Effects of Salt-Controlled Self-Assembly of Triblock Copolymers F68

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Salt-Controlled Self-Assembly of Triblock Copolymers-F68 on Interaction Forces between Oil Drops in Aqueous Solution Hang Jin, Wei Wang, Hongli Chang, Yun Shen, Zhipeng Yu, Yunya Tian, Yang Yu, and Jing Gong Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02925 • Publication Date (Web): 02 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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SaltSalt-Controlled SelfSelf-Assembly ssembly of Triblock Copolymersopolymers-F68 on Interaction Forces Forces between Oil Drops in Aqueous Solution Hang Jin†, Wei Wang*†, Hongli Chang†, Yun Shen†, Zhipeng Yu†, Yunya Tian†, Yang Yu† and Jing Gong*† †National Engineering Laboratory for Pipeline Safety, Beijing Key Laboratory of Urban Oil and Gas Distribution Technology, China University of Petroleum, Beijing. 18# Fuxue Road, Changping District, 102249, Beijing, China. *E-mail: [email protected], [email protected].

ABSTRACT:

Nonionic

triblock

copolymers

surfactant-Pluronic

F68

(PEO76-PPO29-PEO76) are widely used in industrial processes, such as foaming, emulsification and stabilization. The behaviors of triblock copolymers such as the salt-dependent self-assembly in bulk solution and the irreversible adsorption at oil/water interface are mainly focused to explore their effects on the interaction forces between nano-spacing interfaces of oil droplets. In this study, AFM technique was employed to measure the drop interaction forces with different F68 bulk concentrations. All selected bulk concentrations (≥100 µM) of copolymers can ensure the formation of a stable layer structure of stretched polymer chains (“brush”) at oil/water interface, which behaved as mechanical barrier at the interface. This study quantified the forces caused by space hindrance of F68 copolymers both in bulk phase and at the interface of oil/F68 aqueous solution during drop interaction. The effects of monovalent electrolyte (NaCl)-induced self-assembly behavior of triblock copolymers F68 in bulk solution on drop interaction forces were measured through AFM technique. Key Words: Self-Assembly, triblock copolymers, interface of oil/ F68 aqueous solution, electrolyte, AFM, interaction forces

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INTRODUCTION Emulsions stabilized by interfacial active components, including surfactants1,2, polymers3 and nano-particles4,5, widely exist in pharmacy6,7, biochemistry8, food9, new energy10, and petroleum chemical industries11. Especially, the interfacial active polymers with large molecular weight are widely studied due to the diversity of structure, which can generate strong steric barrier to inhibit drop coalescence12,13 and stabilize emulsions. Triblock copolymers, having narrow molecular-weight distribution and nontoxicity, are focused popularly because of their

irreversible

interfacial

adsorption

characters14,15,

salt-controlled

self-assembly behaviors16-19 and controllable molecular structure rigidity and composition20,21,22. Recently, the advances of atomic force microscope (AFM) and surface force apparatus (SFA) in measuring the interaction forces of dispersed droplets23-27 and the real-time film drainage process28,29 provide a systematic understanding for the mechanisms of emulsion stabilization. A consistent theory30-35 was derived to describe non-equilibrium force measurement and coalescence phenomena involving deformable droplets. But for the interfacial active components with large molecular weight (e.g., polymers, polyelectrolytes and mixed systems), their adsorption at oil/water interface or aggregation in bulk phase36-39 impose a more complicate effect on the interaction forces between emulsion drops40-44. In addition, the drop interaction under the effect of triblock copolymers stabilizer was rarely investigated by AFM in previous studies14-19. Manor et al.36 systematically studied the steric effect of a series of Pluronic triblock copolymers adsorbed at interface after washout with aqueous solution, where the effect of self-assembly behavior in bulk phase demands further study. Special attentions have been paid on the adsorption behavior at soft/rigid surface14,15, and the aggregation in solutions16-19,45,46 of triblock copolymers. Nevertheless, there still lacks the quantitative analysis on interaction force under the influence of self-assembly of copolymers in bulk phase. In present study, the AFM technique is used to directly measure the interaction forces between oil drops with F68 aggregates in aqueous solutions. The effect of salinity on the self-assembly behavior is discussed. Based on this study, the physicochemical properties of emulsions stabilized by triblock


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copolymers (PEO-PPO-PEO) can be precisely manipulated through controlling the solution conditions (e.g., salinity, polymer concentration).

EXPERIMENTAL SECTION Materials. Pluronic F68 (average molecular structure PEO76PPO29PEO76) was purchased from Sigma-Aldrich CO., U.S.A., with an average molecular weight of 8400. F68 was used without further purification. F68 solutions were prepared by dissolving quantitative amount of F68 in deionized water with a resistance of 18.3 MΩ·cm. Then the solutions were sonicated for 1.5h to completely dissolve. The prepared solutions were sealed and stored in a thermostatic water bath with 20℃ temperature. NaCl was purchased from Sigma-Aldrich CO., U.S.A. and used (Bioxtra, >99.5%) without further purification. And oil used for experiments was tetradecane (>98%). The experimental substrates were silicon wafers (1.2 × 1.2 cm2) covered with a homogeneous silica layer of thickness 337.1±0.4 nm measured by imaging ellipsometry (Accurion_EP4SE, Germany) at 20±0.5°C (see Fig. S1 in Supplementary Information). Before experiments, the silicon wafers were rinsed with ethanol and water then dried in a square box under a stream of purified nitrogen. The AFM fluid cell and O rings were all cleaned by ethanol and water then dried with purified nitrogen. All measurements were carried out under a solution condition of pH=6. Rectangular tipless AFM cantilevers, purchased from MikroMasch (HQ:CSC38/tipless/Cr-Au), were applied to pick up oil droplets deposited on the substrates. The spring constant of cantilever was obtained through the thermal tune function of the operating software in Bruker multimode 8, which is based on the equations of Hutter and Bechhoefer47. Interfacial Tension. The equilibrium interfacial tension of tetradecane-F68 solution at various NaCl bulk concentrations, c (mM), were measured by the spinning drop method with a Dataphysics SVT20N (Germany) at 20±0.5°C. Contact Angle Measurements. The contact angles of tetradecane drops formed on the silicon wafers in aqueous solution were measured. In excess of 3 wafers with 3 drops per disk were used to obtain the average measured contact angle. The contact angles of


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tetradecane drops in polymer solutions were found to be constant within reasonable measurement error after an equilibration period of 30 min (see Fig. S2 in Supplementary Information). The sensitivity of drop interaction forces to contact angle changes was analyzed with the limiting case for droplet deformation48 (see Fig. S4 in Supplementary Information). AFM Force Measurements. The AFM technique (Bruker, multimode 8) was applied to measure the interaction forces between two oil drops in aqueous solution. For each measurement, a narrow gauge needle syringe (a custom-made ultra-sharp glass pipet) was used to spray oil and generate oil drops on the substrate23,24. Small water drops were added slowly to cover the oil drops spreading over substrate. It was found that several oil drops remain on the substrate although most of them were washed off. To facilitate the transfer of oil droplets from substrate to cantilever, a hydrophobic surface of cantilever is needed. In this study, rectangular cantilevers coated with chrome (thickness of 20 nm) and gold (thickness of 20 nm) were immersed in an ethanol solution of hydroxyl-thiol (11-mercapto-1-decanol, Aldrich 97%) for 18 h to allow the formation of a self-assembled monolayer (SAM) of thiols at surface. Then the droplet can be picked up from the substrate by the hydrophobic cantilever. However, the droplet cannot be transferred from substrate to cantilever in F68 solutions. It’s believed that the copolymer layer adsorbed at drop surface can prevent the drop from adhering to cantilever. To avoid the intercross contamination between the experimental and ambient environment, including solution exchange procedure, the fluid cell and the substrate were sealed by O ring during experiments. Before force measurement, the oil drop attached on cantilever was driven close to the drop on substrate by the engagement process of AFM. The engage process will be stopped when the deflection of cantilever meets the default setpoint. Force curve is taken in the individual measurement mode, spaced by several seconds to ensure independent measurement with a piezo displacement of 2 μm. Before a new measurement, the solution in fluid cell needs to be totally replaced by injecting a new solution of 10 times the cell volume through a syringe pump. In Manor et al.36’s research, the transition from electrostatic to steric stability is proposed at about 10mM NaCl in a F108 solution. So for F68 copolymer (with a shorter chain


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length) in this study, the solution salinities used are larger than 10mM (≥20 mM) to keep the steric effect of adsorbed polymer in dominant.

Results and Discussion Effect of F68 Concentration. In literatures13,14,15, F68 solution with the concentration of 25 µM, corresponds to a “hairpin” conformation where the triblock copolymer molecules are closely packed as “brushes” at the soft interface. So the chosen concentrations of F68 are larger than 25 µM to keep the adsorbed molecules in a dense structure. It’s believed that the adsorption of F68 at oil/water interface is stable and irreversible with a strong desorption barrier13,14,15. A washout experiment is performed to totally replace the solution in AFM fluid cell, while the adsorbed F68 molecules remain on the interface due to the irreversibility of adsorption. For oil drops immersed in F68 solutions, the coalescence does not occur during measurement and the coalescence force is beyond the measured force of AFM in our experimental system. The critical micelle concentration of F68 in deionized water has been proved larger than 1000 µM at 20℃13,15. Thus, the drop interaction forces are measured and compared with F68 concentrations lower than 1000 µM (100 µM, 500 µM). The electric double layer disjoining pressure during drop interaction is shortened with an addition of 20 mM NaCl to make steric barrier of adsorbed F68 dominant. A comparison of measured drop interaction forces (with a piezo displacement 2 µm) stabilized by F68 solutions (20mM NaCl) before and after washout by 20mM pure NaCl solution is shown in Fig. 1. The detailed washing procedure is shown in the schematic map in Fig. 1C. The interaction force between a pair of tetradecane drops immersed in 100 µM F68 solution (20 mM NaCl) is first measured after an equilibration period of 30 min (Fig. 1C-1). Then the system is flushed with 20 mM NaCl solution and equilibrates for 10min before a new force measurement (Fig.1C-2). Next, 10 times cell volume of 500 µM F68 (20 mM NaCl) solution is pumped through the fluid cell and equilibrates for 30 min to conduct a new force measurement (Fig. 1C-3). After that, the fluid cell is flushed again with 20 mM NaCl solution (Fig. 1C-4). It can be concluded that the consistency of the drop radius can be ensured although there might be a minor change in contact angle during measurement.


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Figure 1. Comparison of the interaction forces (V=0.4 µm/s) between two tetradecane drops (Rcan~35 µm, Rsub~32 µm) before and after washing with 20 mM NaCl solution, respectively in (A) 100 µM (20 mM NaCl) and (B) 500 µM F68 (20 mM NaCl) solutions. (C) Schematic map of detailed washing procedure. Comparison of the equilibrium forces (V=100 nm/s) between tetradecane drops (Rcan~30 µm, Rsub~32 µm) immersed in 500 µM F68 (20 mM NaCl) solution measured with (D) force ramp 1 nN and (E) force ramp 5 nN. It can be seen that the repulsive force appears larger when copolymer exist in the bulk solution (before washout). After washout with 20 mM pure NaCl solution, the repulsive force appears smaller due to the displacement of F68 molecules in the bulk phase. With a lower driven velocity V=100 nm/s, the equilibrium forces between two oil drops (500 μM F68, 20 mM NaCl) were measured before and after washout (see Fig. 1D-E). Under this experimental condition, the characteristic oscillatory disjoining pressure is not detected which can be attributed to the smaller amounts of added F68 (0.42%) compared with


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the added solute (larger than 6%) in previous study25. Considering the irreversible adsorption of F68 copolymers at the interface13-15,

36,

the difference between

measured force curves before and after washout is attributed to the existence of F68 copolymers in the bulk phase, which represents the space hindrance in F68 bulk solution.

Figure 2. Theoretical modeling of the force-distance data between drops (Rcan~35 µm, Rsub~32 µm) stabilized in (A) 100 µM F68 (20 mM NaCl) and (B) 500 µM F68 (20 mM NaCl) solutions after washing with 20 mM NaCl solution. The force curve after washout is fitted with the limiting case for droplet deformation48 (see Eq. (1) and Fig. 2).

(1) Where RC is the radius of droplet on cantilever and RS is the radius of droplet on substrate. θC and θS are the contact angles of droplet on cantilever and substrate, respectively. σ is the equilibrium interfacial tension of oil-aqueous solution interface. ΔX is the drop displacement. hf is the constant film thickness when the drop surfaces come into contact with deformation. From Fig. 2, it can be seen that the fitting results are in agreement with the force curve after washout. However, the force curve before washout is hard to analyze theoretically in detail because of the trapped copolymers between interacting drops. The complex interacting mechanisms between trapped copolymers impose a much more complicated effect on drop interaction. From the above analysis, the drop deforms to a larger extent than the compression of polymer brush when drops are coming into contact with each other. Thus, the repulsive interactions are expected due to the space hindrance of adsorbed polymer brush and the deformation of drops. The equilibrium interfacial tensions of tetradecane drops in F68 solutions ACS Paragon Plus Environment

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with 20 mM NaCl are measured to be 14.3±1 mN/m and 11.8±1 mN/m for 100 µM and 500 µM F68 respectively. After washing the drops, the changes in interfacial tension caused by desorption are measured to be lower than 1.5 mN/m. To investigate the effects of interfacial tension changes, the interaction forces after washout with 20 mM pure NaCl solution were compared. It can be seen that the difference in force curves caused by the variations of interfacial tension is limited, as shown in Fig. 3A. The sensitivity of force curves to interfacial tension changes is analyzed as well (see Fig. S3 in Supplementary Information). From the modeling results, it’s found that the minor change of interfacial tension (±1.5 mN/m) has little impact on the gradient of interaction forces. From Fig. 3B and Fig. 3C, it can be seen that the force hysteresis appears larger when copolymers exist in the bulk solution (before washout). After washout with 20 mM pure NaCl solution, the force hysteresis appears smaller due to the displacement of F68 molecules in bulk phase. Considering the irreversible adsorption of F68 copolymers at the interface, the force difference between measured force curves before and after washout can be attributed to the trapped F68 copolymers in bulk phase, which represent the space hindrance of F68 copolymers in bulk solution when drops interact with each other. Also, the force differences during drop interaction were found to be enhanced with the increment of F68 concentrations from 100 µM to 500 µM, which means the space hindrance in 500 µM F68 solution is larger than the one obtained in 100 µM F68 solution. It indicates that the strengthened force difference is caused by the increased number of copolymers per volume solution rather than the increased adsorption at interface.


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Figure 3. (A) Force curves (V=0.4 µm/s) between two tetradecane drops (Rcan~35 µm, Rsub~32 µm) in F68 (20 mM NaCl) solutions after washout with 20 mM pure NaCl solution. (B) and (C) are the force-distance data for the above two cases Effects of Self-Assembly of F68 Molecules in Bulk Phase. It has been proved that the aggregation of Pluronic copolymers in aqueous solutions is strongly salt-dependent, especially on salt type and concentration 13,16-19.

The salt-dependent aggregates of F68 copolymers in bulk phase have much

impact on the mechanical behavior of emulsified droplets. In this part, the effects of NaCl on the self-assembly of F68 molecules in bulk phase are discussed through the measurement of interaction forces between oil drops. Fig. 4 shows the measured interaction force curves between drops immersed in 500 µM F68 (20 mM NaCl) solution and 500 µM F68 (100 mM NaCl) solution. The detailed operations of cell flushing are the same as the operations introduced before. After the drop stabilization respectively in 500 µM F68 (20 mM NaCl) solution and 500 µM F68 (100 mM NaCl) solution, little interaction difference is found between two cases. It indicates that the self-assembly of 500 µM F68 under salinity below 100mM has little impact on the drop interaction forces.


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Figure 4. Force curves (V=0.4 µm/s) between two tetradecane drops (Rcan~30 µm, Rsub~32 µm) in 500µM F68 and NaCl mixed solution before washout. The ion-induced conformational changes of triblock copolymers have been investigated by Nambam et al.49. It should also be noted that the increased salinity will directly increase the adsorption of F68 at interface and decrease the interfacial tension13,14. Thus, at higher salinity (≥100 mM), the influence of high-salt induced changes of interfacial tension on drop dynamics needs to be clarified by comparing the measured interaction forces under different salinities after washout with pure salt solution. Meanwhile, effect of the self-assembly of F68 in bulk phase on the drop interaction can also be illustrated by contrasting the differences in interaction force measured before and after washout with pure salt solution. During the experiment, after the equilibrium adsorption in 500 µM F68 solutions with increased salinity of 100 mM, 300 mM and 500 mM NaCl respectively, the loading-washout with pure NaCl solutions is undertaken to flush F68 copolymers out of the fluid cell. The salinities of flushing salt solutions are 100 mM, 300 mM and 500 mM NaCl correspond to the salinities of equilibrium F68 solutions. Then force measurements are achieved with different driven velocities. The detailed washing procedure is shown in the schematic map in Fig. 5. In Fig. 5C, the measured interaction force between the same pair of drops after washout with pure NaCl solution is demonstrated. The measured interfacial tensions of tetradecane-F68 solutions with 100 mM, 300 mM and 500 mM NaCl were 11.1±1 mN/m, 9.9±1 mN/m and 8.0±1 mN/m respectively. It can be


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concluded that the difference in force curves caused by the variations of interfacial tension is limited.

Figure 5. Detailed washing procedure is shown in (A). Time dependent force curves (V=0.4µm/s) between two tetradecane drops (Rcan~30 µm, Rsub~31 µm) (B) equilibrated in 500 µM F68 solutions with increasing salinity before washout and (C) after washout respectively with 100 mM, 300 mM and 500 mM NaCl solution. (D) and (E) are the force-distance data for the above two cases. Fig. 5B and Fig. 5D are the measured force curves between two drops immersed in 500 µM F68 solutions with different salinities (before washout) under driven velocity V=0.4 µm/s. By increasing the salinity from 100 to 500 mM, higher repulsion force is detected significantly, which is directly induced by the increased size of F68 aggregates under strengthened salinity. The effect of salinity on the aggregation of PEO-PPO-PEO type copolymer can be attributed to the change of hydrogen-bonding structure in water under the influence of strong polarization of added salt16-19. To become hydrated, the added salt must compete


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with the hydrophilic copolymer chains for the water molecules, where the competition enhances the self-hydration of salts through hydrogen bonding and weakens the interaction of copolymer molecules with water. It results in the reduced solubility of copolymer and further induces the salt-dependent self-assembly (a “salting out” effect)16-19. For F68 solutions, the solubility of F68 decreases when higher amount of NaCl is dissolved in water. Consequently, the F68 copolymers tend to adsorb at oil-water interface (Fig. 5C), and self-assemble in bulk phase (Fig. 5B). In Fig. 6A, the force curves after washout are fitted with the limiting case for droplet deformation48. Similar constant film thicknesses are obtained compared with the case in 100 µM F68 solution. From the fitting results, the space hindrance of adsorbed polymer brush in different cases keeps the same, which indicates that the adsorbed copolymers are still closely packed as “brush” at interfaces with increasing salinity.

Figure 6. (A) Theoretical modeling of the force-distance data between drops (Rcan~30 µm, Rsub~31 µm) stabilized in 500 µM F68 solutions with increasing salinity. (B) Equilibrium forces (V=100 nm/s) between tetradecane drops (Rcan~30 µm, Rsub~32 µm) immersed in 500 µM F68 solutions (300 mM and 500 mM NaCl) measured with force ramp 1 nN. The equilibrium forces between two oil drops immersed in 500 μM F68 solutions (300 mM and 500 mM NaCl) were measured before and after washout with a lower driven velocity V=100 nm/s (see Fig. 6B). However, the characteristic depletion force25 is not detected. It can be attributed to the smaller amount of added F68 (0.42%) which cannot form enough self-assembled aggregates for detecting depletion and structural forces during drop interaction. The effect of the self-assembly induced space hindrance on the drop interaction


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forces can be characterized by the difference in measured force curves before and after washout with pure NaCl solution, as shown in Fig. 5. Self-Aggregate of F68 Molecules at Silica surface. During the experiment, the precipitated F68 aggregates (≥100mM) were found at the surface of immersed silica. The topographic AFM images of silica surface immersed in 500 µM F68 solutions with different salinities (100, 200, 300 and 500 mM NaCl) are achieved by AFM with ScanAsyst mode after 30 min equilibration. As shown in Fig. 7, in the height images of silica surface morphology, the increased sizes of F68 aggregates are detected. To clearly show the precipitated F68 aggregates in solution with 100 mM NaCl, a smaller surface area (5×5μm2) is chosen in Fig. 7A to achieve the height image by AFM.

Figure 7. AFM height images of silica surface immersed in 500µM F68 solutions with different NaCl concentrations: (A) 100mM (5×5μm2), (B) 200mM (20×20μm2), (C) 300mM (20×20μm2), and (D) 500mM (20×20μm2) at 25±1℃. The height of randomly distributed aggregates or “particles” increases from 3.1 to 259.9 nm with increased salinity from 100 to 500 mM. From previous


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research13, the adsorption of PEO-PPO-PEO molecules at silica surface is induced by hydrogen bridges between the surface silanol groups and the oxygen of PEO, which is strongly affected by the dissociation degree of silanol groups on silica. Since the dissociation degree of silanol groups in aqueous solution increases with ionic strength13, the stable hydrogen bridges between the surface silanol groups and the oxygen of PEO segments cannot form spontaneously, which will decrease the adsorption of F68. In the meantime, the solubility of F68 copolymer molecules in bulk phase decreases as well with increased ionic strength. As a result, F68 molecules are more inclined to form larger aggregates at silica surface to reduce the contact between F68 and silica surface, and the size of aggregates enlarges with increased ionic strength.

CONCLUSIONS The interaction forces between tetradecane droplets stabilized in Pluronic F68 copolymer solutions with different salinity were directly measured using drop probe AFM technique. The increase in F68 bulk concentrations lead to larger repulsive force measured between the same pair of droplets, which can be attributed to the increased number of copolymers per volume solution. It has been proved that the assembling structures of F68 copolymer in bulk phase is deeply influenced by NaCl. Enforced repulsive forces were detected between drops immersed in 500 µM F68 solutions with NaCl concentrations larger than 100 mM, while little difference is observed at lower salinity (≤100 mM). It was also found that the increased salinity will directly enforce the adsorption of F68 at oil/water interface and lower the interfacial tension. After increasing salinity (>100mM) of 500 µM F68 solution, the precipitated F68 aggregates with larger size were found from AFM images at surface of immersed silica. We propose that the trapped copolymer aggregates between two interacting interfaces could further stabilize the drops and can be manipulated precisely through controlling the solution conditions.

ASSOCIATED CONTENT Supporting Information Thickness of the silica layer covered on silicon wafers measured by ellipsometry.


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Measured contact angles of tetradecane drops formed on the silicon wafers in F68 solutions. Sensitivity of force curves to interfacial tension changes and contact angle changes calculated with the limiting case for droplet deformation.

AUTHOR INFORMATION Corresponding Author *Wei Wang, E-mail: [email protected]. *Jing Gong, E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors wish to thank the National Natural Science Foundation of China (51774303, 51422406, 51534007), the National Science and Technology Specific Project (2016ZX05028-004-001), the Henry Fok Foundation (142021), and the Science Foundation of China University of Petroleum, Beijing (C201602) for providing financial support for this research.

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