Water dynamics-The effects of ions and nanoconfinement

Sungnam Park, David E. Moilanen, M. D. Fayer

Research output: Contribution to journalArticle

139 Citations (Scopus)

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 2O. The vibrational echo experiments measure spectral diffusion, and the pump-probe experiments measure orientational relaxation. Both experimental observables are directly related to the tructural 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 ∼3 in ∼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.

Original languageEnglish
Pages (from-to)5279-5290
Number of pages12
JournalJournal of Physical Chemistry B
Volume112
Issue number17
DOIs
Publication statusPublished - 2008 May 1
Externally publishedYes

Fingerprint

Micelles
Ions
Water
micelles
water
Hydrogen bonds
ions
hydrogen bonds
echoes
Pumps
pumps
encapsulating
probes
Experiments
Carbon Monoxide
Hydroxyl Radical
Salts
Spectroscopy
Polarization
salts

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

Water dynamics-The effects of ions and nanoconfinement. / Park, Sungnam; Moilanen, David E.; Fayer, M. D.

In: Journal of Physical Chemistry B, Vol. 112, No. 17, 01.05.2008, p. 5279-5290.

Research output: Contribution to journalArticle

Park, Sungnam ; Moilanen, David E. ; Fayer, M. D. / Water dynamics-The effects of ions and nanoconfinement. In: Journal of Physical Chemistry B. 2008 ; Vol. 112, No. 17. pp. 5279-5290.
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