The environmental response and stability of pectin and poly-l-lysine multilayers

The environmental response of multilayers consisting of pectin and poly-l-lysine (PLL) to changes in pH and ionic strength was studied using Fourier transform infrared-attenuated total reflection spectroscopy (FTIR-ATR) and a quartz crystal microbalance with dissipation monitoring (QCMD). The swelling of the multilayer increased with increasing ionic strength, and was attributed to the screening effect of salt on the attractive forces within the system. Below pH 7.0, reversible swelling decreased with decreasing pH, until at pHs < 3.6 solubilisation of the multilayer occurred. The pH-dependent ionization of the anhydrogalacturonate residues of pectin in the multilayer was examined by FTIR-ATR. It was found there was a substantial shift in their pKa to more acidic pHs. The deswelling of the multilayer with decreasing pH is associated with small decreases in the ionization of anhydrogalacturonate residues. The solubilisation of the multilayer is associated with a loss of charge on the pectin component.