We have used X-ray reflectivity and grazing-incidence X-ray diffraction to
study amphiphilic crown ethers 1 and 2 at the air-water interface. A new sy
nthetic route was employed, which substantially reduced the level of impuri
ties. Unlike previous studies of 1, we saw no evidence of a first-order lyi
ng-down to standing-up transition of the alkyl tails. We did observe in bot
h compounds a gradual reorientation of the tails away from the interface as
the film was compressed, followed by an irreversible collapse to a multila
yer structure. The monolayer films have a fluid-like inplane structure, whi
le the multilayer films are crystalline.