Nanoscopic water layers adsorbed onto mesoporous silica aggregates and their effect on the stability and the static yield stress of their dispersion in liquid paraffin

Nanoscopic liquid-like water films were deposited onto silica mesoporous aggregates from vapour phase at 30 °C and at 0.9 P/P0 or from water–paraffin emulsions. The water layers, surround entirely the silica aggregates, reduced their specific surface area from 136 ± 5 m2/g to 80 ± 4 m2/g, and leaved substantially unchanged their meso-porosity. No formation of nano-pores smaller than 1 nm was observed.The dispersions in liquid paraffin of these powders, without applying any shear rate, result different from the correspondent suspensions obtained with dry silica meso-pores aggregates especially when the dispersions reach the gel-like status, i.e. after the percolation threshold θ*, set at 8.2% (v/v). For θ values less the θ* the water films act as insulation to reduce the stability of the dispersions, i.e. to reduce the net attractive forces between the silica surfaces and the paraffin.In the gel-like regime, the adsorbed water molecules play a new and emerging role. As the solid volume concentration is increased the silica aggregates covered with water layers come in to contact and form a “liquid menisci” between neighbouring films that allow the formation of a “pendular state” because of capillary forces. These capillary/viscous forces cause a second generation of gel agglomerates, few millimetres in dimension, that are interconnected between them and somewhat separated by thin voids. By comparison with the gel status formed by the dry silica agglomerate dispersions, it has been derive that the paraffin molecules migrate inside the void spaces created throughout the “pendular state”. Maximum static yield stress and separation energies equal to 2500 Pa and 3 × 10−18 J, respectively, were measured and compared with values of 1190 Pa and 3 × 10−19 J for dispersions without adsorbed water layers.

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► Effect of water liquid-like layers on the rheology of silica–paraffin dispersions.
► Silica–paraffin attractive forces reduction by water films adsorbed in viscous status.
► Pendular state structure due to water layer on silica clusters in gel-like conditions.
► Role of capillary forces on the stability of silica–paraffin concentrated dispersions.