A study of nanoconfined water in halloysite

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 H ₂ O 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 H ₂O 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 H ₂O. H ₂ O 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 H ₂O 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 Si ₂Al ₂O ₅(OH) ₄ (H ₂O) _2(ILW)(H ₂O) _{3.4 – 5.0(UCW+LUW)}. LT-DSC revealed that about 2.31 ± 0.09 moles _H2O/mole _kaol of the 5.4–7.0 moles _H2O/mole _kaol remain unfrozen down to −75 °C, consistent with 2.0 moles _H2O/mole _kaol 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 H ₂O 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 H ₂O, also termed hole water, to a higher frequency with a concomitant narrowing. Compared to unconfined water, which was indistinguishable from bulk water, these isolated H ₂ O molecules trapped in siloxane ditrigonal cavities are more hydrogen bonded and more ordered.