Thixotropic gelation of nanoparticles in strongly acidic solution

Abstract

Colloid system of dispersed particles in highly acidic solution with a large amount of protons and anions shows different behavior from those in solutions with a small amount of ions. This work focuses on the gelation behavior of the silica suspension in concentrated sulfuric acid solution of 1.5 and 5.0 M concentration. The gelation behavior of silica suspension was investigated rheologically by small amplitude dynamic oscillation measurement. The suspension was prepared from fumed silica ($Aerosil^®$ 200) or commercial colloidal silica ($Ludox^®$ series), and sulfuric acid. Its gel strength and gelation rate were measured from the time evolution of the storage modulus. The gel with a large amount of silica or high sulfuric acid concentration has high gel strength and fast gelation rate. Gel prepared from the silica particles with a small diameter showed high gel strength and fast gelation rate. It implies that the high gel strength is achieved with a low content of silica with a small diameter. The gels prepared from the colloidal silica with the same total surface area in a unit volume has the similar gelation behavior such as gel strength and gelation rate within the silica content range of the same gel structure. The deionized commercial $Ludox^®$ was used in order to achieve high gel strength. This means that sodium ion in the $Ludox^®$, which plays a stabilizing role, should be removed in order to form a strong gel. The positively charged surface and chloride stabilizing ion resulted in the low gel strength. The gelation time was determined by measuring the time evolution of the ratio of loss modulus and storage, in which Winter`s criterion was applied. The short gelation time and the high gel strength were shown at high temperature. The apparent activation energies were evaluated from the temperature dependence of the gelation time. The values are in the range of 25 to 27 kJ/mol and are invariant with silica content and sulfuric acid concentration. They...