title: Hydrogen-bonded monolayers and interdigitated multilayers at the air-water interface.
abstract: Crystalline monolayers of octadecylsulfonate amphiphiles (C18S) separated by hydrophilic guanidinium (G) spacer molecules were formed at the air-water interface at a surface coverage that was consistent with that expected for a fully condensed monolayer self-assembled by hydrogen bonding between the G ions and the sulfonate groups. The surface pressure-area isotherms reflected reinforcement of this monolayer by hydrogen bonding between the G ions and the sulfonate groups, and grazing incidence X-ray diffraction (GIXD) measurements, performed in-situ at the air-water interface, revealed substantial tilt of the alkyl hydrophobes (t = 49 degrees with respect to the surface normal), which allowed the close packing of the C18 chains needed for a stable crystalline monolayer. This property contrasts with behavior observed previously for monolayers of hexadecylbiphenylsulfonate (C16BPS) and G, which only formed crystallites upon compression, accompanied by ejection of the G ions from the air-water interface. Upon compression to higher surface pressures, GIXD revealed that the highly tilted (G)C18S monolayer crystallites transformed to a self-interdigitated (G)C18S crystalline multilayer accompanied by a new crystalline monolayer phase with slightly tilted alkyl chains and disordered sulfonate headgroups. This transformation was dependent on the rate of compression, suggesting kinetic limitations for the "zipper-like" transformation from the crystalline monolayer to the self-interdigitated (G)C18S crystalline multilayer.
authors.affiliation: Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA.
authors.forename: Stephen M
authors.initials: SM
authors.lastname: Martin
authors.forename: Kristian
authors.initials: K
authors.lastname: Kjaer
authors.forename: Markus J
authors.initials: MJ
authors.lastname: Weygand
authors.forename: Isabelle
authors.initials: I
authors.lastname: Weissbuch
authors.forename: Michael D
authors.initials: MD
authors.lastname: Ward