Evaporation rate & electric field

[jangheej]

why is evaporation rate (e.g. of water molecules) faster in the presence of the electric field?
I understand that the water molecules become polarized and hence feel electric force...

can anyone explain more specifically?
thnx! =D

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[Drakkith]

Does this depend on the type of electic field or orientation at all?

 

[SpectraCat]

why is evaporation rate (e.g. of water molecules) faster in the presence of the electric field?
I understand that the water molecules become polarized and hence feel electric force...

can anyone explain more specifically?
thnx! =D

I am not familiar with the phenomenon you describe, and so I am not certain the the following rationalization is correct, but it is at least somewhat sensible given what is known about H-bonding dynamics in liquid water, and the interaction of polar molecules with electric fields.

Water molecules in liquid water are held together by a network of transient hydrogen bonds ... on average each water molecule is an acceptor for two H-bonds, and a donor for two H-bonds (4 total). However, the lifetime of these H-bonds is very short ... on the order of a few fs, so they are switching all of the time.

When you switch on the field, it interacts with the dipole moments of the molecules, so that they have an energetically preferred orientation in the lab frame. In terms of the H-bonding picture above, this means that now H-bonds pointing in certain directions will be stabilized by the field, while those pointing in other directions will be destabilized. Overall the net effect of this in the bulk is basically negligible, however at the surface it makes a difference for static field orientations normal to the surface. This is because the lowest energy orientations for a water molecule at the surface of liquid water are those where one O-H bond lies in the surface plane, while the other points down into the liquid. When the electric field is normal to the surface, the dipole moment of the water molecules (which lies on the axis of rotation between the two OH bonds) wants to be aligned with the field, which destabilizes the H-bonding network at the surface. The end result is faster evaporation.