Jesús Sigifredo Valencia Ríos

Synthetic high-charge organomica: effect of the layer charge and alkyl chain length on the structure of the adsorbed surfactants.

A family of organomicas was synthesized using synthetic swelling micas with high layer charge (Na(n)Si(8-n)Al(n)Mg(6)F(4)O(20)·XH(2)O, where n = 2, 3, and 4) exchanged with dodecylammonium and octadecylammonium cations. The molecular arrangement of the surfactant was elucidated on the basis on XRD patterns and DTA. The ordering conformation of the surfactant molecules into the interlayer space of micas was investigated by (13)C, (27)Al, and (29)Si MAS NMR. The arrangement of alkylammonium ions in these high-charge synthetic micas depends on the combined effects of the layer charge of the mica and the chain length of the cation. In the organomicas with dodecylammonium, a transition from a parallel layer to a bilayer-paraffin arrangement is observed when the layer charge of the mica increases. However, when octadecylammonium is the interlayer cation, the molecular arrangement of the surfactant was found to follow the bilayer-paraffin model for all values of layer charge. The amount of ordered conformation all-trans is directly proportional of layer charge.

Formation of Organo-Highly Charged Mica

The interlayer space of the highly charged synthetic Na-Mica-4 can be modified by ion-exchange reactions involving the exchange of inorganic Na(+) cations by surfactant molecules, which results in the formation of an organophilic interlayer space. The swelling and structural properties of this highly charged mica upon intercalation with n-alkylammonium (RNH(3))(+) cations with varying alkyl chain lengths (R = C12, C14, C16, and C18) have been reported. The stability, fine structure, and evolution of gaseous species from alkylammonium Mica-4 are investigated in detail by conventional thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), in situ X-ray diffraction (XRD), and solid-state nuclear magnetic resonance (MAS NMR) techniques. The results clearly show the total adsorption of n-alkylammonium cations in the interlayer space which expands as needed to accommodate intercalated surfactants. The surfactant packing is quite ordered at room temperature, mainly involving a paraffin-type bilayer with an all-trans conformation, in agreement with the high density of the organic compounds in the interlayer space. At temperatures above 160 °C, the surfactant molecules undergo a transformation that leads to a liquid-like conformation, which results in a more disordered phase and expansion of the interlayer space.

EFECTO DE LAS CONDICIONES DE SÍNTESIS EN MEDIO ACUOSO PARA LA OBTENCIÓN DE ÓXIDOS POLICATIÓNICOS

En este trabajo se obtuvieron dos oxidos tipo perovskita basados en el sistema La0.75Sr0.25Co0.5Fe0.5O3, mediante el uso de acido citrico y propionico, a fin de evaluar el potencial de cada especie para formar compuestos de coordinacion y su efecto en la produccion de los oxidos ceramicos finales. Los resultados del modelamiento para establecer y controlar las principales especies quimicas en solucion acuosa en funcion del pH, confirmaron que este parametro es crucial para establecer las condiciones ideales dehomogeneidad. Los analisis de difraccion de rayos X, confirman la obtencion de materiales con tamanos de cristalito diferentes, de 10.0 nm para el caso de la muestra conseguida con acido citrico y de 42.0 nm para la muestra con acido propionico. El refinamiento de los resultados de difraccion y la a en las bases de datos de la ICSD, permitieron identificar la fase cristalina con un alto grado de homogeneidad y pureza en ambos casos. Los resultados de microscopia electronica de barrido y de microanalisis por EDS, mostraron algunas similitudes a nivel superficial y textural entre los dosmetodos de obtencion, principalmente relacionadas con el tamano de cristal y con el grado de densificacion de los oxidos.