Protein Adsorption at the Air-Water Interface by a Charge Sensing Interferometric Technique

Area of Science:

• Interfacial Science
• Physical Chemistry
• Biophysics

Background:

• Understanding protein and surfactant adsorption at interfaces is crucial for various applications.
• Traditional methods struggle to quantify adsorption at very low concentrations.
• Air-water interfaces are dynamic systems influenced by molecular interactions.

Purpose of the Study:

• To introduce and validate a novel differential interferometric technique for studying interfacial adsorption.
• To investigate the adsorption kinetics and equilibrium of proteins (BSA, lysozyme) and an ionic surfactant (SDS) at air-water interfaces.
• To correlate interfacial charge variations with adsorption phenomena at low concentrations.

Main Methods:

• Utilized a differential interferometric technique to monitor capillary wave amplitudes and frequencies on millimeter-sized air bubbles.
• Excited capillary waves using an electric field (approx. 10 V/cm).
• Measured resonant modes (radial and shape) to assess interfacial properties and surface charge.

Main Results:

• The technique successfully measured capillary wave amplitudes as low as 10⁻⁹ m.
• Observed changes in bubble resonant mode amplitudes preceding frequency shifts due to adsorbed charged molecules.
• Quantified adsorption kinetics for BSA and lysozyme, identifying a slow, energy barrier-limited process.
• Determined equilibrium surface excess concentrations (Γ) for BSA and SDS at sub-CMC levels.

Conclusions:

• The developed interferometric technique offers high sensitivity for studying interfacial adsorption at low concentrations.
• Adsorption of charged molecules significantly impacts interfacial dynamics, affecting mode amplitudes before frequencies.
• The method provides reliable measurements of adsorption kinetics and equilibrium, complementing existing surface tension isotherm models.