Mazen Al-Ghoul

Cosynthesis, Coexistence, and Self-Organization of α- and β-Cobalt Hydroxide Based on Diffusion and Reaction in Organic Gels

We report the cosynthesis of highly stable laminated single crystal alpha- and beta-Co(OH) 2 using the reaction and diffusion of a hydroxide solution into a gel containing Co(II). The obtained alpha-Co(OH) 2, which is known to be thermodynamically unstable and transforms in a short period of time to the beta form, has been stabilized in the gel medium for weeks. The system also exhibits Liesegang banding where complicated spatial dynamics during the formation of the two polymorphs are shown to take place.

Characterization of internal structure of hydrated agar and gelatin matrices by cryo‐SEM

There has been a considerable interest in recent years in developing polymer gel matrices for many important applications such as 2DE for quantization and separation of a variety of proteins and drug delivery system to control the release of active agents. However, a well‐defined knowledge of the ultrastructures of the gels has been elusive. In this study, we report the characterization of two different polymers used in 2DE: Gelatin, a naturally occurring polymer derived from collagen (protein) and agar, a polymer of polysaccharide (sugar) origin. Low‐temperature SEM is used to examine the internal structure of these gels in their frozen natural hydrated states. Results of this study show that both polymers have an array of hollow cells that resembles honeycomb structures. While agar pores are almost circular, the corresponding Gaussian curve is very broad exhibiting a range of radii from nearly 370 to 700 nm. Gelatin pores are smaller and more homogeneous reflecting a narrower distribution from nearly 320 to 650 nm. Overall, these ultrastructural findings could be used to correlate with functions of the polymers.

Surface-functionalized silica aerogels and alcogels for methylene blue adsorption

Surface-functionalized silica aerogels and alcogels prepared via a two-step sol–gel process through the combination of different silicon precursors were used in the adsorption of methylene blue dye molecules from aqueous media. The effect on the adsorption in batch reactors of the nature of precursors, the solvent used in the adsorbents synthesis, and the pH of the dye solution was monitored. Phenyl-functionalized silica materials revealed the highest adsorption capacity. Two phenyl-modified silica aerogels were widely tested in adsorption under various experimental conditions where the effect of pH, temperature, contact time, initial dye concentration, and adsorbent dose were investigated. The synthesis solvent was found to have a clear effect on the behavior of the adsorbent. Optimal conditions were found at pH 8 and 9 where the adsorbent–adsorbate surface charge interactions and the π–π stacking are most favourable. The adsorption followed a pseudo-second order kinetics, indicative of a co-existing chemisorption and physisorption processes. The adsorption data fitted the Sips isotherm and exhibited for the best aerogel a maximum adsorption capacity of 49.2 mg of dye per gram of adsorbent. The thermodynamic study revealed the adsorption of methylene blue onto phenyl-functionalized silica aerogels to be an exothermic and ordered adsorption process.

Band Propagation, Scaling Laws and Phase Transition in a Precipitate System. I: Experimental Study

In the first part of this work, we present an experimental study of the precipitation/redissolution reaction-diffusion system of initially separated components in two distinct organic gels: agar and gelatin. The system is prepared by diffusing a concentrated ammonia solution into a gel matrix that contains nickel sulfate. In agar, the system exhibits a pulse propagation due to the concomitant precipitation reaction between Ni(II) and hydroxide ions and redissolution due to ammonia. At a later stage of propagation, a transition to Liesegang banding is shown to take place. The dynamics of the distance traveled by the precipitation pulse, its width, and mass are shown to exhibit power laws. Moreover, the mass of the bands is shown to oscillate in time, indicating the emergence of a complex mass enrichment mechanism of the formed Liesegang bands. At the microscopic level, we show evidence that the system undergoes a continuous polymorphic transition concomitant with a morphological change whereby the solid in the pulse, which consists of nanospheres of α-nickel hydroxide transforms to form the bands, which consists of larger platelets of β-nickel hydroxide. This clearly indicates the existence of a dynamic Ostwald ripening mechanism that underlies the dynamics on both scales. On the other hand, in gelatin, although we can still obtain similar power laws as in the case of agar, no transition to bands was observed. It is shown that in this case, the propagating pulse is made of nanoparticles of α-nickel hydroxide with an average diameter ~50 nm.

Cadmium–Aluminum Layered Double Hydroxide Microspheres for Photocatalytic CO2 Reduction

We report the synthesis of cadmium-aluminum layered double hydroxide (CdAl LDH) using the reaction-diffusion framework. As the hydroxide anions diffuse into an agar gel matrix containing the mixture of aluminum and cadmium salts at a given ratio, they react to give the LDH. The LDH self-assembles inside the pores of the gel matrix into a unique spherical-porous shaped microstructure. The internal and external morphologies of the particles are studied by electron microscopy and tomography revealing interconnected channels and a high surface area. This material is shown to exhibit a promising performance in the photoreduction of carbon dioxide using solar light. Moreover, the palladium-decorated version shows a significant improvement in its reduction potential at room temperature.

Transition from rings to spots in a precipitation reaction–diffusion system

We report for the first time the transition from rings to spots with squared/hexagonal symmetry in a periodic precipitation system, which consists of sulfide/hydroxide ions diffusing into a gel matrix containing dissolved cadmium(II) ions. A phase diagram delineating the onset of the transition and the regions of various patterns is presented. The transition threshold, wavelength, and size of the resulting spots are shown to be controllable by adjusting the initial concentrations of the diffusing electrolytes. A scenario analogous to spinodal decomposition using the Cahn–Hilliard equation is shown to capture the experimental results.

Superdiffusive cusp-like waves in the mercuric iodide precipitate system and their transition to regular reaction bands.

We report a two-dimensional (2D) reaction-diffusion system that exhibits a superdiffusive propagating wave with anomalous cusp-like contours. This wave results from a leading precipitation reaction (wavefront) and a trailing redissolution (waveback) between initially separated mercuric chloride and potassium iodide to produce mercuric iodide precipitate (HgI2) in a thin sheet of a solid hydrogel (agar) medium. The propagation dynamics is accompanied by continuous polymorphic transformations between the metastable yellow crystals and the stable red crystals of HgI2. We study the dynamics of wavefront and waveback propagation that reveals interesting anomalous superdiffusive behavior without the influence of external enhancement. We find that a transition from superdiffusive to subdiffusive dynamics occurs as a function of outer iodide concentration. Inner mercuric concentrations lead to the transition from the anomalous cusp-like to cusp-free regular bands. While gel concentration affects the speed of propagation of the wave, it has no effect on its shape or on its superdiffusive dynamics. Microscopically, we show that the macroscopic wave propagation and polymorphic transformations are accompanied by an Ostwald ripening mechanism in which larger red HgI2 crystals are formed at the expense of smaller yellow HgI2 crystals.

Alternating metastable/stable pattern in the mercuric iodide crystal formation outside the Ostwald Rule of Stages.

We report a reaction-diffusion system in which two initially separated electrolytes, mercuric chloride (outer) and potassium iodide (inner), interact in a solid hydrogel media to produce a propagating front of mercuric iodide precipitate. The precipitation process is accompanied by a polymorphic transformation of the kinetically favored (unstable) orange, (metastable) yellow, and (thermodynamically stable) red polymorphs of HgI2. The sequence of crystal transformation is confirmed to agree with the Ostwald Rule of Stages. However, a region is found of initial inner iodide concentration, where a stationary pattern of alternating metastable/stable crystals is formed. A theoretical model based on reaction diffusion coupled to a special nucleation and growth mechanism is proposed. Its numerical solution is shown to reproduce the experimental results.

Chemical waves in heterogeneous media.

Formation of precipitation patterns and wave propagation in excitable media have attracted considerable scientific interest in the context of nonlinear chemical kinetics because of a new approach to micro and nanofabrication, in addition to some biological aspects. All precipitation patterns share common morphological characteristics, namely the formed patterns are stationary and no dynamical patterns can be observed in these classical precipitation systems (e.g., Liesegang phenomenon). However, it has been recently shown that in several circumstances dynamic patterns (chemical waves) can exist in purely inorganic precipitation systems similar to the well-known and studied (excitable) waves in Belousov-Zhabotinsky reaction. In this study, we show how to fine-tune the pattern characteristics in precipitation systems, such as the wavelength and the pattern morphology by changing the concentrations of the reagents, and we demonstrate chemical waves on a moving 3D spherical precipitation layer. We show that such precipitation waves have anomalous transport property, specifically superdiffusive nature, and it can be controlled by the initial concentration of the inner electrolyte. Moreover, we present several precipitation systems in which chemical wave propagation inside a moving precipitation layer can emerge. This observation points out the generality and robustness of similar behavior in diffusion-precipitation systems.

Propagating Fronts in Thin Tubes: Concentration, Electric, and pH Effects in a Two-Dimensional Precipitation Pulse System

In this paper, we studied the dynamics of a CaCO3 precipitate deposition pulse in a thin, long tube connecting two reservoir sinks of coprecipitates. The pulse profile, as well as the time t(c) and distance x(c) of the first appearance of precipitate, is studied as a function of the initial concentration of CO(3)(2-) in the right reservoir, [CO(3)(2-)](0), and later as a function of an applied external electric field at different voltages. The time variations of the pulse location and the pH at the center of the tube are determined. The distance from the calcium chloride sink (x) at any fixed time decreases as [CO(3)(2-)](0) increases. The time evolution of the front location exhibits a crossover between an early time regime and a late time regime. The pH-time curve shows a marked resemblance with a sigmoid shape. At any time, the pH consistently increases with [CO(3)(2-)](0). In the presence of a constant electric field applied across the tube (fixed voltage), t(c) decreased with the field strength, whereas x(c) exhibited a correlated increase. Irregularities in the variation of distance with the applied voltage (at a fixed time) were noted. The pH experiences a slight increase with the applied voltage. The pulse width exhibits a nonlinear time dependence, of the form w = a + bt(1/6). The shape of the deposition pulse deviates from a Gaussian distribution. This study is of special interest in the experimental simulation and modeling of precipitate deposition and potential clogging in microcapillary channels.