Micellar and bicontinuous microemulsion structures show different solute–solvent interactions: a case study using ultrafast nonlinear infrared spectroscopy

Using aqueous and organic probes to simultaneously explore the structural dynamics of reverse micellar and bicontinuous microemulsion structures.

Kinetics of exchange between zero-, one-, and two-hydrogen-bonded states of methyl and ethyl acetate in methanol

It has recently been shown that the ester carbonyl stretching vibration can be used as a sensitive probe of local electrostatic field in molecular systems. To further characterize this vibrational probe and extend its potential applications, we studied the kinetics of chemical exchange between differently hydrogen-bonded (H-bonded) ester carbonyl groups of methyl acetate (MA) and ethyl acetate (EA) in methanol. We found that, while both MA and EA can form zero, one, or two H-bonds with the solvent, the population of the 2hb state in MA is significantly smaller than that in EA. Using a combination of linear and nonlinear infrared measurements and numerical simulations, we further determined the rate constants for the exchange between these differently H-bonded states. We found that for MA the chemical exchange reaction between the two dominant states (i.e., 0hb and 1hb states) has a relaxation rate constant of 0.14 ps(-1), whereas for EA the three-state chemical exchange reaction occurs in a predominantly sequential manner with the following relaxation rate constants: 0.11 ps(-1) for exchange between 0hb and 1hb states and 0.12 ps(-1) for exchange between 1hb and 2hb states.

Infrared Pump-Probe Study of Nanoconfined Water Structure in Reverse Micelle

The influence of nanoconfinement on water structure is studied with time- and frequency-resolved vibrational spectroscopy of hydrazoic acid (HN3) encapsulated in reverse micelle. The azido stretch mode of HN3 is found to be a promising infrared probe for studying the structure and local hydrogen-bond environment of confined and interfacial water in reverse micelle due to its narrow spectral bandwidth and large transition dipole moment. The results show a clear separation between the core and shell spectral components, making it advantageous over the previously studied infrared probes. The measured vibrational lifetimes appear to be substantially different for the interfacial and bulk-like environments but show no remarkable size dependency, which indicates that water structures around this IR probe are distinctively different in the core and shell regions. The influence of local hydrogen bond network in the first and higher solvation shells on the vibrational dynamics of HN3 is further discussed.