Monovalent and divalent aqueous electrolytes confined in negatively charged porous silica are studied by means of molecular simulations including free energy calculations. Owing to the strong cation adsorption at the surface, surface charge overcompensation (overscreening) occurs which leads to an effective positive surface next to the Stern layer, followed by a negatively charged diffuse layer. A simple Poisson-Boltzmann model in which the single-ion potential of mean force is introduced is shown to capture the most prominent features of ion density profiles near an amorphous silica surface. Nevertheless, due to its mean-field nature, which fails to account for correlations, this simple model does not predict overscreening corresponding to charge inversion at the surface. Such an overscreening drastically affects the transport of confined electrolytes as it leads to flow reversal when subjected to an electric field. A simple continuum theory is shown to capture how the electro-osmotic flow is affected by overscreening and by the apparent enhanced viscosity of the confined electrolytes. Comparison with available experimental data is discussed, as well as the implications of these phenomena for ζ-potential measurements.

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We investigate the Stern layer of charged silica–water interfaces by calculating the ion–surface interaction from molecular dynamics simulations. The McMillan–Mayer potentials of mean force between a charged oxygen site and a lithium or cesium cation have been calculated. Contact ion pairs (CIPs) are important for the adsorption and desorption of ions, especially for lithium. An activation energy appears, which can result in a large estimated relaxation time. In the case of lithium, time scales needed to bind or unbind ions to and from the surface are found to be very long (up to the order of seconds for some surfaces), which implies that molecular dynamics cannot always be fully equilibrated. This work provides a new image of the Stern layer: it is not a continuous layer but a set of Bjerrum pairs. As a matter of fact, quantitative (macroscopic) treatments of such systems with localized surface charges require a three-dimensional model, contrary to the more commonly used one- or two-dimensional theoretical treatments.

We present a simulation and modeling study of electro-osmotic flow of an aqueous cesium chloride solution confined in a charged amorphous silica slot. Contrasting traditional models of the electric double layer, molecular dynamics simulations indicate that there is no stagnant layer, no Stern layer conduction, and no outer Helmholtz layer. The description of the interface requires two considerations. First, a distinction has to be made between free and surface-bonded ions. The latter do not form a physical layer but rather a set of ion–surface contact pairs. Second, the mobility of the free ions is reduced relative to their bulk value. This hydrodynamic effect needs to be included. These two concepts, coupled to simple macroscopic equations, are sufficient to describe surface conductivity and electro-osmotic flow in the frame of classical mean-field treatment. We show that surface conduction is negative at high concentration, and the Bikerman formula is only valid at low concentration.

Published liquid-liquid extraction studies of Pa(V) were interpreted with aqueous mono-, di-and trications. B3LYP DFT is applied here to such cations surrounded by two explicit hydration layers: Linear or tetrahedral geometries are found for the Pa(V) aquo ions. PaO 2 + is similar to the other AnO 2 + cations, but has strong apical bonds, resulting from the highly negative O yl charge, which decreases along the An(V) series. This explains the instability of PaO 2 + in water, and the differences with the heavier An(V). PaO 2 + diprotonates to give Pa(OH) 2 3+ and can further dihydrolyse to give T d -Pa(OH) 4 + , which might very well be the most stable Pa(V) monocation. PaOOH 2+ is confirmed to be the Pa(V) aqueous dication invoked in the literature for pH r 1.4 AE 0.7. PaO 3+ is confirmed in sulfate solution, with a bond length close to 180 pm. Pa(OH) 2 3+ cannot be excluded in other conditions. The strong influence of the solvent was not fully taken into account in most previous theoretical studies that focused only on bare or partially solvated PaO 2 + . Toraishi et al. have studied hydrated Pa(V) and our work confirms this study and its qualitative interpretation. The new tetrahedral Pa(OH) 4 + geometry that is shown here to be important opens the field to further quantum chemical studies of Pa(V) and other f-elements. As a test for the two-shell model approach for Pa(V), fluoride coordination to Pa(V) is studied and compared with published EXAFS data: an excellent fit is obtained with the well-established species PaF 7 2À , but most other stoichiometries tested are precluded.

Realistic models of amorphous silica surfaces with different silanol densities are built using Monte Carlo annealing. Water-silica interfaces are characterized by their energy interaction maps, adsorption isotherms, self-diffusion coefficients, and Poiseuille flows. A hydrophilic to hydrophobic transition appears as the surface becomes purely siliceous. These results imply significant consequences for the description of surfaces. First, realistic models are required for amorphous silica interfaces. Second, experimental amorphous silica hydrophilicity is attributed to charged or uncharged defects, and not to amorphousness. In addition, autoirradiation in nuclear waste glass releases hydrogen atoms from silanol groups and can induce such a transition.

The Free-Electron Laser for Infrared Experiments (FELIX) was used to study the wavelength-resolved multiple photon photodissociation of discrete, gas-phase uranyl (UO22+) complexes containing a single anionic ligand (A), with or without ligated solvent molecules (S). The uranyl antisymmetric and symmetric stretching frequencies were measured for complexes with general formula [UO2A(S)n]+, where A was hydroxide, methoxide, or acetate; S was water, ammonia, acetone, or acetonitrile; and n = 0-3. The values for the antisymmetric stretching frequency for uranyl ligated with only an anion ([UO2A]+) were as low or lower than measurements for [UO2]2+ ligated with as many as five strong neutral donor ligands and are comparable to solution-phase values. This result was surprising because initial DFT calculations predicted values that were 30-40 cm(-1) higher, consistent with intuition but not with the data. Modification of the basis sets and use of alternative functionals improved computational accuracy for the methoxide and acetate complexes, but calculated values for the hydroxide were greater than the measurement regardless of the computational method used. Attachment of a neutral donor ligand S to [UO2A]+ produced [UO2AS]+, which produced only very modest changes to the uranyl antisymmetric stretch frequency, and did not universally shift the frequency to lower values. DFT calculations for [UO2AS]+ were in accord with trends in the data and showed that attachment of the solvent was accommodated by weakening of the U-anion bond as well as the uranyl. When uranyl frequencies were compared for [UO2AS]+ species having different solvent neutrals, values decreased with increasing neutral nucleophilicity.

The sonoluminescence (SL) spectra of OH(A(2)Σ(+)) excited state produced during the sonolysis of water sparged with argon were measured and analyzed at various ultrasonic frequencies (20, 204, 362, 609, and 1057 kHz) in order to determine the intrabubble conditions created by multibubble cavitation. The relative populations of the OH(A(2)Σ(+)) v' = 1-4 vibrational states as well as the vibronic temperatures (T(v), T(e)) have been calculated after deconvolution of the SL spectra. The results of this study provide evidence for nonequilibrium plasma formation during sonolysis of water in the presence of argon. At low ultrasonic frequency (20 kHz), a weakly excited plasma with Brau vibrational distribution is formed (T(e) ~ 0.7 eV and T(v) ~ 5000 K). By contrast, at high-frequency ultrasound, the plasma inside the collapsing bubbles exhibits Treanor behavior typical for strong vibrational excitation. The T(e) and T(v) values increase with ultrasonic frequency, reaching T(e) ~ 1 eV and T(v) ~ 9800 K at 1057 kHz.