THERMODYNAMIC STUDY FOR THE PROTONATION OF HALLOYSITE

The Halloysite (Al2Si2O5(OH)4·2H2O) is an abundant and cheap clay and isconsidered one of the most promising nano structured and naturallyoccurring clay mineral. Large deposits of this material are present in France,Belgium, China, New Zealand and USA [1,2]. Among the spheroidal,tubular or platy morphologies, the tubular is the most common and abundantone. Typically, halloysite nanotubes (HNTs) are formed by 15 – 20aluminosilicate layers, having a length of 1 ± 0.5 μm, and inner and outerdiameters of 10 – 15 nm and 50 -70 nm, respectively [1]. In each layer, theSiOH and the AlOH groups are disposed on the external and the internalsurfaces, respectively. As consequence, in each nanotube and in a large pHrange, there is a charge separation between the inner and the outer surfaces.In particular, the inner surface is positively charged because of protonationof AlOH groups and the outer surface is negatively charged because of thegradual deprotonation of SiOH groups at pH higher than 2.This charge separation is one of the most important features for the HNTsversatility and is strictly dependent on the acid – base properties of theirfunctional groups. In this work, the protonation / deprotonation of SiOH andAlOH groups of HNTs is evaluated by ISE-H+ potentiometric titrations ofHNTs aqueous suspensions in different ionic media and ionic strengths(0.025 ≤ I / mol L-1 ≤ 0.750) and in a wide pH range (2 – 11), at T =25°C.ISE-H+ potentiometric titration data have been processed by using thesame models successfully used in a previous work for carboxylic and phenolgroups of humic and fulvic substances [3]. Moreover, the change ofsuperficial charge with the changing of pH was also evaluated by ζ potentialmeasurements.Correlations between the ζ potential trend and thedistribution of protonated and deprotonated species of HNTs were found.