Swelling and Stability of Polyelectrolyte Multilayers in Ionic Liquid Solutions

Swelling of polyelectrolyte multilayers with aqueous solution of ionic liquid (IL) is examined with a dissipative quartz crystal microbalance. Multilayers are prepared with different combinations of polyelectrolyte, employing PEI(PSS/PAH)(4)PSS and PEI(PSS/PDADMAC)(4)PSS. An enhancement of mass coverage and dissipation is found for films in contact with aqueous solution of IL, i.e., 1-hexyl-3-methylimidazolium chloride and 1-methyl-3-methylimidazolium chloride, suggesting incorporation of IL, accompanied by film swelling. Swelling increases with increasing IL concentration. It is strictly reversible up to a stability limit of IL concentration, above which irreversible layer decomposition starts. Each swelling step can be divided into a fast and a slow process, resulting from IL uptake and chain reorganization or layer decomposition, respectively. While absolute IL uptake and swelling limits differ, the concentration dependence is similar in all systems investigated. The swelling is attributed to hydrophobic interactions, since we find a correlation of the swelling with hydrophobicity of IL.