Limiting values for bacterial potentials

Limiting values for bacterial potentials

Limiting values for bacterial potentials

In aqueous media the electrostatic potential (w) on
bacterial cell surfaces influences the distribution of
charge in the surrounding medium and ions of opposite charge (counterions) are attracted toward the surface. The electric energy of binding ions at the surface
is zFw, where z is the valency of the ion and F is Faraday’s constant. Hence, for z = 1 and w = 100 mV the
binding energy is approximately 104
J/mol of ions,
which is, at room temperature, about 4 times larger
than their Brownian motion energy. As a consequence,
a large fraction of the countercharge will be bound
directly at the surface. That fraction obviously increases with an increasing value of w. Furthermore, for
a given fraction of bound counterions, the number of
ions bound per unit surface area increases with ionic
strength. This, in turn, lowers the z potential (i.e., z
potentials become less negative). In a “hard particle”
model the z potentials are essentially zero when the
ionic strength of the medium exceeds 0.1–>0.2M,
while in a “soft particle” model usually small residual
z potentials of around −10 mV are measured at elevated ionic strength because of the presence of fixed
membrane charges.

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