Dilational Viscoelastic Properties of WaterâFuel ... - ACS Publications

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Dilational viscoelastic properties of water-fuel interfaces in single and binary surfactant systems Qian Zhang, Yanxiang Li, Lixia Cao, Lei Li, Kun Huang, Wangliang Li, and Chuanfang Yang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00517 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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Energy & Fuels

Dilational

viscoelastic

properties

of

water-fuel

interfaces in single and binary surfactant systems

Qian Zhang,†,‡ Yanxiang Li,† Lixia Cao,† Lei Li,† Kun Huang,*,§ Wangliang Li,*,† and Chuanfang Yang*,†,‖ †

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

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

University of Chinese Academy of Sciences, Beijing 100049, China

§

School of Metallurgical and Ecological Engineering, University of Science &

Technology of Beijing, Beijing 100083, China ‖

Current address: 17368 Rosalla Dr, Eden Prairie, MN 55346, USA

[email protected]

Abstract: Interfacial rheology of a surfactant film surrounding a water-in-oil emulsion influences the stability of the emulsion significantly. In this research, we chose monoolein of molecular weight of 356.54 and Pentaerythritol Oleate (PETO-B) of molecular weight of 1193.93 as the surfactants, and dissolved them in clay-treated ultralow sulfur diesel fuel separately and combiningly to mimic single and binary surfactant systems. The water/fuel interfacial viscoelastic properties were than studied using the oscillation pendant drop method subject to different surfactant concentrations and oscillation frequencies. Surfactant coverage on the interface and

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surfactant molecule migration rate were calculated to investigate the viscoelastic phenomenon of the surfactant film. The migration rate of surfactant in the bulk was found to be crucial for the surfactant to effectively compensate the interfacial tension (IFT) gradient caused by interfacial area expansion and compression. Faster moving monoolein molecules could rapidly compensate the IFT gradient as oscillation was triggered, resulting in much higher elastic modulus in comparison to PETO-B molecules with lower migration rate. Higher surfactant concentration also produced faster migration rate resulting in higher film elastic modulus. Lower oscillation frequency promoted the formation of a film with lower elasticity and higher viscous modulus, while higher oscillation frequency led to lower film viscous modulus. The calculated occupying area per surfactant molecule on the interface indicated that monoolein had a much tighter arrangement at the interface than PETO-B. The binary surfactant system was constructed with monoolein and PETO-B with various weight ratios. The interfacial rheology of such a system was decided by the faster migrating surfactant that built the fundamental framework of the film, and adding PETO-B to monoolein-containing fuel could therefore decrease the interfacial elastic modulus, and hence the emulsion’s stability, to benefit emulsified water separation.

1. INTRODUCTION Ultralow sulfur diesel fuel (ULSD) for transportation is added with lubricity, anti-wear and fuel stabilizer additives to maintain its usability for inner combustion engines. However, some of the additives have surface activities that significantly


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increase the stability of emulsified water formed in the fuel during engine operation. As discussed in our other study,1 the diameter of emulsified water droplets in ULSD is 4-35 μm while the diameter of the spray nozzles that un-separated water droplets would pass through is 2-5 μm. High pressure common rail system is widely used in modern diesel engines and the pressure in the system can be as high as 200 MPa. At such high pressure, emulsified water can damage engine spray nozzle by causing discontinuity of the lubricant layer on the inner wall of the nozzle that leads to severe wear, and by corroding the nozzle and other engine parts, resulting in engine malfunction and even breakdown.2-4 Therefore, this kind of water together with other free water in ULSD must be removed. As previously reported, coalescence separation with non-woven filter media is by far the most economical, effective and feasible method to tackle the emulsified water separation problem.1,5 However, the effectiveness of coalescence is dependent on various factors including media structure, surface wettability, operating conditions, and more importantly, the stability of the emulsion itself that is directly related to the interfacial rheology of the emulsion. Surfactants accumulate on the interface forming an interfacial film which has malleability and intensity. When two water droplets in fuel collide, their films squeeze mutually and deform to an extreme state. If the films are highly elastic, the droplets will bounce off to the opposite direction and restore the films’ initial state. If the surfactant films have low elasticity to begin with, then the droplets will likely break through the barrier after collision and merge to form a larger droplet.6 Elastic modulus, one of the interfacial properties, is a telling indicator of an interfacial film’s


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ability to store energy and resist deformation. Viscous modulus, on the other hand, is another interfacial property that describes the energy loss induced by intermolecular interaction during the film relaxation process. In fact, elastic modulus and viscous modulus are two crucial properties of the surfactant film of a water-oil emulsion, which have huge effects on the separability of the emulsion by media coalescence and other means.7 The essence of the viscoelastic modulus refers to the relaxation process of surfactants near the interface. Surfactants have multiple relaxations on and around the oil-water interface: diffuse-exchange between the interface and oil bulk, monolayer rearrangement, and the configuration changes of molecules.8-10 Low molecule weight surfactant typically experiences two relaxation processes: one is the diffuse-exchange as a fast relaxation process, and the other

is the conformational change as a slow

relaxation process. However, for large molecule weight surfactant, especially macromolecules, their migration rates are extremely slow to complete the diffuse-exchange between the interface and the bulk in a constrained time. So, as reported, only the exchange of monomers between different regions of the surface layer can be considered, such as a slow relaxation process of the inner strains of a polymer chain, or a fast relaxation process of the exchange of the chain as a whole.6-8 Wang et al.11 investigated the relaxation processes of AM/POEA block copolymer without and with SDS surfactant at the octane-water interface. They observed the fast relaxation process between different regions in the interfacial layer and the slow relaxation process of conformational changes of AM/POEA block polymer chain. The


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added SDS surfactant may either compete with the copolymer at the interface or combine with it to form micelles, and both situations have influence on the interfacial rheology properties. Zhou et al.12 indicated that a tight surfactant film would be weakened by insertion of other active components at the crude oil/water interface. This proved that surfactants in a binary system may have a lower viscoelastic modulus than a single system. Such a behavior may be taken advantage of when designing an additive package for diesel fuel to not only enhance the needed fuel property, but also purposely reduce the emulsion stability for easier fuel/water separation. Although there is increasing number of on-going works to design coalescence filter materials for emulsified water separation from diesel fuel on board a vehicle, barely any research has taken the effort to investigate the interfacial rheology of fuel/water systems. Interfacial tension (IFT) has been generally used as an easy-to-measure fuel property in standard test. It was also thought that IFT is representative enough to describe the level of difficulty for separating emulsified water from diesel fuel. However, this is far from the truth as to what indeed controls an emulsion’s stability and how we can respond with a proper separation solution with confidence using both lab test data and field trial results. For this reason, in this work, we selected monoolein and PETO-B as the typical fuel additives/surfactants to study the interfacial rheology of fuel/water systems using oscillation pendant drop method. The emulsion stability can be deduced from the viscoelastic properties, as reported by Wang et al.13 and Georgieva et al.,14 who declared that the interfacial dilational


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elasticity is a key factor of demulsification. With the same pendant drop method but for water/crude oil and brine/crude oil systems, Perles C.E. et al have reported that temperature, oscillation frequency and salt content in water all influenced the interfacial film’s viscoelastic modulus.15 So hereby we chose to perform our experiments with different surfactant concentration and oscillation frequency to understand their influences on the viscoelastic properties in our systems, as have been done to other systems by other researchers.16-18 Both a single surfactant system and a binary surfactant system were investigated to correlate the interfacial rheology in addition to IFT with the emulsion’s stability. With the understanding gained in this work, it is anticipated that better or alternative means can be innovated to achieve more effective fuel/water separation for engine protection.

2. EXPERIMENTAL SECTION 2.1.

Materials.

Monoolein (MW: 356.54 g/mol, purity: 50%) was purchased from Dalian Meilun Biotech Co. Pentaerythritol Oleate (PETO-B) (MW: 1193.93 g/mol, purity: 99.8%) was purchased from Liaocheng Ruijie Chemical Co., Ltd. Monoolein and PETO-B are liposoluble and nonionic surfactants. Both can be used as fuel lubricity improver and anti-abrasion agent to reduce engine wear. In addition, monoolein is specified as the required additive for SAE J1488 and ISO 16332 test standards for emulsified water separation from diesel fuel. The choice of PETO-B is based on its much higher molecular weight and more complex molecular structure as another extreme case in


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Energy & Fuels

contrast to monoolein, for the sake of interfacial rheology studies. ULSD (ultra-low Sulphur diesel) was purchased from a local diesel station. 2.2.

Preparation of additized ULSD and measurement of water/fuel separation efficiency.

The purchased ULSD was treated with active clay to remove the original additives as described in our previous work.1 Monoolein and PETO-B were respectively dissolved into the surfactant-deprived ULSD to obtain a surfactant concentration range of 20-300 ppm. The molar concentrations of monoolein (Purity 50%) are 2.4, 6.0, 12.1, 24.1, 36.210-5 mol/L according to its mass concentration which are 20, 50, 100, 200 and 300 ppm. The molar concentrations of PETO-B are 1.4, 3.6, 7.2, 1.4, 2.2 10-5 mol/L corresponding to the same mass concentration range. To prepare the fuels with binary surfactants, monoolein and PETO-B were mixed with a weight ratio of 1:3, 1:1 and 3:1 first, then respectively dissolved in the treated ULSD to obtain the same total surfactant concentration range as for a single surfactant system. The method of emulsified water separation from diesel fuel by a stainless steel felt as the coalescence media was discussed in our previous work.1 Briefly, the felt was washed and dried first, and then was mounted on a filter holder. The water in fuel emulsion was generated by adding precisely 50 μL of deionized water into 50 mL of ULSD additized with a surfactant, mechnically stirred at 25000 rpm for 30 seconds sharp on an EMCEE Electronics instrument defined as MSEP according to ASTM D7261. The prepared emulsion was automatically pumped through the filter, and the


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filtrate was collected to measure the unseparated water content with an automatic volumetric Karl-Fischer titrator AKF-1 (Shanghai Hegong Scientific Instrument, Inc.) to calculate the separation efficiency. Each experiment was conducted 3 times to minimize the experimental error. 2.3.

Viscoelastic property measurement

The rheology parameters were investigated by oscillation pendant method using Kruss DSA K100. Briefly, a fresh water droplet was generated from the needle tip (needle inner diameter 1.81mm) of the instrument and immediately immersed in the ULSD prefilled in a 2 cm ⅹ 2 cm ⅹ 2 cm transparent quartz cuvette. By diffusion, the fuel dissolved surfactant quickly assembled around the fuel/water interface to attain adsorption equilibrium. The droplet was then allowed to oscillate with a sinusoidal signal at a controlled frequency of 0.1-0.5 Hz. The oscillated amplitude in surface area was normalized as 1 and the surface deformation rate given by the software was below 10%. It denotes that the force exerted by the pump to expand and contract the droplet does not produce extra force to cause droplet deformation. The variation of surface area at 0.1 Hz slowly increases and decreases respectively with droplet expansion and contraction, providing enough time for surfactant migration. In comparison, The variation of surface area at 0.5 Hz is much faster that forces rapid response of surfactant to the interfacial tension gradient, which is required in many cases considering engine vibration, flow fluctuation, and so on. Oscillation frequency larger than 0.5 Hz will cause the droplet interfacial deformation rate to exceed 10% thatinduces significant experimental errors due to the limitation of the instrument used


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Energy & Fuels

for the study. Therefore, the frequency range in our research was chosen as 0.1-0.5 Hz. The experiment was performed at a constant room temperature of 18 ºC. Since in a real scenario on a diesel-powered vehicle, the lifetime of a water droplet is in the range of seconds to tens of seconds before it is removed, plus the emulsion preparation time for our water-fuel coalescence experiments was fixed at 30s per the procedure of EMCEE equipment,1 so we chose the first 30 seconds after oscillation was initiated to study the interfacial behavior of fuel and water with monoolein and PETO-B as the fuel additives. In the experiment, it took 1 s for the formation of the fresh water droplet. And the oscillation started right after the fresh water droplet was dispensed. A 29.18 s video was recorded immediately after the oscillation was triggered, and therefore the experiment time was recorded as 30 s. The variation of IFT and interfacial area was analyzed by the built-in drop shape analysis program. The elastic modulus, loss modulus, and viscous modulus were then calculated using the Fourier analysis software associated with the instrument. The static IFT was measured on a separate tensiometer, Drop Master (DM-701, Kyowa, Japan). Each experiment was triplicated to minimize the experimental error. In fact, the error bars of the data points are too small to be seen in the graphs, so we put the error range in the supporting tables as the Supplementary Information.

3. THEORETICAL BACKGROUND Surfactant molecules transferred to the oil-water interface form a film that can store energy and resist deformation when subject to compression and expansion.


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Interfacial dilational rheology obtained by the oscillation pendant drop method is defined as the variation of interfacial tension versus that of interfacial area： 𝑑γ

ε = 𝑑ln𝐴 = |ε|exp(iθ)

(1)

where ε is the complex interfacial dilational viscoelastic modulus, γ is the interfacial tension，A is the water-oil interfacial area and θ is the phase angle.1 The expansion and compression of the interface allow the surfactant to have a relaxation process between the interface and the bulk of the oil.19 The time difference between the variation of interfacial area and that of interfacial tension can be described by a phase angle θ as depicted in Figure 1. The dilational viscoelastic modulus can also be written as: ε = εd + iεη = εd + iωηd

(2)

As Equation 2 shows, ε consists of two parts, a real part εd and an imaginary part iεη. The real part is the dilational elastic modulus and the imaginary part is the viscous modulus. The elastic modulus represents the film’s ability to store energy, and the viscous modulus describes its ability to dissipate energy in the relaxation process. ω is the oscillation frequency, ηd is dilational viscosity, subscript d stands for dilational, i is the unit imaginary number, solution of the equation x2=-1.


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Energy & Fuels

Figure 1. The phase angle between the variation of interfacial area and interfacial tension measured in 50 ppm monoolein additized fuel with an oscillation frequency of 0.5 Hz

The phase angle can be calculated by the following equation: εη

(3)

tan θ = εd

It is a function of the ratio of elastic and viscous modulus, which describes the viscoelastic characteristics of the surfactant film. The interfacial film behaves as a purely elastic body when θ = 0 °. To calculate surfactant excess concentration on the water-fuel interface, the Gibbs adsorption equation can be used in the most general form as: dγ = -∑𝑖𝛤idμi

(4)

where dγ represents the variation in interfacial tension. Γi is the surface excess concentration of any surface phase (phase in between bulk fuel and water in our case) consisting of surfactant of the system. dμi is the change in chemical potential of any surface phase of the system. Surface excess concentration is defined as the excess surfactant per unit area of the interface to compensate the interfacial tension gradient. In a uniform two phase


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system, when the interface does not attain the saturated interface adsorption capacity, the surface excess concentration is approximately identical to the value of surfactant coverage of the interface. By definition, the variation of the interfacial tension can also be described by Gibbs adsorption equation:20-23 dγ = -RT Γ1d lnC1 = -2.303RT Γ1d logC1

(5)

where R= 8.31 J/(mol·K), Γ1 is the surface concentration of the surfactant in mol/cm2, γ is interfacial tension in mN/m, C1 is the molar concentration of surfactant in the liquid phase at adsorption equilibrium in mol/L, and T is the absolute temperature, in our case, 291 K. By calculation, the number of surfactants at adsorption equilibrium in the fuel bulk is 103~104 times as much than that on the interface. So the value of C1 can be approximately equal to the original surfactant concentration (2.4~36.2 10-5 mol/L for monoolein, 14~2.2 10-5 mol/L for PETO-B). Many surfactants are adsorbed as a monolayer at the interface that fit for. Langmuir adsorption.24 The saturation adsorption capacity Γm per area on the interface can be calculated as: 1 Γ1

k

1

(6)

= C1Γ + Γm m

The plot of 1/Γ1 versus 1/C1 is a straight line with a slope of k/ Γm and an intercept of 1/Γm, where k is a constant (= 55.3exp(ΔG˚/RT)) in mol/L at absolute temperature T, ΔG˚ is the free energy of adsorption at infinite dilution.22 Therefore, the area that a molecule occupies at the interface can be calculated by


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Energy & Fuels

Equation 7: 1

(7)

𝑎 = NΓ1 where N is the Avogadro’s number.22

4. RESULTS AND DISCUSSION 4.1. Critical micelle concentration of Monoolein and PETO-B in fuel. The IFT of water and fuel containing monoolein and PETO-B at the studied concentration range of 20-300 ppm is obtained by the Drop Master tensiometer. The relationship between IFT and log C is plotted as shown in Figure 2, where C is the surfactant concentration in mol/L. Neither of the surfactants reaches its critical micelle concentration (CMC) with a constant IFT, indicating that both surfactants are dissolved and distributed in the fuel without aggregation even at the highest concentration of 300 ppm. The CMC of monoolein and PETO-B are respectively 620.4 ppm and 341.2 ppm, as calculated by the Gibbs adsorption equation and the fitting equation of log C against IFT.

Figure 2. The relationship between interfacial tension and surfactant concentration log C


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4.2. The separation efficiency of emulsified water from fuels containing monoolein and PETO-B. The separation efficiency of monoolein stabilized emulsion was found to decrease with the increase of monoolein concentration. However, as indicated in Table 1, at the largest surfactant concentration of 300 ppm, even the lowest separation efficiency of the emulsion stabilized with PETO-B reaches 91.8%, while that with monoolein as the surfactant is merely 17.5 %. The result implies that emulsions with monoolein are much more stable than those with PETO-B, which has been reflected by the IFT difference shown in Table 2. The question hereby is that, how do the interfacial rheology properties of the two kinds of emulsions correlate with the separation efficiency, do they agree with what IFT predicts?

Table 1. The coalescence separation efficiency (%) of water-in-fuel emulsions by stainless steel felt

Surfactant

20 ppm

50 ppm

100 ppm

200 ppm

300 ppm

Monoolein

95.3±0.0

92.8±0.0

41.6±0.1

34.5±0.1

17.5±0.0

PETO-B

96.9±0.0

93.7±0.0

93.9±0.0

93.3±0.0

91.8±0.0

4.3. Surfactant parameters (Γ, d̅, a) on the interface 4.3.1. Surface surfactant coverage concentration (Γ) on the interface at static and oscillation state. Surfactant migrates to the water-fuel interface and reduces the interfacial tension accordingly until an equilibrium surfactant concentration is reached at the interface


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Energy & Fuels

that stabilizes the interfacial tension. For a certain surfactant, its saturated adsorption capacity (Γm) on the interface is a constant that can be obtained by Equation (6). By calculation, the saturated adsorption capacity of monoolein is 7.84 ⅹ 10-6 mol/m2 at 620.4 ppm (CMC) and that of PETO-B is 0.65 ⅹ 10-6 mol/m2 at 341.2 ppm (CMC). That is to say that the number of monoolein molecules needed to saturate the same water/fuel interface is over an order of magnitude larger than that of PETO-B. Surface surfactant coverage concentration (Γ) at the interface per unit area can be approximately calculated using Gibbs adsorption equation as aforementioned, also as has been reported by other researchers as well,4 and the result is displayed in Figure 3. Γ increases with surfactant concentration until it attains the saturated adsorption capacity (Γm). The surface adsorption capacity of monoolein increases from 1.5 to 6.210-6 mol/m2 as the surfactant concentration is increased, and accordingly, the separation efficiency of water from monoolein additized fuels decreases from 95.3% to 17.5%. In contrast, with the increase in surfactant concentration, the surface adsorption capacity of PETO-B increases from 0.13 to 0.63 10-6 mol/m2 and the emulsified water separation efficiency from PETO-B additized fuels only slightly decreses, dropping from 96.9% to 91.8%. Obviously, the Γs of monoolein are much larger than those of PETO-B while the separation efficiencies with monoolein are much lower than that with PETO-B, especially at higher surfactant concentrations. Therefore, compared with PETO-B molecules, we speculate that monoolein molecules are more closely aligned on the interface to better stabilize the emulsion it helped to form.


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At static pendent drop state, the surface adsorption capacities of monoolein are 0.51, 2.2, 2.9, 4.6, 5.3 10-6 mol/(m2·s) and that of PETO-B are 0.39, 0.45, 0.48, 0.51, 0.5410-6 mol/(m2·s) in the studied concentrations (respectively 20, 50,100,200 and 300 ppm). At oscillation state, the surface adsorption capacities of monoolein and PETO-B increase from 3.00 and 0.4710-6 mol/m2 to 4.10 and 0.50 10-6 mol/m2 at 0.25 Hz, respectively. Oscillation creates disturbance and accelerates the surfactant migration rate from the fuel bulk to the water-fuel interface.19 Compared with a static pendant drop (Γ ranges from 0.52ⅹ10-6 to 5.35ⅹ10-6 mol/m2), oscillation at 0.1-0.5 Hz can enhance Γ of monoolein. But for PETO-B, the increase in Γ(ranging from 0.40ⅹ10-6 to 0.52ⅹ10-6 mol/m2) due to oscillation only appears at higher surfactant concentrations compared with the static state, as can be better distinguished in the inset of Figure 3. In the following, we will choose to use the characteristic parameters of monoolein and PETO-B at 0.25 Hz oscillation for more detailed analyses.

Figure 3. Surface surfactant coverage concentration at static state and oscillation state with frequencies of 0.1 – 0.5 Hz.


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As Table 2 illustrates, as the surfactant concentration rises from 20 ppm upto 300 ppm, the surface surfactant coverage concentration (Γ) increases and IFTdecreases. At the same concentration, the water-monoolein-fuel interface has lower IFT than the water-PETO-B-fuel interface, which generally implies that the emulsion formed with monoolein is more stable and more difficult to separate. Moreover, the amount of each surfactant adsorbed at the interface does not attain their saturated adsoption capacity (Γm). This proves that both monoolein and PETO-B resulted in a Langmuir-type of monolayer adsorption at a concentration range of 20 - 300 ppm. The largest surface coverage with 300 ppm monoolein (Γ/Γm) at 0.25 Hz is 78.4%, and that for 300 ppm PETO-B at the same frequency is 97.0%. This is to say that the interfacial film formed with PETO-B is almost saturated when the bulk concentration reaches 300 ppm, while that with monoolein is still far away from being saturated.


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Table 2. Interfacial tension, surface surfactant coverage concentration (Γ), and Γ/Γm (%) of monoolein and PETO-B at oscillation frequency of 0.25Hz Monoolein

PETO-B

Surfactant

IFT

Γ

Γ/Γm

IFT

Γ

Γ/Γm

concentration

(mN/m)

(ⅹ10-6mol/m2

(%)

(mN/m)

(ⅹ10-6mol/m2

(%)

)

)

20 ppm

35.4±0.4

1.6±0.0

20.0±0.0

41.2±0.0

0.3±0.0

47.7±0.0

50 ppm

32.0±0.5

2.8±0.0

36.0±0.0

40.5±0.1

0.4±0.0

64.6±0.0

100 ppm

25.2±0.3

4.1±0.0

52.0±0.3

39.6±0.0

0.5±0.0

76.9±0.0

200 ppm

16.9±0.4

5.4±0.0

68.9±0.0

38.8±0.1

0.5±0.1

89.2±0.0

300 ppm

12.4±0.2

6.2±0.1

78.4±0.0

37.0±0.1

0.6+0.0

97.0±0.0

4.3.2 The average migration rate (d̅) of surfactant in 30 s of oscillation Surfactant average migration rate (d̅) from the fuel bulk to the water-fuel interface is a crucial factor of interfacial tension reduction when a fresh water droplet is put in the fuel. Generally speaking, higher surfactant concentrations lead to a larger concentration gradient between the fuel bulk and the interface, therefore resulting in higher migration rates. At the same mass concentration, surfactant with smaller molecule weight has more molecules while surfactant with larger molecule weight has less. In our system, as the molecular weight of monoolein is only 30% of PETO-B, the number of available monoolein molecules is therefore 3 times larger than that of PETO-B. The average migration rate at the static state and oscillation state are


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respectively obtained by Equation (8) and the results are given in Table 3 and Figure 4.25 d̅ static= Γi/30; d̅ osc = Γi/30

(8)

where Γi is the surfactant excess concentration corresponding to a certain surfactant concentration, and 30s is the time period in second of each measurement. Table 3. The average migration rate of surfactants at static state and oscillation state with 0.25 Hz frequency.

Average migration rate

Monoolein

PETO-B

(ⅹ10-8 mol/(m2·s))

(ⅹ10-8 mol/(m2·s))

Surfactant conc

d̅ static

d̅ osc

d̅ static

d̅ osc

20 ppm

1.7±0.0

5.2±0.0

1.3±0.0

1.0±0.0

50 ppm

7.2±0.0

9.4±0.0

1.5±0.0

1.4±0.0

100 ppm

9.8±0.0

13.7±0.0

1.6±0.0

1.7±0.0

200 ppm

15.4±0.0

18.0±0.0

1.7±0.0

1.9±0.0

300 ppm

17.8±0.0

20.5±0.0

1.7±0.1

2.2±0.0

As the data in Table 3 indicates that the average migration rate of monoolein increases from 1.7 to 17.8 10-8 mol/(m2·s) as monoolein concentration is increased from 20 to 300 ppm in static state; during oscillation state with a frequency of 0.25 Hz,

the migration rate increases from 5.2 to 20.5 10-8 mol/(m2·s) . The average

migration rate of PETO-B increases from 1.3 to 1.7 10-8 in static state and 1.0 to 2.2 10-8 mol/(m2·s) at 0.25 Hz oscillation. Surfactant migration rate increases with the


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surfactant concentration both in static and oscillation systems. As expected, oscillation causes monoolein to migrate much faster, but such an effect is not significant for PETO-B. And at the same concentration, the migration rate in oscillation system is larger than that in static system except that of PETO-B at 20 ppm and 50 ppm. Complicated molecular structure and large molecule size/weight create more obstacles for surfactant migration. Such a phenomenon has also been reported elsewhere that disturbance accelerates the motion of small molecule weight surfactant but offers more obstacles for large molecule weight surfactant.15, 26, 27 The ratio of average migration rate of monoolein and PETO-B is 5.08 at 20 ppm and 9.33 at 300 ppm at 0.25 Hz oscillation. In static state, the ratio becomes 1.3 at 20 ppm and 10.3 at 300 ppm. Therefore, monoolein migrates much faster than PETO-B, the difference is more dramatic at higher surfactant concentrations. What this means is that, when the water-fuel interface expands and contracts, monoolein has faster response to the interfacial area variation than PETO-B, which has been reflected by the interfacial tension intensity and should be mirrored also by interfacial rheology properties.

Figure 4. The average migration rate of monoolein and PETO-B at static state and oscillation state


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with frequency of 0.1-0.5 Hz.

Although disturbance from the external environment can accelerate surfactant migration, at the same concentration, the average migration rates at different oscillation frequency (0.1-0.5Hz) have no significant difference, particularly for monoolein, as shown in Figure 4. For PETO-B, stronger oscillation accelerates molecule migration only when the surfactant concentration is relatively low as can be seen from the inset of Figure 4.

4.3.3. The occupying area by a single surfactant molecule on the interface The average area of a single molecule occupying the water-fuel interface is determined by the area of the hydrophilic group but not the hydrophobic groups.22 As shown in Figure 5, monoolein has a smaller polar head as the hydrophilic group and a non-polar alkyl tail as the hydrophobic group, while PETO-B has a larger polar head which is introuduced into the molecule, and four non-polar tails, thus offering a much larger steric hindrance. The cross-sectional area of an aliphatic chain oriented perpendicular to the interface is about 20 Å2 and that of a benzene ring is 25 Å2. The cross sectional area of a -CH2- group lying flat in the interface is 7 Å2. Usually, the alkyls chains with a hydrophilic group at the end of the molecule does not lie flat on the interface but tilit with an angle. So it is obvious that the hydrophobic chain does not tightly pack at saturation adsorption but has some space for relaxation proscess.22 The average area of a single molecule occupying at the interface is calculated by Equation (7) and the results are given in Table 4 :


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Table 4. The area of a single surfactant molecule occupying at the interface: a (Å2, square angstrom)

Surfactant concentration

amonoolein (Å2)

aPETO-B (Å2)

20 ppm

105.9±0.3

543.7±0.4

50 ppm

59.0±0.2

399.6±0.2

100 ppm

40.5±0.5

333.3±0.5

200 ppm

30.8±0.2

285.3±0.7

300 ppm

27.0±0.1

242.3±0.5

As Table 4 shows, the average area decreases with surfactant concentration. This indicates that the surfactant molecules arrange more tightly at higher surfactant concentrations. The total area of the polar head of monoolein is 67 Å2 referring to the basic data of Chemical Spider.28 It can be seen that only at 20 ppm, the area of a single monoolein molecule occupying at the interface exceeds this value.29-31 Therefore, the alkyls chains of monoolein at 20 ppm may lie on the interface and tilit with an angle contributing to a part of the cross-sectional area on the interface. As more surfactant molecules come to the interface, they have to make rooms to accommodate each other.


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Figure 5. The molecular structure of monoolein (a) and PETO-B (b).

In Figure 6, the round head “ball” represents the hydrophilic group and the tail drawn as

a “rod” represents the hydrophobic group. Above the “bule water-fuel

interface” is the oil phase and below it is the water phase. At a low surfactant concentration, 20 ppm, for example, the alkyl chains of monoolein lie on the interface forming a loose arrangement as Figure 6 monoolein (a) depicts. When the concentration increases to 50 ppm or even higher, the area of a single molecule on the interface is reduced to smaller than 67 Å2. This indicates that every molecule is orienting in a way that its polar head is in full contact with water, forming a tight and vertical molecule arrangement, as illustrated in Figure 6 monoolein (b) and (c). For PETO-B, however, the scenario is totally different. Even at the highest concentration of 300 ppm, where the area a single PETO-B molecule occupies is the smallest (242.3 Å2), it is still much larger than the total polar area of this molecule, which is 105 Å2 as

referred to the basic data of Chemical Spider.29-32 Mukerjee et al. found that when the number of carbon atom in a hydrophobic group’s straight-chain exceeds 16 at the aqueous-hydroarbon interface, the surfactant’s adsorption at the interface significantly decreases.33 This is due to the long alkyl chain coil and the increased


cross-sectional

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area of the molecule at the interface. In our case, the carbon number of the main chain and the branch chain of PETO-B are all beyond 16, the average area of a single PETO-B molecule occupying at the interface has a signifcant increase due to the contribution of the alkyls chains. This denotes that PETO-B molecules spread on the interface with a loose molecule arrangement, as illustrated by Figure 6 PETO-B(d), (e) and (f).

Figure 6. Illustration of molecule arrangement [20 ppm(a), 100ppm(b), 300 ppm(c) monoolein; 20 ppm(d), 100 ppm (e), 300 ppm (f) PETO-B] on the water-fuel interface. (below the water-fuel interface is the water phase and above it is the oil bulk)

At a low concentration, surfactant molecules tumble or loosely align at the interface because the quantity of surface active species is small. As the concentration increases, surfactant molecules arrange straightly and pack tightly on the interface. Table 2 shows that the surface adsorption capacity of monoolein and PETO-B respectively is increased

from 1.6 and 0.3 to 6.2 and 0.6 ⅹ10-6 mol/m2 when

surfactant concentration is increased from 20 ppm to 300 ppm. And the water


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separation efficiencies respectively decreases from 95.3% and 96.9% to 17.5% and 91.8% as is shown in Table 1. Therefore, compared with monoolein, PETO-B has a loosened arrangement at the interface due to

its large molecule weight and side

chains, resulting in a unstable emulsion. In our system, the arrangement structure of monoolein and PETO-B on the interface matched with the separation efficiency results. It indicates that interfacial rheological property measurement is interesting and essential to understand the difference in emulsion stability related to interface occupation with different surfactants, as will be discussed next. The rule of thumb could be: a tightly packed surfactant layer resistes interface deformation and produces a stable emulsion, while a loose surfactant layer makes the emulsion less stable but benefits coalescence.14 However, this is true only for the same surfactant molecules; it is not necessarily true when referring to different surfactants, as has been reported previously2. In fact, we have chosen Trimer acid as another surfactant and added it to the ULSD for the same experiment. As Trimer acid has ethylenic bond, carboxyl groups and four branches chains, the arrangement of Trimer acid molecules on the water-fuel interface becomes more complex. Therefore, it is reasonable to say that the effect of different surfactants on interfacial tension and rheology needs to be analyzed specifically, generization of the rules seems challenging. 4.4. Dilational viscoelastic modulus of a single surfactant component at water-fuel interface 4.4.1. The relaxation process during oscillation The existence of interfacial dilational viscoelastic property is due to surfactant


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relaxation process on the interface and in the bulk of the liquid. The relaxation process consists of three parts: surfactant rearrangement on the interface, diffuse-exchange between the interface and the oil bulk and change in surfactant configuration,11 and the latter usually occurs when the interfacial tension becomes constant.9 When the sinuous oscillation is induced, the surfactant’s hydrophilic groups concentrate on the interface and the alkyl chains are squeezed out of the surface layer under compression and pulled in under expansion.34 Also, surfactant exchanges between the oil-bulk and the interface within several tens of seconds as the fast relaxation progresses. The relaxation in surfactant configuration is a slow relaxation process of inner strains of a polymer chain, in a time frame of 1000s to several tens of seconds.11, 34 And this occurs when surfactant has reached its equilibrium adsorption with a constant interfacial tension. As the experiment duration in our system is 30s, it is obvious that monoolein rapidly experiences the fast relaxation process while PETO-B does it slowly. Surfactants’ different responses to the interfacial deformation will result in different viscoelastic properties of the interfacial films. As the definition indicates, there is a time shift between the variation of interfacial area and interfacial tension at the peak values as the droplet expands and compresses with the sinusoidal signal. The dilational elastic modulus represents the ability of the interface to restore its initial shape when oscillation occurs. Higher elastic modulus connotes the surfactant film can store more energy and prevent deformation. Lower elastic modulus indicates the film is readily to break up. The viscous modulus elucidates the irreversible loss of energy of surfactant molecules due


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to the flow resistance and molecules interaction during the oscillation. 4.4.2 Interfacial elastic modulus of a single surfactant system. Surfactants accumulate at the oil-water interface forming a thin film. The property of the film depends on the density and intensity of surfactant arrangement. When two emulsion droplets collide, their films squeeze onto each other and deform to an extreme state. At that state, both films create a weakest point where the surfactant layers break up, offering an extraordinary opportunity for them to coalesce. If the films had intensive elasticity inhibiting the film to break up, then the droplets would bounce off and fail to coalesce. However, if the films have low elasticity, the droplets would merge into one bigger droplet achieving coalescence.6 Therefore, emulsion with highly elastic surfactant layers is relatively stable that works against coalescence, and the opposite is also true. Effect of Concentration: At a low concentration, surfactant tends to spread on the interface to form a two-dimension structure with weak intermolecular force. These characteristics of arrangement typically give the film low interfacial elastic and viscous modulus. As the surfactant concentration increases, surfactant molecules gradually align straightly with their heads contacting water and their tails stretching into the oil bulk, forming a three-dimension structure.10 Such arrangement makes the surfactant film more difficult to break up and induces large energy loss due to intermolecular interaction subject to oscillation. As has been discussed, IFT gradient exists in two situations when oscillation occurs: (1) between the old and the new-born interfaces, and (2) between the bulk and


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the interface. Accordingly, surfactant compensates the IFT gradient on the interface by two ways: re-arrange at the interface by Marangoni effect, and exchange between the interface and the bulk instantly.13 Therefore, surfactants with fast migration rate can afford to compensate the IFT gradient and maintain the stability of the film;23 while surfactants with low migration rate will likely fail to do so. Figure 7 shows that the elastic modulus increases with the increase of surfactant concentration in a single surfactant system for both monoolein and PETO-B, due to the faster migration rate and higher interface surfactant coverage. Obviously, at each oscillation frequency, the interfacial film formed by monoolein has much higher elastic modulus than that of PETO-B. This means the water-in-fuel emulsion formed with monoolein is more stable than that with PETO-B, which is consistent with the trend of the measured interfacial tension. At the same surfactant concentration, the interfacial tension of monoolein-additized-fuel

and

water

is

much

lower

than

that

of

PETO-B-containing-fuel and water, as indicated in Table 2. Effect of Oscillation Frequency: For monoolein, the increase in oscillation frequency raises the elastic modulus similarly at each surfactant concentration, as is shown in Figure 7. This behavior is much anticipated, but the reason needs to be carefully examined, as the increase in oscillation frequency does not statistically promote the surfactant migration rate in the bulk of the fuel within the scope of our experiments, as has been illustrated in Figure 4. For PETO-B, the changing trend of film elasticity induced by oscillation behaves a bit differently. Lower frequency leads to higher elastic modulus, especially at low


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surfactant concentrations, and the trend can still be observed but not so obviously at high concentrations. Phrased in other words, stronger disturbance does not necessarily promote the film strength of PETO-B.

Figure 7. The Elastic modulus of fuel/water interfaces consisting of monoolein and PETO-B under different oscillation frequencies

Normally, higher frequency produces larger interface deformation rate that requires surfactant to compensate the local interfacial tension variation quickly to maintain the film strength. The increased elastic modulus of monoolein with increased oscillation frequency comes from the more rapid rearrangement of interfacial molecules due to the molecule’s small size and flexibility that respond quickly to disturbance. These molecules can move freely on the interface because the surface is not saturated with surfactant. Higher oscillation frequency enhances PETO-B’s migration rate when the concentration of the surfactant is small; when the concentration is large, the difference is dismissed as shown in Figure 7. For PETO-B, the change in film elasticity under oscillation is also mainly due to local interfacial molecule arrangement because of the


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low migration rate in the bulk on average. However, due to the large molecule size and the steric hindrance created by the molecule itself, plus the more saturated interface particularly at high surfactant concentrations, higher frequency causes the molecules to tumble into each other, leading to random molecule arrangement that reduces the film’s elastic strength. On contrary, low oscillation frequency offers sufficient time for PETO-B molecules to orderly re-arrange themselves at the interface to compensate the interfacial gradient,35 and hence resulting in higher elastic modulus. 4.4.3. Interfacial viscous modulus of single surfactant system. Dilational viscous modulus is induced by irreversible surfactant intermolecular interaction. Surfactant concentration and oscillation frequency both have a great impact on it. Effect of Concentration: At a low concentration of 20 ppm, the head of monoolein molecules interacts with water molecules and their tails spread along the interface. The intermolecular interaction between the surfactant molecules is weak resulting in low viscous modulus, as is shown in Figure 8. As the surface surfactant coverage concentration (Γ) of monoolein increases due to concentration increase, their tails stretch into the fuel bulk and the intermolecular interaction between the alkyl chains strengthens. Therefore, the viscous modulus of monoolein has a steep increase and then flattens out as monoolein concentration reaches beyond 20 ppm, where the molecules arrange more tightly on the interface. For PETO-B film, the viscous modulus slightly increases with the concentration attributed to the loose arrangement


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of molecules in the concentration range.13

Figure 8. The viscous modulus of fuel/water interfaces consisting of monoolein and PETO-B under different oscillation frequencies

Effect of Oscillation Frequency: As shown in Figure 8, for both monoolein and PETO-B films, the viscous modulus increases with decreasing oscillation frequency. This phenomenon embodies that low oscillation frequency offers a longer response time that allows the intermolecular interaction to become stronger.

4.4.4 Phase angles of the two surfactants The phase angle is the ratio of the viscous modulus versus the elastic modulus that represents the quantitative characterization of the interfacial film. As indicated in Figure 9, the phase angle of monoolein film increases quickly when the surfactant concentration rises from 20 ppm to 50 ppm. This is due to the change in interfacial film structure, from a loose arrangement to a tight arrangement, as has been discussed earlier. At 300 ppm, the film of monoolein presents a highly elastic character at 0.5 Hz since the phase angle is the smallest compared with other oscillation frequencies.


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For PETO-B, at low concentration and high frequency (0.25 and 0.5 Hz), the film has more energy loss, as reflected by the higher phase angles observed than other situations. As a tighter surfactant film is formed with the increase in surfactant concentration, the phase angle becomes stable. At the meanwhile, all the phase angles are smaller than 10°, meaning the films are more elastic than viscous.

Figure 9. The phase angle of monoolein and PETO-B films

4.5. Dilatational viscoelastic modulus of binary surfactant system at water-oil interface. When the systems contain two surfactants, the situation becomes more complicated, because the two surfactants mutually block each other when they migrate in the bulk and compete for spaces on the water-fuel interface. Here we chose different M/P (monoolein/PETO-B) mass ratios to fabricate binary surfactant systems with total surfactant concentration of 20 to 300 ppm, in accordance with the single surfactant systems. The mass M/P ratios are 1:3, 1:1, 3:1 and the corresponding molar ratios are 0.56:1, 1.67:1 and 5.03:1 respectively. The viscoelastic modules are


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measured the same way using the oscillation pendant drop method and the results are given and discussed below.

4.5.1 The elastic modulus of Monoolein/PETO-B system

Figure 10. The Elastic modulus of monoolein-PETO-B binary surfactant system with different mass ratio ( M:P 1:3 (a), M:P 1:1 (b), M:P 3:1 (c) ).

As discussed for the single surfactant systems, surface surfactant coverage has not reached its saturation adsorption capacity within the surfactant concentration range studied. This is also the case for the binary surfactants systems, meaning surfactants adsorbed on the interface form a monolayer. Effect of Concentration: As Figure 10 illustrates, the elastic modulus of M:P 1:3 tends to be a constant above 100 ppm and the elastic modulus of the M:P 1:1 and 3:1 binary surfactant system increases with total surfactant concentration, similar to what is observed for the single surfactant systems. Larger surfactant concentrations create larger concentration gradients, resulting in a faster compensation when oscillation occurs. As the content of monoolein increases, the elastic modulus of the binary system also increases but with a few exceptions, likely due to experimental


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errors. When the oscillation is initiated, monoolein rapidly compensates the interfacial gradient while PETO-B has a slow migration rate. By comparing Figure 10 with Figure 7, it is seen that the elastic modulus of the binary surfactant system even with the highest M/P ratio of 3:1, is close to, but not exceeds that of the single monoolein system. Also, for the system with the lowest M/P ratio of 1:3, the elastic modulus still exceeds the modulus of pure PETO-B system. All these obviously indicate that the elastic properties of the binary surfactant system mainly come from the contribution of monoolein. Effect of Oscillation frequency:

The data in Figure 10 a-c show that the elastic

modulus of M:P binary surfactant system has an increase at higher oscillation frequencies. It may imply that stronger oscillation-induced disturbance may promote the surfactant migration rate from the fuel bulk to the interface on one hand, but on the other hand, it forces the molecules already on the interface to move more rapidly around and compensate the surface tension gradient created by the oscillation. In the M:P system, monoolein and PETO-B compete for the occupied area on the interface. The migration rate of monoolein is much faster than that of PETO-B at the same surfactant concentration, as indicated in Table 3. It is therefore reasonably to speculate that in all the three conditions (M:P 1:3, 1:1, 3:1), more rapidly transferred monoolein molecules construct the basic framework of the surfactant layer, and PETO-B molecules fill the vacancies.

4.5.2. Monoolein/PETO-B system viscous modulus


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Figure 11. The viscous modulus of M:P1:3, M:P1:1, M:P3:1 versus total surfactant concentrationunder oscillation frequency of 0.1 - 0.5 Hz.

As shown in Figure 11, the viscous modulus of the interfacial film formed by the binary surfactants increases with surfactant concentration increase from 20 to 100 ppm, and then becomes flat at 200 and 300 ppm. This is because at high surfactant concentrations, the surfactant films are already tightly compacted that further increase in concentration does not elevate intermolecular interaction. From this figure, it is also seen that at and below 100 ppm, the viscous modulus at all oscillation frequencies ranks as: M:P 1:3 < M:P 1:1 < M:P 3:1. It indicates that when more monoolein molecules transfer to the interface, more viscous energy dissipation occurs. And this phenomenon also proves that monoolein plays a dominant role in determining the film’s rheological properties that decide the emulsion’s stability. Increased oscillation frequency decreases the viscous modulus for the binary surfactant system at all surfactant concentrations, again this coincides with the observation for the single surfactant systems. The reason is that lower oscillation


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frequency provides surfactants molecules more time and opportunities to interact among themselves, rearrange on the interface and exchange between the bulk and the interface.36 Zhou et al.12 indicated that a tight surfactant film would be weakened by insertion of other active components at the crude oil/water interface. The spots occupied by PETO-B offer steric hinderance and loosen up the arrangement of monoolein layer. Compared with single monoolein surfactant system, a loosened arrangement of monoolein and PETO-B molecules leads to a lower elastic modulus. Therefore, the elastic modulus of a binary system is lower than that of a single surfactant system.

4.5.3. Monoolein/PETO-B system phase angles

Figure 12. The phase angle of M:P1:3, M:P1:1, M:P3:1 systems under different oscillation frequency ( 0.1 Hz (a), 0.25 Hz (b), 0.5 Hz (c) )

As shown in Figure 12, the phase angle of the binary surfactant systems ranges from 1° to 6°. It tends to decrease when monoolein content is higher in the binary composition, indicating a more elastic surfactant film, which is attributed to the larger contribution of monoolein. Also, it is interesting that the phase angle is not significantly affected by the oscillation frequency. This makes sense as an emulsion’s stability is a thermodynamic property, the interfacial film’s strength should not


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depend on external physical or mechanical forces.22 The stability of an emulsified water-surfactant-fuel system is the result of spontaneous movement of surfactants to and from the interface that is thermodynamically controlled. External mechanical forces, for example, can facilitate the transfer of surfactant to the interface but once the surfactant molecules are in position of forming an interfacial film, such forces will not be able to cause the destabilization of the emulsion unless thermo or chemical effects can be accompanied with these forces, such as microwave or ultrasonication.

5. CONCLUSIONS. Interfacial rheology is an important measure to evaluate an emulsion stability, and the elastic modulus of the surfactant film formed either by monoolein or PETO-B correlates well with interfacial tension measurement in this study. The strength of the interfacial film mainly depends on the nature of the surfactant molecules. Monoolein has small molecule weight without branches and PETO-B has large molecule weight with branches offering steric hinderance. Surface surfactant coverage, average migration rate and the area occupied by a single molecule on the water-fuel interface subject to external disturbances are calculated to analyze the viscoelastic modulus. Surface surfactant coverage of monoolein increases with its concentration increase, indicating that more surfactant molecules have transferred to the interface forming a more tightly packed surfactant layer. Surfactant molecules with fast migration rate such as monoolein have rapid response to interfacial tension gradient and vice versa. The occupied area of a single molecule on the water-fuel interface also proves that monoolein has a tight arrangement on the interface while PETO-B has a loose one. A loose arrangement of surfactant leads to a low elastic modulus that is unable to resist


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droplet deformation, while a tightly packed arrangement such as monoolein performs the opposite. Oscillation frequency has a great influence on the film’s viscous modulus as it induces different levels of intermolecular interactions such as internal molecule friction and resistance to flow. For both the single and binary surfactant systems studied herein, increased surfactant concentration endows the interfacial film with stronger elastic property than viscous property as indicated by the phase angles. For binary systems, the fundamental structure of the film is mainly decided by the surfactant with a faster migration rate. The elastic modulus of binary system is lower than that of single surfactant system, which provides an opportunity to alleviate the challenge of emulsion separation by re-designing additive packages for diesel fuels.

ACKNOWLEDGEMENT

This work is supported by the National Key Research and Development Program of China under the contract number of 2017YFB0308000 and the National Natural Science Foundation of China under the grant number of 21476237

REFERENCES (1) Zhang, Q.; Li, L.; Li, Y.; Cao, L.; Yang, C. Surface Wetting-Driven Separation of Surfactant-Stabilized Water–Oil Emulsions. Langmuir 2018, 29 (29). (2) Wu, T. H.; Gong, R. J.; Zhang, X. G.; Sheng, C. X. A Conductivity-Based Sensor for Detecting Micro-Water in On-Line Oil Analysis. Advanced Materials Research 2014, 850-851, 279-283. (3) Narayan, S.; Moravec, D. B.; Hauser, B. G.; Dallas, A. J.; Dutcher, C. S. Removing Water from Diesel Fuel: Understanding the Impact of Droplet Size on Dynamic Interfacial Tension of Water-in-Fuel Emulsions. Energy Fuels 2018, 32 (7), 7326-7337.


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