Persistent nucleation and size dependent attachment kinetics produce monodisperse PbS nanocrystals.

Modern syntheses of colloidal nanocrystals yield extraordinarily narrow size distributions that are believed to result from a rapid "burst of nucleation" (La Mer, JACS, 1950, 72(11), 4847-4854) followed by diffusion limited growth and size distribution focusing (Reiss, J. Chem. Phys., 1951, 19, 482). Using a combination of in situ X-ray scattering, optical absorption, and 13C nuclear magnetic resonance (NMR) spectroscopy, we monitor the kinetics of PbS solute generation, nucleation, and crystal growth from three thiourea precursors whose conversion reactivity spans a 2-fold range. In all three cases, nucleation is found to be slow and continues during >50% of the precipitation. A population balance model based on a size dependent growth law (1/r) fits the data with a single growth rate constant (k G) across all three precursors. However, the magnitude of the k G and the lack of solvent viscosity dependence indicates that the rate limiting step is not diffusion from solution to the nanoparticle surface. Several surface reaction limited mechanisms and a ligand penetration model that fits data our experiments using a single fit parameter are proposed to explain the results.

Coagulation behavior of spherical particles embedded in laminar shear flow in presence of DLVO-and non-DLVO forces.

HYPOTHESIS: Complex and coupled interaction between coagulation mechanisms results in a nonlinear variation of coagulation rate with shear rate. CALCULATIONS: Coagulation behavior of colloidal dispersions is investigated in a laminar shear flow by solving the Fokker-Plank equation for pair probability density function, simultaneously incorporating the effect of (i) Brownian diffusion, (ii) fluid flow, (iii) van der Waals attraction and (iv) double layer repulsion force. Furthermore, analysis foremost studies the effect of non-DLVO solvation force. FINDINGS: Theoretical analysis with experimental validation reveals that, coagulation rate varies non-linearly with the shear rate in presence of double-layer repulsion force, due to the strong coupling of coagulation mechanisms. This is observed by an occurrence of coagulation minima at intermediate shear rate. Increase in double layer repulsion force either facilitates or hinders the occurrence of redistribution of particles, as observed by early or late fall of coagulation rate. Systems with different ionic strengths show a crossover of coagulation rates, whereas those with different solvation forces do not show similar trend. Analysis also shows that, ad hoc additivity assumption of individual Brownian and shear coagulation rates does not hold in case of laminar shear flow, against to that observed in extensional flow [Melis et al. (AIChE J., 999, 1383)].

Stability Characteristics of Nanoparticles in a Laminar Linear Shear Flow in the Presence of DLVO and Non-DLVO Forces.

Stability characteristics of colloids in a laminar flow have been studied by solving the Fokker-Plank equation for the pair probability density function. This was done by simultaneously incorporating, for the first time, the effects of (i) Brownian motion, (ii) fluid convection, (iii) the van der Waals force, and (iv) double-layer repulsion. Furthermore, this work, for the first time, studies the effects of solvation and steric repulsion on a colloid's stability. This work interestingly finds that the stability of colloids initially increases with an increase in particle size followed by a decrease. In contrast to that, stability is observed to increase in an accelerating manner with an increase in surface potential. Similarly, it is found that stability shows a relatively stronger dependence on the solvation potential pre-exponential coefficient as compared to that found with the solvation potential decay length or surface potential. In the case of the coating thickness, though stability shows increasing behavior, the rate of increase is decelerating. The coating density is found to be the single most important parameter in controlling stability as observed by even higher self-accelerating rate of increase of stability as compared to that observed with particle size or the surface or solvation potential. Importantly, analysis reveals that stability to secondary minimum coagulation is not affected by either solvation or steric potential.