title: Origin of the Instability of Octadecylamine Langmuir Monolayer at Low pH.
abstract: It has been reported that an octadecylamine (ODA) Langmuir monolayer becomes unstable at low pH values with no measurable surface pressure at around pH 3.5, suggesting significant dissolution of the ODA molecule into the subphase solution (Albrecht, Colloids Surf. A 2006, 284-285, 166-174). However, by lowering the pH further, ODA molecules reoccupy the surface, and a full monolayer is recovered at pH 2.5. Using surface sum-frequency spectroscopy and pressure-area isotherms, it is found that the recovered monolayer at very low pH has a larger area per molecule with many gauche defects in the ODA molecules as compared to that at high pH values. This structural change suggests that the reappearance of the monolayer is due to the adsorbed Cl(-) counterions to the protonated amine groups, leading to partial charge neutralization. This proposition is confirmed by intentionally adding monovalent salts (i.e., NaCl, NaBr, or NaI) to the subphase to recover the monolayer at pH 3.5, in which the detailed structure of the monolayer is confirmed by sum frequency spectra and the adsorbed anions by X-ray reflectivity.
authors.forename: Zaure
authors.initials: Z
authors.lastname: Avazbaeva
authors.forename: Woongmo
authors.initials: W
authors.lastname: Sung
authors.forename: Jonggwan
authors.initials: J
authors.lastname: Lee
authors.forename: Minh Dinh
authors.initials: MD
authors.lastname: Phan
authors.forename: Kwanwoo
authors.initials: K
authors.lastname: Shin
authors.affiliation: Ames Laboratory and Department of Physics and Astronomy, Iowa State University , Ames, Iowa 50011, United States.
authors.forename: David
authors.initials: D
authors.lastname: Vaknin
authors.forename: Doseok
authors.initials: D
authors.lastname: Kim
mesh_terms: D000588:Amines / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D012965:Sodium Chloride / Q000737:chemistry; D013499:Surface Properties; D014867:Water / Q000737:chemistry
chemical_list: D000588:Amines; D014867:Water; D012965:Sodium Chloride; C009317:stearylamine