Hydrogen-Bonded Monolayers and Interdigitated Multilayers at the Air−Water Interface

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° 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.