ENERGY MODELING OF COMPETITION BETWEEN TUBULAR AND PLATY MORPHOLOGIES OF CHRYSOTILE AND HALLOYSITE LAYERS
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ENERGY MODELING OF COMPETITION BETWEEN TUBULAR AND PLATY MORPHOLOGIES OF CHRYSOTILE AND HALLOYSITE LAYERS ANDREI A. KRASILIN * 1
Ioffe Institute, 26 Politekhnicheskaya st., St. Petersburg 194021, Russia
Abstract—The present study considered the problem of halloysite nanoscroll synthesis by energy modeling of the formation of chrysotile and halloysite particles. The main aim of the study was to reveal an energy preference between scrolled and platy morphologies of the particles. Both hydrosilicates possess the ability to scroll spontaneously but relatively facile hydrothermal synthesis of the nanoscrolls is available only to the former, whereas halloysite forms mainly plates under the same conditions. This issue was investigated by a phenomenological energy model, taking into account: (1) strain energy due to the size difference between metal oxide and silica sheets; (2) surface-energy difference on the opposite sides of the layer; and (3) adhesion energy. Calculations showed that the halloysite layer had a significant scrolling potential due to the first energy component, but the surface-energy difference acted in the opposite direction and tried to unbend the layer. In contrast, these two actions were co-directional in chrysotile layers. In both cases, the formation of multi-layered plates became more energy favorable when the specific surface energy of the edges decreased. In the range 0.5–3 J/m2 for the specific surface energy, only halloysite layers showed an energy preference for platy particles over nanoscrolls, especially at small layer sizes. Certain processes, such as hydration, could reduce the corresponding specific surface energy value and, as a result, could stabilize the platy morphology of halloysite at the earliest stages of particle growth under hydrothermal conditions. Keywords—Chrysotile . Energy modeling . Halloysite . Nanoscroll . Nanotube . Surface energy INTRODUCTION The last decade (2010–2020) has been marked by a new wave of interest in clays and clay minerals. Many applications have been found, in fields ranging from adsorption and water remediation to catalysis and medicine, and these are due to the natural abundance and morphological and structural diversity of clay minerals. Among these minerals, hydrosilicates are an interesting group which are able to scroll spontaneously: Al2SiO3(OH)4 imogolite (Paineau et al. 2016; Shafia et al. 2016; Arancibia-Miranda et al. 2017; Picot et al. 2018), Mg3Si2O5(OH)4 chrysotile (Bloise et al. 2010; Korytkova et al. 2011; Lafay et al. 2016; Krasilin et al. 2017; Maslennikova and Gatina 2017; López-Salinas et al. 2019; Bian and Kawi 2020), and Al2Si2O5(OH)4 halloysite (Cataldo et al. 2018; Krasilin et al. 2019b; Lvov et al. 2019). Their crystal structures combine two covalently bonded sheets of metal-oxygen octahedra and silicon-oxygen tetrahedra (joined in a network or separated as in the case of imogolite). Substantial differences in the size of the sheets and in terms of their structure give rise to a bending momentum, which transforms the layers i
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