M. K. Durbin

Tilted phases of fatty acid monolayers

X‐ray diffraction data from water‐supported monolayers of fatty acids with chain lengths from 19 to 22 is presented. The structures of the tilted mesophases L2’, L2, and Ov are characterized in detail. The contributions to the unit cell distortion from the tilt and the ordering of the backbone planes of the molecules are separated. It is shown that at the swiveling transition L2’–L2, not only the tilt azimuth but also the packing of the backbone planes change discontinuously. We demonstrate that the tilting transition LS–L2 is accompanied by the ordering of the backbone planes and may be discontinuous. Evidence is presented for a herringbone ordering transition within the L2 region. The distortions are related to symmetry of the phases and described by the order parameters responsible for tilt and herringbone ordering of the backbone planes of the molecules.

Transitions to a new chiral phase in a Langmuir monolayer

Isotherms and x-ray diffraction studies of eicosanoic acid Langmuir monolayers show a phase in which the molecular tilt is intermediate between nearest-neighbor (NN) and next-nearest-neighbor (NNN) directions. The transition from this “I” phase to an NN-tilted structure is first order, with a ∼60° change in the tilt direction, while the transition to an NNN-tilted structure is apparently continuous. These results can be explained using a Landau-type theory for uniaxially distorted lattices, which is a modification of an existing theory for hexagonal lattices [J. V. Selinger and D. R. Nelson, Phys. Rev. Lett. 61, 416 (1988)].

Specular x-ray reflectivity studies of microstructure and ordering in self-assembled multilayers

Specular x-ray reflectivity has been used to probe the structures of self-assembled monolayers and multilayers deposited using a three-step siloxane-based self-assembly technique that is repeated to form periodic multilayers. In films containing up to ten trilayers, it is found that the film thickness increases linearly as a function of the number of trilayers with no observable change in the surface roughness. Bragg peaks corresponding to the inter-trilayer spacing are observed. Both of these results indicate high structural regularity in these self-assembled multilayers. In self-assembled films with different constituent molecular building blocks, substantial and unexpected changes in the film structure occur as a result of subtle changes in the layers.

Order in Langmuir-Blodgett films of lead and cadmium stearate: an X-ray diffraction study

Abstract We have studied monolayer and multilayer Langmuir-Blodgett films of lead stearate and cadmium stearate using grazing incidence X-ray diffraction. We find that although these two types of films have similar in-plane structures, lead stearate forms better ordered films: the correlation length increases with the number of layers in lead stearate but levels off at the three-layer value in cadmium stearate. The lattices of adjacent lead ion layers are correlated, but adjacent cadmium layers are not.

Isotherm and X-ray diffraction studies of mixed monolayers

We have studied Langmuir monolayers of saturated fatty acid-alcohol mixtures (same chain length) and acid-acid mixtures (different chain length) using isotherms and X-ray diffraction. The diffraction data confirm that the constituents are miscible, and that at zero pressure the mixed acid-alcohol monolayer areas are less than the areas of the pure components. We have determined the phase diagrams of a number of different mixtures. In any given phase, the intermolecular distances measured perpendicular to the molecules are independent of the pressure and of the constituents of the mixture. However, the location of some phase boundaries is a strong function of the composition of the mixture.