Porous glasses are leaching products of phase-separated alkali borosilicates. The first phase is an alkali-rich borate phase, soluble in hot mineral acids, water or alcohols. The second phase is almost pure silica. The pores can be formed during the phase separation process in a wide size range from 0.3 to 1000 nm in diameter, depending on glass composition, time and temperature of phase separation and leaching conditions. A quenching of the initial glass melt followed by a short-time treatment ...
Porous glasses are leaching products of phase-separated alkali borosilicates. The first phase is an alkali-rich borate phase, soluble in hot mineral acids, water or alcohols. The second phase is almost pure silica. The pores can be formed during the phase separation process in a wide size range from 0.3 to 1000 nm in diameter, depending on glass composition, time and temperature of phase separation and leaching conditions. A quenching of the initial glass melt followed by a short-time treatment at lower temperatures results in microporous materials. Their surface is characterized by hydroxyl groups with concentration between 4 and 8 OHs nm-2.
This kind of membranes possesses, in comparison with other porous inorganic solids, high thermal stability, chemical resistance, high optical transparency, very flexible geometric forms, very reactive surface and good accessibility to eventually available active sites inside the porous structure. Moreover, this type of membranes are characterized by low permeability of air, a low surface acidity caused by very weakly acidic silanol and weakly acidic boranol groups. They are of interest for molecular sieve applications.
Porous glasses are applied in biotechnology, membrane technology, micro-reaction engineering, dental industry, in heterogeneous catalysis between others applications.
Nanoporous glass membranes properties are studied with help of an electrodialysis cell. There are 4 different porous sizes available: 15, 30, 50 and 100 nm. The cell includes 4 compartments: 2 electrode rinse ones, 1 dilute and 1 concentrate stream compartments. There is a cathode and an anode in each of the ends of the cell followed by an anionic and a cationic membrane; the nanoporous glass membrane is placed in the middle of the cell. Two Luggin capillaries are used for measuring the potential difference across the glass membrane.
In a 0.01 M NaCl solution, no limiting current was observed in the case of 100 nm and 15 nm porous glass membranes. In a 0.001 M NaCl solution, the limiting current for all studied membranes was detected. This behaviour was predicted by Yaroshchuk A. (2012) model for nanoporous membranes: the limiting current may occur in more dilute solutions and may not occur in more concentrated ones. However, in a 0,0001 M NaCl solution no limiting current was observed due to high resistivity of the system.
In the graph below the experimental I-V curves 0.001 M NaCl solutions are shown for a 15, 30, 50 and 100 nm porous glasses membranes and also for a CMX-SB homogeneous membrane to comparison.
Generally, it can be seen that the highest limiting current is observed for the 50 nm membrane, it decreases in the range: 50 > 30 > 15 > CMX-SB. This range may be explained by increasing membrane permselectivity: lower pore size reduces the fraction of pore space filled with uncharged solution. The case of 100 nm membrane presents an exception to the rule.
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