Salt-induced fast aggregation of polystyrene latex.

Abstract

We study fast, salt-induced aggregation of polystyrene spheres by means of low-angle static light scattering covering two decades of scattered wave vector q. The measurements are taken at a fixed salt concentration, varying the monomer concentration from ${\mathit{c}}_{0}$=1\ifmmode\times\else\texttimes\fi{}${10}^{9}$ to 5\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. During each run, the fractal dimension ${\mathit{d}}_{\mathit{f}}$ of the clusters and the time evolution of the average radius 〈${\mathit{R}}_{\mathit{G}}$〉 and average mass 〈M${\mathrm{〉}}_{\mathit{w}}$ are determined. At larger concentrations we consistently find ${\mathit{d}}_{\mathit{f}}$=1.61\ifmmode\pm\else\textpm\fi{}0.02, but, as the concentration is decreased, ${\mathit{d}}_{\mathit{f}}$ grows to 1.83\ifmmode\pm\else\textpm\fi{}0.02. The value of ${\mathit{d}}_{\mathit{f}}$ is determined both from the asymptotic behavior of the scattered intensity I(q) and from the relation between the average mass and radius, I(0)\ensuremath{\propto}${\mathit{R}}_{\mathit{G}\mathit{f}}^{\mathit{d}}$. Good agreement is found between the two sets of data. The earlier portions of the curves for ${\mathit{R}}_{\mathit{G}}$ as a function of time t are compatible with a power-law growth and collapse onto a master curve when the reduced time T=${\mathit{c}}_{0}$t is used. Noticeable deviations from power-law growth are, however, observed at later times. Finally, we show that for the lowest concentrations the reactions stop when the clusters attain a maximum diameter. We present arguments showing that reaction termination is due to sedimentation, the time required to diffuse across intercluster distance becoming longer than the settling time through the sample due to sedimentation.