8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P5.188Wed 711:10-14:00The effect of absolute particle size on the metastability ofthe liquid phaseCharles Zukoski 1 and Ryan J. Larsen 11 Department of Chemical and Biomolecular Engineering, University of Illinois atUrbana-Champaign, 114 Roger Adams Lab, 600 S. Matthews 61801, Urbana, IL,United States of AmericaWhen a gas of attractive spheres is cooled, the system exhibits a phase transition. For manymolecular systems, this transition involves the formation of a liquid phase. However, in somesystems the gas/liquid phase transition is metastable with respect to a gas/crystal phase transition,and can only be observed when the cooling is performed rapidly. The phase behavior of a givensystem depends on the shape of the pair potential. In molecular systems described by the Leonard-Jones potential, the shape of the potential is independent of size of the molecule. By contrast,the shape of a van der Waals interaction potential of a colloidal particle depends on the absolutesize of the particle. This is because the colloidal potential is obtained from an integration ofdipoles within the particle, and the contribution of the shorter-ranged repulsions decreases relativeto the attractions as the size of the particle increases. An increase in particle size therefore has theeffect of ”exposing” shorter-range attractions. We demonstrate that the accompanying increase inthe particle attractions causes the liquid-liquid phase separation to become more metastable withrespect to the liquid-solid phase separation, and that this mechanism operates independently ofchanges to the range of the attraction. We also show that the existence of an equilibrium liquidstate is controlled by the ratio of two energy scales: the potential energy at particle contact, whichis characteristic of the solid state, and the integral of the potential, which is characteristic of theliquid state. Our results suggest the possibility of controlling metastability through small changesin the length scale the repulsion, and establish that the absolute particle size is a critical variable indetermining the metastable states of nanoparticle systems.188