Surface Characterization of Colloidal Silica Nanoparticles by Second Harmonic Scattering: Quantifying the Surface Potential and Interfacial Water Order

Abstract

The microscopic description of the interface of colloidal particles in solution is essential to understand and predict the stability of these systems, as well as their chemical and electrochemical reactivity. However, this description often relies on the use of simplified electrostatic mean field models for the structure of the interface, which give only theoretical estimates of surface potential values and do not provide properties related to the local microscopic structure, such as the orientation of interfacial water molecules. Here we apply polarimetric angle-resolved second harmonic scattering (AR-SHS) to 300 nm diameter SiO 2 colloidal suspensions to experimentally determine both surface potential and interfacial water orientation as a function of pH and NaCl concentration. The surface potential values and interfacial water orientation change significantly over the studied pH and salt concentration range, whereas zeta-potential (ζ) values remain constant. By comparing the surface and ζ-potentials, we find a layer of hydrated condensed ions present in the high pH case, and for NaCl concentrations ≥1 mM. For milder pH ranges (pH < 11), as well as for salt concentrations <1 mM, no charge condensation layer is observed. These findings are used to compute the surface charge densities using the Gouy–Chapman and Gouy–Chapman–Stern models. Furthermore, by using the AR-SHS data, we are able to determine the preferred water orientation in the layer directly in contact with the silica interface. Molecular dynamics simulations confirm the experimental trends and allow deciphering of the contributions of water layers to the total response.

A New Imaginary Term in the Second-Order Nonlinear Susceptibility from Charged Interfaces

“…3b. The " (*) values obtained in this fashion are within a factor of 2 of those published for other charged aqueous interfaces, such as those reported recently by the Roke group for silica suspensions, 51 albeit for much lower ionic strengths. Point estimates of " (*) reported in that work were reported for salt concentrations of 10 mM and pH 10 to be around 2 x 10 -22 m 2 V -2 , while negative point estimates were reported for ionic strengths of 0.1 mM and below at pH 10 and pH 5.7.…”

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

“…This comparison indicates either that the diffuse layer waters are the dominant contributors to the SFG signal, or that the surface water structure is changing in such a way to yield the observed similarity between 3 and the integrated intensity. However, as recent studies have shown, 24,31,59 the surface water structure likely exhibits an ionic strength dependence, which cannot be ascertained from a comparison of 3 and the integrated intensity. To deconvolute the (2) and χ (3) 3 responses and achieve a deeper understanding of these structural changes with salt addition, the phase of the SFG signal (i.e.…”

Water at charged interfaces

“…when, however, other components in the system exhibit a net orientation, these can also contribute to the signal, as is, for instance, the case of lipid monolayers at the air-water interface 254 . Non-resonant sHG has been used to learn about the surface potential and charge of mineral surfaces as well as membranes 80,157,[271][272][273] , has been refined using heterodyne detection 268 and has been shown, in scattering geometry, to provide information on φ 0 from the surface of particles, droplets and vesicles 267,274,275 . using sHG microscopy, the aligned water molecules have been used as reporters of surface heterogeneity and dynamics at mineral interfaces 90 , asymmetrical lipid bilayer membranes 88 and neuronal membranes 276 .…”