https://doi.org/10.1007/s100510050152
Microscopic measurement of the linear compressibilities of two-dimensional fatty acid mesophases
1
Service de Physique de l'État Condensé,
CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
2
LURE, 91405 Orsay Cedex, France
3
Laboratoire Physico-Chimie Curie (CNRS, UMR 168) , Institut Curie,
11 rue Pierre et Marie
Curie, 75231 Paris Cedex 05, France
Corresponding author: a daillant@spec.saclay.cea.fr
Received:
29
July
1997
Revised:
14
October
1997
Accepted:
23
October
1997
Published online: 15 January 1998
The linear compressibility of two-dimensional fatty acid mesophases
has been determined by grazing incidence X-ray diffraction.
The unit cell parameters of the ,
,
, S and
phases of
behenic acid
and of the
phase of myristic
acid
were determined as a function of surface
pressure and temperature. Surface pressure versus molecular area isotherms
were reconstructed from these measurements, and the linear compressibility
(relative distortion along a given direction for a two-dimensional
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 be related
to the reorganization of the headgroup hydrogen-bounded network, whose
role should be revalued.
Intermediate compressibilities are observed in phases with quasi long-range
order (directions normal to the molecular tilt in the
or
phases, S phase, and could be related to the ordering of these phases.
The lowest compressibilities are observed in the solid untilted
phase and for
one direction of the S and
phases. They are similar to the
compressibility of crystalline polymers and correspond to the interactions
between methyl groups in the crystal. Finally, negative compressibilities
are observed in the transverse plane for the
and
phases and can be
traced to subtle reorganizations upon untilting.
PACS: 61.10-i – X-ray diffraction and scattering / 68.10.Et – Interface elasticity, viscosity and viscoelasticity / 68.60.Bs – Mechanical and acoustical properties
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 1998