Are Keggin's POMs Charged Nanocolloids or ... - ACS Publications

Article Cite This: Langmuir 2018, 34, 2026−2038

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Are Keggin’s POMs Charged Nanocolloids or Multicharged Anions? Alla Malinenko, Alban Jonchère, Luc Girard, Sandra Parrès-Maynadié, Olivier Diat, and Pierre Bauduin* ICSM, CEA, CNRS, ENSCM, Univ Montpellier, Marcoule, France

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ABSTRACT: Owing to their multiple charges and their nanometric size, polyoxometalates (POMs) are at the frontier between ions and charged colloids. We investigated here the effect of POM−POM electrostatics repulsions on their self-diffusion in water by varying POM and supporting salt concentrations. The self-diffusion coefficients of two Keggin’s POMs [silicotungstate (SiW12O404−) and phosphotungstate (PW12O403−)] were determined by dynamic light scattering (DLS) and 1H/31P DOSY NMR, whereas POM−POM electrostatic repulsions were investigated by the determination of the static structure factors using small-angle X-ray scattering (SAXS). The self-diffusion coefficients for the two POMs and for different POM/background salt concentrations were collected in a master curve by comparing the averaged POM−POM distance in solution to the Debye length. As for classical charged colloids, we show that the POM’s counterions should not be considered in the calculation of the ionic strength that governs POM−POM electrostatic repulsions. This result was confirmed by fitting the POM−POM structure factor by considering a pair potential of spherical charged particles using the well-known Hayter mean spherical approximation (MSA). These Keggin POMs also behave as (super)chaotropic anions (i.e., they have a strong propensity to adsorb on (neutral polar) surfaces, which was also investigated) here on the surface of octyl-β-glucoside (C8G1) micelles. The variations of (i) the chemical shift of 1H/31P NMR signals and (ii) the self-diffusion coefficients obtained by DOSY 1H/31P NMR of PW3− and of C8G1 were in good agreement, confirming the strong adsorption of POMs on the micelle polar surface from static and dynamic points of view. We concluded that Keggin’s POMs behave (i) as anions because they adsorb on surfaces as chaotropic anions and (ii) as colloids because they can be described by a classical colloidal approach by dynamic and static scattering techniques (i.e., by the investigation of their interparticle electrostatic structure factor and self-diffusion without considering the POM’s counterions in the ionic strength calculation). This work highlights the dynamic properties of POMs at soft interfaces compared to bulk aqueous solution, which is essential in the understanding of functional properties of POMs, such as (photo)catalysis and the rational design of POM-based hybrid nanomaterials from soft templating routes (i.e., in aqueous solutions at room temperature).

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INTRODUCTION The general interest in polyoxometalate (POM) nanometric ions has spawned from their wide variety of compositions and structural versatility.1−3 The presence of transition-metal atoms in a high oxidation state confer to POMs an extensive range of chemical and physical properties such as catalytic,4−6 redox,2,7 magnetic,8 dielectric,9 photoluminescence, and photochromic.10−12 These many features allow POMs to be used in a wide range of applications such as chemical synthesis, materials science, electrochemistry, and medicine.1,13−16 As surface effects are essential for most of the POM applications (e.g., ref 7), numerous investigations have focused on their adsorption on soft or solid supports or via (hybrid) selfassembly.17−24 Nevertheless, in contrast to the extensive interest in these unique characteristics, the investigation of the dynamic properties of POMs in aqueous solution has gained much less attention,25−33 while they are essential in transport or reaction phenomena as well as in the self-assembly or building of complex 2D or 3D architectures containing POMs. Among the dynamic properties, the self-diffusion coefficient (D) is of © 2017 American Chemical Society

particular significance for better understanding molecular processes and interactions in solutions. The self-diffusion coefficients of the most common Keggin’s POMs obtained from previous works are gathered in Table 1 and are discussed in the following paragraphs. Some of the pioneering works on the diffusion of POMs in aqueous solution were performed by Pope and co-workers.25,26 Their first study on the subject focused on the investigation of the D of two isomorph pairs of POMs, molybdosilicate (SiMo12O404−) and tungstosilicate (SiW12O404−), as well as molybdocobaltiate (CoMo6O213−) and molybdochromiate (CrMo6O213−). Using a method based on a specific radioactive marker of the POM solute, the authors demonstrated that POM self-diffusion was independent of molecular weight, and for the large species, it is rather influenced by the solvent surrounding the POM. Although their second work26 was focused on the reduction properties of POMs, the diffusion Received: October 19, 2017 Revised: December 15, 2017 Published: December 26, 2017 2026

DOI: 10.1021/acs.langmuir.7b03640 Langmuir 2018, 34, 2026−2038

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Table 1. Type of POM, POM Anion Molecular Weight (IW), Charge of POM (z), POM Concentration ([POM]), Medium, Viscosity of the Medium (η), Temperature (T), Diffusion Coefficient (D), Hydrodynamic Radius (Rh), Ionic Strength (I), and Technique Reported in the Referenced Publications POM

IW, g/mol

z

[POM], mM

[CoMo6O21]3− [CrMo6O21]3− [SiMo12O40]4− [SiW12O40]4−

971 964 1820 2875

3 3 4 4

8−21 8−21 3−11 3−11

[PW12O40]3− [SiW12O40]4−

2877 2875

3 4

0.05−5 0.05−5

[PVW11O40]3−

2744

3

1

η, mPa·s

medium

Reference 25 H2O + NaClO4 I = 1

2741

4

1

[AlVW11O40]5−

2740

5

1

D, m2/s × 1010 ± ± ± ±

Rh, nm

0.2a 0.2a 0.2a 0.2a

techniques

30 30 30 30

7.4 7.4 6.2 6.1

Reference 26 H2O + 1 M H2SO4 1.2 H2O + 0.9 M Na2SO4 1.5

25 25

3.36 ± 0.15a 2.56 ± 0.15a

0.56 0.56

polarographic method

Reference 30 H2O/TBA (2:3 v/v)b

60

2.38−2.78

0.62−0.72e

single-potential-step chronoamperometry

c

[SiVW11O40]4−

T, °C

H2O H2O/TBA (2:3 v/v)b H2Oc H2O/ TBA (2:3 v/v)b H2Oc

[PW12O40]3− [SiW12O40]4− [AlW12O40]5−

2877 2875 2873

3 4 5

54 54 54

H2O

[SiW12O40]4−

2875

4

0.5

H2O + HClO4

[SiW12O40]4−

2875

4

at infinite dilution

H2O

0.89 0.89 0.89

Reference 29 0.89 0.89 0.89

25 60 25 60 25

2.08−2.56 1.64−1.79

open capillary method

0.57 0.68−0.83e 0.56 0.96−1.03e 0.59

25 25 25

1.8−2.0 1.1−1.6 1.1−1.7

MD

25

4.5−4.9d 5.1a

square wave voltammetry voltammetry + MSA-transport theory

10

2.4a

conductivity measurements

18 25 30 50

3.2a 3.9a 4.8a 7.0a

Reference 33

Reference 32 1.306 1.053 0.89 0.797 0.547 Reference 27

[PW12O40]3−

2877

3

[PW12O40]3−

2877

3

∼94

H2O

H2O

4.6 Reference 28 0.89

2.4a 3.2a

0.48

PGSE-NMR

MD (two models)

a

Diffusion coefficient at infinite dilution; TBA, tert-butyl alcohol. bAddition of XCl, where X = Li, Na, K. The concentrations of added salt depend on X and POM and vary in the range from 85 to 202 mM. cAddition of XCl, where X = Li, Na, K. The exact concentrations of added salts were not specified. dVaries with the concentration of added HClO4 (0.11 M < [HClO4] < 1M) eEffective radius

coefficients of tungstophosphate (PW12O403−) and tungstosilicate (SiW12O404−) were deduced from electrochemistry measurements. Different diffusion coefficients were obtained for these two POMs, and the authors suggested that this difference could originate from the different viscosities of the supporting electrolytes. In 2000, Grigoriev et al. investigated ion pair formation between POM and different counterions in various media using chronoamperometry.30 In this study, they determined the diffusion coefficients of POMs in order to estimate their apparent radii, and they did not observe ion pairs in water. In

contrast, ion pairs between POMs and cations were observed in water/alcohol mixtures, as expected from the lower dielectric constant of these media compared to that of pure water.30 Furthermore, using a molecular dynamics (MD) approach, Leroy et al. have analyzed the self-diffusion of Keggin-type POMs with different counterions in water,29 and their results have shown some consistency with the data published by Grigoriev et al. One of the conclusions drawn from this simulation work was that the diffusion coefficient dependence on the charge of POM as well as on the size of the counterion is not trivial. It depends on the subtle interplay between direct 2027


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Table 2. Experimental Results from the Present Work: Type of POM, POM Anion Molecular Weight (IW), Charge of POM (z), POM Concentration ([POM]), Medium, Viscosity of the Medium (η), Temperature (T), Diffusion Coefficient (D), Hydrodynamic Radius (Rh), Ionic Strength (I), and Technique POM [SiW12O40]

4−

IW, g/mol

z

[POM], mM

2875

4

10 15 20 25 30 35 10

20 [PW12O40]3− a

2877

3

30

η, mPa·s

medium H2O

H2O H2O H2O H2O H2O H2O H2O

0.89

+ + + + + + +

50 mM NaCl 75 mM NaCl 100 mM NaCl 250 mM NaCl 50 mM NaCl 100 mM NaCl 300 mM NaCl

T, °C

D, m2/s × 1010

25

9.0 11.1 12.1 11.7 12.8 11.2 5.93 5.21 4.87 4.69 7.19 5.85 3.25

0.90 ± 0.01

1.30a

25

Rh, nm

technique DLS

0.50 0.52

0.52

DOSY NMR

Viscosity of 30 mM H3PW in the presence of 300 mM NaCl.

dispersion forces (induced dipole−induced dipole) in addition to classical electrostatics effects.38 In a previous work, we highlighted that Keggin’s POMs have a strong tendency to adsorb, through noncovalent interactions, on soft hydrated surfaces in water media.22 Indeed, it was shown that PW12O403− and SiW12O404− adsorb strongly on nonionic surfactant micelles and at the water/air interface covered by nonionic surfactants, i.e., polyethoxy- or sugar-based surfactants. It was proposed that the process of POM adsorption originates mainly from an entropically driven process through the partial dehydration of the POMs and of the interface. The ability of POMs to adsorb on hydrated surfaces was further demonstrated on hydrophilic oligomers [poly(ethylene glycol), PEG] in bulk water.21 Indeed, it was shown that POMs build soluble POM−PEG nanoassemblies in water. POMs were assigned within the anion classification of Hofmeister to the term superchaotropic anions because of their adsorption properties and their ability to increase strongly the cloud point of nonionic surfactants.22 Chaotropic anions are indeed usually referred to as large and polarizable (low charge density), which makes Keggin POM archetypes of superchoatropic anions. The general superchaotropic behavior of POMs, and more specifically their adsorption on hydrated polar interfaces, appears to be essential to the design of hybrid materials using soft methods, which is a field of growing interest.12,21,39−41 Moreover, it seems that the superchaotropic behavior is not exclusive to POMs but is a more general property of nanoions with delocalized charges, such as boron clusters (dodecaborate42), which was recently described as a superchaotrope, or metallacarboranes.43−45 Considering the IUPAC’s definition of a colloidal system,46 which is based on a unique criterion of size, i.e. “...particles dispersed in a medium have at least in one direction a dimension roughly between 1 nm and 1μm”, Keggin’s POMs (∼1 nm) are borderline cases to be called colloids. However, it is commonly accepted that van der Waals (VdW) forces are determinant in colloidal systems whereas ions are mostly controlled by pure electrostatics. Indeed, VdW forces are at least of the same order of magnitude as electrostatics for (large) charged colloids, whose equilibrium of forces is well picked up by the classical DLVO theory.47 However, earlier evidence has shown that ions of 2−3 nm in size, such as gold nanoparticles covered by SAM (self-assembled monolayers), terminated with charged carboxylate groups,48 or Keplerate’s POMs,49−51 do

POM−cation electrostatic interactions and also on the stability of the solvation shell of the ions. Meanwhile, Olynyk et al. studied the diffusion coefficients of 12-tungstosilicate by cyclic voltammetry in the presence of an excess of supporting electrolyte to suppress the migration of the electroactive species.33 They obtained the diffusion coefficient at infinite dilution by extrapolation of the experimental values from the mean spherical approximation (MSA) transport calculations in agreement with conductivity results. This work suggests that there is no ion association (ion pairs) in water,34 which is in agreement with previous works.29,30 Ion pairing between Keggin’s POM monovalent cations has so far been observed only in the special case of niobate-based POMs, such as [SiNb12O40]16− and [GeNb12O40]16−, due to their extremely high charge density.35,36 In 2003, diffusion coefficients of 12-tungstosilicate in acid form at low POM concentrations and at various temperatures were calculated by Horky et al. via the ion mobility obtained from conductivity measurements.32 At room temperature, the D value obtained in this latter study was around 20% lower than the one obtained by Olynyk et al.33 More recently, Poulos with co-workers investigated the diffusion of 12-tungstophosphate in the confined hydrophilic region of lyotropic lamellar phases by pulsed gradient spin echo (PGSE) NMR, and they reported a strong reduction of the diffusion coefficient compared to those determined in bulk water at the same concentration. The authors explained this effect by the adsorption of POMs onto the nonionic surfactant bilayers.27 A recent MD study on 12-tungstophosphate showed that discrepancies in the diffusion coefficient values exist depending on the chosen charge model.28 Many factors, such as concentration, nature and concentration of the supporting electrolyte, viscosity of the solution, and temperature, influence the self-diffusion of charged solutes. The POM/electrolyte concentrations are the most influential parameters with respect to the POM self-diffusion through the change in the POM−POM electrostatic interactions. POM nanometric anions are much larger than standard anions, such as chloride, for example, and can therefore be seen as small (nano)colloids or as large (macro)ions.37 They have many delocalized charges, but their overall charge densities are low and their polarizabilities are therefore expected to be high. As a consequence, it is likely that POMs interact through 2028


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were chosen as representatives of the Keggin POMs because (i) they have been well studied in the literature (Table 1), (ii) they have an isomorphic structure with a nearly spherical shape, and (iii) they have different number of charges which enable us to investigate specifically the effect of charge density on the electrostatic interactions. To the best of our knowledge, the self-diffusion of free POMs in water has so far never been investigated by dynamic light scattering. As POMs have shown the general property of a (super)chaotropic anion, i.e., with the propensity to adsorb on (hydrated) surfaces, the dynamic and static properties of POMs were not only investigated in bulk water but rather at the surface of nonionic micelles made of a sugar-based surfactant: n-octyl-β-monoglucoside (C8G1).

not behave like colloids because VdW forces are typically 2 orders of magnitude lower than electrostatic interactions for these systems. Recent simulations have shown for Kelplerate’s POMs (2−3 nm) that the increase in the electrical charge could produce the condensation of counterions, which subsequently leads to Keplerate’s POMs’ self-assembly.49 Counterion condensation, induced by increasing surface charge density, is a common (pure electrostatic) effect in colloids and surface science; for example, it is observed with charged surfactant monolayers by increasing the surface concentration52 or in polyelectrolytes (known as Manning condensation). For Keggin’s POMs (∼1 nm), the situation seems to be very much different. Indeed, a recent work by Bera et al. has investigated Keggin’s POMs with three, four, and five charges, namely, PW12O403−, SiW12O404−, and AlW12O405−, by SAXS experiments using synchrotron radiation.53 They concluded that Keggin’s POMs with higher charges have stronger electrostatic repulsions but also that a POM of lower charge density (PW12O403−) forms nanoaggregates of POMs. As a consequence, there is an apparent contradiction concerning the effect of the electrical charge on the self-assembly (aggregation) of POMs, with Keplerate’s POMs on one side and Keggin’s POMs on the other side, suggesting a different origin for the POM aggregation. Keplerate’s POMs show a classical electrostatic effect which induces counterion condensation (and selfassembly) presumably due to their high charge density whereas the aggregation of low-charge-density Keggin’s POMs, e.g., PW12O403−, may be related to their superchaotropic behavior. This is supported by a previous investigation showing that the superchaotropic behavior of PW12O403− was more pronounced than that of SiW12O404−.22 The apparent dual behavior of POMs between salt and colloid was carefully discussed by Tianbo Liu in a recent contribution to this journal.17 On one hand, it was pointed out that POMs, because of their delocalized charges, cannot be described by classical electrostatic theory, such as Debye− Hückel which assumes point charges. On the other hand, POMs are soluble species, which makes them different from colloidal particles stabilized by the combination of repulsive and attractive interactions as described in the DLVO theory. The previous contribution and discussion by Liu focused on much larger POMs of Keplerate’s type, i.e., POMs which have a wheel structure with typically >100 metal centers and at least 10 charges. As stated above, Keplerate’s POMs have the general tendency to self-assemble in large blackberry vesicles mediated by counterion condensation17,54−56 while on the contrary Keggin’s POMs are fully dissociated from their (monocharged) counterions and generally show no self-assembly in water.33 In the present work, we tackle a similar question as raised previously by Liu on the ion or colloid nature of POMs, but we focus here on the general properties of Keggin’s POMs in water and at interfaces with a different approach based on the investigation of the dynamic and static properties of POMs. To understand the behavior of POMs more deeply and to determine if they behave as charged colloids or as salts, we followed an experimental approach that is classical in investigating charged colloids. The self-diffusion of POMs and the POM−POM structure factor, that both account for POM−POM electrostatic interactions, were investigated by using dynamic light scattering (DLS), diffusion-ordered spectroscopy nuclear magnetic resonance (DOSY NMR), and small-angle X-ray scattering (SAXS) techniques. Tungstophosphate (PW12O403−) and tungstosilicate (SiW12O404−)

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EXPERIMENTAL SECTION

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METHODS

Materials. Silicotungstic acid (H4SiW12O40·xH2O, H4SiW, MW = 2878.17 g/mol, >99.9%), phosphotungstic acid (H3PW12O40·xH2O, H3PW, MW = 2880.05 g/mol, >99.9%), n-octyl-β-D-monoglucoside, also known as C8G1 (MW = 272.37 g/mol, purity >98%, cmc = 25 mM), and sodium chloride (NaCl, >99.5%) were purchased from Sigma-Aldrich. The thermal gravimetric analysis (TGA) measurements showed that the water content in POMs equals up to 10 and 5% for H4SiW and H3PW, respectively. The molar concentration of POM was converted to volume fraction using the density of the POMs, which is around 5.39 g/mL.57 All chemicals were used as received. Milli-Q water (18.2 MΩ·cm at 25 °C) was used for sample preparation. Sample Preparation. All POM solutions were prepared by the solubilization of POM powders in Milli-Q water. For the DLS experiments, all POM solutions were filtered just before the measurements using polytetrafluoroethylene or a cellulose acetate membrane with a pore size of 0.4 μm. The POM solutions in the presence of NaCl were prepared by adding salt solutions at certain concentration to the POM powder. The solutions containing POM and C8G1 with and without NaCl were prepared by the solubilization of POM powders in a solution containing surfactant/NaCl at the desired concentration. The pH of the POM solutions was measured and was always below 4.0, i.e., in the range of the chemical stability of the POMs in water.58,59

SAXS measurements using Mo radiation (λ = 0.071 nm) were performed on a bench built by XENOCS. Collimation was applied using a 12:∝ multilayer Xenocs mirror (for Mo radiation) coupled to two sets of scatterless FORVIS slits providing a 0.8 × 0.8 mm2 X-ray beam at the sample position. The scattered beam was recorded using a large online scanner detector (diameter 345 mm, from MAR Research). A large q range (0.2 to 40 nm−1) was covered thanks to off-center detection. Preanalysis of data was performed using FIT2D software. The scattering intensities are expressed versus the magnitude of scattering vector q = [(4π)/λ] sin(θ/2)), where λ is the wavelength of incident radiation and θ is the scattering angle. Two millimeter quartz capillaries were used as sample containers for the solutions. The usual corrections for background (empty cell and detector noise) subtractions and intensity normalization using a high-density polyethylene film as a standard were applied. The experimental resolution was Δq/q = 0.05. To analyze the SAXS spectra, SASfit software was used.60 Diffusion coefficients of POMs were determined by dynamic light scattering using an ALV-CGS3 goniometer equipped with a 22 mW HeNe laser (632.8 nm) and an APD-based single-photon detector coupled to an ALV/LSE-5004 autocorrelator. It has a minimum realtime sampling time of 0.1 μs and a maximum of about 50 s. For all experiments, the temperature was maintained at 25 °C. The experimental autocorrelation function was measured at different angles from 30 to 150° with a 10° step. The time correlation function g2(q, τ) − 1 showed two modes for unfiltered H4SiW solutions and one mode for filtered ones. The autocorrelation functions were fitted by the least-squares method by applying eqs S1 2029


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Langmuir and S2 and by using the Solver in Microsoft Excel. For H3PW samples, two modes were always observed, even after filtration. The origin of the second slow mode is likely due to the presence of nanoaggregate as observed by Bera et al. for H3PW at high acidicity.53 The correlation functions were first analyzed using two exponentials, and only the faster mode, related to the free POMs in solution, was used to determine the diffusion coefficient. NMR measurements were recorded at 25 °C on a Bruker 400 Avance III spectrometer operating at 400.13 MHz for 1H and at 161.98 MHz for 31P. It was equipped with a z-gradient 5 mm BBFO probe. Chemical shifts for 1H and 31P NMR are reported in parts per million with respect to tetramethylsilane (TMS) and 85% phosphoric acid. The 1H spectra were collected with a 3.98 s acquisition time, a 30 s relaxation delay, and a 7.5 μs 90° pulse width. The 31P spectra were collected with a 0.51 s acquisition time, a 120 s relaxation delay, and a 9.5 μs 90° pulse width. DOSY experiments were performed with the bipolar gradient pulse pair longitudinal eddy-current delay sequence (ledbpgp2s). Methanol was used to calibrate the temperature of the probe. The gradient strength was calibrated with a D2O sample. The diffusion coefficients of PW3− and C8G1 were obtained from corresponding peaks located at around −15.3 ppm for 31P NMR spectra and at around 4.4, 1.2, and 0.8 ppm for 1H NMR spectra.

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RESULTS AND DISCUSSION POMs as Charged Nanocolloids. Self-Diffusion of POMs by DLS and NMR. The analysis of the dynamic light scattering (DLS) results obtained from the H4SiW filtered solution yielded a monomodal correlation function with a fast mode indicating the presence of small particles (Figure S1 in the SI). The average exponential decay, Γ, of the autocorrelation functions has a linear q2 dependence which is the sign of a purely diffusive process and can be attributed to the selfdiffusion of POMs. The self-diffusion coefficients are plotted in Figure 1a (black squares) as a function of H4SiW concentration. The POM diffusion coefficient varies only slightly by increasing the H4SiW concentration, except for the lowest (measurable) concentration of 10 mM, and equals on average to 12 × 10−10 m2/s. This value is almost twice as high as those previously published; see Table 1 for comparison. This discrepancy can be attributed to the high electrostatic interactions between POMs in pure water. Indeed, it is well known that electrostatic repulsions between particles lead to increased D values compared to those of noninteracting particles.61 This assumption can be tested by two experimental methods: the first one is to dilute the system to decrease POM−POM interactions; however, further dilution below 10 mM (0.5% v/ v) would lead to a scattering intensity that is too low, below the sensitivity limit of the DLS apparatus. The second approach is to screen electrostatic interactions by adding a supporting electrolyte, such as sodium chloride. In Figure 1b, the diffusion coefficient D for 10 and 20 mM H4SiW was plotted as a function of added salt concentration. The D values obtained for 20 mM are higher than for 10 mM, as expected from stronger POM−POM electrostatic repulsions. The addition of salt leads to a decrease in the D value which can be attributed to the vanishing of the repulsive interactions. At around 100 mM NaCl, full screening of the electrostatic repulsions is reached as D tends toward a constant value (4.9 × 10−10 m2/s). As the volume fraction is only of 0.5% v/v at 10 mM, hard sphere repulsions can be safely considered as negligible, meaning that this D value is the one expected at infinite dilution, D0. This value was reported in Figure 1a (empty crossed square) and can be also retrieved from the diffusion coefficient measurements without added salt by the extrapolation of D to zero H4SiW concentration as shown in Figure 1a by the dotted guide line.

Figure 1. Diffusion coefficient D of SiW4− as a function of the concentration in water (a) and as a function of the sodium chloride concentration for [H4SiW] = 10 and 20 mM (b). The value of D for [H4SiW] = 10 mM with 100 mM NaCl is assumed to be the value at infinite dilution and is reported in (a) at [H4SiW] = 0 (empty crossed square).

Thus, the D0 value of H4SiW in the presence of sufficient salt concentration or by extrapolation at low H4SiW concentration is now in good agreement with published values (Table 1, refs 32 and 33). The conversion of this value of D0 into hydrodynamic radii (Rh) from the Stokes−Einstein relation gives an Rh value of around 0.50 nm, which is close to the geometrical radius (R ≈ 0.45 nm) and hydrodynamic radii obtained in the publications mentioned in Table 1 or determined from small-angle X-ray scattering experiments.2,22 Note that the viscosity of 10 mM H4SiW in the presence of salt [(0.90 ± 0.01) × 10−3 Pa/s] was used to calculate the hydrodynamic radius. It is then confirmed that the high values of D obtained here are due to significant POM−POM electrostatic repulsive interactions. For POM, this effect is significant even at low concentrations since H4SiW is fully dissociated in aqueous solutions with SiW carrying four negative charges. For H3PW, two modes in the autocorrelation functions were obtained (Experimental Section). Though the fast one can be attributed to the POM self-diffusion process, the origin of the slow mode is unclear. It may result from the presence of small aggregates of PW3−, as observed in a highly acidic medium by 2030


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Langmuir Bera et al. in a recent contribution.53 These authors also investigated Keggin’s POMs with higher electrical charges, SiW4− and AlW5−, for which no aggregation was detected. In a recent investigation, it was shown that the superchaotropic behavior of PW3− is more pronounced compared to that of SiW4−, which suggests here that the superchaotropic behavior may be related to aggregate formation for PW3−.22 Indeed, the superchaotropic property of Keggin’s POMs is related to their high polarizability, which may induce strong POM−POM attractions (dispersion forces)62,63 able to fight against electrostatic repulsions and ultimately lead to POM aggregation. It was shown theoretically for nitrate (NO3−), a typical chaotropic anion, that significant anion−anion attraction forces emerge from dispersion forces at high concentrations (in the molar range) but without aggregation formation.62,64 Nevertheless, from the fast mode and for a solution containing 100 mM NaCl, the D0 and Rh values obtained for H3PW are close to those of H4SiW, 4.3 m2/s (0.57 nm) compared to 4.9 m2/s (0.50 nm), respectively. This was expected from the same size and ionic weight of SiW4− and PW3−, and this is in agreement with previous measurements. (See Table S1 and the Experimental Section for more details.) 31 P DOSY NMR measurements were carried out on this system by keeping in mind that the H3PW self-diffusion coefficient value obtained from DLS measurements can be affected by the presence of the second (slow) mode contribution to the time correlation function. The diffusion coefficient for H3PW in water was measured by Poulos et al. using 31P PGSE-NMR at 5 %v/v (∼94 mM).27 The selfdiffusion of H3PW was investigated with the same procedure but here at a lower concentration (30 mM, 1.6% v/v) and in the presence of 300 mM NaCl to prevent the effect of electrostatic interactions on D. Here a value of 3.3 × 10−10 m2/s was determined, corresponding to an apparent hydrodynamic radius of 0.52 nm, taking into account the relatively high viscosity of the solution (1.30 × 10−3 Pa/s). This value is very close to the one obtained for H4SiW by DLS (0.50 nm) and in agreement with the size of POM and earlier results (Table 1). POM−POM Electrostatic Interactions. From Figure 1b, it was shown that D depends strongly on both the POM and the salt concentrations as expected from electrostatics. An attempt was made here to rationalize the variations in the D values according to electrostatic considerations. The aim is to collect in a master curve all of the experimental data (D) obtained for different compositions: changing the type of POM (SiW4− or PW3−) and the POM/background salt concentrations. To fulfill this goal, electrostatic effects are evaluated by comparing the POM−POM average distance in solution to the Debye lengths that characterize long-range interactions in the system. Debye lengths λD were calculated from eq 1 ⎛ ε ε k T ⎞1/2 λD = ⎜ 0 r B2 ⎟ ⎝ 2NAe I ⎠

Debye length, is the one at the midplane between colloids, i.e., the ionic strength equidistant between the charged particles. Therefore, for diluted charged colloids, for which the average colloid−colloid distance is large, only the background salt (and not the colloid’s counterions) contributes to the ionic strength of the medium. However, we express in the following text the ionic strength for the general case as I = Ibkg +

1 (α[H+]z H+ 2 + [Na +]z Na+ 2 + [Cl−]zCl−2) 2 (2)

where Ibkg is the ionic strength of Milli-Q water (which is typically ∼10−5 M for air-equilibrated water), zi is the charge number of ion i, and α is a factor ranging from 0 to 1 that represents the contribution of the POM’s counterions to I. For α = 1, POM’s counterions contribute fully to I and for α = 0, POM’s counterions do not contribute to I, as is classically considered for charged colloids. In order to estimate the strength of POM−POM interactions, we define a unitless ratio d/2λD, where d is the average distance between the surfaces of POMs for a given concentration, assuming that POMs are distributed on a cubic lattice. Therefore, d = dPOM−POM − 2RPOM, with dPOM−POM being the average POM−POM distance and RPOM being the POM radius. For d/2λD > 1, POMs are considered to be too far from each other to interact, and for d/2λD < 1, the double layers overlap, which implies that significant electrostatic repulsions take place. In Figure 2, D/D0 ratios are plotted as a function of d/2λD for the different compositions: [H4SiW] = 10 mM with [NaCl] = 0, 20, 50, 75, 100, 250 mM and [H4SiW] = 20 mM with [NaCl] = 0, 20, 50, 100, and 150 mM and [H3PW] = 10 mM in the presence of 100 mM NaCl. The self-diffusion coefficient at infinite dilution D0 for H4SiW and H3PW was assumed to be equal to the diffusion coefficient of H4SiW (10 mM) screened by 250 mM NaCl and the diffusion coefficient of H3PW (10 mM) screened by 100 mM NaCl. λD was calculated with different values of α = 0 and 1, shown respectively in Figure 2a,b. For α = 0, all points are set on a master curve except for the two points with the lowest d/2λD values (close to 0). These two lower d/2λD values in Figure 2a were obtained with [H4SiW] = 10 and 20 mM, with their low values arising from the high Debye lengths, which diverge at low ionic strength. This discrepancy at very low d/2λD values is likely to arise from a too strong influence of the repulsive interactions on the diffusion coefficient. The mutual diffusion coefficient should then be considered rather than the self-diffusion coefficient.65 A log−log representation of Figure 2, given in SI (Figure S9), makes a break (for α = 0) more apparent between the two regions for d/2λD < 1 (D > D0) and d/2λD > 1 (D ≈ D0), i.e., between the concentrated and diluted regimes where POM− POM interactions play and do not play a role in the selfdiffusion of POMs, respectively. For α = 1, Figure 2b, no self-agreement among all of the experimental data was obtained, i.e., no master curve was obtained. Therefore, POM’s counterions should not be taken into account in the calculation of I, as is the case for diluted and charged colloids. As a conclusion, by considering POMs to be (nano)colloids, basic electrostatic considerations enable us to rationalize the variation of the POMs’ diffusion coefficient for different POMs and concentrations of POM and supporting salt.

(1)

where ε0 is the vacuum permittivity, εr is the relative permittivity, kB is Boltzmann’s constant, T is the temperature, NA is Avogadro’s number, e is the elementary charge, and I is the ionic strength. The values of Debye lengths depend on the ionic strength that can be calculated in different ways. Indeed, the POM’s counterions may or may not be taken into account in the calculation of I. The general consensus is that the ionic strength, which should be considered in the calculation of the 2031


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Figure 2. D/D0 as a function of d/2λD for [H4SiW] = 10 mM with [NaCl] = 0, 20, 50, 75, 100, and 250 mM, [H4SiW] = 20 mM with [NaCl] = 0, 20, 50, 100, and 150 mM, and [H3PW] = 10 mM in the presence of 100 mM NaCl. D0 is the self-diffusion coefficients at infinite dilution. We made the assumption that D0 values for H4SiW and H3PW equal those obtained for [H4SiW] = 10 mM with [NaCl] = 250 mM and [H3PW] = 10 mM with [NaCl] = 100 mM, respectively. The curves are calculated for α = 0 (a) and α = 1 (b). For α = 0, a master curve is observed with all of the experimental data obtained here except for d/2λD close to zero.

The investigation of the self-diffusion of POM using the DLS technique emphasized the electrostatic interactions between POMs. The next step was to investigate POM−POM interaction under static conditions using SAXS. In our previous works, SAXS spectra of POMs in solution have been shown, either free in solution21,22 or adsorbed on micelles.22 The previous SAXS analysis was performed by considering POMs to be homogeneous spheres. However, here we focused on the POM−POM interactions through the specific POM structure factor, S(q), and its evolution as a function of POM concentration and ionic strength. In Figure 3a, the SAXS spectra of H4SiW are plotted in absolute intensity, at different POM concentrations in water. In such isotropic media, the scattering intensity by POMs as a function of q can be simply expressed as I(q) = ΦPOM VPOM(Δρ) 2 P(q , RPOM) S(q)

Figure 3. SAXS spectra of H4SiW at different concentrations: 10, 15, 20, 25, 30, and 35 mM (a) and at a fixed concentration of 10 mM by varying the NaCl concentration: 25, 50, 75, 100, and 250 mM (b). The corresponding S(q) profiles are in (c) and (d).

radius assuming POM to be a sphere, and Δρ is the electronic contrast, which equals to the difference in scattering length densities between POM and water, ρ POM and ρ water , respectively. P(q, RPOM) is the sphere form factor and S(q) is

(3)

where ΦPOM is the POM volume fraction, VPOM is the volume 3 of the POM ion with VPOM = 4 πRPOM 3, RPOM is the POM 2032


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Langmuir the structure factor that accounts for POM−POM interactions in solution. The plot of I(q)/ΦPOM vs q (Figure S4) shows first a superposition of the spectra at large q values (q > 2 nm−1). In this q range, the form factor of POM, P(q), is probed and is independent of POM concentration. Second, the decrease in scattering intensity at low q values (q < 2 nm−1), which gets stronger as the POM concentration increases, is related to the structure factor and can be attributed to growing electrostatic repulsions between POMs. Therefore, the shape of the spectrum is related only to P(q) when S(q) = 1, i.e., when POM−POM repulsive interactions are negligible. This condition is met either when the POM is diluted or when salt is added at a concentration sufficiently high to screen electrostatic repulsions fully. The SAXS instrument is not sensitive enough to obtain a good scattering signal at high POM dilution. Therefore, SAXS spectra of 10 mM H4SiW solutions were collected at different salt concentrations (Figure 3b). The increase in the scattering intensity at low q values is observed by adding salt until saturation is reached ([NaCl] > 100 mM), indicating the full screening of the repulsive interactions between POMs (Figure 3b). The scattering curve at 250 mM NaCl can be well fitted using a P(q) of homogeneous spheres characterized by a radius of RPOM = 0.46 nm, as already determined in previous studies. Then, S(q) values were obtained for different POMs and salt concentrations by dividing all of the scattering spectra by ΦPOMVPOM(Δρ)2 P(q, RPOM = 0.46 nm) (Figure 3c,d). As expected from repulsive particles, the structure factors are equal to 1 at large q values and decrease at lower q values. These profiles were analyzed considering a pair potential of spherical charged particles such as the one developed by Verwey− Overbeek66 and by using the well-known Hayter mean spherical approximation (Figure 4).67 This model requires four parameters: the effective charge of the colloid (zeff), the effective radius of the sphere (Reff), the volume fraction of the particle Φ, and the ionic strength (or salt concentration for a monovalent salt) I. zeff and Reff were fitted, whereas Φ and I were considered to be input parameters and were set equal to their experimental values. An attempt was made to fit the experimental S(q) by the rescaled MSA model, but it gave much less agreement, being always out of the model convergence. The fit of S(q) for [H4SiW] = 10 mM without added salt shows good agreement with the experimental data (Figure 4a), with only two fitting parameters, zeff = 3.6 and Reff = 0.77 nm. These values are reasonable compared to the nominal ones, zPOM = 4 and RPOM = 0.46 nm. For higher POM concentrations (Figure 4b), the adjustment of the model to the experimental S(q) is more critical and it is then difficult to achieve the convergence of the fit even if similar zeff and Reff values as for the system at 10 mM are obtained from the fitting process (Table 3). When the ionic strength is varied (Figure 4 c), the model fits much better to the experimental data. However, a slight increase in the effective charge as well as a more pronounced variation in the size is found by increasing the salt concentration. This deviation from reasonable Reff values shows here the limit of this model for nano-objects characterized by a very high surface charge density under highly dilute conditions. It is also possible that some part of the POM forms nanoaggregates, as noticed by Bera et al. in a previous contribution (only) for H3PW in a highly acidic medium.53 The presence of a small number of POM nanoassemblies could

Figure 4. Structure factors as a function of q for (a) [H4SiW] = 10 mM in Milli-Q water; data were fitted using the MSA Hayter model with two fitting parameters, the effective charge (zeff = 3.6) and the effective radius (Reff = 0.77 nm), by setting the POM volume fraction and salt concentration equal to their experimental values. (b) [H4SiW] = 20 and 30 mM in Milli-Q water, in which the fitting process gave zeff = 4 and 4.4 and Reff = 0.77 and 0.78 nm. (c) [H4SiW] = 10 mM with [NaCl] = 25 and 100 mM; the fitting process gave zeff = 3.8 and 4.4 and Reff = 0.86 and 1.05 nm.

also explain the deviation from reasonable Reff values, compared to the radius of a POM anion. These results are in agreement with those obtained by DLS experiments and point out that the electrostatic repulsions between the POMs are almost completely screened for salt concentrations above 100 mM. Moreover, it is confirmed that (i) POMs are fully dissociated from their counterions, here H+, as already concluded from previous conductivity measurements,33 and that (ii) POMs behave as highly charged nanocolloids, as deduced in the previous section. The SAXS analysis was also applied for H3PW (Figure S5a). The fit of the scattering spectrum of the full screened system, 2033


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Langmuir Table 3. Parameters Resulting from the Fits of S(q), Obtained from SAXS, by Using a Pair Potential of Spherical Charged Particles within the Hayter Mean Spherical Approximation fitting parameters

input parameters

[H4SiW], mM

[NaCl], mM

zeff

Reff, nm

I, mM

Φ, %v/v

10 20 30 10 10

0 0 0 25 100

3.6 4 4.4 3.8 4.4

0.77 0.77 0.78 0.86 1.05

0 0 0 25 100

0.52 1.04 1.56 0.52 0.52

10 mM POM with 100 mM NaCl, using a sphere form factor gives a radius of 0.47 nm. This result is very similar to the one obtained for H4SiW, as expected from the isostructure of PW3− and SiW4− and in agreement with the literature.22,27,68 Meanwhile, the structure factor profiles differ from those obtained with H4SiW (Figure S5b) and show two steps versus q whatever the ionic strength in the system. The addition of salt leads to a screening effect of the electrostatic repulsions as for H4SiW. However, for H3PW, a salt concentration of 100 mM is sufficient to obtain a full screening, meaning that the effective charge of H3PW is weaker than for H4SiW, as expected from the lower formal charge of PW3− compared to that of SiW4−. The peculiarity in the profile of the H3PW structure factors may be related to POM cluster formation, which could be at the origin of the slow mode observed in DLS. POMs as (Superchaotropic) Anions: Adsorption on Neutral Micellar Surfaces. In a recent study by Naskar et al., it was shown that POMs, H3PW, and H4SiW spontaneously adsorb at the surface of nonionic micelles.22 The POM adsorption process on the micelle affects the monomer−micelle equilibrium as observed in the evolution of the critical micellar concentration (cmc) of the surfactant by the addition of POM; see the results for C8G1 with H3PW and H4SiW in Figure S6. H4SiW slightly increases the cmc, whereas H3PW has the opposite effect. This result suggests that H4SiW shifts the monomer−micelle equilibrium toward the formation of monomer, whereas H3PW favors micellization due to a stronger affinity of PW3− for the surface of the micelle. Indeed, the investigation of the micelles by SAXS and the variation in the cloud points of a polyethoxy surfactant with POMs have shown that PW3− adsorbs more strongly on the micellar surface than does SiW4−.22 In the present work, we investigate the evolution of the interactions between H3PW and H4SiW with C8G1 micelles using 31P NMR and 1H/31P DOSY NMR for H3PW and 1H DOSY NMR for H4SiW. The advantage of combining 31P and 1 H DOSY NMR is to obtain information on the dynamics of both H3PW and C8G1. For H4SiW, only 1H DOSY could be conducted on C8G1. DOSY NMR provides information on the dynamics of the systems, i.e., self-diffusion of the POM and the surfactant micelle, and enables us to investigate the effect of adsorption on the self-diffusion of the POM. The chemical shift of the 31P NMR signal of PW3− for [H3PW] = 30 mM was recorded with increasing C8G1 concentration from 0 up to 500 mM; see Figure 5a. In this concentration range, C8G1 mostly forms micelles, as the cmc ranges from 10 to 26 mM (Figure S6), and PW3− adsorbs on the micelles, as clearly monitored by SAXS (Figure S7). The SAXS intensity profiles indeed show large oscillations indicating the presence of the core−shell.

Figure 5. (a) Chemical shift of the 31P peak from PW3− as a function of C8G1 concentration. (b) Fraction of H3PW adsorbed on the micellar surface versus added C8G1 concentration (i) experimental data estimated using eq 4 (black squares), (ii) fit for K using the Langmuir isotherm (red line), and (iii) fit obtained from the Langmuir isotherm substituting K stemmed from the 1:1 binding model (blue line).

The NMR signal becomes constant for [C8G1] > 200 mM, meaning that above this concentration the chemical environment around the POM does not change with further addition of the surfactant. This indicates that all POMs are adsorbed on micelles when [C8G1] > 200 mM. Therefore, at this specific concentration, [C8G1] = 200 mM, the average C8G1/PW3− ratio (6.7) corresponds to the lowest accessible ratio in micelles, i.e., the maximum coverage of PW3− on the micellar surface. This value is in good agreement with the C8G1/PW3− ratios determined by SAXS (4.3) and by an ion flottation experiment (5.4) in a previous study.22 As a consequence, when the C8G1 concentration increases above 200 mM, the surfactant/PW3− ratio increases, which means that the micelle surface charge, brought about by the adsorbed PW3−, decreases. The mole fraction of PW3− adsorbed at the micellar surface, ads f POM, can be simply expressed as a function of the chemical shift as 2034


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Langmuir ads = f POM

(δ − δ0) (δads − δ0)

(4)

by assuming that (i) the chemical shift without C8G1 (δ0 = 15.298 ppm) corresponds to the free PW3− and that (ii) the maximum chemical shift obtained for [C8G1] > 200 mM (δads = 15.427 ppm) corresponds to PW3− adsorbed at the micellar surface. By assuming that f ads POM corresponds to the surface coverage on the micelles by POM, with f ads POM = 1 in the case of maximum coverage, then a plot of f ads POM vs [C8G1] (Figure 5b) can be assimilated to an adsorption isotherm. A simple Langmuir model for monolayer adsorption was applied to fit the f ads POM vs [C8G1] curve; see Figure 5b. However, the significant deviation from the Langmuir model, obtained at low surfactant concentrations, is likely to arise from PW3−−PW3− electrostatics repulsions at the micellar surface. Therefore, the best fit gave only a rough estimate of the adsorption constant (K ≈ 40 M−1) of PW3− on the micellar surface. Moreover, at low surfactant concentrations the surfactant monomers, involved in the monomer−micelle equilibrium, may compete and artificially decrease the adsorption constant. This latter effect should vanish at higher surfactant concentrations well above the cmc (10 mM), i.e. [C8G1] ≫ cmc. A 1:1 stochiometric binding model69 (see the SI for more details) was also applied and gave a good fit of the data with K = 9 M−1; see Figure S8. However, this model assumes the formation of a 1:1 PW3−/C8G1 complex, which is not relevant here as no well-defined stoichiometry is expected. Nevertheless, this model gives the order of magnitude of the association constant. The simulation of a Langmuir model with K = 9 M−1 was plotted in Figure 5b (blue line). It produces f POM values much lower than the ones obtained experimentally for the high surfactant concentrations, but good agreement is obtained in the low surfactant concentrations. Consequently, the adsorption constant of PW3− on the C8G1 micellar surface lies in the range of 9 < K < 40 M−1, confirming that POMs strongly adsorb on polar neutral surfaces in aqueous phases, as was qualitatively demonstrated by SAXS in a previous study.22 The diffusion coefficient values of PW3− (DPW3−) and C8G1 (DC8G1) as a function of surfactant concentration, for 30 mM H3PW, obtained respectively by using 31P and 1H DOSY NMR, are presented in Figure 6. DPW3− strongly decreases from 3.48 × 10−10 to 7.5 × 10−11 m2/s for 0 < [C8G1] < 200 mM, and then it only slightly decreases for [C8G1] > 200 up to a value of 3.67 × 10−11 m2/s for [C8G1] = 300 mM. This result is in full agreement with the evolution in the 31P chemical shift (Figure 5), which provided information about the maximum coverage of micelles by PW3− for [C8G1] > 200 mM, i.e., for C8G1/PW3− > 6.7. The evolution of DC8G1 by increasing [C8G1] has a similar shape as for DPW3−; see Figure 6. However, DC8G1 is much smaller than DPW3− for [C8G1] → 0, as expected from the large difference in size between C8G1 micelles (R > 2 nm)70,71 and PW3− (R = 0.46 nm). By increasing [C8G1] from 0 to 200 mM, DPW3− becomes comparable to DC8G1 indicating that PW3− sticks strongly on the micelles. The diffusion coefficients of C8G1 in water, i.e., without PW3−, were also determined for comparison (Figure 6 inverted triangles) and show a decrease by increasing [C8G1]. Nilsson et al. have reported that the decrease in the diffusion coefficient was related to the increase in the C8G1 micelle size by increasing the surfactant concentration.71 However, it was also shown that the adsorption of PW3− on

Figure 6. Diffusion coefficients of PW3− (dark squares), C8G1 in the presence of [H3PW] = 30 mM (red circles), and C8G1 in pure water (magenta inverted triangles) as a function of surfactant concentration. Simulations of D using eq 5 with two different values of adsorption constants: K = 9 M−1 (solid line) and K = 40 M−1 (dashed line). All data were obtained at 25 °C.

C8G1 micelles leads to a decrease in the micellar size and the formation of spherical micelles.22 The fraction of POM adsorbed on the surface of micelles can be estimated from the PW3− diffusion coefficient, with the same assumptions made for eq 4 ads f POM =

(D − D0) (Dads − D0)

(5)

where D is the diffusion coefficient of PW3− at a given C8G1 concentration, D0 is the diffusion coefficient of PW3− without surfactant, and Dads is the diffusion coefficient of PW3− when all of the POM is adsorbed on the micelles. The simulations of the D vs [C8G1] curves by using the Langmuir isotherm model with the K values found above, 9 and 40 M−1, were added to Figure 6. The agreement between the two simulation curves and the experimental data is comparable to the one obtained above for the fit of the 31P chemical shifts. This result indicates that the range of K values obtained above is valid. Therefore, the evolution in the 31P chemical shifts and in the C8G1/PW3− diffusion coefficients with [C8G1] are in good agreement. Comparison H4SiW and H3PW. The diffusion coefficients of C8G1 (DC8G1) as a function of [C8G1] in the presence of 30 mM H4SiW and 30 mM H3PW are presented in Figure 7. Both H4SiW and H3PW lead to decreases in the C8G1 diffusion coefficient, but the decrease is more pronounced with H3PW. As already stated, H3PW has a stronger affinity for the C8G1 micelle surface than does H4SiW.22 This difference in their surface affinity is reflected in the evolution of the C8G1 cmc by the addition of POMs (Figure S7): (i) H4SiW shifts the monomer−micelle equilibrium toward the formation of monomers, i.e., it increases the cmc, whereas (ii) H3PW, due to its higher affinity for the micellar surface, decreases the cmc, promoting micelle formation. Moreover, the DC8G1 values, as measured by 1H DOSY NMR, are averaged values between the contributions of C8G1 as monomers (Dmonomer ) and as micelles C8G1 monomer micelle (Dmicelle ), with D > D due to the larger size of the C8G1 C8G1 C8G1 micelle compared to the size of the monomer. Therefore, the contribution of monomers to DC8G1is more significant in the 2035


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from their size (close to 1 nm), which lies at the border of the IUPAC definition of colloidal systems, and they can therefore be investigated using classical experimental approaches for charged colloids, e.g., by investigating the POM−POM structure factor in solution as determined by SAXS.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b03640. Dynamic light scattering, Debye length, SAXS measurements, adsorption constant, and 1:1 binding model (PDF)

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Figure 7. Diffusion coefficients as a function of surfactant concentration for 30 mM H3PW without surfactant (black squares); C8G1 in the presence of 30 mM H3PW (red squares), C8G1 in the presence of 30 mM H4SiW (blue squares), and C8G1 alone (magenta inverted triangles). All data were obtained at 25 °C.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Luc Girard: 0000-0003-2300-4875 Pierre Bauduin: 0000-0001-6128-7971

low surfactant concentration range, [C8G1] < 200 mM, for H3PW compared to H4SiW. This effect explains the origin of the stronger decrease in the diffusion coefficient observed with H3PW compared to H4SiW. For higher surfactant concentrations, [C8G1] > 200 mM, the contribution of monomers to DC8G1 becomes negligible for both H3PW and H4SiW. However, the DC8G1values remain lower with H3PW than with H4SiW. This difference is likely to arise from the higher charge of the micelles covered by SiW4− compared to PW3−. Indeed, stronger repulsive interactions between micelles, arising from the POMs adsorbed at their surface, are expected to increase the micelle diffusion coefficient. Unfortunately, the zeta potential measurement, giving information on the micellar surface charge, could not be conducted because of electrochemical reactions of POMs at the electrodes, resulting in the formation of blue-dark coloration due to the presence of reduced POMs species.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Pr. J-F. Dufrêche, Dr. S. Gourdin, and Th. Buchecker for fruitful discussions. National funding ANR-12BS08-0017 (CELADYCT) supported this work.

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ABBREVIATIONS POM, polyoxometalate; SiW 4− , silicotungstate; PW 3− , phosphotungstate; H4SiW, silicotungstic acid; H3PW, phosphotungstic acid; C8G1, octyl-β-glucoside; PEG, poly(ethylene glycol); DLS, dynamic light scattering; SAXS, small-angle X-ray scattering; PGSE NMR, pulsed gradient spin echo nuclear magnetic resonance; DOSY NMR, diffusion-ordered spectroscopy nuclear magnetic resonance; MSA, mean spherical approximation; MD, molecular dynamics

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CONCLUSIONS The POM−POM interactions as well as the adsorption of POM on a neutral micellar surface were studied by combining dynamic and static approaches. This comprehensive analysis demonstrated the importance of electrostatic repulsions between POMs and its impact on the dynamic behavior of POM anions in aqueous solutions. It was shown that the selfdiffusion of SiW 4− and PW 3− , at different POM/salt concentrations, could be rationalized on a master, based on electrostatic considerations and without taking into account POM’s counterions in the Debye length calculation, as for classical charged colloids. The electrostatic inter-POM interactions obtained by SAXS confirmed that a classical colloidal approach can be used to describe POMs in solution. The strength of the spontaneous adsorption of POMs on polar surfaces, i.e., their behavior as (chaotropic) anions, was also evaluated by fitting the combined data obtained by 1H/31P NMR and 1H/31P DOSY NMR by adsorption isotherm models. This confirmed that the stickiness of POMs toward polar (hydrated) interfaces is very high and that POMs can be classified as superchaotropic. As a conclusion Keggin’s POMs exhibit dual behavior as anions and as colloids: (i) POMs behave as anions because they adsorb at surfaces as chaotropic anions, but (ii) POMs can also be considered to be colloids

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REFERENCES

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