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Size Effect of Silica Shell on Gas Uptake Kinetics in Dry Water Yong Li, Diwei Zhang, Dongsheng Bai, Shujing Li, Xinrui Wang, and Wei Zhou Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01918 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on July 3, 2016
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Size Effect of Silica Shell on Gas Uptake Kinetics in Dry Water Yong Li, Diwei Zhang, Dongsheng Bai, Shujing Li, Xinrui Wang, Wei Zhou* (Department of Chemistry, School of Science, Beijing Technology and Business University, Beijing 100048)
Micro-shell Water Core
Guest Gas
Water Core
DW-D10 Particle
Nano-shell Water Core Gas Hydrate Gas Molecule
Hydrophobic Silica Shell
DW-R202 Particle
The DW with micro-shell possesses larger uptake capacity, whereas the DW with nano-shell has faster uptake rate. The larger size of component silica shell contributes to higher stability of DW, there are more intact dry water particles for accepting CO2. However, CO2 will encounter more steric hindrance and consume more energy and time for entering water core of DW with micro-shell.
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Size Effect of Silica Shell on Gas Uptake Kinetics in Dry Water Yong Li, Diwei Zhang, Dongsheng Bai, Shujing Li, Xinrui Wang, Wei Zhou* (Department of Chemistry, School of Science, Beijing Technology and Business University, Beijing 100048)
* Corresponding Author:
[email protected] ABSTRACT: Two kinds of dry water (DW) is prepared by mixing water and hydrophobic silica particles with nanometer or micrometer dimension, and two DW are found to have similar size distribution regardless of silica shell’s size. CO2 uptake kinetics of DW with nanometer (nano-shell) and micrometer shells (micro-shell) are measured respectively, and both uptake rate and capacity show obvious size effect of silica shell. The DW with micro-shell possesses larger uptake capacity, whereas the DW with nano-shell has faster uptake rate. By comparing the uptake kinetics of soluble NH3 with CO2 further, the micro-shell is identified to enhance the stability and the dispersion degree of DW, and the nano-shell offers a shorter path for guest gas transiting into water core. Further, molecular dynamics simulation is introduced for illustrating nano-size effect of silica shell on the initial step of gas uptake. It is found that the concentration of gas molecules close to silica shell is higher than that in bulk water core. With the increase of the size of silica shell, the amount of CO2 in silica shell decreases, and the gas uptake is easier to reach steady state. 1. INTRODUCTION Dry Water (DW) is a water-in-air emulsion produced by mixing water with hydrophobic silica particles at high speed.1 Because of containing over 95 wt% water, DW looks like a powder but exhibits a free-flowing property. In comparison to the bulk water, DW has much higher surface-to-volume ratio，which greatly enhanced kinetics of clathration in a gaseous system.2, 3 It is well-known that the kinetics of gas hydrate formation is slow due to low solubility and small diffusion coefficients of guests in water.4, 5 As a result, gas hydrates tend to form at gas/water interfaces rather than inner aqueous phase. So in bulk water, the hydrate formation was restricted by limited gas-water interface. However, enormous water/powder interface in DW offer large amounts of ‘gas acceptors’, and gases can be incorporated more quickly and stably into DW to form gas hydrates.6 Base on efficient adsorptivity, DW was considered as a potential material for the storage, transport, separation, and capture of gases, such as CO2, CH4, and Kr.7-9 At present, improving both stability of DW and formation rate of gas hydrate are still key issues for practical application of DW .10, 11 For the preparation of stable DW, strong stirring is required to initiate droplet formation prior to their coating, while high shear leads to size reduction. 12 The contact angle of hydrophobic silica particle correlates to the quality of the final product. Contact angle close to 105 º leads to mousse formation whereas a slightly higher value of approximately 118 º allows dry water formation.13,14 The effect of porous silica in foam system on hydrate kinetics was investigated: a fast hydrate growth rate and improved hydrate formation with good reproducibility were due to the highly connected interstitial pores.15,16 A formation temperature of 278 K was found to be optimal with the CH4 uptake capacity and rate falling off both above and below this temperature, and the incorporation of a hydrocolloid gelling agent was reported to increase the recyclability of the DW system.9 Recently, a high capacity and improved reversibility was reported for a mixed colloidal system made of hydrogel particles and dry water particles.17 Besides used as surfactant in DW, hydrophobic particle was also found to be kinetic promoters for gas hydrate formation due to the increase of mass transfer rates at the gas/water interface and the formation of partial clathrates in
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the vicinity of hydrophobic surfaces.18,19 Through comparing optional kinetic promoters, the maximum uptake of the dry K2CO3 is lower but the kinetics of CO2 absorption is much faster than dry diethanolamine.20 In this work, we focused on size effect of hydrophobic silica on gas uptake rate and capacity of DW. The sizes of two kinds of silica particles, with similar static contact angles (apparent contact angles), were chose at micrometer and nanometer respectively. Firstly, the particle distribution and optical image of DW consist of different silica particle were measured and compared in order to clarify the different basic characteristics between two DWs. Subsequently, we investigated CO2 and NH3 uptake kinetics in two DWs under different conditions. As typical insoluble and soluble gases, the combination of CO2 and NH3 can distinguish the respective role of solid shell and aqueous core of DW. Finally, we speculated the effect mechanism of nanometer shell on gas uptake kinetics in DW by means of molecular dynamic simulations. 2. EXPERIMENTALS 2.1. Materials. Pure carbon dioxide gas (99.99%) and ammonia gas (99.999%) were supplied by Beijing YangLilai Chemical Gas Co., Ltd (China), ammonia solution (AR, 25%) and cupric chlorides (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd (China). Hydrophobic silica particles, AEROSIL R202 and SIPERNAT D10, were supplied by EVONIK (Germany). The related properties of silica particles to this work were listed in Table 1. The static contact angles of D10 and R202 are higher than 110 º, which is a basic requirement for forming stable dry water in a high shear mixing process. 13 2.2. Synthesis and Characterization. DW was prepared by mixing 5 g of hydrophobic silica with 95 g deionized water at about 19000 rpm for 90 s in a domestic blender. According to the trade name of silica particle, the corresponding DW were abbreviated as DW-R202 and DW-D10, respectively. By the same procedure, the dry water with the cupric chloride solution as core was prepared. In order to make difference from DW, the dry CuCl2 solution is abbreviated to DC. Two kinds of DW were screened using a set of standard sieve (Zhejiang Sieves Factory, China) into five parts with a diameter range of smaller than 98 µm, 98~224 µm, 224~335 µm, 335~500 µm, and larger than 500 µm. The mass distribution and topography of DW were measured by analytical balance (Denver Instrument TP-1102, Beijing) and optical microscopy (Leica DM500, China) equipped with a digital camera (Canon，EOS 60D), respectively. 2.3. Kinetics Measurement. DW (22.0 g) was loaded into a vessel with an internal volume of 40 mL. A circulator bath was connected to the vessel for providing a constant temperature. The vessel was then pressurized with guest gas as required and sealed. For NH3, experimental temperature and initial pressure were 298 K and 0.38 MPa, and for CO2 were 276 K and 3.50 MPa. Gas uptake kinetics in DW was studied by observing gas pressure change as a function of time. Additionally, DC-D10 and DC-R202 with same weight were added respectively into a glass column with an inner diameter of 0.5 cm，and the height of each filling DC was adjusted to equal scale of column. Then two columns were placed together into a closed desiccator, which contained a bottle of ammonia solution as provider of low pressure NH3. 3. MODEL AND SIMULATION METHOD The molecular dynamics (MD) simulations were performed by using LAMMPS,
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source program for massively parallel simulations. The initial configuration containing a water/silica/carbon dioxide three-phase system was placed into a simulation box with a size of 10 nm × 10 nm × 40 nm, as is shown in Figure 1. Since a dry-water particle is usually in a micrometer scale, far greater than the scale of the classical MD simulations, we represent a dry-water particle by a pair of parallel silica layers and several water molecules between them. The silica layers were prepared by packing the amorphous silica spheres to a face-centered cubic (fcc) motif (Figure 1), while the dangling bonds on the surfaces of each silica sphere were saturated by–CH3 groups, showing a hydrophobic properties. The diameters of 1.0, 2.0, 2.5, 4.0, 5.0, 8.0 and 10.0 nm of silica spheres were considered to investigate the effects on the dynamics of gas diffusion. A certain amount of water molecules were added between the two silica layers (region A in Figure 1) with the density of 0.997 g/cm3, and several CO2 molecules were also added into the outside part of the two silica layers (region B) to reach the specified value of the pressure of CO2 gas (i.e. 5 MPa). The amount of water and carbon dioxide added in different systems is summarized in Table 2. In our simulations, the TIP4P water model22 was used and the rigidity of water molecules was restricted with the SHAKE algorithm.23 Carbon dioxide molecules were represented by the EPM2 model.24 The hydroxylated silica model25 was adopted for the inner part of the silica layers, while the surface part (i.e. the –CH3 groups) was represented by a single point model.26 The unlike parameters of Lennard-Jones interactions were obtained by the Lorentz-Berthelot mixing rule, while the parameters for the H2O−CO2 interaction were taken from ref 27. A cutoff radius of 12 Å was utilized for the short-ranged interactions, while the long-ranged Coulomb interactions were evaluated by the pppm algorithm.28 Periodic boundary conditions were imposed in all three Cartesian directions. In this work, temperature was maintained at 278 K by using the Nosé-Hoover algorithm29-31 with the relaxation parameter of 0.2 ps. During the simulation process, the amount of CO2 molecules in region B is maintained as the initial value via artificially adding/deleting molecules. A whole simulation run includes two parts. First, an NVT relaxation of 1 ns was performed at 278 K to eliminate the effect of the initial configuration. Then, a simulation run on the timescale of 50 ns was performed with time step of 2 fs to study the gas adsorption and diffusion process. During the simulations, the position of silica molecules was fixed. 4. RESULTS AND DISCUSSION 4.1. Characterization of DW. By means of measuring the mass of DWs with different particle sizes, as shown in Figure 2, DW-D10 and DW-R202 were found to have similar maximum and minimum value at 98~224 µm and >500 µm, respectively. Compared with DW-R202, DW-D10 has larger distribution at
