A relationship between US healthcare worker smallpox vaccination rates in 2003 and presidential election results in 2004.

A statistical relationship exists between state per capita smallpox vaccination rates of healthcare workers in 2003 and state presidential election results in 2004. The potential implications of political influence on national biosecurity decision making are discussed.

Structure, wrinkling, and reversibility of Langmuir monolayers of gold nanoparticles.

The assembly of nanoparticles into large, two-dimensional structures provides a route for the exploration of collective phenomena among mesoscopic building blocks. We characterize the structure of Langmuir monolayers of dodecanethiol-ligated gold nanoparticles with in situ optical microscopy and X-ray scattering. The interparticle spacing increases with thiol concentration and does not depend on surface pressure. The correlation lengths of the Langmuir monolayer crystalline domains are on the order of five to six particle diameters. Further compression of the monolayers causes wrinkling; however, we find that wrinkled monolayers with excess thiol can relax to an unwrinkled state following a reduction of surface pressure. A theoretical model based on van der Waals attraction and tunable steric repulsion is adopted to explain this reversibility.

Morphological characterization of self-assembled peptide nucleic acid amphiphiles.

Peptide nucleic acid amphiphiles (PNAA) are a promising set of materials for sequence-specific separation of nucleic acids from complex mixtures. To implement PNAA in micellar separations, the morphology and size of PNAA micelles in the presence and absence of a sodium dodecyl sulfate (SDS) cosurfactant have been studied by small-angle X-ray scattering and dynamic light scattering. We find that a 6-mer PNAA with a 12-carbon n-alkane tail forms ellipsoidal micelles (a = 5.15 nm; b = 3.20 nm) above its critical micelle concentration (CMC) of 110.9 microM. On addition of a stoichiometric amount of complementary DNA, PNAA hybridizes to DNA, suppressing the formation of PNAA micelles. At a ratio of 19:1 SDS/PNAA (total concentration = 20 mM), spherical micelles are formed with outer radius Rs = 2.67 nm, slightly larger than spherical micelles of pure SDS. Capillary electrophoresis studies show that PNAA/DNA duplexes do not comicellize with SDS micelles. No such effects are observed using noncomplementary DNA. The shape and size of the PNAA micelles is also verified by dynamic light scattering (DLS) studies. These results provide an interesting case study with competing electrostatic, hydrophobic, and hydrogen-bonding interactions in micellar systems and make possible the use of PNAA in micellar separations of DNA oligomers.

Structure of the interface between two polar liquids: nitrobenzene and water.

Synchrotron X-ray reflectivity is used to study the electron density as a function of depth through the bulk nitrobenzene-water interface at four different temperatures. The measured interfacial width differs from the predictions of capillary wave theory with a progressively smaller deviation as the temperature is raised. Computer simulations suggest the presence of both molecular layering and dipole ordering parallel to the interface. Either layering or a bending rigidity, that can result from dipole ordering, can explain these measurements.

Ion distributions near a liquid-liquid interface.

Mean field theories of ion distributions, such as the Gouy-Chapman theory that describes the distribution near a charged planar surface, ignore the molecular-scale structure in the liquid solution. The predictions of the Gouy-Chapman theory vary substantially from our x-ray reflectivity measurements of the interface between two electrolyte solutions. Molecular dynamics simulations, which include the liquid structure, were used to calculate the potential of mean force on a single ion. We used this potential of mean force in a generalized Poisson-Boltzmann equation to predict the full ion distributions. These distributions agree with our measurements without any adjustable parameters.

X-ray studies of the interface between two polar liquids: neat and with electrolytes.

We demonstrate the use of X-ray reflectivity to probe the electron density profile normal to the interface between two polar liquids. Measurements of the interfacial width at the neat nitrobenzene/water and the neat water/2-heptanone interfaces are presented. These widths are consistent with predictions from capillary wave theory that describe thermal interfacial fluctuations determined by the tension and bending rigidity of the interface. Variation of the temperature of the water/nitrobenzene interface from 25 degrees C to 55 degrees C indicates that the role of the bending rigidity decreases with increasing temperature. X-ray reflectivity measurements of the electrified interface between an aqueous solution of BaCl2 and a nitrobenzene solution of TBATPB demonstrate the sensitivity of these measurements to the electrolyte distribution at the interface. A preliminary analysis of these data illustrates the inadequacy of the simplest, classical Gouy-Chapman theory of the electrolyte distribution.