Thermodynamic versus non-equilibrium stability of palmitic acid monolayers in calcium-enriched sea spray aerosol proxy systems

Abstract

Of the major cations in seawater (Na⁺, Mg²⁺, Ca²⁺, K⁺), Ca²⁺ is found to be the most enriched in fine sea spray aerosols (SSA). In this work, we investigate the binding of Ca²⁺ to the carboxylic acid headgroup of palmitic acid (PA), a marine-abundant fatty acid, and the impact such binding has on the stability of PA monolayers in both equilibrium and non-equilibrium systems. A range of Ca²⁺ conditions from 10 μM to 300 mM was utilized to represent the relative concentration of Ca²⁺ in high and low relative humidity aerosol environments. The CO₂⁻ stretching modes of PA detected by surface-sensitive infrared reflection–absorption spectroscopy (IRRAS) reveal ionic binding motifs of the Ca²⁺ ion to the carboxylate group with varying degrees of hydration. Surface tensiometry was used to determine the thermodynamic equilibrium spreading pressure (ESP) of PA on the various aqueous CaCl₂ subphases. Up to concentrations of 1 mM Ca²⁺, each system reached equilibrium, and Ca²⁺:PA surface complexation gave rise to a lower energy state revealed by elevated surface pressures relative to water. We show that PA films are not thermodynamically stable at marine aerosol-relevant Ca²⁺ concentrations ([Ca²⁺] ≥ 10 mM). IRRAS and vibrational sum frequency generation (VSFG) spectroscopy were used to investigate the surface presence of PA on high concentration Ca²⁺ aqueous subphases. Non-equilibrium relaxation (NER) experiments were also conducted and monitored by Brewster angle microscopy (BAM) to determine the effect of the Ca²⁺ ions on PA stability. At high surface pressures, the relaxation mechanisms of PA varied among the systems and were dependent on Ca²⁺ concentration.