Phase behavior of magnetic colloid-polymer mixtures: 2. A magnetic sensing coil study

The effects of dipolar interaction and isotropic attraction on the phase behavior of ferrofuids are separately investigated by studying a ferrofluid, containing oleic acid grafted magnetite particles, mixed with nonadsorbing polymer. The presence of polymer causes an effective isotropic attraction (depletion attraction) between the magnetic particles, with a strength determined by the polymer concentration. The magnetic interaction was modified by applying a magnetic field. A phase diagram is measured using a magnetic sensing coil, which allows measurement of the magnetic particle concentration in the separate phases without disturbing the sample. Moreover, it is shown that the sensing coil in principle also allows determination of the polymer concentration in the separate phases. In practice, however, this determination in complicated by the fact that phase separation can cause significant fractionation: particle sizes in the dilute phase were up to 33% lower than those in the unfractionated ferrofluid. In the absence of polymer, the ferrofluid is stable at all concentrations and field strengths. Phase separation, even in zero field, is observed above polymer concentrations of, typically, 50 g L-1. Applying a magnetic field of 28 kA m(-1) lowers the amount of polymer at which the ferrofluid destabilizes by approximately 20%. Excess oleic acid at a concentration of 160 g L-1 was also found to induce phase separation.