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Aged porous 13 wt% silica hydrogel beads were characterized by measuring the freezing point depression of water in silica gel pores, which provides information about the ice crystal radius (Rn), the ice-pore volume (Vn), the pore shape (F), and the ice-surface area (An). Hydrothermal treatment at high temperatures increases both the pore size and apparent pore volume of not previously dried hydrogel beads. The ice crystal radius, is increased from 8.6 to 27 nm after ageing for 7 h at 473K, while the apparent ice-pore volume increases from 0.9 to 1.4 ml/g. The pore size and pore volume decrease upon drying. At Rn values of 8.6 and 27 nm, the drop ΔRn on drying is 5.1 and 7 nm, respectively, and the decrease of Vn, ΔVn, is 0.4 and 0.6 ml/g, which amounts to 44 and 38%, respectively. The exceptionally small decrease of pore volume for a gel upon drying suggests that this measurement is underestimated by thermoporometry. The soft gel is compressed as liquid is drained from the network during freezing. The apparent increase of the pore volume of the hydrogel must be interpreted to indicate the effect of stiffening (through ageing) on the amount of compression during the measurement. Based on the probably real pore volume Vn of the hydrogel of 1.4 ml/g, instead of 0.9 ml/g, the pore volume decrease at Rn = 8.6 nm would be 0.9, not 0.4 ml/g, so the percentage change would be 64%, rather than 44%. By contrast, at a Rn value of 10.9 and 25 nm, the thermoporometric surface area An* of the hydrogels, which is the surface area of the ice crystals corrected for a non-freezing layer of water, is equal to the BET surface areas of the xerogel measured by nitrogen sorption, which indicates that no pores are enclosed. The increase in pore volume of the aged xerogels by both mercury penetration and thermoporometry is real. It reflects the coarsening and stiffening by ageing, which leads to less shrinkage during drying. The xerogels are probably stiff enough that they experience little contraction on application of thermoporometry. |
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