Why the Gas Uptake Behavior of Dry Salt Water Is Vastly Different

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Why the Gas Uptake Behavior of Dry-Salt Water is Vastly Different above 279 K? A Dynamics-Controlled Process and Can be Intensified by Cooling Stimulation Method Jingpeng Hou, Wei Zhou, Xinrui Wang, and Dongsheng Bai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07612 • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 23, 2018

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The Journal of Physical Chemistry

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Why the Gas Uptake Behavior of Dry-Salt Water is Vastly Different

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above 279 K? A Dynamics-Controlled Process and Can be Intensified

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by Cooling stimulation Method

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Jingpeng Houa, Wei Zhoua, Xinrui Wanga, Dongsheng Baia*

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a

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Industry, Beijing Technology and Business University, Beijing 100048, PR China

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* [email protected]

Department of Chemistry, School of Science / Key Laboratory of Cosmetic, China National Light

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Abstract

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The effect of temperature on CO2 uptake behavior of dry-salt water was investigated by

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kinetics measurements and molecular dynamics simulations. The gas uptake capacity was found

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to be dramatically decreased at 279 K by direct cooling adsorption procedure. By means of

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simulations, it was realized that the orientation of water and the hydrogen-bond network structure

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near the silica surface are sensitive to temperature. With the temperature decreased, the water layer

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becomes local-structured by forming more hydrogen bonds, facilitating the capture of CO2 by

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forming hydrate precursor easily. The change of the structure is irreversible once the CO2 uptake

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process occurred, hence the gas uptake can carry on continuously even if the temperature rises. A

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cooling stimulation procedure was found to efficiently trigger gas uptake process at temperature

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higher than 279 K. By using this method, gas adsorption can be induced due to the energy barrier

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of nucleation is reduced. This technology can save a certain amount of energy used to keep the low

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temperature. Therefore, it might be a potential industrial method for efficient gas storage.

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1. Introduction

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The efficient gas storage and transport, such as energy gas storage, become more and more

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important in recent years since the consumption of traditional fossil energies is greatly accelerated

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by the industrialization process. Porous materials, such as activated carbon, silica, zeolite, metal

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organic frameworks (MOF), porous polymer networks (PPN), can be introduced to enhance the

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adsorption of gases.1-4 Recent years have witnessed an ever-growing interest in gas/water interfaces

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stabilized by particles because it contributes to new concepts and materials such as dry water.5 Dry

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water (DW) is a water-in-air inverse foam produced by mixing water with hydrophobic silica

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nanoparticles.6,7 Because it contains more than 95 wt% water, DW looks like a powder but exhibits

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a free-flowing property. In comparison to bulk water and liquid marbles,8 DW has a much higher

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surface-to-volume ratio. As such, the finely dispersed water droplets lead to greatly enhanced

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kinetics of adsorption in a gaseous system,9,10 resulting in increased rate of methane hydrate

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formation in comparison to bulk water.11-13 On the basis of efficient adsorptivity, thus, DW can be

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considered as a material for gas capture, storage and transport, such as CH4, CO2 and Kr.7,11,14 More

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importantly, compared with porous materials such as MOF, covalent organic frameworks (COF)

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and PPN, DW has the advantages of cheap raw materials (only water and silica), simple preparation

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process, complete desorption of gas after destruction of its structure, and low cost on re-preparation.

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Therefore, it is considered to be the most likely gas storage material for industrial use.

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At present, improving both the adsorption capacity and the formation rate of gas hydrate are

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still key issues for practical application of DW,15,16 and many methods have been intensively

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investigated to increase the formation rate of hydrate. Such as the use of vigorous mixing devices,

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the addition of promoting materials, and increasing the gas-water contact area by using finely

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ground ice particles.17,18 In addition, a mixed colloidal system made of hydrogel particles and DW

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particles has been reported with a high adsorption capacity.19 Recently, a dry-K2CO3-containing

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water prepared by surface modified mesoporous silica particles can enhance the CO2 capture in both

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adsorption rate and capacity.20 Moreover, cyclodextrin was also found to be a kinetic promoter for

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gas hydrate formation in both simulation21 and our previous experimental study.22 We should note

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that although progressive advances made so far, gas storage by these methods is still hard to be

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applied in industrial fields.

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In energy industry field, the practical application of DW is constrained to both raw materials


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supply and energy saving.23 Since dry water is a free-flowing powder composed of water droplets,

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numerous water is needed as raw material. If DWs can be prepared by mixing seawater (or non-

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purified water containing electrolytes similar to seawater) with hydrophobic silica nanoparticles and

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show an uptake capacity similar to that prepared by pure water, lots of cost on water purification

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will be saved on the industrial scale. Furthermore, the effect of temperature on CH4 and CO2

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adsorption kinetics has been reported,11,22 and only the temperature range from 273 K to 277 K was

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considered to be optimal, which means a lot of energy will be consumed to keep the low temperature

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during the whole gas adsorption period.24 If DWs have an acceptable uptake capacity when it works

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at higher temperature, a large amount of energy consumption will be reduced in industrial field. As

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far as we know, almost no related study concerning with these problems has been reported.

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In this work, experiments have been carried out to investigate the CO2 uptake kinetics of DW

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under different conditions. Considering that seawater in which the total amount of Na+ and Cl- is

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~3wt%25 are abundant in earth, we prepared DWs by using sodium chloride solution (3 wt%) instead

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of pure water. The DWs prepared with NaCl solutions are named dry-salt water (DSW). We found

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that the gas uptake capacity is dramatically decreased to 0 when temperature is higher than 279 K.

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Molecular dynamics (MD) simulations were performed to illustrate the microscopic mechanism.

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Moreover, based on the results of temperature-variation simulations, a cooling stimulation method

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was suggested for gas uptake by DSW, so that the gas adsorption can be easily induced at

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temperature higher than 279 K, saving a certain amount of energy. It provides a new insight for the

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industrial application of DW in gas storage.

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2. Experimental Section

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2.1. Materials and Synthesis

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SIPERNAT D10, hydrophobic silica particles with the static contact angle of 119°, were

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supplied by EVONIK (Germany). Pure carbon dioxide gas (99.99%) was supplied by Beijing

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Yanglilai Chemical Gas Co., Ltd. (China), and sodium chloride (AR, 99.5%) was purchased from

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Sinopharm Chemical Reagent Co., Ltd. (China).

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The sodium chloride solution with concentration of 3 wt% was prepared firstly, and DSW was

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prepared by mixing 5 g of hydrophobic D10 silica powders with 95 g of sodium chloride solution

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at ~19000 rpm for 90 s in a domestic blender.



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2.2. Kinetics Measurement

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The prepared DSW of 25.0 g was loaded into a miniature high-pressure reactor (SLM-200, 300

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mL, China), which is connected to a circulator bath (DTY-10B, China) for temperature controlling.

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When DSW sample was cooled to a specified temperature, the reactor was pressurized to 3.5 MPa

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with pure CO2 and sealed. Gas uptake kinetics in DSW was studied by observing the time evolution

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of gas pressure. The temperature and pressure were recorded every 15 minutes until the adsorption

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equilibrium is reached. To provide a qualitative assessment of the kinetics, we measured the CO2

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uptake kinetics in DSW under different temperatures.

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3. Model and Simulation Method

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To understand the microscopic mechanism of gas uptake, we performed molecular dynamics

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(MD) simulations by using LAMMPS.26 As is shown in Figure 1, the initial configuration contains

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a NaCl solution/silica/carbon dioxide three-phase system, which was placed into a simulation box

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with 20 × 8 3 × 16 nm3. In our simulations, we represent a DSW particle by a layer of

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hydrophobic silica spheres with a certain amount of water and NaCl molecules, because DSW

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particle is far greater than the scale of the classic MD simulations. The silica layers were prepared

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by packing the amorphous silica spheres with diameter of 4 nm to a face-centered cubic motif

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(Figure 1), whereas the dangling bonds on the surfaces of each silica sphere were saturated by –CH3

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groups, showing hydrophobic property. In the initial configuration, 62800 water molecules and 600

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Na+ and Cl- ions were added to the central part of the box to keep the 3 wt% salt solution has a

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density of 0.997 g/cm3, and 2450 CO2 molecules were also added to the left side of the silica layer

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and the right side of the salt solution to reach the pressure of 5 MPa (slightly higher than the

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experiments). In this way, we can investigate gas uptake process at two kinds of interfaces at the

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same time in same conditions: DSW interface and gas-liquid interface. We note that the direct gas-

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liquid phase interface at the right side is just designed to compare the difference between DSW

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interface and free liquid water interface. For a DSW particle, CO2 can only enter the aqueous water

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core through silica pores, and there is no direct water-gas interface in perfect structure of DSW.


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Figure 1. Initial configuration of the simulation system at the projection on (a) xy plane and (b) yz plane. The

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silica, water and CO2 molecules are represented by the stick models, NaCl ions are represented by purple and

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green spheres, whereas hydrogen bonds are denoted by blue-dashed lines. Note that in panel (b), the water, CO2

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and NaCl molecules are not shown for clarity.

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In our simulations, the TIP4P/2005 water model27 in which the rigidity was restricted with

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SHAKE algorithm28 was used. CO2 molecules were represented by the EPM2 model.29 The

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hydroxylated silica model30 was adopted for the inner core of the silica spheres, whereas the –CH3

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groups on the surface were represented by a single-point model.31 The potential parameters were

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taken from ref. 32 to represent Na+ and Cl-. The unlike parameters of Lennard-Jones interactions

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were obtained by the Lorentz-Berthelot mixing rule. The short-range interactions were cut at 12 Å,

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whereas the long-range Coulomb interactions were evaluated using the PPPM algorithm.33 Periodic

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boundary conditions were imposed in all three Cartesian directions.

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A whole simulation run includes two steps: An NVT relaxation of 1 ns was performed at a

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specified temperature of 260 K, 265 K, 270 K, 275 K, and 280 K, respectively, to eliminate the

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effect of the initial configuration; then, a simulation at the same temperature with 500 ns was

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performed with a time step of 2 fs to study the gas uptake mechanism. The temperature was

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maintained by using the Nosé-Hoover algorithm,34-36 with a relaxation parameter of 0.2 ps. During

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the simulations, the position of the silica spheres was fixed.

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4. Results and Discussion

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Like DW, the prepared DSW shows a good dispersibility, stability and flowability. Since there

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is a certain amount of salt in the water core, the structure and the hydrogen-bond network of the



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inner water will be different from pure water in low temperature, which may change the mechanism

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and kinetics of gas uptake.

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4.1. Uptake Kinetics of DSW at Constant Temperature

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Many studies have shown that the gas uptake capacity is affected by temperature.11,22,37-39 Take

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methane as an example, owing to the hydrate formation, the optimal temperature is about 273~277

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K, in which it shows the largest uptake capacity.11 Similar to methane, carbon dioxide is also an

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insoluble gas that can form hydrate too. We investigated the temperature effect for CO2 uptake in

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DSW, and the gas uptake kinetics is shown in Figure 2. In this paper, the gas uptake capacity of

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DSW is defined volumetrically, i.e. the volume of adsorbed gas in standard condition divided by

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the volume of 25.0 g DSW we used (40 cm3), V/V. Considering the deviation from ideality, the

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molar number of adsorbed gas was calculated first by using the Peng-Robinson equation of state

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(EOS) according to the current experimental pressure and temperature,40 and then the volume of gas

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in standard condition was converted by ideal gas EOS. We note that the compressibility factor of

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CO2 in our experimental conditions (273~288 K, 3.0~3.5 MPa) is between 0.68 and 0.79 calculated

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by Peng-Robinson EOS. One can see clearly that the optimal temperature (with the maximum

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uptake capacity) is ~277 K in our experimental system, consistent with that in pure DW.22 The rapid

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uptake at the initial stage at low temperature such as 273 K might be attributed to the hydrate

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formation.

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Figure 2. CO2 uptake kinetics in DSW at different temperatures. The initial pressure of CO2 in the reactor is 3.5

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MPa.


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Surprisingly, when temperature is higher than 279 K, without warning, the gas uptake capacity

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of DSW is dramatically decreased to 0. And we found that at 279 K, experiments under the same

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conditions show completely different results: in most cases (about 75%) there is no adsorption in a

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long time, and in a small part of cases (25%) there is a strong adsorption (~25 V/V), as marked in

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the green band in Figure 2. We speculate that the CO2 adsorption in DSW is a dynamics-controlled

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process at the moderate temperature of 279 K. The energy barrier restricts the adsorption process,

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and the induction time for gas uptake probably increased obviously due to the uncertainty of

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nucleation. By the aid of MD simulations, we explored the source of energy barrier in details.

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4.2. Nucleation Mechanism for Gas Uptake

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To illustrate the microscopic mechanism of gas uptake near the DSW interface, we performed

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a series of MD simulations at 260 K, 265 K, 270 K, 275 K, and 280 K, respectively. We note that

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the temperature we used does not match the experimental temperature, because the force field of

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TIP4P/2005 represents a lower melting point of 252.1 K for water molecules.27 In addition, one

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should note that during CO2 uptake process, there are two aspects that might need to be taken into

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account to quantify the total adsorbed CO2: the uptake of CO2 by water based on the Henry constant,

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and the acid-base reaction of carbon dioxide in water. Classical MD simulation cannot account for

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the second one. Since the ionization equilibrium constants of carbonate and bicarbonate are so small

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in the experimental conditions (~10-7 and ~10-11, respectively), the amount of ions formed during

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CO2 uptake can be negligible, and it is acceptable to study the microscopic mechanism of CO2

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uptake by using classical MD.



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Figure 3. Density profile of CO2 at the end of the simulation time.

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The density profiles of CO2 at the end of 500 ns are shown in Figure 3. One can see clearly

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that at lower temperature, more CO2 molecules entered into the deep inside of the DSW. However,

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at 275 K and 280 K, a large amount of CO2 is accumulated only near the silica particles to decrease

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its surface free energy due to its hydrophobic nature, and is unable to enter the inner core of DSW.

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We define the area of x range from -5 nm to -1 nm as region A (as is marked in Figure 3), and the

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gas uptake dynamics in region A (i.e. the CO2 molecules get into region A from the left gas phase

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by passing through the gap between silica spheres) is shown in Figure 4. At the initial stage, the

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adsorption rate of each system is very similar. After ~50 ns, the low temperature systems can still

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adsorb CO2 molecules, while the adsorption rate of the high temperature systems decreased

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obviously until the equilibrium is reached at ~200 ns.


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Figure 4. Time evolution of the number of CO2 adsorbed in region A.

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We think that the structure of water layers near the silica interface will be changed at different

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temperatures, which can affect the mass transfer of CO2. Therefore, we calculated the orientation

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distribution of water molecules in the first hydration layer of silica particles (i.e. water molecules

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within 3.7 Å from the surface of silica, obtained from RDF curve shown in Figure 5). The orientation

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of water molecule is defined as the angle between the water dipole and the connection between

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centroid of silica and oxygen atom of the water, and the results are shown in Figure 5(a). At 275 K

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and 280 K, the hydrogen atoms of water are more likely to point towards silica since the hydrophobic

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property of its surface. Hence, the hydrogen bond network between water molecules is difficult to

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establish, which can be seen from Figure 6. A hydrogen bond is identified when the distance of two

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oxygen atoms of water molecules (or the distance of a carbon atom of –CH3 groups on the silica

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surface and an oxygen atom of water) is within 3.5 Å and the H−O···O angle (or the H−C···O angle)

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is less than 30°.41 When temperature decreases, more hydrogen bonds formed between water

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molecules to reduce the energy of the system. Thus, the orientation of water relative to the silica

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particle becomes disordered. The establishment of hydrogen bond network of water at the interface

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area is a key factor to capture large amount of CO2: if the CO2 molecules close to the interface are

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surrounded by local-structured water molecules with hydrogen bonds, they are easier to be captured

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to form cage-like precursor structures of gas hydrates.42



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Figure 5. (a)Orientation distribution of water molecules in the first hydration layer of silica particles at the initial

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stage of the simulations. (b)Radial distribution function of silica sphere centroid-oxygen of water.

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Figure 6. Hydrogen bonds per water molecule in the first hydration layer of silica particles at the initial stage of

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the simulations.

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For the direct gas-liquid interface located at the right side of the simulation box, we calculated

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the orientation distribution of both water and CO2 molecules near the interface at different

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temperatures. Here the orientation of water molecule is defined as the angle between the water

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dipole and the direction of the normal vector of the interface pointing into liquid water, while the


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orientation of CO2 is defined as the C-O vector relative to the normal vector of the interface pointed

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into bulk CO2. The results are presented in Figure 7(a) and (b), which are very similar to the study

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by Singer et al.43 Different from the dipole distribution induced by local curvature at the silica

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interface, the dipole of water molecules is almost parallel to the gas-liquid interface. Note that for

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CO2 in bulk water phase, owing to the electrostatic interactions, a preferred configuration between

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CO2 and its neighbor water molecule is shown in Figure 7(c). However, at gas-liquid interface, the

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arrangement of water and CO2 is different from that in bulk system, which may prevent the capture

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of CO2 molecules.

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Figure 7. Orientation distribution of water and CO2 molecules in the gas-liquid interface at the final stage of the

225

simulations.

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To investigate the temperature sensitivity on the structure of the silica interface, we performed

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two additional simulations with temperature variation: (i) an NVT simulation of 200 ns at 275 K is

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followed by a 100 ns simulation at 270 K, then temperature rises back to 275 K for 400 ns; (ii) same

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as simulation (i) but the position of CO2 is fixed and the interactions between CO2 and other

230

molecules are set to zero during the first 350 ns (equivalent to no CO2). Then, at 350 ns, to avoid

231

the overlaps of atoms, a relaxing process of 10 ps with soft pairwise interaction was performed by

232

using the command “pair_style soft” in LAMMPS. The amount of CO2 adsorbed in region A and

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the orientation distribution of water molecules are shown in Figure 8. It can be seen that the dipole

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distribution of water induced by interface curvature is sensitive to temperature: the hydrophobic

235

effect is the main factor at high temperature while it is controlled by the hydrogen-bond network



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when temperature decreased. If there are no CO2 molecules, part of hydrogen bonds is broken, and

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water molecules return to the hydrophobic-controlled state when temperature rises. Once the CO2

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is captured at low temperature, it will be surrounded by water molecules, which will change the

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hydrogen-bond distribution of water. When temperature increases, the orientation distribution of the

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interfacial water cannot be reversibly returned back, so that the gas uptake can carry on continuously.

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Figure 8. Time evolution of the number of CO2 adsorbed in region A (a) and the orientation distribution of water

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molecules (b) in temperature-variation simulations. In panel (a), the hollow dots indicate that the temperature is

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275 K, and the solid dots indicate 270 K.

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4.3. Cooling Stimulation for Gas Uptake The temperature sensitivity of the interfacial properties (the dynamic barrier) in temperature-


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variation simulations provides us with a way to enhance gas capture. We consider that accelerating

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the nucleation through cooling stimulation may be a good method to eliminate the barrier. So, based

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on the constant temperature uptake experiment (scheme I in Figure 9), we designed the following

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experimental schemes. Firstly, same as scheme I, sample DSW of 25.0 g was cooled down to 279

252

K and kept at this temperature, and the reactor was then pressurized with pure CO2 to 3.5 MPa and

253

sealed. At this point, gas uptake is not occurring in most cases. Subsequently, the temperature is

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adjusted to 277 K (cooling stimulation) and remained in a period of time for the starting of CO2

255

adsorption. Finally, the temperature is adjusted back to 279 K for gas uptake. The modified process

256

is marked as scheme II.

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Figure 9. Experimental schemes used in our experiments.

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The gas uptake kinetics by the cooling stimulation method is shown in Figure 10(a). At the

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initial stage, almost no gas adsorption was observed (less than 1 V/V), showing the existence of the

261

barrier. When temperature decreased from 279 K to 277 K at 45 min, gas uptake occurs immediately.

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When the CO2 uptake reaches ~15 V/V, we adjust the temperature back to 279 K. The uptake

263

process of CO2 goes on continuously, and the final gas uptake capacity reached 25 V/V, same as



264

the capacity of the experiments carried out at the constant temperature of 279 K (Figure 2), although

265

the two processes are different.

266

267 268

Figure 10. CO2 uptake kinetics in DSW by cooling stimulation. In the upper panel of (a) and (b), the hollow dots

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indicate that the temperature is 279 K, and the solid dots indicate 277 K, which can also be seen in the lower panel.

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As the simulation results suggested, the cooling stimulation changed the local structure of

271

DSW, thus the adsorption process is initiated. In scheme II, if the adsorption quantity is high enough

272

(more than 25 V/V) during the stimulation period of 277 K, we found that the gas will be desorbed

273

from the DSW when temperature is back to 279 K. The result is shown in Figure 10(b). As we

274

expected, the desorption process reaches an equilibrium after the volumetric gas capacity decreased

275

to ~25 V/V. Therefore, we considered that 25 V/V is the saturated uptake capacity of DSW at 279

276

K, which is controlled by thermodynamics. If there is no cooling stimulation, however, the uptake


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277

process will be difficult to start because of the barrier controlled by dynamics. Besides, in several

278

cooling stimulation experiments, we found that there is an obvious induction time for the CO2

279

uptake when temperature decreased to 277 K, and the results are shown in Figure 11. This is the

280

same as the viewpoint of classical nucleation theory: macroscopic transition can take place only

281

when the critical nuclei are formed, which requires a certain amount of time to cross the energy

282

barrier.

283 284

Figure 11. CO2 uptake kinetics in DSW with induction time by cooling stimulation. The hollow dots indicate that

285

the temperature is 279 K, and the solid dots indicate 277 K. Note that the three experiments in each panel are under

286

the same conditions.

287

To verify the simulation results of scheme (ii) in Figure 8, i.e. the effect of temperature on the

288

interface of DSW is reversible when there is no gas, we designed a similar experiment according to

289

scheme III shown in Figure 9: after the sample DSW underwent the cooling stimulation, pure CO2

290

is added to the reactor. However, no adsorption occurred, indicating that the structure of DSW is

291

very sensitive to the temperature. Only the special structure of DSW at low temperature can initiate

292

the adsorption process when it is exposed to CO2. In other words, the gas adsorption is strongly

293

related to the morphology of water in the inner core of DSW.

294

4.4. Applicability of Cooling Stimulation Method

295

Based on the cooling stimulation method, we further performed several experiments at other



296

temperatures. For example, at 280 K, DSW can uptake ~19 V/V of CO2 by cooling stimulation to

297

277 K. The undercooling of the system to 277 K is sufficient to provide the driving force for CO2

298

hydrate nucleation. After the nuclei formed, the gas uptake will continue even if the temperature

299

rises to 280 K. When temperature is higher than 283 K, however, the cooling stimulation method is

300

invalid: the gas captured by stimulation will be fully desorbed after the temperature rises to 283 K.

301

We note that 283 K is the upper limit of temperature that can be used by the cooling stimulation

302

method. The CO2 uptake capacity obtained by constant temperature method (scheme I) and cooling

303

stimulation method (scheme II) at different temperatures is shown in Figure 12.

304 305

Figure 12. Gas uptake capacity by scheme I and II at different temperatures.

306

Finally, by using cooling stimulation method, one can save a certain amount of energy. We

307

should note that cooling stimulation at higher temperature (e.g. at 280 K) can save more energy, but

308

the final uptake capacity of gas will be reduced. The optimal operating condition should be at a

309

temperature which has the lowest energy consumption per unit mass of CO2 adsorbed. The energy

310

consumption is related to the stimulation method used. In our experiments, we used a simple

311

circulator bath for temperature controlling, and 279 K is the best operating condition for cooling

312

stimulation method.

313

5. Conclusion

314

In this paper, we explored the capture process of small insoluble gas molecules such as CO2

315

by DSW. The DSW can be prepared by simple process with cheap raw materials. We found that


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there is a dramatic change in the gas uptake capacity at temperature higher than 279 K: DSW has

317

almost no gas adsorption capacity by constant temperature experiments. This is because the low

318

undercooling increases the induction time of nucleation. MD simulation results show that near the

319

silica surface, the orientation of water and the hydrogen-bond network structure are very sensitive

320

to temperature. With the temperature decreased, the water layer near the interface becomes local-

321

structured by forming more hydrogen bonds, which facilitated the capture of CO2 by forming

322

hydrate precursor easily. The change of the water layer structure is irreversible once the CO2 uptake

323

process occurred at low temperature, hence the gas uptake can carry on continuously even though

324

the temperature rises back. By using the cooling stimulation, gas adsorption can be induced because

325

the energy barrier of nucleation is reduced. However, the saturated uptake capacity of DSW is

326

controlled by thermodynamics, which is only related to the operating temperature, such as ~25 V/V

327

in our experiments at 279 K.

328

Finally, since the formation of gas hydrate precursor at low temperature is very common for

329

conventional insoluble small gas molecules, we note that the cooling stimulation technology can

330

also be used for the storage of a large class of gas. And compared with the direct cooling adsorption

331

process, it can save a certain amount of energy used to keep the low temperature. The DSW prepared

332

by NaCl solution (rather than by pure water) shows a good uptake capacity, which means the direct

333

usage of non-purified water containing electrolytes is possible, and hence the cost on water

334

purification will be also saved. Therefore, it might be a potential industrial method for efficient gas

335

storage. The key to the application of this method is how to intensify hydrate formation at higher

336

temperatures. Adding suitable additives might be a possible way, which will be explored in our

337

subsequent research.

338 339

Conflicts of interest

340

There are no conflicts to declare.

341 342

Acknowledgement

343

This work is supported by National Natural Science Foundation of China (No. 21403009).

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Why the Gas Uptake Behavior of Dry Salt Water Is Vastly Different

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