Control of the molecular packing in guanidinium monolayers through binding with aqueous polycarboxylates

By using newly developed guanylating agents, octadecyl- and dioctadecylguanidinium amphiphiles were synthesized. The interaction between each guanidinium monolayer and polycarboxylates in the subphase was investigated on the basis of the pi-A isotherm, FT-IR spectroscopy, and XPS measurements. When linear dicarboxylates were used, the molecular areas of the monolayer increased, as the length of the methylene chain between the carboxylate groups increased. The expansion of the molecular area was greater for the octadecylguanidinium monolayers than for the dioctadecylguanidinium monolayers. The molecular packing was affected by the shape of polycarboxylate molecules in the case of phthalate, cis-1,2-cyclohexanedicarboxylate, and 1,1-cyclohexanediacetate. It is clear that, the molecular packing in the complexed monolayers is governed by the distance and relative orientation of the two carboxylate groups in a polycarboxylate. With all of the dicarboxylates, excluding oxalate, FT-IR and XPS measurements of the LB films indicated the formation of 1 : 1 guanidinium/carboxylate pairs with hydrogen bonding interactions. Oxalate produced an asymmetric complex where one guanidinium was bound to oxalate through hydrogen bonding, and the other guanidinium existed as a non-hydrogen bonded counter ion. These results are useful for the development of two-dimensional molecular patterns.