Chemisorption of lanthanide ions on succinate-functionalized mesoporous silica: An in situ characterization by fluorescence.

Chemisorption of Eu3+ and Tb3+ on SBA-15 functionalized with succinic groups has been studied by in situ steady-state fluorescence measurements. The enhancement of the emission sensitive bands indicates that both ions adsorb forming inner-sphere surface complexes. Adsorption is a fast process that attains equilibrium in about 5min. The variation of the peaks maxima (I592 and I616, for europium, and I490 and I545, for terbium) with the total ion concentration is accounted for by the sum of the contributions due to the adsorbed and free ions. The former contribution is langmuirian. At pH 4.5, the respective adsorption constants are 5×105 and 3×105M-1, and the maximum adsorption capacities are 5.10×10-4 and 5.23×10-4molg-1. The mismatch between the latter values and the number of attached carboxylic groups is discussed. Comparison with other functionalized mesoporous silicas indicates that the anchored succinic groups are very efficient for removing lanthanide ions from aqueous solutions.

Simple thiol-ene click chemistry modification of SBA-15 silica pores with carboxylic acids.

A straightforward approach for anchoring tailored carboxylic groups in mesoporous SiO2 colloidal materials is presented. The thiol-ene photochemical reaction between vinyltrimethoxysilane precursors and various thiocarboxylic acids which has, click chemistry features (i.e. high conversion yields, insensitivity to oxygen, mild reaction conditions), results in carboxylated silane precursors that can be readily used as surface modifiers. The carboxylic groups of acetic, undecanoic and succinic acid were immobilized on the silica mesopore walls of SBA-15 powders employing the synthesized silane precursors. Post-grafting has been confirmed through infrared spectrometry (FTIR), energy dispersive X-ray spectroscopy (EDS), elemental analysis (EA) and zeta potential measurements. Detailed field-emission gun scanning electron microscopy (FESEM) images and small angle X-ray scattering (SAXS) data revealed parallel mesopores and ordered mesostructures. It is shown that the immobilized COOH groups are chemically accessible for acid-base reactions as well as copper adsorption. Immobilization of easily synthesized tailored carboxylic modified alkoxide precursors within mesoporous systems provides a unique chemical nanoenvironment within these ordered frameworks.

Halide exchange on Mg(II)-Al(III) layered double hydroxides: exploring affinities and electrostatic predictive models.

The crystalline chloride form of layered double hydroxide (LDH) with the formula Mg0.75Al0.25(OH)2Cl0.25·mH2O was gradually exchanged with F(-), Br(-), or I(-) up to a total displacement of Cl(-). For the three anions, both the exchange isotherms as well as the structural changes were inspected along the whole range of chloride displacement. The bulkier Br(-) and I(-) followed an ideal exchange behavior isotherm while F(-) denoted strong deviations from the ideal regime as well as phase segregation. The exchange constants recorded herein were contrasted with bibliographic data belonging to an analogous LDH host, revealing a strong linear free energy correlation. Higher Al(III) to Mg(II) ratios, or layer charge densities, favor a stronger selectivity for smaller halides. For both hosts, the exchange free energy was satisfactorily described in terms of strictly electrostatic-based models.

A real time in situ ATR-FTIR spectroscopic study of glyphosate desorption from goethite as induced by phosphate adsorption: effect of surface coverage.

The desorption of glyphosate from goethite as induced by the adsorption of phosphate was investigated by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy in combination with adsorption isotherms. Desorption of glyphosate was very low in the absence of phosphate. Addition of phosphate promoted glyphosate desorption. At low initial surface coverages, added phosphate adsorbed on free surface sites, mainly, displacing a small amount of glyphosate. At high initial surface coverages, on the contrary, phosphate adsorption resulted in a significant glyphosate desorption. In the latter conditions, the ratio desorbed glyphosate to adsorbed phosphate was 0.60. The desorption process can be explained by assuming that phosphate adsorbs first forming a monodentate mononuclear complex, which rapidly evolves into a bidentate binuclear complex that displaces glyphosate.

Temperature response of luminescent tris(bipyridine)ruthenium(II)-doped silica nanoparticles.

Nanoparticle-based temperature imaging is an emerging field of advanced applications. Herein, the sensitivity of the phosphorescence of tris(bipyridine)ruthenium(II)-doped silica nanoparticles towards temperature is studied. 130 nm size particles were prepared by a modification of Stöber's method, that allows the incorporation of Ru[(bpy)(3)](2+) into the outer particle shell. The entrapped Ru[(bpy)(3)](2+) retains its photophysical properties, yet the emission of the particles is not affected by the presence of O(2), neither by anionic quenchers; quenching by MV(2+), on the other hand, is strongly dependent on pH. Between 20 and 60°C, the steady-state emission of the particles decreases linearly with increasing temperature. The slope of the straight line diminishes slightly on thermal cycling, but soon stabilizes. Fluorescence measurements by scanning confocal microscopy indicate that the silica nanoparticles doped with Ru[(bpy)(3)](2+) can indeed be employed to probe thermal processes in micro-environments.

Complexation at the edges of hydrotalcite: the cases of arsenate and chromate.

Sorption of CrO(4)(2-) and HAsO(4)(2-) by hydrotalcite, in its chloride form, was studied as a function of anion concentration. In both cases, the shape of the isotherms is langmuirian. The maximum uptake of CrO(4)(2-) equals the ion-exchange capacity of the solid, whereas sorption of HAsO(4)(2-) saturates at a higher value. Chloride ions inhibit the uptake of both anions, the amount of sorbed CrO(4)(2-) declining rapidly to zero. The uptake of HAsO(4)(2-), however, attains a constant value at high chloride concentrations. The excess of arsenate uptake follows, at constant pH, a langmuirian dependence with equilibrium concentration and decreases with increasing pH, depicting a marked change in slope at pH≈pQ(a3). CrO(4)(2-) and HAsO(4)(2-) have notable, albeit different, effects on the electrophoretic behavior of hydrotalcite; the positive particle charge is screened almost completely by CrO(4)(2-), whereas sorption of HAsO(4)(2-) produces charge reversal. These results reflect the formation of inner-sphere arsenate surface complexes at the edges of hydrotalcite particles. The underlying rationale is discussed in terms of the crystal structure of hydrotalcite surfaces.

Amylase-functionalized mesoporous silica thin films as robust biocatalyst platforms.

A robust biocatalyst was produced by immobilization of alpha-amylase into mesoporous silica thin films with local order pore structure and 11 nm pore diameter, supported on glass stripes. The activity of this novel catalyst was evaluated for direct starch degradation. The catalyst films show excellent activity, and enhanced stability with respect to free enzyme at extreme conditions of pH and temperature. In addition, they can be easily separated from the reaction media and reused several times.

Coating liposomes with yttrium basic carbonate: making hybrid nanocapsules.

Novel yttrium basic carbonate hybrid nanocapsules have been prepared using liposome templates. The method resorts to urea hydrolysis to increase the precipitation driving force of Y(OH)CO(3)xH(2)O homogeneously. The head groups at the outer vesicle layer act as adsorption sites for hydrolyzing Y(III) ions, thus assisting heterogeneous nucleation. The criteria behind the synthesis, as well as the potentialities of the advanced procedure, are stressed.

Homogeneous precipitation of layered Ni(II)-Cr(III) double hydroxides.

An adequate account of the hydrolytic properties of Cr3+ and Ni2+ allows setting the conditions for homogeneous nucleation of layered Ni(II)-Cr(III) double hydroxides; water exchange and hydrolysis rate constants indicate that, at very high rates of base dosing, formation of heteronuclear Cr(III)-Ni(II) hydroxo species should prevail over precipitation of active Cr(OH)3. This is realized by the urea method under microwave-assisted hydrothermal conditions. This approach yields crystalline Ni1-xCrx(OH)2(CO3)x/2nH2O (x approximately 0.32-0.36) in less than 5 min at 453 K; higher degrees of crystallinity are obtained at higher temperatures and/or longer aging times. Formation of Ni(II)-Cr(III) LDHs upon microwave-assisted hydrothermal aging of freshly coprecipitated Ni(OH)2+Cr(OH)3 mixtures takes longer, due to a different operating mechanism. The implications of the advanced rationale for the design of synthesis procedures are stressed. It is proposed that homogeneous nucleation of Ni(II)-Cr(III) LDHs involves the edge-on condensation of planar heteronuclear Cr(III)-Ni(II) hydroxo trimers. Ordered aggregation of primary particles leads to the final platelet crystals, a process that involves the exchange of CO2-(3) ions dangling at the crystallites' edges by bridging OH-.

Anion-exchange equilibrium and phase segregation in hydrotalcite systems: intercalation of hexacyanoferrate(III) ions.

The intercalation of hexacyanoferrate(III) in chloride-hydrotalcite was studied as a function of the composition of the equilibrated aqueous phase, the relevant solution variable being the ratio (C(Fe(CN)(6)(3-))(1/3)/C(Cl)-. The isotherm shows that the exchange is markedly nonideal and reveals the segregation of two solid phases of fixed compositions at (C(Fe(CN)(6)(3-))(1/3)/C(Cl)- = 17.1 M(-2/3). This is the first case of solid phase segregation during anion exchange in hydrotalcite systems diagnosed from thermodynamic measurements. In agreement, PXRD patterns of the partially exchanged solids indicate that these phases, with distinct basal spacings, coexist in the range 0.21 < or = x < or = 0.86. The observed behavior is accounted for by classical thermodynamic formalisms of ion exchange. The limitations of other representations commonly used to describe anion exchange in layered double hydroxide systems are stressed. A rationale of the structural changes accompanying the exchange is also offered.

Thermal decomposition of crystalline Ni(II)-Cr(III) layered double hydroxide: a structural study of the segregation process.

A structural study of the thermal evolution of Ni(0.69)Cr(0.31)(OH)(2)(CO(3))(0.155) x nH(2)O into NiO and tetragonal NiCr(2)O(4) is reported. The characteristic structural parameters of the two coexisting crystalline phases, as well as their relative abundance, were determined by Rietveld refinement of powder x-ray diffraction (PXRD) patterns. The results of the simulations allowed us to elucidate the mechanism of the demixing process of the oxides. It is demonstrated that nucleation of a metastable nickel chromite within the common oxygen framework of the parent Cr(III)-doped bunsenite is the initial step of the cationic redistribution. The role that trivalent cations play in the segregation of crystalline spinels is also discussed.

Delamination and restacking of hybrid layered double hydroxides assessed by in situ XRD.

Delamination and restacking of dodecyl-sulfate-intercalated hydrotalcite in CCl4 and toluene were followed by in situ low-angle XRD. It is shown that exfoliation of hybrid layered double hydroxides must be rationalized in terms of the miscibility of the solvent and the aliphatic chains of the intercalated amphiphilic anions. The macroscopic appearance of diluted systems, however, is determined by the interaction between the edges of the exfoliated lamella.