Long-chain mercury carboxylates relevant to saponification in oil and tempera paintings: XRPD and ssNMR complementary study of their crystal structures

0554813 - ÚACH 2023 RIV GB eng J - Journal Article
Barannikov, Ruslan - Kočí, Eva - Bezdička, Petr - Kobera, Libor - Mahun, Andrii - Rohlíček, Jan - Plocek, Jiří - Švarcová, Silvie
Long-chain mercury carboxylates relevant to saponification in oil and tempera paintings: XRPD and ssNMR complementary study of their crystal structures.
Dalton Transactions. Roč. 51, č. 10 (2022), s. 4019-4032. ISSN 1477-9226. E-ISSN 1477-9234
R&D Projects: GA MŠMT(CZ) LM2018124; GA ČR(CZ) GA19-05259S
Grant - others:AV ČR(CZ) StrategieAV21/23; AV ČR(CZ) StrategieAV21/10
Program: StrategieAV; StrategieAV
Institutional support: RVO:61388980 ; RVO:61389013 ; RVO:68378271
Keywords : cross-polarization * adiabatic pulses * thermal-behavior * chemical-shift * metal soaps * NMR
OECD category: Inorganic and nuclear chemistry; Polymer science (UMCH-V); Condensed matter physics (including formerly solid state physics, supercond.) (FZU-D)
Impact factor: 4, year: 2022
Method of publishing: Limited access
https://doi.org/10.1039/D1DT04160F

Saponification, resulting from pigment-binder interactions, is one of the most endangering phenomena affecting the appearance and stability of painted works of art. The crystallization of metal carboxylates (soaps) in paint layers is recently assumed as the most critical point for the development of undesirable changes induced by saponification, however, the factors triggering it are not fully understood. The red pigment cinnabar (HgS) has been suspected of contributing to saponification, however, the paucity of reliable reference structural data limited the experimental research of its effect at the molecular level. Within this study we synthesized mercury(ii) carboxylates of the formula Hg(C16)(x)(C18)(2-x) (x = 0.0, 0.2, 0.5, 0.8, 1.0, 1.2, 1.5, 1.8, 2.0) where C16 and C18 are hexadecanoate (palmitate) and octadecanoate (stearate), respectively, and characterize them by combination of X-ray powder diffraction (XRPD) and C-13 and Hg-199 solid state NMR (ssNMR). For a more detailed interpretation of their structural and thermal behavior, Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) were used. The crystal structure of the studied mercury carboxylates was described on the basis of complementary ssNMR and XRPD measurements, Rietveld refinement and DFT calculations. All the subjected compounds crystallize in a monoclinic lattice of the C2/c symmetry. Mercury atoms are arranged in a slightly distorted square antiprismatic geometry and are monodentatically bonded to carboxylate anions. The structural disorder at the aliphatic end of the stearic acid chains was detected in the mixed carboxylates. Within the paper, the structural (dis)similarity with the corresponding lead carboxylates is discussed. The synthesized and characterized mercury carboxylates were applied to describe neo-formed mercury soaps in a model experiment simulating an egg-based paint system.
Permanent Link: http://hdl.handle.net/11104/0329459


Research data: CCDC