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

To gain global understanding of the complexinteractions marine organics participate in at the surface of seaspray aerosols (SSAs), thermodynamic parameters are needed asinputs for atmospheric models. Traditional surface studies useLangmuirfilms that exist in pseudoequilibrium and requireassumptions to obtain thermodynamic properties. To addressthese challenges, we have developed a new application forequilibrium spreading pressure (ESP), an experimental valuebased on the true thermodynamic equilibrium between afilmand its solid form at the aqueous interface. By changing theequilibrium phase state of a marine relevant palmitic acid/palmitate(PA) monolayer as a function of temperature (7.0-20.2 degrees C) andsodium chloride concentration (1-1000 mM), we can exper-imentally capture the thermodynamic values of three-dimensional (3D) solid to two-dimensional (2D) monolayer spreading. Coolertemperatures present more unfavorable and disordered conditions for spreading PA into a 2Dfilm. However, the addition of NaCl tothe solution does not follow a monotonical trend in thermodynamic values, exhibited by an anomalous 100 mM NaCl condition,which is similar to 20 and similar to 35% more enthalpically favorable than 10 and 1000 mM NaCl, respectively. Our results represent some of thefirst thermodynamic data for spreading PA from 3D solid to 2Dfilm given atmospheric conditions and thefirst study to utilize ESPto determine the thermodynamic properties of PA-Na+interactions. Our true equilibrium-based approach helps to inform on the3D to 2D phase-state transition of organic coatings, presenting an essential input for climate models and global understanding ofSSA interfaces.