Molecular dynamics

Molecular dynamics in confined geometry

Example for a surfactant stabilized microemulsion droplet. Water and oil may be replaced by various hydrophilic or hydrophobic substances, respectively.
Example for a surfactant stabilized microemulsion droplet. Water and oil may be replaced by various hydrophilic or hydrophobic substances, respectively.

When geometrical confinement on the nanometer scale is imposed on a glass forming liquid, usually by imbibing the liquid into a nanoporous material, molecular dynamics changes significantly. In many cases a pronounced decrease of the glass transition temperature Tg and a broadening of the α-relaxation peak is observed. One of the aims in studying glass formation in restricted geometry is to identify a characteristic lengthscale of the glass transition. One expects that as soon as the lengthscale of the confinement falls below such a characteristic length, pronounced deviations of the dynamics from bulk behaviour can be identified. However, as a combination of surface, pressure and finite-size effects play a role in these systems, experiments are usually difficult to interpret.

At that point glassforming microemulsions are particularly interesting. Microemulsions are formed out of (e.g.) droplets of a hydrophillic substance (simplest: water) in a hydrophobic environement, usually an oil of some kind. The droplets are stabilized by a layer of surfactent molecules. When suitable substances are combined such a microemulsion can be stable down to the glass transition of its constituents and dynamics of the molecules confined in the droplets can be studied. If the glass transition temperature Tg of the droplets is above that of the continuous medium (i.e. Tg > TG) then dynamics in soft confinement can be studied and depending on the relation of Tg and TG surface, pressure and finite size effects may be separated. As experimental methods we apply amongst others dynamic light scattering, dielectric spectroscopy, differential scanning calorimetry, small-angle X-ray scattering, NMR and neutron scattering.

For further information please contact Dr. T. Blochowicz