J. Alsnielsen et al., PRINCIPLES AND APPLICATIONS OF GRAZING-INCIDENCE X-RAY AND NEUTRON-SCATTERING FROM ORDERED MOLECULAR MONOLAYERS AT THE AIR-WATER-INTERFACE, Physics reports, 246(5), 1994, pp. 252-313
The advent of well collimated, high intensity synchrotron X-ray source
s and the consequent development of surface-specific X-ray diffraction
and fluorescence techniques have recently revolutionized the study of
Langmuir monolayers at the air-liquid interface. These methods allowe
d for the first time the determination of the in-plane and vertical st
ructure of such monolayers with a resolution approaching the atomic le
vel. We briefly describe these methods, including grazing incidence X-
ray diffraction, specular reflectivity, Bragg rods, standing waves and
surface fluorescence techniques, and review recent results obtained f
or Langmuir films from their use. The methods have been successfully a
pplied for the elucidation of the structure of crystalline aggregates
of amphiphilic molecules at the water surface such as alcohols, carbox
ylic acids and their salts, alpha-amino acids and phospholipids. In ad
dition, it became possible to monitor by diffraction the growth and di
ssolution of the crystalline self-aggregates as well as structural cha
nges occurring by phase transitions. Furthermore, via the surface X-ra
y methods, new light is shed on the structure of the underlying attach
ed solvent or solute ionic layer. Examples are given where singly or d
oubly charged ions bound to the two-dimensional (2D) crystal form eith
er an ordered or diffuse counter-ionic layer. Finally, the surface dif
fraction methods provide data on transfer of structural information fr
om 2D clusters to 3D single crystals which had been successfully accom
plished by epitaxial-like crystallization both in organic and inorgani
c crystals.