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SignificanceOver 150 years ago, Faraday discovered the presence of a water layer on ice below the bulk melting temperature. This layer is important for surface chemistry and glacier sliding close to subfreezing conditions. The nature and thickness of this quasi-liquid layer has remained controversial. By combining experimental and simulated surface-specific vibrational spectroscopy, the thickness of this quasi-liquid layer is shown to change in a noncontinuous, stepwise fashion around 257 K. Below this temperature, the first bilayer is already molten; the second bilayer melts at this transition temperature. The blue shift in the vibrational response of the outermost water molecules accompanying the transition reveals a weakening of the hydrogen bond network upon an increase of the water layer thickness. On the surface of water ice, a quasi-liquid layer (QLL) has been extensively reported at temperatures below its bulk melting point at 273 K. Approaching the bulk melting temperature from below, the thickness of the QLL is known to increase. To elucidate the precise temperature variation of the QLL, and its nature, we investigate the surface melting of hexagonal ice by combining noncontact, surface-specific vibrational sum frequency generation (SFG) spectroscopy and spectra calculated from molecular dynamics simulations. Using SFG, we probe the outermost water layers of distinct single crystalline ice faces at different temperatures. For the basal face, a stepwise, sudden weakening of the hydrogen-bonded structure of the outermost water layers occurs at 257 K. The spectral calculations from the molecular dynamics simulations reproduce the experimental findings; this allows us to interpret our experimental findings in terms of a stepwise change from one to two molten bilayers at the transition temperature.

The ability to prepare single-crystal faces has become central to developing and testing models for chemistry at interfaces, spectacularly demonstrated by heterogeneous catalysis and nanoscience. This ability has been hampered for hexagonal ice,Ih —a fundamental hydrogen-bonded surface—due to two characteristics of ice: ice does not readily cleave along a crystal lattice plane and properties of ice grown on a substrate can differ significantly from those of neat ice. This work describes laboratory-based methods both to determine theIh crystal lattice orientation relative to a surface and to use that orientation to prepare any desired face. The work builds on previous results attaining nearly 100% yield of high-quality, single-crystal boules. With these methods, researchers can prepare authentic, single-crystal ice surfaces for numerous studies including uptake measurements, surface reactivity, and catalytic activity of this ubiquitous, fundamental solid.

On the surface of water ice, a quasi-liquid layer (QLL) has been extensively reported at temperatures below its bulk melting point at 273 K. Approaching the bulk melting temperature from below, the thickness of the QLL is known to increase. To elucidate the precise temperature variation of the QLL, and its nature, we investigate the surface melting of hexagonal ice by combining noncontact, surface-specific vibrational sum frequency generation (SFG) spectroscopy and spectra calculated from molecular dynamics simulations. Using SFG, we probe the outermost water layers of distinct single crystalline ice faces at different temperatures. For the basal face, a stepwise, sudden weakening of the hydrogen-bonded structure of the outermost water layers occurs at 257 K. The spectral calculations from the molecular dynamics simulations reproduce the experimental findings; this allows us to interpret our experimental findings in terms of a stepwise change from one to two molten bilayers at the transition temperature.

Ice surface molecular structures determine reactions and interactions on ice. Observation of macroscopic (etch pit) and microscopic (electron backscatter diffraction orientation density function) images definitively shows the molecular-level termination and the layer structure. Identification of the molecular-level structure enables a statistical model that accounts for observation of contrasting growth from the vapor and growth from the liquid. At the solid–vapor interface, the surface is dominated by primary prism and basal faces. In contrast, the secondary prism face dominates at the solid–liquid interface. A statistical model suggests a basis for this contrast: growth at the liquid–solid interface is enthalpically driven, whereas poor thermal transport in the vapor suppresses growth of the secondary prism face. Physics and chemistry of ice surfaces are not only of fundamental interest but also have important impacts on biological and environmental processes. As ice surfaces—particularly the two prism faces—come under greater scrutiny, it is increasingly important to connect the macroscopic faces with the molecular-level structure. The microscopic structure of the ubiquitous ice I h crystal is well-known. It consists of stacked layers of chair-form hexagonal rings referred to as molecular hexagons. Crystallographic unit cells can be assembled into a regular right hexagonal prism. The bases are labeled crystallographic hexagons. The two hexagons are rotated 30° with respect to each other. The linkage between the familiar macroscopic shape of hexagonal snowflakes and either hexagon is not obvious per se. This report presents experimental data directly connecting the macroscopic shape of ice crystals and the microscopic hexagons. Large ice single crystals were used to fabricate samples with the basal, primary prism, or secondary prism faces exposed at the surface. In each case, the same sample was used to capture both a macroscopic etch pit image and an electron backscatter diffraction (EBSD) orientation density function (ODF) plot. Direct comparison of the etch pit image and the ODF plot compellingly connects the macroscopic etch pit hexagonal profile to the crystallographic hexagon. The most stable face at the ice–water interface is the smallest area face at the ice–vapor interface. A model based on the molecular structure of the prism faces accounts for this switch.

Physics and chemistry of ice surfaces are not only of fundamental interest but also have important impacts on biological and environmental processes. As ice surfaces—particularly the two prism faces—come under greater scrutiny, it is increasingly important to connect the macroscopic faces with the molecular-level structure. The microscopic structure of the ubiquitous ice Ih crystal is well-known. It consists of stacked layers of chair-form hexagonal rings referred to as molecular hexagons. Crystallographic unit cells can be assembled into a regular right hexagonal prism. The bases are labeled crystallographic hexagons. The two hexagons are rotated 30° with respect to each other. The linkage between the familiar macroscopic shape of hexagonal snowflakes and either hexagon is not obvious per se. This report presents experimental data directly connecting the macroscopic shape of ice crystals and the microscopic hexagons. Large ice single crystals were used to fabricate samples with the basal, primary prism, or secondary prism faces exposed at the surface. In each case, the same sample was used to capture both a macroscopic etch pit image and an electron backscatter diffraction (EBSD) orientation density function (ODF) plot. Direct comparison of the etch pit image and the ODF plot compellingly connects the macroscopic etch pit hexagonal profile to the crystallographic hexagon. The most stable face at the ice–water interface is the smallest area face at the ice–vapor interface. A model based on the molecular structure of the prism faces accounts for this switch.

Equilibrium is a major topic in both introductory and physical chemistry curricula. Shultz discusses how this important concept might be conveyed to students.

Award in the Course and Curriculum Development (CCD) program for FY1994.

Nonlinear spectroscopy, both second harmonic and sum frequency generation — SHG and SFG — have proven to be powerful techniques for probing a variety of interfaces from the very dynamic, high vapor pressure liquid–air surface to buried interfaces between hydrophobic and hydrophilic phases to irregular and amorphous solid surfaces. With the advent of off-the-shelf laser systems, it has become easier and easier to collect nonlinear spectra. The major impediments to wide spread usage of nonlinear spectroscopy are the challenges in interpretation of the spectra produced. This work begins with an introduction to nonlinear spectroscopy based on an optical-geometrical view of the interaction between the probe beams and molecules in the interfacial region. The introduction serves as a basis for exploration of recent developments and current issues. Two case studies are included: examination of ions at the aqueous interface including evidence for H3O+ at the interface and investigation of molecular interactions on nonmetallic, nanostructured interfaces.

Students in science, technology, engineering and math (STEM) courses are often exposed to pictures, animations, and displays that are intended to convey complex concepts and interactions. These types of presentations have been termed visualizations, which embodies the idea they are external representations that convey information in an interpretable form. In many cases, well-designed visualizations help make visible the kinds of processes and relationships that normally are unobservable to the naked eye. Much of the content of STEM coursework proves inaccessible in this way, either because the critical elements under study are so microscopic or temporally expanded that seeing them is