The partitioning of volatile and semi-volatile organic species between condensed and vapor phases is important in a wide range of natural and engineered systems, including organic solvents, fuel mixtures, and atmospheric organic aerosol. The vapor pressure of a substance determines its saturation pressure above a substance in its pure state. In mixtures, the vapor pressure scales with the concentration of each component, with non-ideal effects leading to complex mixing rules. Additional complications arise when vapor pressures of solid systems are explored, as these may exist in equilibrium (i.e. a crystal) or non-equilibrium states (e.g. an amorphous sub-cooled liquid), leading to a wide range of possible vapor pressures.<br/><br/>In this work, a linear quadrupole electrodynamic balance was used to probe the evaporation rate of single levitated particles to characterize vapor pressures. Using Mie resonance spectroscopy, the precise evolution of the size of spherical particles was measured, while in the case of non-spherical particles, the electrostatic force required to balance the weight was used to estimate evolving mass. We incorporate a heating element to control temperature from ambient conditions up to ~373K, allowing vapor pressures and the enthalpy of vaporization to be determined for even low volatility substances. We specifically explore the vapor pressures of linear chain terminal dicarboxylic acid molecules, which are found in atmospheric aerosol and engine oils, for example, as a result of oxidation processes. These acids show distinct behavior based on the number of carbon atoms, and may exist in both crystalline or sub-cooled states under dry conditions. Through these measurements we highlight the thermodynamic properties describing the liquid/solid-vapor equilibrium of malonic, glutaric, succinic and adipic acid particles.

Vapor pressure measurements of low-volatility particles containing organic mixtures

ACS Spring 2022

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