Surface Activity, Spreading and Aggregation Behavior of Eco-friendly

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Surface Activity, Spreading, and Aggregation Behavior of Ecofriendly Perfluoropolyether Amide Propyl Betaine in Aqueous Solution Jixian Shen,† Yanyun Bai,† Xiumei Tai, Wanxu Wang, and Guoyong Wang* China Research Institute of Daily Chemical Industry, Taiyuan 030001, P. R. China ABSTRACT: The hydrophobic−oleophobic properties of perfluoropolyether (PFPE) are similar to those of fluorocarbons with seven or more long carbon chains, and it’s biodegradable properties make it a good alternative to conventional perfluorooctyl derivatives. In this study, a novel environmentally friendly fluorinated surfactant perfluoropolyether amide propyl betaine (PFPE-B) was synthesized and characterized by Fourier transform infrared spectroscopy and 1 H and and 19F NMR. Its surface activity, micellization, and adsorption performance at the gas−liquid interface and wettability and aggregation behavior in aqueous solution were systematically investigated by surface tension, electrical conductivity, dynamic surface tension, dynamic contact angle, dynamic light scattering (DLS), and transmission electron microscopy (TEM) techniques. The γcmc and critical micelle concentration (CMC) value for PFPE-B are about 16.87 mN·m−1 and 0.07 mmol·L−1, far less than that of cocoamidopropyl betaine (CAB), demonstrating that surfactant PFPE-B has higher surface activity than surfactants with a hydrocarbon chain. And, it was confirmed that the kinetics of adsorption at the interface increases with an increase of concentration. Contact angle measurements conducted on low-energy paraffin film and polytetrafluoroethylene surfaces demonstrated that PFPE-B exhibited efficient spreading above the CMC. Furthermore, DLS and TEM studies revealed that PFPE-B self-assembled in aqueous solution to form spherical vesicles. KEYWORDS: Fluorinated surfactant, Perfluoropolyether amide propyl betaine, Perfluoropolyether, Aggregation

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INTRODUCTION Fluorinated surfactants are special and important amphiphilic molecules, which are known as “super surfactants”.1−3 Fluorinated surfactants exhibit excellent chemical and thermal stability, biological inertia, high gas-dissolving capacities, and hydrophobic−oleophobic properties as well as high surface activities (both in terms of level and efficiency).4,5 For instance, the fluorinated surfactant can reduce remarkably the surface tension of water to 15−20 mN·m−1, much less than the hydrocarbon surfactants (30−40 mN·m−1) and siloxane surfactants (20−25 mN·m−1); in other words, fluorinated surfactants present the highest surface activity among all types of surfactants.6−8 These unique properties of fluorinated surfactants make them irreplaceable by other types of surfactants in many industrial applications, such as coatings, inks, insecticides, textiles, fire-fighting foams, electroplating processes, soil and stain repellents, etc.9,10 Fluorinated surfactants are also largely used for preparing reverse micelles or vesicles that can be used as carriers in biological fluids.11 The most commonly used fluorinated surfactants are perfluorooctanesulfonate (PFOS) and the ammonium salt of perfluorooctanoate (PFOA).12,13 However, the long-chain perfluoroalkyl perfluorinated surfactants, which had been listed as persistent organic pollutants (POPs), are difficult to degrade in the environment and can be accumulated in the food chain.14 The Fluoro-Council has agreed that fluorinated surfactants © XXXX American Chemical Society

could transfer from long-chain perfluoroalkyls to short-chain alternatives and novel fluorinated groups such as the perfluoropolyether (PFPE).15,16 However, the present alternatives are not so satisfactory.17 Hence, alternative research on novel fluorinated surfactants is extremely essential. Many strategies about synthesizing non-bio-accumulable alternatives to PFOA regard short-chain products. Wen et al.18 synthesized a series of hybrid surfactants containing separate fluorocarbon and hydrocarbon chains. Yoshimura et al.19 reported a cationic gemini surfactant with partially fluorinated spacers and studied its aggregation behavior in solution. But the research on perfluoropolyether alternatives is generally more poor than that on short-chain fluorocarbon surfactants. Perfluoropolyether surfactants have been proven to have better flexibility, biocompatibility, water solubility, and lower Kraft points than perfluoroalkyl perfluorinated surfactants. Mele et al.20 investigated the phase behavior of homologous perfluoropolyether surfactants, whose structures were composed by PFPE carboxylates with hydrophobic chain terminated by Cl−C3F6O and either sodium or ammonium counterions, via optical microscopy, small-angle x-ray scattering (SAXS), and 19F NMR. Caboi et al.21 studied the microReceived: December 27, 2017 Revised: March 12, 2018 Published: March 23, 2018 A

DOI: 10.1021/acssuschemeng.7b04895 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Sodium hydroxide (0.40 mmol, 1.60 g) and chloroacetic acid (0.40 mmol, 3.78 g) with a molar ratio 1:1 were dissolved in water to prepare sodium chloroacetate. PFPE-B was synthesized through a quaternization reaction between the intermediate and sodium chloroacetate. The intermediate and sodium chloroacetate were mixed in absolute ethanol and stirred at 50 °C for 10 h. After the insoluble byproduct and excessive sodium chloroacetate were filtered off, the solvent of the separated liquid was removed with the helping of rotary evaporation apparatus. The products were dissolved in acetone, filtered, evaporated, and dried under vacuum drying conditions to give PFPE-B. The yield was 20.20 g (79.9%). Characterization of PFPE-B. The chemical structures of PFPF, intermediate, and PFPE-B were characterized by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (NMR). A Bruker Vertex-70 spectrometer was used to measure FT-IR spectra. 1H and 19F NMR spectra of the synthesized perfluoropolyether surfactant and intermediate were obtained using a Varian INOVA-400 MHz instrument with C3D6O as solvent. Measurements of the Properties of PFPE-B in Aqueous Solution. Equilibrium Surface Tension. The equilibrium surface tension of the aqueous solutions of the surfactant PFPE-B was measured with a Krüss K12 tensiometer (Krüss Company, Germany) by the De-Nuöy ring method at 25.0 ± 0.1 °C. Before the measurement, the prepared solutions were stored for 24 h to keep equilibrium. For the samples of each concentration, the surface tension value is an average value measured three times to minimize data errors. Electrical Conductivity. The electrical conductivity was prepared by a conductivity analyzer (model DDS-11A, Shanghai Leici-Chuangyi Instrument and Meter Co., Ltd., Shanghai, China). The electrical conductivity measurements were controlled by a thermostatic water bath with the uncertainty of ±0.1 °C. Dynamic Surface Tension. The dynamic surface tension (DST) was measured using a Krüss BP100 bubble-pressure tensionmeter (Krüss Company, Germany, accuracy ±0.01 mN·m−1) by a method that involves the measurement of the maximum pressure necessary to blow a bubble in a liquid from the tip of a capillary at 25.0 ± 0.1 °C. The measurements were conducted with effective surface ages ranging from 10 ms to 200 s. Contact Angle. The compound PFPE-B’s wettability was measured with contact angle using a Krüss DSA 25 instrument (Krüss Company, Germany). The polytetrafluoroethylene (PTFE) film and paraffin film were used as base plates on which to measure the contact angle of the PFPE-B solution. The temperature and environmental humidity were kept constant at 25.0 ± 0.1 °C, 50 ± 5%, respectively, by the watercirculating bath (Haake Company, Germany). Dynamic Light Scattering (DLS). DLS was performed in a Zeta Plus Particle Size Analyzer instrument (Brookhaven, USA), to investigate the effective diameter and size distribution of the PFPE-B aggregates in aqueous solution. Transmission Electron Microscopy (TEM). The micromorphology of PFPE-B aggregates in aqueous solutions were analyzed in negatively stained transmission electron micrographs obtained with a JEM-1011 transmission electron microscope (Jeol Company, Japan) at 100 kV. Samples were stained with 2 wt % phosphotungstic acid on a carboncoated copper grid before TEM scanning.

structure of the PFPE ammonium carboxylates at high concentration. Although some research on the phase behavior of perfluoropolyether surfactants has been made by scholars, few reports are reported about synthesis, wettability, adsorption, and aggregation. In this paper, a new type of betaine amphoteric surfactant was synthesized using N,N-dimethyl-1,3-propyldiamine, sodium hydroxide, chloroacetic acid, and 2,5-bis(trifluoromethyl)-3,6dioxaundecafluorononanoyl fluoride (PFPF) as raw materials. Scheme 1 shows the synthesis route of the perfluoropolyether Scheme 1. Synthetic Routes and Acronyms of Perfluoropolyether Amide Propyl Betaine

amide propyl betaine (PFPE-B). The structure of the product was characterized by Fourier transform infrared spectroscopy and 1H and 19F NMR. We describe physicochemical properties such as equilibrium surface tension, electronic conductivity, dynamic surface tension, dynamic contact angle, and aggregation behavior of PFPE-B. Furthermore, we expect the present work could promote to extend the scope of the knowledge of fluorocarbon surfactant systems, e.g., wettability and aggregation behavior. Besides, the PFPE-B surfactant has the potential to replace PFOS surfactants in aqueous filmforming foam (AFFF) for spontaneous spreading and film formation.22

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EXPERIMENTAL DETAILS AND METHODS

Materials. 2,5-Bis(trifluoromethyl)-3,6-dioxaundecafluorononanoyl fluoride (98%, PFPF) was purchased from Lisheng Regent Biochemistry Technology Co., Ltd. (Guangdong, P.R. China). N,N′Dimethyl-1,3-propyldiamine (99%) was obtained from Aladdin. Triethylamine (99%), sodium hydroxide (99.5%), and chloroacetic acid (98%) were all obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, P.R. China). Diethyl ether, acetone, and ethyl alcohol absolute were all purchased from Tianjin Shentai Chemical Reagent Co. (Tianjin, P.R. China). The water we used in all experiments was deionized water (18.2 MV). Synthesis of PFPE-B. 2,5-Bis(trifluoromethyl)-3,6-dioxaundecafluorononanoyl fluoride (0.40 mmol, 20.01 g) was dropwise added to a stirred solution of N,N′-dimethyl-1,3-propyldiamine (0.41 mmol, 4.18 g) in diethyl ether containing triethylamine (0.41 mmol, 4.14 g) at 0 °C for 2 h. Then, the mixture refluxed for over 5 h under the protection of nitrogen at 45 °C. After the solvent and excess triethylamine were evaporated under reduced pressure, the residue, which was the impure intermediate of PFPE-B, was washed five times with deionized water and dried under vacuum for 24 h to yield the intermediate in the form of yellow oily liquid (21.61 g).

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RESULTS AND DISCUSSION Synthesis and Characterization of PFPE-B. PFPE-B was synthesized from 2,5-bis(trifluoromethyl)-3,6-dioxaundecafluorononanoyl fluoride (PFPF), sodium hydroxide, chloroacetic acid, and N,N′-dimethyl-1,3-propyldiamine according to Scheme 1. The structures of reactant, intermediate, and product were characterized by FT-IR and 1H and 19F NMR. Figure 1 shows the FT-IR spectra of the reactant (PFPF), intermediate, and product (PFPE-B). The carbonyl stretching vibration absorption peak of PFPF is at a wave number of 1884 cm−1 in Figure 1 (1), which moves to 1710 cm−1 in Figure 1 (2, 3) because the fluorine atom leaving in acyl fluoride group B


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Figure 1. FT-IR spectra of (1) PFPF, (2) the intermediate, and (3) PFPE-B.

Figure 3. 19F-NMR of PFPF, the intermediate, and PFPE-B.

Surface Activity of PFPE-B Solutions. PFPE-B surfactant shows good water solubility at 25 °C. The surface activity of PFPE-B was determined by static surface tension measurement. Figure 3 shows the surface tension (γ) versus concentration (C) plots obtained for the investigated PFPE-B aqueous solution at 25 °C. It is clearly that the static surface tension decreases significantly with the increasing concentration of PFPE-B, above which a near plateau region of surface tension appears, suggesting that the surface adsorption reached equilibrium. The breakpoint in the γ versus C plots is assigned to the critical micelle concentration (CMC), which is consistent with commercial surfactant behavior. From Figure 4, it is observed that PFPE-B exhibits relatively low CMC values of 0.07 mmol·L−1 and low γcmc values of ∼16.81 mN· m−1.

reduces the electron-withdrawing inductive effect of the carbon−oxygen double bond. From Figure 1, it was observed that the sharp peaks at 2965−2850 cm−1 and around 1465 cm−1 are the characteristic peaks of −CH3 and −CH2−. And, the presence of the wide adsorption at 3398 cm−1 is the stretching vibration for N−H of the group −CONH−, which had a clear absorption peak at 1708 and 1537 cm−1. Therefore, complete PFPE-B was identified by the appearance of the peak at 1634 cm−1. Figure 2 shows the 1H NMR spectra of the intermediate and PFPE-B (CD3COCD3, trifluoroacetic acid as an internal standard). The strong peak at 2.04 ppm is attributed to the CD3COCD3.

Figure 2. 1H NMR of the intermediate and PFPE-B.

Figure 4. Surface tension plots as a function of concentration for PFPE-B in aqueous solutions at 25 °C.

Figure 3 gives the 19F NMR spectra of PFPF, the intermediate, and PFPE-B (CF3COOH). The external standard method was used in the 19F NMR spectra measurement, and the strong peaks at 0 ppm are attributed to the CF3COOH. It can be clearly identified that the functional group of −COF has completely disappeared in the intermediate and product by analyzing the signal at δ = 102.19 ppm. There were no significant differences in 19F NMR spectra of the intermediate and PFPE-B. So, the results of FT-IR and NMR confirmed that PFPE-B has been synthesized successfully.

For a deeper discussion the surface activity of PFPE-B, the characteristic parameter of the surfactant were compared with other types surfactants reported in literatures, cocoamidopropyl betaine (CAB) which had the same chemical structure just different in hydrophobic group and sodium perfluoropolyether carboxylate (PFPE-Na) with the same hydrophilic fluorocarbon chain was just different in hydrophilic group. The chemical structure of CAB and PFPE-Na are described in Scheme 2.23,24 The values of the characteristic parameter of the surfactants are summarized in Table 1. C


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superior in effectiveness and efficiency of surface tension reduction. This also demonstrated that the perfluoropolyether surfactant has the predominant surface performance than hydrocarbon surfactant with similar structure. The results can be ascribed to a stronger hydrophobic interactions from the perfluoropolyether chain having the strong intramolecular bonds and weak intermolecular interactions. The result of γcmc and CMC also compared to PFPE-B and PFPE-Na discovered that the surface activity of the former is higher. This indicates that hydrophilic groups have an effect on the surface activity and the critical micelle concentration.27 From the table, the maximum surface excess concentration, Γmax, and the minimum area per molecule at the air−liquid interface, Amin, can also be noted. Comparing to the PFPE-Na, the Γmax value of PFPE-B is higher than PFPE-Na while the Amin value is lower, indicating a more densely arrangement of PFPE-B molecules at the air−liquid interface. Different hydrophilic head groups lead to different molecular arrangement even with the same hydrophobic group. The electrostatic repulsion between head groups may contribute to the higher Γmax and lower Amin values of PFPE-B. It is well denoted that the electrostatic repulsion between head groups of PFPE-B is less than that of PFPE-B’s. Micellization of PFPE-B Solutions. Electrical conductivity measurements were carried out to characterize the micellization thermodynamic properties of PFPE-B.28,29 Figure 5 shows the

Scheme 2. Chemical Structure of PFPE-B, CAB, and PFPENa

To explore the effectiveness of surface tension reduction, Πcmc and pC20 of the PFPE-B at the air−water interface could be obtained on account of the surface tension plots. It is known that C20 refers to the concentration of surfactant when the surface tension of pure solvent was reduced by 20 mN·m−1. The parameters, Πcmc and pC20, can be determined as follows:25 1 pC20 = lg C20 (1)

∏ = γ0 − γcmc

(2)

cmc

Where γ0 is the surface tension of deionized water and γcmc corresponds the surface tension at concentration CMC at 25 °C. The maximum surface excess concentration (Γmax) at the air−water interface was calculated using the Gibbs adsorption isotherm.26 Γmax = −

⎛ ∂γ ⎞ 1 ⎜ ⎟ 2.303nRT ⎝ ∂lgC ⎠

(3)

T −1

−1

where R is the gas constant (8.314 J·mol ·K ), T denotes the absolute temperature, γ is the surface tension of the surfactant aqueous solution, and C is the concentration of the surfactant aqueous solution. From the maximum surface excess concentration, the area occupied per surfactant molecule (Amin) at the air−water interface is obtained by using the following expression. 1 A min = NA Γmax (4)

Figure 5. Electrical conductivity as a function of concentration for PFPE-B in aqueous solutions at different temperature: ■ = 25, ● = 35, ▲ = 45 °C.

variation in conductivity with concentration at the different temperature. It is obvious that the trend of conductivity with concentration is consistent with two straight lines with different slopes and the breakpoint of the curves corresponds to the critical micelle concentration (CMC). All the values of the thermodynamic parameters at different temperature are summarized in Table 2. The value of CMC obtained by conductivity method and the surface tension measurement is in

Here, NA is Avogadro’s constant (6.022 × 1023 mol−1). The estimated surface property parameters for PFPE-B, together with the previously reported data of CAB are all listed in Table 1. As observed in Table 1, the γcmc and CMC values of PFPE-B are both lower than those of CAB, revealing that PFPE-B is

Table 1. Aggregation and Adsorption Parameters for PFPE-B Aqueous Solutions at 25 °C

a

surfactant

CMC (mmol·L−1)

γcmc (mN·m−1)

pC20

Γmax (μmol·m−2)

Amin (nm2)

PFPE-B CABa PFPE-Nab

0.07 3.65 1.28

16.87 26.76 22.60

5.52

2.16

0.77

2.06

0.81

Reported in ref 23. bReported in ref 24. D


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ACS Sustainable Chemistry & Engineering Table 2. Thermodynamic Parameters of Micellization for PFPE-B at the Different Temperature T (°C)

103 CMC (mmol·L−1)

β

ΔGθm (kJ·mol−1)

ΔHθm (kJ·mol−1)

−TΔSθm (kJ·mol−1)

25 35 45

73.12 79.33 83.25

0.14 0.17 0.20

−37.38 −40.34 −42.56

−5.48 −6.01 −6.54

−31.90 −34.33 −36.02

good agreement at 25 °C. From Figure 5, it can be seen that the CMC values increase with temperature. The standard Gibbs free energy of micelle formation (ΔGθm) can be calculated from the following equation. The standard enthalpy of micellization (ΔHθm) and standard entropy of micellization (ΔSθm) also can be obtained. ΔGθm = (1 + β)RT ln Xcmc ΔH θm = −(1 + β)RT 2

dln Xcmc dT

ΔSθm = (ΔHmθ − ΔGmθ )/T

(5)

(6) (7)

Where Xcmc stands for the value of CMC converted to the volume mole fraction and can be calculated from Xcmc = CMC/ 55.4. β represents the degree of counterion binding at micelle/ solution interface, expressed by β = 1 − α (α can be obtained by the ratio between the slopes of conductivity curves above and below the CMC). As shown in Table 2, all the ΔGθm values are negative in the different temperature range, implying that the micellization of PFPE-B in aqueous solution is a spontaneous process. It found that the value of ΔHθm is significantly less than the −TΔSθm, in the considered temperature ranges, which indicates the entropic driving is a primary contributor to micellization. In the aggregation process, the hydration water around the hydrophobic tail chain is released, which leads to the increase of entropy. Furthermore, the value of ΔGmθ decreases with increasing temperature, manifesting that increasing temperature can hinder the formation of micelles in the investigated temperature range. Adsorption Kinetics of PFPE-B at the Air−Water Interface. The dynamic surface tension was performed using a maximum bubble−pressure technique to investigate the kinetic adsorption at the air−water interface for different concentrations.30 The variations of the dynamic surface tension of the surfactant PFPE-B as a function surface age PFPE-B at different concentrations in Figure 6 is shown. The gradual decreasing in surface tension with surface age represents the diffusion of PFPE-B molecules to the air−water interface and approach to equilibrium conditions. As can be seen in Figure 6, the dynamic surface tension reduction was faster and greater as the PFPE-B concentration increased, indicating that the effect of elevated concentration is helpful to decrease the adsorption time at the air−liquid interface. The effect of concentration on adsorption behavior is consistent with the Fick’s first law, the higher concentration of bulk phase results in better diffusion rate and more evident change of dynamic surface tension. There are usually two sequential steps to realize the adsorption process: First, surfactant molecules diffusing from the bulk aqueous phase to the subsurface caused by the concentration gradient, and in the following step, the surfactant molecules adsorb to the air−liquid interface from the subsurface. Generally, the adsorption process can be reasonably assumed to be diffusion controlled, due to the diffusion rate being much slower than the adsorption rate. For the aqueous

Figure 6. Dynamic surface tensions with surface age for PFPE-B aqueous solutions at 25 °C: (black) = 0.01, (red) = 0.02, (light blue) = 0.06, (green) = 0.11, (pink) = 0.22, (yellow) = 0.48, (dark blue) = 1.11 mmol·L−1.

solutions at pre-CMC concentrations, the Word−Tordai equation can describe the diffusion-controlled adsorption well on the fresh surface to illustrate the adsorption process:31 Γ(t ) = 2C0

Dt D −2 π π

∫0

π

Csd( t − τ )

(8)

Where Γ(t) is the surface excess concentration at time t, D is apparent diffusion coefficient, C0 is the bulk concentration, Cs is the concentration at the subsurface when t = 0, and τ refers to the dummy variable. The 2C0 Dt /π refers to the migration of molecules from bulk phase to the subsurface, and the π

2 D/π ∫ Csd( t − τ ) denotes that molecules diffuse 0 from subsurface back to bulk phase with the increase of subsurface concentration. Thus, then finally eq 8 can be simplified for two cases of short time and long time, as shown in eqs 9 and 10, respectively.

Short time γ(t )t → 0 = γ0 − 2nRTC0

Dt π

(9)

Long time γ(t )t →∞ = γeq +

2 nRT Γ eq

2C0

π Dt

(10)

where T is the absolute temperature, γ0 is the surface tension of pure solvent (water), γ(t) is the dynamic surface tension at time t, γeq is the equilibrium surface tension at infinite time, Γeq is equilibrium surface excess concentration obtained from the equilibrium surface tension measurement, and n is considered to be 2 for an ionic surfactant system. The plots of γ versus t1/2 based on eq 5 and γ versus t−1/2 based on eq 6 show a linear relationship, calculating the apparent diffusion coefficient from the slope of data plots. The values of the diffusion coefficients E


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ACS Sustainable Chemistry & Engineering Table 3. Apparent Diffusion Coefficients of PFPE-B at 25 °C concentration (mmol·L−1) 0.01 0.02 0.06

gradient (kt→0) −6.78 −9.17 −18.23

DS (m2·s−1) −8

1.47 × 10 6.72 × 10−9 2.95 × 10−9

gradient (kt→∞)

DL (m2·s−1)

DS/DL

271.70 220.18 136.64

5.67 × 10−11 2.17 × 10−11 6.25 × 10−12

259.26 309.68 472.00

hydrophobic surface was used as the substrate to determine the wettability of different PFPE-B concentrations of aqueous solutions.

at short-time (DS) and long-time (DL) of PFPE-B at pre-CMC concentrations are calculated by eqs 9 and 10, which are listed in Table 3. For the surfactant PFPE-B, the absolute gradient’ value of the plots in Figure 7 increased with concentration, which is

Figure 7. Dynamic surface tension as a function of t1/2 and t−1/2 for PFPE-B.

consistent with the fact indicated in Figure 6 that the time required to attain the equilibrium surface tension decreases with the increasing concentrations. From Table 3, both the diffusion coefficients DS and DL decrease with the increases of the PFPE-B concentration. This is because the intermolecular electrostatic repulsion is stronger with the increasing concentration and, thus caused the free direction of the surfactant movement being impeded. When the value of DS/DL was small, the adsorption process was controlled by a diffusion process and the adsorption barrier did not exist. However, the numeric range of DS/DL is from 259.26 to 472.00, meaning that the adsorption process is controlled by mixed diffusion−kinetic adsorption. Wettability of PFPE-B Solutions on Low Energy Surface. Wettability is a very important surface property in daily life and production, such as washing, flotation, decontamination, oil recovery, paint, coating, and deposition.32 The contact angle is a preferable criterion with which to measure wettability. The dynamic contact angle with surfactant PFPE-B aqueous solutions by the sessile drop method is presented in Figure 8. The PTFE and paraffin film as the

Figure 8. Contact angles versus time with different concentrations on Parafilm and PFPE at 25 °C: (square) = 0.01, (circle) = 0.02, (uppointing triangle) = 0.06, (down-pointing triangle) = 0.11, (diamond) = 0.22, (left-pointing triangle) = 0.48, (right-pointing triangle) =1.11 mmol·L−1.

It is obvious that the trends of contact angle on paraffin film at different concentrations are consistent with that on PTFE substrate, their contact angle decrease with the continuous of time. Indeed, the wetting (θ ≤ 90°) was observed on both PTFE and paraffin film at a concentration above CMC or at least very close to CMC. It is noteworthy that it seems that the contact angle is static without an obvious trend of decline at a concentration as low as 0.01 mmol·L−1. This may be because of the large surface tension along the interface of the gas/liquid and almost no adsorption on the interface of liquid/solid at the low concentration. For comparison, the new fluorocarbon surfactant PFPE-B has a larger contact angle on PTFE than on paraffin film at the same concentration. This is because PTFE substrate has a lower surface energy and a higher surface energy F


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unimodal distribution. In the concentration 0.3 wt %, the unimodal peaks can be observed with diameters from 150 to 296 nm. And the aggregate diameters of PFPE-B ranges from 160 to 360 nm. The result of sizes distribution of aggregate was much larger than the spherical micelles whose diameter is around 5 nm, indicating that large aggregates such as vesicles might exist in these solutions. To gather more detailed properties of PFPE-Bs aggregates in detail, TEM was used to visualize their ultrastructure. Figure 10

can offer conductive thermodynamic condition for wetting. The spreading coefficient S is introduced to analyze the wetting process. S = γsg − (γsl + γlg)

(11)

Where γsg, γsl, γlg are the solid/gas, solid/liquid, and liquid/gas surface tensions, respectively. Adsorption of surfactant on the substrate can decrease both the γsl and γlg and in this way increase the spreading coefficient. From Figure 8, it can be seen that the contact angle decrease rapidly between 0−30 s at high concentration. Initially, the adsorption on both liquid/gas and liquid/solid interface promotes spreading. Moreover, the paraffin film can be completely wetted by aqueous solutions of PFPE-B above concentration 1.11 mmol·L−1, a capability which is not available for almost all the common hydrocarbon surfactants. Aggregation Behavior of PFPE-B in Solution. As with hydrocarbon surfactants, fluorocarbon surfactants in aqueous solution can self-assemble into a variety of aggregates, such as micelles, vesicles, and liquid crystals.33 To investigate the aggregation behavior of PFPE-B at concentrations above the CMC in detail, dynamic light scattering (DLS) and negativestaining TEM measurements were used to further investigate the self-assembly of PFPE-B aqueous solutions. The dynamic light scattering (DLS) was used to analyze the hydrodynamic diameters distribution of the aggregate formed at different concentrations for PFPE-B and the intensity-weighted distribution graphs are shown in Figure 9. For the two concentrations, the sizes distribution of aggregates shows

Figure 10. Negatively stained transmission electron microscopy (TEM) images of aggregates formed in PFPE-B: (a) 0.3 and (b) 0.5 wt %.

shows TEM images of PFPE-B aqueous solutions at two different concentrations. The micrographs in Figures 10 clearly show the existence of vesicles, whose structure is a hollow sphere. In the electron micrographs, the presence of vesicles with a wide range of diameters from 150 to 360 nm were observed. In addition, the sizes of the aggregates observed by TEM were in agreement with those derived by DLS. The aggregation process of surfactant is mainly driven by hydrophobic forces in aqueous solution.34 Fluorocarbon surfactant has more hydrophobic properties. Israelachvili and Mitchel proposed a critical packing parameter theory, which is defined as34 P = v /α0lc

(12)

Where P is the critical accumulation parameter, ν is the volume of the hydrophobic moiety, α0 corresponds to the area of the hydrophilic head in a closely arranged monolayer, and lC is the average chain length of hydrophobic chains. Generally speaking,

Figure 9. Intensity-weighted size distributions of PFPE-B surfactant: (a) 0.3 and (b) 0.5 wt %. G


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the P value of ordinary hydrocarbon surfactant (single-chain molecule) with a large hydrophilic headgroup compared to its tail usually was less than 0.5, which means that aggregation is usually in the state of micelles. When the P value in the range of 0.5 to 1, surfactant molecules are more likely to aggregate into vesicles. In our study, the calculation of P is difficult because the molecular volume and chain length of perfluoro-polyether tails cannot be obtained exactly. However, the hydrophobic group of the perfluoro-polyether tail is larger in volume and hydrophobic than hydrocarbon surfactant, rendering vesicle formation favorable.

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CONCLUSION A novel amphoteric perfluoropolyether surfactant (PFPE-B) was synthesized and characterized by FT-IR and 1H and 19F NMR. PFPE-B exhibited excellent surface activities (γcmc = 16.87 mN·m−1, CMC = 0.07 mmol·L−1). The electrical conductivity results demonstrated that the micellization of PFPE-B in water is a spontaneous process and CMC increases with the temperature increase. It was shown that the kinetics of adsorption at the air−water interface was significantly improved by concentration. However, the diffusion coefficient of the monomer for the PFPE-B was decreasing with the increasing of concentration. The wetting ability of the surfactant was quite good, as even the hydrophobic surface of the wax film could reach completely wetted. Furthermore, the contact angle could be the stable situation within 40 s, indicating that it would take 40 s to achieve adsorption equilibrium at liquid−solid interface. Aggregates of the surfactant PFPE-B formed in aqueous solutions are vesicles as suggested by DLS and TEM. Furthermore, the hollow vesicle structure may be widely utilized in situations that call for micro-sized dimentions, the release of drugs, gene engineering, and biomineralization. So, PFPE-B could be used as a vehicle for drug delivery, as a model system of a biomembrane, or a wetting agent in the pesticide or printer ink industry.

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AUTHOR INFORMATION

Corresponding Author

*Mailing address: China Research Institute of Daily Chemical Industry, 34 Wenyuan Street, Taiyuan, Shanxi Province, 030001 P.R. China. E-mail: [email protected]. ORCID

Guoyong Wang: 0000-0002-8308-4443 Author Contributions †

J.S. and Y.B. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is funded by National key R & D plan (grant no. 2017YFB0308704) and Shanxi Science and Technology Department (grant no. 201705D211008). We would also like to express our appreciation for JALA Research Found Projects.

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REFERENCES

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Surface Activity, Spreading and Aggregation Behavior of Eco-friendly

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