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Probing the interaction mechanism between oil droplets with asphaltenes and solid surfaces using AFM
Journal of Colloid and Interface Science 558 (2020) 173–181
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Probing the interaction mechanism between oil droplets with asphaltenes and solid surfaces using AFM Chen Shi 1, Lei Xie 1, Ling Zhang, Xi Lu, Hongbo Zeng ⇑ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada
g r a p h i c a l a b s t r a c t Wetting phenomena of oil/water/solid systems at the nanoscale under the effect of salinity, oil type, asphaltenes concentration and surface hydrophobicity.
a r t i c l e
i n f o
Article history: Received 29 July 2019 Revised 23 September 2019 Accepted 24 September 2019 Available online 25 September 2019 Keywords: Surface forces Oil droplets Wetting Hydrophobic interaction DLVO theory AFM
a b s t r a c t Wetting phenomena of oil/water/solid systems are fundamentally governed by the stability of confined water film and interaction mechanism between oil droplet and solid surface in water. Herein, droplet probe AFM was used to quantify the surface forces of model oil droplets including toluene and heptol in presence of interfacial asphaltenes interacting with mica surfaces of varied hydrophobicity in different water environments. It was found that adsorption of asphalenes at oil/water interface could result in the enhanced electrical double layer (EDL) repulsion at low salinity while strengthen the steric repulsion at high salinity, both of which contributed to a more stable water film between oil droplets and mica surfaces, inhibiting oil droplet attachment. Addition of heptane strengthened the repulsive EDL force and steric hindrance since more asphaltenes were adsorbed onto the interface. For hydrophobized mica surface, the attractive hydrophobic interaction could overcome steric hindrance due to interfacially adsorbed asphaltenes, thereby inducing strong attachment and adhesion of oil droplet. Our results demonstrate the nanomechanical mechanism underlying the interactions between oil droplets and solid surfaces in presence of interfacial materials, which can further explain the wetting of oil/water/solid systems in many engineering applications such as oil fouling and corrosion, and oil/water separation. Ó 2019 Elsevier Inc. All rights reserved.
⇑ Corresponding author. 1
E-mail address: [email protected] (H. Zeng). Chen Shi and Lei Xie contributed equally to this work.
https://doi.org/10.1016/j.jcis.2019.09.092 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
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1. Introduction Wetting phenomena of a solid surface play critical roles in many conventional and modern technologies, such as crude oil recovery by water flooding [1–3], water-based bitumen extraction [4–6], 3D printing [7,8], oil/water separation [9,10], surface coating and cleaning and many others [11–13]. In oil/water/solid system, the wetting phenomena are largely governed by the stability of the confined water film, with a thick and stable water film corresponding to the water-wet surface while an unstable water film leading to the oil-surface contact [14–17]. Intrinsically, the thickness and stability of the confined water film are governed by the surface interactions between oil and solid across a water film. Typically, strong electrical double layer (EDL) repulsion can sustain a thick and stable water layer [3,17–19], while attractive van der Waals (VDW) and hydrophobic interactions will result in rupture of the water layer and attachment of oil onto the solid surface [20–22]. Therefore, direct quantitative measurements of these surface forces and interaction mechanisms in oil/water/solid systems will facilitate fundamental understanding and practical prediction of the wettability of solid surfaces under complex solution conditions. In oil industries, understanding the wettability of oil/water/solid systems and the related effects on oil recovery has long been a challenging issue for engineers and researchers [2,23–26]. In practice, these three phases involved are usually comprised of complex components, and thus their characteristics are influenced by a convoluted interplay among different factors (e.g. temperature, pressure, adsorption of interfacially active components) [27–29]. Of all the components in crude oil, asphaltenes are believed to be the major cause of most critical issues encountered in oil production [26,30–33]. With the hydrophobic polyaromatic cores coupled with hydrophilic polar groups, asphaltenes can spontaneously adsorb onto the interfaces, which could significantly alter the properties and behaviors of the interfaces and influence the surface forces in the oil/water/solid system [3,34–37]. Adsorption of asphaltenes on solids was reported to alter the wettability of the surfaces [24,29,37–40], and that on oil/water interfaces could change the surface potential, interfacial tension and some other properties [3,25,31,35,41]. On the other hand, the water chemistry (e.g. salinity, salt type and pH) also critically impacts the surface forces in the oil/water/solid system [3,25,41,42]. Therefore, a quantitative unraveling of the impacts of asphaltenes on the surface interactions of oil/water/solid systems with complex phases is of great importance, which will eventually help predict their wetting behavior under specified conditions. During recent years, much attention has been paid to quantify the intermolecular forces involving asphaltenes and asphaltenes models in various media using various nanomechanical tools involving surface forces apparatus (SFA), atomic force microscope (AFM), etc. [25,43–47]. The force measurements between asphaltenes-coated silica and bare silica surface in aqueous solutions using AFM showed that the adsorbed asphaltenes could alter the surface potential of the silica surface, which also varied with the salinity and pH of the solution [4,25]. SFA was also applied to quantify the intermolecular forces of asphaltenes interacting with mica surface in brine solutions, which were correlated with the wettability of mica by oil and brine [2,23]. Recently, the droplet/ bubble probe AFM was developed for quantifying the surface forces involving deformable oil droplets and air bubbles in various systems [48–58]. This technique was utilized to quantify the surface interactions between oil (or water) droplets across water (or oil) medium in presence of asphaltenes, and the measured interaction mechanism of emulsion droplets provided significant implication on elucidating the stabilization mechanism of the emulsions with asphaltenes at the nanoscale [3,42].
In this work, we quantitatively probed the interaction mechanism between oil droplet and solid surface in presence of asphaltenes using the droplet probe AFM. The freshly cleaved mica and hydrophobized mica due to their molecular smoothness were used as the model clay with different wettability. The influence of salinity, oil type, asphaltenes concentration and surface hydrophobicity on the oil droplet-solid surface interaction in water medium were investigated. The measured force data were interpreted using the theoretical calculations on the basis of Reynolds lubrication theory and Young-Laplace equation. The study provides the nanomechanical insight into the wettability of oil/water/solid system with interfacially active materials, and this methodology can be versatilely applied to a broad range of engineering systems to decipher the intrinsic wetting mechanism. 2. Materials and methods 2.1. Materials The HPLC grade of toluene (99.8%), heptane (99%) and dichloromethane (DCM) were purchased from Fisher Scientific. Highestpurity octadecyltrichlorosilane (OTS, ACROS Organics), 1dodecanethiol (Sigma Aldrich) and sodium chloride (NaCl, Fisher Scientific) were purchased. 2.2. Preparation of asphaltenes solution The extraction of asphaltenes from crude oil samples (Shell) follows a reported ASTM IP143 procedure [3,59]. The asphaltenes solution was prepared in toluene or 1:1 heptol (heptane and toluene mixture 1:1 v/v). After sonicating for 30 min, the solution was stored in fridge and re-sonicated for 30 min to facilitate the well dispersion of asphaltenes prior to each experiment. 2.3. Interfacial tension (IFT) measurement A tensiometer was applied to measure the IFT of asphaltenes solution in aqueous phase by analyzing and fitting the drop shape. These values at equilibrium were used for theoretically analyzing the force data measured using droplet probe AFM. The interfacial tension between a toluene droplet and pure water was measured to be 35.5 ± 0.2 mN/m, which was very stable with time, revealing the purity and stability of the toluene/water system. For measurements of asphaltenes solutions, the interfacial tension was found to gradually decrease with time. The values at equilibrium were used for theoretical analysis of the force results measured using droplet probe AFM. The toluene/water interfacial tension with addition of 0, 10, and 100 mg/L asphaltenes was measured to be 35.5, 34.0, and 32.0 mN/m, respectively. The heptol/water interfacial tension with addition of 0, 10 and 100 mg/L asphaltenes was measured to be 40.1, 39.2 and 37.0 mN/m, respectively. 2.4. Preparation of hydrophilic and hydrophobic mica surfaces Freshly cleaved mica was used as model hydrophilic solid surface for its molecular smoothness, and the mica surface was also hydrophobized using OTS through a previously reported vapor deposition method [20]. Briefly, freshly prepared mica surfaces were exposed to OTS vapor for about 3 days to achieve a water contact angle of about 90°. 2.5. Droplet probe AFM measurements Fig. 1 shows a schematic of force quantification of an oil droplet interacting with a solid surface using droplet probe AFM equipped
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The augmented Young-Laplace equation shows the oil droplet deformation attributed to the Laplace pressure, disjoining pressure P and hydrodynamic pressure p:
c @
r @r
r
@h 2c ¼ pP @r R
ð2Þ
where R is the radius of oil droplet and c is the IFT of oil/water interface. The disjoining pressure P is arising from various intermolecular forces involving VDW, EDL, hydrophobic interaction, and so on. The disjoining pressure arising from VDW and EDL interaction between oil droplet and solid surface can be described as Eqs. (3) and (4), respectively.
PVDW ½hðr; tÞ ¼
Fig. 1. Experiment setup of droplet probe AFM for measuring the interaction forces between an oil droplet of radius R and a solid surface in aqueous solution.
with the inverted microscopy. For each experiment, a controlled de-wetting method was applied to generate and immobilize oil droplets on the AFM glass disk, and solid surfaces were then placed into the fluid cell after oil droplets generation [3,18,60]. The custom-made tipless silicon cantilever (400 70 2 lm) with a circular gold patch (diameter 65 lm, thickness 30 nm) at the end of the cantilever was used. The gold patch was hydrophobized by dodecanethiol for firmly immobilizing the hydrophobic oil droplet in aqueous solutions. The oil droplet used for force measurements typically has the radius of 45–80 lm, which allows the immobilized oil droplet to be spherical in geometry and ensures the force measurements not to be affected by the droplet deformation induced by the immobilization. An oil droplet probe was created by picking up an oil droplet with a tipless AFM cantilever. Afterwards, the oil droplet probe was moved over a solid surface, and then force measurements were performed by lowering and raising the cantilever toward the surface. It is known that the hydrodynamic effect is weaker at a reduced velocity. The hydrodynamic interaction dominates the overall interactions at a very high velocity (>30 lm/s) [20,49]. Unver a low velocity (1 lm/s), the hydrodynamic interaction is sufficiently weak that surface forces dominate the overall interactions [20,49]. In this work, the driving velocity of the probe was chosen as ~1 lm/s to investigate the influence of surface forces. For the experiments with asphaltenes, interaction forces were measured after 30 min following oil droplet generation, which was consistent with the IFT measurements. It is noted that the piezo displacement represents the relative movement of the cantilever, and the zero piezo displacement is arbitrarily defined as the position where the maximum attraction during the retraction is reached or the coalescence is observed. 2.6. Theoretical model The force data was interpreted using theoretical calculations on the basis of Reynolds lubrication theory and Young-Laplace equation [61–63]. Reynolds lubrication theory indicates the dynamic evolution of thin water film confined between the oil droplet and solid surface:
@h 1 @ 3 @p ¼ rh @t 12lr @r @r
ð1Þ
where t is the time, r is the radial coordinate, p is the hydrodynamic pressure, l is the viscosity of water, and h is the thickness of confined water film. According to recent reports, both water/oil and water/solid interfaces are considered non-slippery [48–58].
PEDL ðhÞ ¼
AH
ð3Þ
3
6p h
2e0 ej2 ½ðeþjh þ ejh uo us ðu2o þ u2s Þ ðeþjh ejh Þ
j ¼ ð2q0 e2 =e0 ekB TÞ
1=2
2
ð4Þ
ð5Þ
where AH is the Hamaker constant, uo and us are the surface potential of oil/water interface and water/solid interface, e0 is the vacuum permittivity, e is the dielectric constant of the medium, e is the fundamental charge, and q0 is the number density of ions. In 1 mM NaCl, the Debye length is ~9.6 nm and the EDL interaction is very strong, which can be calculated based on Eq. (4). When measuring the interaction in 100 mM NaCl, the EDL interaction is significantly suppressed with the Debye length of ~0.96 nm, and thus the EDL interaction can be neglected during the analysis of force results [14]. The overall interactions F(t) can be summerized by:
Z FðtÞ ¼ 2p
0
1
½pðr; tÞ þ PðhÞrdr
ð6Þ
More detailed theoretical calculations can be found in previous reports [48–58]. 3. Results and discussion 3.1. Interaction between oil droplet and hydrophilic mica in 1 mM NaCl Measured interaction results between a toluene droplet and a hydrophilic mica surface in 1 mM NaCl with presence of various concentrations of asphaltenes are shown as green symbols in Fig. 2. Positive values of measured force refer to repulsive force, while negative values indicate the attraction. The arrows represent the movement of the toluene droplet during force measurements. Strong repulsion was detected in Fig. 2A when a pure toluene droplet was driven to approach a mica surface until a maximal repulsion of ~40 nN was measured. Afterwards, the toluene droplet was retracted from the mica surface and the repulsive force decreased until a weak attraction was measured attributed to ‘‘hydrodynamic suction” effect. During the approach-retraction cycle, the toluene droplet remained stable without the oil-solid attachment as observed from the optical microscopy, even if higher force load was applied. It is known that both toluene and mica carry negative charges in 1 mM NaCl solution at natural pH ~5.6 [3,64,65], and thus the EDL interaction is repulsive and may be sufficient to sustain a stable water layer between the oil and mica surface. The surface potential of toluene/water interface can be adopted from previous reports as 35 ± 5 mV [3]. The Hamaker constant for VDW force between toluene and mica in water can be calculated to be 1:36 1020 J by considering the physical parameters of
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Fig. 2. Interaction forces (A–C) and profiles at maximum loading force (D–F) of the toluene droplet-hydrophilic mica interaction with various concentrations of asphaltenes in 1 mM NaCl at natural pH of ~5.6. (r = 60 lm for A and D, r = 50 lm for B and E, r = 48 lm for C and F, Green symbols: experiment data, black lines: fitting results). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
toluene, mica and water [14], and the positive value suggests that VDW interaction is attractive. With these parameters, the aforementioned theoretical model was used to fit the measured force data. Clearly, the measured interaction results (green symbols) could be well fitted by the theoretical calculations (black curve), based on which the surface potential of mica is determined to be 70 ± 8 mV, agreeing with previously reported value [64,65]. The calculated profiles in Fig. 2D show [1–4] the central droplet region is flattened attributed to the balance between the Laplace pressure inside the toluene droplet and the external EDL repulsion. The calculated water film thickness was 32.9 nm, indicating that the EDL repulsion sustained a thick water layer that effectively prevented the droplet attachment onto the mica surface under this condition. The measured results with asphaltenes addition in toluene are also listed as green symbols in Fig. 2(B and C), respectively. The theoretical model (black curves) was also used to calculate the interaction forces using the previously obtained surface potential of mica and literature reported potential values of toluene droplet [3]. The excellent consistency between the measured force data and calculated results verified the validity of fitted surface potential of mica, and suggested the interaction mechanism involved could be well explained by the theoretical model. It is noted that the surface potential values of oil droplets obtained from the force profiles are consistent with their zeta potential values under the same solution conditions [3]. The more negative surface potential with addition of asphaltenes was due to the adsorption of asphaltenes at toluene/water interface, which could further strengthen the EDL repulsion between toluene droplet and mica surface and thereby sustain a thicker water film. The calculated profiles of toluene droplets at maximal force load clearly demonstrate that the confined water film thickness increased to 34.8 nm with 10 mg/L asphaltenes and 40.9 nm with 100 mg/L asphaltenes as compared to that of the pure toluene. The calculated disjoining pressure profile of the toluene/water/mica system is also shown in Fig. 3. It is evident that the EDL repulsion plays the significant role in sustaining water film and inhibiting the toluene droplet attachment. And increasing asphaltenes concentration in
Fig. 3. Different disjoining pressure profiles between a toluene droplet with various concentrations of asphaltenes and a hydrophilic mica surface in 1 mM NaCl.
oil droplet can strengthen the EDL repulsion and lead to thicker confined water film. The interaction forces between 1:1 heptol droplet and mica surface in 1 mM NaCl with presence of various concentrations of asphaltenes were also investigated to investigate the solvent effects on the interaction. Addition of heptane (poor solvent) is known to destabilize the asphaltenes in oil phase and facilitate the migration of asphaltenes to the interface. The theoretical model incorporated with fitted surface potential of mica was used to fit the measured force results (Fig. 4), with the surface potential of pure heptol droplet obtained as 37 ± 4 mV, which was close to that of toluene droplet. With 10 and 100 mg/L asphaltenes addition, the surface potential became 60 ± 5 and 85 ± 7 mV, respectively. The change of surface potential at heptol/water interface is slightly larger than that at toluene/water interface, which could be attributed to increased amount of asphaltenes adsorbed at the interface with heptane addition.
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Fig. 4. Interaction forces (A–C) and profiles at maximum loading force (D–F) of the heptol droplet-hydrophilic mica interaction with various concentrations of asphaltenes in 1 mM NaCl at natural pH of ~5.6. (r = 60 lm for A and D, r = 48 lm for B and E, r = 50 lm for C and F, Green symbols: experiment data, black lines: fitting results). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Interaction forces of toluene droplet-hydrophilic mica interaction (A, B and C) and heptol droplet-hydrophilic mica interaction (D and E) with various concentrations of asphatenes in 100 mM NaCl at natural pH ~5.6. (r = 50 lm for A, r = 45 lm for B, r = 55 lm for C, r = 80 lm for D, r = 75 lm for E, Green symbols: experiment data, black lines: fitting results). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.2. Interaction between oil droplet and hydrophilic mica in 100 mM NaCl Fig. 5A shows the interactions between a pure toluene droplet and a hydrophilic mica surface in 100 mM NaCl. A weak jump-in behavior, i.e. a sudden decrease of the measured force, was observed with the repulsion reaching about 3 nN, which suggested the attachment of the toluene droplet onto the mica surface. After
jump-in, the interaction force first turned slightly attractive due to the capillary bridging of the toluene droplet, and then became repulsive again as the cantilever further compressed the droplet. When the droplet was retracted, adhesion was registered before the full droplet detachment from the surface. It was noted that a small satellite droplet of toluene was always left on the mica surface after detachment. In 100 mM NaCl, the EDL repulsion is highly compressed and thus the VDW attraction governs the interaction
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forces, thereby inducing the attachment of toluene droplet onto the mica surface. Theoretically calculated force curve by considering the VDW attraction was found to agree excellently with the measured force curve (Fig. 5A). It should be noted that the mica surface is highly hydrated in water, and the hydrated ions have been known to cause the short-range repulsion between mica surfaces in water [64]. In our case, it is likely that a certain amount of hydrated ions may be trapped between the toluene droplet and the mica surface even after the observed jump-in behavior. And the hydration repulsion can also prevent the full spreading of toluene droplet on mica surface. With addition of asphaltenes, attachment of toluene droplet on mica surface was found to be largely inhibited (Fig. 5(B–E)). For 10 mg/L asphaltenes in toluene, the jump-in behavior could not be detected when the toluene droplet approached the mica surface, but a weak adhesive force of ~5 nN, indicated by the sudden jumpout behavior, was measured when the toluene droplet was retracted from the mica surface. When more asphaltenes were added (100 mg/L), the force results became similar to those in Fig. 4 and no adhesion was measured during retraction. The effects of solvent type were also studied. As shown in Fig. 5(D and E), even addition of low concentration of asphaltenes (10 mg/L) in heptol can effectively inhibit droplet attachment with no adhesion measured. The inhibition of droplet attachment could be due to the protection from the adsorbed asphaltenes and asphaltenes aggregates at the oil/water interface. It has been reported that the interfacial asphaltenes and asphaltenes aggregates can form a robust film at the interface, resulting in the steric repulsion that can inhibit the coalescence between oil droplets [26,30–33] as well as possibly droplet attachment in our case here. When the asphaltenes concentration is low (10 mg/L here), the adsorbed asphaltenes at the interface was insufficiently high to provide the complete protection, and thereby partial droplet attachment onto mica surface occurred and slight adhesion was detected. However, when the concentration is high, more asphaltenes adsorb onto the oil/water interface, which fully inhibits the droplet attachment behavior. Since more asphaltenes are forced to migrate to the oil/water inter-
face in heptol as compared to toluene, even low concentration of asphaltenes is enough to provide the complete protection against droplet attachment. Our previous studies applied the Alexander de Gennes (AdG) model to describe the steric repulsion induced by asphaltenes, and successfully fitted the toluene droplet-toluene droplet interaction in water with asphaltenes by incorporating the AdG model into the aforementioned theoretical model [3,66,67]. The steric repulsion in the AdG model can be described as
kT Psteric ðhÞ 3 s
" 3=4 # 9=4 2d h for D < 2L h 2d
ð7Þ
where d is the characteristic length, s is the mean distance between asphaltenes anchoring sites, and kT is the product of Boltzmann constant and temperature. Here we use the AdG model to tentatively calculate the measured force curves in Fig. 5(E–G). By using s ~ 3 nm and d ~ 3 nm adapted from literature [3], the calculated force curve agreed reasonably well with the measured force curve for 100 mg/L asphaltenes in toluene, indicating the AdG model could satisfactorily describe the steric repulsion induced by asphaltenes. Slightly larger d ~ 3.2 and 3.4 nm were used for the 10 and 100 mg/L asphaltenes in heptol cases to better fit the force curves. The slight increment in d might be due to more aggregation induced by addition of heptane. 3.3. Interaction between oil droplet and hydrophobized mica in 100 mM NaCl Mica surfaces hydrophobized with OTS were used as model hydrophobic surfaces to investigate the interaction mechanism between oil droplet and hydrophobic solid. The green squares in Fig. 6A show the measured results between a pure toluene droplet and a hydrophobized mica surface in 100 mM NaCl. A rapid jumpin behavior indicated the toluene droplet attachment was detected with the repulsion reaching about 1.5 nN, which was much less
Fig. 6. Interaction forces of toluene droplet-hydrophobized mica interaction (A–C) and heptol droplet-hydrophobized mica interaction (D and E) with various concentrations of asphatenes in 100 mM NaCl at natural pH ~5.6. (r = 45 lm for A, r = 55 lm for B, r = 45 lm for C, r = 42 lm for D, r = 74 lm for E, Green symbols: experiment data, black lines: fitting results). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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than that of hydrophilic mica surface shown in Fig. 5A. After jumpin, the interaction force rapidly decreased to negative maximum without subsequent increase, and the toluene droplet was detached from the cantilever and immobilized on the surface during retraction. Both of the phenomena suggested strong adhesion of the toluene droplet onto the hydrophobized mica surface. Since the VDW interaction is only weakly attractive to induce weak attachment as shown in Fig. 5A, the rapid attachment and strong adhesion between toluene droplet and hydrophobized mica surface are expected to be induced by the strong hydrophobic attraction here. A previously reported exponential equation for hydrophobic interaction potential involving deformable liquid droplets was incorporated within the theoretical model to calculate the forces between toluene droplet and hydrophobized mica surface, which was described as [20,48]:
W HB ðhÞ ¼ cð1 coshw Þexpðh=DH Þ
ð8Þ
where hw is the solid’s water contact angle in oil and DH is the decay length. And hence the disjoining pressure arising from hydrophobic interaction can be calculated as
PHB ðhÞ ¼ cð1 coshw Þ=DH expðh=DH Þ
ð9Þ
Here, the water contact angle of hydrophobized mica surface in toluene was measured to be ~120° using a goniometer and the characteristic length DH was adapted from previous reports to be 1 nm. The calculated force curve was found to agree quite well with the measured force data, confirming the determining role of hydrophobic interaction in attachment of toluene droplet. The influence of asphaltenes concentration on the interactions between oil droplet and hydrophobized mica surface was shown in Fig. 6(B–E). With addition of asphaltenes, both force curves in Fig. 6(B and C) showed sudden jump-in behaviors with the repulsion reaching about 1.5 nN, which was similar to that in Fig. 6A. The theoretically calculated force results by incorporating the effects of hydrophobic interaction were consistent with measured force data. As mentioned previously, asphaltenes was found to lead to steric repulsion to inhibit the attachment of toluene droplet onto hydrophilic mica surface in 100 mM NaCl. However, the similarity between the force curves with and without asphaltenes in toluene shown in Fig. 6(A–C) indicates the hydrophobic attraction dominates over other surface forces, which induces the oil droplet attachment on hydrophobized mica surfaces. Fig. 6(D and E) show the interaction of heptol droplet with asphaltenes addition. It was interesting to note that weak jump-in was first detected after a slight repulsion, and a second severe jump-in was measured after the linear increment of interaction force. This double jump-in behavior might be attributed to the steric hindrance of more asphaltenes and asphaltenes aggregates at the heptol/water interface. Asphaltenes can form more aggregates in presence of heptane, leading to stronger steric hindrance. Therefore, when the heptol droplet approaches the surface, the hydrophobic asphaltene aggregates may first attach to the surface, corresponding to the first jump-in. The attached asphaltenes aggregates can provide some steric repulsion to prevent further approach of heptol droplet, leading to the increase of interaction force after the first jump-in. Thereafter, the heptol droplet is further compressed towards the surface, and the hydrophobic attraction finally triggers full attachment of the heptol droplet on the hydrophobized mica surface. 4. Conclusions In this study, for the first time we quantitatively understand the significant role of salinity, oil type, asphaltenes concentration and
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surface hydrophobicity in modulating the oil-solid interaction mechanism in aqueous medium, particularly at the nanoscale. The EDL repulsion was found to prevent the oil droplet attachment onto hydrophilic mica surface in low salinity solution. Adsorption of asphaltenes at oil/water interface could result in the enhanced EDL repulsion at low salinity while strengthen the steric repulsion at high salinity, both of which contributed to a stable water film between oil droplet and hydrophilic mica surface, thereby inhibiting oil droplet attachment in water with both low and high salinity. It was also found that addition of poor solvent led to more adsorption of asphaltenes at the interface, which strengthened the EDL repulsion and steric repulsion. For the surface interactions between oil droplet and hydrophobized mica surface, the hydrophobic interaction could induce strong attachment and adhesion of oil droplet by overcoming the steric hindrance. Using oil droplet probe AFM, we proposed that the oleophilicity of hydrophilic solids in water can be enhanced by increasing the salinity and hydrophobicity of solid surfaces as well as decreasing poor solvent content and interfacial materials. Previously, the quantitative force measurements of asphaltenes were mainly conducted by driving an asphlantenes-coated substrate to interact with solid surfaces such as another asphaltenes-coated substrate and bare silica in both aqueous and organic solutions [25,43–47]. These force measurements were performed using the simple solid/liquid/solid system and asphaltenes were immobilized on solid substrates, which is different from the practical oil production with the complex oil/water/solid system and asphaltenes are mostly mobilized at the oil/water interface. The oil droplet probe AFM could help realize the quantitative measurements of an oil droplet interacting with a solid surface in water with interfacially mobilized asphaltenes. This study provides insights into unraveling the wetting mechanism of oil/water/solid systems in oil production, which could be extended to investigate the molecular mechanism of other engineering systems such as oil fouling and corrosion, and oil/water separation. For future study, the correlation between the nanoscopic interaction mechanism of oil/water/solid systems with interfacial materials (e.g., polymers, nanoparticles, proteins) and the macroscopic performance in different applications could be established. In some specific systems (e.g., porous membranes), the characterization of surface wettability is particularly challenging, while droplet probe AFM could provide useful data to help optimize the structure toward high oil/ water separation efficiency. Acknowledgements This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), the Alberta Advanced Education & Technology Small Equipment Grants Program (AET/SEGP), and the Canada Research Chairs Program.
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Probing the interaction mechanism between oil droplets with asphaltenes and solid surfaces using AFM Chen Shi 1, Lei Xie 1, Ling Zhang, Xi Lu, Hongbo Zeng ⇑ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada
g r a p h i c a l a b s t r a c t Wetting phenomena of oil/water/solid systems at the nanoscale under the effect of salinity, oil type, asphaltenes concentration and surface hydrophobicity.
a r t i c l e
i n f o
Article history: Received 29 July 2019 Revised 23 September 2019 Accepted 24 September 2019 Available online 25 September 2019 Keywords: Surface forces Oil droplets Wetting Hydrophobic interaction DLVO theory AFM
a b s t r a c t Wetting phenomena of oil/water/solid systems are fundamentally governed by the stability of confined water film and interaction mechanism between oil droplet and solid surface in water. Herein, droplet probe AFM was used to quantify the surface forces of model oil droplets including toluene and heptol in presence of interfacial asphaltenes interacting with mica surfaces of varied hydrophobicity in different water environments. It was found that adsorption of asphalenes at oil/water interface could result in the enhanced electrical double layer (EDL) repulsion at low salinity while strengthen the steric repulsion at high salinity, both of which contributed to a more stable water film between oil droplets and mica surfaces, inhibiting oil droplet attachment. Addition of heptane strengthened the repulsive EDL force and steric hindrance since more asphaltenes were adsorbed onto the interface. For hydrophobized mica surface, the attractive hydrophobic interaction could overcome steric hindrance due to interfacially adsorbed asphaltenes, thereby inducing strong attachment and adhesion of oil droplet. Our results demonstrate the nanomechanical mechanism underlying the interactions between oil droplets and solid surfaces in presence of interfacial materials, which can further explain the wetting of oil/water/solid systems in many engineering applications such as oil fouling and corrosion, and oil/water separation. Ó 2019 Elsevier Inc. All rights reserved.
⇑ Corresponding author. 1
E-mail address: [email protected] (H. Zeng). Chen Shi and Lei Xie contributed equally to this work.
https://doi.org/10.1016/j.jcis.2019.09.092 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
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1. Introduction Wetting phenomena of a solid surface play critical roles in many conventional and modern technologies, such as crude oil recovery by water flooding [1–3], water-based bitumen extraction [4–6], 3D printing [7,8], oil/water separation [9,10], surface coating and cleaning and many others [11–13]. In oil/water/solid system, the wetting phenomena are largely governed by the stability of the confined water film, with a thick and stable water film corresponding to the water-wet surface while an unstable water film leading to the oil-surface contact [14–17]. Intrinsically, the thickness and stability of the confined water film are governed by the surface interactions between oil and solid across a water film. Typically, strong electrical double layer (EDL) repulsion can sustain a thick and stable water layer [3,17–19], while attractive van der Waals (VDW) and hydrophobic interactions will result in rupture of the water layer and attachment of oil onto the solid surface [20–22]. Therefore, direct quantitative measurements of these surface forces and interaction mechanisms in oil/water/solid systems will facilitate fundamental understanding and practical prediction of the wettability of solid surfaces under complex solution conditions. In oil industries, understanding the wettability of oil/water/solid systems and the related effects on oil recovery has long been a challenging issue for engineers and researchers [2,23–26]. In practice, these three phases involved are usually comprised of complex components, and thus their characteristics are influenced by a convoluted interplay among different factors (e.g. temperature, pressure, adsorption of interfacially active components) [27–29]. Of all the components in crude oil, asphaltenes are believed to be the major cause of most critical issues encountered in oil production [26,30–33]. With the hydrophobic polyaromatic cores coupled with hydrophilic polar groups, asphaltenes can spontaneously adsorb onto the interfaces, which could significantly alter the properties and behaviors of the interfaces and influence the surface forces in the oil/water/solid system [3,34–37]. Adsorption of asphaltenes on solids was reported to alter the wettability of the surfaces [24,29,37–40], and that on oil/water interfaces could change the surface potential, interfacial tension and some other properties [3,25,31,35,41]. On the other hand, the water chemistry (e.g. salinity, salt type and pH) also critically impacts the surface forces in the oil/water/solid system [3,25,41,42]. Therefore, a quantitative unraveling of the impacts of asphaltenes on the surface interactions of oil/water/solid systems with complex phases is of great importance, which will eventually help predict their wetting behavior under specified conditions. During recent years, much attention has been paid to quantify the intermolecular forces involving asphaltenes and asphaltenes models in various media using various nanomechanical tools involving surface forces apparatus (SFA), atomic force microscope (AFM), etc. [25,43–47]. The force measurements between asphaltenes-coated silica and bare silica surface in aqueous solutions using AFM showed that the adsorbed asphaltenes could alter the surface potential of the silica surface, which also varied with the salinity and pH of the solution [4,25]. SFA was also applied to quantify the intermolecular forces of asphaltenes interacting with mica surface in brine solutions, which were correlated with the wettability of mica by oil and brine [2,23]. Recently, the droplet/ bubble probe AFM was developed for quantifying the surface forces involving deformable oil droplets and air bubbles in various systems [48–58]. This technique was utilized to quantify the surface interactions between oil (or water) droplets across water (or oil) medium in presence of asphaltenes, and the measured interaction mechanism of emulsion droplets provided significant implication on elucidating the stabilization mechanism of the emulsions with asphaltenes at the nanoscale [3,42].
In this work, we quantitatively probed the interaction mechanism between oil droplet and solid surface in presence of asphaltenes using the droplet probe AFM. The freshly cleaved mica and hydrophobized mica due to their molecular smoothness were used as the model clay with different wettability. The influence of salinity, oil type, asphaltenes concentration and surface hydrophobicity on the oil droplet-solid surface interaction in water medium were investigated. The measured force data were interpreted using the theoretical calculations on the basis of Reynolds lubrication theory and Young-Laplace equation. The study provides the nanomechanical insight into the wettability of oil/water/solid system with interfacially active materials, and this methodology can be versatilely applied to a broad range of engineering systems to decipher the intrinsic wetting mechanism. 2. Materials and methods 2.1. Materials The HPLC grade of toluene (99.8%), heptane (99%) and dichloromethane (DCM) were purchased from Fisher Scientific. Highestpurity octadecyltrichlorosilane (OTS, ACROS Organics), 1dodecanethiol (Sigma Aldrich) and sodium chloride (NaCl, Fisher Scientific) were purchased. 2.2. Preparation of asphaltenes solution The extraction of asphaltenes from crude oil samples (Shell) follows a reported ASTM IP143 procedure [3,59]. The asphaltenes solution was prepared in toluene or 1:1 heptol (heptane and toluene mixture 1:1 v/v). After sonicating for 30 min, the solution was stored in fridge and re-sonicated for 30 min to facilitate the well dispersion of asphaltenes prior to each experiment. 2.3. Interfacial tension (IFT) measurement A tensiometer was applied to measure the IFT of asphaltenes solution in aqueous phase by analyzing and fitting the drop shape. These values at equilibrium were used for theoretically analyzing the force data measured using droplet probe AFM. The interfacial tension between a toluene droplet and pure water was measured to be 35.5 ± 0.2 mN/m, which was very stable with time, revealing the purity and stability of the toluene/water system. For measurements of asphaltenes solutions, the interfacial tension was found to gradually decrease with time. The values at equilibrium were used for theoretical analysis of the force results measured using droplet probe AFM. The toluene/water interfacial tension with addition of 0, 10, and 100 mg/L asphaltenes was measured to be 35.5, 34.0, and 32.0 mN/m, respectively. The heptol/water interfacial tension with addition of 0, 10 and 100 mg/L asphaltenes was measured to be 40.1, 39.2 and 37.0 mN/m, respectively. 2.4. Preparation of hydrophilic and hydrophobic mica surfaces Freshly cleaved mica was used as model hydrophilic solid surface for its molecular smoothness, and the mica surface was also hydrophobized using OTS through a previously reported vapor deposition method [20]. Briefly, freshly prepared mica surfaces were exposed to OTS vapor for about 3 days to achieve a water contact angle of about 90°. 2.5. Droplet probe AFM measurements Fig. 1 shows a schematic of force quantification of an oil droplet interacting with a solid surface using droplet probe AFM equipped
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The augmented Young-Laplace equation shows the oil droplet deformation attributed to the Laplace pressure, disjoining pressure P and hydrodynamic pressure p:
c @
r @r
r
@h 2c ¼ pP @r R
ð2Þ
where R is the radius of oil droplet and c is the IFT of oil/water interface. The disjoining pressure P is arising from various intermolecular forces involving VDW, EDL, hydrophobic interaction, and so on. The disjoining pressure arising from VDW and EDL interaction between oil droplet and solid surface can be described as Eqs. (3) and (4), respectively.
PVDW ½hðr; tÞ ¼
Fig. 1. Experiment setup of droplet probe AFM for measuring the interaction forces between an oil droplet of radius R and a solid surface in aqueous solution.
with the inverted microscopy. For each experiment, a controlled de-wetting method was applied to generate and immobilize oil droplets on the AFM glass disk, and solid surfaces were then placed into the fluid cell after oil droplets generation [3,18,60]. The custom-made tipless silicon cantilever (400 70 2 lm) with a circular gold patch (diameter 65 lm, thickness 30 nm) at the end of the cantilever was used. The gold patch was hydrophobized by dodecanethiol for firmly immobilizing the hydrophobic oil droplet in aqueous solutions. The oil droplet used for force measurements typically has the radius of 45–80 lm, which allows the immobilized oil droplet to be spherical in geometry and ensures the force measurements not to be affected by the droplet deformation induced by the immobilization. An oil droplet probe was created by picking up an oil droplet with a tipless AFM cantilever. Afterwards, the oil droplet probe was moved over a solid surface, and then force measurements were performed by lowering and raising the cantilever toward the surface. It is known that the hydrodynamic effect is weaker at a reduced velocity. The hydrodynamic interaction dominates the overall interactions at a very high velocity (>30 lm/s) [20,49]. Unver a low velocity (1 lm/s), the hydrodynamic interaction is sufficiently weak that surface forces dominate the overall interactions [20,49]. In this work, the driving velocity of the probe was chosen as ~1 lm/s to investigate the influence of surface forces. For the experiments with asphaltenes, interaction forces were measured after 30 min following oil droplet generation, which was consistent with the IFT measurements. It is noted that the piezo displacement represents the relative movement of the cantilever, and the zero piezo displacement is arbitrarily defined as the position where the maximum attraction during the retraction is reached or the coalescence is observed. 2.6. Theoretical model The force data was interpreted using theoretical calculations on the basis of Reynolds lubrication theory and Young-Laplace equation [61–63]. Reynolds lubrication theory indicates the dynamic evolution of thin water film confined between the oil droplet and solid surface:
@h 1 @ 3 @p ¼ rh @t 12lr @r @r
ð1Þ
where t is the time, r is the radial coordinate, p is the hydrodynamic pressure, l is the viscosity of water, and h is the thickness of confined water film. According to recent reports, both water/oil and water/solid interfaces are considered non-slippery [48–58].
PEDL ðhÞ ¼
AH
ð3Þ
3
6p h
2e0 ej2 ½ðeþjh þ ejh uo us ðu2o þ u2s Þ ðeþjh ejh Þ
j ¼ ð2q0 e2 =e0 ekB TÞ
1=2
2
ð4Þ
ð5Þ
where AH is the Hamaker constant, uo and us are the surface potential of oil/water interface and water/solid interface, e0 is the vacuum permittivity, e is the dielectric constant of the medium, e is the fundamental charge, and q0 is the number density of ions. In 1 mM NaCl, the Debye length is ~9.6 nm and the EDL interaction is very strong, which can be calculated based on Eq. (4). When measuring the interaction in 100 mM NaCl, the EDL interaction is significantly suppressed with the Debye length of ~0.96 nm, and thus the EDL interaction can be neglected during the analysis of force results [14]. The overall interactions F(t) can be summerized by:
Z FðtÞ ¼ 2p
0
1
½pðr; tÞ þ PðhÞrdr
ð6Þ
More detailed theoretical calculations can be found in previous reports [48–58]. 3. Results and discussion 3.1. Interaction between oil droplet and hydrophilic mica in 1 mM NaCl Measured interaction results between a toluene droplet and a hydrophilic mica surface in 1 mM NaCl with presence of various concentrations of asphaltenes are shown as green symbols in Fig. 2. Positive values of measured force refer to repulsive force, while negative values indicate the attraction. The arrows represent the movement of the toluene droplet during force measurements. Strong repulsion was detected in Fig. 2A when a pure toluene droplet was driven to approach a mica surface until a maximal repulsion of ~40 nN was measured. Afterwards, the toluene droplet was retracted from the mica surface and the repulsive force decreased until a weak attraction was measured attributed to ‘‘hydrodynamic suction” effect. During the approach-retraction cycle, the toluene droplet remained stable without the oil-solid attachment as observed from the optical microscopy, even if higher force load was applied. It is known that both toluene and mica carry negative charges in 1 mM NaCl solution at natural pH ~5.6 [3,64,65], and thus the EDL interaction is repulsive and may be sufficient to sustain a stable water layer between the oil and mica surface. The surface potential of toluene/water interface can be adopted from previous reports as 35 ± 5 mV [3]. The Hamaker constant for VDW force between toluene and mica in water can be calculated to be 1:36 1020 J by considering the physical parameters of
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Fig. 2. Interaction forces (A–C) and profiles at maximum loading force (D–F) of the toluene droplet-hydrophilic mica interaction with various concentrations of asphaltenes in 1 mM NaCl at natural pH of ~5.6. (r = 60 lm for A and D, r = 50 lm for B and E, r = 48 lm for C and F, Green symbols: experiment data, black lines: fitting results). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
toluene, mica and water [14], and the positive value suggests that VDW interaction is attractive. With these parameters, the aforementioned theoretical model was used to fit the measured force data. Clearly, the measured interaction results (green symbols) could be well fitted by the theoretical calculations (black curve), based on which the surface potential of mica is determined to be 70 ± 8 mV, agreeing with previously reported value [64,65]. The calculated profiles in Fig. 2D show [1–4] the central droplet region is flattened attributed to the balance between the Laplace pressure inside the toluene droplet and the external EDL repulsion. The calculated water film thickness was 32.9 nm, indicating that the EDL repulsion sustained a thick water layer that effectively prevented the droplet attachment onto the mica surface under this condition. The measured results with asphaltenes addition in toluene are also listed as green symbols in Fig. 2(B and C), respectively. The theoretical model (black curves) was also used to calculate the interaction forces using the previously obtained surface potential of mica and literature reported potential values of toluene droplet [3]. The excellent consistency between the measured force data and calculated results verified the validity of fitted surface potential of mica, and suggested the interaction mechanism involved could be well explained by the theoretical model. It is noted that the surface potential values of oil droplets obtained from the force profiles are consistent with their zeta potential values under the same solution conditions [3]. The more negative surface potential with addition of asphaltenes was due to the adsorption of asphaltenes at toluene/water interface, which could further strengthen the EDL repulsion between toluene droplet and mica surface and thereby sustain a thicker water film. The calculated profiles of toluene droplets at maximal force load clearly demonstrate that the confined water film thickness increased to 34.8 nm with 10 mg/L asphaltenes and 40.9 nm with 100 mg/L asphaltenes as compared to that of the pure toluene. The calculated disjoining pressure profile of the toluene/water/mica system is also shown in Fig. 3. It is evident that the EDL repulsion plays the significant role in sustaining water film and inhibiting the toluene droplet attachment. And increasing asphaltenes concentration in
Fig. 3. Different disjoining pressure profiles between a toluene droplet with various concentrations of asphaltenes and a hydrophilic mica surface in 1 mM NaCl.
oil droplet can strengthen the EDL repulsion and lead to thicker confined water film. The interaction forces between 1:1 heptol droplet and mica surface in 1 mM NaCl with presence of various concentrations of asphaltenes were also investigated to investigate the solvent effects on the interaction. Addition of heptane (poor solvent) is known to destabilize the asphaltenes in oil phase and facilitate the migration of asphaltenes to the interface. The theoretical model incorporated with fitted surface potential of mica was used to fit the measured force results (Fig. 4), with the surface potential of pure heptol droplet obtained as 37 ± 4 mV, which was close to that of toluene droplet. With 10 and 100 mg/L asphaltenes addition, the surface potential became 60 ± 5 and 85 ± 7 mV, respectively. The change of surface potential at heptol/water interface is slightly larger than that at toluene/water interface, which could be attributed to increased amount of asphaltenes adsorbed at the interface with heptane addition.
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Fig. 4. Interaction forces (A–C) and profiles at maximum loading force (D–F) of the heptol droplet-hydrophilic mica interaction with various concentrations of asphaltenes in 1 mM NaCl at natural pH of ~5.6. (r = 60 lm for A and D, r = 48 lm for B and E, r = 50 lm for C and F, Green symbols: experiment data, black lines: fitting results). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Interaction forces of toluene droplet-hydrophilic mica interaction (A, B and C) and heptol droplet-hydrophilic mica interaction (D and E) with various concentrations of asphatenes in 100 mM NaCl at natural pH ~5.6. (r = 50 lm for A, r = 45 lm for B, r = 55 lm for C, r = 80 lm for D, r = 75 lm for E, Green symbols: experiment data, black lines: fitting results). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3.2. Interaction between oil droplet and hydrophilic mica in 100 mM NaCl Fig. 5A shows the interactions between a pure toluene droplet and a hydrophilic mica surface in 100 mM NaCl. A weak jump-in behavior, i.e. a sudden decrease of the measured force, was observed with the repulsion reaching about 3 nN, which suggested the attachment of the toluene droplet onto the mica surface. After
jump-in, the interaction force first turned slightly attractive due to the capillary bridging of the toluene droplet, and then became repulsive again as the cantilever further compressed the droplet. When the droplet was retracted, adhesion was registered before the full droplet detachment from the surface. It was noted that a small satellite droplet of toluene was always left on the mica surface after detachment. In 100 mM NaCl, the EDL repulsion is highly compressed and thus the VDW attraction governs the interaction
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forces, thereby inducing the attachment of toluene droplet onto the mica surface. Theoretically calculated force curve by considering the VDW attraction was found to agree excellently with the measured force curve (Fig. 5A). It should be noted that the mica surface is highly hydrated in water, and the hydrated ions have been known to cause the short-range repulsion between mica surfaces in water [64]. In our case, it is likely that a certain amount of hydrated ions may be trapped between the toluene droplet and the mica surface even after the observed jump-in behavior. And the hydration repulsion can also prevent the full spreading of toluene droplet on mica surface. With addition of asphaltenes, attachment of toluene droplet on mica surface was found to be largely inhibited (Fig. 5(B–E)). For 10 mg/L asphaltenes in toluene, the jump-in behavior could not be detected when the toluene droplet approached the mica surface, but a weak adhesive force of ~5 nN, indicated by the sudden jumpout behavior, was measured when the toluene droplet was retracted from the mica surface. When more asphaltenes were added (100 mg/L), the force results became similar to those in Fig. 4 and no adhesion was measured during retraction. The effects of solvent type were also studied. As shown in Fig. 5(D and E), even addition of low concentration of asphaltenes (10 mg/L) in heptol can effectively inhibit droplet attachment with no adhesion measured. The inhibition of droplet attachment could be due to the protection from the adsorbed asphaltenes and asphaltenes aggregates at the oil/water interface. It has been reported that the interfacial asphaltenes and asphaltenes aggregates can form a robust film at the interface, resulting in the steric repulsion that can inhibit the coalescence between oil droplets [26,30–33] as well as possibly droplet attachment in our case here. When the asphaltenes concentration is low (10 mg/L here), the adsorbed asphaltenes at the interface was insufficiently high to provide the complete protection, and thereby partial droplet attachment onto mica surface occurred and slight adhesion was detected. However, when the concentration is high, more asphaltenes adsorb onto the oil/water interface, which fully inhibits the droplet attachment behavior. Since more asphaltenes are forced to migrate to the oil/water inter-
face in heptol as compared to toluene, even low concentration of asphaltenes is enough to provide the complete protection against droplet attachment. Our previous studies applied the Alexander de Gennes (AdG) model to describe the steric repulsion induced by asphaltenes, and successfully fitted the toluene droplet-toluene droplet interaction in water with asphaltenes by incorporating the AdG model into the aforementioned theoretical model [3,66,67]. The steric repulsion in the AdG model can be described as
kT Psteric ðhÞ 3 s
" 3=4 # 9=4 2d h for D < 2L h 2d
ð7Þ
where d is the characteristic length, s is the mean distance between asphaltenes anchoring sites, and kT is the product of Boltzmann constant and temperature. Here we use the AdG model to tentatively calculate the measured force curves in Fig. 5(E–G). By using s ~ 3 nm and d ~ 3 nm adapted from literature [3], the calculated force curve agreed reasonably well with the measured force curve for 100 mg/L asphaltenes in toluene, indicating the AdG model could satisfactorily describe the steric repulsion induced by asphaltenes. Slightly larger d ~ 3.2 and 3.4 nm were used for the 10 and 100 mg/L asphaltenes in heptol cases to better fit the force curves. The slight increment in d might be due to more aggregation induced by addition of heptane. 3.3. Interaction between oil droplet and hydrophobized mica in 100 mM NaCl Mica surfaces hydrophobized with OTS were used as model hydrophobic surfaces to investigate the interaction mechanism between oil droplet and hydrophobic solid. The green squares in Fig. 6A show the measured results between a pure toluene droplet and a hydrophobized mica surface in 100 mM NaCl. A rapid jumpin behavior indicated the toluene droplet attachment was detected with the repulsion reaching about 1.5 nN, which was much less
Fig. 6. Interaction forces of toluene droplet-hydrophobized mica interaction (A–C) and heptol droplet-hydrophobized mica interaction (D and E) with various concentrations of asphatenes in 100 mM NaCl at natural pH ~5.6. (r = 45 lm for A, r = 55 lm for B, r = 45 lm for C, r = 42 lm for D, r = 74 lm for E, Green symbols: experiment data, black lines: fitting results). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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than that of hydrophilic mica surface shown in Fig. 5A. After jumpin, the interaction force rapidly decreased to negative maximum without subsequent increase, and the toluene droplet was detached from the cantilever and immobilized on the surface during retraction. Both of the phenomena suggested strong adhesion of the toluene droplet onto the hydrophobized mica surface. Since the VDW interaction is only weakly attractive to induce weak attachment as shown in Fig. 5A, the rapid attachment and strong adhesion between toluene droplet and hydrophobized mica surface are expected to be induced by the strong hydrophobic attraction here. A previously reported exponential equation for hydrophobic interaction potential involving deformable liquid droplets was incorporated within the theoretical model to calculate the forces between toluene droplet and hydrophobized mica surface, which was described as [20,48]:
W HB ðhÞ ¼ cð1 coshw Þexpðh=DH Þ
ð8Þ
where hw is the solid’s water contact angle in oil and DH is the decay length. And hence the disjoining pressure arising from hydrophobic interaction can be calculated as
PHB ðhÞ ¼ cð1 coshw Þ=DH expðh=DH Þ
ð9Þ
Here, the water contact angle of hydrophobized mica surface in toluene was measured to be ~120° using a goniometer and the characteristic length DH was adapted from previous reports to be 1 nm. The calculated force curve was found to agree quite well with the measured force data, confirming the determining role of hydrophobic interaction in attachment of toluene droplet. The influence of asphaltenes concentration on the interactions between oil droplet and hydrophobized mica surface was shown in Fig. 6(B–E). With addition of asphaltenes, both force curves in Fig. 6(B and C) showed sudden jump-in behaviors with the repulsion reaching about 1.5 nN, which was similar to that in Fig. 6A. The theoretically calculated force results by incorporating the effects of hydrophobic interaction were consistent with measured force data. As mentioned previously, asphaltenes was found to lead to steric repulsion to inhibit the attachment of toluene droplet onto hydrophilic mica surface in 100 mM NaCl. However, the similarity between the force curves with and without asphaltenes in toluene shown in Fig. 6(A–C) indicates the hydrophobic attraction dominates over other surface forces, which induces the oil droplet attachment on hydrophobized mica surfaces. Fig. 6(D and E) show the interaction of heptol droplet with asphaltenes addition. It was interesting to note that weak jump-in was first detected after a slight repulsion, and a second severe jump-in was measured after the linear increment of interaction force. This double jump-in behavior might be attributed to the steric hindrance of more asphaltenes and asphaltenes aggregates at the heptol/water interface. Asphaltenes can form more aggregates in presence of heptane, leading to stronger steric hindrance. Therefore, when the heptol droplet approaches the surface, the hydrophobic asphaltene aggregates may first attach to the surface, corresponding to the first jump-in. The attached asphaltenes aggregates can provide some steric repulsion to prevent further approach of heptol droplet, leading to the increase of interaction force after the first jump-in. Thereafter, the heptol droplet is further compressed towards the surface, and the hydrophobic attraction finally triggers full attachment of the heptol droplet on the hydrophobized mica surface. 4. Conclusions In this study, for the first time we quantitatively understand the significant role of salinity, oil type, asphaltenes concentration and
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surface hydrophobicity in modulating the oil-solid interaction mechanism in aqueous medium, particularly at the nanoscale. The EDL repulsion was found to prevent the oil droplet attachment onto hydrophilic mica surface in low salinity solution. Adsorption of asphaltenes at oil/water interface could result in the enhanced EDL repulsion at low salinity while strengthen the steric repulsion at high salinity, both of which contributed to a stable water film between oil droplet and hydrophilic mica surface, thereby inhibiting oil droplet attachment in water with both low and high salinity. It was also found that addition of poor solvent led to more adsorption of asphaltenes at the interface, which strengthened the EDL repulsion and steric repulsion. For the surface interactions between oil droplet and hydrophobized mica surface, the hydrophobic interaction could induce strong attachment and adhesion of oil droplet by overcoming the steric hindrance. Using oil droplet probe AFM, we proposed that the oleophilicity of hydrophilic solids in water can be enhanced by increasing the salinity and hydrophobicity of solid surfaces as well as decreasing poor solvent content and interfacial materials. Previously, the quantitative force measurements of asphaltenes were mainly conducted by driving an asphlantenes-coated substrate to interact with solid surfaces such as another asphaltenes-coated substrate and bare silica in both aqueous and organic solutions [25,43–47]. These force measurements were performed using the simple solid/liquid/solid system and asphaltenes were immobilized on solid substrates, which is different from the practical oil production with the complex oil/water/solid system and asphaltenes are mostly mobilized at the oil/water interface. The oil droplet probe AFM could help realize the quantitative measurements of an oil droplet interacting with a solid surface in water with interfacially mobilized asphaltenes. This study provides insights into unraveling the wetting mechanism of oil/water/solid systems in oil production, which could be extended to investigate the molecular mechanism of other engineering systems such as oil fouling and corrosion, and oil/water separation. For future study, the correlation between the nanoscopic interaction mechanism of oil/water/solid systems with interfacial materials (e.g., polymers, nanoparticles, proteins) and the macroscopic performance in different applications could be established. In some specific systems (e.g., porous membranes), the characterization of surface wettability is particularly challenging, while droplet probe AFM could provide useful data to help optimize the structure toward high oil/ water separation efficiency. Acknowledgements This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), the Alberta Advanced Education & Technology Small Equipment Grants Program (AET/SEGP), and the Canada Research Chairs Program.
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