Responsive Emulsions Stabilized by Amphiphilic Supramolecular

Apr 23, 2018 - described in Figure S3. The interface of toluene containing .... taken after 24 h of storage at the room temperature. Scheme 2. Illustr...
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Responsive Emulsions Stabilized by Amphiphilic Supramolecular Graft Copolymers Formed in Situ at the Oil−Water Interface Huazhang Guo,†,‡ Pin Liu,†,‡ Huaming Li,†,‡ Chong Cheng,*,§ and Yong Gao*,†,‡ †

College of Chemistry and ‡Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education; Key Laboratory of Polymeric Materials & Application Technology of Hunan Province, Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, Xiangtan University, Xiangtan, Hunan Province 411105, China § Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States S Supporting Information *

ABSTRACT: Amphiphilic supramolecular graft copolymers which can stabilize oil-in-water (o/w) emulsions and enable responsive demulsification were demonstrated in this study. Linear poly[(N,N-dimethylacrylmide)-stat-(3-acrylamidophenylboronic acid)] (PDMA-stat-PAPBA) copolymers with phenylboronic acid (PBA) groups and linear polystyrene homopolymers with cis-diol terminals (PS(OH)2) were synthesized by reversible addition−fragmentation chain transfer polymerization. By the homogenization of the biphasic mixtures of an alkaline water solution of PDMA-stat-PAPBA copolymer and a toluene solution of PS(OH)2 homopolymer, stable o/w emulsions could be generated, although neither PDMA-stat-PAPBA nor PS(OH)2 alone was able to stabilize the emulsion. It was verified that the dispersed oil droplets in the emulsions were stabilized by the amphiphilic PDMA-stat-PAPBA-g-PS supramolecular graft copolymers, which were formed in situ at the oil−water interface by the complexation between the lateral PBA groups of PDMA-stat-PAPBA and the diol terminals of PS(OH)2 during homogenization. These emulsions showed pH- and glucose-responsive demulsification because of the reversible B−O bonds between the PDMAstat-PAPBA backbones and the PS side chains. The effects of polymer concentrations on emulsion formation were also investigated. The current study provides an alternative method for the facile preparation of responsive polymeric emulsifiers, which potentially may be extended to other polymer pairs containing PBA and cis-diol groups.

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INTRODUCTION Emulsion is defined as a class of disperse system consisting of two immiscible liquids.1 In the field of scientific research, emulsions occupy an important position owing to their diverse applications in many areas, such as emulsion polymerization, material synthesis, coating and encapsulation, food processing, cosmetics, and so on.2−7 Emulsifier is a crucial component for the formation of the emulsion. Low molecular weight (MW) surfactants and amphiphilic polymers with different topologies are the main candidates for emulsifiers.8−10 In addition, solid particles or organic polymer soft particles exhibiting suitable wettabilities can be used as Pickering emulsifiers.11 Relative to low MW surfactants, amphiphilic polymeric emulsifiers offer greater opportunities in terms of structural flexibility, diversity, and functionality because of their designable molecular architecture and adjustable composition and MW. 12−14 Although significant efforts have been concentrated on the development of emulsifiers to improve the stability of emulsions, as a matter of fact, the demulsification of emulsions © XXXX American Chemical Society

is also very important in the industry. For example, the highly viscous heavy crude oil often needs to form less viscous emulsion with water to facilitate transportation through pipelines, and the emulsion must be highly stabilized by surfactants to avoid phase separation during the transportation, whereas the emulsion stability is no longer desired once the crude oil reaches its destination.15,16 Therefore, a facile demulsification approach is highly preferred for the crude oilbased emulsion. In many other cases in which only the temporary stability is needed, such as the industrial waste-water treatment,17 emulsion polymerization,18 and so on, responsive emulsions are also highly preferred. Facile demulsification can make the separation of products and the recycling of emulsifiers much easier. Moreover, responsive emulsions are critically Received: February 11, 2018 Revised: April 23, 2018

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DOI: 10.1021/acs.langmuir.8b00476 Langmuir XXXX, XXX, XXX−XXX

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dimethylacrylmide)-stat-(3-acrylamidophenylboronic acid)]-gpolystyrene (PDMA-stat-PAPBA)-g-PS copolymers. Unlike conventional amphiphilic graft copolymers, PDMA-statPAPBA-g-PS copolymers were formed in situ at the oil− water interface by means of the complex interaction of the PBA lateral groups of PDMA-stat-PAPBA dissolved in the water with the diol terminals of PS(OH)2 dissolved in the oil phase during homogenization. The formation of the supramolecular graft copolymers were verified by interfacial tension measurements and Fourier transform infrared (FT-IR) characterization, as well as size-exclusion chromatography (SEC) analysis. The pH- and glucose-responsive demulsification of the emulsion was also carefully investigated.

important for controlled release when the emulsions are used as delivery vehicles.19 An essential strategy for the preparation of responsive emulsions is to use emulsifiers with switchable interfacial activities. Regarding this aspect, amphiphilic copolymers have been broadly studied because of the fact that their interfacial activities can be flexibly switched by the suitable change of the appropriate physicochemical parameters, such as polymeric emulsifier structure and composition, temperature, pH, the wavelength of light, and so on. 13,15,20−26 Amphiphilic copolymers with dual responsivities have attracted considerable interest and can allow sophisticated control of the structures and structure-dependent properties of the corresponding emulsions or other dispersed systems through diverse stimuli.27,28 Generally, it is challenging to design and synthesize amphiphilic copolymers with specific combinations of dual responsivities, although such copolymers can be very relevant to certain applications. For instance, amphiphilic copolymers with both pH- and glucose-responsivities may potentially be utilized for drug delivery through emulsions or other types of vehicles because a number of common diseases, such as cancer29 and diabetes,30 are involved with disease-specific variations of acidity or glucose levels in biological systems. However, there are few reports on such amphiphilic copolymers with interfacial activities that can be tuned by pH and glucose concentration.31,32 An amphiphile exhibiting optimal emulsifying properties usually possesses a hydrophilic part and a hydrophobic part, which are linked by covalent bonding. In addition, an amphiphilic emulsifier could be formed by means of supramolecular interaction. The efforts made in this direction can be traced to the early 1940s, when Schulman and Cockbain33 developed “nujol” to stabilize emulsions. As charged complexes, such “nujol” emulsifiers were formed in situ at the oil−water interfaces through the electrostatic interactions between the oilsoluble agent (cholesterol, cetyl alcohol, etc.) and water-soluble sodium cetyl sulfate. Nowadays, protein−polysaccharide electrostatic complexes are often used as emulsifiers for the preparation of Pickering emulsions in the food industry.34−36 In contrast to a conventional amphiphile, a supramolecular amphiphile generally consists of a hydrophilic part and a hydrophobic part that possess complementary recognition units for noncovalent interactions. However, there are very limited reports on the emulsions, except Pickering emulsions,37−39 stabilized by in situ formed supramolecular amphiphiles. Only very recently, Wang and co-workers reported the oil-in-waterin-oil (o/w/o) double emulsions by means of the interaction between PEG and iodine at the oil−water interface.40 Noteworthily, the interfacial formation of polymeric complexes has been considerably studied,41 especially for the preparation of capsules.42−44 In such systems, the components for the formation of the complexes are interfacially active and result in network structures at interfaces for biphasic stabilization. Dynamic covalent bonds (DCBs), also called reversible covalent bonds, refer to any covalent bonds possessing the capacity to be formed and broken under equilibrium control.45 DCBs combine the robustness of classic covalent bonds with the reversibility of noncovalent bonds. At present, DCBs have been an important tool for the fabrication of smart materials and widely employed in several areas, such as self-healing materials, triggered drug delivery, and so on.46−50 In this study, we report a novel system of responsive o/w emulsion, in which the emulsion droplets are stabilized by poly[(N,N-dimethyla-

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EXPERIMENTAL SECTION

Materials. N,N-Dimethylacrylamide (DMA) and styrene (St) were passed through a basic alumina column and then distilled under reduced pressure prior to polymerization, respectively. 2-(Ethylthiocarbonothioylthio)-2-methylpropanoate (EMP), 3-acrylamidophenyl boronic acid (APBA), and 2,2-bis(hydroxymethyl)butyl 2(ethylthiocarbonothioylthio)-2-methylpropanoate (EMP-(OH)2) were synthesized according to the previously reported methods, respectively.24,51 2-Bromoisbutyric acid ethyl ester and D-glucose were purchased from Aladdin (China) and used as received. Azobisisobutyronitrile (AIBN) was recrystallized twice from ethanol prior to use. Synthesis of the PS(OH)2 Homopolymer. St (1.04 g, 10.0 mmol), EMP-(OH)2 (85.2 mg, 250 μmol), and AIBN (13.7 mg, 83.0 μmol) were charged in a round-bottom flask. After being purged with nitrogen for 40 min, the flask was sealed and immersed in an oil bath at 60 °C. The polymerization reaction was stopped after 6 h by cooling to room temperature. The polymerization solution was diluted with CH2Cl2 and precipitated in diethyl ether. After multiple operation cycles involving dissolution, precipitation, and filtration, the purified polymer product was dried under vacuum at room temperature. Synthesis of the PDMA-stat-PAPBA Copolymer. DMA (4.00 g, 40.0 mmol), APBA (3.63 g, 19.0 mmol), EMP (224 mg, 1.00 mmol), AIBN (16.4 mg, 100 μmol), and DMF/H2O (v/v, 19:1, 7.50 mL) were charged in a round-bottom flask equipped with a magnetic stir bar. After being purged with nitrogen for 40 min, the flask was first sealed and then transferred to an oil bath at 70 °C. After 2 h, the polymerization reaction was stopped by cooling to room temperature. The polymerization solution was diluted with CH2Cl2 and precipitated in cold diethyl ether. After multiple cycles of dissolution, precipitation, and filtration, the purified polymer product was dried under vacuum at room temperature. Generation of the o/w Emulsion. In a typical experiment, PDMA-stat-PAPBA and PS(OH)2 were first dissolved in basic water solution and toluene, and the concentrations of the polymers were 0.015 and 0.05 wt %, respectively. The final pH of the PDMA-statPAPBA aqueous solution was adjusted to 11 by dialysis against pH 11 NaOH aqueous solution (unless otherwise stated, all pHs shown in the text were the final values of the solutions). Then, 4 mL of PDMA-statPAPBA aqueous solution and 2 mL of PS(OH)2 solution in toluene were sequentially charged into a 10 mL glass vial. Homogenization was performed using an XHF-D high speed disperser (Ningbo Scientz Biotechnology Co., Ltd, China) at a stirring rate of 12 000 rpm for 1 min. The pHs prior and after emulsification were essentially unchanged. The droplets of emulsion were observed by an optical microscope after 24 h of standing at room temperature. A few drops of the diluted emulsions were placed on a glass slide and viewed. The same emulsion preparation procedure was employed for the generation of emulsions stabilized at other polymer concentrations. pH-and Glucose-Responsiveness of the o/w Emulsion. The pH-triggered demulsification of the o/w emulsion was performed by the addition of a known volume (36.0 μL) of 1 mol/L of HCl aqueous solution into the emulsion. For the destabilized emulsion, a known volume (11.0 μL) of 1 mol/L NaOH aqueous solution was added. The re-emulsification was then performed by homogenization at a stirring B

DOI: 10.1021/acs.langmuir.8b00476 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Scheme 1. Synthesis Routes for PS(OH)2 and PDMA-stat-PAPBA

rate of 12 000 rpm for 1 min. The glucose-triggered demulsification of the o/w emulsion was performed by the addition of solid glucose into emulsion to reach a known concentration of glucose at room temperature, accompanying with a gentle stirring. Characterization. 1H NMR spectra were recorded on a Bruker AV-400 NMR spectrometer at room temperature using CDCl3 or CD3OD as the solvent. FT-IR spectra of samples were recorded on a PerkinElmer Spectrum One FT-IR spectrometer using the KBr pellet method. The number-average MW (Mn) and MW distributions of polymers were determined by SEC measurements, which were performed on a Waters 1515 SEC setup equipped with a Waters 2414 differential refractive index detector in DMF at 80 °C with a flow rate of 1.0 mL/min. Narrowly dispersed polystyrenes were used as calibration standards. The pendant PBA groups of PDMA-stat-PAPBA copolymers were protected through the esterification reaction with pinacol prior to the SEC measurement. Interfacial tension measurements were performed by the Wilhelmy plate method. The experiments were performed with a Kruss tensiometer K20 equipped with a Wilhelmy slide. The platinum plate was cleaned and heated to a red/orange color with a spirit lamp before use. The solution was allowed to stand for 10 min to equilibrate before taking a measurement. The mean value of three measurements was taken as the interfacial tension values. Dynamic light scattering (DLS) measurements were carried out using a Brookhaven BI-200SM instrument equipped with a 50 mW solid-state laser operating at 532 nm. Optical micrographs were collected with an optical microscope (Leica, DM 4500P). The average size of droplets was obtained by the measurement of the sizes of 100 randomly selected droplets, followed by statistical analysis.

Figure 1. SEC traces of PS(OH)2 and PDMA-stat-PAPBA obtained by DMF SEC (80 °C, 1.0 mL/min).

monomodal elution peak with a narrow polydispersity index (PDI) for each of the polymers, suggesting good synthetic control in the corresponding RAFT process. On the basis of the calibration of the SEC instrument using linear polystyrenes, PS(OH)2 has a Mn of 3600 g/mol with a PDI of 1.09 and PDMA-stat-PAPBA has a Mn of 44 800 g/mol with a PDI of 1.20. Chemical structures of PS(OH)2 and PDMA-stat-PAPBA were verified by 1H NMR analysis (Figure S1). Specifically, 22 mol % of APBA monomer units in PDMA-stat-PAPBA was revealed. The Mn of PS(OH)2 from 1H NMR analysis was almost similar to that from SEC analysis, whereas the Mn of PDMA-stat-PAPBA based on 1H NMR analysis was ∼19 300 g/ mol, which was much smaller than that from SEC analysis (S1, Supporting Information), presumably because of the differences between hydrodynamic volumes of polystyrene standards and PDMA-stat-PAPBA copolymers with the same MWs in DMF. Investigations on the Interfacial Activities of Polymers. The interfacial activities of the polymers were evaluated by the interfacial tension experiments. The interfacial tension of the toluene−water interface was determined by the Wilhelmy plate method. Prior to measurements, the solubility of PDMAstat-PAPBA in water was investigated. The presence of an equilibrium between the uncharged trigonal state and the charged tetragonal form for PBA compounds in aqueous media has been reported, with the pKa of PBA of about 9.36 High pH can shift the equilibrium from the unchanged state to the charged form. Accordingly, the formation of more charged tetragonal species can improve the water solubility of PDMAstat-PAPBA copolymers. The hydrodynamic diameter (Dh) of

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RESULTS AND DISCUSSION Synthesis of PS(OH)2 and PDMA-stat-PAPBA by Reversible Addition−Fragmentation Chain Transfer Polymerization. As shown in Scheme 1, PS(OH)2 and PDMA-stat-PAPBA copolymers were synthesized at 70 °C by AIBN-initiated reversible addition−fragmentation chain transfer (RAFT) polymerization using EMP-(OH)2 and EMP as RAFT agents, respectively. For the preparation of PS(OH)2, St was used as the monomer in the bulk polymerization for 6 h ([St]0/[EMP-(OH)2]0/[AIBN]0 = 40:1:0.33). For the preparation of PDMA-stat-PAPBA, DMA and APBA were used as comonomers in DMF/H2O (v/v, 19:1) solution for polymerization for 2 h ([DMA]0/[APBA]0/[EMP]0/[AIBN]0 = 40:19:1:0.1). MW and MW distribution of PS(OH)2 and PDMA-statPAPBA polymers were analyzed by SEC using DMF as the eluent. As shown in Figure 1A, SEC revealed a symmetrical C

DOI: 10.1021/acs.langmuir.8b00476 Langmuir XXXX, XXX, XXX−XXX

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phase. Therefore, with different structural components interacting with water and toluene, respectively, PS(OH)2 possesses moderate surfactant properties. Because of the absence of the interfacial activity of PS, the system with water containing 0.015 wt % of PDMA-stat-PAPBA−toluene at pH 11 and toluene with 0.05 wt % of PS (entry 6, Table 1) showed a γ very close to that of entry 3. However, after the replacement of PS with PS(OH)2 in toluene, the γ value displayed a significant reduction and reached 30.8 mN/m (entry 7, Table 1). On the basis of earlier reports,33,53 this significant reduction in γ can be ascribed to the interactions between the diol groups of PS(OH)2 and the negatively charged PBA groups of PDMA-stat-PAPBA PS(OH)2 in the biphasic system. As discussed above, there was a significant presence of the diol terminal groups of PS(OH)2 in the water side of the interface. Driven by the complex interaction between the negatively charged PBA group with the cis-diol group,54,55 the PDMA-stat-PAPBA copolymer in the water phase would be shifted toward the oil−water interface and then complexed with the diol groups of PS(OH)2. As a result, the amphiphilic supramolecular PDMA-stat-PAPBA-g-PS copolymer was formed in situ at the water−toluene interface. The amphiphilic supramolecular copolymer was “locked” at the oil− water interface and acted as the surfactants, leading to the remarkable reduction in γ of the water−toluene interface. Both FT-IR and SEC analysis provided evidence for the formation of PDMA-stat-PAPBA-g-PS. As illustrated in Figure S4A, strong absorption peaks at 1068 and 1250 cm−1, corresponding to the stretching absorption of B−O−C,56 appeared in the FT-IR spectrum of PDMA-stat-PAPBA-g-PS, and they were not observed in the spectra of both PS(OH)2 and PDMA-statPAPBA. Figure S4B displays SEC traces of the PS(OH)2, PDMA-stat-PAPBA, and the polymer sample isolated from the emulsion. Three elution peaks were observed in the SEC trace of the polymer sample isolated from the emulsion. Among these three peaks, two of them were assigned to PDMA-statPAPBA and PS(OH)2, respectively; the minor peak with the smallest elution time, corresponding to the highest MW among all polymer species, was attributed to PDMA-stat-PAPBA-g-PS. Although SEC analysis provided evidence for the formation of supramolecular graft copolymer, it should be noted that because the extent of supramolecular interactions depends on conditions of media, the SEC results obtained using DMF as the eluent would not quantitatively reveal the in situ conversion of PDMA-stat-PAPBA-g-PS at the oil−water interface in the emulsion. The outcomes of emulsification experiments also agreed with the results of interfacial tension measurements. For the emulsion preparations, the volume ratio of water to the toluene was 2, and the polymer concentrations in the water and the toluene were 0.015 and 0.05 wt %, respectively. The emulsifying results are displayed in Figure 2. Stable emulsions could not be formed under the conditions indicated in the entries of 3, 4, 5, and 6 of Table 1, respectively, as shown in Figure 2A−D. In the case of system of entry 7 in Table 1, a stable toluene-in-water emulsion was generated, as indicated in Figure 2E. The generation of the emulsion was attributed to the formation of the amphiphilic PDMA-statPAPBA-g-PS supramolecular graft copolymers at the toluene− water interface during the homogenization. The supramolecular graft copolymers covered on the surface of the dispersed oil droplets. The hydrophilic PDMA-stat-PAPBA backbone was essentially present in the water phase, and the hydrophobic PS

PDMA-stat-PAPBA in water at different pHs was used to interpret its water solubility. The DLS curves of PDMA-statPAPBA in water at different pHs are displayed in Figure S2. With the increase of pH, the Dh of PDMA-stat-PAPBA copolymers decreased gradually, corresponding to the enhanced water solubility and the reduced extent of intermolecular assembly. The Dh of the copolymer of only ∼2 nm was observed at pH 11, suggesting that the copolymer was unimolecularly dissolved under this condition. The concentrations of the PDMA-stat-PAPBA copolymer in water at pH 11 and the PS(OH)2 homopolymer in toluene were 0.015 and 0.05 wt %, respectively. The specific interfacial tension (γ) values of the toluene−water interface at different conditions are listed in Table 1. Table 1. γ Values of the Toluene−H2O Interface at Different Conditions entry 1 2 3 4 5 6 7

oil−water interface toluene−water (pH 7) toluene−water (pH 11) toluene−water (pH 11) toluene−water (pH 11) toluene−water (pH 11) toluene−water (pH 11) toluene−water (pH 11)

polymer components in the water and toluene

γ (mN/m)

none

3652

none

42.7 ± 0.8

0.015 wt % of PDMA-stat-PAPBA in water 0.015 wt % of PS(OH)2 in toluene

42.3 ± 0.9

0.05 wt % of PS in toluene

43.0 ± 0.1

0.015 wt % of PDMA-stat-PAPBA in water + 0.05 wt % of PS in toluene 0.015 wt % of PDMA-stat-PAPBA in water + 0.05 wt % of PS(OH)2 in toluene

42.9 ± 0.4

40.8 ± 0.1

30.8 ± 0.4

The γ value was 42.7 mN/m for the interface of toluene with water at pH 11 (entry 2, Table 1), which is higher than the reference value of 36 mN/m for the interface of toluene with water at pH 7 (entry 1, Table 1).52 The presence of 0.015 wt % of the PDMA-stat-PAPBA copolymer in water at pH 11 (γ = 42.3 mN/m, entry 3, Table 1) did not result in a considerable change of interfacial tension, indicating that the negatively charged PDMA-stat-PAPBA copolymer was essentially present in the water bulk phase rather than the water−oil interface. The interface of toluene with 0.05 wt % of PS(OH)2 and water at pH 11 (entry 4, Table 1) showed a γ of 40.8 mN/m, corresponding to a moderate decrease of the γ value (by ∼2 mN/m) as compared with that of the interface of pure toluene with water at pH 11. This phenomenon can be attributed to the amphiphilic structure of PS(OH)2. The co-existence of a hydrophilic diol terminal and a hydrophobic PS backbone in PS(OH)2, endowing it with a certain level of interfacial activity. To further verify the interfacial activity of PS(OH)2, the PS homopolymer without a diol terminal was synthesized and used as a control synthesis, and SEC characterization of PS is described in Figure S3. The interface of toluene containing 0.05 wt % PS homopolymer and water at pH 11 (entry 5, Table 1) exhibited essentially the same γ as that of the interface of toluene and water at pH 11 (entry 2, Table 1). Thus, it is evident that the hydrophilic diol terminal of PS(OH)2 plays a critical role in its interfacial activity and can affiliate effectively with the water phase. At the toluene−water interface, the hydrophilic diol terminal faced toward the water phase and the hydrophobic PS backbone was stretched toward the toluene D

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of 0.05 wt % in the toluene and are displayed in Figure 3A−D. The concentrations of PDMA-stat-PAPBA copolymers in water

Figure 3. (A−D) Photographs of the toluene−water (pH 11) biphasic systems 24 h after homogenization, with containing varied concentrations of PDMA-stat-PAPBA copolymers [(A) 0.005; (B) 0.01; (C) 0.015; and (D) 0.02 wt %] and toluene containing a fixed 0.05 wt % of PS(OH)2. Panels (E−G) are the optical microscopy photographs of emulsion droplets (scale bar: 25 μm), corresponding to the emulsions shown in (B−D), respectively. The volume ratio of water to toluene was 2. The photographs and optical microscopy images were taken after 24 h of storage at the room temperature.

Figure 2. (A−E) Photographs of the different toluene−water (pH 11) biphasic systems 24 h after homogenization: (A) containing 0.015 wt % of PDMA-stat-PAPBA copolymer in water; (B) containing 0.05 wt % of PS(OH)2 in toluene; (C) containing 0.05 wt % of PS in toluene; (D) containing 0.015 wt % of PDMA-stat-PAPBA copolymer in water and 0.05 wt % of PS in toluene; and (E) containing 0.015 wt % of PDMA-stat-PAPBA copolymer in water and 0.05 wt % of PS(OH)2 in toluene. (F,G) Optical microscopy images of emulsion droplets (scale bar: 25 μm) shown in (E) after different placement periods: (F) 24 h and (G) 2 months. The volume ratio of water to toluene was 2. The photographs and optical microscopy images, except image G, were taken after 24 h of storage at the room temperature.

phase were 0.005, 0.01, 0.015, and 0.02 wt %, corresponding to ∼1.5:1, ∼3:1, ∼5:1, and ∼6:1 of the total molar ratios of PBA/ diol, respectively. As shown in Figure 3A, only a thin emulsion layer was formed for the case with only 0.005 wt % of PDMAstat-PAPBA copolymer in water phase. When the content of PDMA-stat-PAPBA copolymer in water phase was 0.01, 0.015, and 0.02 wt %, toluene-in-water emulsion could be formed, as displayed in Figure 3B−D, and the corresponding optical microscopy images of the emulsion droplets corresponding to these systems are shown in Figure 3E−G. It was found that the average sizes of droplets decreased with the increase of the PDMA-stat-PAPBA concentration in the water. The average droplet sizes were almost 12.0 ± 9.0, 11.0 ± 6.4, and 9.9 ± 5.4 μm, corresponding to 0.01, 0.015, and 0.02 wt % of PDMA-statPAPBA content in water phase, respectively. There were two possible reasons for the failure of the formation of emulsion at 0.005 wt % of PDMA-stat-PAPBA. First, the copolymer concentration might be too low to allow the formation of a

side chains were located in the oil phase. The PDMA-statPAPBA-g-PS copolymer was “locked” at the oil−water interface and stabilized the oil droplets, as illustrated in Scheme 2. Figure 2F shows the optical microscopy images of emulsion droplets after standing for 24 h. The average sizes of droplets were 11.0 ± 6.4 μm. The emulsion showed a long-term stability. After the placement of the emulsion for 2 months at room temperature without any disturbances, the average sizes of droplets were 11.8 ± 3.3 μm, as shown in Figure 2G. Study on the Effect of the Polymer Concentrations on Emulsion Formation. The effect of the polymer concentrations on the formation of the emulsion was investigated in this work. Emulsions were formed by the homogenization of water at pH 11 containing different concentrations of PDMAstat-PAPBA copolymer and the fixed PS(OH)2 concentration

Scheme 2. Illustration of the Formation of the Toluene-in-Water Emulsion Stabilized by Amphiphilic PDMA-stat-PAPBA-g-PS Supramolecular Graft Copolymers Formed in Situ at the Oil−Water Interface

E

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PS(OH)2 in the oil phase reached 0.02 wt %, with the average size of oil droplets of 19.0 ± 7.5 μm. Upon the further increase of the concentration of PS(OH)2 in the oil phase to 0.03 and 0.05 wt %, the resulting emulsions possessed almost the same thickness of emulsion layer. However, the average droplet size decreased with the increase of the PS(OH)2 concentration. For example, for the systems with 0.03 and 0.05 wt % of PS(OH)2 in toluene, the average size of oil droplets was 16.0 ± 3.3 and 11.0 ± 6.4 μm, respectively. The reduction of the size of the dispersed droplets indicated the increase of the interfacial area stabilized by the in situ formed PDMA-stat-PAPBA-g-PS graft copolymers. With the increase of the molar ratio of diol/PBA from ∼1/23 to ∼1/5, the yield of PDMA-stat-PAPBA-g-PS may significantly increase, and the amphiphilicity of PDMA-statPAPBA-g-PS became more balanced. Both factors may contribute to the increase of the stabilized interfacial area with the increase of the molar ratio of diol/PBA. pH- and Glucose-Responsive Demulsification. For the present toluene-in-water emulsions, it was confirmed that the dispersed oil droplets were stabilized by the PDMA-statPAPBA-g-PS supramolecular graft copolymers formed in situ at the oil−water interface. The PS side chains were tethered on the PDMA-stat-PAPBA backbone through the B−O DCBs. It is well-known that the reversible B−O DCBs exhibit pH- and glucose-responsibilities.51,55 The destruction of the B−O bond can result in the detachment of PS side chains from the PDMAstat-PAPBA backbone. Therefore, any factor that can destroy the B−O DCBs, such as pH and glucose, would be the triggers for the demulsification of the emulsions. An HCl aqueous solution (36.0 μL, 1 mol/L) was added to the emulsion shown in Figure 5A, which was prepared from water (pH 11) with 0.015 wt % of PDMA-stat-PAPBA and toluene with 0.05 wt % of PS(OH)2 to adjust pH from 11 to 6.7. The emulsion was completely demulsified within 30 min at rest, as indicated in Figure 5B. The demulsification was the result of the destruction of B−O DCBs in the acidic medium. Amphiphilic PDMA-statPAPBA-g-PS was disassociated to PDMA-stat-PAPBA copolymer and PS(OH)2 homopolymer. Both the PDMA-stat-PAPBA copolymer and PS(OH)2 homopolymer had been proved to be incapable of stabilizing the dispersed oil droplets, as revealed in Figure 2A,B. During the demulsification, the addition of HCl solution resulted in the formation of NaCl salt. Owing to the charge shielding effect, the solubility of the PDMA-stat-PAPBA copolymer was significantly decreased in NaCl aqueous solution. DLS measurements indicated that PDMA-statPAPBA aggregates with larger sizes were found in the 8.97 mM of NaCl aqueous solution (note: this concentration of NaCl was equal to that produced during demulsification). Also, it was found that the resulting PDMA-stat-PAPBA formed

sufficient amount of PDMA-stat-PAPBA-g-PS graft copolymers required for stabilizing emulsion. Second, because of the high amount of PS(OH)2 relative to PDMA-stat-PAPBA in the system, the resulting PDMA-stat-PAPBA-g-PS might be too hydrophobic and not optimal for stabilizing emulsion. With the increase of the concentration of PDMA-stat-PAPBA in the water phase, more PDMA-stat-PAPBA-g-PS graft copolymers with balanced amphiphilicity were formed at the oil−water interface for effective emulsion stabilization, leading to the decreasing of the droplet size. Figure 4 shows the photographs and the corresponding optical microscopy images of emulsions that were prepared

Figure 4. (A−D) Photographs of the toluene−water (pH 11) biphasic systems 24 h after homogenization, with water containing a fixed 0.015 wt % of PDMA-stat-PAPBA and toluene containing varied PS(OH)2 concentration ((A) 0.01 ; (B) 0.02 ; (C) 0.03; and (D) 0.05 wt %). Panels (E−G) are optical microscopy photographs of emulsion droplets (scale bar: 25 μm), corresponding to the emulsions shown in (B−D), respectively. The volume ratio of water to toluene was 2. The photographs and optical microscopy images were taken after 24 h of storage at the room temperature.

using the water phase at pH 11 with a fixed 0.015 wt % of PDMA-stat-PAPBA copolymer and toluene with varied concentrations of PS(OH)2. The concentrations of PS(OH)2 in the oil phase were 0.01, 0.02, 0.03, and 0.05 wt %, corresponding to ∼1:23, ∼1:12, ∼1:8, and ∼1:5 of the total molar ratios of diol/PBA, respectively. As shown in Figure 4A, a thin oil layer was observed above the emulsion layer for the case with 0.01 wt % of PS(OH)2 in the toluene phase. Toluene could be completely emulsified when the concentration of

Figure 5. (A) Photograph of emulsion formed by the homogenization of pH 11 water with 0.015 wt % of PDMA-stat-PAPBA and toluene with 0.05 wt % of PS(OH)2; (B) photograph of the broken emulsion triggered by adjusting pH from 11 to 6.7; and (C) photograph of the emulsion generated from the broken emulsion by adjusting pH back to 11 and followed by homogenization. Panels (D,E) are the optical microscopy images of emulsion droplets (scale bar: 25 μm), corresponding to the emulsions shown in (A,C), respectively. The photographs and optical microscopy images were taken after 24 h of storage at the room temperature. F

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Figure 6. (A−C) Photographs of the emulsions with different concentrations of glucose of (A) 0.4; (B) 0.5; and (C) 0.6 mM. (D−G) Optical microscopy images of the emulsion droplets at different concentrations of glucose: (D) 0; (E) 0.25; (F) 0.4; and (G) 0.5 mM. Scale bar: 25 μm. The photographs and optical microscopy images were taken after 24 h of storage at the room temperature.

concentration in the water phase reached 0.4 mM (∼1.6:1 of molar ratio of glucose/PBA), the average droplet size remarkably increased to 22.0 ± 2.6 μm. A thin layer of toluene oil was observed to be isolated from the emulsion layer when the glucose concentration reached 0.5 mM (Figure 6B), corresponding to ∼2:1 of molar ratio of glucose/PBA. On further increasing the glucose concentration to 0.6 mM (∼2.4:1 of molar ratio of glucose/PBA), the emulsion presented a rapid demulsification within a shorter time period (∼10 min) (Figure 6C). Glucose-triggered demulsification was originated from the replacement of the PBA/PS(OH)2 complexes by the PBA/ glucose complexes, because glucose with cis-diol structure exhibited a stronger complex capability with PBA.57 The newly formed PDMA-b-PAPBA/glucose complexes were highly hydrophilic and showed no capability to stabilize the emulsion. Theoretically, if the complex reaction of glucose with PBA groups can occur quantitatively, stoichiometric glucose is sufficient to demulsify emulsion. However, only a minor change of the emulsion was observed within the observation period (24 h) when the stoichiometric glucose was introduced into the water phase, indicating the limited reaction extent. A higher dosage of glucose is required to promote the competitive complex reaction and to result in obvious demulsification.

aggregates in the NaCl aqueous solution when the pH was adjusted back to 11 (Figure S5, Supporting Information). On the other hand, for the broken emulsion (Figure 5B), adjusting pH back to 11 by the addition of NaOH aqueous solution, toluene could be totally re-emulsified after homogenization. The resulting emulsion (Figure 5C) showed a high stability, but a decreased emulsion layer thickness and an increased average size of droplets relative to the original one (Figure 5A). The average size of the droplets was increased from 11.0 ± 6.4 μm (Figure 5D) to 14.3 ± 2.8 μm (Figure 5E). On the other hand, a control experiment of directly adding NaCl to an emulsion stabilized by PDMA-stat-PAPBA-g-PS did not show any considerable change of the average size of the emulsion droplets (Figure S6), indicating that the supramolecular graft copolymers were not noticeably salt-responsive under the emulsion conditions. For the demulsified emulsion, after tuning the pH back to 11, our controlled experiment indicated that the stable o/w emulsion could also be regenerated when the original toluene solution of PS(OH)2 was replaced by toluene without PS(OH)2 (Figure S7, Supporting Information). It was difficult to clarify whether PDMA-stat-PAPBA aggregates could be dissolved at the toluene−water interface. Even if a proportion of PDMA-stat-PAPBA aggregates was disassociated to form individual molecules at the toluene−water interface, however, PDMA-stat-PAPBA could not act alone as the polymeric emulsifier to stabilize the oil droplets in the absence of PS(OH)2, as indicated in Figure 2. Therefore, the regenerated o/w emulsion should be stabilized by PDMAstat-PAPBA aggregates rather than PDMA-stat-PAPBA-g-PS copolymers. It was possible that some PS(OH)2 chains were also grafted on the PDMA-stat-PAPBA aggregates by virtue of the complexion between PBA and PS(OH)2. However, the graft efficiency of PS(OH)2 must be very low because most of PBA groups were embedded in the inner domains of aggregates. Figure 6 shows the glucose-induced changes of the emulsion prepared from pH 11 water with 0.015 wt % of PDMA-statPAPBA and toluene with 0.05 wt % of PS(OH)2. The average size of oil droplets of the original emulsion displayed in Figure 6A was about 11.0 ± 6.4 μm. After the addition of the glucose, the average droplet size increased (Figure 6E−G). It was found that the stability of the emulsion strongly depended on the concentration of the glucose. For example, the average droplet size showed a slight increase from 11.0 ± 6.4 to 12.0 ± 3.4 μm within 24 h when the concentration of glucose was 0.25 mM, corresponding to ∼1:1 of molar ratio of glucose/PBA (based on 22 mol % of PBA in PDMA-stat-PAPBA). When the glucose

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CONCLUSIONS

By virtue of the complexation between the PBA groups of PDMA-stat-PAPBA and the diol terminal of PS(OH)2, amphiphilic supramolecular PDMA-stat-PAPBA-g-PS graft copolymers were formed in situ at the toluene−water interface during homogenization. The PDMA-stat-PAPBA-g-PS copolymers could effectively stabilize the interface, resulting in the formation of toluene-in-water emulsion. The polymer concentrations showed remarkable effects on the emulsion formation. With a fixed 0.05 wt % of PS(OH)2 in oil phase, the average size of oil droplets decreased with the increase of the PDMAstat-PAPBA concentration in the water. The lowest concentration of PDMA-stat-PAPBA copolymer in pH 11 water phase was 0.01 wt % for the formation of the stable toluene-in-water emulsion. With a fixed 0.01 wt % of PDMA-stat-PAPBA copolymer in water phase, the concentration of PS(OH)2 in the oil phase was required to be at least 0.02 wt % for the complete emulsification of toluene. The formed emulsions showed pHand glucose-responsive demulsification behavior owing to the reversible B−O linkages between PDMA-stat-PAPBA backbones and PS side chains. This method of in situ formation of G

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supramolecular copolymers potentially can be applied to other polymer pairs with PBA units and the diol terminal.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b00476. 1 H NMR spectra of PS(OH)2 and PDMA-stat-PAPBA; the solubility of PDMA-stat-PAPBA in alkaline water at different pHs; the synthesis procedure of PS and SEC characterization; experimental proofs of the formation of PDMA-stat-PAPBA-g-PS at the water−toluene interface; the deduction of the water solubility of PDMA-statPAPBA at different conditions; effect of NaCl on the stability of emulsion stabilized by PDMA-stat-PAPBA-gPS; and the photograph and optical microscopy images of the regenerated emulsion at pH 11 upon the replacement of toluene solution of PS(OH)2 by toluene without PS(OH)2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: ccheng8@buffalo.edu (C.C.). *E-mail: [email protected] (Y.G). ORCID

Yong Gao: 0000-0001-7962-7196 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial supports from the National Natural Science Foundation of China (21574112 and 21174118), Hunan Provincial Natural Science Foundation of China (2017JJ2243), Open Project of Hunan Provincial University Innovation Platform (15K123), and U.S. National Science Foundation (CHE-1412785).

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