Controlling Colloidal Interactions with Charged Nanoparticles

As published in the June 23, 2009, Journal of Physical Chemistry, a team of researchers, led by Professor Paul Van Tassel in the Department of Chemical Engineering, and in collaboration with John Walz of Virginia Polytechnic Institute, are discovering how they might be able to predict, control, and exploit colloidal interactions.

 

Colloids - objects of size intermediate between the molecular and macroscopic worlds – include examples such as proteins, cells, and particles. The forces that act on colloidal objects govern many important industrial and medical applications, ranging from colloidal crystallization, as used to fabricate photonic and bio-materials, to bioassay, as employed in diagnostics and drug development.

 

Two of the governing forces in colloidal systems are well known: electric double layer repulsion, the phenomenon responsible for preventing milk fat from coagulating, and van der Waals attraction, an important force governing condensed phases. Collectively, these two contributions are referred to as the DVLO interaction.

 

A third contribution to the colloidal force has gained increasing attention over the past few decades. The so-called depletion interaction is observed when a third component, whose size is larger than the solvent molecules (typically water) but smaller than the colloidal object, is added to the system. The force results from the unbalanced osmotic pressure in the region near contact between two colloidal objects due to the altered concentration and structure of the newly added component.

 

“What makes this feature exciting,” says Van Tassel,” is the extent to which the depletion force, more so than the other forces, can be controlled.” By changing the size of the added component – in this case, a charged nanoparticle – one can alter the range of the interaction, while the strength of the interaction can be manipulated by changing the concentration of the added component. To date, most of the attention has been focused on uncharged additives; the researchers seek here to understand the depletion force for charged systems – capable of producing a much more pronounced effect.

 

Using atomic force microscopy and molecular computer simulation, they have begun to investigate the influence of charged nanoparticles on the force between colloidal objects, with the goal of gaining nanoscale control over colloidal interactions. An interesting discovery, according to Van Tassel, has been the unusual layered structure of the nanoparticles observed between two neighboring colloidal particles and its influence on the resulting force. In fact, through a statistical mechanical order parameter, they have observed structural order intermediate between solid and liquid – reminiscent of glassy systems. These findings could ultimately lead to nanoparticle additive-based methods toward tailored colloidal crystalline materials and/or high affinity bioassays.