Is Ice Nucleation by Organic Crystals Nonclassical? An Assessment of the Monolayer Hypothesis of Ice Nucleation

Abstract

Potent ice nucleating organic crystals display an increase in nucleation efficiency with pressure and memory effect after pressurization that set them apart from inorganic nucleants. These characteristics were proposed to arise from an ordered water monolayer at the organic−water interface. It was interpreted that ordering of the monolayer is the limiting step for ice nucleation on organic crystals, rendering their mechanism of nucleation nonclassical. Despite the importance of organics in atmospheric ice nucleation, that explanation has never been investigated. Here we elucidate the structure of interfacial water and its role in ice nucleation at ambient pressure on phloroglucinol dihydrate, the paradigmatic example of outstanding ice nucleating organic crystal, using molecular simulations. The simulations confirm the existence of an interfacial monolayer that orders on cooling and becomes fully ordered upon ice formation. The monolayer does not resemble any ice face but seamlessly connects the distinct hydrogen-bonding orders of ice and the organic surface. Although large ordered patches develop in the monolayer before ice nucleates, we find that the critical step is the formation of the ice crystallite, indicating that the mechanism is classical. We predict that the fully ordered, crystalline monolayer nucleates ice above −2 °C and could be responsible for the exceptional ice nucleation by the organic crystal at high pressures. The lifetime of the fully ordered monolayer around 0 °C, however, is too short to account for the memory effect reported in the experiments. The latter could arise from an increase in the melting temperature of ice confined by strongly icebinding surfaces.

The role of structural order in heterogeneous ice nucleation

“…In fact, the formation of more-or-less ordered monolayers of water at the interface between the ice-nucleating agent and the water phase has been recently debated in the context of whether or not the nucleation of ice follows a classical pathway in the case of e.g. phloroglucinol dihydrate monolayers 92 as well as wurtzite-structured surfaces. 93 …”

Unraveling the Mechanism of Ice Nucleation by Mica (001) Surfaces

“…Good ice nuclei can induce freezing at temperatures as warm as ∼−2 °C . Identifying substances that can serve as effective ice nuclei, as well as exploring the mechanisms of heterogeneous nucleation has been the aim of a good deal of research, involving both experiments ,− and more recently molecular simulations. − …”

Accurate prediction of ice nucleation from room temperature water

“…We note that this is not meant to act as direct competition to IcePic, as it is not generally reasonable to expect humans to match performance levels of machine learning models in regression. However, given that the water contact layer is widely discussed ( 36 , 37 ) and explicitly used to explain heterogeneous ice nucleation in the literature ( 20 , 21 , 23 , 26 , 34 , 38 – 44 )—discussion dates as far back as the 1940s with the origin of substrate lattice match ( 32 , 45 )—it is reasonable to expect some level of human expertise here. Even if it cannot match IcePic, assessing human performance can uncover gaps in our knowledge that might prove useful to address and thereby give new physical insights into ice nucleation.…”

“…It therefore remains necessary to determine each material’s effect on ice nucleation on a case-by-case basis. Moreover, building a detailed understanding of the ice nucleating ability of a single material often requires both experiment and simulation ( 20 – 29 ). The field is thus in a difficult situation: one cannot determine nucleation behavior a priori; thus, it must be established on a case-by-case basis, but even this can be extremely difficult and time consuming.…”