Rheology of silica dispersions in organic liquids: New evidence for solvation forces dictated by hydrogen bonding

Dispersions of hydrophilic fumed silica are investigated in a range of polar organic media. The silica forms stable, low-viscosity sols exhibiting sheer thickening behavior in a host of liquids, including ethylene glycol and its oligomers and short-chain alcohols, such as n-propanol. In contrast, the silica flocculates into colloidal gels in other liquids, such as glycols with methyl end-caps and longer-chain alcohols. We suggest that there is a causal relationship between the hydrogen-bonding ability of the liquid and the colloidal microstructure observed. In strongly hydrogen-bonding Liquids, a solvation layer is envisioned to form on the silica surface through hydrogen bonding between liquid molecules and surface silanol groups (Si-OH). This gives rise to short-range, non-DLVO repulsions ("solvation forces") which stabilize the silica particles. In contrast, in the case of liquids with Limited hydrogen-bonding ability, silanols on adjacent silica particles are envisioned to interact directly by hydrogen banding. This leads to particle flocculation and ultimately to gelation. Our study further fuels the debate regarding the existence of solvation forces in dispersions.