Phase behavior of entropically controlled model colloidal dispersions

Michael David Heying, Purdue University


The fundamental understanding of physical phenomena has always been of importance to the utilization and discovery of new technologies. While the desired properties may change, the need to continually characterize the behavior of various systems does not diminish. Here, we discuss initial investigations of the thermophysical properties of various model colloidal dispersions. In several colloidal systems, the interactions that develop between colloidal particles are due mainly to entropic forces. Hence, these systems are conveniently modeled as fluids of non-interacting hard particles to which non-idealities can be later added. Despite their apparent simplicity, hard particle systems exhibit a wide variety of complex behaviors, many of which are seen in real systems. By continuing to refine the theoretical approaches to the study of hard particle fluids, we will better understand entropic forces in colloidal dispersions. We begin by examining a general binary hard particle system with various non-idealities. In one dimension, we exactly determine all the thermophysical properties of this system. We also investigate the effect that non-idealities, like van der Waals attractions, have on the phase behavior of this model system. This study emphasizes the need for accurate description of hard particle fluids in higher-dimensional systems. We next focus on improving the theoretical description of the hard particle fluid. The well-established statistical geometric approach of scaled particle theory (SPT) is chosen as the starting point due to its reliance on exact equations and intuitively physical arguments. We derive new additional exact equations for SPT, one of which contains a three-body correlation function that must be approximated. We generate a simple but accurate closure to this function. The resulting SPT predictions are now the order of accuracy of current equations of state. Finally, we extend the SPT approach to a hard sphere mixture. All of the exact equations for the pure component fluid are valid for the mixture. The resultant predictions are an improvement over the previous SPT formulations, though the range over which a solution of the equations can be obtained is limited by the particles' diameter ratio. For certain properties, SPT is now comparable to other current equations of state.




Corti, Purdue University.

Subject Area

Chemical engineering

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