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.