On the Origin of the Plateau in Surface-Pressure Isotherms of Aromatic Carboxylic Acids

Langmuir monolayers of 4‘-(4-methylphenyl)-5‘-phenyl-1,1‘:3‘,1‘ ‘-terphenyl-4-carboxylic acid (p-tolyl-PTCA) show a well-defined surface isotherm indicative of a fluid phase followed by a pronounced and broad plateau in which the molecular area decreases by about a factor of 2. The origin of this plateau has been investigated using fluorescence and Brewster angle microscopy (BAM) as well as Fourier transform infrared reflectionabsorption spectroscopy (FT-IRRAS). Whereas the surface structure is monomolecular at pressures below the onset of the plateau, the experimental results point to the formation of structures that are no longer monomolecular in the plateau region: As the plateau is reached, changes in the film structure are observed in fluorescence micrographs whereas FT-IRRAS indicates a change in the hydration of the carboxyl groups. A 4-fold increase in reflectivity observed in BAM across the plateau corresponds to an increase in film thickness by a factor of 2. This may correspond to the formation of a laterally homogeneous double layer or to a surface structure that is inhomogeneous on a submicrometer length scale, in which monolayer regions coexist with areas covered with more than two monolayers of material. Subsidiary AFM results indicate the presence of triple-layer patches scattered randomly over a smooth monolayer along the plateau. This is in contradiction to the earlier hypothesis of molecular tilting, which was thought to be more probable than a multilayer formation (Dynarowiczatka, P.; Dhanabalan, A.; Cavalli, A.; Oliveira, O. N., Jr. J. Phys. Chem. B 2000, 104, 1701). It thus appears that although polyphenyl carboxylic acids (PTCA and its derivatives) do not show liquid crystalline properties their surface structure resembles more closely that of molecular mesogen films rather than that of monolayers formed by conventional amphiphilic compounds.