Binary Mixtures

The chemistry of liquids and solutions continues to fascinate scientists. Besides the practical importance of liquids as the medium for most useful chemical reactions, the role of the solvent in mediating various chemical reactions is a complex and important question of broad scientific interest. This is particularly true of aqueous solutions, which are fundamentally involved in the chemical processes of life - all of biological chemistry takes place in an aqueous environment. Aqueous solutions are perplexing since they adopt some of the eccentric behavior of water.

The eccentricities of aqueous solutions are closely tied to the structure and the dynamics of the liquid. For instance, the dynamics of water slow down markedly as a solute is added, as manifest by a variety of dynamical measures. For instance, the reorientational correlation times, the diffusion coefficients, and the dielectric behavior of water all become more sluggish upon mixing. Because many of the interactions that distinguish liquids occur at low frequencies, the far-infrared and low frequency infrared regions of the spectrum are a fruitful place to study the dynamics of the mixtures.

Our work on binary mixtures is directed towards understanding how polar organic molecules affect the structure and the dynamics of hydrogen bonding liquids -- water and methanol in particular. The collective reorientational motions of liquids are manifest in the microwave and far-infrared regions of the spectrum, as are most librational motions. The librational motions of the hydrogen atoms in water, and the hydroxyl hydrogen on methanol, are found at much higher frequencies than heavy atom librations. Our research shows that the band occurring near 700 wavenumbers is a sensitive probe of the hydrogen bonding environment in the mixtures. Through a combination of dielectric analysis of the spectra and comparison to molecular dynamics simulations, we have been able to interpret changes in these bands upon mixing.

The simulations simultaneously give us dynamical and structural information, allowing us to relate changes in one to changes in the other. For instance, this figure shows how the spatial distribution function (SDF) of oxygen atoms from neighboring water molecules changes upon mixing. The surfaces around the central molecule indicate locations that have a high probability of finding an oxygen atom from another water molecule. Part (a) shows water-rich compositions, in which the tetrahedral structure of water is evident. Upon dilution, however, the tetrahedral positions of hydrogen bond donors are lost to a more dipolar position, as shown in part (b).