Long-Term Evaluation of the Antifouling Performance of Ionic Liquid-Based Coatings on Marble and Tufa Probes Against Spontaneous Colonization: A Five-Year Monitoring

The long-term antifouling performance of cholinium-based ionic liquid (IL) coatings applied to marble and tufa stone surfaces for cultural heritage protection was studied. A bilayer system consisting of a nano-silica consolidant (NanoEstel®, NE) and ILs was evaluated over a five-year period (2019–2024) under controlled environmental conditions. The coatings’ ability to prevent spontaneous microbial colonization was assessed through macroscopic observation, ImageJ analysis, light microscopy (LM), and field-emission scanning electron microscopy (FE-SEM). The results demonstrated that specific ILs, particularly 3 and 3a, exhibited sustained antifouling effectiveness, maintaining low surface coverage after five years (as low as 4.5% on tufa and 2.1% on marble). Microbiological analysis confirmed minimal fungal structures and the absence of culturable organisms on the most effective coatings. These findings highlight a clear relationship between IL lipophilicity and antifouling performance, with more lipophilic species providing better protection. Overall, these results confirm that NE/IL-based coatings offer a durable, environmentally friendly, and effective strategy for long-term protection of stone materials, supporting their application in the sustainable conservation of cultural heritage. A coating based on a cholinium IL formulation was also tested for in situ application over five years (2021–2026).