Nanometer-Size Polyoxometalate Anions Adsorb Strongly on Neutral

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Nanometer-Size Polyoxometalate Anions Adsorb Strongly on Neutral Soft Surfaces Bappaditya Naskar, Olivier Diat, Véronique Nardello-Rataj, and Pierre Bauduin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06273 • Publication Date (Web): 17 Aug 2015 Downloaded from http://pubs.acs.org on August 24, 2015

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The Journal of Physical Chemistry

Nanometer-Size Polyoxometalate Anions Adsorb Strongly on Neutral Soft Surfaces Bappaditya Naskar,a≠ Olivier Diat,a Véronique Nardello-Rataj,b Pierre Bauduina*

a

ICSM, UMR 5257 (CEA, CNRS, UM, ENSCM) CEA Marcoule, BP 17171, 30207 Bagnols-sur-Cèze, France Fax: (+33) 466 797 611 E-mail: [email protected] ≠

Current address: Department of Chemistry, Sundarban Hazi Desarat College, University of Calcutta, Pathankhali, PIN-743611, India

b

Université Lille 1, UCCS UMR 8181, F-59655 Villeneuve d’Ascq, France

1

ACS Paragon Plus Environment


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Abstract Polyoxometalates (POMs) are nanometric metal-oxide anions with unique chemical and physical properties. During last decade, significant efforts have been made to give POMs surface activity and self-assembly properties that are essential for catalysis applications and for producing organic-inorganic hybrid materials. In this work, POMs based surfactants are produced spontaneously through non-covalent interactions in water by mixing non-ionic surfactants with POM. The most common POMs of tungstosilicate and tungstophosphate, have indeed an unexpected strong tendency to adsorb on polar and neutral interfaces. Micelles in water and water/air interfaces were investigated by SAXS and ion flotation showing the POM anions adsorbed at the micelle surface and on monolayers of non-ionic surfactants. This general property of POM provides a unique opportunity for deeper understanding of many medicinal effects of POMs, i.e. their antiviral and antitumor activities that involves their specific adsorption on biological surfaces.

Introduction Interest in POM chemistry has been constantly growing since the 90’s because of their uniquely versatile molecular structures that give rise to a variety of physical and chemical properties.1-3 Many of the applications of POMs, in various domains such as in catalysis, material science, electrochemistry, protection against corrosion, metallic patterning, lithography and medicine, involve at some stage contact with surfaces/interfaces.4-5 Therefore an increase in the concentration of the POM at interfaces is essential and desired for most applications. In this context many attempts have been made to induce surfactant properties, i.e. surface activity and self-assembly, to POMs for the development of innovative systems in molecular nanoscience. Electrostatic coupling of POMs with cationic surfactants has been first proposed to tackle this 2


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challenge, but this procedure mostly resulted in the formation of water-insoluble solids that were used for example to form solid thin films6-8 or to produce controlled self-assembled systems exclusively in non-aqueous polar solvents.9 It was also reported that POM based nanoparticles could be easily produced in water by simply mixing POMs with a cationic surfactant10 or with micelles of an anionic surfactant.11 Such POM nanoparticles were found to be surface active at the oil/water interface leading to stabilize water-in-oil emulsions (Pickering emulsions) used as reaction media with the nanoparticles entrapped POM acting as a catalyst.10 An alternative, and successful, way to induce surfactant properties to POMs was proposed by chemically grafting alkyl chains to POMs,12-15 the POM playing the role of the surfactant polar head. This approach resulted in hydrophilic POM-surfactants that were used to form microemulsions13 or smart stimuli responsive micelles in water15 with high potential for homogeneous catalysis. Nevertheless this approach requires the elaboration of time-consuming and multi-step synthesis of POM grafted surfactants that can be subject to chemical degradation such as hydrolysis. In the present contribution, we show that POM-surfactants are produced spontaneously in aqueous solutions by a self-assembly process between POMs and micelles of non-ionic surfactants. Therefore the above mentioned drawbacks related to the synthesis of POMsurfactants are avoided. This work points out a general physico-chemical property of POM polyanions in water, i.e. that they strongly adsorb through non-covalent interactions on polar electrically neutral surfaces covered by non-ionic surfactants. A polar interface refers here to a water/oil (or air) interface covered by polar moieties, such as here ethoxy- or sugar-moieties. Adsorption of ions at interfaces is given a particular attention in the discussion of specific ion effects (SIE) in biological and physico-chemical processes.16-20 Therefore the present results were discussed in terms of the SIE in order to obtain information on the intermolecular forces driving the POMs to adsorb on polar surfaces. For the last two centuries, salts and more specifically ion effects have been intensively investigated due to their large impact in many applications and processes. General trends in the specific effects of ions have been highlighted and are often referred to as the ion Hofmeister’s series. In these classifications, ions are usually ordered from salting-out ions (also sometimes described as small, hydrophilic, i.e. high charge density, kosmotropic, water structure-maker ions) to salting-in ions (large, sticky, chaotropic, water structure-breaker, polarizable ions), such as citrate3- >SO42- >F- >Cl- >Br- >I- >ClO4- >SCNtypically for anions. POM poly-anions (≈ 1 nm), that are much larger than classical anions (≈ 3



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0.2-0.3 nm), have so far never been studied in the context of SIE and classified in the Hofmeister’s series. Nevertheless they appear to be good candidates, regarding their properties: large size, charge delocalization and low charge density, to ascertain the commonly accepted idea that salting-in anions have a strong propensity to adsorb on surfaces due to their high polarizability and facilitated dehydration. We present here a systematic and refined study on the adsorption (salting-in property) of POMs on soft, neutral and polar surfaces with resolved structures at the nanoscale. Two model surfaces were investigated, i.e. micelles and water surfaces covered by non-ionic surfactants, by means of two independent techniques small angle X-ray scattering (SAXS) and ion flotation i.e. ion extraction process by a foam. The extraction of POMs by foams, produced by bubbling aqueous solutions of the non-ionic surfactants studied, is used, as a macroscopic measurement, to confirm the adsorption of POM at polar interfaces. Micelles arising from the self-assembly of POMs and non-ionic micelles can be described as organic-inorganic hybrid nano-assemblies.

Experimental Section Materials: H4SiW12O40.12H2O and H3PW12O40.12H2O were obtained from Aldrich. n-octyl –β-Dmonoglucoside (C8G1) and polyoxylene (10) oleyl ether (C18:1E10, commercially known as Brij O10) was obtained from Anatrace (purity >97%) and Fluka, respectively. AKYPO® RO 90 VG (nonaoxyethyleneoleylether carboxylic acid, R-O-(CH2CH2O-)nCH2COOH, R = C16/C18, n ≈ 9) from Kao Chemicals was used as received (88.5% purity, MW = 722 g.mol-1). C8E4 was synthesized in the laboratory (for details see SI). The salts NaCl, NaSCN and Na2WO4 obtained from Sigma-Aldrich and purity as provided from the manufacture is 99%. All the chemicals used as received unless otherwise stated. Doubly distilled water (κ = 5−6 µS cm−1 at 25 °C) was employed for solution preparation. pH values of the solutions were measured and were always below 4.0 i.e. in the range of stability of the POMs in water.28 Methods: SAXS measurements using Mo radiation (λ=0.071 nm) were performed on a bench built by XENOCS. For the flotation experiment, desired concentration of surfactant and POM solutions was poured into the flotation column and N2 gas was injected continuously. (for details see SI 4


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and figure S4). The POM and surfactant concentration before and after flotation were analyzed by X-ray fluorescence (XEPOS Spectro, Germany) and Shimadzu TOC-VCSH analyzer, respectively. The cloud point of surfactant solution in presence of POMs was measured by taking the solution in a thin glass tube and the point of clouding was detected visually by comparing with the original solution against an illuminated background.

Results and discussion Choice of the model systems Salts of classical Keggin-type hetero-polyoxometalates anions, SiW12O404- (SiW4-) and PW12O403- (PW3-), and non-ionic polyethoxylated and sugar based surfactants were chosen as model systems (figure 1a). SiW4- and PW3- were selected due to their high stability and fully symmetric structures. Moreover the change in the central heteroatom from Si to P enables to tune the overall charge without affecting the size of the POMs (i.e. isostructural). Two representatives of the most commonly used non-ionic surfactants, i.e. polyethoxy- and sugar based surfactants, were chosen: chemically pure tetraethyleneglycol monooctyl ether, C8E4 (figure 1b) since it shows a clouding point in water that enables an easy classification of salt effects (see below), and n-octyl-β-D-monoglucoside, C8G1 (figure 1c), because of its interest in biology as cell adhesion and cell recognition are mostly mediated by glycolipids at the cell surface. These surfactants have the same hydrocarbon chain length but differ from their head group structures.

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Figure 1: Optimized structures of (a) [SiW12O40]4- (SiW4-) or [PW12O40]3- (PW3-); n = 4 (Si) and 3 (P), (b) tetraethyleneglycol monooctyl ether (C8E4), and (c) n-octyl-b-D-monoglucoside (C8G1). Adsorption of POMs on micelles: a SAXS study SAXS is particularly well adapted to investigate the micellar systems studied herein as it is sensitive to electron density inhomogeneity from the atomic to the sub-nanometer distances. Owing to their large sizes and high electron densities, due to the presence of tungsten atoms (Z = 72), POMs produce a very high contrast when dissolved in liquids such as water. Therefore adsorption of POMs on micellar systems can be easily monitored by SAXS. The SAXS spectra of C8E4 (60 mM), SiW4- (10 mM) and their mixtures are shown in Figure 2A. The concentration of C8E4 is above its critical surfactant concentration (CMC = 8.5 mM) therefore C8E4 micelles are present in solution. The scattered intensity is plotted in absolute scale (see SI) as a function of wave vector q, defined as q = 2π / λ sin(θ / 2) where θ is the scattering angle and λ is the X-ray wavelength. Because of the low scattering signal of C8E4 micelles in water, their signal was undetectable with our lab set-up. On the contrary SiW4- in pure water showed at only 10 mM a scattering pattern that is typical of the scattering of dispersed spherical objects that can be well fitted (full line in Figure S1, the description of the model is given in SI) SAXS with a radius RSiW = 0.44 nm that is in agreement with the molecular size.21 The mixture of C8E4 4−

and SiW4-, respectively at 60 and 10 mM, revealed a completely different scattering pattern, showing large and intense oscillations, that is a typical signature of a core-shell system with an electron density excess in the shell. This intense signal appears in the lower q-range, q < 3 nm-1, corresponding to distances in real space larger than 2 nm i.e. larger that the POMs size. Consequently, the SAXS pattern seems to indicate that SiW4- adsorbs at the surface of C8E4 micelle producing a high electron density shell around the micelles. This was confirmed and quantified by fitting the whole scattering curves using a core-shell model (see below). POMs adsorption seems to be a general process on micelles of non-ionic polyethoxylated surfactants like for example the industrial grade surfactant Brij O10 (C18:1E10) as shown in SI, Figure S2. It is interesting to note that by addition of the POMs the micelle signal becomes visible in SAXS, the POMs acting as X-ray contrast agents by illuminating the micelles. SAXS measurement for the mixture of C8E4 (60 mM) and PW3- (10 mM) was not performed due to a low solubility limit 6


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of the surfactant in the presence of POM for [PW3-] above 4 mM. Raman spectra seem to indicate that no direct bond is involved between POMs and the surfactants (see SI, Figure S1). SiW4- shows that the characteristic band at 996 cm-1 (stretching vibration of W-O)22 remains unchanged in the presence of surfactant. This suggests that the adsorption of POMs has a physico-chemical origin and is not due to chemical binding (covalent bond) between the POMs and the surfactant. At low q values, q < 0.6 nm-1, the SAXS spectra of the SiW4- and of the C8E4-SiW4- micelles show a clear depression in the scattering intensity that can be ascribed to strong inter-POMs and inter-micelle repulsions, respectively. Addition of salt, NaCl, clearly leads to suppress these strong repulsions as an increase in the scattered intensity in the low q-range is observed (see Figure 2B). This proves that these repulsions are of electrostatics origin. POM salts dissociate from their counter-ions in water producing an apparent charge on the POM that is easily observed by conductivity measuring.23 Therefore, the charge of POMs induces electrostatics repulsions between the POMs in water as well as between the C8E4 micelles that are charged due to the POMs adsorbed on their surfaces. The nonionic micelles become artificially ionic in the presence of POMs meaning the surfactant becomes more hydrophilic, this is in agreement with the increase in the C8E4 cloud point by adding POMs (see below). The study was extended to another type of non-ionic micellar system by varying the nature of the surfactant polar head using a glucose unit (C8G1) instead of a polyethoxylated moiety. SAXS also indicates that SiW4- adsorbs on the surface of C8G1 micelles as it was observed with C8E4 micelles, see Figure 1C. The influence of the charge density of POMs were also varied by 4SAXS = 0.46 nm, see SI) compared to SiW investigating PW3- that has a similar size ( RPW (Figure 3−

2C). C8G1-PW3- shows a slight shift in the position of the oscillation and an increase in the scattered intensity in the low q-regime compared to the C8G1-SiW4-. SiW4- and PW3- have the same electron density, therefore they produce a similar electronic contrast. Consequently the increase in the scattered intensity observed with PW3- is related to its stronger adsorption on C8G1 micelles. SAXS experiments were also carried out with other ethoxylated and carboxylic acid surfactant with SiW4- (see in SI Figure S2). They all lead to a similar conclusion proving that the adsorption effect of the POM on polar (strongly hydrated) non-ionic micellar surfaces is a general process.

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The adsorption of POM on C8G1 and C8E4 micelles was also highlighted by dynamic light scattering (DLS) experiments (see supplementary information). Addition of POMs clearly shows the decrease in the micellar size for C8G1 and C8E4 respectively from 8 to 4 nm (SiW and PW) and from 5 to 2.6 nm (SiW). The decrease in size of C8G1 and C8E4 micelles upon adsorption of POM on their surface is likely to be related to the subsequent (i) increase in the surfactant polar head, decreasing the surfactant critical parameter, and (ii) increase in the electrostatic repulsions between the polar heads.

Figure 2: SAXS spectra of (A) C8E4, SiW4- and their mixtures in aqueous medium; (B) C8E4, SiW4- and their mixtures in presence of salt; (C) C8G1, SiW4-, PW3- and their mixtures in 100 mM NaCl solution. The ordinate axis is the absolute X-ray intensity. The fits are shown as black solid lines. All the experiments were performed at 23°C.

8


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The SAXS spectra have been fitted in order to have a quantitative description of the micelles and the POM distribution at the micellar level. The scattered intensity was considered as a sum of contributions from micelles, POMs and background as: I (q ) = I POM + I micelles + Bkg

Eq. (1)

The contributions of the POMs and micelles were expressed in absolute value as: 2

(

2 I (q ) = nPOM VPOM (SLDPOM − SLDwater ) Psphere (RPOM , q )S HS q,ϕ POM , RHSPOM

+ nmicelles Pcore−shell⋅⋅ sphere (R, dr, SLDcore , SLDshelle , q )S HS (q,ϕ , RHS ) + Bkg

) Eq. (2)

with n, the particle concentrations, V, the volume of the particles, SLD, the scattering length density which is related to the electron density, P and S, the form and structure factors that take into account the shape of the particles and the inter-particles interactions. A spherical form factor, Psphere , and a spherical core-shell form factor, Pcore−shell⋅sphere , were used to describe the shapes of (i) the POMs with a radius, RPOM, and (ii) the micelles with a radius of the core, R, and a shell thickness, dR. The fitting procedure was performed on spectra of solutions in the presence of 100 mM NaCl in order to screen electrostatics repulsions between the particles. Therefore, interactions between the POMs were taken into account with a hard-sphere structure factor,

(

POM S HS q, ϕ POM , R HS

)

and S HS (q, ϕ , RHS ) respectively between POMs and between micelles, with

RHS, the hard-sphere radius, and ϕ , the volume fraction. The radii and the SLD of the POMs were determined independently by fitting the SAXS spectra of the POMs in 100 mM NaCl aqueous solution (see the results in Table 1, and the fits and procedure in SI). For micellar systems the fitting procedure was constrained by keeping 3 some of the parameters constant: RPOM, VPOM simply expressed as VPOM = 4 / 3πRPOM , SLDPOM,

SLDwater, SLDcore and nPOM being calculated from the total POM concentration. nMicelles is a constrained parameter expressed as nMicelles = ([Surfactant]-cmc]/Nagg with cmc the surfactant critical micellar concentration and Nagg the aggregation number, i.e. the number of surfactant per micelle, expressed as Nagg= Vmicelles/Vsurfactant=(4/3πR3)/Vsurfactant with R the core radius being fitted. The other parameters (Bkg, SLDshell, R, dR, RHS, and ϕ ) were fitted. It is noteworthy that the much higher SLD values of the POMs compared to the SLDs of the other components, surfactant and water, (see Table S1 in SI) imply that (i) SLDshell is mainly influenced by the POMs concentration in the shell and (ii) dR is therefore close to the size of a POM. Consequently the most influent parameters on the scattering spectra are here R, dR and SLDshell listed in Table 9



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1, along with SLDcore. This model was able to fit properly the experimental SAXS spectra for C8E4-SiW4-, C8G1-SiW4- and C8G1-PW3-, see the full lines in figure 2B and 2C. C8E4-SiW4- micelles have an overall size of 2.41 nm (Rcore+Rshell = 1.61 + 0.80 nm) that is in agreement with the size of (spherical) C8E4 micelles in water obtained from dynamic light scattering, RH = 2.5 nm.24 The micelle radius is very close to the surfactant length in an extended conformation (≈ 2.6 nm) implying that SiW4- is embedded in the polethoxylated (PEO) parts of the surfactant in the micelle as drawn in figure 3A. The penetration of PW3- in PEO moities has been observed previously in lamellar phases made of a polyethoxylated surfactant.25

Table 1: Results of the fit of SAXS of POMs and with surfactants in 100 mM NaCl solution at 23 0C.

10 mM SiW410 mM PW360 mM C8E4 +10 mM SiW4100 mM C8G1 +10 mM SiW4100 mM C8G1 +10 mM PW3-

R nm 0.44 0.46 1.61

dR nm na na 0.8

SLDcore cm-2 .10-11 9.63 8.42 0.91

SLDshell cm-2 .10-11 na na 1.69

[surfactant] [POM]

1.60

0.86

0.99

1.20

10.4

1.35

0.9

0.99

1.28

4.3

na na 5.3

For C8G1-SiW4- micelles the situation is somewhat different as the overall radius of the micelle (Rcore+Rshell = 1.60 + 0.86 nm) corresponds exactly to the sum of the length of C8G1 in an extended conformation, around 1.6 nm, and the diameter of the POMs, around 0.88 nm. Therefore SiW4- is located at the micelle surface without penetrating the sugar moiety, as drawn in Figure 3B. A previous investigation by SAXS using synchrotron radiation revealed that C8G1 micelles have a bi-axial ellipsoid shape, with an aspect ratio ranging from 3.5 to 5.9 in the surfactant concentration range 50-150 mM, that is nearly independent of the presence of salts (LiNO3, Nd(NO3)3) up to high salinity, i.e. in the molar range.26 Consequently adsorption of SiW4- on C8G1 micelles has a dramatic effect, and a strong ion specific effect, on the micelle size. The decrease in the micelle size is subsequent to SiW4- adsorption that induces an increase in the average area occupied by the surfactant polar head and an increase in the repulsive interactions 10


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between the surfactant polar head. The difference in the penetration behavior between C8E4 and C8G1 is likely to be related to (i) the stronger hydration of glucose compared to the one of polyethoxylated (PEO) part; the glucose part has many hydroxyl groups that are both donor and acceptor of H-bonds whereas PEO part has mainly ethereal oxygens that are only H-bond acceptor; or (ii) to some conformational entropy effect due to the chain flexibility of polytehoxylated part compared to the glucose moiety that is rigid.27 POMs can easily penetrate loosely hydrated PEO moiety but it is hindered by the strong hydration of the sugar moiety. For C8G1-PW3- micelles the core radius (Rcore = 1.35 nm) was found to be significantly smaller compared to the one obtained with C8G1-SiW4- micelles (Rcore = 1.6 nm) meaning that PW3- partially penetrates in the glucose part of C8G1 micelles.

(A)

(B)

Figure 3: Schematic representation of the POM distribution in the micelle with based on coreshell model of SAXS fitting results. Yellow and blue region represents of the micelle’s core and shell, respectively, whereas R is the radius of the core and dR is the thickness of the shell. SiW4distribution on C8E4 micelle (A) and C8G1 micelle (B).

Adsorption of POMs at the water surface: an ion flotation study The POMs adsorption behavior on electrically neutral interface was further exemplified by performing ion flotation experiments. Ion flotation is an extraction/separation technique for recovering and removing metal ions from dilute aqueous solutions.28 In this process an ionic surfactant is used to make (metal) ions surface-active so that they adsorb at the air-aqueous phase 11



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interface. Foam is produced by bubbling gas through the solution and the surfactant-ion couple is removed thereafter by skimming the foam. Consequently pure electrostatic interactions govern the adsorption of ions at the water-air interface in the foam by fulfilling electro-neutrality between the ionic surfactant and its counterion. Ions are therefore not expected to be extracted in a flotation experiment using non-ionic surfactants, which do not contain any ion complexing groups in the structures. Therefore ion flotation experiment with non-ionic surfactants offers an unequivocal proof of the POM adsorption. We proposed here to use non-ionic surfactants, a PEO-surfactant (Brij® O10; C18:1E10) and C8G1, in flotation experiments in the presence of POMs, SiW4- or PW3- (see the experimental procedure in SI). The surfactant concentration was kept below the CMC value, [Brij] = 0.5 mM (CMCBrij = 0.9 mM) and [C8G1] = 5 mM (CMCC8G1 = 19 mM), in order to study ion adsorption only at the water-air surface and to avoid interference with the POMs adsorption on micelles. The efficiency of the extraction, see Table 2, was calculated from the POMs concentrations at the initial and final (residual) stages of the flotation process as well as from the POMs concentration in the foamate, i.e. the solution obtained from the collapsed foam. The extraction rates are significant, around 20%, for all flotation experiments meaning that 20% of the POMs in solution were extracted by the water-air interface of the foam. Note that similar experiments conducted with classical Hofmeister salts (NaCl, NaSCN…) led to non-detectable effects on the extraction rate, i.e. the difference in the ion concentration between the initial and final stages are below the experimental errors (1% of the initial ion concentration). The high range of extraction rates obtained here with the POMs is a direct proof of the strong POM adsorption at the water-air interface covered by neutral surfactant molecules. It should be noted that it is possible to complete extraction of POMs by continuous supplying/feeding of surfactant solution in the column.

Table 2: Flotation results of POMs and surfactants at 23 0C Extraction%a

[surfactant] [POM]

0.5 mM Brij O10 + 0.5 mM SiW4-

18.0

5.14

0.5 mM Brij O10 + 0.5 mM PW3-

22.2

4.30

12


b

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a,

extraction% =

5 mM C8G1 + 5 mM SiW4-

25.2

3.90

5 mM C8G1 + 5 mM PW3-

19.7

5.38

[POM ]initial − [POM ]residual 100 b, surfactant [Surfactant ] = [POM ]initial POM [POM ]

initial initial

− [Surfactant ]residual − [POM ]residual

Interestingly, the surfactant/POMs ratio in the surfactant film at the water surface, expressed as a function of POMs and surfactant bulk concentrations (see Table 2), gives values, from 4 to 5, in the same order of magnitude and the same ratio at the micellar surface, from 4 to 10 (see Table1), deduced from the SAXS data treatment. Moreover, the SAXS and flotation experiments suggest also that POMs adsorb independently of the surface curvature.

Cloud point measurement and discussion on specific ion effects The solubility of non-ionic surfactants decreases with increasing temperature by way of dehydration of the ethoxy moiety (loosening of hydrogen bonding between the ethereal oxygens of ethoxy group and water), and polar-nonpolar conformation change of the ethoxy group.29 Upon heating, aqueous surfactant solutions overcome a liquid-liquid phase transition at a temperature called the cloud point (CP). Adding salts to non-ionic surfactants decreases or increases their cloud points, i.e. their aqueous solubility, according to the salting-in and saltingout ability of the salts. This effect has been used to classify ions, which results for most surfactant systems in the usual Hofmeister’s series.18, 30-32 An attempt was made here to put the POMs in this ion classification by determining the evolution of the CP of C8E4 as a function of salt concentration and by comparing these evolutions with some other more classical salts. The CP of C8E4 at 60 mM was found 40 °C in pure water which is in agreement with the literature report.33 Figure 4 shows the evolution of the CP as a function of concentration of different salts by varying the anion: WO42-, a parent component of the studied POMs, Cl-, SCN-, SiW4- and PW3-. A classical trend in the CP evolution is obtained: WO42- > Cl- > SCN- with Clhaving a rather neutral effect and WO42- (comparable to SO42-) and SCN- acting as salting-out and salting-in agents, respectively by decreasing and increasing the CP of C8E4 in a narrow temperature window of ± 5°C. The addition of POMs led on the contrary to a tremendous 13



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increase in the CP up to more than 90°C at only very low POM concentrations. Consequently the salt effect on the CP of C8E4 gives the following classification of anions, from salting-out to salting-in: WO42- > Cl- > SCN- >> POM4- > POM3-. It was shown by Glatter et al.34 that the cloud point phenomenon of CiEj surfactants was resulting from the change in the micelle shape in solution. Dehydration of the ethoxy moieties, induced by an increase in the temperature, leads to a sphere to rod-like transition in the micelle shape. This shape transition is concomitant with a dramatic increase in inter-micellar attractions resulting ultimately in phase demixion. Therefore the strong increase in the CP by adding POM can be attributed here to the adsorption of POM on micelles, revealed by SAXS (see the discussion above), that induces both inter-micellar repulsions of electrostatic origin and a decrease in the micellar size. Compared to POM, classical Hofmeister’s salts have only a slight effect on the micellar size of non-ionic surfactants.26 The increase in the CP observed with POM addition corresponds, in other words, to an increase in the hydrophilicity of the surfactant which is not surprising as C8E4 is getting charged through the POM adsorption. POM adsorption results then to the spontaneous formation of POM surfactant in solution. Regarding their effect on the cloud point of C8E4 and their strong propensity to adsorb on polar neutral surfaces, POMs can be qualified as super-chaotropic anion.

Figure 4: Cloud points of a 60 mM C8E4 solution in the presence of POMs: SiW4- and PW3-, and sodium salts with various representative anions of the Hofmeister’s series: SCN- (salting-in), WO4- (salting-out) and Cl- (intermediate between salting-in and salting-out). Inset: ion containing surfactant solution for temperatures below (left) and above (right) the cloud point. The cloudy 14


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appearance of the solution arises from the liquid-liquid phase demixion of a diluted- and a concentrated-surfactant phase. The self-assembly process between the POM and polar surfaces is not due to pure electrostatic attraction, as observed generally between two species of opposite charges. In other words no ion pair between the POM anion can form with the non-ionic surfactant head. The POMs and the studied polar surfaces are both oxygen rich and highly hydrated and have therefore no apparent reason to associate at low concentrations in water. What is then the reason for the strong propensity of POMs to adsorb, apparently not specifically, on polar surfaces? For many decades SIE have been ascribed to the salt-induced changes of water properties and to the ability of ions to bind at charged interfaces.16-17, 20 The terms salting-out and salting-in ions, or water structure-maker and -breaker, were generally used to refer respectively to small, highly hydrated, non-adsorbing ions and to large, polarizable, loosely hydrated and adsorbing ions. Many attempts have been made in the past to correlate SIEs with ion properties, such as the size, polarizability, free energy/entropy of hydration of ions.19 However this approach only had a limited success mainly because it did not consider the surface. Only recently the polarity (or charge) of the interface has been proposed to be at the origin of some exception or reversal of specific ion effects.35-36 The authors could conclude that (i) hydrophilic ions are attracted to hydrophilic surfaces,36 because hydrated ions have the opportunity to shed off some water of hydration when they adsorb, whereas (ii) large hydrophobic ions, such as tetraphenylborate ions, adsorb exclusively on hydrophobic surfaces and not on hydrophilic ones.35 From all these considerations it can be concluded that SIE results in a complex and subtle balance between ionwater/water-surface interactions that involves hydrophobic effects, ion/surface dehydration, and ion polarizabilities with both enthaplic and entropic origins.19 The adsorption of an ion on a surface implies that some water molecules sweep off from the hydration shells of the surface and of the ion, as shown in figure 5. This partial dehydration process has a high enthalpy cost which is supposedly higher for small salting-out anions compared to large salting-in anions of low charge density. On the opposite the ion adsorption process promotes the system entropy by releasing many water molecules into the bulk phase. This latter effect is supposed to be prominent over the enthalpy cost for POM anions, owing to their large size (surface). On the contrary for small salting-in anions, showing a much weaker tendency to adsorb on the surface, it seems that the entropic benefit does not to compensate the 15



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enthalpy loss as much as for the POMs. Moreover the delocalized charges of POMs and their low charge densities, which are primarily due to their large sizes (∼ 1 nm) compared to the more classical anions (≈ 0.2-0.3 nm), are likely to confer to them high polarizability values, low dehydration enthalpy and therefore a strong propensity to adsorb. The high polarizability of the POMs, in addition to their large sizes, is expected to reinforce van der Waals forces, i.e. permanent dipole-induced dipole interactions, established with polar surfaces. It seems that this hypothesis agrees comparing PW3- and SiW4-. PW3- has indeed a lower charge density, and therefore an expected higher polarizability, compared to SiW4- and displays the strongest saltingin effect on the CP. This stronger chaotropic effect of PW3- over SiW4- is also supported by the SAXS results on the micelles (see Fig. 2) showing a deeper penetration of PW3- in the micelle shell. A question arises here: Are polar moieties, ethoxy- or sugar- moieties, at water/oil (or air) interfaces needed for the POMs to adsorb? To answer this question, interfacial tension measurements were conducted at the water/dodecane interface, i.e. an “apolar” interface which is not covered by hydrophilic moieties, by adding POMs. No significant variation in the water/dodecane interfacial tension was measured upon addition of POM proving that POMs do not adsorb on apolar surfaces. This indicates that polar hydrophilic moieties are needed at interfaces for the POMs to adsorb.

Figure 5: Schematic representation of the adsorption a POM anion on a hydrophilic interface covered here by the poly-ethoxylated surfactant C8E4. Both the surface and the POM undergo

16


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partial dehydration with an enthalpic cost being counterbalanced by a significant entropic gain related to the release of many water molecules in the bulk aqueous phase.

Conclusions and outlook This work has shown that the extension of the classical Hofmeister’s series towards super chaotropic anions is accessible to large nanometric multi-charged ions. In this regard POMs emerged as model representatives of super chaotropic anions. In view of these results, POMs appear to have a general tendency to adsorb on hydrophilic (highly hydrated) curved and planar surfaces. Therefore mixing a non-ionic surfactant with a POM leads to the spontaneous formation of a POM-surfactant through a self-assembly process in water. This process is supposedly driven by a significant increase in the system entropy which is related to the release of many water molecules in the bulk during the POM adsorption, with both the POM and the surface undergoing partial dehydration. Considering the extreme diversity of biological interfaces from protein surfaces, being locally polar, apolar, neutral and charged depending on the amino-acid sequence and on the protein secondary structure, to viruses, bacteria or cell surfaces it is expected that the strong propensity of POMs to adsorb on hydrophilic surfaces is involved in biological processes of many kinds. POMs have been found indeed to have antiviral, antibacterial, antitumoral activities37-38 and to have, under given conditions, the ability to cross cell membranes.37 During the last decade POMs have also been found to strongly influence blood glucose level in model animals.37,

39

It was

clearly shown that POMs have a potential in diabetes treatment while their mode of action is so far not fully understood. One might expect that the physiological effect of POMs on blood sugar level is related to the strong interaction between POMs and surface sugar moieties as highlighted here. As regards the development of new cancer treatments protein kinases represent some of the most promising drug targets.40 In the quest for new selective inhibitors of the protein kinase CK2, POMs have unexpectedly emerged as good candidates because they adsorb specifically on one of the protein subunits.41 Also noteworthy is the strong tendency of POMs to form complexes with cations. POMs have been shown for example to be remarkably selective in removing lanthanide and actinide anions from nuclear wastes.42 This specific complexation property could be useful to tune ion separation 17



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in ion flotation processes by using well suited non-ionic surfactant/POM combinations. The current results should stimulate further studies into these biological and physico-chemical effects.

Acknowledgements This work was supported by the Agence Nationale de la Recherche (ANR CATASURF, Project ANR-10-CD21-001 and ANR CELADYCT ANR-12-BS08-0017). The authors would like to thank Prof. A. Proust, Dr. G. Guillemot and Dr. V. Jallet for fruitful discussion on polyoxometalate chemistry; A. Jonchère and B. Corso for their assistance with Raman spectroscopy and SAXS measurements, respectively.

Supporting Information SAXS fitting procedure, some fitted SAXS spectra and fitting parameters are shown. Some other SAXS spectra and Raman spectra are also shown. Details of C8E4 synthesis and experimental procedure are given. This material is available free of charge via the Internet at http://pubs.acs.org

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Nanometer-Size Polyoxometalate Anions Adsorb Strongly on Neutral

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