G S. Park Acceleration of collective orientation in nano confined water of DMPC

Acceleration of collective orientation in nanoconfined water of DMPC

Kihoon Eom1, Jeongmin Jang1, and Seonmyeong Kim1, Philipp Honegger2, Othmar Steinhauser2 , Devis Di Tommaso3, Gun-Sik Park1 1Seoul National University, 08826 Seoul, South Korea 2University of Vienna, 1090 Vienna, Austria 3Queen Mary University of London, E1 4NS London, UK

Abstract— The effect of confinement on the hydrogen bonding networks of water molecules encapsulated in phospholipid multilamellar vesicles is investigated by dielectric relaxation spectroscopy. Acceleration of collective orientational dynamics is confirmed experimentally.

I. INTRODUCTION

The physical and chemical properties of liquid water are profoundly governed by its hydrogen-bonding networks [1]. In the bulk liquid water, the hydrogen-bonded water molecules form an approximately tetrahedral structure that continuously fluctuates on the picosecond timescale. However, the structure and dynamics of water bodies are affected strongly upon introduction of an interface or confinement[2]. A wealth of studies has been mainly conducted on the nature of hydration, and they describe a slow hydration shell consisting of up to several layers of water from the surface [3]. For a nanoconfined water, if the confinement length scale is larger than the hydration layer thickness, the water molecules beyond form a bulk liquid-like phase, giving rise to a simple model of retarded hydration water and an unperturbed bulk-like water beyond the hydration layer [4]

Yet in nanoconfined water environments, some peculiar phenomena have been reported. For instance, the diffusion rate of water in carbon nanotubes (CNT) increases as the CNT diameter decreases, but it is almost identical between graphene sheets regardless of the surface-to-surface distance [5]. As another example, the self-dissociation reaction of water is inhibited in CNT, but it is enhanced between graphene sheets [6]. Analogously, water-hydrogen exchange of biomolecules is rapid in bulk water but drastically slowed down in confinement [7]. These observations indicate that the two-component model of the slow hydration shell and the bulk-like water lacks some descriptive qualities.

A major shortcoming of conventional single-molecular spectroscopic methods, such as vibrational spectroscopy IR, NMR and scattering, nuclear quadrupole resonance, for investigating the nanoconfined water is the lack of information about correlated motion among water molecules, which tends to align their permanent dipoles into the tetrahedral hydrogen bonding network. how they report on the dynamics of a water molecule or a pair of molecules. They may reveal intermolecular phenomena, but necessarily lack important cross-correlations and. Thus they do not report on some properties arising from the collective interactions of the molecules forming a piece of matter, such as the dielectric constant

In contrast, dielectric relaxation spectroscopy (DRS) is a collective spectroscopic method tracking the temporal evolution of the sum dipole of the entire sample instead of the molecular ones. Hence DRS includes not only the temporal behaviour of the molecular dipoles, but also how they move with regards to each other.

II. RESULTS Our dielectric relaxation measurements show that its collective dynamics are much faster, not slower; at the long range, water dipoles align far more anti-parallel than in bulk-liquid water; the cause behind this drastic change is not the interaction with the confining surface as one would suspect, but arises from geometrical confinement itself.

Dielectric loss

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Fig. 1: The dielectric loss spectrum of DMPC MLVs and its decomposition of relaxation modes for the nanoconfined water in the MLVs in DMPC/water

III. SUMMARY

This study reports how the entire water phase behaves differently when nanoconfined, even beyond the surface hydration layer. A counter-intutive collective dynamics acceleration of water due to nanoconfinement was observed, which has been invisible in NMR, scattering, and vibrational spectroscopies.

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