Counterion Effect on Vibrational Relaxation and the Rotational Dynamics of Interfacial Water and an Anionic Vibrational Probe in the Confined Reverse Micelles Environment.

Vibrational relaxation and the rotational dynamics of water molecules encapsulated in reverse micelles (RMs) have been investigated by ultrafast infrared (IR) spectroscopy and two-dimensional IR (2D IR) spectroscopy. By changing the counterion of the hydrophilic headgroup in the RMs formed by Aerosol-OT (AOT) from Na+ to K+, Cs+ and Ca2+, we could determine the specific counterion effects on the rotational dynamics of water molecules. The orientational relaxation time constant of water decreases in the order Ca2+ > Na+ > K+ > Cs+. The SCN- anionic probe and counterion can form ion pairs at the interfacial region of the RMs. The rotational dynamics of SCN- anion significantly decreases because of the synergistic effects of confinement and the surface interactions in the interfacial region of the RMs. The results can provide a new understanding of the cationic Hofmeister effect at the molecular level observed in biological studies.