Dynamic and static light scattering were applied to the determination of the stability ratio and fractal dimension of kaolinite (KGa-2) at different kaolinite or/and electrolyte concentrations at pH 9.5. Dynamic light scattering was used to measure the kinetics of early stage aggregation to determine the stability ratio, W, as well as the cluster sizes which determine the fractal regime. Static light scattering was used to measure the fractal dimension, D(f). Results show that the two classes of "universality" (Lin et al. Nature 1989, 339, 360) characterizing the diffusion- and reaction-limited regimes of cluster-cluster aggregation do apply to colloidal kaolinite as limit cases when W approximately 1 or W > 100, respectively. In the intermediate regime where 5 < W < 100, the growth of the aggregate radius showed a power-law behavior similar to diffusion-limited cluster aggregation. For the intermediate aggregation regime, a scaling relation between fractal dimension and stability ratio, reflecting a continuous increase in particle packing density in the aggregate as the sticking probability of particles was reduced, was demonstrated.

Copolymerization of ST (styrene) with EDM (ethylene glycol dimethacrylate) gives nonlinear polymers. The average molecular weight and polydispersity of copolymers composed of ST and EDM were measured using the GPC (gel permeation chromatography) technique, and an exponential increase in both quantities with reaction time was observed. 1 H and 13 C NMR spectra of the copolymers were taken to prove the structure and purity of the products and to calculate the composition and pendant content of the copolymers. The ratio of the pendant double bonds of the copolymers was determined from the 1 H NMR spectra. A remarkable increase in the ratio of the pendant double bonds with reaction time was observed. The mechanism for the chain propagation was studied and found to follow the terminal model. Reacitvity ratios were found to be r1 ) 0.452 ( 0.031 for monomer ST and r2 ) 0.297 ( 0.027 for monomer EDM. According to the size distribution of the copolymers measured by DLS (dynamic light scattering) and the average radii of gyration measured by SLS (static light scattering) techniques, the samples showed strong polydispersity and an increase in molecular size with reaction time. Unusually high molecular weight values were observed after a certain reaction time.

Medium generation PAMAM dendrimers are extensively researched as drug delivery vehicles therefore detailed knowledge of their physico-chemical properties in solution is vital. We have selected ethylenediamine core, generation five poly(amidoamine) (PAMAM_E5) dendrimers (amine terminated PAMAM_E5.NH 2 and its succinamic acid derivative PAMAM_E5.SAH, respectively) as model compounds to study their dynamic behavior in water as a function of pH and concentration. Diffusion coefficients of water and the dendrimers were determined in deuterium oxide using pulse-field gradient stimulated echo (PGSE) NMR. The diffusion rate for PAMAM_E5.NH 2 increased monotonously with increasing pH values while for PAMAM_E5.SAH a maximum was found at an isoelectric value of pH ¼ 5.7. The apparent diffusion coefficients of dendrimers decreased linearly with increasing concentration measured at their self-pH (pH ¼ 9.4 for PAMAM_E5.NH 2 and pH ¼ 5.7 for PAMAM_E5.SAH) in the absence of added salts. The observations could be explained by considering hard sphere interactions between strongly hydrated dendrimer molecules. The average hydrodynamic radii of dendrimers were determined by extrapolating the measured diffusion coefficients to zero dendrimer concentration and applying the Stokes-Einstein equation. The calculated values were R H ¼ 3.05 AE 0.04 nm for PAMAM_E5.NH 2 and R H ¼ 3.37 AE 0.08 nm for PAMAM_E5.SAH respectively. Measured diffusion coefficients of water (D 2 O) also decreased linearly with increasing dendrimer concentration. From the concentration dependence, the average number of water molecules that form one dynamic unit with one macromolecule could be calculated on the basis of three different obstruction models. It has been concluded that hydrated PAMAM dendrimers in aqueous solutions behave as soft colloids against solvent molecules but as hard-sphere colloids against each other. Their equivalent hard-sphere radii were found to be equal to the measured hydrodynamic radii.

The aggregation rate was determined for the < 0.2 microm size fraction of kaolinite (KGa-2) using simultaneous static and dynamic light scattering at pH 9.5. It was found that method suggested by Holthoff et al. [Langmuir 1996, 12, 5541] is suitable for determination of the absolute aggregation rate constant of a clay dispersion without using the particle optical factors. The determined fast aggregation rate constant is k11,fast = (3.7 +/- 0.2) x 10(-18) m3 s(-1). Stability behavior of kaolinite colloids was studied as a function of concentration of sodium chloride by simultaneous static and dynamic scattering. The critical aggregation concentration was found to be 0.085 +/- 0.005 mol dm(-3). When calculating the relationship between the stability ratio and the electrolyte concentration using the DLVO theory, the best fit to the experimental data was achieved with a Hamaker constant of A = (4.7 +/- 0.2) x 10(-20) J.

No abstract