Water Dynamics at the Interface in AOT Reverse Micelles

Abstract

The orientational dynamics of water molecules at the interface in large Aerosol-OT (AOT) reverse micelles are investigated using ultrafast infrared spectroscopy of the OD stretch of dilute HOD in H2O. In large reverse micelles(~9 nm diameter or larger), a significant amount of the nanoscopic water is sufficiently distant from the interface that it displays bulk-like characteristics. However, some water molecules interact with the interface and have vibrational absorption spectra and dynamics distinct from bulk water. The different characteristics of these interfacial waters allow their contribution to the data to be separated from the bulk. The infrared absorption spectrum of the OD stretch is analyzed to show that the interfacial water molecules have a spectrum that peaks near 2565 cm−1 in contrast to 2509 cm−1 in bulk water. A two component model is developed that simultaneously describes the population relaxation and orientational dynamics of the OD stretch in the spectral region of the interfacial water. The model provides a consistent description of both observables and demonstrates that water interacting with the interface has slower vibrational relaxation and orientational dynamics. The orientational relaxation of interfacial water molecules occurs in 18 ± 3 ps in contrast to the bulk water value of 2.6 ps.

Reduced coupling of water molecules near the surface of reverse micelles

“…40,41,81,82 Isotopic dilution leads to cancelation of both inter-and intramolecular couplings between OH (or OD) oscillators. In this case, transient anisotropy experiments on an OH/OD oscillator would provide information explicitly on the orientational dynamics of water molecules.…”

“…the extent of probed distances from the interface) at charged interfaces is still under debate. 37 On the other hand, ultrafast vibrational spectroscopy studies also evidenced a substantial deceleration of population relaxation rates 38 and a decrease in the induced anisotropy decay [39][40][41][42] of water vibrations in the lipid hydration layer. This implies completely different hydrogen-bond dynamics and effect of intermolecular couplings at the water-lipid interface.…”

Anomalous Acid-Base Equilibria in Biologically Relevant Water Nanopools

“…[5][6][7][8][9][10][11][12][13] For example, the dielectric constant of a water nanopool has been reported to be much lower than that of bulk water (ε = 78. 5) 17 but similar to that of an alcohol (ε = [30][31][32][33][34][35][36][37][38][39][40].…”

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Water plays a crucial role in many principal biological phenomena such as enzymatic catalysis and proton pumping through a membrane protein channel.1-4 Moreover, in biological systems, water is usually contained in a small pocket of a membrane, 5-8 and such confined water, which is generally called a water nanopool, [8][9][10] shows peculiar properties differing considerably from the properties of bulk water.

8-16The confinement effect and the enclosing interfacial surfaces of waterpools are the main factors to determine the properties, such as polarity, viscosity, and H-bonding ability, of water nanopools.5-13 For example, the dielectric constant of a water nanopool has been reported to be much lower than that of bulk water (ε = 78.5)17 but similar to that of an alcohol (ε = 30-40).18,19 On the other hand, biological processes often take place based on proton relay along a hydrogen (H)-bonded chain, [1][2][3][4][20][21][22][23][24] and the dynamics of biological proton relay is determined by the size, the structure, and the motion of a water cluster which is the prime agent in most of biological systems. 3,[13][14][15][24][25][26] Thus, for better understanding of cellular dynamics, it is necessary to investigate the properties of a biologically relevant water nanopool as a biomimetic system of water confined in a cell membrane.

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“…[1][2][3][4] Moreover, in biological systems, water is usually contained in a small pocket of a membrane, [5][6][7][8] and such confined water, which is generally called a water nanopool, [8][9][10] shows peculiar properties differing considerably from the properties of bulk water.…”