Effects of Inorganic Ions on Ice Nucleation by the Al Surface of Kaolinite Immersed in Water

Abstract:

Ice nucleation can occur by either homogeneous or heterogeneous nucleation. In the atmosphere, a large fraction of ice is initiated by heterogeneous ice nucleation with the presence of ice-nucleating particles. Several mineral dusts are recognized as efficient ice-nucleating particles. Here we focus on one of the common mineral dusts – kaolinite.

At high relative humidity, inorganic coatings take up water, leading to mineral dust particles coated with water and inorganic ions. In our work, molecular dynamics simulations were employed to investigate the influence of inorganic salts on ice nucleation by the Al-surface of kaolinite, terminated with hydroxyl groups. Seven salt solutions LiI, LiCl, NaI, NaCl, KI, KCl and NH₄I are considered. Simulations performed at 300 K show no specific ion adsorption at the kaolinite surface. At a supercooling of 26 K (taking account of freezing point depression), 1 M salt solutions slowed ice nucleation by a factor of 2-3 compared with pure water, and significantly reduced the rate of ice growth after nucleation. All salt solutions had similar effects on ice nucleation, and no specific ion effect were identified. Ice nucleation simulations for 1 M NaI, NaCl, KI, KCl, and LiI solutions were performed for a range of temperatures. In all cases, the supercooling required for ice nucleation was lower by 1~6 K than that required for pure water.