pH-Responsive Emulsions with Supramolecularly Assembled Shells

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Separations

pH-responsive emulsions with supramolecularly assembled shells Li Hao, Cengiz Yegin, I-Cheng Chen, Jun Kyun Oh, Shuhao Liu, Ethan Scholar, Luhong Zhang, Mustafa Akbulut, and Bin Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00984 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 18, 2018

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Industrial & Engineering Chemistry Research

pH-responsive emulsions with supramolecularly assembled shells Li Hao,a,b Cengiz Yegin,c I-Cheng Chen,b Jun Kyun Oh,b Shuhao Liu,c Ethan Scholar,b Luhong Zhang,a Mustafa Akbulut,b,c,d* Bin Jiang,a**

a

School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China; b

Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843-3122, USA;

c

Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843-3003, USA; d

Texas A&M Energy Institute, Texas A&M University, College Station, TX 77843-3372, USA;

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

ABSTRACT: A novel and simple pH-switchable oil-in-water emulsion system is prepared by using amino-amide in combination with a trace amount of citric acid as stabilizer. The highly pH-responsive supramphiphile is assembled with amino-amide and citric acid via electrostatic interaction, which is characterized by attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, zeta potential, dynamic light scattering (DLS), surface tension and interfacial tension. The emulsification-demulsification with various oils can be switchable via tuning the pH through the addition of minor acid or base. Emulsion is ultrastable at pH of 2.2 but highly localized phase separation completes rapidly at pH of 5.5, while upon adjusting pH back to 2.2, stable emulsion is re-emulsified and recreated due to the re-assembly of supramphiphle. The emulsification and demulsification process are dependent on the reversible assembly and disassembly of the supramphiphile. Such pH-triggered emulsification and demulsification can be switched at least five times. Such intriguing properties of the novel pH-switchable emulsions indentify their potential in application to enhanced oil recovery and provide an effective approach to forming responsive emulsions and completing rapid demulsification.

KEYWORDS: emulsion, demulsification, pH-switchable, reversible, self-assemble, supramphiphile.

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1. Introduction

Emulsification plays a significant role in a wide range of industrial processes, such as food storage, pharmaceutical, material coating, cosmetics, drug delivery, and petroleum process, for which long-term emulsion stability is critical.1,2 However, phase separation is also necessary in the field of heavy oil transportation, enhanced oil recovery, and emulsion polymerization, where only temporary emulsion stability is desired.3,4,5 In general, chemical demulsification has been widely employed to break up emulsion through the addition of demulsifiers, which is often complex with large energy consumption or generates extra additives causing environmental concerns and subsequent pollutions.6,7 Thus, smart or switchable emulsions with on/off property are desired. Such emulsions can be prepared with switchable surfactants, emulsifiers, or stabilizers, which can be transformed between surface-active and surface-inactive forms via appropriate triggers.8,9,10

In recent years, ever-increasing research interests have been attracted by the field of stimuli-responsive materials for the controlled formation of emulsions that can undergo reversible changes in physical-chemical or colloidal properties in response to external or internal stimuli, such as pH, temperature, light, electrolytes, redox, and CO2.11,12,13,14,15,16 To date, among these triggers, CO2/N2 triggers have been intensively investigated. For example, Jessop and co-workers have synthesized and reported a series of switchable surfactants or solvents, which were dependent on the switchability of tertiary-amines or amidine groups by protonation and deprotonation

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in the presence and absence of CO2, respectively, so that the on and off property can be achieved.17,18,19 Liang et al.20 grafted ligands with CO2-responsive terminal groups on silica particles to stabilize oil-in-water emulsions. They were also able to alter the stability of such emulsions by exposing the system to CO2 gas, which increases the surface energy of ligands and favors the disassembly of functionalized-silica shells from emulsion. Amines and fatty acids have been proven to be good CO2-switchable chemicals.21,22,23,24 Recently, considerable attention has been devoted to develop more convenient

and

eco-friendly

pH-triggered

surfactants

or

emulsifiers.

The

pH-responsive emulsions based on acid-base reactions tend to suffer from the loss of performance owing to the accumulation of neutralization products, newer strategies involving simple, inexpensive, and easy-to-implement are still needed.24 In this context, for instance, Ren et al.25 reported the utilization of dynamic chemical interactions between polyethylenimine (PEI) and PEI benzaldehyde (B) to prepare pH-switchable emulsions. The coupling of aromatic ring on PEI via imine formation improved the hydrophobicity of emulsion shell and prevented the detachment of molecules from emulsion shell to water phase. The chemical coupling between these molecules and the resultant interfacial characteristics could be controlled by manipulating solution pH. Armes et al.26,27 described the cross-linking of amino groups and carboxylic-acid-based polymers to achieve pH-responsive emulsion properties. However, it is still challenging to explore pH-responsive surfactants or emulsifiers formed by non-covalent interactions.

Supramphiphiles are generally formed by non-covalent dynamic interactions or 4


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multiple weak intermolecular interactions such as

hydrogen bonding, dispersion

forces, hydrophobic interactions, π-π stacking, and Coulomb interactions.28,29,30 Due to the controllable behavior and reversibility of non-covalent interaction, the molecular self-assembly and disassembly of supramphiphiles can be adjusted by external stimuli. Xu et al.

31

prepared a supramolecular system involving electrostatic

interactions of oleic acid and Jeffamine D230, which is a bifunctional amphiphile, to accomplish highly CO2-responsive characteristics, which were utilized for preparing oil-in-water emulsions that are sensitive to CO2. Usually, in this case, supramphiphiles can be simply fabricated without further purification and the transition is more rapid and simple.32 While only a few studies have reported the formation of stimuli-responsive supramphiphiles so far,33,34,35 switching between emulsification and demulsification using pH-responsive supramphiphiles in a reversible fashion has a lot of potentials in various industrial applications. Additionally, fatty acids derived from plants, which can be utilized as sustainable and renewable building blocks in supramolecular assembly processes, are a particularly attractive intermediate chemicals because of their high biodegradability, low cost, environmental friendliness, and superior biocompatibility. 36,37,38

In this work, for sustainable purpose, we utilized stearic acid and N,N-dimethyl-1,3-propanediamine as starting materials to prepare a class of amino-amide/citric

acid

(marked

as

AACA)

based

highly

pH-responsive

supramphiphile and investigated the pH-switchable properties in emulsification and demulsification.

Stable

oil-in-water

emulsions

were

5


prepared

by

use

of


amino-amide/citric acid supramphiphile as stabilizer. Such emulsions could complete rapid phase separation (i.e. demulsification) and re-emulsification for seconds by adjusting pH value through the addition of minor acids or bases. The emulsification-demulsification process was switchable due to the reversible assembly and disassembly of supramphiphile. These intriguing features make such a system a highly smart material for enhanced oil recovery and effective oil/water phase separation without adding any demulsifiers.

2 Experimental Section

2.1 Materials

N,N-dimethyl-1,3-propanediamine (DMPDA, ＞99%), sodium fluoride (NaF, 99%),n-hexane (≥98.5%, ACS grade), and maleic acid (＞98%) were purchased from Alpha Aesar (Ward Hill, MA). Aluminum oxide (Al2O3) (activated, neutral, 150 mesh), stearic acid (95%), citric acid (99%), hydrochloric acid (HCl, 37%), sodium hydroxide (NaOH, ≥97.0%), and toluene (≥99.5%, ACS grade) were procured from Sigma Aldrich (St. Louis, Missouri). Paraffin oil was acquired from Fluka (St. Louis, MO).Dichloromethane (≥99.5%) was procured from TCI America (Portland, OR). All the reagents were used as received. Milli-Q water (resistivity of∼18.2 MΩ·cm−1) was used in all the experiments.

2.2 Synthesis of N-[3-(dimethylamino)propyl] stearamide

Figure 1 illustrates the reaction scheme for the formation of amino-amide via the

6


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condensation of DMPDA and stearic acid, which is commercially available, natural, long-chain fatty acid. The reaction was carried out by heating the mixtures under reflux at 160 °C. The product was prepared in the presence of argon atmosphere and in the absence of any solvent for 10 hours as described in references.33,39,40 Here, NaF was used as catalyst to accelerate the reaction while Al2O3 was utilized to absorb water produced during the condensation reaction. Upon the completion of the reaction, the unreacted precursors were washed away with cold acetone. Finally, the product was

dried

under

vacuum

at

40

°C

overnight

to

obtain

N-[3-(dimethylamino)propyl]stearamide. O H3C

(CH2)16

C

stearic acid

CH3 OH

+ H2N

(CH2)3

DMPDA

N CH3

NaF Al2O3

CH3 +

H3 N

N

(CH2)3

O

Heat Dehydration

H3C

CH3

(CH2)16

C

CH3 NH

(CH2)3

amino/amide

N

+ H2O CH3

+ O H3C

(CH2)16

C

O-

Figure 1. The reaction scheme for the formation of long-chain amino-amide from the condensation of stearic acid and N,N-dimethyl-1,3-propanediamine via NaF catalysis. 2.3 Preparation of pH-responsive emulsions

The supramolecular complexation of citric acid and amino-amide is the main principle behind the formation of pH-responsive emulsions. A typical preparation procedure started with the mixing of oil (2 mL) and water (4 mL) at 1:2 volume ratio in a glass vial. As oil phase; one of toluene, n-hexane, dichloromethane, and paraffin oil was used to produce emulsion systems with varying polarity, density, and viscosities. Then, 0.005 mmol/L of amino-amide was added into the vial to achieve a concentration of 0.05 wt% in the oil/water mixture. Subsequently, 70 µL of 1 mol/L citric acid solution was added into the vial. The mixture was homogenized by a vortex 7



mixer (MX-S, Scilogex LLC, Rocky Hill, CT) and vigorous shaking for several minutes at room temperature until the system became turbid. Afterwards, 70 µL of 1 mol/L HCl solution was added into as-prepared emulsion to adjust pH and enhance the colloidal stability. To determine if the formed emulsions are water-in-oil or oil-in-water in nature, aliquots of the prepared emulsions were added in water or oil and observed under an optical microscope (Olympus BX51, Japan).

The supramolecular shell of emulsions involving the complexation of amino-amide and citric acid could be reversibly assembled and disassembled by adjusting pH from acidic (＜5.5) conditions to neutral and basic conditions (≥5.5) and vice versa. This was accomplished by adding a few drops of 1 mol/L NaOH or 1 mol/L HCl solutions in the suspension and shaking the whole system for several minutes. Five cycles between emulsified and demulsified states were feasible in this system. With a higher number of cycles, salts forming due to the neutralization of NaOH and HCl with their repeated additions into the suspension disrupted the supramolecular assembly forming the shell of emulsions.

Attenuated total reflectance Fourier-transform infrared spectrometry (ATR-FTIR, IRPrestige-21, Shimadzu Scientific Instruments Inc., Columbia, MD) was utilized to investigate the interactions between amino-amide and citric acid. Measurements were an average of 12 repeats for each ligand and were done in the transmission mode. IRsolution software package (Version 1.40, Shimadzu) was used to analyze the obtain data.

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2.5 Characterization of interfacial properties and size of emulsions

The surface tension of aqueous solutions of amino-amide amphiphile and amino-amide/citric acid mixtures was measured at various pH values (pH 2-11) in ambient air using pendant droptensiometry (PDT). Likewise, the interfacial tension of these solutions was characterized in hexane and paraffin oil via PDT, which relied on capturing digital images of droplets suspended from a needle and the analysis of drop shape to calculate the values.41,42,43 Special attention was paid to the droplet size to ensure that it is large enough so that the Bond number is not close to zero, increasing the accuracy of shape analysis.

The transmittance of the solution involving amino-amide, citric acid, water, and oil was monitored using the UV-vis spectroscopy (UV-1800, Shimadzu, Japan) as a function of pH at 680 nm where appeared maximum transmittance for each sample. The particle size distribution and zeta potential of emulsions with supramolecular shells were measured using dynamics light scattering (DLS) and laser Doppler electrophoresis (LDE) via a Zetasizer Nano ZS 90 (MalvernInstruments, Ltd., Westborough, MA) at various pH values in the range of 3 and 11. Emulsions larger than 5 µm were characterized with an optical microscope instead of light scattering (Olympus BX51, Japan).

3 Results and discussion

3.1 Chemical interactions of building blocks of supramolecular system

9



ATR-FTIR spectroscopy was used to investigate the complexation of amino-amide amphiphile and citric acid (Figure 2a&b). Bare amino-amide amphiphile displayed a characteristic stretching vibration of C=O at 1638 cm-1, corresponding to amido group, and peak at 3308 cm-1and 1547 cm-1, associated with the stretching and bending vibrations of N–H, respectively. The strong bands in the region of 2800-2900 cm-1 were attributed to the stretching vibration of C–H while the peaks at 1460 cm-1 and 1375 cm-1 were C–H scissoring and methyl rock bands. The main IR peaks of citric acid were observed at 3497, 3273 cm-1 (OH str.), 1740, 1688 cm-1 (C=O str.), 1427, 1383, 1352 cm-1 (C–O–H bend), 1233, 1213, 1192, 1167, 1130 cm-1 (C–O str.), 1078, 1045cm-1 (C–O–H str.), 932, 899, 876, 816 cm-1 (C=O bend), which were consistent with literature. 44

Compared to pure amino-amide and citric acid, the complexed formed of amino-amide and citric acid demonstrated several distinct characteristics. First, two distinct peaks at 1730 cm-1 and 1604 cm-1 corresponding to stretching vibration of C=O and COO- appeared in the spectrum of aqueous solution of amino-amide/citric acid complexation (Figure 2c), which were formed by the deprotonation of COOH and indicated the formation of carboxylate. Additionally, two obvious strong peaks appeared in C–O–H stretching vibration region for the complexation (1153 and 1028 cm-1). All above characteristic peaks confirmed the formation of carboxylate and the corresponding complexation of amino-amide and citric acid. Different from conventional surfactants, complexation of amino-amide and citric acid was formed through electrostatic interaction, as illustrated in Figure 2c, which could promote the 10


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formation of micelles and lead to pH reversible property.33 (a)

(b)

(c)

CH3

O H3C

(CH2)16

C

NH

(CH2)3

NH+ O-

CH3 CH3

O H3C

(CH2)16

C

NH

(CH2)3

CH3 CH3

O H3 C

(CH2)16

C

NH

(CH2)3

O O

NH+

OH O-

O O-

NH+ CH3

adaptable amphiphile

Figure 2. ATR-FTIR spectra of amino-amide and citric acid systems in the range of (a) 1800-800 cm-1, and (b) 3800-2400 cm-1, (c) adaptable amphiphile formed by amino-amide and citric acid. 3.2 Effect of pH on size, zeta potential, and stability of complexation

To better understand the effect of pH on supramolecularly complexation, first, we have investigated the transmittance of such complexation as a function of pH 11



(Figure 3). Transmittance of amino-amide/citric acid complexation suspension was mostly constant (about 29.45±1.38%) in the pH range of 7 to 12 and strongly increased with increasing acidity in the range of 2 to 6, including the formation of intermolecular associations below pH 6. These trends can be explained by the protonation of amino groups and deprotonation of carboxyl groups, which can induce the electrostatic complexation. Specifically, the amino groups were protonated at pH of 2.0 (pKa=2.40 of adaptable amphiphile), and two of the carboxyl groups of citric acid were dissociated at pH of 5.5 and the degree of dissociation for carboxyl groups of citric acid completed totally at pH 7 because of its pKa values (pKa1=3.13, pKa2=4.76 and pKa3=6.39). By addition of HCl solution (1 mol/L), the complexation solution would become transparent again upon gentle mechanical agitation, which indicated the complex system possessed a pH-tunable transition. It should be mentioned that quaternary amine played a significant role in the supramolecular assembly of amino-amide and citric acid and enhanced the pH-responsiveness of the system.

Figure 3. Transmittance at 680 nm of complexation of long-chain amino-amide with citric acid as a function of pH. 12


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The amino-amide should be positively charged at the amino group below pH 5.5 due to its pKa of 9.40, which was consistent with zeta potential measurement (as shown in Figure 4). In this case, the zeta potential of amino-amide suspensions was positive below pH 7 owing to possible protonation of the tertiary amine groups in aqueous solutions. The system exhibited an isoelectric point (IEP) at approximately pH 5.5. It was worthy to note that when pH was above 5.5, the complexation of amino-amide and citric acid was less favorable due to weaker electrostatic contribution of intermolecular interactions induced by smaller surface charges. Besides, the salt formation would decrease the surface charge and thus affect the stability and aggregation behavior of amino-amide. As displayed in Figure 4, the complexation with amino-amide and citric acid still possessed higher positive charges even with the addition of 200 mmol/L NaCl, which indicated salt-tolerant property of the complex system.

Figure 4. Zeta potential changes of amino-amide/citric acid aqueous solutions as a function of pH, and effect of salt concentration at pH 2.0 and 3.8 with the addition of 0, 50, 100, 150, 200 mmol/L NaCl.

13



3.3 pH-switchable toluene-in-water emulsion stabilized by AACA

As mentioned above, amino-amide system exhibited good responsiveness and water-soluble switchability to pH value, therefore it can be applied for the preparation of pH-responsive emulsions, which would achieve the rapid switch between emulsification and demulsification upon adjusting pH value. The emulsification and demulsification process were presented by digital photographs in Figure 5 (a-g). Specifically saying, amino-amide dissolving in water would formed the turbid solution with pH 9.4 as shown in Figure 5a. From DLS, it showed that the hydrodynamic size for amino-amide was 0.54±0.03 µm around 30 ºC in Fig.S1. The reported hydrodynamic size from DLS is based on the assumption of a spherical shape, which only for a comparison purpose. And then it appeared two immiscible phases once adding toluene as oil phase (Fig.5b). Subsequently, by adding citric acid into the liquids and stirring the system, the interfaces between the two immiscible phases disappeared fast and it transformed into a kind of homogeneous emulsion with pH of 2.2 immediately (Fig.5c). The droplet size in AACA-based toluene emulsion was measured to be 2.24±0.24 µm from optical microscope image (Fig.5i). The drop test confirmed that the emulsion was O/W type. Afterwards, by tuning pH to 3.8, it can be still observed the toluene droplets formed in the emulsion from Fig. d&i, and can be calculated the droplet size of 3.17±0.61 µm. Upon increasing pH value gradually, the toluene emulsion started to be broken and partial phase separation was observed once pH close to 4.3, and the demulsification and phase separation was obtained completely while pH increasing to near 5.5 with oil-water interface 14


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appearing obviously and clear toluene phase without droplets (Fig.5e-g, j). The reversible emulsions switched by the pH values were readily achieved. It appeared as the homogeneous emulsion again upon adding HCl solution and shaking or stirring to adjust pH of 2.2. Furthermore, the responsive process of the toluene/AACA/water system switched by tuning pH was very fast, even after adding one drop of acid, the system immediately became emulsion.

Figure 5.(a-g) Digital photographs of emulsification and demulsification process of AACA-based toluene-in-water emulsions: (a) amino-amide dispersed in water (pH 9.4), (b) toluene/AACA/water system appearing at two immiscible phases before citric acid addition, (c) toluene/AACA/water system forming homogeneous emulsion after adding citric acid (pH 2.2), (d) toluene-in-water emulsion by addition of NaOH switching pH to 3.8), (e) toluene-in-water emulsion switched off by addition of NaOH (pH 4.3), (f) toluene-in-water emulsion switched off by addition of NaOH (pH 5.2), (g) demulsification induced by addition of NaOH and adjusted pH to 5.5, appearing two 15



immiscible phases; (h-j) Optical microscope images for toluene/AACA/water system during different periods at pH values of 2.2, 3.8, 5.5. 3.4 pH-switchable emulsions forming with different oil phases

In order to further investigate the application of AACA compound as emulsifier, four different oils with various viscosities and densities (toluene, n-hexane, dichloromethane, and paraffin oil) were chosen as oil phases for preparing pH-switchable emulsions. In these series of experiments, four stable emulsions were acquired and the emulsification-demulsification process can be switched by tuning pH, as presented in Figure 6(a-d). The optical microscopic images of these emulsions were shown in Figure 6(e-h). The droplet sizes of the emulsions were calculated from micrographs to be approximately 2.76±0.72 µm, 3.70±0.87 µm, 5.65±1.73 µm, and 15.06±8.14 µm for toluene, n-hexane, dichloromethane (CCl2), and paraffin oil, respectively, among which the size of the emulsion using paraffin oil as oil phase was estimated to be larger than those of other three emulsions due to its high viscosity8. Therefore, AACA compound emulsifier could stabilize various types of oils in water, which render it a promising material in many applications.

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Figure 6. Digital photographs of AACA-based emulsification-demulsification process for (a) toluene, (b) n-hexane, (c) dichloromethane, (d)paraffin oil; Optical micrographs of AACA-based emulsions using different oil phases: (e) toluene-in-water emulsion, (f) n-hexane-in-water emulsion, (g) dichloromethane-in-water emulsion, (h) paraffin oil-in-water emulsion. 3.5 Reversibility and stability of AACA-based emulsions

The reversibility of AACA-based emulsion switched by pH was shown in Figure 7. It can be seen that the emulsion was switched easily several times to realize emulsification-demulsification process and behaved as a pH-responsive system. The system rapidly formed a stable emulsion at pH 2.2 upon adding HCl solution, whereas the demulsification took place rapidly by adding NaOH solution, resulting in two phase separation layers. The same emulsification-demulsification process can be repeated more than five times without obvious changes, indicating that AACA compound acted as an excellent emulsifier in response to external pH stimuli. The emulsion droplet size increased from 2.76±0.72 µm to 5.04±0.82 µm with increasing

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the number of cycles.

The AACA-based emulsions using different oil phases were very stable against aggregation. Zeta potential results also indicated the stability of AACA-based emulsions, as summarized in Table 1. Initially, amino-amide dispersed in water and amino-amide/citric acid aqueous solution presented similar positive zeta potentials of 10.66±2.55 and 10.89±2.91 mV, respectively, which were ascribed to the presence of quaternary ammonium (N+). The addition of acid to toluene/AACA/water system increased the zeta potential to 65.67±5.43 mV, indicating the formation of stable emulsion, presumably owing to strong electrostatic repulsion45,46. Meanwhile, the differences of viscosity and permittivity among four different oil phases led to various zeta potential values during the formation of emulsions. Three of them indicated good colloidal stability, but AACA-based dichloromethane-in-water emulsion showed weak electrostatic repulsion mainly due to its high permittivity whose stability was not as good as other three emulsions. Table 1. Zeta potential of AACA-based emulsions using different oils at various stages of the formation. Type Amino-amide dispersed in water Amino-amide/citric acid aqueous solution AACA-based toluene-in-water emulsion AACA-based hexane-in-water emulsion AACA-based dichloromethane-in-water emulsion AACA-based paraffin oil-in-water emulsion

Zeta potential/mV 10.66±2.55 10.89±2.91 65.67±5.43 34.60±4.81 8.36±0.30 37.47±2.96

The emulsions stabilized by AACA-based supramphiphile and formed at pH around 2.2 can be stable for more than 60 days without obvious change, as

18


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comparatively displayed in Figure 7c, which exhibited excellent long-term stability against coalescent and also suggested supramphiphile assemble of amino-amide/citric acid effectively stabilized emulsions. Additionally, the as-prepared emulsion only utilized a small amount of amino-amide (0.05 wt%). Indeed, amino-amide can complex with citric acid and then form a kind of amphiphilic compound. The chemical nature of the switchability is the reversible quaternization of the tertiary amine group in the amino-amide.

Figure 7. (a) Digital photographs of AACA-based toluene-in-water emulsions switched between emulsification-demulsification process for five cycles, (b) toluene emulsion droplet size distribution changes during emulsification-demulsification cycles, (c) digital photographs and optical microscope images of initial AACA-based toluene emulsion and emulsion after standing for two months. NaCl

concentration

will

rise

with

an

increasing

number

of

emulsification-demulsification process cycles owing to the accumulation of NaCl by

19



the addition of HCl and NaOH solution. Hence, the effect of NaCl concentration of the amino-amide/citric acid supramphiphile solution on the average droplet size of the emulsion was investigated (Figure 8). The average emulsion droplet size increased from 2.24±0.24 to 5.57±0.03 µm with the increase of NaCl concentration, so the increasing of salt concentration may account for the increased droplet size upon multiple emulsification-demulsification cycles (Fig.7b). The increased amount of salt will decrease the Debye length and reduce the range of electrostatic interactions between interacting species, thereby hindering the supramolecular assembly processes. A smaller screening length also implies that colloidal suspension will have higher tendency to aggregate and higher chance to sediment owing to increased size. The gradual buildup of background salt would form electrostatic screening which suppressed

pH-induced

property

and

hence

prevented

the

emulsification-demulsification cycle process24-25.

μm

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

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Figure 8. Average droplet size of toluene emulsion as a function of NaCl concentration in amino-amide solutions. 3.6 Possible mechanism of emulsification-demulsification induced by pH-stimuli 20


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The surface tension of supramphiphile solutions at pH 2.2 was determined to be 42.24±0.85 mN/m, which indicated that the supramolecular assemble may behave as long-chain surfactants and could be excellent emulsifier or stabilizer for emulsions (Supporting Information Table S1). Nevertheless, upon adjusting pH above 5.5, surface tension of supramphiphile solution was increased to 68.51±1.54 mN/m, similar to that of water (72.38±0.84 mN/m), which illustrated that supramolecular assemble of amino-amide and citric acid at pH above 5.5 could not behave like typical surfactants because it cannot lower the surface tension of pure water (Supporting Information Table S1 and Figure S3). The results of interfacial tension (IFT) measurement at different pH values showed that supramphiphile solution at pH 2.2 can decrease IFT to a low value of 4.42±1.52 and 10.91±1.08 mN·m-1 at paraffin oil/water and hexane/water interfaces, respectively, indicating excellent adsorption capacity and high interfacial activity of supramphiphiles at oil/water interfaces. Upon increasing the pH of supramphiphile solution from 2.2 to 5.5, the IFT increased to 21.33±1.02 and 21.23±1.35 mN·m-1 at paraffin oil/water and hexane/water interfaces, which indicated that the interfacial activity of supramphiphile can be controlled by tuning pH values (as shown in Table 2). Amino-amide aqueous solution alone does not possess surface and interfacial activity (Figure S3&S4). Table 2. Interfacial tension of supramphiphile assembled by amino-amide with citric acid at oil/water interfaces under different pH values. Droplet phase

Continuous phase

IFT (mN·m-1)

amino-amide aqueous solution

paraffin oil

52.50±4.46 (pH=9.4)

21



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supramphiphile aqueous solution

paraffin oil

4.42±1.52 (pH=2.2)

21.33±1.02 (pH=5.5)

water

paraffin oil

43.92±2.15 (pH=2.2)

51.70±1.25 (pH=5.5)

amino-amide aqueous solution

hexane

supramphiphile aqueous solution

hexane

10.91±1.08 (pH=2.2)

21.23±1.35 (pH=5.5)

water

hexane

42.12±3.15 (pH=2.2)

49.73±2.97 (pH=5.5)

50.39±3.52 (pH=9.4)

A possible mechanism for pH-induced demulsification is proposed in Figure 9. The switchable behavior is explained by the protonation of amino groups and deprotonation of carboxyl groups and the resultant complexation processes. Amino-amide and citric acid can form a building block for the supramolecular assembly via electrostatic interactions between N+ of amino-amide and COO- of citric acid. Around pH 2.2, amino-amide molecules are at fully protonated state, (i..e the cationic form) which can have strong Columbic forces against citric acid and form the supramphiphile enhancing the surface activity. The electrostatic interaction of quaternary ammonium group (-N+) and the anionic (-COO-) can stabilize the emulsions. It can be explained that the supramphiphile molecules adsorb at the oil/water interface with the opportunity to touch H+ and OH- ions which leads to responsibility to the pH stimuli. The adsorption of adaptable amphiphile of amino-amide/citric acid at the oil/water interface forms a stable interfacial film to stabilize the droplets due to interfacial activity which results in the formation of stable O/W emulsions. The number of protonated tertiaryamine group of amino-amide decreases as pH increasing to 5.5, resulting in the salt bridges significantly decreasing in number. Thus, amino-amide/citric acid supramphiphile becomes dissociated to 22


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interfacial inactive amino-amide due to the break of electrostatic interaction and the hydrolysis of supramphiphile, which leads to quaternary ammonium group (NH3+) and the anionic (COO-) electrostatic interaction weakened and desorption from oil/water interface, undergoing the immediate coalescence of the oil droplets followed by completed phase separation, i.e., demulsification. With removal of OH- by adding H+, the amino-amide reacts with citric acid to reassemble supramphiphile, giving rise to reversible transitions between emulsified and demulsified states.

(a) H3C

O NH

(CH2)3

+

N

HO

OH

CH3

(CH2)16

C

OH

O

CH3

O H3C

OH O

O

CH3

C

(CH2)16

NH

NH+

(CH2)3

O-

CH3

H3C

(CH2)16

C

O

CH3

O NH

O

NH+

(CH2)3

OH O-

CH3 CH3

O H3C

(CH2)16

C

NH

O O-

NH+

(CH2)3

CH3

adaptable amphiphile CH3

O H3C

(CH2)16

C

NH

(CH2)3

N+

+

electrostatic interaction

O CH2

C

-

O-

CH3

(b) oil

-

O/W emulsion

emulsification (H+)

+

oil demulsification (OH-)

-

+

water +

+ -

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


water

Figure 9. Proposed emulsification and demulsification mechanism: (a) the synthesis protocol for the formation of the pH-responsive supramphiphile, (b) emulsification and demulsification process induced by adaptable supramphiphile. 23



We anticipate that such pH-responsive emulsions with supramolecularly assembled shells can find intriguing uses industrial applications, in particular cosmetics, medical, food industries, and enhanced oil recovery. For example, one of the advantages for enhanced oil recovery is that the oil and water emulsion formed from the displacement fluid can be quickly demulsified in response to external stimuli. The simplest is to adjust the switching between emulsification and demulsification by pH intelligently without any additional demulsifier. Besides, the raw materials for amino-amide are environmental-friendly, which only are needed a trace amount to form a stable O/W emulsion.

4 Conclusions

In this work, an effecient approach was proposed to design and prepare a pH-switchable emulsion system involving amino-amide. Although amino-amide and citric acid individually cannot stabilize emulsions, the self-assembly of amino-amide with citric acid provided a convenient pathway to obtain pH-responsive supramphiphile via electrostatic interaction without tedious procedures, which made it use as stabilizer or emulsifier with a trace amount (0.05 wt%) to form a stable O/W emulsion. The transmittance of the supramphiphile aqueous solution declined as pH increase and the lowest transmittance point occurred at pH 5.5. Besides, the supramphiphile system exhibited an isoelectric point at approximately pH 5.5 and above this point, the complexation of amino-amide and citric acid lost most of its charge and the electrostatic contribution of intermolecular interactions gradually

24


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decreased and became weak. The O/W emulsions with various oil phases were stable to coalescence at pH 2.2 but can be demulsified rapidly and completely at pH 5.5. The AACA-based O/W emulsions could undergo at least five successive cycles of emulsification-demulsification process by tuning pH values and had good long-term stability. Because of the accumulation of salt in the system and thus buildup of background salt after more number of reversible cycles, which partially screened the electrostatic interaction among supramphipiles and suppressed the pH-induced property, thereby increasing the droplet size gradually.

AUTHOR INFORMATION Corresponding Authors *Telephone: 979-847-8766. Fax: 979-845-6446. E-mail: [email protected] (Mustafa Akbulut), Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843-3122, United States. **E-mail: [email protected] (Bin Jiang), School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China.

Notes

The authors declare no competing financial interest.

Supporting information

25



Fig. S1: Hydrodynamic size of supramolecular system involving amino-amide as a function of temperature.

Fig. S2: Hydrodynamic diameter of amino-amide solution with various concentrations at different pH values.

Fig. S3: Surface tension of amino-amide solution with various concentrations at different pH values.

Fig. S4: Interfacial tension between amino-amide solution with various concentrations and paraffin oil at different pH values.

Fig. S5: Digital photographs of AACA-based toluene-in-water system switched between emulsification and demulsification process by use of different acids.

Table S1: Surface tension of aqueous solution of amino-amide/citric acid under different pH.

ACKNOWLEDGEMENT

The authors gratefully appreciate the financial support from the China Scholarship Council (CSC).

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Captions of figures Figure 1. The reaction scheme for the formation of long-chain amino-amide from the condensation of stearic acid and N,N-dimethyl-1,3-propanediamine via NaF catalysis. Figure 2. ATR-FTIR spectra of amino-amide and citric acid systems in the range of (a) 1800-800 cm-1, and (b) 3800-2400 cm-1, (c) adaptable amphiphile formed by amino-amide and citric acid. Figure 3. Transmittance at 680 nm of complexation of long-chain amino-amide with citric acid as a function of pH. Figure 4. Zeta potential changes of amino-amide/citric acid aqueous solutions as a function of pH, and effect of salt concentration at pH 2.0 and 3.8 with the addition of 0, 50, 100, 150, 200 mmol/L NaCl. Figure 5.(a-g) Digital photographs of emulsification and demulsification process of AACA-based toluene-in-water emulsions: (a) amino-amide dispersed in water (pH 9.4), (b) toluene/AACA/water system appearing at two immiscible phases before citric acid addition, (c) toluene/AACA/water system forming homogeneous emulsion after adding citric acid (pH 2.2), (d) toluene-in-water emulsion by addition of NaOH switching pH to 3.8), (e) toluene-in-water emulsion switched off by addition of NaOH (pH 4.3), (f) toluene-in-water emulsion switched off by addition of NaOH (pH 5.2), (g) demulsification induced by addition of NaOH and adjusted pH to 5.5, appearing two immiscible phases; (h-j) Optical microscope images for toluene/AACA/water system during different periods at pH values of 2.2, 3.8, 5.5. Figure 6. Digital photographs of AACA-based emulsification-demulsification process for (a) toluene, (b) n-hexane, (c) dichloromethane, (d)paraffin oil; Optical micrographs of AACA-based emulsions using different oil phases: (e) toluene-in-water emulsion, (f) n-hexane-in-water emulsion, (g) dichloromethane-in-water emulsion, (h) paraffin oil-in-water emulsion. Figure 7. (a) Digital photographs of AACA-based toluene-in-water emulsions switched between emulsification-demulsification process for five cycles, (b) toluene emulsion droplet size distribution changes during emulsification-demulsification cycles, (c) digital photographs and optical microscope images of initial AACA-based toluene emulsion and emulsion after standing for two months. Figure 8. Average droplet size of toluene emulsion as a function of NaCl concentration in amino-amide solutions. Figure 9. Proposed emulsification and demulsification mechanism: (a) the synthesis protocol for the formation of the pH-responsive supramphiphile, (b) emulsification and demulsification process induced by adaptable supramphiphile. 30


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Captions of tables Table 1. Zeta potential of amino-amide-based emulsions using different oils at various stages of the formation. Table 2. Interfacial tension of supramphiphile assembled by amino-amide with citric acid at oil/water interfaces under different pH values.

31



Table of Contents Graphic

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pH-Responsive Emulsions with Supramolecularly Assembled Shells

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