Controlled delivery of microemulsion via a PVA-PAA double-network hydrogel for safe removal of smoke deposits from murals

A PVA-PAA hydrogel encapsulates microemulsions for non-invasive mural smoke removal. • PVA:PAA 7:3 hydrogel has high strength (0.66 MPa, 700%) and 67% liquid retention. • This hydrogel restores mural shape and color, enabling non-destructive cleaning. As a significant heritage of human civilization, murals often develop hydrophobic smoke deposits on their surfaces due to the combustion of incense, candles, and Yak butter, posing a serious threat to their long-term preservation. Traditional cleaning methods fail to meet the stringent requirements of cultural relic conservation, particularly with regard to safety, controllability, and non-destructiveness, thereby highlighting the urgent need for a gentle, safe, and controllable cleaning strategy. To address this challenge, this study developed a polyvinyl alcohol-polyacrylic acid (PVA-PAA) double-network hydrogel loaded with an oil-in-water (O/W) microemulsion (composed of AEO-9, SDBS, n-pentanol, liquid paraffin, and water) via a solvent displacement method. The hydrogel forms a three-dimensional network through the synergistic interaction between PVA and PAA, achieving stable cross-linking primarily through hydrogen bonding. The influence of varying PVA:PAA mass ratios on hydrogel performance was systematically investigated. Results demonstrated that the hydrogel with a mass ratio of 7:3 exhibited optimal mechanical properties(the tensile stress is 0.66 MPa and the strain is 700%.), swelling capacity(110%), and liquid retention(the water retention rate is 67% after working at 60 degrees celsius for 4 h.). The cleaning evaluation demonstrates that following treatment with the system, the surface morphology, color difference, and contact angle of the simulated mural samples with smoke deposits mural closely resemble those of the original sample. This indicates that the system can gently penetrate the contamination layer, effectively promote the emulsification and dispersion of pollutants, and preserve the structural integrity of the underlying mural substrate. This research presents a novel, controllable, and non–destructive cleaning strategy for the removal of complex hydrophobic contaminants from cultural relic surfaces, offering strong potential for practical application.