# Superchaotropic ion flotation: A new concept for the extraction and separation of nanometer-sized ions by non-ionic surfactant-based foams

Klaudia Skorzewska, Alban Jonchère, Coralie Pasquier, Luc Girard, Pierre Bauduin

<figure><img src="image_1.png" /></figure>

## ► To cite this version:

Klaudia Skorzewska, Alban Jonchère, Coralie Pasquier, Luc Girard, Pierre Bauduin. Superchaotropic ion flotation: A new concept for the extraction and separation of nanometer-sized ions by non-ionic surfactant-based foams. Separation and Purification Technology, 2023, 323, pp.124284. 10.1016/j.seppur.2023.124284. hal-04962134

HAL Id: hal-04962134

https://hal.science/hal-04962134v1

Submitted on 22 Feb 2025

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

<figure><img src="image_2.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
1 Superchaotropic ion flotation: a new concept for the

2 extraction and separation of nanometer-sized ions by

3 non-ionic surfactant-based foams

4 Klaudia Skorzewska, Alban Jonchère, Coralie Pasquier, Luc Girard and Pierre Bauduin*

5 ICSM, CEA, CNRS, ENSCM, Univ Montpellier, Marcoule, France

6

7 ABSTRACT: Ion flotation is a separation technology for recovering ions from dilute aqueous
8 solutions. All ion flotation processes known so far make use of ionic foaming agents
9 (surfactants) to selectively extract ions of opposite charge by electrostatic interactions. Recently,
10 it was shown that nanometric-sized ions (nano-ions) with low charge density, such as certain
11 polyoxometalates (POMs), strongly adsorb to neutral hydrated surfaces. In this study, we provide
12 proof-of-concept for a method that we call "superchaotropic ion flotation," which uses non-ionic
13 surfactant foams to selectively recover and separate superchaotropic POMs from other ions,
14 including non-superchaotropic ions. Specifically, we investigated the extraction of
15 isopolyoxomolybdates, formed by pH and concentration variation of molybdate aqueous
16 solutions, using foams produced with a commercial polyethoxylated surfactant (BrijO10). The
17 speciation of polyoxomolybdates was investigated by Raman spectroscopy and small angle X-
18 ray scattering (SAXS). SAXS has also confirmed the superchaotropic behaviour of the
polyoxomolybdates species via the characterization of their strong adsorption on surfactant micelles. The flotation recovering is high and maximal at pH 1, a pH for which Mo₃₆O₁₁₂⁸⁻, an anion with a low charge density, is the predominant molybdate species. It was found that the surfactant has a strong influence on the molybdate speciation. It was also demonstrated that molybdate may be separated from tungstate at pH 2 by superchaotropic ion flotation. In conclusion, we propose a novel ion separation method via non-ionic surfactant aqueous foams and we show that the superchaotropic effect makes it possible to foresee various applications in separation science.

KEYWORDS: Superchaotropicity, ion flotation, foam, POM, polyoxomolybdate, speciation

## 1. INTRODUCTION

Molybdenum is an essential plant nutrient used in plant fertilizers and a widely used strategic metal in electronics, aerospace, industry and steel production.¹,²,³,⁴ It is also found in spent nuclear fuel; its behaviour in solution is studied because it can be found up to 20 wt.% in insoluble residues of dissolution.⁵ The primary source of molybdenum is high-grade sulphide ore, molybdenite, while other minerals like wulfenite, powellite, ferrimolybdite, and molybdite also contain molybdenum. In the world's mine production of molybdenum, the production from primary high-grade molybdenum resources (50-57%) has decreased between 1995 and 2011 from 59% to 46%.⁶ The remainder originates from secondary low-grade sources as a co- or by-product of copper, uranium⁷,⁸ or tungsten extraction (25-45% Mo).⁹ With primary sources becoming scarcer and the potential environmental pollution from industrial discharge, it is crucial to develop recycling and unconventional recovery methods for molybdenum.¹⁰ However, current extraction and separation techniques are expensive and environmentally
1 unfriendly,¹¹,¹² highlighting the need for cheaper alternatives to increase molybdenum recycling.

2 Various methods for the separation and purification of molybdenum from solution may be

3 applicable to leach solutions from molybdenum ores and raw materials or from various

4 molybdenum-contaminated effluents. These methods are solvent extraction, ion exchange,

5 membrane separation and precipitation.⁶ Solvent extraction and membrane separation have the

6 advantage of high selectivity while ion exchange allows purification and joint recovery of other

7 base metal impurities, although the latter method is less suitable on an industrial scale.⁶

8 Molybdenum is sometimes a by-product of certain mining operations, as in the case of

9 tungsten ores such as Scheelite, in which case-selective extraction processes must be

10 developed.¹³,¹⁴ This separation poses a global challenge due to their similar chemical properties.

11 Existing techniques are complex, expensive, and environmentally hazardous due to pollutant

12 generation.¹²,¹⁵,¹⁶,¹⁷ Also due to the high exploitation of ores containing more molybdate than

13 tungstate, discovering better separation methods is increasingly crucial to address these issues.

14 Ion flotation, first described by Sebba,¹⁸ is an ion concentration technology for removing

15 inorganic ion from dilute aqueous solutions (c < 10⁻⁴ M).¹⁹,²⁰,²¹,²² Several publications have

16 summarized its applications, including analytical chemistry,²³ wastewater treatment,²⁴,²⁵,²⁶

17 precious metals recovery,²⁷,²⁸ rare earth element pre-concentrating,²⁹,³⁰ and selective ions

18 separation.³¹ Zaho et al suggested its use in removing molybdate and arsenate from aqueous

19 solutions by.³² In the classical ion flotation process, ionic surface-active agents are added to the

20 aqueous solution to attract oppositely charged non-surface-active inorganic ions at the water-air

21 interface. A gas is injected through the column to produce foam concentrating the surfactant-ion

22 pair, which are then skimmed off. The extraction and separation of ions in classical ion flotation

23 rely on electrostatic interactions between the foaming agents and the oppositely charged ions.
In 2015 Naskar et al.³³ and Assaf et al.³⁴ highlighted the superchaotropic behaviour of nanometric-sized ions with low charge density referred to as nano-ions, which readily adsorb/bind to neutral hydrated surfaces or non-charged polar organic molecules.³³ This property arises when the volume charge density of nano-ions is below approximately 12 charges/nm³.

eteropolyoxometalate anions, like Keggin or Dawson types, exhibit significantly higher superchaotropicity compared to classical chaotropic anions such as iodide (I⁻) or thiocyanate (SCN⁻).³³,³⁵ Polyoxometalates (POMs) can form under specific conditions of pH, concentration, and background salts, displaying a wide diversity of sizes, structures, and chemical compositions.³⁶,³⁷,³⁸ Molybdenum is one of the extensively studied elements for POM formation.

The speciation diagram of molybdate in aqueous medium reveals various polymolybdate species depending on pH and molybdenum concentration. For concentration of molybdenum higher than 10⁻⁴, the formation of the following polymolybdate species is observed: MoO₄²⁻ (pH > 6-8), HMo₇O₂₄⁵⁻ and H₂Mo₇O₂₄⁴⁻ (pH 1.5-5), α-Mo₈O₂₆⁴⁻ (pH 2-4), β-Mo₈O₂₆⁴⁻ (pH 2-3), Mo₃₆O₁₁₂⁸⁻ and MoO₂²⁺ (pH 0-2).¹⁵,³⁹,⁴⁰ Some of these species, such as Mo₃₆O₁₁₂⁸⁻, the so-called Krebbs anion, present a very low charge density and therefore are likely to exhibit superchaotropicity.

Indeed, Mo₃₆O₁₁₂⁸⁻ has a very low volume charge density of 0.83 charge/nm³ (or 0.054 e³/atom).

For comparison, the classical superchaotropic Keggin POMs ions, α-SiW₁₂O₄₀⁴⁻ and α-PW₁₂O₄₀³⁻, have respectively a volume charge density of 8.7 and 6.5 charge/nm³ (or 0.075 and 0.057 e³/atom).⁴¹ Naskar et al.³³ and Hohenschutz et al.⁴² show that α-SiW₁₂O₄₀⁴⁻ and α-PW₁₂O₄₀³⁻ adsorb at the water-air surface covered by non-ionic surfactants: the polyoxyethylene (10) oleyl ether (BrijO10, C₁₈:₁E₁₀) and the *n*-Octyl-β-D-monoglucoside (C8G1). Keggin POMs with higher charge density such as α-BW₁₂O₄₀⁵⁻ and α-H₂W₁₂O₄₀⁶⁻ (charge density ≈ 10.9 and 13.0 charge/nm³) show respectively weak and no detectable binding to γ-cyclodextrin by ITC, i.e.
weak and no superchaotropic behaviour.⁴³ The other isopolyoxometalate species: HMo₇O₂₄·⁵⁻
(charge density ≈ 20.4 charge/nm³), H₂Mo₇O₂₄·⁴⁻ (16.3), α/β-Mo₈O₂₆·⁴⁻ (14.3) have a charge
density above roughly 12.5 charge/nm³ and therefore are not expected to be superchaotropes.

In this work, we put forward the proof-of-concept for “superchaotropic ion flotation”, a
process that selectively extracts and separates superchaotropic nano-ions from other non-
superchaotropic (nano-)ions using non-ionic surfactant foams. To this end, we studied the
flotation of isopolyoxomolybdate anions, formed by adjusting pH and concentration of sodium
molybdate solutions, using BrijO10 as the non-ionic surfactant. X-ray fluorescence was utilized
to estimate the extraction rate and assess the efficiency of the flotation process by measuring
metalate concentration before and after flotation. We then examined the separation of tungstate
and molybdate to evaluate the selectivity of superchaotropic ion flotation. The speciation of the
polyoxomolybdates species with and without BrijO10 was studied through small-angle X-ray
scattering (SAXS) and Raman spectroscopy in bulk solution. SAXS measurements on surfactant
solutions containing molybdates at various pHs and concentrations confirmed the
superchaotropic behaviour of isopolyoxometalates. As demonstrated previously, superchaotropic
POMs exhibit strong adsorption at the surface of non-ionic micelles, detected by SAXS due to
the high contrast between POMs and water or organic surfactants.³³,⁴⁴ Superchaotropic flotation
offers potential selectivity advantages over conventional classical ion flotation, which is already
environmentally friendly⁴⁵ compared to other separation methods such as solvent extraction.

## 2. EXPERIMENTAL

### 2.1 Materials and reagents

Commercial non-ionic surfactant Brij®O10, also known as poly(oxyethylene) 10 oleyl ether
(C₁₈H₃₅(OCH₂CH₂)ₙOH, n ~10) (BrijO10) (purity > 99%, surfactant content 100%, Mw= 709
1 g/mol), Sodium molybdate (Na₂MoO₄, purity ≥ 98%, Mw= 205.92 g/mol), and Sodium tungstate
2 dihydrate (Na₂WO₄·2H₂O, purity ≥ 98%, Mw= 329.85 g/mol) were purchased from Sigma-
3 Aldrich. Milli-Q water (18 MΩ Millipore with conductance 5-6 µS cm⁻¹ at 25 °C), was used for
4 the preparation of all solutions. pH was adjusted by adding hydrochloric acid 37% from Carlo
5 Erba Reagents (purity > 97%).

2.2 Determination of Mo and W concentrations

The molybdenum and tungsten concentrations in flotation solutions were determined by X-ray fluorescence spectroscopy (XRF) on a SPECTRO XEPOS III spectrometer. Prior to the measurements, the foaming solutions were diluted in milli-Q water to reach a final metal concentration of approximately 20 mM, due to the measurement limitation of the device. The measurements were performed on 5 mL of the diluted solutions. All spectra were recorded under helium atmosphere and analysed with X-LabPro® software. The effect of surfactant on the detection of molybdate by XRF, i.e. on the calibration curve, was evaluated to take into account any matrix effect. Each measurement was repeated 3 times.

2.3 Small-angle and wide-angle X-ray scattering (SAXS/WAXS)

Small-angle X-ray and wide-angle scattering (SAXS/WAXS) measurements were performed
on a bench built by XENOCS using Mo radiation (λ = 0.71 Å). The samples were in 2 mm
diameter glass capillaries and placed at a fixed sample-to-detector distance of ~77.2 cm, giving
access to a q-range of approximately 0.2 to 30 nm⁻¹. Usual corrections for background (empty
cell and detector noise) and intensity normalization using a high density polyethylene as a
standard were applied.⁴⁶ Experimental resolution was Δq/q = 0.05. Spectra of Mo₈O₂₆⁴⁻ were
simulated from atom coordinates in solid state with Crysol program.⁴⁷

2.4 Raman spectroscopy
Raman spectroscopy measurements were conducted at room temperature using a Horiba Jobin Yvon LabRAM Aramis confocal Raman microscope in backscattering geometry configuration, using an excitation wavelength of 532 nm, with a laser spot size of ~1 µm. The incident laser power was around 45 mW. The LabRam ARAMIS software was used to handle the data. The following parameters were applied for each measurement: x10 objective, hole width of 1000 µm, slit width of 200 µm, acquisition time 40-60 s and accumulation between 3-10 times. Before each measurement, a calibration was made on a silicon wafer. The samples were prepared in 2 mm diameter glass capillaries and were mounted on a sample-holder to ensure that all the capillaries were at the same distance and position. In order to subtract the background on the spectrum, an empty capillary, a capillary with water and a capillary with surfactant solution were measured.

## 2.5 Ion foam flotation experiments

### 2.5.1 Flotation setup and method

Flotation experiments were performed by pouring 60 mL of flotation solution (initial solution) into the column shown in Fig. 1 (28 mm inner diameter and 210 mm height). Nitrogen gas was injected continuously with a flow-rate of 5 mL.min⁻¹ through a sintered glass with 16 - 40 µm pores at the bottom of the glass column. The foam was collected in a 1 L round-bottom flask at a height of 210 mm, corresponding to an 80 mm foam column above 130 mm of solution. The round-bottom flask was weighed before and after each experiment to calculate the transport (see below for the detail of the calculation). Experiments ended when the foam stopped overflowing into the round-bottom flask (around 6-7 hours after the beginning of the experiment, i.e. 6-7 hours after the foam started to flow into the round-bottom flask). Samples were collected at the
1 beginning of the flotation in the initial solution, at the end in the residual solution, and in the

2 collapsed foam solution. Each experiment was repeated three times.

<figure><img src="image_3.png" /></figure>

3

4 Fig. 1. Description of the ion flotation process. Azote was injected through a sintered glass to

5 produce foam from a solution containing BrijO10 surfactant and metalate (molybdate and/or

6 tungstate). The foam produced was collected in a round bottom flask and the flotation process

7 was finished when the solution stopped foaming. The different (concentration, volume) terms are

8 defined in the text.

9

10 **2.5.2 Finding suitable surfactant concentration for flotation**

11 In ion flotation experiments, production of dry foams is desired to obtain a low transport

12 factor, T, typically below 10%.⁴⁸ Transport factor in ion flotation is defined as the volume of
1 solution obtained from the collected foam at the end of the flotation experiment over the volume
2 of the initial foaming solution:

$$T = \frac{V^f}{V^0} \qquad (\text{Eq. 1})$$

4 with, $V^0$ and $V^f$ respectively the volume of the initial foaming solution and the volume of the
5 solution collected from the collapsed foam. T is one of the criteria to consider for evaluating the
6 efficiency of a flotation process. In a flotation process, the extraction/separation of species takes
7 place at the water-air surface in the foam. Therefore, a maximum amount of surface should be
8 produced and a minimum amount of solution should be transferred in foam, i.e., low T.
9 Typically, a highly efficient flotation process shows a low transport factor (formation of a dry
10 foam), which indicates an efficient extraction with a low volume of secondary waste production.
11 The concentration of surfactant has a strong influence on T and therefore it must be optimized in
12 order to have good foaming (foam stability and foamability). Typically, T increases when the
13 surfactant concentration is increased as the amount of foam produced increases.⁴⁹ The effect of
14 the industrial surfactant BrijO10 concentration on the transport was studied (see Fig-SI. I). The
15 best compromise between good foamability and low transport (T < 10%) was obtained for a
16 BrijO10 concentration of 1.5 mM. Higher BrijO10 concentrations lead to T > 10%. For instance,
17 7 mM of BrijO10 leads to a transport factor of 30%. Therefore, flotation experiments in the
18 present study were all conducted with a fixed BrijO10 concentration of 1.5 mM.

### 2.5.3 Flotation conditions and evaluation

20 Foaming solutions were prepared by dissolving BrijO10 and sodium molybdate in water to
21 reach concentrations of respectively 1.5 mM and 20-180 mM. The initial pH of the solution was
22 adjusted to the desired values 1, 3 and 8 by adding concentrated hydrochloric acid (37% in
1 weight). The flotation experiments were conducted at a constant room temperature of 21 ± 1°C
2 in an air-conditioned laboratory.

3 Three factors are considered to characterize and evaluate the efficiency of the ion flotation
4 process.²² One of the most important is the recovery (Ri), of the species i, which quantifies the
5 decrease in the metal concentration in the foaming solution at the end of the flotation experiment.
6 The recovery informs on how much molybdate was extracted by the foam. For example, a
7 recovery of 20% means that 20% of molybdate was extracted by the foam. Considering the
8 accuracy of the experiments and measurements, we can estimate that an ion flotation experiment
9 is significant for a recovery percentage higher than 10%.⁴⁸

$$
10 \qquad Ri = 1 - \frac{V^r c_i^r}{V^0 c_i^0} \qquad (\text{Eq. 2})
$$

11 with $V^0$ and $V^r$ respectively the volume of the initial and residual foaming solutions (i.e. before
12 and after the flotation process), $C_i^0$ and $C_i^r$ respectively the solute concentration in the initial and
13 residual foaming solutions.

14 The second important efficiency factor is the transport factor (T), which was already defined
15 and discussed, see above and in Eq. 1.

16 The third factor being considered, the interfacial recovery factor (Ii), corresponds to the total
17 mole fraction of the solute adsorbed at the water/air interface and transferred into the collected
18 foam.²²,⁵⁰

$$
19 \qquad I_i = 1 - \frac{c_i^r}{c_i^0} \qquad (\text{Eq. 3})
$$

20 Ri and Iᵢ are interrelated by the following generalized equation:

$$
21 \qquad R_i = I_i + T \frac{c_i^r}{c_i^0} \qquad (\text{Eq. 4})
$$

22 Therefore, Ri, Ii and T are the important factors that we will discuss in the following sections.
1 **3. RESULTS AND DISCUSSION**

2 **3.1. Flotation**

3 The efficiency factors (Ri, T and Ii) obtained for the flotation experiments with 1.5 mM
4 BrijO10 for concentration of molybdate ranging from 0 to 180 mM at pH 1, 3 and natural pH
5 (from pH 5.5 to 8.5 depending on the molybdate concentration) are presented in Fig. 2.

<figure><img src="image_4.png" /><figcaption>7 **Fig. 2.** Efficiency factors of the flotation of molybdate with BrijO10 foam (1.5 mM in initial solution) at pH 1 (diamonds), pH 3 (squares) and natural pH (triangles): recovery factor Ri,</figcaption></figure>
transport factor T and interfacial recovery factor $Ii$ (error bars are calculated by the standard deviation on triplicates, the solid lines are guides to the eyes).

3

At natural pH, the recovery $Ri$ is not significant (5 to 7%) for the different molybdate concentrations. At pH 3, $Ri$ becomes significant but still relatively low (10 to 20%). $Ri$ increases steadily with molybdate concentration to reach a maximum at a molybdate concentration of 100 mM. Above 100 mM, $Ri$ decreases down to 12% at 180 mM. At pH 1, $Ri$ has the same evolution with the molybdate concentration as at pH 3, with also a maximum for 100 mM of molybdate, but $Ri$ increases to much higher values, up to 30%.

At natural pH, $T$ slightly increases from 7 to 8% with the molybdate concentration, presumably due to the increase of pH, from 7 to 8.5 (for, respectively, 20 to 180 mM molybdate). Under acidic conditions (pH 1 and 3) (see Fig. 2), $T$ decreases with the molybdate concentration to reach a minimum of 3.4% (for pH 1) for 100 mM.

At natural pH, negative interfacial recovery factors, $Ii$ are measured (see Fig. 2). A negative $Ii$ means that the molybdate concentration in the residual solution is increased compared to the concentration in the initial solution. It indicates depletion of molybdate ions from the air/water interface i.e. negative surface excess.⁵¹ Depletion of ions at interfaces is a well-known phenomenon in the context of Hofmeister specific ion effect.⁵² In ion flotation experiments, depletion of $Li^+$ ions was observed by Micheau et al. with an anionic sulfactant.⁵¹ At pH 3, $Ii$ is significant (> 10%) between 80 and 120 mM of molybdate with a maximum ($Ii_{max}$ = 12.8%) at 100 mM. At pH 1, $Ii$ is significant between 40 and 170 mM of molybdate with a maximum at 100 mM ($Ii_{max}$ = 26.9%). As the minimum $T$ value coincides with the maximum $Ri$ value, a maximum $Ii$ was expected according to the generalized equation (Eq. 4).
Thus, the flotation results show that molybdate species are extracted in the foam, i.e. adsorbed at water/air interface in the foam, under acidic condition (pH 1 and 3). The decrease in *Ii* (and *Ri*) above 100 mM is presumably related to the saturation of the foam interface by the extracted molybdate species. Indeed, once the saturation of the interface is reached, i.e. once the maximal molybdate concentration ($C_i^0 - C_i^r$) that be extracted by a given concentration of surfactant is reached, further increase in the molybdate concentration ($C_i^0$) is expected to lead to a decrease in the *Ri* and *Ii*, see Eq. 3 and 4. Efficient extraction was expected at pH 1 as, according to the speciation of molybdate in aqueous medium, the low charge density $Mo_{36}O_{112}^{8-}$ is the most abundant species formed at this pH. However, no superchaotropic ion was expected at pH 3, yet an interfacial recovery was observed, although weaker than for pH 1. This result suggests that superchaotropic species form at pH 1 but also at higher pH, likely due to the presence of the surfactant. Therefore, in order to identify the polyoxomolybdate species, the effect of the surfactant on the speciation of molybdate was further studied using SAXS and Raman spectroscopy, as we will discuss.

Investigating the effect of temperature on POM (Polyoxometalate) adsorption at the water-air surface in foam would have been highly interesting as it could provide valuable information about the thermodynamics of the process. However, increasing the temperature poses significant challenges due to excessive evaporation. Moreover, elevated temperature can potentially alter the speciation of POMs and their binding constants, making it difficult to distinguish between these two effects and draw clear conclusions.

## 3.2 The adsorption of molybdate species on BrijO10: SAXS experiment.

### 3.2.1 Speciation of molybdate as a function of pH
SAXS is a well-established non-destructive method for probing the size, shape, reactivity and interactions of dissolved species. This method is one of the techniques used to obtain information on the speciation of POM solutions¹⁵,⁵³,⁵⁴,⁵⁵ as well as supramolecular assemblies of POM.⁵⁶ POMs contain high-Z elements, here Mo, with high electron density, and therefore strongly scatter X-rays in aqueous media. SAXS is also interesting for investigation of POMs because theoretical spectra can be easily simulated from their chemical structure.⁴⁷

SAXS spectra were measured on solutions of 100 mM of molybdate for different pHs (see **Fig. SI. 2**). SAXS spectra on a solution of 100 mM of molybdate at natural pH were also compared with the one on pure water (see **Fig-SI. 3**). The spectra of Mo₃₆O₁₁₂⁸⁻ and Mo₈O₂₆⁴⁻ were simulated from their atomic coordinates in solid state (see **Fig. 3**) and compared to the

11 experimental spectra at pH 1 and

3.

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** SAXS-spectra of 100 mM of molybdate speciation in water at different pHs (in acidic medium HCl) and simulated spectra of Mo₈O₂₆⁴⁻ from atom coordinates in solid state with Crysol⁴⁷.</figcaption></figure>
Above pH 5 there is no sign of formation of large species (see Fig-SI. 2). According to the literature,¹⁵ at pH 8 in aqueous solutions, only the species MoO₄²⁻ is present. Because of its small size, MoO₄²⁻ produces a weak scattering and therefore it is not detected experimentally by SAXS. By decreasing pH, an increase in the excess scattering intensity is observed at q < 10 nm⁻¹ (see Fig-SI. 2 and Fig. 3), indicating the formation of large species under acidic conditions. Between pH 2 and 5, we observed the scattering produced by the species Mo₇O₂₄⁶⁻ or Mo₈O₂₆⁴⁻ as confirmed by the comparison with the simulated spectra of Mo₈O₂₆⁴⁻ (Fig. 3), although Mo₇O₂₄⁶⁻ and Mo₈O₂₆⁴⁻ cannot be distinguished from each other (see Fig-SI. 2 and Fig. 3).⁵⁷ At pH 1, the stronger excess scattering, showing an oscillation at around 5-8 nm⁻¹, is associated with the formation of larger species which can be attributed to Mo₃₆O₁₁₂⁸⁻, as confirmed by comparing the experimental spectrum with the simulated spectrum of Mo₃₆O₁₁₂⁸⁻ (Fig. 3).

### 3.2.2 Investigation of the superchaotropic behaviour of the molybdate species in the presence of surfactant and as a function of pH

In a next step, the influence of surfactant on the molybdate species was investigated using SAXS. SAXS analysis of molybdate solutions in the presence of non-ionic surfactants yields valuable insights in two aspects: (i) it reveals any potential change in POM speciation compared to pure water (or HCl aqueous solutions), and (ii) it provides information on the superchaotropic behaviour of POMs in the solution. Indeed, the adsorption of superchaotropic POMs onto the micellar surface produces a distinct signature in the SAXS spectra, characterized by a core-shell structure representative of decorated micelles with a pronounced excess electron density in the shell.³³ This signature is evident through a significant increase in scattering intensity at low wave vector, q, and a broad oscillation within the intermediate q range.
The spectra of solutions containing 100 mM of molybdate and 20 mM of BrijO10 at natural pH, pH 1 and pH 3 are shown in **Fig. 4.a, c & d**. Moreover, the spectra of solutions containing 100 mM of molybdate at pH 1 (**Fig. 4.b**) and 20 mM of BrijO10 (**Fig. 4.e**) are given as references for comparison and to support the discussion.

At natural pH, 20 mM of BrijO10 (**Fig. 4.e**) gives a low but distinguishable scattering signal showing a large oscillation centred at 1 nm⁻¹. Such a scattering pattern is typical for non-ionic micelles with excess electron density in the corona (composed of the polar heads), see for example in ref⁵⁸ for C8G1 micelle or ref³⁵ for C8E4 micelle. The formation of micelles is expected as the BrijO10 concentration is well above its critical micellar concentration (cmc = 0.04 mM⁵⁹). The addition of 100 mM molybdate to 20 mM BrijO10 (**Fig. 4.d**) has a slight effect on the scattering, as it only adds a background level compared to the scattering of 20 mM BrijO10 (**Fig. 4.e**). This indicates that there is no change in the micellar structure and that molybdate does not scatter, as observed in water (see **Fig-SI. 2** at natural pH and pH 6).

<figure><img src="image_6.png" /></figure>
1 **Fig. 4.** SAXS-spectra of several solutions of BrijO10 and molybdate at different pHs: a) 100 mM molybdate + 20 mM BrijO10 pH 1; b) 100 mM molybdate pH 1; c) 100 mM + 20 mM BrijO10 pH 3; d) 100 mM molybdate + 20 mM BrijO10 pH natural; e) 20 mM BrijO10 pH natural.

4 At pH 3, the spectrum Fig. 4.c is mostly comparable to the scattering of 100 M molybdate
5 without surfactant (Fig. 3 at pH 3), apart from the presence of a broad and weak oscillation in the
6 q regime 0.5-2 nm⁻¹. This spectrum at pH 3 is similar to the sum of the spectra of a 100 mM
7 BrijO10 micellar solution and of a solution of 100 mM of molybdate at pH 3 (see **Fig-SI. 4**).

8 Therefore, in **Fig. 4.c** the scattering is mostly due to the scattering of the isopolyoxomolybdates,
9 Mo<sub>7</sub>O<sub>24</sub><sup>6-</sup> and Mo<sub>8</sub>O<sub>26</sub><sup>4-</sup>, as in water at pH 3, and the scattering of the micelles is weak compared
10 to the one of the POMs in these ranges of POM/surfactant concentration. At pH 3, we can
11 conclude that (i) there is no apparent change in the POM structure upon addition of BrijO10, and
12 that (ii) no superchaotropic POM species adsorb on the micellar surface. This latter statement
13 contradicts the flotation results at pH 3 that showed a non-negligible extraction by the foam (Fig.
14 2) and therefore suggested the presence of superchaotropic POMs. However, we can note here
15 that the local conditions in the foam, that is high surfactant concentrations and planar
16 water/surface, are different from the ones in the bulk solution, i.e. lower concentrations and
17 strongly curved surface of micelles. This difference may explain the discrepancy observed here
18 between the SAXS (bulk properties) and flotation (macroscopic interface) results. Another
19 reason for this discrepancy could be that the concentration of superchaotropic POMs is too low
20 to be detected by SAXS, meaning that the scattering of micelles is not negligible compared to the
21 scattering of POMs in these experimental conditions. Therefore, at pH 3 there is no evidence of
22 the presence of superchaotropic species at the surface of the micelles.
At pH 1 (Fig. 4.a and b), the addition of the surfactant 20 mM BrijO10 to 100 mM molybdate gives a strong excess of scattering in the low q region (see the dark arrow in Fig. 4) with a large oscillation (centred at 1.3 nm⁻¹). This scattering pattern is the typical signature of core shell micelles with a large excess electron density in the shell, as previously observed and quantified with adsorbed superchaotropic Keggin POM on non-ionic micelles.<sup>33,41,44</sup> This result indicates that a polyoxomolybdate species with superchaotropic behaviour formed in solution and adsorbed on BrijO10 micelles. The nearly perfect overlap of the scattering spectra a and b depicted in Fig. 4, at q > 2 nm⁻¹, where the characteristic scattering of POMs is observed, suggest that (i) the concentration of Mo<sub>36</sub>O<sub>112</sub><sup>8-</sup> is similar in the presence and absence of BrijO10, and, thus (ii) Mo<sub>36</sub>O<sub>112</sub><sup>8-</sup> is likely the superchaotropic POM species decorating the BrijO10 micelles.

To further investigate the identification of POM species and on the potential impact of surfactant on Mo speciation, Raman spectroscopy was performed.

It is important to note that the surfactant concentrations utilized in the SAXS study (up to 100 mM) and Raman study (up to 200mM, see in the next section) are significantly higher than those used in the foam (1.5 mM). As a result, one may question the comparability of the concentration ranges between the flotation and SAXS experiments. However, it should be considered that the surfactant concentration in the liquid phase of the foam is much higher, at least by a factor of 10, compared to the bulk concentration of 1.5 mM. Therefore, the surfactant concentration employed in the SAXS investigation falls within the same range as that present in the liquid phase of the foam during a flotation experiment.

**3.3 Investigation of molybdate species with and without BrijO10 at different pHs: Raman experiment**
The flotation results have shown a significant extraction of molybdate species at pH 3, suggesting that superchaotropic species are formed and extracted by the foams. However, Mo<sub>7</sub>O<sub>24</sub><sup>6-</sup> and Mo<sub>8</sub>O<sub>26</sub><sup>4-</sup>, the species found at pH 3 in HCl aqueous solutions, are not expected to be superchaotropic due to their charge density, too high to exhibit superchaotropicity. Moreover, SAXS did not evidence the presence of superchaotropic species on the micelles at pH 3. In order to shed light on this apparent contradiction, the molybdate speciation in the presence of BrijO10 surfactant was investigated further by Raman spectroscopy. Raman spectroscopy is a useful technique for studying the speciation of molybdenum and molybdate species, as it is sensitive to the various local atomic configurations.<sup>16,60,61,62</sup>

First, Raman spectroscopy was used on 100 mM molybdate solutions at different pHs without surfactant, see Fig. 5. At natural pH, MoO<sub>4</sub><sup>2-</sup>, which is the major species in basic and slightly acidic solutions, is identified by a peak at 897 cm<sup>-1</sup>. This signal is decreased significantly as the pH decreases down to 5.0, whereas a peak at 946 cm<sup>-1</sup> grows. This evolution is attributed to the polycondensation of MoO<sub>4</sub><sup>2-</sup> in heptamolybdate species (or paramolybdates) Mo<sub>7</sub>O<sub>24</sub><sup>6-</sup>, HMo<sub>7</sub>O<sub>24</sub><sup>5-</sup> and H<sub>2</sub>Mo<sub>7</sub>O<sub>24</sub><sup>4+</sup>. At pH 4.0, (i) the signal of MoO<sub>4</sub><sup>2-</sup> is not detectable and (ii) two new peaks are observed at 960 cm<sup>-1</sup> and 970 cm<sup>-1</sup> which are attributed to the octamolybdate species, α-Mo<sub>8</sub>O<sub>26</sub><sup>4-</sup> and β-Mo<sub>8</sub>O<sub>26</sub><sup>4+</sup> respectively. At pH 3.0, the signal of heptamolybdate species (at 946 cm<sup>-1</sup>) has nearly disappeared and α-Mo<sub>8</sub>O<sub>26</sub><sup>4-</sup> and β-Mo<sub>8</sub>O<sub>26</sub><sup>4+</sup> are the major species. At pH 1.0, three peaks at 902, 954 and 983 cm<sup>-1</sup>, which are typical of the Mo<sub>36</sub>O<sub>112</sub><sup>8-</sup> anions, become dominant.<sup>15,16,61</sup>
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Raman spectra of 100 mM molybdate in aqueous solutions as a function of pH (an offset was added for clarity).</figcaption></figure>

In a next step, the influence of the surfactant on the speciation of molybdate ions was investigated. The Raman spectra were recorded on solutions of 100 mM of molybdate at pH 1, 3 and natural pH with 20 mM of BrijO10 and compared to the spectra without surfactant (see Fig. 6). At natural pH the Raman spectra is not modified by the presence of BrijO10 micelles. At pH 3, a new peak arises in the presence of surfactant at 986 cm⁻¹, which could be attributed to Mo₃₆O₁₁₂⁸⁻. However, the two other typical vibration modes of Mo₃₆O₁₁₂⁸⁻, especially the one at around 902 cm⁻¹, are not observed, which casts doubt on the attribution of the mode at 986 cm⁻¹ to Mo₃₆O₁₁₂⁸⁻. At pH 1, the three vibrational modes attributed to Mo₃₆O₁₁₂⁸⁻ are nearly unaffected when surfactant is added, except the vibrational mode at higher wavenumber that is not observed at 982 cm⁻¹ but at 986 cm⁻¹ in presence of the surfactant. In order to shed light on these unexplained effects at pH 3 and 1, further Raman spectroscopy experiments were conducted to better understand the effect of surfactant on the Mo speciation. However, at this stage, the conclusions on Raman spectroscopy at pH 1 aligns with the SAXS results, which suggested that
1 Mo₃₆O₁₁₂⁸⁻ is (i) the species adsorbed on the micelles, and therefore it is (ii) the species that is

2 extracted in the foam during flotation experiments with BrijO10.

<figure><img src="image_8.png" /></figure>

3

4 **Fig. 6.** Raman spectra of 100 mM molybdate in aqueous solution without (···) and with (—) 20

5 mM BrijO10 at pH 1, 3 and natural pH (an offset was added for clarity).

6 Raman spectra were recorded for 100 mM of molybdate at pH 3 with increasing concentrations

7 of BrijO10 from 0 to 220 mM (see Fig. 7). Interestingly, the solutions turned from colourless to

8 yellow for surfactant concentrations above 20 mM (see insert in Fig. 7). The signals associated

9 with the HMo₇O₂₄⁵⁻/H₂Mo₇O₂₄⁴⁻ and α-Mo₈O₂₆⁴⁻ clearly vanish as the BrijO10 concentration

10 increases, while at the same time a signal centred at 986 cm⁻¹ emerges to become sharp and

11 intense. This suggests that the surfactant has a strong effect on the isopolyoxometalate

12 speciation. Based on this hypothesis, the surfactant could apparently convert HMo₇O₂₄⁵⁻

13 /H₂Mo₇O₂₄⁴⁻ and α-Mo₈O₂₆⁴⁻ into another species which is not Mo₃₆O₁₁₂⁸⁻ because the two

14 vibration modes of Mo₃₆O₁₁₂⁸⁻ at around 954 and 902 cm⁻¹ are not observed. We propose to

15 attribute the emergence of the signal centred at 986 cm⁻¹ to the formation of Mo₆O₁₉²⁻ anions. In

16 addition to the different isopolyoxomolybdate species already listed above, the yellow-coloured

17 Mo₆O₁₉²⁻ (Lindqvist type) is known in the solid state and also in aqueous solutions but only at
high content (> 30%) of polar organic solvents, such as, for example, acetone, ethanol or acetonitrile.<sup>15,63</sup> Recently, Falaise et al. have shown that Mo<sub>6</sub>O<sub>19</sub><sup>2-</sup> could be stabilized in water (acidified with HCl in the pH range 1 < pH < 2.5) upon addition of γ-cyclodextrin (γ-CD) as host-guest complexes, {Mo<sub>6</sub>O<sub>19</sub>@-CD}<sup>2-</sup>, resulting from its chaotropic effect.<sup>57</sup> Therefore, it appears here that addition of non-ionic surfactant has the same effect. It is important to note that the concentration of BrijO10 necessary to observe a significant increase of Mo<sub>6</sub>O<sub>19</sub><sup>2-</sup> species is well above the concentration of surfactant used for the flotation (1.5 mM). However, the concentration of surfactant in the foam is much higher (at least 15 to 20 times) than the concentration in the initial solution and it presumably becomes sufficiently high to initiate the formation of Mo<sub>6</sub>O<sub>19</sub><sup>2-</sup>. Therefore, it is likely that at pH 3 the Mo<sub>6</sub>O<sub>19</sub><sup>2-</sup> species is formed at sufficiently high concentrations in the foam and that it is the species extracted in flotation experiments at pH 3. Indeed, Mo<sub>6</sub>O<sub>19</sub><sup>2-</sup> anion has a low charge density (10.3 charge / nm<sup>3</sup>) and it is therefore expected to exhibit a superchaotropic behaviour and to adsorb on the micelles surface.<sup>64</sup>

To confirm the Raman observations, SAXS measurements were performed on 100 mM molybdate solution at pH 3 with concentrations of BrijO10 ranging from 10 to 120 mM (see Fig. SI, 5). No significant change in the scattering of the POMs, i.e. in the *q* regime above 3 – 4 nm<sup>-1</sup>, was observed upon increasing surfactant concentration. The contribution of BrijO10 micelles is clearly observed in the lower *q* regime, i.e. large oscillations, and has a lower contribution at high *q* values. Consequently, drawing conclusions regarding the possible conversion of hepta- or octamolybdate species to Mo<sub>6</sub>O<sub>19</sub><sup>2-</sup> at pH 3 based on SAXS data becomes challenging.
<figure><img src="image_9.png" /></figure>

1

2 **Fig. 7.** Raman spectra of 100 mM molybdate at pH 3 in aqueous solution with increasing

3 BrijO10 concentrations in mM: 0, 60, 80, 100, 120, 180 and 220 mM (from g to a). For the sake

4 of clarity, the green arrows show the effect of the increase in the BrijO10 concentration at

5 different places in the Raman spectra. In the 850-860 cm⁻¹ region the signal of the surfactant is

6 observed. The inserted photos display a range of sample concentrations, ranging from 1.5 to 80

7 mM, and clearly exhibit a yellowish appearance in the solutions with surfactant concentrations

8 above 20 mM.

9 The effect of surfactant on the Mo speciation was also investigated at pH 1 by Raman

10 spectroscopy for 100 mM of molybdate with increasing concentration of BrijO10 (10-100 mM)

11 (see **Fig. 8**). As already observed at pH 3, the solutions, at pH 1, also turned from colourless to

12 yellow at around 20 mM BrijO10 (see insert in **Fig. 8**). The addition of surfactant leads to a

13 decrease in the Mo₃₆O₁₁₂⁸⁻ Raman signals, clearly observed on the signal centred at 902 cm⁻¹, in

14 favour of the Mo₆O₁₉²⁻ signal at around 986 cm⁻¹. SAXS measurements of 100 mM molybdate

15 solution with 0, 30 and 60 mM BrijO10 (**Fig-SI. 7**) clearly indicate that Mo₃₆O₁₁₂⁸⁻ is converted

16 into the formation of smaller species, presumably Mo₆O₁₉²⁻, from 30 to 60 mM BrijO10.
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Raman spectra of 100 mM molybdate at pH 1 in aqueous solution with increasing BrijO10 concentrations in mM: 10, 30, 80 and 100 mM (from d to a). The inserted photos display a range of sample concentrations, ranging from 1.5 to 80 mM, and clearly exhibit a yellowish appearance in the solutions with surfactant concentrations above 20 mM.</figcaption></figure>

The observed alteration in the molybdate speciation can indeed be interpreted as the conversion of POM species into other species, triggered by an increase in surfactant concentration. This conversion results from the stabilization of the newly formed superchaotropic species. Indeed POMs are formed through a self-assembly process of molybdate ions, which can be described by a chemical equilibrium with a specific equilibrium constant. When chaotropic POM species adsorb onto the micellar or water-air surfaces in the foam, an additional adsorption equilibrium comes into play, alongside the equilibria governing POM formation. As a result, the relative concentrations of different POM species are altered.

Furthermore, the surfactant-covered micellar surface or water-air surface has a different physicochemical environment from the bulk solution, which potentially affects the standard chemical potential of POM species. Consequently, certain POM species may become stabilized at the
1 interfaces. For instance, $HMo_7O_{24}^{5-}/H_2Mo_7O_{24}^{4-}$ and $\alpha-Mo_8O_{26}^{4-}$ species may then convert into

2 $Mo_6O_{19}^{2-}$ species while being stabilized through the surfactant.

3 In the present work, we establish a correlation between the adsorption of POM at highly curved

4 micellar surface (section 3.2 SAXS and 3.3 Raman) and planar water surface covered with

5 surfactant in the foam experiment (section 3.1 Flotation). The question of whether this

6 correlation is valid was partially addressed in a previous study,³³ where we estimated the

7 surfactant-to-POM ratio at both the micellar surface and the water-air surface in foams. We

8 observed that the surfactant/POM ratios for both types of interfaces ranged between 4 and 5.

9 Consequently, based on these experiments, we can conclude that curvature does not significantly

10 influence the POM adsorption process, and a correlation between bulk and surface adsorption

11 can be established.

12 Moreover, it is important to note that the surfactant concentrations used in the SAXS study are

13 higher, reaching up to 100 mM, compared to the concentration of 1.5 mM used in the foam

14 experiments. This raises the question of whether conclusions can be drawn, given this disparity

15 in concentration ranges between the flotation and SAXS experiments. However, it is crucial to

16 consider that the surfactant concentration in the liquid phase of the foam is significantly higher,

17 at least by a factor of 10, compared to the bulk concentration of 1.5 mM. Therefore, the

18 surfactant concentration used in the SAXS investigation falls within the same range as the liquid

19 phase of the foam during a flotation experiment.

20 **3.4 Tungstate/Molybdate separation**

21 In order to assess the ability of superchaotropic ion flotation to selectively separate ions, we

22 made preliminary experiments to evaluate the separation of tungstate and molybdate. Tungstate

23 has a complex speciation in acidic media due to many simultaneous equilibria, some of them
with slow kinetics and leading to the formation of metastable species. Therefore, tungstate differs strongly from molybdate in terms of their speciation diagram.¹⁵ At low pH, for example, the equivalent species of the Krebbs anion (Mo₃₆O₁₁₂⁸⁻) for tungstate was never reported. Instead, it is well known that tungstic acid (H₂WO₄), which a low solubility in water, precipitates below pH 2.⁶⁵ Therefore, we carried out flotation from several aqueous solutions with different concentrations of sodium molybdate and sodium tungstate and 1.5 mM BrijO10 at pH 2. Tungstate, unlike molybdate, is not expected to form superchaotropic ions at pH 2, because all the isopolyoxotungstate species known in water have too high a charge density. Therefore, separation of molybdate from tungstate by flotation using BrijO10 is expected at pH 2. The results of the flotation experiments are given in **Table 1**.

**Table 1.** Results of flotation experiments of tungstate and molybdate mixtures at pH 2 by flotation using BrijO10 at 1.5 mM.

<table><thead><tr><th rowspan="2">Experiment</th><th colspan="2">[Mo]<br>(mM)</th><th colspan="2">[W]<br>(mM)</th><th colspan="2">Recovery<br>Ri (%)</th><th colspan="2">Transport<br>T (%)</th><th colspan="2">Interfacial<br>Recovery Ii</th></tr><tr><th>Initial</th><th>Residual</th><th>Initial</th><th>Residual</th><th>Mo</th><th>W</th><th>Mo</th><th>W</th><th>Mo</th><th>W</th></tr></thead><tbody><tr><td>a</td><td>101.3</td><td>84.1</td><td>/</td><td>/</td><td>20.2</td><td>/</td><td>3.9</td><td>/</td><td>17.0</td><td>/</td></tr><tr><td>b</td><td>/</td><td>/</td><td>100.7</td><td>98.1</td><td>/</td><td>10.3</td><td>7.9</td><td>/</td><td>2.7</td><td>/</td></tr><tr><td>c</td><td>51.0</td><td>46.0</td><td>/</td><td>/</td><td>14.9</td><td>/</td><td>5.6</td><td>/</td><td>9.8</td><td>/</td></tr><tr><td>d</td><td>/</td><td>/</td><td>50.1</td><td>51.0</td><td>/</td><td>6.5</td><td>8.1</td><td>/</td><td>-1.8</td><td>/</td></tr><tr><td>e</td><td>50.8</td><td>46.8</td><td>51.1</td><td>53.0</td><td>16.5</td><td>6.0</td><td>9.5</td><td>/</td><td>-3.8</td><td>/</td></tr><tr><td>f</td><td>23.0</td><td>22.2</td><td>80.6</td><td>81.0</td><td>11.3</td><td>7.5</td><td>8.0</td><td>/</td><td>-0.5</td><td>/</td></tr><tr><td>g</td><td>101.4</td><td>85.8</td><td>99.7</td><td>98.9</td><td>24.2</td><td>11.2</td><td>10.4</td><td>/</td><td>0.8</td><td>/</td></tr><tr><td>h</td><td>49.2</td><td>45.7</td><td>200.9</td><td>196.7</td><td>17.5</td><td>13.0</td><td>11.2</td><td>/</td><td>2.1</td><td>/</td></tr></tbody></table>

14
Flotation experiments were first conducted on molybdate and on tungstate separately at 50 and 100 mM. As expected, and in contrast to molybdate, tungstate was not extracted by flotation, as reflected by the very low values *Ii* of -1.81 and 2.66 % for 50 and 100 mM tungstate solutions.

The flotation of equimolar tungstate/molybdate mixtures, around 50 mM/50 mM and 100/100, gives interfacial recovery *Ii* values that are comparable to the flotation experiments on separate tungstate/molybdate. This tendency is also observed for *Ri*, although it is less pronounced, which is likely to be due to a higher ion concentration in the mixtures compared to the experiments where the metalates are investigated separately. Moreover, it is worth mentioning that the transport factor *T* is significantly higher in the presence of tungstate (9-11%), than with Mo alone (4-5.6%). Therefore, the non-negligible recovery factor (*Ri*) of tungstate in flotation of Mo/W mixtures is due to high transports and not due to the adsorption of tungstate species at the water/air interface. Indeed, the tungstate interfacial recovery *Ii* remains very low (at most 2.1 %) although *Ii* for molybdate is unchanged compared to the flotations performed with the same molybdate concentration without tungstate.

The maximum in Mo/W separation was obtained for the 100 mM/100 mM molybdate/tungstate mixture for which the interfacial recovery for molybdate was 15.5 % and for tungstate only 0.8 %, giving a Mo/W separation factor of roughly 20. It is interesting to note that the flotation of molybdate species from molybdate/tungstate mixtures is comparable to the flotation of molybdate without tungstate, suggesting that tungstate does not interfere with the flotation of molybdate. Therefore, we show here that the separation of molybdate from tungstate by the superchaotropic flotation works efficiently. Flotation of tungstate/molybdate mixtures in excess of tungstate over molybdate (80/23 and 200/49) were also conducted. In these conditions (W/Mo ≈ 4/1), the Mo/W separation appears to be less efficient as the interfacial recovery of Mo
1 is divided by a factor of two compared to the flotation results in equimolar condition (W/Mo =
2 1).

3 The further identification of specific polyoxometalate species extracted by flotation in the
4 foam can be challenging, especially in tungstate/molybdate mixtures where mixed species may
5 form. However, the successful separation of tungstate and molybdate observed in this study,
6 consistent with the flotation of each metalate individually, suggests that the formation of mixed
7 species is not significant, particularly under equimolar conditions.

8 Therefore, under the given experimental conditions and without optimisation, it is feasible to
9 extract molybdate and separate it from tungstate using a non-ionic surfactant. However, the
10 separation of the two ions is not complete due to high transport. Enhancing drainage can mitigate
11 this issue, either by reducing the gas flow rate or increasing the height of the flotation column.
12 Optimization of these parameters can lead to improved separation efficiency.
13

13

14 **4. Conclusions**

15 This study presents a proof-of-concept for superchaotropic flotation, utilizing non-ionic
16 surfactants to extract and separate superchaotropic polyoxometalates nano-ions. Notably, at pH 1
17 with 1.5 mM of BrijO10 and a Mo concentration of 100 mM, the interfacial recovery reaches up
18 to 25%, indicating the significant potential of this extraction process. Our analysis of flotation,
19 Raman spectroscopy and SAXS data, reveals that polyoxomolybdate species, specifically
20 Mo<sub>36</sub>O<sub>112</sub><sup>-</sup> and Mo<sub>6</sub>O<sub>19</sub><sup>2-</sup>, adsorb at the air-water interface in foams. Additionally, at pH 3, the
21 stabilisation of Mo<sub>6</sub>O<sub>19</sub><sup>2-</sup> at the surface of non-ionic micelles or at the water/air interface (in
22 foams) highlights the influence of surfactants on the speciation of molybdate ions. Indeed, our
23 results suggest that the surfactant promotes the conversion of the non-superchaotropic species,
1 Mo<sub>7</sub>O<sub>24</sub><sup>6-</sup> and Mo<sub>8</sub>O<sub>26</sub><sup>4-</sup>, into the superchaotropic Mo<sub>6</sub>O<sub>19</sub><sup>2-</sup> species by its adsorption on the
2 surfactants. This later result goes in line with a recent study on the formation of Mo<sub>6</sub>O<sub>19</sub><sup>2-</sup> in the
3 cavity of γ-cyclodextrin. Importantly, we have successfully demonstrated the selective separation
4 of molybdate from tungstate at pH 2 by generating polyoxomolybdate species and implementing
5 the innovative concept of superchaotropic flotation. Based on this new concept of
6 superchaotropic flotation, we reached a Mo/W separation factor of roughly 20 using the
7 commercial polyethoxylated surfactant BrijO10.

8

9 ASSOCIATED CONTENT

10 Supporting Information.

11 Efficiency factors of the flotation of molybdate for the system: Na₂MoO₄ with BrijO10 foam
12 (pH= 1) after 6-7 hours flotation at different surfactant concentrations: 1.5, 3 and 7 mM.; SAXS-
13 spectra of 100 mM molybdate at natural pH and water.; SAXS-spectra of 100 mM molybdate at
14 different pH ; SAXS-spectra of 100 mM molybdate at pH 3, 20 mM of BrijO10 at natural pH ;
15 SAXS-spectra of 100 mM molybdate at pH 3, with 10, 30, 60 and 120 mM BrijO10 and
16 simulated spectra of Mo8O264- from atom coordinate in solid state with Crysol. ; SAXS-spectra
17 of 100 mM molybdate at pH 1, 20 mM of BrijO10 at natural pH; Raman peaks table from
18 literature⁶¹ (Mo concentration at 350 mM) and from this work (Mo at 100 mM) upon decreasing
19 pH.; SAXS-spectra of 100 mM molybdate solutions with 0, 30 and 60 mM BrijO10.

20

21 AUTHOR INFORMATION

22 **Corresponding Author**
1 *(P. B.). E-mail: pierre.bauduin@cea.fr

2 Author Contributions

3 The manuscript was written through contributions of all authors. All authors have given

4 approval to the final version of the manuscript.

5 Funding Sources

6 CNRS through the international research project “Nano-ions at soft interfaces” (IRP 466

7 NISI)

8 CEA through the project CEA RTA/RCHIM

9 ANR through the project CHAOPOM (grant number ANR-22-CE92-0043)

10 Notes

11 The authors declare no competing financial interest.

12 ACKNOWLEDGEMENTS

13 We thankfully acknowledge Philipp Schmid, Max Hohenschutz and O. Diat for scientific
14 exchanges. We would like to thank the Agence National pour la Recherche for funding the
15 CHAOPOM project (grant number ANR-22-CE92-0043).

16

17 ABBREVIATIONS

18 BrijO10 C₁₈:₁(EO)ₙ with n ≈ 10; Ii interfacial recovery; POMs polyoxometalates; Ri recovery;

19 SAXS small-angle X-ray scattering; T Transport factor; XRF X-ray fluorescence spectroscopy

20 REFERENCES
1 (1) Shields HA Hr. *Overview Refractory Metal Markets*; 2000.

2 (2) Wadsworth, J.; Nieh, T. G.; Stephens, J. J. Recent Advances in Aerospace Refractory

3 Metal Alloys. *Int. Mater. Rev.* **1988**, *33* (1), 131–150.

4 (3) Lennartson, A. Made by Molybdenum. *Nature Chemistry*. Nature Publishing Group 2014, p 746.

6 (4) Chen, X. R.; Zhai, Q. J.; Dong, H.; Dai, B. H.; Mohrbacher, H. Molybdenum Alloying in Cast Iron and Steel. *Adv. Manuf. 2020*, *8* (1), 3–14.

8 (5) Kaczmarek, T.; Szenknect, S.; Claparède, L.; Cabié, M.; Le Goff, X.; Mesbah, A.; Podor, R.; Dacheux, N. Impact of Ruthenium Metallic Particles on the Dissolution of UO2 in Nitric Acid. *npj Mater. Degrad.* **2022**, *6* (1), 1–12.

11 (6) Lasheen, T. A.; El-Ahmady, M. E.; Hassib, H. B.; Helal, A. S. Molybdenum Metallurgy Review: Hydrometallurgical Routes to Recovery of Molybdenum from Ores and Mineral Raw Materials. *Miner. Process. Extr. Metall. Rev.* **2015**, *36* (3), 145–173.

14 (7) Frascoli, F.; Hudson-Edwards, K. A. Geochemistry, Mineralogy and Microbiology of Molybdenum in Mining-Affected Environments. *Minerals*. MDPI AG February 2018.

16 (8) Laird, K. R.; Das, B.; Cumming, B. F. Enrichment of Uranium, Arsenic, Molybdenum, and Selenium in Sediment Cores from Boreal Lakes Adjacent to Northern Saskatchewan Uranium Mines. *Lake Reserv. Manag.* **2014**, *30* (4), 344–357.

19 (9) Prasad, P. M.; Mankhand, T. R.; Prasad, A. J. Molybdenum Extraction Process: An Overview. *NML Technical Journal*. 1997, pp 39–58.
1 (10) Huang, Y. H.; Tang, C.; Zeng, H. Removing Molybdate from Water Using a Hybridized Zero-Valent Iron/Magnetite/Fe(II) Treatment System. *Chem. Eng. J.* **2012**, *200–202*, 257–263.

4 (11) Pan, Y.; Sun, X.; Zhang, Y. Separation of Vanadium and Molybdenum from Aqueous Solution Using PEG2000 + Sodium Sulfate + Water Aqueous Two-Phase System. *SN Appl. Sci.* **2019**, *1* (11).

7 (12) Du, J.; Li, J.; He, D.; Xu, M.; Zhang, G.; Cao, Z.; Wu, S. Green Separation and Recovery of Molybdenum from Tungstate Solution Achieved by Using a Recyclable Vulcanizing Agent. *J. Clean. Prod.* **2021**, 278, 123930.

10 (13) Coca, J.; Díez, F. V.; Morís, M. A. Solvent Extraction of Molybdenum and Tungsten by Alamine 336 and DEHPA. *Hydrometallurgy* **1990**, *25* (2), 125–135.

12 (14) Wei, Y.; Salih, K. A. M.; Rabie, K.; Elwakeel, K. Z.; Zayed, Y. E.; Hamza, M. F.; Guibal, E. Development of Phosphoryl-Functionalized Algal-PEI Beads for the Sorption of Nd(III) and Mo(VI) from Aqueous Solutions – Application for Rare Earth Recovery from Acid Leachates. *Chem. Eng. J.* **2021**, *412* (October 2020).

16 (15) Gumerova, N. I.; Rompel, A. Polyoxometalates in Solution: Speciation under Spotlight. *Chemical Society Reviews*. Royal Society of Chemistry November 2020, pp 7568–7601.

18 (16) P Griffith, B. W.; B Lesniak, P. J. *1931, R15*, 223; 1969; Vol. 75.

19 (17) Zhao, Z.; Gao, L.; Cao, C.; Li, J.; Chen, X.; Chen, A.; Liu, X.; Sun, P.; Huo, G.; Li, Y.; et al. Separation of Molybdenum from Tungstate Solution—Scavenging Thiomolybdate by Copper Compound. *Metall. Mater. Trans. B* **2012**, *43* (6), 1284–1289.
1 (18) Sebba, F. Concentration by Ion Flotation. *Nat. 1959 1844692 1959*, 184 (4692), 1062–1063.

3 (19) Liu, Z.; Doyle, F. M. Modeling Metal Ion Removal in Alkylsulfate Ion Flotation Systems. *Mining, Metall. Explor. 2001*, 18 (3), 167–171.

5 (20) Polat, H.; Erdogan, D. Heavy Metal Removal from Waste Waters by Ion Flotation. *J. Hazard. Mater. 2007*, 148 (1–2), 267–273.

7 (21) Thalody, B.; Warr, G. G. Ion Flotation: A Laboratory Experiment Linking Fundamental and Applied Chemistry. *J. Chem. Educ. 1999*, 76 (7), 956.

9 (22) Micheau, C.; Schneider, A.; Girard, L.; Bauduin, P. Evaluation of Ion Separation Coefficients by Foam Flotation Using a Carboxylate Surfactant. *Colloids Surfaces A Physicochem. Eng. Asp. 2015*, 470, 52–59.

12 (23) Foam Flotation: Theory and Applications (Chemical Industries) (Volume 11) by Clarke, A. N. and Wilson, D. J.: *Good (1983) | Anybook Ltd.*

14 (24) Ďorďovič, V. Self-Assembly of Boron Cluster Compounds and Their Coassembly with Polymers, Univerzita Karlova, 2017.

16 (25) Wan Nafi, A.; Taseidifar, M. Removal of Hazardous Ions from Aqueous Solutions: Current Methods, with a Focus on Green Ion Flotation. *J. Environ. Manage. 2022*, 319, 115666.

19 (26) Xu, J.; Qiao, H.; Yu, K.; Chen, M.; Liu, C.; Richtering, W.; Zhang, H. Cu2+ Tunable Temperature-Responsive Pickering Foams Stabilized by Poly (N-Isopropylacrylamide-Co-Vinyl Imidazole) Microgel: Significance for Cu2+ Recovery via Flotation. *Chem. Eng.*
1 J. 2022, 442 (April).

2 (27) Hernández-Expósito, A.; Chimenos, J. M.; Fernández, A. I.; Font, O.; Querol, X.; Coca,

3 P.; García Peña, F. Ion Flotation of Germanium from Fly Ash Aqueous Leachates. *Chem.*

4 Eng. J. 2006, 118 (1-2), 69-75.

5 (28) He, X. C. Ion Flotation of Rhodium(III) and Palladium(II) with Anionic Surfactants.

6 Talanta 1991, 38 (3), 319-323.

7 (29) Chirkst, D. E.; Lobacheva, O. L.; Dzhevaga, N. V. Ion Flotation of Lanthanum(III) and

8 Holmium(III) from Nitrate and Nitrate-Chloride Media. *Russ. J. Appl. Chem. 2012*, 85 (1),

9 25-28.

10 (30) Chirkst, D. E.; Lobacheva, O. L.; Dzhevaga, N. V. Ion Flotation of Rare-Earth Metals

11 with Sodium Dodecyl Sulfate. *Russ. J. Appl. Chem. 2011*, 84 (9), 1476-1482.

12 (31) Bahri, Z.; Rezai, B.; Kowsari, E. Selective Separation of Gallium from Zinc Using

13 Flotation: Effect of Solution PH Value and the Separation Mechanism. *Miner. Eng. 2016*,

14 86, 104-113.

15 (32) Zhao, Y.; Zouboulis, A. I.; Matis, K. A. Removal of Molybdate and Arsenate from

16 Aqueous Solutions by Flotation. *Sep. Sci. Technol. 1996*, 31 (6), 769-785.

17 (33) Naskar, B.; Diat, O.; Nardello-Rataj, V.; Bauduin, P. Nanometer-Size Polyoxometalate

18 Anions Adsorb Strongly on Neutral Soft Surfaces. *J. Phys. Chem. C* **2015**, *119* (36),

19 20985-20992.

20 (34) Assaf, K. I.; Ural, M. S.; Pan, F.; Georgiev, T.; Simova, S.; Rissanen, K.; Gabel, D.; Nau,

21 W. M.; Assaf, K. I.; Ural, M. S.; et al. Water Structure Recovery in Chaotropic Anion
1 Recognition: High-Affinity Binding of Dodecaborate Clusters to γ-Cyclodextrin. Angew.

2 *Chemie* **2015**, *127* (23), 6956–6960.

3 (35) Buchecker, T.; Schmid, P.; Grillo, I.; Prévost, S.; Drechsler, M.; Diat, O.; Pfitzner, A.;

4 Bauduin, P. Self-Assembly of Short Chain Poly-N-Isopropylacrylamid Induced by

5 Superchaotropic Keggin Polyoxometalates: From Globules to Sheets. *J. Am. Chem. Soc.*

6 **2019**, *141* (17), 6890–6899.

7 (36) Pope, M. T. *FROM TOPOLOGY VIA SELF-ASSEMBLY TO APPLICATIONS*
*Polyoxometalate Chemistry From Topology via Self-Assembly to Applications Edited By.*

9 (37) Long, D.-L.; Tsunashima, R.; Cronin, L. Polyoxometalates: Building Blocks for
Functional Nanoscale Systems. *Angew. Chemie Int. Ed.* **2010**, *49* (10), 1736–1758.

11 (38) Long, D.-L.; Burkholder, E.; Cronin, L. Polyoxometalate Clusters, Nanostructures and
Materials: From Self Assembly to Designer Materials and Devices. *Chem. Soc. Rev.* **2007**,
13 36 (1), 105–121.

14 (39) Petrus, E.; Bo, C. Unlocking Phase Diagrams for Molybdenum and Tungsten
15 Nanoclusters and Prediction of Their Formation Constants. *J. Phys. Chem. A* **2021**, *125*
16 (23), 5212–5219.

17 (40) Petrus, E.; Segado, M.; Bo, C. Nucleation Mechanisms and Speciation of Metal Oxide
18 Clusters. *Chem. Sci.* **2020**, *11* (32), 8448–8456.

19 (41) Buchecker, T.; Schmid, P.; Renaudineau, S.; Diat, O.; Proust, A.; Pfitzner, A.; Bauduin, P.
20 Polyoxometalates in the Hofmeister Series. *Chem. Commun.* **2018**, *54* (15), 1833–1836.

21 (42) Hohenschutz, M.; Grillo, I.; Dewhurst, C.; Schmid, P.; Girard, L.; Jonchère, A.; Diat, O.;
1 Bauduin, P. Superchaotropic Nano-Ions as Foam Stabilizers. *J. Colloid Interface Sci.*

2 **2021**, *603*, 141–147.

3 (43) Yao, S.; Falaise, C.; Ivanov, A. A.; Leclerc, N.; Hohenschutz, M.; Haouas, M.; Landy, D.;

4 Shestopalov, M. A.; Bauduin, P.; Cadot, E. Hofmeister Effect in the Keggin-Type

5 Polyoxotungstate Series. *Inorg. Chem. Front.* **2021**, *8* (1), 12–25.

6 (44) Hohenschutz, M.; Grillo, I.; Diat, O.; Bauduin, P. How Nano-Ions Act Like Ionic

7 Surfactants. *Angew. Chemie - Int. Ed.* **2020**, *59* (21), 8084–8088.

8 (45) Doyle, F. M. Ion Flotation - Its Potential for Hydrometallurgical Operations. *Int. J. Miner.*

9 *Process.* **2003**, *72* (1–4), 387–399.

10 (46) Malinenko, A.; Jonchère, A.; Girard, L.; Parrès-Maynadié, S.; Diat, O.; Bauduin, P. Are

11 Keggin's POMs Charged Nanocolloids or Multicharged Anions? *Langmuir* **2018**, *34* (5),

12 2026–2038.

13 (47) Svergun, D.; Barberato, C.; Koch, M. H. J. CRY SOL – a Program to Evaluate X-Ray

14 Solution Scattering of Biological Macromolecules from Atomic Coordinates. *J. Appl.*

15 *Crystallogr.* **1995**, *28* (6), 768–773.

16 (48) Galvin, K. P.; Nicol, S. K.; Waters, A. G. Selective Ion Flotation of Gold. *Colloids and*

17 *Surfaces* **1992**, *64* (1), 21–33.

18 (49) Rujirawanich, V.; Chavadej, S.; O’Haver, J. H.; Rujiravanit, R. Removal of Trace Cd2+

19 Using Continuous Multistage Ion Foam Fractionation: Part I-The Effect of Feed SDS/Cd

20 Molar Ratio. *J. Hazard. Mater.* **2010**, *182* (1–3), 812–819.

21 (50) Micheau, C.; Diat, O.; Bauduin, P. Ion Foam Flotation of Neodymium: From Speciation
1 to Extraction. *J. Mol. Liq.* **2018**, *253*, 217–227.

2 (51) Micheau, C.; Schneider, A.; Girard, L.; Bauduin, P. Evaluation of Ion Separation

3 Coefficients by Foam Flotation Using a Carboxylate Surfactant. *Colloids Surfaces A*

4 *Physicochem. Eng. Asp.* **2015**, *470*.

5 (52) Petrache, H.; Kimchi, I.; Harries, D.; Parsegian, V. Measured Depletion of Ions at the

6 Biomembrane Interface. *J. Am. CHEM. SOC.* **2005**, *127*, 11546–11547.

7 (53) Nyman, M. Small-Angle X-Ray Scattering to Determine Solution Speciation of Metal-

8 Oxo Clusters. *Coord. Chem. Rev.* **2017**, *352* (November), 461–472.

9 (54) Nyman, M.; Fullmer, L. Small Angle X-Ray Scattering of Group V Polyoxometalates. In

10 *Trends in Polyoxometalates Research*; Ruhlmann, L., Schaming, D., Eds.; 2015; pp 151–

11 170.

12 (55) Dufaye, M.; Duval, S.; Stoclet, G.; Loiseau, T. Crystal Chemistry and SAXS Studies of an

13 Octahedral Polyoxoarsenotungstate Nanocluster Encapsulating Four Unprecedented

14 Thorium Arsenate Fragments ({Th3As2On} – n = 25 or 26). *Eur. J. Inorg. Chem.* **2019**,

15 2019 (42), 4500–4505.

16 (56) Buchecker, T.; Le Goff, X.; Naskar, B.; Pfitzner, A.; Diat, O.; Bauduin, P.

17 Polyoxometalate/Polyethylene Glycol Interactions in Water: From Nanoassemblies in

18 Water to Crystal Formation by Electrostatic Screening. *Chem. - A Eur. J.* **2017**, *23* (35),

19 8434–8442.

20 (57) Falaise, C.; Moussawi, M. A.; Floquet, S.; Abramov, P. A.; Sokolov, M. N.; Haouas, M.;

21 Cadot, E. Probing Dynamic Library of Metal-Oxo Building Blocks with γ-Cyclodextrin. *J.*
1 Am. Chem. Soc. **2018**, *140* (36), 11198–11201.

2 (58) Bauer, C.; Bauduin, P.; Girard, L.; Diat, O.; Zemb, T. Hydration of Sugar Based

3 Surfactants under Osmotic Stress: A SAXS Study. *Colloids Surfaces A Physicochem. Eng.*

4 Asp. **2012**, *413*.

5 (59) Liu, F.; Wang, Z.; Sun, D.; Wei, X.; Zhou, W.; Li, G.; Zhang, G. Adsorption Kinetics of

6 Brij 97 at the Air/Solution Interface. *J. Dispers. Sci. Technol.* **2006**, *27* (5), 657–663.

7 (60) Popov, V. V.; Menushenkov, A. P.; Yastrebtsev, A. A.; Molokova, A. Y.; Pisarev, A. A.;

8 Khramov, E. V.; Zubavichus, Y. V.; Shchetinin, I. V.; Ponkratov, K. V.; Tsarenko, N. A.;

9 et al. The Synthesis and Studies of Crystal/Local Structures and Morphology of Hydrated

10 Molybdenum Oxides. *J. Solid State Chem.* **2021**, *301* (February), 122356.

11 (61) Zhang, J. L.; Hu, J. T.; Zhang, L. F. Raman Studies on Species in Single and Mixed

12 Solutions of Molybdate and Vanadate. *Chinese J. Chem. Phys.* **2016**, *29* (4), 425–429.

13 (62) Dement'ev, I. A.; Kozin, A. O.; Kondrat'ev, Y. V.; Korol'kov, D. V.; Proyavkin, A. A.

14 Mononuclear, Polynuclear, and Cluster Complexes of Molybdenum and Their Reactions

15 as Models of Biochemical Systems and Processes. *Russ. J. Gen. Chem.* **2007**, *77* (5), 822–

16 843.

17 (63) Himeno, S.; Ishii, N.; Hasegawa, M.; Saito, A.; Hori, T. Spectrophotometric Studies on

18 the Hexamolybdate Ion in Aqueous Organic Solutions. *Inorganica Chim. Acta* **1987**, *131*

19 (1), 11–13.

20 (64) Girard, L.; Naskar, B.; Dufrêche, J.-F.; Lai, J.; Diat, O.; Bauduin, P. A Thermodynamic

21 Model of Non-Ionic Surfactants' Micellization in the Presence of Polyoxometalates. *J.*
1 *Mol. Liq. 2019*, 293.

2 (65) Lei, Y.; Sun, F.; Liu, X.; Zhao, Z. Understanding the Wet Decomposition Processes of

3 Tungsten Ore: Phase, Thermodynamics and Kinetics. *Hydrometallurgy* **2022**, *213* (July

4 2021), 105928.

5

6
1

2 Superchaotropic flotation: an innovative method and

3 a new concept for the extraction and separation of

4 ions by foams.

5 Klaudia Skorzewska,‡ Alban Jonchère,‡ Coralie Pasquier,‡ Luc Girard‡ and Pierre Bauduin‡**

6 ‡ICSM, CEA, CNRS, ENSCM, Univ Montpellier, Marcoule, France

7 **SUPPLEMENTARY:**
<figure><img src="image_11.png" /><figcaption>**Fig-S1.** 1. Efficiency factors of the flotation of molybdate for the system: Na₂MoO₄ with BrijO10 foam (pH= 1) after 6-7 hours of flotation at different surfactant concentrations: 1.5, 3 and 7 mM.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig-SI. 2. SAXS-spectra of 100 mM molybdate at different pH</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig-SI. 3 SAXS-spectra of 100 mM molybdate at natural pH and water.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Fig-SI. 4.** SAXS-spectra of 100 mM molybdate at pH 3, 20 mM of BrijO10 at natural pH, the sum of the two spectra and the spectra of a solution of 100 mM molybdate with 20 mM of BrijO10 at pH3.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig-SI. 5.** SAXS-spectra of 100 mM molybdate at pH 3, with 10, 30, 60 and 120 mM BrijO10 and simulated spectra of Mo<sub>8</sub>O<sub>26</sub><sup>4-</sup> from atom coordinate in solid state with Crysol.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>**Fig-SI. 6.** SAXS-spectra of 100 mM molybdate at pH 1, 20 mM of BrijO10 at natural pH, the sum of the two spectra and the spectra of a solution of 100 mM molybdate with 20 mM of BrijO10 at pH 1</figcaption></figure>

<table><caption>Table-SI. 1. Raman peaks table from literature<sup>61</sup> (Mo concentration at 350 mM) and from this work (Mo at 100 mM) upon decreasing pH (from the top down to the bottom of the table).</caption><thead><tr><th>Mo Concentration [mM]</th><th>pH</th><th>Mo speciation</th><th>Raman shift [cm<sup>-1</sup>]</th></tr></thead><tbody><tr><td>100</td><td>7.5</td><td>MoO<sub>4</sub><sup>2-</sup></td><td>897</td></tr><tr><td>350</td><td>7</td><td>MoO<sub>4</sub><sup>2-</sup></td><td>897</td></tr><tr><td>100</td><td>6</td><td>MoO<sub>4</sub><sup>2-</sup></td><td>897</td></tr><tr><td>350</td><td>6</td><td>MoO<sub>4</sub><sup>2-</sup></td><td>896</td></tr><tr><td>100</td><td>6</td><td>not observed</td><td>940</td></tr><tr><td>350</td><td>6</td><td>Mo<sub>7</sub>O<sub>24</sub><sup>6-</sup></td><td>940</td></tr><tr><td>100</td><td>5</td><td>Almost disappeared MoO<sub>4</sub><sup>2-</sup></td><td>897</td></tr><tr><td>100</td><td>5</td><td>The shift of the peak at 940 cm<sup>-1</sup>, HMo<sub>7</sub>O<sub>24</sub><sup>5-</sup> / H<sub>2</sub>Mo<sub>7</sub>O<sub>24</sub><sup>4-</sup></td><td>946</td></tr><tr><td>100</td><td>4</td><td>The shift of the peak at 940 cm<sup>-1</sup>, HMo<sub>7</sub>O<sub>24</sub><sup>5-</sup> / H<sub>2</sub>Mo<sub>7</sub>O<sub>24</sub><sup>4-</sup></td><td>946</td></tr><tr><td>100</td><td>4</td><td>α-Mo<sub>8</sub>O<sub>26</sub><sup>4-</sup></td><td>957</td></tr><tr><td>350</td><td>4</td><td>The shift of the peak at 940 cm<sup>-1</sup>, HMo<sub>7</sub>O<sub>24</sub><sup>5-</sup> / H<sub>2</sub>Mo<sub>7</sub>O<sub>24</sub><sup>4-</sup></td><td>948</td></tr><tr><td>350</td><td>4</td><td>Almost disappeared MoO<sub>4</sub><sup>2-</sup></td><td>897</td></tr><tr><td>100</td><td>3</td><td>β-Mo<sub>8</sub>O<sub>26</sub><sup>4-</sup></td><td>970</td></tr><tr><td>100</td><td>3</td><td>α-Mo<sub>8</sub>O<sub>26</sub><sup>4-</sup></td><td>957</td></tr><tr><td>100</td><td>2</td><td>HMo<sub>7</sub>O<sub>24</sub><sup>5-</sup> / H<sub>2</sub>Mo<sub>7</sub>O<sub>24</sub><sup>4-</sup></td><td>946</td></tr><tr><td>100</td><td>2</td><td>α-Mo<sub>8</sub>O<sub>26</sub><sup>4-</sup></td><td>957</td></tr><tr><td>100</td><td>2</td><td>β-Mo<sub>8</sub>O<sub>26</sub><sup>4-</sup></td><td>970</td></tr><tr><td>350</td><td>2</td><td>α-Mo<sub>8</sub>O<sub>26</sub><sup>4-</sup></td><td>957</td></tr><tr><td>350</td><td>2</td><td>β-Mo<sub>8</sub>O<sub>26</sub><sup>4-</sup></td><td>908</td></tr></tbody></table>
<table><tr><td>350</td><td>2</td><td>β-Mo<sub>8</sub>O<sub>26</sub><sup>4-</sup></td><td>970</td></tr><tr><td>100</td><td>1</td><td>The intensity of the peak increases Mo<sub>36</sub>O<sub>112</sub><sup>-</sup></td><td>957</td></tr><tr><td>100</td><td>1</td><td>Mo<sub>36</sub>O<sub>112</sub></td><td>902</td></tr><tr><td>100</td><td>1</td><td>Mo<sub>36</sub>O<sub>112</sub></td><td>983</td></tr><tr><td>350</td><td>1</td><td>The intensity of the peak increases Mo<sub>36</sub>O<sub>112</sub><sup>-</sup></td><td>957</td></tr><tr><td>350</td><td>1</td><td>Mo<sub>36</sub>O<sub>112</sub></td><td>902</td></tr><tr><td>350</td><td>1</td><td>Mo<sub>36</sub>O<sub>112</sub></td><td>983</td></tr></table>

<figure><img src="image_17.png" /><figcaption>**Fig-SI. 7.** SAXS-spectra of 100 mM molybdate solutions with 0, 30 and 60 mM BrijO10. The arrow shows where the scattering intensity shifts to higher q, i.e. formation of smaller objects by increasing the BrijO10 concentration from 30 to 60 mM while the intensity does not change between 0 and 30 mM. Moreover, the intensity at lower q decreases, which also indicates that the concentration of Mo36 species decorating the micelles decreases.</figcaption></figure>