On the phase state of aerosol nanoparticles from a detailed geometric analysis of their free volume accessible to small penetrants

Using Delaunay tessellation followed by Monte Carlo integration, we geometrically analyze atomistically-detailed model structures of aerosol nanoparticles to connect their free volume to their phase state. Nanoparticles investigated consist of water, organic molecules (such as cis-pinonic acid) and inorganic species (such as sulfate and ammonium ions). Our emphasis is on the effects of relative humidity and organic content on nanoparticle free volume, and its spatial distribution within the nanoparticle. Our analysis provides information for the distribution of empty pores in the nanoparticle, the available free volume that a guest molecule (e.g., water) can reside, and the connectivity of such pockets of accessible volume. Indirectly, our geometric analysis provides exact measures of the shape, surface area and volume of the nanoparticle.

It is found that with increasing organic concentration, the total unoccupied as well as the total accessible volume to a hypothetical penetrant in the nanoparticle increase. It is also found that the unoccupied and accessible volumes in the organic islands of the nanoparticle or at its surface are always larger compared to those in its aqueous or inorganic domains. Pores accessible to a water molecule are mainly found in the intermediate and outer areas of the nanoparticle which are dominated by organic molecules.

The largest pores accessible to a water molecule were discovered in the nanoparticle with the highest organic mass fraction and the lowest relative humidity (RH). With increasing RH, the presence of additional water molecules disturbs these cavities since organic mass is pushed to the outer regions of the nanoparticle. Simultaneously, at these highest-RH nanoparticles, the pure inorganic volume vanishes and the same happens with its organic-inorganic interfacial domains, implying a complete separation of organic molecules from inorganic ions (with the latter showing a strong preference to accumulate in the internal areas of the nanoparticle). Under the same conditions, the cis-pinonic acid was found to form a single island inside the nanoparticle characterized by a density almost identical to that of bulk cis-pinonic acid, indicative of a liquid-like phase. In contrast, the inorganic mass prefers to form a single large island whose density is very similar to that of ammonium sulfate; this indicates a solid-like phase at the core of the NP. This finding agrees with another finding that domains dominated by inorganic ions are rather dense having no cavities wherein any realistic penetrant with a radius greater than 1 Å could be accommodated. Water, on the other hand, prefers to reside in several islands, each of which of the same volume (practically) when RH is kept at low levels. In contrast, at higher levels of RH, water prefers to form a big island with numerous smaller water droplets around it.