Contraction and Coagulation of Spherical Polyelectrolyte Brushes in

Sep 30, 2016 - Effect of the presence of an additional salt (NaNO3) on Ag+-induced shrinking of SPB(HMEM), effect of sample history on Ag+-induced shr...
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Contraction and Coagulation of Spherical Polyelectrolyte Brushes in the Presence of Ag+, Mg2+, and Ca2+ Cations Anna Ezhova and Klaus Huber* Physikalische Chemie, Universität Paderborn, 33098 Paderborn, Germany S Supporting Information *

ABSTRACT: Unlike Na+ cations, which interact with fully neutralized poly(acrylic acid) as purely electrostatic entities, cations like Ag+, Mg2+, and Ca2+ exhibit specific interactions with the COO− residues of anionic polyacrylates. The present work analyzes the interaction of all four cations with a layer of polyacrylate chains grafted onto spherical polystyrene core as an outer shell. First and foremost the analysis answers the question on how these specific interactions influence the nature of the shell and solution behavior of the spherical polyelectrolyte brushes. It could be unambiguously demonstrated that Ag+, Mg2+, and Ca2+ cations induce a drastic shrinking of the polyacrylate shells at cation concentrations lower by 2−3 orders of magnitude compared to the transition of a fully stretched osmotic brush to a shrunken salted brush accomplished with Na+ cations. Ag+ induces a particularly abrupt contraction, which comes close to the annealing of such brushes achieved with a neutralization by protons. Finally, the solution behavior of the brushes in the presence of Ag+ and Ca2+ cations is compared with the respective pattern of molecularly dissolved linear polyacrylate chains in terms of phase diagrams.

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INTRODUCTION Spherical polyelectrolyte brushes (SPB) establish an interesting class of polyelectrolytes, both from the fundamental point of view and with respect to their potential in the field of material science.1 These composite particles have a highly regular shape with a compact sphere-like colloid as a core and an enveloping shell of linear polyelectrolyte chains. The polyelectrolyte chains are with one end grafted onto the spherical surface and emanating toward the outer fringes of the brush with the other end. The properties of the shell in terms of the length of the polyelectrolyte chains, their grafting density,2 and the chemical nature of their ionic residues3 can be controlled by synthesis. The number of ionizable residues per brush typically adopts an order of magnitude of 105. In the case of complete dissociation such a large number would create a huge electrostatic potential. This is effectively avoided by the brush due to a territorial binding of counterions, which results in a neutralization of more than 90% of its charges.4 However, the nature of the polyelectrolyte turns the shell into a responsive compartment acting on salinity and presence of specifically interacting cations and solvent quality. These properties make SPBs highly interesting objects. The most prominent and fundamental feature of SPBs is the impact of simple monovalent counterions with pure electrostatic interactions on the shell of polyelectrolyte chains. Without any further salt added to the solution, the counterions confined to the shell exhibit an osmotic pressure in the shell which causes the shell to expand. Polymer elasticity and polymer excluded volume interactions oppose this expansion, and equilibrium swelling is reached once the opposing force of © XXXX American Chemical Society

the polymer layer balances the expansion driven by the osmotic pressure from the counterions. Addition of a monovalent salt builds up an osmotic pressure in solution and gradually relaxes the difference to the osmotic pressure of the confined counterions once the concentration of the added salt outside of the brushes reaches the level of the local salt concentration inside of the brushes. The resulting brush is called a salted brush to distinguish it from the above introduced state of a fully extended brush, which is called an osmotic brush. This pattern was postulated first by Pincus and Ross5,6 and by Zhulina et al.7−10 and verified by a set of experiments on planar and spherical brushes,2,3,11−16 which also include first hints of deviations from theoretical expectations due to specific interactions between counterions and charged residues of the polyelectrolyte chains.16 As has been demonstrated by experiments with La3+ cations in SPBs based on sodium polyacrylate chains (NaPA), multivalent counterions relax the osmotic pressure inducing a collapse of the shell due to an entropically driven replacement of three confined monovalent cations by one trivalent cation and due to (partial) binding of those trivalent cations to the anionic residues of the polyelectrolyte chains.17,18 Further evidence for interactions that go beyond simple electrostatic forces have been provided by Tirell et al.19−22 with La3+ and Ru(NH3)63+ but using spherical19 and planar20−22 brushes based on polystyrenesulfonate (NaPSS) as polyelectrolyte Received: June 15, 2016 Revised: September 19, 2016

A

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enables a closer look on the impact of Ag+ as a monovalent SIC and of two representative alkaline earth cations anticipated as bivalent SICs.25,26 Since for Ag+ and Ca2+ cations detailed knowledge already exists on the corresponding solution behavior of linear NaPA chains,25−29 a meaningful comparison with our new results on SPBs will become accessible. As a particularly interesting aspect we will address the shrinking of a SPB induced by Ag+ as monovalent SIC, in comparison to the impact of Na+ as a typical representative of a monovalent EIC. In more detail, we prepare SPBs with a dense polystyrene core covered by a layer of grafted NaPA coils.30 With these SPBs a threshold where maximum shrinking or contraction occurs is first established at variable SPB content for Ag+, Mg2+, and Ca2+ cations and compared with the shrinking behavior induced in diluted linear NaPA coils with Ag+ and Ca2+ cations. Successively, a coagulation threshold of the SPBs with Ag+ and Ca2+ cations shall be identified and compared with the respective phase behavior observed with linear NaPA coils. The impact of Ag+ cations on the morphology of the brush layer and the solubility of the SPBs is highly relevant in light of its potential use in acting as a host for silver nanoparticles formed in the respective brush layers.28,31

instead. Those authors used a bridging mechanism between oppositely charged moieties belonging to different chains to interpret the observed abrupt collapse of the shell and the attractive potential among two approaching shells.19−22 Noteworthy, subtle differences observed between La3+ and Ru(NH3)63+ in their impact on the NaPSS shells supported the existence of specific interactions.21 However, such direct or specific binding of a counterion to negative residues of the polyelectrolyte chains is still far from being fully understood. If it is the dominating effect, the respective counterions are hereafter denoted as specifically interacting cations (SIC). SICs have to be distinguished from purely electrostatically interacting counterions (EIC) responsible for the osmotic swelling and for a contraction due to osmotic deswelling, a distinction also suggested by Konradi and Rühe.23 The most prominent example for a specific interaction is the annealing of a spherical brush of NaPA chains in the shell with (monovalent) protons.2,3 NaPA in particular is known to exhibit specific interactions also with a large number of other cations like alkaline earth cations24−26 or silver cations.23,27 Whereas alkaline earth cations are bivalent and their interaction with the polyelectrolyte shell may thus show an interplay of SIC behavior and of multivalent EIC behavior, Ag+ cations are monovalent cations, which, if interacting only electrostatically (i.e., via pure EIC behavior), should show a pattern indistinguishable from monovalent sodium counterions. Silver salts are therefore particularly suitable to investigate the impact of specific interactions with molecularly dispersed polyelectrolyte chains as well as with polyelectrolyte chains confined in a SPB shell. Specific interactions of cations with polyelectrolyte chains can be characterized by means of their solution behavior. For NaPA chains in the presence of alkaline earth cations a precipitation threshold exists which obeys the linear relation24−26 [Mn +]c = m + r0[COO−]c

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EXPERIMENTAL SECTION

Materials. Sodium nitrate (NaNO3), magnesium sulfate hexahydrate (MgSO4·6H2O), sodium chloride (NaCl), and calcium chloride hexahydrate (CaCl2·6H2O) from Fluka (Buchs, Switzerland), silver nitrate (AgNO3) from Sigma-Aldrich (St. Louis, USA), potassium persulfate (KPS) from Sigma-Aldrich (Taufkirchen, Germany), sodium dodecyl sulfate (SDS) from Sigma-Aldrich (Tokyo, Japan), 4-hydroxybenzophenone from Sigma-Aldrich (Shanghai, China), triethylamine from Sigma-Aldrich (Diegem, Belgium), benzoin from Sigma-Aldrich (Taufkirchen, Germany), dimethylaniline from SigmaAldrich (Bangalore, India) and 2-hydroxy-4′-hydroxyethoxy-2-methylpropiophenone from Sigma-Aldrich (St. Louis, USA) with a purity >99% were used as received. Styrene from Sigma-Aldrich (Taufkirchen, Germany), acrylic acid from Sigma-Aldrich (Zwijndrecht, Netherlands), acryloyl chloride from Fluka (Buchs, Switzerland), and methacryloyl chloride from Fluka (Buchs, Switzerland) were used as received. These liquids were destabilized by a flush column charged with basic aluminum oxide Woelm B-Super I from Woelm Pharma (Bad Honnef, Germany) before use. Acetone, chloroform, diethyl ether, ethanol, toluene, tert-butanol, and pyridine with a purity >99% were used as received. Silica gel 60 from Fluka (Buchs, Switzerland) was used for column chromatography. Bidistilled water with a conductivity 1 mM. Only above the just mentioned level at [Ag+] ∼ 6 mM the dense aggregates precipitate with the exact value of [Ag+] depending on the polymer concentration [COO−], thus establishing a precipitation threshold. Motivated by this knowledge, the present work investigates the contraction and coagulation/precipitation behavior of SPBs furnished with a NaPA shell. The contraction and coagulation/ precipitation of SPBs are studied in the presence of Ag+, Mg2+, and Ca2+ cations and related to the respective solution behavior in aqueous NaNO3 or NaCl as an inert salt. The investigation B

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Macromolecules Table 1. Characteristic Parameters of the SPB and Its Constituents

a

SPB (used photoinitiator)

Rh,PS (nm)

Rh,SPB (nm)

L (nm)

Mη (g mol−1)

σ (nm−2)

Rh,PA‑chain (nm)

D (nm)

Lc (nm)

SPB(HMEM) SPB(ABP) SPB(BA)

59.2 71.7 70.0

218.9 163.1 220.0

159.7 91.4 150

61642 34837 56946

0.055 0.044 0.046

8.0 5.8 7.8

4.9 5.4 5.3

163.5 93.6 153.6

The parameters are introduced in the paragraph “Preparation and Characterization of the SPBs” of the Experimental Section.

core−shell particles, i.e., the SPBs, were purified by serum replacement.32,33 Values of hydrodynamic radii of the PS cores (Rh,PS) and of the SPBs (Rh,SPB) were established via dynamic light scattering (DLS) measurements in salt-free solutions. In the case of SPBs the pH of the solutions was set to 10. The thickness of the shell L is expressed as the difference in hydrodynamic radius of the complete SPB and of the corresponding polystyrene core as

L = R h,SPB − R h,PS

(2)

To provide a complete and consistent characterization of the SPBs, complementary characterization of the grafted PA hairs was performed as follows. A small fraction of each purified SPB sample was separated and subdued to complete cleavage of the grafted PA chains by hydrolyzing the ester bond of the photoinitiator under basic conditions (2 M NaOH). Viscosimetry of the resulting NaPA chains in dilute aqueous 0.5 M NaCl provides the intrinsic viscosity [η] therein, and use of the corresponding Mark−Houwink relation34 which reads [η] = 0.0186Mη0.72 yields a viscosity-average Mη of the molecular weight and along with this a hydrodynamic radius Rh,PA‑chain = (3Mη[η]/10πNA)1/3. A contour length Lc = 0.254(Mη/Mo) in nm was estimated, with Mo the molecular weight of a monomeric sodium acrylate unit. The entire amount of PA grafted onto the SPBs was determined by conductometric titration using a hand-held conductivity meter Cond 340i (WTW) equipped with a standard conductivity cell TetraCon 325 (WTW). The amount of grafted chains and their length Lc then yielded the grafting density σ and the average distance D = 4/(πσ)1/2 between two neighboring grafting points in a straightforward manner. Further details of the synthesis and the characterization are presented in refs 2, 3, 30, and 32. Table 1 gives an overview of the parameters of all SPBs under consideration. Preparation of SPB Solutions for Dynamic Light Scattering. For characterization of the SPBs (Table 1) a drop (ca. 0.05 mL) of purified latices SPB(HMEM), SPB(ABP), and SPB(BA) was diluted with water (50 mL) to a final concentration of ca. 5−10 ppm. The pH value of such salt-free solutions was adjusted to the value of ca. 10 using ca. 0.05−0.1 mL of 0.1 M NaOH, which guarantees complete conversion to NaPA chains in the shells. Solutions were stirred overnight and characterized by DLS on the following day. In order to compare the effects of Na+ and Ag+ ions on the shrinking of the PA-shell (experiment 1, Figure 1), four 50 mL flasks with NaNO3 solutions of the concentrations 0.001, 0.01, 0.1, and 1 M and four 50 mL flasks with AgNO3 solutions of the concentrations 0.00005, 0.0001, 0.0005, and 0.001 M were prepared first. A drop of purified SPB(HMEM) latices was dissolved in each flask. Na+−SPB solutions were kept at a pH of ca. 11 and Ag+−SPB solutions at a pH of ca. 6.5. All pH values were carefully adjusted using ca. 0.05−0.1 mL of 0.1 M NaOH. All solutions were stirred overnight and characterized by DLS on the following day. In order to investigate the effect of Ag+ ions on the shrinking of the PA layer at variable pH (experiment 2, Figure 2 and Figure S2a), two stock solutions of SPB(HMEM) with [COO−] = 0.1 mM using 0.01 M NaNO3 as a solvent were prepared first. Solutions were stirred for a few hours, and the pH values of both solutions were established as 4.4. One SPB(HMEM) stock solution remained to be used without further pH adjustment. The pH in the second solution was adjusted to 6.5 by adding ca. 0.05 mL of 0.1 M NaOH and stirred overnight. With these two stock solutions, two series of experiments were established: one at pH = 4.4 and another one at pH = 6.5. In each series the overall ionic strength was set to [AgNO3] + [NaNO3] = 0.01 M while the desired [Ag+] was varied. This was achieved by mixing equal amounts of the SBP(HMEM) solution and a binary salt solution at variable [Ag+] with

Figure 1. Thickness L of the PA layer in SPB(HMEM) solutions at a pH of 6.5 as a function of AgNO3 concentration and at a pH of 11 as a function of NaNO3 concentration.

Figure 2. Thickness L of the PA layer of SPB(HMEM) in solutions at two values of pH as a function of AgNO3 concentration in 0.01 M NaNO3. The concentration of [COO−] is 0.05 mM. The black horizontal line corresponds to the plateau value Lpl = 40 nm of the contracted PA layer. The shaded area indicates the standard deviation of Lpl which is ±5 nm. Black and red arrows indicate the silver cation concentrations [Ag+]shr where shrinking of the PA layer is completed at a pH of 6.5 and at a pH of 4.4, respectively. The red and black bars on the x-axis indicate standard deviation of the two [Ag+]shr values. [AgNO3] + [NaNO3] = 0.01 M in the scattering cuvette before analysis by DLS. In order to investigate the effect of Ag+ ions on the shrinking of the PA layer at variable ionic strength (experiment 3a, Figure S1), two stock solutions of SPB(HMEM) with [COO−] = 0.2 mM were prepared. One SPB stock solution was prepared using 0.01 M NaNO3 as a solvent, and the other one was based on salt-free water. In both SPB stock solutions the pH was adjusted to 6.5 by adding ca. 0.05 mL of 0.1 M NaOH. Both solutions were stirred overnight. Using aqueous solutions of 0.01 M AgNO3, 0.01 M NaNO3, and salt-free water, two series of experiments were established. In one series an amount of SPB C

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In order to ensure stability of the samples and identify aggregation thresholds, all samples shown in Figure 1, 2, 5, and 6 were measured again after 1, 2, and 3 days of shelf life. As is shown in the Supporting Information (Figure S2a) for the data from Figure 2 as an example, none of the values changed significantly, thus proving stability of the samples with both intermediate and final degrees of deswelling. Absence of any aggregation caused by nonequilibrium states during the mixing of an SPB solution with the SIC solution is corroborated by a direct comparison of selected static light scattering curves and of the corresponding q2-dependencies of D, which were recorded from the dilute SPBs before and after mixing with the Ag+ solutions (Figure S2b). Dynamic Light Scattering Characterization of SPBs. DLS experiments were performed with the Model ALV-CGS 5000E from ALV-Laser Vetriebsgesellschaft (Langen, Germany) equipped with a He−Ne laser from Soliton with 35 mW (Gilching, Germany) at a wavelength of 632.8 nm at 25 °C in the range of the scattering angles 30° < θ < 150°. Data evaluation of the DLS measurements was based on the cumulant analysis of the electric field-time autocorrelation, g1(τ), according to35,36

in salt-free solution was mixed with an equal amount of aqueous AgNO3 at variable [Ag+] in the scattering cuvettes and monitored by DLS. In the other series of experiments [Ag+] was varied at the constant overall ionic strength of [Na+] + [Ag+] = 0.01 M in a binary salt solution. This was achieved by mixing the SPB(HMEM) solution in 0.01 M NaNO3 with an equal amount of the binary salt solution at variable [Ag+] with [AgNO3] + [NaNO3] = 0.01 M in the scattering cuvette before analysis by DLS. In order to study the impact of the SPB concentration, the experimental series at 0.01 M NaNO3 was complemented with further series at three different values of [COO−], establishing experiment 3b (Figure 3a and Figure S3a). The

ln(g1(τ )) = A − Γτ + k 2τ 2

(3)

where τ is the delay time, Γ is the first cumulant, and k2 is the second cumulant. Extrapolating Γ to zero scattering vector q = 0 gives the zaverage diffusion coefficient Dz according to Dapp =

Γ = Dz(1 + CR g 2q2) q2

(4) 2

where C is a shape sensitive constant, Rg is the z-averaged radius of gyration, and

4πn sin q=

( θ2 )

λ

(5)

is the scattering vector determined by the scattering angle θ, n the refractive index of the solvent, and λ the wavelength of the incident light. An effective average hydrodynamic radius, Rh, for the scatterers was obtained from Dz by using Stokes−Einstein relation

Figure 3. Threshold concentrations of silver cations [Ag+]shr for complete shrinking (Table S1) with SPB in 0.01 M NaNO3 at pH = 6.5 for SPB(HMEM), SPB(ABP) and SPB(BA) (a) and threshold concentrations of calcium cations [Ca2+]shr in for complete shrinking (Table S3) in 0.01 M NaCl at pH = 6.5 for SPB(HMEM) (b).

Rh =

kBT 1 6πη Dz

(6)

where T is the absolute temperature and η is the viscosity of the solvent. A refractive index n = 1.3357 and a viscosity η = 0.948 mPa·s was used for aqueous solution in 0.01 M NaNO3 as well as in 0.01 M NaCl solution.

concentration of COO− groups was varied between 0.05 and 0.25 mM. The analyzed range of counterion concentrations was 0.01 ≤ [Ag+] ≤ 10 mM. A variation of the Ag+ content at [COO−] of 0.05− 0.25 mM as has been performed within experiment 3b was also carried out with SPB(ABP) and SPB(BA), thus enabling a comparison of three SPBs (Figure S3) which differ in the parameters of their respective NaPA shells (Table 1). It has to be stressed that all manipulations and measurements with Ag+−SPB solutions were performed in the dark in order to avoid reduction of Ag+ ions and formation of Ag nanoparticles.27,28 The effect of Ca2+ ions on the shrinking of the PA shell was investigated by means of analogous experiments as performed in experiments 3a and 3b with Ag+ and SPB(HMEM) at pH = 6.5 (Figures 3b and 5, Figure S5b). CaCl2 salt was used as a source of Ca2+ ions and NaCl was used instead of NaNO3 in order to keep the overall concentration of the positive charges at 2[Ca2+] + [Na+] = 0.01 M. The analyzed range of [Ca2+] was 0.05 ≤ [Ca2+] ≤ 1.5 mM. In one further experiment the impact of Mg2+ cations was analyzed with SPB(HMEM) at the two SPB concentration of [COO−] = 0.1 and 0.15 mM and at a pH = 6.5 (Figure S5a). The overall salinity was kept constant at 2[Mg2+] + [Na+] = 0.01 M by means of NaCl. The analyzed range of counterion concentrations was 0.1 ≤ [Mg2+] ≤ 3 mM. The procedure applied was analogous to the two corresponding dilution series applied for Ca2+.

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RESULTS AND DISCUSSION Three different SPB samples with a core of PS and a shell of linear NaPA chains have been prepared in order to analyze the impact of specifically interacting cations on the solution behavior of polyelectrolyte brushes. As depicted in Table 1, the SPB samples differ in their core size expressed as the hydrodynamic radius Rh,PS of the polystyrene core and molar mass of the grafted NaPA chains. Nomenclature of the SPB samples is adapted to the respective photoinitiator used to attach the NaPA chains to the core. Whereas sample SPB(HMEM) is used as a representative sample to illustrate the main results of the present work, data from the other two samples further confirm and supplement these results. Discussion shall be started by comparing how dilute aqueous SPB(HMEM) respond to the addition of AgNO3 and of NaNO3. Whereas the pH of Na+ containing solutions was close to 11, we had to keep the pH as low as 6.5 once adding Ag+ cations in order to avoid precipitation of AgOH. However, this shift in pH had no significant influence3 on the observed trends D

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Macromolecules with salt variation since the thickness L of the SPB shells at the respective salt-free state, expressed by eq 2, was close to 150 nm at both pH’s, indicating highly stretched NaPA chains. As is indicated in Figure 1, the shrinking induced by Ag+ ions corresponds to an abrupt contraction extending over 10−4 M < [Ag+] < 10−3 M and is completed at a roughly stoichiometric aliquot. Shrinking induced by Na+ ions is more gradual and proceeds over 3 decades of increasing [Na+]. This is a clear proof of the specific nature of the interactions of Ag+ cations with COO− residues, which are thus comparable to the impact of protons.3 The drastic shrinking of SPB(HMEM) induced by Ag+ ions shall be investigated in more detail and successively compared with the impact of two alkaline earth cations also expected to predominantly interact specifically with the COO− residues. All experiments discussed in the following were performed under conditions of a constant amount of positive charges [+] in solution, which was achieved by adding a certain amount of SIC, denoted as Mn+, via replacing the corresponding amount of Na+ at [+] = n[Mn+] + [Na+] = 0.01 M. Impact of Ag+ Cations. Detailed discussion of the impact of Ag+ cations on the morphology and solution behavior of SPBs at variable concentrations of SPB shall be preceded by a brief description of the influence of pH and salinity on this impact. Accordingly, Figure 2 represents typical shrinking experiments at two pH values (4.4 and 6.5), where the shell thickness L is recorded as a function of the concentration of Ag+ cations at constant [COO−] = 0.1 mM. In the present work, such trends are characterized in terms of a characteristic concentration of SIC, here Ag+, denoted as [Ag+]shr where the final shell thickness Lpl has been approached and hence shriking is completed. Values for [Ag+]shr have been established by connecting the first data point which is part of the final plateau in L with the data point next to it toward decreasing [Ag+]. The value of [Ag+]shr is identified as the center of this connecting line. The width of the connecting line projected onto the [Ag+]-axis is used as an estimation for the uncertainty of the respective [Ag+]shr value. Strikingly, a drop in the pH from 6.5 to 4.4 shifts [Ag+]shr from 0.25 to 0.625 mM, suggesting a strong competition between monovalent protons and Ag+. Contrary to this strong influence of H+ on the shrinking of the shell, entire removal of the sodium ions has hardly any effect. As is shown in the Supporting Information (Figure S1), the trend observed at [+] = 0.01 M and pH = 6.5 is fully reproduced if performed also in salt-free solution, indicating that the exchange pressure from Na+ cations at an ionic strengths of 0.01 mM is negligible. Along this line, the impact of Ag+ cations on dilute solutions of SPB brushes shall now be investigated with all three SPB samples at four or three different SPB concentrations, respectively. A pH = 6.5 and a salinity of [+] = 0.01 M are used as solvent conditions. To begin with, the shrinking induced by Ag+ in brush sample SPB(HMEM) is analyzed at variable concentration of SPB denoted as the molar concentration of carboxylate functions [COO−]. The trends of L versus [Ag+] are summarized in the Supporting Information (Figure S3a and Table S1). The corresponding shrinking thresholds [Ag+]shr are correlated with the respective SPB content expressed as [COO−] in Figure 3a, indicating a clear increase of the threshold value [Ag+]shr with the SPB content. To further confirm this result, a series of experiments have been carried out also with the samples SPB(ABP) and SPB(BA) at three different SPB contents. A

detailed overview on the shrinking curves is again given in the Supporting Information (Figure S3 and Table S1). The characteristic shrinking values [Ag+]shr thereof are summarized in Figure 3a. SPB(ABP) and SPB(BA) in fact revealed the same increase of [Ag+]shr with [COO−] as has been revealed with sample SPB(HMEM). Extrapolation of the trends to [COO−] = 0 yields intercepts close to ca. 0.2 mM, which corresponds to a lower limit of [Ag+] required to initiate the exchange of the counterions originally present in the respective SPB particles with Ag+ ions. The established threshold line which denotes the complete shrinking of the SPBs with NaPA shells in the presence of Ag+ ions shall be complemented with the phase boundary separating solutions of stable SPB colloids from a regime in which the SPB colloids coagulate. The coagulation threshold observed with the experimental series of SPB(HMEM) at a concentration of [COO−] = 0.1 mM and a pH = 6.5 (Figure 4)

Figure 4. Results from the visual observations ([COO−] > 0.25 mM) and the DLS experiments ([COO−] < 0.25 mM) on the solution behavior of Ag+−SPB(HMEM) in 0.01 M NaNO3 at a pH of 6.5: PAshell shrinking threshold (full squares) with data from Figure 3 and coagulation threshold (empty squares) with data from Table S2. Solid lines indicate phase boundaries of solutions of linear Ag+−PA in 0.01 M NaNO3 (ref 27): precipitation threshold (3), separation line between dense unstable and dense stable aggregates (2), aggregation threshold (1).

lies at [Ag+]c = 8 mM. This threshold is 40 times higher than the shrinking threshold, indicating a broad regime of stable solutions of single SPBs in their maximally shrunken state. The coagulation threshold could be identified with light scattering by the onset of aggregation at SPB contents up to a concentration of [COO−] = 0.25 mM. For larger SPB contents, solutions became increasingly turbid even in their stable colloidal state, and light scattering was no longer possible. Therefore, we could not establish the solution behavior of the present Ag+−SPB in the same range of NaPA concentrations as we had been able to do in the case of linear NaPA chains,27 where the range extended over 0.64 mM < [COO−] < 5.12 mM. In order to still compare these two polyelectrolyte systems, we establish a coagulation threshold [Ag+]c versus [COO−] by visual inspection of milky SPB solutions at the concentration range of 1 mM < [COO−] < 4 mM, where coagulation becomes transparent by the appearance of solid-like flakes of precipitating Ag+−SPB. To this end we prepared a matrix of 4 × 9 samples covering a concentration regime of 1 mM ≤ [COO−] ≤ 4 mM and 7 mM < [Ag+] ≤ 15 mM. Each E

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Macromolecules sample was scrutinized with respect to coagulation and classified as either “stable” or “coagulating”. Details are given in the Supporting Information (Figure S4). The resulting threshold values for coagulation [Ag+]c are indicated in Figure 4 and compared with the values [Ag+]shr where maximal shrinking of the shell could be observed. In addition, Figure 4 compares the solution behavior of SPBs with the one established for linear NaPA coils in the presence of Ag+.27 The comparison reveals a few interesting analogies and differences between SPBs and linear NaPA. The coagulation threshold of the SPBs lies close to the precipitation threshold of linear chains being only 3 mM lower on average, with both threshold lines having similar slopes. Whereas in the case of the SPB this slope corresponds to 0.6 Ag+ cations per COO− residue, a value of 0.45 Ag+ cations per COO− had been established for the linear NaPA chains. The coagulation threshold and the precipitation threshold for these two PA systems, which are considered to be correspondent, border on a broad regime of stable solutions. However, in the case of linear NaPA chains the stable entities are dense stable aggregates,27 and in the case of SPBs we are dealing with stable single colloids having dense shells. A striking difference appears if these broad stable regimes are approached from below with increasing [Ag+]. Whereas in the case of linear NaPA chains already a few Ag+ cations per 1000 COO− residues suffice to induce an aggregation, no such instability is observed with the SPBs. The reason for this difference may lie in the fact that the shells of the SPBs are built of closely arranged linear NaPA chains with a large overall mass. While the molecularly dispersed linear NaPA coils first aggregate to such large entities and the aggregates eventually condense, the shell forming NaPA coils have already reached the state of a large mass right from the onset, thus surpassing aggregation and solely undergoing densification in the shell once a certain regime of ∼0.3 Ag+ ion per COO− residue is reached. In this sense, the line separating dense stable from dense unstable aggregates of linear chains (line 2 in Figure 4) may correspond to the shrinking threshold of the shells of the SBPs in Figure 4. Impact of Mg2+ and Ca2+ Cations. The impact of Mg2+ and Ca2+ cations on the solution behavior of SPBs with a NaPA shell has been solely performed with SPB(HMEM). The system Mg2+−SPB was analyzed by means of a variation of [Mg2+] at two different SPB contents in [+] = 0.01 M at a pH = 6.5. Four series at four different SPB contents in [+] = 0.01 M at a pH = 6.5 were used to investigate the system Ca2+−SPB. An additional experiment has been carried out to analyze the sensitivity of the Ca2+−SPB to a competing inert salt. Details of the shrinking experiments performed with Mg2+ at the two different contents of SPB(HMEM) are summarized in Figure S5a. The resulting L versus [Ca2+] curves are represented in Figure S5b, and the corresponding concentrations [Ca2+]shr where shrinking is completed are shown in Figure 3b. A representative example given in Figure 5 for the system Ca2+−SPB indicates similar behavior as in the case of Ag+ ions. However, two differences to the Ag+ system stand out: (i) completion of the shrinking occurring at an overall concentration of positive charges of [+] = 0.01 mM requires 4 times as many charges for bivalent Ca2+ cations than for monovalent Ag+ cations, indicating a significantly weaker interaction of the Ca2+ cations with COO− residues; (ii) in line with this, a decrease of the shrinking limit [Ca2+]shr by 0.5 mM when switching to the salt-free solution of Ca2+−SPBs suggests a slightly larger sensitivity toward an ion exchange by

Figure 5. Thickness L of the PA layer on SPB(HMEM) in Ca2+−SPB solutions at a pH of 6.5 as a function of CaCl2 concentration in a saltfree solution and in 0.01 M NaCl. The concentration of [COO−] is in both cases 0.1 mM. The data measured in 0.01 M NaCl are a reproduction of the respective experimental series shown in Figure S5b. The black horizontal line corresponds to the plateau value Lpl = 25 nm of the maximally shrunken PA layers. The shaded area indicates the standard deviation of Lpl which is ±5 nm. Arrows denote estimated [Ca2+]shr and [Ca2+]c. The blue and black bars on the x-axis indicate the experimental uncertainties of [Ca2+]shr. Both samples coagulate at [Ca2+]c = 1.5 mM.

the Na+ cations than Ag+ cations do, with the latter being essentially insensitive toward such an exchange pressure. An insight into the impact of all three cations on the shell thickness of SPB(HMEM) is given in Figure 6 where shrinking experiments with all three cations, performed under identical conditions of [COO−] = 0.15 mM, [+] = 0.01 M, and pH = 6.5, are depicted. Among the three cations, completion of the shrinking is achieved with the lowest concentration in the case

Figure 6. Thickness L of the PA layer in Mn+−SPB(HMEM) solutions at a pH of 6.5 in 0.01 M NaNO3/NaCl versus the concentration of positive charges n[Mn+], where n[Mn+] = 2[Ca2+], 2[Mg2+], and [Ag+]. The concentration of [COO−] is 0.15 mM. The dashed horizontal columns represent plateau values for the maximally shrunken PA layers Lpl: 25 nm (Ca2+), 40 nm (Ag+), and 52 nm (Mg2+) with standard deviation of Lpl which is ±5 nm. Their extensions represent the stable noncoagulated Mn+−SPB solutions. The full squares denote n[Mn+]shr, and the arrows show the coagulation limit n[Mn+]c. F

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Macromolecules of the Ag+ ions. Noteworthy, the impact of Ca2+ is stronger than of Mg2+, since full shrinking of the shell is reached already at a level of Ca2+ cations [Ca2+]shr, which is one-third of that observed with Mg2+ cations. It is instructive to consider a suggestion by Konradi and Rühe23 to classify SICs into two types of strongly interacting cations: SICs where binding causes dehydration of polyanion and cation (dh-type) and SICs which involve bridges between negative residues from the same or different polyanionic chains (br-type). In the case of Ag+, binding is strong enough to be completed already after addition of stoichiometric amounts of these cations. The impact of Ca2+ on the NaPA-shells is also significantly stronger than of Na+. Here, the particularly large extent of shell shrinking observed with Ca2+ cations deserves additional consideration. The shells of the SPB(HMEM) finally reach a thickness as low as 25 nm, which corresponds to a segment density in the shell larger than 10 g/L. This is in full accord with other findings in dilute aqueous solutions of linear NaPA coils in the presence of Ca2+: Linear NaPA chains collapse to compact spherical particles in the presence of amounts of Ca2+ cations close to the precipitation threshold,29 and upon crossing the threshold those spheres coalesce to larger droplets.37 The result of this coalescence process most likely corresponds to the separation of a dense liquid phase in aqueous solution, which also plays a pivotal role in the polymerinduced liquid-precursor (PILP) process reported by Gower et al.38 This particularly strong impact of Ag+ and Ca2+ cations on the shrinking of the spherical NaPA shells under present consideration may be reconciled with the fact that the two cations are of dh-type.23,39 Binding according to the dh-type is driven by the entropy gain accomplished by the liberation of water molecules when SICs are binding, which makes the polymer increasingly hydrophobic and hence less soluble with increasing decoration with cations. In addition to the shrinking threshold, a further threshold line appears at a considerably higher level of Ca2+, which represents the onset of coagulation. Identification of this coagulation process as well as the shell shrinking was accessible to light scattering only in extremely dilute solutions. However, the coagulation threshold could be extended into the regime of turbid solutions by means of a visual inspection with respect to flocculation of a matrix of samples with varying ratio [Ca2+]/ [COO−] as has also been done for Ag+. A detailed overview on the flocculation screening and the evaluation of the coagulation thresholds [Ca2+]c is given in Figure S6. The coagulation data together with the shrinking threshold make accessible the same type of diagram shown for Ag+ ions in Figure 4 now for the Ca2+ ions. Such a diagram does not only describe the solution behavior of the SPB with an NaPA shell in the presence of Mn+ but also makes accessible an instructive comparison of the SPB pattern with the respective behavior of linear NaPA chains. In comparing the coagulation line of Ca2+ in Figure 7 with the corresponding precipitation threshold of linear NaPA chains, a similar shift of ∼2 mM between two roughly parallel lines appears, as in the case of Ag+ in Figure 4. Also, the trend of the shrinking threshold observed at low [COO−] and expressed as [Ca2+]shr versus [COO−] lies significantly below the coagulation threshold [Ca2+]c versus [COO−] (Figure 7). Yet, the data suggest that the borderline of shrinking eventually approaches the coagulation threshold in the case of Ca2+. No such hint exists in the case of the Ag+ ions. Unfortunately, such an approach of the shrinking threshold to

Figure 7. Results from the visual observations ([COO−] > 0.25 mM) and the DLS experiments ([COO−] < 0.25 mM) on Ca2+−SPB solutions in 0.01 M NaCl at a pH of 6.5: PA shell shrinking threshold (full squares) with data from Figure 3b and Table S3 and coagulation threshold (empty squares) with data from Table S4. Dashed lines connect experimental data points and serve as a guide to the eye. The solid line indicates the precipitation threshold of Ca2+−PA solutions in 0.01 M NaCl from ref 29.

the coagulation threshold, if existing, could only be verified with small-angle X-ray or neutron scattering.

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SUMMARY Spherical polyelectrolyte brushes with a shell of grafted linear NaPA chains dissolved in dilute aqueous solution were exposed to three different cations: Ag+ as a monovalent transition metal cation and Mg2+ and Ca2+ as two representatives of bivalent alkaline earth metal cations. Addition of Ag+ cations to dilute solutions of the SPBs induces a drastic contraction of the shell already at a level of added Ag+ cations equivalent to the amount of dissolved COO− residues. No difference could be discerned between the shrinking process induced in salt-free solution and at an ionic strength of 0.01 M achieved with additional NaNO3. An extent of shrinking similar to that achieved with the Ag+ ions can be accomplished with Na+ ions only if an amount 3 orders of magnitude larger had been applied. Just as striking is the trend of the shell thickness versus the content [Mn+], which is much steeper in the case of Ag+ than in the case of Na+. This clearly demonstrates that the impact of Ag+ cations on the swollen NaPA shell of the SPBs under present consideration has to be reconciled with a purely specific nature of the interactions of the Ag+ cations with the COO− residues in the shell, leading to a quantitative neutralization via complete binding, very much in the same way as protons would do. A similar pattern has been observed by Konradi and Rühe23 with planar brushes. It is even more striking that the NaPA-based SPBs show a much simpler phase behavior in the presence of Ag+ cations than long linear NaPA chains do.27 In the case of SPBs, the shrinking threshold borders a broad regime of stable Ag+−SPB, without any aggregation of colloids. Apparently, the inherent crowding of the NaPA chains in the shell bypasses the aggregation step, which extends over a broad regime of Ag+ concentration in the phase diagram of linear NaPA chains.27 This finding is highly relevant in the light of the capability of SPBs with NaPA shells to generate and host Ag nanoparticles, leading to hybrid colloids with interesting new applications.31,40 G

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Macromolecules

(2) Wang, X.; Wu, S.; Li, L.; Zhang, R.; Zhu, Y.; Ballauff, M.; Lu, Y.; Guo, X. H. Synthesis of Spherical Polyelectrolyte Brushes by Photoemulsion Polymerization with Different Photoinitiators. Ind. Eng. Chem. Res. 2011, 50, 3564−3569. (3) Guo, X.; Ballauff, M. Spherical Polyelectrolyte Brushes: Comparison between Annealed and Quenched brushes. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2001, 64, 051406. (4) Das, B.; Guo, X.; Ballauff, M. The Osmotic Coefficient of Spherical Polyelectrolyte Brushes in Aqueous Salt-free Solution. Prog. Colloid Polym. Sci. 2006, 121, 34−38. (5) Pincus, P. Colloid Stabilization with Grafted Polyelectrolytes. Macromolecules 1991, 24, 2912−2919. (6) Ross, R. S.; Pincus, P. The Polyelectrolyte Brush: Poor Solvent. Macromolecules 1992, 25, 2177−2183. (7) Borisov, O. V.; Birshtein, T. M.; Zhulina, E. B. Collapse of Grafted Polyelectrolyte Layer. J. Phys. II 1991, 1, 521−526. (8) Zhulina, E. B.; Borisov, O. V.; Birshtein, T. M. Structure of Grafted Polyelectrolyte Layer. J. Phys. II 1992, 2, 63−74. (9) Zhulina, E. B.; Borisov, O. V. Polyelectrolytes Grafted to Curved Surfaces. Macromolecules 1996, 29, 2618−2626. (10) Zhulina, E. B.; Birshtein, T. M.; Borisov, O. V. Curved Polymer and Polyelectrolyte Brushes beyond the Daoud-Cotton Model. Eur. Phys. J. E: Soft Matter Biol. Phys. 2006, 20, 243−256. (11) Ahrens, H.; Förster, S.; Helm, C. Charged Polymer Brushes: Counterion Incorporation and Scaling Relations. Phys. Rev. Lett. 1998, 81, 4172−4175. (12) Muller, P.; Guenoun, P.; Delsanti, M.; Demé, B.; Auvray, L.; Yang, J.; Mays, J. W. Spherical polyelectrolyte block copolymer micelles: Structural change in the presence ofmonovalent salt. Eur. Phys. J. E: Soft Matter Biol. Phys. 2004, 15, 465−472. (13) Balastre, M.; Li, F.; Schorr, P.; Yang, J.; Mays, J. W.; Tirrell, M. V. A study of Polyelectrolyte Brushes Formed from Adsorption of Amphiphilic Diblock Copolymers Using the Surface Forces Apparatus. Macromolecules 2002, 35, 9480−9486. (14) Biesalski, M.; Johannsmann, D.; Rühe, J. Electrolyte-induced Collapse of a Polyelectrolyte Brush. J. Chem. Phys. 2004, 120, 8807− 8814. (15) Dingenouts, N.; Ballauff, M.; Pontoni, D.; Narayanan, T.; Goerigk, G. Counterion Distribution around a Spherical Polyelectrolyte Brush Probed by Anomalous Small-Angle X-ray Scattering. Macromolecules 2004, 37, 8152−8159. (16) Mei, Y.; Ballauff, M. Effect of Counterions on the Swelling of Spherical Polyelectrolyte Brushes. Eur. Phys. J. E: Soft Matter Biol. Phys. 2005, 16, 341−349. (17) Mei, Y.; Lauterbach, K.; Hoffmann, M.; Borisov, O.; Ballauff, M.; Jusufi, A. Collapse of Spherical Polyelectrolyte Brushes in the Presence of Multivalent Counterions. Phys. Rev. Lett. 2006, 97, 158301. (18) Mei, Y.; Hoffmann, M.; Ballauff, M.; Jusufi, A. Spherical Polyelectrolyte Brushes in the Presence of Multivalent Counterions: The Effect of Fluctuations and Correlations as Determined by Molecular Dynamics Simulations. Phys. Rev. E 2008, 77, 031805. (19) Schneider, C.; Jusufi, A.; Farina, R.; Pincus, P.; Tirrell, M.; Ballauff, M. Stability behavior of anionic spherical polyelectrolyte brushes in the presence of La(III) counterions. Phys. Rev. E 2010, 82, 011401−11. (20) Farina, R.; Laugel, N.; Pincus, P.; Tirrell, M. Brushes of strong polyelectrolytes in mixed mono- and tri-valent ionic media at fixed total ionic strengths. Soft Matter 2013, 9, 10458−10472. (21) Farina, R.; Laugel, N.; Yu, J.; Tirrell, M. Reversible Adhesion with Polyelectrolyte Brushes Tailored via the Uptake and Release of Trivalent Lanthanum Ions. J. Phys. Chem. C 2015, 119, 14805−14814. (22) Brettmann, B. K.; Laugel, N.; Hoffmann, N.; Pincus, P.; Tirrell, M. Bridging Contributions to Polyelectrolyte Brush Collapse in Multivalent Salt Solutions. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 284−291. (23) Konradi, R.; Rühe, J. Interaction of Poly(Methacrylic acid) Brushes with Metal Ions: Swelling Properties. Macromolecules 2005, 38, 4345−4354.

The impact of the two alkaline earth cations on the solution behavior of the SPBs with a NaPA shell is slightly weaker than that of the Ag+ cations. Whereas the amount of cationic charges from Ca2+ required to complete shrinking of the NaPA shells of the brushes is 4 times larger than the required amount of Ag+, this factor increases to 16 if the same comparison is done with Mg2+. Since the impact of Ca2+ on the shrinking behavior of COO−-based SPBs does not overlay with the one of the homologous Mg2+, the interactions exerted by Ca2+ cations are more specific in nature than those exerted by Mg2+ cations. Similar qualitative differences between Ca2+ and Mg2+ have already been pointed out by Ikegami and Imai.25 They have varied the pH of diluted poly(acrylic acid) in the presence of these cations and observed that Ca2+ cations obey trends according to eq 1 already at a lower degree of neutralization of the poly(acrylic acid) chains than Mg2+ cations do.25 However, the fact that the Mg2+-induced shrinking sets in at a cation concentration more than 2 orders of magnitude lower than in the case of Na+ and that Mg2+ cations exhibit a precipitation threshold with NaPa according to eq 1 points to specific interactions even in the case of Mg2+. Noteworthy, Tirell et al.41 just published a study in which they have exposed NaPSS based SPBs to Y3+, Ba2+, Ca2+, Mg2+, and Na+ and observed the same trend in terms of the impact of shell shrinking induced with Ca2+, Mg2+, and Na+ as obtained in the present work for NaPAbased SPBs. However, care has to be taken in such a direct comparison since unlike the sulfonate residues in the study by Tirell et al.41 the present work uses COO− groups as negative charges, and specifity of interactions of cations with ionic residues does depend on both components.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01286. Effect of the presence of an additional salt (NaNO3) on Ag+-induced shrinking of SPB(HMEM), effect of sample history on Ag+-induced shrinking of SBB(HMEM), Ag+induced shrinking of SBPs for three different types of SPBs at variable SBP content, Ag+- and Ca2+-induced coagulation of SBP(HMEM) at variable SBP contents, Mg2+- and Ca2+-induced shrinking of SBP(HMEM) at variable SBP contents (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(K.H.) E-mail [email protected]; Tel (+49) 5251 602125; Fax (+49) 5251 604208. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Deutsche Forschungsgemeinschaft (DFG) within the Research Training Group “Micro- and Nanostructures in Optoelectronics and Photonics” (GRK 1464).

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REFERENCES

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