Excited state intramolecular charge transfer reaction in non-aqueous reverse micelles: Effects of solvent confinement and electrolyte concentration#

Identical Diffusion Distributions and Co-Cluster Formation Dictate Azeotrope Formation: Microscopic Evidences and Experimental Signatures

What selects azeotropic pairs and governs the azeotropic conditions (composition and temperature) is an open and intriguing question. A combined simulation and experimental work presented here investigates this by considering ethanol-water mixtures. We find identical distributions of center-of-mass diffusion coefficients for ethanol and water molecules under the azeotropic condition (95.5 wt % ethanol +4.5 wt % water, Tazeo = 351.1K). Moreover, the particle displacements show strong interspecies correlations at Tazeo. Interestingly, simulated reorientation time distributions become identical at Tazeo but at a composition different from that at which the translational diffusion distributions overlapped. Cluster analyses indicate that solutions at Tazeo with xwater ≤ 15 wt % are more microheterogeneous than those with higher water content, although no anomaly in the composition-dependent solution structural properties was detected. Ethanol-water and ethanol-ethanol interaction energies show pronounced nonideal composition dependence, but the size of the relative fluctuations in them remained small (∼0.5kBT). Rare water-water H-bonding, predominant water-ethanol H-bonding, and a sizable population of "free" water molecules characterize the azeotropic solutions. The red edge excitation spectroscopic (REES) measurements with a dissolved anionic fluorescent dye, coumarin343 (C343), support the predicted solution microheterogeneity by showing a nonmonotonic composition dependence of the excitation energy-induced changes in the fluorescence emission spectral frequencies and bandwidths, the largest changes being under the azeotropic condition. Subsequent dynamic anisotropy measurements reveal a nonmonotonic composition dependence of C343 rotation times with a peak under the azeotropic condition. In summary, equalization of the component translational diffusion coefficients and solution microheterogeneity with regular composition dependence of the solution structure appear to characterize the ethanol-water azeotrope.

Photoisomerization and reorientational dynamics of DTDCI in AOT/alkane reverse micelles containing non-aqueous polar liquids

Molecular mobility of the symmetric carbocyanine fluorophore DTDCI was studied in AOT/alkane reverse micelles containing non-aqueous polar liquids DMF, formamide, ethylene glycol and glycerol by monitoring both the torsional photoisomerization and rotational reorientation, both of which were sensitive to microviscosity of the local environment. The DTDCI fluorophore resides completely within the AOT-polar liquid reverse micelle nano-droplets, where its dynamics were found to be significantly retarded irrespective of the polar liquid taken, due to a combination of electrostatic and hydrophobic forces that induce the guest DTDC(+) cation to attach to the AOT molecules of the host droplet. The addition of strong hydrogen-bond donating polar liquids like formamide, ethylene glycol and glycerol causes a systematic enlargement of the droplets. Rotational dynamics of the fluorophore inside the nano-droplets was characterized by a diffusion coefficient comparable to that in highly viscous solvents like ethylene glycol.