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Re: [ESPResSo-users] Bjerrum Length for Explicit Solvent

From: Stefan Kesselheim
Subject: Re: [ESPResSo-users] Bjerrum Length for Explicit Solvent
Date: Sun, 6 Oct 2013 11:43:22 +0200


On Oct 6, 2013, at 11:22 AM, Peter Košovan <address@hidden> wrote:
> To this end, I do not see inconsistency between what Stefan and Marcus have 
> written.

To make it clear: 
In my opinion the Bjerrum length can not be measured, and is not a physical 
quantity. It is a theoretical concept that is used to combine measurements of 
the bulk (!) dielectric permittivity and the temperature of a medium into a 
single quantity that is helpful to keep formulas brief. 

Markus' opinion is that the Bjerrum length can be measured: You use to charged 
particles of known charge, hold them in appropriate tweezers (that can be 
tricky though) measure the force as a function of distance, integrate that 
force and where this integrated curve cuts the $k_B T$ line you read of the 
Bjerrum length.

Both definitions are possible, but not compatible. The first is a (bulk) 
material property at a particular temperature, the second is a property of ion 
pairs in a particular medium at a particular temperature.

Interestingly in the second definition the salt concentration will play a role 
was well. 


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