Oscillating electric field effects on adsorption of methane-water system

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Oscillating electric field effects on adsorption of methane-water system on kaolinite surface Yudou Wang, Bo Liao, Zhaoyang Kong, Zhigang Sun, Li Qiu, and Diansheng Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02961 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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Energy & Fuels

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Oscillating electric field effects on adsorption of

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methane-water system on kaolinite surface

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Yudou Wang*, Bo Liao, Zhaoyang Kong, Zhigang Sun, Li Qiu, Diansheng Wang

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School of Science, China University of Petroleum, Qingdao, 266580, China

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ABSTRACT: A quantitative understanding of oscillating electric field effects on adsorption of

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methane-water system on kaolinite surfaces is vital for enhancing methane desorption and

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forecasting gas production. We have performed non-equilibrium molecular dynamics simulations

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to investigate the adsorption behaviors of methane-water on the kaolinite (0 0 1) surface applying

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oscillating electric fields in the frequency range of 0-100 GHz and the amplitude of 0-0.25 V/Å.

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The simulated results demonstrate that water will preferentially adsorb onto the surface, forming a

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water layer and preventing the adsorption of methane, and thus, leading to the reduced adsorption

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of methane. The applied oscillating electric fields contribute to thicker water layer and smaller

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amount of adsorbed methane for water-methane system on kaolinite surface. Furthermore, higher

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oscillating frequency and stronger intensity of the applied electric fields facilitate the desorption of

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methane. These phenomena associate with that the applied oscillating fields reduces hydrogen bond

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amounts on the interface of kaolinite, and higher frequencies or stronger intensities further break

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hydrogen bonds. Meanwhile, the interaction energy of water-kaolinite and self-diffusion

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coefficient of water increase with the frequency and the intensity of the applied electric fields. This

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study helps understanding the mechanism how the oscillating electric fields affecting the 1 ACS Paragon Plus Environment

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adsorption behaviors of methane-water system on kaolinite surfaces and is of applicable

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importance to boosting the gas production.

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Keywords: Methane; Adsorption; Kaolinite; Water; Oscillating electric field; Molecular

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simulation

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1

Introduction

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As one of the alternatives of conventional fossil energy and clean energy, shale gas is attracting

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increasing attentions.1-3 The adsorbed gas content could account for 20 % to 85 % of the shale gas

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reservoirs due to large internal surface areas of micro and nano-scale pores existing in organic

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matters (kerogens) and clay minerals in shales.4-6 Therefore, research on methane adsorption in

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shale gas reservoir is one of the primary concerns to evaluate gas-in-place and forecast gas

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production. Kerogens and clay minerals are the main adsorbents of the shale gas.7-11 Gas adsorption

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in kerogen may significantly contribute to gas-in-place in shale gas reservoirs. In addition to

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organic matters, clay minerals may provide additional adsorption capacity, which can be

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comparable to that adsorbed on kerogens, due to high internal surface area12,

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enhancing gas desorption from clays can significantly improve shale gas recovery. Methane

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adsorption in clay minerals depends on their chemical compositions, pores structures and water

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content.3 Water is considered ubiquitous in shale gas systems.14 The adsorption performances on

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mineral−water interface were studied experimentally.15-18 Experimental studies have indicated that

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moist equilibrated clays hinder the gas adsorption on them,12, 18 and the adsorption amounts of

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methane would reduce about 40%–90% compared with the dry condition.9, 18, 19 However, the

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related experimental data are still inadequate, especially for adsorption on pure clay minerals. In

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addition to experimental studies, the grand-canonical Monte Carlo (GCMC) and molecular

13.

Therefore,

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dynamics (MD) simulations were used to study the effects of water on gas sorption in clay minerals

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or organic matters and to examine the underlying sorption mechanism.8, 20-22 Most researches focus

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on methane adsorption on montmorillonite, kaolinite, coals, and kerogen, and effects of water

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content, pressure, and temperature on gas adsorption and clays swelling were studied.7, 12, 22-24

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Recent years, external electrical fields were introduced into hydrogen production, degradation of

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environmentally harmful organic compounds, or in petroleum industry to enhance liquids or gases

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desorption.25-31. The effects of applied electric fields on water or gas adsorption on surfaces of clays

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or energy materials are of key interests for these applications. The thermal effects of electric or

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electromagnetic fields on kerogens or clays are critical to shale gas desorption. Wang et al.32

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proposed a method to enhance shale gas desorption by electrical resistance heating. Moreover, the

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non-thermal field effects of electric field on matter are another factor. For example, the adsorption

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of water, a typical polar molecule, is significantly affected by oscillating electric fields. Therefore,

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the applied oscillating fields might potentially disrupt the interaction between clay and

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water/methane and could possibly change the fluid properties, such as diffusivity. Researches on

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effects of electrical fields on liquid or gas adsorption have been done in some fields. Futera and

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English have investigated the effects of external static electric fields applied to a wide variety of

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TiO2/water interfaces using non-equilibrium molecular-dynamics techniques.33 Adsorption of

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formaldehyde molecule on the pristine or transition metal doped graphene by controlling the

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external electric fields are theoretically investigated using density functional theory method.34 Li

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et al. have studied the influences of the electrostatic fields on the process of methane adsorption on

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the surfaces of coal under various conditions.35 The effects of the properties of a specific protein

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on its adsorption to metal surfaces in the presence of external electric potential was investigated.36

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Although the mechanism of the water adsorption on carbon, graphene, and coals have been studied 3 ACS Paragon Plus Environment

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for many years,11-13, 18 the effects of oscillating electrical fields on water adsorption is still not

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completely understood. And there is no report on oscillating electric fields effects on adsorption of

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gas-water system on clays.

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In this work, we used molecular dynamics (MD) simulations to investigate the effects of

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oscillating electric fields on methane-water sorption and diffusion in clay nanopores. Since

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kaolinite is one of the main clay minerals in shale and kaolinite interacts strongly with water

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through hydrogen bonds, kaolinite was selected as the clay nanopores framework in this study. We

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assumed that the nanopores surface of clay is completely hydrophilic and selected the kaolinite (0

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0 1) surface as the nanopores surface. We mainly analyzed the density distribution and diffusion

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characteristic of the mixed fluid consisting of water and methane by changing the electric fields

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frequency and intensity and water content. In order to thoroughly understand the relationship

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between clay minerals and methane or water, the hydrogen bonds, interaction energy, and radial

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distribution functions between the components were carefully examined in this study.

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Methodology

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All MD simulations are carried out with LAMMPS (Large-scale Atomic/Molecular Massively

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Parallel Simulator) code.37 The system simulated consists of kaolinite, water, and methane. The

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water molecule uses the SPC/E model, and SHAKE algorithm was applied to the water molecules

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to constrain the O-H bond length and the H-O-H bond angle. The length and the angle were

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calculated to be 0.100 nm and 10956, respectively, as shown in Figure 1a. An all-atom methane

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model is used; the bond length and the bond angle of methane were calculated to be 1.090 nm and

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10947, respectively, as shown in Figure 1b. Kaolinite is a 1:1 clay mineral. The dioctahedral

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layers are composed of two sheets connected by common oxygen atoms and include a tetrahedral

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sheet consisting of SiO4 tetrahedra sharing corners and an octahedral sheet consisting of AlO6 4 ACS Paragon Plus Environment

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octahedra sharing edges. The clay model is based on the kaolinite unite cell structure

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(Si4Al4O10(OH)8). The structure of kaolinite used here was obtained from Bish et al.38 The space

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group symmetry C1 presents the following kaolinite lattice parameters: a = 0.515  nm, b = 0.893

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 nm, c = 0.738  nm, α = 91.93°, β = 105.04°, and γ = 89.79°. The kaolinite structure model is shown

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in Figure 1c. Our simulation box consists of two double tetrahedral-octahedral (TO) layers, and

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each double TO layer contains 40 kaolinite unit cells (5 × 4 × 2 supercell) with the dimensions of

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2.575 nm, 3.572 nm, and 1.404 nm along the x, y, and z directions, respectively, as shown in Figure

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1d. In our simulations, all atoms of the TO layers remain fixed. The initial configuration for

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hydrated kaolinite is achieved by introducing a certain number of water molecules into the

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simulation box. In this study, the number of water molecules for simulations are 80, 160 and 240,

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corresponding to 3.2 wt%, 6.4 wt% and 9.6 wt% water content, respectively. All molecules were

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placed in a void space of the periodic simulation box which is large enough to accommodate the

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kaolinite (0 0 1) surface and water.

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Figure 1. Snapshot of (a) water, (b) methane, (c) kaolinite and (d) simulation model with the color

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scheme: O, red; Al, pink; H, white; Si, yellow; metahne and water, grey and blue.

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The dynamic properties of methane and water on the kaolinite (0 0 1) surface under the effects

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of oscillating electric fields are studied by non-equilibrium molecular dynamics (NEMD). After 5 ACS Paragon Plus Environment

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the equilibrium simulation, an oscillating electric fields parallel to the (0 0 1) surface is applied

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across the entire space of simulation box. The applied oscillating electric fields are expressed as

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follows:

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(1)

𝐸(𝑡) = 𝐸𝑚𝑎𝑥𝑠𝑖𝑛 (𝜔𝑡)

5

where ω=2πƒ indicates oscillation frequency, and Emax is the amplitude of the oscillating electric

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fields. Herein, the effects of Emax (0.05 V/Å, 0.10 V/Å, 0.15 V/Å, 0.20 V/Å, and 0.25 V/Å) and

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oscillating frequency ƒ (10.0 GHz, 25.0 GHz, 33.3 GHz, 50.0 GHz, and 100 GHz, which is

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corresponding to vibration periods T = 100 ps, 40 ps, 30 ps, 20 ps, and 10 ps, respectively) were

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explored.

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The CLAYFF force field39 and OPLS all atom (OPLS-AA) force field40 are applied to describe

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kaolinite and methane molecules, respectively. The non-bond interactions among all atoms in the

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system are described by:41

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𝑈(𝑟𝑖𝑗) = 𝑈𝐿𝐽 + 𝑈𝐶 = 4𝜀𝑖𝑗

𝜎𝑖𝑗 12

𝜎𝑖𝑗 6

𝑟𝑖𝑗

𝑟𝑖𝑗

[( ) ― ( ) ] +

𝑞𝑖𝑞𝑗 4𝜋𝜀0𝑟𝑖𝑗

+ 𝑞𝑖𝒓𝑖 ∙ 𝑬,

(2)

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where rij is the distance between the centers of i and j atoms, qi and qj are the partial charges of the

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atoms i and j, respectively, and ε0 is the dielectric permittivity of vacuum. The vector ri is the

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dislocation of the charged atom, and E is the external electric fields. The parameter εij controls the

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strength of the short-range interactions, and the LJ diameter σij is used to set the length scale. The

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LJ parameters σij and εij are deduced from the conventional Lorentz−Berthelot combining rules: 𝜎𝑖 + 𝜎𝑗

19 20 21

(3) 𝜎𝑖𝑗 = 2 , (4) 𝜀𝑖𝑗 = 𝜀𝑖𝜀𝑗. The parameters used in this study were listed in Table 1. The cutoff of nonbonded interaction is

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set to be 10.0 Å. Long-range electrostatic interaction was calculated using the particle-particle

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particle-mesh (PPPM) summation algorithm with a convergence parameter of 10-4. 6 ACS Paragon Plus Environment

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The MD simulations based on velocity Verlet algorithm in the canonical (NVT) ensemble were

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used with a time step of 1.0 fs, while the temperature was controlled by the Nose-Hoover thermostat

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(relaxation time 0.1 ps) at 333.15 K. The trajectories of 2.0 ns were collected for data sampling and

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analysis for all the cases without electric fields applying after 2.0 ns of equilibrium simulation. As

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charged particles are affected by the oscillating electric fields continuously, the system needs a

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relatively longer time to reach steady state, and thus, 5 ns NEMD simulation was used to stabilize

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the system, and then the final 4 ns was used for sampling and analysis of the non-equilibrium data.

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Table 1. Lennard-Jones parameters and atomic charge. atom

ε (kcal/mol) σ (Å)

q (e)

C

0.066

3.50

-0.240

H

0.03

2.50

0.060

O

0.1553

3.1660 -0.8476

H

0

0

Methane

Water

0.4238

Kaolinite

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Al

1.3297×10-6 4.2713 1.575

Si

1.8402×10-6 3.3020 2.100

Ob

0.1554

3.1655 -1.050

Oh

0.1554

3.1655 -0.950

H

0

0

0.425

Ob: bridging oxygen; Oh: hydroxyl oxygen.


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Results and discussion

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3.1 Effects of water content on methane adsorption

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The effects of water content on methane adsorption is discussed before electrical field is introduced

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into the system. The densities of methane and water molecules distributed in a 3 nm pore with

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different water content at 333.15 K and 25 MPa, which is collected in bins of 0.2 Å widths, are

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shown in Figure 2. It is clear that the first adsorption peak of the methane molecules can be found

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at z = 2.41 Å from the pore walls without preloaded water because of the interaction between

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methane and kaolinite. There is also clearly a second adsorption layer forming at z = 6.27 Å from

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the pore walls, as shown in Figure 2a. And 60.58 % of the methane molecules adsorbed on the

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kaolinite (0 0 1) surface with 0 wt% water content. The kaolinite (0 0 1) surface is formed from

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hydroxyl groups which allows the formation of hydrogen bonds with the polar water molecules.

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The interaction between kaolinite and water due to the hydrogen bonds is much stronger than the

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interaction between kaolinite and methane. Thus water is more competitive to adsorb on the

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surfaces of kaolinite. When water presents in the nanopores, water molecules preferentially form

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an adsorption layer, preventing methane adsorption onto the surface. As a result, methane

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adsorption is greatly reduced in clay nanopores, as shown in Figure 2b, 2c and 2d. An extremely

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strong peak of water density distribution curve indicating the water adsorption layer is formed at z

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= 1.34 Å from the surface of the walls, which is more close to the surface than methane. We can

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also observe that the density of the first adsorption layer of methane drastically decreases with

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water content, but the density of the second adsorption layer of methane only slightly decreases.

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The proportion of methane molecules adsorbed on the surface of kaolinite decreases from 60.58 %

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to 52.60 %, 46.52 % and 39.87 % when water content increases from 0.0 wt% to 3.2 wt%, 6.4 wt%

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and 9.6 wt%, respectively. Thus, due to stronger interaction between water and the kaolinite 8 ACS Paragon Plus Environment

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surfaces, large amounts of methane molecules present as bulk phase and occupy the void fraction

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of the nanopore, rather than adsorbed onto the surfaces. Also, the adsorption layer of methane is

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gradually moving away from the surface of kaolinite with the increase of water content due to more

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water molecules adsorbed on the surfaces of kaolinite. The position of the first layer of methane is

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2.35 Å, 2.46 Å, 2.53 Å, and 2.54 Å when water content is 0 wt%, 3.2 wt %, 6.4 wt %, and 9.6 wt

6

%, respectively. Actually, the first adsorption layer of the methane nearly disappears when water

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content is 9.6 wt%.

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Figure 2. Configuration snapshots (left) and density distributions of H2O and CH4 along the z-

11

direction (right) with different water content: (a) 0 wt%; (b) 3.2 wt%; (c) 6.4 wt%; (d) 9.6 wt%

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with the color scheme: Clay framework, bule; Water, red; Methane, grey.

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There are two kinds of hydrogen bonds formed between hydroxyl groups on the surface and

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water molecules. One of them was formed between the water proton and the oxygen atom of one

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hydroxyl, whereas another one was formed between the water oxygen atom and the surface 9 ACS Paragon Plus Environment

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protons.23 The radial distribution function (RDF) g(r) of atom B with respect of atom A , which is

2

defined as follows:42 𝑑𝑁

3

𝑔𝐴𝐵(r) = 4𝜋𝜌

,

4

is used to describe the distributions of the two forms of hydrogen bonds by calculating atomic

5

density which varies as a function of the distance from one particular atom. Where dN is the number

6

of atom B within a distance of r to 𝑟 + 𝑑𝑟 away from the reference atom A, and 𝜌𝐵 is the density

7

of atom B. From the RDFs, it is possible to acquire information of the density by computing the

8

absolute value of the RDFs, while its shape reflects the structure and interaction force, which can

9

reflect the strength of the intermolecular interaction force.

2 𝐵𝑟 𝑑𝑟

(5)

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The RDFs between hydrogen of hydroxyl on the surface of kaolinite and oxygen of water (HSur-

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OW) and oxygen of hydroxyl on the surface of kaolinite and hydrogen of water (OSur-HW) are shown

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in Figure 3. Considering the cutoff of nonbonded interaction (10.0 Å) and the width of the pore

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(half of the width is 15.0 Å), the RDFs were shown in the range of 12.5 Å. The first peaks of the

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two RDFs represent the two types of hydrogen bonds formed between hydroxyl and water

15

molecules. And the other peaks describe the distribution of hydrogen or oxygen atom of water

16

around the oxygen or hydrogen atom of hydroxyl, as shown in Figure 3a and Figure 3b. The first

17

peak of HSur-OW RDF is relatively stronger than the first peak of OSur-HW RDF. The reason is that

18

it is more difficult to form an OSur-HW hydrogen bond due to the position of oxygen of hydroxyl is

19

lower than the hydrogen, as shown in Figure 3. With the increase of water content, more water

20

molecules adsorbed on the surface of kaolinite. Thus, the first peaks of both RDFs decrease with

21

the increase of water content.

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Figure 3. Radial distribution functions (RDFs) between water and atoms in kaolinite with different

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water content at 333.15 K. (a) Osur-Hw; (b) HSur-OW.

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The effects of water on the adsorption of methane can be further investigated by analyzing the

5

interaction energy between methane or water and kaolinite. It is noted that the interaction energy

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is the summation of hydrogen bond energy and Van der Waals energy, as well as electrostatic

7

energy.42, 43 The stability of adsorbate on the surface can be represented by the interaction energy.

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Negative values mean that the adsorbate can be adsorbed on the surface.44 The interaction energy

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can be calculated as,24, 42, 45

10 11

𝐸𝑖𝑛𝑡𝑒𝑟𝑎𝑐𝑡𝑖𝑜𝑛 = 𝐸𝐴𝐵 ―(𝐸𝐴 + 𝐸𝐵),

(6)

where EAB is the energy of A B complex, EA and EB are the energies of A and B isolate.

12

From Figure 4, we can see that the interaction energy between methane and kaolinite is much

13

higher than that between water and kaolinite, indicating that interaction between water and

14

kaolinite is stronger and water is difficult to desorb from the surface of kaolinite. On the other hand,

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the interaction energy between methane and kaolinite become less negative when the water content

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increases from 0 wt% to 9.6 wt%, indicating that the presence of water reduces the interaction

17

between methane and kaolinite. Thus, the reduction in sorption capacity of methane in the presence 11 ACS Paragon Plus Environment

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of water could therefore be explained by the fact that water presence reduces the interaction

2

between the methane and kaolinite. It is consistent with the previous conclusion that the first peak

3

of the methane adsorption layer moves away from the surface of kaolinite with the increase of

4

water content. And the methane molecules would desorb from the surface of kaolinite with the

5

increase of water content. Moreover, thicker adsorbed water layer with higher water content makes

6

the interaction energy between water and kaolinite increases.

7 8

Figure 4. The interaction energy of CH4-kaolinite and H2O-kaolinite at 333.15 K without electric

9

fields applying.

10

To explore the transport properties of the methane-water system in the kaolinite nanopore, the

11

self-diffusion coefficients of methane and water were calculated from the time limit of its mean-

12

square displacement (MSD). The slope of the mean-squared displacement versus time is

13

proportional to the diffusion coefficient of the diffusing molecules. The self-diffusion coefficient

14

of water and methane was obtained as the limit of mean-square displacement using the Einstein

15

relation:33

16

D=

〈|𝑟(𝑡0 + 𝑡) ― 𝑟(𝑡0)| 1 lim 𝑡 4 𝑡 →∞

〉 .

2

(7)


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Figure 5 shows the effects of water content on self-diffusion coefficients of methane and water.

2

The diffusion coefficient of water increases with the increase of water content. Because more

3

methane molecules are desorbed from the surface of kaolinite and present as bulk phase when more

4

water molecules are preloaded in it, the diffusion coefficient of methane increases with the increase

5

of water content.

6 7

Figure 5. Self-diffusion coefficients of CH4 and H2O at 333.15 K and 25MPa without applying

8

electric fields.

9

3.2 Effects of the frequency of oscillating electric field on adsorption

10

To investigate the effects of the frequency of oscillating electric fields on adsorption of methane-

11

water on kaolinite surface, we present the density distribution of methane and water in Figure 6. It

12

is obvious that the density of the adsorption layer of water decreases, and the adsorption layer of

13

water thickens due to the increased frequency. With the frequency of 10 GHz, 33 GHz, 50 GHz,

14

and 100 GHz, the peak of the water density decreases from 2.296 g/cm3 to 1.764 g/cm3, 1.526

15

g/cm3, 1.452 g/cm3, and 1.282 g/cm3 in kaolinite pore, respectively, and the thickness of water

16

adsorption layer increases from 4.60 Å to 4.80 Å, 5.59 Å, 6.20 Å, and 8.00 Å, respectively, as

17

shown in Figure 6a. Although the methane molecules, which is non-polar, cannot be affected

18

directly by the electric fields, the distribution of methane is affected by collisions from water 13 ACS Paragon Plus Environment

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molecules, of which rotations and oscillations occur due to the oscillated electric fields, in the

2

methane-water system. Another factor affecting methane adsorption is that parts of the adsorption

3

sites of methane were occupied by desorbed water molecules. Thus, the influence of oscillating

4

electric field on the adsorption of methane is exerted through the effect of electric field on water.

5

The results of methane show that both of the density profiles of second adsorption layer and bulk

6

phase are affected by introducing oscillating electric fields, although the density profile of first

7

adsorption layer of methane is little affected, as shown in Figure 6b. With the increase of frequency

8

from 0 GHz to 50 GHz, the second peak of adsorbed phase density of methane decreases from

9

0.172 g/cm3 to 0.129 g/cm3, and the peak of second adsorbed phase density of methane disappears

10

with frequency increasing to 100 GHz. Furthermore, the average values of bulk density of methane

11

were up to 0.109 g/cm3, 0.117 g/cm3, 0.121 g/cm3 and 0.127 g/cm3 from 0.102 g/cm3 with the

12

corresponding frequency, respectively. The proportion of adsorbed methane molecules decreases

13

from 43.27 % to 41.58 %, 37.00 %, 34.75 % and 7.12 % when frequency increases from 0 GHz to

14

10 GHz, 33 GHz, 50 GHz and 100 GHz, respectively. And the second adsorption layer of methane

15

is gradually moving away from the surface with the increase of frequency. The position of the

16

second peak of methane is 5.35 Å, 5.53 Å, 5.70 Å and 5.88 Å when frequency of oscillating electric

17

fields is 0 GHz, 10 GHz, 33 GHz, 50 GHz and 100 GHz, respectively.


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1 2

Figure 6. Density distribution of (a) water and (b) methane for 6.4 wt% water content with different

3

frequency at 333.15 K and 0.25 V/Å.

4

We can intuitively understand the specific changes in the density of methane from Figure 7.

5

These results clearly substantiate that the distribution of methane changes remarkably in the range

6

of oscillating electric fields applied. From Figure 7b, we can judge that part of the adsorbed

7

methane on the surface of kaolinite was desorbed from the surface to the center of the pore when

8

10 GHz electric field was introduced. With the increase of the frequency, more adsorbed methane

9

was desorbed from the surface of kaolinite. When the frequency increases to 100 GHz (Figure 7f),

10

the density of the adsorption layer is much lower than that of bulk phase. It is consistent with the

11

discussion above. It is easy to find that the adsorption behaviors of methane-water can be greatly

12

influenced by the applied electric fields. Expressly, the oscillating electric fields can improve the

13

desorption of methane on kaolinite surface in the presence of water.

14


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Page 16 of 36

1 2

Figure 7. Density distributions of methane on x-y plane in the kaolinite pore for 6.4 wt% water

3

content with different frequency at 333.15 K and 0.25V/Å: (a) 0 GHz; (b) 10 GHz; (c) 25 GHz; (d)

4

33 GHz; (e) 50 GHz; (f) 100 GHz.

5

Water molecule has a highly polarized molecule structure so that the behavior of water molecule

6

will be greatly influenced by the applied oscillating electric fields. The electric fields were applied

7

along the y direction of the simulation boxes; thus, the average dipole moment along the y direction,

8

μy, was calculated to show the degree of molecular alignment with the electric fields. μy was

9

calculated by 1

10

𝑁

𝜇𝑦 = 𝑁∑𝑖 = 1𝜇𝑦,𝑖,

(8)

11

where N is the total number of water molecule. μy,i represents the y-component of the individual

12

dipole moment of each molecule. The results in Figure 8 for μy of water in oscillating electric fields

13

with Emax = 0.25 V/Å show that water molecules rotate following the oscillating electric fields

14

according to the frequency of applied electric fields, leading to dipolar alignment with the fields.

15

There is a broadly slight decrease of peak alignment of water molecules with the increase of

16

frequency of electric fields. 16 ACS Paragon Plus Environment

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1 2

Figure 8. y-component of average dipole moment for 6.4 wt% water content in 0.25 V/Å fields.

3

To discuss water molecules aligned with the applied electric fields in detail, the percentage of

4

dipole alignment was calculated to infer the effectiveness of molecular reorientation with electric

5

fields:46 〈𝑚𝑎𝑥|𝜇𝑦|〉 . 𝑁〈𝜇0,𝑦〉

6

α=

7

This parameter defines the proportion of all dipoles aligned with the applied electric fields. The

8

percentage of dipole alignment with electric fields for different frequency were shown in Figure 9.

9

The results show that higher frequency leads to lower alignments. But more water molecules

10

(9)

desorbed from the surface of kaolinite by higher frequency electric fields.

11 12

Figure 9. Percentage of dipole alignment for 6.4 wt% water content in 0.25 V/Å fields. 17 ACS Paragon Plus Environment

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Page 18 of 36

1

We can discuss water desorption mechanism enhanced by oscillating electric fields using the

2

number of hydrogen bonds. As mentioned above, water molecules can form hydrogen bonds with

3

hydroxyl groups on the surface of kaolinite, and it is the main reason why water has a stronger

4

interaction with kaolinite compared with methane. Nevertheless, the reorientation of water

5

molecules following the oscillating electric fields should affect hydrogen bonding, which is

6

strongly dependent on water molecular orientation. Thus, the variation in the total number of

7

hydrogen bonds upon application of oscillating electric fields were calculated (Figure 10). The

8

results in Figure 10a show that the number of hydrogen bonds decreases remarkably with the

9

increase of frequency. Figure 10b shows the proportion of broken hydrogen bonds due to electrical

10

fields. It is therefore concluded that hydrogen bonds are more prone to electric fields with higher

11

frequency. More than 40 % of hydrogen bonds are destroyed by 100 GHz electric fields. That is

12

why more water molecules were desorbed from kaolinite surface by higher frequency electric fields.

13

But the change speed of hydrogen bonds is smoother in higher frequency.

14 15

Figure 10. (a) The number of hydrogen bonds and (b) The percentage changes in the total number

16

of hydrogen bonds for 6.4 wt% water content in 0.25 V/Å fields.

17


18

OW) and oxygen of hydroxyl on the surface of kaolinite and hydrogen of water (OSur-HW) with 18 ACS Paragon Plus Environment

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1

different frequency at 333.15 K are shown in Figure 11. It is clearly that the intensity of the first

2

peak of RDFs decreases with the increase of frequency of applied electric fields, but the position

3

of the peak remains unchanged, for both HSur-OW and OSur-HW interactions. That means the higher

4

frequency electric fields can lead to a bigger decrease in the number of hydrogen bonds. It is

5

consistent with the conclusion above.

6 7

Figure 11. The radial distribution function (RDFs) between (a) OSur and HW, (b) HSur and OW with

8

different frequency at 333.15 K and 0.25 V/Å.

9

The effects of frequency of oscillating electric field on adsorbed water and methane on the

10

surface of kaolinite can be better investigated by analyzing the interaction energy between water

11

or methane and kaolinite, as shown in Figure 12. These results clearly show that the interaction

12

energy between water and kaolinite increases with the increase of frequency of electrical fields. It

13

means that the interaction between water and kaolinite becomes weaker when higher frequency

14

electrical fields are introduced and more water molecules would desorb from the surface, which is

15

consistent with the discussion above. The increase speed of interaction energy of water in the low

16

frequency region is faster than that in the high frequency, which indicates that the dominating

17

effects of frequency becomes less apparent at higher frequencies. On the other side, with the 19 ACS Paragon Plus Environment

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Page 20 of 36

1

decrease of water content, the interaction energy becomes more negative. The decreasing energies

2

of absolute value reflect weaker interactions between the kaolinite and water molecules with

3

increasing water content. We also observed that the interaction energy between methane and

4

kaolinite has little change with the increase of frequency compared with the interaction energy of

5

water and kaolinite due to nonpolar methane molecule.

6 7

Figure 12. Interaction energy of CH4-kaolinite, H2O-kaolinite with different frequency at 333.15

8

K and 0.25 V/Å.

9 10

Figure 13. Diffusion coefficients of water molecules with different water content and frequency at

11

333.15 K and 0.25 V/Å. 20 ACS Paragon Plus Environment

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1

Figure 13 shows the effects of frequency of electrical fields on self-diffusion coefficients of water

2

for different water content. It is obvious that the self-diffusion coefficients of water increase with

3

the increase of frequency of electric fields whatever water content is, which implies that the

4

mobility of molecules increases with the increase of electric fields frequency.47 It is interesting that

5

the self-diffusion coefficients of water with low water content becomes higher than that of high

6

water content when frequency exceeds about 28 GHz. It means that the mobility of water molecules

7

gets more improvement by high frequency electric fields in the case of low water content. The self-

8

diffusion coefficient is mainly determined by the space of movement and the density of water.

9

Broader movement space results in higher self-diffusion coefficient. But higher density of the fluids

10

results in lower mobility. At lower frequency, the thickness of adsorption layer increases and the

11

density of water decreases with the increase of water content, as shown in figure 14a. The self-

12

diffusion coefficient of water is mainly dominated by the thickness of the adsorption layer at lower

13

frequency, which makes the self-diffusion coefficients higher in higher water content case. But the

14

thickness of adsorption layer is almost same under the action of higher frequency oscillating

15

electric field for all water content cases, as shown in figure 14b. And the self-diffusion coefficient

16

of water is mainly dominated by the density of water. Thus, the self-diffusion coefficient decreases

17

with the increase of water content, which is similar to the tendency of water density in adsorption

18

layer.


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1 2

Figure 14. Density distribution of water with different water content in (a) 10GHz frequency and

3

(b) 100GHz electric field at 333.15 K.

4

3.3 Effects of the intensity of oscillating electric field on adsorption

5

The effects of the intensity of oscillating electric fields on adsorption of methane-water on

6

kaolinite surfaces were shown in the density distribution profiles of methane and water in Figure

7

15. The peak of the adsorbed phase density of water decreases from 2.296 g/cm3 with zero field to

8

2.077 g/cm3, 1.491 g/cm3, 1.379 g/cm3 and 1.282 g/cm3 when amplitude of 0.05 V/Å, 0.10 V/Å,

9

0.15 V/Å and 0.25 V/Å of electric fields were introduced, respectively. And the corresponding

10

thickness of water adsorption layer increases from 5.80 Å to 6.40 Å, 6.80 Å, 7.60 Å and 8.00 Å

11

when oscillating electric fields with Emax= 0.05 V/Å, 0.10 V/Å, 0.15 V/Å and 0.25 V/Å were

12

introduced, respectively, as shown in Figure 15a. It indicates that more water molecules gradually

13

dissociate from the surface of kaolinite with the increase of intensity. As a result, the density profile

14

of methane changes with the increase of intensity due to the interaction between water and methane

15

molecules. The second peak of adsorbed phase density of methane decrease from 0.172 g/cm3 to

16

0.126 g/cm3 when the amplitude of the electric filed increase from 0 V/Å to 0.10 V/Å, and the peak

17

of second adsorbed phase density of methane almost disappears when amplitude increase over 0.15 22 ACS Paragon Plus Environment

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1

V/Å. Furthermore, the average density of methane in bulk phase were up to 0.106 g/cm3, 0.119

2

g/cm3, 0.124 g/cm3 and 0.127 g/cm3 from 0.102 g/cm3 with the corresponding intensity,

3

respectively. We can also observe that the stronger the intensity, the smaller the increasement of

4

the desorption amount of methane.

5 6

Figure 15. Density distribution of (a) water and (b) methane with different amplitude at 333.15 K

7

and 100 GHz.

8

The effects of intensity of electric fields on changes of density of methane in the nanopore were

9

shown in Figure 16. When electric field of 0.05 V/Å was introduced into the system, a slight change

10

of methane distribution is observed. Part of adsorbed methane molecules desorb from the surface

11

and present in the bulk phase. When amplitude increases to 0.10 V/Å, the adsorbed layer of

12

methane almost disappears and a large increasement of density of methane is observed. With the

13

continuous increase of intensity of electric fields, a slight increasement of the density of bulk phase

14

in the center of the pore in detected. This is in agreement with the conclusions above.


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Page 24 of 36

1 2

Figure 16. Density distribution of methane in the pore with different intensity of electric fields

3

(333.15 K, 100 GHz): (a) 0 V/Å, (b) 0.05 V/Å, (c) 0.10 V/Å, (d) 0.15 V/Å, (e) 0.20 V/Å, (f) 0.25

4

V/Å.

5

Figure 17 shows the effects of intensity of electric fields on alignment with the field of 100 GHz.

6

The results show that water molecules rotate following the oscillating electric fields according to

7

the frequency of applied electric fields, and the peak of y-component of the average dipole moment

8

of water increases with the increase of intensity, meaning that stronger applied electric fields can

9

better overcome the interaction between water molecule and other molecules and lead to a better

10

alignment. The percentage of dipole alignment increases with the increase of intensity of electric

11

fields. Dipole alignment of water is more responsive when Emax is less than 0.10 V/Å. The

12

percentage of dipole alignment increases slightly when Emax is bigger than 0.10 V/Å.


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Energy & Fuels

1 2

Figure 17. (a) System y-component of the total dipole moment and (b) Percentage of dipole

3

alignment for 6.4 wt% water content in 100 GHz fields.

4

5 6

Figure 18. (a) The number of hydrogen bonds and (b) The percentage changes in the total number

7

of hydrogen bonds for 6.4 wt% water content in100 GHz fields.

8

The variation in the total number of hydrogen bonds is affected by different intensities of

9

oscillating electric fields, as shown in Figure 18a. The results show that the number of hydrogen

10

bonds decreases remarkably with the increase of intensity. Figure 18b shows the proportion of

11

hydrogen bonds destroyed by electrical fields. The number of destroyed hydrogen bonds is more


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Page 26 of 36

1

sensitive to the intensity at lower electric fields intensity. Only 10% more hydrogen bonds were

2

broken when Emax increases from 0.10 V/Å to 0.25 V/Å.

3


4

OW) and oxygen of hydroxyl on the surface of kaolinite and hydrogen of water (OSur-HW) with

5

different intensity at 333.15 K are shown in Figure 19. It is clearly that the intensity of the first

6

peak of RDFs decreases with the increase of intensity of applied electric fields, but the position of

7

the peak remains unchanged, for both HSur-OW and OSur-HW interactions. It indicates that the

8

density of water molecules on the surface of kaolinite decreases, and the interaction of kaolinite

9

and water declines with the increase of intensity of oscillating electric fields.

10 11

Figure 19. Radial distribution function (RDFs) between (a) OSur and HW, (b) HSur and OW with

12

different intensity at 333.15 K and 100 GHz.

13

The effects of intensity of oscillating electric-filed on the interaction energy between water or

14

methane and kaolinite are shown in Figure 20. The results clearly show that the intensity of

15

oscillating electric fields have minor effects on methane molecules due to methane molecule is

16

non-polar. And the interaction energy between water and kaolinite increases with the increase of

17

intensity of electric fields. It means that more adsorbed water desorbed from the surface of the 26 ACS Paragon Plus Environment

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Energy & Fuels

1

kaolinite with the increase of intensity of the fields. The intensity of oscillating electric fields has

2

a considerable effects on the interaction energy between water and kaolinite in the amplitude range

3

of 0.05-0.15 V/Å for all water content cases. And the dominating effects of intensity becomes less

4

apparent with the increase of the electric fields intensity in high intensity range. Finally, with the

5

decrease of water content, the interaction energy becomes more negative, and the reason have

6

explained in above section.

7 8

Figure 20. Effects of intensity on interaction energy between water, methane and kaolinite (333.15

9

K, 100 GHz).

10 11

Figure 21. Self-diffusion coefficients of water with different intensity at 333.15 K and 100 GHz.


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Page 28 of 36

1

The effects of intensity of electric fields on self-diffusion coefficients of water follow a complex

2

pattern. Electric fields with larger amplitude facilitates the self-diffusion of methane-water system,

3

regardless of the water content, implying that higher mobility of the liquid system will be aquired,47

4

as shown in Figure 21. Similar as the effects of frequency of fields, the self-diffusion coefficients

5

of water with low water content becomes higher than that of high water content when intensity is

6

high enough. The mechanisms of this phenomenon are similar as the effects of frequency on self-

7

diffusion coefficients. It means that the mobility of water molecules with low water content gets

8

more improvement by high frequency or intensity of electric fields. Thus, the effects of frequency

9

and intensity of electric fields on self-diffusion coefficients are related to water content in the

10

nanopore.

11

4

Conclusions

12

Molecular dynamic simulation was used to study the effects of oscillating electric fields on

13

adsorption of methane-water system on kaolinite surfaces. Introducing electric fields with different

14

frequencies and intensities, the density distribution and diffusion characteristics of the mixed fluid

15

consisting of water and methane with different water/methane ratio were analyzed. The

16

mechanisms of methane-water adsorption on kaolinite surface in oscillating electric fields were

17

explored by hydrogen bonds, interaction energy, and radial distribution functions. The main

18

conclusions are as follows.

19

1. methane-water adsorption on kaolinite surface is an important factor to evaluate gas-in-place

20

of shale gas and to find a new method to improve shale gas recovery due to the ubiquity of water

21

in shale gas reservoir. The mount of adsorbed methane decreases with the increase of water content

22

because of water molecules preferentially adsorb onto the surface of kaolinite.


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Energy & Fuels

1

2. Theoretically, introducing oscillating electric fields is an efficient method to enhance methane

2

desorption on kaolinite (0 0 1) surface in the presence of water. The influence of oscillating electric

3

field on the adsorption of methane is exerted through the effect of electric field on water

4

3. Water molecules were preferentially adsorbed onto oxygen and hydrogen atoms in kaolinite

5

by forming hydrogen bonds. The adsorption of methane on kaolinite decreases with the increase

6

of water content without electric fields applying.

7

4. The adsorption of water decreases, and the adsorption layer thickens by introducing oscillating

8

electric fields. The desorption of methane can be enhanced by the oscillating electric fields in the

9

presence of water. The adsorption of methane decreases with the increase of frequency or intensity

10

of the electric fields.

11

5. The applied oscillating fields reduces number of hydrogen bonds on the interface of kaolinite

12

due to water molecules re-orientates following the oscillating electric fields. Electric fields of

13

higher frequency or stronger intensity breaks more hydrogen bonds.

14

6. The interaction energy of H2O-kaolinite, self-diffusion coefficient of H2O increase with

15

frequency and intensity of the applied electric fields.

16

AUTHOR INFORMATION

17

Corresponding Author

18

*E-mail: [email protected].

19

ORCID

20

Yudou Wang: 0000-0002-2691-9969

21

Notes


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

The authors declare no competing financial interest.

2

Acknowledgements

3

This research was supported financially by the National Natural Science Foundation of China

4

(51574268), and the Shandong Province Natural Science Foundation (ZR2014EEM050).

5

Reference

6 7 8 9 10

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