Inhomogeneous dynamics in confined water nanodroplets

Abstract

The effect of confinement on the dynamical properties of liquid water was studied by mid-infrared ultrafast pump-probe spectroscopy on HDO:D2O in reverse micelles. By preparing water-containing reverse micelles of different well defined sizes, we varied the degree of geometric confinement in water nanodroplets with radii ranging from 0.2 to 4.5 nm. We find that water molecules located near the interface confining the droplet exhibit slower vibrational energy relaxation and have a different spectral absorption than those located in the droplet core. As a result, we can measure the orientational dynamics of these different types of water with high selectivity. We observe that the water molecules in the core show similar orientational dynamics as bulk water and that the water layer solvating the interface is highly immobile.AOT ͉ confinement ͉ infrared ͉ mid-infrared pump-probe spectroscopy ͉ reverse micelle T here are many examples in the fields of biology (1, 2), geochemistry (3), tribology (4), and nanofluidics (5), where water molecules are not present as a bulk liquid, but in small numbers and confined geometries. The presence of an interface is known to influence the structure and dynamics of liquid water. Near a surface, ordering of water molecules into layers occurs (6-8), as was shown by steady-state surface-sensitive techniques like x-ray diffraction. This ordering was found to extend up to several molecular diameters into the liquid. In the case of small water droplets, the confinement is three-dimensional, and the overall structure and dynamics of the water may be affected.A suitable model system for studying confined water nanodroplets are reverse micelles (9, 10). A solution of nanometersized droplets forms when preparing an emulsion of water in an apolar solvent by addition of a surfactant. The anionic lipid surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) is known to form micelles that are reasonably monodisperse (Ϸ15%) (11,12). The size of the water droplets can be easily varied by changing the molar water-to-AOT ratio, conventionally denoted by the parameterIn previous studies, two-component models have been proposed to describe both the linear absorption (13) and the vibrational energy relaxation (14, 15) of confined water, to reflect the different properties of water located at the surface and in the core of the nanodroplets. As for the structural hydrogen bond rearrangements of liquid water, the effects of nanoconfinement are under much debate. So far, no inhomogeneities were observed in the molecular orientational motions of the water molecules throughout the droplets (14), although such inhomogeneities have been predicted by molecular dynamics simulations (16). A decrease in the average mobility of water in nanodroplets has been observed by several techniques (17-20), although so far no distinction could be made between water molecules at the surface layers of the droplets and those in the core. Therefore, from these studies it remained unclear whether the mobility of water in nanodro...

Reduced coupling of water molecules near the surface of reverse micelles

“…The initial 30% decrease of anisotropy at 300 fs time scale originates from the population exchange due to intramolecular coupling between the symmetric and asymmetric stretches belonging to the same water molecule, 45 although some contribution from librations cannot be completely ruled out. 43,44 The subsequent slow decay of the anisotropy with a 20 ps time constant points towards extremely slow rotational diffusion of water molecules and general immobility of interfacial water. Similar long-lived polarization memory was also observed for water molecules trapped in a weakly hydrated membrane.…”

“…Quite surprisingly, the behavior observed in these experiments for isotopically pure water appears to be similar to the one in water isotopic mixtures, where inter-and intramolecular couplings are artificially broken by isotopic dilution. 43,44 Nonetheless, experiments which are capable of a more direct elucidation of the interand intramolecular dynamics 45 in the membrane hydration layer (for example, by selectively addressing dynamics at different vibrational frequencies) have not been reported yet.…”

“…One way to avoid the aforementioned problem in exploring the dynamics at the frequencies of different spectral signatures is to use selective excitation, either by using coherent multidimensional spectroscopy [57][58][59][60][61][62] or by narrowing the bandwidth of the excitation pulses. 43 In this paper, we investigate the extent of the intermolecular vibrational coupling between the OH stretching modes of water molecules in the lipid hydration shell. The vibrational anisotropy dynamics are selectively induced by a (relatively) narrow-band IR excitation of the OH stretching modes of water in small (diameter of 1 nm) reverse micelles where most water molecules are interface-bound, and compare its behaviour with that observed in the large (diameter of 10 nm) ones.…”

Validation and Divergence of the Activation Energy Barrier Crossing Transition at the AOT/Lecithin Reverse Micellar Interface

“…[15][16][17][18][19] Mid-infrared ultrafast pumpprobe spectroscopy of RMs of different well-defined sizes revealed slow vibrational relaxation dynamics of water at the interface. 16 Using the picosecond-resolved fluorescence spec-…”

Dynamics of water nanodroplets and aqueous protons in non-ionic reverse micelles

“…The remaining water molecules, whose hydrogens are involved in intermolecular connectivity should be the ones that contribute to the lower frequency branch. Concerning spectral shifts, we remark that ultrafast infrared pump-probe spectroscopy experiments have revealed the existence of such frequency shifts in the case of ionic micelles such as in Aerosol-OT (AOT) 8,39 or in the cetyltrimethylammonium bromide micelle (CTAB), 7 which are usually explained in terms of the influence of confinement and ionic charges on the structure of the hydrogen-bond network. In our case, three different spectral shifts were observed: (i) Do B 30 cm À1 for both interfacial and bulk-like water; (ii) Do B À25 cm À1 for water at the interface and (iii) Do B À40 cm À1 for bulk-like water.…”