Stern and Diffuse Layer Interactions During Ionic Strength Cycling

TL;DR

This study combines second harmonic generation measurements and atomistic modeling to explore how the Stern and diffuse layers at silica-water interfaces interact during ionic strength changes, revealing conditions of coupled and decoupled dynamics and the molecular origins of hysteresis.

Contribution

It provides new insights into the structural and electrostatic behavior of interfacial layers during ionic strength cycling using combined experimental and computational approaches.

Findings

Stern and diffuse layer dynamics can be decoupled or coupled depending on ionic strength change magnitude.

Contact ion pairs play a key role in the Stern layer structure.

Water polarization influences nonlinear optical signals within 2 nm of the interface.

Abstract

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.