(113c) Direct Simulation Approach for Computing the Interfacial Free Energy for Nucleation From Solution

Authors: 
Duff, N., University of California, Santa Barbara
Peters, B., University of California Santa Barbara

Nucleation from solution is important in many pharmaceutical
crystallization processes, biomineralization, materials synthesis, and self-assembly1.  Simulation methodology has progressed rapidly for nucleation in pure-component and implicit solvent systems; however there has been relatively little progress for simulating nucleation from a solution with explicit solvent molecules.  The impasse stems from problems in combining the specialized methods for simulating rare barrier-crossing events, with specialized particle insertion algorithms for maintaining a constant chemical potential driving force (supersaturation) during nucleation2.  At present there are no algorithms that simultaneously address these two central aspects of nucleation from solution.  We present a lattice gas - Potts model (LGP)3 to aid in the development of new simulation strategies for nucleation from solution.  To our knowledge the LGP is the first general lattice model to capture common crystallization phase diagram features like a eutectic point and solute/solvent melting points.  Simulations of nucleation in the LGP reveal a competition between nucleation of amorphous and crystalline particles.  A simple explanation for this commonly observed phenomenon is developed from classical nucleation theory.  Finally, we demonstrate a new "mitosis" simulation method for directly computing the interfacial free energy of a nucleus with the solution.  The method entirely avoids the need for particle insertion or particle identity changes, so it should be applicable for atomistic simulations of real crystal nucleation processes in solution.  The new approach should
provide a path forward for using simulations to help engineer solvent additives
to direct polymorph selection and self-assembly.

1.         Lovette MA, Browning AR, Griffin DW, Sizemore JP, Snyder RC, Doherty MF.
Crystal Shape Engineering. Industrial & Engineering Chemistry Research. 2008;47(24):9812-9833.

2.         Frenkel
D, Smit B. Understanding Molecular Simulation. 2nd ed. London: Academic
Press, Inc.; 2002.

3.         Chaikin
PM, Lubensky TC. Principles of condensed matter physics. New York:
Cambridge University Press; 2000.