A study of nanoconfined water in halloysite

Abstract

Recent years have witnessed a significant level of interest on the properties and applications of halloysite. Despite the widespread interest, little is known about the nature of nanoconfined H2O and its role in governing the dehydration of halloysite (10 Å) to halloysite (7 Å). This is because most studies have utilized halloysite that has irreversibly transformed from fully hydrated halloysite (10 Å) to halloysite (7 Å). This study examined a naturally occurring fully hydrated halloysite (10 Å) that was collected in a wet state and preserved in a sealed desiccator. Thermogravimetric (TGA/DTG) and low-temperature differential scanning calorimetry (LT-DSC) were used to quantify the amount of H2O present in halloysite (10 Å) as interlayer water (ILW), lumen water (LUW) and unconfined water external to the halloysite tubes (UCW). The LT-DSC results, in particular, identified two exothermic freezing peaks and two endothermic melting peaks that could distinguish between unconfined and lumen H2O. H2O confined in the lumen is characterized not only by a depression in the freezing temperature, but also by a significant reduction in the freezing enthalpy. Consistent with work on other phyllosilicates, interlayer H2O in halloysite (10 Å) remains unfrozen, as detected by DSC scans down to −75 °C. For fully hydrated halloysite (10 Å), the half-unit cell formula is Si2Al2O5(OH)4 (H2O)2(ILW)(H2O)3.4–5.0(UCW+LUW). LT-DSC revealed that about 2.31 ± 0.09 molesH2O/molekaol of the 5.4–7.0 molesH2O/molekaol remain unfrozen down to −75 °C, consistent with 2.0 molesH2O/molekaol assigned to the interlayer and 0.31 assigned to the δ-layer on the hydrophilic surface of the inner lumen. Dehydration experiments using LT-DSC, XRD and ATR-FTIR showed that unconfined + lumen H2O was lost rapidly, with a slower release of ILW + δ-layer. ATR-FTIR spectra of dehydrated halloysite exhibited a significant shift in the δ(H-O-H) bending band of sorbed H2O, also termed hole water, to a higher frequency with a concomitant narrowing. Compared to unconfined water, which was indistinguishable from bulk water, these isolated H2O molecules trapped in siloxane ditrigonal cavities are more hydrogen bonded and more ordered.