Molecular Dynamics Study of Hexadecane Droplets: Kinetics and Mechanism of Freezing

Abstract

This work presents results, aimed at identifying the crystallization mechanism of surfactant-stabilized hexadecane-in-water droplet with 15 nm diameter at the molecular level. The question is addressed by atomistic molecular dynamics simulations of models consisting of ca. 2 million atoms. To represent as closely as possible real-world systems, a procedure for constructing droplets of different sizes is developed, allowing control of the surfactant surface coverage. Two crystallization protocols are applied: slow freezing, in which the surface solidifies first, followed by relaxation and subsequent cooling to induce crystallization in the bulk, and fast freezing, in which the droplet is directly cooled to the final temperature, leading to simultaneous surface and core crystallization. A few key results may be outlined. The droplet surface always freezes first. Nucleation occurs stochastically at multiple independent sites, gradually propagating over the surface. Both shell and core undergo heterogeneous nucleation initiated by surfactant molecules. The nuclei in the core typically form close to the surface. The cooling procedure affects the spatial freezing pathway and molecular ordering in the drop core. The preferred orientation of bulk crystallites relative to the originating surface is nearly perpendicular. Once solidified, the surface alone is sufficient to induce droplet deformation to a triangular prism-like shape, additionally stabilized by the bulk crystallites. The deformation is a clear indication of a rotator phase, stable over hundreds of nanoseconds, further confirmed by the fraction of gauche conformations, P2 order parameters, and radial distribution functions. These findings agree with and complement experimental data and provide molecular-level verification of the experimentally observed fundamental difference in the stability of hexadecane rotator phase: transient in bulk and stable under micro- and nanoconfinement. This is the fundamental knowledge on microscopic freezing mechanisms of hexadecane and similar even-parity alkane-based materials at interfaces.