Nanoconfined Water in Magnesium-Rich 2:1 Phyllosilicates
NW Ockwig and JA Greathouse and JS Durkin and RT Cygan and LL Daemen and TM Nenoff, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 131, 8155-8162 (2009).
Inelastic neutron scattering, density functional theory, ab initio molecular dynamics, and classical molecular dynamics were used to examine the behavior of nanoconfined water in palygorskite and sepiolite. These complementary methods provide a strong basis to illustrate and correlate the significant differences observed in the spectroscopic signatures of water in two unique clay minerals. Distortions of silicate tetrahedra in the smaller-pore palygorskite exhibit a limited number of hydrogen bonds having relatively short bond lengths. However, without the distorted silicate tetrahedra, an increased number of hydrogen bonds are observed in the larger-pore sepiolite with corresponding longer bond distances. Because there is more hydrogen bonding at the pore interface in sepiolite than in palygorskite, we expect librational modes to have higher overall frequencies (i.e., more restricted rotational motions); experimental neutron scattering data clearly illustrates this shift in spectroscopic signatures. It follows that distortions of the silicate tetrahedra in these minerals effectively disrupt hydrogen-bonding patterns at the silicate-water interface, and this has a greater impact on the dynamical behavior of nanoconfined water than the actual size of the pore or the presence of coordinatively unsaturated magnesium edge sites.
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