Armando Domínguez

Capillary condensation in heterogeneous mesoporous networks consisting of variable connectivity and pore-size correlation

Heterogeneous three-dimensional mesoporous networks (A. J. Ramirez-Cuesta, S. Cordero, F. Rojas, R. J. Faccio and J. L. Riccardo, J. Porous Mater., 2001, 8, 61, ) constructed under the premises of the dual site–bond model have been used as probe substrates to study the effects of variable connectivity and pore-size correlation on the aspects of both hysteresis loops and primary sorption scanning curves. The shapes of the hysteresis loops obtained from sorption simulation in networks of diverse morphologies are compared and discussed. It is found that the precursor structural parameters of the Monte Carlo simulated networks together with the sorption algorithm used in this work, can lead to IUPAC types H1, H2 and H3-like hysteresis loops, depending on the values chosen for the pore-size distribution parameters and mean connectivity. Network morphology also influences greatly the mechanisms of sorption processes in poorly or highly size correlated porous substrates. Sorption results on these 3-D porous specimens help to visualize the extents of pore blocking (vapour percolation) and delayed adsorption (liquid percolation) phenomena and also to foresee the most appropriate methods to ascertain the structure of porous materials.

A fractal-like kinetics equation to calculate landfill methane production

Abstract Landfill appears as a convenient choice to get rid of municipal solid waste while providing energy, due to methane generated through anaerobic fermentation. However, without capture and treatment landfill gas is considered an important source of atmospheric methane. The control and use of this gas require knowledge of both, current yield and long-term accumulative production. These values are usually calculated with mathematical expressions that consider 100% of conversion, and homogeneous chemical reactivity inside the fill. Nevertheless, fermentation in landfills is erratic and spatially heterogeneous. This work introduces a fractal-like chemical kinetics equation to calculate methane generation rate from landfill, QCH4 (m3/year), in the way: Q CH 4 =L 0 ∑ j ∑ i M ij C ij 0 k i (t j ) −d s /2 exp [−k i t j ], where fermentable wastes are partitioned in readily, moderately and slowly biodegradable categories, L0 is the potential of methane yield of refuse (m3/tonne under standard conditions), ds is the solid-phase fracton dimension, ki is the reaction kinetics constant of waste category i (year−1), and tj is the time from the year of burying j (year), Cij0 (kg/tonne) and Mij (kg) are the initial concentration and the mass of waste category i landfilled in year j, respectively. The idea behind this equation is that methane production kinetics is limited by the diffusion of hydrolyzed substrate into a heterogeneous solid-phase towards discrete areas, where methanogenesis occurs. A virtual study for a hypothetical case is developed. The predictions from this fractal approach are contrasted with those coming from two equations broadly used in the industrial work. The fractal-like kinetics equation represents better the heterogeneous nature of the fermentation in landfills.

Effect of the Drug-Excipient Ratio in Matrix-Type-Controlled Release Systems: Computer Simulation Study

The main objective of this work is to study the drug release behavior from inert matrix systems by using computer simulation. This study allowed us to propose a new statistical method to evaluate the drug percolation threshold as a function of the exposed surface area of the device. The matrix system was simulated as a simple cubic lattice. The sites of the lattice were randomly occupied at various drug–excipient ratios. By simulating a diffusive process, the drug was delivered from the matrix system. The obtained release profiles were fitted to two different models: near the excipient percolation threshold, the square root of the time was well fitted, whereas close to (but above) the drug percolation threshold, the power law described accurately the release data. A relationship between the initial drug load and the amount of drug trapped inside the matrix system at infinite time was found. This relationship was conveniently described by an error function. Percolation thresholds in the matrix systems were determined from the latter relationship by using a nonlinear regression method. The assessment of percolation thresholds depends on the exposed surface area of the matrix systems. Moreover, estimated percolation thresholds were in agreement with the predicted values stated in the percolation theory.

Mixed wettability: a numerical study of the consequences of porous media morphology

Abstract Four types of 2-D numerical networks have been used as models of porous media, in order to derive capillary pressure curves in the case of two immiscible fluids competing for the possession of the porous space under mixed-wetting conditions. The porous networks were constructed using a variation of the dual site bond model (DSBM) framework, which allows a substrate to have an adequate geometrical and topological distribution of its pore elements. For the same pore-size distribution, the DSBM can provide simulated porous media of different topological structures. Just taking into account thermodynamics aspects, it is possible to calculate some wetting indices for the special case involving spontaneous water imbibition, forced water drive, oil imbibition, and forced oil drive within each one of the above topologically different substrata, i.e. the main flooding sequences characteristic of wettability experiments have been studied in a variety of porous media. In this work, Amott–Harvey and US Bureau of Mines wetting indices are calculated from simulated capillary pressure curves describing the water–oil and oil–water immiscible displacements occurring in these topologically different porous structures.

Pore-blocking and pore-assisting factors during capillary condensation and evaporation

Abstract Thirty-four years ago Everett [The Solid–Gas Interface, Vol. 2, Marcel Dekker, New York, 1967, p. 1055] proposed a pore-blocking factor when establishing the foundations of a non-independent domain theory (IDT) of sorption hysteresis. Such pore-blocking 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.

Is the Alexander–Orbach Conjecture Suitable for Treating Diffusion in Correlated Percolation Clusters?

How does a particle diffuse inside a percolation cluster? This question is of both scientific and practical importance, e.g. in drug-controlled release and vapour adsorption. Diffusion in fractal media is characterized by the fracton dimension, ds. The Alexander and Orbach conjecture indicates that ds = 4/3 for diffusion in classical percolation clusters and, after much research on the subject, it is still provides a very good approximation for ds in the case of uncorrelated percolation cluster structures. However, what happens to the value of ds when a particle is moving inside a correlated percolation cluster? In this work, this problem is studied via Monte Carlo computer simulation. Our results show that the Alexander and Orbach conjecture is not always fulfilled.

Virtual study of wettability effects on bubble growth by solute diffusion in correlated porous networks

With the aim of analysing the consequences of both uniform and mixed-wetting conditions on bubble growth by solute diffusion inside correlated porous networks, a numerical study has been performed by means of a 2-D automaton built under a set of hypotheses derived from experimental observations. Results show that different wetting conditions imply varied and significant consequences on bubble growth. This phenomenon is also deeply affected by the spatial size correlation existing between the pore entities that conform a void network.

On the universal behavior of sorption isotherms in disordered mesoporous solids

Abstract 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.

Twofold description of the morphology of colloid aggregates and other complex structures

Abstract A new statistical theory, formerly used in the fields of porous media and heterogeneous adsorbent surfaces, is applied to describe the morphology of products of aggregation and gelation.

Mercury Intrusion Effects Modelled in Pores with Axial Symmetry and Attenuated Cross Section

The study of Hg intrusion in axially symmetric pores modelled from a circular function, including a cross-sectional attenuation, unveils or ratifies some interesting effects. One of them is the influence of the pore-wall angle of inclination on the intrusion pressure and meniscus radius of curvature. Another one is that Hg penetration in pores of this sort cannot proceed gradually but only in a jump-like manner. Virtual penetration curves indicate Laplace stable or unstable states as well as the virulence of the irreversible penetration of pores with a varying cross section. It is also important to point out that, in structures depicting a sinuous cavity to throat inter-connections, a single meniscus can be transformed into more than one meniscus by a sort of snap-off mechanism. Hysteresis in these structures can be expected to be more intense as the pore entities become more sinuous. Finally, it is also important to mention that the onset of liquid penetration is not always occurring at the minimum cross section of a pore channel, but at a specific point beyond this occlusion.