure 3. Normalized swollen thickness of the PMAPS brush in aqueous solutions determined by AFM observation. The swollen thickness was normalized by the thickness in salt-free water. The error bars show standard deviation.

Cation Specificity in Hydration States of PMAPS Brushes


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NR profiles of a PMAPS brush (PMAPS brush_02 in Table S1) in salt-free D2O, NaCl, and TMAC aqueous solutions, SLD profiles determined by curve fitting of the NR profiles, and polymer volume fraction profiles calculated from the SLD profiles are shown in Figure 4.

Figure 4. (I) NR profiles of a PMAPS brush (PMAPS brush_02) in (a) salt-free D2O, (b) 10 mM NaCl, (c) 100 mM NaCl, (d) 10 mM TMAC and (e) 100 mM TMAC-D2O solutions. Intensities are offset by arbitrary factors. (II) SLD profiles obtained by curve fitting of NR profiles. (III) Polymer volume fraction profiles calculated from the SLD profiles.


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The NR profiles depend on the salt concentration in the NaCl and TMAC solutions, but cation specificity was largely absent. The weak oscillation remained even in the 100 mM TMAC solutions, whereas the oscillation was smeared in 100 mM NaSCN solutions. The tetramethyl ammonium and sodium cations induced similar hydration efficiencies of the PMAPS brush. The association number and lifetimes of the SB/cation associations were determined by molecular dynamics simulations.35 Cations interact with the sulfonate group in the SB unit, but the force of interaction is so weak that the cation specificity can be hardly recognized. Electrons are tightly bound to the positively charged cations leading to high charge density and small ionic radii. Thus, cations are normally more strongly hydrated compared to anions. Because the interaction force between strongly hydrated cations and bulky sulfate anions is intrinsically weak, the counter anions are preferably bound to the quaternary ammonium cation in SB. This competitive association of solutes results in anion specificity rather than cation specificity. In this case, the binding of chloride anions is preferable to the binding


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of both sodium and tetramethyl ammonium cations, reducing the cation specificity. The absence of cation specificity was observed in the anionic polyelectrolyte of poly(3-sulfopropyl methacrylate) brushes in a previous QCM-D study.36 The mass of the hydrated anionic polyelectrolyte brushes does not depend on the cation species coexisting with the common chloride counter anion. The ion-specific interactions between the ions and SB is illustrated in Figure 5 with consideration of the above discussion.

Figure 5. Schematic representation of the ion-specific hydration states of PMAPS brushes.

Ion-Specific Aggregation States of PMAPS in Aqueous Solutions


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The aggregation states of the PMAPS chains in aqueous solutions were verified by turbidity testing. The temperature dependent transmittance of PMAPS (PMAPS_04 in Table S1) aqueous solutions are shown in Figure 6. The 0.1 wt% PMAPS salt-free aqueous solution became cloudy at 15 °C, then transparent at 40 °C. The temperature range was broader than other thermo-sensitive polymers, such as poly(N-isopropylacrylamide)37, that exhibits a rapid hydration entropy driven coil–globule transition. Because the SB couples dissociate via thermal activation, the aggregated PMAPS chains disengage to dissolve in the solutions upon heating. The upper critical solution temperature (UCST) and temperature range for the phase transition associate with the binding efficiency of the SB units in the PMAPS chains. The UCST decreased in the 10 mM NaCl solution, indicating that the cohesive interaction force in the SBs was reduced due to charge screening by the bound ions. Also, the ion binding induced net charge augmentation results in the hydration of SB units leading to the UCST reduction. The 10 mM NaSCN solution exhibited a more significant reduction in the UCST compared to the 10 mM NaCl solution, remaining transparent even at 5 °C,


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indicating that the efficient binding of weakly hydrated thiocyanate anions substantially promotes SB couple disengagement. The preferentially bound thiocyanate anions impart a negative net charge to the PMAPS chains resulting in enhanced hydration. Meanwhile, the 10 mM TMAC solution exhibited almost identical thermo-responses to the 10 mM NaCl aqueous solution. Consequently, the UCST of the 10 mM salt solutions followed the order: NaCl ≅ TMAC >> NaSCN. The ion-specific response which was more significant for the anion species, is consistent with the NR results.


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Figure 6. Temperature dependent transmittance of (I) 0.1 wt% and (II) 1.0 wt% PMAPS aqueous solutions at 550 nm. (a) Salt-free water, (b) 10 mM NaCl, (c) 10 mM NaSCN and (d) 10 mM TMAC solutions.

The 1.0 wt% PMAPS solutions exhibited significant UCST increase and rapid transition in comparison with the 0.1 wt% solutions. The UCST shift indicates that the aggregation of PMAPS chains is promoted in high concentration condition.


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The concentration dependent UCST increase associate with the layered polymer volume fraction profile in the PMAPS brushes. The lateral chain density in the polymer brush is intrinsically ununiform, and has gradient from the bottom to top because of the molecular weight distribution. The PMAPS brush chains in the middle layer are relatively crowded due to the high lateral chain density, leading to the profound aggregation to produce higher polymer volume fraction layers. Meanwhile, the PMAPS chains in the top layer exhibit significant hydration probably because of the reduced lateral chain density.

The PMAPS brush consists of crowded surface-bound PMAPS chains with a relatively high lateral chain density, and are confined to a narrow cylinder around the surface attachment point. Thus, it is plausible that the PMAPS brush chains behave quite differently from the unbound chains in solutions. The main scaling regimes of polyelectrolyte brushes were identified in the pioneering works of Pincus38 and Borisov39 and coworkers. The thickness of the charged polymer brushes depends on the salt concentration that balances the Gaussian elasticity and osmotic force of the polymer brushes, while the scaling regimes exhibits


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specific crossover salt concentration. The short-range excluded-volume interactions, long-range electrostatic interactions, and translational entropy of the charged chains and ions complicate the behavior of the charged polymer brushes in salt solutions.40 For PSB brushes, the swollen thickness depends on the dipolar cohesive interaction of the SB moieties, deviating from the classic theory for polyelectrolytes. The -concentration (Cs,) of PMAPS in NaCl aqueous solution may increase at the inner region of the PMAPS brush because of chain crowding. Meanwhile, the charge crowding also enhances osmotic pressure, which facilitates hydration of PMAPS brushes. As shown in the turbidity test, the PMAPS 10 mM NaCl solution got transparent at a lower temperature than the salt-free aqueous solution. However, PMAPS brushes exhibit no obvious hydration state variation in salt-free water and 10 mM NaCl solution. The inconsistency in the PMAPS solutions and PMAPS brushes indicates that the PMAPS brushes have a threshold concentration that the hydration state is unaffected by the ions as predicted by theoretical studies in polyelectrolyte brushes.40 Meanwhile, the thiocyanate anion induces the hydration state variation even in 10 mM concentration, indicating that


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the thiocyanate ions accessed the inside of the PMAPS brushes and induced hydration state variation similar to that of the unbound PMAPS chains. Thus, the threshold concentration for ion penetration depends on the ion species, and the more polarized bulky anions has the lower threshold concentration.



CONCLUSION

The ion-specific hydration of zwitterionic PMAPS brushes was examined using NR, AFM, and turbidity testing. Detailed insights into the asymmetric hydration states of the swollen PMAPS brushes and associated ion-specific modulation were described herein. The PMAPS brushes adopt asymmetric swollen structures with clear interfaces under salt-free condition due to the dipole–dipole cohesive interactions of their SB groups. The addition of salt ions promotes the hydration to produce a diffusive tail due to charge screening by the bound ions followed by disengagement of the SB pairs. Salt-induced hydration is apparent for the anions, but cations exhibit weak ion specificity. The anion/cation specificity can


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be explained by the ion-specific interactions between the charged groups in SB and the salt ions. The ion-specific hydration of PMAPS brushes was largely consistent with the ion-specific aggregation state of the PMAPS chains in aqueous solutions. The insights into the hydration state of the charged polymer brushes described here could be useful to tailor the response of surface-tethered charged polymer assemblies in the presence of various ions.

ASSOCIATED CONTENT

Supporting Information.

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

Characteristics of the PMAPS brushes and free (unbound) PMAPS, preparation of PMAPS brushes and free (unbound) PMAPS, conditions for size exclusion chromatography and ellipsometry, NR profiles and SLD profiles of PMAPS brushes


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under dry nitrogen gas flow, NR fitting parameter values in MOTOFIT for swollen PMAPS brushes, typical AFM topographic images of PMAPS brushes in aqueous solutions (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +81-92-802-2518. Tel: +81-92-8022516.

*E-mail: [email protected]. Fax: +81-92-802-2518. Tel: +81-92-8022517.

Present Addresses †Department

of Integrated Science and Technology, Faculty of Science and

Technology, Oita University, 700 Dannoharu, Oita 870-1192, Japan. E-mail: [email protected]

Author Contributions


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All authors contributed equally to the writing of this manuscript, and all authors have approved the final version of the manuscript. A.T. and Y.H. conceived of and directed the project. T.S., Y.I., K.I., and K.H. performed the experiments and analyzed the results. N.Y. contributed to the NR experiments.

ACKNOWLEDGMENT

This work was supported by JSPS KAKENHI Grant Number JP18K05218. 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 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 in part by the “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” (MEXT) and “Integrated Research Consortium on Chemical Sciences”


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(MEXT). NR measurements were performed on BL-16 in the Materials and Life Science Facility (MLF), J-PARC, Japan (program no. 2014S08, and 2017L2501).

ABBREVIATIONS PMAPS, poly(3-(N-2-methacryloyloxyethyl-N,N-dimethyl)ammonatopropane sulfonate); SB, sulfobetaine; SLD, scattering length density; NR, neutron reflectivity.

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TOC graphic


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Ion-Specific Hydration States of Zwitterionic Poly(sulfobetaine

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