Print this page 29 August 2006 2006/32

Hairy colloids stick better

For the first time a team of German and Dutch researchers has succeeded in explaining the molecular mechanism behind gelation of a colloidal suspension. The molecules at the surface of the colloids are ordering together with the solvent when lowering the temperature. In this way the colloids stick together and a gel developes. The surface change involves an enormous transformation in the viscosity of the suspension. This is the first time that scientists have been observing the role of molecules at the surface of colloids as to the behaviour of these colloids. The researchers have published the results in the scientific magazine Proceedings of the National Academy of Sciences (PNAS) of 24 August 2006.

Colloids are tiny particles the size of some nanometers to about one millimeter. In solution these particles may exhibit different phases, varying from a flowing gaseous suspension to solid crystalline structures. Highly viscous, amorphous gels also appear frequently. Colloidal particles become visible under a microscope because of their size. Therefore, physicists use colloidal suspensions - solutions of these particles - as a model system for studying phase transitions in matter such as in melting and freezing. Besides, colloidal suspensions serve as 'markers' for biomolecular interactions. Colloids that have specific proteins at the surface are, for one, sensitive to antibodies. In a vicinity of antibodies the suspension will form a gel.

Influences of interface between particles
The interface between the particles determines the behaviour and the phase of the suspension. The surface molecules and the solvent in their turn, may influence the interface between the particles. So, by modifying the surface of the colloids researchers are able to influence and to stabilize the phase of the suspension.

Despite the influence of the surface on the phase behaviour of colloids, most of the methods of measurement that map this phase behaviour are not sensitive to changes to that surface. Usually, changes at a macroscopic scale are being investigated, such as loss of heat or changes in viscosity. At an even smaller scale researchers are also investigating the colloids under a microscope, so that the joint transformations can be observed in detail. Although these colloids are very tiny, the surface molecules are even more decreasing by a factor of one hundred.

Hairy colloids
For a long time researchers have been studying a model system that consists of 'hairy colloids'. These are glass colloids covered with long, organic molecules (see figure 1). In literature the role of the 'hairs' (the surface molecules in the phase behaviour of the colloidal suspension) has been brought up for discussion many times. Recently, German and Dutch researchers¹⁾have developed a surface-specific spectroscopic method that enables them to follow how the hairs are complying, ordering and directing within the suspension during a change of phase. The researchers cool down the fluid suspension so that it developes into a highly viscous gel.

The transformation into a different phase changes the vibrations that are caused by the surface molecules. The new method is sensitive to changes in the vibrations. The transformation from a fluid suspension into a highly viscous gel seems to be caused by major changes in the confirmation and orientation of the hairs (see figure 2). In forming a gel the hairs are largely ordering, at which the solvent fills up the hairless spaces. This enlarges the interface between the particles. Eventually, the particles stick together and a gel is being formed. Furthermore, the experiments show that it will take some days before the hair will finally reach their highly-ordered structure.

For the first time researchers have been linking up the molecular behaviour at the surface of colloids and the macroscopic phase behaviour of the suspension. This may be the start of new prospects for research, particularly on biomolecular interfaces to particles that have a relevance to say, drug delivery.

¹⁾ The article is entitled Surface molecular view of colloidal gelation. The authors are Sylvie Roke (Max-Planck Institute for Metals Research, Duitsland), Otto Berg (Leiden University), Johan Buitenhuis (Research Center Jülich, Duitsland), Alfons van Blaaderen (Utrecht University) and Mischa Bonn (Leiden University and FOM-Institute for Atomic and Molecular Physics, Amsterdam).
Sylvie Roke has been taken her doctoral degree in 2004 in the pay of the Foundation for Fundamental Research on Matter (FOM) and she is the winner of the Minerva-Award 2006.

For more information, please contact professor Mischa Bonn, FOM-Institute for Atomic and Molecular Physics, Amsterdam, phone: (020) 608 12 34.

For the electronic pictures, please contact Annemarie Zegers, public relations department, FOM, phone: (030) 600 12 18,

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Figure 1. Hairy colloids

Scientists are examining hairy colloids, small glass particles (100 nanometer in diameter) containing long organic molecules at the surface (alkyl, CH2 chains), represented in the figure by zigzag lines. Actually, the diameter of the particles is about one hundred times the length of the hairs.
Figure 2. From suspension to gel

A suspension of small, hairy glass particles of about 100 micrometer in diameter is being cooled down. Then, the fluid suspension becomes a gel, at which the particles are sticking together, as schematically indicated in the figure. The proces is reversible. The upper part of the figure shows what happens with the molecules at the surface of the colloids when a gel becomes a fluid suspension, or the other way around. You see how the molecules are ordering at low temperatures. Then, the suspension is forming a gel. The lower part shows the vibration spectra of the surface molecules. The vibrations are considerably stronger to the gel, which indicates a high level of ordering in the surface molecules. The signal is different in shape and very weak to the disordered hairs in the suspension state. The researchers are able to detect from the spectra quantitative data on the molecular (dis)order.