Stable ordering in Langmuir-Blodgett films.

Defects in the layering of Langmuir-Blodgett (LB) films can be eliminated by depositing from the appropriate monolayer phase at the air-water interface. LB films deposited from the hexagonal phase of cadmium arachidate (CdA2) at pH 7 spontaneously transform into the bulk soap structure, a centrosymmetric bilayer with an orthorhombic herringbone packing. A large wavelength folding mechanism accelerates the conversion between the two structures, leading to a disruption of the desired layering. At pH > 8.5, though it is more difficult to draw LB films, almost perfect layering is obtained due to the inability to convert from the as-deposited structure to the equilibrium one.

Dynamics of spheroid self-assembly in liquid-overlay culture of DU 145 human prostate cancer cells.

The in vitro self-assembly of multicellular spheroids generates highly organized structures in which the three-dimensional structure and differentiated function frequently mimic that of in vivo tissues. This has led to their use in such diverse applications as tissue regeneration and drug therapy. Using Smoluchowski-like rate equations, herein we present a model of the self-aggregation of DU 145 human prostate carcinoma cells in liquid-overlay culture to elucidate some of the physical parameters affecting homotypic aggregation in attachment-dependent cells. Experimental results indicate that self-aggregation in our system is divided into three distinct phases: a transient reorganization of initial cell clusters, an active aggregation characterized by constant rate coefficients, and a ripening phase of established spheroid growth. In contrast to the diffusion-controlled aggregation previously observed for attachment-independent cells, the model suggests that active aggregation in our system is reaction-controlled. The rate equations accurately predict the aggregation kinetics of spheroids containing up to 30 cells and are dominated by spheroid adhesive potential with lesser contributions from the radius of influence. The adhesion probability increases with spheroid size so that spheroid-spheroid adhesions are a minimum of 2.5 times more likely than those of cell-cell, possibly due to the upregulation of extracellular matrix proteins and cell-adhesion molecules. The radius of influence is at least 1.5 to 3 times greater than expected for spherical geometry as a result of ellipsoidal shape and possible chemotactic or Fröhlich interactions. Brownian-type behavior was noted for spheroids larger than 30 microm in diameter, but smaller aggregates were more motile by as much as a factor of 10 for single cells. The model may improve spheroid fidelity for existing applications of spheroids and form the basis of a simple assay for quantitatively evaluating cellular metastatic potential as well as therapies that seek to alter this potential.

Microstructure determination of AOT + phenol organogels utilizing small-angle X-ray scattering and atomic force microscopy.

Dry reverse micelles of the anionic twin-tailed surfactant bis(2-ethylhexyl) sulfosuccinate (AOT) dissolved in nonpolar solvents spontaneously form an organogel when p-chlorophenol is added in a 1:1 AOT:phenol molar ratio. The solvents used were benzene, toluene, m-xylene, 2,2,4-trimethylpentane (isooctane), decane, dodecane, tetradecane, hexadecane, and 2,6,10,14-tetramethylpentadecane (TMPD). The proposed microstructure of the gel is based on strands of stacked phenols linked to AOT through hydrogen bonding. Small-angle X-ray scattering (SAXS) spectra of the organogels suggest a characteristic length scale for these phenol-AOT strands that is independent of concentration but dependent on the chemical nature of the nonpolar solvent used. Correlation lengths determined from the SAXS spectra indicate that the strands self-assemble into fibers. Direct visualization of the gel in its native state is accomplished by using tapping mode atomic force microscopy (AFM). It is shown that these organogels consist of fiber bundle assemblies. The SAXS and AFM data reinforce the theory of a molecular architecture consisting of three length scales-AOT/phenolic strands (ca. 2 nm in diameter) that self-assemble into fibers (ca. 10 nm in diameter), which then aggregate into fiber bundles (ca. 20-100 nm in diameter) and form the organogel.

Mechanisms and kinetics of self-assembled monolayer formation.

Recent applications of various in situ techniques have dramatically improved our understanding of the self-organization process of adsorbed molecular monolayers on solid surfaces. The process involves several steps, starting with bulk solution transport and surface adsorption and continuing with the two-dimensional organization on the substrate of interest. This later process can involve passage through one or more intermediate surface phases and can often be described using two-dimensional nucleation and growth models. A rich picture has emerged that combines elements of surfactant adsorption at interfaces and epitaxial growth with the additional complication of long-chain molecules with many degrees of freedom.

Shear-induced molecular precession in a hexatic Langmuir monolayer.

Liquid crystalline behaviour is generally limited to a select group of specially designed bulk substances. By contrast, it is a common feature of simple molecular monolayers and other quasi-two-dimensional systems, which often possess a type of in-plane ordering that results from unbinding of dislocations-a 'hexatic' liquid crystalline phase. The flow of monolayers is closely related to molecular transport in biological membranes, affects foam and emulsion stability and is relevant to microfluidics research. For liquid crystalline phases, it is important to understand the coupling of the molecular orientation to the flow. Orientationally ordered (nematic) phases in bulk liquid crystals exhibit 'shear aligning' or 'tumbling' behaviour under shear, and are described quantitatively by Leslie-Ericksen theory. For hexatic monolayers, the effects of flow have been inferred from textures of Langmuir-Blodgett films and directly observed at the macroscopic level. However, there is no accepted model of hexatic flow at the molecular level. Here we report observations of a hexatic Langmuir monolayer that reveal continuous, shear-induced molecular precession, interrupted by occasional jump discontinuities. Although superficially similar to tumbling in a bulk nematic phase, the kinematic details are quite different and provide a possible mechanism for domain coarsening and eventual molecular alignment in monolayers. We explain the precession and jumps within a quantitative framework that involves coupling of molecular orientation to the local molecular hexatic 'lattice', which is continuously deformed by shear.

Alignment of hexatic langmuir monolayers under shear.

We have studied the structural changes that fatty acid monolayers in the Ov phase undergo when a simple shear flow is imposed. A strong coupling is revealed by the changes in domain structure that are observable using Brewster angle microscopy, suggesting the possibility of shear alignment. The dependence of the alignment on the molecular polar tilt proves that the mechanism is different than in nematic liquid crystals. We argue that the degenerate lattice symmetry lines of the underlying pseudohexagonal lattice align in the flow direction, and we explain the observed alignment angle using geometrical arguments.

Liquid to hexatic to crystalline order in Langmuir-Blodgett films.

Atomic force microscope images of zinc arachidate (ZnA2) Langmuir-Blodgett films show that three- and five-layer films are "hexatic," with long-range bond-orientational order and short-range positional correlations of three to five lattice repeats. The monolayer in contact with the substrate is disordered. Films of seven or more layers of ZnA2 are crystalline. A population of dislocations, most likely originating at the substrate, disrupts the positional but not the orientational order of the lattice, leading to hexatic layers intermediate between crystal and liquid. The influence of the substrate propagates farther into ZnA2 films than into cadmium arachidate films because the molecular cohesion is much weaker in ZnA2 than in cadmium arachidate, as evidenced by a less dense molecular packing.

Langmuir-Blodgett films.

The controlled transfer of organized monolayers of amphiphilic molecules from the airwater interface to a solid substrate was the first molecular-scale technology for the creation of new materials. However, the potential benefits of the technology envisioned by Langmuir and Blodgett in the 1930s have yet to be fully realized. Problems of reproducibility and defects and the lack of basic understanding of the packing of complex molecules in thin films have continued to thwart practical applications of Langmuir-Blodgett films and devices made from such films. However, modern high-resolution x-ray diffraction and scanning probe microscopy have proven to be ideal tools to resolve many of the basic questions involving thin organic films. Here, studies are presented of molecular order and organization in thin films of fatty acid salts, the prototypical system of Katharine Blodgett. Even these relatively simple systems present liquid, hexatic, and crystalline order; van der Waals and strained layer epitaxy on various substrates; wide variations in crystal symmetry and interfacial area with counterions; modulated superstructures; and coexisting lattice structures. The wide variety of possible structures presents both a challenge and an opportunity for future molecular design of organic thin-film devices.

Strained-layer van der Waals epitaxy in a Langmuir-Blodgett film.

Atomic force microscope images of Langmuir-Blodgett films of lead and manganese fatty acid salts show that these monolayers have long-range order and are oriented with respect to the mica substrate, although the lattice symmetries of the monolayers and substrate are dramatically different. The surface lattice of sequentially thicker films evolves toward the bulk structure while retaining the substrate alignment. This behavior is in distinct contrast to films of cadmium fatty acid salts on mica, or all films on amorphous silicon oxide, in which the monolayer structure is disordered and a three-layer-thick film displays the bulk structure.

Surface order and stability of langmuir-blodgett films.

Angstrom-resolution atomic force microscope images of Langmuir-Blodgett monolayers and multilayers of cadmium arachidate in air and under water show a dramatic change from a disordered arrangement to a crystalline lattice by the addition or removal of a single layer of molecules. The disordered surface is less stable than the ordered one to mechanical stresses such as atomic force microscopy tip forces or at the air-water contact line during contact angle measurements. The difference in the degree of order in the alkyl chains is attributed to the strong attractive interaction between headgroups in the presence of the divalent cation.