The Role of Ions on the Surface-Bound Water Structure at the Silica/Water Interface: Identifying the Spectral Signature of Stability

Abstract

We explore the influence of salt addition on the structure of water interacting closely with a charged silica surface. Isolating these surface effects is challenging, even with surface-specific techniques like sum frequency generation (SFG), because of the presence of aligned water nanometers to microns away from the charged silica. Here we combine zeta potential and SFG intensity measurements with the maximum entropy method and reported heterodyne second harmonic and sum frequency generation results to deconvolute from the total signal intensity the SFG spectral contributions of the waters adjacent to the surface. Deconvolution reveals that at very low ionic strength the surface water structure is similar to that of a neutral silica surface near the point of zero charge with waters oriented in opposite directions. This result suggests the known metastability of silica near the PZC and the stability of silica in low ionic strength solutions may originate from the same source, these oppositely oriented surface-bound waters. Orientation changes are induced upon adding salt, which lead to a decrease in the total amount of aligned water at the surface.

Water Dipole Populations in the Electrical Double Layer and Their Contributions to the Total Interfacial Potential at Different Surface Charge Densities

“…The former can be approximated by measuring the  potential, while the latter is known from the literature. 46,54,56 Fig. 1b shows that our observed  potentials decreased monotonically in magnitude from high to low pH, consistent with an isoelectric point below pH 2 for this type of silica and salt concentration.…”

“…The centrosymmetry of bulk water is broken in the electrical double layer by four phenomena: hydrogen bonding of water molecules with the underlying surface, the ordering of water around specifically and non-specifically adsorbed ions, the alignment of water permanent dipoles with the electric field emanating from charged sites at the surface, and the polarization of water molecules in said field. The first (hydrogen bonding with the surface) is expected to be largely responsible for the water structure in the Stern layer, [47][48]54 which we attribute to the second-order susceptibility 𝜒 𝑆 (2) . The SFG intensity is then given by the following equation:…”

The influence of silica nanoparticle geometry on the interfacial interactions of organic molecules: a molecular dynamics study

“…6(b), at pH 9-10) on the interface of cuboid nanoparticles which in turn brings more water molecules at the interface due to the hydration of sodium ions. 35 However, such an enhanced water density on cuboid surface at pH 9-10 in the presence of 0.5 M NaCl is not observed (Fig. S3, ESI †).…”

Charge-Induced χ(3) Susceptibility in Interfacial Nonlinear Optical Spectroscopy Beyond the Bulk Aqueous Contributions: The Case for Silica/Water Interface

“…SHG and SFG-VS has been used extensively in studying the charged silica/water interface. 1,4,14,[18][19][20][21][22][23][24][25][26][27] The interpretations based on non-resonant SHG and SFG-VS data and by considering of the  (3) contributions exhibited substantial disagreements and have been puzzling the community. 4,15 After reviewing the significantly different SHG signal data from the charged silica/water and the charged Langmuir monolayer covered air/water interface, [1][2][3] the necessity to consider the bulk silica  (3) contributions at the charges silica/water interface was recognized, and the details are yet to be hammered out.…”

“…7 These efforts not only led to the corrected description of the charge induced  (3) contributions of the polarized water molecules in general, [5][6][7] but also resulted to a series systematic treatment of the spectral lineshapes of the phase-resolved SFG-VS spectra of the charged aqueous interfaces involving the  (3) contributions from polarized bulk water molecules near the charged interface. [8][9][10][11][12][13][14] With the explicit definition of the two phase angles, one of the charge induced  (3) term and another between the  (3) and the  (2) ， which are both experimentally measurable, 12 Geiger and co-workers recently discovered that for the SHG from the charged silica/water interface there exists an additionally new imaginary non-resonant  (3) term other than the traditionally considered  (3) contributions from polarized bulk water molecules near the charged interface. 15 We shall show that the  (3) contributions from the bulk phase is actually complex and likely more complicated than what has been considered.…”