Marcello Merli

Montmorillonite nanodevices for the colon metronidazole delivery

The adsorption profiles of the antibiotic metronidazole (MNE) into the K10-montmorillonite (MMT-K10) clay and the subsequent release have been investigated as a function of pH and MNE/MMT-K10 ratio, in order to evaluate the potential of the MNE/MMT-K10 hybrids as controlled drug delivery system. The adsorption mechanism has been first elucidated by performing complementary equilibrium and kinetic studies and through the X-ray diffractometry (XRD) characterization of the obtained composite materials. The gathered results allowed us to propose a mechanism consisting of a multi-step pathway involving the neutral and the cationic form of the drug, which interact with different sites of the clay surfaces, i.e. the interlayer region and the faces of the lamella. In a second step the drug release kinetics has been studied under physiological pH mimicking conditions simulating the oral drug administration and delivery. For the sake of comparison the commercial formulation has also been employed for the release studies. The investigation of the release profiles and the comparison with the commercial formulation of the drug reveal that the new-tailor made formulation could be fruitful exploited for successfully prolonged the action of drug in the desired site.

Local Structural Environment of Calcium in Garnets: A Combined Structure-Refinement and XANES Investigation

Comparative crystal-chemical analysis of natural garnets allowed the compositional gap at Gr 45÷70% in the (Py, Alm)-Gr join to be related to the presence of two different structural arrangements around the X site in pyralspitic and ugranditic composition, respectively. A combined investigation (single-crystal X-ray structure-refinement (SREF), Ca K-edge XANES spectroscopy and multiple-scattering calculation of XANES spectra) carried out on a series of natural garnets in the (Py, Alm)-Gr join, allowed the structural modifications of the Ca local environment at increasing grossular content to be followed.The differences in the experimental XANES spectra as a function of the Ca content were shown to depend only on the configuration of the ligand shells surrounding the absorbing atom, and not on the nature of adjacent dodecahedral cations. Both experimental XANES spectra and their theoretical simulations confirm that Ca at the X site, when less abundant than (Fe+Mg), is not in the same structural configuration as in grossular, but adapts itself to that imposed by the dominant (Fe, Mg) cations. In the same way, (Fe, Mg) adapt themselves to the X-site configuration observed in grossular when Ca is the dominant cation. The deformation of the X site due to progressive Ca → (Fe, Mg) substitution is gradual but not linear along the solid solution; it shows different slopes on the two sides of the observed (Py, Alm)-Gr compositional gap.Best performances in reproducing the XANES spectra were obtained in the framework of the one-electron fullmultiple-scattering theory, using the real Hedin-Lundqvist exchange-correlation potential and clusters of 83 atoms (i.e. for a coordination sphere with 7 Å radius from the central Ca absorber); smaller clusters resulted inadequate in simulating some XANES features, indicating that a high number of single and multiple scatteringpaths contribute in garnets to the first part of the absorption spectrum.

In situ analysis of garnet inclusion in diamond using single-crystal X-ray diffraction and X-ray micro-tomography

A single crystal of garnet enclosed in a diamond from the Jericho kimberlite (Slave Craton, Canada) has been investigated using X-ray diffraction and X-ray micro-tomography. The novel experimental approach allowed us to determine the crystal structure of the garnet. The unit-cell edge a and fractional atomic coordinates of oxygen were used to determine the composition via an updated Margules model for garnets. The composition is Pyr0.41(5)Alm0.36(7)Gro0.22(1)Uva0.01(1), which is indistinguishable from the eclogitic garnets found in other Jericho diamonds. We also demonstrated that residual pressures on the inclusion of up to 1 GPa do not affect significantly the determination of the garnet composition by structure refinement.

Leverage analysis and structure refinement of minerals

Abstract Leverage analysis allows detection of the reflections that have the greatest influence on the estimate of a refined variable. We have applied leverage analysis in the structure refinement of some rock-forming minerals, in the attempt of settling the best procedures to obtain accurate and reliable results. Attention has been focused on those variables (e.g., the refined site-scatterings) which are fundamental to the determination of site populations, which in turn are the basis for the study of order-disorder processes and petrogenetic modeling. A garnet (pyrope) and an amphibole (tremolite) were used to test the procedure, being illustrative of different symmetries and structural complexity. As the omission of reflections with high leverage on a particular variable may strongly decrease the accuracy in its estimate, the results indicate that systematic procedures for data truncation [either on the basis of (low or high) sin θ/λ, high ΔF/θFo or of low I/σI] are potentially dangerous. The choice of a correctly ionized model is shown to be critical to obtain accurate estimates of the refined site-scatterings. The set of reflections with the highest leverage value with respect to a peculiar group of variables was found to be nearly invariant within isomorphous solid-solutions; this implies that the results of this work are valid throughout the amphibole and garnet compositional spaces. The warnings proposed to the treatment and refinement of X-ray diffracted data may be of general utility in the structure refinement of minerals.

Determination of site population in olivine: Warnings on X-ray data treatment and refinement

Abstract Leverage analysis enables identification of reflections with the greatest influence on the estimate of each refined variable, and thus may be an important tool to improve standard structure-refinement procedures, especially in the case of minerals with complex composition. In this work, leverage analysis was used to investigate in detail the influence of each reflection in high-resolution X-ray diffraction data collected from olivine, a mineral often used to model order-disorder processes and to calculate temperatures of closure and cooling rates of host rocks. Particular attention was paid to the estimate of the scattering power at the cation sites, that are crucial for the above studies. Various procedures for data correction and refinement were also investigated, and the different possible choices were compared to choose the strategy that provides the best results. The presence of high-leverage weak reflections in olivine strongly suggests that systematic data truncation according to intensity threshold should be avoided. The estimates of the site-scatterings obtained under the different conditions tested are very close (always ≤3 σ); they are often smaller than those which may be obtained from electron-microprobe analysis under different experimental conditions or on inhomogeneous (zoned) crystals. Chemical data should thus not be routinely used to constrain the refinement procedure and/or to optimize final site-populations, provided that appropriate errors are given; on the other hand, they are valuable to appreciate the presence and the amounts of very minor, sometimes unexpected, substituents. Our tests show that precision better than 0.001 in site-occupancy determination (sometimes claimed in the literature) is probably not achieved.

Experimental and robust modeling approach for lead(II) uptake by alginate gel beads: influence of the ionic strength and medium composition.

Systematic kinetic and equilibrium studies on the lead ions removal ability by Ca-alginate gel beads have been performed by varying several internal parameters, namely, number of gel beads, nature and composition of the ionic medium and pH, which allowed us to model a wastewater in order to closely reproduce the composition of a real sample. Moreover, the effects brought about the different ionic species present in the reacting medium have been evaluated. Differential Pulse Anodic Stripping Voltammetry (DP-ASV), has been systematically used to perform kinetic and equilibrium measurements over continuous time in a wide range of concentration. Kinetic and equilibrium data have been quantitatively analyzed by means of robust approach both for the non-linear regression and the subsequent residuals analysis in order to significantly improve the results in terms of precision and accuracy. Alginate gel beads have been characterized by SEM and an investigation on their swelling behavior has also been made. Removal efficiency of the calcium-alginate gel beads has been calculated and results obtained have showed a relevant dependence on ionic strength, composition of ionic media, pH of solution and number of gel beads. The number of gel beads takes part as key crucial components, i.e., the higher the number of beads the greater the amount of Pb(II) species removed from the sample, the lower the time needed to reach the maximum removal efficiency of 90%.

Preparation and characterization of bio-organoclays using nonionic surfactant

The present study was aimed at the preparation and characterization of tailor made hybrid materials, whose peculiar hosting capability could be exploited in biotechnological applications. With this purpose, the modification of K10 montmorillonite by intercalation of Tween 20 surfactant, was accomplished. The influence of two internal parameters, namely pH and surfactant/clay ratio, on the surfactant uptake ability by clay was investigated. The adsorption mechanism was elucidated on the basis of complementary kinetic and equilibrium studies and, then, corroborated by the useful information provided by the FT-IR, TGA and XRD characterization. The gathered results allow to draw the conclusion that the whole surfactant uptake is the result of two contributions: a site-limiting component, governed by negative cooperative interactions, which takes into account for the Tween 20 adsorption onto the pristine clay, and a non-specific linear partitioning component, related to the adsorption of the surfactant onto the in situ prepared organo-clay. Moreover, at strongly acidic pH, a mechanism consisting of two-steps pathways involving two non-energetically equivalent binding sites of the clay surfaces, was proposed, while, on increasing the pH, the clay interlayer becomes the sole available site for the surfactant uptake. In the light of the interesting results obtained, among the plethora of potential biotechnological applications, the present paper suggests the exploitation of the prepared organo-clays to improve the performance of either hydrophilic or hydrophobic drug carriers systems.

Bader's analysis of the electron density in the Pbca enstatite- Pbcn protoenstatite phase transition

The Bader’s topological analysis of the electron density obtained via ab initio quantum mechanical simulation at Hartree–Fock and DFT level has been performed for experimentally in situ heated structures of enstatite–protoenstatite MgSiO 3 . The measurements have been performed in the temperature range 1200–1400 K. The work was aimed at characterizing both the evolution of the electron arrangement in the crystal and that of the crystal-structure at the enstatite–protoenstatite phase transition in terms of topology of the electron density, with particular care about the Mg–O and the O–O bonds. The observed breaking of some chemical bonds with increasing temperature and/or due to the phase transformation have been explained, for the first time, in the light of a topological mechanism based on the Catastrophe Theory applied to the framework of the Bader’s theory.

Topological analysis of the electron density of the clinopyroxene structure by the maximum entropy method: an exploratory study

In the framework of a series of methodological studies to test the reliability of the reconstruction of the electron density distribution (EDD) by the maximum entropy method (MEM), we have attempted to check the possibilities of getting further information about the structural changes as a function of the chemical composition occurring in low-Ca clinopyroxenes by the analysis of the topology of the MEM EDD. The results of this study show that the topological analysis of MEM EDD yields results comparable to the bond critical points characteristics obtained in ground state EDD in minerals. Notable differences related to solid solutions have been found only at the M2 site of clinopyroxene structure. An attempt to describe the bond character at this site in the presence of significant solid solutions has shown differences related to chemical substitution that can be described in terms of variable ionicity. This result could be ultimately related to the different thermodynamic behaviour of the P 2 1 / c ↔ C 2/ c phase transitions shown by chemically different clinopyroxenes. Since MEM can deal with experimental data on solid solutions, which cannot be treated with other available methods to reconstruct the experimental electron density, this approach opens new possibilities for the study of changes in the EDD topology of minerals with complex chemical substitutions.

Deconvolution procedure of the UV-vis spectra. A powerful tool for the estimation of the binding of a model drug to specific solubilisation loci of bio-compatible aqueous surfactant-forming micelle

UV-vis-spectra evolution of Nile Red loaded into Tween 20 micelles with pH and [Tween 20] have been analysed in a non-conventional manner by exploiting the deconvolution method. The number of buried sub-bands has been found to depend on both pH and bio-surfactant concentration, whose positions have been associated to Nile Red confined in aqueous solution and in the three micellar solubilisation sites. For the first time, by using an extended classical two-pseudo-phases-model, the robust treatment of the spectrophotometric data allows the estimation of Nile Red binding constant to the available loci. Hosting capability towards Nile Red is exalted by the pH enhancement. Comparison between binding constant values classically evaluated and those estimated by the deconvolution protocol unveiled that overall binding values perfectly match with the mean values of the local binding sites. This result suggests that deconvolution procedure provides more precise and reliable values, which are more representative of drug confinement.