Dissolution of solid NaCl nanoparticles embedded in supersaturated water vapor probed by molecular dynamic simulations

The dissolution process for small, on the order of 1000 atoms, crystalline NaCl particles with defects embedded in highly supersaturated water vapor was studied by the molecular dynamics (MD) simulation method. We found that a breakdown of the crystal lattice does not occur unless (1) the thickness of water layer covering the surface of salt particles exceeds several molecular layers and (2) there are a considerable number of defects in the crystal. The collapse of the crystal lattice starts when the amount of water taken up by a salt particle reaches about half ( approximately 50%) of the amount of salt in this particle. The number of defects required to initiate subsequent dissolution of the NaCl crystal on the time scale accessible by our simulations ( approximately 40 ns) is in the range of 10 to 12%. We also report the estimates for the time required to form supersaturated aqueous solutions of NaCl from originally crystalline particles as a function of the number of defects in the crystal.

A new approach to studying aqueous reactions using diffuse reflectance infrared Fourier transform spectrometry: application to the uptake and oxidation of SO2 on OH-processed model sea salt aerosol

Diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) is a powerful technique for analyzing solid powders and for following their reactions in real time. We demonstrate that it can also be applied to studying the uptake and reactions of gases in liquid films. Within the DRIFTS cell, a 10%(w/w) mixture of MgCl(2) x 6H(2)O in NaCl was equilibrated with air at 50% RH, which is above the deliquescence point of the magnesium salt but below that of NaCl. This mixture of NaCl coated with an aqueous magnesium chloride solution was then reacted with gas phase OH to generate hydroxide ions via a previously identified interface reaction. This treatment, hereafter referred to as OH-processing, was sufficient to convert some of the magnesium chloride to Mg(OH)(2) and Mg(2)(OH)(3)Cl x 4H(2)O, making the aqueous film basic and providing a reservoir of alkalinity. Subsequent addition of SO(2) to the basic processed mixture resulted in its uptake and conversion to sulfite which was measured by FTIR. The sulfite was simultaneously oxidized to sulfate by HOCl/OCl(-) that was formed in the initial OH-processing of the salt. Further uptake and oxidation of SO(2) in the aqueous film took place when the salt was subsequently exposed to O(3). These studies demonstrate that DRIFTS can be used to study the chemistry in liquid films in real time, and are consistent with the hypothesis that the reaction of gaseous OH with chloride ions generates alkalinity that enhances the uptake and oxidation of SO(2) under these laboratory conditions.

Void-induced dissolution in molecular dynamics simulations of NaCl and water

To gain a better understanding of the interaction of water and NaCl at the surface during dissolution, we have used molecular dynamics to simulate the interface with two equal-sized slabs of solid NaCl and liquid water in contact. The introduction of voids in the bulk of the salt, as well as steps or pits on the surface of the NaCl slab results in a qualitative change of system structure, as defined by radial distribution functions (RDFs). As an example, the characteristic Na-Na RDF for the system changes from regularly spaced narrow peaks (corresponding to an ordered crystalline structure), to a broad primary and smaller secondary peak (corresponding to a disordered structure). The change is observed at computationally short time scales of 100 ps, in contrast with a much longer time scale of 1 mus expected for complete mixing in the absence of defects. The void fraction (which combines both bulk and surface defects) required to trigger dissolution varies between 15%-20% at 300 K and 1 atm, and has distinct characteristics for the physical breakdown of the crystal lattice. The void fraction required decreases with temperature. Sensitivity studies show a strong dependence of the critical void fraction on the quantity and distribution of voids on the surface, with systems containing a balanced number of surface defects and a rough surface showing a maximum tendency to dissolve. There is a moderate dependence on temperature, with a 5% decrease in required void fraction with a 100 K increase in temperature, and a weak dependence on water potential model used, with the SPC, SPC/E, TIP4P, and RPOL models giving qualitatively identical results. The results were insensitive to the total quantity of water available for dissolution and the duration of the simulation.

Experimental study of the sampling artifact of chloride depletion from collected sea salt aerosols

Sampling artifact of chloride depletion from collected sea salt particles was studied, based on simultaneous measurements of size distribution measurements by a 10-stage Micro-Orifice Uniform Deposit Impactor (MOUDI) and of PM2.5 measurements by a Compact Porous Metal Denuder Sampler (PMDS) at a coastal site in Hong Kong on May 7, 8, 9, 11, and 29, 1998. The ambient concentrations of SO2, HNO3, HNO2, and NH3 were also measured by the PMDS. PM2.5 measurements by the PMDS, which is equipped with denuders and nylon back filters, are compared with the PM1.8 and PM3.1 measurements by the MOUDI. The percentages of chloride depletion from sea salt aerosols in PM1.8 and PM3.1 were 4-45% higher than that in PM2.5. This suggests that chloride evaporation in PM1.8 and PM3.1 collected on Teflon filters of the MOUDI during sampling was present. From the sum of the contributions of particles on the Teflon and nylon filters of the PMDS, nitrate formation almost completely accounts for chloride depletion in PM2.5 prior to collection since the equivalent ratio of [Na+] to ([NO3-] + [Cl-]) is close to the seawater ratio of 0.85. However, it was found that 22-74% of nitrate and 45-86% of chloride in the collected particles on the Teflon filter of the PMDS evaporated during sampling. Excess chloride depletion unexplained by NO3- and nss-SO4(2-) was found in the collected particles on the Teflon filter of the PMDS. Similarly, an amount of 3.7-27.2 nequiv m-3 of excess depleted chloride (equivalent to 8-55% of total chloride depletion) was found in supermicron particles collected by the MOUDI. In the 1.8-3.1 microns particles, the excess depleted chloride is positively correlated to the chloride evaporated from the deposited particles. Particle--particle interactions are proposed to explain the evaporation of nitrates and chlorides in the PMDS and MOUDI measurements. The observed chloride depletion from seasalt aerosols was partially attributed to sampling artifact due to particle--particle interactions.