  Water interactions with organic surfaces are of central importance in
biological systems and many Earth system processes. Here we describe
experimental studies of water collisions and uptake kinetics on liquid and
solid butanol from 160 to 200 K. Hyperthermal D2O molecules (0.32 eV) undergo
efficient trapping on both solid and liquid butanol, and only a minor fraction
scatters inelastically after an 80% loss of kinetic energy to surface modes.
Trapped molecules either desorb within a few ms, or are taken up by the butanol
phase during longer times. The water uptake and surface residence time increase
with temperature above 180 K indicating melting of the butanol surface 4.5 K
below the bulk melting temperature. Water uptake changes gradually across the
melting point and trapped molecules are rapidly lost by diffusion into the
liquid above 190 K. This indicates that liquid butanol maintains a surface
phase with limited water permeability up to 5.5 K above the melting point.
These surface observations are indicative of an incremental change from solid
to liquid butanol over a range of 10 K straddling the bulk melting temperature,
in contrast to the behavior of bulk butanol and previously studied materials.
