Molecular recognition of a dissolved carboxylate by amidinium monolayers at the air-water interface

Abstract

The specific properties of a molecular recognition system based on the acid-base interactions of amidinium and carboxylate components are studied. The cationic heptadecylbenzamidinium chloride spread as a Langmuir monolayer is the “host” component and sodium phenylacetate dissolved in the aqueous subphase is the “guest” component. The interaction of the heptadecylbenzamidinium chloride with the dissolved carboxylate causes drastic changes in the surface properties. The specific features of the surface film of the amidinium-carboxylate complex are characterised by coupling the results of the surface pressure-area (π-A) isotherms with the Brewster angle microscopy and atomic force microscopy studies. In the plateau region of the π-A isotherms of the amidinium-carboxylate monolayer, round domains consisting of numerous filigree branches are formed. At higher resolution, strings of molecular thickness are observed which with further compression increasingly overgrow with a second molecular layer.