Rogelio Rodríguez

Adsorption properties of silica sols modified with thiol groups

Abstract The surfaces of silica particles, prepared by the sol–gel method, were chemically modified with thiol groups to change the adsorption properties of the metal ions. The flocculation kinetics of silica sols, with and without the thiol groups, were measured in presence of various metal ions in alcoholic solution by monitoring their growth with time using dynamic light scattering. The flocculation experiments were carried out by adding small amounts of metal ions until a critical flocculation concentration was reached. The amounts of metal ions adsorbed on the surface of silica and thiol-modified silica particles were determined by using atomic absorption. The flocculation process was improved when the silica surface was modified with thiol groups. The silica particles were observed with transmission electron microscopy, Fourier transform infra-red and Raman spectroscopy.

Synthesis and spectroscopic characterization of Eu3+-doped poly(acrylic acid)

Abstract Eu3+-doped poly(acrylic acid) was synthesized by polymerization in aqueous solution of the monomer partially neutralized with different amounts of Eu2O3 (0.5–5.0 mol% of Eu3+) using potassium persulfate as initiator. The Eu3+-doped polymer was characterized by 1 H - and 13 C -NMR, Fourier transform infrared (FT-IR) and luminescent spectroscopy. The NMR and FT-IR results show clear evidence of the coordination between Eu3+ cations and the carboxylate anions which result of the partial neutralization of the monomer prior to polymerization. On the other hand, the polymer phosphorescence intensity was observed to decrease with the increasing of the Eu3+ concentration due to the complex formation. Additionally, the homogeneous broadening and the relative intensity of the 5 D 0 → 7 F 1,2 transitions indicate that the dopant ions are uniformly distributed in low symmetry coordination sites.

Dynamic light scattering studies of the stability and growth of silica particles

Abstract The kinetics of silica sol growth under different chemical conditions was observed by measuring the particle size of solds size profile shows damped and periodic oscillations under a steady state condition is reached. This oscillatory behavior of the profile is associated with the unstable character of the chemical reaction. The instability arises because one reaction of the process is auto-catalytic. This instability produces a wide variety of possible structures that are formed during this part of the reaction. By appropriatly changing some chemical parameters such as pH, concentration, temperature, etc., it was possible to obtain a stable reaction from the beginning of the process. All the predictions of the effect of growth parameters on the stability of the reaction were fulfilled, this giving good support to the assumptions about the unstable character of the chemical reaction.

Master behaviour for gelation in fluoride-catalyzed gels

Abstract Master behavior for the gelation time in a sol-gel system catalyzed with ammonium fluoride is reported. This method allows for the determination in a reproducible way of the gelation profile in a sol-gel system, when the amount of this salt is known. The silica gel was prepared by mixing tetraethyl orthosilicate, water and ethanol; the reaction was carried out under reflux conditions. The kinetics of aggregation of silica particles was followed by using a dynamic light scattering technique, from the beginning of the chemical reaction. The unstable behaviour of the reaction at the beginning of the process was obtained in all cases. After this unstable regime, the system reaches a steady-state regime characterized by a constant value of the particle size. When the system approaches gelation, the particle size grows fast and this rate of growth depends on the pH of the system.

Time dependence of particle size and particle number of silica sols under reflux conditions

Abstract In this paper some results of work on the kinetics of aggregation of silica particles obtained by the sol-gel method are reported. The silica gel was prepared by mixing tetraethyl orthosilicate, ethanol, water, and a cobalt salt (cobalt (II) acetylacetonate). The chemical reaction was carried out under reflux conditions (T = 75°C). The aggregation was followed using dynamic light scattering from the onset of the chemical reaction. A systematic and reproducible behavior of the growth of the silica sols was obtained. The system begins with sols of relatively small sizes (around 100 nm), but within a short time the particle size increases to around 1 μm. After this increase, damped oscillations in size are observed until the radius remains constant. The particle size in this regime is similar to the initial values of the sols. The oscillations in the particle size profile can be explained as being a non-linear effect in the chemical process because one stage of the reaction is autocatalytic.

Aggregation profiles of silica sols in a sol-gel process

Abstract The kinetics of aggregation of sols in a sol-gel system was followed by measuring the particle size profile of sols from the start of the chemical reaction. The particle size was measured using dynamic light scattering. The particle size profiles show three different regions. The first one corresponds to an unstable regime characterized by reproducible oscillations in the size of the sols. This is due to a wide variety of different dissipative structures formed during the early stage of the reaction because of the autocatalytic character of the polymerization reaction. The second region is a steady-state regime which is characterized by the fact that the particle size remains practically constant for a long period of time. In this regime, the relative scattered intensity grows in an approximately linear way with the reaction time, i.e. the number of sols is increasing with time. In the last region, the particle size profile shows a rapid increase of the size of sols, and this corresponds to gelation. The number concentration of sols is so large that they begin to touch each other to form the gel. This is the mechanism by which the system reaches gelation.

Flocculation process of alumino-silicate particles using different metal ions in aqueous and alcoholic solutions

Abstract Silica–alumina particles prepared by the sol–gel method were used to adsorb metal ions in aqueous and alcoholic solutions. The flocculation kinetics of particles with a Si:Al molar composition 1:1 were studied in the presence of several metal ions by monitoring the particle growth with time using dynamic light scattering (DLS). Flocculation experiments were carried out following the two different routes: (a) adding small amounts of metal ions until the critical flocculation concentration was reached; (b) injecting the critical flocculation concentration all in one shot. The flocculated samples were characterized by atomic absorption spectroscopy (AA).

A study of the kinetics of gelation of silica particles induced by lead ions in alcoholic solution

A study of the kinetics of aggregation of silica particles induced by the addition of lead ions in alcoholic solution is reported. The metal ions were added sequentially at regular time intervals until gelation was reached. The aggregation process was followed by monitoring the growth of the aggregates as a function of time by using dynamic light scattering. The active sites on the surface of particles produce an adsorption of the metal ions, which promotes the aggregation processes of silica particles accelerating gelation. Several experiments were performed by changing the sol and the lead ions concentrations.

Mechanical Properties of the Composite Asphalt-Styrene-Butadiene Copolymer at High Degree of Modification

Abstract The morphology and elastic moduli of the composite styrene-butadiene copolymer-asphalt (SBR-A) were obtained at different degrees of modification from 0 to 40% by weight. Even when the 40% of rubber is far above the usual amount of rubber used for asphalt modification, the purpose of this work is to study the composite morphology at high degree of modification and to obtain a prediction for the elastic response of the material. The morphology of the samples was obtained by scanning and transmission electron microscopy. The mechanical properties of the samples obtained at different chemical compositions were determined at three different strain rates: 5, 50 and 500 mm/min. For low SBR concentrations, the elastic modulus of the as prepared materials was well modeled by using the Kerners model, however for concentrations larger than around 20%, the mechanical response of the material is higher than the predicted by this model. By introducing a percolation model that takes into account the connectiv...

Percolation modeling of the mechanical behavior of the composite polybutadiene-cellulose diacetate

Abstract Elastic modulli of the composite polybutadiene-cellulose diacetate (PBd-DAC) were obtained for different DAC concentrations within the range 0 to 30%. For low DAC concentrations, the mechanical modulus is well modeled by using the Kerners model; however, for concentrations larger than 10%, the mechanical response of the material is higher than the predicted by this model. By introducing a percolation model that takes into account physical and/or chemical links between the domains of the discrete phase, it is possible to obtain a good agreement with the experimental data for DAC concentrations up to 30%.