Measuring molecular weight by atomic force microscopy

Absolute-molecular-weight distribution of cylindrical brush molecules were determined using a combination of the Langmuir Blodget (LB) technique and Atomic Force Microscopy (AFM). The LB technique gives mass density of a monolayer, i.e., mass per unit area, whereas visualization of individual molecules by AFM enables accurate measurements of the molecular density, i.e., number of molecules per unit area. From the ratio of the mass density to the molecular density, one can determine the absolute value for the number average molecular weight. Assuming that the structure of brush molecules is uniform along the backbone, the length distribution should be virtually identical to the molecular weight distribution. Although we used only brush molecules for demonstration purpose, this approach can be applied for a large variety of molecular and colloidal species that can be visualized by a microscopic technique.     

Microfluidic platform for the generation of organic-phase microreactors

Rapid prototyping photolithography of a thiolene-based resin was used to fabricate microfluidic devices stable to aliphatic and aromatic organic solvents. The swelling of the cross-linked polymer matrix in various organic solvents was quantified, and the solvent resistance properties of these microfluidic devices are described. Discrete droplets of hexanes and toluene of uniform size were generated in microfluidic devices inside a water matrix containing SDS surfactant (SDS = sodium dodecyl sulfate). Variation of water and organic flow rates in the fluidic channels was used to control droplet size and separation. Droplet composition could be controlled by varying flow rates of two joined organic streams. Organic-phase synthetic reactions within the droplets were demonstrated with the bromination of alkenes inside benzene droplets.     

Molecular motion in a spreading precursor film

Spreading of a polymer drop on a solid substrate was monitored with molecular resolution. Three characteristic rates, i.e., the spreading rate of the precursor film D(spread)=(3.9+/-0.2)x10(3) nm(2)/s, the flow-induced diffusion rate of molecules within the film D(induced)=1.3+/-0.1 nm(2)/s, and the thermal diffusion coefficient of single molecules D(therm)D(induced), the plug flow of polymer chains was identified as the main mass-transport mechanism of spreading with an insignificant contribution from the molecular diffusion.     

Formation constants of mercury(II) with some buffer/masking agents and the formation of mixed-ligand complexes

Values are given for the formation constants of complexes of mercury(II) with some buffer/masking agents. The mixed-ligand complex formation of mercury(II)-chelates with other complexing agents has also been studied.