Christelle Alié

Preparation of low-density xerogels through additives to TEOS-based alcogels

A new process for preparing silica xerogels with similar textural properties to silica aerogels by drying under vacuum has been studied. The xerogels are produced by adding, before gelation, 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS) to tetraethylorthosilicate (TEOS)-based alcogels, synthesised in a single base-catalysed (NH3) step. It is hypothesized that EDAS acts as a nucleation agent leading to silica particles with a hydrolysed EDAS core and a shell principally made of hydrolysed TEOS. The EDAS concentration and the basicity of the aqueous NH3 solution are important parameters influencing the resistance of the gel to drying stress. A decreasing EDAS/TEOS ratio or an increasing concentration of NH3 at constant EDAS content leads to less shrinkage during drying and so the final pore volume is larger. Gels prepared with a low EDAS/TEOS ratio (about 0.03) contain large particles (∼20 nm) due to the nucleation process by EDAS, thus the pores between those particles are also large and the drying stress is reduced.

Mercury porosimetry: applicability of the buckling-intrusion mechanism to low-density xerogels

Mineral materials can be either crushed or invaded by mercury during mercury porosimetry experiments. It has been shown here that many low-density xerogels exhibit the two volume variation mechanisms successively, compaction followed by intrusion, when submitted to mercury porosimetry and that a unimodal pore size distribution can be obtained by applying Pirards collapse model below the pressure of transition Pt and Washburns intrusion theory above Pt. To confirm the validity of the use of the buckling law, one low-density xerogel was wrapped in a tight membrane (intrusion is prevented and the sample is crushed during the whole porosimetry experiment). The analysis of the mercury porosimetry data of the wrapped sample by the buckling law leads to a continuous unimodal distribution similar to the distribution of the unwrapped sample obtained by applying the buckling law below Pt and the intrusion law above Pt. The position of Pt is characteristic of the tested material: when submitted to mercury pressure, aerogels and low-density xerogels only collapse in case of very small aggregates whereas they are crushed and then intruded in case of larger silica aggregates. The fact that compacted slabs of monodisperse non-aggregated silica spheres (of the same size range as the xerogels and aerogels) show only intrusion during mercury porosimetry experiments implies that the particles need to be aggregated so that the compaction mechanism takes place. The position of the change of mechanism from crushing to intrusion is not directly related to the size of the elementary particles but is linked to the size of the aggregates of silica particles.

Preparation of low-density xerogels by incorporation of additives during synthesis

Abstract Low-density xerogels were prepared by incorporation of an additive to alcogels prior to gelation. The additives studied are 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS), 3-aminopropyltrimethoxysilane (AMS), propyltrimethoxysilane (PMS), tetramethylorthosilicate (TMOS) and 3-aminopropyltriethoxysilane (AES) using tetraethylorthosilicate (TEOS) as main silica precursor. Samples were also prepared with EDAS as additive and TMOS as main silica reagent. When the additive contains methoxy groups, it reacts first, forms nuclei on which the main reagent TEOS reacts to form the silica particles. The nucleation mechanism by the additive occurs only in case of a difference of reactivity between additive and main silica precursor. The other group of the additive (amine, alkyl group, …) influences only the gelation time. In case of ethoxy groups (series AES/TEOS) or methoxy groups (series EDAS/TMOS) for both additive and main reagent, there is no nucleation by the additive.

Characterization of porous texture of hyperporous materials by mercury porosimetry using densification equation

Abstract The purpose of this paper is to propose a method of analyzing the mercury porosimetry data in the case of materials called hyperporous. This class of material does not undergo intrusion by mercury; instead, it shrinks under the mercury isostatic pressure and its density increases. The phenomenon is partially or completely irreversible. The proposed method enables computing the pore volume distribution as a function of the pore size in the same way as Washburns method does in the case of mercury intrusion.

The use of additives to prepare low-density xerogels

Abstract Low-density xerogels have been prepared by incorporation of an additive directly during the synthesis of the gel. The silica precursors (main reagent) were: tetramethylorthosilicate (TMOS), tetraethylorthosilicate (TEOS) and tetrapropylorthosilicate (TPOS) and the additives were: 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS) and 3-aminopropyltriethoxysilane (AES). With EDAS as additive, a nucleation mechanism by the additive takes place and exactly the same properties (pore volume, specific surface area, particle and aggregate size) are obtained either with TEOS or TPOS as main reagent. The nucleation mechanism is related to the difference in reactivity between additive and main reagent. With the AES/TPOS series, pore volumes up to 17 cm 3 / g have been obtained with pore sizes up to nearly 10 μm. The particle size is more than 100 nm. The couple AES–TPOS seems not to give rise to nucleation. It is likely that the difference in reactivity between ethoxy groups and propoxy groups is not sufficient to generate the nucleation mechanism by the additive.

Preparation of low-density xerogels from mixtures of TEOS with substituted alkoxysilanes. I. 17O NMR study of the hydrolysis–condensation process

Low-density xerogels were synthesised by incorporation of an additive to base catalysed tetraethylorthosilicate (TEOS) alcogels directly during the preparation of the sol. The nucleation mechanism by the additive was established by experiments during sol–gel transition. 17O NMR spectroscopy on TEOS–ethanol–water, 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS)–ethanol–water and EDAS–TEOS–ethanol–water solutions shows that the hydrolysis–condensation of EDAS is much faster than that of TEOS. Consequently it can be assumed that EDAS forms nuclei, onto which TEOS condenses later to form the silica particles.

Textural properties of low-density xerogels

The extent of shrinkage during drying is controlled by the balance between the capillary pressure developed in the pore liquid and the modulus of the solid network. One first method to obtain low-density xerogels consists in strengthening TEOS-based alcogels by providing new monomers to the alcogel after gelation. In the second method, low-density xerogels are produced by surface modification (silylation) of the wet gel with trimethylchlorosilane. The capillary pressure is reduced and the presence of non-reactive species on the surface makes the shrinkage reversible. A reduction of the capillary pressure can be achieved by introduction of a substituted alkoxide 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS) to a TEOS-based alcogel, synthesised in a single base-catalysed step. This additive acts as a nucleation agent leading to big silica particles (∼20 nm) with a low EDAS/TEOS ratio (∼0.03). The pores between those particles are also large and the drying stress is reduced. The textural properties of those three materials are compared: bulk densities of the samples modelled on the first and third method are varying in the same range (0.25–0.35 g/cm3) while xerogels obtained by the surface modification process are less dense (0.1–0.15 g/cm3). The biggest pores are observed in the third method.

The role of the main silica precursor and the additive in the preparation, of low-density xerogels

The incorporation of an additive during sol–gel synthesis reduces shinkage during ambient drying. The following additives have been studied: 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS), 3-aminopropyltriethoxysilane (AES) and 3-(2-aminoethylamino)propyltriethoxysilane (EDAES) and the main silica precursors were tetraethylorthosilicate (TEOS) and tetrapropylorthosilicate (TPOS). When the additive contains methoxy groups (EDAS), it acts as a nucleation agent of the silica particles and exactly the same properties (pore volume, specific surface area, particle and aggregate size) are obtained whether the main reagent is TEOS or TPOS. The nucleation mechanism is based on the difference in reactivity between additive and main reagent. In case of nucleation by the additive, the nucleation agent fixes the properties whatever the main silica precursor is. When both the additive and the main reagent contain ethoxy groups (series AES–TEOS and EDAES–TEOS), there is no nucleation mechanism by the additive, and the silica particle size remains nearly constant. With less reactive main reagent (series AES–TPOS and EDAES–TPOS), pore volumes up to 17 cm3/g have been obtained with pore sizes up to nearly 10 μm and very big particles (∼100 nm). The absence of nucleation by the additive for the couples AES–TPOS and EDAES–TPOS could be due to the fact that the difference in reactivity between ethoxy groups and propoxy groups is not sufficient to initiate the nucleation mechanism by the additive. In the absence of nucleation by the additive, the main reagent plays a role: highly porous materials with very large pores are prepared with TPOS.

Mercury porosimetry applied to porous silica materials: successive buckling and intrusion mechanisms

Abstract Some silica low-density xerogels exhibit two successive volume variation mechanisms, compaction and intrusion when submitted to mercury porosimetry. The position of the pressure of transition P t between the two mechanisms is characteristic of the tested material and allows to compute the buckling constant used to determine the pore size distribution in the compaction part of the experiment. The analysis of the mercury porosimetry data of a low-density xerogel wrapped in a tight membrane by the buckling law (intrusion is prevented and the sample is crushed during the whole porosimetry experiment) leads to a continuous unimodal distribution similar to the distribution of the unwrapped sample obtained by applying the buckling law below P t and the intrusion law above P t . This experiment confirms the validity of the use of the buckling law. The behaviour of the low-density xerogels can be related to one of their morphological characteristics. As the size of the aggregates of silica particles increases, the strength towards crushing increases and the change of mechanism from crushing to intrusion takes place at a lower pressure.

Preparation of low-density xerogels from mixtures of TEOS with substituted alkoxysilanes. II. Viscosity study of the sol-gel transition

Abstract Mixtures of TEOS with substituted methoxysilanes generate low-density xerogels due to a nucleation mechanism involving the substituted alkoxysilane. The sol–gel transition of these mixtures was followed by rheological characterisation. The transition from sol to gel takes place in a few minutes at ambient temperature. For the series exhibiting nucleation by the additive, the gel time goes through a slight minimum when the ratio of additive/main reagent increases. The elastic modulus increases with increasing ratio of additive/main reagent as the particle size decreases because of the nucleation mechanism by the additive. Samples with smaller particles exhibit the highest modulus for equal silica concentrations.