From Silica Nano-Particles to Silica Gels and Beyond - Salt Induced Aggregation of Silica Nano-Particles and the Stability of Resultant Gels

Abstract: Aqueous silica nanoparticle suspensions are widely available and used within a number of industries. A relatively new area of application is as a grouting material for sealing narrow fractures in tunnels. While silica sols/gels have been used successfully to grout sections of tunnels the continued reliability of the grouting requires knowledge of gel formation, long time stability and functionality. Although much research has been done for silica nanoparticle interactions with monovalent cations, the effect of anions and gelling in salt mixtures has not been thoroughly investigated. Ionic interactions with the silica surface were investigated by potentiometric titrations and gel time tests. The strength of cation interaction with the silica surface is found to be controlled by cation and anion interactions in bulk salt solution in the following order Cl- ≈ ClO4- < ClO3- < NO3- < SO4-. Anions to the left in the ranking lead to shorter gel times and higher surface charge density, indicating stronger cation-surface interactions. In salt mixtures with divalent and monovalent ions generally the cations interaction with silica surface follows the direct Hofmesiter series. However, there are considerable differences seen in the kinetics of gel formation. Strongly interacting cations in a mixture of monovalent cations and divalent cations, determine the gelling kinetics. For divalent cations an unexpected shift in the Hofmeister series was observed for Mg2+ at pH > 8. It is expected that Mg2+ due to its strong hydration should follow the direct Hofemister series as Li+ does i.e., weakly interacting with silica surface due to strong hydration, than Ca2+, but this is not the case. However, at pH < 8 the direct Homeister series was observed. The plausible explanation for this unusual strong interaction of Mg2+ with negatively charged silica surface compared to Ca2+ is its ability to polarize the hydrating water molecules leading to strong interaction with silica surface. The effect of temperature and particle size on the aggregation behaviour is investigated using gel time tests, rheological measurements, and electrospray scanning mobility particle sizer. Smaller average particle size and increased temperature lead to faster aggregation due to increased Brownian motion causing higher number of particle collisions in the sols. The formation of a gel network is sudden, leading to an exponential increase in complex viscosity. The average number of particles contained in an aggregate of average size at the vii gel point was found to be three, indicating that large numbers of particles are not incorporated in the gel network at the gel point. To test the long-time stability of silica gels new test equipment was designed and constructed. Waters of different ionic composition and pH were pushed through gels and leachates were collected for maximum 488 days and were analysed by inductively coupled plasma atomic emission spectroscopy for metal concentrations. It was found that much of the salt such as NaCl used to generate the gels exits with the water. The amount of salt exiting follows the direct Hofmeister series for monovalent cations i.e., Na+ leaching more than K+. Increased pH of the water entering the gels does not lead to increased silica dissolution since the silica gels buffer the water down to pH ≈ 9-10. Using a simple numerical method the collected data is used to predict the lifetime of the grouted silica gels. The lifetime is calculated to between 200 and 400 years depending on different factors such as flow rate through the gels and salt used to form the gels.