Functionalized self-assembled monolayers (SAMs) are the focus of ongoing investigations because they can be chemically tuned to control their structure and dynamics for a wide variety of applications, including electrochemistry, catalysis, and as models of biological interfaces. Here we combine reflection 2D infrared vibrational echo spectroscopy (R-2D IR) and molecular dynamics simulations to determine the relationship between the structures of functionalized alkanethiol SAMs on gold surfaces and their underlying molecular motions on timescales of tens to hundreds of picoseconds. We find that at higher head group density, the monolayers have more disorder in the alkyl chain packing and faster dynamics. The dynamics of alkanethiol SAMs on gold are much slower than the dynamics of alkylsiloxane SAMs on silica. Using the simulations, we assess how the different molecular motions of the alkyl chain monolayers give rise to the dynamics observed in the experiments.self-assembled monolayer | dynamics | 2D IR spectroscopy | MD simulation S elf-assembled monolayers (SAMs) on planar metal surfaces enable the tailoring of interfacial properties by functionalization of the alkyl chains. SAMs formed by alkanethiol chains on gold surfaces are of particular interest due to the ordered packing of the chains, chemical stability, and facile methods of preparation, as well as the diverse array of chemical functionalization that can be added (1). The properties of SAMs on gold have led to applications including electrochemical devices (2), surface patterning (3), model biological surfaces (4), and heterogeneous catalysis (5). In many of these applications, the interfacial properties of the monolayer are determined largely by the particular head group linked at the terminal site of the alkyl chain.The structure of SAMs on gold has been well characterized by scanning probe microscopy (6), helium diffraction (7), X-ray photoelectron spectroscopy (8), sum-frequency generation spectroscopy (SFG) (9), and linear infrared spectroscopy (10). However, to determine how the physical and chemical properties of SAMs are related to their microscopic dynamics and structure and the influence of head groups present in most applications, fast time-resolved experimental techniques, with sufficient selectivity and sensitivity, are required to measure the structural dynamics of a monolayer of molecules on the appropriate picosecond (ps) timescale.Two-dimensional infrared vibrational echo spectroscopy (2D IR) provides the necessary observables by measuring spectral diffusion, i.e., the time-dependent evolution of the probe vibrational frequency in response to structural fluctuations of the chemical environment (11-13). To use 2D IR to investigate monolayer dynamics requires selectivity for the interfacial region. One method to achieve this is to combine 2D IR spectroscopy with SFG (14, 15), which requires a vibrational mode that has both a large IR transition dipole and a large Raman cross-section. However, for a monolayer functionalized with the vibra...

Abstract. We present the first clearly resolved observations of surfaces of growing and ablating hexagonal ice crystals using variable-pressure scanning electron microscopy. The ice surface develops trans-prismatic strands, separated from one another by distances of 5-10 µm. The strands are present at a wide range of supersaturations, but are most pronounced at temperatures near the frost point. Pyramidal facets consistent with Miller-Bravais indices of 1011, and possibly also 2021, are associated with ice growth under these conditions. A molecular-dynamics model of a free-standing ice I h nanocolumn containing 8400 water molecules does not develop trans-prismatic strands, suggesting these features originate at larger spatial or temporal scales. The possible relevance of these surface features to cirrus ice is discussed.

We present an Arrhenius analysis of self-diffusion on the prismatic surface of ice calculated from molecular dynamics simulations. The six-site water model of Nada and van der Eerden was used in combination with a structure-based criterion for determining the number of liquid-like molecules in the quasi-liquid layer. Simulated temperatures range from 230 K-287 K, the latter being just below the melting temperature of the model, 289 K. Calculated surface diffusion coefficients agree with available experimental data to within quoted precision. Our results indicate a positive Arrhenius curvature, implying a change in the mechanism of self-diffusion from low to high temperature, with a concomitant increase in energy of activation from 29.1 kJ mol(-1) at low temperature to 53.8 kJ mol(-1) close to the melting point. In addition, we find that the surface self-diffusion is anisotropic at lower temperatures, transitioning to isotropic in the temperature range of 240-250 K. We also present a framework for self-diffusion in the quasi-liquid layer on ice that aims to explain these observations.