The Rebirth of Waste Cooking Oil to Novel Bio-based Surfactants

Waste cooking oil (WCO) is a kind of non-edible oil with enormous quantities and its unreasonable dispose may generate negative impact on human life and environment. However, WCO is certainly a renewable feedstock of bio-based materials. To get the rebirth of WCO, we have established a facile and high-yield method to convert WCO to bio-based zwitterionic surfactants with excellent surface and interfacial properties. The interfacial tension between crude oil and water could reach ultra-low value as 0.0016 mN m−1 at a low dosage as 0.100 g L−1 of this bio-based surfactant without the aid of extra alkali, which shows a strong interfacial activity and the great potential application in many industrial fields, in particular, the application in enhanced oil recovery in oilfields in place of petroleum-based surfactants.

washed with water and then dried. In the first step (as shown in Fig. 1), the mixture of fatty acids from WCO was prepared using alkaline hydrolysis in yield of 98.0%. The methyl trans-esterification products of WCO were analyzed using GC-MS chromatography. The main fatty acid composition of WCO was presented in Table 1. The composition of WCO would change in different area or different diet custom. Nonetheless, the modification of fatty acids mixtures according to our previous work is applicative 21 . The chemical modification of instable double bonds in oleic/linoleic/linolenic acid molecules was first carried out through Friedel-Crafts alkylation reaction using benzene as alkylation reagent and AlCl 3 as catalyst. Then the chemical modification of the carboxyl group of all fatty acids was accomplished  Surface properties. Variation of the surface tensions (SFT) with the concentration of PFAPMB at 25.0 o C was illustrated in Fig. 2. Surface properties (calculation method and formula were listed in Supplementary Information, Section "Surface properties") of PFAPMB were listed in Table 2. As seen in Fig. 2, SFTs were decreased gradually with the increase of surfactant concentrations. An obvious inflection point was obtained in the plot of SFT versus surfactant concentration, as indicated e.g. by the arrow in Fig. 2, which is named as the critical micelle concentration (CMC). The CMC of PFAPMB was extremely low as 0.734 mg L −1 with the SFT at CMC (SFT CMC ) of 28.4 mN m −1 . The CMC of PFAPMB was three orders of magnitude lower than other bio-based surfactants and the max surface excess concentration was almost two times of others' 18 .
Interfacial properties. Dynamic interfacial tensions (IFT) between Daqing crude oil (the composition and fundamental properties of the crude oil were listed in Supplementary Information, section "Material") and water at the different concentrations of PFAPMB in water solutions at 50.0 o C (the average temperature of the stratum in Daqing oil field, China) were shown in Fig. 3(a). When the interfacial tension between the surfactant solution and crude oil arrives at ultra-low level (<10 −2 mN m −1 ), the efficiency of oil recovery will be remarkably enhanced 22,23 . As Fig. 3(a) showed, the minimum interfacial tension between oil and different concentration solutions of PFAPMB decreased gradually with the increase of concentrations. The minimum of (IFT min ) could reach 0.0016 mN m −1 at the concentration of 0.100 g L −1 PFAPMB. Considered in the application, achieving ultra-low interfacial tension is the primary goal, the lower usage of the surfactants is also preferred. So, the subsequent experiment selected the concentration of 0.020 g L −1 as the research basis, at which concentration the IFT min was 0.0032 mN m −1 . There is an interesting phenomenon, when the concentration of PFAPMB was much lower than single surfactant in our precious work 21 , better reduction of IFT could be obtained. This is due to the synergism among the surfactant analogues. Mixture-chain surfactants are known to have superior surface/interfacial performance to single-chain counterparts owing to the enhanced ability of molecular self-assembly 18,19 . In the reaches of surface chemistry, several surfactants were frequently compounded together 24,25 , in order to reduce the dosage of surfactants and to achieve lower surface/  interfacial tensions. In this case, the starting material was the mixture of fatty acids; the final product was also a mixture, and PFAPMB is a naturally surfactant compound system showing great synergism. The effect of adding extra Ca 2+ /NaCl on dynamic interfacial tensions between Daqing crude oil and 0.020 g L −1 PFAPMB solutions at 50.0 o C were tested respectively and the dynamic interfacial tensions were shown in Fig. 3(b) and (c). The influence of electrolyte on interfacial properties of PFAPMB were presented in Table 3. From the results of Fig. 3(b), 3(c) and Table 3, PFAPMB has strong electrolyte tolerance. When the concentration of Ca 2+ was below 300 mg L −1 , the dynamic interfacial tensions between Daqing crude oil and 0.020 g L −1 PFAPMB solutions could remain in ultralow range. When the concentration of NaCl was below 10.0 g L −1 , viz. the total dissolved substance was below 15000 mg L −1 , PFAPMB solutions still showed excellent interfacial properties. Salinity and divalent ions have important effect on the interfacial properties of surfactants. The application of anion surfactants has been limited due to the complexation between surfactants and ions in high-salinity and high-calcium oil fields. In China, the salinities of most oil fields are commonly below 16000 mg L −1 and the concentrations of Ca 2+ are below 300 mg L −1 26 . The present results suggested that PFAPMB had strong electrolyte tolerance, and it could remain good interfacial properties in a wide range of salinity. This observation had provided the possibility for its application in most oil fields of China.
Wetting, emulsification and foaming properties. The contact angle θ average of 0.500 g L −1 PFAPMB solution was 38.79 o , which was measured after adding the solution for 60 s. The emulsification property was determined by recording the time taken for separation aqueous phase solutions from adequately blending mixture of liquid paraffin and 0.500 g L −1 PFAPMB solution. The time for separation 3 mL and 5 mL aqueous phase solutions were 170 s and 302 s, respectively. The foaming property of 0.500 g L −1 PFAPMB solution was obtained according to Ross-Miles test at 40 o C. The initial foam height and foam height after 10 min were 73 mm and 70 mm, respectively. Biodegradation estimation. Ultimate biodegradation scores for bio-based zwitterionic surfactants in PFAPMB were from 2.64 to 2.92. The bigger score indicates that the expected total degradation speed is faster. Scores above 2.8 are in the order of "weeks", and scores between 2.0 to 2.8 were in order of "months". The expected total degradation time was in the order of "months" for PFAPMB. These results also demonstrate that PFAPMB are biodegradable, and the degradation time is suitable for EOR, in which the stability of surfactants for several months is needed.

Discussion
In this work, the rebirth of WCO by converting it to bio-based zwitterionic surfactants was achieved via a facile and high-yield chemical modification. It is satisfactory that PFAPMB exhibits excellent interfacial properties, and the interfacial tensions between Daqing crude oil and 0.100 g L −1 PFAPMB solutions could reach ultralow value, as 0.0016 mN m 1 . Ultra-low IFT between oil and water is necessary for EOR in oilfields in the absence of alkali. As a comparison, alkali, was added with petroleum-based surfactants to reduce oil-water interfacial tension to ultralow values and enhance oil recovery currently [27][28][29] , which has resulted in well bore scaling, stratum damage and permeability decline 30,31 . Thus the bio-based zwitterionic surfactants derived from WCO could be considered as a potent alternative in oil recovery to increase oil displacement efficiency and to reduce the dosage of alkali in oil fields. Additionally, the solution of PFAPMB showed good spreadability on the hydrophobic solid substrates, stable emulsification property, strong foaming power and appropriate biodegradability. These results all demonstrate that PFAPMB has wide potential applications. In view of abundant starting material, outstanding surface/interfacial properties and biodegradability, bio-based zwitterionic surfactants derived from WCO will be of great interests for many surfactants application fields, such as detergents, cosmetics, bioremediation, etc, and more specifically in enhanced oil recovery. At the same time, the negative impact of WCO to environment and human could be considerably reduced. Waste cooking oil is not waste anymore, it could get its gorgeous rebirth and create more values.

Methods
Pretreatment of WCO. WCO collected from canteens and restaurants was firstly pretreated through filtration and centrifugation. The upper oil phase was acidified, filtered and washed with water. The starting material was obtained after dehydration. 20.00 g WCO (saponification value: 177.83 mg KOH g −1 ) and 30 mL NaOH methanol/water solution (3 M, v methanol /v water = 2:1) were added into a round bottom flask. The reaction mixture was refluxed at 70 °C for 6 h. After the hydrolysis, the mixture was neutralized and further acidized using hydrochloric acid (6 M), until the pH value of the solution was about 2. Then 50 mL hot water was added to separate fatty acids and glycerol. The aqueous phase was removed. The oil phase was washed by hot water for 5 times, and the pH value of the aqueous phase was about 6. Then the oils phase was dried using cyclohexane as water-carrying agent.

Synthesis of bio-based zwitterionic surfactants derived from WCO.
The acid values before and after the hydrolysis process were 1.37 mg KOH g −1 and 174.33 mg KOH g −1 . So the hydrolysis efficiency was 98.0%. 5.00 g (18 mmol) of mix fatty acids and 2.40 g (18 mmol) of AlCl 3 were added to 25 mL benzene in a round-bottom flask. The reaction mixture was refluxed at 65 °C, and anhydrous CaCl 2 was placed in a drying tube above the reactor. After 6 h of reaction, the product was washed by 15 mL hydrochloric acids (1 M) for 3 times, then excess benzene was removed on a rotary evaporator giving Phenyl fatty acids (PFA), 5.90 g, yielded 98.0%. A solution of PFA (5.90 g, 18 mmol) in trichloromethane (25 mL) was added dropwise to a drying flask with 1.45 mL SOCl 2 (20 mmol) in it, and the mixture was stirred at 40 °C for 2 h. Then the mixture was distilled to remove the solvent and excess SOCl 2 . The residue was dissolved in 10 mL acetone, and 2.30 mL N,N-dimethyl-1,3-propanediamine (20 mmol) was slowly added in it at 0 °C. The reaction mixture was then warmed to 40 °C and stirred for 2 h. The excess diamine and solvent were removed on a rotary evaporator giving N-phenyl fatty amidopropyl-N,N-dimethylamine (PFAPMA), 6.76 g, yielded 91.5%. PFAPMA were quaternized using sodium chloroacetate with a molar ratio of 1:1.25 in a solvent mixture of methanol and water (v methanol / v water = 1:4). The final products were refluxed at 75 °C for about 12 h; the mixture was vaporized off under reduced pressure, then dissolved in ethanol and filtered. The filter liquor was distilled to remove ethanol and purified by recrystallization in ethyl acetate. 7.29 g precipitate, N-phenyl fatty amidopropyl-N, Characterization. GC-MS spectra were recorded on an Agilent 6890N network GC system and 5975 inert Mass Selective Detector. ESI HRMS spectra were recorded on the Waters LCT Premier XE Mass Spectrometers. 1 H NMR spectra of all the intermediates and final surfactants were recorded on a Bruker Avance 400 spectrometer (400 MHz) in CDCl 3 at room temperature.
Surface tension measurement. The surface tensions of the aqueous solution at different surfactant concentrations were measured by using a DCAT 21 tensiometer (Dataphysics, Germany) with the plate method and the temperature was controlled at 25.0 ± 0.1 °C. Before each measurement, the quartz plate was briefly heated above an alcohol burner until glowing. In order to attain equilibrium conditions, the quartz plate should be cooled to room temperature and dipped in the solution for a while. The measurement should be repeated three times and an average value was obtained. The surface tension between air and double distilled water was 71.8 mN m −1 at 25 °C.   Interfacial tension measurement. Interfacial tensions between Daqing crude oil and water were measured by the spinning-drop method at 50 ± 0.1 °C using SVT 20 tensiometer (Dataphysics, Germany). Equilibrium was affirmed to be obtained when successive values taken at 5 min intervals agreed to within 0.001 mN m −1 . The volumetric ratio of water to oil in the spinning-drop tensiometer is about 2000. Daqing crude oil was used as oil phase. Surfactant solutions were prepared with Daqing oil field simulated formation water and the concentration of the surfactants was 0.020 g L −1 . The formation water consists of 112.7 mg L −1 CaCl 2 , 42.8 mg L −1 MgCl 2 , 1597.1 mg L −1 NaCl, 17.0 mg L −1 Na 2 SO 4 , 381.6 mg L −1 Na 2 CO 3 and 3176.0 mg L −1 NaHCO 3 . The total dissolved substance is 5327.2 mg L −1 . The interfacial tension between Daqing crude oil and simulated formation water was 9.70 mN m −1 at 50 °C.
Contact angle measurement. The contact angles were measured by using the sessile drop technique. The apparatus consisted of a camera with a micro lens and a horizontal platform. The platform was connected to the thermostat to control temperature. Before measurement, the platform environment was preheated at 25 ± 1 °C by switching on the thermostat. When the temperature reached 25 °C, the solid substrate was placed in middle of the platform. A 2 μL solution drop was introduced onto the solid substrate through a microsyringe. The contact angles were measured by taking photographic images after adding the solution for 60 s. The contact angles were determined directly from the photographs by drawing tangent lines between the liquid drop and the solid surface. The measurements were repeated 5 times for every samples and the average angle value was figured out. The average contact angle on the three phase contact gas/double distilled water/solid was 92.04 o at 25 °C.
Emulsification property measurement. The emulsification properties of bio-based zwitterionic surfactants were obtained at room temperature. It was determined by giving vigorous downward stokes to a mixture of 10 mL 0.500 g L −1 bio-based zwitterionic surfactants solution and 10 mL liquid paraffin in a graduated test tube. The time taken for separation of 3 mL and 5 mL aqueous phase solution was recorded (t/s). The recorded time was repeated for 3 imes.
Foaming property measurement. The foaming property of bio-based zwitterionic surfactants was obtained according to the Ross-Miles test at 40 o C 19 . The foam heights at 0 min and 10 min were recorded (h/mm). The recorded height was repeated for 3 times.
Ultimate biodegradation estimation. Ultimate biodegradation estimation of PFAPMB was determined by EPI Suite, BIOWIN3 biodegradation model, which is frequently used to estimate the degradation of organic chemicals 32 .