Structure Control of Ion Exchange in N-Octadecanoyl-l-alanine Langmuir−Blodgett Films Studied by FTIR Spectroscopy

Abstract

Ion transport and ion exchange in two-dimensional N-octadecanoyl-l-alanine Langmuir−Blodgett (LB) films were studied through FTIR spectroscopy. At the intrinsic pH values of the ion-containing solutions (without any adjustment), the metal ions penetrating into the LB films are selectively exchanged with the carboxylic acid groups of film-forming molecules. This behavior is different from the interactions between the metal ions in aqueous subphases and the corresponding monolayers at the air−water interface. Ion exchange in the N-octadecanoyl-l-alanine LB films is controlled by the structure of the intermolecular hydrogen-bonding network. Ion exchange is favorable to Zn2+and Ag+ ions, as it promotes an increase of intermolecular hydrogen bonds and a decrease of intermolecular distance. Ion exchange is unfavorable to Ca2+, Cd2+, and Ni2+ ions, as it is suppressed by the occurrence of intramolecular hydrogen bonds and the increase of intermolecular distance. The reverse exchange process by protons is consistent with the above results. Ion exchange is unfavorable to zinc and silver salts, as it is suppressed by the weakening of intermolecular hydrogen bonds and the increase of intermolecular distance, and it is extremely favorable to calcium, cadmium, and nickel salts in converting from intramolecular to intermolecular hydrogen bonds.