Anion effects on monolayers of a new amphiphilic styryl-pyridinium dye at the air–water interface

The effect of various salts in the aqueous subphase on monolayer properties of a new methylthio-styryl-pyridinium dye has been investigated by measuring surface pressure–area and surface potential–area isotherms as well as by reflection spectroscopy and Brewster angle microscopy. At any area per molecule the surface pressure increases in the series KClO4 < KBr < KCl < KF, that follows the Hofmeister series. No chromophore association upon monolayer compression is observed by reflection spectroscopy, however, after passing a phase transition at an area of about 0.7 nm2, tilting of the chromophores is found. This behavior differs considerably from that of an analog dye with two methoxy groups instead of the methylthio group in the styryl part of the chromophore.