Aging effects in the electrokinetics of colloidal iron oxides.

We analyze in this contribution the effect of aging on the electrokinetic properties of magnetite (Fe(3)O(4)) and hematite (alpha-Fe(2)O(3)). In both cases, high-purity commercial samples and monodisperse synthetic particles were studied. Commercial magnetite showed a rather erratic dependence of its electrophoretic mobility u(e) with the concentration of NaCl. Furthermore, sufficient concentrations of the latter were able to change the sign of the mobility. When KNO(3) solutions were used, although no such change was observed, no clear effect of [KNO(3)] on the mobility was found, and, in addition, an intense aging effect was detected, as the mobility became increasingly positive in suspensions that were stored over 1 day. The picture was radically different with synthetic magnetite spheres, as the expected overall decrease of u(e) with either NaCl or KNO(3) concentration was measured. However, also in this case the aging effect was clearly observed: u(e) tended in this case to more negative values upon suspension storage, and a steady value of the mobility was reached only after 5 days in NaCl (and even longer in KNO(3) solutions). Because of the crystal structure similarities between magnetite and maghemite (gamma-Fe(2)O(3)), it has been shown that the final step of magnetite oxidation is maghemite. This is confirmed in the present study, as the mobility-pH trends of magnetite progressively approach those of maghemite after about 7 days of storage. Since hematite is chemically more stable than magnetite, our study focused in this case on the comparison between commercial and synthetic particles. The former showed a negative mobility at pH 5.5 under all conditions, suggesting an isoelectric point well below the value accepted for hematite (>/=7). The effect of aging on commercial samples was again very significant, as u(e) decreased in absolute value, apparently without limit as the time since preparation was longer. In contrast, synthetic hematite showed a more predictable dependence on ionic strength, and more limited aging effects, as u(e) reached equilibrium values after around 5 days in NaCl; longer times were required in KNO(3) solutions.