  Second harmonic generation amplitude and phase measurements are acquired in
real time from fused silica:water interfaces that are subjected to ionic
strength transitions conducted at pH 5.8. In conjunction with atomistic
modeling, we identify correlations between structure in the Stern layer,
encoded in the total second-order nonlinear susceptibility, chi(2)tot, and in
the diffuse layer, encoded in the product of chi(2)tot and the total
interfacial potential, phi(0)tot. chi(2)tot:phi(0)tot correlation plots
indicate that the dynamics in the Stern and diffuse layers are decoupled from
one another under some conditions (large change in ionic strength), while they
change in lockstep under others (smaller change in ionic strength) as the ionic
strength in the aqueous bulk solution varies. The quantitative structural and
electrostatic information obtained also informs on the molecular origin of
hysteresis in ionic strength cycling over fused silica. Atomistic simulations
suggest a prominent role of contact ion pairs (as opposed to solvent-separated
ion pairs) in the Stern layer. Those simulations also indicate that net water
alignment is limited to the first 2 nm from the interface, even at 0 M ionic
strength, highlighting water's polarization as an important contributor to
nonlinear optical signal generation.
