Institute of Environmental Engineering , University of Pannonia , P.O. Box 158, Veszprem 8201 , Hungary.
Department of Chemistry and Biochemistry , Montana State University , P.O. Box 173400, Bozeman , Montana 59717 , United States.
Inorg Chem. 2018 Jun 18;57(12):7151-7167. doi: 10.1021/acs.inorgchem.8b00877. Epub 2018 Jun 6.
A procedure is developed for defining a compositionally and structurally realistic, atomic-scale description of exfoliated clay nanoparticles from the kaolinite family of phylloaluminosilicates. By use of coordination chemical principles, chemical environments within a nanoparticle can be separated into inner, outer, and peripheral spheres. The edges of the molecular models of nanoparticles were protonated in a validated manner to achieve charge neutrality. Structural optimizations using semiempirical methods (NDDO Hamiltonians and DFTB formalism) and ab initio density functionals with a saturated basis set revealed previously overlooked molecular origins of morphological changes as a result of exfoliation. While the use of semiempirical methods is desirable for the treatment of nanoparticles composed of tens of thousands of atoms, the structural accuracy is rather modest in comparison to DFT methods. We report a comparative survey of our infrared data for untreated crystalline and various exfoliated states of kaolinite and halloysite. Given the limited availability of experimental techniques for providing direct structural information about nano-kaolinite, the vibrational spectra can be considered as an essential tool for validating structural models. The comparison of experimental and calculated stretching and bending frequencies further justified the use of the preferred level of theory. Overall, an optimal molecular model of the defect-free, ideal nano-kaolinite can be composed with respect to stationary structure and curvature of the potential energy surface using the PW91/SVP level of theory with empirical dispersion correction (PW91+D) and polarizable continuum solvation model (PCM) without the need for a scaled quantum chemical force field. This validated theoretical approach is essential in order to follow the formation of exfoliated clays and their surface reactivity that is experimentally unattainable.
开发了一种从叶蜡石族层状铝硅酸盐中剥离粘土纳米颗粒的组成和结构真实的原子尺度描述的方法。通过使用配位化学原理，可以将纳米颗粒内的化学环境分离为内球、外球和周球。以验证的方式对纳米颗粒的分子模型边缘进行质子化，以实现电荷中性。使用半经验方法（NDDO 哈密顿和 DFTB 形式）和带有饱和基组的从头算密度泛函进行结构优化，揭示了以前忽略的由于剥离而导致形态变化的分子起源。虽然对于处理由数万原子组成的纳米颗粒，使用半经验方法是可取的，但与 DFT 方法相比，结构精度相当低。我们报告了对未处理的结晶态和各种剥离态高岭石和埃洛石的红外数据的比较调查。考虑到实验技术提供有关纳米高岭石的直接结构信息的有限可用性，振动光谱可以被认为是验证结构模型的重要工具。实验和计算的伸缩和弯曲频率的比较进一步证明了首选理论水平的使用是合理的。总体而言，可以使用 PW91/SVP 理论水平（带有经验色散校正（PW91+D）和极化连续溶剂化模型（PCM）），而无需比例量子化学力场，针对理想纳米高岭石的无缺陷、静止结构和势能面曲率来构建缺陷自由的理想纳米高岭石的最佳分子模型。这种经过验证的理论方法对于跟踪剥离粘土的形成及其在实验上无法获得的表面反应是必不可少的。
