Suppression of sub-surface freezing in free-standing thin films of a coarse-grained model of water

• Published in: Physical chemistry chemical physics : PCCP Vol. 16; no. 47; pp. 25916 - 25927
• Main Authors: Haji-Akbari, Amir, DeFever, Ryan S, Sarupria, Sapna, Debenedetti, Pablo G
• Format: Journal Article
• Language: English
• Published: England 01.01.2014
• Subjects: Asphericity; Atmospherics; Free energy; Freezing; Models, Chemical; Molecular dynamics; Molecular Dynamics Simulation; Nuclei; Sampling; Surface Properties; Thin films; Water - chemistry
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c4cp03948c

Freezing in the vicinity of water-vapor interfaces is of considerable interest to a wide range of disciplines, most notably the atmospheric sciences. In this work, we use molecular dynamics and two advanced sampling techniques, forward flux sampling and umbrella sampling, to study homogeneous nucleation of ice in free-standing thin films of supercooled water. We use a coarse-grained monoatomic model of water, known as mW, and we find that in this model a vapor-liquid interface suppresses crystallization in its vicinity. This suppression occurs in the vicinity of flat interfaces where no net Laplace pressure in induced. Our free energy calculations reveal that the pre-critical crystalline nuclei that emerge near the interface are thermodynamically less stable than those that emerge in the bulk. We investigate the origin of this instability by computing the average asphericity of nuclei that form in different regions of the film, and observe that average asphericity increases closer to the interface, which is consistent with an increase in the free energy due to increased surface-to-volume ratios. An 850-molecule crystalline nucleus identified in simulations of a 5 nm film at 235 K showing liquid-like and solid-like molecules in blue and fuchsia, respectively.