Patching up dipoles: can dipolar particles be viewed as patchy colloids?

Abstract

We investigate whether the liquid–vapour phase transition of strongly dipolar fluids can be understood using a model of patchy colloids. These consist of hard spherical particles with three short-ranged attractive sites (patches) on their surfaces. Two of the patches are of type A and one is of type B. Patches A on a particle may bond either to a patch A or to a patch B on another particle. Formation of an AA (AB) bond lowers the energy by ε _AA (ε _AB ). In the limit , this patchy model exhibits condensation driven by AB-bonds (Y-junctions). Y-junctions are also present in low-density, strongly dipolar fluids, and have been conjectured to play a key role in determining their critical behaviour. We map the dipolar Yukawa hard-sphere (DYHS) fluid onto this 2A + 1B patchy model by requiring that the latter reproduce the correct DYHS critical point as a function of the isotropic interaction strength ε_Y. This is achieved for sensible values of ε _AB and the bond volumes. Results for the internal energy and the particle coordination number are in qualitative agreement with simulations of DYHSs. Finally, by taking the limit , we arrive at a new estimate for the critical point of the dipolar hard-sphere fluid, which agrees with extrapolations from simulation.

Keywords:

• liquid–vapour transition
• self-assembly
• magnetic fluids

Acknowledgement

Financial support from the Foundation of the University of Lisbon and the Portuguese Foundation for Science and Technology (FCT) under Contracts nos. POCI/FIS/55592/2004, POCTI/ISFL/2/618 and PTDC/FIS/098254/2008, as well as through Pluriannual contracts with CFTC, is gratefully acknowledged.