  Iso-electric point(IEP) is the PH, at which the $\zeta$ potential is measured
to be zero. The occurrence of IEP has been understood due to the neutralization
of surface charge density (SCD) at the solid-liquid interface. In this work, we
use the potential trap model to study the sources of the surface charge density
at verious PC and PH, by taking the water-silica system as an example. It is
revealed that in the case of $PH<8$, the SCD is mainly originated from the
dissociation of water molecules. And the bulk ions trapped at the interface can
dominate the SCD when $PH>9$. Due to the mass action law, the dissociation of
water molecules is suppressed at the PH close to IEP, leading to a zero surface
charge density. In this way, zero $\zeta$ potential is obtained at the IEP. It
has also been obtained that the increase of the salt concentration in the water
can decrease the $\zeta$ potential, but increase the surface charge density.

Soft Condensed Matter;Chemical Physics