Dynamics of supercooled water in highly compacted clays studied by neutronscattering

Abstract

The freezing behavior of water confined in compacted charged and unchargedclays (montmorillonite in Na- and Ca-forms, illite in Na- and Ca-forms, kaoliniteand pyrophyllite) was investigated by neutron scattering. Firstly, the amount offrozen (immobile) water was measured as a function of temperature at the IN16backscattering spectrometer, Institute Laue–Langevin (ILL). Water in uncharged, partlyhydrophobic (kaolinite) and fully hydrophobic (pyrophyllite) clays exhibited asimilar freezing and melting behavior to that of bulk water. In contrast, waterin charged clays which are hydrophilic could be significantly supercooled. Toobserve the water dynamics in these clays, further experiments were performedusing quasielastic neutron scattering. At temperatures of 250, 260 and 270 K thediffusive motion of water could still be observed, but with a strong reduction in thewater mobility as compared with the values obtained above 273 K. The diffusioncoefficients followed a non-Arrhenius temperature dependence well described by theVogel–Fulcher–Tammann and the fractional power relations. The fits revealed that Na- andCa-montmorillonite and Ca-illite have similar Vogel–Fulcher–Tammann temperatures(TVFT, often referred to as the glass transition temperature) of∼120 K and similar temperatures at which the water undergoes the ‘strong–fragile’ transition,Ts∼210 K. On the other hand, Na-illite had significantly larger values ofTVFT∼180 Kand Ts∼240 K. Surprisingly, Ca-illite has a similar freezing behavior of water to that ofmontmorillonites, even though it has a rather different structure. We attribute this to thestronger hydration of Ca ions as compared with the Na ions occurring in the illiteclays.