Biomedical subjects

F González-Caballero

Publications and source records attributed to F González-Caballero.

12 recordsLinked to original sources

PubMed2005

Stability and magnetic characterization of oleate-covered magnetite ferrofluids in different nonpolar carriers.

This work describes the preparation and stability evaluation of suspensions consisting of hydrophobic magnetite nanoparticles dispersed in different organic solvents. The ferrite particles are covered by a shell of chemisorbed oleate ions following a procedure that is described in detail. The oleate-covered particles were dispersed in different organic solvents with dielectric constants, epsilon(r), ranging between 1.8 and 9, and the centrifugal field strength needed to remove particle aggregates formed during the synthesis was determined for the different liquid carriers used. A thermodynamic analysis demonstrated that the observed stability of the suspensions in liquids with epsilon(r) < 5 is well correlated with the very low lyophobic attraction between the particles. This can easily be surmounted by thermal agitation, since the van der Waals attraction is negligible. In contrast, for liquids with epsilon(r) > 9, the suspensions become unstable because of the combined action of the van der Waals and lyophobic attractions, the latter being dominant for very polar solvents. Finally, a complete magnetic characterization of the oleate-magnetite powder, as well as of several stable ferrofluids prepared with it, was carried out. From this characterization, the magnetic diameters and magnetic moments of the particles immersed in the different liquid carriers were estimated and compared to those corresponding to the dry magnetic particles. This made it possible to estimate the thickness of the nonmagnetic layer on the particles.

Journal Article↗

PubMed2005

Preparation and sedimentation behavior in magnetic fields of magnetite-covered clay particles.

This work is devoted to the preparation of magnetite-covered clay particles in aqueous medium. For this purpose, magnetite nanoparticles were synthesized by a coprecipitation method. These magnetic particles are adhered to sodium montmorillonite (NaMt) particles in aqueous suspensions of both materials, by appropriate control of the electrolyte concentrations. The best condition to produce such heteroaggregation corresponds to acid pH and approximately 1 mol/L ionic strength, when the electrokinetic potentials (zeta-potential) of both NaMt and Fe3O4 particles have high enough and opposite sign, as demonstrated from electrophoresis measurements. When a layer of magnetite re-covers the clay particles, the application of an external magnetic field induces a magnetic moment in clay-magnetite particles parallel to the external magnetic flux density. The sedimentation behavior of such magnetic particles is studied in the absence or presence of an external magnetic field in a vertical direction. The whole sedimentation behavior is also strongly affected by the formation of big flocculi in the suspensions under the action of internal colloidal interactions. van der Waals and dipole-dipole magnetic attractions between magnetite-covered clay particles dominate the flocculation processes. The different relative orientation of the clay-magnetite particles (edge-to-edge, face-to-edge, and face-to-face) are discussed in order to predict the most favored flocculi configuration.

Journal Article↗

PubMed2000

Polarization of the Electrical Double Layer. Time Evolution after Application of an Electric Field.

Electrophoresis is one of the electrokinetic phenomena most widely investigated, both from a fundamental point of view and as a research tool in academia and industry. However, the dependence between electrophoretic mobility and zeta potential is, in a general case, far from simple, because of the many physical processes involved. In this work, we first describe qualitatively and (in some cases) quantitatively the time behavior of the dipole moment induced in the electrical double layer by an applied electric field. Further, a simple relationship is deduced between the dipole moment and the electrophoretic mobility. Through the analysis of the time dependence of the former, it is possible to resolve the different contributions to the stationary values of the mobility. Three characteristic relaxation times are distinguished in the time evolution of the dipole moment: tau(H) (the time needed for hydrodynamic flows to be established), tau(MW) (time for ionic electromigration to develop), and tau(VD) (after this time, diffusion flows are established in the system, and the double layer polarization is complete). This means that different mechanisms are operating on the double layer for different times after the application of the field, and that computing the mobility at such different times is equivalent to calculating the steady-state electrophoretic mobility under different approximations. A comparison is shown between estimated and computed mobility values as functions of time and of zeta potential, confirming the validity of the asymptotic calculations. Copyright 2000 Academic Press.

Journal Article↗

PubMed2000

Rheological and Electrokinetic Properties of Sodium Montmorillonite Suspensions.

In this article, we describe the rheology of Na montmorillonite suspensions as a function of pH, at constant ionic strength. The observed behavior is discussed quantitatively in terms of the potential energy of interaction between particles, keeping in mind the anisotropic nature of clay particles. The extended DLVO model that includes electrostatic, van der Waals, and polar acid-base contributions to the total energy is used. It is found that face-to-face interactions are virtually independent of pH, whereas edge-to-edge interactions are most attractive at the isoelectric point of edges (pH approximately 7). The most significant variations occur in face-to-edge potential energy, with strong attractions at pH<7. Steady-state viscometry showed that the yield stress decreases up to an order of magnitude between pH 3 and pH 7, with a much slower rate of decrease in the 7-11 pH interval. Concerning oscillatory measurements, it is found that both the elastic (G') and viscous (G") moduli are practically independent of frequency. It is also demonstrated that G'>G", the difference being larger at acid pH values. These results, in addition to potential energy calculations, suggest the existence of an elastic, coagulated structure up to pH 7, whereas as the pH is increased such structure is more relaxed because of electrostatic repulsions. Similar conclusions are reached when creep-recovery data are analyzed. Copyright 2000 Academic Press.

Journal Article↗

PubMed2000

Dynamics of the Electric Double Layer: Analysis in the Frequency and Time Domains.

No rigorous theory of electrokinetic phenomena is conceivable without properly accounting for double layer polarization under the action of external fields. Since processes leading to such polarization need a finite time to develop, an analysis of the behavior of the quantities of interest (potential and ion concentration profiles, particle or fluid velocity, and so on) as a function of time should be extremely illustrative. In this work, we analyze how those quantities evolve in the nanosecond to microsecond time range after the application of an electric field. The network method is proposed (in which, essentially, an electric circuit simulator program is used to solve the differential equations involved, after their proper interpretation in terms of fluxes and forces) to gain information about the evolution with time of the potential, counterion, and co-ion perturbations, the particle velocity, and the fluid velocity profile. The performance of the method is first ckecked in the frequency domain, for which rigorous solutions exist, and then the procedure is used in the time domain. Reasons are discussed for the observed time dependencies of the analyzed quantities. Copyright 2000 Academic Press.

Journal Article↗

PubMed1999

Deposition of Colloidal Zinc Sulfide on Glass Substrate.

The effect of colloidal forces involved in the deposition of spherical zinc sulfide colloidal particles on a packed bed of glass has been studied. Experiments were performed by pumping a suspension of monodisperse colloidal ZnS particles through a cylindrical plug of ground glass, and by continuous determination of the outgoing suspension concentration. The flux density of adhered particles, jexp (number of particles deposited per unit time and unit surface area of glass collector), decreased with both pH and ionic strength of the aqueous electrolyte solution. Qualitative explanation of the experiments has been given in terms of the total energy of interaction between the dispersed particles and the substrate, and between the particles themselves, computed from the extended DLVO theory, including acid-base interactions. The contributions to the total free energy of interaction were determined from the zeta potential and surface free energy of ZnS and glass, measured under different experimental conditions. It was found that at pH 4 (below the isoelectric point of ZnS) the efficiency of the deposition of ZnS on glass was maximum. At higher pH values the amount of ZnS deposited on glass clearly decreased. Increasing NaCl concentration at fixed pH (>/=6) decreased the efficiency of the deposition. Adhesion experiments were also performed at pH 4 in the presence of increasing concentrations of CaCl2 or La(NO3)3 in the dispersion medium. In these cases, the rate of adhesion was qualitatively well correlated with the computed ZnS-glass interactions. Copyright 1999 Academic Press.

Journal Article↗

PubMed1997

Expanding forces in aqueous outflow pathways of a nonaccommodating mammal: an approach via comparative dynamic morphology.

Six dog eyes were fixed by intracameral perfusion of fixative at pressures of 0, 5, 10, 15, 20, and 25 cm of water. Eight dog eyes were fixed after the injection in both ocular chambers of a number of cholinergic agents, either singly or in combination. Under the effect of miotics and under increased ocular pressure, the aqueous pathways expand. An analysis of the forces involved in expansion of the exit pathways reveals the primary role of the detached ciliary body in nonaccommodating mammals. Two mechanisms appear to have been conserved in dogs and humans throughout evolution. The first is an active mechanism: the opening of the trabecular meshwork as a consequence of the combined action of the ciliary muscle and the iris and its insertion ligaments-the uveoscleral trabeculate-in dogs, and the longitudinal portion of the ciliary muscle and scleral spur in humans. The second is a passive mechanism: the infundibular arrangement of the drainage structures assisted by the traction on the zonular ligament of the lens, which responds to an increase in pressure in the anterior chamber by widening the pathways, thus favoring outflow.

Accommodation, Ocular↗

PubMed1995

Role of cameral mucous gel in primary narrow angle and closed angle glaucoma: a pathogenic clue.

The cameral mucous gel (CMG) has been described as a layer of glycoprotein-enriched hyaluronic acid lining the anterior surface of the iris, covering the trabecular meshwork, and spreading over the posterior surface of the cornea. The CMG is thought to exert a colloid-osmotic effect on the hydrostatic forces involved in the circulation of the aqueous humour which may help our understanding of the pathophysiology of open angle and angle closure glaucomas. The CMG was precipitated in two normal human eyes, one with an artificially shortened anterior chamber and the other with an open chamber. In the eye with a narrow angle, the CMG was seen to fill the iridocorneal gap completely, blocking access to the trabecular meshwork from the central anterior chamber. The CMG may be implicated in the pathogeny of narrow angle and closed angle glaucoma. The two types of glaucoma may share a common mechanism depending on the thickness of the layer of CMG that precedes the exit pathways. Pretrabecular CMG thickness is a decisive determinant of the colloid-osmotic resistance of the gel to aqueous outflow, and this thickness is governed by, among other factors, the position of the iris relative to the posterior surface of the cornea. The formation of a thick layer of CMG in the narrow chamber angle prevents the normal anterior chamber pressure from exerting a backward displacement effect on the peripheral iris. Unopposed posterior chamber pressure may therefore force the peripheral iris forward, making angle closure likely.

Anterior Chamber↗

PubMed1990

A study of the electrokinetic and stability properties of nitrofurantoin suspensions. I: Electrokinetics.

An experimental investigation is described of the electrokinetic properties of nitrofurantoin. The experiments are based on measurements of electrophoretic mobility in suspensions of this drug and the calculation of its zeta potential (zeta) following two different procedures. A simple mechanism is proposed for explaining the sign of the surface charge of nitrofurantoin and its increase with the pH of the medium. The effect on zeta of changes in the concentration of NaCl, CaCl2, or AlCl3 is studied for different pH values of the suspensions. The behavior of zeta in the presence of NaCl shows evidence of Cl- adsorption on the particles, especially at low electrolyte concentrations. The AlCl3 salt was capable of reversing the sign of the surface charge, changing it to positive when a given, pH-dependent concentration was added to the suspension. The effect of this salt on zeta for different pH values was probably due to the hydrolysis of the cation Al3+. A significant increase of the negative surface charge was observed when a small amount (0.1%) of a synthetic polymer, Carbopol 934, was present in NaCl or AlCl3 systems. This appears to indicate that the large polymer molecules adsorb on nitrofurantoin particles as part of their stabilizing mechanism.

Acrylic Resins↗

PubMed1990

A study of the electrokinetic and stability properties of nitrofurantoin suspensions. II: Flocculation and redispersion properties as compared with theoretical interaction energy curves.

The stability and redispersion properties of nitrofurantoin dispersions are experimentally studied using simple but reproducible techniques. Solutions of different electrolyte concentrations and pH values are employed as dispersing media. The pH appears to be a determinant factor in the properties studied. Thus, with all the electrolytes considered (NaCl, CaCl2, and AlCl3, almost optimum redispersion was achieved when the pH was maintained close to neutrality. However, the important effect of AlCl3 as compared with the other electrolytes is also clearly demonstrated. Especially interesting are the results obtained when 0.1% Carbopol is added to the suspensions. The important effect of this polymer on the surface charge of nitrofurantoin, mainly at pH 7, manifests in excellent redispersion of the suspensions with either of the electrolytes employed. Using simple expressions that approximate the equations proposed in the DLVO theory of interaction between colloidal particles, the variation of the total interaction energy between nitrofurantoin particles is calculated as a function of interparticle distance. The results are compared with the experimental determinations of the sedimentation and redispersion properties of nitrofurantoin. Although some discrepancies between theory and experiments are found, the results indicate a reasonable agreement between the general features of both.

Acrylic Resins↗

PubMed1990

A network thermodynamic method for numerical solution of the Nernst-Planck and Poisson equation system with application to ionic transport through membranes.

Simple techniques of network thermodynamics are used to obtain the numerical solution of the Nernst-Planck and Poisson equation system. A network model for a particular physical situation, namely ionic transport through a thin membrane with simultaneous diffusion, convection and electric current, is proposed. Concentration and electric field profiles across the membrane, as well as diffusion potential, have been simulated using the electric circuit simulation program, SPICE. The method is quite general and extremely efficient, permitting treatments of multi-ion systems whatever the boundary and experimental conditions may be.

Biological Transport↗

PubMed1989

Simulation of concentration polarization in electrokinetic processes by network thermodynamic methods.

Simple techniques of network thermodynamics are used to study the influence of concentration polarization on the determination of electrokinetic properties of physical and biological membrane systems. A network model of the polarization phenomenon resulting from discontinuities in transport numbers between the membrane and their adjacent solutions is proposed. The concentration profiles and potential drop across the membrane in plugs of monodisperse polystyrene particles and in giant algal cells have been simulated using the electrical circuit simulation program SPICE.

Electrophysiology↗