Ageing in the Inorganic Nanoworld: Example of Magnetite Nanoparticles in Aqueous Medium*

Changes in the inherent solid phase and aqueous interfacial properties of magnetite nanoparticles in a Massart type magnetic fluid were investigated. Magnetite nanoparticles were synthesized, purified, dialyzed against 0.001 mol dm–3 HCl and then stored at 4 °C for 6 years. The solid phase transformation of magnetite to maghemite and formation of akageneite shell on the magnetic core, as well as a slight increase in particle size were proved, causing no change in the superparamagnetic features of nanoparticles after storage as long as 6 years. The aqueous interfacial properties, however, were altered in a strange way. Surprisingly, neither the surface charge density nor the particle charge in acidic region showed significant changes, except for a definite decrease in pHpzc and pHiep from pH » 8 to » 7. Coalescence of akageneite shell during recrystallization resulted in a significant decrease of the specific surface area of the aged sample, and probably contributed to its declining colloidal stability.