On the Utilization of the Stockmayer Model for Ferrocolloids: Phase Transition at Zero External Field

Abstract

The possibility of interpreting the phase separation between a dilute phase and a dense phase, observed in ferrocolloid suspensions at zero external magnetic field, by using a simple one-component model is investigated. The molecules of the suspending solution are not explicitly taken into account; we thus deal with a model concerning only the colloidal particles. The model we consider is the Stockmayer fluid, where the interaction between the particles includes the short-range Lennard-Jones potential and the long-range dipole-dipole potential. In the present case the latter describes the interaction between the magnetic moments of the ferroparticles. The advantage of this model is that we can investigate precisely the effect of the dipolar interaction, which in any case is present in the system. Our aim is mainly to examine whether this long-range interaction can be responsible for the peculiarities of the phase transition on which we focus. The coexistence curve for the Stockmayer fluid is determined from a second-order virial expansion, and this is found to be accurate enough on the basis of a comparison with recent Monte Carlo results. It is found that although the 1/ r 3 part of the potential does not play any direct role, because of the average on the orientations, the form of the potential plays a significant role through the transformation one must perform to relate the variables of the model to the "true" variables of the actual system. Then it is found that the experiments cannot be interpreted without requiring the size polydispersity, which is introduced here only at a very qualitative level.