Relaxation and the reentrant appearance of phases in molecular monolayer

The structure of monolayers of CH{sub 3}(CH{sub 2}){sub 22}COOH on water, along {similar to}21 {degree}C isotherms between 18 and 35 A {sup 2}/molecule, has been studied with glancing-angle x-ray diffraction and specular reflection. Following an incremental area change, in some regions of the isotherm, the diffraction pattern evolves until the surface pressure {pi} is relaxing {approx lt}0.01 dyn/cm h. Reentrant behavior between tilted and nontilted phases is observed. Phase coexistence is indicated by differences between the deposited area/molecule and the area of the x-ray-determined unit cell.