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

Langmuir monolayers consisting of fatty acids with relatively short alkyl chains (C 14 H 29 COOH (pentadecanoic acid), C 15 H 31 COOH (palmitic acid), and C 16 H 33 COOH (heptadecanoic acid)) are stable at a neutral pH (pH ≈ 6) but become unstable at a high pH (pH ≈ 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 CH x and OH ranges. Fatty acids with longer alkyl chains needed less amount of MgCl 2 to recover the monolayer at a high pH. A much lower concentration of Mg 2+ as compared to Ca 2+ 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.