Research Projects

Properties of the interior of aqueous reverse micelles

Solvation in Supercritical Fluids. In supercritical fluids (SCFs), density can be varied from dilute gas-like to liquid-like. The solubility of various solutes at a given temperature above the critical temperature, T_c, can be changed considerably by varying the pressure. At temperatures that are not much higher than T_c, the fluid compressibility is large at densities in the vicinity of the critical density, r_c. This leads to a local density augmentation around solutes that are more attractive to the solvent than are other solvent molecules. Because of this, many measurable solute properties that are sensitive to its surroundings exhibit a plateau as a function of solvent density r in the density range where augmentation occurs. The existence of the plateau signals the fact that the local environment of the solute changes less than one would predict from the bulk density variation. The extent of the local density enhancement that contributes to a given observable depends on solute-solvent interactions as well as on the solution property that is measured as a function of r.

Figure 1. A comparison of the local density augmentation predicted from the first shell coordination numbers, N(R) (red symbols), and from the frequency shifts in emission spectra (black symbols) for dipolar diatomic solutes in CO₂ at 1.05 T_c. The change in solute-solvent interactions associated with solute electronic excitation is modelled as a change in solute charge distribution from quadrupolar to dipolar.

Our research is aimed at elucidating the effects of local density augmentation on on several measurable properties related to solvation and reactivity. These include solvatochromic shifts in electronic absorption and emission spectra, vibrational energy relaxation rates, and time-evolution of fluorescence depolarization and Stokes shift. We are using theory and molecular dynamics (MD) computer simulation to determine how these properties depend on the local solvation structure and on its time-evolution in response to solute-induced perturbations.

Fig. 1 shows our results for the difference Dr_eff= r_eff- r between the effective local density, r_eff, and the bulk CO₂ solvent density r for a dipolar solute in CO₂solvent. In addition to Dr_eff obtained from the first solvation shell coordination numbers, the density augmentation estimated from the solute electronic emission frequency shifts Dn_em is shown. We found that solute-solvent orientational correlations enhance Dn_em beyond the amount predicted on the basis of local solvent clustering.

Properties of the interior of aqueous reverse micelles. Aqueous reverse micelles (RMs) are surfactant aggregates in nonpolar solvents that enclose packets of aqueous solution in their interior. The size of the enclosed water droplet can be tuned by varying w₀ = [H₂O]/[surfactant]. This makes it possible to study how the properties of water change with the extent of confinement and leads to many technological applications of RMs, for example to their use as reaction media in the production of nanoparticles whose size and shape are controlled by w₀ and surfactant properties. We are using molecular modelling, molecular dynamics (MD) simulation and quasielastic neutron scattering (QENS) to study the properties of RMs, focusing primarily on their interior region. This work involves a collaboration with Professor Nancy Levinger's group.

In the initial steps of our research, we developed a model of the RM interior region which combines a continuum representation of the surfactant head groups and of the nonpolar phase with atomistic representation of the head groups, counterions and water molecules. We have used this model and to examine how the water structure and dynamics depend on RM size (the diameter is proportional to w₀), distance from the interface and counterion type. We have also used the model to investigate solvation dynamics of solutes that are electrostatically attracted and repelled by the head groups and to predict the QENS spectrum S(Q,w) of water hydrogens within RMs and comparing it to experimental results on RMs formed by deuterated aerosol-OT (AOT) surfactant in perdeotero-isoctane. We are now in the process of developing and testing RM models which include atomistic representation of the surfactant tails and of the nonpolar phase.

Fig. 2 shows snapshots of model RMs designed to represent model Na⁺-AOT and K⁺-AOT at w₀ = 2. Our MD results show that water interfacial structure and mobility are sensitive to counterion size. We find that interfacial water is significantly more moblie in the presence of K⁺ than in the presence of Na⁺.


Figure 2. Snapshots of the RM interior for model Na⁺AOT (left) and K⁺AOT (right) corresponding to w₀= 2. Key: blue—head group anions, red—water oxygen, white—water hydrogen, green—K⁺, yellow—Na⁺. Note the change from 3-fold to 4-fold counterion-headgroup coordination in going from Na⁺AOT to K⁺AOT.

Solvation dynamics. An important step in solution-phase reaction dynamics is the rate of solvent reorganization in response to a change in the solute-solvent potential. This process, called solvation dynamics, is experimentally-accessible by several ultrafast spectroscopic methods, the most commonly used one being fluorescence upconversion which monitors the time-evolution of the Stokes shift in the flourescence spectrum of a dissolved chromophore. We are using molecular theory and MD simulation to determine the molecular mechanisms of the solvation response in a variety of solvent media, including one-component liquids, liquid mixtures, supercritical fluids, surfactant assemblies and liquid-solid interfaces. One theme of our research is to discover the effects of local density or concentration inhomogeneities and of solute proximity to the interface on solvation dynamics. Another theme is improve the modelling of the change in solute-solvent interactions resulting from solute electronic excitation by using as input into MD simulation solute partial charges and geometries obtained from polarizable continuum model electronic structure calculations. We are also working on molecular theory and simulation designed to determine the molecular processes contributing to the new ways of detecting solvation dynamics via solute-pump/solvent probe optical Kerr effect and terahertz spectroscopies.

Fig. 3 illustrates schematically solvation dynamics as detected by time-resolved fluorescence spectroscopy and our MD simulation results for solvation response, S(t), in an acetonitrile-benzene mixture. MD simulation provides access to the contributions of the two solvent components. It also makes it possible to assign the slowly decaying portion of S(t) to the build-up of acetonitrile local concentration enhancement as a result of the increase in solute polarity on electronic excitation.


Figure 3. On the left is a schematic illustration of solvation dynamics in response to the increase in the solute dipole as a result of S₀ ® S₁ electronic transition. On the right are our MD results for solvation dynamics in a room-temperature acetonitrile-benzene mixture at the acetonitrile mole fraction x_ac = 0.375.