Molecular dynamics study on the surface tension of succinic acid-water nano-aerosol droplets

Abstract

The surface tension plays a significant role in the hygroscopicity of aerosol particles on a nanoscale. However, it cannot be obtained by using the existing measurement techniques. In this study, we simulate the hygroscopic growth of one single succinic acid (SA) particle by using the molecular dynamic (MD) method. Based on the MD simulation results, the surface tension of the stable SA-water droplet is calculated by using a numerical model. Furthermore, the influencing mechanisms of temperature, diameter and concentration of SA on the surface tension of the nanoscale droplet are investigated. The results show that with the temperature increasing from 260 K to 320 K, the surface tension of the droplet decreases, which is mainly caused by the weakening of the intermolecular forces inside the droplet. Besides, the sensitivity of the surface tension to the temperature increases with the increasing SA concentration, which can be explained by the effect of the temperature and the SA concentration on the radial distribution of SA molecules. With the increase of the particle diameter, the surface tension of droplet first increases and then tends to be constant. The normal components of the Irving-Kirkwood pressure tensors are calculated to explain the effect of diameter and SA on the surface tension. In addition, when the SA concentration is increased, the particle diameter range which has an obvious effect on the surface tension is reduced. Moreover, the surface tension of the nanodroplet is negatively correlated with the SA concentration, and the correlation fits into the logarithmic function form, especially for droplet with a diameter smaller than 6.12 nm. The Szyszkowski equation is employed to fit the relationship between SA concentration and the surface tension of droplet. These findings can provide parameter support for improving the theoretical model of particle hygroscopicity and related kinetic processes. This study emphasizes further research on the surface tension of nano-droplets with more complex components.