Imogolite flocculation under alkaline conditions

Abstract

Imogolite flocculates under alkaline conditions even though the net charge is considerably negative. To clarify the reason for this contradiction, we measured the charge characteristics of imogolite with an ion-exchange method using sodium ions (Na þ ) and chloride ions (Cl À ) as index ions, and estimated its electric field intensity at the outer surface using Gauss' law and the structural model of imogolite. The result showed that the electric field intensity of negative charge at the outer surface was about half of that at the inner surface. Further, we discussed that if cations could easily enter the imogolite tube, the electric field of negative charge would be drastically weakened due to the neighboring cations and, consequently, the outer surface would become electrically indifferent. If so, not only the reason for imogolite flocculation under alkaline conditions, but also the reason for imogolite dispersion at the point of zero net charge would be explained consistently. The reason for imogolite dispersion at the point of zero net charge was considered to be that the colloidal stability of imogolite would be affected only by the positive charges that appear on the outer surface of the tube. Finally, the hydrated sodium ions were considered to move in and out of the imogolite tube freely.

Experimental determination of the curvature-induced intra-wall polarization of inorganic nanotubes

“…For an experimentally determined difference of 0.7 V, σ e /ε r = 9.0 mC⋅m -2 . Assuming ε r = 5 as in mica, 43 we obtain σ e = 45 mC ⋅m -2 which is in excellent agreement with the estimation of Gustaffson (42 mC⋅m -2 ). 15 Thus, IMO-OH would have a capacitance of 48 F⋅g -1 , corresponding to an accumulated electrostatic energy of roughly 10 kJ ⋅kg -1 .…”

Stability of Halloysite, Imogolite, and Boron Nitride Nanotubes in Solvent Media

“…Moreover, it was observed that imogolite forms sediments at an alkaline pH, even if the net charge is highly negative, and this leads to fibrous particle aggregation to form thick bundles [71]. To clarify this aspect, Ma and Karube [72] measured the charge features of INT by calculating the intensity of the electric field at the external surface, considering the model of imogolite structure and the Gauss' law.…”

“…To clarify this aspect, Ma and Karube [72] measured the charge features of INT by calculating the intensity of the electric field at the external surface, considering the model of imogolite structure and the Gauss' law.…”

“…It was impossible to explain the flocculation, because the cation exchange capability (CEC) of imogolite was high under alkaline conditions. For instance, assuming cation distribution inside the tube, or inner cation exchange, the measured CEC would correspond to the negative charge at the inner surface, and the electric field intensity at the external surface would become half that at the internal surface [72]. This means that the electric field, due to the negative charges, would be balanced by the counter-ions that entered the tube, and the external surface would be neutral, thus explaining that imogolite flocculates at alkaline pH and its dispersed at the point of zero net charge, or at pH = 6.0 [72].…”