Molecular-scale description of interfacial mass transfer in phase-separated aqueous secondary organic aerosol

Abstract

Abstract. Liquid–liquid phase-separated (LLPS) aerosol particles are known to exhibit increased cloud condensation nuclei (CCN) activity compared to well-mixed ones due to a complex effect of low surface tension and non-ideal mixing. The relation between the two contributions as well as the molecular-scale mechanism of water uptake in the presence of an internal interface within the particle is to date not fully understood. Here we attempt to gain understanding in these aspects through steered molecular dynamics simulation studies of water uptake by a vapor–hydroxy-cis-pinonic acid–water double interfacial system at 200 and 300 K. Simulated free-energy profiles are used to map the water uptake mechanism and are separated into energetic and entropic contributions to highlight its main thermodynamic driving forces. Atmospheric implications are discussed in terms of gas–particle partitioning, intraparticle water redistribution timescales and water vapor equilibrium saturation ratios. Our simulations reveal a strongly temperature-dependent water uptake mechanism, whose most prominent features are determined by local extrema in conformational and orientational entropies near the organic–water interface. This results in a low core uptake coefficient (ko/w=0.03) and a concentration gradient of water in the organic shell at the higher temperature, while entropic effects are negligible at 200 K due to the association-entropic-term reduction in the free-energy profiles. The concentration gradient, which results from non-ideal mixing – and is a major factor in increasing LLPS CCN activity – is responsible for maintaining liquid–liquid phase separation and low surface tension even at very high relative humidities, thus reducing critical supersaturations. Thermodynamic driving forces are rationalized to be generalizable across different compositions. The conditions under which single uptake coefficients can be used to describe growth kinetics as a function of temperature in LLPS particles are described.

The molecular scale mechanism of deposition ice nucleation on silver iodide

“…A large number of molecular dynamics-based studies addressed the impact of these properties on immersion freezing on surfaces like feldspar, carbon, kaolinite, AgI or model lattices. 11,[13][14][15][16][17][18][19][20][21][22][23][24][25][26] These simulations revealed a connection between the formation of subcritical ice clusters and the preordering of liquid water to the kinetics of ice nucleation as well as to the formation of crystalline polymorphs. 12,27,28 Most published simulation studies were performed in immersion freezing mode implying the presence of a macroscopic interface between the ice nucleating surface and water, i.e.…”

“…This is achieved by gradually activating or deactivating their interactions with the rest of the system over multiple Monte Carlo (MC) moves. 50,51 However, in this work, we follow the same insertion/deletion method of David et al and Lbadaoui-Darvas et al 25,26 The MC/MD ratios tested are 500/100, 500/200, and 500/225, all conducted for a vapor pressure of 60 kPa. The two latter ratios speed up convergence and favor crystallization.…”

“…The two latter ratios speed up convergence and favor crystallization. 26 For the vapor pressure of 6 kPa, an MC/MD ratio of 5 is used. As for the GCMC/MD simulations, the Lennard-Jones and electrostatic interactions are handled just as with the MD simulations, but the shake algorithm 52 is used to constrain the O-H bond length and H-O-H angle of the water molecules.…”

“…To tackle the above complexity, we adapted a hybrid Grand Canonical Monte Carlo/Molecular Dynamics (GCMC/MD) method which so far has only been used to study deposition ice nucleation in a handful of very recent studies. 25,26 Our model substrate is silver iodide (AgI), a conventionally used cloud seeding agent and one of the best investigated inorganic ice nucleating substances, as it has been recognized as one of the most potent. 30 Several experimental 31,32 and computer simulation studies 19,33,34 have investigated ice nucleation on AgI surfaces, but none of them related interfacial water adsorption to the deposition IN activity of AgI.…”

“…Developing such mechanistic understanding is crucial for the long-needed modernization of the theoretical modeling framework to describe heterogeneous ice nucleation for atmospheric model simulations, 26,38,39 which to date rely on the almost the century-old classical nucleation theory, with known signicant shortcomings. 26,40 In this study, we use GCMC/MD and MD simulations to investigate deposition ice nucleation on AgI surfaces at the molecular level and we suggest a comprehensive deposition ice nucleation mechanism based on adsorption theory, which is consistent with experimental data and model calculations.…”