Fernando Rojas - Academia.edu (original) (raw)

Papers by Fernando Rojas

Particle & Particle Systems Characterization, 2004

The description of disordered porous media through the Dual Site‐Bond Model (DSBM) is reviewed an... more The description of disordered porous media through the Dual Site‐Bond Model (DSBM) is reviewed and updated with the inclusion of new results. After a brief discussion of the DSBM, the percolation properties of networks are studied. These present novel features with respect to classical percolation in non‐correlated networks. These properties are then used in discussing different processes in disordered porous media, namely, invasion percolation and adsorption‐desorption hysteresis. Finally, a method for the characterization of pore size distributions from experimental adsorption‐desorption isotherms is proposed.

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004

Adsorption-desorption isotherms in disordered mesoporous solids, described by the Dual Site-Bond ... more Adsorption-desorption isotherms in disordered mesoporous solids, described by the Dual Site-Bond Model, are obtained through Monte Carlo simulations and their behavior is correlated to the topological properties of the porous networks and to their percolation properties, extending previous results to the general case of variable connectivity networks. A quasi-universal curve is found which may be useful in the problem of obtaining pore size distributions from the analysis of experimental Adsorption-desorption isotherms.

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004

The independent non-intersecting cylindrical pore nature of SBA-15 materials represents, in princ... more The independent non-intersecting cylindrical pore nature of SBA-15 materials represents, in principle, a very adequate experimental system to analyze the N 2 sorption scanning behavior according to the theorems of the Independent Domain Theory of Sorption Hysteresis. In this work, SBA-15 materials of two general types have been synthesized: (i) substrates made by hexagonal arrays of cylinders of about the same sizes, and (ii) solids including a relatively wide distribution of cylindrical pores of varying diameters arranged in a fairly distorted hexagonal packing. Boundary and primary N 2 sorption scanning curves have been experimentally determined on all these materials in order to observe the possible irruption of cooperative processes during the filling and emptying of tubular SBA-15 pores. Pore domain complexion diagrams (i.e. graphs indicating the amount of pore sizes that are occupied by condensate or vapor) have been constructed by means of Non-Local Density Functional Theory calculations of the pore-size distributions that are incumbent to both boundary and primary scanning curves. It has been inferred that two phenomena branded as advanced adsorption and single pore-blocking can take place in reason of the undulating cross-section nature of some SBA-15 tube materials; the extents of the above irreversible processes depending on the degree of sinuosity of the cross sections of the tubes.

Applied Surface Science, 2002

Thirty-four years ago Everett [The Solid-Gas Interface, Vol. 2, Marcel Dekker, New York, 1967, p.... more Thirty-four years ago Everett [The Solid-Gas Interface, Vol. 2, Marcel Dekker, New York, 1967, p. 1055] proposed a poreblocking factor when establishing the foundations of a non-independent domain theory (IDT) of sorption hysteresis. Such poreblocking factor was defined as the ratio between two desorbed volumes within the same pressure range. The first volume arose from a non-independent pore structure. The second quantity was a virtual one since it represented the volume desorbed if the pores of the substrate had acted as independent domains. In fact, Everett calculated the ratio between pore-blocking factors, while not their absolute values, from experimental data proceeding from sorption results on porous glasses. The astonishing conclusion of all this preliminary work, was that blocking factors depended upon the total amount of condensate at a certain stage of a desorption process rather than on the distribution of it within the porous network. In this way, a unique pore-blocking factor curve ensued from different sorption processes such as boundary and scanning curves. Now, through the aid of simulated heterogeneous 3-D porous networks and the sorption curves thereon developed, an assessment of the above mentioned important assertion has been undertaken. Besides, a pore-assisting factor that may arise during an ascending sorption process has been treated under a similar context.

Adsorption Science & Technology, 2001

The textural properties of sulphated ZrO 2-SiO 2 catalytic supports of different compositions wer... more The textural properties of sulphated ZrO 2-SiO 2 catalytic supports of different compositions were investigated through N 2 sorption and X-ray diffraction (conventional and SAXS) methods. The materials were synthesised by the sol-gel procedure from a mixture of zirconium(IV) tert-butoxide and tetraethyl orthosilicate. The final products were obtained after drying at 125ºC followed by calcination at 400ºC. The porous structures of the substrates were found to depend on their composition: at low zirconia contents these were formed by slit cavities while at high ZrO 2 concentrations the void structure became open; both cavities and necks were of similar size. In the first case, the pore size distribution function could be obtained from an analysis of the ascending boundary curve of the N 2 isotherm since adsorption occurred on a flat surface. In the second case, a unique pore-size distribution was obtained either from the ascending or descending boundary curves of the N 2 isotherm. A tubular geometry seemed appropriate for the description of the sorption characteristics of these substrates. Primary descending scanning curves traced within the hysteresis loops of samples having high ZrO 2 contents offered further textural information. X-Ray diffraction methods provided compound identification, evidence of phase segregation between ZrO 2 and SiO 2 , and particle and pore-size distributions.

SOILS AND FOUNDATIONS, 2005

Soil suction is an essential parameter for describing the mechanical behavior of unsaturated soil... more Soil suction is an essential parameter for describing the mechanical behavior of unsaturated soils. For analytical purposes, its value is usually inferred from the soil-water characteristic curve. However, the expressions developed up till now to describe this curve, do not properly account for the physical facts involved in the hysteresis phenomenon. It is generally accepted that hysteresis arises because pores of different sizes are interconnected generating the "inkbottle" effect. In this paper, a twofold model that simulates the porous structure of soils is used and the mathematical expressions describing the hysteresis of the soil-water characteristic curves are obtained. This model considers that the voids in the soil mass are composed of two kinds of elements: the sites (cavities) and the bonds (throats), each kind showing its proper size distribution. Based on these premises and making use of the probability theory, the equations corresponding to the main hysteresis loop during wetting or drying are established and it is shownthat the extent of the hysteretic behavior depends on the parameters describing the size distributions. In addition, the equations describing the specific internal (scanning) paths for cyclic wetting and drying are obtained. Finally, these results are compared with experimental results that include the main hysteresis loop, some scanning curves and the poresize distribution.

Revista internacional …, 2010

... Fernando Rojas-González Universidad Auto´noma Metropolitana-Iztapalapa, Mexico City, Mexico .... more ... Fernando Rojas-González Universidad Auto´noma Metropolitana-Iztapalapa, Mexico City, Mexico ... particles are used to modify the characteristics (especially the thermal and mechanical properties) of either an epoxy resin (ER) or a ZCOOH-functionalised ER (FER) substrate. ...

Particle & Particle Systems Characterization, 2004

The description of disordered porous media through the Dual Site‐Bond Model (DSBM) is reviewed an... more The description of disordered porous media through the Dual Site‐Bond Model (DSBM) is reviewed and updated with the inclusion of new results. After a brief discussion of the DSBM, the percolation properties of networks are studied. These present novel features with respect to classical percolation in non‐correlated networks. These properties are then used in discussing different processes in disordered porous media, namely, invasion percolation and adsorption‐desorption hysteresis. Finally, a method for the characterization of pore size distributions from experimental adsorption‐desorption isotherms is proposed.

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004

Adsorption-desorption isotherms in disordered mesoporous solids, described by the Dual Site-Bond ... more Adsorption-desorption isotherms in disordered mesoporous solids, described by the Dual Site-Bond Model, are obtained through Monte Carlo simulations and their behavior is correlated to the topological properties of the porous networks and to their percolation properties, extending previous results to the general case of variable connectivity networks. A quasi-universal curve is found which may be useful in the problem of obtaining pore size distributions from the analysis of experimental Adsorption-desorption isotherms.

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2004

The independent non-intersecting cylindrical pore nature of SBA-15 materials represents, in princ... more The independent non-intersecting cylindrical pore nature of SBA-15 materials represents, in principle, a very adequate experimental system to analyze the N 2 sorption scanning behavior according to the theorems of the Independent Domain Theory of Sorption Hysteresis. In this work, SBA-15 materials of two general types have been synthesized: (i) substrates made by hexagonal arrays of cylinders of about the same sizes, and (ii) solids including a relatively wide distribution of cylindrical pores of varying diameters arranged in a fairly distorted hexagonal packing. Boundary and primary N 2 sorption scanning curves have been experimentally determined on all these materials in order to observe the possible irruption of cooperative processes during the filling and emptying of tubular SBA-15 pores. Pore domain complexion diagrams (i.e. graphs indicating the amount of pore sizes that are occupied by condensate or vapor) have been constructed by means of Non-Local Density Functional Theory calculations of the pore-size distributions that are incumbent to both boundary and primary scanning curves. It has been inferred that two phenomena branded as advanced adsorption and single pore-blocking can take place in reason of the undulating cross-section nature of some SBA-15 tube materials; the extents of the above irreversible processes depending on the degree of sinuosity of the cross sections of the tubes.

Applied Surface Science, 2002

Thirty-four years ago Everett [The Solid-Gas Interface, Vol. 2, Marcel Dekker, New York, 1967, p.... more Thirty-four years ago Everett [The Solid-Gas Interface, Vol. 2, Marcel Dekker, New York, 1967, p. 1055] proposed a poreblocking factor when establishing the foundations of a non-independent domain theory (IDT) of sorption hysteresis. Such poreblocking factor was defined as the ratio between two desorbed volumes within the same pressure range. The first volume arose from a non-independent pore structure. The second quantity was a virtual one since it represented the volume desorbed if the pores of the substrate had acted as independent domains. In fact, Everett calculated the ratio between pore-blocking factors, while not their absolute values, from experimental data proceeding from sorption results on porous glasses. The astonishing conclusion of all this preliminary work, was that blocking factors depended upon the total amount of condensate at a certain stage of a desorption process rather than on the distribution of it within the porous network. In this way, a unique pore-blocking factor curve ensued from different sorption processes such as boundary and scanning curves. Now, through the aid of simulated heterogeneous 3-D porous networks and the sorption curves thereon developed, an assessment of the above mentioned important assertion has been undertaken. Besides, a pore-assisting factor that may arise during an ascending sorption process has been treated under a similar context.

Adsorption Science & Technology, 2001

The textural properties of sulphated ZrO 2-SiO 2 catalytic supports of different compositions wer... more The textural properties of sulphated ZrO 2-SiO 2 catalytic supports of different compositions were investigated through N 2 sorption and X-ray diffraction (conventional and SAXS) methods. The materials were synthesised by the sol-gel procedure from a mixture of zirconium(IV) tert-butoxide and tetraethyl orthosilicate. The final products were obtained after drying at 125ºC followed by calcination at 400ºC. The porous structures of the substrates were found to depend on their composition: at low zirconia contents these were formed by slit cavities while at high ZrO 2 concentrations the void structure became open; both cavities and necks were of similar size. In the first case, the pore size distribution function could be obtained from an analysis of the ascending boundary curve of the N 2 isotherm since adsorption occurred on a flat surface. In the second case, a unique pore-size distribution was obtained either from the ascending or descending boundary curves of the N 2 isotherm. A tubular geometry seemed appropriate for the description of the sorption characteristics of these substrates. Primary descending scanning curves traced within the hysteresis loops of samples having high ZrO 2 contents offered further textural information. X-Ray diffraction methods provided compound identification, evidence of phase segregation between ZrO 2 and SiO 2 , and particle and pore-size distributions.

SOILS AND FOUNDATIONS, 2005

Soil suction is an essential parameter for describing the mechanical behavior of unsaturated soil... more Soil suction is an essential parameter for describing the mechanical behavior of unsaturated soils. For analytical purposes, its value is usually inferred from the soil-water characteristic curve. However, the expressions developed up till now to describe this curve, do not properly account for the physical facts involved in the hysteresis phenomenon. It is generally accepted that hysteresis arises because pores of different sizes are interconnected generating the "inkbottle" effect. In this paper, a twofold model that simulates the porous structure of soils is used and the mathematical expressions describing the hysteresis of the soil-water characteristic curves are obtained. This model considers that the voids in the soil mass are composed of two kinds of elements: the sites (cavities) and the bonds (throats), each kind showing its proper size distribution. Based on these premises and making use of the probability theory, the equations corresponding to the main hysteresis loop during wetting or drying are established and it is shownthat the extent of the hysteretic behavior depends on the parameters describing the size distributions. In addition, the equations describing the specific internal (scanning) paths for cyclic wetting and drying are obtained. Finally, these results are compared with experimental results that include the main hysteresis loop, some scanning curves and the poresize distribution.

Revista internacional …, 2010

... Fernando Rojas-González Universidad Auto´noma Metropolitana-Iztapalapa, Mexico City, Mexico .... more ... Fernando Rojas-González Universidad Auto´noma Metropolitana-Iztapalapa, Mexico City, Mexico ... particles are used to modify the characteristics (especially the thermal and mechanical properties) of either an epoxy resin (ER) or a ZCOOH-functionalised ER (FER) substrate. ...