Heterogeneous Nucleation of Butanol on NaCl: A Computational Study of Temperature, Humidity, Seed Charge, and Seed Size Effects

Using a combination
of quantum chemistry and cluster size distribution
dynamics, we study the heterogeneous nucleation of n-butanol and water onto sodium chloride (NaCl)10 seeds
at different butanol saturation ratios and relative humidities. We
also investigate how the heterogeneous nucleation of butanol is affected
by the seed size through comparing (NaCl)5, (NaCl)10, and (NaCl)25 seeds and by seed electrical charge
through comparing (Na10Cl9)+, (NaCl)10, and (Na9Cl10)− seeds.
Butanol is a common working fluid for condensation particle counters
used in atmospheric aerosol studies, and NaCl seeds are frequently
used for calibration purposes and as model systems, for example, sea
spray aerosol. In general, our simulations reproduce the experimentally
observed trends for the NaCl–BuOH–H2O system,
such as the increase of nucleation rate with relative humidity and
with temperature (at constant supersaturation of butanol). Our results
also provide molecular-level insights into the vapor–seed interactions
driving the first steps of the heterogeneous nucleation process. The
main purpose of this work is to show that theoretical studies can
provide molecular understanding of initial steps of heterogeneous
nucleation and that it is possible to find cost-effective yet accurate-enough
combinations of methods for configurational sampling and energy evaluation
to successfully model heterogeneous nucleation of multicomponent systems.
In the future, we anticipate that such simulations can also be extended
to chemically more complex seeds.