Interfacial Enrichment of Lauric Acid Assisted by Long-Chain Fatty Acids, Acidity and Salinity at Sea Spray Aerosol Surfaces

Surfactant monolayers at sea spray aerosol (SSA) surfaces regulate various atmospheric processes including gas transfer, cloud interactions, and radiative properties. Most experimental studies of SSA employ a simplified surfactant mixture of long-chain fatty acids (LCFAs) as a proxy for the sea surface microlayer or SSA surface. However, medium-chain fatty acids (MCFAs) make up nearly 30% of the FA fraction in nascent SSA. Given that LCFA monolayers are easily disrupted upon the introduction of chemical heterogeneity (such as mixed chain lengths), simple FA proxies are unlikely to represent realistic SSA interfaces. Integrating experimental and computational techniques, we characterize the impact that partially soluble MCFAs have on the properties of atmospherically relevant LCFA mixtures. We explore the extent to which the MCFA lauric acid (LA) is surface stabilized by varying acidity, salinity, and monolayer composition. We also discuss the impacts of pH on LCFA-assisted LA retention, where the presence of LCFAs may shift the surface-adsorption equilibria of laurate—the conjugate base—toward higher surface activities. Molecular dynamic simulations suggest a mechanism for the enhanced surface retention of laurate. We conclude that increased FA heterogeneity at SSA surfaces promotes surface activity of soluble FA species, altering monolayer phase behavior and impacting climate-relevant atmospheric processes.


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Complete list of systems studied with molecular dynamics.The abbreviations UC and TC stand for untilted condensed and tilted condensed phases, respectively.such that the isotherm will align with that of the tertiary mixture if lauric acid is completely absent from the interface.We can then quantify the retention of lauric acid by the equation where the APLs are extracted from the liquid condensed (LC) phase.Plots of the resulting P-A isotherms with the corrected and uncorrected traces are provided below.Results correspond to 50%, -10%, and 30% LA retention for pH 2, 5.6, and 8.2, respectively.

Figure S2 .
Figure S2.Visual description of protonated binary lauric acid (LA, green licorice): palmitic acid (PA, purple licorice) monolayer molecular dynamics system in the presence of 0.4 M NaCl.Top: Snapshot from molecular dynamics simulation of protonated LA over a 0.4 M subphase.Bottom: Density profile of water (red), sodium (green), LA carboxylic acid headgroups (orange), and PA carboxylic acid headgroups (blue).This figure indicates that the headgroups of the FA stagger in such a way that PA headgroups are more solvated than LA headgroups.

Figure S3 .
Figure S3.Split violin plots representing H-bonds per residue for LA:PA monolayers at 23 Å 2 /molecule over 0.4 M NaCl (coral) and pure water (purple) subphases.H-bonds were calculated using the H-Bonds analysis tool in VMD v1.9.4a57 with a donor-acceptor distance of 2.9 Å and an angle cut-off of 20 degrees.

Figure S4 .
Figure S4.Radial distribution functions, G(r), between C1 headgroup carbons at low pressure (23 Å 2 /molecule, dashed line) and high pressure (20 Å 2 /molecule, solid line).The distance cutoffs used to determine connected molecules is given by the minimum between the first and second density peaks are labeled.

Figure S5 .
Figure S5.Equimolar LA:PA isotherms at pH 5.6 over a pure water (A) and 0.4 M NaCl (B) aqueous subphase.The profiles, in comparison to PA over pure water, indicate that LA is more surface-stabilized by PA over a salt water subphase than over pure water.The profile of the binary mixture in (A) has significant PA character, with LA contributing to increased compressibility from MMA 15-45 Å 2 /molecule.In contrast, over salt, the profile less closely matches pure PA as the LA is stabilized by the salt and the mixture remains miscible across a broader MMA range.

Figure S6 .
Figure S6.Surface pressurearea isotherms of binary mixtures of lauric acid (LA) and palmitic acid (PA) in a 3:1 molar ratio (black trace) and a 1:3 molar ratio (purple trace).The fatty acid mixtures are spread on pure water at pH 5.6.Greater LA molar contributions lead to a more fluidized monolayer, and increasing the PA molar contribution causes the monolayer to become more rigid due to greater dispersion forces between the long-chain fatty acids.

Figure S7 .
Figure S7.Schematic representing the H-bonding interactions influencing mixing behavior between FAs with mixed protonation states.