Revealing Silica’s pH-Dependent Second Harmonic Generation Response with Overcharging

Abstract

Isolating the contribution of silica in second harmonic generation (SHG) studies at the silica/water interface remains a challenge. Herein, we compare SHG intensities with previously measured zeta potentials and vibrational sum frequency generation (SFG) intensities to deconvolute the silica contribution in the SHG measurements. Under conditions that promote overcharging, the zeta potential and the SFG measurements follow a similar trend; however, SHG yields the opposite behavior. The results can only be rationalized by considering a significant pH-dependent increase in the silica contribution. Using a simplistic, yet physically motivated model, we demonstrate that silica can interfere either constructively or destructively with water. By computing the hyperpolarizabilities of neutral and deprotonated silica clusters with density functional theory [CAM-B3LYP/6-31+G­(d,p)], we reveal that one potential source of this pH-dependent response of silica is a change in the hyperpolarizability upon the deprotonation of surface sites, suggesting that SHG is directly sensitive to surface charging. The direct sensitivity of SHG to the surface charge density of the substrate suggests that SHG would be a powerful tool in studying other mineral oxides such as alumina and titania.

Variable-Angle Surface Spectroscopy Reveals the Water Structure in the Stern Layer at Charged Aqueous Interfaces

“…It is worth noting that significant differences in the surface potential determined using SHG from planar silica and colloidal nanoparticles have been attributed to the inherent symmetries of the interfaces in the two types of experiments . Finally, it is also interesting to note that SHG experiments on planar silica surfaces have found χ SL (2) to be positive in our concentration range (1 μM to 1 mM NaCl) at neutral pH. , However, since the comparison of SHG results far from water resonance and SFG results on resonance at 3150 cm –1 is not straightforward, , we focus on the interpretation of SFG data.…”

“…The basic principle is that the relative angles of the visible and infrared beams remain fixed, while the sample and detector are rotated about a common axis. The infrared beam is tuned to ω IR = 3150 cm –1 as it corresponds to a strong water resonance, thereby minimizing other contributions to χ (2) . , An angle scan from θ vis = 62–72° in steps of 0.4° was performed at each ionic strength of interest at neutral pH 6.0–6.4. The crucial step is successfully removing all angle-dependent contributions to the SFG intensity (shown in Figure a) other than those associated with second-order susceptibility χ total (2) .…”

“…The theory behind this analysis has been further developed and refined by several groups in recent years. ,− In particular, the Geiger group demonstrated that measurement of the χ total (2) phase in an off-resonance SHG experiment enables the surface potential and χ SL (2) to be determined. − ,, Another major contribution to the field has been the development of second harmonic scattering (SHS) experiments and the accompanying theory by the Roke group to obtain the surface potential and χ SL (2) for colloidal nanoparticles in solution. ,− The Wen group developed a scheme based on off-resonance SHG where scanning the wavelength was an effective means of varying the coherence length if the nonlinear susceptibility remains constant, thereby facilitating the separation of χ SL (2) and χ DL (2) and extracting the surface potential . However, there is a selectivity advantage to probing the water response on resonance with a water-specific vibrational mode as in SFG experiments. , Hence, a two-angle SFG method has been proposed to separate the two EDL components and was recently experimentally realized . It is worth noting that such χ (2) models of the EDL can be more readily analyzed if the imaginary component of χ total (2) is available, either from heterodyne-detected SFG experiments ,,, or through phase retrieval approaches such as maximum entropy-based methods. − …”

Quantification of Stern Layer Water Molecules, Total Potentials, and Energy Densities at Fused Silica:Water Interfaces for Adsorbed Alkali Chlorides, CTAB, PFOA, and PFAS

“…Using a larger deprotonated fraction (30% from Gmür et al or Dove and Craven) still results in a 10-fold-larger hyperpolarizability for the ions than the Stern layer water molecules. More likely is a pH-dependent change in the second-order nonlinear susceptibility of the silica surface, which recent experimental and theory work by the Gibbs group has shown to increase considerably in magnitude with the deprotonation state of the surface …”

Does the ionic distribution in the electrical double layer modify second harmonic scattering?

“…23 Second harmonic generation (SHG) and second harmonic scattering (SHS) have also proven to be highly effective optical techniques for studying the EDL. [24][25][26][27][28][29][30][31] Both techniques are sensitive to non-centrosymmetry in a medium, therefore being ideal to probe surface phenomena and the interfacial region: SHG is used to investigate planar interfaces, while the interfaces of submicron-sized colloids dispersed in liquids can be probed through SHS.…”