Micellization and aggregation properties of sodium

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friendly surfactant, PFPE-Na is a perfect substitute of NaPFO. Electrical conductivity measurements implied that the micellization of PFPE-Na in aqueous solution ...
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Micellization and aggregation properties of sodium perfluoropolyether carboxylate in aqueous solution Qiwen Yin, Wei Xue, Yanyun Bai, Wanxu Wang* , Xiaoyuan Ma, Zhiping Du, Guoyong Wang* China Research Institute of Daily Chemical Industry, Taiyuan 030001, PR China

A R T I C L E I N F O

Article history: Received 26 May 2016 Received in revised form 18 July 2016 Accepted 22 July 2016 Available online xxx Keywords: Surfactant Sodium perfluoropolyether carboxylate Micellization Aggregation Properties

A B S T R A C T

Sodium perfluoropolyether carboxylate (PFPE-Na) was synthesized via hydrolyzing the corresponding hexafluoropropylene oxide oligomer (PFPF). The structure of PFPE-Na was characterized by FT-IR and 19F NMR. The micellization and aggregation properties of PFPE-Na surfactant in aqueous solution were studied systematically using equilibrium surface tension, electrical conductivity, dynamic surface tension, steady-state fluorescence, transmission electron microscopy (TEM) and contact angle methodologies. The results of equilibrium surface tension at 25  C showed that the critical micelle concentration (CMC) and the surface tension at CMC (gCMC) of PFPE-Na aqueous solution are lower than sodium perfluorooctanoate [NaPFO, C7F15COONa], which revealing that as a kind of environmentfriendly surfactant, PFPE-Na is a perfect substitute of NaPFO. Electrical conductivity measurements implied that the micellization of PFPE-Na in aqueous solution was an exothermic and entropy-driven process in the range of temperature investigated. Steady-state fluorescence and transmission electron microscopy (TEM) may indicate that PFPE-Na self-assemble in aqueous solution to form larger spherical aggregates with the increase of concentration. In addition, dynamic surface tension measurements of PFPE-Na solution showed an extremely efficient adsorption at concentrations above CMC while the determination of contact angle of PFPE-Na showed the wetting ability was general. ã 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Introduction Fluorocarbon surfactants are seen as special surfactants compared with traditional surfactants [1] for they possess unique properties called “three-high” and “two-loathe”, which mean high surface activity, great heat resistant, chemical resistant, oil repellent and water proof [2]. Fluorocarbon surfactants are widely used not only in numerously industrial and household fields, but also in the fields of enlarged applications where hydrocarbon surfactants cannot be used [3]. Fluorocarbon surfactants have motivated a worldwide interest due to their excellent properties [4]. However, long perfloroalkyl chain of fluorocarbon surfactants like perfluorooctanoic acid (PFOA) are hard to biodegrade leading to the accumulation in nature environment which can become potential carcinogens to human.

* Corresponding author at: China Research Institute of Daily Chemical Industry, 34 Wenyuan Street, Taiyuan, Shanxi Province 030001, PR China. Fax: +86 351 4040802. E-mail address: [email protected] (G. Wang).

So PFOA has been banned in many industry areas. Polyfluoroalkylpolyether compounds have low-melting point and great degradability. The insertion of oxygen atoms into the polyfluoroalkyl chain not only increase the elastic characteristic but also the solubility of the chain [5]. Perfluoropolyether surfactants have low surface energy, high stability, and low toxicity. Furthermore, the components after degradation have no negative effects to the environment. So they have attracted much attentions in basic industry and cutting-edge scientific fields such as military, aerospace and nuclear industry [6,7]. Developing a new type of perfluoropolyether surfactant to replace PFOA has a profound significance and application prospect. Perfluoropolyether carboxylic acid is one of the primary choices of PFOA alternatives. As an important intermediate, perfluoropolyether carboxylate can satisfy the synthesis of downstream productions. However, the micellization and aggregation properties of these surfactants in aqueous solutions were scarce. In this work, PFPE-Na was synthesized and its properties were investigated systematically, such as micellization thermodynamic, adsorption properties, aggregation behaviors and wetting ability.

http://dx.doi.org/10.1016/j.jiec.2016.07.026 1226-086X/ã 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Please cite this article in press as: Q. Yin, et al., Micellization and aggregation properties of sodium perfluoropolyether carboxylate in aqueous solution, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.07.026

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The main purpose of this article is to enrich our knowledge of fluorocarbon surfactant and to provide a more comprehensive foundation of PFPE-Na.

of the maximum pressure which was necessary to blow a bubble in a liquid from the tip of a capillary. The measurements were conducted with effective surface ages ranging from 10 to 200,000 ms.

Experimental Materials 2,5-Bis(trifluoromethyl)-3,6-dioxaundecafluorononanoyl fluoride (PFPF) was obtained from Lisheng Regent Biochemistry Technology Co., Ltd. (Guangdong, P.R. China). Diethyl ether was purchased from Tianjin Kemel Chemical Reagent Co., Ltd. (Tianjin, P.R. China). Sodium hydroxide and sodium chloride were obtained from Shentai Reagent Co., Ltd. (Tianjin, P.R. China). Pyrene (99%) was achieved from Alfa Aesar. Benzophenone was purchased from China National Medicine Group Shanghai Chemical Reagent Company. (Shanghai, P.R. China). Water we used in all experiments was deionized water (18.2 MV). Synthesis of PFPE-Na PFPE-Na was prepared according to the procedures reported previously by the neutralization method [8,9]. The completely hydrolyzed PFPF (4.98 g) in deionized water was neutralized by 2.00 mol/L NaOH aqueous solution (30 mL) at room temperature and coprecipitated with saturated salt water, then filtered and dried sequentially to obtain a product. The product solubilized in diethyl ether was filtered off insoluble inorganic salt and evaporated the solvent of diethyl ether under depression and then dried under a vacuum for 48 h to get a solid product (yield, 93.6%). Structural analyses The chemical structures of PFPF and PFPE-Na were characterized by nuclear magnetic resonance spectroscopy (NMR) and fourier transform infrared spectroscopy (FT-IR). 19F NMR spectra were recorded on a Varian INOVA-400 Hz spectrometer using CDCl3 (PFPF) and D2O (PFPE-Na) as the lock solvent and CF3CO2H as the reference. FT-IR spectra were obtained using a Bruker Vertex70 spectrometer. Measurements Equilibrium surface tension measurements The equilibrium surface tension of PFPE-Na aqueous solutions were measured with Krüss K12 (Krüss Company, Germany) Processor Tensiometer by Wilhelmy plate technique at (25  0.1)  C. Before the measurements, each PFPE-Na aqueous solution was equilibrated for 24 h.

Steady-state fluorescence measurements Fluorescence measurements were carried out using Flspectorophotomet (F-4600, Hitachi, Japan)at (25  0.1)  C. The slit widths for emission and excitation were fixed at 2.5 and 5.0 nm, respectively and pyrene concentration was kept at 5.00  106 mol/L for each solution. I1/I3 was used to indicate the intensity ratio of the first (373 nm) and the third (384 nm) vibronic peaks of pyrene. Transmission electron micrographs (TEM) To investigate micro-morphology of PFPE-Na aggregates in aqueous solutions, TEM was performed with a JEM-1011 transmission electron microscope (Jeol Company, Japan) at 100 kV. All sample solutions were deposited on a carbon-coated copper grid and negatively stained with 2 wt% phosphotungstic acid. Contact angle measurements The wetting ability of PFPE-Na was completed under air by using Krüss DSA 25 instrument (Krüss Company, Germany) to measure the contact angle. A paraffin film was chosen as the solid substrates. The temperature and environmental humidity were kept constant at (25  0.1)  C, (50  5) %, respectively. Results and discussion Characterization of PFPE-Na PFPE-Na was synthesized by hydrolysis reaction with PFPF and sodium hydroxide solution. The structures of PFPF and PFPE-Na were characterized by 19F NMR and FT-IR. Fig. 1 gives the 19F NMR spectra of PFPF and PFPE-Na. The signal at d = 101.9 ppm is the characteristic absorption peak of COFi and the signal between d = 2 to 3 ppm is the peak of CF3a. Influenced by adjacent groups, the signals of CF3e and CF3h move to the peaks between d = 4 to 7. The peak at d = 53.5 ppm is CF2b and the peaks between d = 54 to 55 ppm are CFd and CFg. The characteristic peaks of CF2f and CF2c move to low field in near d = 9 to 10 because of the effect of adjacent oxygen. It can be clearly identified that the functional group of COFi has completely disappeared in the product by analyzing the signal at d = 101.9 [10]. Fig. 2 shows the FT-IR spectra of PFPF and PFPE-Na. FT-IR results show little difference between raw material and product. The carbonyl

Electrical conductivity measurements Measurements of electrical conductivity were performed with a conductivity analyzer (model DDS-11A, Shanghai Leici-Chuangyi Instrument and Meter Co., Ltd., Shanghai, China). All surfactant solutions were prepared with distilled water putting at least 24 h before the measurements. The temperature was controlled by a thermostatic water bath and each electrical conductivity data is an average value measured three times to minimize data errors. Dynamic surface tension measurements The dynamic surface tension was determined by a Krüss BP100 bubble-pressure tensionmeter (Krüss Company, Germany, accuracy 0.01 mN/m) at (25  0.1)  C to achieve the measurement

Fig. 1.

19

F NMR spectra of PFPF and PFPE-Na.

Please cite this article in press as: Q. Yin, et al., Micellization and aggregation properties of sodium perfluoropolyether carboxylate in aqueous solution, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.07.026

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Table 1 Surface property parameters of PFPE-Na aqueous solutions at 25  C. Surfactant

gCMC (mN/m)

CMC (mol/L)

1010Γmax(mol/cm2)

Amin(Å2)

PFPE-Na NaPFOa AE3C-Nab

22.6 24.7 32.5

1.28  102 3.12  102 55.0 mg/g

2.06 2.92 –

80.81 56.89 –

a b

Fig. 2. FT-IR spectra of PFPF and PFPE-Na.

stretching vibration absorption peak moves from nearby 1883 cm1 to 1780 cm1 which is because the leaving of fluorine atom in acyl fluoride group reduces the electron-withdrawing inductive effect of carbon–oxygen double bond. From the 19F NMR and FT-IR spectra results, the target product of PFPE-Na was successfully obtained by the hydrolysis reaction [11]. Equilibrium surface tension The equilibrium surface tension is the commonest way to evaluate the surface activity of surfactants. Fig. 3 shows the surface tension (g ) versus concentration (c) plots obtained from the investigated PFPE-Na at 25  C. It is showed obviously that the surface tension decreases initially with the increasing of PFPE-Na concentration then up to a nearly constant value. The concentration and surface tension of the breakpoint is assigned as critical micelle concentration (CMC) and surface tension at CMC (gCMC), respectively. Obviously, no minimums appear around CMC indicating the high purity of PFPE-Na. The obtained CMC and gCMC values of PFPE-Na aqueous solutions are summarized in Table 1, together with those reported for sodium perfluorooctanoic acid (NaPFO) and hydrocarbon surfactants of polyether carboxylate [AE3C-Na, C12H25O(CH2CH2O)3COONa]. In Table 1, the gCMC values of PFPE-Na and AE3C-Na are 22.6 and 32.5 mN/m, respectively, which demonstrates that the ability of fluorocarbon surfactant to reduce surface tension is stronger than similar structure hydrocarbon surfactants. Fluorine is the most negative-electronic element, so negative charges gathered around fluorine atoms and electron cloud covered densely in fluorocarbon, which leads to decrease interaction force between the fluorocarbon chains and increasing hydrophobic effect to fluorocarbon chain than hydrocarbon chain. These two factors make fluorocarbon surfactant molecules tend to escape from aqueous solution

Fig. 3. The equilibrium surface tension as a function of log C for PFPE-Na at 25  C.

Reported in Ref. [12]. Reported in Ref. [13].

intensively. The CMC of PFPE-Na is 1.28  102 mol/L, much lower than the structure similar NaPFO. Although hydrophilic oxygen atoms are inserted into fluorine carbon chain, the value of CMC does not increase. Obviously, hydrophobicity is not degrading as fluorine carbon chain inserted with hydrophilic oxygen atoms. This is probably because the strong inductive effect of the perfluoroalkyl adjoining oxygen leads to the significant decrease of electron density of the covalent oxygen atoms, which prevent the hydration of oxygen [14,15]. To study the adsorption property of PFPE-Na at the air-solution interface, the maximum surface excess (Gmax) and the minimum surface area per surfactant molecule (Amin) can be estimated according to the Gibbs adsorption isotherm [16]:   1 dg Gmax ¼  ð1Þ 2:303nRT dlog C T

Amin ¼

1016 NA Gmax

ð2Þ

where R is the gas constant (8.314 J/mol1 K1), T is the absolute temperature, NA is the Avogadro’s constant, and (dg /dlog C)T is the slop of the surface tension g vs. log C below the CMC at a constant temperature T. The parameter n is the number of depended on the solute species whose concentration at the interface changes with the surfactant concentration. In this paper, n is taken as 2 based on an anionic surfactant made up of a monovalent surfactant ion without added electrolyte [16]. Both Gmax and Amin values obtained through these equations are summarized in Table 1. As is well known, a higher Gmax or a lower Amin value implies a closer arrangement of surfactant molecules at the air–liquid interface. Compared with NaPFO, the lower Gmax and higher Amin values of PFPE-Na indicate that the arrangement of its molecules at the air– water interface is looser.

Fig. 4. Plots of electrical conductivity versus concentration of PFPE-Na in aqueous solution at different temperatures.

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Table 2 Thermodynamic parameters of micellization process for PFPE-Na at different temperatures. T ( C)

CMC (mmol/ L)

b

DGum (kJ/mol) DHum (kJ/mol) T DSum (kJ/mol)

25 35 45 55

12.28 13.02 14.08 14.45

0.22 0.21 0.23 0.21

25.42 25.81 27.04 27.23

5.27 5.57 6.08 6.34

20.15 20.20 20.96 20.90

Electrical conductivity measurements The electrical conductivity of the PFPE-Na aqueous solutions at different temperatures were measured to characterize the thermodynamic properties of micellization and to evaluate the effect of temperature on their aggregation behavior [17]. Fig. 4 shows the variation of electrical conductivity at different temperatures. The experimental results fit into two straight lines with a substantial change of slope. The breakpoint of the curves corresponds to the CMC. The CMC values at different temperatures are summarized in Table 2. Obviously, the CMC value of PFPE-Na at 25  C obtained from the electrical conductivity method agrees well with those of surface tension measurements. It can be observed that the CMC values increase with the increasing of temperature. The impacts of temperature on CMC fall into two aspects. The formation of micelle is easier by decreasing the hydration of hydrophilic group with the increase of temperature. Meanwhile, the high temperature will against forming micelle by destroying the hydro-structure of surrounding hydrophobic carbon chains, which reveals that the latter factor plays a key role in the micellization for the PFPE-Na. According to the pseudo-phase model of micellization, for 1:1 type ionic surfactants, the standard Gibbs free energy of micellization DGum can be estimated by the following equation [18].

DGum ¼ RTð1 þ bÞInX cmc

ð3Þ

Where b is the degree of counterion binding of micelle which can be obtained by ratio between the slopes of the electrical conductivity versus concentration plots above and below CMC, R is the ideal gas constant, T is the absolute temperature, and XCMC is the CMC in molar fraction. Here the CMC values obtained by conductivity measurement are used to calculate the thermodynamic parameters of micellization for PFPE-Na. The micellization

Fig. 5. Dynamic surface tensions with surface age for different concentrations of PFPE-Na aqueous solution at 25  C.

exothermic. Moreover, it is obvious that the contribution of the T DSum term to the negative DGum is much larger than that of the

DHum term (Table 2), revealing that the micellization for the PFPENa in aqueous solution is entropy-driven in the temperature range investigated [19]. Dynamic surface tension measurements Dynamic surface tension is a frequently-used method to investigate the surfactant adsorption kinetics at the air-water interface [20–23]. It is more important than the equilibrium surface tension in applications which need fast adsorption [24,25]. The dynamic surface tensions of PFPE-Na in serial concentrations were performed by the maximum bubble-pressure technique which can achieve the measurement in milliseconds. Fig. 5 shows the variations of the dynamic surface tensions of PFPE-Na at different concentrations. It can be readily observed that the dynamic surface tension of all solutions decrease initially then approach an equilibrium value over time, The time required to attain the equilibrium decreases with the increasing of surfactant concentration. The surface tensions are very low at the beginning and decrease more rapidly with the increase of the concentration which implies that fluorocarbon surfactants diffuse fast and adsorb effectively at the air/water interface.

standard enthalpy DHum and the micellization standard entropy

DSum can be obtained by the Gibbs–Helmholtz equation as following [19]. DHum ¼

@DGum =T @1=T



DS ¼ u

ð4Þ

DGum  DHum



T

dInX cmc dT

DHum ¼ RT 2 ð1 þ bÞ

ð5Þ

ð6Þ

Estimated values of b and thermodynamic parameters of micelle formation are given in Table 2. It can be observed that the

DGum values of PFPE-Na and the DHum values are both negative in the investigated temperature range, which indicates that their micellization process in the aqueous solution is spontaneous and

Fig. 6. Plots of I1/I3 of pyrene versus the concentration of PFPE-Na at 25  C.

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Fig. 7. Negative-stained TEM images for PFPE-Na solutions (0.10 mol/L).

Aggregation behavior of PFPE-Na in aqueous solution The aggregation behavior of PFPE-Na in water was investigated using pyrene as fluorescent probes and TEM measurements by negative staining techniques. The value of I1/I3 emission intensity ratio for pyrene (I1 and I3 are the intensities of the first and third bands in the pyrene fluorescence spectrum as shown by 1 and 3 in the inset of Fig. 6) is very sensitive to the polarity of the medium surrounding pyrene molecules [26], i.e., the larger the ratio, the bigger the polarity of the medium. Fig. 6 shows a typical evolution of I1/I3 ratio as a function of PFPE-Na concentration. This ratio first drops slightly with increase in PFPE-Na concentration, and then decreases substantially above a critical concentration. The intersection of the two straight lines at the I1/I3 value is thought to be the CMC value [27,28]. As is seen in Fig. 6. the CMC value (1.26  102 mol/L) is very close to the result of surface tension experiment. The CMC measurements have provided evidence for the micellization process of PFPE-Na in water. The forming of unimolecular micelles in water below CMC is due to the weak interaction of carboxylic complex. Above CMC, the unimolecular micelles spontaneously aggregate together to form spherical vesicle-like aggregates (Fig. 7 clearly shows the presence of spherical aggregates with diameters of 50–500 nm in 0.10 mol/L solutions) leading to the greatly increased solubility of the fluorescent probes inside the hydrophobic domains of the aggregates, as similar reported earlier [29–31]. Contact angle measurements Many industrial processes essentially involve wetting processes, such as oil recovery, printing, liquid coating and spray quenching [25,32]. Dynamic contact angle is one of the most versatile methods to characterize the wetting ability of surfactant

aqueous solutions, the smaller contact angle, the better spreading and wetting ability [33]. The contact angles were measured by forming one droplet of PFPE-Na aqueous solutions on a paraffin film at 25  C. Fig. 8 shows the changes of contact angles of PFPE-Na aqueous droplets at different concentrations. It is observed that the contact angle decreases slowly with the increasing of surfactant concentration. This may be caused by the influence of water proof and oil repellent properties of fluorocarbon chain and no additional electrostatic attraction with solid surface. This is similar to previous reports that the contact angle on nonpolar solid of fluorocarbon surfactant solution is greater than hydrocarbon surfactant at the same surface tension [12]. Conclusions In summary, micellization and aggregation properties of PFPENa aqueous solution were investigated by various techniques including equilibrium surface tension, electrical conductivity, dynamic surface tension, steady-state fluorescence, TEM and contact angle. The research results of equilibrium surface tension and steady-state fluorescence have shown that the surface activity of PFPE-Na is much better than NaPFO, indicating PFPE-Na is a good substitute of NaPFO as an important chemical intermediates to synthesize perfluoroalkylpolyethers derivatives. The electrical conductivity results show that the CMC values increase with the increase of temperature, micellization process of PFPE-Na in aqueous solution is exothermic and entropy-driven in the temperature range investigated. Steady-state fluorescence and transmission electron microscopy (TEM) may indicate that PFPENa self-assemble in aqueous solution to form larger spherical aggregates with the increase of concentration. Measurements of dynamic surface tension at the air-water interface and contact angle on a paraffin film revealed PFPE-Na exhibited outstanding adsorption but general wetting ability at concentrations above the CMC as a special surfactant. This work will help us to find its potential application in pesticides, ink and coatings additives. Acknowledgments This project is funded by the National Science & Technology Pillar Program during the Twelfth Five-year Plan Period (Grant No 2014BAE03B03) and Natural Science Found of Shanxi Province (Grant No 2014011014-1 and201605D211008). We would also like to express our gratitude to Guojin Li of China Research Institute of Daily Chemical Industry for TEM observations. References

Fig. 8. Evolution of dynamic contact angle of droplets of aqueous PFPE-Na at various concentrations on parafilm at 25  C.

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Please cite this article in press as: Q. Yin, et al., Micellization and aggregation properties of sodium perfluoropolyether carboxylate in aqueous solution, J. Ind. Eng. Chem. (2016), http://dx.doi.org/10.1016/j.jiec.2016.07.026