A rheological study on temperature dependent microstructural changes of fumed silica gels in dodecane

Abstract

The effect of temperature on colloid systems of hydrophobic and hydrophilic fumed silica in dodecane was investigated mainly by means of dynamic rheology. Dynamic frequency sweeps showed frequency-independent elastic modulus (G′), indicating the presence of gel networks in both dispersions. Dynamic temperature sweeps consisting of a heating and cooling cycle were also conducted. For the hydrophobic fumed silica gel, G′ slowly decreased with rising temperature owing to the increased particle mobility and decreased strength of hydrogen bonds due to the higher thermal energy. However, the hydrophilic fumed silica gel showed a rather unusual temperature dependent behavior involving a transition temperature (T_c) and an irreversible increase in G′ during the cooling cycle. This unusual behavior was believed to be related to the restructuring of nanoparticle chain aggregates (NCA) of fumed silica in gels as also revealed by FTIR results. Further analysis on this behavior showed that elevated temperature tended to increase the fractal dimension (D_f) of aggregates, resulting in more compact NCAs and thereby a denser gel network with a higher G′. These results suggest that temperature is an important external variable that can be regarded as an optional simple way to tailor the viscoelastic properties and to control the microstructure of colloidal systems such as fumed silica dispersions in dodecane.