Unified Description of Diffusion Coefficients from Small to Large Molecules in Organic-Water Mixtures.

Diffusion coefficients in mixtures of organic molecules and water are needed for many applications, ranging from the environmental modeling of pollutant transport, air quality, and climate, to improving the stability of foods, biomolecules, and pharmaceutical agents for longer use and storage. The Stokes-Einstein relation has been successful for predicting diffusion coefficients of large molecules in organic-water mixtures from viscosity, yet it routinely underpredicts, by orders of magnitude, the diffusion coefficients of small molecules in organic-water mixtures. Herein, a unified description of diffusion coefficients of large and small molecules in organic-water mixtures, based on the fractional Stokes-Einstein relation, is presented. A fractional Stokes-Einstein relation is able to describe 98% of the observed diffusion coefficients from small to large molecules, roughly within the uncertainties of the measurements. The data set used in the analysis includes a wide range of radii of diffusing molecules, viscosities, and intermolecular interactions. As a case study, we show that the degradation of polycyclic aromatic hydrocarbons (PAHs) by O3 within organic-water particles in the planetary boundary layer is relatively short (≲1 day) when the viscosity of the particle is ≲102 Pa s. We also show that the degradation times predicted using the Stokes-Einstein relation and the fractional Stokes-Einstein relation can differ by up to a factor of 10 in this region of the atmosphere.

A Molecular Mechanism of Ice Nucleation on Model AgI Surfaces.

Heterogeneous ice nucleation at solid surfaces is important in many physical systems including the Earth's atmosphere. AgI is one of the best ice nucleating agents known; however, why AgI is such an effective ice nucleus is unclear. Using molecular dynamics simulations, we show that a good lattice match between ice and a AgI surface is insufficient to predict the ice nucleation ability of the surface. Seven faces modeled to represent surfaces of both ß-AgI and γ-AgI, each having a good lattice match with hexagonal and/or cubic ice, are considered, but ice nucleation is observed for only three. Our model simulations clearly show that the detailed atomistic structure of the surface is of crucial importance for ice nucleation. For example, when AgI is cleaved along certain crystal planes two faces result, one with silver ions and the other with iodide ions exposed as the outermost layer. Both faces have identical lattice matches with ice, but in our simulations ice nucleation occurred only at silver exposed surfaces. Moreover, although hexagonal ice is often the only polymorph of ice considered in discussions of heterogeneous ice nucleation, cubic ice was frequently observed in our simulations. We demonstrate that one possible mechanism of ice nucleation by AgI consists of particular AgI surfaces imposing a structure in the adjacent water layer that closely resembles a layer that exists in bulk ice (hexagonal or cubic). Ice nucleates at these surfaces and grows almost layer-by-layer into the bulk.

Ion solvation in a water-urea mixture.

We employ molecular dynamics simulations and the reference interaction site model (RISM) integral equation theory to study the solvation structure and solvation thermodynamics of the transfer process from water to a water-urea mixture. Simple positive and negative ions together with uncharged species of the same size are used as crude models for the hydrophilic and hydrophobic groups of a protein. We find that urea preferentially solvates positively charged species. The solvation free energies obtained indicate that larger solutes favor the transfer from water to a water-urea mixture. The decomposition of the transfer free energy into the energetic and entropic terms shows that the energetic part is much larger than the entropic one and tends to dominate the transfer process, supporting the direct mechanism of urea-denaturation. In addition, the effect of urea on the water liquid structure is discussed from the viewpoint of solvation entropy.

Structure and interaction in aqueous urea-trimethylamine-N-oxide solutions.

The structural and energetic properties of solutions containing water, urea, and trimethylamine-N-oxide (TMAO) are examined using molecular dynamics simulations. Such systems are of interest mainly because TMAO acts to counter the protein denaturing effect of urea. Even at relatively high concentration, TMAO is found to fit well into the urea-water structure. The underlying solution structure is influenced by TMAO, but these perturbations tend to be modest. The TMAO-water and TMAO-urea interaction energies make an important contribution to the total energy in solutions where counter-denaturing effects are expected. TMAO-water and TMAO-urea hydrogen bonds have the largest hydrogen-bond energies in the system. Additionally, TMAO cannot hydrogen bond with itself, and hence it interacts strongly with water and urea. These observations suggest that the mechanism of TMAO counter denaturation is simply that water and urea prefer to solvate TMAO rather than the protein, hence inhibiting its unfolding.