Understanding the Interaction between Trivalent ... - ACS Publications

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Understanding the Interaction between Trivalent Lanthanide Ions and Stereoregular Polymethacrylates through Luminescence, Binding Isotherms, NMR, and Interaction with Cetylpyridinium Chloride Ksenija Kogej,*,† Sofia M. Fonseca,‡ José Rovisco,‡ M. Emília Azenha,‡ M. Luísa Ramos,‡,§ J. Sérgio Seixas de Melo,‡ and Hugh D. Burrows*,‡ †

Department of Chemistry and Biochemistry, Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, P.O. Box 537, SI-1000 Ljubljana, Slovenia ‡ Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal § Centre for Neuroscience and Cell Biology (CNC), University of Coimbra, 3004-517 Coimbra, Portugal S Supporting Information *

ABSTRACT: Complexation of isotactic, syndiotactic, and atactic poly(methacrylic acid), PMA, with trivalent lanthanide ions has been studied in water at a degree of neutralization 0.5. Metal ion binding is shown by quenching of cerium(III) fluorescence, enhancement of Tb(III) luminescence, and lanthanide-induced line broadening in the PMA 1H NMR spectra. Comparison with lanthanide−acetate complexation suggests carboxylate binds in a bidentate fashion, while Ce(III) luminescence quenching suggests an ≈3:1 carboxylate:metal ion stoichiometry, corresponding to charge neutralization. The presence of both free and bound Ce(III) cations in PMA solutions is confirmed from luminescence decays. Studies of Tb3+ luminescence lifetime in H2O and D2O solutions show complexation is accompanied by loss of 5−6 water molecules, indicating that each bidentate carboxylate replaces two coordinated water molecules. The behavior depends on pH and polyelectrolyte stereoregularity, and stronger binding is observed with isotactic polyelectrolyte. Binding of cetylpyridinium chloride, CPC, in these systems is studied by luminescence, NMR, and potentiometry. NMR and Tb3+ luminescence lifetime studies show the strongest binding with the isotactic polymer. Binding of surfactant to poly(methacrylate) in the presence of lanthanides is noncooperative, i.e., it binds to the free sites; binding isotherms in the presence of lanthanides are shifted to higher free surfactant concentrations, compared with sodium ions, have lower slopes and show a clear two-step binding mechanism. While CPC readily replaces the Na+ ions of poly(methacrylate) and binds very strongly (low critical association concentrations), exchange is much more difficult with the strongly bound trivalent lanthanide ions. Effects of tacticity are seen, with surfactant interacting most strongly with isotactic chains in the initial stages of binding, while in the final stages of binding the interaction is strongest with atactic poly(methacrylate).

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INTRODUCTION Although the interaction between metal cations and anionic polyelectrolytes has been studied for over 60 years,1−4 there are still major challenges in understanding the types of interactions involved and the driving forces for counterion binding.5 This is of both considerable theoretical and practical interest because of its role in many systems and processes of chemical and biological importance. Although emphasis is often placed on the role of electrostatic interactions as a driving force in the association process, 6 other factors, such as counterion dehydration7−10 and the entropy gain in replacing a number of monovalent metal ions by one high valent ion,11 are also involved. In addition, although many models of binding consider a rather diffuse interaction, with the counterion having significant mobility within the ion atmosphere of the © 2013 American Chemical Society

polyelectrolyte, there is now strong evidence that counterions, particularly higher valent ones, may bind to specific sites of the polyions through formation of inner-sphere complexes.10,12,13 The interaction of anionic polyelectrolytes with higher valent metal ions is of relevance in areas as diverse as rheology control,14 scale inhibitors,7 metal ion sequestering,15 delayedrelease drug formulations,16 and DNA compaction.17 The trivalent lanthanides form a series of metal cations with very similar chemical behavior but distinct spectroscopic and magnetic properties.18 They can also model the behavior of biologically relevant, but spectroscopically silent, cations, such Received: September 18, 2013 Revised: October 27, 2013 Published: October 31, 2013 14429

dx.doi.org/10.1021/la4036024 | Langmuir 2013, 29, 14429−14437

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as Ca(II) and Zn(II),18 which make them good probes for metal ion interactions with polyelectrolytes in solution.19,20 The electronic transitions from the majority of the lanthanide ions involve the spin and Laporte forbidden f → f transitions.18 Although their study is complicated by very low molar absorption coefficients, they show fine structure associated with the various J levels arising from spin−orbit coupling, often providing a sensitive probe of local environment.18 In addition, vibronic coupling with X−H stretching modes (X = O, N, C) provides an efficient nonradiative pathway to excited state deactivation,21,22 leading to a quenching of the lanthanide luminescence. This has been used to develop a reliable method for measuring the number of water molecules coordinated to the lanthanide ions by comparison of the luminescence lifetimes of ions such as Tb(III) in H2O and D2O solutions.21,22 In contrast, the lowest energy electronic transition in cerium(III) involves the fully allowed transition between the 4f and 5d orbitals. This leads to a broad absorption band with a reasonable molar absorption coefficient23 and an emission which is highly sensitive to coordination. In particular, the emission is partially or fully quenched upon complexation with carboxylates.24−26 The study of effects of polyelectrolytes on both terbium(III) and cerium(III) luminescence has been shown to provide detailed information on lanthanide ion binding in aqueous solution.20,27 Information on lanthanide binding can also be obtained from the line broadening and changes in chemical shift in the 1H NMR spectrum induced by paramagnetic ions.28,29 In this paper we report a study of the application of these techniques to obtain information at the molecular level on the binding of trivalent lanthanide ions to poly(methacrylate)s in aqueous solutions. Poly(methacrylic acid) (PMA) exists as different stereoisomers, the highly stereoregular syndiotactic and isotactic forms, s- and i-PMA, and the randomly substituted atactic form, a-PMA. Solution properties,30−39 intermolecular association,37,40,41 and binding of species, such as oppositely charged surfactants,41 may all be affected by the polymer tacticity. For example, while s-PMA and a-PMA will dissolve in water, and show similar solution behavior,31,32 in part because a-PMA is largely syndiotactic, i-PMA is insoluble in water below a certain critical degree of neutralization of carboxylic acid groups, αcrit,31,32,38 and behaves as a weaker acid than the other forms over the whole range of the carboxylic acid dissociation.31,32,34,38 In addition, for the same degree of neutralization in aqueous 0.1 M NaCl solution, studies of binding isotherms show that at the onset of cooperative binding the association of cetylpyridinium chloride (CPC) is stronger with i-PMA than with a-PMA.41 However, more surfactant is bound by a-PMA in the region where the polyelectrolyte is saturated with CP+ ions. This has been interpreted in terms of the greater hydrophobicity and possibly higher charge density of i-PMA competing with the greater flexibility of a-PMA.41 Because of this importance of the stereoregularity of the polymer upon binding, we complement the luminescence and NMR spectroscopy measurements with the determination of the binding isotherms of PMA samples of different tacticity with trivalent lanthanum(III) ions. We also report the interaction of CPC with these systems. We believe that these studies provide an insight into the binding of cations to oppositely charge polyelectrolytes at both the microscopic and macroscopic levels.

Article

EXPERIMENTAL SECTION

Materials and Methods. The surfactant N-cetylpyridinium chloride (CPC, Kemika, Zagreb, Croatia) was purified as reported earlier.42 Atactic, isotactic, and syndiotactic poly(methacrylic acid), aPMA, i-PMA, and s-PMA, respectively, were prepared and characterized according to previous procedures.39,41 i-PMA and sPMA were prepared by hydrolysis of their parent poly(methyl methacrylates). The stereoregular compositions in triad content of all samples are reported in Table 1. Two samples of the isotactic form of

Table 1. Molar Mass Values and Stereoregular Compositions (in Atactic, Syndiotactic, and Isotactic Triad Content) of PMA Isomers isomer

M (g mol−1)

% atactic triads

% syndiotactic triads

% isotactic triads

a-PMA i-PMAa i-PMAb s-PMA

131 000 593 000 138 000 37 500

39 ∼4 2

49 ∼4 4 ∼81

12 92 94 ∼19

a

A commercial sample supplied by Aldrich as poly(methyl methacrylate) and hydrolyzed (employed in potentiometric and fluorescence measurements). bSample synthesized in the Laboratory of Polymer Chemistry, University of Leuven (employed in fluorescence measurements). PMA were used in fluorescence measurements, whereas only one was employed in binding studies. For further details on sample preparation see refs 40 and 41. All PMAs were used at a degree of neutralization of carboxyl groups, αN, equal to αN = 0.5, which was achieved by the addition of 0.1 M NaOH to stock solutions of acid forms at αN = 0 in water. LaCl3·7H2O (puris, pa) was from Fluka. Cerium(III) and terbium(III) perchlorates (Aldrich) were of the purest grade available and were used as received. Deuterium oxide (Aldrich, 99.9%) was used in NMR and Tb(III) luminescence lifetime measurements. Solutions were prepared in either triply distilled water (Ljubljana) or Milli-Q Millipore water (Coimbra). Binding isotherms for CPC binding to a-, i-, and s-PMA were determined by the potentiometric titration method using a surfactantselective membrane electrode as reported previously.41 All measurements were performed in 0.1 mM LaCl3 and αN = 0.5 at 25 °C and at a PMA concentration equal to 0.5 mM (given in moles of monomer groups per volume). Absorption and luminescence spectra were recorded on Shimadzu UV-2100 and Jobin-Ivon SPEX Fluorolog 3-22 spectrometers, respectively. For Tb(III) lifetime measurements, an ISA/Jobin-Ivon Spex Fluorolog 1934D3 accessory with a 150 W pulsed xenon lamp was used. Cerium(III) fluorescence decays were measured using a home-built TCSPC apparatus with an IBH NanoLED (281 nm) excitation source, Jobin-Ivon excitation and emission monochromators, two photomultiplier tubes (start: Hamamatsu IP21; stop: Philips XP2020Q photomultiplier), a Canberra time-to-amplitude converter (TAC), an analog-to-digital converter (ADC), two discriminators, and a multichannel analyzer (MCA).43 After pulse excitation, the TAC generates an output pulse whose amplitude is directly proportional to the time between the start and stop pulses. The TAC output is first converted into a digital signal, and then this signal is sent to the MCA, which determines the memory channel corresponding to the delay time between the start and stop pulses. The probability of a stop pulse to occur at a given time is directly proportional to the probability of the emission of a photon. As a consequence, when many flashes are produced and analyzed, the distribution of the generated events in the MCA channels will generate a histogram, which is the experimental decay curve.44 The decays were obtained with alternate measurements (1000 counts per cycle, cpc) of the pulse profile (obtained with emission identical to the excitation wavelength) and the sample emission wavelength until 5 × 104 counts at the maximum were reached. The fluorescence decays were analyzed using the modulation 14430


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functions method of Striker45 with automatic correction for the photomultiplier “wavelength shift”. 1H NMR spectra were obtained in D2O solutions on a Varian UNITY-500 NMR spectrometer, as described in detail elsewhere.46,47 Chemical shifts (ppm) are given relative to tetramethylsilane using tert-butyl alcohol (1.30 ppm) as external reference.

Table 2. Number of Bound Carboxylate Groups per Polymer from Fluorescence Titrations with Cerium(III)

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RESULTS AND DISCUSSION Cerium(III) Fluorescence Studies. Absorption and fluorescence spectra were run on aqueous solutions of cerium(III) perchlorate alone and in the presence of various concentrations of PMA of different tacticity. These were adjusted to the appropriate pH to give a degree of neutralization of carboxyl groups, αN, equal to αN = 0.5. The absorption and fluorescence spectra in the absence of poly(methacrylate) are identical to those presented previously23,26 and are assigned to the hydrated species [Ce(H2O)8]3+ and [Ce(H2O)9]3+. As has previously been reported for interaction of cerium(III) in the presence of carboxylates,24−26,48 on addition of PMA, quenching of the luminescence was observed. However, there was no significant change in either the emission maximum or the shape of the spectrum. Similar quenching behavior was observed with all four PMA samples, and representative data for i-PMA are shown in Figure 1.

polymer

VPMA (μL)

[PMA] (M)

αN

107nCe (mol)

N, no. of bound carboxylates

i-PMAa i-PMAb s-PMA a-PMA

75 115 100 100

0.0222 0.0154 0.0180 0.0181

0.48 0.48 0.48 0.48

3 3 3 3

2.7 2.8 2.9 2.9

a

A commercial sample supplied by Aldrich as poly(methyl methacrylate) and hydrolyzed (employed in potentiometric and fluorescence measurements). bSample synthesized in the Laboratory of Polymer Chemistry, University of Leuven (employed in fluorescence measurements).

assign this to Ce(III) bound to the carboxylate groups of PMA. While our results do not allow us to identify the mode of binding, the fact that effects on luminescence spectra and lifetimes are identical with that for cerium(III) in the presence of acetate suggests similar behavior. In that case molecular mechanics, NMR,26 and FTIR49 studies suggest bidentate binding of the carboxylate ligand to the lanthanide ion. The two cerium(III) luminescence lifetimes seen upon addition of the polyelectrolyte are independent of PMA concentrations (Figure 2b), indicating coexistence of free and bound cations. However, the amplitude (which mirrors the concentration of the species at time zero, i.e., in the ground state) of the bound Ce(III) component increases and that of free Ce(III) decreases with polymer concentration (Figure 2c). With s-PMA, more complex behavior was observed at the highest carboxylate:Ce(III) ratio (4:1), where the decay could only be fitted to three exponentials; it is possible that this resulted from some precipitation. NMR spectroscopy was used to obtain further information on the Ce(III) binding. 1H NMR spectra were run of 5 mM solutions of i-PMA, s-PMA, and a-PMA in D2O alone and in the presence of Ce3+. The spectra of the polymers showed the expected signals in the region 1.0−1.3 ppm due to the methyl group and 1.8−2.1 ppm assigned to the methylene protons, in agreement with literature data, where they have been assigned to the various stereoregular sequences possible.50−52 Upon addition of Ce(III), no significant shifts in these peaks were observed in any of the polymers, but there was a slight broadening of the signals in agreement with the binding of PMA to the paramagnetic (4f1) metal ion. Typical results for aPMA are given as Supporting Information (Figure S1). In our previous 1H NMR study on the interaction of acetate ion with the paramagnetic Ce3+,26 both broadening of the bands and a shift of the methyl signal were seen on complexation. However, in that case the methyl group is adjacent to the coordinating carboxylate group, whereas with PMA the methyl group and coordinating carboxylate are separated by one carbon atom. Terbium(III) Luminescence Studies. The interaction between lanthanides and polymethacrylates was also studied using Tb(III). The 1H NMR spectrum of PMA was broadened and some shifts in the peaks were observed due to binding to the strongly paramagnetic (4f8) Tb3+ (Supporting Information, Figure S1). The interaction was studied using Tb(III) luminescence, and an increase in intensity was seen upon addition of PMA, with similar behavior observed in all cases. Results for i-PMA are given in Figure 3. The emission arises from a transition within the 4f orbitals, which is markedly affected by the presence of coordinated water molecules due to vibronic coupling with O−H

Figure 1. Luminescence spectra of Ce(III) in the absence (black line) and in the presence of various concentrations of i-PMA (3.69 × 10−5− 6.11 × 10−4 M). Inset: Stern−Volmer plot for the luminescence quenching of Ce(III) by i-PMA.

Stern−Volmer plots were found to be nonlinear and in all cases followed a titration curve. Results for i-PMA are shown as an inset in Figure 1. Analysis of these titration curves allowed the estimation of the number of bound carboxyl groups per cerium(III) ion from the molar ratio of PMA repeat units to Ce(III) at the plateau region. Results are shown in Table 2 and suggest a value approximately three, corresponding to charge neutralization. Time-resolved luminescence measurements on aqueous solutions of Ce(III) in the presence of a-PMA showed two fluorescence decay components. Similar behavior was observed with i-PMA and s-PMA, and typical data are given for i-PMA in Figure 2a. The longest component has a lifetime around 45 ns, identical to that found with this metal ion in aqueous solution and attributed to the hydrated cerium(III) ion.23,26 The other component has a shorter lifetime of 1.7 ns, very close to that found with the cerium(III)−acetate complex in water.26 We 14431


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Figure 3. Luminescence spectra of 1 mM Tb(III) solution in water (black line) and D2O (red line) and 0.2 mM Tb(III) solution with 1 mM i-PMA in water (blue line). Inset: luminescence decays of 0.2 mM Tb(III) solutions with 1 mM i-PMA in water (a) and D2O (b). The luminescence lifetimes in water and D2O are respectively 0.846 and 2.82 ms.

the Tb(III) excited states in these systems involves energy transfer to vibrational levels of bound water molecules. Because of differences in energy of the O−H and O−D stretch vibrations, the efficiency of this process is different for H2O and D2O solutions and forms the basis of a method for determining the number of coordinated water molecules by measuring the luminescence lifetimes in these two solvents.21,22 A typical luminescence decay is shown as an inset in Figure 3. We have determined the number of bound water molecules by determining the luminescence lifetimes in these two solvents and using the equations from ref 22. These are presented in Table 3. Table 3. Number of Coordinated Water Molecules (nH2O) of Tb(III) in Aqueous PMA Solutions (αN = 0.5, cp = 1 mM) without and with Added CPC at a Surfactant (S) to Polyion (P) Charge Ratio S/P = 0.4a nH2O

a

Figure 2. (a) Typical fluorescence decay for a 4:1 ratio of an iPMA:Ce(III) solution, obtained with λexc = 281 nm; the fluorescence decay times τi and pre-exponential factors (Ai,j), together with the weighted residuals, autocorrelation function (A.C.) and χ2 (χ2) values are also presented for a better judgment of the quality of the fit. (b) Fluorescence lifetimes (τ1: squares; τ2: circles) as a function of iPMA:Ce(III) ratio. (c) Pre-exponential factors (a1: squares; a2: circles) as a function of i-PMA:Ce(III) ratio. Cerium(III) concentration 0.2 mM, decays observed at 365 nm.

isomer

without CPC

with CPC

i-PMA s-PMA a-PMA

3.3 3.6 3.9

3.2 3.6 3.4

This S/P ratio is highlighted on the isotherms by ★; see Figure 6.

The results show that 5−6 water molecules are lost on Tb(III) binding to PMA. This is similar to what is observed when this metal ion binds to poly(acrylate),19,20 poly(vinyl sulfate),27 single-strand DNA8 or RNA54 and is consistent with each bidentate carboxylate group replacing two water molecules on complexation. The experimental uncertainty in the number of bound water molecules (nH2O) determined by this method has been discussed elsewhere22 and is estimated to be approximately ±0.5. The number of bound water molecules appears to be slightly less with i-PMA than with s-PMA or aPMA, although the difference is close to the experimental uncertainty. This is in qualitative agreement with the report by Luján-Upton and Okamoto55 on terbium(III) binding to isotactic and syndiotactic PMA at a different degree of ionization (αN = 0.75). In addition, it is also in agreement

modes,21,22 which contrasts with the case of Ce(III), where the high energy 5d → 4f does not have such a nonradiative pathway and is relatively unaffected by coordinated water. The increase in Tb(III) emission intensity due to loss of bound water increases the luminescence lifetime. Lanthanide ions normally have 8−9 coordinated water molecules.53 Nonradiative decay of 14432


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with stronger binding of protons, as shown by the weaker acidity of i-PMA,31 and with stronger binding of copper(II) ion toward i-PMA compared with s-PMA.56 We note, however, that Mg2+ appears to bind more strongly to the syndiotactic isomer, which has been explained in terms of the binding being more ionic.56 In that case, it is possible that the cation is not specifically bound at a carboxylate site. In our results at αN = 0.5, the number of bound water molecules when Tb3+ binds to PMA is greater than that reported by Luján-Upton and Okamoto at a higher degree of ionization.55 To test the dependence of the number of bound water molecules on αN, studies have also been made of the effect of pH using i-PMA (Figure 4). The pH was corrected by the equation pH = pD − 0.87 for D2O solutions.

Figure 5. Structure and 1H NMR spectra and peak assignment for cetylpyridinium chloride in D2O. The effect of binding to i-PMA, aPMA, and s-PMA in the pyridinium region is shown as an inset: (a) CPC, 0.7 mmol dm−3; (b) CPC/a-PMA, 5:2.5 mmol dm−3; (c) CPC/ s-PMA, 5:2.5 mmol dm−3; (d) CPC/i-PMA, 5:2.5 mmol dm−3.

potentiometric and fluorescence studies, which show that CPC binds more strongly to i-PMA than to a-PMA.41 Studies have been made of the effect of cetylpyridinium chloride (CPC) on the binding of lanthanides with the PMA systems by looking at the effect on the number of coordinated water molecules determined from the Tb(III) luminescence lifetimes (Table 3) and by studying the binding isotherms. With the luminescence data there are no significant effects on the number of coordinated water molecules, with the possible exception of a-PMA. This suggests noncooperative binding of CPC and lanthanides to the PMA. Figure 6 shows binding isotherms (i.e., plots of the degree of binding β vs free surfactant concentration, cSf, in solution; β is Figure 4. Number of coordinated water molecules (nH2O) of Tb(III) in aqueous i-PMA solutions as a function of pH.

Although caution is necessary in interpreting the effect of pH in these results, since there are difficulties with its definition in the two solvents, and hydrolysis of Tb(III) will occur at high pH (first hydrolysis constant pK1 values around 8.0 at 25 °C),57 there is a definite trend for a decrease in the number of coordinated waters with increasing pH, which is likely to be associated with increased ionization of the polymer and changes in the lanthanide ion. This fully explains the observed differences between the number of bound water molecules determined in this study at αN = 0.5 and in previous work55 at αN = 0.75. Studies of CPC Binding. We have extended these studies to the binding of cetylpyridinium chloride (CPC) to poly(methacrylic acid). Previous studies have shown that the stereoregularity of the polymer chain has a significant effect on this interaction.41 The 1H NMR spectrum was run of CPC in D2O alone (Figure 5), and in the presence of i-PMA, s-PMA, and a-PMA. The spectrum of CPC is in good agreement with literature reports.58 Upon addition of the polymer, new bands were observed in the aliphatic region, associated with the methyl and methylene groups of PMA. However, the biggest changes were seen in the region of the pyridinium ring (8.1− 9.1 ppm). With i-PMA and s-PMA, these signals were lost completely, suggesting strong binding leading to reduced mobility. In contrast, while these signals were broadened with a-PMA (inset, Figure 5), they were still observed, albeit with reduced intensity. This is in complete agreement with

Figure 6. Binding isotherms for CPC binding by a-, i-, and s-PMA with αN = 0.5 in 0.1 mM LaCl3 and 10 mM NaCl at 25 °C. The encircled points indicate the onset of turbidity (○, △) and the formation of flakes (▷; see text), and the symbol ★ designates the solutions used in fluorescence measurements (see text). The concentrations of negative (c−, originating from COO− groups) and positive charges (c+, originating from La3+ ions) and the total PMA concentration (cPMA) are also designated. The arrows point to the associated y-axis.

expressed as the number of bound surfactant ions per number of ionized carboxyl groups on PMA chains) for CPC binding to a-, i-, and s-PMA with cPMA = 0.5 mM and αN = 0.5 in 0.1 mM LaCl3. For comparison, previous results for the same degree of neutralization of a- and i-PMA in 10 mM NaCl solutions are included.41 Binding isotherms in 0.1 mM LaCl3 are shifted to higher free surfactant concentrations in comparison with those in 10 mM NaCl. This results in higher critical association concentration, CAC, values in LaCl3 solutions. This may be 14433


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associated with a change in PMA conformation on binding to the trivalent lanthanides. The estimated CAC values are reported in Table 4. Furthermore, the slope of binding Table 4. CAC and βsat Values for CPC Binding by a-, i-, and s-PMA with αN = 0.5 in Aqueous 0.1 mM LaCl3 and 10 mM NaCl (Data from Ref 42) at 25 °C CAC × 106/mol L−1

βsat,1 (βsat,2)

βsat

isomer

0.1 mM LaCl3

10 mM NaCl

0.1 mM LaCl3

10 mM NaCl

i-PMA s-PMA a-PMA

1.4 56 7.1

0.8

∼0.10 (∼0.30) ∼0.15 (∼0.24) ∼0.05 (∼0.42)

0.74

4.4

0.93

Figure 7. Binding isotherms for CPC binding by a-, i-, and s-PMA with αN = 0.5 in 0.1 mM LaCl3 and 10 mM NaCl at 25 °C plotted as a function of the ratio cSf/CAC. Symbols are the same as in Figure 6.

isotherms is lower in the presence of LaCl3 and the isotherms exhibit a clear two-step binding mechanism (for a discussion of the two-step isotherms in 10 mM NaCl; see ref 41). The saturation in the first step is achieved at βsat,1 values below 0.16 and the second one at βsat,2 values between 0.20 and 0.42, depending on the isomer form of PMA (see values in Table 4). Clearly, the saturation degree of binding in the presence of 10 mM NaCl is considerably higher. These results show that CPC binding by PMAs is weaker in the presence of LaCl3, although the ionic strength of 0.1 mM LaCl3 is lower than that of 10 mM NaCl. The binding of an oppositely charged surfactant ion to the polyion usually induces precipitation of the polyelectrolyte/ surfactant complex due to charge neutralization when approaching the 1:1 charge ratio. In Figure 6, the encircled points (○, △) on the isotherms in LaCl3 solutions indicate the onset of precipitation (i.e., the appearance of a turbid milky solution) in the a- and i-PMA case. In the case of s-PMA no precipitation was observed. Upon increasing the CPC concentration above this point, mixed i-PMA/CPC/LaCl3 solutions remained milky, whereas in a-PMA/CPC/LaCl3 ones flakes of the precipitated complex formed in an otherwise clear solution (cf. the second encircled (▷) point on the aPMA/CPC/LaCl3 isotherm). These observations are parallel with the binding sequence indicated by βsat,2 values: in the final stage of binding the interaction of CPC is the strongest with aPMA and the weakest with s-PMA. In the initial stage of binding, however, CPC interacts most strongly with i-PMA and least strongly with s-PMA (see CAC values in Table 4). The strong initial interaction of CP+ with the isotactic chain was previously attributed to a higher local charge density of the helical i-PMA chain conformation.41 For an easier mutual comparison, the isotherms are also plotted on a relative concentration axis, i.e., as a function of the ratio cSf/CAC (Figure 7). We can see that the initial slope (cf. the first binding step below βsat,1 in 0.1 mM LaCl3) depends strongly on the ionic strength of the medium and on the PMA tacticity: it is higher in 10 mM NaCl than in 0.1 mM LaCl3 and increases in the direction i-PMA < a-PMA < s-PMA. The slope of the isotherm is a measure of the degree of cooperativity of interaction in a certain surfactant−polyelectrolyte pair. The results in 0.1 mM LaCl3 show that the cooperativity in this step is the highest with s-PMA, in contrast to the previous result that the interaction with s-PMA is the weakest, both at the onset of binding (high CAC values) and in the second binding step (lowest βsat,2 values). These observations can be explained as follows. Trivalent La3+ ions are much more strongly bound by the PMA chain than the monovalent sodium cations. This can be justified

thermodynamically12 and is likely to be entropy driven due both to counterion exchange of three monovalent cations by one trivalent one17 and to changes in hydration.10 This will lead to a considerably lower effective charge density of the polyion,59 which governs the binding in the initial stage immediately above the CAC. When CP+ is gradually added to the polyelectrolyte, it easily replaces Na+ and binds very strongly (low CAC values) with a high degree of cooperativity, whereas this exchange is much more difficult with the strongly bound trivalent La3+ ions (higher CAC values). Low slopes of binding isotherms, supported by studies of the number of coordinated water molecules using Tb(III) luminescence, suggest that the binding of CP+ is more or less noncooperative in the presence of La3+ ions: surfactant ions bind to the free binding sites on the polyion without replacing La3+. Since La3+ ions are obviously the least strongly bound by s-PMA, this leads to a comparatively higher effective charge density of the syndiotactic polyion and results in the highest βsat,1 values and degree of cooperativity with s-PMA in the first stage. Thus, intermediate saturation is achieved when around 15 (s-PMA), 10 (i-PMA), and 5% (a-PMA) of COO− groups are occupied by CP+. In the second binding step, CP+ aggregates grow; they start replacing La3+ ions and may even act as “cross-links” for the association between PMA chains. If the interaction is stronger (as with a-PMA), the aggregate is fairly hydrophobic due to more effective charge neutralization and will eventually precipitate. Stronger interaction between CP+ and a-PMA in the saturation binding region has been attributed to a more flexible atactic chain.2,3 If the interaction is weaker (like with sPMA), the interchain association may be similar to some physical gelation process (such as in carrageenans). One could suppose that the aggregate between surfactant and s-PMA retains some residual charge, which is the basis for its solubility in water. The regularly alternating syndiotactic placement of carboxyl groups may be the basis for this. i-PMA, which is known to have a locally helical conformation in solution,35 demonstrates an intermediate behavior with respect to solubility of the complex with surfactant and to βsat,2 values. Note, however, that the slope of isotherms in the second binding step is similar for all three cases, which may indicate a similar mechanism of interaction. The hypothesis on noncooperative CP+ binding in the presence of La3+ ions is confirmed by fluorescence results on the number of hydration water molecules retained by Tb3+ ions 14434


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importance of water loss in driving counterion binding to polyelectrolytes.7,10 A pH dependence is observed in the number of coordinated water molecules lost when trivalent lanthanide ions bind to PMA, possibly due to acid−base effects on both lanthanide ion and PMA. The demonstration of carboxylate displacing coordinated water molecules on polyelectrolyte binding to the metal ion may have broader implications for both materials and biological research. These include the effects of counterion binding on nanostructuring, such as the formation of biological coronas on nanoparticles for biomedical applications.62 The binding of CPC to PMA in the presence of trivalent lanthanide ions was studied and was shown to be noncooperative and follow a two-step binding mechanism. When CPC is added to the PMA, it readily replaces Na+ ions and binds very strongly, with the strongest binding seen by NMR to be with i-PMA, in agreement with previous potentiometric studies using a surfactant-sensitive electrode.41 However, exchange of surfactant is much more difficult with the strongly bound trivalent lanthanide ions, shifting binding isotherms to higher free surfactant concentrations and increasing critical association concentrations. At high surfactant concentrations, surfactant aggregates start to grow, and marked effects of tacticity of the polymer are observed: with a-PMA precipitation occurs, while with s-PMA, where interaction is weaker, the interaction may be similar to gelation. i-PMA shows intermediate behavior.

in both pure PMA and PMA/CPC mixed solutions. The charge ratio between CP+ cations and negatively charged COO− groups used in these measurements lies in the region of the second binding step, just below the final saturation of the chain (i-PMA and a-PMA: see the points on the isotherms designated by the stars) or above it (s-PMA: this condition is well above the saturation limit at β ≈ 0.8 and is not indicated in this case). The free trivalent lanthanide ions, such as Tb3+, are normally coordinated by approximately 9 water molecules. However, the above results all show that La3+, Ce3+, and Tb3+ are all strongly bound by PMAs, independent of their stereoregularity. This is clearly demonstrated by an extensive loss of water molecules from the hydration sheet of the lanthanide ion and confirmed by luminescence studies on carboxylate binding with Ce3+. Upon binding, Tb3+ ions lose between 5 and 6 hydration water molecules (see the data on nH2O) with a simultaneous increase in fluorescence intensity. The added CP+ cations do not displace the bound Tb3+ (or La3+) ions. Although the difference is within the estimated experimental uncertainty, the data on nH2O suggest that binding of Tb3+ may be slightly stronger if CP+ is present: for example, in the presence of CP+, nH2O appears to decreases from 3.9 to 3.4 for a-PMA and from 3.3 to 3.2 for i-PMA, respectively, whereas it remains the same for sPMA (see the data in Table 3). At the same time, the fluorescence intensity decreases in comparison with PMA solutions in the absence of CPC.

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CONCLUSIONS A detailed study has been carried out of the binding of trivalent lanthanide ions to isotactic, syndiotactic, and atactic poly(methacrylate) in aqueous solutions under conditions where the polyelectrolyte is half neutralized. Steady state and timeresolved luminescence studies using cerium(III) show complexation and indicate that around three carboxylate groups bind to each metal ion, corresponding to charge neutralization. It is suggested, based on the behavior with cerium(III) acetate in aqueous solution,26 that this involves bidentate coordination of metal ion by each carboxylate group. Binding of trivalent lanthanides by three carboxylate groups is expected to lead to marked conformational changes in the polymer. This is supported by previous studies with methyl methacrylate− methacrylic acid copolymers labeled with the fluorescent 9vinylanthracene group, 60 where a marked increase in fluorescence intensity is seen when Tb(III) binds because the anthracene moiety is going to a more hydrophobic environment as the polyelectrolyte adopts a more compact conformation. Polymer compaction is also seen by an abrupt decrease in the viscosity of solutions when Tb(III) is added to aqueous solutions of polyacrylate.19 Loss of coordinated water from lanthanide ions on binding to PMA is seen by an increase in Tb(III) luminescence intensity and deuterium isotope effects in luminescence lifetimes. These show 5−6 water molecules are lost on binding; i.e., each carboxylate group displaces roughly two water molecules. This is consistent with water loss being one of the driving forces for interaction. Complexation depends on the stereoregularity of the polymer, and stronger binding is suggested with i-PMA than with s-PMA (or a-PMA). Studies on the thermodynamics of complexation of lanthanide ions with poly(methacrylic acid) and its copolymers61 have shown that it is largely entropy driven and suggest that dehydration of metal ion and polymer plays a major role in this. Our results are fully consistent with this and are in agreement with the

ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectrum of a-PMA in D2O alone and in the presence of Ce3+ and Tb3+. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (K.K.). *E-mail [email protected] (H.D.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Slovenian Research Agency and GRICES for financial support for the collaboration between Ljubljana and Coimbra (through the Program P1-0201 and project BIPT/04-06-012). S.M.F. acknowledges a Postdoctoral grant from the Fundaçaõ para a Ciência e a Tecnologia (FCT), the Portuguese agency for scientific research, (FCT/SFRH/BPD/ 34703/2007). The Coimbra group thanks the FCT for financial support (PEst-C/QUI/UI0313/2011). M.L.R. also acknowledges financial support from FEDER through COMPETE program, the FCT (PEst-C/SAU/LA0001/2013-2014), and Rede Nacional de RMN (REDE/1517/RMN/2005), the Portuguese NMR Network, for spectrometer facilities.

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REFERENCES

(1) Alfrey, T.; Berg, P. W.; Morawetz, H. J. The counterion distribution in solutions of rod-shaped polyelectrolytes. J. Polym. Sci. 1951, 7, 543−547. (2) Fouss, R. M.; Katchalsky, A.; Lifson, S. The potential of an infinite rod-like molecule and the distribution of the counter ions. Proc. Natl. Acad. Sci. U. S. A. 1951, 37, 579−589.

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(3) Oosawa, F. A simple theory of thermodynamic properties of polyelectrolyte solutions. J. Polym. Sci. 1957, 23, 421−430. (4) Manning, G. S. Limiting laws and counterion condensation in polyelectrolyte solutions. I. Colligative properties. J. Chem. Phys. 1969, 51, 924−933. (5) Dautzenberg, H.; Jaeger, W.; Kötz, J.; Philipp, B.; Seidel, Ch.; Stscherbina, B. Polyelectrolytes. Formation, Characterization and Application; Hanser: Munich, 1994. (6) Levin, Y. Electrostatic correlations: from plasma to biology. Rep. Prog. Phys. 2002, 65, 924−933. (7) Sinn, C. G.; Dimova, R.; Antonietti, M. Isothermal titration calorimetry of the polyelectrolyte/water interaction and binding of Ca2+: Effects determining the quality of polymeric scale inhibitors. Macromolecules 2004, 37, 3444−3450. (8) Costa, D.; Burrows, H. D.; Miguel, M. G. Changes in hydration of lanthanide ions on binding to DNA in aqueous solution. Langmuir 2005, 21, 10492−10496. (9) Romsted, L. Do amphiphile aggregate morphologies and interfacial compositions depend primarily on interfacial hydration and ion-specific interactions? The evidence from chemical trapping. Langmuir 2007, 23, 414−424. (10) Burrows, H. D.; Costa, D.; Ramos, M. L.; Miguel, M.; da, G.; Teixeira, M. H.; Pais, A. A. C. C.; Valente, A. J. M.; Bastos, M.; Bai, G. Does cation dehydration drive the binding of metal ions to polyelectrolytes in water? Phys. Chem. Chem. Phys. 2012, 14, 7950− 7953. (11) Mattai, J.; Kwak, J. C. T. Binding of La3+ ions by dextran sulfate polyanions in aqueous solutions containing excess sodium chloride. J. Phys. Chem. 1984, 88, 2625−2629. (12) Nagata, I.; Okamoto, Y. Investigation on ion binding in synthetic polyelectrolyte solutions using rare earth metal fluorescent probes. Macromolecules 1983, 16, 749−753. (13) Horváth, J.; Nagy, M. Thermodynamic characterization of rare earth salts of strong polyacid complexes. J. Phys. Chem. B 2007, 111, 5140−5145. (14) Pierre, A.; Mercier, R.; Foissy, A.; Lamarche, J. M. Cementthinner adsorption on calcium-carbonate. Adsorpt. Sci. Technol. 1989, 6, 179−246. (15) De Stefano, C.; Gianguzza, A.; Piazzese, D.; Sannartano, S. Quantitative parameters for the sequestering capacity of polyacrylates towards alkaline earth metal ions. Talanta 2003, 61, 181−194. (16) Cilurzo, F.; Gennari, C. G. M.; Selmin, F.; Vistoli, G. Effects of metal ions on entero-soluble poly(methacrylic acid-methylmethacrylate) coating: A combined analysis by ATR-FTIR spectroscopy and computational approaches. Mol. Pharmaceutics 2010, 7, 421−430. (17) Widom, J.; Baldwin, R. L. Cation-induced toroidal condensation of DNA. Studies with Co3+(NH3)6. J. Mol. Biol. 1980, 144, 431−453. (18) Bünzli, J. C. G.; Choppin, G. R. Lanthanides Probes in Life, Chemical and Earth Sciences; Elsevier: Amsterdam, 1989; Chapters 6 and 9. (19) Okamoto, S.; Vyprachticky, D.; Furuya, H.; Abe, A.; Okamoto, Y. Ion binding properties of polycarboxylates using terbium(III) as a fluorescent probe: Viscosities and coordinated water molecules in polycarboxylate-terbium(III) complexes in aqueous solutions. Macromolecules 1996, 29, 3511. (20) Burrows, H. D.; Encarnaçaõ , T.; Ramos, M. L.; Valente, R.; ́ Palomar, D.; Tapia, M. J. A interacçaõ entre lantanideos trivalentes e aniõ es poli(acrilatos) em soluçaõ aquosa. Coloides e Interfases; Velázquez Salicio, Ma. M., Dolores Merchán Moreno, Y. Ma., Eds.; Ediciones Universidad Salamanca: Salamanca, 2005; pp 299−304. (21) Horrocks, W.; De, W.; Sudnick, D. R. Lanthanide ion probes of structure in biology - laser-induced luminescence decay constants provide a direct measure of the number of metal-coordinated watermolecules. J. Am. Chem. Soc. 1979, 101, 334−340. (22) Beeby, A.; Clarkson, I. .; Dickens, R. S.; Faulkner, S.; Parker, D.; Royle, L.; de Sousa, A. S.; Williams, J. A. G.; Woods, M. Non-radiative deactivation of the excited states of europium, terbium and ytterbium complexes by proximate energy-matched OH, NH and CH oscillators:

an improved luminescence method for establishing solution hydration states. J. Chem. Soc., Perkin Trans. 2 1999, 493−503. (23) Kaizu, Y.; Miyakawa, K.; Okada, K.; Kobayashi, H.; Sumitani, M.; Yoshihara, K. Aqua ligand dissociation of [Ce(OH2)9]3+ in the 5d4f excited-state. J. Am. Chem. Soc. 1985, 107, 2622−2626. (24) Blasse, G.; Schipper, W.; Hamelink, J. J. On the quenching of the luminescence of the trivalent cerium ion. Inorg. Chim. Acta 1991, 189, 77−80. (25) Marques, E. F.; Burrows, H. D.; Miguel, M.; da, G. The Structure and Thermal Behaviour of Some Long Chain Cerium(III) Carboxylates. J. Chem. Soc., Faraday Trans. 1998, 94, 1729−1736. (26) Azenha, M. E.; Burrows, H. D.; Fonseca, S. M.; Ramos, M. L.; Rovisco, J.; Seixas de Melo, J.; Sobral, A. J. F. N.; Kogej, K. Fluorescence from cerium(III) acetate complexes and the nature of carboxylate binding of trivalent lanthanides to carboxylates in aqueous solution. New J. Chem. 2008, 32, 1531−1535. (27) Tapia, M. J.; Burrows, H. D. Cation polyelectrolyte interactions in aqueous sodium poly(vinyl sulfonate) as seen by Ce3+ to Tb3+ energy transfer. Langmuir 2002, 18, 1872−1876. (28) Barry, C. D.; North, A. C. T.; Glasel, J. A.; Williams, R. J. P.; Xavier, A. V. Quantitative determination of mononucleotide conformations in solution using lanthanide ion shift and broadening NMR probes. Nature 1971, 232, 236−245. (29) Geraldes, C. F. G. C.; Sherry, A. D. Reference 13, Chapter 4. (30) Loebl, E. M.; O’Neil, J. J. Solution properties of isotactic polymethacrylic acid. J. Polym. Sci. 1960, 45, 538−540. (31) Crescenzi, V. Some recent studies of polyelectrolyte solutions. Adv. Polym. Sci. 1968, 5, 358−386. (32) Nagasawa, M.; Murase, T.; Kondo, K. Potentiometric titration of stereoregular polyelectrolytes. J. Phys. Chem. 1965, 69, 4005−4012. (33) Leyte, J. C.; Arbouw-van der Veen, H. M. R.; Zuiderweg, L. H. Irreversible potentiometric behavior of isotactic poly(methacrylic acid). J. Phys. Chem. 1972, 76, 2559−2561. (34) Leyte, J. C.; Mandel, M. Potentiometric behavior of polymethacrylic acid. J. Polym. Sci., Part A 1964, 2, 1879−1891. (35) Muroga, Y.; Noda, I.; Nagasawa, M. Investigation of local conformations of poly-electrolytes in aqueous-solution by small-angle X-ray-scattering. 2. Local conformations of stereoregular poly(sodium methacrylates). Macromolecules 1985, 18, 1580−1582. (36) Kogej, K.; Berghmans, H.; Reynaers, H.; Paoletti, S. Unusual behavior of atactic poly(methacrylic acid) in aqueous solutions monitored by wide-angle light scattering. J. Phys. Chem. B 2004, 108, 18164−18173. (37) Van den Bosch, E.; Keil, Q.; Filipcsei, G.; Berghmans, H.; Reynaers, H. Structure formation in isotactic poly (methacrylic acid). Macromolecules 2004, 37, 9673−9675. (38) Jerman, B.; Kogej, K. Fluorometric and potentiometric study of the conformational transition of isotactic and atactic poly(methacrylic acid) in mixed solvents. Acta Chim. Slov. 2006, 53, 264−273. (39) Jerman, B.; Breznik, M.; Kogej, K.; Paoletti, S. Osmotic and volume properties of stereoregular poly(methacrylic acids) in aqueous solution: Role of intermolecular association. J. Phys. Chem. B 2007, 111, 8435−8443. (40) Eliassaf, J.; Silberberg; Katchalsky, A. Negative thixotropy of aqueous solutions of polymethacrylic acid. Nature 1955, 176, 1119. (41) Vlachy, N.; Dolenc, J.; Jerman, B.; Kogej, K. Influence of stereoregularity of the polymer chain on interactions with surfactants: Binding of cetylpyridinium chloride by isotactic and atactic poly(methacrylic acid). J. Phys. Chem. B 2006, 110, 9061−9071. (42) Škerjanc, J.; Kogej, K.; Vesnaver, G. Polyelectrolyte surfactant interactions - enthalpy of binding of dodecylpyridinium and cetylpyridinium cations to poly(styrenesulfonate) anion. J. Phys. Chem. 1988, 92, 6382−6385. (43) Seixas de Melo, J.; Fernandes, P. F. Spectroscopy and photophysics of 4- and 7-hydroxycoumarins and their thione analogs. J. Mol. Struct. 2001, 565−566, 69−78. (44) Seixas de Melo, J. The influence of oxygen on the lifetime of luminescent probes. A simple device for degassing solutions for fluorescence measurements. Chem. Educ. 2005, 10, 29−35. 14436


Langmuir

Article

(45) Striker, G.; Subramaniam, V.; Seidel, C. A. M.; Volmer, A. Photochromicity and fluorescence lifetimes of green fluorescent protein. J. Phys. Chem. B 1999, 103, 8612−8617. (46) Burrows, H. D.; Knaapila, M.; Monkman, A. P.; Tapia, M. J.; Fonseca, S. M.; Ramos, M. L.; Pyckhout-Hintzen, W.; Pradhan, S.; Scherf, U. Structural studies on cationic poly{9,9-bis[6-(N,N,Ntrimethylammonium)alkyl]fluorene-co-1,4-phenylene} iodides in aqueous solutions in the presence of the nonionic surfactant pentaethyleneglycol monododecyl ether (C12E5). J. Phys. C: Condens. Matter 2008, 20, 104210. (47) Justino, L. L. G.; Ramos, M. L.; Abreu, P. E.; Carvalho, R.; Sobral, A. J. F. N.; Scherf, U.; Burrows, H. D. Conformational Studies on Poly(9,9-dialkylfluorene)s in Solution using NMR Spectroscopy and Density Functional Theory Calculations. J. Phys. Chem. B 2009, 113, 11808−11821. (48) Frey, S. T.; Horrocks, E. DeW. Complexation, luminescence, and energy transfer of Ce3+ with a series of multidentate amino phosphonic acids in aqueous solution. Inorg. Chem. 1991, 30, 1073− 1079. (49) Tackett, J. E. FT-IR characterization of metal acetates in aqueous solution. Appl. Spectrosc. 1989, 43, 483−489. (50) Bovey, F. A. Polymer NMR spectroscopy. VIII. The influence of the pH of the polymerization medium on the stereochemical configuration of polymethacrylic acid. J. Polym. Sci., Part A 1963, 1, 843−848. (51) Klesper, E.; Johnsen, A.; Gronski, W. NMR study of configurational sequences in polymethacrylic acid. J. Polym. Sci., Polym. Lett. 1970, 8, 369−375. (52) Schriever, J.; Leyte, J. C. 1H NMR measurements on aqueous solutions of poly(methyacrylic acid). Polymer 1977, 18, 1185−1189. (53) Ohtaki, H.; Radnai, T. Structure and dynamics of hydrated ions. Chem. Rev. 1993, 93, 1157−1204. (54) Costa, D.; Ramos, M. L.; Burrows, H. D.; Tapia, M. J.; Miguel, M.; da, G. Using lanthanides as probes for polyelectrolyte-metal ion interactions. Hydration changes on binding of trivalent cations to nucleotides and nucleic acids. Chem. Phys. 2008, 352, 241−248. (55) Luján-Upton, H.; Okamoto, Y. Use of terbium(III) as a probe for the ion-binding properties of tactic polyacids and triacidic model compounds. J. Fluoresc. 1998, 8, 355−362. (56) Morcellet, M. Effect of tacticity on the association of poly(methacrylic acid) with divalent ions. J. Polym. Sci., Polym. Lett. 1985, 23, 99−102. (57) Yakubovich, Yu. Ya.; Alekseev, V. G. Hydrolysis constants of tervalent lanthanum and lanthanide ions in 0.1 M KNO3 solutions. Russ. J. Inorg. Chem. 2012, 57, 911−915. (58) Chiarini, M.; Bunton, C. A. Oxidation of thioanisole by peroxomolybdate in alcohol-modified micelles of cetylpyridinium chloride. Langmuir 2002, 18, 8806−8812. (59) Sarraguça, J. M. G.; Skepö, M.; Pais, A. A. C. C.; Linse, P. Structure of polyelectrolytes in 3:1 salt solutions. J. Chem. Phys. 2003, 119, 1621−1628. (60) Tiera, M. J.; De Oliveira, V. A.; Burrows, H. D.; Miguel, M. G.; Neumann, M. G. Fluorescence study of the interaction between metal ions and methacrylic acid copolymers in aqueous solutions. Colloid Polym. Sci. 1998, 276, 206−212. (61) Nishide, H.; Izushi, T.; Arai, H.; Yoshioka, N.; Tsichida, E. Complexation constants of lanthanide ions with poly(methacrylic acid) and its copolymers. J. Macromol. Sci., Chem. 1987, A24, 343−351. (62) Monopoli, M. M.; Åberg, C.; Salvati, A.; Dawson, K. A. Biomolecular coronas provide the biological identity of nanosized materials. Nat. Nanotechnol. 2012, 7, 779−786.

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