Fluorescence Dynamics of Coumarin C522 on Reduced-Charge Montmorillonite in Aqueous Dispersion

Solvation is an important phenomenon, especially in association with heterogeneous phase interactions. Coumarin C522, C14H12NO2F3, is used as a fluorophore probe to study the interaction between coumarin and a reduced-charge montmorillonite (RCM) surface. Such hydrophilic and hydrophobic interactions are of interest for sorption processes in confined environments. The prepared RCM series with 0.00, 0.12, 0.26, 0.43, 0.66, and 0.97 Li+ molar fractions provide different surface charges. The aqueous dispersion of the C522/water/RCM system is studied by using steady-state and time-resolved fluorescence spectroscopies. Both the Stokes shift and the dynamics of the solvation process varied as a function of surface charge. Steady-state fluorescence spectroscopy reveals that the C522 Stokes shift varies from 5115 cm−1 for the 0.00 Li+ molar fraction to 3988 cm−1 for the 0.97 Li+ molar fraction. Time-resolved fluorescence spectroscopy determines that the decay time T (1) varies from 1.0 ps for the 0.00 Li+ molar fraction to 3.6 ps for the 0.97 Li+ molar fraction. Within the range of a few picoseconds, the dynamics of the water solvation shell may be described with H-bond rearrangement, modified with the different RCM surface charges. Two models illustrating the interactions between C522 and RCM in water are proposed which qualitatively describe the dynamics. To the best of our knowledge, this experiment is the first measurement of solvation dynamics on a montmorillonite structure surface using ultrafast laser fluorescence spectroscopy.