Molecular Treatment of Nano-Kaolinite Generations

Abstract

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.

Effect of structural iron on nanoscroll formation via exfoliation of a high iron-content kaolin

“…Li and coworkers determined with theoretical calculation that the minimum interior diameter for a single kaolinite sheet to be scrolled is about 9.08 nm (R = 45.4 Å), the optimal is 24.30 nm (R = 121.5 Å), and the maximum is 100 nm (R = 500 Å) [67]. In our previous study [52], the optimal curvature for an optimal, defect-free nanokaolinite (G 2 model, PW91 + D/SVP/PCM) was determined to be around 67.6 Å (13.52 nm), which is less than the experimentally observed optimal curvature of 121.5 Å (Table 2) [35,59]. The calculated minimal curvature value for the G 2 models is 22.7 Å, with the optimum of 67.6 Å and maximum of 200 Å.…”

“…2/3620-3695 cm −1 , 910-940 cm −1 ) and Si-O groups of the T-sheet (1000-1115 cm −1 , 800-740 cm −1 ) [50,51]. The νOH region of kaolinite is quite unique, having 4 bands at 3692, 3667, 3652 cm −1 (assigned to inner surface hydroxides), and 3620 cm −1 (assigned to inner hydroxides) [52]. The distinctive bands at ca.…”

“…Only subtle changes can be observed after the exfoliation of kaolinite in the structural vibrations regions [52], such as the slight broadening of νSi-O vibrations and of the inner surface hydroxide [Fig. 2(e-g)].…”

Thermodynamic Stability of Nano-grained Alloys Against Grain Coarsening and Precipitation of Macroscopic Phases

“…Now, let us divide Eq. [1] by n A (mole), which is the amount of matter in the phase considered in Eq. [1] [95] :…”

“…NANO-MATERIALS play an increasing scientific and social role. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] In the past, nano-materials were simplified to single nano-particles. However, it has been clear for some decades that the real use of nanomaterials is expected if they form macroscopic articles with nano-structure inside.…”