Aging Effects in the Electrokinetics of Colloidal Iron Oxides

Abstract

We analyze in this contribution the effect of aging on the electrokinetic properties of magnetite (Fe 3 O 4 ) and hematite (α-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 (γ -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. C 2002 Elsevier Science

Synthesis and characterization of magnetite particles covered with α-trietoxysilil-polydimethylsiloxane

“…The decreased saturation magnetization can be attributed to surface effects, such as magnetically inactive layer producing disordered surface [31]. The oxidation of magnetite particles during the reaction with PDMS-TES could also occur [32]. Low hysteresis values were observed for all three samples (Ma, Ma-PDMS 1 , Ma-PDMS 2 ) and their characteristic curves were reversible at 300 K with coercivity around 1 G.…”

Stabilization of magnetorheological suspensions by polyacrylic acid polymers

“…The material not adhered to the flask wall in contact with the permanent magnet (B = 0.42 T) was pipetted off and substituted with ethanol. Water was not used in this step because the aging of magnetite surface in aqueous solutions can produce magnetite (γ -Fe 2 O 3 ), thus modifying the isoelectric point (see below) of the particles [19]. The process was considered finished after 15 cleaning cycles with ethanol.…”

Deposition of hematite particles on polypropylene walls in dynamic conditions

“…Thus, the iron oxide particles are charged positively and negatively below and above pH 6.2, respectively. The PZC/IEP values of hematite reported in the literature vary in a wide pH interval from 5.5 to 9.5 [28,29]. The factors that most often influence those values are impurities, the presence of specifically adsorbed ions, and temperature.…”