Probing the micelle/water interface by a rapid laser-induced proton pulse

Area of Science:

• Physical Chemistry
• Biophysical Chemistry
• Chemical Kinetics

Background:

• The laser-induced pH jump technique offers high time resolution for studying rapid chemical reactions.
• Macromolecular surfaces, like proteins, are complex for studying surface group kinetics.
• Micelles provide a model system with controllable surface properties for kinetic studies.

Purpose of the Study:

• To measure the kinetics of protonation-deprotonation of surface groups on macromolecules using the laser-induced pH jump.
• To investigate the influence of micellar surface charge and structure on indicator protonation/deprotonation rates.
• To determine the pK shifts of adsorbed pH indicators and correlate them with kinetic changes.

Main Methods:

• Utilized the laser-induced pH jump technique to measure reaction kinetics.
• Employed neutral detergent micelles (Brij 58) as a model high molecular weight structure.
• Varied micellar charge by adding sodium dodecyl sulfate and used adsorbed pH indicators (bromocresol green, neutral red).

Main Results:

• Proton dissociation from adsorbed bromocresol green was slower than from the free indicator, attributed to micellar stabilization.
• Adsorbed bromocresol green showed a pK shift on neutral micelles, explained by decreased proton dissociation rates.
• Neutral red, being more lipid-soluble in its alkaline form, did not exhibit similar kinetic or pK changes upon adsorption.
• Protonation reactions were diffusion-controlled, with rates modulated by micellar charge.
• Calculated micellar charge using kinetic data was consistent with equilibrium measurements.
• The technique allowed estimation of proton diffusion coefficients within the micellar hydrated shell.

Conclusions:

• The laser-induced pH jump technique is effective for studying kinetics of protonation-deprotonation on model surfaces like micelles.
• Micellar surface properties significantly influence the kinetics and thermodynamics (pK) of adsorbed indicator reactions.
• The study demonstrates the utility of this technique for probing interfacial phenomena and estimating diffusion coefficients.