Phases of Surface-Parallel Molecules Discovered in Hg-Supported Langmuir Monolayers
- PDF / 89,833 Bytes
- 1 Pages / 612 x 792 pts (letter) Page_size
- 10 Downloads / 179 Views
only to amorphous networks. The researchers confirmed a well-defined, ordered mesostructure by scanning and transmission electron microscopy studies. They also found that the solid monoliths obtained in this way were transparent but birefringent when observed between crossed polarizers. Saravanamuttu and Andrews proposed a model based on bicontinuous microemulsions to explain the unusual submicrometer periodicity of the composites. Microemulsions are ternary systems composed of organic, H2O, and surfactant phases. The researchers said that in their system, a microemulsion forms upon generation of amphiphilic oligosiloxanes through condensation reactions of alkoxysilanes. Because they possess relatively short organic chains, sterical limitations, and non-ionic head groups, these oligomers cannot form micellar structures. Instead, they are likely to self-assemble into minimal-energy, amphiphilic monolayers with low curvature and bending rigidity that occupy the interfaces between immiscible organic (alkoxysilane) and H2O phases. According to the researchers, this could lead to a thermodynamically stable bicontinuous microemulsion in which the oligosiloxane monolayers partition organic and aqueous phases into discrete volumes. This microstructure could then be “frozen” by photopolymerization of the acrylate moieties, they said. ANDREI A. ELISEEV
Phases of Surface-Parallel Molecules Discovered in Hg-Supported Langmuir Monolayers Langmuir monolayers (LMs) are formed by amphiphilic molecules like alcohols and fatty acids on a liquid surface. The structure and properties of these monolayers strongly depend on the architecture of the constituent molecules and their inter- and intramolecular interactions. Until recently, however, LMs were studied on aqueous subphases, where the hydrophobic repulsion aligns the molecules normal to the surface. In the November 15 issue of Science, researchers from Bar-Ilan University, Israel; Brookhaven National Laboratory; and Harvard University have reported angstrom resolution x-ray measurements of the coverage dependence of the structure of LMs formed by stearic acid on a liquid Hg subphase.
M. Deutsch of Bar-Ilan and co-workers said that the resolution is possible because the high surface tension of Hg results in a very low surface roughness (~1 Å). Under the combined action of the alkyl chain-Hg attraction and the weak but important carboxyl head group-Hg subphase bond (~1.5 kJ/mol), a small change in the coverage induces a drastic change in the orientation of the molecules relative to the surface. At high coverage, from 19.5 Å2/molecule to ~25 Å2/molecule, a monolayer showing two phases of surface-normal molecules was found, similar to those found on water. At low coverage, from 57 Å2/molecule to 120 Å2/molecule, two hitherto unobserved phases of single and double layers of surface-parallel molecules were found. These flat-lying phases consist of molecular dimers packed side by side to form stripes. Long-range in-plane order is found to exist in the direction normal to the stripes, b
Data Loading...
