Hi Frank, Well, my mistake was not correcting the reference to the Tyndall Effect. Although one should separate the Faraday-Tyndall Cone from the general Tyndall Effect, which does hold that the incident light changes colour as the particle size is increased, from blue through to white for polychromatic illumination. I made no reference to intensity, but of course you are correct, the reflected light will be more intense with large particles and lose intensity as particle size reduces, until it cannot be found at all with particle size of about 1nm(?).
Gold particles are indeed red at around 13nm, certainly for those produced by chemical reduction. There are many papers that attest so, and the production of gold sols (mainly for the microbiology labs) is a rather large industry. I cannot comment upon your experiences, other than to say your gold colloid product is indeed ruby red. It is true that the colour of a colloid is not determined by particle size alone, this is a function of size, shape, dispersion, plasmid excitation and extinction, and lie within the Rayleigh scattering and Mie theories. However, all other things being equal, particle size does determine colour. As Basab Chaudhuri and Syamal Raychaudhuri put it: 'All gold colloids display a single absorption peak in the visible range between 510 and 550 nm. With increasing particle size, the absorption maximum shifts to a longer wavelength, while the width of the absorption spectra relates to the size range. The smallest gold colloids (25 nm) are yellow-orange, midrange particles (1020 nm) are wine red, and larger particles (3064 nm) are blue-green. Smaller gold particles are basically spherical, while particles in the range of 3080 nm show more shape eccentricity related to the ratio of major to minor axes.' Particle size is determined by a number of methods, the most common benchmark being microscopy (TEM or SEM)in the papers I have read. Regards Ivan. > -----Original Message----- > From: Frank Key [mailto:[email protected]] > Sent: Thursday, 25 July 2002 3:17 a.m. > To: [email protected] > Subject: Re: CS>Particle size vs. color > > > Ivan wrote: > > > No. The Tyndall Effect is only manifest when the particle is much > > smaller than the wavelength of light. For instance, gold > colloids look > > red when the particles are about 13nm in diameter. > > > > Regards > > Ivan. > > That statement is not true. TE intensity of reflected light > is maximum when > the particle size is one wavelength of the illumination. > For a 632 nm laser, > the highest intensity of reflected light will be for > particles that are 632 > nm in diameter. The intensity of the reflected light > diminishes as the > particle get smaller than the wavelength of the illumination source. > > Your comment on the color of colloidal gold being red when > the particles are > about 13 nm is false. Mesogold, a colloidal gold product, > is ruby red and > has particles that have been measured to be 1.4 nm. > > The color of a colloid is not solely determined by particle > size. It is also > influenced by the particle dispersion which is effected by > zetapotential. A > small ionic change can change the zetapotential and make a > sol consisting of > the same sized particles to appear as: red, yellow, blue, > brown, green, and > violet. We have proven this in a lab that is capable of > accurate measurement > of particle size. > > At the Colloidal Science Lab we use the Malvern Zetasizer > 3000 HSA, which is > a photon corellation spectrometer designed to measure > colloidal particles in > the nanometer range. > > I would be interested to know how the particle size is > being determined by > those who are making authoritative sounding statements > about particle size > properties. . > > frank key > > > > > > > -- > The silver-list is a moderated forum for discussion of > colloidal silver. > > Instructions for unsubscribing may be found at: > http://silverlist.org > > To post, address your message to: [email protected] > > Silver-list archive: > http://escribe.com/health/thesilverlist/index.html > > List maintainer: Mike Devour <[email protected]> > > --- > Incoming mail is certified Virus Free. > Checked by AVG anti-virus system (http://www.grisoft.com). > Version: 6.0.377 / Virus Database: 211 - Release Date: 15/07/2002 > --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.377 / Virus Database: 211 - Release Date: 15/07/2002
