S. F. J. Cox

Crystal Nucleation in Liquids: Open Questions and Future Challenges in Molecular Dynamics Simulations

The nucleation of crystals in liquids is one of nature’s most ubiquitous phenomena, playing an important role in areas such as climate change and the production of drugs. As the early stages of nucleation involve exceedingly small time and length scales, atomistic computer simulations can provide unique insights into the microscopic aspects of crystallization. In this review, we take stock of the numerous molecular dynamics simulations that, in the past few decades, have unraveled crucial aspects of crystal nucleation in liquids. We put into context the theoretical framework of classical nucleation theory and the state-of-the-art computational methods by reviewing simulations of such processes as ice nucleation and the crystallization of molecules in solutions. We shall see that molecular dynamics simulations have provided key insights into diverse nucleation scenarios, ranging from colloidal particles to natural gas hydrates, and that, as a result, the general applicability of classical nucleation theory has been repeatedly called into question. We have attempted to identify the most pressing open questions in the field. We believe that, by improving (i) existing interatomic potentials and (ii) currently available enhanced sampling methods, the community can move toward accurate investigations of realistic systems of practical interest, thus bringing simulations a step closer to experiments.

Molecular radical models for the muonium centres in solids

Abstract Molecular radical models are presented for the various muonium-related paramagnetic centres observed in the elemental and compound semiconductors, and for those which are currently being reported in other crystalline solids, notably halides. The measured coupling constants are shown to place tight constraints on the intrinsic geometries which need be considered, i.e. on the crystallographic sites and local relaxation of the lattice, as well as on the nature of the singly occupied molecular orbital. The role of tunnelling states is emphasized.

The Many Faces of Heterogeneous Ice Nucleation: Interplay Between Surface Morphology and Hydrophobicity

What makes a material a good ice nucleating agent? Despite the importance of heterogeneous ice nucleation to a variety of fields, from cloud science to microbiology, major gaps in our understanding of this ubiquitous process still prevent us from answering this question. In this work, we have examined the ability of generic crystalline substrates to promote ice nucleation as a function of the hydrophobicity and the morphology of the surface. Nucleation rates have been obtained by brute-force molecular dynamics simulations of coarse-grained water on top of different surfaces of a model fcc crystal, varying the water-surface interaction and the surface lattice parameter. It turns out that the lattice mismatch of the surface with respect to ice, customarily regarded as the most important requirement for a good ice nucleating agent, is at most desirable but not a requirement. On the other hand, the balance between the morphology of the surface and its hydrophobicity can significantly alter the ice nucleation rate and can also lead to the formation of up to three different faces of ice on the same substrate. We have pinpointed three circumstances where heterogeneous ice nucleation can be promoted by the crystalline surface: (i) the formation of a water overlayer that acts as an in-plane template; (ii) the emergence of a contact layer buckled in an ice-like manner; and (iii) nucleation on compact surfaces with very high interaction strength. We hope that this extensive systematic study will foster future experimental work aimed at testing the physiochemical understanding presented herein.

Shallow donor state of hydrogen in indium nitride

The nature of the electron states associated with hydrogen in InN has been inferred by studying the behavior of positive muons, which mimic protons when implanted into semiconductors. The muons capture electrons below 60 K, forming paramagnetic centers with a binding energy of about 12 meV. Together with an exceedingly small muon-electron hyperfine constant indicative of a highly delocalized electron wave function, the results confirm the recently predicted shallow-donor properties of hydrogen in InN.

Molecular simulations of heterogeneous ice nucleation. I. Controlling ice nucleation through surface hydrophilicity

Ice formation is one of the most common and important processes on earth and almost always occurs at the surface of a material. A basic understanding of how the physicochemical properties of a materials surface affect its ability to form ice has remained elusive. Here, we use molecular dynamics simulations to directly probe heterogeneous ice nucleation at a hexagonal surface of a nanoparticle of varying hydrophilicity. Surprisingly, we find that structurally identical surfaces can both inhibit and promote ice formation and analogous to a chemical catalyst, it is found that an optimal interaction between the surface and the water exists for promoting ice nucleation. We use our microscopic understanding of the mechanism to design a modified surface in silico with enhanced ice nucleating ability.

Muon level-crossing spectroscopy of organic free radicals

Abstract Muon level-crossing spectroscopy has been applied to the study of muonium-substituted radicals formed in liquid benzene, hexadeuterobenzene, furan, 2-methylpropene, 2,3-dimethyl-2-butene, and gaseous ethene. The magnitudes and signs of the proton and deuteron hyperfine constants are reported, and are discussed in terms of isotope effects and intramolecular motion.

Molecular simulations of heterogeneous ice nucleation. II. Peeling back the layers

Coarse grained molecular dynamics simulations are presented in which the sensitivity of the ice nucleation rate to the hydrophilicity of a graphene nanoflake is investigated. We find that an optimal interaction strength for promoting ice nucleation exists, which coincides with that found previously for a face centered cubic (111) surface. We further investigate the role that the layering of interfacial water plays in heterogeneous ice nucleation and demonstrate that the extent of layering is not a good indicator of ice nucleating ability for all surfaces. Our results suggest that to be an efficient ice nucleating agent, a surface should not bind water too strongly if it is able to accommodate high coverages of water.

Hyperfine constants for the ethyl radical in the gas phase

Abstract Muon spin rotation and level-crossing spectroscopy have been used to measure the muon, proton, deuteron and 13C hyperfine coupling constants for the isotopically substituted ethyl radicals CH2CH2Mu, CD2CD2Mu and 13CH213CH2Mu in the gas phase.

Structure and intramolecular motion of muonium-substituted cyclohexadienyl radicals

Abstract Hyperfine coupling constants of isotopically substituted cyclohexadienyl radicals have been measured as a function of temperature by muon spin rotation and level-crossing spectroscopy. Data are presented for the muon, proton and deuteron hyperfine couplings of the methylene groups in C6H6Mu and C6D6Mu, and also for all the 13C hyperfine couplings of 13C6H6Mu. Comparison of the results with semi-empirical calculations supports a planar ring configuration with complex motion of the methylene substituents.

Intramolecular motion in the tert-butyl radical as studied by muon spin rotation and level-crossing spectroscopy

Abstract Muon spin rotation and muon level-crossing spectroscopy have been used to determine muon ( A μ ) and proton ( A p ) hyperfine coupling constants for the muon-substituted tert-butyl radical (CH 3 ) 2 CCH 2 Mu over a wide range of temperature in isobutene. A p (CH 3 ) is almost constant, but A μ (CH 2 Mu) falls and A p (CH 2 Mu) rises with increasing temperature, consistent with a preferred conformation of the methyl group in which the CMu bond is coplanar with the symmetry axis of the radical orbital. The A μ data cover the temperature range from 297 K down to 43 K, where the solution is frozen. There is a discontinuity in A μ at the melting point, as well as a change in temperature dependence. It is suggested that the potential barrier for methyl group rotation is lower in the liquid due to simultaneous inversion at the radical centre, and that the inversion mode is somewhat inhibited in the solid. The best fit of the liquid-phase data indicates a V 2 barrier of 1.8 kJ mol −1 , and is consistent with a long CMu bond and a tilt of the CH 2 Mu group in the direction that brings the Mu atom closer to the radical centre.