|Dendrimers and Mixed micelles!|
|Dr. Gavin Kirton's Home Page|
|Welcome to my home page!|
|In the transition period, an old subject of interest - the wonderful world of dendrimers will continue to be presented. I aim to provide a view on my current research into mixed micelle systems. Enjoy, and check for updates!|
|So, what are dendrimers?|
|Dendrimers form a new class of macromolecules or polymers (big molecules, like plastics, proteins, and so on) which have a regular branching structure, like an ideal tree. Other names for these molecules are cascade, starburst, cauliflower, hyperbranched or fractal polymers. Conventional polymers, for example, polystyrene (foam packaging), polyethyleneterephthalate (PET, plastic drink bottles), and polyvinylchloride (PVC) are basically long chains, ie linear polymers.
A schematic illustration is given below:
|Conventional linear polymer.|
|A culinary way to picture the difference is conventional polymers as "spaghetti" and dendrimers as "meatballs".|
|What's so special about dendrimers?|
|Dendrimers, when produced by stepwise synthesis (linking branches in stages, from the edge, or leaves down to the core, or root) are monodisperse. This means that these polymers have a precise molecular weight and size. Normal polymer synthesis results in a mixture in which chains are of differing lengths, and so there is a distribution of molecular weights and sizes. So what?, you may ask. Well, when we wish to study the physical properties of polymers, having a mixture of different size polymers makes the situation rather messy, and the theory gets horribly complicated. So it's always a good idea to start with something simple to figure out what the heck is going on.
Another special feature of dendrimers, especially when they are large, is that they tend to form nice spherical shapes with a well-defined interior and exterior. This results in some of their peculiar properties, in particular in having low specific viscosities. They have also been found to act as molecular micelles, in which they become special containers for other molecules. Combined with their large size and geometry (DNA is found to wrap around particular dendrimers), they are being investigated as drug delivery and anti-cancer agents.
|Why am I interested in them?|
|My interest in dendrimers stems (pardon the pun) from the surface activity of particular dendrimer branches. This arises from the hydrophobic (water-fearing) edge parts, and the hydrophilic (water-loving) core, so that these branches tend to stand up on a water surface like a nano-forest, cores (roots) going into the water, and the branches going up into the air. If a small amount of these dendrimers is spread, then they will form an extremely thin molecular film, only one molecule thick (a monolayer).
If the water surface area is reduced, then the molecules will be clumped together, and then the monolayer gets squeezed. Because of this, there will be a mechanical resistance, which is registered as a change in surface tension. If the monolayer is compressed enough, then it must relieve the stress by moving from two dimenisons to three. This means that the molecules either go into the water, or go up into the air making thicker films (multilayers).
My the aim of my research is to find out the how the monolayer collapses, and what structures are formed as they are squeezed, as well as how I can relate this to the structure of the dendriemr molecules themselves. This will aid our understanding of surface-active polymers, and to surface science in general. Surfaces are very important, everything tends to happen at the surface!
|So how are dendrimer films studied?|
|As is the rule for science, a range of techniques must be used to obtain a good picture of what's going on. The primary tool of study for molecules on liquid surfaces is the Languir trough. It really is just a small Teflon bath, with a moterised barrier (to change the area), and some means of measuring the surface tension. Usually this is by measuring the "pull" of the surface on a small strip of paper. By plotting the surface pressure (directly related to surface tension) versus the surface area, one obtains a so called pi-A isotherm. These are characteristic of the surface-active molecules, and useful to find out the physical properties and "footprint" of the moelcules.
A second technique is reflectometry (using neutrons or X-rays). A beam of particles from some source (for example an X-ray generator, a nuclear reactor or a spallation target) is directed at a small, glancing angle across the surface to be studied, and the reflected beam is registered at a counter. Due to the differing reflectivies of different beam wavelengths (an analogy is different colours of visible light) from surface structure, a reflectogram (reflectivity spectrum) is obtained. This technique is sensitive to the vertical profile of very thin surface films, in terms of thickness of scattering materials.
Combining the techniques gives a molecular picture or explanation of the processes that are detected in using the Langmuir trough.
|Other personal links:|
|Where I am currently working:-|
|Where I completed my Doctoral thesis:-|
|Research School of Chemistry, ANU|
|Rocky Mountain College|
|I am an Assistant Professor at Rocky Mountain College. My teaching is specialized in general, analytical and instrumental chemistry. My interest continues to be in the field of surfactant and polymer science. I have undergraduate students starting projects in the properties of mixed surfactant micelle propert|
Dr. Gavin Kirton
Department of Chemistry
Rocky Mountain College
1511 Poly Drive
Billings, Montana 59102
United States of America
|© 1996 Prof. Gavin F. Kirton|
|Last modified: 25 June 2008|