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    Recovery of Fatty Acid Monolayers by Salts Investigated by Sum-Frequency Generation Spectroscopy

    Year: 2022

    Journal: J. Phys. Chem. B, Volume 126, JAN 27, page 643–649

    Authors: Sam, Sokhuoy; Krem, Sona; Lee, Jaejin; Kim, Doseok

    Organizations: NRF [2019R1A2C1004292, 2018R1A6A1A03024940]; Korea Basic Science Institute (National Research Facilities and Equipment Center) grant (Advanced Bio-Interface Core Research Facility) [2020R1A6C101A192]

    Langmuir monolayers consisting of fatty acids with relatively short alkyl chains (C14H29COOH (pentadecanoic acid), C15H31COOH (palmitic acid), and C16H33COOH (heptadecanoic acid)) are stable at a neutral pH (pH approximate to 6) but become unstable at a high pH (pH approximate to 11). Further addition of a small amount of divalent salt in subphase water was found to recover the monolayer at a high pH because binding of the divalent cations to the carboxylic headgroups renders the molecule more stable against dissolution in subphase water. This revival of the monolayer was observed via a pressure-area isotherm measurement and sum-frequency generation spectrum in the CHx and OH ranges. Fatty acids with longer alkyl chains needed less amount of MgCl2 to recover the monolayer at a high pH. A much lower concentration of Mg2+ as compared to Ca2+ is required to revive fatty acid molecules to the surface. Monovalent and trivalent salts were compared with the above divalent salts on the ability to recover the fatty acid monolayers.
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