Ion specific tuning of nanoparticle dispersion in an ionic liquid: a structural, thermoelectric and thermo-diffusive investigation.

Dispersions of charged maghemite nanoparticles (NPs) in EAN (ethylammonium nitrate) a reference Ionic Liquid (IL) are studied here using a number of static and dynamical experimental techniques; small angle scattering (SAS) of X-rays and of neutrons, dynamical light scattering and forced Rayleigh scattering. Particular insight is provided regarding the importance of tuning the ionic species present at the NP/IL interface. In this work we compare the effect of Li+, Na+ or Rb+ ions. Here, the nature of these species has a clear influence on the short-range spatial organisation of the ions at the interface and thus on the colloidal stability of the dispersions, governing both the NP/NP and NP/IL interactions, which are both evaluated here. The overall NP/NP interaction is either attractive or repulsive. It is characterised by determining, thanks to the SAS techniques, the second virial coefficient A2, which is found to be independent of temperature. The NP/IL interaction is featured by the dynamical effective charge ξeff0 of the NPs and by their entropy of transfer SNP (or equivalently their heat of transport ) determined here thanks to thermoelectric and thermodiffusive measurements. For repulsive systems, an activated process rules the temperature dependence of these two latter quantities.

Design of concentrated colloidal dispersions of iron oxide nanoparticles in ionic liquids: Structure and thermal stability from 25 to 200 °C.

HYPOTHESIS: Some of the most promising fields of application of ionic liquid-based colloids imply elevated temperatures. Their careful design and analysis is therefore essential. We assume that tuning the structure of the nanoparticle-ionic liquid interface through its composition can ensure colloidal stability for a wide temperature range, from room temperature up to 200 °C. EXPERIMENTS: The system under study consists of iron oxide nanoparticles (NPs) dispersed in ethylmethylimidazolium bistriflimide (EMIM TFSI). The key parameters of the solid-liquid interface, tuned at room temperature, are the surface charge density and the nature of the counterions. The thermal stability of these nanoparticle dispersions is then analysed on the short and long term up to 200 °C. A multiscale analysis is performed combining dynamic light scattering (DLS), small angle X-ray/neutron scattering (SAXS/SANS) and thermogravimetric analysis (TGA). FINDINGS: Following the proposed approach with a careful choice of the species at the solid-liquid interface, ionic liquid-based colloidal dispersions of iron oxide NPs in EMIM TFSI stable over years at room temperature can be obtained, also stable at least over days up to 200 °C and NPs concentrations up to 12 vol% (≈30 wt%) thanks to few near-surface ionic layers.

Thermodiffusion anisotropy under a magnetic field in ionic liquid-based ferrofluids.

Ferrofluids based on maghemite nanoparticles (NPs), typically 10 nm in diameter, are dispersed in an ionic liquid (1-ethyl 3-methylimidazolium bistriflimide - EMIM-TFSI). The average interparticle interaction is found to be repulsive by small angle scattering of X-rays and of neutrons, with a second virial coefficient A2 = 7.3. A moderately concentrated sample at Φ = 5.95 vol% is probed by forced Rayleigh scattering under an applied magnetic field (up to H = 100 kA m-1) from room temperature up to T = 460 K. Irrespective of the values of H and T, the NPs in this study are always found to migrate towards the cold region. The in-field anisotropy of the mass diffusion coefficient Dm and that of the (always positive) Soret coefficient ST are well described by the presented model in the whole range of H and T. The main origin of anisotropy is the spatial inhomogeneities of concentration in the ferrofluid along the direction of the applied field. Since this effect originates from the magnetic dipolar interparticle interaction, the anisotropy of thermodiffusion progressively vanishes when temperature and thermal motion increase.

Colloidal dispersions of oxide nanoparticles in ionic liquids: elucidating the key parameters.

The combination of ionic liquid and nanoparticle properties is highly appealing for a number of applications. However, thus far there has been limited systematic exploration of colloidal stabilisation in these solvents, which provides an initial direction towards their employment. Here, we present a new and comprehensive study of the key parameters affecting the colloidal stability in dispersions of oxide nanoparticles in ionic liquids. Twelve diverse and representative ionic liquids are used to disperse iron oxide nanoparticles. The liquid interface of these nanoparticles has been carefully tuned in a molecular solvent before transferring into an ionic liquid, without passing through the powder state. Multiscale-characterisation is applied, on both the micro and the nano scale, incorporating both small angle X-ray scattering and dynamic light scattering. The results show the surface charge of the nanoparticles to be a crucial parameter, controlling the layering of the surrounding ionic liquid, and hence producing repulsion allowing efficient counterbalancing of the attractive interactions. For intermediate charges the strength of the repulsion depends on the specific system causing varying levels of aggregation or even none at all. Several samples consist of sufficiently repulsive systems leading to single dispersed nanoparticles, stable in the long term. Thanks to the magnetic properties of the chosen iron oxide nanoparticles, true ferrofluids are produced, appropriate for applications using magnetic fields. The strength and breadth of the observed trends suggests that the key parameters identified here can be generalised to most ionic liquids.

Inversion of thermodiffusive properties of ionic colloidal dispersions in water-DMSO mixtures probed by forced Rayleigh scattering.

Thermodiffusion properties at room temperature of colloidal dispersions of hydroxyl-coated nanoparticles (NPs) are probed in water, in dimethyl sulfoxide (DMSO) and in mixtures of water and DMSO at various proportions of water, [Formula: see text]. In these polar solvents, the positive NPs superficial charge imparts the systems with a strong electrostatic interparticle repulsion, slightly decreasing from water to DMSO, which is here probed by Small Angle Neutron Scattering and Dynamic Light Scattering. However if submitted to a gradient of temperature, the NPs dispersed in water with ClO4- counterions present a thermophilic behavior, the same NPs dispersed in DMSO with the same counterions present a thermophobic behavior. Mass diffusion coefficient [Formula: see text] and Ludwig-Soret coefficient [Formula: see text] are measured as a function of NP volume fraction [Formula: see text] at various [Formula: see text]. The [Formula: see text]-dependence of [Formula: see text] is analyzed in terms of thermoelectric and thermophoretic contributions as a function of [Formula: see text]. Using two different models for evaluating the Eastman entropy of transfer of the co- and counterions in the mixtures, the single-particle thermophoretic contribution (the NP's Eastman entropy of transfer) is deduced. It is found to evolve from negative in water to positive in DMSO. It is close to zero on a large range of [Formula: see text] values, meaning that in this [Formula: see text]-range [Formula: see text] largely depends on the thermoelectric effect of free co- and counterions.

Thermodiffusion of citrate-coated γ-Fe2O3 nanoparticles in aqueous dispersions with tuned counter-ions - anisotropy of the Soret coefficient under a magnetic field.

Under a temperature gradient, the direction of thermodiffusion of charged γ-Fe2O3 nanoparticles (NPs) depends on the nature of the counter-ions present in the dispersion, resulting in either a positive or negative Soret coefficient. Various counter-ions are probed in finely tuned and well characterized dispersions of citrate-coated NPs at comparable concentrations of free ionic species. The Soret coefficient ST is measured in stationary conditions together with the mass-diffusion coefficient Dm using a forced Rayleigh scattering method. The strong interparticle repulsion, determined by SAXS, is also attested by the increase of Dm with NP volume fraction Φ. The Φ-dependence of ST is analyzed in terms of thermophoretic and thermoelectric contributions of the various ionic species. The obtained single-particle thermophoretic contribution of the NPs (the Eastman entropy of transfer sNP) varies linearly with the entropy of transfer of the counter-ions. This is understood in terms of electrostatic contribution and of hydration of the ionic shell surrounding the NPs. Two aqueous dispersions, respectively, with ST > 0 and with ST < 0 are then probed under an applied field H[combining right harpoon above], and an anisotropy of Dm and of ST is induced while the in-field system remains monophasic. Whatever the H[combining right harpoon above]-direction (parallel or perpendicular to the gradients and ), the Soret coefficient is modulated keeping the same sign as in zero applied field. In-field experimental determinations are well described using a mean field model of the interparticle magnetic interaction.

Thermoelectricity and thermodiffusion in charged colloids.

The Seebeck and Soret coefficients of ionically stabilized suspension of maghemite nanoparticles in dimethyl sulfoxide are experimentally studied as a function of nanoparticle volume fraction. In the presence of a temperature gradient, the charged colloidal nanoparticles experience both thermal drift due to their interactions with the solvent and electric forces proportional to the internal thermoelectric field. The resulting thermodiffusion of nanoparticles is observed through forced Rayleigh scattering measurements, while the thermoelectric field is accessed through voltage measurements in a thermocell. Both techniques provide independent estimates of nanoparticle's entropy of transfer as high as 82 meV K(-1). Such a property may be used to improve the thermoelectric coefficients in liquid thermocells.

Microstructure of colloidal dispersions in the ionic liquid ethylammonium nitrate: influence of the nature of the nanoparticles' counterion.

In order to better identify the key parameters governing colloidal stability in ionic liquids we probe the influence of the nature of the initial counterion of citrate-coated maghemite nanoparticles (NP), with Na(+), Li(+) and ethylammonium (EA(+)) on their dispersions in ethylammonium nitrate (EAN). Chemical analysis shows that sodium and lithium counterions remain at the nanoparticle surface after their transfer from water to EAN, despite their low concentration compared with EA(+). Macroscopically, all suspensions are stable over the range of volume fractions ΦNP tested (∼ 1% to 8%). A microstructural study coupling small angle scattering and magneto-optic birefringence measurements shows that nanoparticles are perfectly dispersed with sodium counterions and interact through weak repulsions. Conversely, small clusters of a few nanoparticles are formed with lithium counterions, with the aggregation number increasing with ΦNP. However, such clusters are fragile; evidence that the attractions responsible for aggregation are of weak amplitude. Suspensions with EA(+) counterions show an intermediate behaviour. Our results demonstrate the determining role of initial counterions of the nanoparticles on the microstructure of colloidal dispersions in ionic liquids and therefore, the essential role of the interfacial zone between the solid and the liquid.

Tuning the colloidal stability in ionic liquids by controlling the nanoparticles/liquid interface.

To shed light on the origin of colloidal stability in ionic liquids,we focus on a model colloidal system (maghemite nanoparticles) in which surface charge and counterion nature can be controlled at will. We thus evidence the crucial role of interfacial features on dispersion quality in a standard ionic liquid, ethylammonium nitrate.

Ion-selective electrode for dodecyldimethylamine oxide: a "twice-Nernstian" slope for the determination of a neutral component

An electrode originally sensitive to dodecyltrimethylammonium (DTA+) was proven to be sensitive to dodecyldimethylamine oxide (DDAO), a surfactant with acidobasic properties. The response of the electrode was tested from pH 2 to 9.3. Its slope is Nernstian when the surfactant is entirely protonated. At a pH where the molecule is mainly under the neutral form, the electrode responds with a "twice-Nernstian" slope around 120 mV/decade. The validity of this electrode for measurements was checked by confronting the evolution of the critical micelle concentration of DDAO vs pH with data already published and by determining the complexation constant of DDAO and beta-cyclodextrin. A possible explanation of the "twice-Nernstian" slope, using a dimer of DDAO is proposed.

The Use of Thermodynamics for the Indirect Determination of Neutral Surfactants Activities in Mixtures of Potassium Polyvinyl Sulfate (PVSK), Dodecyltrimethylammonium Bromide (DTABr), and N-Dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB12).

Mixtures of zwitterionic and cationic surfactants were studied in the presence of an anionic polyelectrolyte. We used N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB12), dodecyltrimethyl ammonium bromide (DTA+Br-), and polyvinyl sulfate (potassium salt) (PVS-K+). The composition of the system was determined by measuring the activity of DTABr with an ion-selective electrode, and deducing the activity of SB12 from thermodynamic equations. This method provides information about the distribution of all species (activities of both surfactants and composition of the micelles), by experimental determination of only one quantity. SB12 affected the adsorption of DTABr onto PVSK. Copyright 1999 Academic Press.

Stability of a Nanometric Zirconia Colloidal Dispersion under Compression: Effect of Surface Complexation by Acetylacetone

The stability of a colloidal dispersion of nanometric zirconia particles has been studied during a compression process. Using the osmotic stress method, cycles of compression and reswelling were applied to the dispersion to test the reversibility of the process. Original dispersions are stable in a very limited pH range (0.5-2). At pH 3, the bare particles aggregate irreversibly under compression as checked by osmotic pressure and light and X-ray scattering measurements. To improve the stability, small organic complexing molecules (acetylacetone) were added to the original dispersion. The adsorbed monolayer on the particle surfaces acts as a steric barrier and prevents the two colloids from contacting. As a consequence, the dispersion becomes more compressible and the compression cycle is totally reversible. The experimental data are quantitatively reproduced with a classical theory of statistical mechanics of liquids based on a DLVO-like colloid-colloid potential.