Abstract
We propose an analytical approximation for the dependence of the effective
charge on the bare charge for spherical and cylindrical macro-ions
as a function of the size of the colloid and salt content, for the
situation of a unique colloid immersed in a sea of electrolyte (where
the definition of an effective charge is non-ambiguous). Our approach
is based on the Poisson-Boltzmann (PB) mean-field theory. Mathematically
speaking, our estimate is asymptotically exact in the limit kappaa
>> 1, where a is the radius of the colloid and kappa is the inverse
screening length. In practice, a careful comparison with effective
charge parameters, obtained by numerically solving the full nonlinear
PB theory, proves that our estimate is good down to kappaa similar
to 1. This is precisely the limit appropriate to treat colloidal
suspensions. A particular emphasis is put on the range of parameters
suitable to describe both single and double strand DNA molecules
under physiological conditions.
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