- Email: [email protected]
A fatty acid solvent of switchable miscibility
Accepted Manuscript A fatty acid solvent of switchable miscibility Qianqian Chen, Lei Wang, Gaihuan Ren, Qian Liu, Zhenghe Xu, Dejun Sun PII: DOI: Reference:
S0021-9797(17)30667-7 http://dx.doi.org/10.1016/j.jcis.2017.06.011 YJCIS 22438
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
25 March 2017 5 June 2017 5 June 2017
Please cite this article as: Q. Chen, L. Wang, G. Ren, Q. Liu, Z. Xu, D. Sun, A fatty acid solvent of switchable miscibility, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.06.011
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A fatty acid solvent of switchable miscibility *
Qianqian Chen a, Lei Wang a, Gaihuan Ren a, Qian Liu b, Zhenghe Xu c,d, Dejun Sun a, a
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan,
Shandong 250100, PR China b
School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing
100083, PR China c
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9,
Canada d
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 1000084, PR China
*Corresponding author. Tel. +86-531-88364749, Fax. +86-531-88364750 E-mail: [email protected].
Abstract Ion pair interactions were explored to design a fatty acid solvent of switchable miscibility with water. Fatty acids of medium length chains are immiscible with water but become miscible with water when ion pairs are formed with amines. The ion pairs become phase separated after bubbling CO2 into the solution due to the dissociation of the fatty acid-amine complexes. Ion pairs of caprylic acid (C8) and low toxic poly(oxypropylene) diamine (Jeffamine D-230) were characterized by FT-IR and 1H NMR. Log Kow values and surface activity were used to understand the switchable solvent mechanism in removing and recovering oily contaminants. More importantly, the ion pairs show a negligible adsorption on solid surfaces. Furthermore, both C8 and D-230 were recycled during the washing process. Thus the fatty acid as switchable solvent could be applied for oily contaminant removal from oily solid wastes. Keywords CO2 switchable, fatty acid-amine complex, ion pair interaction, oil contaminants removal, oily solid wastes.
Introduction In many chemical production processes (e.g., reactions, extractions, and/or separations), solvents with switchable miscibility and environmentally benign features are advantageous over traditional solvents [1–6]. For example, such solvents can be used to remove or recover oily contaminants from oily drill cuttings. Compared with the traditional treatment methods for oily drill cuttings, such as solidification and stabilization [7], microwave treatments [8], supercritical fluid extraction [9], photodegradation [10], and bioremediation [11], the use of switchable solvents could significantly reduce power consumption (e.g., distillation) without strict requirement of high pressure equipment, thereby reducing both capital and operating costs. Furthermore, the switchable nature of the solvents opens the doors for the recycling of solvent, which further reduces the net cost of treatment. Solvents of switchable solubility [12–15] are miscible with oily compound under air but not with that under CO2. Such switchable solvents were first reported by Jessop et al. [15]. Further studies on solvents of switchable polarity [14] or switchable miscibility [16–20] have been reported in recent years. Most of these solvents are based on organic molecules of amidine or amine groups. The amine as switchable solvents are usually toxic [21] and require complex synthesis procedures [16]. One problem encountered when using the amidine solvent is its loss due to the ease of its hydrolysis [22]. When amine solvent [23] is used to extract oil from solid surfaces, amine is adsorbed on solids to generate secondary wastes. To bring the residual amine content to a desired level, carbonated water is often used to wash the treated solids. There is a strong interest in designing a green solvent [24–26] with low toxicity and negligible adsorption on solid surfaces. Fatty acids with good biocompatibility and abundance in nature are ideal candidates for designing such switchable solvents [27,28]. Many complexes formed by fatty acids and amines have shown CO2-switchable behaviors [29–31].
Studies on CO2-responsive micelle morphology [32–36] have been reported. In a recent study, we demonstrated the preparation of a CO2-responsive emulsion using a highly interfacial active long-chain fatty acid-based emulsifier [37]. Water immiscible fatty acids have been shown to become water soluble [38, 39] when their carboxylate groups become complexed with amine [17,40–42]. It is hypothesized that a fatty acid as switchable solvent can be obtained if the complexation between the fatty acid and amine can be made reversible by adding and removing CO2． In this study, we describe a successful search for a novel fatty acid solvent of switchable miscibility. Hydrophobic fatty acid can be transformed to be water soluble after reacting with amine to form fatty acid-amine complexes, and it becomes water immiscible (phase separated) after bubbling with CO2 (Fig. 1). With this unique property, such fatty acid solvent was used to remove oil from oily drill cuttings as an example of its potential environmental application.
Fig. 1. Switchable phase behaviors of fatty acid.
2. Materials and Experimental Methods 2.1 Materials and instruments A primary diamine with hydrophobic poly(oxypropylene) (n ≈ 2-3) as its backbone, poly(oxypropylene) diamine (Jeffamine D-230), of molecular weight ≈ 230 was purchased from Aldrich. Caprylic acid (C8, AR) was obtained from Sinopharm Chemical Reagent Co. Ltd., China. Paraffin oil (Trade Name: Marcol 52) was purchased from Exxon Mobil. FT-IR
(NEXUS 670, Thermo Nicolet) and 1H NMR (Bruker Avance 400 spectrometer, 400 Hz, CDCl3) were used to determine the formation of C8-D-230 (C8-D). 2.2 Measuring the switchable miscibility of fatty acids Prediction of log Kow values The octanol/water partition coefficient (log Kow) provides a quantitative measure of hydrophobicity. The log Kow values of all compounds were predicted using the ALOGPS 2.1 software [43, 44]. Measuring the switchable behaviors of different fatty acids To confirm the switchable miscibility, a series of fatty acids and amines were used to prepare their complexes. Methylamine (pKa = 10.6) [45], ethylamine (pKa = 11.0) [46], DETA (pKa1 = 10.1, pKa2 = 9.1, pKa3 = 4.4) [34], or D-230 (pKa1 = 10.9, pKa2 = 9.8) [47] aqueous solutions (5 wt%) were mixed with originally water immiscible fatty acids (chain lengths from C6 to C9) at a molar ratio of n(-NH2)/n(-COOH) = 1:1. Solutions were mixed vigorously with a magnetic stirrer for 10 min. The solutions were found to be transparent, suggesting the complete miscibility of the fatty acids with amine aqueous solutions. When CO2 was bubbled into the solution at 400 mL/min for 30 min, the fatty acid-amine complex was classified as CO2 switchable if the solution became biphasic (i.e., phase separated). 2.3 Surface activity measurement The surface tension of a fatty acid-amine complex (C8-D) aqueous solution was measured using an automated surface tension meter (JYW-200, China) at 25°C. Prior to each measurement, the glass cell was cleaned with ethanol and washed repeatedly with water, and the platinum ring was flamed to burn off any organic contaminants. The measurement was repeated three times for each solution. The dynamic interfacial tension (IFT) of the C8-D aqueous solutions was measured using
the pendant drop method on a commercial drop shape analyzer (Tracker, France Teclis) at 25°C. A drop of aqueous solution was created at the end of a syringe needle in a cuvette filled with paraffin oil. Images of the drop were captured and the Laplace equation was used to obtain the interfacial tension. 2.4 Oil removal by C8 Measuring the efficiency of C8 in removing oil from oily drill cuttings The model oily drill cuttings used in this study were prepared by immersing dry drill cuttings to paraffin oil. The oil content of original oily drill cuttings was tested with an Infrared Oil Content Analyzer (OIL460, Huaxia, China). First, a standard curve was obtained by using a series of standard solutions with variable paraffin oil/CHCl3 concentrations ranging from 5 to 25 wt%. Then, the model oily drill cuttings (2 g) were treated with 15 mL of CHCl3 and the supernatant (oil+CHCl3) was decanted into a 25-mL volumetric flask for analysis. The oil content of the model oily drill cuttings was 15 wt%, which is within the reported range [48]. The oil content was calculated using the formula below:
To investigate the efficiency of C8 in removing oil from drill cuttings, four samples were prepared by mixing C8 (from 0.5 to 3 g) with oily drill cuttings (2 g). Samples were vigorously shaken with a mechanical shaker (BF-1F, Jintan, China) at 150 r/min and 25°C for 30 min. Then, 5 wt% (Fig. S1) D-230 solutions were added to the four mixtures and shaken for 20 min. After filtering, the solid residuals were completely washed with deionized water before drying at room temperature. Next, the oil content of the residual solids was measured using the Infrared Oil Content Analyzer. The efficiency of normal water in washing the oily drill cuttings was also tested for comparison. Measuring the adsorption of C8-D on the drill cuttings
To determine the adsorption of C8-D on the drill cuttings after washing, oily drill cuttings (20 g) were washed with C8 (10 g) as described above. After adding 5 wt% D-230 solution to the mixture and shaking for 20 min, the drill cuttings were filtered and dried under natural conditions. Next, D2O was used to dissolve C8-D on the drill cuttings and the content of C8-D was analyzed by 1H NMR spectra using TEAB as the internal standard substance.
3 Results and discussion 3.1 Selection of fatty acid solvent To remove oily contaminants, fatty acid as a washing solvent should be a liquid with good oil miscibility. The fatty acid-amine complexes prepared from fatty acids with different chain lengths and various amines were tested for the desired phase behaviors (Fig. 1). The complex was easily obtained by neutralizing the fatty acid with the amine (Eq. (1)). Hydrophobic fatty acid can be transformed into a water soluble complex through the formation of an ion pair with an amine, whereas the bubbling of CO2 promotes the dissociation of the fatty acid-amine complex (Eq. (2)). After the removal of CO2, the water soluble complex reforms as a result of the increased pH (Eq. (2)).
Table 1 shows the switchable behaviors of a series of water soluble complexes prepared by mixing fatty acids with amine aqueous solutions. When CO2 was bubbled into the fatty acid-amine solution, H+ and HCO3- slowly formed, causing the protonation of ionized fatty acids by H+. The water immiscible un-ionized fatty acid then became separated from the aqueous solution. If an amine shows a strong basicity in solution, such as for methylamine and ethylamine, the pH of the solution could remain high after bubbling CO2 (pHmethylamine-fatty
acid
and pHethylamine-fatty acid were at 6.9 and 7.0, respectively). Fatty acids with pKa values of
4.86 (C8), 4.80 (C7), 4.88 (C6), and 4.90 (C9) remain dissociated under this condition [49]; therefore, only D-230 [47] and DETA [34] with weak basicity are suitable for preparing the desired complex. Complexes of amines with fatty acids of longer alkane chain length usually show a higher interfacial activity. In this case, the oil is easily emulsified [37], making the subsequent oil separation process very difficult. Fig. 2a shows photos of the paraffin oil mixture with 5 wt% C8-D (caprylic acid-D-230) and C9-D (nonanoic acid-D-230) aqueous solutions. The samples prepared with C9-D become emulsified, whereas the samples containing C8-D display a clear boundary between the two phases. Such differences can be ascribed to the different interfacial activities of the two fatty acid-amine complexes. The surface tension (γ) of the C8-D aqueous solution is 62 mN/m, which is slightly lower than that of pure water (72 mN/m) (Fig. 2b). The dynamic interfacial tension (IFT) of paraffin oil and 0.01 mol/L C8-D aqueous solution is 18 mN/m (Fig. S2). Under this condition, the paraffin oil/C8-D aqueous solution emulsions are unstable. This makes C8 a suitable switchable solvent for effective oil-water separation process. Table 1 Different chain length fatty acids tested for switchable behaviors log Kow
Fatty acids
1.9 Hexanoic acid (C6) 2.4 Ocnanthic acid (C7) 3.25 Caprylic acid (C8) 3.5 Nonanoic acid (C9) “Y”: switchable, “N”: not switchable
methylamine N N N N
ethylamine N N N N
DETA
D-230
Y Y Y Y
Y Y Y Y
To evaluate the safety and environmental effects of the fatty acids and amines, LD50 (oral, rat) data are compared in Table 2. Compared to DETA, D-230 is superior as the incorporated poly(oxypropylene) in its backbone decreases its eco-toxicity [16]. In consideration of the toxicity of the fatty acids, C8 is more appropriate when compared with switchable solvents
prepared using N,N-dimethylcyclohexanamine (DMCHA) [21] and other fatty acids. This evidence suggests that the C8 solvent can be used as a green switchable solvent. In the following sections, the switching characteristics and oil removal capability of the C8 solvent will be discussed.
Fig. 2. (a) Paraffin oil/water emulsions (1:1 w/w) containing 5 wt% C8-D (upper) and 5 wt% C9-D (lower) taken at 5 min, 30 min and 24 h after preparation. (b) Surface tension of C8-D (black) and C9-D (red) aqueous solutions at 25°C as a function of concentration.
Table 2 Properties of chemicals. LD50 (oral, rat) (mg kg−1) a C8 240 10080 ＞100 C7 223 110 7000 C6 205 102 3000 DMCHA 118 7.22 348 D-230 2880 DETA 207 94 1080 a Data obtained from the MSDS of each component, as supplied by Sigma–Aldrich. Substance
Boiling point (°C)
Flash point (°C)
Melting point (°C) 17 −7.5 −3 −60 −39
3.2 Switchable behaviors of C8 solvent The switchable behaviors of the C8 solvent are demonstrated in Fig. 3. Mixtures of C8 (1 mL) and paraffin oil (4 mL) were colored with Oil Red O. The total volume of the oil phase was 5 mL (Fig. 3a). After adding D-230 aqueous solution (nearly 10 wt%), the oil phase was reduced to 4 mL (Fig. 3b), which suggests that C8 (1 mL) was mostly transferred to the aqueous phase. Although C8 is oil miscible, it has a finite solubility in water (0.068 g C8 can
dissolve in 100 g water at 20°C). As a result, a small amount of colored C8 dissolved in water produced a reddish color in the water phase. After bubbling CO2 at an atmospheric pressure for 30 min, the oil phase increased to 5 mL, indicating that C8 became water insoluble again and transferred back to the oil phase (Fig. 3c). Removing CO2 by bubbling N2 at 60°C reduced the oil phase back to 4 mL (Fig. 3d), clearly demonstrating the switchable miscibility of C8 in oil to C8-D in water.
Fig. 3. Switchable behaviors of C8: (I) C8 (1 mL) in paraffin oil (4 mL). After mixing with D-230 aqueous solution, the oil phase was reduced by 1 mL; (II) After bubbling CO2, the oil phase increased to 5 mL; (III) Upon removal of CO2, the oil phase was reduced to 4 mL again.
When C8 is mixed with D-230 aqueous solution, the two chemicals associate with each other by ion pair interaction to form a water soluble ion pair complex (C8-D). After bubbling CO2 into the aqueous solution, the increased H+ promote the dissociation of the ion pair complex, resulting in the formation of water immiscible C8 and water soluble protonated D-230 (D-2302+). As a result, the oil phase returned to the original volume (5 mL) (Fig. 3c). When CO2 was removed by N2 bubbling at 60°C [50], causing the dissociation of primary polyamine bicarbonate and formation of D-230 [37]. Then, the C8 and free D-230 molecules associated to reform the water soluble complexes, reducing the oil phase back to approximately 4 mL (Fig. 3d). The results clearly demonstrate that C8 can successfully switch between the oil miscible and water miscible forms through the reversible ion pair reaction.
The C8-D complexes were studied using FT-IR and 1H NMR spectroscopy. The FT-IR spectrum of C8-D is compared with the spectra of C8 and D-230 in Fig. 4. The peak at 1712 cm−1 corresponds to the carbonyl (-C=O-) peak of C8. After neutralization by D-230, this band shifts to 1538 cm−1, indicating the formation of carboxylate. For D-230, -NH2 shows its characteristic asymmetric and symmetric stretching vibrations at 3368 cm-1 and 3290 cm-1 and bending vibration at 1580 cm-1. However, the -NH2 stretching band in C8-D disappears due to the formation of -NH3+, which has a weak bending vibrational peak at 1685 cm−1. The results demonstrate the binding of C8 and D-230 to form C8-D.
Fig. 4. FT-IR spectra of C8, D-230 and C8-D.
Fig. 5 shows the 1H NMR spectra of C8, D-230 and C8-D in CDCl3. In the spectrum of C8-D, the peak at δ = 11.8 ppm for the -COOH group disappears, and the peak at δ = 8.2 ppm indicates the formation of -NH3+. The 1H chemical shifts of 1st and 2nd -CH2 groups near the C=O group in C8-D moved up-field due to the increased electron density. This indicates that proton transfers from -COOH to -NH2, which further confirms the formation of C8-D.
1
Fig 5. H NMR spectra of C8-D, C8 and D-230.
To investigate the switchable behaviors of binding in C8-D, CO2 and N2 were alternately bubbled into its aqueous solution and four cycles of this process were performed (Fig. 6). The pH of the C8-D aqueous solution is 7.2. At this pH, C8 is mainly present in the ionized form [49], while the dominant D-230 species is the divalent cation (D-2302+), with both primary amine groups protonated. When CO2 was bubbled, the ionized C8 was protonated by H+ to form un-ionized C8. The conductivity κ of the aqueous phase increased, accompanied by a decrease of pH. This transition is attributed to the presence of HCO3 - and H+ by the excess CO2 bubbling. Upon bubbling N2 at 60°C for 1 h to remove CO2, D-230 was reformed and reacted with C8 to form a water miscible ion pair complex. Thus the κ and pH returned to their original values.
Fig. 6. pH (blue squares) and conductivity κ (black stars) changes in 5 wt% C8-D aqueous solution in response to repeated cycles of bubbling or removing CO2.
The polarity switching of C8 solvent was also investigated with the variation in the wavelength of maximum absorbance (λmax) of Nile Red dye (Fig. S3). As a result of the low polarity of C8, the λmax of water-saturated C8 is at 580 nm. After interacting with D-230, C8 becomes water soluble in the form of a C8-D complex with the λmax–C8-D at 623 nm [22]. 3.3 Oil separation and solvent recycling A fatty acid solvent of switchable miscibility can be used to remove oil components from oil-contaminated soil or solids such as oily drill cuttings. The model oily drill cuttings used in this study were prepared by immersing dry drill cuttings in paraffin oil. The process of oil removal from the oily drill cuttings is shown in Fig. 7. First, water immiscible C8 was completely mixed with the oily drill cuttings to extract oil. Then, the low-density oil was separated from the solvent to the upper phase after the addition of D-230 aqueous solution (Fig. 7I). After filtering the solids, the C8 was separated from D-230 in the filtrate (C8-D aqueous solutions) by bubbling CO2 into the recovered aqueous phase (Fig. 7II). Protonated D-230 (D-2302+) becomes deprotonated by bubbling N2 to remove CO2 [37] (Fig. 7III). Both C8 and D-230 can then be recycled.
Fig. 7. Process of oil removal from oily drill cuttings using C8: (I) oil separation process; (II) C8 recycling process; and (III) D-230 recycling process (red dashed line represents the recycle process, “ ” represents the drill cuttings).
In the oil separation process (Fig. 7I), if a portion of C8 remains in the oil phase, it will cause the loss of solvent and secondary pollution of the recovery oil. 1H NMR spectroscopy was used to determine the separation of paraffin oil from C8. The C8 and paraffin oil were shaken together at a 1:1 (w/w) ratio at room temperature. 1H NMR spectroscopy shows the peaks of C8 and paraffin oil (Fig. 8b). The boundary between the two liquid phases starts to appear with the addition of D-230 solution. A comparison of spectra b and c in Fig. 8 shows a negligible amount of C8 in the separated oil phase, confirming that C8 can be effectively separated from the paraffin oil.
1
Fig. 8. H NMR spectra of (a) pure paraffin oil, (b) a mixture of C8 and paraffin oil , and (c) paraffin oil after the removal of C8 by D-230 solution.
To confirm the recycling of C8 (Fig. 7II), CO2 was bubbled into the C8-D solution, a phase
boundary was observed. FT-IR (Fig. S4) was used to characterize the oil phase, while the κ and pH of the water phase were measured after bubbling N2. The peak at 1712 cm−1 belongs to -COOH, which indicates the regeneration of C8. However, the peaks corresponding to the bending vibrational of carboxylate at 1461 and 1532 cm−1 remain (although they are much weaker) and the -OH out-of-plane vibration peak of C8 at 938 cm−1 disappears. This indicates that a small amount of C8 associates with protonated D-230 (D-2302+) to form complexes (C8-D) through hydrogen bonding [51–53]. Although the recycled C8 sample contained a small amount of C8-D complexes, it is not considered to be an issue as all the original materials (C8 and D-230) remain in the recycled system, as shown in Fig. 7. As shown in Fig. 9, the κ and pH of the D-2302+ aqueous solution phase were 2300 μs/cm and 6.0, respectively, after bubbling CO2 into the C8-D solution. Bubbling N2 at 60ºC to remove CO2 caused a concomitant decrease in κ and increase in pH, indicating the deprotonation of D-2302+ (Fig. 7III). Bubbling N2 cannot completely deprotonate the protonated D-230 (D-2302+) molecules [24]. The final pH of the D-230 solution was around 9.1, which was slightly lower than the initial value (pH 10.2).
2+
Fig. 9. Conductivity and pH of D-230 solution when bubbling N2.
Furthermore, in order to verify the generality of C8 in removing oily contaminants, trichloroethylene (Fig. S5) and toluene (Fig. S6) were also tested as model oils in place of
paraffin oil. Trichloroethylene (3 mL) was mixed with C8 (1 mL) and colored with Oil Red O. A single phase was observed, which suggests good miscibility of C8 with trichloroethylene. After D-230 solution was added, the volume of lower red oil phase decreased to 3 mL (Fig. S5), suggesting an excellent separation of C8 from trichloroethylene. After bubbling CO2, the oil phase returned to 4 mL, indicating that C8 becomes water immiscible, thus demonstrating the switchable miscibility of C8 with water. 3.4 Oil removal from oily drill cuttings We performed a preliminary evaluation on the C8 solvent of switchable miscibility for removing oil from contaminated drilling cuttings (Fig. S7). The original oil content of the drill cuttings was 15 wt%. The residual oil on the drill cuttings remained at around 10 wt% after washing with only water (Table 4). In contrast, the residual oil content on the drilling cuttings was reduced to 0.87 wt% after washing with the C8 and D-230 aqueous solutions. The recycled C8 showed a similar efficiency, with a residual oil content of 0.92 wt% on the washed drilling cuttings. These findings suggest that the C8 can be effectively recycled and reused to wash oily drill cuttings without degradation. In a large-scale test, 25 g C8 was used to wash 30 g oily drill cuttings. The average mass of oil recovered by this process was 4.18 g, which suggests that 93% of the oil was successfully removed. Table 4 Washing efficiency under different conditions. solvent C8
H2O
m (solvent)/ m (oily drill cuttings) 0.5:2 1:2 2:2 3:2 5:2
Oil content after washing with C8 (%) 1.300 0.870 0.875 0.879 10
Percent oil removal (%) 91.3 94.2 94.1 94 33.3
The residual C8-D content on the filtered solids after the treatment was determined by 1H NMR spectroscopy with TEAB as the internal standard (Fig. S8). The solids contained 1.2 wt% (~1200 ppm) C8-D (Table 5). After a simple washing of the filtered solids with deionized
water, the content of C8-D was reduced to 0.2 wt% (~200 ppm), which is below the required level of 300 ppm for safe disposal. This result shows great advantages of C8 over the amine solvents as carbonated water is needed to bring the residual amine content of the treated solids to the level required for safe disposal [23].
Table 5 Residual solvent content of solids after washing with water. No. of washings with water C8-D on the solid (%) 0 1.2 1 0.5 2 0.2 A 20 g sample of contaminated drill cuttings containing 1.2% C8-D, washed with 100 mL water for the times indicated.
4. Conclusions In our previous study, a long-chain fatty acid-based emulsifier with high interfacial activity was designed to prepare CO2-responsive emulsions [37]. Here a fatty acid of medium chain length (caprylic acid, C8) was used as switchable solvents. The resulting ion pairs with D-230 exhibited a low interfacial activity, facilitating C8 solvent separation from oily contaminants after washing. The reversible ion pair interactions led to switchable miscibility: oily caprylic acid (C8) became water miscible when forming ion pairs with D-230, while the ion pairs became phase separated after bubbling CO2 due to the dissociation of fatty acid-amine complexes. It is worth pointing out that previous research on fatty acid-based ion pairs has focused mainly on stimuli-responsive phase behaviors and emulsions [32–36], fatty acid as switchable solvents have not been previously studied. Compared with traditional amine or amidine as switchable solvents [16, 21–23], C8 shows lower toxicity and better biocompatibility. In addition, ion pairs exhibit less adsorption (1.2%) than amine (6.4%) on solid surfaces [23]. Therefore, most of the C8 and D-230 can be easily recovered and reused. Our study provides a new type of fatty acid solvent with switchable
miscibility. Moreover, this solvent can be considered as a green solvent for removing oil from contaminated solids.
Notes The authors declare no competing financial interest.
Acknowledgement
This work is supported by Natural Science Foundation of China (21333005) and National Science and Technology Major Project of China (2016ZX05040-005). References [1] L. Phan, H. Brown, J. White, A. Hodgson, P.G. Jessop, Green Chem. 11 (2009) 53. [2] G. Lasarte-Aragones, R. Lucena, S. Cardenas, M. Valcarcel, J. Sep. Sci. 38 (2015) 990. [3] G. Lasarte-Aragones, R. Lucena, S. Cardenas, M. Valcarcel, Talanta, 131 (2015) 645. [4] A.R. Boyd, P. Champagne, P.J. McGinn, K.M. MacDougall, J.E. Melanson, P.G. Jessop, Biotechnol. Tech. 118 (2012) 628. [5] C. Samorì, D. López Barreiro, R. Vet, L. Pezzolesi, D.W.F. Brilman, P. Galletti, E. Tagliavini, Green Chem. 15 (2013) 353. [6] D. Fu, S. Farag, J. Chaouki, P.G. Jessop, Biotechnol.Tech. 154 (2014) 101. [7] S.A. Leonard, J.A. Stegemann, J. Hazard. Mater. 174 (2010) 484. [8] J.P. Robinson, S.W. Kingman, C.E. Snape, S.M. Bradshaw, M.S.A. Bradley, H. Shang, R. Barranco, Chem.Eng.Res.Des. 88 (2010) 146. [9] C.G. Street, S.E. Guigard, J. Can. Petrol. Technol. 48 (2013) 26. [10] G.D. Ji, Y.S. Yang, Q. Zhou, T. Sun, J.R. Ni, Environ. Inter. 30 (2004) 509. [11] J.M.A.a.P.P.A. Reuben N. Okparanma, Afr. J. Environ. Sci. Technol. 3 (2009) 131. [12] T. Yamada, P.J. Lukac, M. George, R.G. Weiss, Chem. Mater. 19 (2007) 967. [13] D.J. Heldebrant, C.R. Yonker, P.G. Jessop, L. Phan, Energ.Environ.Sci. 1 (2008) 487. [14] D.J. Heldebrant, P.G. Jessop, C.A. Thomas, C.A. Eckert, C.L. Liotta, J. Org. Chem. 70 (2005) 5335. [15] P.G. Jessop, D.J. Heldebrant, X. Li, C.A. Eckert, C.L. Liotta, Nature, 436 (2005) 1102. [16] C. Samorì, L. Pezzolesi, D.L. Barreiro, P. Galletti, A. Pasteris, E. Tagliavini, RSC Adv. 4 (2014) 5999. [17] J.R. Vanderveen, J. Durelle, P.G. Jessop, Green Chem. 16 (2014) 1187. [18] J. Durelle, J.R. Vanderveen, P.G. Jessop, Phys. Chem. Chem. Phys. 16 (2014) 5270. [19] A.D. Wilson, F.F. Stewart, RSC Adv. 4 (2014) 11039. [20] L. Phan, J.R. Andreatta, L.K. Horvey, C.F. Edie, A.-L. Luco, A. Mirchandani, D.J. Darensbourg, P.G. Jessop, J. Org. Chem. 73 (2008) 127.
[21] P.G. Jessop, L. Kozycz, Z.G. Rahami, D. Schoenmakers, A.R. Boyd, D. Wechsler, A.M. Holland, Green Chem. 13 (2011) 619. [22] P.G. Jessop, L. Phan, A. Carrier, S. Robinson, C.J. Dürr, J.R. Harjani, Green Chem. 12 (2010) 809. [23] A. Holland, D. Wechsler, A. Patel, B.M. Molloy, A.R. Boyd, P.G. Jessop, Can. J. Chem. 90 (2012) 805. [24] P.G. Jessop, Green Chem. 13 (2011) 1391. [25] P.G. Jessop, D.A. Jessop, D. Fu, L. Phan, Green Chem. 14 (2012) 1245. [26] P. Pollet, E.A. Davey, E.E. Ureña-Benavides, C.A. Eckert, C.L. Liotta, Green Chem. 16 (2014) 1034. [27] I. Johansson, M. Svensson, Curr. Opin. Colloid Interface Sci. 6 (2001) 178. [28] M. Svensson, Surfactants from renewable resources (2010) 3. [29] E. Ceschia, J.R. Harjani, C. Liang, Z. Ghoshouni, T. Andrea, R.S. Brown, P.G. Jessop, RSC Adv. 4 (2014) 4638. [30] Y. Zhang, Y. Feng, J. Colloid Interface Sci. 447 (2015) 173. [31] W. Xu, H. Gu, X. Zhu, Y. Zhong, L. Jiang, M. Xu, A. Song, J. Hao, Langmuir 31 (2015) 5758. [32] Z. Chu, Y. Feng, Chem. Commun. 46 (2010) 9028. [33] Y. Feng, Z. Chu, Soft matter, 11 (2015) 4614. [34] C. Zhou, X. Cheng, O. Zhao, S. Liu, C. Liu, J. Wang, J. Huang, Soft matter, 10 (2014) 8023. [35] K. Sakai, K. Nomura, R.G. Shrestha, T. Endo, K. Sakamoto, H. Sakai, M. Abe, Langmuir 28 (2012) 17617. [36] Y. Zhang, Y. Feng, J. Colloid Interface Sci. 447 (2015) 173. [37] P. Xu, Z. Wang, Z. Xu, J. Hao, D. Sun, J. Colloid Interface Sci. 480 (2016) 198. [38] Y. Wang, H. Xu, X. Zhang, Adv. Mater. 21 (2009) 2849. [39] X. Zhang, C. Wang, Supramolecular amphiphiles, Chem. Soc. Rev., 40 (2011) 94. [40] Y. Zhang, Y. Zhang, C. Wang, X. Liu, Y. Fang, Y. Feng, Green Chem. 18 (2016) 392. [41] X. Su, M.F. Cunningham, P.G. Jessop, Chem. commun. 49 (2013) 2655. [42] X. Su, T. Robert, S.M. Mercer, C. Humphries, M.F. Cunningham, P.G. Jessop, Chem.Eur.J. 19 (2013) 5595. [43] I.V. Tetko, J. Gasteiger, R. Todeschini, A. Mauri, D. Livingstone, P. Ertl, V.A. Palyulin, E.V. Radchenko, N.S. Zefirov, A.S. Makarenko, V.Y. Tanchuk, V.V. Prokopenko, Comput.-Aided Mol. Des. 19 (2005) 453. [44] I.V. Tetko, V.Y. Tanchuk, J. Chem. Inf. Comput. Sci. 42 (2002) 1136. [45] U.-J. Kim, S. Kuga, J. Chromatogr. A 919 (2001) 29. [46] M.J. Potter, M.K. Gilson, J.A. McCammon, J. Am. Chem. Soc. 116 (1994) 10298. [47] L. Wang, S. Liu, T. Wang, D. Sun, Colloid Surf. A 381 (2011) 41. [48] J.P. Robinson, S.W. Kingman, C.E. Snape, S.M. Bradshaw, M.S.A. Bradley, H. Shang, R. Barranco, Chem. Eng. Res. Des. 88 (2010) 146. [49] S.-s. Chang, M. Redondo-Solano, H. Thippareddi, Int. J. Food Microbiol. 144 (2010) 141. [50] S.M. Mercer, T. Robert, D.V. Dixon, C.S. Chen, Z. Ghoshouni, J.R. Harjani, S. Jahangiri, G.H. Peslherbe, P.G. Jessop, Green Chem. 14 (2012) 832. [51] S. Karlsson, J. Päivärinta, R. Friman, A. Poso, M. Hotokka, S. Backlund, J. Phy. Chem. B 105 (2001) 7944. [52] J. Päivärinta, S. Karlsson, A. Poso, M. Hotokka, Chem. Phy. 263 (2001) 127. [53] S. Karlsson, S. Backlund, R. Friman, Colloid Polym. Sci. 278 (2000) 8
Graphical abstract
S0021-9797(17)30667-7 http://dx.doi.org/10.1016/j.jcis.2017.06.011 YJCIS 22438
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
25 March 2017 5 June 2017 5 June 2017
Please cite this article as: Q. Chen, L. Wang, G. Ren, Q. Liu, Z. Xu, D. Sun, A fatty acid solvent of switchable miscibility, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.06.011
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A fatty acid solvent of switchable miscibility *
Qianqian Chen a, Lei Wang a, Gaihuan Ren a, Qian Liu b, Zhenghe Xu c,d, Dejun Sun a, a
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan,
Shandong 250100, PR China b
School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing
100083, PR China c
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9,
Canada d
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 1000084, PR China
*Corresponding author. Tel. +86-531-88364749, Fax. +86-531-88364750 E-mail: [email protected].
Abstract Ion pair interactions were explored to design a fatty acid solvent of switchable miscibility with water. Fatty acids of medium length chains are immiscible with water but become miscible with water when ion pairs are formed with amines. The ion pairs become phase separated after bubbling CO2 into the solution due to the dissociation of the fatty acid-amine complexes. Ion pairs of caprylic acid (C8) and low toxic poly(oxypropylene) diamine (Jeffamine D-230) were characterized by FT-IR and 1H NMR. Log Kow values and surface activity were used to understand the switchable solvent mechanism in removing and recovering oily contaminants. More importantly, the ion pairs show a negligible adsorption on solid surfaces. Furthermore, both C8 and D-230 were recycled during the washing process. Thus the fatty acid as switchable solvent could be applied for oily contaminant removal from oily solid wastes. Keywords CO2 switchable, fatty acid-amine complex, ion pair interaction, oil contaminants removal, oily solid wastes.
Introduction In many chemical production processes (e.g., reactions, extractions, and/or separations), solvents with switchable miscibility and environmentally benign features are advantageous over traditional solvents [1–6]. For example, such solvents can be used to remove or recover oily contaminants from oily drill cuttings. Compared with the traditional treatment methods for oily drill cuttings, such as solidification and stabilization [7], microwave treatments [8], supercritical fluid extraction [9], photodegradation [10], and bioremediation [11], the use of switchable solvents could significantly reduce power consumption (e.g., distillation) without strict requirement of high pressure equipment, thereby reducing both capital and operating costs. Furthermore, the switchable nature of the solvents opens the doors for the recycling of solvent, which further reduces the net cost of treatment. Solvents of switchable solubility [12–15] are miscible with oily compound under air but not with that under CO2. Such switchable solvents were first reported by Jessop et al. [15]. Further studies on solvents of switchable polarity [14] or switchable miscibility [16–20] have been reported in recent years. Most of these solvents are based on organic molecules of amidine or amine groups. The amine as switchable solvents are usually toxic [21] and require complex synthesis procedures [16]. One problem encountered when using the amidine solvent is its loss due to the ease of its hydrolysis [22]. When amine solvent [23] is used to extract oil from solid surfaces, amine is adsorbed on solids to generate secondary wastes. To bring the residual amine content to a desired level, carbonated water is often used to wash the treated solids. There is a strong interest in designing a green solvent [24–26] with low toxicity and negligible adsorption on solid surfaces. Fatty acids with good biocompatibility and abundance in nature are ideal candidates for designing such switchable solvents [27,28]. Many complexes formed by fatty acids and amines have shown CO2-switchable behaviors [29–31].
Studies on CO2-responsive micelle morphology [32–36] have been reported. In a recent study, we demonstrated the preparation of a CO2-responsive emulsion using a highly interfacial active long-chain fatty acid-based emulsifier [37]. Water immiscible fatty acids have been shown to become water soluble [38, 39] when their carboxylate groups become complexed with amine [17,40–42]. It is hypothesized that a fatty acid as switchable solvent can be obtained if the complexation between the fatty acid and amine can be made reversible by adding and removing CO2． In this study, we describe a successful search for a novel fatty acid solvent of switchable miscibility. Hydrophobic fatty acid can be transformed to be water soluble after reacting with amine to form fatty acid-amine complexes, and it becomes water immiscible (phase separated) after bubbling with CO2 (Fig. 1). With this unique property, such fatty acid solvent was used to remove oil from oily drill cuttings as an example of its potential environmental application.
Fig. 1. Switchable phase behaviors of fatty acid.
2. Materials and Experimental Methods 2.1 Materials and instruments A primary diamine with hydrophobic poly(oxypropylene) (n ≈ 2-3) as its backbone, poly(oxypropylene) diamine (Jeffamine D-230), of molecular weight ≈ 230 was purchased from Aldrich. Caprylic acid (C8, AR) was obtained from Sinopharm Chemical Reagent Co. Ltd., China. Paraffin oil (Trade Name: Marcol 52) was purchased from Exxon Mobil. FT-IR
(NEXUS 670, Thermo Nicolet) and 1H NMR (Bruker Avance 400 spectrometer, 400 Hz, CDCl3) were used to determine the formation of C8-D-230 (C8-D). 2.2 Measuring the switchable miscibility of fatty acids Prediction of log Kow values The octanol/water partition coefficient (log Kow) provides a quantitative measure of hydrophobicity. The log Kow values of all compounds were predicted using the ALOGPS 2.1 software [43, 44]. Measuring the switchable behaviors of different fatty acids To confirm the switchable miscibility, a series of fatty acids and amines were used to prepare their complexes. Methylamine (pKa = 10.6) [45], ethylamine (pKa = 11.0) [46], DETA (pKa1 = 10.1, pKa2 = 9.1, pKa3 = 4.4) [34], or D-230 (pKa1 = 10.9, pKa2 = 9.8) [47] aqueous solutions (5 wt%) were mixed with originally water immiscible fatty acids (chain lengths from C6 to C9) at a molar ratio of n(-NH2)/n(-COOH) = 1:1. Solutions were mixed vigorously with a magnetic stirrer for 10 min. The solutions were found to be transparent, suggesting the complete miscibility of the fatty acids with amine aqueous solutions. When CO2 was bubbled into the solution at 400 mL/min for 30 min, the fatty acid-amine complex was classified as CO2 switchable if the solution became biphasic (i.e., phase separated). 2.3 Surface activity measurement The surface tension of a fatty acid-amine complex (C8-D) aqueous solution was measured using an automated surface tension meter (JYW-200, China) at 25°C. Prior to each measurement, the glass cell was cleaned with ethanol and washed repeatedly with water, and the platinum ring was flamed to burn off any organic contaminants. The measurement was repeated three times for each solution. The dynamic interfacial tension (IFT) of the C8-D aqueous solutions was measured using
the pendant drop method on a commercial drop shape analyzer (Tracker, France Teclis) at 25°C. A drop of aqueous solution was created at the end of a syringe needle in a cuvette filled with paraffin oil. Images of the drop were captured and the Laplace equation was used to obtain the interfacial tension. 2.4 Oil removal by C8 Measuring the efficiency of C8 in removing oil from oily drill cuttings The model oily drill cuttings used in this study were prepared by immersing dry drill cuttings to paraffin oil. The oil content of original oily drill cuttings was tested with an Infrared Oil Content Analyzer (OIL460, Huaxia, China). First, a standard curve was obtained by using a series of standard solutions with variable paraffin oil/CHCl3 concentrations ranging from 5 to 25 wt%. Then, the model oily drill cuttings (2 g) were treated with 15 mL of CHCl3 and the supernatant (oil+CHCl3) was decanted into a 25-mL volumetric flask for analysis. The oil content of the model oily drill cuttings was 15 wt%, which is within the reported range [48]. The oil content was calculated using the formula below:
To investigate the efficiency of C8 in removing oil from drill cuttings, four samples were prepared by mixing C8 (from 0.5 to 3 g) with oily drill cuttings (2 g). Samples were vigorously shaken with a mechanical shaker (BF-1F, Jintan, China) at 150 r/min and 25°C for 30 min. Then, 5 wt% (Fig. S1) D-230 solutions were added to the four mixtures and shaken for 20 min. After filtering, the solid residuals were completely washed with deionized water before drying at room temperature. Next, the oil content of the residual solids was measured using the Infrared Oil Content Analyzer. The efficiency of normal water in washing the oily drill cuttings was also tested for comparison. Measuring the adsorption of C8-D on the drill cuttings
To determine the adsorption of C8-D on the drill cuttings after washing, oily drill cuttings (20 g) were washed with C8 (10 g) as described above. After adding 5 wt% D-230 solution to the mixture and shaking for 20 min, the drill cuttings were filtered and dried under natural conditions. Next, D2O was used to dissolve C8-D on the drill cuttings and the content of C8-D was analyzed by 1H NMR spectra using TEAB as the internal standard substance.
3 Results and discussion 3.1 Selection of fatty acid solvent To remove oily contaminants, fatty acid as a washing solvent should be a liquid with good oil miscibility. The fatty acid-amine complexes prepared from fatty acids with different chain lengths and various amines were tested for the desired phase behaviors (Fig. 1). The complex was easily obtained by neutralizing the fatty acid with the amine (Eq. (1)). Hydrophobic fatty acid can be transformed into a water soluble complex through the formation of an ion pair with an amine, whereas the bubbling of CO2 promotes the dissociation of the fatty acid-amine complex (Eq. (2)). After the removal of CO2, the water soluble complex reforms as a result of the increased pH (Eq. (2)).
Table 1 shows the switchable behaviors of a series of water soluble complexes prepared by mixing fatty acids with amine aqueous solutions. When CO2 was bubbled into the fatty acid-amine solution, H+ and HCO3- slowly formed, causing the protonation of ionized fatty acids by H+. The water immiscible un-ionized fatty acid then became separated from the aqueous solution. If an amine shows a strong basicity in solution, such as for methylamine and ethylamine, the pH of the solution could remain high after bubbling CO2 (pHmethylamine-fatty
acid
and pHethylamine-fatty acid were at 6.9 and 7.0, respectively). Fatty acids with pKa values of
4.86 (C8), 4.80 (C7), 4.88 (C6), and 4.90 (C9) remain dissociated under this condition [49]; therefore, only D-230 [47] and DETA [34] with weak basicity are suitable for preparing the desired complex. Complexes of amines with fatty acids of longer alkane chain length usually show a higher interfacial activity. In this case, the oil is easily emulsified [37], making the subsequent oil separation process very difficult. Fig. 2a shows photos of the paraffin oil mixture with 5 wt% C8-D (caprylic acid-D-230) and C9-D (nonanoic acid-D-230) aqueous solutions. The samples prepared with C9-D become emulsified, whereas the samples containing C8-D display a clear boundary between the two phases. Such differences can be ascribed to the different interfacial activities of the two fatty acid-amine complexes. The surface tension (γ) of the C8-D aqueous solution is 62 mN/m, which is slightly lower than that of pure water (72 mN/m) (Fig. 2b). The dynamic interfacial tension (IFT) of paraffin oil and 0.01 mol/L C8-D aqueous solution is 18 mN/m (Fig. S2). Under this condition, the paraffin oil/C8-D aqueous solution emulsions are unstable. This makes C8 a suitable switchable solvent for effective oil-water separation process. Table 1 Different chain length fatty acids tested for switchable behaviors log Kow
Fatty acids
1.9 Hexanoic acid (C6) 2.4 Ocnanthic acid (C7) 3.25 Caprylic acid (C8) 3.5 Nonanoic acid (C9) “Y”: switchable, “N”: not switchable
methylamine N N N N
ethylamine N N N N
DETA
D-230
Y Y Y Y
Y Y Y Y
To evaluate the safety and environmental effects of the fatty acids and amines, LD50 (oral, rat) data are compared in Table 2. Compared to DETA, D-230 is superior as the incorporated poly(oxypropylene) in its backbone decreases its eco-toxicity [16]. In consideration of the toxicity of the fatty acids, C8 is more appropriate when compared with switchable solvents
prepared using N,N-dimethylcyclohexanamine (DMCHA) [21] and other fatty acids. This evidence suggests that the C8 solvent can be used as a green switchable solvent. In the following sections, the switching characteristics and oil removal capability of the C8 solvent will be discussed.
Fig. 2. (a) Paraffin oil/water emulsions (1:1 w/w) containing 5 wt% C8-D (upper) and 5 wt% C9-D (lower) taken at 5 min, 30 min and 24 h after preparation. (b) Surface tension of C8-D (black) and C9-D (red) aqueous solutions at 25°C as a function of concentration.
Table 2 Properties of chemicals. LD50 (oral, rat) (mg kg−1) a C8 240 10080 ＞100 C7 223 110 7000 C6 205 102 3000 DMCHA 118 7.22 348 D-230 2880 DETA 207 94 1080 a Data obtained from the MSDS of each component, as supplied by Sigma–Aldrich. Substance
Boiling point (°C)
Flash point (°C)
Melting point (°C) 17 −7.5 −3 −60 −39
3.2 Switchable behaviors of C8 solvent The switchable behaviors of the C8 solvent are demonstrated in Fig. 3. Mixtures of C8 (1 mL) and paraffin oil (4 mL) were colored with Oil Red O. The total volume of the oil phase was 5 mL (Fig. 3a). After adding D-230 aqueous solution (nearly 10 wt%), the oil phase was reduced to 4 mL (Fig. 3b), which suggests that C8 (1 mL) was mostly transferred to the aqueous phase. Although C8 is oil miscible, it has a finite solubility in water (0.068 g C8 can
dissolve in 100 g water at 20°C). As a result, a small amount of colored C8 dissolved in water produced a reddish color in the water phase. After bubbling CO2 at an atmospheric pressure for 30 min, the oil phase increased to 5 mL, indicating that C8 became water insoluble again and transferred back to the oil phase (Fig. 3c). Removing CO2 by bubbling N2 at 60°C reduced the oil phase back to 4 mL (Fig. 3d), clearly demonstrating the switchable miscibility of C8 in oil to C8-D in water.
Fig. 3. Switchable behaviors of C8: (I) C8 (1 mL) in paraffin oil (4 mL). After mixing with D-230 aqueous solution, the oil phase was reduced by 1 mL; (II) After bubbling CO2, the oil phase increased to 5 mL; (III) Upon removal of CO2, the oil phase was reduced to 4 mL again.
When C8 is mixed with D-230 aqueous solution, the two chemicals associate with each other by ion pair interaction to form a water soluble ion pair complex (C8-D). After bubbling CO2 into the aqueous solution, the increased H+ promote the dissociation of the ion pair complex, resulting in the formation of water immiscible C8 and water soluble protonated D-230 (D-2302+). As a result, the oil phase returned to the original volume (5 mL) (Fig. 3c). When CO2 was removed by N2 bubbling at 60°C [50], causing the dissociation of primary polyamine bicarbonate and formation of D-230 [37]. Then, the C8 and free D-230 molecules associated to reform the water soluble complexes, reducing the oil phase back to approximately 4 mL (Fig. 3d). The results clearly demonstrate that C8 can successfully switch between the oil miscible and water miscible forms through the reversible ion pair reaction.
The C8-D complexes were studied using FT-IR and 1H NMR spectroscopy. The FT-IR spectrum of C8-D is compared with the spectra of C8 and D-230 in Fig. 4. The peak at 1712 cm−1 corresponds to the carbonyl (-C=O-) peak of C8. After neutralization by D-230, this band shifts to 1538 cm−1, indicating the formation of carboxylate. For D-230, -NH2 shows its characteristic asymmetric and symmetric stretching vibrations at 3368 cm-1 and 3290 cm-1 and bending vibration at 1580 cm-1. However, the -NH2 stretching band in C8-D disappears due to the formation of -NH3+, which has a weak bending vibrational peak at 1685 cm−1. The results demonstrate the binding of C8 and D-230 to form C8-D.
Fig. 4. FT-IR spectra of C8, D-230 and C8-D.
Fig. 5 shows the 1H NMR spectra of C8, D-230 and C8-D in CDCl3. In the spectrum of C8-D, the peak at δ = 11.8 ppm for the -COOH group disappears, and the peak at δ = 8.2 ppm indicates the formation of -NH3+. The 1H chemical shifts of 1st and 2nd -CH2 groups near the C=O group in C8-D moved up-field due to the increased electron density. This indicates that proton transfers from -COOH to -NH2, which further confirms the formation of C8-D.
1
Fig 5. H NMR spectra of C8-D, C8 and D-230.
To investigate the switchable behaviors of binding in C8-D, CO2 and N2 were alternately bubbled into its aqueous solution and four cycles of this process were performed (Fig. 6). The pH of the C8-D aqueous solution is 7.2. At this pH, C8 is mainly present in the ionized form [49], while the dominant D-230 species is the divalent cation (D-2302+), with both primary amine groups protonated. When CO2 was bubbled, the ionized C8 was protonated by H+ to form un-ionized C8. The conductivity κ of the aqueous phase increased, accompanied by a decrease of pH. This transition is attributed to the presence of HCO3 - and H+ by the excess CO2 bubbling. Upon bubbling N2 at 60°C for 1 h to remove CO2, D-230 was reformed and reacted with C8 to form a water miscible ion pair complex. Thus the κ and pH returned to their original values.
Fig. 6. pH (blue squares) and conductivity κ (black stars) changes in 5 wt% C8-D aqueous solution in response to repeated cycles of bubbling or removing CO2.
The polarity switching of C8 solvent was also investigated with the variation in the wavelength of maximum absorbance (λmax) of Nile Red dye (Fig. S3). As a result of the low polarity of C8, the λmax of water-saturated C8 is at 580 nm. After interacting with D-230, C8 becomes water soluble in the form of a C8-D complex with the λmax–C8-D at 623 nm [22]. 3.3 Oil separation and solvent recycling A fatty acid solvent of switchable miscibility can be used to remove oil components from oil-contaminated soil or solids such as oily drill cuttings. The model oily drill cuttings used in this study were prepared by immersing dry drill cuttings in paraffin oil. The process of oil removal from the oily drill cuttings is shown in Fig. 7. First, water immiscible C8 was completely mixed with the oily drill cuttings to extract oil. Then, the low-density oil was separated from the solvent to the upper phase after the addition of D-230 aqueous solution (Fig. 7I). After filtering the solids, the C8 was separated from D-230 in the filtrate (C8-D aqueous solutions) by bubbling CO2 into the recovered aqueous phase (Fig. 7II). Protonated D-230 (D-2302+) becomes deprotonated by bubbling N2 to remove CO2 [37] (Fig. 7III). Both C8 and D-230 can then be recycled.
Fig. 7. Process of oil removal from oily drill cuttings using C8: (I) oil separation process; (II) C8 recycling process; and (III) D-230 recycling process (red dashed line represents the recycle process, “ ” represents the drill cuttings).
In the oil separation process (Fig. 7I), if a portion of C8 remains in the oil phase, it will cause the loss of solvent and secondary pollution of the recovery oil. 1H NMR spectroscopy was used to determine the separation of paraffin oil from C8. The C8 and paraffin oil were shaken together at a 1:1 (w/w) ratio at room temperature. 1H NMR spectroscopy shows the peaks of C8 and paraffin oil (Fig. 8b). The boundary between the two liquid phases starts to appear with the addition of D-230 solution. A comparison of spectra b and c in Fig. 8 shows a negligible amount of C8 in the separated oil phase, confirming that C8 can be effectively separated from the paraffin oil.
1
Fig. 8. H NMR spectra of (a) pure paraffin oil, (b) a mixture of C8 and paraffin oil , and (c) paraffin oil after the removal of C8 by D-230 solution.
To confirm the recycling of C8 (Fig. 7II), CO2 was bubbled into the C8-D solution, a phase
boundary was observed. FT-IR (Fig. S4) was used to characterize the oil phase, while the κ and pH of the water phase were measured after bubbling N2. The peak at 1712 cm−1 belongs to -COOH, which indicates the regeneration of C8. However, the peaks corresponding to the bending vibrational of carboxylate at 1461 and 1532 cm−1 remain (although they are much weaker) and the -OH out-of-plane vibration peak of C8 at 938 cm−1 disappears. This indicates that a small amount of C8 associates with protonated D-230 (D-2302+) to form complexes (C8-D) through hydrogen bonding [51–53]. Although the recycled C8 sample contained a small amount of C8-D complexes, it is not considered to be an issue as all the original materials (C8 and D-230) remain in the recycled system, as shown in Fig. 7. As shown in Fig. 9, the κ and pH of the D-2302+ aqueous solution phase were 2300 μs/cm and 6.0, respectively, after bubbling CO2 into the C8-D solution. Bubbling N2 at 60ºC to remove CO2 caused a concomitant decrease in κ and increase in pH, indicating the deprotonation of D-2302+ (Fig. 7III). Bubbling N2 cannot completely deprotonate the protonated D-230 (D-2302+) molecules [24]. The final pH of the D-230 solution was around 9.1, which was slightly lower than the initial value (pH 10.2).
2+
Fig. 9. Conductivity and pH of D-230 solution when bubbling N2.
Furthermore, in order to verify the generality of C8 in removing oily contaminants, trichloroethylene (Fig. S5) and toluene (Fig. S6) were also tested as model oils in place of
paraffin oil. Trichloroethylene (3 mL) was mixed with C8 (1 mL) and colored with Oil Red O. A single phase was observed, which suggests good miscibility of C8 with trichloroethylene. After D-230 solution was added, the volume of lower red oil phase decreased to 3 mL (Fig. S5), suggesting an excellent separation of C8 from trichloroethylene. After bubbling CO2, the oil phase returned to 4 mL, indicating that C8 becomes water immiscible, thus demonstrating the switchable miscibility of C8 with water. 3.4 Oil removal from oily drill cuttings We performed a preliminary evaluation on the C8 solvent of switchable miscibility for removing oil from contaminated drilling cuttings (Fig. S7). The original oil content of the drill cuttings was 15 wt%. The residual oil on the drill cuttings remained at around 10 wt% after washing with only water (Table 4). In contrast, the residual oil content on the drilling cuttings was reduced to 0.87 wt% after washing with the C8 and D-230 aqueous solutions. The recycled C8 showed a similar efficiency, with a residual oil content of 0.92 wt% on the washed drilling cuttings. These findings suggest that the C8 can be effectively recycled and reused to wash oily drill cuttings without degradation. In a large-scale test, 25 g C8 was used to wash 30 g oily drill cuttings. The average mass of oil recovered by this process was 4.18 g, which suggests that 93% of the oil was successfully removed. Table 4 Washing efficiency under different conditions. solvent C8
H2O
m (solvent)/ m (oily drill cuttings) 0.5:2 1:2 2:2 3:2 5:2
Oil content after washing with C8 (%) 1.300 0.870 0.875 0.879 10
Percent oil removal (%) 91.3 94.2 94.1 94 33.3
The residual C8-D content on the filtered solids after the treatment was determined by 1H NMR spectroscopy with TEAB as the internal standard (Fig. S8). The solids contained 1.2 wt% (~1200 ppm) C8-D (Table 5). After a simple washing of the filtered solids with deionized
water, the content of C8-D was reduced to 0.2 wt% (~200 ppm), which is below the required level of 300 ppm for safe disposal. This result shows great advantages of C8 over the amine solvents as carbonated water is needed to bring the residual amine content of the treated solids to the level required for safe disposal [23].
Table 5 Residual solvent content of solids after washing with water. No. of washings with water C8-D on the solid (%) 0 1.2 1 0.5 2 0.2 A 20 g sample of contaminated drill cuttings containing 1.2% C8-D, washed with 100 mL water for the times indicated.
4. Conclusions In our previous study, a long-chain fatty acid-based emulsifier with high interfacial activity was designed to prepare CO2-responsive emulsions [37]. Here a fatty acid of medium chain length (caprylic acid, C8) was used as switchable solvents. The resulting ion pairs with D-230 exhibited a low interfacial activity, facilitating C8 solvent separation from oily contaminants after washing. The reversible ion pair interactions led to switchable miscibility: oily caprylic acid (C8) became water miscible when forming ion pairs with D-230, while the ion pairs became phase separated after bubbling CO2 due to the dissociation of fatty acid-amine complexes. It is worth pointing out that previous research on fatty acid-based ion pairs has focused mainly on stimuli-responsive phase behaviors and emulsions [32–36], fatty acid as switchable solvents have not been previously studied. Compared with traditional amine or amidine as switchable solvents [16, 21–23], C8 shows lower toxicity and better biocompatibility. In addition, ion pairs exhibit less adsorption (1.2%) than amine (6.4%) on solid surfaces [23]. Therefore, most of the C8 and D-230 can be easily recovered and reused. Our study provides a new type of fatty acid solvent with switchable
miscibility. Moreover, this solvent can be considered as a green solvent for removing oil from contaminated solids.
Notes The authors declare no competing financial interest.
Acknowledgement
This work is supported by Natural Science Foundation of China (21333005) and National Science and Technology Major Project of China (2016ZX05040-005). References [1] L. Phan, H. Brown, J. White, A. Hodgson, P.G. Jessop, Green Chem. 11 (2009) 53. [2] G. Lasarte-Aragones, R. Lucena, S. Cardenas, M. Valcarcel, J. Sep. Sci. 38 (2015) 990. [3] G. Lasarte-Aragones, R. Lucena, S. Cardenas, M. Valcarcel, Talanta, 131 (2015) 645. [4] A.R. Boyd, P. Champagne, P.J. McGinn, K.M. MacDougall, J.E. Melanson, P.G. Jessop, Biotechnol. Tech. 118 (2012) 628. [5] C. Samorì, D. López Barreiro, R. Vet, L. Pezzolesi, D.W.F. Brilman, P. Galletti, E. Tagliavini, Green Chem. 15 (2013) 353. [6] D. Fu, S. Farag, J. Chaouki, P.G. Jessop, Biotechnol.Tech. 154 (2014) 101. [7] S.A. Leonard, J.A. Stegemann, J. Hazard. Mater. 174 (2010) 484. [8] J.P. Robinson, S.W. Kingman, C.E. Snape, S.M. Bradshaw, M.S.A. Bradley, H. Shang, R. Barranco, Chem.Eng.Res.Des. 88 (2010) 146. [9] C.G. Street, S.E. Guigard, J. Can. Petrol. Technol. 48 (2013) 26. [10] G.D. Ji, Y.S. Yang, Q. Zhou, T. Sun, J.R. Ni, Environ. Inter. 30 (2004) 509. [11] J.M.A.a.P.P.A. Reuben N. Okparanma, Afr. J. Environ. Sci. Technol. 3 (2009) 131. [12] T. Yamada, P.J. Lukac, M. George, R.G. Weiss, Chem. Mater. 19 (2007) 967. [13] D.J. Heldebrant, C.R. Yonker, P.G. Jessop, L. Phan, Energ.Environ.Sci. 1 (2008) 487. [14] D.J. Heldebrant, P.G. Jessop, C.A. Thomas, C.A. Eckert, C.L. Liotta, J. Org. Chem. 70 (2005) 5335. [15] P.G. Jessop, D.J. Heldebrant, X. Li, C.A. Eckert, C.L. Liotta, Nature, 436 (2005) 1102. [16] C. Samorì, L. Pezzolesi, D.L. Barreiro, P. Galletti, A. Pasteris, E. Tagliavini, RSC Adv. 4 (2014) 5999. [17] J.R. Vanderveen, J. Durelle, P.G. Jessop, Green Chem. 16 (2014) 1187. [18] J. Durelle, J.R. Vanderveen, P.G. Jessop, Phys. Chem. Chem. Phys. 16 (2014) 5270. [19] A.D. Wilson, F.F. Stewart, RSC Adv. 4 (2014) 11039. [20] L. Phan, J.R. Andreatta, L.K. Horvey, C.F. Edie, A.-L. Luco, A. Mirchandani, D.J. Darensbourg, P.G. Jessop, J. Org. Chem. 73 (2008) 127.
[21] P.G. Jessop, L. Kozycz, Z.G. Rahami, D. Schoenmakers, A.R. Boyd, D. Wechsler, A.M. Holland, Green Chem. 13 (2011) 619. [22] P.G. Jessop, L. Phan, A. Carrier, S. Robinson, C.J. Dürr, J.R. Harjani, Green Chem. 12 (2010) 809. [23] A. Holland, D. Wechsler, A. Patel, B.M. Molloy, A.R. Boyd, P.G. Jessop, Can. J. Chem. 90 (2012) 805. [24] P.G. Jessop, Green Chem. 13 (2011) 1391. [25] P.G. Jessop, D.A. Jessop, D. Fu, L. Phan, Green Chem. 14 (2012) 1245. [26] P. Pollet, E.A. Davey, E.E. Ureña-Benavides, C.A. Eckert, C.L. Liotta, Green Chem. 16 (2014) 1034. [27] I. Johansson, M. Svensson, Curr. Opin. Colloid Interface Sci. 6 (2001) 178. [28] M. Svensson, Surfactants from renewable resources (2010) 3. [29] E. Ceschia, J.R. Harjani, C. Liang, Z. Ghoshouni, T. Andrea, R.S. Brown, P.G. Jessop, RSC Adv. 4 (2014) 4638. [30] Y. Zhang, Y. Feng, J. Colloid Interface Sci. 447 (2015) 173. [31] W. Xu, H. Gu, X. Zhu, Y. Zhong, L. Jiang, M. Xu, A. Song, J. Hao, Langmuir 31 (2015) 5758. [32] Z. Chu, Y. Feng, Chem. Commun. 46 (2010) 9028. [33] Y. Feng, Z. Chu, Soft matter, 11 (2015) 4614. [34] C. Zhou, X. Cheng, O. Zhao, S. Liu, C. Liu, J. Wang, J. Huang, Soft matter, 10 (2014) 8023. [35] K. Sakai, K. Nomura, R.G. Shrestha, T. Endo, K. Sakamoto, H. Sakai, M. Abe, Langmuir 28 (2012) 17617. [36] Y. Zhang, Y. Feng, J. Colloid Interface Sci. 447 (2015) 173. [37] P. Xu, Z. Wang, Z. Xu, J. Hao, D. Sun, J. Colloid Interface Sci. 480 (2016) 198. [38] Y. Wang, H. Xu, X. Zhang, Adv. Mater. 21 (2009) 2849. [39] X. Zhang, C. Wang, Supramolecular amphiphiles, Chem. Soc. Rev., 40 (2011) 94. [40] Y. Zhang, Y. Zhang, C. Wang, X. Liu, Y. Fang, Y. Feng, Green Chem. 18 (2016) 392. [41] X. Su, M.F. Cunningham, P.G. Jessop, Chem. commun. 49 (2013) 2655. [42] X. Su, T. Robert, S.M. Mercer, C. Humphries, M.F. Cunningham, P.G. Jessop, Chem.Eur.J. 19 (2013) 5595. [43] I.V. Tetko, J. Gasteiger, R. Todeschini, A. Mauri, D. Livingstone, P. Ertl, V.A. Palyulin, E.V. Radchenko, N.S. Zefirov, A.S. Makarenko, V.Y. Tanchuk, V.V. Prokopenko, Comput.-Aided Mol. Des. 19 (2005) 453. [44] I.V. Tetko, V.Y. Tanchuk, J. Chem. Inf. Comput. Sci. 42 (2002) 1136. [45] U.-J. Kim, S. Kuga, J. Chromatogr. A 919 (2001) 29. [46] M.J. Potter, M.K. Gilson, J.A. McCammon, J. Am. Chem. Soc. 116 (1994) 10298. [47] L. Wang, S. Liu, T. Wang, D. Sun, Colloid Surf. A 381 (2011) 41. [48] J.P. Robinson, S.W. Kingman, C.E. Snape, S.M. Bradshaw, M.S.A. Bradley, H. Shang, R. Barranco, Chem. Eng. Res. Des. 88 (2010) 146. [49] S.-s. Chang, M. Redondo-Solano, H. Thippareddi, Int. J. Food Microbiol. 144 (2010) 141. [50] S.M. Mercer, T. Robert, D.V. Dixon, C.S. Chen, Z. Ghoshouni, J.R. Harjani, S. Jahangiri, G.H. Peslherbe, P.G. Jessop, Green Chem. 14 (2012) 832. [51] S. Karlsson, J. Päivärinta, R. Friman, A. Poso, M. Hotokka, S. Backlund, J. Phy. Chem. B 105 (2001) 7944. [52] J. Päivärinta, S. Karlsson, A. Poso, M. Hotokka, Chem. Phy. 263 (2001) 127. [53] S. Karlsson, S. Backlund, R. Friman, Colloid Polym. Sci. 278 (2000) 8
Graphical abstract