Surface Wetting-Driven Separation of Surfactant-Stabilized WaterâOil

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Surface wetting driven separation of surfactant-stabilized water/oil emulsions Qian Zhang, Lei Li, Yanxiang Li, Lixia Cao, and Chuanfang Yang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04248 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 28, 2018

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Surface wetting driven separation surfactant-stabilized water/oil emulsions

of

Qian Zhang,†,‡ Lei Li,† Yanxiang Li,† Lixia Cao,† Chuanfang Yang*,† †

CAS Key Laboratory of Green Process & Engineering, Institute of Process

Engineering, Chinese Academy of Sciences, Beijing 100190, PR China ‡

University of Chinese Academy of Sciences, Beijing 100049, PR China

ABSTRACT: Four fluorocarbon polymers including PTFE and PVDF were coated respectively on a stainless steel felt to separate emulsified water droplets from ultralow sulfur diesel fuels (ULSD). The original fuel treated with clay to remove additives

was

additized

again

with

four

known

surfactants

including

Pentaerythrityoleate, (octadecadienoic acid) dipolymer, (octadecadienoic acid) tripolymer and monoolein individually. The different surfactants adsorbed on the fuel-water interface reduce the interfacial intension with different intensity. The separation efficiency at various surfactant concentrations was used to evaluate the coalescence effect exerted by these coatings. It was found the separation was both surfactant and coating dependent. A fluoro-polyurethane coating (FC1) stood out to counteract the adverse effect of all the surfactants. Solid free energy was then measured using Acid-Base and Kaelble-Uy adhesion theories for all the coatings, but its correlation with coalescence was not found at all. Coating aging in surfactant additized fuel on the coating’s water wettability was also examined to better understand how historical wetting affects separation. A tumbled model for 1

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fluorocarbons was identified that well explained the continuous decline of water contact angle on the FC1 coating in fuel. Subject to the challenge of foreign environment, the fluoroalkyl chains of the polymer tilt to expose the carbonyl groups underneath, resulting in favored coalescence separation in the presence of surfactants.

KEYWORDS: fluorocarbon polymer; water-in-oil emulsions; ultralow sulfur diesel; surfactants; coalescence separation; surface energy, tumbled model

2


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INTRODUCTION The separation of emulsified water from ultralow sulfur diesel (ULSD) is a difficult problem on board of a diesel-powered vehicle due to the presence of fuel additives. ULSD as a clean energy has been commonly applied in recent years in many countries. However, to maintain the usability and stability of the diesel after the desulphurization process in a refinery, different kinds of surfactants/additives such as lubricant, corrosion-resistant agent, and anti-oxidation agent are added to the fuel. When water is emulsified in the fuel due to pump shearing, the surfactants can transfer to the fuel-water interface and make the water droplets to suspend more stably in the fuel, rendering their separation more difficult. On the other hand, as the development of diesel engine advances, high pressure common rail system is widely used. In such system, the pressure can be as high as 200 MPa and the fuel needs to pass through a series of spray nozzles with clearance of 2-5 µm only. The nozzles are easily blocked by solid particles and damaged by water-induced corrosion, causing the engine to malfunction. In addition, free water including emulsified water will reduce the lubricating property of the fuel leading to engine wear. In cold temperatures, these water droplets will crystallize to become ice particles to block the fuel filter, resulting in engine fuel starvation. Therefore, effective and continuous separation of water from diesel fuel is a must for engine protection. There are three forms of water in diesel fuel: free, dissolved and emulsified water. They can be separated by gravity settling, centrifugation, stripping and coalescence separation respectively. When the diameter of emulsified water is beyond 100 µm, the 3


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water can be separated by gravity sedimentation effectively.1 However, gravity has very little effect on the separation of emulsified water droplets smaller than 100µm. In modern engines, the size of emulsified water in ULSD is typically around 4-35 µm, which can be barely addressed by gravity settling. As centrifugation is difficult to apply to diesel engine systems, separation by a stripping or a coalescence filter becomes the ideal technology for emulsified water removal. However, stripping these small droplets by a porous water barrier has been proved repeatedly ineffective due to the increased stability of the droplets and the required dust holding capacity of the filter, which cannot be simply met by a microfiltration media. As a result, coalescence filtration has become the simplest and most economical means to meet the requirement of both water separation and dust loading. During a coalescence process, droplets collide and merge into bigger droplets by overcoming the surface tension force.2 Along the flow channel of a coalescing filter, the droplets are enlarged first and then settled by gravity to achieve water-oil separation. In the process, the ability of the filter to capture emulsified water is a crucial step for effective coalescence. Theoretically, the contact angle of water on a surface of 0° and 180 ° under fuel denotes respectively total water spreading to form a thin film on the surface, and no attachment of water droplet to the filter at all. Both situations occurring on a surface illustrate a filter’s inability to effectively coalesce water droplets, unless a third situation is met where the contact angle is delivered as between 90° and 140°.3 Stuti S. R.2 studied the separation of water from diesel fuel by PVAc (polyvinyl acetate) and PVP (polyvinylpyrrolidone) nanofiber-coated glass fiber media with high 4


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hydrophobicity. The separation efficiency of the nanofiber coating is 96.6%.2 Hydrophilic glass fiber is also used to separate emulsified water, however, it fails to show good advantage for the separation although mechanical vibration of the filter reinforces the process.4 It occurred that these works did not take into consideration the effect fuel activities on separation, and it is the additives that cause the disarming of many fuel/water separators. In a water-fuel-solid system, the motion behavior of surfactants or surfactant-type of fuel additives is complicated. Surfactants tend to assemble at the fuel-water interface quickly to reduce the interfacial tension, get adsorbed on the solid-fuel interface changing the arrangement of surface molecules, or move randomly in the bulk of the fuel and accumulate to form reverse micelles. All of these dynamic behaviors of surfactants influence the coalescence separation of emulsified water using a filter media. The surface property of a filter media is also critical in achieving and maintaining the necessary water separation efficiency. This often times cannot be delivered by the filter media alone, although packing the media with different original surface properties in layers sometimes function well by coincidence. More often surface treatment is applied to the original media to deal with the separation challenge surfactants bring in. Nevertheless, the selection of certain chemistry to treat the media is usually secretive and the effectiveness is emulsion system dependent. A universal solution, although difficult to find, is always desired. To this end, fluorocarbon polymers/chemicals are seriously considered as the promising candidates due to their inactiveness to surfactant adsorption, which was thought to be the key for sustainable performance of a media to separate 5


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surfactant-stabilized emulsified water. Fluorocarbon polymers are characterized as hydrophobic, stain, corrosion and high temperature

resistant

in

many

industrial

applications.

The

polyurethane

fluorochemical [FC1, (CF)n=4] identified in our previous work has excellent performance in promoting water-in-fuel emulsion separation with monoolein as the fuel additive.5 In order to acquire a greater insight into the role that fluoroalkyl group plays in coalescence when challenged by various surfactants including monoolein, hereby we carried out a thorough study on several common fluorocarbon polymers involving PTFE(polytetrafluoroethylene), PVDF(polyvinylidene fluoride), FC1, and FC2. PTFE is a full fluorocarbon polymer, PVDF is a partially fluorocarbon polymer， and FC2 is also a full fluorocarbon resin whose characteristic is similar to that of PTFE. The resultant coalescence experiments showed that PTFE, PVDF and FC2 all had poor separation of emulsified water at high surfactant concentration, proving that fluoroalkyl group is not a key factor affecting the coalescence. The surface energy of all the coatings was also measured, which showed no correlation with the effectiveness of separation either. The only experimental evidence that related to the separation was the coating’s response to water wetting over time under the fuel. The continuous water spreading on FC1 coating in the presence of different surfactants was explained by a tumbled model for fluoropolymers with C-F chain length equal to or smaller than 6, subject to the exposure to foreign environment.

6


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EXPERIMENTAL Materials PTFE dispersion and PVDF resin were provided by Shanghai 3F New Materials Co., Ltd. The other two fluorochemical dispersions FC1 and FC2 were supplied by a local company. Stainless steel fiber felt used as the base material for coalescence separation was purchased from Filter Company (Zhejiang, China). ULSD was purchased from a diesel station in Beijing. Active clay was supplied by Huangshan Baiyue Activated clay Co., Ltd. Isopropanol alcohol (IPA) and n-methyl pyrrolidone (NMP) were purchased from Aldrich. Monoolein was offered by Dalian Meilun Biotech Co. Pentaerythrity oleate (PETO-B) was purchased from Liaocheng Ruijie Chemical Co., Ltd. High purity (octadecadienoic acid) dipolymer (C36 Dimer acid) and (octadecadienoic acid) tripolymer (C54 Trimer acid) were provided by Jiangxi Aturex Co., Ltd. All these surfactant types of additives can be used as fuel lubricants. (octadecadienoic acid) dipolymer and (octadecadienoic acid) tripolymer are also used as wear-resisting agents, and monoolein can be used as a fuel stabilizer as well. Table 1 lists the chemical formula and the molecular weight of each surfactant.

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Table 1. Surfactant specification Surfactant

Structural formula

Molecular weight

Pentaerythrityoleate

C(CH2OOCC17H33)4

1193.93

HOOCC8H15(C9H17)(C8H17)C9H17COOH

564.93

(Octadecadienoic acid)

HOOCC9H17(C8H15)CH(C6H13)C3H5(C7H14

846.37

tripolymer

COOH)CH(C8H17)C9H17COOH

Monoolein

C17H33COOC2H3OHCH2OH

(PETO-B) (Octadecadienoic acid) dipolymer

356.54

Preparation of surfactant-deprived ULSD 15 g/L active clay was added to a bottle containing the original ULSD. The bottle was then put on a constant speed thermostatic shaker set at 180 r/min and 25℃ for 24h to remove the original additives/surfactants in the ULSD. The surfactantdeprived ULSD was subsequently obtained by filtering out the clays. The interfacial tension of water and diesel was measured before and after clay-treatment of the ULSD to evaluate the effectiveness of surfactant removal. PETO-B, high purity (octadecadienoic acid) dipolymer, (octadecadienoic acid) tripolymer and monoolein were added to the surfactant-deprived ULSD separately with varying concentrations of 10, 20, 50, 100, 200, and 500 ppm. Then the interfacial tension of water and diesel was measured to evaluate the influence of surfactant addition. 8


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Coalescence experiments The stainless steel fiber felts were washed with detergent, alcohol, and de-ionized water respectively, and then dried in an oven at 50℃ before each use. PTFE, PVDF, FC1, FC2 were dip-coated on the felts and cured at the same temperature as done for each coating on the glass slide. The coated felt was then mounted in a filter holder. Precisely 50 mL ULSD additized with a surfactant was mixed with 50 µL de-ionized water in a graduated syringe by a mechanical stirrer preset at 25000 rpm on an EMCEE Electronics instrument, as defined as MSEP according to ASTM D7261. The prepared emulsion was automatically pumped through the filter, and the filtrate was collected to measure the un-separated water content with an automatic volumetric Karl-Fischer titrator AKF-1 (Shanghai Hegong Scientific Instrument, Inc.). The coalescence separation efficiency of the filter is calculated as5:

C 0 − Cf ×100% C0

(1)

where C0 is the initial water content in the emulsion, and Cf is the un-separated water content in the filtrate, sampled typically in the middle portion of the filtrate collected in a transparent test tube. The initial water content was controlled constant as: 50×10-3/ (50×10-3+50×0.835) × 106 = 1196 ppm. Each experiment was conducted three times to average the data. Preparation of fluoropolymer coatings on glass slides Glass slides were cut to a size of 25 mm×25 mm×1 mm and washed with detergent, alcohol and de-ionized water respectively using an ultrasonic cleaning machine, and then they were dried in an oven. PTFE dispersion (60 wt %) was diluted 9


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to a solid content of 20% in de-ionized water and spin-coated on the glass slides by a spin coater. The coated glass slides were put into a Muffle furnace oven at 330℃ for 30 min to cure the polymer. PVDF was dissolved in NMP with a weight ratio of 15:85, and then the mixture was stirred mechanically at 300 rpm for 2 h in a 40℃water bath. After that, it was put into a vacuum oven to exhaust air bubbles at 50℃ for 2 h. Next the polymer was cast on the glass slides and put into a drying oven for 12 h at 60℃ to form the coating films. FC1 and FC2 were respectively dispersed in IPA and de-ionized water at a weight ratio of 10:5:85, and then spin-coated on each single glass slide twice. FC1 coated glass slides were put into a vacuum oven at 140℃ for 12 h, and the FC2 coated glass slides at 100℃ for 12 h to cure the coating completely. Contact angle. The contact angles of water and USLD on the four coating surfaces were measured with a contact analyzer (Drop Master DM-701, Kyowa, Japan). The contact angles of diiodomethane and ethylene glycol on the coatings were also obtained to calculate the solid surface energy. In addition, water sliding angles on all the coating surfaces were measured to aid the analysis of the surface properties. Contact angle under ultralow sulfur diesel. ULSD with a certain surfactant concentration was first put into a transparent quartz container, and then the fluoropolymer coated glass slide was immersed into the ULSD and aligned parallelly with the flat bottom of the container. A fresh water droplet was generated from the instrument to contact the coating surface for immediate contact angle measurement. Contact angle at different surface aging times in the ULSD was also recorded by 10


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dispensing a fresh water drop at different time intervals and allowing it to contact with the coated surface. The aging time studied includes 2, 5, 8, 10 and 15 min. The objective is to understand if surfactant adsorption changes the surface wetting behavior given sufficient time for the surface to adsorb surfactant from the fuel. In addition, the variation of water contact angle on the FC1 coating after 10 min immersion in ULSD was also monitored to examine if surface aging changes the wetting behavior. For PTFE, PVDF and FC2 coated surfaces, similar experiments were performed.

RESULTS AND DISCUSSION The dynamic interfacial tension of water and ULSD As shown in Figure 1, dynamic interfacial tension measurement result indicates the interfacial tension of water and the original ULSD decreases from 35 to 29 mN/m2 in 5 min, while that of water and the clay-treated ULSD remains at 40 mN/m2. This is an indication that the surfactants in the original ULSD have been removed sufficiently by the active clay.

Figure 1. Behavior of dynamic interfacial tension of water and fuel 11


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As water and oil are immiscible phases, the molecules at the oil-water interface are attracted by the same phase molecules and repelled by the other phase, rendering high interfacial tension in the system. So the interfacial tension of water in surfactant-free ULSD is higher, while that in the original ULSD is lower and decreases with time. This is obviously due to the bridge effect of surfactant, where the hydrophobic group extends to the oil phase while the hydrophilic group interacts with the water molecules to lower the interfacial tension, making the system more stable.6 After clay-treatment, four known kinds of surfactants were added to the fuel respectively to make a series of new fuels with surfactant concentration ranging from 10 to 500 ppm. The interfacial tension was measured again to evaluate the impact of the different surfactants and their concentrations. Apparently, the interfacial tension of water in the ULSD containing PETO-B, high purity (octadecadienoic acid) dipolymer, (octadecadienoic acid) tripolymer and monoolein all decreases as the concentration of the surfactants increases. Moreover, monoolein has the strongest effect on interfacial intension reduction, followed by high purity (octadecadienoic acid) tripolymer and high purity (octadecadienoic acid) dipolymer, while PETO-B has the weakest effect. Low interfacial tension would enhance the deformability of the water droplets and makes the droplets to adapt to the surrounding environment more easily, which in turn causes more difficulty in coalescence separation with a filter media. However, the fact that different surfactants have different effects on interfacial tension is complicated. This might be associated with the type and quantity of the head groups, the length of alkyl chain, the structure of surfactant molecules and so on. 12


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The type and quantity of the head groups of the surfactants determine the strength of their interaction with water. Aleksey et al7 clarified that the hydrogen bonding of –COOH and water is more intensive than that of –CH2OH and water. As a result, on the solid-fuel interface, surfactants with carboxyl groups as the head groups arrange in linear row with ordered tail arrangement, while surfactants with hydroxyl groups as the head groups array with disordered tail and penetrate into fuel molecules. The intensity of the hydrophilic groups is described as: -COOH > -OH > -OCOH.7 For the long-tailed surfactant, the hydrocarbon n-alkanes are flexible and bend easily, which allows the surfactant to stack intricately on the interface and increases the steric hindrance intensively. In addition, the branched chains of surfactants that fail to stretch straightly with a regular order also produce a steric hindrance and disturb the arrangement of alkyl chains in the space, weakening their effect on interfacial tension and the interaction among the hydrocarbon tails. The more bendy the alkane chain is, the larger the steric hindrance it will provide. Thus a surfactant containing ethylene linkage with a bend angle of 30° also presents steric hindrance that decreases its influence on the interfacial tension. In Figure 2, the influence of the four surfactants on interfacial tension is compared as: Monoolein > (Octadecadienoic acid) tripolymer > (Octadecadienoic acid) dipolymer > PETO-B. These surfactants, with alkyl chains as their hydrophobic groups and carbonyl, hydroxyl or ester groups as their hydrophilic groups, are nonionic and liposoluble. The intermolecular interaction between the densest tail groups of the surfactants is due to van der Waals force, and the interaction between 13


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the head groups is polar interaction.8 The steric hindrance enhances with increased length of alkyl chains. So the steric effect provided by the alkyl length of different surfactant is described as: PETO-B > (Octadecadienoic acid) tripolymer > (Octadecadienoic acid) dipolymer > Monoolein. PETO-B has four linked ester groups and its backbone has two side chains, each of which has an ester group and an ethylene linkage. So the molecule of PETO-B is highly bendy, and its steric hindrance is therefore the largest. High purity (octadecadienoic acid) dipolymer has two carbonyl groups and an ethylene linkage, while (octadecadienoic acid) tripolymer has three carboxyl groups and an ethylene linkage. Both of them have branched chains shorter than that of PETO-B. Monoolein, on one hand, has one ester group, two hydroxyl groups and an ethylene linkage. On the other hand，the molecular mass of monoolein is the smallest among all the surfactants, so more molecules and hydrophilic groups exist when the surfactant concentration is the same. Therefore, the combining intensity of head groups with water at the same surfactant concentration is: Monoolein > (Octadecadienoic acid) tripolymer > (Octadecadienoic acid) dipolymer > PETO-B. The steric effect provided by the bending degree of each surfactant, however, follows the order of PETO-B > (Octadecadienoic acid) tripolymer > (Octadecadienoic acid) dipolymer > Monoolein. That is to say that PETO-B has the least intensity on interfacial tension reduction and monoolein has the largest, which agrees with the experimental observation.

14


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Figure 2. Water-fuel interfacial tension evolution caused by the presence of PETO-B (a), (Octadecadienoic acid) dipolymer (b), (Octadecadienoic acid) tripolymer (c), Monoolein (d).

Characteristics of fluoropolymer coatings Characteristics of fluoropolymer coatings on glass slides. PTFE is a full fluorocarbon polymer and FC2 is a nonionic full fluorocarbon resin, while PVDF is partially fluorinated and FC1 is a urethane type of fluorochemical that contains four C-F structures as its tail group. The data in Table 2 show the water contact angle on the four fluorocarbon polymers as: PTFE > FC2 > FC1 > PVDF. Apparently, all of the fluorocarbon coatings are hydrophobic, their static water contact angles in air are between 96° and 130°, and the sliding angles are all smaller than 10° as measured. On the other hand, the comparison of oil contact angle on the coating surfaces shows that FC1 > FC2 > PTFE > PVDF. Obviously, FC1 and FC2 are much more oleophobic

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than PTFE and PVDF. Moreover, the water contact angles on PTFE, PVDF, FC2 under the fuel are 162.0°, 156.8°, 159.7° , while that of FC1 is 120° and decreases with time, as observed.

Table 2. Original water contact angle and oil contact angle on four coating surfaces. PTFE

PVDF

FC1

FC2

WCA in air

130.5±0.2°

95.7±0.4°

114.1±0.3°

117.8±0.2°

OCA in air (original fuel)

24.4±1.8°

6.3±0.3°

77.3±0.3°

75.9±0.2°

OCA in air (clay-treated fuel)

36.0±0.3°

4.8±1.2°

76.5±0.5°

74.4±0.3°

WCA under clay-treated fuel

162.0±0.2°

156.8±0.4°

120.3±2.7°

159.7±0.4°

Diiodomethane CA

106.0±0.3°

56.8±0.2°

93.5±0.3°

100.3±0.3°

Ethylene glycol CA

117.3±0.3°

58.0±0.4°

102.5±0.3°

103.8±0.2°

As fluorine atom has intensive electronegativity, the energy of C-F bond is so strong that fluoroalkyl group comparatively is weak in polarity. The strong electron withdrawing ability of fluorine atoms draws electrons from the adjacent carbons, resulting in a highly deficient state of electron density. So the fluorine atoms pack tightly with the attached carbon. Then the electrons of the neighboring carbon-carbon bonding and its attached hydrogen atoms move to compensate the deficiency of the electron density, which therefore enhances the acidity of the polymer and the dispersion interaction of the solid surface.9 PTFE is a full fluorocarbon polymer whose backbone carbons are all at an electron deficiency state. High electron density around fluorine atoms shield electrons from exchanging with the external 16


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environment. The repellency between the fluorine atoms makes PTFE to possess a helical chain structure and a hydrophobic characteristic.10 The monomer of PVDF arranges in the order of fluorocarbon bonds followed by carbon hydrogen bonds alternately. Replacing fluorine atoms with hydrogen atoms makes PVDF more hydrophilic than PTFE. PVDF has some degree of acidity by an obvious shift of electrons from the adjacent carbon-hydrogen bond to the fluorine-carbon bond. This movement of electrons enhances the dispersion interaction of PVDF, resulting in a lower water contact angle.9 FC1, which was studied in our previous work in detail, is a urethane type of fluorochemical containing both hydrophilic sites and hydrophobic sites.5 The structure of FC1 is described as: X-[CH2CH2O]nCH2CH2O-Y-NH-COO-CH2CF2CF2CF2CF3 Where X, Y are undefined segments Fluoroalkyl and ester groups make the distribution of the electron cloud density heterogeneous that improves the water wettability on this polymer. Its coating on glass slide shows a microstructure of bicontinuous particles and wrinkles.5 According to the characteristics of water repellency and oil resistance of FC2, we infer that the outermost fluorine atoms of FC2 coating have a crystalline structure.11-12 The difference of the four polymers in wettability in air plays an important role in explaining the wetting phenomena under the fuel, yet it alone is not sufficient to explain everything observed. Therefore, the contact angles of diiodomethane and ethylene glycol on the four surfaces were obtained to calculate the solid free energy of each coating surface. 17


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The solid surface free energy. Drop Master can give the measurement result of solid surface energy if the contact angles of water, diiodomethane, ethylene glycol on the surface are known. Theories of surface free energy are involved to divide the energy into components that are determined by the intermolecular force. Usually, Van der Waals force and hydrogen bonding force define the intermolecular force. Van der Waals force consists of orientation, inductive and dispersion forces. The hydrogen bonding force represents the interaction between a hydrogen atom and an atom of high donatives.13 The most widely used acid-base theory divides the component of solid free energy into Lifshits-van der Waals (LW) and Lewis acid-base (AB). And the component of Lewis acid-base (AB) is composed of electron acceptor component (γ+) and electron donor component (γ-). Kaleble-Uy theory divides surface free energy into dispersion component (d) and polar component (p). However, this theory does not describe the contribution of hydrogen bonding force. Both theories can be used to evaluate the interaction of water with a solid surface and therefore applied specifically in this work to calculate the surface free energy. The equations associated with the process of measurement are described as: The Young’s equation: γS = γLcos ߠ + γSL

(2)

The Dupre’s equation: γS + γL = WSL + γSL

(3)

Work of adhesion: WSL = γL (1 +cos ߠ )

(4)

Where γS is surface free energy of solid, γL is surface free energy of liquid, γSL is solid-liquid interfacial free energy, ߠ is the liquid-solid contact angle and WSL is the required energy to separate a solid and a liquid at their interface. 18


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As shown in Figure 3, the solid free energy follows the order of PVDF > FC1 > FC2 ≈ PTFE. Meanwhile, the water-solid interface free energy follows PTFE ≈ FC2 > FC1 > PVDF and the work of adhesion PVDF > FC1 > FC2 > PTFE. Apparently, the comparison result of solid free energy and water-solid interface free energy is absolutely the opposite. PTFE has the lowest surface free energy while PVDF the highest. The γ- represents the ability of surface molecules to donate electrons, and the γ+ represents the ability of electron acceptation. The data in Table 3 show that γ- and γ+ of PTFE, FC1 and FC2 are all zero, and those of PVDF are 0.2 and 0.8 respectively. It indicates that PVDF has extremely weak ability of donating and accepting electrons, and all others have none. Such a characteristic of PVDF makes its water contact angle smaller than others. In addition, the significant γ- and γ+ values of PVDF demonstrate that the polymer’s fluorine atoms have strong screening effect on electron donation and acceptation for the polymer to remain stable.14 The data calculated with Kaelble-Uy theory also prove that the nonpolar interaction of these polymers contributes far more to the solid free energy than the polar interaction, as shown in Table 4. However, in literatures, the water contact angle on PTFE is about 120° and the solid free energy is around 18-20 mN/m2.11 The deviation of the values here may be due to the enhanced roughness of PTFE coating caused by the PTFE particles used to form the coating at high curing temperature. The increased roughness can elevate the contact angle of the original hydrophobic surface and thus lower the solid free energy by calculation15-16. The data of FC2 are similar to those of PTFE, as both materials are full fluorocarbon polymers. The solid free energy of FC1 is slightly 19


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higher than PTFE and FC2 due to the contribution of fluorine atoms. It implies that the outermost molecules of FC1 coating surface are fluorine atoms while carbonyl groups are hidden underneath. Van Dame et al12 proposed a tumbled model of fluorocarbon polymer to explain the reduction of contact angle with time after a rapid heating-cooling procedure that deteriorated the crystalline structure of the polymer. At normal situation, the fluoroalkyl chain orients vertically to the surface. However, when disturbed by the outer polar environment, it tilts and lowers the screening effect of fluorine atoms, exposing the carbonyl groups to contact with water molecules. However, the tiny difference between the solid free energy of FC1 and PTFE might be an indication of a negligible tilt of FC1 fluoroalkyl chain in air environment.10

Figure 3. The comparison of solid free energy (S.F.E.), interfacial free energy (I.F.E.), work of adhesion (W.A.) of water on PTFE, PVDF, FC1, and FC2 coating surfaces based on Acid-based theory(a) and Kaelble-Uy theory(b).

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Table 3. The component of solid free energy of four coating surfaces calculated based on the Acid-Base mechanism (mN/m). Component of solid surface free energy

PTFE

PVDF

FC1

FC2

γLW

8.6

30.4

10.9

8.9

γ+

0

0.8

0

0

γ-

0

0.2

0

0

Total

8.6

31.4

10.9

8.9

Table 4. The component of solid free energy of four coating surfaces calculated based on the Kaelble-Uy mechanism (mN/m). Component of solid surface free energy

PTFE

PVDF

FC1

FC2

d

9.3

29.3

10.4

8.1

p

0

1.1

0.8

0.5

Total

9.3

30.4

11.2

8.6

The result shown in Figure 3 indicates that the adhesion ability of PVDF is the largest and PTFE is the smallest. Smaller work of adhesion implies that the intermolecular interaction at the water-solid interface is less intensive, or phrased in other words, the hydrogen bonding between water and the outermost molecules of the coating surface is weaker. So the water droplet is expected to readily roll down from the PTFE surface when the surface is inclined. The measurement result of the sliding angle supports this point (Table 5). The work of adhesion of FC1 is higher than that of PTFE and FC2 and lower than that of PVDF, consistent with the contact angle result. In this work, as all the fluorocarbon polymers are hydrophobic, the difference of

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interfacial free energy among them is not remarkable. According to the lowest energy principle in water-vapor-solid system, the solid free energy is the driving force of water spreading.17 So the hydrophobic characteristic of these four coating surfaces can be explained by that: a lower free energy solid surface is difficult to be replaced by a higher free energy solid-water interface. Sliding angle. To confirm the reorganization of the outermost solid surface molecules, sliding angle was measured to each coating surface. Obviously, as shown in Table 5, the sliding angles of PTFE and FC2 are 3.7° and 5.0° respectively, and those of FC1 and PVDF are 7.5° and 7.0°, larger than the former two coatings. Note that the solid free energy of FC1 (11mN/m) is approximate to PTFE and FC2, much lower than that of PVDF (32mN/m), yet the sliding angle of FC1 (7.5º) is higher than

that of

PVDF (7.0º) which is counter-intuitive. The adhesive energy follows the order of FC1 > PVDF > FC2 > PTFE. It denotes that water droplet adheres to FC1 surface better than to PVDF surface when the surfaces are inclined.167 Since fluoroalkyl chain has strong shielding effect preventing the polar groups from exposing and interacting with water molecules,18 it is reasonable to believe that the FC1’s fluoroalkyl chain at the outermost solid surface may have reorganized to increase the wettability of water on the surface.12

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Table 5. The measurement result of adhesion energy and sliding angle Coatings

PTFE

PVDF

FC1

FC2

E (Adhesive energy) (mJ/m2)

0.32±0.0

0.40±0.1

0.50±0.1

0.38±0.0

Sliding angle (°)

3.7±0.6

7.0±1.4

7.5±0.7

5.0±0.0

In fact, as Kojj19-20 and his coworkers suggested, when the number of fluorine atoms of the main unit of a fluorocarbon polymer is smaller than 6 and larger than 2, the fluoroalkyl group is mobile and has no crystallization conformation. This can induce the surface molecules to reorganize easily and cause water contact angle to decrease with time. In the structure of FC1, whose molecule contains O=C(NH)-Ogroup and repeated -CH2CH2-O- units, the number of fluorine atom is four which falls into the criteria defined by the model. So the FC1’s fluoroalkyl chain has mobility and is readily to be induced by the outer polar environment to tilt, exposing the carbonyl group to water to enhance water spreading, or rather, adhesion, on the coating surface. Such characteristic of FC1 can also explain why the water contact angle slightly decreases with time in the ULSD free of surfactants. Further, whether this mechanism can be used to explain the phenomenon that water spreads on FC1 coating surface immersed in surfactant-rich ULSD is worth to elaborate on later. Coalescence separation results Coalescence separation of an emulsion by a fibrous media can be divided into three steps: the approach of dispersed water droplets to the filter, the attachment and coalescence of the droplets, and the release of the enlarged droplets from the filter.21 23


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Regardless of the approaching process which is typically fluid mechanics and media physical pore size related, the second step is so important that it determines the ability of the filter media to capture and grow the emulsified water droplets. The adsorption of surfactants on the coating surface normally increases the difficulty level of coalescence. However, if desorption happens easily, the overall process should favor the separation because of the possible reduced emulsion stability and the refreshed media surface for continuous droplet capture and confluence. It has been illustrated that the favorable water contact angle on a surface for effective coalescence ranges from 90° to 140°.3 If this is true, surfaces coated with PTFE and FC2 should have poor efficiency of water separation. The bi-layer structure of the stainless steel felt with an aperture of the upper layer of 5 µm and that of the bottom layer of 10 µm is beneficial for the enlarged droplets to release. This is partially verified with the coalescence experimental result hereby, especially when surfactant exists in the emulsion system. In our previous work, we merely focused on monolein as the surfactant, ignoring the other types of surfactants that are also used as the fuel lubricant additives. Here we carried out similar separation experiments using ULSD doped with PETO-B, high purity (octadecadienoic acid) dipolymer and high purity (octadecadienoic acid) tripolymer and the results are shown in Figures 4a, 4b and 4c.

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Figure 4. The coalescence separation efficiency of surfactant stabilized emulsified water from ULSD using stainless fiber felts coated with PTFE, PVDF, FC1, and FC2 respectively ULSD+ PETO-B (a), ULSD+(octadecadienoic acid) dipolymer (b), ULSD+(octadecadienoic acid) tripolymer (c), ULSD+monoolein (d).

For the ULSD additized with PETO-B, as shown in Fig. 4a, the separation efficiency of the original felt is not affected by the presence of surfactant; it is maintained at around 94-95% regardless of the increase in surfactant concentration. This corresponds to the fact that PETO-B has the least influence on fuel-water interfacial tension as depicted in Fig. 2a. For this scenario, applying coating to the material surface appears not necessary unless the coating can boost the separation, which is physically impossible due to the adverse effect of surfactant. If required, the best coating should be something that does not reduce the efficiency of separation as surfactant concentration rises. FC1 is such a coating that retains the original felt’s efficiency at all surfactant concentrations. Using 100 ppm monoolein additized fuel as 25


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an example, after water was emulsified in it, it was found with microscopic observation that the initial droplet size ranged from 2 to 38 µm, and averaged 7µm. The filtrate with the uncoated felt had a droplet size of 2-130 µm and averaged 15-20

µm, while the filtrate through the FC1 coated felt had only a few measurable big droplets of 80-100 µm5; No other droplets could be identified by the microscope, as they became so big to be observed by naked eyes in the dimension of 0.5-2 mm. And more evidently, most water was separated as a water layer accumulated at the bottom of the collector, an obvious result of coalescence. In fact, the collection tube of the filtrate serves another purpose as a separation device, which provides a static environment for the coalesced droplets to settle quickly to the bottom of the container. This separation time was pre-determined as 1 min before the sample was taken for water content analysis and droplet size observation. PVDF coating causes the efficiency to decrease a bit at higher PETO-B concentration, but PTFE and FC2 coatings make the separation much worse, the efficiency decreases to around 70-74%. When the ULSD is additized with (octadecadienoic acid) dipolymer, as shown in Fig.4b, similar separation results are obtained for the original felt and the FC1 coating. All the other coatings decrease the efficiency to a deeper extent compared with the case of PETO-B, correlated to the stronger interfacial effect of (octadecadienoic acid) dipolymer as shown in Fig.2b. Fig. 4c starts to show the surfactant effect on the original felt when (octadecadienoic acid) tripolymer was added to the fuel. The filter’s efficiency drops from 95% to 84% when the concentration of this surfactant reaches 200 ppm. On the contrary, FC1 coated felt 26


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retains the 95% efficiency at all surfactant concentrations. The performance of the other three coatings is even worse than the case with the dipolymer as the surfactant, since the tripolymer has a stronger interfacial effect as demonstrated earlier in Fig.2c. The most interesting case is shown in Fig. 4d, where monoolein is used as the surfactant. The original felt loses its performance even when the surfactant concentration is increased to 100 ppm. Only FC1 coated felt roughly sustains its ability to separate monoolein stabilized emulsion at all the surfactant concentrations studied. The rest three coatings behave just like the felt without coatings, the separation efficiency is reduced from 95% to below 40%. The reason is that monoolein is the strongest surfactant; it reduces the interfacial tension dramatically, especially when its concentration is high, making the emulsion too stable to be separated. FC1 shows a positive effect on the separation because it leads to special water wettability in fuel as will be discussed next. PVDF is better than full fluorocarbon polymer PTFE and FC2 because of its alternative array of carbon hydrogen bonds and fluorine carbon bonds in its structure. What is worth mentioning here is that, the coalescence test technique applied in this work should not be considered as a standard test for real-world media development. It is used as a quick tool to screen potential coalescence media and identify those techniques that show positive effects for fuel/water separation. The downside is its inability to test long-term performance of a media, which should be addressed by standardized tests using either the most recent version of SAE J1488 or ISO 16332, by bearing in mind that even these standards have a long way to go down the road for 27


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improvement by new scientific discoveries. The measurement of water contact angle under monoolein additized ULSD Since monoolein22 is the strongest surfactant studied in this work, it is necessary to better understand how the coating surfaces interact with it to either maintain the coalescence media’s separation efficiency, or to deteriorate the media’s performance. Such understanding should be helpful in identifying better coating materials or chemistries for stable media performance in the long term. The absorption of surfactants on the solid surface under the ultralow sulfur diesel. Figure S1 describes the technique used to measure the dynamic water contact angle (water spreading against time) on different coating surfaces under the ULSD doped with surfactant at varying concentrations. And Figure 5 depicts the specific method of water contact angle measurement after a certain period of surface aging in fuel. In a typical experiment, a quartz pool 60×40×30 mm in dimension is filled with 40 mL monolein additized ULSD at a certain concentration and the coated slide is put into the bottom of the quartz pool seriatim. The concentration of the surfactant ranges from 10 to 500ppm and the measurement is conducted to record the initial contact angle. At the time after the slide is immersed in the pool for 2, 5, 8, 10 and 15min, a fresh water droplet is dispensed to contact a fresh spot of the coating, and the initial water contact angle on the coating surface is measured. The purpose of such measurement is to evaluate how surface aging affects water wetting. It is speculated that if sufficient length of time is allowed at a given surfactant concentration, the surface of the coating will be saturated with surfactant by adsorption so the wetting 28


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behavior will be consistent; or if the surface is resistant to surfactant adsorption at all. As Figure 6 illustrates, as monoolein concentration increases from 0 to 500 ppm, the initial water contact angles on PTFE and FC2 under the fuel all range from 150° to 170°, while that of PVDF is increases from 127° to 170° and that of FC1 decreases from 125° to 70° . The contact angle on PTFE does not change significantly with respective increase of the aging time and the surfactant concentration. The same trend applies to FC2 coating. The capture mechanism of coalescence filtration clarifies that a material will have no effect on coalescence when the contact angle of the dispersed phase on the material surface is 180°.3 And as observed during the experiment, water droplets roll on PTFE and FC2 freely and randomly, indicating the coating materials are not able to capture the droplets. Hence, PTFE and FC2 coatings will not aid the coalescence separation of monoolein stabilized emulsified water from ULSD. On the contrary, they may deteriorate the separation delivered by the original filter media as proved by our experiments.

Figure 5. Schematic diagram of water contact measurement on a coating surface immersed in fuel at different immersion times, a new water drop is used to contact an unused spot of the surface for each measurement.

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Figure 6. Initial water contact angle on PTFE (a), PVDF (b), FC1 (c), FC2 (d) coating surfaces under ULSD with monoolein at aging times of 2 min, 5 min, 8 min, 10 min, 15 min respectively.

The water contact angle on PVDF coating surface increases from 127° to 170° as the concentration of monoolein increases, however, it changes little over time at the same monoolein concentration. When the concentration of monoolein is the lowest, the contact angle on the PVDF coating is minimal, indicating some extent of interaction between the polarized hydrocarbon and the water molecules. As the concentration of monoolein increases, more surfactants come to the water-fuel interface, enhancing the deformability of water droplet and increasing the contact angle. Yet this should have happened to PTFE and FC2 coating surfaces as well but no similar contact angle trend is identified, the opposite trend is observed instead. It implies that surfactant adsorption on PVDF coating may play a role; while such adsorption on PTFE and FC2 is negligible unless the surfactant concentration is way 30


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high and the surfactant orientation on the surface becomes unusual to cause the contact angle to become smaller. As can be seen from Figure 6c, the contact angles on FC1 shows a dramatically opposite trend when the surfactant concentration is increased. First of all, when no surfactant is present, the contact angle (125º) is smaller compared with all the other coatings, indicating better water wetting to start with. Second, the contact angle reduces as more surfactant is in the fuel, indicating stronger interaction of the surface with the surfactant that exposes more polar groups of the coating to the water. At 500 ppm monoolein concentration, the average water contact angle becomes 80º, and the initial contact angle drops from 90° to 70° when aging time arrives at 15 minutes. And as illustrated in Figure S2, the contact angle on the four fluorocarbon polymer surfaces shows similar tendencies when the coatings were immersed in (octadecadienoic acid) tripolymer additized ULSD. This demonstrates that the special water wetting phenomenon on FC1 appears to be universal for different surfactants, which is very important for separation enhancement because of the unknown nature of additives in fuel in reality. After each coating surface was immersed in 200 ppm monoolein-additized fuel for 10 min to allow for surfactant adsorption, the dynamic water contact angle was measured in fuel and the result is shown in Figure 7.

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Figure 7. The variation of water contact angle on PTFE, PVDF, FC1, FC2 coating surfaces aged in ULSD with 200ppm monoolein for 10 min.

It can be seen that the contact angles on PTFE, PVDF and FC2 coatings maintain around 160° over 2 minutes while that of FC1 decreases from 118° to 65°. This is certainly due to the more surface adsorption of surfactant molecules creating opportunities for water to interact with the hydrophilic segments of the coating. To further test if such contact angle decline continues, we purposely put a few water droplets on the FC1 coating surface immersed in monoolein additized ULSD, and astonishingly, allowing for sufficient time, each individual water droplet gradually spread on the surface and the adjacent droplets joined together to form a water film, as depicted in Figure S3. This phenomenon illustrates not only the affinity of the surface to water, but also how easy the coalescence could occur simply on a surface submersed in fuel. To validate if this kind of wetting or non-wetting happens to other surfactants, another set of experiments was conducted in 200 ppm (octadecadienoic acid) tripolymer additized ULSD and the result is given in Figure S4. The water contact angle on PTFE, PVDF, FC2 coating surfaces is all around 150°~170° in 2 min without 32


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any change, indicating the surfaces have barely any coalescence ability. As a comparison, the contact angle on FC1 coating surface decreases from 160° to 60° in 2 min, repeating the situation occurred to monoolein additized fuel. The special phenomenon can also be explained by the tumbled model. Our previous QCM experiments proved that FC1 coating has intensive absorption of monoolein from the ULSD that favors instant demulsification5. Fuel molecules and surfactant molecules adsorbed on the solid surface disturb the arrangement of the mobile fluoroalkyl chains of the polymer and cause them to tilt. These chain tilts suddenly expose the underneath carbonyl groups of the polymeric molecules, creating greater prerequisite conditions for the polymer to strengthen its adsorption of surfactants and interact with the water by hydrogen bonding, as depicted in Figure 8. The longer time the coating is immersed in the ULSD, the more monoolein molecules will be adsorbed on the surface until adsorption equilibrium is reached. This process induces more tilted fluoroalkyl chains to expose more carbonyl groups. The bonding force of the adsorbed surfactant and the coating surface is weak so that the adsorption is reversible. When water spreads on the surface, the accumulated surfactants at the junction of fuel-water-solid phase would be adsorbed by the adjoined unsaturated FC1 surfaces, or transferred to the new-born fuel-water interfaces, or to the bulk of the fuel oil. Meanwhile, there is fierce competition between the fuel-water interface and the solid-fuel interface to adsorb these surfactant molecules. However, the distribution of surfactants in the water-fuel-solid system will attain a new equilibrium rapidly created by the Marangoni effect. If sufficient time is allowed, the droplet would absolutely 33


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spread on the solid surface. This characteristic of FC1 is favorable for capturing water droplets allowing them to coalesce. Reduced surfactants on water-fuel interface due to competitive adsorption onto the unsaturated FC1 coating surface mitigate the physical barriers for emulsified water droplets to combine.5

Figure 8. Schematic description of fluoroalkyl polymer’s (FC1) behavior in strengthening surfactant adsorption and water wettability based on the tumbled model.

The activity of surfactant in the water-fuel-solid system. The activity of surfactant in a water-fuel-solid system is complicated. Surfactant moves randomly in the fuel bulk at low concentration while forms reverse micelles by self-assembling at high concentration. The self-assembly of surfactant occurs when the concentration of surfactant is beyond CMC (critical micelle concentration). For a hydrophobic surface immersed in the ULSD, surfactant would adsorb on the solid surface by the alkyl group via Van der Waals force.23 So the adsorption is reversible and the alkyl group of surfactant will lie on the surface at low concentration. As the

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concentration of surfactant increases, the aggregation of alkyl group on the solid surface would perform vertical orientation as depicted in the tumbled model.10 If the alkyl chain is long enough, the fuel molecules will penetrate in the arrangement of these chains. When a fresh water drop is dispensed into the ULSD bulk immersed with a solid surface, immediately, a competition for surfactant adsorption between the fuel-water interface and the solid-fuel surface starts. As has been discussed, the water-fuel interfacial tension depends on the head groups, the alkyl chain length, and the structure of the surfactant. The adsorption of surfactant on the solid surface depends on the characteristics of the solid surface molecules and the characters of the surfactant. PTFE coating immersed in the ULSD only slightly absorbs monolein because of the intensive electronegativity of fluorine atoms and the low solid free energy of the surface. This characteristic of PTFE keeps most of monolein molecules in the fuel instead of being preferentially absorbed on the solid surface. When a fresh water droplet is dispensed to the fuel, monolein in the fuel would transfer to the fuel-water interface reducing the interfacial tension. As the water droplet gets much closer to the solid surface, the surfactant molecules will still compact tightly around the droplet. When the water drop contacts with the solid surface, a small amount of surfactant molecules and fuel molecules will be squeezed out from the water-fuel-solid interface, resulting in an approximately 160° water contact angle on the PTFE surface. Further, as indicated in Figures 6-7, the unchanged contact angle versus time and monolein concentration demonstrates that the wettability of PTFE is not enhanced because 35


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PTFE has no exposed polar groups, even though the surrounding environment disturbs the arrangement of the outermost fluorine atoms. The variation trend of contact angle on FC2 is similar to that of PTFE. However, FC2 solidifies at 110ºC while PTFE at 330ºC that leads to more heat consumption. In addition, FC2 has smaller resin particle size than PTFE, which results in a smoother coating surface. So FC2 is a better replacement of PTFE. In the case of PVDF, the contact angle slightly increases with the increase of aging time but becomes larger in general as monoolein concentration increases until 100 ppm. At low monoolein concentration, water droplet can spread on the solid surface a little because of the stagger array of C-F bonds and C-H bonds of PVDF. Lower density arrangement of C-F bonds allows the water contact angle under the ULSD with monoolein to gradually increase with monoolein concentration. The screening effect of fluorine atoms and the tightly compacted surfactant molecules around the droplet make the contact angle to attain a value as high as 160° at the 500ppm monoolein concentration. For FC1 coating surface, the situation is much more intricate. As discussed earlier, the special structure of this fluorochemical fits tumbled model description pretty well in a water-fuel-solid system. As such, it not only enhances the adsorption of surfactant, but also promotes water spreading and reduces the barrier for better coalescence to occur. It is expected that there exist some other fluorochemicals like this that are worth pursuing. Conclusions: Fluorocarbon polymers, with excellent hydrophobicity and sometimes oleophobicity, are thought to be good candidates for enhancing coalescence separation of 36


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water-in-oil emulsions. The perception has been proved wrong in this work unless such polymer has unique chemistry such as FC1 identified earlier in our work. As specified as a full fluorocarbon polymer, FC2 has similar hydrophobicity and oleophobicity to FC1, but it does not help the separation at all. PTFE and PVDF are not in any means good separation aids either. This indicates that fluoroalkyl group is not the crucial factor for tuning positively the performance of coalescence separation when surfactants are present in the fuel. Solid free energy, adhesive energy and sliding angle are all unable to differentiate the performance of the four fluorocarbon polymer coatings studied in this work as to where the separation is headed. The only observable parameter that distinguishes FC1 from other polymers is the under fuel water wetting behavior, a close approximate to the real-case scenario, where the dispersed water must be in contact with the material to be collected and grow. The influence of surface aging in surfactant-saturated fuel on the coating’s wettability indicates that the exposing foreign environment causes FC1 to behave dramatically differently than other polymers, rendering the surface water wettable. Such a phenomenon can be well explained by the existent tumbled model in that, the outer polar environment induces a tilt of the fluoroalkyl chains of FC1 to expose the underneath carbonyl groups, thus enhancing water wettability and strengthening surfactant adsorption. Competitive and reversible adsorption of surfactant by the coating surface and the water-fuel interface allows for water confluence and spreading on the surface, which subsequently counteracts the adverse effect brought in by surfactants for coalescence separation. 37


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ASSOCIATED CONTENT Supporting Information Graphic of contact angle measurement on coating surfaces under ULSD; water contact angle on aged surfaces under ULSD containing (octadecadienoic acid) tripolymer; water film forming on FC1 coating surface in monoolein additized ULSD; variation of water contact angle on coating surfaces aged in (octadecadienoic acid) tripolymer additized ULSD ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China under the grant number of 21476237 and the National Key Research and Development Program of China under the contract number of 2017YFB0308002.

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Sulfur Diesel Using Novel Polymer Nanofiber-Coated Glass Fiber Media. Acs Appl. Mater. Interfaces 2016, 8 (33), 21683-21690. 3.

Basu, S. A study on effect of wetting on mechanism of coalescence in a model

coalescer. J Colloid Interface Sci. 1993, 159 (1), 68-76. 4.

Yang, X.; Wang, H.; Chase, G. G. Performance of hydrophilic glass fiber media

to separate dispersed water drops from ultra low sulfur diesel supplemented by vibrations. Sep. Purif. Technol. 2015, 156, 665-672. 5.

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