  We develop and verify a phase-sensitive second harmonic generation
spectroscopic scheme that allows for direct determination of the absolute
surface charge density and surface potential of a water interface without need
of prior interfacial information. The method relies on selective probing of
surface-field-induced reorientation order of water molecules in the electrical
double layer and is, hence, independent of the interfacial molecular bonding
structure. Application of this technique to a mixed surfactant monolayer on
water suggests the manifest effect of the chain-chain interactions among the
monolayer on adsorption of soluble ionic surfactants. We also deduce the
third-order nonlinear susceptibility of bulk water and prove its applicability
to analysis of charges of various water interfaces. In addition, we show that
the Debye-H\"uckle theory should be avoided in the spectroscopic analysis for
its potential significant error, as evidenced experimentally and theoretically.
