Imogolite properties

Allophane and Imogolite. AUophane is an amorphous clay that is essentially an amorphous soHd solution of sUica, alumina, and water (82). In allophane less than one-half of the aluminum is held in tetrahedral coordinations and the Si02 to AI2O2 ratio typically varies between 1.3 and 2.0, but values as low as 0.83 have been reported. The typical morphology of allophane is cylindrical (37). AUophane may be associated with haUoysite, smectite minerals, or it may occur as a homogeneous mixture with evansite, an amorphous soHd solution of phosphoms, alumina, and water. Its composition, hydration, and properties vary. Chemical analyses of two allophane samples are given in Table 5. 

Table 7-4 Selected properties of phyllosilicates, allophane, and imogolite"... 

The same group has also obtained composite materials based on imogolite and water-soluble polymers such as hydroxypropylcellulose (HPC) and poly (vinylalcohol) (PVA) [32]. HPC is a rigid rod polymer that has a cholesteric phase, whereas PVA is a random coil amorphous polymer. Significantly enhanced mechanical properties were only obtained in the case of the HPC composite materials. 

Guimaraes, L., Pinto, YJ4., Lourenco, M.P., Duarte, H.A., 2013. Imogolite-like nanotubes structure, stability, electronic and mechanical properties of the phosphorous and arsenic derivatives. Physical Chemistry Chemical Physics 15, 4303—4309. 

Alvarez-Ramirez, F. (2007) Ab initio simulation of the structural and electronic properties of aluminosilicate and aluminogermanate natotubes with imogolite-like structure. Physical Review B, 76. 

Bursill, L.A., Peng, J.L. Bourgeois, L.N. (2000) Imogolite An aluminosilicate nanotube material. Philosophical Magazine A, Physics of Condensed Matter Structure Defects and Mechanical Properties, 80,105-117. 

Guimaraes, L., Enyashin, A.N., Frenzel, J., Heine, T., Duarte, H.A. Seifert, G. (2007) Imogolite nanotubes Stability, electronic, and mechanical properties. ACS Nano, 1, 362-368. 

Theng, B.K.G., Russell, M., Ghurchman, G.J. Parfitt, R.L. (1982) Surface-properties of allophane, halloysite, and imogolite. Clays and Clay Minerals, 30,143-149. 

Theoretical methods have been used since 2002 to investigate the electronic, structural and mechanical properties of imogolites aiming to contribute to the understanding of these systems at atomic level. Most of the theoretical investigations at that time were based on force fields specially developed for aluminosilicate or aluminogermanate systems . The large size of NT unit cell is still a limitation for the use of more sophisticated quantum mechanical methods. 

The present chapter is structured as follows we will discuss the synthesis and formation mechanism of imogolites at Section 2, whereas Section 3 features the computational aspects of the approximate method Self-Consistent-Charge Density-Functional Tight-binding and its parameterization steps. The theoretical analysis for NT structures and stability will be presented at Section 4, followed by the electronic and mechanical properties at Section 5. Final remarks will be presented at Section 6. 

It is worthwhile to mention that the functionalization of the NTs inner and outer surface has been successfully accomplished, broadening, then, the frontiers of imogolites application. They are envisaged for the development of nanoreactors, controlled release devices, ion conductors for batteries, gas storage and separation systems. Computer modelling of such functionalized systems have already been reported providing insights about their structural, electronic and mechanical properties. ... 

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