Combining experiment and theory to probe salt aerosol deliquescence

Atmospheric aerosols represent an interesting and practical way for students to learn about chemical thermodynamics. This paper describes a laboratory experiment designed for physical chemistry students in which they learn about phase transitions of salt aerosols. The aerosols are generated by atomizing a salt solution, and then they are passed through a gas cell in a transmission-mode Fourier transform infrared spectrometer. Utilizing the O-H stretch of liquid water, the relative quantity of water in the aerosols can be measured. The deliquescence phase transition in which the solid aerosol particle dissolves to form a liquid droplet is evidenced by an abrupt increase in the water content of the aerosols. Multiple salts are suitable for analysis, including sodium chloride and ammonium bisulfate. Additionally, comparisons to model calculations can be made using the Extended Aerosol Inorganics Model Web site. This lab exposes students to important thermodynamic concepts such as chemical potential, activity, and phase transitions and forces them to critically interpret spectra.