Mario Visca

Preparation of uniform colloidal dispersions by chemical reactions in aerosols. I. Spherical particles of titanium dioxide

Abstract Spherical titanium dioxide particles were prepared by hydrolysis of aerosols consisting of liquid titanium(IV) compounds. The droplets of the latter were nucleated with AgCl in a falling film generator and subsequently hydrolyzed in specially designed chambers kept at different temperatures. When titanium(IV) ethoxide was used as starting material, TiO2 spheres of narrow size distribution were obtained with modal diameters ranging between 0.06 and 0.6 μm. The size of the particles could be altered by changing the temperature of the falling film generator, the flow rate of the carrier gas, and the AgCl nuclei concentration. The titanium dioxide particles so prepared were essentially amorphous. Titanium tetrachloride was also used as the liquid aerosol material. Owing to the vapor pressure characteristics of this compound, much higher concentration of the droplets was obtained in the same equipment. However, the titanium dioxide powder, resulting from the hydrolysis of TiCl4 aerosols, consisted of spheres of broader size distributions. The crystallinity of the solid particles could be changed by heating of the solid aerosol stream at different temperatures. The titanium dioxide powders, prepared by the aerosol technique, could be readily dispersed in water. The particle charge was pH dependent and showed an electrophoretic isoelectric point between pH 4.5 and 5.5.

Cation adsorption at the TiO2-water interface

Abstract The interactions between hydrated Mn 2+ or Cu 2+ and the surface of porous TiO 2 have been investigated by using the electron spin resonance (ESR) technique in conjunction with classical colloid chemistry. Mn(H 2 O) 2+ 6 is specifically adsorbed at the surface. It shows a strong dependence on the surface potential since the interacting Mn 2+ amount sharply increases at a pH higher than the isoelectric point (i.e.p.); on the contrary, noninteracting Mn 2+ is only found at pH 2 surface. Four different ESR signals are identified, corresponding to different Cu 2+ environments. An anomalous freezing behavior of water is found after adsorption into the pores of charged TiO 2 .

Nitroxide radicals as probes for water mobility in perfluorinated membrane

Abstract In this paper the properties of water and of large organic ions or neutral molecules confined within the perfluorinated membrane Nafion are studied by electron spin resonance of nitroxide probes. The membrane was used in conditions of full hydration and after exchange with mono- and di-positive ions. The analysis was mainly based on the ESR lineshapes and the correlation times for the motion of positive, neutral, and negative radicals. The dependence of the lineshape on the charge of the probe and on the charge of metal ions was analyzed as a function of temperature. The behavior of adsorbed water was similar to that observed in other systems in which the liquid was finely dispersed. The decreased polarity of sorbed water as a consequence of the lower strength of the hydrogen bond, suggested by A. Falk (Canad. J. Chem. 58, 1495 (1980)) was confirmed from the AN values of nitroxides. These were located in ionic clusters and were exposed to the interactions with pendant fluorocarbon chains. The extent of these interactions depended on the charge of either the nitroxides or the exchanged metal ion. The ESR patterns invariantly arose from the superposition of many absorptions due to radicals showing different motional rates and in slow exchange among different sites. This indicated the presence of ionic clusters of variable sizes in which water moved with different diffusion coefficients. The comparison between the ESR results and the previously reported NMR results confirmed the nonuniformities in the membrane structure.

Interactions in mixed fluorocarbon latex-hematite dispersions

Abstract Interactions of a monodispersed polymer latex (polytetrafluoroethylene) and spherical hematite particles of narrow size distribution of comparable size were studied as a function of pH and ionic strength over a wide range of surface potentials. The stability of single and mixed dispersions was determined in the presence of varying concentrations of KNO3. Selective coagulation was observed when the particles were of comparable surface (zeta) potentials, while heterocoagulation took place when these potentials differed significantly. The theoretical calculations were in agreement with the behavior of the mixed systems.

Mass transport electrophoresis: An improved apparatus

Abstract A mass-transport electrophoresis apparatus is proposed by which high stability with time of the impressed current is achieved. This is obtained by introducing a salt bridge between the inner electrode compartment and the chamber where enrichment of the colloid occurs. This disposition allows the use of concentrated solutions of electrolytes in which reversible behavior of the employed electrodes is easily obtained; in the mean time, it is possible to avoid both the growth of films at the electrode surface and the electrical transport of the ions produced by the electrode reaction into the colloidal suspension, thus preventing possible coagulation phenomena of the colloid.