Photon correlation spectroscopy (PCS), a dynamic light-scattering technique for particle size measurement, was used to determine the coagulation rates of aqueous dispersions of relatively monodisperse South Carolina Peerless kaolinite, Silver Hill, Montana, illite, Wyoming montmorillonite, and Florida palygorskite. This technique allows quantitative measurement of the rate of coagulation for clay particles where the traditional turbidity method gives only a qualitative measure. The critical coagulation concentrations for KCl at pH = 10.0 were: 0.199 M for kaolinite, 0.202 M for illite, 0.290 M for montmorillonite, and 0.034 M for palygorskite. The effective Hamaker constants, calculated using Derjaguin-Landau-Verwey-Overbeek theory, were: 3.1 × 10−20 J for kaolinite, 2.5 × 10−20 J for illite, 2.2 × 10−20 J for montmorillonite, and 1.63 × 10−19 J for palygorskite. Stern potentials at the critical coagulation concentration at pH 10.0 were: −42.7 mV for kaolinite, −40.7 mV for illite,‒21.2 mV for montmorillonite, and −66.9 mV for palygorskite.

Mica domains have received less attention in the literature than smectite quasi-crystals. This study was conducted to determine whether mica crystals form domains in suspension, the conditions in which those domains exist, and the distribution of adsorbed Na and Ca ions in the domains. Particle size distributions and electrophoretic mobilities (EM) of Silver Hill illite in suspension densities of 0.5 g liter−1 were determined by photon correlation spectroscopy (PCS). Solutions at salt concentration from 2 to 10 mmolc liter−1, sodium adsorption ratio (SAR) from 0 to ∞ (mmol liter−1)0.5, and pH values 5, 7, and 9 were used to prepare the clay suspensions. The particle size of Silver Hill illite suspensions showed a bimodal distribution. Through PCS measurements at low angles, the second peak of the bimodal distribution of the illite was found to be associated with the rotational movement of the b-dimension of the particles. Illite domains broke down in the range of SAR 10 to 15 (mmol liter−1)0.5 equivalent to exchangeable sodium percentages (ESP) of 13 to 18. Illite thus demonstrates a similar stability to smectites that require ESP ≈ 15 to disaggregate quasi-crystals. The EM of the illite particles increased drastically when the SAR increased from 2 to 10 (mmol liter−1)0.5. This increase in EM could not be explained exclusively by the change in the particle size. Cation demixing is required to explain the increase of the zeta potential at the shear plane. The EM of the Silver Hill illite was doubled when the pH increased from 5 to 9 at SAR > 15, but no pH effect was found when SAR < 15. The effect of pH on the EM at SAR values > 15 can be understood if we consider that at SAR > 15 most of the particles are single platelets. The relative importance of variable charge on single platelets or crystals is apparently greater than on domains because the pH affected the mobility of the individual crystals but not the mobility of the domains. The combination of particle size distribution and EM data gives additional information about the zero point of charge of the variable charge, also called point of zero net proton charge (PZNPC) of the clay. For Silver Hill illite, we estimate a PZNPC value between 5 and 7.

Sedimentation Field Flow Fractionation (SdFFF) was combined with Photon Correlation Spectroscopy (PCS), to characterize changes in the structure of the colloidal particles of reconstituted skim milk of diameter >50 nm (aggregates of casein and calcium phosphate known as casein micelles) with the changes in partitioning (with the addition of salt) of calcium (Ca), inorganic phosphate (Pi) and casein between the serum and colloidal phases of the milk. The number weighted particle size distributions are determined. These are well represented by a log-normal distribution. Methods are presented for estimating the relative contributions of scattering and absorbance to the SdFFF detector signal and for taking both into account when analysing SdFFF data. The values found for the effective density of the casein micelles were in good agreement with the literature and ranged from (1·06–1·08 g cm−3) according to the composition of micelles. The changes in the scattering intensity as determined by PCS correlated with the changes in the particle composition. Although the concentrations of colloidal calcium phosphate (CCP) (1·1–3·5 g/kg milk) and micellar casein (18·1–27·2 g/kg milk) varied considerably only small changes in the size distribution of particles >50 nm diameter were observed except for milk to which 30 mmol Pi+10 mmol Ca/kg milk had been added where the particle size distribution shows a swelling of the particles consistent with a lower than expected value for the particle density. These observations suggest that the micelles have the ability to both lose (depleted micelles) and accommodate (enriched micelles) more casein, calcium and inorganic phosphate in their interior, thus confirming the model of the micelles which postulates an open structure allowing freedom of movement of casein and small ions.