Assembling Xanthan Gum at the Air–Water Interface and Disentangling It with Ionic Liquids

Abstract

Xanthan gum is a biopolymer used in a wide range of products in the food and cosmetic industries. As ionic liquids (ILs) have emerged as antimicrobial molecules, they can be used as preservatives for this polymer. Hence, it is important to understand the interaction of ionic liquids with Xanthan gum. In this investigation, polymer self-assembly at the air–water interface has been studied in the presence of ionic liquids floating at the air–water interface. From the surface pressure–area isotherm, it is shown that the electrostatic interaction drives the polymer to the interface, resulting in a viscoelastic film. In-plane dilation rheology has determined the storage and loss moduli of the film, which are found to depend on the concentration of the polymer dissolved in the water subphase. Additionally, a synchrotron-based X-ray reflectivity study has produced the electron density profile across the interface, depicting the structure of the film and suggesting that the negatively charged side groups of Xanthan gum attach to the positively charged headgroup of ionic liquids. This assembly drives the polymer to disentangle, which has been further verified in an aqueous solution of the polymer showing a non-newtonian shear-thinning behavior. The storage and loss moduli curves show two crossover frequencies, manifesting an elastic plateau width that decreases in the presence of IL in the solution. This, in turn, is a signature of the disentangling effect of the IL.