C. Fradin et al., MICROSCOPIC MEASUREMENT OF THE LINEAR COMPRESSIBILITIES OF 2-DIMENSIONAL FATTY-ACID MESOPHASES, EUROPEAN PHYSICAL JOURNAL B, 1(1), 1998, pp. 57-69
The linear compressibility of two-dimensional fatty acid mesophases ha
s been determined by grazing incidence X-ray diffraction. The unit cel
l parameters of the L-2, L-2', L-2'', S and CS phases of behenic acid
CH3-(CH2)(20)-COOH and of the L-2 phase of myristic acid CH3-(CH2)(12)
-COOH were determined as a function of surface pressure and temperatur
e. Surface pressure versus molecular area isotherms were reconstructed
from these measurements, and the linear compressibility (relative dis
tortion along a given direction for a two-dimensional isotropic applie
d stress) was determined both in the sample plane and in a plane norma
l to the aliphatic chain director (transverse plane). The linear compr
essibilities range over two orders of magnitude from 0.1 to 10 m/N and
are distributed depending on their magnitude in 4 different sets whic
h we are able to associate with different molecular mechanisms. The la
rgest compressibilities (10 m/N) are observed in the tilted phases. Th
ey are apparently independent on the chain length and could be related
to the reorganization of the headgroup hydrogen-bounded network, whos
e role should be revalued. Intermediate compressibilities are observed
in phases with quasi long-range order (directions normal to the molec
ular tilt in the L-2 or L-2' phases, S phase, and could be related to
the ordering of these phases. The lowest compressibilities are observe
d in the solid untilted CS phase and for one direction of the S and L-
2'' phases. They are similar to the compressibility of crystalline pol
ymers and correspond to the interactions between methyl groups in the
crystal. Finally, negative compressibilities are observed in the trans
verse plane for the L-2' and L-2'' phases and can be traced to subtle
reorganizations upon untilting.