Second Harmonic Scattering Reveals Ion-Specific Effects at the SiO2 and TiO2 Nanoparticle/Aqueous Interface
Marie Bischoff1, Denys Biriukov2,3, Milan Předota3
1Laboratory for fundamental BioPhotonics (LBP), Institute of Bioengineering (IBI), and Institute of Materials Science (IMX), School of Engineering (STI), École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
Summary
Ion-specific effects influence colloidal systems. This study quantifies ion adsorption mechanisms on SiO2 and TiO2 nanoparticles, revealing cation- and surface-specific preferences for inner- vs. outer-sphere adsorption.
Area of Science:
- Colloid and Interface Science
- Surface Chemistry
- Nanoparticle Interactions
Background:
- Ion-specific effects are critical for colloidal system stability and interfacial processes.
- Understanding the electrostatic structure of solid/water colloidal interfaces is essential but challenging.
- Current knowledge of ion-specific effects at these interfaces remains limited.
Purpose of the Study:
- To quantify interfacial water orientation and electrostatic surface potential around SiO2 and TiO2 nanoparticles.
- To elucidate ion adsorption mechanisms at low ionic strengths (<1 mM).
- To investigate cation- and surface-specific adsorption preferences (inner- vs. outer-sphere).
Main Methods:
- Polarimetric angle-resolved second harmonic scattering was used to measure interfacial properties.
- Experiments were conducted on 100 nm SiO2 and TiO2 colloidal particles.
- Molecular dynamics simulations were performed on crystalline SiO2 and TiO2 surfaces.
Main Results:
- Ion adsorption mechanisms were established based on interfacial water orientation and surface potential.
- Distinct cation- and surface-specific adsorption behaviors were observed.
- Ca2+ showed preferential outer-sphere adsorption on SiO2, contrasting with dominant inner-sphere adsorption on TiO2.
Conclusions:
- The study provides molecular-level insights into the electrostatic environment of colloidal nanoparticles.
- Ion-specific effects, particularly cation adsorption, were quantified with micromolar sensitivity.
- Experimental and simulation data confirm cation- and surface-specific adsorption preferences.
