Magnetically induced phase condensation with asimptotic critical temperature in an aqueous magnetic colloid

V. Socoliuc - D. Bica - L. Vékás

"Petru Poni" Institute of Macromolecular Chemistry, 41A Grigore Ghica Voda Alley, 700487 Iasi, Romania }{ Laboratory of Magnetic Fluids, Center for Fundamental and Advanced Technical Research, Romanian Academy -- Timisoara Branch, Bv. M. Viteazu 24, Timisoara RO-300223, Romania

Abstract
We investigated the phenomenon of magnetically induced phase condensation in a water-based magnetic nanocolloid with magnetite nanoparticles of 5 nm in diameter, sterically stabilized by a double surfactant layer of myristic acid (MA) and dodecyl-benzenesulphonic acid (DBS). It is found that the sample presents an asymptotic critical temperature T_∞ ≈ 40^oC, above which no condensation occurs, no matter how intense the external magnetic field is. Below the asymptotic critical temperature there is a temperature dependent critical field H_c (T), below which no condensation occurs and above which the amount and the size (in the micron range) of the condensed phase drops increase with the field intensity. The critical field exponentially increases as the temperature approaches T_∞. From DLS investigations it was found that the magnetic nanoparticles spontaneously agglomerated, most likely due to van der Waals and/or hydrophobic interactions, into clusters whose population diminished with the increasing temperature. Figs 3, Refs 9.