Effects of counter ions of clay platelets on the swelling behavior of nanocomposite gels

The effects of replacing the native Na+ counter ions associated with the clay platelets by various other cations on the swelling behavior of nanocomposite (NC) gels consisting of an organic (polymer)/inorganic (clay) network were investigated. The negative surface charge of the clay platelet conferred an ionic nature on the NC gels making them a type of polyelectrolyte gel; consequently, the swelling behavior of the NC gels was strongly influenced by the valence of the co-existing counter ions. NC gels containing monovalent cations such as Na+, K+ and Li+ exhibited large swellings and subsequent deswelling in water after attaining maximum degrees of swelling. In contrast, introduction of multivalent cations such as Ca2+, Mg2+, and Al3+ into NC gels depressed markedly both the swelling and subsequent deswelling. The decreased swelling and suppressed deswelling with multivalent ions were strongly influenced by the initial gel state and result from the formation of additional cross-links through ionic interactions between the clay platelets and the multivalent cations. Also, the similar swelling behaviors were observed for all NC gels with different clay concentration. Further, reversible absorption/desorption and selective absorption of multivalent cations were observed for the NC gels examined. (C) 2012 Elsevier Inc. All rights reserved.