Re: What is the smallest possible average particle size for a powdered solid?

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