Chemistry is often performed in heterogeneous environments, such as in microemulsions, which are commonly used as a type of nanoreactor for controlling the morphology and size of polymers and nanoparticles. The role of surfactant-water interface plays in the thermodynamics and dynamics of chemical reactions is critical to understanding complex confined environments. Reverse micelles (RMs) offer an ideal model system to understand complex nano-scale environments. Here we present studies of the picosecond hydrogen-bond dynamics at the oil/water interface in RMs ultrafast two-dimensional infrared (2D IR) spectroscopy and modeling through MD simulations. We probe the interfacial effects of monomers and polymers encapsulated within the ~100 nm aqueous environment of the reverse micelles. In particular, the experiments are focused on characterizing the interfacial hydrogen-bond dynamics of non-ionic sorbitan monostearate surfactants with acrylamide (monomer) or polyacrylamide (polymer, 40,000 M<sub>n</sub>) encapsulated within the RMs. The results show that the hydrogen-bond populations at the interface remain unchanged with the encapsulation of either polymer or monomer species. The solvation dynamics at the reverse micelle interface with no encapsulants is ~1 ps, the inclusion of monomer species does not affect the interfacial dynamics, however, with the inclusion of the polymer species at only 5wt% a threefold slowdown in water dynamics is observed.  The change in solvation dynamics is due to the localization of the polymer species at the surfactant-water interface, whereas, the monomer does not localize to the interface. These results have important implications for controlling chemical reactions in confined environments, namely, the reactants and products experience different local environments.

Polymer modulated dynamics at the oil/water interface

ACS Spring 2022

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