Pickering Emulsion Gel Based on Funori Biopolymer and Halloysite Nanotubes: A New Sustainable Material for the Cleaning of Artwork Surfaces

Abstract

Within the cultural heritage field, colloidal systems based on ecofriendly products represent alternatives to toxic solvents as cleaners for surface artworks. In this paper, we developed an innovative protocol to prepare a Pickering emulsion gel based on Funori biopolymer, halloysite clay nanotubes (HNTs), and isooctane as confined oil phase. The filling of oil droplets within the Funori network might reduce the amounts of hydrocarbons necessary for surface cleaning applications. Moreover, the specific interactions between Funori biopolymer and halloysite stabilized isooctane droplets could increase the colloidal stability of the Pickering emulsions and favor the removal efficiency toward hydrophobic contaminants. First, we optimized the preparation of Funori gel in aqueous media by the addition of variable amounts of Ba 2+ ions as cross-linkers. To this purpose, we studied the biopolymer cross-linking process by the investigation of the rheological properties and the gelation kinetics. Then, we prepared a hybrid gel based on Funori and Pickering emulsions stabilized by HNTs to achieve sustainable materials with hydrophobic microdomains that are useful for surface cleaning applications. We investigated the colloidal stability and the rheological properties, which were strictly correlated to the specific Funori/halloysite interactions. The peculiar effects of Funori on the HNTs Pickering emulsion were assessed by studying the energetics of detaching of halloysite at isooctane/water interface. Remarkably, the presence of the biopolymer enhanced the interfacial activity of HNTs and, consequently, the colloidal stability of the Pickering emulsion. Finally, we explored the surface cleaning efficiency of the Pickering emulsion gel toward Regalrez 1126, which is a hydrophobic contaminant that can be found on artworks. Remarkably, both contact angle measurements and optical investigations revealed a complete removal of Regalrez 1126 from the surface after 2 min of gel application. Further cleaning tests on a real artwork (an oil on canvas coming from a private collection) confirmed the suitability of the Funori/HNTs/isooctane gel for restoration purposes.