| Popis: |
The surface and porous properties of New Zealand metahalloysites, and the manner in which these properties are affected by chemical and physical treatments, have been studied by adsorption methods. The clay mineral surface has a considerable influence on the nature of the adsorption, process, polar water and ethyl chloride molecules giving rise to low pressure hysteresis not observed for the adsorption of nitrogen and argon. Low pressure hysteresis is attributed to structuring effects in the interfacial layer, desorption occurring from a layer more densely packed than during adsorption. Treatment of the mineral with solutions of metal chlorides altered the specific surface area to a small degree while treatment with trimethylchlorosilane was found to have a considerable effect on the surface properties. Particle morphology is found to have a significant influence on the effects which take place upon heating metahalloysite, larger changes in specific surface area occuring for material having an elongated particle shape. Vacuum outgassing at raised temperatures causes small changes only in the specific surface area and pore size distribution of a metahalloysite prepared by dehydrating an halloysite from TePuke. Dehydroxylation gave rise mainly to small pores of diameter 30 - 100Å. The surface area changes, which occur as the outgassing temperature is increased, are explained by a combination of three effects, loss of strongly adsorbed water, particle coalescence and dehydroxylation. Gravimetric water vapour sorption measurements were made on homoionic metahalloysite saturated with the exchange cations Li+, Na+, K+, Rb +, Cs +, Mg 2+, Ca 2+ and Sr 2+. The size and range of the hysteresis loop is found to vary with the nature of the exchangeable ion. The relative effect of the various cations depends upon their respective polarizabilities. Li+ and Mg2+ ions are anomalous in their behaviour, an effect attributed to their small size and consequent high energy of interaction with the mineral surface. This high interaction energy retards the ion hydration below that expected for the high polarizability of these ions. A molecular adsorption model is advanced which assumes specific site adsorption, the sites being intimately connected with the exchange cations. The completion of the various stages postulated in the model correspond closely to the points, on Jura- Harkins’ plots of the experimental data, at which distinct slope changes occur, and which indicate ‘phase’ changes in the adsorbed film. Small highly charged ions stabilise a molecular arrangement believed to be similar to the Hendricks-Jefferson ‘nett’ structure. The larger ions disrupt this structure and for these ions a ‘phaset’ change is observed which corresponds with the formation of a closest-packed adsorbed layer. The proposed adsorption model is essentially the same as that successfully used for the kaolinite-water system. That similar models can be employed to predict the nature of the adsorption of water vapour on both kaolinite and metahalloysite is an indication of the resemblance between the two mineral surfaces. |
