Kočí Eva, Rohlíček Jan, Kobera Libor, Plocek Jiří, Švarcová Silvie, Bezdička Petr
Institute of Inorganic Chemistry of the Czech Academy of Sciences, ALMA Laboratory, Husinec-ŘeŽ 1001, 250 68 Husinec-ŘeŽ, Czech Republic.
Institute of Physics of the Czech Academy of Sciences, Na Slovance 1999/2, 182 21 Prague 8, Czech Republic.
Dalton Trans. 2019 Sep 7;48(33):12531-12540. doi: 10.1039/c9dt02040c. Epub 2019 Jul 31.
Long-chain lead carboxylates, on the one hand, represent compounds for versatile industrial applications in high-tech industries, while on the other hand, they are predominant constituents of secondary products of saponification of paint layers in works of art. Affecting significantly the appearance and stability of painted works of art, saponification is one of the most serious problems of preservation of cultural heritage objects. Despite their versatility as well as hazardousness, there is a paucity of single-crystal X-ray structures of long-chain carboxylates, due to difficulties in preparing single crystals of sufficient quality. We studied the crystal structure of polycrystalline mixed lead carboxylates of the formula Pb(C16)(C18) (x = 0; 0.25; 0.5; 0.75; 1; 1.5; 2), where C16 and C18 stand for hexadecanoate (palmitate) and octadecanoate (stearate) anions, respectively, by complementary X-ray powder diffraction (XRPD) and C and Pb solid state NMR (ssNMR). Mixed lead carboxylates consisting of hexadecanoate and octadecanoate are relevant to the formation of soaps in egg yolk and/or oil-based binders combined with lead-based pigments, which belong to the most common pigments in history. Combining an advanced XRPD analysis with a comparative analysis of ssNMR parameters, we described the structural model of mixed lead carboxylates. We revealed that both hexadecanoate (C16) and octadecanoate (C18) chains are present in one crystal structure, creating the statistical disorder at the ethyl end of the chains. Based on the Pb ssNMR spectra, we revealed two distinct local environments of lead atoms, corresponding to the symmetrically (i.e., (C16)-Pb-(C16) and/or (C18)-Pb-(C18) and asymmetrically (i.e., (C16)-Pb-(C18)) substituted lead carboxylates, and we confirmed the formation of a holo-directed structure for both the structural motifs. The structural models were applied to identify the neo-formed crystalline lead soap in a model experiment simulating the simplified historic paint consisting of the pigment lead tin yellow type I and emulsion binder prepared from egg yolk and linseed oil. We identified the secondary product as a mixed lead carboxylate of the composition Pb(C16)(C18).
一方面，长链羧酸铅是高科技产业中具有多种工业应用的化合物，另一方面，它们是艺术品中漆层皂化副产物的主要成分。皂化对彩绘艺术品的外观和稳定性有重大影响，是文化遗产保护中最严重的问题之一。尽管长链羧酸盐用途广泛且具有危险性，但由于难以制备出质量足够好的单晶，其单晶X射线结构的研究较少。我们通过互补的X射线粉末衍射（XRPD）以及碳和铅的固态核磁共振（ssNMR）研究了化学式为Pb(C16)(C18)（x = 0；0.25；0.5；0.75；1；1.5；2）的多晶混合羧酸铅的晶体结构，其中C16和C18分别代表十六烷酸（棕榈酸）和十八烷酸（硬脂酸）阴离子。由十六烷酸和十八烷酸组成的混合羧酸铅与蛋黄和/或油基粘合剂与铅基颜料结合形成的肥皂有关，铅基颜料是历史上最常见的颜料之一。通过将先进的XRPD分析与ssNMR参数的对比分析相结合，我们描述了混合羧酸铅的结构模型。我们发现十六烷酸（C16）和十八烷酸（C18）链存在于一个晶体结构中，在链的乙基末端产生了统计无序。基于铅的ssNMR光谱，我们揭示了铅原子的两种不同局部环境，分别对应于对称取代（即（C16）-Pb-（C16）和/或（C18）-Pb-（C18））和不对称取代（即（C16）-Pb-（C18））的羧酸铅，并且我们证实了这两种结构基序均形成了全定向结构。这些结构模型被应用于在模拟由I型铅锡黄颜料和由蛋黄与亚麻籽油制备的乳液粘合剂组成的简化历史涂料的模型实验中识别新形成的结晶性铅皂。我们将该副产物鉴定为组成为Pb(C16)(C18)的混合羧酸铅。
