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Reversibly Switching Wormlike Micelles Formed by Selenium-containing Surfactant and Benzyl Tertiary Amine Using CO2/N2 and Redox Reaction Yongmin Zhang, Lian Liu, Xuefeng Liu, and Yun Fang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03837 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 23, 2018
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Reversibly Switching Wormlike Micelles Formed by a Selenium-containing Surfactant and Benzyl Tertiary Amine Using CO2/N2 and Redox Reaction Yongmin Zhang, Lian Liu, Xuefeng Liu* and Yun Fang
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Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical & Materials Engineering, Jiangnan University, Wuxi 214122, P. R. China
ABSTRACT: Multi-responsive wormlike micelles (WLMs) remains a significant challenge in the construction of smart soft materials based on surfactants. Herein, we report the preparation of a viscoelastic wormlike micellar solution based on a new 8 redox-responsive surfactant, sodium dodecylselanylpropyl sulfate (SDSePS), and commercially available benzyl tertiary amine (BTA) in the presence of CO2. In this system, the SDSePS can be reversibly switched on (selenide) and off (selenox10 ide) by a redox reaction, akin to that previously reported for benzylselanyl- or phenylselanyl- surfactants. By alternately adding H2O2 and N2H4H2O, WLMs can be reversibly broken and formed due to transformation of the hydrophilic headgroup of 12 SDSePS, originating from the reversible formation of selenoxide. Moreover, WLMs can also be switched on and off by cyclically bubbling CO2 and N2, due to variation of the binding interaction between SDSePS and BTA, resulting from the re14 versible protonation of BTA. This interesting and unique multi-responsive behavior makes the current WLMs a potential candidate for smart control of the “sol–gel” transition or substantial thickening of solutions. 6
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INTRODUCTION
Surfactants in solution can self-assemble into diverse ordered structures, such as spherical micelles, rodlike micelles, vesicles, and so on. Among these, wormlike mi20 celles (WLMs) are long, flexible, threadlike micelles, typically with diameters of several nanometers, and lengths of 22 hundreds or thousands of nanometers. Due to their unique structure and prominent rheological response, WLMs have 24 become a highly promising focus of recent developments in soft matter over the past decade, and show greatly potential 1-5 26 for applications. 18
Over the past decade, controlling the viscoelasticity of surfactant solutions through switching WLMs by a minor external stimulus has attracted considerable attention from 30 scientists and engineers. To date, the switching of WLMs by an individual physical or chemical stimulus, such as 6-12 32 pH, temperature,13-15 light,16-19 magnetic field,20 CO2,21-27 27-30 redox, and so on, has been extensively reported, where34 as multi-responsive WLMs have been less well documented. Thus, many recent efforts have been devoted to the con36 struction of multi-responsive WLMs to achieve a wider controllable range or higher precision. 28
coelastic WLMs were formed and ruptured by the alternate addition of N2H4H2O and H2O2 (1.1 equivalents with re52 spect to BSeAS), because of a transformation of the molecular structure. Moreover, when BSeAS was in the reduced 54 form, the WLMs showed a reversible response to the bubbling and removal of CO2, reflecting a transition between a 56 gel-like fluid and a water-like fluid due to the formation and collapse of a pseudo-gemini structure. Very recently, 58 Cui et al reported another CO2 and redox dual responsive worm.30 60 Notably, the selenium atom is very close to the benzene ring in BSeAS, which is generally believed to be critical 62 for reversible redox switchability of selenium according to Zhang and Xu’s previous studies on selenium-containing 31-33 64 polymers. Furthermore, selenium-containing surfactants reported to date have mainly been limited to those 66 with similar hydrophobic tails (such as benzyl or phenylselanylalkyl),27,28,34,36 in which the selenium atom is usually 68 located at the end of hydrophobic tail, far from the hydrophilic headgroup. Conventionally, when the selenium atom 70 is separated from the benzene ring by more than two CH2 unites, the compounds exhibit irreversible oxidation from 37 72 selenide to selenone. The question then arises as to whether this applies to similar surfactants with the seleni74 um atom far from the benzene ring, or for those without a benzene ring. 50
Redox and CO2 are a particularly intriguing pair because of their promising applications in physiological environ40 ments. Redox processes are constantly occurring in all organisms (formation and destruction of inflamed cells).31-34 Besides, for a chemical redox process, the solvent and 76 42 On the other hand, CO2 gas, as a product of metabolism, by-product of redox reaction are inevitably produced and possesses excellent biocompatibility and is environmental78 accumulated in the system, which may lead to a dramatic 35 44 ly benign. In this context, our team recently pioneered a change in the viscosity of WLMs. In the previously report27,28 CO2 and redox dual responsive WLM,27 which was formed 80 ed redox-responsive WLMs, the reductant and oxidant 46 from a selenium-containing surfactant (sodium benzylsewere generally applied at 1 equivalent or more. Thus, a lanylundecyl sulfate, BSeAS) and a commercial diamine 82 further question that arises is whether redox-responsive in a molar 48 (N,N,N’,N’-tetramethyl-1,2-ethanediamine) ratio of 2:1. When the system was saturated with CO2, visACS Paragon Plus Environment 38
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behavior can be realized using substoichiometric amounts of reductant or oxidant.
Herein, we report a novel selenium-containing surfactant, sodium dodecylselanylpropyl sulfate (SDSePS, Scheme 1), in which the redox-sensitive group is close to the hydro88 philic headgroup. SDSePS can reversibly respond to the addition of N2H4H2O and H2O2, switching between sele90 nide and selenoxide. Based on SDSePS and commercial benzyl tertiary amine (BTA, Scheme 1), novel redox and 92 CO2 dual responsive WLMs have been fabricated. 86
(3) Sodium dodecylselanylpropyl sulfate (SDSePS) Under N2 atmosphere, a solution of NaBH4 (5.67 g) in deionized water was added to a solution of DC12Se (15.00 g) o 134 in THF at 0 C. The mixture was stirred for 20 min at room temperature, and then a solution of SBrPS (16.01 g) in de136 ionized water was added. The reaction was allowed to proceed at room temperature for ca. 24 h. the solvent was then 138 removed under reduced pressure, and the crude product was purified by column chromatography, and recrystalliza140 tion. SDSePS (18.60 g, yield 75.80%) was obtained as a white solid. 1 142 H NMR (400 MHz, CD3OD, Figure 1A), /ppm: 0.90-0.94 (t, J=8.00 Hz, 3H), 1.31-1.43 (m, 18H), 1.65-1.69 (m, 2H), 144 1.98-2.05 (m, 2H), 2.58-2.62 (t, J=8.00 Hz, 2H), 2.64-2.68 (t, J=8.00 Hz, 2H), 4.08-4.11 (t, J=6.00 Hz, 2H). 77Se NMR 146 (600 MHz, D2O, Figure S1), δ/ppm: 154.81. ESI HRMS (Figure 1B): Calcd: 387.2 (M + H+). Found: m/z = 387.2. 132
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Scheme 1 Redox-induced transition of sodium dodecylselanylpropyl sulfate (SDSePS) and CO2/N2-induced transition of benzyl tertiary amine (BTA).
EXPERIMENTAL SECTION 98
Materials
Selenium powder, sodium borohydride, bromododecane, 3bromopropanol, chlorosulfuric acid, hydrazine hydrate, 30 wt% H2O2, and all organic solvents used in this study were 102 analytical-grade products from Shanghai Chemical Reagent Co., Ltd. Water was triply distilled by a quartz water puri104 fication system. 100
Synthesis of selenium-containing anionic surfactant The anionic surfactant, sodium dodecylselanylpropyl sulfate (SDSePS), was prepared by a simple three-step pro108 cess. 106
(1) 1,2-didodecyldiselane (DC12Se)
Characterization 1
H NMR spectra were recorded at room temperature on a Bruker Avance 400 spectrometer at 400 MHz. Chemical shifts are expressed in ppm downfield from TMS as inter77 152 nal standard. Se NMR spectra were recorded at room temperature on an Agilent DD2 600 spectrometer at 600 154 MHz. ESI-MS spectra were obtained with the Bruker Daltonics Data Analysis 3.2 system. 150
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Sample preparation
A concentrated stock solution was prepared by dissolving SDSePS (150 mmol), benzyl tertiary amine (BTA, 100 mmol) into distilled water (1 L), followed by magnetic 160 agitation for several minutes (referred to as “SDSePSBTA”). Low concentration of samples were obtained by 162 diluting this stock solution with distilled water. All of the samples were prepared at the same fixed molar ratio of 164 SDSePS/BTA, 3:2, unless stated otherwise. The concentrations of the mixed solution refer to that of SDSePS. 158
CO2 was bubbled into an SDSePS-BTA mixed solution at ambient temperature at a fixed flow rate of 0.1 L/min at a 168 pressure of 0.1 MPa, leading to a viscoelastic fluid (referred to as “SDSePS-BTA-CO2”). Thereafter, the sample 170 was kept in a sealed vessel to avoid contacting with air. To remove CO2, N2 was bubbled into the SDSePS-BTA-CO2 172 solution at 50 °C at the same flow rate as CO2 until equilibrium was reached, resulting in a water-like solution (re174 ferred to as “SDSePS-BTA”). A small amount of H2O2 (0.4 equiv. with respect to SDSePS, ca. 0.1 wt% of the total 176 mass) was added to SDSePS-BTA-CO2 at room temperature, resulting in a water-like fluid (referred to as 178 “SDSePS-BTA-CO2-Ox”) in less than 10 min. The reverse process could be achieved after 16 h by adding the requisite 180 amount of N2H4⋅H2O (1.0 equiv. with respect to H2O2, ca. 0.22 wt% of the total mass) at 35 °C. All of the samples 182 obtained were kept at 25 °C for about 24 h prior to the measurements. 166
DC12Se was synthesized according to the previously reported method.27,28 Briefly, aqueous solution of sodium 112 borohydride was slowly added into Se power suspension at 0 oC. After stirring for 20min, one additional equiv of Se 114 powder was added and was continuously reacted for 15 min at room temperature and then heated to 70 oC for an116 other 20 min. After cooling, a solution of bromododecane in THF was added, and the mixture was stirred for ca. 18 h o 118 at 50 C. The crude product was extracted with dichloromethane, and the solid was recrystallized from ethyl ace120 tate. DC12Se (40.82 g, yield 71.60%) was obtained as yellow needle-like crystals. 122 (2) Sodium 3-bromopropyl sulfate (SBrPS) 110
chlorosulfuric acid (19.5 g) was added under mechanical stirring into a solution of 3-bromopropanol (20.12g) in dichloromethane, and the temperature was maintained at 0 o 126 C. The resulting mixture was aged for 30min and then added to saturated Na2CO3 solution. The solvents were 128 removed under reduced pressure, and the product was recrystallized from ethanol. SBrPS (24.04 g, yield 71.25%) 130 was obtained as white solid. 124
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Rheology
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Rheological measurements were carried out on a Physica MCR 301 (Anton Paar, Austria) rotational rheometer
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Langmuir
equipped with a concentric cylinder geometry CC17 device (ISO3219). Samples were equilibrated at 25 °C for no less than 20 min prior to the experiments. Dynamic frequency 190 spectra were performed in the linear viscoelastic regime, previously determined from dynamic stress sweep meas192 urements. All of the experiments were carried out in stress-controlled mode, and CANNON standard oil was 194 used to calibrate the instrument before the measurements. The temperature was set at 25 ± 0.01 °C with a Peltier tem196 perature control device, and a solvent trap was used to minimize solvent evaporation during the measurements. 188
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Cryo-TEM observation
Cryo-TEM observation of the solutions was performed in a controlled environment vitrification system. The chamber temperature was 25−28 °C, and the relative humidity was 202 kept close to saturation to prevent evaporation during the preparation. Aliquots (5 µL) of solutions pre-heated at 25 204 °C were placed on a carbon-coated holey film supported by a copper grid, and gently blotted with a piece of filter paper 206 to obtain a thin liquid film (20−400 nm) on the grid. The grid was rapidly quenched with liquid ethane at −180 °C 208 and then transferred into liquid nitrogen (−196 °C) for storage. The vitrified specimen stored in liquid nitrogen was 210 then transferred to a JEM1400 cryo-microscope using a Gatan CP3 cryo-holder, controlled by its workstation. The 212 acceleration voltage was 120 kV, and the working temperature was kept below −170 °C. The images were recorded 214 digitally with a charge-coupled device camera (Gatan 832) under low-dose conditions with an under-focus of approx216 imately 3 µm. 200
Dynamic light scattering (DLS)
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DLS measurements were performed on a ALV/DLS/SLS5022F (HOSIC LIMITED, Germany) with a back220 scattering angle of 90° and an He–Ne laser (λ= 633 nm). Samples were filtered through a 0.2-µm filter of mixed 222 cellulose acetate to remove any interfering dust particles. To obtain the apparent hydrodynamic radius (Rh,app), intenautocorrelation functions were analyzed using 224 sity CONTIN. 218
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Surface tension measurement
The surface tension (γ) of the systems was determined on a drop volume tensionmeter at 25 ± 0.1oC. The outer radius of the glass capillary was 0.58 mm. In the procedure for γ 230 measurements, a sufficient aging time is necessary for the pendant drop surface to reach an equilibrium state. Finally, 232 the drop volume was corrected by the Harkins–Brown method. 228
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RESULTS AND DISCUSSION 236
Redox-responsive behavior of SDSePS
Changes in molecular structure generally govern variations in microscopic ordered structures, and thereby affect the macroscopic properties. Therefore, the redox-responsive 240 behavior of SDSePS was first analysed at the molecular level by means of spectroscopic techniques. 238
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Figure 1 Evidences at the molecular level for the redox response of SDSePS. (A) 1H NMR spectra using D2O as solvent, and (B) ESI-MS spectra.
When SDSePS was oxidized with equimolar H2O2, the H NMR signals of four groups of protons (R-CH2-Se-, Se-CH2-, -Se-CH2-CH2- and R-CH2-CH2-Se-) adjacent to 250 the selenium atom showed obvious downfield shifts (Figure 1A), indicating that the environment about these pro38-40 252 tons was switched from apolar to polar. In particular, the peaks assigned to R-CH2-Se- and -Se-CH2- were split 254 into two symmetrical doublets, respectively, indicative of an asymmetrically substituted selenium atom after oxida31,41 256 tion, i.e., selenoxide. The introduction of one oxygen atom on the selenium atom prevents free rotation of the C258 Se-C bond, resulting in geminal couplings of R-CH2-Seand -Se-CH2-. Meanwhile, the formation of selenoxide 260 provides a more polar environment for the neighboring protons, accounting for the downfield shifts in the signals 262 of R-CH2-Se-, -Se-CH2-, -Se-CH2-CH2- and R-CH2-CH2Se-. These changes in the chemical shifts were fully re264 versed after adding equimolar N2H4H2O, implying a reversible redox process. Moreover, 77Se NMR analysis 266 (Figure S1) revealed that the peak assigned to the selenium atom underwent a considerable shift from 154.80 ppm to 268 857.37 ppm, further confirming the formation of selenoxide after oxidation with H2O2.28,31 Additionally, ESI-MS 270 data (Figure 1B) showed that the addition of H2O2 led to an increase in the molecular ion peak from 387.2 (MH+) to + 272 403.2 (MH ), whereas the addition of N2H4H2O led to a 248
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reversion from 403.2 (MH+) to 387.2 (MH+), indicating 274 that only one oxygen atom became attached to the selenium atom after oxidation which could be removed by reduction. 276 All of the above results suggest that the addition of H2O2 caused selenide to be oxidized into selenoxide, not sele278 none, and that the selenoxide could be reduced by adding N2H4H2O, as previously observed for compounds with a 280 selenium atom adjacent to a benzene ring. Therefore, SDSePS can reversibly respond to the addition of 282 N2H4H2O and H2O2. Preparation of WLMs Based on SDSePS and BTA As is well known, anionic surfactants alone are generally difficult to self-assemble into viscoelastic WLMs. Similar286 ly, one-component SDSePS solution (120 mM, far above the cmc) proved to be of low viscosity at room temperature 288 (Figure S2). When SDSePS was mixed with BTA in a molar ratio of 3:2, the dispersion (SDSePS-BTA) still showed 290 high flowability (Figure S2), behaving a typical Newtonian fluid. The zero-shear viscosity (η0) obtained by extrapolat292 ing the viscosity curve along the Newtonian plateau to a zero-shear rate was as small as 1.2 mPas (Figure 2), just 294 above that of pure water. Moreover, over the whole concentration range, SDSePS-BTA consistently showed New296 tonian fluid behaviors with low viscosity. This suggests that the dispersions may be dominated by small spherical 298 micelles, or short, non-entangled rodlike micelles. 284
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Figure 2 Zero-shear viscosity (η0) plotted as a function of SDSePS-BTA at room temperature.
However, 120 mM SDSePS-BTA immediately converted into a transparent viscoelastic ‘‘gel’’ (SDSePS-BTA304 CO2) when it was saturated with CO2. Therefore, steady and oscillatory experiments were carried out to further sub306 stantiate these visual observations. Plotting the η0 of SDSePS-BTA-CO2 as a function of surfactant concentra308 tion (C), two clear break points were seen in the η0-C curve: 18.6 mM and 75 mM (Figure 2). The first critical point is * 42 310 generally defined as the overlapping concentration (C ). * At C
