Selective Cationic Binding at the Air−Water Interface by Thin Films of Rigid Amphiphiles Bearing Laterally Attached Crown Ether Moieties

Abstract

Three 4,4-didecyloxy-p-terphenyl derivatives with laterally attached crown ether units of different ring size [(1,4,7,10-tetraoxacyclododecyl)-,(1,4,7,10,13-pentaoxacyclopentadecyl)-, and (1,4,7,10,13,16-hexaoxacyclooctadecyl)-2-methyl 2,5-bis(4-decyloxyphenyl)benzoate] have been synthesized, and their Langmuir films were investigated by means of a film balance. They form well-ordered thin films when spread at the air-water interface which are characterized by two sharp breaks in their surface pressure-area isotherms separated by a large plateau. The lateral pressure in the plateau region-in which the molecules undergo a first-order phase transition with formation of a defined triple layer-significantly increases on enhancing the number of oxyethylene units in the crown ether units. Additionally, it depends on the type and concentration of alkali metal ions (Li + , Na + , K + , Rb + , Cs + ) in the subphase. 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 alkali metal ions to the crown ether units. This shows that facial amphiphiles could represent an interesting new platform for the investigation of guest-host interactions and molecular recognition processes at the air-water interface.