Water and butanol are used as working fluids in condensation
particle
counters, and condensation of a single vapor onto an ion can be used
as a simple model system for the study of ion-induced nucleation in
the atmosphere. Motivated by this, we examine heterogeneous nucleation
of water (H2O) andn-butanol (BuOH) vapors
onto three positively (Li+, Na+, K+) and three negatively charged (F–, Cl–, Br–) ions using classical nucleation theory and
computational quantum chemistry methods. We study phenomena that cannot
be captured by Kelvin–Thomson equation for small nucleation
ion cores. Our quantum chemistry calculations reveal the molecular
mechanism behind ion-induced nucleation for each studied system. Typically,
ions become solvated from all sides after several vapor molecules
condense onto the ion. However, we show that the clusters of water
and large negatively charged ions (Cl–and Br–) thermodynamically prefer the ion being migrated to
the cluster surface. Although our methods generally do not show clear
sign-preference for ion–water nucleation, we identified positive
sign-preference for ion–butanol nucleation caused by the possibility
to form stabilizing hydrogen bonds between butanol molecules condensed
onto a positively charged ion. These bonds cannot form when butanol
condenses onto a negatively charged ion. Therefore, we show that ion
charge, its sign, as well as vapor properties have effects on the
prenucleation and critical cluster/droplet sizes and also on the molecular
mechanism of ion-induced nucleation.