Influence of apolar group structure on the properties of Langmuir monolayers of polyphenyl carboxylic acids

Langmuir monolayers of purely aromatic carboxylic acids are investigated through surface pressure (π), surface potential (ΔV)–area (A) isotherms and Brewster angle microscopic studies. The monolayer characteristic of the basic compound, namely 5′-phenyl-1,1′:3′,1″-terphenyl-4-carboxylic acid, abbreviated as PTCA, is compared with its derivatives containing hydrophilic (nitro) or hydrophobic (phenyl) substituents attached to the symmetrical triphenylbenzene core. The nature of the substituent as well as its position (2′ or 4′) has profound influence on the monolayer organization. Also, chemical modification leading to rotation restrictions of the hydrophobic moiety of PTCA was found to alter significantly its characteristic as Langmuir monolayer. The results are discussed in view of different orientation and different molecular packing arrangements.