Giustetto Roberto, Llabrés I Xamena Francesc X, Ricchiardi Gabriele, Bordiga Silvia, Damin Alessandro, Gobetto Roberto, Chierotti Michele R
Dipartimento di Scienze Mineralogiche e Petrologiche, University of Turin, via V. Caluso 35, 10125 Turin, Italy.
J Phys Chem B. 2005 Oct 20;109(41):19360-8. doi: 10.1021/jp048587h.
Maya Blue pigment, used in pre-Colombian America by the ancient Mayas, is a complex between the clay palygorskite and the indigo dye. The pigment can be manufactured by mixing palygorskite and indigo and heating to T > 120 degrees C. The most quoted hypothesis states that the dye molecules enter the microchannels which permeate the clay structure, thus creating a stable complex. Maya Blue shows a remarkable chemical stability, presumably caused by interactions formed between indigo and clay surfaces. This work aims at studying the nature of these interactions by means of computational and spectroscopic techniques. The encapsulation of indigo inside the clay framework was tested by means of molecular modeling techniques. The calculation of the reaction energies confirmed that the formation of the clay-organic complex can occur only if palygorskite is heated at temperatures well above the water desorption step, when the release of water is entropically favored. H-bonds between the clay framework and the indigo were detected by means of spectroscopic methods. FTIR spectroscopy on outgassed palygorskite and freshly synthesized Maya Blue samples showed that the presence of indigo modifies the spectroscopic features of both structural and zeolitic water, although no clear bands of the dye groups could be observed, presumably due to its very low concentration. The positions and intensities of delta(H2O) and nu(H2O) modes showed that part of the structural water molecules interact via a hydrogen bond with the C=O or N-H groups of indigo. Micro-Raman spectra clearly evidenced the presence of indigo both in original and in freshly synthesized Maya Blue. The nu(C=O) symmetric mode of Maya Blue red-shifts with respect to pure indigo, as the result of the formation of H-bonds with the nearest clay structural water. Ab initio quantum methods were applied on the indigo molecule, both isolated and linked through H-bonds with water, to calculate the magnitude of the expected vibrational shifts. Calculated and experimental vibrational shifts appeared to be in good agreement. The presence of a peak at 17.8 ppm and the shift of the N-H signal in the 1H MAS NMR spectrum of Maya Blue provide evidence of hydrogen bond interactions between indigo and palygorskite in agreement with IR and ab initio methods.
玛雅蓝颜料在哥伦布发现美洲大陆之前被古代玛雅人使用，它是一种粘土坡缕石和靛蓝染料之间的复合物。这种颜料可以通过将坡缕石和靛蓝混合并加热到T > 120摄氏度来制造。最常被引用的假设是，染料分子进入渗透粘土结构的微通道，从而形成稳定的复合物。玛雅蓝表现出显著的化学稳定性，大概是由靛蓝和粘土表面之间形成的相互作用引起的。这项工作旨在通过计算和光谱技术研究这些相互作用的本质。通过分子建模技术测试了靛蓝在粘土骨架内的包封情况。反应能量的计算证实，只有当坡缕石在远高于水脱附步骤的温度下加热时，粘土 - 有机复合物的形成才会发生，此时水的释放从熵的角度来看是有利的。通过光谱方法检测到了粘土骨架和靛蓝之间的氢键。对脱气后的坡缕石和新合成的玛雅蓝样品进行的傅里叶变换红外光谱（FTIR）表明，靛蓝的存在改变了结构水和沸石水的光谱特征，尽管没有观察到染料基团的明显谱带，大概是由于其浓度非常低。δ(H₂O)和ν(H₂O)模式的位置和强度表明，部分结构水分子通过氢键与靛蓝的C = O或N - H基团相互作用。显微拉曼光谱清楚地证明了原始玛雅蓝和新合成的玛雅蓝中都存在靛蓝。由于与最近的粘土结构水形成氢键，玛雅蓝的ν(C = O)对称模式相对于纯靛蓝发生红移。对孤立的以及通过氢键与水相连的靛蓝分子应用了从头算量子方法，以计算预期振动位移的大小。计算得到的和实验测得的振动位移似乎吻合得很好。玛雅蓝的¹H MAS NMR谱中17.8 ppm处的峰以及N - H信号的位移提供了靛蓝与坡缕石之间氢键相互作用的证据，这与红外光谱和从头算方法一致。
