Phase State and Thermodynamic Properties of Proxy Sea Spray Aerosol Interfaces Derived from Temperature-Dependent Equilibrium Spreading Pressure

To gain global understanding of the complex interactions marine organics participate in at the surface of sea spray aerosols (SSAs), thermodynamic parameters are needed as inputs for atmospheric models. Traditional surface studies use Langmuir films that exist in pseudoequilibrium and require assumptions to obtain thermodynamic properties. To address these challenges, we have developed a new application for equilibrium spreading pressure (ESP), an experimental value based on the true thermodynamic equilibrium between a film and its solid form at the aqueous interface. By changing the equilibrium phase state of a marine relevant palmitic acid/palmitate (PA) monolayer as a function of temperature (7.0–20.2 °C) and sodium chloride concentration (1–1000 mM), we can experimentally capture the thermodynamic values of three-dimensional (3D) solid to two-dimensional (2D) monolayer spreading. Cooler temperatures present more unfavorable and disordered conditions for spreading PA into a 2D film. However, the addition of NaCl to the solution does not follow a monotonical trend in thermodynamic values, exhibited by an anomalous 100 mM NaCl condition, which is ∼20 and ∼35% more enthalpically favorable than 10 and 1000 mM NaCl, respectively. Our results represent some of the first thermodynamic data for spreading PA from 3D solid to 2D film given atmospheric conditions and the first study to utilize ESP to determine the thermodynamic properties of PA–Na+ interactions. Our true equilibrium-based approach helps to inform on the 3D to 2D phase-state transition of organic coatings, presenting an essential input for climate models and global understanding of SSA interfaces.