Heating and Evaporation of Aqueous NaCl Solution under Microwave Irradiation

This study combined in situ experiments and molecular simulations to investigate the heating behavior of sodium chloride (NaCl) solutions, up to 1 M, irradiated by microwaves. MW-induced evaporation rate, before reaching boiling, of pure water was obtained at 2.6 × 10-4 g/(s·cm2), which was more than 10 times the water evaporation under nonheating conditions. This evaporation rate increased with rising NaCl concentrations, along with higher temperatures in the outer water layer. At a NaCl concentration of 1 M, the MW-induced evaporation rate increased by 110%, compared with pure water. Notably, the heating rates varied with both NaCl concentration and distance from the air/water surface. Within 1 mm of the surface, the heating rate of a 1 M NaCl solution increased by 90%, compared with pure water. However, the heating rate was almost independent of NaCl concentration at 5 mm from the surface. Molecular dynamics simulations were conducted on a 9 nm-thick water slab under an oscillating electric field of 1 V/nm to enable a qualitative comparison. Despite differences in field amplitude and physical scale, the simulations show a trend similar to the experimental observations, in which both heating and evaporation rates increase with NaCl concentration. In particular, the obtained evaporation rates before boiling were in the same order of magnitude. The simulation provides molecular insights into the impacts of NaCl and microwaves on water dynamics, including diffusion (prior to boiling) and water-water interactions. The combined experimental and simulation findings provide useful parameters for optimizing microwave-assisted evaporation.