A comparative study of cluster-surface collisions: Molecular-dynamics simulations of (H2O)1000 and (SO2)1000

Dynamics and Sorption Kinetics of Methanol Monomers and Clusters on Nopinone Surfaces

Organic–organic interactions play important roles in secondary organic aerosol formation, but the interactions are complex and poorly understood. Here, we use environmental molecular beam experiments combined with molecular dynamics simulations to investigate the interactions between methanol and nopinone, as atmospheric organic proxies. In the experiments, methanol monomers and clusters are sent to collide with three types of surfaces, i.e., graphite, thin nopinone coating on graphite, and nopinone multilayer surfaces, at temperatures between 140 and 230 K. Methanol monomers are efficiently scattered from the graphite surface, whereas the scattering is substantially suppressed from nopinone surfaces. The thermal desorption from the three surfaces is similar, suggesting that all the surfaces have weak or similar influences on methanol desorption. All trapped methanol molecules completely desorb within a short experimental time scale at temperatures of 180 K and above. At lower temperatures, the desorption rate decreases, and a long experimental time scale is used to resolve the desorption, where three desorption components are identified. The fast component is beyond the experimental detection limit. The intermediate component exhibits multistep desorption character and has an activation energy of E_a = 0.18 ± 0.03 eV, in good agreement with simulation results. The slow desorption component is related to diffusion processes due to the weak temperature dependence. The molecular dynamics results show that upon collisions the methanol clusters shatter, and the shattered fragments quickly diffuse and recombine to clusters. Desorption involves a series of processes, including detaching from clusters and desorbing as monomers. At lower temperatures, methanol forms compact cluster structures while at higher temperatures, the methanol molecules form layered structures on the nopinone surface, which are visible in the simulation. Also, the simulation is used to study the liquid–liquid interaction, where the methanol clusters completely dissolve in liquid nopinone, showing ideal organic–organic mixing.

Cluster-induced desorption investigated by means of molecular dynamics simulations-Microsolvation in clusters of polar and non-polar constituents

The desorption of surface-adsorbed molecules induced by cluster-surface collisions of neutral molecular clusters, in particular, SO₂ clusters, was investigated by means of molecular dynamics simulations. The desorption efficiency was found to be in general much higher for clusters of polar molecules when compared to nonpolar cluster constituents, for both nonpolar and polar adsorbates. In all cases, desorption is shown to proceed via dissolvation of the analyte in the cluster. In systems with nonpolar cluster constituents, the process is mainly driven by the increase in the entropy of the dissolved analyte in a larger cluster fragment. The latter process is enhanced by polar cluster constituents since the respective clusters show lower fragmentation at comparable kinetic energy and thus provide in average larger cluster fragments for the analytes to be dissolved in. In systems with clusters of polar constituents and polar adsorbates, the process is most efficient due to the additional energetic stabilization of the desorbed molecule in the solvation shell formed in the cluster fragment.

Role of cohesive energy in droplet fragmentation

Using molecular-dynamics simulation, we investigate the fragmentation behavior of droplets after collision with a wall. We demonstrate that the ratio of the impact to the cohesive energy E(coh) of the droplet is the key quantity characterizing the droplet fragmentation process. To show this both van der Waals-bonded Ar and N(2) droplets and polar H(2)O droplets are studied. If the impact energy per molecule E<(0.35-0.4)E(coh), the droplet is reflected without fragmenting. Beyond that impact energy fragmentation of the droplet abruptly starts. At E=E(coh), the fragmentation process already results in a fine dispersal of the droplet into daughter droplets; the maximum fragment contains only less than 4% of the initial droplet mass and around one-third of the droplet has been shattered into isolated molecules. The disintegration process continuously increases with collision energy. These findings are relevant for the process of droplet fragmentation as used in the method of impact desolvation of electrosprayed microdroplets mass spectrometry.