pH Switchable Emulsions Based on Dynamic Covalent Surfactants

Subscriber access provided by University of Newcastle, Australia

Article

pH Switchable Emulsions Based on Dynamic Covalent Surfactants Gaihuan Ren, Lei Wang, Qianqian Chen, Zhenghe Xu, Jian Xu, and Dejun Sun Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04546 • Publication Date (Web): 10 Mar 2017 Downloaded from http://pubs.acs.org on March 12, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

pH Switchable Emulsions Based on Dynamic Covalent Surfactants Gaihuan Ren, § Lei Wang, § Qianqian Chen,§ Zhenghe Xu, †‡* Jian Xu, §* and Dejun Sun§* §

Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Shandong University, Jinan, Shandong, 250100, P. R. China †

Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta Canada T6G 2V4 ‡

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, P. R. China 1000084

*Corresponding author. Tel. +86-531-88364749, Fax. +86-531-88364750, E-mail: [email protected].

ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

ABSTRACT: Dynamic covalent surfactants were designed to prepare pH switchable emulsions.

A

dynamic

covalent

bond

between

nonamphiphilic

building

blocks

(polyethylenimine (PEI) and benzaldehyde (B)) was introduced to form the dynamic covalent surfactant PEI-B. The dynamic nature of covalent bond in PEI-B was confirmed by 1H NMR and fluorescence probe analysis. Stable emulsions were successfully prepared with interfacial active PEI-B at pH 7.8 with various water/paraffin oil ratios under sonication. When lowering the pH to 3.5, a complete phase separation was observed as a result of breaking dynamic covalent bond in the interfacial active PEI-B. After tuning the pH back to 7.8, stable emulsion was obtained again due to the formation of the dynamic covalent bond and hence interfacial active PEI-B. The emulsification and demulsification were dependent on the formation and breaking of dynamic covalent bond in PEI-B. Such pH triggered emulsification and demulsification can be switched for at least three times. Application of dynamic covalent surfactants will open up a novel route for preparing responsive emulsions.


Page 3 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

1. INTRODUCTION Emulsions are widely used in commercial products and industrial processes. For some applications, such as food storage and bitumen emulsification, long-term emulsion stability is critical. However, in other cases, such as oil recovery, heavy oil transportation, and emulsion polymerization,1 only temporary emulsion stability is desired. In general, breaking stable emulsions can be achieved through chemical methods (e.g., adding demulsifiers2,3 and salt4,5) or physical methods (e.g., applying high electric field6 and centrifugation7,8). However, these methods are usually either energy intensive or may cause consequential or/and subsequent pollutions. To avoid large energy consumption and subsequent pollutions, emulsions with on and off property (switchable emulsions) were desired. Such emulsions can be prepared with switchable surfactants. Switchable surfactants undergo reversible interconversions between active and inactive forms in response to specific external stimuli, such as pH,9,10 CO2,1,11-14 temperature,15 light irradiation,16 and redox.17 Many studies reported switchable surfactants. Jessop and co-workers1,18-22 have studied a series of switchable surfactants, with tertiary-amine or amidine groups. In the presence of CO2, these compounds protonate and become water-soluble surfactants (active form). In the absence of CO2, these molecules deprotonate and become hydrophobic tertiary-amine or amidine (inactive form), so that the on and off property of emulsions can be achieved. However, traditional switchable surfactants suffer serious drawbacks. Synthesis of tertiary-amine and amidine, for example, is complicated and time-consuming.1,18,23 Hence, there are clear needs to develop more convenient and eco-friendly dynamic covalent surfactants. Dynamic covalent surfactants are formed by introducing the dynamic covalent


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

bonds between building blocks. Among dynamic covalent bonds, dynamic imine bond is especially attractive due to its simple synthesis and pH responsive nature, i.e., stable in alkaline environment but unstable in acidic environment.24-28 Because dynamic imine bond can be reversibly controlled by changing pH, dynamic imine surfactants (amphiphilic molecules/polymers) have been widely used in building stimuli-responsive micelles,29-32 vesicles,33 and capsules.34

However, few reports on dynamic covalent surfactants based

switchable emulsions have been found. In this work, we report pH switchable emulsions prepared using an imine bond based dynamic covalent surfactant (PEI-B). The PEI-B was formed by connecting benzaldehyde and PEI though dynamic imine bond. PEI is a hydrophilic polymer that consists of primary (25%), secondary (50%) and tertiary amines (25%).35,36 PEI has pKa of 8.7-9.7.37,38 PEI has been widely used in many fields, such as artificial enzymes and gene delivery after simple modification of its amines by alkylation, acylation and imine formation.35 Introducing benzene ring to PEI through imine formation was found to increase its hydrophobicity and thus emulsification ability. Our current work shows that by adjusting the pH of PEI-B aqueous solutions, the interfacial activity of PEI-B can be switched between active and inactive states. As a result, PEI-B based emulsions (O/W) can be stabilized or destabilized (or phase separated) by changing pH of the continuous aqueous phase, which provides a new route for preparing pH responsive emulsions.

2. EXPERIMENTAL SECTION 2.1 Chemicals and Materials. A branched polyethylenimine (PEI, Mw 10 000) was obtained from Alfa Aesar. Benzaldehyde (AR), trifluoroacetic acid (TFA, CR), sodium


Page 4 of 26

Page 5 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

hydroxide (AR), tetrahydrofuran (AR), salicylaldehyde (AR) and hydrogen chloride (36.5 wt%)

were

purchased

from

Sinopharm

Chemical

Reagent

Co.

Ltd.,

China.

P-butoxybenaldehyde (99%) was obtained from J&K Chemical. Nile red was supplied by Aladdin Reagents of China. Paraffin oil was received from Exxon Mobil. All the chemicals were used as received without further purification. Deionized water was used in all the experiments. 2.2 Preparation and Characterization of PEI-B. PEI and benzaldehyde were mixed in CH3OH with molar ratio of primary amine to aldehyde of 1:1. The mixture was stirred at 500 rpm and room temperature (20±2°C) for around 10 min to obtain a homogeneous solution. PEI-B was prepared by the formation of dynamic covalent bonds between the aldehyde groups in the benzaldehyde and the primary amine groups in the PEI through Schiff Base reaction (Scheme 1). After completion of reaction, the solvent (CH3OH) was removed by rotary evaporation at a reduced pressure with the residue being the product (PEI-B). The PEI-B obtained as such was characterized by FTIR and 1H NMR. FTIR spectra were obtained using a BRUKER ALPAH-T spectrometer at a spectral resolution of 4 cm-1. FTIR samples were prepared by “sandwiching” the samples between KBr windows. For 1H NMR characterization, a BRUKER AVANCE 400 spectrometer was used. The samples were dissolved in CD3OD, and the acidic environment of the solutions was achieved by the addition of TFA. 2.3 Fluorescence Measurement. The CMC of PEI-B was determined by fluorescence spectrophotometer (Hitachi F-7000) with Nile red as a fluorescent probe.29,39 Nile red is a hydrophobic molecule whose florescence is significantly influenced by the polarity of probing


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

environment.40,41 Initially, Nile red was dissolved in THF to 2 mM concentration as a stock solution. Then 5 µL Nile red solutions were added to 10 mL of PEI-B aqueous solutions (pH 7.8) under sonication. The samples were left standing for 12 hours at room temperature prior to fluorescence measurement. The fluorescence measurements were conducted at the excitation wavelength of 550 nm. The maximum emission wavelength (λmax) of PEI-B aqueous solutions of different concentrations was measured by the fluorescence spectrophotometer. The pH reversible behavior of PEI-B aqueous solution was also characterized by the fluorescence measurements. Nile red is a kind of solvatochromic dye which is an indicator of the solvent polarity. Its λmax shifts toward longer wavelengths with increasing environmental polarity.41 This shift of λmax is used to confirm the decomposition/re-formation of dynamic covalent surfactants (PEI-B) upon tuning pH.31 2.4 Dynamic Interfacial Tension (IFT) Measurement. The dynamic IFT of PEI-B aqueous solution with different pH values and paraffin oil was measured using a Drop Profile Analysis Tentiometer (Tracker, France). Images were captured during the formation of a droplet (3 µL, PEI-B aqueous solution) in paraffin oil. The interfacial tension was calculated using Laplace equation from the curvature of the droplet obtained by analyzing the images captured. 2.5 Emulsion Preparation. To prepare emulsions, 10 mL freshly prepared PEI-B aqueous solution (pH 7.8) was transferred to a 50 mL glass vial containing 10 mL paraffin oil. The mixtures were sonicated by using a SCIENTZ JTY92-IIN Sonicator (the probe size U = 6 mm) in ice bath for 200 s (400 w, 5 s off and 5 s on). The emulsions were sealed and stored at room


Page 6 of 26

Page 7 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

temperature to determine their long term stability. 2.6 Emulsion Characterization. The type of emulsions was determined by adding a drop of emulsion into water or paraffin oil. Stability of emulsions was assessed by visual inspection and Dynamic Light Scattering (DLS, Brookhaven BI-200SM instrument) measurements. The DLS measurements were performed at a fixed scattering angle of 90° using a green laser (λ= 532 nm) of variable intensity. Hydrodynamic diameter was calculated from autocorrelation functions which were analyzed with the CONTIN method. 2.7 pH-induced Emulsification and Demulsification. To test the pH switchable behavior of emulsions, pH of the prepared emulsions was adjusted using 1 M HCl or 1 M NaOH solution. After complete phase separation at pH 3.5, the content of benzaldehyde that transferred to oil phase was determined by a TU-1810 type UV-vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.) at 247 nm.

3. RESULTS AND DISSCUSSION 3.1 FTIR and 1H NMR Characterization of PEI-B. The FTIR spectra of benzaldehyde, PEI and PEI-B were compared in Figure 1. FTIR spectrum (a) in Figure 1 shows a characteristic vibration of benzaldehyde (-C=O) at 1711 cm-1.42 In the PEI FTIR spectrum (Figure 1b), the peaks at 3350 cm-1 and 3285 cm-1 are ascribed to asymmetric and symmetric stretching vibration of -NH2, respectively. Compared to benzaldehyde and PEI, a distinct strong peak at 1650 cm-1 corresponding to stretching vibration of -C=N bond42 appears, while the peaks corresponding to -C=O and -NH2 symmetric and symmetric stretching vibrations disappear in the spectrum of PEI-B (Figure 1c). It should be mentioned that a similar small


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

peak (at about 3330 cm-1) still exists in the spectrum of PEI-B, it could be attributed to contribution of -NH- stretching vibration. These spectral features confirm the formation of -C=N bond and the corresponding PEI-B. Scheme 1. Synthesis of dynamic covalent PEI-B.

Figure 1. Comparison of the FTIR spectra: (a) benzaldehyde (black), (b) PEI (red) and (c) PEI-B (blue). Different from conventional surfactants, PEI-B is formed through dynamic imine bond. Therefore, pH reversible property is expected.29,43-45 To confirm this hypothesis, 1H NMR spectra of benzaldehyde and PEI-B were characterized as shown in Figure 2. The proton signal at 9.9 ppm in Figure 2a is assigned to -CHO in benzaldehyde. In the spectrum of PEI-B (Figure 2b), the -CHO signal disappears while a new peak at 8.3 ppm appears, indicating the formation of dynamic imine bond (-CH=N-). More interestingly, after tuning PEI-B solution to acidic environment, the imine bond signal at 8.3 ppm disappears while -CHO signal at 9.8


Page 8 of 26

Page 9 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

ppm reappears (Figure 2c), confirming that the dynamic covalent bonded PEI-B is decomposed to benzaldehyde and PEI. The results of 1H NMR confirmed not only the formation of dynamic covalent bond (-CH=N-) and hence new compound PEI-B, but also the pH responsive nature of the dynamic imine bonded PEI-B. In 1H NMR measurement, CH3OD was used as the solvent with the addition of TFA to break the dynamic covalent bond by hydrolysis after change to acidic environment. It turns out to be very difficult to change the solution to alkaline environment. So 1H NMR measurement is difficult to confirm reforming of dynamic covalent bond in PEI-B. Nevertheless, the formation-breaking-reformation of dynamic covalent bond in PEI-B was confirmed using fluorescence spectroscopy with Nile red as the fluorescence probe to determine λmax of PEI-B aqueous solutions with pH cycles, as discussed below.

Figure 2. 1H NMR spectra of (a) benzaldehyde, (b) PEI-B, and (c) PEI-B in (b) with added TFA. 3.2 CMC Determination and pH Switchable Behavior of PEI-B Aqueous Solution. To illustrate the surface activity of PEI-B, the CMC of PEI-B was determined using fluorescence emission spectroscopy with Nile red as the fluorescent probe. The results in Figure 3 show that at PEI-B concentrations smaller than 0.022 mg·mL-1, λmax of Nile red was detected at 651 ± 2 nm, which corresponds to a typical λmax of Nile red in a polar water environment. As


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PEI-B concentration increases, a significant shift of λmax to shorter wavelength was found at about 0.022 mg·mL-1. The shift43 indicates that Nile red molecules are solubilized in an apolar environment, implying the formation of PEI-B micelle. Therefore, the concentration of 0.022 mg·mL-1 can be considered to be CMC of PEI-B.

Figure 3. λmax of Nile red in various concentrations of PEI-B solutions at pH 7.8 and 25°C for the emission wavelength of 550 nm. The pH switchable characteristic of PEI-B aqueous solutions (above CMC) was demonstrated by the shift of λmax during acid or base addition (Figure 4). At pH 7.8, PEI-B molecules are in the form of micelles, and the detected λmax of Nile red in such apolar environment is 619 ± 2 nm. This finding is consistent with the results presented in Figure 3. However, a red shift of λmax from 619 ± 2 nm to 651 ± 2 nm is observed when the lowering the solution pH from 7.8 to 3.5. Such shift31 indicates the disassembly of micelle and dissociation of PEI-B at pH 3.5. After tuning the pH back to 7.8, λmax returned to 619 ± 2 nm. This shift of λmax reveals the appearance of micelles, suggesting the reformation of amphiphilic PEI-B. After 3 times of pH cycles, the λmax of Nile red returns to its original value (619 ± 2 nm) without loss in efficiency, indicating the excellent pH responsiveness of PEI-B. In the control experiment, fluorescence emission spectrum of PEI or benzaldehyde aqueous solution alone was measured. For PEI or benzaldehyde aqueous solution, the λmax of Nile red ACS Paragon Plus Environment

Page 10 of 26

Page 11 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

probe remained at 651 ± 2 nm when the pH changed from 7.8 to 3.5 (Figure S1, Figure S2), same as the λmax of Nile red probe in polar water environment. So PEI or benzaldehyde aqueous solution alone does not possess surface activity and switchable behavior.

Figure 4. λmax of Nile red in 0.45 mg·mL-1 PEI-B aqueous solutions in response to pH cycle process at emission wavelength of 550 nm and 25°C (■ pH 7.8, ● pH 3.5). 3.3 Dynamic Interfacial Tension at different pH values. In order to investigate the interfacial activity of PEI-B, the interfacial tension of paraffin oil-PEI-B aqueous solutions at different pH values was measured at 25°C. The results in Figure 5 show a low IFT of 12 mN·m-1 after a 15-min equilibration at pH 7.8, indicating high interfacial activity of PEI-B. Such amphipathicity of PEI-B is attributed to the introduction of hydrophobic benzene ring through the formation of dynamic covalent bond. When lowering the pH of PEI-B aqueous solution from 7.8 to 3.5, the IFT increased to 32.9 mN·m-1, which is close to the interfacial tension paraffin oil-PEI aqueous solution, and paraffin oil-benzaldehyde aqueous solution at the same pH (Figure S3). These results suggest the decomposition of PEI-B at pH 3.5 and nonamphiphilic properties of PEI and benzaldehyde. Upon increasing the pH back to 7.8, the equilibrium IFT returned back to 12 mN·m-1, indicating the reformation of interfacial active PEI-B. PEI itself is almost interfacial inactive at both pH 7.8 and pH 3.5 as shown by little


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

change in the dynamic interfacial tension of PEI aqueous solutions with changing pH from 7.8 to 3.5 (Figure S4). This confirms that the reversible interfacial activity change (Figure 5) is not caused by PEI or benzaldehyde itself. Results of IFT measurement at different pH and PEI-B concentrations are summarized in Table S1. The dynamic IFT of paraffin oil and PEI-B aqueous solutions as a function of PEI-B concentration at pH 7.8 is shown in Figure S5. The equilibrium IFT in Table S1 is shown to decrease with increasing PEI-B concentration. At pH 3.5, the results in Table S1 show an almost constant dynamic IFT of oil-PEI-B aqueous at 34 mN·m-1 for all the concentrations studied. The results of interfacial tension measurements in paraffin oil-PEI-B aqueous solution systems show that the interfacial activity of the PEI-B at given PEI-B concentration can be controlled by tuning pH values.

Figure 5. Dynamic IFT of paraffin oil-PEI-B aqueous solutions (0.45 mg·mL-1) at different pH values (■ pH 7.8, ○ lowering pH to 3.5, and □ tuning pH back to 7.8). 3.4 Emulsions based on PEI-B. Homogenous emulsion was obtained after mixing PEI-B aqueous solution (pH 7.8) and paraffin oil under sonication (Figure S6a). The drop test confirmed that all the prepared emulsions are O/W type. In the control experiment, attempt was made to prepare emulsions with PEI or benzaldehyde aqueous solutions at pH 7.8, but no


Page 12 of 26

Page 13 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

homogeneous emulsions was obtained, and complete phase separation was observed after 10 hours of storage (Figure S6b, 6c). This finding suggests that it is PEI-B rather than PEI or benzaldehyde that effectively stabilized paraffin oil-in-water emulsions. The effect of PEI-B concentration (above CMC) on the formation of paraffin oil-in-water emulsions was investigated (Figure 6). At 0.1 wt% PEI-B concentration, emulsion can be prepared, but the system was unstable (Figure 6a), most likely due to insufficient amount of PEI-B in the system to effectively stabilize the emulsion.46 When PEI-B concentrations were larger than 0.10 wt%, homogeneous and stable emulsions were obtained with the droplet size in the range of 600-700 nm (Figure 6b). Increasing PEI-B concentration from 0.25 wt% to 1.50 wt% was found to decrease the size of emulsion droplets with a narrower droplet size distribution (Figure 6b). The observed trend agrees well with the results reported by Abismail et al.47


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. (a) Photograph and (b) droplet size distributions of paraffin oil-in-water emulsions (water to paraffin oil volume ratio = 1:1) prepared at pH 7.8 using different concentrations (wt%) of PEI-B aqueous solution. The photograph and DLS measurement were taken 24 hours after preparation. The emulsion prepared with 0.80 wt% PEI-B aqueous solution (pH 7.8) is very stable without any noticeable phase separation (Figure 8a, 8b). The droplet size was measured by DLS, the droplet size distribution remain unchanged after 7 days of storage (Figure 7), indicating that PEI-B is an effective emulsifier to stabilize paraffin oil-in-water emulsions.

Figure 7. Droplet size distribution curves for the emulsions prepared with 0.80 wt% PEI-B at pH 7.8 (water to paraffin oil volume ratio = 1:1) freshly prepared (initial) and 7 days after.


Page 14 of 26

Page 15 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

3.5 Emulsions at Different pH values. To illustrate the switchable nature of O/W emulsions based on dynamic covalent PEI-B, the effect of pH on the stability of the emulsions was investigated (Figure S7). As discussed above, dynamic covalent PEI-B became dissociated at acidic pH conditions. As a result the PEI-B stabilized emulsions became unstable upon lowing pH under gentle stirring. At pH 4.5, for example, partial phase separation was observed but complete phase separation was obtained after 1 week. At pH ≤ 3.5, complete phase separation was obtained within 5 min. Considering rapid demulsification and keep the emulsion system as close as possible before and after tuning pH, pH 7.8 and 3.5 were selected and studied in this work. There is no direct correlation between with the pKa (8.7-9.7) of PEI and pH selected for this study.

3.6 pH Switchable Behavior of Emulsions. According to the results of fluorescence probe analysis and dynamic interfacial tension measurement, switching of PEI-B stabilized emulsions between stable state and phase separation state is anticipated by tuning the pH between 7.8 and 3.5. At pH 7.8, homogenous and stable emulsion was indeed obtained as shown above (Figure 8a, 8b). An instant phase separation appeared upon decreasing pH from 7.8 to 3.5 (Figure 8c). The separated upper oil phase was clear, and the bottom aqueous phase was nearly clear after standing for 24 hours (Figure 8d). No stable emulsion was formed at pH 3.5 after re-sonication (Figure 8e). Furthermore, a homogenous and stable emulsion was formed again following sonication after raising the pH of the solution to 7.8 (Figure 8f). The experimental results here suggest that the emulsion stability can be effectively controlled by simply tuning the pH of emulsions. The emulsification and demulsification cycle can be repeated three times without any loss in reversibility.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8. pH switchable process of paraffin oil-in-water emulsions (water to paraffin oil volume ratio = 1:1) stabilized by 0.80 wt% PEI-B. (a) stable emulsion at pH 7.8; (b) emulsion of (a) standing for 7 days; (c) phase separated system after tuning the pH of (a) from 7.8 to 3.5 for 5 min; (d) phase separated system after tuning the pH of (a) from 7.8 to 3.5 and standing for 24 h; (e) phase separated system after re-sonication of (d) at pH 3.5; (f) stable emulsion after re-sonication of (d) after tuning the pH back to 7.8. A mechanism for pH-induced demulsification is present in Figure 9. At pH 7.8, PEI-B adsorbs to the oil-water interface due to its interfacial activity (as proved in Figure 5). The adsorbed PEI-B molecules form a stable interfacial film to stabilize the droplets (Figure 9a). As pH decreases from 7.8 to 3.5, PEI-B becomes dissociate to interfacial inactive PEI and benzaldehyde (as indicated in Figure S3) due to the break of dynamic imine bond. PEI is a highly hydrophilic polymer, so it transferred from droplet interface into the aqueous phase. As for benzaldehyde, due to the logKow (octanol/water partition coefficient) of benzaldehyde is 1.48, we estimate that a benzaldehyde would partition both into aqueous phase and oil phase. From the result obtained from UV-vis spectrophotometer, about 48.82 wt% of benzaldehyde migrated into upper oil phase. So about half of benzaldehyde transferred into oil phase and half of benzaldehyde transferred into aqueous phase. Therefore, at pH 3.5, there is no interfacial active substance remained in the oil-water interface, allowing the intense coalescence of the droplets, which results in complete phase separation (Figure 9b). The


Page 16 of 26

Page 17 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

essence of the demulsification is the decomposition of dynamic covalent surfactant PEI-B.

Figure 9. Proposed emulsification and demulsification mechanism. (a) Emulsion stabilized by PEI-B at pH 7.8; (b) Phase separation after pH decreased from 7.8 to 3.5. PEI moved from interface to aqueous phase and benzaldehyde migrated both in aqueous phase and oil phase with the partition ratio of about 1:1. The result of UV-vis absorption demonstrates that about half of benzaldehyde is transferred into the oil phase after demulsification. However, stable emulsion can still be obtained under sonication by replacing the separated oil with pristine paraffin oil and adjusting pH of the remaining aqueous phase to 7.8 (Figure S8). To understand this observation, additional experiments were conducted as follows. A series of PEI-B surfactants were prepared with varying primary amine to aldehyde molar ratios from 40:1 to 1:1. These PEI-B surfactants were used to prepare emulsions under sonication. At the ratio of 40:1, no stable emulsion was found. Over the ratios from 9:1 to 1:1, stable emulsions were obtained as shown in Figure S9. By replacing the separated oil with pristine paraffin oil, the molar ratio of primary amine to aldehyde in the new system is 2:1. This is still within the range of primary amine to aldehyde molar ratios of 9:1 to 1:1 that stable emulsions can be formed as tested in the above experiment. Theoretically the operation of replacing the separated oil phase with pristine paraffin oil can be repeated for three times. Even after 3 cycles (replacing upper separated oil with pristine paraffin oil), the separated aqueous phase could be still reused when adding new benzaldehyde to the system.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

The effect of water to paraffin oil volume ratio (WOR) on the switching characteristics was also studied. Homogenous and stable emulsions are obtained with WOR value from 1:1 to 4:1. The rapid demulsification was also observed when tuning pH from 7.8 to 3.5 (Figure S10). The results demonstrate an outstanding pH switchable character of emulsions even at higher WOR (4:1). In order to demonstrate the generality of our result, other dynamic covalent surfactants were

also

prepared

with

PEI

and

aromatic

aldehydes

(salicylaldehyde

and

4-butoxybenzaldehyde). In all cases, pH switchable emulsions were successfully obtained by using the formed dynamic covalent surfactants (Figure S11). Therefore we conclude that it is a general method to prepare pH switchable emulsions based on dynamic covalent surfactants. The emulsions based on dynamic covalent surfactants are sensitive to pH, instant phase separation can be achieved once the pH changed to 3.5. 4. CONCLUSIONS In this work, a new surfactant of the dynamic covalent PEI-B was designed and synthesized without tedious synthesis procedures. The pH-dependent formation and decomposition of PEI-B was confirmed by 1H NMR and fluorescence probe analysis. The result of dynamic IFT measurement also indicated that the interfacial activity of PEI-B can be tuned by changing aqueous phase pH values. We take the advantage of the pH responsive PEI-B to reversibly modulate the stability of paraffin oil-in-water emulsions. The homogeneous and stable emulsions prepared at pH 7.8 showed instant complete phase separation upon decreasing pH to 3.5, while neutralizing pH back to 7.8 resulted in formation of stable emulsions again. The key factor of the pH switchable behavior of emulsions depends


Page 19 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

on the pH reversible nature of dynamic imine bond. The stabilization and destabilization of emulsions could be repeated at least three cycles by simply changing pH values. The switchable behavior based on dynamic covalent bond will open up a novel route for preparing switchable emulsions.

ASSOCIATED CONTENT Supporting Information. Fluorescence emission spectroscopy of PEI and benzaldehyde (Figure S1, Figure S2). Dynamic IFT of paraffin oil-aqueous solution (Figure S3-5); Photographs of emulsions stabilized by PEI, benzaldehyde, and superamphiphile PEI-B (Figure S6); Photographs of emulsions at different pH (Figure S7); Emulsification and demulsification of emulsions by replacing upper oil phase with pristine paraffin oil (Figure S8); Effect of primary amine to aldehyde ratio on the stability of emulsions (Figure S9); Effect of water to oil ratio on the pH switchable behavior of emulsions (Figure S10); Emulsification and demulsification behaviors of emulsions stabilized by dynamic covalent surfactant PEI-S (Figure S11).

AUTHOR INFORMATION Corresponding Author *E-mail [email protected]. Tel +86-531-88364749. Fax +86-531-88364750.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work is supported by Natural Science Foundation of China (NSFC, 21333005).


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

REFERENCES (1) Liu, Y.; Jessop, P. G.; Cunningham, M.; Eckert, C. A.; Liotta, C. L. Switchable Surfactants. Science 2006, 313, 958-960. (2) Wu, J.; Xu, Y.; Dabros, T.; Hamza, H. Effect of Demulsifier Properties on Destabilization of Water-in-oil Emulsion. Energy & fuels 2003, 17, 1554-1559. (3) Feng, X.; Mussone, P.; Gao, S.; Wang, S.; Wu, S.-Y.; Masliyah, J. H.; Xu, Z. Mechanistic Study on Demulsification of Water-in-diluted Bitumen Emulsions by Ethylcellulose.

Langmuir 2009, 26, 3050-3057. (4) Klaus, A.; Tiddy, G. J.; Solans, C.; Harrar, A.; Touraud, D.; Kunz, W. Effect of Salts on the Phase Behavior and the Stability of Nano-emulsions with Rapeseed oil and an Extended Surfactant. Langmuir 2012, 28, 8318-8328. (5) Cendejas, G.; Arreguín, F.; Castro, L. V.; Flores, E. A.; Vazquez, F. Demulsifying Super-heavy Crude oil with Bifunctionalized Block Copolymers. Fuel 2013, 103, 356-363. (6) Yang, C. Electrochemical Coagulation for oily Water Demulsification. Sep. Purif. Technol. 2007, 54, 388-395. (7) Vold, M. J. Mechanism for the Ultracentrifugal Demulsification of O/W Emulsions.

Langmuir 1985, 1, 74-78. (8) Krebs, T.; Schroën, C.; Boom, R. Separation Kinetics of an oil-in-water Emulsion Under Enhanced Gravity. Chem. Eng. Sci. 2012, 71, 118-125. (9) Chu, Z.; Feng, Y. pH-switchable Wormlike Micelles. Chem. Commun. 2010, 46, 9028-9030. (10) Maçon, A. L.; Rehman, S. U.; Bell, R. V.; Weaver, J. V. Reversible Assembly of pH


Page 20 of 26

Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Responsive

Branched

Copolymer-stabilised

Emulsion

via

Electrostatic

Forces.

Chem.Commun. 2016, 52, 136-139. (11) Liang, C.; Harjani, J. R.; Robert, T.; Rogel, E.; Kuehne, D.; Ovalles, C.; Sampath, V.; Jessop, P. G. Use of CO2-triggered Switchable Surfactants for the Stabilization of oil-in-water Emulsions. Energy & Fuels 2011, 26, 488-494. (12) Liu, P.; Lu, W.; Wang, W.-J.; Li, B.-G.; Zhu, S. Highly CO2/N2-Switchable Zwitterionic Surfactant for Pickering Emulsions at Ambient Temperature. Langmuir 2014, 30, 10248-10255. (13) Su, X.; Cunningham, M. F.; Jessop, P. G. Switchable Viscosity Triggered by CO2 Using Smart Worm-like Micelles. Chem. Commun. 2013, 49, 2655-2657. (14) Xu,

P.;

Wang,

Z.;

Xu,

Z.;

Hao,

J.;

Sun,

D.

Highly

Effective

Emulsification/demulsification with a CO2-switchable Superamphiphile. J. Colloid Interface

Sci. 2016, 480, 198-204. (15) Tang, J.; Berry, R. M.; Tam, K. C. Stimuli-Responsive Cellulose Nanocrystals for Surfactant-Free Oil Harvesting. Biomacromolecules 2016, 17, 1748-1756. (16) Takahashi, Y.; Fukuyasu, K.; Horiuchi, T.; Kondo, Y.; Stroeve, P. Photoinduced Demulsification of Emulsions Using a Photoresponsive Gemini Surfactant. Langmuir 2013,

30, 41-47. (17) Datwani, S. S.; Nguyen Truskett, V.; Rosslee, C. A.; Abbott, N. L.; Stebe, K. J. Redox-Dependent Surface Tension and Surface Phase Transitions of a Ferrocenyl Surfactant: Equilibrium and Dynamic Analyses with Fluorescence Images. Langmuir 2003, 19, 8292-8301.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(18) Scott, L. M.; Robert, T.; Harjani, J. R.; Jessop, P. G. Designing the Head Group of CO2-triggered Switchable Surfactants. RSC Advances 2012, 2, 4925-4931. (19) Mihara, M.; Jessop, P.; Cunningham, M. Redispersible Polymer Colloids Using Carbon Dioxide as an External Trigger. Macromolecules 2011, 44, 3688-3693. (20) Fowler, C. I.; Jessop, P. G.; Cunningham, M. F. Aryl Amidine and Tertiary Amine Switchable Surfactants and Their Application in the Emulsion Polymerization of Methyl Methacrylate. Macromolecules 2012, 45, 2955-2962. (21) Harjani, J. R.; Liang, C.; Jessop, P. G. A Synthesis of Acetamidines. J. Org. Chem. 2011,

76, 1683-1691. (22) Fowler, C. I.; Muchemu, C. M.; Miller, R. E.; Phan, L.; O’Neill, C.; Jessop, P. G.; Cunningham, M. F. Emulsion Polymerization of Styrene and Methyl Methacrylate Using Cationic Switchable Surfactants. Macromolecules 2011, 44, 2501-2509. (23) Zhu, Y.; Jiang, J.; Liu, K.; Cui, Z.; Binks, B. P. Switchable Pickering Emulsions Stabilized by Silica Nanoparticles Hydrophobized in Situ with a Conventional Cationic Surfactant. Langmuir 2015, 31, 3301-3307. (24) Belowich, M. E.; Stoddart, J. F. Dynamic imine chemistry. Chem. Soc. Rev. 2012, 41, 2003-2024. (25) Layer, R. W. The Chemistry of Imines. Chem. Rev. 1963, 63, 489-510. (26) Xin, Y.; Yuan, J. Schiff's Base as a Stimuli-responsive Linker in Polymer Chemistry.

Polymer Chemistry 2012, 3, 3045-3055. (27) Godoy‐Alcántar, C.; Yatsimirsky, A. K.; Lehn, J. M. Structure‐stability Correlations for Imine Formation in Aqueous Solution. J. Phys. Org. Chem. 2005, 18, 979-985.


Page 22 of 26

Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(28) Jia, Y.; Li, J. Molecular Assembly of Schiff Base Interactions: Construction and Application. Chem. Rev. 2015, 115, 1597-1621. (29) Wang, G.; Wang, C.; Wang, Z.; Zhang, X. H-Shaped Supra-Amphiphiles Based on a Dynamic Covalent Bond. Langmuir 2012, 28, 14567-14572. (30) He, L.; Jiang, Y.; Tu, C.; Li, G.; Zhu, B.; Jin, C.; Zhu, Q.; Yan, D.; Zhu, X. Self-assembled Encapsulation Systems with pH Tunable Release Property Based on Reversible Covalent Bond. Chem. Commun. 2010, 46, 7569-7571. (31) Wang, C.; Wang, G.; Wang, Z.; Zhang, X. A pH‐Responsive Superamphiphile Based on Dynamic Covalent Bonds. Chemistry-A European Journal 2011, 17, 3322-3325. (32) Minkenberg, C. B.; Homan, B.; Boekhoven, J.; Norder, B.; Koper, G. J. M.; Eelkema, R.; van Esch, J. H. Responsive Wormlike Micelles from Dynamic Covalent Surfactants.

Langmuir 2012, 28, 13570-13576. (33) Minkenberg, C. B.; Li, F.; van Rijn, P.; Florusse, L.; Boekhoven, J.; Stuart, M. C.; Koper, G. J.; Eelkema, R.; van Esch, J. H. Responsive Vesicles from Dynamic Covalent Surfactants.

Angew. Chem., Int. Ed. 2011, 123, 3483-3486. (34) Jia, Y.; Fei, J.; Cui, Y.; Yang, Y.; Gao, L.; Li, J. pH-responsive Polysaccharide Microcapsules Through Covalent Bonding Assembly. Chem. Commun. 2011, 47, 1175-1177. (35) Ahn, C.-H.; Chae, S. Y.; Bae, Y. H.; Kim, S. W. Biodegradable Poly (ethylenimine) for Plasmid DNA Delivery. J. Controlled Release 2002, 80, 273-282. (36) Bai, L.; Zhu, H.; Thrasher, J. S.; Street, S. C. Synthesis and Electrocatalytic Activity of Photoreduced Platinum Nanoparticles in a Poly (ethylenimine) Matrix. ACS Appl. Mater.

Interfaces 2009, 1, 2304-2311.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(37) Sakata, M.; Kunitake, M. Chromatographic Removal of DNA From Protein Solutions by Cationic Polymer Beads. Curr. Pharm. Anal. 2007, 3, 170-179. (38) Martins, N. C. T.; Freire, C. S. R.; Pinto, R. J. B.; Fernandes, S. C. M.; Neto, C. P.; Silvestre, A. J. D.; Causio, J.; Baldi, G.; Sadocco, P.; Trindade, T. Electrostatic Assembly of Ag Nanoparticles onto Nanofibrillated Cellulose for Antibacterial Paper Products. Cellulose 2012, 19, 1425-1436. (39) Wang, J.; Chen, X.; Cui, W.; Yi, S. pH-responsive Vesicles from Supra-amphiphiles Based on Dynamic Imine bond. Colloids Surf., A 2015, 484, 28-36. (40) Greenspan, P.; Fowler, S. D. Spectrofluorometric Studies of the Lipid Probe, Nile red. J.

Lipid Res. 1985, 26, 781-789. (41) Greenspan, P.; Mayer, E. P.; Fowler, S. D. Nile red: a Selective Fluorescent Stain for Intracellular Lipid Droplets. J. Cell Biol. 1985, 100, 965-973. (42) Zhou, L.; Chen, M.; Guan, Y.; Zhang, Y. Multiple Responsive Hydrogel Films Based on Dynamic Schiff Base Linkages. Polym. Chem. 2014, 5, 7081-7089. (43) Wang, G.; Wu, G.; Wang, Z.; Zhang, X. Asymmetric and Symmetric Bolaform Supra-amphiphiles: Formation of Imine Bond Influenced by Aggregation. Langmuir 2014, 30, 1531-1535. (44) Wang, G.; Wang, C.; Wang, Z.; Zhang, X. Bolaform Superamphiphile Based on a Dynamic Covalent Bond and its Self-assembly in Water. Langmuir 2011, 27, 12375-12380. (45) Minkenberg, C. B.; Hendriksen, W. E.; Li, F.; Mendes, E.; Eelkema, R.; Van Esch, J. H. Dynamic Covalent Assembly of Stimuli Responsive Vesicle Gels. Chem. Commun. 2012, 48, 9837-9839.


Page 24 of 26

Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(46) Liu, H.; Wang, C.; Zou, S.; Wei, Z.; Tong, Z. Simple, Reversible Emulsion System Switched by pH on the Basis of Chitosan without Any Hydrophobic Modification. Langmuir 2012, 28, 11017-11024. (47) Abismaı̈ l, B.; Canselier, J. P.; Wilhelm, A. M.; Delmas, H.; Gourdon, C. Emulsification by Ultrasound: Drop size Distribution and Stability. Ultrasonics Sonochemistry 1999, 6, 75-83.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Graphical Abstract


Page 26 of 26

pH Switchable Emulsions Based on Dynamic Covalent Surfactants

Recommend Documents