Lack of Aggregation of Molecules on Ice Nanoparticles

Multiple molecules adsorbed on the
surface of nanosized ice particles
can either remain isolated or form aggregates, depending on their
mobility. Such (non)­aggregation may subsequently drive the outcome
of chemical reactions that play an important role in atmospheric chemistry
or astrochemistry. We present a molecular beam experiment in which
the controlled number of guest molecules is deposited on the water
and argon nanoparticles in a pickup chamber and their aggregation
is studied mass spectrometrically. The studied molecules (HCl, CH3Cl, CH3CH2CH2Cl, C6H5Cl, CH4, and C6H6)
form large aggregates on argon nanoparticles. On the other hand, no
aggregation is observed on ice nanoparticles. Molecular simulations
confirm the experimental results; they reveal a high degree of aggregation
on the argon nanoparticles and show that the molecules remain mostly
isolated on the water ice surface. This finding will influence the
efficiency of ice grain-mediated synthesis (e.g., in outer space)
and is also important for the cluster science community because it
shows some limitations of pickup experiments on water clusters.