Microscopic measurement of the linear compressibilities of two-dimensional fatty acid mesophases

The linear compressibility of two-dimensonial fatty acid mesophases has been determined by grazing incidence x-ray diffraction. The unit cell parameters of the L2, L ′ 2, L ′′ 2 , S and CS phases of behenic acid CH3 − (CH2)20 − COOH and of the L2 phase of myristic acid CH3 − (CH2)12 − COOH were determined as a function of surface pressure and temperature. Surface pressure vs molecular area isotherms were reconstructed from these measurements, and the linear compressibility (relative distortion along a given direction for isotropic applied stress) was determined both in the sample plane and in a plane normal to the aliphatic chain director (transverse plane). The linear compressibilities range over two orders of magnitude from 0.1 to 10 m/N and are distributed depending on their magnitude in 4 different sets which we are able to associate with different molecular mechanisms. The largest compressibilities (10 m/N) are observed in the tilted phases. They are apparently independent on the chain length and could