Session S31: Ice Nucleation, Amorphous Ices and the Role of Interfaces

The Many Faces of Ice and Nonlinear Interferometry.

Ice is likely the most ubiquitous solid in the Universe, yet even here on Earth its surface contains many mysteries. At atmospheric pressure, the stable form of ice is hexagonal ice; known as $I_{h}$. This contribution will present data about (i) equilibrium growth at the ice-water interface, (ii) procedures to generate \textit{any} targeted ice face, and (iii) vibrational spectra of the ice-air interface. Contrary to common belief, the stable ice-water interfaces does not consist of the basal face; rather it consists of pyramidal or prism faces. Growth results from a balance between the molecular density and the top half-bilayer configuration. Arguments reminiscent of Pauling's residual entropy of ice generate the configurational contribution. Prism faces are favored due to greater entropy. Ice grows cryptomorphologically: the macroscopic sample does not reveal the crystalline axes. Locating the crystal axes as well as generating authentic faces for fundamental studies use a combination of the birefringence of ice and etch profiles. Surface vibrational spectroscopy supports an ice model consisting of extended, cooperative motion and beyond-bonding-partner determination of hydrogen bond strength. The surface vibrational spectrum is probed with the nonlinear spectroscopy sum frequency generation (SFG). Currently, nonlinearity limits use of SFG to diagnose interactions. This limitation can be circumvented by measuring the full, complex spectrum. We will report initial results from a newly invented nonlinear interferometer that reveals the full complex spectrum.   

Direct Calculation of the Rate of Homogeneous Ice Nucleation for a Molecular Model of Water

Ice formation is ubiquitous in nature, with important consequences in many systems and environments. However, its intrinsic kinetics and mechanism are difficult to discern with experiments. Molecular simulations of ice nucleation are also challenging due to sluggish structural relaxation and the large nucleation barriers, and direct calculations of homogeneous nucleation rates have only been achieved[1-2] for mW, a monoatomic coarse-grained model of water. For the more realistic molecular models, only indirect estimates have been obtained by assuming the validity of classical nucleation theory[3]. Here, we use a coarse-grained variant of a path sampling approach known as forward-flux sampling to perform the first direct calculation of the homogeneous nucleation rate for TIP4P/Ice, which is the most accurate water model for studying ice polymorphs. By using a novel topological order parameter, we are able to identify a freezing mechanism that involves a competition between cubic and hexagonal ice polymorphs[4]. In this competition, cubic ice wins as its growth leads to more compact crystallites[4]. [1] Li, et al., PCCP, 13, 19807 (2011) [2] Haji-Akbari, et al., PCCP, 16, 25916 (2014) [3] Sanz et al., JACS 135, 15008 (2013) [4] Haji-Akbari, Debenedetti, PNAS, 112, 10582 (2015)   

Scratching the surface of ice: Interfacial phase transitions and their kinetic implications.

The surface structure of ice maintains a high degree of disorder down to surprisingly low temperatures. This is due to a number of underlying interfacial phase transitions that are associated with incremental changes in broken symmetry relative to the bulk crystal. In this talk I summarize recent work attempting to establish the nature and locations of these different phase transitions as well as how they depend on external conditions and nonequilibrium driving. The implications of this surface disorder is discussed in the context of simple kinetic processes that occur at these interfaces. Recent experimental work on the roughening transition is highlighted.   

Exploring the coupling between surface crystallinity and surface hydrophilicity in heterogeneous ice nucleation

Heterogeneous ice nucleation has significant influence in a variety of fields ranging from global climate change to intracellular freezing. Although its prevalence can be explained quantitatively by the classical nucleation theory [1], there is a lack of molecular level understanding of the key factors governing ice nucleation at the interface between water and ice nucleator. Here, by employing advanced molecular simulation, we show [2] that heterogeneous ice nucleation on graphitic surface is controlled by the coupling of surface crystallinity and surface hydrophilicity. Molecular level analysis shows that the crystalline graphitic surface with an appropriate hydrophilicity templates ice basal plane forming in the contact layer, thus significantly enhances its ice nucleation efficiency. Remarkably, the templating effect is found to transit from within the first contact layer of water to the second as the hydrophilicity increases, yielding an oscillating distinction between the crystalline and amorphous graphitic surfaces in their ice nucleation efficiencies. Our study sheds new light on the long-standing question of what constitutes a good ice nucleator. 1 R. Cabriolu and T. Li, Physical Review E 91, 052402 (2015). 2 Y. Bi, R. Cabriolu, and T. Li, arXiv:1510.01371 (2015).   

Two Dimensional Ice crystals intercalated between graphene and mica.

The physics and chemistry of the interfacial contact between water and solid surfaces are of the highest fundamental and practical interest in environmental sciences, many biological systems and corrosion effects. Water intercalated between graphene and mica has recently received much interest, even amplified by intriguing intercalation effects and by the evolution of fractals. These confined water layers are argued to be ice-like at room temperature. Due to its good thermal isolation from the environment, as a result of poor perpendicular heat transport through both mica and graphene, this system is uniquely suited for studying the consequences of heat transport, due to latent heat effects, during growth and melting of 2D ice crystals. The enigmatic growth of ice crystals poses a longstanding fundamental problem and its solution is possibly hidden in influences of heat and particle transport. Indeed, we find that heat and particle transport play a crucial role in the growth of ice crystals under high-temperature and high supersaturation conditions.   

2D ice from first principles: structures and phase transitions

Despite relevance to disparate areas such as cloud microphysics and tribology, major gaps in the understanding of the structures and phase transitions of low-dimensional water ice remain. Here we report a first principles study of confined 2D ice as a function of pressure. We find that at ambient pressure hexagonal and pentagonal monolayer structures are the two lowest enthalpy phases identified. Upon mild compression the pentagonal structure becomes the most stable and persists up to ca. 2 GPa at which point square and rhombic phases are stable. The square phase agrees with recent experimental observations of square ice confined within graphene sheets. We also find a double layer AA stacked square ice phase, which clarifies the difference between experimental observations and earlier force field simulations. This work provides a fresh perspective on 2D confined ice, highlighting the sensitivity of the structures observed to both the confining pressure and width.   

Using nanoscale amorphous solid water films to create and study deeply supercooled liquid water at interfaces

Molecular beam vapor deposition of water on cryogenic substrates is known to produce amorphous solid films. When heated above their glass transition these films transform into deeply supercooled liquid water. These nanoscale liquid films can be used to study kinetic processes such as diffusion, isotope exchange, crystallization, and solvent mediated reactions in unprecedented detail. This talk will highlight our recent advances in this area. My colleagues Yuntao Xu, Chunqing Yuan, Collin Dibble, R. Scott Smith, Nick Petrik, and Greg Kimmel made important contributions to this work.This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. The research was performed using EMSL, a national scientific user facility sponsored by DOE's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, which is operated by Battelle, operated for the U.S. DOE under Contract DE-AC05-76RL01830.   

Local Structure in~\textit{Ab Initio} Liquid Water: Signatures of Amorphous Phases

Within the framework of density functional theory, the inclusion of exact exchange and non-local van der Waals/dispersion interactions is crucial for predicting a microscopic structure of ambient liquid water that quantitatively agrees with experiment [1]. In this work, we have used the local structure index (LSI) order parameter to analyze the local structure in such highly accurate \textit{ab initio} liquid water. At ambient conditions, the LSI probability distribution, P($I$), was unimodal with most water molecules characterized by more disordered high-density-like local environments. With thermal excitations removed, the resultant bimodal P($I$) in the inherent potential energy surface (IPES) exhibited a 3:1 ratio between high- and low-density-like molecules, with the latter forming small connected clusters amid the predominant population. By considering the spatial correlations and hydrogen bond network topologies \textit{among} water molecules with the same LSI identities, we demonstrate that the signatures of the experimentally observed low- and high-density amorphous phases of ice are present in the IPES of ambient liquid water [2]. [1] DiStasio \emph{et al.}, J. Chem. Phys. \textbf{141}, 084502 (2014). [2] Santra \emph{et al.}, Mol. Phys. \textbf{113}, 2829 (2015).