Langmuir monolayers of polyphenyl carboxylic acids

A series of 4'-substituted (methyl, phenyl, biphenyl, toluilyl, nitro, cyan o, and p-nitrophenyl) 5'-phenyl-m-terphenyl-4-carboxylic acids (PTCAs) have been synthesized, and their Langmuir monolayers were characterized using s urface pressure and surface potential isotherms under a variety of experime ntal conditions, A long plateau appears at the pressure-area isotherms for most compounds, which is attributed to the tilting of molecules upon compre ssion. A Model is presented for the molecular arrangements which explains t he decrease in dipole moment in the plateau region. As expected, the monola yer stability was higher for derivatives with more hydrophobic substituents , whereas the introduction of an additional hydrophilic group, viz. -CN or -NO2, in the parent PTCA molecule yielded negative surface potentials since the latter groups contributed negatively, in opposition to the positive co ntribution from the COOH group.