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

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 dodecylbenzenesulphonic acid (DBS). It is found that the sample presents an asymptotic critical temperature T∞ ≈ 40°C, 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 Hc(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.