Dicationic-Type Quaternary Ammonium Salts as Candidates of Desiccants for an Air-Conditioning System

Liquid desiccant air conditioners contribute to sustainable living by reducing the electricity consumption. They can be driven with continuous ventilation and thus can prevent airborne transmission of infectious diseases like COVID-19. In order to develop efficient desiccant materials for liquid desiccant air conditioners, we have investigated the dehumidification capability of 21 types of ammonium salts, many of them being ionic liquids. The hygroscopicity of dicationic quaternary ammonium bis(dimethyl or diethylphosphate) is found to increase with the carbon chain length of the spacer group -(CH2)(n)- that bridges the two terminal cationic moieties of the dication, that is, (CH2)(2) < (CH2)(3) < (CH2)(6). This trend remains unchanged when methyl substituents on the cation and the phosphate anion are replaced by ethyl groups. A high dehumidification capability is obtained for N-1,N-1,N-1,N-6,N-6,N-6-hexamethylhexane-1,6-diaminium bis(dimethylphosphate) ([HMC6]-[DMPO4](2)) and 1,6-diethyl-1,1,6,6-tetramethylhexane-1,6-diaminium bis( diethylphosphate) ([DETMC6][DEPO4](2)); their capability per gram reaches almost twice that of CaCl2, and their dehumidification rate is superior to popular solid desiccants. A favorable equilibrium water vapor pressure for the liquid desiccant air-conditioning system is attained for an 80% (w/w) aqueous solution of [HMC6][DMPO4](2). In addition, the 80% aqueous solution of [HMC6][DMPO4](2) affords no corrosive effect on steel, aluminum, and stainless steel and a very weak effect on copper. Dicationic quaternary ammonium bis(dimethyl or diethylphosphate)s are stable compounds; their aqueous solutions produce no odor after storing for over 1 year under ambient conditions. Molecular dynamics simulations are performed to gain insights into the solvation structure, energetics, and transport behavior of water in aqueous solutions of [HMC6][DMPO4](2) and related salts.