Experimental and theoretical evidence for bilayer-by-bilayer surface melting of crystalline ice
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 hy…
Structure and Dynamics of the Quasi-Liquid Layer at the Surface of Ice from Molecular Simulations
We characterized the structural and dynamical properties of the quasi-liquid layer (QLL) at the surface of ice by molecular dynamics simulations with a thermodynamically consistent water model. Our simulations show that for three low-index ice surfaces only the outermost molecular layer presents short-range and mid-range disorder and is diffusive. The onset temperature for normal diffusion is much higher than the glass temperature of supercooled water, although the diffusivity of the QLL is higher than that of bulk water at the corresponding temperature. The underlying subsurface layers impose an ordered template, which produces a regular patterning of the ice/water interface at any tempera…
Interaction of Charged Amino-Acid Side Chains with Ions: An Optimization Strategy for Classical Force Fields
Many well-established classical biomolecular force fields, fitted on the solvation properties of single ions, do not necessarily describe all the details of ion pairing accurately, especially for complex polyatomic ions. Depending on the target application, it might not be sufficient to reproduce the thermodynamics of ion pairing, but it may also be necessary to correctly capture structural details, such as the coordination mode. In this work, we analyzed how classical force fields can be optimized to yield a realistic description of these different aspects of ion pairing. Given the prominent role of the interactions of negatively charged amino-acid side chains and divalent cations in many …
Combined Experimental and Theoretical Investigation of Heating Rate on Growth of Iron Oxide Nanoparticles
Thermal decomposition is a promising route for the synthesis of highly monodisperse magnetite nanoparticles. However, the apparent simplicity of the synthesis is counterbalanced by the complex interplay of the reagents with the reaction variables that determine the final particle size and dispersity. Here, we present a combined experimental and theoretical study on the influence of the heating rate on crystal growth, size, and monodispersity of iron oxide nanoparticles. We synthesized monodisperse nanoparticles with sizes varying from 6.3 to 27 nm simply by controlling the heating rate of the reaction. The nanoparticles show size-dependent superparamagnetic behavior. Using numerical calcula…