Infrared spectroscopic study of chemically induced dewetting in liquid crystalline types of self-assembled monolayers

Self-assembled monolayers (SAMs) were prepared by chemisorption of n-hexadecanoic acid (1), 4-hexadecyloxybenzoic acid (2), and 4'-hexadecyloxybiphenyl-4-carboxylic acid (3), molecules with liquid crystalline properties, onto air-exposed silver substrate to produce films with essentially identical CO2-/Ag interfaces. Brief H2S exposure leads to immediate -CO2H formation and film reorganization. Both the 1 and 2 SAMs dewet into discrete crystallites with their alkyl chain c-axes aligned near parallel to the substrate plane. In contrast, the 3 SAM reorganizes into a uniform, quasi-2-D, network of hydrogen-bonded CO2H group at the film/substrate interface in which the c-axes remain near perpendicular to the substrate plane. Upon extended storage in air, oxidation of the sulfided Ag surface layer induces a near complete return of the 1 SAM to its original state, while the 2 and 3 SAMs show only partial reversal. These results demonstrate the interplay between intermolecular and interfacial interaction forces and their effects on dynamic restructuring of highly organized monolayers under chemical stress.