Use of isoelectric point and pH to evaluate the synthesis of a nanotubular aluminosilicate

Abstract

To follow the synthesis of imogolite, transmission electron microscopy is needed. In this paper, the isoelectric point (IEP) and the aging pH are proposed as alternative methods. Two synthetic procedures were used (S-I and S-II), both involving a co-precipitation followed by an aging treatment where the aluminosilicate evolves from proto-imogolite (detected after the co-precipitation step), to imogolite; its formation is reached after 120 h (S-I) or 168 h (S-II) of aging, depending on the co-precipitation method used. In S-I the isoelectric point increases from 7.1 to 10.5, while in S-II it increases from 6.6 to 9.2 during the aging treatment. Additionally, a linear relationship between the IEP and the pH at different aging steps was found. That relationship may be used to follow the process of synthesis by simply measuring the pH, becoming an alternative to more complex methods.

Nanotubular Aluminosilicates: A Case Study for Science and Industry

“…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 surface and structural changes as consequence of the aging process can be detected through surface charge measurements carried out by electrophoretic migration (ME) 11,41 , which allows the determination of the isoelectric point (IEP) of samples. The IEP changes during the synthesis and aging processes, allowing identifying the product changes while reaction takes place 11,12 .…”

“…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.…”

“…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 .…”