Diffusive transport within complex environments is a critical piece of the chemistry occurring in such diverse membrane systems as proton exchange and bilayer lipid membranes. In the present study, fluorescence correlation spectroscopy was used to evaluate diffusive charge transport within a strong polyelectrolyte polymer brush. The fluorescent cation rhodamine-6G was used as a counterion probe molecule, and the strong polyelectrolyte poly(styrene sulfonate) was the polymer brush. Such strong polyelectrolyte brushes show promise for charge storage applications, and thus it is important to understand and tune their transport efficiencies. The polymer brush demonstrated preferential solvation of the probe counterion as compared to solvation by the aqueous solvent phase. Additionally, diffusion within the polymer brush was strongly inhibited, as evidenced by a decrease in diffusion constant of 4 orders of magnitude. It also proved possible to tune the transport characteristics by controlling the solvent pH, and thus the ionic strength of the solvent. The diffusion characteristics within the charged brush system depend on the brush density as well as the effective interaction potential between the probe ions and the brush. In response to changes in ionic strength of the solution, it was found that these two properties act in opposition to each other within this strong polyelectrolyte polymer brush environment. A stochastic random walk model was developed to simulate interaction of a diffusing charged particle with a periodic potential, to show the response of characteristic diffusion times to electrostatic field strengths. The combined results of the experiments and simulations demonstrate that responsive diffusion characteristics in this brush system are dominated by changes in Coulombic interactions rather than changes in brush density. More generally, these results support the use of FCS to evaluate local charge transport properties within polyelectrolyte brush systems, and demonstrate that the technique shows promise in the development of novel polyelectrolyte films for charge storage/transport materials.
Silicate perovskite and ferropericlase are thought to be the primary constituents of the lower mantle, whereas silicate post-perovskite is more likely found in the lowermost mantle. Because these minerals contain certain amounts of iron, their properties and, consequently, those of the deep mantle, are strongly influenced by iron's spin and valence states. A high-spin to low-spin crossover in ferropericlase has been observed to occur in the middle part of the lower-mantle conditions. Recent Mössbauer results consistently show that Fe 2+ predominantly exhibits extremely high quadrupole splittings in perovskite and post-perovskite, whereas a high-spin to low-spin transition of Fe 3+ in the octahedral site occurs at high pressures. These results provide a new venue for discussion of the effects of the spin and valence states of iron on the physical and chemical properties of the lower mantle.
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