Oriented aggregation of lepidocrocite and impact on surface charge development

Philipp A. Kozin, German Salazar-Alvarez, and Jean-François Boily

Langmuir 30 (2014) 9017–9021

DOI: 10.1021/la500507e

Abstract

The impact of lepidocrocite (γ-FeOOH) nanoparticle aggregation on mineral surface charge development was resolved in aqueous solutions of NaCl and NaClO4. Synthetic rod-like particles exhibiting charged edge (100) and neutrally/low-charged (010) faces self-aggregated in salt-free solutions. Aggregation was notably imaged by high-resolution transmission electron microscopy, and inferred by decreases in N2(g)-B.E.T. specific surface area from 94 m2/g to 77 m2/g after 12 months, and to 66 m2/g after 33 months storage. Potential determining (H+, OH–) ions loadings in the 4–11 pH range were unchanged only if the particles remained aggregated in NaCl but only if they were disaggregated in NaClO4. These differences, alongside molecular simulations and experimental ion loadings resolved in other studies from our group, point to important controls on background electrolyte ion identity on the aggregation and charge development in lepidocrocite. These results may apply further to other mineral surfaces of comparable surface (hydr)oxo populations.

Hamaker Constants of Iron Oxide Nanoparticles

http://pubs.acs.org/doi/abs/10.1021/la201387d

Langmuir, 2011, 27 (14), pp 8659–8664
Bertrand Faure, German Salazar-Alvarez, and Lennart Bergström

DOI: 10.1021/la201387d

Abstract:

The Hamaker constants for iron oxide nanoparticles in various media have been calculated using Lifshitz theory. Expressions for the dielectric responses of three iron oxide phases (magnetite, maghemite, and hematite) were derived from recently published optical data. The nonretarded Hamaker constants for the iron oxide nanoparticles interacting across water, A_1w1 = 33 – 39 zJ, correlate relatively well with previous reports, whereas the calculated values in nonpolar solvents (hexane and toluene), A₁₃₁ = 9 – 29 zJ, are much lower than the previous estimates, particularly for magnetite. The magnitude of van der Waals interactions varies significantly between the studied phases (magnetite < maghemite < hematite), which highlights the importance of a thorough characterization of the particles. The contribution of magnetic dispersion interactions for particle sizes in the superparamagnetic regime was found to be negligible. Previous conjectures related to colloidal stability and self-assembly have been revisited on the basis of the new Lifshitz values of the Hamaker constants.