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scratching nano's sweet surface

As a species, we have been searching for better ways to turn bulk materials into powders since prehistoric times. Grain into flour for bread. Charcoal into black powder for fireworks. Stones into pigment for paint and cosmetics. And perhaps tastiest of all, basil into pesto for pasta.


Wherever you are at this moment, if you look around, you will be hard pressed to find a manufactured object that has not been reduced in size, at some point in its life. The steel in your desk was produced from finely ground iron particles. Even your wooden bookcase came from Mother Nature's naturally comminuted concoction called soil. In addition to serving the wondrous human gifts of whispers and kisses, our mouths also serve the mundane chore of grinding our food before we swallow to improve digestion and nutrition. Or so we tell our kids.


At base, this ancient and continuing quest for fineness is neither cosmic accident nor human contrivance. It is Nature's way of making materials more available for exploitation. As Mother Nature has it, most physical and chemical events happen on the surfaces of materials, not in the mass. When you pop a square of chocolate into your mouth, your taste buds sing in response to the chemical reaction that takes place when the flat of the square dissolves on your tongue. Liquid phase chocolate meets solid phase taste bud. In lab lingo the point where chocolate meets tongue is called an interface. Whether you knew that or not, you intuitively know that when it comes to chocolate bars, more surface means more flavor. So you crush the morsel between your molars and the slab's original six surfaces, suddenly become hundreds of smaller surfaces, much to the glee of the neglected taste buds on your cheeks, lips and every other bud-bearing cranny in your oral cavity. Suddenly your whole mouth is singing like a boy's choir. Why? Because you created more places for chemical reactions to take place - more interfaces. You increased the surface area of the chocolate by liberating its internal mass. Mmmmmm. Same mass - more surface area tastes better!


Same thing in nanoscience - except much more consequential. Mother Nature's cookbook says that as particle size decreases surface area increases geometrically. To illustrate, imagine this: first, spill the contents of a five- pound bag of flour on a gymnasium floor and spread it out as thin as possible. Then slice the empty bag open and lay it flat, next to the powdery mess you just made. The surface area of the bag would be equal to the surface area of the flour, if the flour were a solid block instead of a powder. At this point, it should be pretty easy to conclude that the surface area of the flour on the floor would be much larger than that of the laid out bag. If we count the sides and bottoms of each and every flour particle, the difference in surface areas becomes astounding. Let's take a look at an extreme scientific example.


Aerogels are synthetically produced materials that have the highest surface area of any known solid material. They contain up to 99.99% empty space. Cotton candy is a good bulk world starting point for thinking about aerogels. The first silicon dioxide aerogel was produced almost 75 years ago, in 1931. Aerogels are made up of rigid chains of 3-4 nanometer spherical clusters of molecules, separated by 30-40 nanometer voids or pores. But unlike cotton candy, and contrary to intuition, the strength of an aerogel's molecular bonds makes it fairly rigid. At the same time the empty pores cause the material to weigh next to nothing - 3 grams per cubic meter. A dollar bill weighs about a gram and a cubic meter is roughly the size of a refrigerator. So, to compare apples to apples, five pounds of aerogel would require a bag big enough to hold more than seven hundred refrigerators. Looking at it another way, aerogels can have as much as 12,000 square meters of surface area per gram. At this point, the gymnasium floor becomes too small to imagine spreading anything out. Think football stadiums and airports. Aerogels hold great promise for applications such as thermal and sound insulation as well as atmospheric filters. NASA has used them to capture particles of space dust.


But the benefits of high surface area are not limited to exotic or synthetic materials. The property of high surface area is basic to the scientific inquiry and commercial development of all things nano.


In the land of hard things, like minerals and metals, particles are made up of multiple crystals, which are formed from very orderly arrays of atoms - some on the surface - others in the middle. The surface atoms in particles are the ones that react with other surface atoms in other substances. The ones in the middle don’t do much socializing - they have jobs to do - but it's sort of back room stuff, like maintaining the electrical, heating and lighting systems. At nanoscale, the ratio of surface atoms to internal atoms becomes larger as particles get smaller. In other words, smaller particles mean more surface atoms, hence, more opportunities for interactions with other atoms. This phenomenon has great import for all chemical reactions. For example, catalysts are substances that accelerate chemical reactions. They do this by lowering the activation energy of the materials they work on. Activation energy is the energy it takes to get a chemical reaction going. Ever notice how hard it is to start a campfire compared to how easy it is to keep it going? Activation energy is the difference between the ommph it takes to start the fire and the ahhh it takes to keep it going. Every substance in the world has a threshold of activation - and so far, it doesn't look as though, nano-or-no-nano, anybody's likely to change that anytime soon. But at nanoscale, so many more surface atoms become available that the aggregate of nearly concurrent interactions can intensify a result like a choir singing a Halleluiah. Nobody's singing very loudly, but the effect is grand.


In the land of soft things, like living cells, increased surface area can mean higher efficiencies and lower doses for pharmaceuticals. When high surface area is exploited in conjunction with small particle size, non-soluble particles can be suspended in a transport liquid and, due to their small size, cross cell membranes, as though they were dissolved.


Using techniques of surface functionalization, scientists are able to modify the chemical composition of materials by adding new atomic and molecular components, thereby creating new performance characteristics. The vast increases in surface area facilitated by nanoscale particles make the fruits of surface functionalization potentially more abundant than ever before.


Gram for gram: whatever the force, strong or weak, chemical or physical; whichever the operative, electrons, atoms, molecules, crystals, grains, or polymers, the smaller the particles, the more sites for interaction. The implications of high surface area for scientific innovation are immense. The exploration and exploitation of that place where two substances meet - the surfaces that define the interface - constitute a proverbial sweet spot for the future of nanotechnology.


This Article first appeared in Tom’s column, Views from the Bottom, on Nanotechnology.com

© Copyright 2005, Thomas P. Imerito / dba Science Communications


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©2009 Science Communications
thomas@science-communications.com