External electric fields were used to amplify thermal fluctuations at the interface between two thin liquid films. Similar to the results shown previously for the enhancement of fluctuations at the polymer/air interface, interfacial fluctuations having a well-defined wavelength were enhanced with a characteristic growth rate. A simple theoretical framework to describe the experimental observations is presented. Both experiment and model calculation show a substantial reduction in feature size as a result of the change in surface/interfacial energy when going from the thin film to the bilayer case. Experimentally, features develop nearly 50 times faster for the bilayers in comparison to the polymer/air case. These results point to a simple route by which the nanoscopic feature can be easily and rapidly produced or replicated.

The free solution electrophoretic mobilities of poly(styrenesulfonate), ss-DNA, and duplex DNA are measured by capillary electrophoresis across a range of ionic strengths and, for poly(styrenesulfonate) and ss-DNA, across a range of chain lengths. The data are then compared with mobilities reported in the literature and predicted by theory. For ionic strengths below 0.1 M, the capillary method is more accurate and rapid than previous techniques; it also provides a distribution of mobility values for polyelectrolyte mixtures. A maximum of the free solution mobility with respect to chain length is discovered in the oligomer range for both poly(styrenesulfonate) and ss-DNA; lowering ionic strength accentuates this unexplained phenomenon. In the large chain limit, where the mobility is independent of chain length, the ionic strength dependences of mobility for all three polymers are remarkably similar. These dependences can only be explained by models that incorporate nonlinear electrostatic effects into the description of the counterion cloud. The Manning model (with relaxation correction) best approximates the dependence of mobility on ionic strength.

ABSTRACT:To understand the reversible gelation and subsequent aging of hydrogels prepared by freeze/thaw processing of poly(vinyl alcohol) (PVOH) solutions, the microstructures of gels prepared by different freeze/thaw protocols and aged to varying extents are studied by cryogenic transmission electron microscopy, solid-state nuclear magnetic resonance, X-ray scattering, and differential scanning calorimetry (DSC). As discussed in the literature, gelation by the freeze/thaw process occurs as a homogeneous aqueous poly(vinyl alcohol) solution is cycled, perhaps multiple times, between temperatures above 0°C and well below 0°C. The current investigation has determined that a few percent of chain segments crystallize during the first cycle, organizing themselves into 3-8 nm primary crystallite junctions separated on an irregular mesh by an average spacing of ϳ 30 nm. Aging or imposition of additional freeze/thaw cycles augments the level of crystallinity and transforms the as-formed liquid-like microstructure, characterized in the electron microscope by rounded ϳ 30 nm pores, into a fibrillar network. Observation that the transformation occurs at fixed mesh spacing and approximately constant average crystallite size suggests the formation of secondary crystallites that do not affect network connectivity. Dendritic ice crystallization and possibly spinodal decomposition superimpose on this nanoscale structure a matrix of much larger pores.

Linear DNA molecules are visualized while undergoing Brownian motion inside media patterned with molecular-sized spatial constraints. The media, prepared by colloidal templating, trap the macromolecules within a two-dimensional array of spherical cavities interconnected by circular holes. Across a broad DNA size range, diffusion does not proceed by the familiar mechanisms of reptation or sieving. Rather, because of their inherent flexibility, DNA molecules strongly localize in cavities and only sporadically "jump" through holes. Jumping closely follows Poisson statistics. By reducing DNA's configurational freedom, the holes act as molecular weight-dependent entropic barriers. Sterically constrained macromolecular diffusion underlies many separation methods and assumes an important role in intracellular and extracellular transport.

The dissolution and dissolved molecular state of cytochrome c were investigated in the room temperature ionic liquid ethylmethylimidazolium ethylsulfate, [EMIM][EtSO4], by viscometry, optical and vibrational spectroscopies, and peroxidase activity. In dilute mixtures, viscometry demonstrated true molecular dissolution of cytochrome c in the ionic liquid and uncovered a molecular size larger than that in aqueous buffer, suggesting altered solvation or slight denaturation. The protein's heme unit absorbs light outside the spectral range masked by [EMIM], enabling conformational assessments by UV-visible and circular dichroism spectroscopies. Adding trends from fluorescence and Fourier transform infrared spectroscopy, unchanged secondary but perturbed tertiary structures were determined, consistent with the appreciable peroxidase activity measured. Different than in aqueous buffers, denaturation is not accompanied by aggregation. Results are relevant to the proposed application of ionic liquids as media for room temperature preservation of biomacromolecules.