Effect of Salt Concentration on the pH Responses of Strong and Weak

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Effect of Salt Concentration on the pH Responses of Strong and Weak Polyelectrolyte Brushes Jian Zhang, Ran Kou, and Guangming Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01395 • Publication Date (Web): 19 Jun 2017 Downloaded from http://pubs.acs.org on June 23, 2017

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Effect of Salt Concentration on the pH Responses of Strong and Weak Polyelectrolyte Brushes Jian Zhang, Ran Kou, Guangming Liu* Department of Chemical Physics, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, P. R. China 230026

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Abstract. Strong polyelectrolyte brushes (SPB) and weak polyelectrolyte brushes (WPB) have different origins in response to pH, which makes their pH-responsive properties sensitive to salt concentration in different ways. Herein, we have employed poly[2-(methacryloyloxy)ethyl

trimethylammonium

chloride]

(PMETAC)

and

poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) brushes as model systems for SPB and WPB, respectively, to investigate the effect of salt concentration on the pH responses of SPB and WPB using several surface-sensitive techniques. The pH responsive properties of the PMETAC brushes are governed by the reorganization of the inter-chain hydrogen bonds between the grafted chains, whereas the pH response of the PDMAEMA brushes is controlled by the charge of the grafted chains. The response of the properties of the PMETAC brushes including hydration, conformation, and surface wettability becomes weaker with increasing salt concentration induced by the competitive adsorption of counterions to the brushes between OH- and Cl-. The weakening of the pH response of the PMETAC brushes is more remarkable at the relatively high pH values. The pH response of the PDMAEMA brushes also exhibits a salt-concentration dependence. As the salt concentration increases, the weakening of the pH response of the PDMAEMA brushes is attributed to the decrease of osmotic pressure within the brushes at the relatively low pH values.

*To whom correspondence should be addressed. Email: [email protected]

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Introduction Polyelectrolytes can dissociate in aqueous solutions forming a charged polymer backbone.1,2 When polyelectrolyte chains are attached to a surface with a high grafting

density,

the

attached

chains

will

form

polyelectrolyte

brushes.3

Polyelectrolyte brushes with stimuli-responsive properties have attracted great interest due to the extensive applications of their tunable properties in a wide range of fields such as surface wettability, adhesion, friction, and lubrication.4-11 The properties of weak polyelectrolyte brushes (WPB) are strongly sensitive to pH because the charge of the grafted chains is dependent on the dissociation-association of protons.12-16 In contrast, it is widely held that the charge of strong polyelectrolyte chains is insensitive to pH.17 Therefore, the strong polyelectrolyte brushes (SPB) are always considered to be independent of pH.18,19 However, our recent study demonstrated that many important properties of SPB are dependent on pH.20 The reorganization of the inter-chain hydrogen bonds plays a critical role in determining the pH-responsive properties of SPB and the hydrogen bond network can be modulated by the adsorption of OH- (or H3O+) with the positively (or negatively) charged brushes with the variation of pH.20 As the pH response of SPB is closely related to the adsorption of OH- or H3O+ with the brushes, the pH-responsive properties of SPB should be dependent on salt concentration due to the competitive adsorption of OH- or H3O+ to the brushes with the other types of counterions dissolved in the aqueous solutions. In comparison with the OH- and H3O+ which are hydrogen bond donors, the other types of inorganic counterions usually do 3


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not have the ability to form hydrogen bonds with the hydrogen bond acceptors associated with the grafted strong polyelectrolyte chains. Therefore, it is expected that the increase of salt concentration induced competition in the counterion adsorption with SPB will have strong influences on the formation of inter-chain hydrogen bonds, thereby affecting the pH responsive properties of SPB. On the other hand, the pH response of WPB should also be dependent on salt concentration, as the modulation of the properties of WPB by either pH or salt concentration is generally governed by the variation of osmotic pressure within the brushes.3,12,21 Thus, there is an interplay between pH and salt concentration in terms of their effects on the properties of WPB via tuning the osmotic pressure within the brushes. As SPB and WPB have different origins in response to pH, their pH-responsive properties should be sensitive to salt concentration in different ways. In this work, we have employed poly[2-(methacryloyloxy)ethyl trimethylammonium chloride] (PMETAC) and poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) brushes as model systems for SPB and WPB, respectively, to investigate the effect of salt concentration on the pH responses of SPB and WPB. We are interested in how the pH-responsive properties of SPB and WPB are influenced by the salt concentration and how the salt concentration works in different ways on the pH responses between SPB and WPB.

Experimental Section Materials. 2-(Dimethyl amino)ethyl methacrylate (DMAEMA) (99%, Aladdin) and 4


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[2-(Methacryloyloxy) ethyl] trimethylammonium chloride (METAC) [75% (wt%) in H2O, Aladdin] were used after the purification using a basic alumina column. Copper (II) bromide (CuBr2, 99%) was purchased from Sinopharm and used as received. Copper(I) bromide (CuBr) was prepared from CuBr2 by reacting with sodium sulfite. Ethyl 2-bromoisobutyrate (2-EBiB) (98%, Aldrich), 2,2’-bipyridyl (BPy) (98%), methanol (≥ 99.9%), and HCl (37 wt%, Sinopharm) were used as received. ω-Mercaptoundecyl bromoisobutyrate (MUBB) was synthesized according to the procedure reported previously.22 Hexadecane (>99.5%, Aladdin) was used after the purification using a neutral alumina column. All salts (99.99%, metals basis, Aladdin) were used as received. Water was purified by filtration through a Millipore gradient system after pre-distillation, giving a resistivity of 18.2 MΩ·cm. Preparation of polyelectrolyte brushes. The gold-coated substrates were cleaned by the piranha solution containing 3 parts of H2O2 and 7 parts of H2SO4 (v/v) at 60 °C for about 10 min, and then washed with water and dried by N2. Subsequently, the substrates were immersed in an anhydrous ethanol solution containing MUBB (5 mM) for ~ 24 hours to form the initiator layer at room temperature. The PMETAC brushes were successfully prepared by using the surface-initiated atom transfer radical polymerization (SI-ATRP) method.23 Typically, METAC (~ 5.9 g), BPy (~ 0.04 g), and the free initiator 2-EBiB (~ 0.03 g) were dissolved in a mixture of methanol and water [4:1 (v/v), 20 mL]. Then, the flask was degassed by three cycles of freeze-pump-thaw, and backfilled with N2. Afterwards, the initiator-modified substrates and CuBr (~ 0.03 g) were added to the flask quickly under the N2 5


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protection at ~ 25 °C. After the polymerization, the substrates were successively rinsed with water and methanol, followed by soaking in a mixture of methanol and water [4:1 (v/v)] overnight to remove the unreacted monomer and ligand. The PDMAEMA brushes were prepared using a similar method. Briefly, DMAEMA (~ 8.4 g), BPy (~ 0.17 g), and 2-EBiB (~ 1.2 × 10-3 g) were dissolved in methanol (~ 20 mL). Then, the flask was degassed by three cycles of freeze-pump-thaw, backfilled with N2. Afterwards, the initiator-modified substrates and CuBr (~ 0.03 g) were added to the flask quickly under the N2 protection at ~ 25 °C to initiate the polymerization. After the polymerization, the substrates were successively rinsed with water and methanol, followed by soaking in methanol overnight to remove the unreacted monomer and ligand. The dry thicknesses of PMETAC and PDMAEMA brushes in air were ~ 25 nm, as determined by the spectroscopic ellipsometry (M-2000 V, J.A. Woollam, USA). Preparation of pH solutions. According to the previously reported experimental method, KCl instead of NaCl was employed to control the ionic strength of the pH solutions, so that the results obtained here can be compared with the previous study.20 For instance, the pH solutions with an ionic strength of 10 mM were prepared using 10 mM HCl, 10 mM KOH, and 10 mM KCl. When the pH response of polyelectrolyte brushes was investigated at different salt concentrations, the constant salt concentration of the different pH solutions was kept to discount the influence of ionic strength through the addition of KCl.

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Quartz crystal microbalance with dissipation (QCM-D) and ellipsometry measurements. The pH responses of the PMETAC and PDMAEMA brushes were studied by the combination of QCM-D and spectroscopic ellipsometry.24 The quartz crystal sensor with a fundamental resonant frequency (f0) of ~ 5 MHz was mounted in a fluid cell with the sensing side exposed to the pH solutions. Here, only the sensing side of the QCM sensor was grafted with the polyelectrolyte brushes. The sensor had a mass sensitivity constant (C) of 17.7 ng·cm-2·Hz-1.25 When the sensor is excited to oscillate in the thickness shear mode by applying a RF voltage across the electrodes near the resonant frequency, a thin layer adsorbed on the sensor surface can induce a decrease in resonant frequency (f). In vacuum or air, if the adsorbed layer is rigid, evenly distributed and much thinner than the quartz crystal, then the f is related to the mass change (m) of the layer and the overtone number (n = 1, 3, 5….) by the Sauerbrey equation,26 m  

qlq f f0

n

 C

f n

(1)

where q is the specific density of the quartz crystal and lq is the thickness of the quartz crystal. The dissipation factor is defined by.24

D

Ed 2Es

(2)

where Ed and Es are the energy dissipated during one oscillation and the energy stored in the oscillating system, respectively. The measurement of dissipation factor is based on the fact that the voltage over the sensor decays exponentially as a damped sinusoidal when the driving power of a piezoelectric oscillator is switched off.24 In 7


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this work, the Δf and ΔD of the polyelectrolyte brushes in the solution with a salt concentration of 10 mM at pH 2 were used as the reference state. All of the results obtained were from measurements of frequency and dissipation factor at the third overtone (n = 3) and all the experiments were conducted at ~ 25 C. In the ellipsometry measurements, two ellipsometric parameters Ψ and Δ can be expressed by the following equation.27 tan( )ei 

Rp Rs

(3)

where tan(Ψ) is the amplitude ratio of the reflection coefficient of p-polarized light (Rp) to that of s-polarized light (Rs), and Δ is the phase difference. The dry thickness of the polyelectrolyte brushes was determined by treating the polyelectrolyte layer as a single Cauchy layer between the gold surface and air. The wet thickness of the polyelectrolyte brushes was determined in the relevant pH solutions using an ellipsometry-compatible QCM-D module through numerical modeling of the data with a two-layer model. The two layers represented the polyelectrolyte layer and the gold coating. Contact angle measurements. The water contact angles (WCA) and oil contact angles (OCA) on the surface of polyelectrolyte brushes were determined using a contact angle goniometer (CAM 200, KSV, Finland) at 25 ºC. The values of WCA were obtained from the pendant bubble contact angle measurements. The WCA and OCA measurements were performed in situ in the relevant pH solutions.

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Results and Discussion

Scheme 1. (a) Schematic illustration of the possible influence of salt concentration on the pH response of the positively charged PMETAC brushes due to the competitive adsorption of counterions to the brushes between OH- and Cl-. (b) Schematic illustration of the possible influence of salt concentration on the pH response of the positively charged PDMAEMA brushes due to the interplay between pH and salt concentration in terms of their effects on the osmotic pressure within the brushes.

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As reported previously, the pH response of SPB is determined by the reorganization of the inter-chain hydrogen bonds, induced by the adsorption of OH- or H3O+ with the brushes.20 That is, the pH-controlled mole fraction of adsorbed OH- or H3O+ within the brushes plays a crucial role in the pH response of SPB via tuning the formation of inter-chain hydrogen bonds. In fact, the mole fraction of the adsorbed OH- or H3O+ within SPB can be adjusted not only by pH but also by salt concentration. For the case of positively charged SPB as an example, a higher salt concentration would lead to a lower mole fraction of OH- adsorbed within the brushes at a given pH due to the competitive adsorption of counterions to the brushes between OH- and the other types of anions (e.g., Cl-) driven by the counterion condensation (Scheme 1a). Therefore, it is expected that a weaker pH response of SPB would be observed at a higher salt concentration. In comparison with the pH response of SPB, the pH-responsive properties of WPB are determined by the pH-controlled degree of charging of the grafted weak polyelectrolyte chains through the variation of osmotic pressure within the brushes (Scheme 1b). Besides, the WPB are also sensitive to salt concentration because the osmotic pressure within the brushes is decreased with increasing external salt concentration. Thus, it is expected that the increase of salt concentration would also weaken the pH response of WPB. Nevertheless, the salt concentration should have different ways to affect the pH responses between SPB and WPB due to the different origins of the pH responses between them. To test the hypothesis that the pH responses of SPB and WPB can be weakened by the increase of salt concentration and 10


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to clarify how the salt concentration works in different ways on the pH responses between SPB and WPB, we have employed the PMETAC and PDMAEMA brushes as model systems for SPB and WPB, respectively, to investigate their pH-responsive properties at different salt concentrations in the following parts. Effect of salt concentration on the pH response of the PMETAC brushes. Figure 1 shows the pH dependence of frequency shift (Δf) of the PMETAC brushes as a function of the salt concentration. Δf is indicative of a change in mass of the polymer brushes induced by the hydration/dehydration of the brushes.28-33 At a given pH, Δf increases as the salt concentration increases, indicating the dehydration of the PMETAC brushes due to the reduction of osmotic pressure within the brushes with increasing external salt concentration. On the other hand, Δf increases as pH increases, suggesting that the PMETAC brushes dehydrate with increasing pH. Based on the Manning’s counterion condensation theory,34,35 our previous study demonstrated that the mole fraction of the adsorbed OH- within the PMETAC brushes increases as pH is increased.20 Namely, more OH- will associate with the grafted PMETAC chains with increasing pH. Thus, the grafted chains will form more inter-chain hydrogen bonds via the adsorbed OH- and the carbonyl group of the grafted PMETAC chains as pH is increased. The formation of inter-chain hydrogen bonds would lead to the weakening of the interactions between the PMETAC brushes and the water molecules, thereby giving rise to a dehydration of the PMETAC brushes with increasing pH.

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150

fn=3 / Hz

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100 50 0

PMETAC 2

4

6

pH

8

10

12

Figure 1. pH dependence of frequency shift (Δf) of the PMETAC brushes as a function of the salt concentration. Here, KCl is used to control the salt concentration of the pH solutions.

It is evident that the pH dependence of the PMETAC brushes is relatively weak at the lower pH values compared with that at the higher pH values at the salt concentration of 10 mM. This is because the mole fraction of adsorbed OH- within the PMETAC brushes only weakly increases as pH is increased from 2 to 7 and strongly increases as pH is increased from 7 to 12.20 A stronger adsorption of OH- to the PMETAC brushes would give rise to a stronger reorganization of the inter-chain hydrogen bonds, thereby leading to a stronger pH response from pH 7 to 12, particularly for the change of pH from 10 to 12. The pH response of the PMETAC brushes becomes weaker with increasing salt concentration, as reflected by the fact that the pH dependence of change in Δf becomes less obvious as the salt concentration increases. The pH response of the PMETAC brushes is determined by the pH-mediated adsorption of OH- to the brushes. In the pH solutions, there is a 12


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competitive adsorption of counterions to the PMETAC brushes between OH- and Cldriven by the condensation of counterions to the grafted chains.36 At a given pH, the concentration of OH- is fixed. As the salt concentration increases, the Cl- will exhibit a stronger competition with the adsorption of OH- to the PMETAC brushes. Consequently, more Cl- will adsorb to the grafted PMETAC chains and the mole fraction of the adsorbed OH- within the brushes is decreased as the salt concentration is increased. This means that the ability of the grafted chains to form the inter-chain hydrogen bonds decreases with increasing salt concentration. As a result, the pH-mediated reorganization of the inter-chain hydrogen bonds becomes weaker with the increase of salt concentration, leading to a weaker pH response of the PMETAC brushes at a higher salt concentration. In other words, the pH response of the PMETAC brushes, particularly at the high pH values, is suppressed by the high salt concentration. PMETAC

-6

0

Dn=3 / 10

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-5

10 mM 20 mM 30 mM 50 mM 100 mM

-10 -15 -20 2

4

6

pH

8

10

12

Figure 2. pH dependence of dissipation shift (ΔD) of the PMETAC brushes as a function of the salt concentration. Here, KCl is used to control the salt concentration of the pH solutions. 13


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It is known that the dissipation shift (ΔD) is related to either the collapse/swelling or the change in comparative stiffness of the polyelectrolyte brushes.20,29,30,37,38 In Figure 2, ΔD decreases as the salt concentration increases at a given pH, indicating the collapse of the PMETAC brushes induced by the decrease of osmotic pressure within the brushes with increasing salt concentration. This is confirmed by the results in Figure 3, in which the wet thickness of the PMETAC brushes decreases as the salt concentration increases at a given pH. Moreover, the counterion condensation should be influenced by the crowding of the polyelectrolyte chains.39-41 The PMETAC brushes have a crowded environment with a grafting density of ~ 0.4 chains/nm2.20 As the salt concentration increases, the collapse of the PMETAC brushes would make the environment of the brushes more crowded. This would influence the binding of the counterions to the grafted chains, thereby modulating the salt concentration dependence through the adjustment of osmotic pressure. On the other hand, ΔD is also indicative of the change in the comparative softness or rigidity of the PMETAC brushes. As pH increases, the decrease of ΔD indicates that the PMETAC brushes become stiffer with the increase of pH because no obvious collapse or swelling occurs for the brushes during the change of pH, as reflected by the constant wet thickness of the PMETAC brushes with pH (Figure 3). More specifically, the increased adsorption of OH- to the brushes at a higher pH leads to the formation of more inter-chain hydrogen bonds between the grafted chains. The formation of inter-chain hydrogen bonds would physically crosslink the PMETAC brushes, thus, the rigidity of the brushes increases with the increase of pH. Similar to 14


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the Δf, the pH dependence of ΔD also becomes less obvious at a higher salt concentration. This observation can be explained as well by the fact that the pH-mediated reorganization of the inter-chain hydrogen bonds is weakened with the increase of salt concentration. In addition, the value of ΔD at the salt concentration of 10 mM is lower than that at the salt concentrations of 20 and 30 mM at pH 12. This may be because the formation of inter-chain hydrogen bonds at pH 12 has a stronger influence on the dissipation factor than the collapse of the brushes induced by the small increase of the salt concentration.

Thickness / nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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68

PMETAC

64

60 2

4

6

pH

8

10

12

Figure 3. pH dependence of wet thickness of the PMETAC brushes as a function of the salt concentration. Here, KCl is used to control the salt concentration of the pH solutions.

From discussions above, the hydration of the PMETAC brushes is dependent on pH. Therefore, the surface wettability of the PMETAC brushes is expected to be controlled by pH. As both the pH and the salt concentration employed here have no 15


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obvious influences on the surface tension of the solutions (Figure S1, Supporting Information), the measured contact angle should be mainly related to the hydration state of the polyelectrolyte brushes. In Figure 4a, the WCA on the surface of the PMETAC brushes increases as pH is increased. This fact indicates that the surface of the PMETAC brushes becomes less hydrophilic with increasing pH. This is consistent with the results shown in Figure 1. That is, the formation of inter-chain hydrogen bonds between the grafted chains leads to a weakening of the interactions between the PMETAC brushes and the water molecules, making the PMETAC brushes less hydrophilic at a higher pH value. It is evident that the pH dependence of WCA becomes less obvious at a higher salt concentration. This observation is also correlated with the weakening of the reorganization of the inter-chain hydrogen bonds with increasing salt concentration. Additionally, the increase of WCA with the increase of salt concentration at a given pH is attributed to the dehydration of the brushes induced by the reduction of osmotic pressure within the brushes. In Figure 4b, the oil (hexadecane) wettability on the surface of the PMETAC brushes is also dependent on pH. As pH increases, the decrease of OCA means that the surface of the PMETAC brushes becomes more oleophilic. The observed pH dependence of OCA is also related to the hydration of the PMETAC brushes, determined by the pH-mediated reorganization of the inter-chain hydrogen bonds. Similar to the WCA, the OCA also exhibits a weaker pH response at a higher salt concentration. The decrease of OCA with increasing salt concentration at a given pH is attributed to the dehydration of the PMETAC brushes. From the results shown in 16


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Figures 1, 2, and 4, the pH response of the PMETAC brushes becomes weaker at a higher salt concentration. This is because the pH-mediated reorganization of the inter-chain hydrogen bonds is weakened with increasing salt concentration induced by the competitive adsorption of counterions to the brushes between OH- and Cl-.

(a) PMETAC

180 (b) PMETAC

42 168

36 30

10 mM 20 mM 30 mM 50 mM 100 mM

24 2

4

6

pH

8

10

OCA

WCA

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156

10 mM 20 mM 30 mM 50 mM 100 mM

144 2

12

4

6

pH

8

10

12

Figure 4. (a) pH dependence of water contact angle (WCA) on the surface of the PMETAC brushes as a function of the salt concentration. (b) pH dependence of oil (hexadecane) contact angle (OCA) on the surface of the PMETAC brushes as a function of the salt concentration. Here, KCl is used to control the salt concentration of the pH solutions.

Effect of salt concentration on the pH response of the PDMAEMA brushes. It is well-known that the pH-responsive properties of WPB are governed by the charge of the grafted chains, controlled by the association-dissociation of protons.3,12 For the case of positively charged PDMAEMA brushes as an example, the pH-controlled association or dissociation of protons with the dimethylamino groups determines the charge of the grafted chains, thereby making the PDMAEMA brushes pH responsive. In Figure 5, irrespective of the salt concentration, Δf remains almost no changes as pH 17


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increases from 2 to 4, and then sharply increases as pH increases from 4 to 7, followed by a constant Δf with the increase of pH from 7 to 12. It is reported that the grafted PDMAEMA chains has a lower pKa of ~ 6 compared with the free PDMAEMA chains with a pKa of ~ 7 in aqueous solutions owing to the osmotic pressure effect.42-44 Consequently, the degree of charging of the grafted PDMAEMA chains should remarkably decrease around pH 6 as pH increases. At pH 2 and 4, the grafted PDMAEMA chains are highly charged. As pH increases from 4 to 7, the grafted PDMAEMA chains transit from a highly charged into a lowly charged state due to the decrease in the degree of charging of the grafted chains. As a result, the concentration of the free counterions within the PDMAEMA brushes decreases with the increase of pH from 4 to 7, resulting in a reduction of the osmotic pressure within the brushes and the accompanying dehydration of the brushes. This is why Δf sharply increases as pH increases from 4 to 7. From pH 7 to 12, the degree of charging of the grafted PDMAEMA chains only slightly changes, leading to the almost constant value of Δf in this range of pH. Figure 5 also shows that the Δf of the PDMAEMA brushes is dependent on the salt concentration at pH 2 and 4, but is independent of the salt concentration at pH 7, 10, and 12. At pH 2 and 4, Δf increases as the salt concentration increases, suggesting that the PDMAEMA brushes dehydrate due to the decrease of the osmotic pressure within the brushes. This result is similar to the observation shown in Figure 1 for the PMETAC brushes. This is understandable because both the PDMAEMA brushes at the relatively low pH values and the PMETAC brushes are highly charged and their 18


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hydration state can be tuned by the external salt concentration through the modulation of osmotic pressure. At the relatively high pH values from 7 to 12, the PDMAEMA brushes become lowly charged and are insensitive to the salt concentration. Therefore, the pH response of the PDMAEMA brushes as reflected by the pH dependence of Δf becomes weaker at a higher salt concentration.

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600 300 0

PDMAEMA 2

4

6

pH

8

10

12

Figure 5. pH dependence of frequency shift (Δf) of the PDMAEMA brushes as a function of the salt concentration. Here, KCl is used to control the salt concentration of the pH solutions.

Figure 6 shows the pH dependence of ΔD of the PDMAEMA brushes as a function of the salt concentration. At a given salt concentration, no obvious changes in ΔD are observed as pH is increased from 2 to 4, and a sharp decrease in ΔD is observed as pH is increased from 4 to 7, followed by a constant ΔD with increasing pH from 7 to 12. The decrease of ΔD from pH 4 to 7 implies that the PDMAEMA brushes collapse induced by the decrease of the degree of charging of the grafted chains. ΔD has an obvious dependence on the salt concentration at pH 2 and 4, but is independent of the 19


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salt concentration at pH 7, 10 and 12, which agrees with the results of Δf shown in Figure 5. At pH 2 and 4, ΔD decreases as the salt concentration increases, indicating that the PDMAEMA brushes collapse with increasing salt concentration due to the decrease of osmotic pressure within the brushes. In the range of pH from 7 to 12, the lowly charged nature of the grafted chains causes the PDMAEMA brushes to be insensitive to the salt concentration. Obviously, the pH response of the PDMAEMA brushes as reflected by the pH dependence of ΔD also becomes weaker at a higher salt concentration. 10 mM 20 mM 30 mM 50 mM 100 mM

-6

0

Dn=3 / 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-20 -40 -60 -80

PDMAEMA 2

4

6

pH

8

10

12

Figure 6. pH dependence of dissipation shift (ΔD) of the PDMAEMA brushes as a function of the salt concentration. Here, KCl is used to control the salt concentration of the pH solutions.

At a given salt concentration, the wet thickness of the PDMAEMA brushes keeps almost constant as pH is increased from 2 to 4 (Figure 7). As pH increases from 4 to 7, the wet thickness of the PDMAEMA brushes sharply decreases from ~ 63 nm to ~ 31 nm. This is an indication that the PDMAEMA brushes transit from a swollen to a 20


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collapsed state. At the same time, the refractive index of the PDMAEMA brushes increases associated with the collapse of the brushes from pH 4 to 7 (Figure S2, Supporting Information). When pH is increased from 7 to 12, the wet thickness of the PDMAEMA brushes only slightly changes with pH. The change of wet thickness of the PDMAEMA brushes with pH can also be estimated from the QCM results (Figure S3, Supporting Information). At pH 2 and 4, the wet thickness decreases as the salt concentration increases as shown in the inset of Figure 7, which is consistent with the results shown in Figures 5 and 6. By contrast, the wet thickness is independent of the salt concentration at pH 7, 10, and 12. As a consequence, the pH response of the PDMAEMA brushes as reflected by the pH dependence of wet thickness becomes

Thickness / nm

weaker as well at a higher salt concentration.

70

Thickness / nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

64 62 60 2

50

pH

4

10 mM 20 mM 30 mM 50 mM 100 mM

40 30

66

PDMAEMA 2

4

6

pH

8

10

12

Figure 7. pH dependence of wet thickness of the PDMAEMA brushes as a function of the salt concentration. The inset shows a zoom of the salt concentration dependence of wet thickness of the PDMAEMA brushes at pH 2 and 4. Here, KCl is used to control the salt concentration of the pH solutions.

21


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Figure 8a shows the pH dependence of WCA on the surface of the PDMAEMA brushes as a function of the salt concentration. The WCA increases as pH increases from 2 to 12 at a given salt concentration. This is correlated with the dehydration of the PDMAEMA brushes with increasing pH. On the other hand, the WCA increases as the salt concentration increases at the relatively low pH values of 2 and 4, but is insensitive to the salt concentration at the relatively high pH values of 7, 10, and 12. In Figure 8b, the OCA decreases with the increase of pH from 2 to 12 at a given salt concentration. This is also related to the dehydration of the PDMAEMA brushes with increasing pH. The OCA decreases with increasing salt concentration at pH 2 and 4, but is almost independent of the salt concentration in the range of pH from 7 to 12. Thus, it can be concluded that the pH dependence of surface wettability of the PDMAEMA brushes becomes less obvious as the salt concentration increases. 42

(a) PDMAEMA

156

(b) PDMAEMA

150 36

OCA

WCA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30

10 mM 20 mM 30 mM 50 mM 100 mM

24 2

4

6

pH

8

10

144 10 mM 20 mM 30 mM 50 mM 100 mM

138 132

12

2

4

6

pH

8

10

12

Figure 8. (a) pH dependence of water contact angle (WCA) on the surface of the PDMAEMA brushes as a function of the salt concentration. (b) pH dependence of oil (hexadecane) contact angle (OCA) on the surface of the PDMAEMA brushes as a function of the salt concentration. Here, KCl is used to control the salt concentration of the pH solutions. 22


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Comparison between the PMETAC and PDMAEMA brushes. The adsorption of hydroxide to the PMETAC brushes should be a type of physisorption induced by the counterion condensation, whereas the pH-controlled dissociation-association of proton of the PDMAEMA brushes should be a type of chemisorption.45 As discussed above, the pH responses of both the PMETAC and PDMAEMA brushes become weaker at a higher salt concentration. However, the different origins of the pH responses between SPB and WPB make the salt concentration exhibiting different ways to work on the pH responses between the PMETAC and PDMAEMA brushes. Figure 9a shows the ΔD-Δf plot of the PMETAC brushes as a function of the salt concentration during the change of pH. As pH increases from 2 to 12, ΔD decreases with increasing Δf, suggesting the co-occurrence of collapse and dehydration of the PMETAC brushes. As the ΔD-Δf plots at the different salt concentrations have a similar slope, the distance from the start point at pH 2 to the end point at pH 12 in the ΔD-Δf plots reflects the strength of the pH response of the PMETAC brushes at different salt concentrations. Obviously, the pH response of the PMETAC brushes becomes weaker at a higher salt concentration, as reflected by the decrease in the distance with the increase of salt concentration. Furthermore, it can be seen that the weakening of the pH response of the PMETAC brushes with increasing salt concentration mainly occurs at the relatively high pH values. As reported previously, the mole fraction of the adsorbed OH- within the PMETAC brushes has a strong increase as pH increases from 7 to 12, which generates a strong reorganization of the inter-chain hydrogen bond network.20 In other words, 23


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the PMETAC brushes have a stronger response to pH at the relatively high pH values. As the effect of salt concentration on the pH response of the PMETAC brushes is manifested by the competitive adsorption of counterions to the brushes between OHand Cl-. Therefore, the increasing salt concentration induced weakening of the pH response of the PMETAC brushes should be more remarkable at the relatively high pH values.

g pH

-12 -18

0 -6

-6

in

Dn=3 / 10

s ea cr

-6

10 mM 20 mM 30 mM 50 mM 100 mM

Dn=3 / 10

0

in

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50

-40

pH

-60 -80 (b) PDMAEMA

(a) PMETAC 0

inc rea sin g

-20

10 mM 20 mM 30 mM 50 mM 100 mM

100

fn=3 / Hz

150

0

300

600

fn=3 / Hz

900

Figure 9. Dissipation shift (ΔD) versus frequency shift (Δf) of the polyelectrolyte brushes as a function of the salt concentration during the change of pH. (a) The PMETAC brushes. (b) The PDMAEMA brushes. Here, KCl is used to control the salt concentration of the pH solutions.

Because the ΔD-Δf plots of the PDMAEMA brushes at the different salt concentrations also have a similar slope (Figure 9b), the distance between the start point at pH 2 and the end point at pH 12 of the ΔD-Δf plots is indicative of the strength of the pH response of the PDMAEMA brushes at the different salt concentrations. The decrease in the distance with increasing salt concentration indicates that the pH response of the PDMAEMA brushes becomes weaker at a higher 24


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salt concentration. In contrast to the PMETAC brushes, the increase of salt concentration induced weakening of the pH response of the PDMAEMA brushes mainly occurs at the relatively low pH values. The pH response of the PDMAEMA brushes is related to the pH-controlled variation of the degree of charging of the grafted chains. At the relatively low pH values of 2 and 4, the highly charged PDMAEMA brushes are sensitive to the salt concentration through the modulation of osmotic pressure within the brushes. By contrast, the lowly charged PDMAEMA brushes at the relatively high pH values of 7, 10, and 12 are almost independent of the salt concentration. This is why the weakening of the pH response of the PDMAEMA brushes with increasing salt concentration mainly occurs at the relatively low pH values.

Conclusion Herein, we have employed the PMETAC and PDMAEMA brushes as model systems to investigate the effect of salt concentration on the pH responses of SPB and WPB. The pH responses of both the PMETAC and the PDMAEMA brushes become weaker at a higher salt concentration. The weakening of the pH response of the PMETAC brushes is induced by the competitive adsorption of counterions to the brushes between OH- and Cl- with the increase of salt concentration, whereas the weakening of the pH response of the PDMAEMA brushes is related to the modulation of osmotic pressure within the brushes by the salt concentration. The weakening of the pH responses mainly occurs at the relatively high and low pH values for the PMETAC 25


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and PDMAEMA brushes, respectively. Our study demonstrates that the different origins of the pH responses between SPB and WPB give rise to the different ways for the salt concentration to work on the pH responses of them.

Acknowledgements The financial support of the National Natural Science Foundation of China (21374110, 21574121, 21622405), the Youth Innovation Promotion Association of CAS (2013290), and the Fundamental Research Funds for the Central Universities (WK2340000066) is acknowledged.

Supporting Information: Surface tension of the pH solutions, the change of refractive index of the PDMAEMA brushes, and the change of wet thickness of the PDMAEMA brushes calculated from the frequency shift are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

Notes The authors declare no competing financial interests.

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TOC Graphic

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Effect of Salt Concentration on the pH Responses of Strong and Weak

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