Dresden 2006 – scientific programme

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CPP: Chemische Physik und Polymerphysik

CPP 15: Dynamics and Diffusion

CPP 15.4: Talk

Wednesday, March 29, 2006, 14:45–15:00, ZEU 114

Molecular view of a colloidal gelation — •Sylvie Roke — Max-Planck Institute for Metals Research, Heisenbergstr. 3, Stuttgart, Germany

Many colloidal particles are coated physically or chemically by adsorption with a thin outer layer. Often this layer serves different purposes, such as shielding van der Waals interactions, providing so-called steric stabilization and/or to passivate surface states. If molecular transitions take place in the layer, as a function of temperature, pressure or solvent composition, dramatic changes in collective particle behaviour may result.

So far, particle properties have only been studied experimentally by techniques that lack molecular interface specificity. As a result, the molecular surface properties of the colloids that can affect the particle interaction have never been investigated directly.

We have used the recently developed technique vibrational sum frequency scattering to obtain molecular information about octadecyl-coated silica particles dispersed in n-hexadecane that undergo a gelation. The interface molecules are completely disordered in fluid suspension at high temperatures and become ordered prior to gel formation at lower temperatures. The appearance of interfacial order is accompanied by the liberation of heat and densification, and results in increased van der Waals forces between the particles leading to gelation. On a timescale of days the interface becomes completely ordered, which can affect aging in such gels.