posted on 2023-01-14, 00:29authored byTadini
Wenyika Masaya, Fabien Goulay
The surface–bulk partitioning of small saccharide
and amide
molecules in aqueous droplets was investigated using molecular dynamics.
The air–particle interface was modeled using a 80 Å cubic
water box containing a series of organic molecules and surrounded
by gaseous OH radicals. The properties of the organic solutes within
the interface and the water bulk were examined at a molecular level
using density profiles and radial pair distribution functions. Molecules
containing only polar functional groups such as urea and glucose are
found predominantly in the water bulk, forming an exclusion layer
near the water surface. Substitution of a single polar group by an
alkyl group in sugars and amides leads to the migration of the molecule
toward the interface. Within the first 2 nm from the water surface,
surface-active solutes lose their rotational freedom and adopt a preferred
orientation with the alkyl group pointing toward the surface. The
different packing within the interface leads to different solvation
shell structures and enhanced interaction between the organic molecules
and absorbed OH radicals. The simulations provide quantitative information
about the dimension, composition, and organization of the air–water
interface as well as about the nonreactive interaction of the OH radicals
with the organic solutes. It suggests that increased concentrations,
preferred orientations, and decreased solvation near the air–water
surface may lead to differences in reactivities between surface-active
and surface-inactive molecules. The results are important to explain
how heterogeneous oxidation mechanisms and kinetics within interfaces
may differ from those of the bulk.