Tuning the Dielectric Response of Water in Nanoconfinement through Surface Wettability

Abstract

The tunable polarity of water can be exploited in emerging technologies including catalysis, gas storage, and green chemistry. Recent experimental and theoretical studies have shown that water can be rendered into an effectively apolar solvent under nanoconfinement. We furthermore demonstrate, through molecular simulations, that the static dielectric constant of water can be modified by changing the wettability of the confining material. We find the out-of-plane dielectric response to be highly sensitive to the level of confinement and can be reduced up to 40× , in accordance with experimental data. By altering the surface wettability from superhydrophilic to superhydrophobic, we observe a 36% increase for the out-of-plane and a 31% decrease for the in-plane dielectric constants. Our findings demonstrate the feasibility of tunable water polarity, a phenomenon with great potential for scientific and technological impact.

Field theory of structured liquid dielectrics

“…We believe that, beyond the introduction of our approach and first results in the linear regime, our theory is capable of addressing a wide class of continuum models for polarization phenomena in confined solvents, in particular phenomena pertinent to the nano-confined aqueous systems [25,26]. Recent experimental work has shown that the observed oscillatory behavior of electrostatic and hydration forces is largely determined by the bulk ion and polarization charges, while the solution pH essentially determines the boundary conditions [44].…”

“…This type of water structural interaction modeling has been applied to solvation of phospholipid membranes [20], DNA molecules [21], linear polysaccharides [22], or to the solvation of proteins [23,24]. More recent approaches extend the ideas of nonlocal dielectric response of confined water [25] to the order-parameter description beyond the polarization [26], which are able to describe additional features of confined water structuring [27,28]. The order-parameter description can be arguably viewed as a generalization of the Landau-Ginzburg-type free-energy functionals beyond the polarization formulation [29], describing additional degrees of freedom pertinent to the complicated hydrogen-bond phenomenology [30].…”

Microscopic Simulations of Electrochemical Double-Layer Capacitors

“…In contrast to crystalline materials, which have a wellcharacterized structure, porous carbons are much less easily defined. They are generally ordered at the local scale only; each carbon atom adopts either sp 2 or sp 3 hybridization, with a ratio depending on the synthesis pathway. Many porous carbons formed in the Earth crust, such as kerogen, 96 display a large amount of sp 3 carbon atoms, but the materials used in supercapacitors need to have good electrical conductivity, which is achieved only with sufficiently high sp 2 proportion.…”

“…They are generally ordered at the local scale only; each carbon atom adopts either sp 2 or sp 3 hybridization, with a ratio depending on the synthesis pathway. Many porous carbons formed in the Earth crust, such as kerogen, 96 display a large amount of sp 3 This leads to the formation of locally planar, graphene-like sheets. However, the size, arrangement, and packing of these sheets is very disordered and thus difficult to describe in atomistic models.…”

“…The main advantages of supercapacitors are that the distance between the two charged layers becomes microscopic and that the electrode does not need to be planar anymore; its surface area can be greatly increased through the use of porous materials. 2 The dielectric constant, or rather its local equivalent at the microscopic scale, was shown to be much lower than that of the bulk in the case of liquids (and it is also affected by confinement 3 ), but this detrimental effect is largely overcome by the other advantages.…”