Interdisciplinary Applied Mathematics

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11

Water in Nanochannels


Water and its properties in various forms is one of the most actively investigated areas because of its importance in nature. The anomalies that exist in the bulk properties of water make it very interesting and challenging for research, and a vast deal of literature is already available. Even though water has been studied for more than 100 years now, its properties are far from understood. With the advances in fabrication of nanochannels that are only a few molecular diameters in critical dimension, there is now an opportunity for a major breakthrough in understanding the properties of water in confined nanochannels and in validating atomistic simulations. In this chapter, after introducing some definitions and atomistic models for water, we    present    the    static    and    dynamic behavior    of    water    in    confined


nanochannels.

11.1 Definitions and Models


Water    is    composed    of    two    hydrogen    atoms and    one    oxygen    atom.    Fig


ure 11.1 shows a sketch of the water molecule and the various quantities used to characterize it. For an isolated water molecule (e.g., water in gas phase), it has a bond length rOH of 0.95718 A, and a bond angle 0HOH of 104.474°. In the liquid state, both these values are slightly modified by the water—water and/or water—ion interactions. For example, neutron diffraction experiments suggested a bond length of rOH = 0.970 A and a bond angle of 0HOH = 106.00° (Ichikawa et al., 1991) for the liquid state. Because of the higher electronegativity of the oxygen atom compared to that of the

FIGURE 11.1. Sketch of a water molecule and the various quantities used to characterize the water molecule.


hydrogen atom, the oxygen site of the water molecule will appear to be “negatively charged,” and the hydrogen site will appear to be “positively charged.” Such a charge separation then creates an electric dipole. Because of the symmetry of the water molecule, the dipole can be represented by a line that starts from the oxygen atom and bisects the H—O—H angle (the dotted line in Figure 11.1 is the dipole). If a water molecule is represented by several point charges in an atomistic model, the dipole moment of the model water molecule is given by

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