Relaxation and the reentrant appearance of phases in a molecular monolayer

Abstract

The structure of monolayers of ${\mathrm{CH}}_{3}$(${\mathrm{CH}}_{2}$${)}_{22}$COOH on water, along \ensuremath{\sim}21 \ifmmode^\circ\else\textdegree\fi{}C isotherms between 18 and 35 \AA{} $^{2}$/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 \ensuremath{\pi} is relaxing \ensuremath{\lesssim}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.