Kinetic and Surface Study of Single-Walled Aluminosilicate Nanotubes and Their Precursors

Abstract

The structural and surface changes undergone by the different precursors that are produced during the synthesis of imogolite are reported. The surface changes that occur during the synthesis of imogolite were determined by electrophoretic migration (EM) measurements, which enabled the identification of the time at which the critical precursor of the nanoparticles was generated. A critical parameter for understanding the evolution of these precursors is the isoelectric point (IEP), of which variation revealed that the precursors modify the number of active ≡Al-OH and ≡Si-OH sites during the formation of imogolite. We also found that the IEP is displaced to a higher pH level as a consequence of the surface differentiation that occurs during the synthesis. At the same time, we established that the pH of the reaction (pHrx) decreases with the evolution and condensation of the precursors during aging. Integration of all of the obtained results related to the structural and surface properties allows an overall understanding of the different processes that occur and the products that are formed during the synthesis of imogolite.

Nanotubular Aluminosilicates: A Case Study for Science and Industry

“…Figure 3 shows other IR imogolite absorption signals located at 428, 506, 577, and 967 cm -1 ; in addition, close to 3500 cm -1 a band associated to -OH stretching can be observed. In general terms, imogolite IR spectra are more acute tan theses of allophane being the doublet at 577 and 967 cm -1 the most significant difference (see Figure 3, left) 12,34,35 .…”

“…During the aging process the originally flat surface become curved (tile shaped structure) as a consequence of restructuring of ≡Si-OH and ≡Al-OH groups, changing the Al-Si ratio at the external surface being now the ≡Si-OH groups mainly part of the inner surface and the ≡Al-OH mainly part of the external surface, resulting in a significant change of more than two pH units in the IEP of solid 11,12 .…”

“…Also has two types of surfaces, an outer surface dominated by aluminol groups (≡Al 2 -OH, ≡Al-OH) (see Figure 1b), which are positively charged over a wide pH range (3.0-10.5) and an inner surface formed by silanol groups (≡Si-OH) which is preferably negatively charged [11][12][13][14][15][16][17] .…”

“…identification uncertainty related to the presence of imogolite because during aging process proto-imogolite is formed which is hard to differentiate from imogolite through IR and XRD, signals considering that both aluminosilicates have the same functional groups and similar crystalline ordering 11,12,35 .…”

“…The IEP changes during the synthesis and aging processes, allowing identifying the product changes while reaction takes place 11,12 . In the case of imogolite, the results indicate that IEP changes can be associated to surface changes during its formation.…”