Evolution of host/guest interactions with heating in a palygorskite/methyl red (Maya Red) hybrid composite

An exceptionally stable hybrid material, fit for possible use as an innovative, cheap and ecologic pigment in the Cultural Heritage and Materials Science fields, can be obtained by grinding and heating palygorskite clay with the methyl red dye (2 wt%). Due to its multiple analogies with the famed Maya Blue pigment (an ancestor of modern hybrid materials formed by indigo incorporation in palygorskite/sepiolite clay minerals), such a red/purple adduct can be considered an analogous Maya Red composite. As per its renowned blue predecessor, the chemical and photo-thermal stability observed for this red equivalent is achieved through methyl red diffusion and bonding within the palygorskite tunnels, which occurs after heating or evacuation of a properly ground clay/dye mixture. Specific interactions form inside the host pores between the clay framework and dye reactive groups, contributing to ensure the composite stabilization at different temperatures. An innovative, in-line coupled TGA-FTIR-GC-MS, synchrotron XRPD and molecular mechanics approach was performed on both pristine palygorskite and the related composite with methyl red with the aim to monitor the development of the interactions formed between the host and the guest while progressively heating. Such a study evidenced that several kinds of bonds can exist and differently affect this complex stability, each characterized by a specific binding energy and subjected to a dynamic but reversible evolution as a function of the magnitude of the heating treatment. Weak to moderate temperatures (120-300°C) trigger zeolitic H2O loss and methyl red diffusion but do not imply release of Mg-coordinated OH2, which acts as H-bond donor to the dye carboxyl group. More severe heating (300-490°C) causes a two-step structural OH2 loss and triggers a ligand-displacement mechanism which favors straight interactions between octahedral Mg and the dye COOH acceptor atoms (i.e. oxygen). Reversibility and shift between these different host/guest interactions severely affect the dehydration/rehydration process of the host framework, compared to the pristine clay. Several interrelated phenomena mutually interact in a sort of positive feedback: guest incorporation inside the tunnels prevents structural folding typical of pure palygorskite and modifies the release of both zeolitic H2O and structural OH2, consequently influencing both the nature and the strength of the host/guest interactions. Such a situation is further complicated by the different polymorphs of palygorskite (monoclinic and orthorhombic) showing peculiar and distinct behaviors, which concern both their affinity to form specific bonds with the encapsulated dye and the release of structural OH2 while heating. Sheltering granted by incorporation in the host pores dramatically enhances methyl red thermal stability, whose degradation is likely to occur at temperatures sensibly higher than those decaying the isolated dye.