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CO2/pH-Controllable Viscoelastic Nanostructured Fluid Based on Stearic Acid Soap and Bola-type Quaternary Ammonium Salt Yongmin Zhang, Weiwei Kong, Pengyun An, Shuai He, and Xuefeng Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b04459 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on February 29, 2016
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CO2/pH-Controllable Viscoelastic Nanostructured Fluid Based on Stearic Acid Soap and Bola-type Quaternary Ammonium Salt Yongmin Zhang,*† Weiwei Kong,† Pengyun An,† Shuai He‡ and Xuefeng Liu† †
School of Chemical & Materials Engineering, Key Laboratory of Food Colloids and Biotechnology Ministry of Education, Jiangnan University, Wuxi 214122, P. R. China. ‡ 6 College of Chemistry and Environmental Protection Engineering, Southwest University for Nationalities, Chengdu 610041, P.R. China 4
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ABSTRACT: Fatty acid soaps such as sodium stearate (NaOSA) represent a class of cheap, environmentally friendly surfactants; however, their poor solubility seriously challenges their application in various fields. Herein, we describe a CO2/pH-controllable viscoelastic nanostructured fluid, which was developed by simple mixing of the commodity soap NaOSA with a bola-type quaternary ammonium salt (Bola2be) in a 2:1 molar ratio without the need for complex organic synthesis. The introduction of Bola2be increased NaOSA solubility and promoted micelle growth by forming a non-covalent pseudo-Gemini structure, 2NaOSA-Bola2be. Long aggregates are formed with increases in concentration and these become entangled into a three-dimensional network at 10 times that of the critical micelle concentration (0.057 mM), showing strong thickening ability. Micellar branching occurs above 22.38 mM, as deduced by rheology and verified by cryo-transmission electron microscopy. The worm-based fluid formed from the non-covalent pseudo-Gemini surfactant is highly thermo-sensitive, and features a higher flow activation energy of 399.76 kJ⋅mol–1 compared with common worm systems. Because of the pH-sensitivity of NaOSA, the viscoelastic fluid can respond to common pH stimuli or green CO2 gas, and shows a transition between a gel-like wormlike micellar network and a water-like dispersion with precipitate. However, the CO2-responsive behavior is irreversible.
INTRODUCTION
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Over the past decades, smart nanostructured fluids (SNFs) developed from low-weight molecules, have drawn significant attention in the field of soft matter, from both theoreticians and experimentalists, for fundamental studies and industrial applications.1-6 Among the various SNFs studied, stimuliresponsive viscoelastic wormlike micelles (“worms”) are a highly promising area of recent development because of their unique and tunable rheological properties that result from particular self-assembled structures. In general, these worms have a cross-sectional diameter of only several nanometers and a one-dimensional length of hundreds or thousands of nanometers. Above a threshold concentration, the flexible worms start to become entangled and form dynamic three-dimensional network aggregates that display remarkable viscoelasticity in a manner reminiscent of polymer solutions. There is, however, a significant difference between worms and polymer solutions. Worm-based NFs are dynamic equilibrium systems consisting of a forward (break) reaction and a reverse (re-formation) reaction, and thus they have also been called “living” or “equilibrium” polymers.7,8 More importantly, SNFs based on worms can adapt their properties, function, or appearance dynamically in response to external stimuli, including pH,9-13 temperature,11,14-16 CO2 gas,17-22 ultraviolet/visible 23-25 26 irradiation, magnetic fields, and redox reactions.27,28 It is their interesting and controllable rheological properties that make worm-based SNFs potential candidates for versatile applications in cleaning processes,15,29 drug-reducing fluids,30 templating of nanomaterials,31 and oil recovery.15,32,33
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Gemini surfactants comprising two hydrophilic headgroups and two hydrophobic tails that are chemically linked by a
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spacer group at the level of headgroups, or very close to the headgroups, have demonstrated a better thickening ability than the corresponding traditional single-chain surfactants.34-36 However, tedious organic synthesis and complicated purification procedures impede their development and application. Thus, a convenient method to produce Gemini surfactants without the limitation of complex synthesis is highly desirable, especially for the construction of viscoelastic worm-based SNFs. Recently, Feng’s team pioneered a “pseudo-Gemini” concept, and fabricated pH- and CO2-responsive worm-based NFs based on two pseudo-Gemini surfactants,13,18 respectively. This opens up a new, facile approach to produce viscoelastic worm-based SNFs using non-covalent interactions and appropriate building blocks. Inspired by such a pseudo-Gemini concept, we have previously developed a pH-responsive viscoelastic NF based on a pseudo-oligomeric surfactant, which was formed in situ through a neutralization reaction between N-(3(dimethylamino) propyl) palmitamide and citric acid.12 With sequential pH variation, such an NF exhibited bell-shaped sol– gel–sol transitions, and demonstrated continuous morphological transitions, from sphere to worm to lack of aggregates. To the best of our knowledge, however, the most popular worm-based SNFs reported so far are formed by cationic surfactants. Whereas anionic worms are less documented, and stimuli-responsive anionic ones even less so, they are generally believed to be less irritating and lower toxicity than cationic surfactants.37-39 As the most ancient detergents, fatty acid soaps are derived from natural renewable resources, and have been used widely in the industry and in our daily life for thousands of years because of their availability in large amounts in nature and their biocompatibility.40 Nevertheless, viscoelastic SNFs formed from fatty acid soaps have so far only utilized
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unsaturated fatty acid salts,8,10,19 due to good water solubility compared with their saturated counterparts. No efforts to date have been devoted to the development of worm-based SNFs based on saturated fatty acid soaps. Generally, the solubility of a fatty acid soap surfactant in water can be improved by incorporating alkalis or organic counterions, as verified in our previous study.19 Therefore, on the basis of pseudo-Gemini strategy, a bola-type quaternary ammonium salt, ethylidene-α,ωbis(benzyldimethylammonium bromide) (Bola2be, Scheme 1) was chosen to increase the water-solubility of the saturated fatty acid salt, sodium stearate (NaOSA, Scheme 1), and bridge the NaOSA molecules while preparing the pseudoGemini surfactant, 2NaOSA-Bola2be, which was then applied as a viscosity modifier. The formation of an NF of 2NaOSABola2be, and its pH- and CO2-responsive behavior, were characterized by rheological characterization, cryo-transmission electron microscopy (TEM), dynamic light scattering (DLS) and small angle X-ray scattering (SAXS), and such a viscoelastic fluid shows a response to pH and CO2 gas.
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EXPERIMENTAL 108
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Materials Stearic acid (≥ 99.0%), N,N,N’,N-tetramethyl-1,2ethanediamine (≥ 99.0%), and benzyl bromide (≥ 99.0%) were purchased from Admas; Other chemicals with analyticalgrade were used as received without further purification. Deionized water (18.2MΩ⋅cm) was used for all aqueous solutions.
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NaOSA was obtained following a previously-reported procedure20 by saponification of natural stearic acid. The bolatype quaternary ammonium, Bola2be, was synthesized through the reaction of N,N,N’,N-tetramethylethane-1,2diamine with benzyl bromide using ethanol as solvent. And then the product was purified by recrystallization from the mixed solvent of ethyl acetate and ethanol. 1H NMR (400 MHz, D2O, Figure S1 in the Supporting Information), δ/ppm: 3.120 (s, 12H), 3.96 (s, 4H), 4.60 (s, 4H), 7.50-7.58 (m, 10H).
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Sample preparation 182 124
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Concentrated stock solutions were prepared by dissolving 150 mmol of NaOSA and 75 mmol Bola2be into 1 L distilled water, followed by magnetic agitation for several minutes (referred to as “2NaOSA-Bola2be”). Different concentration samples were then obtained by diluting the stock solutions with distilled water. The molar ratio of NaOSA/Bola2be in all the samples solutions were fixed at 2:1, unless otherwise stated. The concentration of the solution is expressed as the concentration of Bola2be. For example, the mixture of 100 mmol
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H NMR spectra were recorded on a Bruker Avance 400 spectrometer at 400 MHz in D2O at room temperature. Chemical shifts are expressed in ppm downfield from TMS as internal standard. Fourier transform infrared (FT-IR) spectrums were obtained from FT-IR spectrophotometer (FTLA2000-104, ABB Inc., Canada) using a KBr disk containing 1% finely ground sample.
Determination of Krafft temperature (TK)
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Scheme 1 Molecular structures of sodium stearate (NaOSA) and ethylidene-α,ω-bis(benzyldimethylammonium bromide) (Bola2be).
NaOSA and 50 mmol Bola2be can be marked as 50 mM 2NaOSA-Bola2be, and so forth. All the samples obtained were kept at experimental temperature for about 24 h prior to the measurements, unless otherwise stated.
Characterization
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The variation of TK of NaOSA with the addition of Bola2be was determined by visual observation following the previously-reported procedures.19 The phase boundary as a function of temperature was determined by noting a transparent sample upon heating. The transition temperatures (clearing points) reported here were reproducible within ± 0.1 °C.
Rheology Rheological properties of mixture solutions were performed on a Physica MCR 301 (Anton Paar, Austria) rotational rheometer equipped with concentric cylinder geometry CC27 (ISO3219), with a measuring bob radius of 13.33 mm and a measuring cup radius of 14.46 mm. Dynamic frequency spectra were conducted in the linear viscoelastic region, as determined from prior dynamic stress sweep measurements. The effects of gas on rheological properties were carried out online. The sketch of the experimental set-up was shown in Figure S2 in the Supporting Information. CO2/N2 was continuously introduced into the measuring cup with a very small flow rate of 0.02 L⋅min-1 under 0.1 MPa. All the measurements were carried out in the stress-controlled mode, and CANNON standard oil was used to calibrate the instrument before the measurements. The temperature was controlled by a Peltier device and a solvent trap was used to minimize water evaporation during the measurements.
Dynamic light scattering (DLS) DLS measurements were performed on a ALV/DLS/SLS5022F (HOSIC LIMITED, Germany) with a 90° back scattering angle and He–Ne laser (λ= 633 nm). Samples were filtered with a 0.2-µm filter of mixed cellulose acetate to remove any interfering dust particles.
Cryo-TEM observation Samples for cryogenic TEM were prepared using a vitrification robot system. A drop of the solution was put on a holey carbon-coated copper grid; the excess of solution was spread to create a thin liquid film over the grid and was then immediately plunged into liquid ethane at its freezing point. Following the vitrification step, samples were transferred to a liquid nitrogen environment by the use of a cold stage unit (Gatan model 626) in the electron microscope, JEM2010, operating at 120 kV. The working temperature was kept below −170°C, and the images were recorded using a Gatan 832 CCD camera.
pH and pKaH measurement 188
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The pH of aqueous solution was monitored by a Sartorius basic pH-meter PB-10 (±0.01) at 30°C. NaOSA dispersion was titrated with 10 mM hydrochloric acid and the pH contin-
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uously monitored with a Sartorius basic pH-meter PB-10 (± 0.01). The pKaH values were obtained by taking the pH values at the mid-point between two pH jumps.
RESULTS AND DISCUSSION
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Phase behavior 196
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In practical use, surfactants generally require good water solubility at room temperature. Because of its saturated C18 tail, NaOSA is poorly soluble in aqueous solution, and has a high Krafft temperature (TK, ~75 oC).41,42 Therefore, the dependence of the TK of NaOSA on the presence of Bola2be was investigated firstly to satisfy the demands of further research and potential applications. As exhibited in Figure 1, when adding Bola2be to a solution of NaOSA, the TK dropped steeply and then to a smaller extent with increasing Bola2be concentrations, which displays a typical “salting-in” character. Combining these TK values yielded a clear temperature line, which represents the solubility boundary between the clear and opaque regions. For example, above the TK, a homogeneous and transparent micellar solution was observed (Figure 1, inset A) and no turbidity occurred; whereas below the TK, a white turbid phase gradually appeared (Figure 1, inset B). A similar “salting-in” effect was also founded in other alkyl carboxylate surfactant systems, such as tetramethyl ammonium bromide sodium erucate43 and triethylamine bicarbonate.20
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Fig. 1 Krafft point plotted as a function of molar ratio of Bola2be to NaOSA, where CNaOSA is fixed at 1 wt%. Photographs A and B, corresponding to the same sample at CBola2be:CNaOSA =1:2, represent the typical clear micellar solution above TK and opaque dispersion below TK, respectively. To gain deeper insight into the salting-in effect of Bola2be, H NMR analysis was used at different molar ratios of Bola2be:NaOSA to study molecular interactions between Bola2be and NaOSA. As shown in Figure 2, both the α-H and βH of NaOSA showed considerable upfield shifts with the addition of Bola2be into the surfactant solution. When the molar ratio increases to 1:2, the resonance peaks of β-H has blended in the bulk CH2 (except α-H and β-H) resonance peak attributable to the hydrophobic tail. Furthermore, the bulk CH2 resonance peaks exhibited an apparent split, in which parts of CH2 resonance peaks shift to a lower chemical shift. These variations indicate that the benzene rings attached at the qua-
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Fig. 2 Effect of Bola2be concentration on 1H-NMR spectra of NaOSA molecules in 40 mM NaOSA micellar systems.
It may be hypothesized that a pseudo-Gemini surfactant (defined as “2NaOSA-Bola2be”) will be formed when the molar ratio of Bola2be:NaOSA is fixed at 2:1. As is well known, Gemini surfactants possess a stronger water thickening ability compared with their single-tail counterparts,35,36 and in the TK measurement discussed above, an apparent viscosity enhancement was also observed. Thus, research into the rheological behaviors of the stearic acid soap NaOSA and the bolatype quaternary ammonium salt Bola2be at a molar ratio of 2:1 will be necessary and desirable. According to Figure 1, to ensure that the samples used in the studies are in a fully solubilized state, all the rheological measurements discussed here were performed at 30 oC unless otherwise stated.
Effects of concentration on the rheological behaviors 266
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ternary ammonium head had penetrated into the hydrocarbon chains. This is analogous to previous observations for the potassium phthalic acid-cetyltrimethylammonium bromide system.9 The insertion of Bolab2be’s benzene ring causes the regular packing of NaOSA tails in the crystalline state to be disrupted, and the headgroups to be separated from each other owing to steric hindrance. Consequently, it is preferable for NaOSA molecules in the presence of Bola2be to adopt a solubilized rather than a crystalline state, i.e., the salting-in effect. Owing to the combination of hydrophobic effects and electrostatic attraction, Na+ as the initial counterions of the NaOSA soap will be substituted progressively by Bola2be quaternary ammonium ions, and the benzene rings will penetrate into the palisade layer of micelles, which not only decreases the TK of NaOSA, but also favors micellar growth.
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The rheological behaviors of 2NaOSA-Bola2be aqueous solutions with different concentration were firstly systematically studied at 30 oC (Figure 3 and S3 in the Supporting Information). From the results shown in Figure 3, one can determine that the zero-shear viscosity (η0)-concentration (C) curve was divided into three regions by two break-points. At low concentrations (
