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

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~6) but become unstable at a high pH (pH ~ 11). The 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 range. Fatty acid with longer alkyl chain needed less amount of MgCl₂ to recover the monolayer at high pH. 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.