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Computational Study of Langmuir Monolayers and Their Properties Using Molecular Dynamics

Undergraduate #45
Discipline: Chemistry and Chemical Sciences
Subcategory: Climate Change

Arielle Hackel - Georgia State University
Co-Author(s): Francesco Paesani and Estefania Rossich-Molina University of California, San Diego, La Jolla, CA



Aerosols are an important component of the atmosphere and influence the climate. However, their properties can be drastically different depending on their chemical composition. It has been shown that sea-spray contain a large amount of organic and biological molecules. These molecules usually coat the aqueous core of sea-spray aerosols which is enriched with inorganic ions. The interfacial region of sea-spray aerosols can thus be described, with a good approximation, by Langmuir monolayers, composed of fatty acid residues, on aqueous subphases. To gain fundamental insights into the properties of sea-spray aerosols, we started a systematic investigation of the structural, thermodynamic, and dynamical properties of single-component monolayers on a pure water subphase. Our hypothesis is that a decrease in membrane volume will lead to a pressure increase that forces the fatty acid monolayers to change from a tilted, liquid form to a solid hexagonal lattice before the monolayer collapses.

Our initial studies are focusing on palmitic acid (C16) and stearic acid (C18) monolayers. In both cases, the monolayer-water systems are built using Packmol. The properties of the monolayers are then investigated through molecular dynamics (MD) simulations performed with the Amber 15 software using atomistic force fields. These simulations allow us to determine the influence of the surface pressure of each monolayer on its physical properties at the air/water interface as well as the hydrogen-bonding properties of interfacial water. Additionally, studying two different fatty acids provides a unique opportunity for understanding the influence of the chain length on the properties of the monolayer.

The results support the hypothesis as the fatty acid monolayers displayed a tilted, liquid conformation at lower pressures and a hexagonal lattice at higher surface pressures. However, monolayer collapse was not witnessed. By using radial distribution functions, we were able to determine that hydrogen bonding interactions between the fatty acid polar heads occurred most often when the polar heads were in a trans conformation. We were also able to determine that the chain length of the fatty acid had little impact on the ability of the fatty acid polar heads to hydrogen bond with water. Future models will include a salinated water model to provide a model closer to oceanic conditions.

Funder Acknowledgement(s): National Science Foundation

Faculty Advisor: Francesco Paesani,

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This material is based upon work supported by the National Science Foundation (NSF) under Grant No. DUE-1930047. Any opinions, findings, interpretations, conclusions or recommendations expressed in this material are those of its authors and do not represent the views of the AAAS Board of Directors, the Council of AAAS, AAAS’ membership or the National Science Foundation.

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