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Influence of metal oxide nanoparticles concentration on their zeta potential

Wang, Nan, Hsu, Chien, Zhu, Lihua, Tseng, Shiojenn, Hsu, Jyh-Ping
Journal of colloid and interface science 2013 v.407 pp. 22-28
carbon dioxide, hydroxides, iron oxides, isoelectric point, nanoparticles, neutralization, pH, titanium dioxide, zeta potential
In an attempt to estimate the zeta potential of various metal oxide nanoparticles (NPs) dispersed in water, it is interesting to observe that both the magnitude and the sign of this property depend highly upon their concentration. For example, in the case of naked TiO₂ at pH 6, the zeta potential increased from −6.7 to 8.2mV as the particle concentration varied from 0.5 to 5mgL⁻¹. As a result, the isoelectric points of naked TiO₂, Fe₃O₄, Fe(OH)₃, and Al₂O₃-coated TiO₂ could deviate ca. one, one, two, and three pH units, respectively, depending upon the particle concentration. We showed that these behaviors arise mainly from that the dissolved ambient CO₂ reacts with the particle surface functional groups to form MOCO₂ ⁻, which neutralizes or even overcompensates the particle surface charge. The surface density of M–OCO₂ ⁻, [M-OCO2(-)](s) depends upon the particle concentration; if it is sufficiently high, [M-OCO2(-)](s) becomes negligible, so is its influence on the zeta potential. We concluded that the zeta potential measurements for the tested NPs are reliable only if their concentration exceeds a certain level. This also applies to other metal oxides or hydroxides, the surface of which reacts appreciably with dissolved CO₂. The results gathered are of practical significance in estimating the surface properties of unknown and/or newly synthesized NPs since conventional measurements are usually made at dilute particle concentrations.