Typically if d < lambda/10 where d is the diameter of the particle and lambda the wavelength of impinging radiation, then the scattering coefficient will be less than one and the particles in colloidal suspension will essentially be invisible in visible radiation. Thus, one sees a transparent, often colored (e.g. gold sol) apparent solution. For example, a TiO2 of 30 - 50 nm will be as transparent as water but much denser for a 20 vol % loading. Play with Philip Laven’s MiePlot to model such scattering behavior.

For more about Mie, see:

The life of Gustav Mie and the development of the Lorenz-Mie solution to Maxwell’s equations http://tinyurl.com/mt97tp3

For more on transparency (and opaqueness) of TiO2, see:

Optical properties of the three forms of titanium dioxide

http://tinyurl.com/pbgf9hj

Typically if d < lambda/10 where d is the diameter of the particle and lambda the wavelength of impinging radiation, then the scattering coefficient will be less than one and the particles in colloidal suspension will essentially be invisible in visible radiation. Thus, one sees a transparent, often colored (e.g. gold sol) apparent solution. For example, a TiO2 of 30 - 50 nm will be as transparent as water but much denser for a 20 vol % loading. Play with Philip Laven’s MiePlot to model such scattering behavior.

For more about Mie, see:

The life of Gustav Mie and the development of the Lorenz-Mie solution to Maxwell’s equations http://tinyurl.com/mt97tp3

For more on transparency (and opaqueness) of TiO2, see:

Optical properties of the three forms of titanium dioxide

http://tinyurl.com/pbgf9hj

Alan, I agree, only 2 aspects need to be considered:

1) a nanoparticle dispersion is not a "solution", while you have a 30 - 50 nm particle site TiO2 in a solvent, this will have to be called "colloidal dispersion", not "solution" (it looks like a solution, but it isn't)

2) 20% loading of a 30 - 50 nm small nanoparticle material is over-critical, maybe already forming a gel? (what is the viscosity of this colloidal system?)

@ Bernhard

1) Yes, a colloidal sol is not a solution. My point was that it appears to be so. Dispersion is a better word than suspension, but still not quite there, as a system may not be fully dispersed.

2) 20 vol% is easily possible for the TIO2 system and is used in some wood varnishes - transparent but a high particle concentration.

@ Alan Thanks, we seem to agree. (I don't doubt that 20% is possible, probably using some dispersion aids; I myself made a lot of highly concentrated colloidal systems, and sure, at such high concentration, there will be agglomerates, but if well dispersed, no aggregates, and the agglomerates will show a very particular 3D structure)

@ Bernhard

Yes, we're on the same page.