Molecular investigation on the anomalous phenomenon at liquid desiccant surfaces for air conditioning

It is necessary to disclose the two-phase interphase behavior in the liquid desiccant dehumidifier/regenerator applicable for air conditioning, but the present investigation is far from enough. In this paper, the surface structure of liquid desiccant solution is analyzed by molecular dynamics simulations. LiBr-H2O is chosen as the working solution with a concentration of 1 M and the system model is built with Gromacs. System temperatures vary from 300 to 350 K covering the temperature range of liquid desiccant dehumidification and regeneration. Density profiles of ions and water molecules are plotted along the vertical directions, and their distribution preferences on the solution surface are discussed. With the molecular simulation, it is found there is an ions-vapor layer with a thickness of 6–9 Å between the saturated vapor and bulk solution, which is not shown in the traditional macroscopic models. The results show that the density of water remains stable in the bulk while decreases sharply on the solution surface. However, the salt ions, i.e. Li+ and Br-, have a peak density on the surface. This ions-vapor layer behaves like a buffer to transfer water molecules from/to the bulk solution. More research will be required to investigate how to control the ions-vapor layer, so that air dehumidification and solution regeneration can be easily operated, which provides significant energy savings for the liquid desiccant air conditioning.