Zinc–Carboxylate Binding in Mixed Octadecanoic Acid and Octadecanol Monolayers on Proxy Seawater Solution Surfaces

Abstract

Organic coatings on sea spray aerosol are largely comprised of fatty acids in addition to a vast array of other organic molecules including fatty alcohols. The seawater from which sea spray aerosol originates contains metal ions that interact with the organic coating at the air–seawater interface, resulting in transport of these metals into the atmosphere. Metal binding within single-substance monolayers on aqueous solutions has been previously studied. However, such binding events within mixed monolayers, especially those of ocean-relevant ratios, are not well-explored. Here, we examine four monolayer ratios of octadecanoic acid (stearic acid) to octadecanol (stearyl alcohol) on aqueous solutions with varying ZnCl2 concentrations and a seawater-relevant NaCl concentration of 0.465 M. Surface pressure–area (Π-A) isotherms and infrared reflection–absorption spectroscopy (IRRAS) are used to quantify the Zn2+–carboxylate surface binding affinities for each monolayer composition. We find that Zn2+–carboxylate binding is enhanced by ∼300 times at the surface when compared to the bulk solution. Addition of 10% octadecanol reduces the apparent surface binding affinity by over 50% from 3.6 × 103 to 1.4 × 103 M–1; this is significantly more than predicted from the slight reduction in viable 1:1 binding sites. Furthermore, 1:2 Zn2+–carboxylate binding is shown to only be viable for the 100 and 95% fatty acid film, whereas 1:1 binding is observed for all film ratios investigated.

Emerging investigator series: surfactants, films, and coatings on atmospheric aerosol particles: a review

“…Baumler and Allen reviewed vibrational spectroscopy in 2018. 89 Since then, Allen and coworkers, [90][91][92] Grassian and coworkers, 93 and Du and coworkers [94][95][96][97][98][99][100] have published many new studies on the use of infrared reection absorption spectroscopy (IRRAS) to probe model surfactant systems relevant to sea spray aerosol. Allen and coworkers have also continued to explore the surfaces of model sea spray systems with vibrational sum frequency generation (SFG) spectroscopy.…”

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

“…Sea spray aerosols (SSAs) present highly dynamic microenvironments in which their interfacial properties are strongly driven by chemical and physical interactions such as carboxylate−metal binding, 1,2 acid−base equilibrium, 3−5 and ion hydration. 6−8 Each of these parameters unlocks important thermodynamic inputs for global climate models, which provide better insights into atmospheric phenomena like ice nucleation and cold cloud formation.…”

“…Carboxylate−metal interactions under oceanic and atmospheric conditions have been well documented in the literature using various spectroscopic techniques. [1][2][3]23 Exposure to high concentrations of NaCl drives the interfacial equilibrium for certain semisoluble acids, demonstrating a need to expand upon Na + studies, such as their thermodynamically driven interactions, at SSA interfaces. 3 However, the thermodynamic properties of these carboxylate−metal interactions persist as nontrivial measurements.…”

“…Sea spray aerosols (SSAs) present highly dynamic microenvironments in which their interfacial properties are strongly driven by chemical and physical interactions such as carboxylate–metal binding, , acid–base equilibrium, − and ion hydration. − Each of these parameters unlocks important thermodynamic inputs for global climate models, which provide better insights into atmospheric phenomena like ice nucleation and cold cloud formation. The key to unraveling these thermodynamically driven interactions is equilibrium-based measurements and calculations. , Herein, we use equilibrium spreading pressure (ESP) measurements to calculate the thermodynamic equilibrium between the three-dimensional (3D) solid phase and a two-dimensional (2D) film of a fatty acid monolayer to inform on the interfacial chemistry of SSA organic coatings…”

“…Our work marks the revived importance of ESP studies for understanding organic coating phase state under atmospheric conditions and determines the thermodynamic preference for Na + to interact with a palmitic acid/palmitate (PA) monolayer, as shown in Figure . Carboxylate–metal interactions under oceanic and atmospheric conditions have been well documented in the literature using various spectroscopic techniques. − , Exposure to high concentrations of NaCl drives the interfacial equilibrium for certain semisoluble acids, demonstrating a need to expand upon Na + studies, such as their thermodynamically driven interactions, at SSA interfaces . However, the thermodynamic properties of these carboxylate–metal interactions persist as nontrivial measurements.…”