My group studies diffusion in liquids, especially coupled diffusion in multicomponent solutions of polymers, micelles, microemulsions and electrolytes. In addition to providing new information about molecular motions and interactions, the results are used to understand the rates of chemical and physical processes, such as diffusion-limited reactions, carrier-mediated transport, solubilization, gas absorption, crystal growth, and chemical waves and oscillations. We use a variety of techniques to study diffusion, including Taylor dispersion flow techniques, laser light scattering techniques, and computer simulations.

Diffusion is investigated in every branch of science and technology, from theoretical physics to waste management. In the vast majority of these studies, Fick's well known equation

is used to relate the solute flux *J *to the solute diffusion coefficient *D *and the solute concentration gradient. Fick's equation was developed to describe diffusion in solutions containing a single solute, such as sugar dissolved in water. But as a rule, transport in systems of practical significance involves simultaneous diffusion of more than one solute. Important examples include the diffusion of:

- chemically reacting solutes
- polydisperse polymers
- mixed electrolytes in seawater and brines
- protein and buffer electrolytes
- surfactant micelles and solubilizates
- oil, water and surfactant in microemulsions
- mixed hydrocarbons in petroleum

These systems and many others are characterized by strongly coupled diffusion. This means that a gradient in the concentration of a substance can drive large coupled flows of other substances. For example, a mole of diffusing protein can transport hundreds of moles of salts or buffer electrolytes. In polymer + surfactant solutions, such as aqueous poly(ethylene glycol) + sodium dodecyl sulfate, thousands of moles of surfactant can be transported per mole of diffusing polymer.

To investigate diffusion in multicomponent solutions, we measure diffusion coefficients for Fick's equations

which include cross-terms, such as -*D*_{12}∇*c*_{2}, to represent the coupled fluxes of solute driven by the concentration gradients in other solutes. The number of *D*_{ik} coefficients required to describe diffusion in a solution of *N *solutes increases as *N *, so the possibilities for new and interesting diffusion behavior increases rapidly with the number of components.

Our current research includes studies of:

- proton-coupled diffusion in protein solutions

- coupled diffusion in solutions of mixed-surfactant and surfactant + solubilizate micelles

- the relation between diffusion coefficients measured by classical macroscopic gradient methods and popular nuclear magnetic resonance and dynamic light scattering techniques

- the role of solution thermodynamics in coupled diffusion

Thermodynamic stability and the origins of incongruent and strongly coupled diffusion in solutions of micelles, solubilizates and microemulsions.

Diffusion in micelle solutions. What does dynamic light scattering measure?

Dalton’s disputed nitric oxide experiments and the origins of his atomic theory.

Usselman, M. C.; Leaist, D. G.; **Watson, K. D.*** ChemPhysChem ***2008**, *9*, 106.

Isotopic fractionation by diffusion in groundwater.

La Bolle, E. M.; Eweis, J. B.; Leaist, D. G.; Fogg, G. E.; Gravner, J. Diffusive fractionation of isotopes in groundwater, *Water *

A comparison of diffusion coefficients for ternary mixed micelle solutions measured by macroscopic gradient and dynamic light scattering techniques.

Das, B.; Maitra, B.; **Mercer, S. M.**; **Everist, M.**; Leaist, D. G. *Phys. Chem. Chem. Phys.*** 2008**, *10*, 3083.

Nernst-Planck analysis of propagating reaction-diffusion fronts in the aqueous iodate-arsenous acid system.

Ternary mutual diffusion coefficients from error-function dispersion profiles: Aqueous solutions of Triton X-100 micelles + poly(ethylene glycol).

An electrostatic mechanism for the coupled diffusion of polymer molecules and ionic micelles. Aqueous poly(ethylene glycol) + sodium dodecyl sulfate solutions.

Diffusion coefficients for binary, ternary and polydisperse solutions from peak-width analysis of Taylor dispersion profiles

Using Taylor dispersion profiles to characterize polymer molecular weight distributions.

A thermodynamic interpretation for the Aexcluded-volume@ effect in coupled diffusion.

Soret coefficients for aqueous polyethylene glycol solutions and some tests of the segmental model of polymer thermal diffusion.

Quaternary mutual diffusion coefficients for aqueous solutions of a cationic-anionic mixed surfactant from moments analysis of Taylor dispersion profiles.

Incongruent diffusion (negative main diffusion coefficient) for a ternary mixed surfactant system.

Relating multicomponent mutual diffusion and intradiffusion for associating solutes. Application to coupled diffusion in water-in-oil microemulsions.

Leaist, D. G. *Phys. Chem. Chem. Phys.*** 2002**, *4*, 4732.

Surfactant diffusion near critical micelle concentrations.

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Taylor dispersion monitored by electrospray mass spectrometry: A novel approach for studying diffusion in solution.

Coupled diffusion of mixed ionic micelles in sodium dodecyl sulfate + sodium octanoate solutions.

D. G. Leaist and **K. MacEwan**, *J. Phys. Chem. B ***2001**, *105*, 690.

Coupled diffusion of mixed methanol + ethanol clusters in carbon tetrachloride solutions.

Leaist, D. G.; **Hao, L.*** J. Phys. Chem. B ***2001**, *105*, 7446.

Measured and predicted ternary diffusion coefficients for concentrated aqueous LiCl + KCl solutions over a wide range of compositions

Leaist, D. G.; **Kanakos, M. ***Phys. Chem. Chem. Phys.*** 2000**, *2*, 1015.

Predicting the diffusion coefficients of concentrated mixed electrolytes from binary solution data. NaCl + MgCl_{2} + H_{2}O and NaCl + SrCl_{2} + H_{2}O at 25 C.

Leaist, D. G.; **Al-Dhaher**,** F. F.**,* J. Chem. Eng. Data ***2000**, *45*, 308.

Hartley-Crank equations for coupled diffusion in concentrated mixed electrolyte solutions.

Leaist, D. G.; **Curtis, N.*** J. Solution Chem. ***1999**, *28*, 341.

Interdiffusion of aqueous silver nitrate and potassium chromate and the periodic precipitation of silver chromate Liesegang bands.

Coupled diffusion in aqueous weak acid + alkanolamine absorbents.

Leaist, D. G.; **Li, Y.**; **Poissant, R.*** J. Chem. Eng. Data ***1998**, *43*, 1048.

Diffusion in gases and liquids.

Leaist, D. G. in *McGraw Hill Encyclopedia of Science and Technology*, 8th Edn., McGraw-Hill, New York, 1997.

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