Structure and dynamics of water in nanoscopic spheres and tubes

Abstract

General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We study the reorientation dynamics of liquid water confined in nanometer-sized reverse micelles of spherical and cylindrical shape. The size and shape of the micelles are characterized in detail using small-angle x-ray scattering, and the reorientation dynamics of the water within the micelles is investigated using GHz dielectric relaxation spectroscopy and polarization-resolved infrared pump-probe spectroscopy on the OD-stretch mode of dilute HDO:H 2 O mixtures. We find that the GHz dielectric response of both the spherical and cylindrical reverse micelles can be well described as a sum of contributions from the surfactant, the water at the inner surface of the reversed micelles, and the water in the core of the micelles. The Debye relaxation time of the core water increases from the bulk value τ H 2 O of 8.2 ± 0.1 ps for the largest reverse micelles with a radius of 3.2 nm to 16.0 ± 0.4 ps for the smallest micelles with a radius of 0.7 nm. For the nano-spheres the dielectric response of the water is approximately ∼6 times smaller than expected from the water volume fraction and the bulk dielectric relaxation of water. We find that the dielectric response of nano-spheres is more attenuated than that of nano-tubes of identical composition (water-surfactant ratio), whereas the reorientation dynamics of the water hydroxyl groups is identical for the two geometries. We attribute the attenuation of the dielectric response compared to bulk water to a local anti-parallel ordering of the molecular dipole moments. The difference in attenuation between nano-spheres and nano-cylinders indicates that the anti-parallel ordering of the water dipoles is more pronounced upon spherical than upon cylindrical nanoconfinement.

On the origin of the extremely different solubilities of polyethers in water

“…It may be noted that the DRS and transient-IR experiments both probe water reorientation dynamics, but the observed relaxation behavior is generally different, because DRS probes the (first-order) orientational correlation function of the total electric-dipole moment of all the water molecules, whereas transient IR probes the (second-order) orientational correlation function of spatially well separated OH (or OD) groups (the spatial separation being due to the isotopic dilution) 46 . Rotation of a hydrogen-bond donating water molecule around the hydrogen-bonded OH group does not change the direction of the OH vector (and hence does not contribute to the OH-stretch anisotropy decay), but does rotate the electric-dipole moment (and hence does contribute to the dielectric relaxation); as a consequence, a fraction of solvating water molecules can cause a residual offset in the anisotropy, and an overall slowdown of the dielectric relaxation 47,48 . An additional difference between the two experiments is that DRS probes the collective motion of all the water molecules, and hence is more sensitive to collective water dynamics 47,48 .…”

Picosecond orientational dynamics of water in living cells

“…Additional spectra of the cells and cytosol mimics in 100% H 2 O were recorded for background subtraction. Polarization-resolved infrared pump-probe experiments were done using a setup described previously 74 . Pump-beam scattering by the samples was eliminated by delaying every second pump pulse by half an optical cycle using a photo-elastic modulator and averaging the signals at these two pump-probe delays 82 .…”

Исследование Эффектов Дейтерозамещения В Полимерной Мембране С Помощью ИК Фурье-Спектрометрии

“…2, а и 2, b, наблю-даемые различия спектральных зависимостей следует связать с разными силами осцилляторов дейтерирован-ной воды внутри нанометровых пор Нафиона в данном спектральном диапазоне. Таким образом, в эксперимен-тах по ИК фурье-спектрометрии можно изучать эффект " конфайнмента" (confinement), который в данном случае проявляется именно в различии спектров пропускания воды с указанным содержанием дейтерия внутри нано-метровых пор Нафиона и в кювете с размером 90 µm (различные проявления эффекта конфайнмента для воды описаны, например, в работах [43,44]). …”