|MadSci Network: Physics|
Hello Jonathan, A very interesting question. I've been working in areas connected with this subject for half of my professional life. First of all: there is no short answer. Few scientists have addressed the problem in the general way you inquire about. Each and every material behaves differently, depending on the type of bonding between its constituents. A good reference is: J.Lyklema, Fundamentals of Interface and Colloid Science, Volume IV: Particulate Matter. Elsevier, Amsterdam etc., 2005. There are almost countless methods to produce fine particles of solids. Powdering in dry state, e.g. in a mortar or ball mill, doesn't get you very far down in particle size. Usually a steady state is reached, where the rate of crystal growth (or aggregation) equals that of comminution. Hard materials with high melting point can be obtained finer than softer ones. Fine particles in a dry state (e.g. silica "Aerosil", or silicon carbide) can be produced by gas phase reaction (hydrolysis, pyrolysis). In the case of Aerosil, you get irregularly shaped aggregates of spheres each of about 10 nm diameter, yet no separable single spheres. Most procedures (chemical reaction or mechanical milling) to obtain regular nanoparticles result in dispersions of the particles in a liquid such as water, often with added substances (dispersants) to prevent aggregation. Colloidal gold spheres (a few nanometres across) are the textbook example. To isolate the particles, the liquid will have to be removed carefully, e.g. by freeze drying, but generally the particles stick together anyway, mostly by physical attraction (van der Waals force) rather than chemical bonding. For particles in the nanometre range, a tremendous amount of energy has to be put in to overcome these forces. Measuring particle size in the nanometre range is another daunting challenge, and a lot of doubful data appear in the literature. Electron microscopy is the most "direct" method, but often the sample preparation and interpretation of the pictures (except for gold spheres) requires a fair amount of experience. As for particles "as small as a single .. molecule": the concept of molecules does not really apply to covalently bonded solids such as silica or diamond: any chunk of such materials represents a single "molecule". Single atoms, on the other hand, are gases by necessity. Best Regards Werner Sieber
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