Surface vibrational structure of colloidal silica and its direct correlation with surface charge density

Area of Science:

• Surface Chemistry
• Spectroscopy
• Nanotechnology

Background:

• Colloidal silica nanoparticles (NPs) are widely used in various applications.
• Accurate determination of surface charge density (SCD) is crucial for understanding NP behavior.
• Traditional methods like potentiometric titration have limitations.

Purpose of the Study:

• To develop and validate a novel spectroscopic method for determining the SCD of colloidal silica NPs.
• To establish a correlation between ATR-FTIR spectral features and SCD.
• To demonstrate the applicability of the method under varying conditions.

Main Methods:

• Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy was employed.
• Identification of Si-O stretch vibrations for neutral (≡Si-OH) and deprotonated (≡Si-O(-)) surface groups.
• Correlation of Si-(OH) vibration shifts with SCD measured by potentiometric titrations.

Main Results:

• A direct correlation was observed between the Si-(OH) stretch vibration wavenumber and the NPs SCD.
• The Si-(OH) peak shifts to lower wavenumbers with increasing deprotonation (higher SCD).
• The method successfully quantified SCD for silica NPs of different sizes in varying electrolyte concentrations and pH.

Conclusions:

• ATR-FTIR spectroscopy is a viable and sensitive method for determining colloidal silica NP SCD.
• The spectroscopic approach offers advantages for in situ measurements and requires minimal sample volume.
• This technique opens possibilities for integration with microfluidic devices and liquid microjets.