Liquid Interface in Ionic Colloidal Dispersions

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Tuning the Solid/Liquid Interface in Ionic Colloidal Dispersions: Influence on their Structure and Thermodiffusive Properties Cleber Lopes Filomeno, Mansour Kouyaté, Veronique Peyre, Gilles Demouchy, Alex Fabiano Cortez Campos, Régine Perzynski, Francisco Augusto Tourinho, and Emmanuelle Dubois J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10280 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Tuning the Solid/Liquid Interface in Ionic Colloidal

Dispersions:

Influence

on

their

Structure and Thermodiffusive Properties Cleber L. Filomenoa, b, Mansour Kouyatéa, Véronique Peyrea, Gilles Demouchya, d, Alex F. C. Camposb,c, Régine Perzynskia, Francisco A. Tourinhob and Emmanuelle Duboisa,* a

Sorbonne Universités, UPMC Univ Paris 06, CNRS, Laboratoire PHENIX, Case 51, 4 place

Jussieu, F-75005 Paris, France b

Grupo de Fluidos Complexos – Instituto de Química, Universidade de Brasília, CP 04478,

70904-970 Brasília (DF), Brazil c

Faculdade UnB Planaltina, Universidade de Brasília, 73345-010, Planaltina (DF), Brazil

d

Département de physique, Université de Cergy Pontoise, 33 Boulevard du port, 95011

Cergy-Pontoise cedex, France Corresponding Author: E-mail address: *[email protected] (E. Dubois) ABSTRACT: We focus here on the relationship between the physico-chemical properties of electrostatically stabilized colloidal dispersions and the nanoparticles/solvent interface. Dispersions of maghemite (γ-Fe2O3) in polar solvents, here water and dimethyl sulfoxide

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(DMSO), are prepared with a new process which enables tuning easily this interface. Departing from the point of zero charge (PZC), the nanoparticles (NPs) are charged in a controlled way by adding acid or base. This pathway enables to control the surface state of the nanoparticles, i.e. the NP’s charge and the nature of the counterions, as well as the amount of free electrolyte in the dispersion. Stable dispersions are obtained thanks to electrostatic repulsion, in water and in DMSO, with electrolyte concentrations up to 20-40 mM. Small Angle X-ray (SAXS) and Dynamic Light (DLS) scattering techniques are here applied to concentrated dispersions in order to understand the nanostructure and quantify the interparticle interactions. Specific ionic effects are evidenced in both solvents. They depend on the nature of the solvent, with a remarkable effect on the Ludwig-Soret coefficient. 1. Introduction The behavior of colloidal dispersions (CDs) of nanoparticles (NPs) is mainly controlled by the interface between the NPs and the solvent. For this reason, electrostatic and/or steric repulsion are generated by interfacial modification in order to stabilize or flocculate colloidal dispersions. In the case of electrostatic repulsion, the repulsive potential is usually considered to depend on the surface charge of the particle and on the ionic strength in the surrounding solvent, as well as on its dielectric constant.1, 2 This latter parameter is seldom considered, as in most studies, the solvent is water. In addition, the nature of the counterions as well as of the free electrolyte have significant influence in such systems. Since the seminal work of Hofmeister in 1888,3 ion-specific effects have been evidenced in numerous studies dealing for instance with viscosity or solubility evolution or with surfactant organization.4,

5

Some recent studies also

reveal ion specific effects in aqueous colloidal dispersions. For instance, in these aqueous systems, the critical coagulation concentration appears to strongly depend on the nature of the


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added salt,6, 7 and the gelation time varies with counterion nature.8 This illustrates that the nature of the ions influences the shape and value of interparticle potential, which was evidenced directly by force measurements, e. g. [9]. Compared to the large body of work dealing with aqueous suspensions, much less studies have been devoted to ion specific effects in non-aqueous polar solvents. Nevertheless, in a recent study,10 strong specific effects on electrolyte solubility have been observed in ethylene carbonate, which is a polar solvent without any hydrogen bonding. The present study analyzes the structure and the thermodiffusion properties of suspensions of NPs dispersed in two polar solvents (water and dimethylsulfoxide) with NPs of different charges in the presence of various counterions. The structural study of the dispersions is performed by small angle X-Ray scattering and by quasielastic light scattering. The thermodiffusion is assessed by measuring the Ludwig-Soret coefficient, which quantifies the ability of particles to diffuse in a temperature gradient. This is tightly related to Seebeck thermoelectric properties, a technologically-relevant quantity11-15 that could be better exploited on the basis of a deep understanding of the main parameters influencing it. Several authors focused on the case of ionic species dispersed in polar solvents at room temperature.16-22 Few results report on the specific ion effect. Vigolo et al.23 and Eslahian et al.24 presented an influence of the nature of the salt on the Ludwig-Soret coefficient ST for negative nanoobjects. The differences were induced by the properties of the co-ion, Cl- or OH-. On the contrary, Putnam et al.18 modified both the nature of the surface groups and the counterions of commercial polystyrene spheres. They evidenced that the nature of the surface strongly influences the thermodiffusion coefficient DT. Furthermore, in the case of a carboxylated surface, a significant influence of counterion nature was also observed. The same kind of effects by changing the counterions has been recently reported in [25] for iron oxide NPs in water. Still, the relationship between interface characteristics, electrostatic


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repulsions and thermo-diffusion remains at present largely unknown and the present paper aims at providing new information on this matter. The chosen NPs are iron oxide NPs synthetized by the authors with a diameter around 10 nm whose water stability features have been extensively studied. Using such particles, from a single synthetic batch, it is possible (i) to reverse the sign of the surface charge (depending on the suspending medium), which can be estimated for dilute samples;26-29 (ii) to investigate in detail the dispersion structure for a wide composition range;30, 31 and (iii) to modify the nature of the counterion, which leads to significant stabilization effects in water.26,

32

Besides water,

dimethylsulfoxide (DMSO) is chosen due to its properties, relevant for the present study. Firstly, it has a high dielectric constant (ε =46),33 which enables the solvation of ionic species. Accordingly, colloidal stabilization through electrostatic repulsion may well be possible.1 Secondly, its properties significantly differ from those of water as it is an H bond acceptor only and solvates cations better than anions. Furthermore, it is used in numerous syntheses for its properties as reaction medium.34-42 DMSO is thus suitable for applications, as it is also both recyclable and biodegradable. Classified as a green grade solvent, its toxicity is low as it is rated in the pharmacological literature as the only dipolar aprotic solvent with minimal harmfulness to humans.43 In contrast with water, much less is known about colloidal dispersions in DMSO. Some authors prepared NPs dispersions in organic solvents in the absence of any additives,37, 38, 42, 44

but dispersion stability was seldom studied. Information about surface charge was only

reported in [44, 45], the influence of ionic strength was studied,46 and interactions between DMSO and solid surfaces were evidenced by IR spectroscopy.37, 38 With the chosen iron oxide NPs, previous tests proved that dispersion in DMSO was possible using perchloric acid.22, 25 Note that we do not take advantage here of the magnetic properties of the NP, too small to chain in zero


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field. They mainly induce specific behaviors under a magnetic field, which will be studied after firstly understanding the systems without field. In such a system, studying the influence of the interface on thermo-diffusion features implies controlling the interfacial characteristics in a well-defined and reproducible way. It then requires tuning and controlling the charge of the NPs, the nature of the associated counterions, the nature of the solvent, the concentration of all ions and the interparticle interactions. We develop a reproducible method, that can be used in both water and DMSO and that allows determining precisely the amount of charged surface groups of the NPs. It additionally enables preparing concentrated dispersions (Φ > 2%), a key feature for performing meaningful thermodiffusion measurements by Rayleigh forced scattering. The effective NPs charge can then be derived from electrophoretic measurements, while nanostructure and interparticle interactions are quantified using Small Angle X-Ray Scattering (SAXS) and Dynamic Light Scattering (DLS), adapted to absorbing samples. 2. Experimental Section 2.1. Synthesis of magnetic nanoparticles in water Spinel iron oxide nanoparticles were obtained by a condensation process of acidic solutions of FeCl2 and FeCl3 in a strongly alkaline medium.47 The origin and purity of the materials as well as a summarized process are given in the supplementary information (SI). At the end maghemite (γ-Fe2O3) nanoparticles are dispersed in an aqueous nitric acid solution, with a pH between 1.5 and 2. Two batches of maghemite NPs named P1 and P2 were used. Their characteristics are detailed later (section 3.1) and presented in Table 1. The NPs surface and their counterions in


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these precursor fluids are modified to produce the studied samples, which is also explained later in section 3.2. Table 1. Dimensional and magnetic characteristics of precursor colloidal dispersions determined from magnetization and SAXS. ms is the magnetization of the material, d0,mag (resp. d0,saxs) and σmagn (resp. σsaxs) are the size characteristics of the lognormal distribution extracted from magnetization (resp. SAXS) measurements; dwsaxs is the weight average diameter, Rg the radius of gyration and 1/3 the diameter associated to the third order moment (see text for details).

Sample

ms (kA/m)

d0,magn (nm)

P1

278

P2

291

σsaxs

dwsaxs (nm)

Rg (nm)

1/3 (nm)

7.6

0.47

20.6

13.5

10.6

5.2

0.5

15.8

10.3

7.6

σmagn

d0,saxs (nm)

9.2

0.4

6.7

0.4

2.2. Characterization of particles and dispersions In the colloidal dispersions, the volume fractions (Φ) of nanoparticles were calculated from the total iron concentration in the sample determined by flame atomic absorption measurements (FAAS). The magnetization of the material ms, as well as the parameters of the size distribution, assumed lognormal48, are determined by room temperature magnetization measurements. SAXS experiments were carried out at the Soleil Synchrotron SWING beamline in order to extract the form factor of NPs P1 and P2, the weight average volume VNP of the NPs as well as the apparent structure factors. For studying translational diffusion properties, light scattering measurements were performed in the range 0.1% < ΦNP < 4% at 25°C, using a Vasco DLS Particle Analyzer from Cordouan Technologies adapted for dark and absorbing media. Note that the viscosity of DMSO (2.003×10-3 Pa s)49 is around twice the viscosity of water (0.89×10-3 Pa s), therefore experimental times in DLS are similar for DMSO and water. The electrophoretic mobilities were measured in dilute colloidal dispersions (Φ = 0.01%) using a Zeta Potential Nano ZS associated with a dip cell from Malvern at 25°C. The supernatants were separated using an Optima


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MAX-XP Ultracentrifuge from Beckman Coulter at 250 000 ×g for 2 hours at 25 °C. Figure 1 shows the initial sample (Φ ~3-4%) on the left, a partial separation in the middle, and a total separation on the right. The quality of the separation is thus easily detected thanks to the color of the NPs. The clear supernatants were then used for quantifying the free species in the colloidal dispersions. The concentrations were deduced from conductivity measurements with a Precision Component Analyzer 6425 from Wayne Kerr at 25°C thanks to calibration curves for the corresponding pure electrolytes. These concentrations are then used for diluting the colloidal dispersions with the corresponding electrolyte of appropriate concentration. All magnetic fluids elaborated in this work were observed by optical microscopy in the absence and presence of an applied magnetic field (H ≈ 160 kA m-1) in order to check whether they were monophasic or not at the microscale. More details on all these measurements can be found in the SI.

Figure 1. Centrifugation tubes before (left), and after (right) separation. The tube in the middle shows an insufficient separation. On the right, at the bottom of the tube, the highly concentrated CD obtained (Φ ~10-20%) can be re-diluted in solvent if necessary.

Water content of pure DMSO and DMSO solutions were determined via Karl Fischer (KF) coulometric titration. As results, we obtain 0.08±0.01 %wt for pure DMSO, 0.37±0.02 %wt for DMSO-HClO4 0.1 M and 2.33±0.02 %wt for DMSO-TBAOH 0.1 M. From these values and the quantities used during the preparation of the dispersions, the maximum amounts of water in


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DMSO dispersions were calculated (see results in Table 2). No KF determination could be done in colloidal dispersions due to parasitic redox reaction between the NPs and the KF reagent. Note that freeze drying cannot be used to remove water as DMSO is also removed. As well, water content cannot be obtained from thermogravimetric analysis. 2.3. Rayleigh Forced Scattering: determination of the Ludwig Soret coefficient The Ludwig-Soret effect is the mass transport induced in a binary fluid or in a colloid within a temperature gradient. In the stationary conditions, the gradient of NPs volume fraction ሬ∇ԦΦ is linked to the gradient of temperature ሬ∇ԦT by: →

→

∇ Φ = − ST Φ ∇ T

(1)

where ST is the Ludwig-Soret coefficient in the colloid. ST was determined in concentrated dispersions using a Rayleigh forced scattering technique, with a simple homemade device.50 Thanks to a powerful Hg lamp, modulated at 100 Hz, the image of a grid is created in a thin optical plate containing the ferrofluid dispersion. Due to the strong NPs optical absorption, spatially periodic gradients of temperature, of typical period 50 µm, appear in the dispersion, modulating the optical index. The Ludwig-Soret effect then produces in a few seconds gradients of concentration with the same spatial period, modulating the absorption. Both gratings (of temperature and of concentration) are detected by the scattering of the non absorbing laser beam. Their temporal response to the modulation differs by several orders of magnitude.17, 50, 51 The array of concentration being stationary, the Ludwig-Soret coefficient was deduced here from the temporal modulation of the intensity scattered by the probe laser beam as in [50]. 3. Results: new preparation process and associated properties


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3.1

Nanoparticles size distributions

A precise knowledge of the size distribution characteristics is required to check the quality of the precursor colloids and to analyze the measurements obtained hereafter by different techniques. Therefore, several determinations are performed and compared in Table 1 for both precursors P1 and P2. As usually done in similar studies,48 a log-normal distribution P(d) of NPs with magnetic diameters d is assumed :

P(d ) =

 ln 2 ( d / d0 )  exp  −  2σ2 d σ 2π   1

(2)

where d0 is the median diameter (ln d0 = ) and σ the polydispersity index of the distribution. As the NPs are not monodisperse, different average diameters are obtained depending on the techniques used, averages that can be linked to the size distribution. The magnetization of the material ms is first determined from magnetization measurements. Its obtained value is coherent with that of similar colloidal dispersions.52 Then d0,magn and σmagn are extracted from the fit of the whole curve. The dipolar interaction parameter associated to this distribution is small so that no spontaneous chaining of NPs occurs in the dispersions. Secondly, from SAXS measurements, d0,saxs and σsaxs are extracted from the P(Q) extrapolated at zero concentration. The values thus

obtained are compatible with d0,magn and σmagn: the polydispersities of the two samples are similar and the mean size of NPs in P1 is larger than in P2. From SAXS, the weight-average volume VNP of Equation (SI-1) can be directly determined from the extrapolation of I(Q) at Φ = 0 and Q = 0. The corresponding diameter dwsaxs determined from this experimental volume VNP =π/6 d3wsaxs


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(given in Table 1) can also be calculated from the determined size distribution with dw = d0,saxs exp(4.5σ2saxs) (not shown). The good agreement between the two values points to the

good quality of the fit. Finally, the radius of gyration Rg can be estimated from the slope of ln[P(Q)] at low Q (with qRg 0.2, without taking into account the tail which is less reliable. We use a mono-exponential whenever possible, i.e. for dilute samples, in order to reduce the number of fitting parameters to only one. When a stretched exponential is necessary, i.e. for part of the more concentrated samples, we fit the same range of times for consistency. The deduced collective translational diffusion coefficients (Dt) are plotted as a function of volume fraction in the insets of Figure 3 (see results in Table 4).


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At low enough volume fraction, Dt versus Φ is, as expected, linear in all systems (see insets of Figure 3 and Table 4). D0, thus RH, and KD are determined in this linear region (see Table 4). All RH values are in the same range (13-16 nm) except for the attractive samples in alkaline medium. For B-TBA-W sample, which is not stable over long times and presents a slow aggregation leading to a gel after several months, RH clearly evidences the presence of small aggregates in the sample just after its preparation. For B-TBA-D sample, RH is very close to the one of the repulsive samples (even if slightly larger), which is consistent with the value S(Qmin) = 1.3, meaning that very small aggregates are present. For the other samples, for which

the average interparticle interaction is repulsive, the RH values are close to or a bit larger than the Rg values estimated for the precursor fluids in water (see Table 1). This is reasonable as we

expect RH/Rg = 1.3 for monodisperse spheres.64 It confirms that the particles are well dispersed in these repulsive sols, which is consistent with the absence of time evolution according to X-Rays patterns measured after several months. The second parameter, the coefficient KD, should necessarily be smaller than KT if Hard Sphere hydrodynamics is the only source of interaction entering in KF. Indeed KD < KT here (within the error bar) for all repulsive samples except A-ClO4-D2 that has already been identified as presenting an unusual nanostructure from SAXS analysis. For the alkaline samples, KD clearly indicates that the interaction is weakly attractive, which confirms both macroscopic and SAXS observations. Note that the only previous determination of KD and KF, coupling SANS and Forced Rayleigh Scattering for an aqueous sample with citrate-coated NPs and Na+ counterions at [Cit3-] = 3×10-2 M, gave a value KF = KT - KD close to the one of hard spheres (+6.55).62 Such a sample is rather close to the observations for N-TBA-W. The result is similar here within the


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error bar of the measurements (see dashed line calculated from SAXS data in the inset of Figure 3(a)). For a comparison, the same thing is presented in the inset of Figure 3(b). The comparison of DLS with SAXS indicates that these DLS measurements, though obtained with samples with a strong optical absorption, are pertinent. These DLS results on series of concentrations properly reproduce the qualitative differences between the different samples: - the influence of the nature of the counterion comparing A-NO3-W with A-ClO4-W or - the influence of ionic strength comparing A-ClO4-D1 with A-ClO4-D2. In the case of alkaline samples, DLS

demonstrates that the interaction is weakly attractive in B-TBA-D, which is consistent with SAXS, and that the attraction is stronger in B-TBA-W, which is consistent with its following evolution towards a gel. 3.6

Soret-Ludwig coefficient from Rayleigh Forced Scattering

The measured Ludwig-Soret ST coefficients (see section 2.3), given in Table 4, are among the first performed with samples prepared by the well controlled method of section SI-2 for exchanging counterions, in these two polar solvents. They are here all determined at Φmax except for sample A-NO3-W. In that latter case, ST is measured at Φ = 1.94 % to be comparable with the determination of A-ClO4-W at 2.26%, since increasing the volume fraction would reduce the absolute value of ST.16,

22, 51, 65

The ST values are either positive or negative and range here

between -0.17 and + 0.17 K-1, a quite standard order of magnitude for (electrostatically stabilized) concentrated magnetic colloids.16, 22, 51, 65 The main observed effects are: (i) a change of ST sign for acidic dispersions while changing the solvent from water to DMSO (with comparable interparticle repulsion) - it illustrates the importance of the nature of the solvent on the ST value;


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(ii) a quite unusual positive sign of ST for sample N-TBA-W, while in aqueous citrated dispersions with Na+ counterions, with comparable repulsion, it is always found negative -50, 65 it illustrates the importance of the nature of the counterion on the ST value;18, 25 (iii) the large reduction of the ST value occurring in acidic DMSO between A-ClO4-D1 and A-ClO4-D2 as the ionic strength is strongly reduced - it illustrates the importance of the strength of the interactions in connection with that of the structure of the dispersion on ST. It should be explored in more details in the future. (iv) the absence of direct correlation between the sign of Zstruct (or equivalently Zeff) and the sign of the Ludwig-Soret coefficient ST that was already pointed out in previous studies,18, 51, 65 is here confirmed. These results enlighten the role of several parameters on the Ludwig-Soret coefficient ST. Indeed, according to the equations given in [25], ST depends on interparticle interactions and, through the enthalpy of transfer66, depends on the Eastman entropy* of transfer Ŝi of all the charged species (i) in the medium.14, 22, 69, 70 These latter quantities are heats of transport divided by temperature and quantify the Seebeck electric field proportional to the temperature gradient in the dispersion. The Eastman entropy of transfer Ŝ+ and Ŝ- of all the small ions, i.e. counterions of the NPs, free anions and cations contribute to ST, but also ŜNP the NP's entropy of transfer. ŜNP is not known a priori and is a characteristic of the NPs/solvent interface. Following [25], its sign depends on the nature of the condensed counterions, "dressing" the NPs and on the nature of the

*

The entropy of transfer considered here is the one derived by De Groot67 and Agar68 from Onsager’s theory. In this

article, we will use Agar’s terminology and refer to the “Eastman entropy of transfer”.


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solvent. The sign of ŜNP determines the sign of ST as can be seen in Table 3. That point clearly reveals that non-standard electrostatic effects are here involved.

Figure 3. Field autocorrelation functions G1(t) versus correlation time t obtained from DLS measurements on (left) the aqueous citrate sample N-TBA-W and (right) the acid DMSO sample A-ClO4-D1 for different volume fractions. Insets show the evolution of the diffusion coefficients Dt versus Φ; solid lines correspond to the linear fit of Dt at low Φ to determine D0 and KTDLS and the dashed lines correspond to the calculated collective translational diffusion coefficient (Dt) with KT obtained from SAXS measurements (hypothesis KF=6.55). See text for details.

4. Discussion As DMSO has a permittivity ε = 46, lower than water, the stabilization of nanoparticles thanks to electrostatic repulsion is expected to be more difficult than in water. Indeed, in the most simple model, the Bjerrum length is larger (1.2 nm, compared to 0.7 nm in water at room temperature), which implies a larger condensation of counterions on the surfaces, thus a lower effective charge. Moreover, for a given ionic strength, the Debye length is lower, thus the range of the repulsive interaction is reduced. Besides, the solubility of salts in DMSO and water differ: larger species are in general more easily dissolved in DMSO than small ones (for example the solubility of NaClO4 is 1.8 mM while it is 1 M for Et4NClO4).55 Therefore while switching from water to DMSO, the interface NPs/solvent should be modified through the surface charge, the


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effective charge, the organization of the counterions and of the solvent around the NPs, which influences interparticle interactions and consequently the properties of the colloidal dispersions. In literature, the existence of an electrostatic contribution in DMSO was evidenced in two cases, at salt concentrations that are much smaller than those used in the present study.44, 45 In the other cases of stable dispersions in DMSO, the origin of the stabilization was not entirely clarified.34, 38, 40 The authors demonstrate strong interactions between solvent and particles, which could contribute to the stabilization. However, as the particles are synthesized in the solvent, some charges on their surface may arise from the reducing agent.71 The results presented here prove that colloidal dispersions can be stabilized even for salt concentrations as large as 20-40 mM with an electrostatic repulsive contribution. Contrary to literature that deals with powders dispersed in DMSO or NPs directly synthesized in the solvent, we proceed via a transfer of the NPs dispersed in water into DMSO without drying. Beginning with uncharged particles in water, we introduce charges in a controlled way by addition of known amounts of either strong acid or strong base in DMSO.72 For bare particles, the initial state is NPs at the point of zero charge, whereas for particles coated with citrate, the initial state is the NPs coated with citric acid at low pH. Among the numerous advantages of this method, the direct determination of the structural charge per surface (once the free electrolyte concentration is known) shows that these charges are close in water and DMSO for similar ionic strength. It means that, in DMSO, the sites on the surface have been charged, which is not obvious because the pKa of species in DMSO can significantly differ from those in water (for example, in the case of acetic acid, pKa (water) = 4.8 whereas pKa (DMSO) = 12.6)72 and we have no values for the acidic groups on the NPs’ surface


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and no value for citric acid. Note that the particles never disperse in DMSO in the initial neutral state. For the particles without citric acid, the positive NPs in acidic medium can be flocculated by addition of TBAOH in DMSO before redispersing them as negative NPs when more base is added. The pKa’s of the surface sites are thus in the accessible pH range in DMSO, even if not precisely known. For the particles coated with citric acid, the ability to stabilize them while adding base from the neutral state in acidic medium means that citric acid is a weak acid as in water, with pKa’s in the pH range accessible in DMSO. Therefore, there is in DMSO a strong repulsive electrostatic contribution, which can be tuned through the electrolyte quantity. If the structural charges are close in water and DMSO for the systems of same composition except the solvent, the effective charges resulting from the condensation of part of the counterion however differ. For a given structural charge, the effective charge is smaller in DMSO as expected due to its lower dielectric constant. In such a polar organic solvent as DMSO, the question of the presence of a small amount of water and of its role is important. Although not measurable inside the final dispersions yet, this amount is minimized in the preparation process and orders of magnitude can be estimated. Some water can be brought by the NP’s surfaces remaining after washing with the solvents: indeed, we have determined that 1 to 2 water layers remain on NPs in the powder obtained after drying at 100 °C, which gives a maximal limit. For precursor particles P1 (see Table 1), the specific surface calculated from size characteristics is around 90 m2 g–1. Therefore, if Φ = 4% and for 3 Å of water on all surfaces (thickness estimated for one water layer), the corresponding water volume fraction in the colloidal dispersion is around 0.5%. On the other hand, some water is introduced inescapably through the preparation by the electrolytes added to introduce the NPs charges. If all this water is localized on the NPs surface, it would correspond to less than one


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water layer in acidic ferrofluids, and to few layers for the other types of dispersions. The probability appears however low that all the water remains at the solid/solvent interface because (i) water and DMSO are totally miscible, (ii) the organization of the interface depends on the relative affinities of water and DMSO for the surface (iii) the determined effective charge differs between DMSO and water (iv) the stability of systems with similar composition differs in water and DMSO. Consequently, even if some water remains, it does not sweep away specificities introduced by DMSO. DLS on dark media and SAXS are here used simultaneously to get information on the structure of the dispersions and interactions between NPs both in water and DMSO. If they agree on the interparticle interaction, DLS measurements on these concentrated absorbing samples can evidence special behaviours difficult to detect with other techniques, here mainly observed in the acidic colloidal dispersions in DMSO. In these samples, long correlation times τtr (thus low Dt) appear in the correlation functions while increasing the volume fraction, as can be seen in Figure 3(b). On the contrary, this phenomenon does not occur in negative particles coated with citrate (as in Figure 3(a)): either it does not exist in these systems, or it occurs at higher volume fractions due to a modified interparticle potential as the surface is modified by the adsorbed citrate. Such long times have been reported by Kleshchanok et al.45 in a system of charged gibbsite platelets electrostatically stabilized in DMSO. However, the long times were sufficiently different from the short ones to be separated, which is not possible here as many sets of parameters can explain the data. Their short time, which was attributed to a collective diffusion coefficient, decreases with Φ due to interparticle repulsion, as we observe here. Their long time slightly increased with Φ, however there was no definitive interpretation. The long times we observe could also be due to correlated structures or small aggregates that form while increasing


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Φ. In our case, τtr increases by a factor around 3 for the concentrated samples, which could correspond to small structures. In the case of sample A-ClO4-D2, which has a very low ionic strength, another specificity occurs: the distance between particles from SAXS λmax = 27.5 nm differs from the average distance λav = 17 nm, obtained with 1/3=7.6 nm (SAXS) within the hypothesis of strongly repulsive isolated particles. DLS and SAXS observations could correspond to aggregates interacting through strong repulsion. Within the hypothesis of monodisperse aggregates, the volume Vaggregates can be estimated using the experimental average distance by λav3=Vaggregates/Φ. It leads to Vaggregates = 969 nm3, which is of the order of the volume of 4 NPs. Such small objects could indeed be stable. In order to better understand the detailed nanostructure of these dispersions at large volume fractions, coupled DLS and SAXS measurements would be necessary, on series of increasing Φ for several ionic strengths in the dispersions. Such DLS measurements are thus an efficient technique to check the colloidal dispersions whatever the volume fraction in order to extract knowledge on interparticle interactions, on their state of dispersion, or on a time evolution. For the studied repulsive samples, results are reproducible and no aging occurs over months. With the well-defined dispersions prepared, both in water and DMSO, reliable ST coefficients are determined. The results clearly show that these ST can be hugely modulated by changing the components of the systems, here the nature of the counterions, that of the solvent, and the electrolyte concentration. Indeed, a spectacular reversal of the sign of ST occurs in two cases: (i) in acidic medium, with perchlorate counterions, a reversal of the sign of ST occurs when exchanging the solvent from water to DMSO (see Table 4); (ii) in aqueous neutral samples, the sign of ST is reversed when replacing TBA+ counterions (value in Table 4) by sodium counterions (values in [25]). Such a reversal by changing the nature of counterions has been


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reported by Putnam18 on commercial charged latex with different counterions and attributed to free ions. The dispersed NPs should be indeed included among these free ions, as macroions of effective charge Zeff, “dressed” with condensed counterions. These condensed counterions strongly influence the interaction NP/solvent, which is characterized by ŜNP the entropy of transfer of the dressed NP. In case (i), the reversal occurs with the same counterion in both solvents and similar interparticle interactions, which has never been seen before. The only possible origin is therefore the solvation of the dressed NPs and of the free small ions by the solvent, which can strongly differ between the two solvents because water can solvate both anions and cations whereas DMSO better solvates cations. These remarkable effects on the Ludwig-Soret coefficient ST evidence the very strong influence of different monovalent counterions, which thus enable a tuning of the properties of colloidal dispersions. This is also observed here on the ability to disperse or not the NPs in the solvent (Table 2). Moreover, when their dispersion is possible, the strength of the interparticle repulsion is modulated by the counterions: (i) in aqueous acidic medium, interparticle repulsion is higher with nitrate than with perchlorate counterions; (ii) in alkaline medium, interparticle repulsion is higher in DMSO than in water with TBA+ counterions (see Table 2). It means that strong specific effects superimpose on the classic electrostatic description based on charge, ionic strength and Debye length. Moreover, not only ions are important but also the solvents, as ion/solvent interactions as well as dressed NPs/solvent interactions are determining. Such an influence of the counterions on the interactions between charged surfaces (see Table 2) has been directly observed by force measurements on alumina surface,9 showing that repulsion is larger with chloride than with bromide. This was consistent with the stability they measured for the alumina colloidal dispersions. Also large variations of the critical coagulation


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concentration indirectly evidenced a modification of interparticle interactions, as observed on positive and negative latex particles with many different counterions6 or gold NPs with the alkali cations series.7 Similarly, the time of gelation of negatively charged silica colloidal dispersion changed by 104 depending on the alkali cation, pointing to a huge modification of the interactions.8 Other indirect evidences have been seen for example in cationic polyelectrolytes, using small angle scattering and NMR, from the difference of behavior between fluoride and bromide counterions: electrostatic interactions were modified due to a larger affinity of the bromide for the polymer chain, thus modifying the interchain interactions.73 Although demonstrated in water, such ion effects are scarcely tackled in other solvents. Nevertheless a recent work on solubility determinations of salts in ethylene carbonate showed specific ion effects on solubility due to ion-solvent interactions, in a solvent very different from water, as it is a non-hydrogen-bonded fluid.10 It is likely that the same kind of specific ion effect explains the huge impact observed on dispersion properties such as interparticle repulsion and ST. Indeed the Eastman entropies of transfer Ŝi depend on the nature of the ion and moreover introduce specific effects on the NPs which are dressed with condensed counter ions. Therefore ŜNP depends on the solvent for a chosen counterion and depends on the counterion for a chosen solvent. This can modulate the thermophilic/thermophobic properties of the NPs. 5. Conclusions Dispersions of NPs in polar solvents, here water and DMSO, are obtained in a reproducible way departing from a point of zero charge and then charging the surface in a controlled way. Not only the counterions can be chosen but also the concentration of electrolyte can be modulated


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and the structural charge directly determined even for high volume fractions, which can be directly prepared (tested here up to 5%). Electrostatic and stable dispersions are obtained in DMSO even with ~25 mM of electrolyte and the behaviors in DMSO are qualitatively similar to those observed in water. However the samples have a better stability in the long run, without evolution, especially in the alkaline medium, due to less pollution, i.e. less carbonatation, less bacteria, and no NPs’ dissolution in acidic medium. Strong specific ion effects are evidenced in both water and DMSO. In the stable dispersions, depending on the details of the systems, the interparticle interactions are tuned from weakly attractive to strongly repulsive, which is quantified by both SAXS and DLS in dark media. The two techniques are in good agreement, which also validates the DLS measurements performed on our strongly absorbing systems (black solutions). DLS measurements on these concentrated samples are thus a valuable tool for an easy and quick estimation of the specific effects of ions and solvents. A reversal of sign of ST is observed with positively charged nanoparticles associated with perchlorate counterions when changing the solvent from water to DMSO. It means that, in these conditions, the positively charged particles dressed with perchlorate counterions move towards hot regions in water (ŜNP < 0) and cold regions in DMSO (ŜNP > 0). This is one of the first observation of this type, which also evidences specific effects of ions in a non-aqueous solvent, phenomena seldom addressed. These effects can only result from a change of organization of solvent, free ions and condensed ions at the charged solid interface of the NPs, resulting from the different interactions between species in the solvents. The ensemble of tools validated in the present work opens the way towards a more detailed study of Ludwig Soret and Seebeck effects in these colloidal dispersions, in order to tune these effects with clearly identified key parameters.


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6. Acknowledgments We would like to thank J. Perez for assistance at the beamline SWING of synchrotron SOLEIL facility and A. Anfry for technical assistance at PHENIX laboratory, S. Nakamae and M. Roger (SPEC, CEA) for discussions. This research is supported by ANR TE-FLIC (Grant nº ANR-12-PRGE-0011-01), contracts CAPES-COFECUB No. 714/11 and PICS/CNRS No. 5939. C. Lopes Filomeno is very grateful for CAPES grant no. 99999.001111/2014-00 and for CAIQ/UnB support. We thank the referees for their helpful comments.


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SAXS structure factors S(Q, ΦNP) for different NPs interfaces. (a) Aqueous acid dispersions. Inset: (circles: NO3 ; triangle: ClO4 ) experimental osmotic compressibility χexp versus effective volume fractions (Φeff) fitted by Carnahan-Starling formalism for hard spheres (line). (b) Acidic DMSO dispersions of different ionic strengths. (c) Citrated NPs in water and in DMSO and alkaline DMSO dispersion. 127x97mm (300 x 300 DPI)


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Liquid Interface in Ionic Colloidal Dispersions

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