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.
Highlights
Descriptions of the Stern and the diffuse layers that comprise the electrical double layer (EDL)[1,2,3,4,5] over a charged aqueous interface remain largely confined to Bragg-Williams approximations.[6]
A related question concerns the molecular origin of hysteresis,[13, 38,39,40] in which a surface may stay in a charged state that is incommensurate with what is expected from bulk equilibrium thermodynamics
Page 9 These questions, and the results presented in Fig. 3, prompted us to carry out classical molecular dynamics simulations followed by computations of χ($) and Φ(0)!"! to explore what structures might recapitulate, even if only qualitatively, the different χ($) and Φ(0)!"! point estimates obtained from the HD-second harmonic generation (SHG) experiments
Summary
Descriptions of the Stern and the diffuse layers that comprise the electrical double layer (EDL)[1,2,3,4,5] over a charged aqueous interface remain largely confined to Bragg-Williams (mean-field) approximations.[6].
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