Abstract
Thermogravimetric study of Al
3+-exchanged montmorillonite after exposure to acetic (ethanoic) acid vapour or liquid reveals two desorption peaks in the range up to 300 °C. The first, centred around 100 °C, is indistinguishable on this account, as our numerous studies have shown, from the similar desorption of water and many other liquids from the original material at about this temperature. The second, centred around 230 °C, is characteristic of the acid-clay system. We establish that the first desorption is also that of acetic acid that is physically sorbed, whereas the higher temperature desorption is associated with chemisorbed acid.
Uptake rate and equilibration studies with acid and with water vapours yield enthalpies of intercalation for the
physically adsorbed species consistent with liquefaction accompanied, in the case of water, by hydrogen bonding to structural hydroxyls and, possibly, in the case of the acid, with cyclic dimerization. Such intercalation is very slow, 10 h being required to achieve equilibrium at 30 °C, for example. Uptake of the
chemisorbed acid is, by contrast, very rapid and reaches a constant value of
ca. 90 mg g
−1 dry clay at all values of vapour pressures, uptake times and temperatures (30–90°C) employed. The acid in this regime, although obviously strongly bound, can be competitively displaced by water vapour when this is in great excess.
Partition coefficients are calculated and these establish that the intercalated weight ratio (at equal partial pressure) is about 1.3:1.0 in favour of water vapour over the temperature range. This establishes that, in acid-clay systems, where only even modest effort is made to exclude residual water, the acid effectively expels all water. Thus, TGA of the acetic acid-clay system provides a novel opportunity of separately determining sorption isotherms for the physically and chemisorbed acid. The preliminary results reported here establish that the chemisorbed acid dominates the primary uptake, to form the equivalent of a single layer. Subsequently, physical sorption proceeds to produce so-called second and third layers intercalated in the montmorillonite.