Phase transitions of pyrogenic silica suspensions: a comparison to model laponite.

Pyrogenic silica is often used as a thickening agent in paints, pastes, adhesives, or resins. Other applications include, e.g., abrasives in chemical mechanical planarization in the microelectronics industry. In all these applications it is essential to control the state of dispersion. Sometimes, phase transitions from the liquid to the solid state are required while in other cases they have to be completely avoided for the whole shelf life. The nature and influencing parameters of the fluid-solid transition for pyrogenic silica have not been investigated so far. Most investigations deal with the phase transitions of small clay particles such as laponite. Here, we dedicate our interest to the behavior of pyrogenic silica suspensions with varying specific surface area and ionic background concentration. To get an impression of the phase transition behavior we compare our results to model laponite suspensions. We apply dynamic light scattering measurements in the backscattering regime to minimize multiple scattering contributions from concentrated pyrogenic silica suspensions. Further on we exert a decomposition of the measured autocorrelation functions into an ergodic and nonergodic contribution. The analysis of the ergodic spectrum yields two different gelation kinetics for both systems, laponite and pyrogenic silica. For laponite these are in accordance with earlier investigations. The kinetics depend on the ionic background and the solids content of the suspensions. Additionally, we used dynamic extinction spectroscopy to follow the phase transitions of pyrogenic silica on a macroscale.