Here, we summarize a few of the most interesting features observed in the behavior of a water cluster. These are:
1) the existence of a structural phase transition at a field value around E c1=1.5 10 7 V/cm, which was found to signify the cluster elongation along the field and the molecular dipole moment alignment from random directions to directions less than 90 degrees with respect to the field vector. A second phase transition at a higher field value, E c2=5 10 7 V/cm was found to correspond to the transition to a quasi-crystalline state. In both cases, i.e. at E=E c1 and E=E c2, structural relaxation times displayed abrupt jumps.
Structural relaxation times (in semi-logarithmic scale) are displayed for a wide range of fields in the following figure:
2) the transition from a solid-like to a liquid-like phase for relatively weak electric fields, about E=0.5 10 7 V/cm and then back to a solid-like structure for stronger fields. This transition was manifested in the enhancement, relatively to the zero field case, of the structural and reorientational relaxation rates, of self-diffusion coefficients and of molecular mean vibrational amplitudes, the same way as it was observed to occur in supercooled water under a relatively weak external pressure.
3) the appearance of a novel structure at moderately low fields, ~1.5 107 V/cm, consisting of a number of incomplete cubes stacked one upon the other at one edge of the cluster and grown along the field direction.
1. ''A Molecular Dynamics Study of Structural Transitions in Small Water Clusters in the Presence of an External Electric Field'', A. Vegiri and S. V. Schevkunov, J. Chem. Phys. 115, 4175 (2001).
++Also Virtual Journal of Biological Physics Research , 2(9)
2."Translational Dynamics of a Cold Water Cluster in the Presence of an External Uniform Electric Field", A. Vegiri J. of Chem. Phys., 116, 8786, (2002)
3.''Reorientational relaxation and rotational-translational coupling in water clusters in a dc external
electric field'', A. Vegiri, Mol. Liq. 110, 155 (2004).
1) A similar structural phase transition, like the first one in water clusters, was also observed to take place at a particular transition field of about 1.0 10 7 V/cm, signifying the flipping of the molecular dipoles to directions along the field. Because of computational demands, the strongest electric field examined was equal to 1.5 10 7 V/cm.
Unlike to clusters, the application of relatively weak electric fields did not seem to enhance the liquid character of water.
2) The dependence of the structural (left frame) and reorientational (right frame) relaxation times of liquid water as a function of the external electric field is displayed in the following figure:
3) Dynamical quantities such as power spectra, mean square displacements and mean square vibrational amplitudes were found to display substantial anisotropy with respect to the field direction. Thus, hydrogen bonds were stiffer along the field than along orthogonal directions. Related to that, mean square vibrational amplitudes for the in-cage motions and diffusion coefficients were found to be more restricted along the field direction. However, hydrogen bonds were found to become weaker with increasing electric field.
We examined the degree of association of the non polar methane molecules in liquid water, as a function of the external electric field, by considering two cases:
methane molecules with zero polarizability, á=0,
methane molecules with á=17.54 a.u.
In the first case, the solute does not interact directly with the field, whereas in the second case methane molecules interact with the field and the adjacent water molecules through dipole (water)-induced dipole (methane) interactions.
The degree of clustering is shown in the following figure. Electric field values are in 10(^7) V/cm.
In the right frame, where polarizability is taken equal to zero, the electric field increases the dispersion of the methane particles, whereas in the left frame, association of all particles into a big cluster is clearly favored.
The next two graphs show the density profiles of the methane-water system for two different electric field values and for the á?0 case. Notice the complete phase separation and crystallization.