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It is known from experiments that the dielectric constant of a solvent might decrease in regions where there is a strong electric field, for example, near a highly charged ion in an infinitely dilute solution. In such a case the dielectric constant of the liquid might be described as function of the distance from the ion (considering a monoatomic ion, so that we can approximate the dielectric function by a spherically symmetric one), and this description is supposed to capture all the effects of the strong electric field on the dielectric medium. What I am trying to understand here and cannot actually find a good theoretical discussion of is why the dielectric constant would be reduced in regions where the electric field is strong, i.e., what is the molecular mechanism of dielectric saturation.

Magnetic saturation usually happens when the external magnetic field is strong enough so that all the magnets are oriented in a manner which will maximize the interactions between the field and the particles. One cannot help to imagine that in dielectric saturation the dielectric medium will polarize itself in a "maximal" way, in order to maximize the interactions with the strong electric field. It is also not hard to imagine that at some saturation point (a certain electric field), the relaxation/reorientation/etc of the solvent in response to the electric field would increase its energy instead of decreasing it, and hence, it becomes unfavorable for the liquid to polarize itself. The most favorable configuration of this system (strong electric field at a small region of a dielectric liquid) would then be attainable by some other kind of reorganization of the solvent which would reduce its effective polarity, thus effectively decreasing its dielectric constant function. If the argument here given is true, then I would be interested in this molecular reorganization that follows the dielectric saturation, although I can imagine it is probably specific to the actual molecules that make up this dielectric medium.

Any insights on this issue will be greatly appreciated.

In order to clarify the origin of the term:

Dielectric saturation is a popular term in solvation chemistry and materials science. It is known to arise at regions in a dielectric medium where there is a strong field, and the liquid/material responds nonlinearly. Effectively, i.e., what experiments find, is that a reduction in the dielectric constant of the system arises at the region where the field is strong. You can find this term easily in continuum solvent models literature, such as in the 1st chapter ("Modern Theories of Continuum Models") of the recent book "Continuum Solvation Models in Chemical Physics", by Mennucci and Cammi, which is available here http://onlinelibrary.wiley.com/doi/10.1002/9780470515235.ch1/summary provided one has access to it.

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Nice question. You've also answered your own doubts correctly. As to how individual molecules get realigned - think of how a dipole behaves in an external electric field. –  user346 Feb 2 '11 at 6:23
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1 Answer

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I never heard about such an effect, but it is now 40 years ago since I studied such things.

From what You tell, I can "tinker" the following thoughts:

  • All reasoning is on the timescale of water molecule reorientation and proton hopping (tunneling?) in water structure.

  • The dielectric is a very polar, protic solvent, presumably water. (few other solvents dissolve ions, polar aprotic almost never, exept ion pairs, but this is a different story)

  • The dielectric constant (DC) of protic solvents is "produced" by orientation polarisation, this is reason for the DCs being very high compared to nonpolar substances. EG benzene about 2.23 , whereas water has DC of 81 !

  • Ions in water have a first shell of water molecules around, which is immobilized, depending on charge density of the ion a second and third layer are attached, with increasing orientational freedom. Exchange of this layers to the bulk of the liquid can be very slow.

  • These water shells will behave not like water, DC will be as high as we know it from aprotic or apolar solvents.

  • This would explain "regions" of lower DC, as You stated, but I would not call that "saturation".

  • There is some nonlinearity in a lot of dielectrics (used in nonlinear optics) but I hesitate to call such small nonlinearities a "saturation". At best those may be the onset of saturation.

I hope there is something that might help You in Your search.

Georg

PS

Somebody doing measurement of DC in aqueous solutions over a big range of frquencies (up to "Microwave") and does not know about water structure or the chemistry of solvatation, might interpret his results naively as dielectric "saturation" of the solvate shell.

BTW where did You read/hear about this?

Addendum:

What You write about the origin of the term, fits well to my answer. I am not happy with the word "saturation" still, but that is of course a matter of convention. As a chemist, I look upon such a stongly solvated ion as an chemical compound, being in equilibrium with the bulk water, not as a "saturation". Manfred Eigen might shiver :=(

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Thank you for the insights. So, if I understood it correctly, the lack of orientational freedom, i.e., rigidity of the first solvation shell molecules, in the presence of the field created by the ion, makes solvent molecules less prone to reorganize themselves, and hence orientational polarization response decreases, reducing the experimentally measured dielectric constant for this same region. I'd say this makes perfect sense. –  Raphael R. Feb 2 '11 at 16:28
    
""..rigidity of the first solvation shell molecules, in the presence of the field created by the ion, makes solvent molecules less prone to reorganize themselves,..."" Not only the field, this would lead to some polarisation, but the external field could "compete". Those solvatation shells are fixed by the lack of space as well. The shells are denser than the bulk liquid, this is reason for deviationes when adding up molar volumes of solutuions. –  Georg Feb 2 '11 at 19:38
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