Novel Insights on Swelling and Dehydration of Laponite

This paper investigates water sorption and dehydration of model synthetic clay (Laponite) and highlights the large differences with natural Na-smectite clays. Measurements were done by combining for the first time thermogravimetric analysis and X-ray diffraction methods on Laponite powders stored in atmospheres with controlled relative humidity. Laponite exhibits a very good reproducibility of the sorption diagram and appears as the most hygroscopic among Na-smectites. The interlayer distance increases from 11 to 20 angstrom when the ratio r = n(H2O)/n(Na) increases from 2 to 22, in agreement with recent numerical simulations. The desorption process, in this range of r, in isothermal conditions follows a first order kinetics with two regimes around r* = 7 which is close to the coordination number of Na+ ions in concentrated NaCl solution. Activation energies and frequency factors are derived for the two regimes of desorption and discussed in relation with stability of the cationic complex.