PARTIAL HYDROLYSIS OF FERRIC-CHLORIDE SALT - STRUCTURAL INVESTIGATION BY PHOTON-CORRELATION SPECTROSCOPY AND SMALL-ANGLE X-RAY-SCATTERING

Speciation resulting from fast hydrolysis of dilute ferric chloride salt (0.1 M) by NaOH at 25-degrees-C was studied versus the hydrolysis ratio n = NaOH/Fe from 0 to 3.0 at different aging time from t = 400 s up to 1 day by using quasi-elastic light scattering and small-angle X-ray scattering. Before the flocculation threshold (n approximately 2.7), large colloids are formed during the hydrolysis. Photon correlation spectroscopy showed that at t = 400 s the size range varied from approximately 10 nm (n = 1.0) up to 700 nm (n = 2.7). At larger times the colloid size rapidly decreased during the first 30 min. The mean sizes were centered around approximately 10 nm. Small-angle X-ray scattering experiments carried out with n = 1.0 to n = '3.0 permitted description of the semilocal structure of the colloids. The scattering curves are characteristic of colloids formed by aggregates of subunits. Whatever the value of n and the time (t less-than-or-equal-to 1 h), the submit size is equal to 16 angstrom in diameter. It is not modified by pH variation for t less-than-or-equal-to 1 h). The semilocal structure of the colloids depends on the time and n. For n = 1.0 and t = 400 s, locally the colloids are formed of linear aggregates. This local shape is originated from long-range magnetic dipolar interactions, which are known to linearize the fractal aggregates. The same sols aged for 1 h exhibited slightly more branched aggregates. From n = 2.0 up to n = 3.0 fractal structures occurred in the suspensions with fractal dimensions D(f), which was varied from approximately 1.7 (n = 2.0 and 2.5) up to approximately 2 (n approximately 3.0).