Water DynamicsThe Effects of Ions and Nanoconfinement

Abstract

Hydrogen bond dynamics of water in highly concentrated NaBr salt solutions and reverse micelles are studied using ultrafast 2D-IR vibrational echo spectroscopy and polarization-selective IR pump-probe experiments performed on the OD hydroxyl stretch of dilute HOD in H(2)O. The vibrational echo experiments measure spectral diffusion, and the pump-probe experiments measure orientational relaxation. Both experimental observables are directly related to the structural dynamics of water's hydrogen bond network. The measurements performed on NaBr solutions as a function of concentration show that the hydrogen bond dynamics slow as the NaBr concentration increases. The most pronounced change is in the longest time scale dynamics which are related to the global rearrangement of the hydrogen bond structure. Complete hydrogen bond network randomization slows by a factor of approximately 3 in approximately 6 M NaBr solution compared to that in bulk water. The hydrogen bond dynamics of water in nanoscopically confined environments are studied by encapsulating water molecules in ionic head group (AOT) and nonionic head group (Igepal CO 520) reverse micelles. Water dynamics in the nanopools of AOT reverse micelles are studied as a function of size by observing orientational relaxation. Orientational relaxation dynamics deviate significantly from bulk water when the size of the reverse micelles is smaller than several nm and become nonexponential and slower as the size of the reverse micelles decreases. In the smallest reverse micelles, orientational relaxation (hydrogen bond structural randomization) is almost 20 times slower than that in bulk water. To determine if the changes in dynamics from bulk water are caused by the influence of the ionic head groups of AOT or the nanoconfinement, the water dynamics in 4 nm nanopools in AOT reverse micelles (ionic) and Igepal reverse micelles (nonionic) are compared. It is found that the water orientational relaxation in the 4 nm diameter nanopools of the two types of reverse micelles is almost identical, which indicates that confinement by an interface to form a nanoscopic water pool is a primary factor governing the dynamics of nanoscopic water rather than the presence of charged groups at the interface.

Ion–water hydrogen-bond switching observed with 2D IR vibrational echo chemical exchange spectroscopy

“…The orientational dynamics of ha are biexponential with a fast component of 0.7 ps due to fast orientational motion within an intact hydrogen bonding configuration (39). The biexponential nature of the orientational dynamics of ha is well described by a wobbling-in-a-cone model (39)(40)(41), and its effects on the exchange process were included in the fitting described above. Because of space limitations, the orientational relaxation data are not shown here.…”

Coherent infrared multidimensional spectra of the OH stretching band in liquid water simulated by direct nonlinear exciton propagation

“…Both neat water and HOD were studied in bulk water 3,[20][21][22][23][24][29][30][31] and in confined environment. [25][26][27][28]32,34 One interesting challenge of nonlinear spectroscopy of water is how to disentangle the influence of orientational dynamics, population relaxation, frequency fluctuations, and wave function delocalization. Using the NEP, we investigate the sensitivity of nonlinear techniques to both the excitonic coupling and the two-exciton coherence in liquid water.…”

Ultrafast dynamics of solvation: the story so far

“…Nonlinear mid-infrared spectroscopy is an ideal technique for studying intramolecular motion of aqueous solvation shells. 57,58 On the other hand, low-frequency nuclear motion of liquids and solutions has been interrogated directly with THz 59-61 and Raman probes. [62][63][64] …”