Rapid, reversible sorption of water from the vapor by a multilayered composite film: A nanostructured humidity sensor

Composite films prepared by alternate adsorption of poly(diallyldimethylammonium chloride) and exfoliated sheets of a synthetic sheet-silicate mineral (Laponite RD) respond in a dramatic way to changes in ambient humidity. The structural order in these films and the ability to prepare them to precisely controlled thicknesses provided unique opportunities to study this behavior. Ellipsometric and gravimetric evidence indicates that initially some water fills void spaces in the films, giving rise to a relatively small increase in overall film thickness; further uptake of water then causes greater swelling of the film. This behavior is rapid: water penetrates to depths greater than 139 nm within seconds and desorbs from the films at a similar rate. The rapid and reversible response of this system makes it interesting from the standpoint of molecular sensing.