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Ultra-stable aqueous foams with multilayer films stabilized by 1-dodecanol, sodium dodecyl sulfonate and polyvinyl alcohol
Chemical Engineering Science 160 (2017) 72–79
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Ultra-stable aqueous foams with multilayer films stabilized by 1-dodecanol, sodium dodecyl sulfonate and polyvinyl alcohol ⁎
Xing Du, Lei Zhao , Xuan He, Hui Chen, Wei Fang, Weixin Li The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Multilayer films Ultra-stable foams Bound water molecules Molecular dynamics simulations
Ultra-stable foams with multilayer films which were similar to the structure of colloidal gas aphrons (CGA) were designed in view of increasing the amount of bound water molecules at molecular level. This liquid films in foams were spontaneously assembled by 1-dodecanol (DDA), sodium dodecyl sulfonate (SDS) and polyvinyl alcohol (PVA) molecular with a remarkable long-term foam lifetime. The states of water molecules as well as the molecular arrangement in foam films have been analyzed via molecular dynamics (MD) simulation to explore microscopic character and stable mechanism of multilayer films of foams. We found that the combined effect of SDS, DDA and PVA could not only increase distribution intensity of bound water molecules in foam film layers, but it also could decrease the gas diffusion rate to balance the pressure of the air layers of the multilayer foams.
1. Introduction Foams have been successfully applied in variety of emerging fields nowadays, especially in the elaboration of porous materials (Schüler et al., 2012; Wong et al., 2011, Deng et al.) and size-controllable synthesis of nanoparticles (Guo et al., 2007; Li et al., 2007). However, foams are conventionally unstable system since film drainage (Carn et al., 2011), coalescence and disproportionation of the bubbles definitely occur to decrease free energy of overall system (Gonzenbach et al., 2006). In order to better foams stability, surfactants (Lim et al., 2012; Chevallier et al., 2012), polymers (Petkovet al. 2012; Zhao et al., 2015), proteins (Engelhardt et al., 2012; Lech et al., 2015), or surface active solid particles (Zhang et al., 2014; Sham and Notley, 2016; Nakayama et al., 2016; Sarkar et al., 2015b, 2015a) were usually added into the foam system. But, despite the easy and quick movement to the gas-liquid interface by the addition of surfactant (Fameau et al., 2011; Grassia et al., 2016; Vitasari et al., 2013), high viscosity of foam films when adding proteins and polymers (SaintJalmes et al., 2005), or elastic layer formed by the aggregation of surface active solid particles (Wege et al.al., 2008; Zhu et al.al., 2015; Bournival et al.al., 2015; Aveyard et al.al., 2003), those additives could not effectively stop the decay of bubbles. Recently, multilamellar tubes formed in foam films were reported to enhance foam stability by limiting film breaking (coalescence) and gas diffusion (disproportionation) through combining the advantages of both the solid particles and the low-molecular weight amphiphiles 12 hydroxy stearic acid (12HSA) (Fameau et al., 2011). Or, via mixing the suspension of colloidal
⁎
particles and the immiscible secondary liquid, the mixed particle/liquid coating was formed with particle-mediated spreading of the minority liquid around the foams to stabilize bubbles against coalescence (Zhang et al., 2014). Besides, precipitating the surfactant crystals on the surfaces of bubbles in the interstices between bubbles was demonstrated to enhance the foam stability with introducing high concentrations NaCl or KCl to sodium dodecyl sulfate (Zhang et al., 2015). Although the tremendous improvements to avoid the coalescence and disproportionation in foams have been worked out, the drainage can not be completely stopped only by raising up viscosity of liquid film on account of the increase of internal friction of liquid flow (Wang and Nguyen, 2016). Hence, this problem should be considered substantially from the point of view at an underlying level. In the foam film, the bound water and free water, which are named according to the interaction between water molecules and solvent molecules and the impact on the drainage process, were considered the most great factors on the stability of foams (Li et al., 2013). The bound water molecule with low freedom degree plays a positive role in the stability of the foam, while the loss of free water molecule in the foam films always gives rise to the drainage phenomenon. Therefore, to increase the amount of bound water molecules in the films could be regarded as the most effective way to impede drainage from this point of view. In this work, we propose to use different compounds with amphiphilic groups, such as 1-dodecanol (DDA), sodium dodecyl sulfonate (SDS) and polyvinyl alcohol (PVA) to construct a foam structure like the colloidal gas aphrons (CGA) (Fuda and Jauregi, 2006) with multilayer liquid films, which aim to increase the amount of bound
Corresponding author. E-mail address: [email protected] (L. Zhao).
http://dx.doi.org/10.1016/j.ces.2016.11.024 Received 21 September 2016; Received in revised form 7 November 2016; Accepted 11 November 2016 Available online 12 November 2016 0009-2509/ © 2016 Elsevier Ltd. All rights reserved.
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2. Materials and methods
Nomenclature
2.1. Materials
Acronyms SDS DDA PVA
Sodium dodecyl sulfonate (SDS, ≥99% purity), 1-dodecanol (DDA, ≥99% purity) were purchased from Sigma Aldrich. Polyvinyl alcohol (PVA, alcoholysis degree: 99.8–100%) were purchased from Aladdin company. Deionized water was supplied from Aquapro Ultra-pure Water System. The chemical structures of SDS, PVA and DDA are shown in Fig. 2.
sodium dodecyl sulfonate 1-dodecanol polyving akohol
water molecules to get an ultra-stable system. These compounds were employed to offer amphiphilic groups in foam solution to construct multilayer liquid films via self-assembly process as shown in Fig. 1(III1-a), which is similar to the structure of CGA. Conventionally, monolayer foams (II-1-a in Fig. 1) could be spontaneously formed by adding SDS due to its amphiphilic characteristic as illustrated in II-1 (Fig. 1). But the monolayer was normally influenced by the air pressure adjacent to itself to result in unstable foams system. Considering that DDA molecules could be arranged with SDS or with PVA molecules at intervals to spontaneously form the monolayer structure (III-1-a in Fig. 1) because of the solubilization of SDS (Malliaris, 1987) or PVA (Yokoi et al., 1986), the hydrophobic groups of SDS, DDA and PVA could get close and combine with each other as far as possible in solution due to the hydrophobic interaction. As a consequence, the multilayer films foams (III-1-b in Fig. 1) could be formed after stirred with adding PVA to the SDS/DDA foam system. The foams were regarded to be more stable than that with monolayer films because the multilayer of foams could normally bear higher fluctuation threshold of air pressure. And even if one layer of film was broken, the others could turn to be stable with unchanged foam structure and gave rise to an ultra-stable system.
2.2. Preparation The foams were prepared in two basic steps. Firstly, 0.1 mol SDS and 0.1 mol DDA were added in to deionized water and heated at 60 °C for 20 min to ensure all the solutes dispersed uniformly. After cooled down, the solution was stirred rapidly to generate foams for 8–10 min by JJ-1 Numerical Show Precise Power Mixer with speed of 1200– 1300 rpm at 25 °C. Secondly, 0.1 mol PVA and 0.1 mol DDA was added into the SDS/DDA foam system and then stirred rapidly to generate foams according to the aforementioned process. 2.3. Measurement and characterization Fluorescence microscope photograph of foams was taken with OLYMPUS-BX51 Fluorescence Microscope. Foam stability in macroscopic was evaluated by measuring the drainage and collapse of bubbles after standing for 1 h, 12 h, 36 h and 96 h at 25 °C. In order to show foam stability better, the macro photographs of foams after standing for 0 h, 1 h, 12 h, 36 h, 96 h and even 3 months were taken by SONY camera TX-10.
Fig. 1. Schematic presentation of compound (SDS, DDA and PVA) in part I, monolayer films foams formed by SDS (II-1) and the designed multilayer films foams (III-1). II-2. The hydrogen-bonding interaction (dashed line) between SDS molecules and water molecules (circled in orange rectangle) in monolayer foams (II-1-a). The dodecyl sulfonate ions and sodium ions were offered by SDS in liquid film with sulfonic group hydrogen bonding with water molecule while the sodium bonding with water molecules by hydration (arrow). III-2. The hydrogen-bonding and hydration interactions of SDS, DDA molecules and water molecules in monolayer foams (III-1-a). III-3. The interaction of SDS, DDA, PVA and water molecules in multilayer foams (III-1-b).
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water model (Berendsen et al., 1987). The long-range electrostatic interactions were described by the Ewald summation method (Ewald, 1921). All the MD simulations were carried out in the NVT ensemble with a time step of 1.0 fs. A Nose-Hoover thermostat (Nosé, 1984; Hoover, 1985) with a decay constant of 0.1 ps was used to control the temperature. The MD software employed in this study was the Materials Studio 8.0. 2.4.2. Simulation model and procedure Foam system generated by SDS was constructed as follow. 16 surfactant molecules of SDS were disposed to construct a surfactant monolayer with spacing suitable for hexagonal close packing in an orthorhombic simulation box with periodic boundary conditions applied for all three spatial directions (Jang and Goddard, 2006). The lattice parameters refers to the most probable surface concentration of SDS used in previous simulation and experimental results (Jang and Goddard, 2006), and the surface area per molecule for SDS is set to 42.44 Å2. Consequently, the SDS surfactant monolayer was constituted correspondingly as shown in Fig. 3. Then, a 24 Å thick water box (the number of water molecules is 545) with the same lateral scale as those of the surfactant box was built of which density was set to be 0.997 kg/ L, corresponding to the theoretical density at 298 K. Two SDS surfactant monolayers were placed on both sides of the water box (Jang and Goddard, 2006) to form the foam system generated by SDS as illustrated in Fig. 4. Foam system with SDS and DDA was built as follow. The mix layer formed by SDS and DDA (Fig. 5a) was based on the radial arrangement of both, that is 8 DDA molecules took the place of the 8 SDS molecules, and cell parameters was in accordance with the SDS layer. Consequently, the SDS/DDA foam system (Fig. 5b) was constructed by the procedure performed in SDS layer as outlined above. SDS/DDA/ PVA foam system was created as follow. The mix layer of PVA and DDA (Fig. 5c) was constructed according to the procedure performed in SDS and DDA mix layer. The liquid film layer made by PVA and DDA (Fig. 5d) was constructed by the procedure performed in SDS layer as outlined above as well. The foam system generated by SDS, DDA and PVA was formed by two double-layer liquid film - SDS/
Fig. 2. Chemical structure of SDS, PVA and DDA used in this study. The atoms drawn as van der Waals spheres are shown as a small colored sphere: C, gray; H, white; O, red; S, yellow; Na+, violet. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.4. Molecular dynamics simulation 2.4.1. Force field and MD parameters In this paper, the MD simulation with a full atomistic force field was conducted. A SPC/E model (Berendsen et al., 1987) was chosen to build the water box. The potential functions and atom interaction parameters were modeled with the compass force field (Sun, 1998). The total potential energy of the wet foam film system is given in Eq. (1):
E total =E vdW +E Q +Ebond +Eangle +E torsion +E inversion
(1)
whereE total,E vdW ,E Q ,Ebond ,Eangle ,E torsion ,E inversion are the total energy, van der Waals, electrostatic, bond-stretching, angle-bending, torsion, and inversion components, respectively. The atomic charges of surfactant, foam stabilizer and accessory ingredient were assigned using the charge equilibration (QEq) method (Rappe and Goddard III, 1991). The atomic charges of the water molecules were taken from the SPC/E
Fig. 3. Schematic preparation of initial configuration of SDS surfactant monolayer.
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Fig. 4. Schematic preparation of the foam system generated by SDS in which two SDS surfactant monolayers place on both sides of the water box. For clarity, the SDS is drawn as van der Waals spheres and water molecules drawn in line style. C, gray; H, white; O, red; S, yellow; Na+, violet. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. (a) Initial hexagonal packing of the SDS and DDA mix layer, (b) simulated configuration of foam system generated by SDS and DDA, (c) initial hexagonal packing of the PVA and DDA mix layer, (d) simulated configuration of liquid film layer made by PVA and DDA, (e) simulated configuration of foam system generated by SDS, DDA and PVA. For clarity, the SDS, DDA and PVA are drawn as van der Waals spheres and water molecules drawn in line style. C, gray; H, white; O, red; S, yellow; Na+, violet. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
DDA liquid film layer and PVA/DDA liquid film layer as shown in Fig. 5e. The Z dimension was kept large enough to avoid interactions between the periodic replicas. Firstly, 5000 steps of geometry optimi-
zation based on smart algorithm were run to avoid the possible molecular overlap occurred in initial built configurations. The systems equilibrated at about 1 ns. As a result, the MD simulations were carried out under isothermal constant volume for 4 ns subsequently. 75
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carried out under isothermal constant volume for 4 ns, and the results of the last 1 ns were utilized to analyze and evaluate. The radial distribution functions (RDFs) of water molecules adjacent to the sulfonate groups and sodium ions of SDS in the inner liquid film layer of multilayer foams were analyzed to understand distribution of bound water molecules (Fig. 8). It clearly observed that both of distribution intensity of bound water molecules around the sulfonate groups and sodium ions of SDS increased in all systems, while the distribution intensity climbed up dramatically in SDS/DDA/ PVA system, nearly as twice as that in SDS and SDS/DDA foam systems. As illustrated in Fig. 1(II-2), the intermolecular interactions were weak due to the reversible ionization reaction to give rise to the emergence of amount of free water in films. In addition, the dynamic fluctuations of SDS molecules would lead to the fast gas diffusion to make foams unstable. When DDA and SDS were added in solution, more dodecyl sulfonate ions as well as sodium ions could be offered to interact with DDA and water molecules (III-2 in Fig. 1) to increase the amount of bound water molecules in inner liquid film. However, water molecules in the monolayer liquid film had a tendency to escape before interacting with solvent molecules. The distribution intensity of bound water molecules at this moment has not been effectively increased. Contrarily, the addition of PVA with multiple hydrophilic groups would enhance the intermolecular interactions between water and solvent molecules because of strong intermolecular hydrogen bonding as shown in Fig. 1(III-3). Besides, the hydrophobic groups in air1 and air2 layers in the formed multilayer foams would constantly push water molecules to the inner liquid film to get more immobilized water molecules in system (III-1-b in Fig. 1). More water molecules in the layer had consequently more chances to interact with solvent molecules to be bound. Therefore, the distribution intensity of bound water molecules climbed up dramatically around the sulfonate groups and sodium ions of SDS in SDS/DDA/PVA system. Moreover, due to the combined action of DDA and PVA with water molecules, the RDF of water molecules around the hydroxyls of DDA in the outer liquid film layer (Fig. 9) exhibited an increase tendency of distribution intensity of bound water as well. Both of these might be in favour of the stability of the liquid film layers. The equilibrated configuration snapshots of foam systems at the end of simulations were shown in Fig. 10. It observed that the staggered arrangements of hydrophobic tails of the absorbed molecules were increased with the addition of DDA in comparing Fig. 10a with Fig. 10b. It indicated that the molecular arrangements in SDS/DDA system were much more disordered than that in SDS system. Such
Thereinto, the results of the last 1 ns were utilized to analyze and evaluate. 3. Results and discussion 3.1. Microstructure of foams Fluorescence microscope images of foams were given in Fig. 6a–c. The bright field represented the air in the foam while the dark field to the liquid films. Double layered bubbles like the CGA with the addition of DDA and PVA to SDS were clearly observed which almost consisted with Fig. 1(III-1-b). Meanwhile, we found that there were contact points from the inner layer to the outer one. These points which might be formed by the polyhydroxy PVA like bridges were assumed to transversely connect the layers to enhance the stability of bubbles. In conventional foams, liquid would spontaneously flow to the Plateau border on account of the surface tension. It usually resulted in thinner liquid films, and broken bubbles at end (Studart et al., 2006). But in our case, a large amount of hydrophobic groups at the border was favorable to stop the drainage since the water molecules were assumed to be immobilized in the outer layer by hydrophobic effect (Fig. 6c–d). 3.2. Foam stability As shown in Fig. 7, the foams showed no significant drainage and collapse even after 96 h of standing at 25 °C. Actually, the cellular network constructed by ultrathin foam films maintained even after 3 months. However, foams produced by SDS alone show a poor stability with a few tens of minutes (Fameau et al., 2011). As a result, it concluded that the addition of DDA and PVA could effectively slow down the drainage, coalescence and disproportionation processes to make such foams such unprecedented long lifetime lived. 3.3. Molecular simulation study of foam films In order to analyze the multilayer foams structure at the molecular level, molecular dynamics simulation of foam films were carried out in this work. The foam systems were constructed (Figs. 3–5) and the configurations set in simulation in accordance with designed foams in II-1-a, III-1-a and III-1-b (in Fig. 1) respectively corresponded to Figs. 4, 5b and e. 5000 steps of geometry optimization were run to avoid the possible molecular overlap occurred in initial built configurations. The molecular dynamics simulations were subsequently
Fig. 6. Fluorescence microscope images of (a) multilayer films foams system, (b) single bubble with multilayer films, (c) border of three bubbles. (d) The border of three designed bubbles.
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Fig. 7. Photographs of multilayer film foams after standing varied periods.
Fig. 8. Radial distribution functions of water molecules around (a) the sulfonate groups (the sulfur atom was chosen as the center) and (b) sodium ions of SDS in SDS, SDS/DDA, and SDS/DDA/PVA foam systems (oxygen atom of water molecules was chosen as the target).
for the coverage of gas-liquid interface with its multiple hydrophilic hydroxyls always spreading into water to reduce the rate of gas diffusion between layers. The air pressure in air1, air2 and outside layers could be balanced for a long time to exhibit outstanding foam stability. Both the RDF analyses and equilibrated configuration snapshots indicated that, in foams with multilayer films, the stability can be markedly improved not only due to the increase of bound water but due to the decrease of rate of gas diffusion with compacted molecule arrangement and the increased coverage of hydrophobic chains on gasliquid interface. In addition, in order to understand the influence of adding order of compounds on the foam structures we added SDS, DDA and PVA simultaneously to prepare foams. It found that the formed multilayer structure (Fig. 11) was similar to that in Fig. 6. It indicated that the multilayer films can be formed by self-assembly of molecules and it only depended on the additions but not the order. The microscopic structure of such foam at molecular level is still under investigation. Fig. 9. Radial distribution function of water molecules around the hydroxyls of DDA in the outer liquid film layer of multilayer foams (oxygen atom of hydroxyls of DDA was chosen as the center; oxygen atom of water molecules was chosen as the target).
4. Conclusions In summary, amphiphilic compounds, such as DDA, SDS and PVA, were creatively employed to elaborate the ultra-stable foams with multilayer films which were similar to the structure of CGA. It could stand without drainage and collapse even after 96 h at 25 °C. Due to the solubilization of SDS and PVA, DDA molecules were arranged with SDS and PVA respectively to form inner and outer liquid film layer, and the hydrophobic groups of SDS, DDA and PVA close to each other to
disordered arrangement was supposed to strengthen the gas-liquid interface and compactness of molecular network formed by solvent molecules at gas-liquid interface. As a result, the gas diffusion rate could be decreased to get stable foam system (Hu et al., 2012). Furthermore, the hydrophobic chains of PVA molecules were benefit 77
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Fig. 10. The equilibrated configuration snapshots at the end of simulations of (a) SDS, (b) SDS/DDA, (c) SDS/DDA/PVA. For clarity, the SDS, DDA and PVA are drawn as van der Waals spheres and water molecules drawn in line style. C, gray; H, white; O, red; S, yellow; Na+, violet. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 11. Fluorescence microscope images of the foams prepared by one step.
constitute multilayer films foams by the hydrophobic interaction. From structural study in microscopic via molecular dynamics simulation, we found that distribution intensity of bound water rose up dramatically around both sulfur atom and sodium ion of sulfonate groups of SDS in SDS/DDA/PVA system because of the enhanced hydrogen bound between water and solvent molecules when adding PVA. Moreover, with the addition of PVA, the gas-liquid interface could be strengthened as well as the compactness of molecular in network. It led to decreased gas diffusion rate to make the foam system stable. Meantime, the multilayer film structure was favorable for the balance of air pressure between air layers in the system with a higher fluctuation threshold comparing to the monolayer film, which resulted in the formation of ultra-stable foams. Acknowledgment The work was financially supported by the China Postdoctoral Science Foundation (2015M572210), the National Natural Science Foundation of China (61604110), the Hubei Provincial Natural 78
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Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces
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Ultra-stable aqueous foams with multilayer films stabilized by 1-dodecanol, sodium dodecyl sulfonate and polyvinyl alcohol ⁎
Xing Du, Lei Zhao , Xuan He, Hui Chen, Wei Fang, Weixin Li The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Multilayer films Ultra-stable foams Bound water molecules Molecular dynamics simulations
Ultra-stable foams with multilayer films which were similar to the structure of colloidal gas aphrons (CGA) were designed in view of increasing the amount of bound water molecules at molecular level. This liquid films in foams were spontaneously assembled by 1-dodecanol (DDA), sodium dodecyl sulfonate (SDS) and polyvinyl alcohol (PVA) molecular with a remarkable long-term foam lifetime. The states of water molecules as well as the molecular arrangement in foam films have been analyzed via molecular dynamics (MD) simulation to explore microscopic character and stable mechanism of multilayer films of foams. We found that the combined effect of SDS, DDA and PVA could not only increase distribution intensity of bound water molecules in foam film layers, but it also could decrease the gas diffusion rate to balance the pressure of the air layers of the multilayer foams.
1. Introduction Foams have been successfully applied in variety of emerging fields nowadays, especially in the elaboration of porous materials (Schüler et al., 2012; Wong et al., 2011, Deng et al.) and size-controllable synthesis of nanoparticles (Guo et al., 2007; Li et al., 2007). However, foams are conventionally unstable system since film drainage (Carn et al., 2011), coalescence and disproportionation of the bubbles definitely occur to decrease free energy of overall system (Gonzenbach et al., 2006). In order to better foams stability, surfactants (Lim et al., 2012; Chevallier et al., 2012), polymers (Petkovet al. 2012; Zhao et al., 2015), proteins (Engelhardt et al., 2012; Lech et al., 2015), or surface active solid particles (Zhang et al., 2014; Sham and Notley, 2016; Nakayama et al., 2016; Sarkar et al., 2015b, 2015a) were usually added into the foam system. But, despite the easy and quick movement to the gas-liquid interface by the addition of surfactant (Fameau et al., 2011; Grassia et al., 2016; Vitasari et al., 2013), high viscosity of foam films when adding proteins and polymers (SaintJalmes et al., 2005), or elastic layer formed by the aggregation of surface active solid particles (Wege et al.al., 2008; Zhu et al.al., 2015; Bournival et al.al., 2015; Aveyard et al.al., 2003), those additives could not effectively stop the decay of bubbles. Recently, multilamellar tubes formed in foam films were reported to enhance foam stability by limiting film breaking (coalescence) and gas diffusion (disproportionation) through combining the advantages of both the solid particles and the low-molecular weight amphiphiles 12 hydroxy stearic acid (12HSA) (Fameau et al., 2011). Or, via mixing the suspension of colloidal
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particles and the immiscible secondary liquid, the mixed particle/liquid coating was formed with particle-mediated spreading of the minority liquid around the foams to stabilize bubbles against coalescence (Zhang et al., 2014). Besides, precipitating the surfactant crystals on the surfaces of bubbles in the interstices between bubbles was demonstrated to enhance the foam stability with introducing high concentrations NaCl or KCl to sodium dodecyl sulfate (Zhang et al., 2015). Although the tremendous improvements to avoid the coalescence and disproportionation in foams have been worked out, the drainage can not be completely stopped only by raising up viscosity of liquid film on account of the increase of internal friction of liquid flow (Wang and Nguyen, 2016). Hence, this problem should be considered substantially from the point of view at an underlying level. In the foam film, the bound water and free water, which are named according to the interaction between water molecules and solvent molecules and the impact on the drainage process, were considered the most great factors on the stability of foams (Li et al., 2013). The bound water molecule with low freedom degree plays a positive role in the stability of the foam, while the loss of free water molecule in the foam films always gives rise to the drainage phenomenon. Therefore, to increase the amount of bound water molecules in the films could be regarded as the most effective way to impede drainage from this point of view. In this work, we propose to use different compounds with amphiphilic groups, such as 1-dodecanol (DDA), sodium dodecyl sulfonate (SDS) and polyvinyl alcohol (PVA) to construct a foam structure like the colloidal gas aphrons (CGA) (Fuda and Jauregi, 2006) with multilayer liquid films, which aim to increase the amount of bound
Corresponding author. E-mail address: [email protected] (L. Zhao).
http://dx.doi.org/10.1016/j.ces.2016.11.024 Received 21 September 2016; Received in revised form 7 November 2016; Accepted 11 November 2016 Available online 12 November 2016 0009-2509/ © 2016 Elsevier Ltd. All rights reserved.
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2. Materials and methods
Nomenclature
2.1. Materials
Acronyms SDS DDA PVA
Sodium dodecyl sulfonate (SDS, ≥99% purity), 1-dodecanol (DDA, ≥99% purity) were purchased from Sigma Aldrich. Polyvinyl alcohol (PVA, alcoholysis degree: 99.8–100%) were purchased from Aladdin company. Deionized water was supplied from Aquapro Ultra-pure Water System. The chemical structures of SDS, PVA and DDA are shown in Fig. 2.
sodium dodecyl sulfonate 1-dodecanol polyving akohol
water molecules to get an ultra-stable system. These compounds were employed to offer amphiphilic groups in foam solution to construct multilayer liquid films via self-assembly process as shown in Fig. 1(III1-a), which is similar to the structure of CGA. Conventionally, monolayer foams (II-1-a in Fig. 1) could be spontaneously formed by adding SDS due to its amphiphilic characteristic as illustrated in II-1 (Fig. 1). But the monolayer was normally influenced by the air pressure adjacent to itself to result in unstable foams system. Considering that DDA molecules could be arranged with SDS or with PVA molecules at intervals to spontaneously form the monolayer structure (III-1-a in Fig. 1) because of the solubilization of SDS (Malliaris, 1987) or PVA (Yokoi et al., 1986), the hydrophobic groups of SDS, DDA and PVA could get close and combine with each other as far as possible in solution due to the hydrophobic interaction. As a consequence, the multilayer films foams (III-1-b in Fig. 1) could be formed after stirred with adding PVA to the SDS/DDA foam system. The foams were regarded to be more stable than that with monolayer films because the multilayer of foams could normally bear higher fluctuation threshold of air pressure. And even if one layer of film was broken, the others could turn to be stable with unchanged foam structure and gave rise to an ultra-stable system.
2.2. Preparation The foams were prepared in two basic steps. Firstly, 0.1 mol SDS and 0.1 mol DDA were added in to deionized water and heated at 60 °C for 20 min to ensure all the solutes dispersed uniformly. After cooled down, the solution was stirred rapidly to generate foams for 8–10 min by JJ-1 Numerical Show Precise Power Mixer with speed of 1200– 1300 rpm at 25 °C. Secondly, 0.1 mol PVA and 0.1 mol DDA was added into the SDS/DDA foam system and then stirred rapidly to generate foams according to the aforementioned process. 2.3. Measurement and characterization Fluorescence microscope photograph of foams was taken with OLYMPUS-BX51 Fluorescence Microscope. Foam stability in macroscopic was evaluated by measuring the drainage and collapse of bubbles after standing for 1 h, 12 h, 36 h and 96 h at 25 °C. In order to show foam stability better, the macro photographs of foams after standing for 0 h, 1 h, 12 h, 36 h, 96 h and even 3 months were taken by SONY camera TX-10.
Fig. 1. Schematic presentation of compound (SDS, DDA and PVA) in part I, monolayer films foams formed by SDS (II-1) and the designed multilayer films foams (III-1). II-2. The hydrogen-bonding interaction (dashed line) between SDS molecules and water molecules (circled in orange rectangle) in monolayer foams (II-1-a). The dodecyl sulfonate ions and sodium ions were offered by SDS in liquid film with sulfonic group hydrogen bonding with water molecule while the sodium bonding with water molecules by hydration (arrow). III-2. The hydrogen-bonding and hydration interactions of SDS, DDA molecules and water molecules in monolayer foams (III-1-a). III-3. The interaction of SDS, DDA, PVA and water molecules in multilayer foams (III-1-b).
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water model (Berendsen et al., 1987). The long-range electrostatic interactions were described by the Ewald summation method (Ewald, 1921). All the MD simulations were carried out in the NVT ensemble with a time step of 1.0 fs. A Nose-Hoover thermostat (Nosé, 1984; Hoover, 1985) with a decay constant of 0.1 ps was used to control the temperature. The MD software employed in this study was the Materials Studio 8.0. 2.4.2. Simulation model and procedure Foam system generated by SDS was constructed as follow. 16 surfactant molecules of SDS were disposed to construct a surfactant monolayer with spacing suitable for hexagonal close packing in an orthorhombic simulation box with periodic boundary conditions applied for all three spatial directions (Jang and Goddard, 2006). The lattice parameters refers to the most probable surface concentration of SDS used in previous simulation and experimental results (Jang and Goddard, 2006), and the surface area per molecule for SDS is set to 42.44 Å2. Consequently, the SDS surfactant monolayer was constituted correspondingly as shown in Fig. 3. Then, a 24 Å thick water box (the number of water molecules is 545) with the same lateral scale as those of the surfactant box was built of which density was set to be 0.997 kg/ L, corresponding to the theoretical density at 298 K. Two SDS surfactant monolayers were placed on both sides of the water box (Jang and Goddard, 2006) to form the foam system generated by SDS as illustrated in Fig. 4. Foam system with SDS and DDA was built as follow. The mix layer formed by SDS and DDA (Fig. 5a) was based on the radial arrangement of both, that is 8 DDA molecules took the place of the 8 SDS molecules, and cell parameters was in accordance with the SDS layer. Consequently, the SDS/DDA foam system (Fig. 5b) was constructed by the procedure performed in SDS layer as outlined above. SDS/DDA/ PVA foam system was created as follow. The mix layer of PVA and DDA (Fig. 5c) was constructed according to the procedure performed in SDS and DDA mix layer. The liquid film layer made by PVA and DDA (Fig. 5d) was constructed by the procedure performed in SDS layer as outlined above as well. The foam system generated by SDS, DDA and PVA was formed by two double-layer liquid film - SDS/
Fig. 2. Chemical structure of SDS, PVA and DDA used in this study. The atoms drawn as van der Waals spheres are shown as a small colored sphere: C, gray; H, white; O, red; S, yellow; Na+, violet. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
2.4. Molecular dynamics simulation 2.4.1. Force field and MD parameters In this paper, the MD simulation with a full atomistic force field was conducted. A SPC/E model (Berendsen et al., 1987) was chosen to build the water box. The potential functions and atom interaction parameters were modeled with the compass force field (Sun, 1998). The total potential energy of the wet foam film system is given in Eq. (1):
E total =E vdW +E Q +Ebond +Eangle +E torsion +E inversion
(1)
whereE total,E vdW ,E Q ,Ebond ,Eangle ,E torsion ,E inversion are the total energy, van der Waals, electrostatic, bond-stretching, angle-bending, torsion, and inversion components, respectively. The atomic charges of surfactant, foam stabilizer and accessory ingredient were assigned using the charge equilibration (QEq) method (Rappe and Goddard III, 1991). The atomic charges of the water molecules were taken from the SPC/E
Fig. 3. Schematic preparation of initial configuration of SDS surfactant monolayer.
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Fig. 4. Schematic preparation of the foam system generated by SDS in which two SDS surfactant monolayers place on both sides of the water box. For clarity, the SDS is drawn as van der Waals spheres and water molecules drawn in line style. C, gray; H, white; O, red; S, yellow; Na+, violet. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. (a) Initial hexagonal packing of the SDS and DDA mix layer, (b) simulated configuration of foam system generated by SDS and DDA, (c) initial hexagonal packing of the PVA and DDA mix layer, (d) simulated configuration of liquid film layer made by PVA and DDA, (e) simulated configuration of foam system generated by SDS, DDA and PVA. For clarity, the SDS, DDA and PVA are drawn as van der Waals spheres and water molecules drawn in line style. C, gray; H, white; O, red; S, yellow; Na+, violet. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
DDA liquid film layer and PVA/DDA liquid film layer as shown in Fig. 5e. The Z dimension was kept large enough to avoid interactions between the periodic replicas. Firstly, 5000 steps of geometry optimi-
zation based on smart algorithm were run to avoid the possible molecular overlap occurred in initial built configurations. The systems equilibrated at about 1 ns. As a result, the MD simulations were carried out under isothermal constant volume for 4 ns subsequently. 75
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carried out under isothermal constant volume for 4 ns, and the results of the last 1 ns were utilized to analyze and evaluate. The radial distribution functions (RDFs) of water molecules adjacent to the sulfonate groups and sodium ions of SDS in the inner liquid film layer of multilayer foams were analyzed to understand distribution of bound water molecules (Fig. 8). It clearly observed that both of distribution intensity of bound water molecules around the sulfonate groups and sodium ions of SDS increased in all systems, while the distribution intensity climbed up dramatically in SDS/DDA/ PVA system, nearly as twice as that in SDS and SDS/DDA foam systems. As illustrated in Fig. 1(II-2), the intermolecular interactions were weak due to the reversible ionization reaction to give rise to the emergence of amount of free water in films. In addition, the dynamic fluctuations of SDS molecules would lead to the fast gas diffusion to make foams unstable. When DDA and SDS were added in solution, more dodecyl sulfonate ions as well as sodium ions could be offered to interact with DDA and water molecules (III-2 in Fig. 1) to increase the amount of bound water molecules in inner liquid film. However, water molecules in the monolayer liquid film had a tendency to escape before interacting with solvent molecules. The distribution intensity of bound water molecules at this moment has not been effectively increased. Contrarily, the addition of PVA with multiple hydrophilic groups would enhance the intermolecular interactions between water and solvent molecules because of strong intermolecular hydrogen bonding as shown in Fig. 1(III-3). Besides, the hydrophobic groups in air1 and air2 layers in the formed multilayer foams would constantly push water molecules to the inner liquid film to get more immobilized water molecules in system (III-1-b in Fig. 1). More water molecules in the layer had consequently more chances to interact with solvent molecules to be bound. Therefore, the distribution intensity of bound water molecules climbed up dramatically around the sulfonate groups and sodium ions of SDS in SDS/DDA/PVA system. Moreover, due to the combined action of DDA and PVA with water molecules, the RDF of water molecules around the hydroxyls of DDA in the outer liquid film layer (Fig. 9) exhibited an increase tendency of distribution intensity of bound water as well. Both of these might be in favour of the stability of the liquid film layers. The equilibrated configuration snapshots of foam systems at the end of simulations were shown in Fig. 10. It observed that the staggered arrangements of hydrophobic tails of the absorbed molecules were increased with the addition of DDA in comparing Fig. 10a with Fig. 10b. It indicated that the molecular arrangements in SDS/DDA system were much more disordered than that in SDS system. Such
Thereinto, the results of the last 1 ns were utilized to analyze and evaluate. 3. Results and discussion 3.1. Microstructure of foams Fluorescence microscope images of foams were given in Fig. 6a–c. The bright field represented the air in the foam while the dark field to the liquid films. Double layered bubbles like the CGA with the addition of DDA and PVA to SDS were clearly observed which almost consisted with Fig. 1(III-1-b). Meanwhile, we found that there were contact points from the inner layer to the outer one. These points which might be formed by the polyhydroxy PVA like bridges were assumed to transversely connect the layers to enhance the stability of bubbles. In conventional foams, liquid would spontaneously flow to the Plateau border on account of the surface tension. It usually resulted in thinner liquid films, and broken bubbles at end (Studart et al., 2006). But in our case, a large amount of hydrophobic groups at the border was favorable to stop the drainage since the water molecules were assumed to be immobilized in the outer layer by hydrophobic effect (Fig. 6c–d). 3.2. Foam stability As shown in Fig. 7, the foams showed no significant drainage and collapse even after 96 h of standing at 25 °C. Actually, the cellular network constructed by ultrathin foam films maintained even after 3 months. However, foams produced by SDS alone show a poor stability with a few tens of minutes (Fameau et al., 2011). As a result, it concluded that the addition of DDA and PVA could effectively slow down the drainage, coalescence and disproportionation processes to make such foams such unprecedented long lifetime lived. 3.3. Molecular simulation study of foam films In order to analyze the multilayer foams structure at the molecular level, molecular dynamics simulation of foam films were carried out in this work. The foam systems were constructed (Figs. 3–5) and the configurations set in simulation in accordance with designed foams in II-1-a, III-1-a and III-1-b (in Fig. 1) respectively corresponded to Figs. 4, 5b and e. 5000 steps of geometry optimization were run to avoid the possible molecular overlap occurred in initial built configurations. The molecular dynamics simulations were subsequently
Fig. 6. Fluorescence microscope images of (a) multilayer films foams system, (b) single bubble with multilayer films, (c) border of three bubbles. (d) The border of three designed bubbles.
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Fig. 7. Photographs of multilayer film foams after standing varied periods.
Fig. 8. Radial distribution functions of water molecules around (a) the sulfonate groups (the sulfur atom was chosen as the center) and (b) sodium ions of SDS in SDS, SDS/DDA, and SDS/DDA/PVA foam systems (oxygen atom of water molecules was chosen as the target).
for the coverage of gas-liquid interface with its multiple hydrophilic hydroxyls always spreading into water to reduce the rate of gas diffusion between layers. The air pressure in air1, air2 and outside layers could be balanced for a long time to exhibit outstanding foam stability. Both the RDF analyses and equilibrated configuration snapshots indicated that, in foams with multilayer films, the stability can be markedly improved not only due to the increase of bound water but due to the decrease of rate of gas diffusion with compacted molecule arrangement and the increased coverage of hydrophobic chains on gasliquid interface. In addition, in order to understand the influence of adding order of compounds on the foam structures we added SDS, DDA and PVA simultaneously to prepare foams. It found that the formed multilayer structure (Fig. 11) was similar to that in Fig. 6. It indicated that the multilayer films can be formed by self-assembly of molecules and it only depended on the additions but not the order. The microscopic structure of such foam at molecular level is still under investigation. Fig. 9. Radial distribution function of water molecules around the hydroxyls of DDA in the outer liquid film layer of multilayer foams (oxygen atom of hydroxyls of DDA was chosen as the center; oxygen atom of water molecules was chosen as the target).
4. Conclusions In summary, amphiphilic compounds, such as DDA, SDS and PVA, were creatively employed to elaborate the ultra-stable foams with multilayer films which were similar to the structure of CGA. It could stand without drainage and collapse even after 96 h at 25 °C. Due to the solubilization of SDS and PVA, DDA molecules were arranged with SDS and PVA respectively to form inner and outer liquid film layer, and the hydrophobic groups of SDS, DDA and PVA close to each other to
disordered arrangement was supposed to strengthen the gas-liquid interface and compactness of molecular network formed by solvent molecules at gas-liquid interface. As a result, the gas diffusion rate could be decreased to get stable foam system (Hu et al., 2012). Furthermore, the hydrophobic chains of PVA molecules were benefit 77
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Fig. 10. The equilibrated configuration snapshots at the end of simulations of (a) SDS, (b) SDS/DDA, (c) SDS/DDA/PVA. For clarity, the SDS, DDA and PVA are drawn as van der Waals spheres and water molecules drawn in line style. C, gray; H, white; O, red; S, yellow; Na+, violet. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 11. Fluorescence microscope images of the foams prepared by one step.
constitute multilayer films foams by the hydrophobic interaction. From structural study in microscopic via molecular dynamics simulation, we found that distribution intensity of bound water rose up dramatically around both sulfur atom and sodium ion of sulfonate groups of SDS in SDS/DDA/PVA system because of the enhanced hydrogen bound between water and solvent molecules when adding PVA. Moreover, with the addition of PVA, the gas-liquid interface could be strengthened as well as the compactness of molecular in network. It led to decreased gas diffusion rate to make the foam system stable. Meantime, the multilayer film structure was favorable for the balance of air pressure between air layers in the system with a higher fluctuation threshold comparing to the monolayer film, which resulted in the formation of ultra-stable foams. Acknowledgment The work was financially supported by the China Postdoctoral Science Foundation (2015M572210), the National Natural Science Foundation of China (61604110), the Hubei Provincial Natural 78
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