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Probing Proton Delivery in Microheterogeneous Water Structures Using a Proton-Dependent Isomerization Reaction of a
Thien Khuu1, Kana Takematsu2, Jahan Dawlaty1
1University of Southern California, Los Angeles, California 90007, United States.
The Journal of Physical Chemistry. A
|June 13, 2025
Summary
Proton dynamics in mixed solvents like acetonitrile and DMSO are influenced by water content, affecting reaction rates. Surfactants can further alter these dynamics by forming micelles.
Area of Science:
- Physical Chemistry
- Chemical Kinetics
- Solvent Effects
Background:
- Water structure and proton dynamics are crucial in complex environments.
- Understanding these dynamics influences reaction thermodynamics, kinetics, and selectivity.
Purpose of the Study:
- Investigate the impact of water microphases on reaction kinetics.
- Determine how water mole fraction affects proton recovery rates in acetonitrile and dimethyl sulfoxide.
Main Methods:
- Studied proton-dependent recovery kinetics of a merocyanine photoacid.
- Varied water mole fractions (χw) in acetonitrile (ACN) and dimethyl sulfoxide (DMSO) mixtures.
- Introduced cetyltrimethylammonium bromide (CTAB) as a model surfactant.
Main Results:
- Recovery rates (kobs) did not linearly scale with water mole fraction.
- DMSO showed slow rates until χw ∼ 0.7, then linear increase, indicating a required hydrogen bond network.
- ACN exhibited three distinct regions based on water content, suggesting isolated molecules, nanopools, and extended networks.
- CTAB addition caused a sharp decline in recovery rates beyond χw ∼ 0.8 due to micelle formation.
Conclusions:
- Reaction rates are sensitive to the water's hydrogen bonding network structure.
- Solvent properties (e.g., hydrogen bond accepting ability) dictate the water mole fraction threshold for network formation.
- Surfactant-induced micelle formation can significantly hinder proton access and alter reactivity.
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