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Dispersions of hydrophilic fumed silica are investigated in a range of polar organic media. The silica forms stable, low-viscosity sols exhibiting shear 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 …