Dynamics of Water near Mica Surface

The study related to dynamical response of interfacial water has application in wide range of areas ranging from biology to geology. Recent experimental studies have shown that even for sub-nanometer confinement, the shear viscosity of water between mica surfaces is only three times that of free water. This high fluidity is attributed to the large exchange rate of water in the hydration shell of potassium (K⁺) ions.¹ Water near hydroxylated silica surfaces exhibit decoupling in the translational and rotational motions and a monotonic increase in the decoupling was observed toward the surface.² Similar decoupling has also been observed for water near biological molecules.³ In this study, the dynamics of interfacial water near mica surface is evaluated using molecular dynamics simulations. The analysis shows that water in the contact layer has a high residence time near the surface. The K⁺ ions have a strong influence on the dynamics of interfacial water. The presence of K⁺ ions, gives rise to a non-monotonic decoupling behaviour in the translational and orientational motions toward the mica surface. It was observed that orientational motions are much more suppressed when compared with translational motions. Despite slowing down of water relaxation near the surface, dynamics of water in the hydration shell of K⁺ ion is comparable to that observed in bulk water. The analysis also shows the presence of orientational jumps in the contact layer of mica surface. Recently these jumps have been observed in molecular dynamics simulations of bulk water.⁴

References

1. Raviv, U., Laurat, P. and Klein, J. Fluidity of water confined to subnanometer films. Nature, 413, 51–54, 2001.

2. Castrillon, S. R.-V., Giovambattista, N., Aksay, I. A. and Debenedetti, P. G. Effect of surface polarity on the structure and dynamics of water in nanoscale confinement. J. Phys. Chem. B, 113, 1438–1446, 2009.

3. Bagchi, B. Water dynamics in the hydration layer around proteins and micelles. Chem. Rev., 105, 3197–3219, 2005.

4. Laage, D. and Hynes, J. T. A molecular jump mechanism of water reorientation. Science, 311, 832–835, 2006.