Ligia Sierra | Universidad Nacional de La Plata (original) (raw)

Papers by Ligia Sierra

Microporous and Mesoporous Materials, 2008

A series of cubic mesoporous silica materials exhibiting large cage-like cavities (up to 12.5 nm)... more A series of cubic mesoporous silica materials exhibiting large cage-like cavities (up to 12.5 nm) interconnected through windows (Im3m symmetry group) and with a mesopore volume as high as 1.30 cm 3 g À1 have been synthesized for the first time by using a new polyalkylene oxide-based triblock copolymer (PEO 140 PPO 39 PEO 140 ). The ability of this new triblock copolymer surfactant to act as a template for well-defined multidirectional pore network cubic silica particles, exhibiting tailored porous characteristics, was shown to depend on the synthesis conditions, in particular the acidity and temperature, as well as the presence of a cationic co-surfactant. These parameters influence the aggregation of nanoparticles either in micron-sized particles with different nanoparticles coalescence degrees or in blocks of aggregated nanoparticles. The aforementioned materials were characterized by small angle X-ray powder diffraction, nitrogen adsorption, scanning and transmission electronic microscopy, while the hydrodynamic diameters of the new triblock copolymer species were determined in situ by dynamic light scattering.

Microporous and Mesoporous Materials, 1999

Microporous and Mesoporous Materials, 2000

Isometric particles of mesoporous silica could be synthesized between 25°C and 45°C from reaction... more Isometric particles of mesoporous silica could be synthesized between 25°C and 45°C from reaction mixtures containing silicic acids, sodium chloride and a non-ionic surfactant (Triton X100). The size decreases from some tens of micrometers to less than 1 lm when the pH increases from 1.85 to 6. At pH lower than 3.5, glassy material forms a cement between the particles. The particle size distribution narrows at higher pH and at long reaction times (several days). A spheroidal shape is favored by low pH and high temperature, and a more polyhedral shape appears at high pH whatever the temperature. This behavior could be related to the polycondensation rate of the silicic acids, to the lifetime and disorder of the micelles and to the micelles±silicic acid interactions. Ó 2000 Elsevier Science B.V. All rights reserved.

Journal of Applied Polymer Science, 2009

Hyperbranched polyesters (HBPs) of fourth and fifth generation were synthesized by procedures inv... more Hyperbranched polyesters (HBPs) of fourth and fifth generation were synthesized by procedures involving one step (HBP4, HBP5), step by step (HBP4P, HBP5P), and combination of both of them (HBP1-4, HBP1-5) using 2,2-bis(methylol)propionic acid (DMPA), pentaerythritol (PE), and acid catalysis. The characterization of the polyesters was done by nuclear magnetic resonance (NMR), rheology on solution, Fourier transform infrared spectroscopy (FTIR), and acid and hydroxyl values. The degree of branching of polyesters (DB) and the average molecular mass and polymerization degree NMR were calculated by Frey and Fréchet methods and (DP)NMR, respectively, being higher for HBP4P sample. The molecular structure of polyesters depends mainly on of the synthesis method. The hydroxyl values were larger than 430 mg/g KOH for all HBPs, and the conversion was controlled between 90 and 93% to avoid excessive cyclization reactions. The steady shear viscosities at different temperatures and at 41.77 s−1 shear rate were measured for all HBPs solutions in dimethyl formamide (DMF) showing a shear thickening behavior. The flow activation energies (Ea) were calculated by Arrhenius equation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Studies in Surface Science and Catalysis, 2004

The relations between the morphology (particle shape and size) of SBA-15 and SBA-16 mesoporous si... more The relations between the morphology (particle shape and size) of SBA-15 and SBA-16 mesoporous silica and the synthesis parameters (composition of the reaction mixtures, nature of the triblock surfactant, fluoride anions, synthesis procedure) were established. This allowed defining the conditions for the formation of materials with micron-sized isometric particles suitable for HPLC applications. The presence of fluoride anions favors the formation of spherical shaped particles, with probably higher weak acidic silanol groups content to give higher hydrophobic materials after silylation, which is an advantage for their use as HPLC reverse phases. The different materials were evaluated in HPLC tests (separation of mixtures of aromatic and polynuclear aromatic molecules).

Journal De Physique Iv, 2004

Studies in Surface Science and Catalysis, 2001

Four mesoporous silicas, formed of isometric particles with 2.8 nm and 5.6 nm pores, were prepare... more Four mesoporous silicas, formed of isometric particles with 2.8 nm and 5.6 nm pores, were prepared by varying the synthesis conditions (silica source, surfactant, pH and temperature). A part of each sample was silylated with C8 chains. The silanol and silyl groups contents were estimated by 29Si NMR. Good separations have been obtained on mixtures of aromatic molecules by normal-phase HPLC with the four materials. In reversed-phase HPLC the results exhibit differences in function of the pore filling by the silyl groups.

Studies in Surface Science and Catalysis, 2005

Mesoporous aluminosilicates are prepared using CTMABr and sodium silicate and aluminum sulfate as... more Mesoporous aluminosilicates are prepared using CTMABr and sodium silicate and aluminum sulfate as Si and Al source, respectively. The materials are characterized by XRD, elemental analysis, N 2 adsorption, 27 Al NMR and Infrared spectroscopy with pyridine sorption. The results show that the use of aluminum sulfate conducts to mesoporous aluminosilicates where the framework charges are compensated by protons giving acidic Brønsted sites. No Lewis acid sites are observed. The Brønsted acidity seems not to be directly related to the number of incorporated tetrahedral Al atoms but to the nature of the Brønsted sites and to the mesoporous organization. Their catalytic activity towards the transformation of α-pinene is due to their Brønsted acidity. A good selectivity at 160°C towards camphene (75%) can be obtained with the catalyst with the lowest Si/Al ratio.

Macromolecular Symposia, 2007

Summary: The mesoporous silica have been considered fascinating materials for many techonological... more Summary: The mesoporous silica have been considered fascinating materials for many techonological applications due to their porous and morphological characteristics. This review focuses on their use as stationary phases for liquid chromatography, supports for immobilizing biomolecules, catalysts, agent for polymer reinforcement and hard templates for the preparation of mesoporous carbons.

Chemical Engineering Science, 2005

The sorption mechanisms in porous materials have been of long standing interest and debate. Concr... more The sorption mechanisms in porous materials have been of long standing interest and debate. Concretely, the involved hysteresis during nitrogen adsorption-desorption processes and its interpretation has represented an important challenge for experimentalists and theoreticians. Moreover, a better understanding of the different observed hysteretic behaviors and the elapsed are as is still remaining. Such a scenario has motivated us to study the pore connectivity and pore size distribution effects upon the hysteresis in nitrogen adsorption-desorption isotherms of a porous solid. Our simulation has been carried out over the entire range of connectivity assuming threedimensional pores randomly inscribed in a two-dimensional lattice with different occupation probabilities, related with pore connectivity, including the percolation threshold. The adsorption and desorption curves as a function of pressure have been simulated taking into account the monolayer-multilayer formation process and the capillary condensation-evaporation phenomena by using the Broekhoff de Boer and Kelvin equations, respectively. Results show that the employed methodology allows reproducing different types of hysteresis experimentally observed where the hysteretic behavior is strongly dependent upon both connectivity and the broadening of the pore size distribution. Specifically, the hysteresis loop area exhibits a maximum at the percolation threshold and then it decreases monotonically above the threshold.

Advanced Materials, 1999

29] We are currently examining the acid-synthesis mechanism.

Microporous and Mesoporous Materials, 2008

Dynamic light scattering (DLS) was used to follow the evolution of the micelles of two triblock c... more Dynamic light scattering (DLS) was used to follow the evolution of the micelles of two triblock copolymer surfactants, Pluronic F127 and Pluronic P123, in diluted aqueous reaction mixtures during the formation of respectively SBA-16 and SBA-15 type mesoporous silica particles at different temperatures, acid and salt concentrations. The silica source (tetraethoxysilane TEOS) is adsorbed by the micelles and undergoes hydrolysis. The resulting siliceous species coat the hydrophilic corona of the micelles, transforming those into composite colloids. The shape of the colloids formed from Pluronic F127 remains spherical but their size increases, whereas the spherical shape of the colloids with Pluronic P123 evolutes into an elongated shape. The decrease of the repulsion energy of these composite colloids allows their aggregation into ''liquid particles'' (spheres or polyhedra), as observed by optical microscopy. The ''liquid particles'', which can be re-dissolved in the case of the SBA-16 precursor by decreasing the temperature, transform into the final solid mesoporous silica particles by siloxane bonds formation within the coalesced silica walls.

Studies in Surface Science and Catalysis, 2008

Journal of Physical Chemistry B, 2003

Microporous and Mesoporous Materials, 2000

Materials with the ZON structure-type were synthesized between 120°C and 180°C from gels with the... more Materials with the ZON structure-type were synthesized between 120°C and 180°C from gels with the composition: v P2O5; w Al2O3; x SiO2; y MeO; a (CH3)4NCl; b HF; c (HOCH2CH2)2NH; 80H2O. Diethanolamine was used as a pH modifier without templating effects. The crystallization of AlPO4 materials needs the presence of F− anions which compensate the M(CH3)4N+ cationic template. The calcined

Studies in Surface Science and Catalysis, 2005

The synthesis of micro-sized spherical core-shell particles with a thin mesoporous silica shell i... more The synthesis of micro-sized spherical core-shell particles with a thin mesoporous silica shell is shown in this work. The dense silica used as the core is prepared by calcination of a mesoporous SBA-3-type silica, in the form of micron-sized spherical particles. The shell is formed by polycondensation, upon the core particles of: a) tetraethylortosilicate in the presence of a triblock copolymer Pluronic (P123 or F127) in acidic medium (CSA samples) or b) Silicic species, which are generated by partial dissolution of the core in basic medium (NH 4 OH) in the presence of cetyltrimethylammonium bromide (CSB samples). Conditions that improve the interaction between the nuclei and the shell precursors in order to form mesoporous shells are discussed. The results of the evaluation of some of these samples as stationary phases in normal HPLC show the interest of these materials for this application.

Solid State Sciences, 2003

Spherical particles (∅ > 3 µm, with 5-10 nm pore size) of SBA15-type mesoporous silica, usable as... more Spherical particles (∅ > 3 µm, with 5-10 nm pore size) of SBA15-type mesoporous silica, usable as stationary phase for HPLC, were prepared with tetraethoxysilane (TEOS) as silica source, triblock copolymer Pluronic P123 (EO 20 PO 70 EO 20 ) as surfactant S 0 and cetyltrimethylammonium bromide (CTMABr) as co-surfactant S + . The synthesis mechanism involves a surfactant-silica species selfassembly process in acidic medium under quiescent conditions, where S 0 H 3 O + X − I + and S + X − I + interactions occur. The syntheses were carried out using two procedures: (a) with one heating step and (b) with two heating steps. Synthesis conditions, such as the dilution, temperature and acidity of the reaction mixture and the temperature and duration of the reaction, influence the porous characteristics as well as the morphology and size of the particles. Spherical particles were obtained with one heating step under conditions that weaken the surfactants-silicate interactions such as high dilution, high temperature and low acidity. The modification of the micelle volume with the temperature and acidity allowed the adjustment of the pore size.

Microporous and Mesoporous Materials, 2007

Dynamic light scattering (DLS) was used to follow the evolution of the micelles of two triblock c... more Dynamic light scattering (DLS) was used to follow the evolution of the micelles of two triblock copolymer surfactants, Pluronic F127 and Pluronic P123, in diluted aqueous reaction mixtures during the formation of respectively SBA-16 and SBA-15 type mesoporous silica particles at different temperatures, acid and salt concentrations. The silica source (tetraethoxysilane TEOS) is adsorbed by the micelles and undergoes hydrolysis. The resulting siliceous species coat the hydrophilic corona of the micelles, transforming those into composite colloids. The shape of the colloids formed from Pluronic F127 remains spherical but their size increases, whereas the spherical shape of the colloids with Pluronic P123 evolutes into an elongated shape. The decrease of the repulsion energy of these composite colloids allows their aggregation into ''liquid particles'' (spheres or polyhedra), as observed by optical microscopy. The ''liquid particles'', which can be re-dissolved in the case of the SBA-16 precursor by decreasing the temperature, transform into the final solid mesoporous silica particles by siloxane bonds formation within the coalesced silica walls.

Advanced Materials, 1999

29] We are currently examining the acid-synthesis mechanism.

Solid State Sciences, 2005

The use of a reaction mixture, characterized by high dilution (H2O/TEOS=495), low to medium acid ... more The use of a reaction mixture, characterized by high dilution (H2O/TEOS=495), low to medium acid concentration (0.4–1.6 M HCl) and the presence of a cosurfactant (CTMABr), allows the synthesis of micron-sized particles of SBA-16 type mesoporous silica with different morphologies and structural characteristics. With a F127/TEOS molar ratio around 0.005 and a temperature between 70 °C and 95 °C, perfect rhombododecahedra (∅ 2–8 μm) are obtained. High acid concentration (1.6 M HCl) and/or temperature higher than 100 °C produce a smoothening of the edges and lead to the formation of spherical particles. This morphology is also favoured by low F127/TEOS molar ratios and corresponds to a disordered mesoporous structure. At a temperature of 40 °C or lower, the gel-like particles with poorly organized mesostructure are difficult to transform into well-organized materials. High F127/TEOS molar ratios lead to the same result. These different behaviours can be rationalized taking into account the strength of the interactions between the surfactant and the silica species which depends on the conformation of the hydrophilic poly-ethylene-oxide chains. Temperature and acid concentration are the main parameters to modify the conformation. But these two parameters are also responsible of the degradation of the Pluronic F127.

Microporous and Mesoporous Materials, 2008

A series of cubic mesoporous silica materials exhibiting large cage-like cavities (up to 12.5 nm)... more A series of cubic mesoporous silica materials exhibiting large cage-like cavities (up to 12.5 nm) interconnected through windows (Im3m symmetry group) and with a mesopore volume as high as 1.30 cm 3 g À1 have been synthesized for the first time by using a new polyalkylene oxide-based triblock copolymer (PEO 140 PPO 39 PEO 140 ). The ability of this new triblock copolymer surfactant to act as a template for well-defined multidirectional pore network cubic silica particles, exhibiting tailored porous characteristics, was shown to depend on the synthesis conditions, in particular the acidity and temperature, as well as the presence of a cationic co-surfactant. These parameters influence the aggregation of nanoparticles either in micron-sized particles with different nanoparticles coalescence degrees or in blocks of aggregated nanoparticles. The aforementioned materials were characterized by small angle X-ray powder diffraction, nitrogen adsorption, scanning and transmission electronic microscopy, while the hydrodynamic diameters of the new triblock copolymer species were determined in situ by dynamic light scattering.

Microporous and Mesoporous Materials, 1999

Microporous and Mesoporous Materials, 2000

Isometric particles of mesoporous silica could be synthesized between 25°C and 45°C from reaction... more Isometric particles of mesoporous silica could be synthesized between 25°C and 45°C from reaction mixtures containing silicic acids, sodium chloride and a non-ionic surfactant (Triton X100). The size decreases from some tens of micrometers to less than 1 lm when the pH increases from 1.85 to 6. At pH lower than 3.5, glassy material forms a cement between the particles. The particle size distribution narrows at higher pH and at long reaction times (several days). A spheroidal shape is favored by low pH and high temperature, and a more polyhedral shape appears at high pH whatever the temperature. This behavior could be related to the polycondensation rate of the silicic acids, to the lifetime and disorder of the micelles and to the micelles±silicic acid interactions. Ó 2000 Elsevier Science B.V. All rights reserved.

Journal of Applied Polymer Science, 2009

Hyperbranched polyesters (HBPs) of fourth and fifth generation were synthesized by procedures inv... more Hyperbranched polyesters (HBPs) of fourth and fifth generation were synthesized by procedures involving one step (HBP4, HBP5), step by step (HBP4P, HBP5P), and combination of both of them (HBP1-4, HBP1-5) using 2,2-bis(methylol)propionic acid (DMPA), pentaerythritol (PE), and acid catalysis. The characterization of the polyesters was done by nuclear magnetic resonance (NMR), rheology on solution, Fourier transform infrared spectroscopy (FTIR), and acid and hydroxyl values. The degree of branching of polyesters (DB) and the average molecular mass and polymerization degree NMR were calculated by Frey and Fréchet methods and (DP)NMR, respectively, being higher for HBP4P sample. The molecular structure of polyesters depends mainly on of the synthesis method. The hydroxyl values were larger than 430 mg/g KOH for all HBPs, and the conversion was controlled between 90 and 93% to avoid excessive cyclization reactions. The steady shear viscosities at different temperatures and at 41.77 s−1 shear rate were measured for all HBPs solutions in dimethyl formamide (DMF) showing a shear thickening behavior. The flow activation energies (Ea) were calculated by Arrhenius equation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Studies in Surface Science and Catalysis, 2004

The relations between the morphology (particle shape and size) of SBA-15 and SBA-16 mesoporous si... more The relations between the morphology (particle shape and size) of SBA-15 and SBA-16 mesoporous silica and the synthesis parameters (composition of the reaction mixtures, nature of the triblock surfactant, fluoride anions, synthesis procedure) were established. This allowed defining the conditions for the formation of materials with micron-sized isometric particles suitable for HPLC applications. The presence of fluoride anions favors the formation of spherical shaped particles, with probably higher weak acidic silanol groups content to give higher hydrophobic materials after silylation, which is an advantage for their use as HPLC reverse phases. The different materials were evaluated in HPLC tests (separation of mixtures of aromatic and polynuclear aromatic molecules).

Journal De Physique Iv, 2004

Studies in Surface Science and Catalysis, 2001

Four mesoporous silicas, formed of isometric particles with 2.8 nm and 5.6 nm pores, were prepare... more Four mesoporous silicas, formed of isometric particles with 2.8 nm and 5.6 nm pores, were prepared by varying the synthesis conditions (silica source, surfactant, pH and temperature). A part of each sample was silylated with C8 chains. The silanol and silyl groups contents were estimated by 29Si NMR. Good separations have been obtained on mixtures of aromatic molecules by normal-phase HPLC with the four materials. In reversed-phase HPLC the results exhibit differences in function of the pore filling by the silyl groups.

Studies in Surface Science and Catalysis, 2005

Mesoporous aluminosilicates are prepared using CTMABr and sodium silicate and aluminum sulfate as... more Mesoporous aluminosilicates are prepared using CTMABr and sodium silicate and aluminum sulfate as Si and Al source, respectively. The materials are characterized by XRD, elemental analysis, N 2 adsorption, 27 Al NMR and Infrared spectroscopy with pyridine sorption. The results show that the use of aluminum sulfate conducts to mesoporous aluminosilicates where the framework charges are compensated by protons giving acidic Brønsted sites. No Lewis acid sites are observed. The Brønsted acidity seems not to be directly related to the number of incorporated tetrahedral Al atoms but to the nature of the Brønsted sites and to the mesoporous organization. Their catalytic activity towards the transformation of α-pinene is due to their Brønsted acidity. A good selectivity at 160°C towards camphene (75%) can be obtained with the catalyst with the lowest Si/Al ratio.

Macromolecular Symposia, 2007

Summary: The mesoporous silica have been considered fascinating materials for many techonological... more Summary: The mesoporous silica have been considered fascinating materials for many techonological applications due to their porous and morphological characteristics. This review focuses on their use as stationary phases for liquid chromatography, supports for immobilizing biomolecules, catalysts, agent for polymer reinforcement and hard templates for the preparation of mesoporous carbons.

Chemical Engineering Science, 2005

The sorption mechanisms in porous materials have been of long standing interest and debate. Concr... more The sorption mechanisms in porous materials have been of long standing interest and debate. Concretely, the involved hysteresis during nitrogen adsorption-desorption processes and its interpretation has represented an important challenge for experimentalists and theoreticians. Moreover, a better understanding of the different observed hysteretic behaviors and the elapsed are as is still remaining. Such a scenario has motivated us to study the pore connectivity and pore size distribution effects upon the hysteresis in nitrogen adsorption-desorption isotherms of a porous solid. Our simulation has been carried out over the entire range of connectivity assuming threedimensional pores randomly inscribed in a two-dimensional lattice with different occupation probabilities, related with pore connectivity, including the percolation threshold. The adsorption and desorption curves as a function of pressure have been simulated taking into account the monolayer-multilayer formation process and the capillary condensation-evaporation phenomena by using the Broekhoff de Boer and Kelvin equations, respectively. Results show that the employed methodology allows reproducing different types of hysteresis experimentally observed where the hysteretic behavior is strongly dependent upon both connectivity and the broadening of the pore size distribution. Specifically, the hysteresis loop area exhibits a maximum at the percolation threshold and then it decreases monotonically above the threshold.

Advanced Materials, 1999

29] We are currently examining the acid-synthesis mechanism.

Microporous and Mesoporous Materials, 2008

Dynamic light scattering (DLS) was used to follow the evolution of the micelles of two triblock c... more Dynamic light scattering (DLS) was used to follow the evolution of the micelles of two triblock copolymer surfactants, Pluronic F127 and Pluronic P123, in diluted aqueous reaction mixtures during the formation of respectively SBA-16 and SBA-15 type mesoporous silica particles at different temperatures, acid and salt concentrations. The silica source (tetraethoxysilane TEOS) is adsorbed by the micelles and undergoes hydrolysis. The resulting siliceous species coat the hydrophilic corona of the micelles, transforming those into composite colloids. The shape of the colloids formed from Pluronic F127 remains spherical but their size increases, whereas the spherical shape of the colloids with Pluronic P123 evolutes into an elongated shape. The decrease of the repulsion energy of these composite colloids allows their aggregation into ''liquid particles'' (spheres or polyhedra), as observed by optical microscopy. The ''liquid particles'', which can be re-dissolved in the case of the SBA-16 precursor by decreasing the temperature, transform into the final solid mesoporous silica particles by siloxane bonds formation within the coalesced silica walls.

Studies in Surface Science and Catalysis, 2008

Journal of Physical Chemistry B, 2003

Microporous and Mesoporous Materials, 2000

Materials with the ZON structure-type were synthesized between 120°C and 180°C from gels with the... more Materials with the ZON structure-type were synthesized between 120°C and 180°C from gels with the composition: v P2O5; w Al2O3; x SiO2; y MeO; a (CH3)4NCl; b HF; c (HOCH2CH2)2NH; 80H2O. Diethanolamine was used as a pH modifier without templating effects. The crystallization of AlPO4 materials needs the presence of F− anions which compensate the M(CH3)4N+ cationic template. The calcined

Studies in Surface Science and Catalysis, 2005

The synthesis of micro-sized spherical core-shell particles with a thin mesoporous silica shell i... more The synthesis of micro-sized spherical core-shell particles with a thin mesoporous silica shell is shown in this work. The dense silica used as the core is prepared by calcination of a mesoporous SBA-3-type silica, in the form of micron-sized spherical particles. The shell is formed by polycondensation, upon the core particles of: a) tetraethylortosilicate in the presence of a triblock copolymer Pluronic (P123 or F127) in acidic medium (CSA samples) or b) Silicic species, which are generated by partial dissolution of the core in basic medium (NH 4 OH) in the presence of cetyltrimethylammonium bromide (CSB samples). Conditions that improve the interaction between the nuclei and the shell precursors in order to form mesoporous shells are discussed. The results of the evaluation of some of these samples as stationary phases in normal HPLC show the interest of these materials for this application.

Solid State Sciences, 2003

Spherical particles (∅ > 3 µm, with 5-10 nm pore size) of SBA15-type mesoporous silica, usable as... more Spherical particles (∅ > 3 µm, with 5-10 nm pore size) of SBA15-type mesoporous silica, usable as stationary phase for HPLC, were prepared with tetraethoxysilane (TEOS) as silica source, triblock copolymer Pluronic P123 (EO 20 PO 70 EO 20 ) as surfactant S 0 and cetyltrimethylammonium bromide (CTMABr) as co-surfactant S + . The synthesis mechanism involves a surfactant-silica species selfassembly process in acidic medium under quiescent conditions, where S 0 H 3 O + X − I + and S + X − I + interactions occur. The syntheses were carried out using two procedures: (a) with one heating step and (b) with two heating steps. Synthesis conditions, such as the dilution, temperature and acidity of the reaction mixture and the temperature and duration of the reaction, influence the porous characteristics as well as the morphology and size of the particles. Spherical particles were obtained with one heating step under conditions that weaken the surfactants-silicate interactions such as high dilution, high temperature and low acidity. The modification of the micelle volume with the temperature and acidity allowed the adjustment of the pore size.

Microporous and Mesoporous Materials, 2007

Dynamic light scattering (DLS) was used to follow the evolution of the micelles of two triblock c... more Dynamic light scattering (DLS) was used to follow the evolution of the micelles of two triblock copolymer surfactants, Pluronic F127 and Pluronic P123, in diluted aqueous reaction mixtures during the formation of respectively SBA-16 and SBA-15 type mesoporous silica particles at different temperatures, acid and salt concentrations. The silica source (tetraethoxysilane TEOS) is adsorbed by the micelles and undergoes hydrolysis. The resulting siliceous species coat the hydrophilic corona of the micelles, transforming those into composite colloids. The shape of the colloids formed from Pluronic F127 remains spherical but their size increases, whereas the spherical shape of the colloids with Pluronic P123 evolutes into an elongated shape. The decrease of the repulsion energy of these composite colloids allows their aggregation into ''liquid particles'' (spheres or polyhedra), as observed by optical microscopy. The ''liquid particles'', which can be re-dissolved in the case of the SBA-16 precursor by decreasing the temperature, transform into the final solid mesoporous silica particles by siloxane bonds formation within the coalesced silica walls.

Advanced Materials, 1999

29] We are currently examining the acid-synthesis mechanism.

Solid State Sciences, 2005

The use of a reaction mixture, characterized by high dilution (H2O/TEOS=495), low to medium acid ... more The use of a reaction mixture, characterized by high dilution (H2O/TEOS=495), low to medium acid concentration (0.4–1.6 M HCl) and the presence of a cosurfactant (CTMABr), allows the synthesis of micron-sized particles of SBA-16 type mesoporous silica with different morphologies and structural characteristics. With a F127/TEOS molar ratio around 0.005 and a temperature between 70 °C and 95 °C, perfect rhombododecahedra (∅ 2–8 μm) are obtained. High acid concentration (1.6 M HCl) and/or temperature higher than 100 °C produce a smoothening of the edges and lead to the formation of spherical particles. This morphology is also favoured by low F127/TEOS molar ratios and corresponds to a disordered mesoporous structure. At a temperature of 40 °C or lower, the gel-like particles with poorly organized mesostructure are difficult to transform into well-organized materials. High F127/TEOS molar ratios lead to the same result. These different behaviours can be rationalized taking into account the strength of the interactions between the surfactant and the silica species which depends on the conformation of the hydrophilic poly-ethylene-oxide chains. Temperature and acid concentration are the main parameters to modify the conformation. But these two parameters are also responsible of the degradation of the Pluronic F127.