Experimental observation of nanophase segregation in aqueous salt solutions around the predicted liquid-liquid transition in water.

The liquid-liquid transition in supercooled liquid water, predicted to occur around 220 K, is controversial due to the difficulty of studying it caused by competition from ice crystallization (the so-called "no man's land"). In aqueous solutions, it has been predicted to give rise to phase separation on a nanometer scale between a solute-rich high-density phase and a water-rich low-density phase. Here we report direct experimental evidence for the formation of a nanosegregated phase in eutectic aqueous solutions of LiCl and LiSCN where the presence of crystalline water can be experimentally excluded. Femtosecond infrared and Raman spectroscopies are used to determine the temperature-dependent structuring of water, the solvation of the SCN- anion, and the size of the phase segregated domains.

Phonon-like Hydrogen-Bond Modes in Protic Ionic Liquids.

Gigahertz- to terahertz-frequency infrared and Raman spectra contain a wealth of information concerning the structure, intermolecular forces, and dynamics of ionic liquids. However, these spectra generally have a large number of contributions ranging from slow diffusional modes to underdamped librations and intramolecular vibrational modes. This makes it difficult to isolate effects such as the role of Coulombic and hydrogen-bonding interactions. We have applied far-infrared and ultrafast optical Kerr effect spectroscopies on carefully selected ions with a greater or lesser degree of symmetry in order to isolate spectral signals of interest. This has allowed us to demonstrate the presence of longitudinal and transverse optical phonon modes and a great similarity of alkylammonium-based protic ionic liquids to liquid water. The data show that such phonon modes will be present in all ionic liquids, requiring a reinterpretation of their spectra.

The entropy of water in swelling PGA/PAH polyelectrolyte multilayers.

We investigated the thermodynamical properties of water exchanged in poly(l-glutamic acid)/poly(allylamine)hydrochloride (PGA/PAH) polyelectrolyte multilayers containing ferrocyanide. Oxidation/reduction of the ferrocyanide in the multilayer caused a reversible swelling/contraction of the film due to the uptake/release of counter ions and water. We used electrochemical quartz crystal microbalance and electrochemical microcalorimetry to correlate the amount of water with the accompanying entropy changes during electrochemical swelling of the multilayer for a series of different anions at different concentrations. The number of exchanged water molecules was highly dependent on the ionic strength and the type of anion in the buffer solution. However, the entropy change per exchanged water molecule was found to be independent of these two parameters. The water molecules in the polyelectrolyte multilayer have reduced the entropy compared to that of bulk water (≈-1 J mol(-1) K(-1)). A comparison of hydration entropies for free polyelectrolytes and PGA/PAH multilayers suggests that such systems are mainly stabilized by water release during multilayer construction.

Preparation of monodisperse silicon nanocrystals using density gradient ultracentrifugation.

We report the preparation of monodisperse silicon nanocrystals (ncSi) by size-separation of polydisperse alkyl-capped ncSi using organic density gradient ultracentrifugation. The ncSi were synthesized by thermal processing of trichlorosilane-derived sol-gel glasses followed by HF etching and surface passivation with alkyl chains and were subsequently fractionated by size using a self-generating density gradient of 40 wt % 2,4,6-tribromotoluene in chlorobenzene. The isolated monodisperse fractions were characterized by photoluminescence spectroscopy and high-angle annular dark-field scanning transmission electron microscopy and determined to have polydispersity index values between 1.04 and 1.06. The ability to isolate monodisperse ncSi will allow for the quantification of the size-dependent structural, optical, electrical, and biological properties of silicon, which will undoubtedly prove useful for tailoring property-specific optoelectronic and biomedical devices.

Frustration vs Prenucleation: Understanding the Surprising Stability of Supersaturated Sodium Thiosulfate Solutions.

Gibbs classical nucleation theory predicts that a supersaturated solution will have transient nuclei that flitter in and out of existence. Only when one of these nuclei becomes larger than a critical size, will the solution crystalize. Recently, nonclassical nucleation theories have invoked the presence of prenuclei possibly associated with a liquid-liquid phase separation. However, there are few experimental observations of such prenuclei. Here, we use ultrafast optical Kerr-effect spectroscopy to measure the temperature-dependent low-frequency (sub-gigahertz to terahertz) anisotropic Raman spectra of supersaturated aqueous sodium thiosulfate solutions. Clear evidence of clusters is obtained in the spectra. However, on the basis of the inferred stability of these clusters, it appears that they frustrate rather than promote the formation of crystals. This would explain the surprising stability of supersaturated sodium thiosulfate and similar solutions.