DIFFRACTION OF LIGHT BY ORDERED SUSPENSIONS

An experiment is described for applying the Bragg equation to determine lattice parameters in ordered colloidal dispersions. Measurements on electrolyte-free monodisperse latexes show diffraction peaks; upon dilution of the dispersions, the peaks become increasingly sharp and the positions of the maxima shift to indicate increased interparticle spacing. The center-to-center particle separation D, the particle diameter D0, and the volume fraction φ obey the relation φ(D/D0)3 = 0.74, the value 0.74 being the volume fraction of spheres in a close-packed array. The variation of peak width with concentration is accounted for quantitatively in terms of the scattering power of homogeneous spheres in a perfect crystalline array. Diffraction peaks were observed at volume fractions as low as 1%, where the particles are four diameters apart. The existence of order in such dilute dispersions is attributed to long-range electrostatic forces.