Highly Functionalized and Precisely Defined Multipodal Silsesquioxanes with 16 Phosphonate Anchors: Hybrid Nanoscale Consolidants, Tailored for Cultural Heritage Preservation Applications

Tangible cultural heritage artifacts (e.g., archeological sites, historical buildings, monuments, and statues) can suffer severe deterioration upon exposure to external stresses (human-, or environment-derived) through the centuries. Hence, the implementation of effective conservation and consolidation strategies is necessary. A distinctive part of these is the design, preparation and application of functional chemical consolidants. In this paper we report on the synthesis and characterization of a multipodal polyhedral oligomeric silsesquioxane (POSS) with 16 protruding phosphonate groups (PHOS–POSS) that are grafted to the Si8O12 cage-like silsesquioxane core. A diphosphonate (PAPTS) containing a polymerizable silanetriol unit has been synthesized, as a small molecule partial analog of PHOS–POSS. PHOS–POSS is a potent consolidant for gypsum stones, as proven by drilling resistance measurements on artificial model gypsum specimens. This mineral was chosen because it has been used since the ancient times as a structural material (e.g., in the Minoan Palace of Knossos, Crete, Greece). It achieved more than a 1000-fold increase in drilling resistance, while exhibiting high penetration depth (>40 mm). PAPTS, on the other hand, is a much weaker consolidant (enhancing drilling resistance by a factor of 2), most likely due to the low number of phosphonate groups (two). Based on structural correlations with several calcium phosphonate networks and structure solution of the compound Ca-methylamino-di(methylenephosphonate) (Ca–C1D), it is proposed that the high consolidation efficiency of PHOS–POSS is ascribed to its high affinity for Ca sites on gypsum (ability to substitute the surface Ca2+-bound water molecules), and to the random substitution of sulfate groups in the structure of gypsum.