Biomedical subjects
Publications and source records attributed to François Fajula.
Pseudomorphic transformation is introduced as an effective and versatile method to independently optimize the morphology of the grains and the structural properties of mesostructured silicas. In this account a short overview is given of the preparation of MCM-41 and MCM-48 mesophases as non-aggregated and homogeneously distributed particles with sizes ranging from 5 to 800 microm.
The computer aided analysis of the EPR spectra of radical surfactant probes inserted in cetyltrimethylammonium bromide micelles provided information on the kinetics of formation of micelle templated silico-aluminas (MTSA) at 343 K, obtained by means of silica and alumina alkaline solutions at different Si/Al ratios (from infinity to 4). Mainly two spectral components were analyzed and relatively quantified in the EPR spectra: (1) the micellar component, due to probes inserted in the surfactant aggregates, whose mobility decreases over the synthesis time, thus reporting on the progressive modification of the micelle structure and the solid condensation; (2) the interacting component, mainly arising from the electrostatic interactions between the surfactant heads and the charged surface sites. This last component increases its relative intensity over the synthesis time, informing about condensation and structuration of the silico-alumina at the micelle surface. X-ray diffraction (XRD), nitrogen sorption isotherms at 77 K, thermogravimetric analysis, TEM and chemical analysis were performed to characterize both as-synthesized and calcined MTSA materials. Nitrogen sorption isotherms allowed us to evaluate the pore diameter, the specific surface area and the pore volume. At Si/Al<15 a decrease in pore volume and specific surface area was interpreted as due to the contemporaneous presence of a hexagonal MTSA and an amorphous material, which was ascertained by means of XRD as the only present at Si/Al=4. The amorphous structure at Si/Al<15 used Na+ as contraions, whereas the surfactants are no more needed to neutralize the negatively charged groups at the solid surface. The hypothesis of a "break" at Si/Al=15 was supported by EPR: the interactions between the surfactant probe heads and the negatively charged surface groups are drastically reduced at Si/Al<15. On the contrary, at Si/Al>15, increasing amounts of alumina slow the kinetics of the synthesis but enhance electrostatic interactions between the surfactant heads and the negatively charged surface groups. Dilution of the synthesis mixture decreased the extent of the interactions, due to partial protonation of the silanol groups, and slowed the synthesis process.
Ordered mesoporous silicas such as micelle-templated silicas (MTS) feature unique textural properties in addition to their high surface area (approximately 1000 m2/g): narrow mesopore size distributions and controlled pore connectivity. These characteristics are highly relevant to chromatographic applications for resistance to mass transfer, which has never been studied in chromatography because of the absence of model materials such as MTS. Their synthesis is based on unique self-assembly processes between surfactants and silica. In order to take advantage of the perfectly adjustable texture of MTS in chromatographic applications, their particle morphology has to be tailored at the micrometer scale. We developed a synthesis strategy to control the particle morphology of MTS using the concept of pseudomorphic transformation. Pseudomorphism was recognized in the mineral world to gain a mineral that presents a morphology not related to its crystallographic symmetry group. Pseudomorphic transformations have been applied to amorphous spherical silica particles usually used in chromatography as stationary phases to produce MTS with the same morphology, using alkaline solution to dissolve progressively and locally silica and reprecipitate it around surfactant micelles into ordered MTS structures. Spherical beads of MTS with hexagonal and cubic symmetries have been synthesized and successfully used in HPLC in fast separation processes. MTS with a highly connected structure (cubic symmetry), uniform pores with a diameter larger than 6 nm in the form of particles of 5 microm could compete with monolithic silica columns. Monolithic columns are receiving strong interest and represent a milestone in the area of fast separation. Their synthesis is a sol-gel process based on phase separation between silica and water, which is assisted by the presence of polymers. The control of the synthesis of monolithic silica has been systematically explored. Because of unresolved yet cladding problems to evaluate the resulting macromonoliths in HPLC, micromonoliths were synthesized into fused-silica capillaries and evaluated by nano-LC and CEC. Only CEC allows to gain high column efficiencies in fast separation processes. Capillary silica monolithic columns represent attractive alternatives for miniaturization processes (lab-on-a chip) using CEC.
In the present work, the corresponding states principle is proposed as a new approach to clarify the comparison between adsorption-desorption isotherms obtained on porous solids. The applicability of this principle at the capillary critical point in adsorption-desorption isotherms is demonstrated. Deviations of the Clausius-Clapeyron curves in reduced coordinates are interpreted in terms of polarizability, and those are perfectly correlated using analogous Pitzer acentric factors in confined environments. New acentric factors are proposed to take confinement effects in mesoporous materials into account. Enthalpic excesses during capillary condensation could be predicted, and those are supported by calorimetric experiments.
The threshold of reversible capillary condensation is a well-defined thermodynamic property, as evidenced by corresponding states treatment of literature and experimental data on the lowest closure point of the hysteresis loop in capillary condensation-evaporation cycles for several adsorbates. The nonhysteretical filling of small mesopores presents the properties of a first-order phase transition, confirming that the limit of condensation reversibility does not coincide with the pore critical point. The enthalpy of reversible capillary condensation can be calculated by a Clausius-Clapeyron approach and is consistently larger than the condensation heat in unconfined conditions. Calorimetric data on the capillary condensation of tert-butyl alcohol in MCM-41 silica confirm a 20% increase of condensation heat in small mesopores. This enthalpic advantage makes easier the overcoming of the adhesion forces by the capillary forces and justifies the disappearing of the hysteresis loop.
A new mesoporous micelle-templated silica (MTS) route for enzyme encapsulation is presented. The pore structure is given by a new association oflecithin (double chain surfactant) and dodecylamine as cosurfactant. To enhance and to well protect the enzyme activity, lactose was loaded in the synthesis. The mixed-micelles give after the addition of tetraethyl orthosilicate a well-ordered mesoporous material with a spongelike rigid structure stable after calcination at 550 degrees C. The size of the pores lies between 30 and 40 A, matching well with the size of the lipases. The activity of this heterogeneous catalyst was tested in the hydrolysis of the ethylthiodecanoate. These new biocatalysts were very active, more than hydrophobic sol-gel materials and commercially available sol-gel encapsulated lipase. This new MTS synthesis route allows one to encapsulate in one-step various enzymes, even those that are very fragile.
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