Seed–Adsorbate Interactions as the Key of Heterogeneous Butanol and Diethylene Glycol Nucleation on Ammonium Bisulfate and Tetramethylammonium Bromide

Condensation
particle counter (CPC) instruments are commonly used
to detect atmospheric nanoparticles. They operate on the basis of
condensing an organic working fluid on the nanoparticle seeds to grow
the particles to a detectable size, and at the size of few nanometers,
their efficiency depends on how well the working fluid interacts with
the seeds under the measurement conditions. This study models the
first steps of heterogeneous nucleation of two working fluids commonly
used in CPCs (diethylene glycol (DEG) and n-butanol)
onto two positively charged seeds, ammonium bisulfate and tetramethylammonium
bromide. The nucleation process is modeled on a molecular level using
a combination of systematic configurational sampling and density functional
theory (DFT). We take into account the conformational flexibility
of DEG and n-butanol and determine the key factors
that can improve the efficiency of nanoparticle measurements by CPCs.
The results show that hydrogen bonding between the seed and the working
fluid molecules is central to the adsorption of the first DEG/n-butanol molecules onto the seeds. However, intermolecular
hydrogen bonding between the adsorbed molecules can also enhance the
nucleation process for the weakly adsorbing vapor molecules. Accordingly,
the heterogeneous nucleation probability is higher for working fluid–nanoparticle
combinations with a higher potential for hydrogen bonding; in this
case, DEG and ammonium bisulfate. Moreover, conformational analysis
and methodology evaluations indicate that the consideration of adsorbate
conformers and step-wise addition of the vapor molecules to the seeds
is not essential for qualitative modeling of heterogeneous nucleation
systems, at least for systems where the adsorbate and seed chemical
properties are clearly different. This is the first molecular-level
modeling study reporting detailed chemical reasons for experimentally
observed seed and working fluid preferences in CPCs and reproducing
the experimental observations. Our presented approach can be likely
used for predicting preferences in similar nucleating systems.