SELECTIVE CATIONIC BINDING AT THE AIR-WATER-INTERFACE BY THIN FILMS OF RIGID AMPHIPHILES BEARING LATERALLY ATTACHED CROWN-ETHER MOIETIES

Three 4,4 ''-didecyloxy-p-terphenyl derivatives with laterally attache d crown ether units of different ring size [(1,4,7,10-tetraoxacyclodod ecyl)-, (1,4,7,10,13-pentaoxacyclopentadecyl)-, and (1,4,7,10,13,16-he xaoxacyclooctadecyl)-2-methyl 2,5-bis(4-decyloxyphenyl)blenzoate] have been synthesized, and their Langmuir films were investigated by means of a film balance. They form well-ordered thin films when spread at t he air-water interface which are characterized by two sharp breaks in their surface pressure-area isotherms separated by a large plateau. Th e lateral pressure in the plateau region-in which the molecules underg o a first-order phase transition with formation of a defined triple la yer-significantly increases on enhancing the number of oxyethylene uni ts in the crown ether units. Additionally, it depends on the type and concentration of alkali metal ions (Li+, Na+, K+, Rb+, Cs+) in the sub phase. Cations which fit the cavity of the appended crown ethers most significantly stabilize the films. It seems that the surface pressure in the plateau region is reminiscent of the binding constants of alkal i metal ions to the crown ether units. This shows that facial amphiphi les could represent an interesting new platform for the investigation of guest-host interactions and molecular recognition processes at the air-water interface.