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Probing electrolyte behavior at the atomic level provides valuable insights into understanding the reactive transport of electrolyte solutions in nanoconfinement and help explain and predict the macroscopic properties of industrial or natural nanoporous materials. In this study, we focused on the behavior of water and ions in silica nanocylinders with a pore diameter of 2.6 nm filled with electrolyte solutions, XCl at 1 M with X = Li, Na, and Cs, monovalent cations presenting various kosmotropic/chaotropic properties. Using a combination of experiments and theoretical modeling, we analyzed the water dynamics based on three primary effects: the confinement, the electrolyte, and the interfacial ion–porous material surface interaction. Comparing the water dynamics obtained with those of divalent cations previously studied by Baum et al. (Dynamical and Structural Properties of Langmuir 35 (2019) 10780–10794. https://doi.org/10.1021/acs.langmuir.9b01434), we highlight that monovalent cations present weaker interactions with silica surfaces than divalent cations. This significantly impacts the water dynamics in addition to the confinement and electrolyte effects. This study pinpoints the importance of ion behavior within the interfacial layer and its impact on water transport in nanoconfinement.

The identification of new colloidal sol–gel routes for the preparation of actinide oxides, which have a homogeneous and accessible porosity that can easily be impregnated by any concentrated actinide solution, opens new perspectives for the preparation of homogeneous nuclear fuel for minor actinide transmutation. This homogeneity allows us to avoid “hot spot” formation due to the local accumulation of more fissile elements. Here, we report the preparation of macro-microporous ThO2materials by a colloidal sol–gel route. Using a thorium salt with 6-aminocaproic acid as a complexing agent at a controlled pH, we were able to pilot the condensation of thorium hydroxo species forming colloids of tuned nanometric size and thus the sol stability. After a freeze-drying process to concentrate colloids and a thermal treatment allowing complexing agent removal and macroporosity formation by a brutal gas release during combustion, a loose packing of ThO2nanoparticles with an ordered distribution of interparticular porosity and a fraction of nanometric crystallites, whose size depends on the initial colloidal size, were obtained. The sols, pastes, and final materials were characterized by small- and wide-angle X-ray scattering to determine the colloidal size and the final structure of the materials, which was also confirmed by transmission electron microscopy. The most promising material was finally successfully impregnated by a simulating minor actinide solution and thermally treated to prepare a mixed actinide oxide material. This safe technology, relying on the colloidal sol–gel process and the formulation of complex fluids forming tunable precursors, opens new perspectives for the reuse of nuclear waste solutions as new fuel.

The study by numerical methods of ionic distributions in charged solid–liquid interfaces allows the interpretation of many concepts and phenomena such as the ζ potential or ion adsorption. Molecular dynamics (MD) can reveal detailed information about electric double layer (EDL), especially the Stern layer, by including electrostatic, van der Waals, and molecular forces. Here, we aim at analyzing the chemical species and the correlations between the ions and the surface including three-body effects, one particle belonging to the surface and two to the solution. Correlations are specified on the basis of interionic distance screening. An extensive description is provided from simulations of three alkaline earth metal chlorides (Mg2+, Ca2+, and Ba2+) aqueous solutions at 0.6 mol·L–1in the centers of negatively charged silica nanochannels. The resulting McMillan–Mayer potentials of mean force (PMF) exhibit a decreasing affinity of deprotonated silanol along with the series Mg2+> Ca2+> Ba2+, while the formation of bulk M2+–Cl–pairs is in reverse order. A similar trend is obtained for the association constants and the residence times. Over 40% of surface-bound Ba2+ions are correlated with surface-bound Cl–, while the other two cations do not show such trend. When the surface-bound and surface-correlated ions are taken apart, the remaining free ion distributions fit the Poisson–Boltzmann equation well, i.e., the Gouy–Chapman model. This work demonstrates the necessity to account for three-body associations on oxide surface at least for divalent ions.

In this study, we related the alteration of mesoporous materials in aqueous solutions to the properties of confined water in the presence of ions, that is, its structure and its dynamics. To reach this goal, we have determined and quantified the evolution of the morphology of hexagonal mesoporous silica-based materials, that is, MCM-41 (mean pore size of 2.9 nm and dense pore wall) and SBA-15 (mean pore size of 6.8 nm and microporous pore wall), during their alteration in water and electrolyte solutions with ions presenting various kosmotropic properties ([XCl2] = 1 M and X = Ba, Ca, Mg) at 50 °C by in situ small and wide angle X-ray scattering. The experimental results and the calculated small-angle X-ray scattering spectra have demonstrated that the alteration behavior is strongly dependent on the type of silica and the water properties (water dynamics at a picosecond time scale and water structure) related to the confinement size, the nature of the cations, and their surface ion excesses. For MCM-41 silica, having reduced water dynamics, the alteration behavior is mainly driven by silica hydrolysis—recondensation/precipitation processes leading to a pore deformation and a possible precipitation of XCl2metastable phases partially clogging the pores. In case of SBA-15, its alteration behavior in aqueous solutions starts with an initial increase of the pore size and the formation of an alteration layer within its microporosity which grows following a diffusive process of the solution. Inside the microporosity, similar alteration processes to MCM-41 silica occur. These recondensation/precipitation processes may continue until a thermodynamically stable silica phase is formed. The results obtained within the scope of this work revealed the importance of the influence of the dynamics and the structure of water in chemical reactions taking place in confined media filled with water and ions.

In this study, we characterized the structure and the dynamics at a picosecond scale of water molecules in aqueous solutions with cations having various kosmotropic properties (XCl2where X = Ba2+, Ca2+, and Mg2+) confined in highly ordered mesoporous silica (MCM-41 and grafted MCM-41) by Fourier transform infrared spectroscopy and quasi-elastic neutron scattering. We pinpointed the critical pore size and the electrolyte concentration at which the influence of the ion nature becomes the main factor affecting the water properties. These results suggest that whatever the ions kosmotropic properties, for pore sizes ϕp< 2.6 nm and [XCl2] ≤ 1 M, the water dynamics is mainly slowed down by the size of the confinement. For pore sizes of 6.6 nm, the water dynamics depends on the concentration and kosmotropic properties of the ion more than on the confinement. The water properties within the interfacial layer were also assessed and related to the surface ion excesses obtained by sorption isotherms. We showed that, for pore sizes ϕp≥ 2.6 nm, the surface ion excess at the pore surface is the main driver affecting the structural properties of water molecules and their dynamics within the interfacial layer.

The impact of nanoconfinement on the microstructure of alkoxysilane layers grafted by supercritical CO2was determined using model system made of silica nanochannels, i.e., planar silica surface spaced by few nanometers. Two types of silica nanochannels of 3 and 5 nm gap size were grafted with alkoxysilanes having different head groups (thiol, amine, and iodo) using the same protocol as in our previous study on open flat silica surface (materials, T, P). Using the same characterization technique, X-ray reflectivity, we have directly compared the results obtained on open planar silica surface with confined ones. We show that the microstructure of grafted layers obtained on nonconfined silica flat surface is not directly transposable on a confined silica planar surface. In our experimental conditions, on a flat surface, the microstructure of the grafted layer is only driven by the alkoxysilane molecule, whereas in the confined planar silica surface, the microstructure is driven by both the confinement size and the nature of the molecule. This may be explained by the modification of the molecules transport and the change of supercritical CO2properties in nanoconfined media.

Silicon oxide surface properties can be easily modified by grafting alkoxysilane molecules. Here, we studied the structure and the morphology of ultrathin layers prepared by the grafting of alkoxysilanes having different head groups (thiol, amine, and iodo) in supercritical carbon dioxide (CO2) on model plane silicon oxide surfaces. Several characterization techniques (X-ray reflectivity, water contact angle, X-ray photoelectron spectroscopy, and atomic force microscopy (AFM)) were used to determine the physicochemical properties of the layers prepared at different temperatures. Moreover, for the first time, AFM peak force measurements were used to delve deeper into the determination of the structure of these ultrathin alkoxysilane layers. The results show that the grafting temperature and the nature of the head group strongly affect the morphology and structure of the grafted layers. Dense monolayers are obtained with 3-(mercaptopropyl)trimethoxysilane at 60 °C, polycondensed layers are always prepared with [3-(aminoethylamino)propyl]trimethoxysilane, and a dense bilayer is synthesized with 3-(iodopropyl)triethoxysilane at 120 °C.

The structural and dynamical properties of water confined in highly ordered mesoporous silica (SBA-15) in the presence of ions having various kosmotropic properties (XCl2with X = Ba2+, Ca2+, and Mg2+) were characterized using thermogravimetric analysis, infrared spectroscopy, and quasi-elastic neutron scattering and were related to the surface density of ions obtained by sorption isotherms. The results highlight that the water network is perturbed by the confinement and that the dynamics of water molecules at a picosecond time scale is slowed by the presence of ions. These results are mainly explained by the presence of the interfacial layer where both the structure of the water network and the water dynamics are influenced by the surface ion excess and the kosmotropic properties of ions.

The relation existing between water transport in glass, topology, chemical elements (network former and charge compensator), and their structural role in glass were investigated through glass topology modeling, glass and water structure analyses, and water diffusion characterization. Two series of aluminosilicate glasses with and without boron and having various ratios of charge compensators CaO/Na2O were used. The glass structure was characterized using Raman spectroscopy and nuclear magnetic resonance. Their topologies, i.e., the density of bottlenecks and the interstitial sites, were obtained by molecular dynamics. For glasses without boron, the substitution of Na by Ca leads to the strengthening of the glass network. The results are similar for glasses with boron except if the amount of Na is not sufficient. In this case, a part of Ca is required as a charge compensator for AlO4and BO4units. This last result is important since it highlights that the presence of boron and the CaO/Na2O ratio drives the roles of Ca inside the glass structure. Moreover, glass topology is driven by boron presence and to a lesser extent by CaO/Na2O ratio. Water transport characterized by the duration of the predominance of the hydration/interdiffusion processes, the apparent water diffusion coefficient, and the water structure in the hydrated glass were studied and determined using X-ray reflectivity and attenuated total reflectance infrared spectroscopy. The results show that the duration of the predominance of the hydration/interdiffusion processes is driven by the ability of Si-O-X bonds to be hydrolyzed as much as the fraction of free water clusters in hydrated glass. Moreover, for aluminosilicate glasses, we show that water transport is mainly driven by glass topology through the role of Ca and its amount in glass. Indeed, Ca strengthens the glass network by decreasing the density of the bottleneck allowing the diffusion of water molecules. When boron is added to the glass, water transport may be mainly driven by the chemical interactions between the water molecules, Ca, and the network former of glass matrix.

The impact of pore size and pore surface composition on water dynamics confined in highly ordered porous silica material (MCM-41) was investigated using neutron scattering for correlation times in the picosecond range. Samples were synthesized by the hydrothermal route and grafted via the hydrolytic surface sol–gel method to obtain pore wall surfaces with Si–OH, Al–OH, or Zr–OH terminations and pore sizes from 2 to 2.7 nm. The samples were characterized after grafting using nitrogen adsorption–desorption isotherms and small-angle X-ray scattering. At room temperature, the analysis of the quasi-elastic neutron scattering data shows a decrease of mean square displacement of the mobile protons with the pore size and when Si–OH surface termination is replaced by Al–OH and Zr–OH. The water translational diffusion coefficients are close to the bulk value whatever the samples. The amounts of fixed protons depend on the pore size and the ability of the grafted layers to immobilize the water molecules which were determined also from thermogravimetric and differential thermal analysis. This last result highlights that pore surface composition could be the predominant parameter affecting the fixed proton content at this time scale.

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