Impact of Surfactant Structure on NAPL Mobilization and Solubilization

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Impact of Surfactant Structure on NAPL Mobilization and Solubilization in Porous Media Gina Javanbakht, and Lamia Goual Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03006 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 26, 2016

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Industrial & Engineering Chemistry Research

Table of Content Graphic Impact of Surfactant Structure on NAPL Mobilization and Solubilization in Porous Media (ie-‐2016-‐03006t) Gina Javanbakht and Lamia Goual

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Impact of Surfactant Structure on NAPL Mobilization and Solubilization in Porous Media Gina Javanbakht and Lamia Goual* Department of Petroleum Engineering, University of Wyoming, 1000 E. University Ave., Laramie, WY 82071, USA *E-‐mail: [email protected], Phone: 307-‐766-‐3278 ABSTRACT Additives such as surfactants are commonly used to enhance the remediation of oil-‐contaminated rocks. Although the majority of crude oils are light non-‐aqueous phase liquids (LNAPLs), they often contain heavy molecules such as asphaltenes that are classified as dense non-‐aqueous phase liquids (DNAPLs). Surfactants are able to reduce the interfacial tension between water and LNAPL and enhance their mobilization. Furthermore, the formation of microemulsions by surfactants can promote the solubilization of DNAPLs and restore the wettability of contaminated surfaces. Numerous studies have indicated that surfactants can promote the cleanup of oil-‐contaminated rocks, however the impact of surfactant structure on solubilization and mobilization is still unclear. In this study, we investigated the remediation of heterogeneous aquifer rocks using four different nonionic surfactants; n-‐dodecyl β-‐D-‐maltoside, Triton X-‐100, Bio-‐soft N1-‐7 and Saponin. The goal was to develop an improved understanding of the role of the surfactant molecular structure on non-‐ aqueous phase liquids (NAPL) removal through solubilization and mobilization. Each of these surfactants has a unique structural characteristic in its hydrophilic or hydrophobic segment. Dodecyl β-‐D-‐maltoside contains hydroxyl groups in its hydrophilic segment, which tend to form strong hydrogen bonds, while Triton X-‐100 has branched-‐chain alkyl groups in its hydrophobic segment that are more soluble in NAPL. Alkyl ethoxylated surfactants such as N1-‐7 display the simplest structure, while saponin with a heavy and complex structure cannot solubilize NAPLs as fast as other surfactants. Through measurements of phase behavior, dynamic interfacial properties, adsorption, spontaneous imbibition, thin sections analysis, and high resolution transmission electron microscopy, we showed that microemulsions formed by these surfactants are able to mobilize LNAPL, especially in the presence of branched-‐chain alkyl groups in the hydrophobic segments (such as those in Triton X-‐100), which promoted higher reduction in the interfacial tension between NAPL and brine. Micellar solubilization, on the other hand, was favored by the hydroxyl groups in hydrophilic segments (such as those in n-‐dodecyl β-‐D-‐maltoside), which were able to form strong hydrogen bonds at interfaces and favor the desorption of DNAPL from mineral surfaces. Following a different trend from the other surfactants, saponin with a higher solubility in brine showed a tendency to self-‐aggregate and form micron-‐size clusters of microemulsions, which slowed down the NAPL remediation.

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Keywords: NAPL, asphaltene, crude oil, aquifer remediation, surfactant structure, solubilization, mobilization 1. INTRODUCTION In-‐situ surfactant flushing of porous rocks is one of the most innovative technologies in removing organic contaminants, compared to other available remediation methods.1 Surfactants are surface-‐ active agents that tend to adsorb at fluid/fluid interfaces and modify the interfacial energies.2 These molecules consist of a hydrophilic head, which has affiliation to aqueous phases such as brine, and a hydrophobic tail, which is soluble in organic phases, generally referred to as non-‐aqueous phase liquids (NAPLs).3 Based on their hydrophilic heads, surfactants are categorized into four main groups: ionic, cationic, amphoteric, and nonionic. Among these groups, nonionic surfactants have the least tendency to adsorb on rock surfaces, making them the most suitable candidates for soil and aquifer cleanup processes.4 The displacement of NAPLs by low-‐concentration surfactant solutions is the result of different mechanisms including mobilization and solubilization. Mobilization is due to a reduction in the interfacial tension (IFT) between brine and NAPL, which often promotes the immiscible displacement of light non-‐aqueous phase liquids (LNAPLs).1,5,6 Micellar solubilization on the other hand involves desorption of contaminants from mineral surfaces and is desirable in the presence of dense non-‐aqueous phase liquids (DNAPLs).7 Although the majority of crude oils are LNAPLs, they often contain heavy organic macromolecules such as asphaltenes that are classified as DNAPLs. Asphaltenes consist of highly polarizable polydisperse surface-‐active components that tend to adsorb on rocks, altering their wettability. In a previous study, we showed that microemulsions formed by n-‐dodecyl β-‐D-‐maltoside surfactant were able to mobilize bulk oil in a porous rock and solubilize adsorbed asphaltenes from its heterogeneous surface. Micellar solubilization in this case was the result of surfactant adsorption on mineral surfaces, causing asphaltenes to detach and form Winsor Type I microemulsions. Spontaneous imbibition tests on the same rock indicated that the ratio of mobilized to solubilized NAPL was about 6:1.8 Note that this ratio varies with the type of surfactant and rock properties. Different rocks will adsorb variable amounts of DNAPL. The amount of mobilized LNAPL also depends on pore structure and wettability. In strongly water-‐wet pores, NAPLs that were occupied in the center of pores may disconnect and trap during the water flushing or flooding process.9 On the other hand, when the rock is oil-‐wet, the capillary pressure becomes negative and brine cannot invade the pores and mobilize LNAPL. Therefore, weakly water-‐wet rocks yield optimum NAPL mobilization.10 Although wettability is an important parameter that controls fluid flow in porous media, it has often been overlooked. A recent work with n-‐decane (as the NAPL phase) and sodium dodecyl sulfate in brine distinguished between the amount of NAPL mobilized through soil columns and dissolved in aqueous solutions. A sound model for surfactant-‐



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enhanced oil recovery was proposed, however it did not account for micellar solubilization since decane did not alter the soil wettability.11 The present work aims at expending our former study on NAPL mobilization and solubilization in porous media using other types of nonionic surfactants. Four well-‐known classes of nontoxic and biodegradable surfactants have been extensively studied in the literature related to enhanced oil recovery and soil washing processes: alkyl polyglucosides, alkyl phenol ethoxylates, alkyl ethoxylates, and biosurfactants. i.

Alkyl polyglucosides such as n-‐dodecyl β-‐D-‐maltoside are sugar-‐based surfactants that are soluble in polar and non-‐polar solvents.12-‐14 Their interfacial properties,15 adsorption,16 and phase behavior17 was studied in the past. These surfactants are strongly surface-‐active due to the hydrogen-‐bonding groups in their hydrophilic heads, enabling them to significantly reduce the IFT and adsorb in bilayers on water-‐wet surfaces.18

ii.

Alkyl phenol ethoxylates (or tergitol) contain polyethylene oxide chains that facilitate their adsorption on NAPL. Those with branched-‐chain alkyl groups, such as Triton X-‐100, were most effective at reducing the IFT.3 However regardless of their alky group topology, these surfactants showed bilayer adsorption on minerals at concentrations above their critical micelle concentration (CMC),19 and consequently altered the wettability of surfaces from oil-‐ wet toward water-‐wet.20

iii.

Alkyl ethoxylated surfactants (or linear alcohol alcoxylates) are also effective in reducing the IFT21 as well as altering the wettability of contaminated surfaces.22 However with this type of surfactants; the ultra-‐low IFT can only be achieved by addition of other agents such as alkali, co-‐surfactants, or co-‐solvents.23 The addition of alkali to a surfactant solution makes it possible to have in-‐situ generation of surfactant and significant reduction of surfactant adsorption.24, 25

iv.

Biosurfactants are mostly nonionic and their applications in NAPL removal from soil have been well established in the literature.26,27 Recently they have also been successfully applied in enhanced oil recovery.28 Saponin, for instance, was used in lieu of synthetic surfactants to assist with emulsion polymerization.29 Similar to alcohol ethoxylates, saponin is able to reduce IFT to ultra-‐low values by addition of salts or alcohols.30

Despite the fact that numerous studies have documented the application of surfactants in aquifer and soil remediation, less attention has been directed towards investigating the relationship between surfactant molecular structure and their surface activities. As an example, the hydrophilic-‐ lipophilic balance number (HLB) of each surfactant can influence the amount of mobilized or solubilized NAPLs. The HLB number shows the tendency of hydrophilic and hydrophobic segments of surfactants to dissolve in aqueous or NAPL phases, respectively. For example, the hydrophobic part of a surfactant with a low HLB number can partition significantly into the NAPL phase and form reverse microemulsions.31 NAPL contamination can produce different types of


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microemulsions with surfactants of different HLB values. However, surfactants with HLB close to 10 tend to form middle phase microemulsions, which promote the mobilization of NAPLs. In this study, we explored the relationship between surfactant molecular structure and its efficiency in NAPL remediation. One surfactant from each of the above-‐mentioned classes (n-‐dodecyl β-‐D-‐ maltoside, Triton X-‐100, Bio-‐soft N1-‐7, and saponin) was selected and its ability to reduce IFT and contact angle (CA) on contaminated rock surfaces was measured and interpreted based on its molecular structure. Novel correlations were then established between the amount of mobilized/solubilized NAPL and the IFT/CA of the surfactant solution. 2. MATERIALS AND METHODS 2.1 Materials 2.1.1 Fluids The NAPL phase used in this study is a medium crude oil from Milne Point formation in Alaska and its properties are provided in Table 1. This oil contains about 9 wt% of asphaltenes precipitated by n-‐heptane according to ASTM D-‐2007. The ratio between the total acid number (TAN) and the total base number (TBN) shows that the NAPL is fairly neutral. The brine used in this study was prepared by mixing 1 M CaCl2 in distilled-‐deionized water with a resistivity of 2.75E04 Ωm. The procedure used for the selection of this salinity is explained in our previous study.8 Each surfactant solution contained 0.2 wt% of one of the following surfactants: n-‐dodecyl β-‐D-‐maltoside (GC grade, >98%, Sigma Aldrich), Triton X-‐100 (laboratory grade, Sigma Aldrich), Bio-‐soft N1-‐7 (Stepan) and Saponin (molecular biology, Sigma Aldrich). These nonionic surfactants are environmentally friendly, biodegradable with low toxicity and CMC. The structure of these surfactants can be found in Table 2. 2.1.2 Rocks Heterogeneous aquifer rock samples were obtained from the Arkose layer of Fountain formation located in east Colorado and Wyoming, which originally formed from Sherman Granite and contains various minerals. The dominant minerals in this rock are quartz, feldspar, and calcite. Quartz and feldspar constitute more than 80% of the rock. We should note that the rock also contains some dolomite in the form of cement. Several core plugs were drilled with a diameter of 1.5 inches (or 38 mm) and dried in an oven for at least 24 hour before measuring their permeability and porosity with an automated permeameter and porosimeter (AP-‐608, Coretest System). The porosity of the rock samples was found to be in the range of 12 – 20 % and their permeability varied between 2 and 25 mD. 2.2 Methods



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2.2.1 Phase Behavior Phase behavior tests were performed by preparing a brine solution (0.2 wt% surfactant in brine) and mixing it with NAPL at a volume ratio of 1:1. The surfactant concentrations were chosen to be higher than the CMC point of each surfactant to ensure that surfactant micelles form microemulsions with NAPL. In each test, the glass tubes were shaken and rested at 25 °C and atmospheric pressure to form a separate microemulsion phase (or rag layer) between of NAPL and the brine. 2.2.2 High Resolution Transmission Electron Microscopy We used Tecnai TF20 S-‐Twin High Resolution Transmission Electron Microscope (HRTEM) from FEI to image surfactant micelles and microemulsions composed of surfactant solutions and NAPL. The instrument and procedure was illustrated in detail in our pervious study.8 Image J software was used for image processing in order to measure the average size of microemulsions.32 2.2.3 Interfacial Tension and Contact Angle Dynamic interfacial tensions between NAPL and brine and equilibrium CA of NAPL/brine/rock systems were measured with and without surfactant by rising/captive bubble tensiometry and a video-‐image digitization technique. The homemade experimental set-‐up was described in a previous study.33 All measurements were conducted at ambient conditions and the density of fluids was measured at 20 °C using Anton Paar density meter. Each NAPL bubble in brine was left for at least four hours to reach equilibrium for IFT measurements with the rising/captive bubble method during which images were captured every 5 minutes. The size of the needle was chosen to achieve a Bond number close to unity. The captured images were analyzed using the Axismetric Drop Shape Analysis (ADSA) by fitting the drop profile to Young-‐Laplace equation.34 A spinning drop tensiometer (SITE100, KRUSS) was also used to measure the IFT between NAPL and surfactant solution, which is more accurate for IFT values below 1 mN/m.35

For CA tests, the rock samples were first vacuumed at 10-‐7 psi for 12 hours and then immersed in NAPL. After aging in NAPL for 7 days at 60 °C, they were gently placed in the IFT/CA cell. Brine (with and without surfactants) was then transferred into the cell until the substrates were fully immersed in the solution. The NAPL inside the thin substrates formed on average 20 droplets on the surface of the rock, as it was released by spontaneous imbibition of brine (cf. Figure 1). Images of these droplets were captured every 30 seconds and their contact angle distribution was determined using the angle tool of ImageJ software. These tests were repeated to insure reproducibility of the data.


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2.2.4 Adsorption The amount of surfactant adsorption on crushed Arkose rock was determined by UV-‐Vis spectroscopy. The UV-‐Vis absorbance of the surfactant solutions with various concentrations was measured before and after exposure to the rock grains. The diameter of the rock grains was sieved to be between 100-‐200 µm. First, 1 g of the grains was mixed with 25 g of surfactant solutions with different concentrations. The mixtures were shaken at 600 strokes/minute for ten hours to reach equilibrium. The mixtures were then centrifuged to separate surfactant solution from the rock grains. The absorbance of the separated surfactant solutions were measured and compared with the reference curves, which were obtained from the surfactant solutions before they were mixed with the rock grains. The amount of surfactant adsorption on the rock grain was calculated and plotted at different surfactant concentrations using Langmuir isotherm.3 2.2.5 Spontaneous Imbibition To perform spontaneous imbibition tests, 38 mm diameter and 50.8 mm length core samples were drilled from Arkose rock and were subsequently polished and dried. The permeability and porosity of each sample were measured using the automated permeameter and porosimeter in order to choose the core samples with similar permeability and porosity for each set of tests. The samples were first vacuumed for 24 hours and then fully saturated with brine. They were subsequently placed in the core flooding system shown in Figure 2. NAPL was injected to the system at low flow rate and the volume of produced brine was monitored until an initial brine saturation of 50% was achieved. After reaching to 50% of water saturation, contaminated core samples were placed in custom-‐made Amott imbibition cells filled with brine solutions (with and without 0.2 wt% surfactants). Each Amott cell included a bottom sealed glass cell to hold the core sample and a top graduation tube to measure the volume of NAPL expelled from the core. This volume was recorded versus time until no more NAPL was produced. All the tests were performed at ambient temperature to better represent aquifer conditions. 2.2.6 Petrographic Thin Section Two sets of thin sections were provided by Wagner petrographic company using the cores from spontaneous imbibition tests with brine alone and surfactant solution (0.2 wt% maltoside). The size of each thin section was 24 × 46 mm. On each thin section, blue epoxy impregnation, calcite stain, K-‐feldspar stain, and plagioclase stain were applied. A petrographic microscope (Zeiss AXIO, Scope.A1) equipped with AXIO vision software were used for image analysis of the thin sections. 3. RESUTS AND DISCUSSION 3.1 Surfactant Screening



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3.1.1 Phase Behavior The phase behavior of the surfactants listed in Table 2 was investigated based on the procedure explained in Section 2.2.1. All the surfactants were able to significantly reduce the brine/NAPL IFT and form Winsor type III microemulsions in the middle phase between brine and NAPL,36 as shown in Figure 3. The presence of these microemulsions can decrease the capillary drawdown inside the pores, promoting more NAPL mobilization.6-‐8 3.1.2 Microemulsion Formation The HRTEM images of surfactant microemulsion prepared by extracting a small amount of the rag layer between NAPL and brine and diluted 40 times in the same brine are shown in Figure 4. The diameters of microemulsions formed by n-‐dodecyl β-‐D-‐maltoside and Triton X-‐100 were the largest (about 800 nm), suggesting that they can trap more NAPL than Bio-‐soft N1-‐7 and saponin. Saponin has a heavier and more complicated molecular structure than the other surfactants, and is more soluble in brine due to its higher HLB number. After imaging the microemulsions formed by fresh saponin solution and NAPL, we left the rag layer for two weeks and imaged it again. Figure 5 reveals the presence of spider-‐like shaped micelles that tend to aggregate between themselves and trap larger amounts of NAPL. Indeed, the average size of microemulsions has increased from 200 nm to about 1 µm after two weeks. The high tendency of saponin to self-‐aggregate is likely due to the large number of hydroxyl groups in its structure that tend to form strong hydrogen bonds. Consequently, microemulsions formed by saponin are expected to grow, trap, and mobilize more NAPL given enough time, leading to enhanced NAPL removal. 3.2 Interfacial Tension The effect of surfactant on the IFT between NAPL and brine was examined at 25°C and atmospheric pressure. The surfactant concentration was higher than their CMC (i.e., 0.2 wt%). For measurements with the pendant drop method, the IFT varied in time for about four hours before reaching equilibrium. Low IFTs on the other hand were measured by spinning drop tensiometry, which is relatively faster. Figure 6 shows that the addition of surfactant to brine resulted in a very significant decrease in the IFT between NAPL and brine from 22.5 mN/m (IFT of brine and NAPL) to less than 3 mN/m (with an error less than 0.1mN/m). As expected, the surfactants with intermediate HLB (i.e., closer to 10) provided the lowest IFTs. At the NAPL/brine interface, the hydrophilic heads of the surfactant tend to remain in the aqueous phase while the hydrophobic tails stretch toward the NAPL phase. As the surfactant layer forms at the interface, the transition between weak dispersion interactions among NAPL molecules and strong polar interaction among water molecules becomes smoother, leading to an increase in the interfacial pressure and a reduction in the interfacial energy and IFT.37,38 The significant reduction in IFT with surfactant caused the formation of Winsor Type III microemulsions, which promoted the mobilization of NAPL from contaminated aquifers. A closer examinations of the results revealed that the IFT values with


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n-‐dodecyl β-‐D-‐maltoside and Triton X-‐100 are the lowest. These two surfactants have similar HLB and microemulsion size and can trap more NAPL within their microemulsions, as discussed in the previous section. The presence of hydroxyl groups in the sugar head of n-‐dodecyl β-‐D-‐maltoside promotes the formation of strong hydrogen bonds.16,18 Although Triton X-‐100 has a less hydrogen bonding head than n-‐dodecyl β-‐D-‐maltoside, the branched alkyl group in its tail is able to adsorb into the brine/NAPL interface and bring down the IFT significantly. This is in line with previous studies where the presence of branched alkyl groups in surfactant tails was shown to enhance their

efficiency in lowering the IFT,3,39,40 due to increased hydrocarbon surface area per surfactant molecule at the interface.41 It is interesting to note that microemulsions formed by Triton X-100 are different from those with other surfactants (cf. Figure 4c). They exhibit small NAPL droplets scattered throughout the microemulsion instead of a core-‐shell structure. 3.3 Wettability Alteration The effect of aging on the wettability alteration of minerals was previously investigated.8 Different clean minerals (quartz, feldspar, and calcite) found in Arkose rock were immersed in NAPL for 1 to 10 days and the CA of brine was measured on these minerals after each day. By increasing the aging time, the CA increased until it remained constant after 3 days, indicating a wettability alteration from strongly to weakly water-‐wet. Thus in this study, we used 7 day-‐aged minerals in NAPL to ensure sufficient wettability alteration. The change in wettability can be explained by the adsorption of asphaltene molecules on mineral surfaces. The thickness of the adsorbed films is in the order of 5-‐8 nm, depending on the type of minerals.42 Previous studies revealed that asphaltene adsorption on calcite is higher than on quartz. Unlike the silanol groups of quartz that usually adsorb asphaltene monolayers, the carbonate groups of calcite interact strongly with asphaltenes, resulting in multilayer adsorption.43,44 Since Arkose rock contains more than 80% of quartz and feldspar, the film thickness is expected to be close to 5 nm, i.e. monolayer coverage. Figure 7 shows a reduction of static CA on oil-‐contaminated surfaces by addition of 0.2 wt% of surfactants with HLB number close to 13. For example, n-‐dodecyl β-‐D-‐maltoside was able to reduce the average CA from 110 to 30 degrees. This implies a wettability alteration of NAPL-‐contaminated rock surfaces from weakly water-‐wet back to water-‐wet, in agreement with previous work.20 The wettability reversal could be explained by the dual adsorption of surfactant molecules via their hydrophobic tails on the thin DNAPL layer and via their hydrophilic heads on the rock surface, causing DNAPL to detach and form Winsor Type I microemulsions.16 To verify this mechanism, we measured the adsorption of these surfactants on Arkose rock by UV-‐Vis spectroscopy according to the procedure described in Section 2.2.4. Figure 8 reveals that hydrogen bonding is the driving force for adsorption. Indeed, n-‐dodecyl β-‐D-‐maltoside with a large number of hydroxyl groups in its sugar head shows a greater amount of adsorption.18 Both Bio-‐soft N1-‐7 and Triton X-‐100 have alcohol ethoxylated heads, however the straight-‐chain alkyl tails of Bio-‐soft N1-‐7 appear to favor



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adsorption more than the branched-‐chain groups of Triton X-‐100, probably due to less steric effects. To study the impact of saponin self-‐assembly on wettability alteration, we measured the CA on contaminated rock samples after 4 days of contact with the surfactant solution. The insert in Figure 7 shows a reduction in static CA by 10 degrees upon increasing the contact time. When nonionic surfactant solutions with high molecular weight and high HLB such as saponin are introduced to the NAPL phase, they start to form hydrogen bonds and self-‐associate instead of quickly adsorbing on NAPL and rock surface. However, if given enough time, they can adsorb at interfaces to a higher extent and promote the micellar solubilization of NAPL. This behavior was not observed with the other surfactants, as their contact angle remained constant in time, suggesting minimal self-‐ association. 3.4 Spontaneous Imbibition Spontaneous imbibition tests were performed on Arkose core samples with similar permeabilities (2-‐3 mD) and porosities (12-‐15%) and containing 50% of initial brine saturation. First, we recorded the amount of NAPL removal due to spontaneous imbibition of brine by placing contaminated core samples in an Amott imbibition cell for at least 150 hours until production of NAPL ceased. The volume of NAPL removal from the core due to spontaneous imbibition of brine is shown in Figure 9a, as a percentage of the original NAPL volume. The amount of NAPL mobilized by brine is about 15 vol%, which is due to the mobilization of LNAPLs in smaller pores that can easily be invaded by brine. Based on our CA measurements, brine alone does not have the ability to desorb DNAPLs from mineral surfaces, which explains why no solubilization was observed. We repeated the spontaneous imbibition tests on contaminated core samples with all four surfactants. The amount of NAPL recovered by the surfactants versus time was recorded. After 150 hours, all surfactants showed more recovery compared to brine. Because of the lower interfacial tension with NAPL, the surfactant solution could invade small pores as well as large pores. Contaminant removal starts by a fast mobilization of LNAPL from the porous rock. As the production curve reaches an inflection point or a distinct jump, solubilization of DNAPL occurs. This jump depends on the solubilization amount and is more obvious in low permeability rocks. Solubilization is indeed slower than mobilization since it is a kinetic process that involves DNAPL desorption by surfactant molecules. The DNAPL desorption can restore the wettability of contaminated surfaces back to their original water-‐wet condition and reduce the threshold capillary pressure needed for brine to invade the pores. Therefore, another stage of NAPL recovery starts. Assuming that the imbibition curves due to mobilization with and without surfactants have the same trend, we can estimate the amounts of mobilization and solubilization for each surfactant, see for instance Reference 8 for n-‐dodecyl β-‐D-‐maltoside. This surfactant, together with Triton X-‐ 100, showed better recoveries than the other surfactants (6 and 5 vol% more than brine). The


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volume of LNAPL mobilized by Triton X-‐100 was slightly higher than n-‐dodecyl β-‐D-‐maltoside due to its slightly lower IFT. Bio-‐soft N1-‐7 showed lower initial NAPL removal, which implied its weaker ability to mobilize NAPL. This is in line with the higher IFT of this surfactant with NAPL compared to other surfactants. On the other hand, the volume of DNAPL solubilized with n-‐dodecyl β-‐D-‐ maltoside was slightly larger than Triton X-‐100 due to its higher ability to alter wettability. In fact, the amount of solubilized DNAPL decreased from n-‐dodecyl β-‐D-‐maltoside to Bio-‐soft N1-‐7 to Triton X-‐100. This result is in good agreement with the wettability alteration trend in Figure 7, and their adsorption propensity in Figure 8. Since saponin behaves differently from the other surfactants, its imbibition curve was provided in a separate figure for two ranges of rock permeability (cf. Figure 9b). In low permeability cores, the production curve of saponin indicates that the early rate of NAPL recovery (i.e., mobilization) is smaller than other surfactants and even that of brine alone. This result was surprising considering that saponin can reduce the IFT of NAPL/surfactant solution but not as much as the other surfactants. A possible reason for this inconsistency is the self-‐assembly behavior of saponin. As mentioned in the previous section, saponin has a greater aggregation tendency enabling the formation of large clusters of microemulsions and hindering their movement in porous media. The clustering of microemulsions is more problematic in rocks with smaller pore sizes and permeability. The solubilization of DNAPL for saponin starts after about 60 hours, which is considerably longer than the other surfactants (around 20 hours). This delay in the solubilization process is related to the self-‐aggregation of microemulsions, as was observed in CA measurements (cf. insert of Figure 7). In this case, DNAPL (i.e., asphaltenes) start to detach from the surface after microemulsion clusters are formed, which explains why more time is needed to reduce CA and reverse the wettability of contaminated surfaces to water-‐wet. Figure 9b reveals that saponin can remove 5% more NAPL than Bio-‐soft N1-‐7 after 120 hours. In addition, similar spontaneous imbibition tests were performed with saponin and rocks having a permeability of 20-‐25 mD, as shown in Figure 9b. The imbibition curves in this case have different trends than before. The larger pore size and permeability results in higher early rate of NAPL recovery than imbibition with brine alone. This is because the size of microemulsion clusters is smaller than the size of the pores therefore there are fewer barriers for their movement through the medium. Overall, both mobilization and micellar solubilization occur in NAPL remediation by all surfactants. Mobilization of LNAPL through formation of Winsor Type III microemulsions takes place first followed by the slow micellar solubilization of DNAPL (i.e., asphaltenes) through formation of Winsor type I microemulsions. The kinetics of these mechanisms was similar except for saponin, which has a higher HLB. Among the surfactants with HLB close to 13, those with more hydroxyl groups (such as n-‐dodecyl β-‐D-‐maltoside) or more lipophilic branched tails (such as Triton X-‐100)



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exhibited a higher solubility in brine and NAPL phase respectively, and could bring down IFT to its lowest values and facilitate LNAPL mobilization the most. Based on our CA/IFT measurements and imbibition tests, linear correlations could be established between the amounts of mobilized and solubilized NAPL by surfactants and their ability to reduce IFT and CA, respectively. Figure 10a indicates that maltoside is able to reduce the CA more than the other surfactants by forming the strongest hydrogen bonds and having the highest adsorption tendency on the rock surface; therefore the amount of solubilized DNAPL with this surfactant is the highest. On the other hand, the presence of branched alkyl groups in Triton X-‐100 causes the surfactant to dissolve more in the NAPL phase, which reduces IFT and promotes mobilization of LNAPL more than other surfactants (cf. Figure 10b). These results suggest that mixtures of surfactants with two structural types can promote both mobilization and micellar solubilization of NAPL in porous media. Both types should have intermediate HLB numbers. Type 1 contains a linear tail and a large hydrogen-‐bonding head, whereas type 2 has a highly branched tail and a smaller hydrogen-‐bonding head. To visualize wettability alteration of contaminated rock through micellar solubilization of adsorbed DNAPL on the mineral surfaces, we prepared thin sections of the core samples after spontaneous imbibition without and with the best surfactant (n-‐dodecyl β-‐D-‐maltoside) and compared them with polarizing petrographic microscopy. As shown in Figure 11a, after imbibition with brine, the mineral surfaces remained covered with a layer of DNAPL. After introduction of surfactant solution to the contaminated channels, most of the mineral surfaces became clean due to asphaltene desorption from the surface (cf. Figure 11b). The only parts of the rock that remained contaminated were mainly channels that contained dolomite cement. This illustrates that the surface roughness plays an important role in wettability alteration. The surfactant could solubilize adsorbed asphaltenes from the mineral grains with smoother surface whereas the cemented areas, which contain crushed and small grains with rough surfaces, retained the adsorbed contaminants. It should be noted here that it was not possible to estimate the amount of mobilized and residual LNAPLs by brine or surfactant solution in the rock channels since fluids in the channels were replaced by epoxy before cutting the samples. 4. CONCLUSION Systematic experimental analyses were performed to investigate the impact of surfactant structure on mobilization and micellar solubilization of NAPLs in porous media. All the surfactants were able to form Winsor type III microemulsions with brine and NAPL that caused significant IFT reduction. HRTEM micrographs revealed that the size of these microemulsions was bigger with n-‐dodecyl β-‐D-‐ maltoside and Triton X-‐100. These two surfactants have distinctive features either in their hydrophilic head or hydrophobic tail. Hydroxyl groups in the sugar head of n-‐dodecyl β-‐D-‐


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maltoside form strong hydrogen bonds and branched-‐chain alkyl groups in Triton X-‐100 cause strong interactions with the NAPL phase. Therefore, at the interface between brine and NAPL, these two surfactants decrease the interfacial tension to a greater extent resulting in better mobilization of LNAPL. Furthermore, CA measurements and spontaneous imbibition tests revealed that the presence of hydroxyl head groups and straight-‐chain alkyl tails in both n-‐dodecyl β-‐D-‐maltoside and Bio-‐soft N1-‐7 facilitated the desorption of DNAPL from rock surfaces and consequently promoted micellar solubilization, as shown in the schematic of Figure 12. Thus two correlations have been established based on this work; one between the amount of solubilized DNAPL and the extent of CA reduction (or adsorption propensity of surfactant), which is favored by the hydrogen bonding ability of its head, and one between the amount of mobilized LNAPL and the extent of IFT reduction by the surfactant, which is promoted by the branching of its tail. Results with a high HLB surfactant such as saponin were also provided to show the impact of surfactant self-‐assembly on remediation efficiency, especially in low permeability rocks. 5. ACKNOWLEDGEMENT The authors would like to thank the National Science Foundation (Career Award #1351296) and Hess Corporation for financial support. The authors are also grateful to Dr. Erwin Sabio for HRTEM imaging, Dr. Fred McLaughlin for his help in collecting rock samples and thin section analyses, Dr. Mohammad Sedghi and Dr. William Welch for insightful discussions, and Deraldo Andrade for his help with IFT and CA measurements. 6. REFERENCES 1. Mulligan, C. N.; Yong, R. N.; Gibbs, B. F. Surfactant-‐Enhanced Remediation of Contaminated Soil: A Review. Engineering Geology 2001, 60 (1–4), 371–380. 2. Laha, S.; Tansel, B.; Ussawarujikulchai, A. Surfactant–soil Interactions during Surfactant-‐ Amended Remediation of Contaminated Soils by Hydrophobic Organic Compounds: A Review. Journal of Environmental Management 2009, 90 (1), 95–100. 3. Rosen, M. J.; Wang, H.; Shen, P.; Zhu, Y. Ultralow Interfacial Tension for Enhanced Oil Recovery at Very Low Surfactant Concentrations. Langmuir 2005, 21 (9), 3749–3756. 4. Wagner, J.; Chen, H.; Brownawell, B. J.; Westall, J. C. Use of Cationic Surfactants to Modify Soil Surfaces to Promote Sorption and Retard Migration of Hydrophobic Organic Compounds. Environ. Sci. Technol. 1994, 28 (2), 231–237. 5. Abdul, A. S.; Ang, C. C. In Situ Surfactant Washing of Polychlorinated Biphenyls and Oils from a Contaminated Field Site: Phase II Pilot Study. Ground Water 1994, 32 (5), 727–734. 6. Harwell, J. H.; Sabatini, D. A.; Knox, R. C. Surfactants for Ground Water Remediation. Colloids and Surfaces A: Physicochemical and Engineering Aspects 1999, 151 (1–2), 255–268. 7. Soerens, T., D. Sabatini, and J. Harwell. Surfactant Enhanced Solubilization of Residual DNAPL: Column Studies. Subsurface Restoration Conference, Dallas, TX, June 1992.



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8. Javanbakht, G.; Goual, L. Mobilization and Micellar Solubilization of NAPL Contaminants in Aquifer Rocks. Journal of Contaminant Hydrology 2016, 185–186, 61–73. 9. Landry, C. J.; Karpyn, Z. T.; Piri, M. Pore-‐Scale Analysis of Trapped Immiscible Fluid Structures and Fluid Interfacial Areas in Oil-‐Wet and Water-‐Wet Bead Packs. Geofluids 2011, 11 (2), 209– 227. 10. Suicmez, V. S.; Piri, M.; Blunt, M. J. Effects of Wettability and Pore-‐Level Displacement on Hydrocarbon Trapping. Advances in Water Resources 2008, 31 (3), 503–512. 11. Tsakiroglou, C. D.; Aggelopoulos, C. A.; Tzovolou, D. N.; Theodoropoulou, M. A.; Avraam, D. G. Dynamics of Surfactant-‐Enhanced Oil Mobilization and Solubilization in Porous Media: Experiments and Numerical Modeling. International Journal of Multiphase Flow 2013, 55, 11– 23. 12. Pilakowska-‐Pietras, D.; Lunkenheimer, K.; Piasecki, A. Synthesis of Novel N,N-‐Di-‐N-‐ Alkylaldonamides and Properties of Their Surface Chemically Pure Adsorption Layers at the Air/water Interface. Journal of Colloid and Interface Science 2004, 271 (1), 192–200. 13. Griffin, W. C. Classification of Surface-‐active Agents by HLB. Journal of the Society of Cosmetic Chemists 1946, 1, 311-‐26. 14. Aramaki, K.; Ozawa, K.; Kunieda, H. Effect of Temperature on the Phase Behavior of Ionic– Nonionic Microemulsions. Journal of Colloid and Interface Science 1997, 196 (1), 74–78. 15. Niraula, B. B.; Chun, T. K.; Othman, H.; Misran, M. Dynamic-‐Interfacial Properties of Dodecyl-‐Β-‐ D-‐Maltoside and Dodecyl-‐Β-‐D-‐Fructofuranosyl-‐Α-‐D-‐Glucopyranoside at Dodecane/water Interface. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2004, 248 (1–3), 157–166. 16. Somasundaran, P.; Zhang, L. Adsorption of Surfactants on Minerals for Wettability Control in Improved Oil Recovery Processes. Journal of Petroleum Science and Engineering 2006, 52 (1–4), 198–212. 17. Kahlweit, M.; Busse, G.; Faulhaber, B. Preparing Nontoxic Microemulsions with Alkyl Monoglucosides and the Role of Alkanediols as Cosolvents. Langmuir 1996, 12 (4), 861–862. 18. Zhang, L.; Somasundaran, P.; Maltesh, C. Adsorption of n-‐Dodecyl-‐Β-‐D-‐Maltoside on Solids. Journal of Colloid and Interface Science 1997, 191 (1), 202–208. 19. Li, W.; Gu, T. Equilibrium Contact Angles as a Function of the Concentration of Nonionic Surfactants on Quartz Plate. Colloid & Polymer Sci. 1985, 263 (12), 1041–1043. 20. Golabi, E.; Seyedeyn Azad, F.; Ayatollahi, S.; Hosseini, N.; Akhlaghi, N. Experimental Study of Wettability Alteration of Limestone Rock from Oil Wet to Water Wet by Applying Various Surfactants. Society of Petroleum Engineers Heavy Oil Conference (SPE-‐157801), Calgary, Alberta, Canada, 12-‐14 June 2012. 21. Galindo, T. A.; Rimassa, S. M. Evaluation of Environmentally Acceptable Surfactants for Application as Flowback Aids. Society of Petroleum Engineers International Symposium on Oilfield Chemistry (SPE-‐164122), The Woodlands, TX, 8-‐10 April 2013.


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22. Mohan, K.; Gupta, R.; Mohanty, K. K. Wettability Altering Secondary Oil Recovery in Carbonate Rocks. Energy & Fuels 2011, 25 (9), 3966-‐3973. 23. Yang, H. T.; Britton, C.; Liyanage, P. J.; Solairaj, S.; Kim, D. H.; Nguyen, Q. P.; Weerasooriya, U.; Pope, G. A. Low-‐Cost, High-‐Performance Chemicals for Enhanced Oil Recovery. Society of Petroleum Engineers Improved Oil Recovery Symposium (SPE-‐129978), Tulsa, OK, 24-‐28 April 2010. 24. Chu, W.; Kwan, C. Y. Remediation of Contaminated Soil by a Solvent/Surfactant System. Chemosphere 2003, 53 (1), 9–15. 25. Hirasaki, G. J.; Miller, A.; Pope, G. A.; Jackson, R. E. Surfactant Based Enhanced Oil Recovery and Foam Mobility Control. Annual report of DE-‐FC26-‐03NT15406 grant, Rice University, 2004, http://www.owlnet.rice.edu/~gjh/Consortium/resources/DOE-‐Surfactant-‐2004-‐Annual.pdf 26. Banat, I. M.; Makkar, R. S.; Cameotra, S. S. Potential Commercial Applications of Microbial Surfactants. Appl. Microbiol. Biotechnol. 2000, 53 (5), 495–508. 27. Makkar, R. S.; Cameotra, S. S. Synthesis of Enhanced Biosurfactant by Bacillus Subtilis MTCC 2423 at 45°C by Foam Fractionation. J. Surfact. and Deterg. 2001, 4 (4), 355–357. 28. Shahri, M. P.; Shadizadeh, S. R.; Jamialahmadi, M. A New Type of Surfactant for Enhanced Oil Recovery. Petroleum Science and Technology 2012, 30 (6), 585–593. 29. Schmitt, C.; Grassl, B.; Lespes, G.; Desbrières, J.; Pellerin, V.; Reynaud, S.; Gigault, J.; Hackley, V. A. Saponins: A Renewable and Biodegradable Surfactant From Its Microwave-‐Assisted Extraction to the Synthesis of Monodisperse Lattices. Biomacromolecules 2014, 15 (3), 856–862. 30. Shahri, M. P.; Shadizadeh, S. R.; Jamialahmadi, M. Applicability Test of New Surfactant Produced from Zizyphus Spina-‐Christi Leaves for Enhanced Oil Recovery in Carbonate Reservoirs. Journal of the Japan Petroleum Institute 2012, 55 (1), 27–32. 31. Griffin, W. C. Calculation of HLB Values of Non-‐Ionic Surfactants. Am. Perfumer. Essent. Oil Rev. 1955, 65, 26–29. 32. Vega, C.; Miguel, E. de. Surface Tension of the Most Popular Models of Water by Using the Test-‐ Area Simulation Method. The Journal of Chemical Physics 2007, 126 (15), 154707. 33. Saraji, S.; Goual, L.; Piri, M.; Plancher, H. Wettability of Supercritical Carbon Dioxide/Water/Quartz Systems: Simultaneous Measurement of Contact Angle and Interfacial Tension at Reservoir Conditions. Langmuir 2013, 29 (23), 6856–6866. 34. Neumann, A. W.; David, R.; Zuo, Y. Applied Surface Thermodynamics, Second Edition, CRC Press, 2011. 35. Vonnegut, B. Rotating Bubble Method for the Determination of Surface and Interfacial Tensions. Review of Scientific Instruments 1942, 13 (1), 6–9. 36. Israelachvili, J. N. Intermolecular and Surface Forces: Revised Third Edition, Academic Press, 2011. 37. Howard, J. D. Patterns of Sediment Dispersal in the Fountain Formation of Colorado. The Mountain Geologist 1966, 3 (4), 147–153.



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38. Mazor, E. Understanding Groundwater Systems of the Southern Laramie Basin, Albany County, Wyoming through Applied Chemical and Physical Data, Report WWWRC-‐90-‐19, Wyoming Water Research Center, 1990. 39. Wu, Y.; Iglauer, S.; Shuler, P.; Tang, Y.; Goddard, W. A. Branched Alkyl Alcohol Propoxylated Sulfate Surfactants for Improved Oil Recovery. Tenside Surfactants Detergents 2010, 47 (3), 152–161. 40. Phan, T. T.; Attaphong, C.; Sabatini, D. A. Effect of Extended Surfactant Structure on Interfacial Tension and Microemulsion Formation with Triglycerides. J. Am. Oil Chem. Soc. 2011, 88 (8), 1223–1228. 41. Varadaraj, R.; Bock, J.; Valint, P.; Zushma, S.; Thomas, R. Fundamental Interfacial Properties of Alkyl-‐Branched Sulfate and Ethoxy Sulfate Surfactants Derived from Guerbet Alcohols. 1. Surface and Instantaneous Interfacial Tensions. J. Phys. Chem. 1991, 95 (4), 1671–1676. 42. Dudášová, D.; Simon, S.; Hemmingsen, P. V.; Sjöblom, J. Study of Asphaltenes Adsorption onto Different Minerals and Clays: Part 1. Experimental Adsorption with UV Depletion Detection. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2008, 317 (1–3), 1–9. 43. Keleşoğlu, S.; Volden, S.; Kes, M.; Sjöblom, J. Adsorption of Naphthenic Acids onto Mineral Surfaces Studied by Quartz Crystal Microbalance with Dissipation Monitoring (QCM-‐D). Energy Fuels 2012, 26 (8), 5060–5068. 44. Rudrake, A.; Karan, K.; Horton, J. H. A Combined QCM and XPS Investigation of Asphaltene Adsorption on Metal Surfaces. Journal of Colloid and Interface Science 2009, 332 (1), 22–31.


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Table 1. Properties of NAPL used in this study ρ20C (g/mL)

0.9214

Reflective Index at 20°C

1.5222

Viscosity (mPa.s)

112.0

C (%)

85.07

H (%)

7.75

N (%)

1.09

O (%)

1.61

S (%)

4.63

H/C

1.1

Asphaltenes (wt%)

9.03

TAN (mg of KOH/g)

1.69

TBN (mg of KOH/g)

2.25

TBN/TAN

1.33



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Table 2. Properties of selected surfactants Name

Structure

Chemical

MW (g/mol)

HLB

CMC (wt%)

C24H46O11

511

13.35

0.02

H3(CH2)10-‐

647

12.9

0.01

formula

n-‐Dodecyl β-‐ D-‐Maltoside

Bio-‐soft N1-‐7

O(C2H4O)7H

Triton X-‐100

C14H22O(C2H4O)10H

625

13.5

0.02

Saponin

C45H73NO15

1650

36.3

0.01


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(a)

(b)

Figure 1. Examples of images used for CA measurement on contaminated rock surfaces during spontaneous imbibition a) with brine, b) with surfactant (maltoside) solution. NAPL droplets have higher CA in the presence of surfactant



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Figure 2. Schematic of the core flooding system used to obtain an initial water saturation of 50 wt% in Arkose core samples


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(a)

(b)

(c)

(d)

Figure 3. Winsor type III microemulsion phase between NAPL and 0.2 wt% surfactant solution in brine, a) n-‐Dodecyl β-‐D-‐maltoside, b) bio-‐soft N1-‐7, c) Triton X-‐100, d) Saponin



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(a)

(b)

800 nm

(c)

400 nm

(d)

800 nm

200 nm

Figure 4. HRTEM micrographs of o/w microemulsions with: (a) n-‐Dodecyl β-‐D-‐maltoside, (b) Bio-‐ soft N1-‐7, (c) Triton X-‐100, (d) Saponin. The average microemulsion size is provided at the bottom of each image


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Figure 5. HRTEM micrographs o/w microemulsions with saponin solution aged for two weeks. The average microemulsion size is about 1 µm.



3 Maltoside Interfacial Tension (mN/m)

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2.5 2

Triton X-‐100L N1-‐7 Saponin

1.5 1 0.5 0

Figure 6. Effect of surfactants on the interfacial tension between NAPL and brine (IFT without surfactants is 22.5 mN/m)


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Figure 7. Effect of surfactants on equilibrium contact angle of brine on NAPL-‐contaminated minerals. Insert represents the effect of aging with saponin on wettability alteration of contaminated surfaces



4.0 3.5 3.0 Adsorption (mg/g)

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2.5 2.0 1.5 Maltoside

1.0

N1-‐7 Triton

0.5 0.0 0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Surfactant Concentration (wt%)

Figure 8. Adsorption of surfactants on rock grains using UV-‐vis spectroscopy


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(a) Maltoside, Triton, N1-‐7 25

NAPL Removal (vol %)

20

k = 2-‐5 mD

Start of solubilization

15

10 Brine N1-‐7 Triton Maltoside

5

0 0

20

40

60

80

100

120

140

160

180

Time (Hours)

(b) Saponin 40

40

k = 2-‐5 mD

30 25 20 15 10 Brine Saponin

5 30

60

90 120 Time (Hours)

150

30 25 20 15 10 Brine Saponin

5

0 0

k = 20-‐50 mD

35 NAPL Removal (vol %)

35 NAPL Removal (vol%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

180

0

30

60 90 120 Time (Hours)

150

180

Figure 9. a) Effect of non-‐associating surfactants on NAPL removal from low permeable (2-‐5 mD) Arkose cores with Swi=50%, and (b) impact of rock permeability on saponin-‐enhanced NAPL removal from Arkose cores with Swi=50%



(a) Solubilization 70 Triton

CA (Degree)

60 50

N1-‐7

40 30

Maltoside

20 10 0 1

2

3

4

5

Solubilized DNAPL (%)

(b) Mobilization 1.8 1.6 IFT (mN/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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N1-‐7

1.4 1.2

Maltoside

1

Triton

0.8 0.6 12

13

14

15

16

Mobilized LNAPL (%)

17

18

Figure 10. a) Amount of solubilized DNAPL vs. contact angle with each surfactant. b) Amount of mobilized LNAPL vs. interfacial tension with each surfactant


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(a) Without surfactant

(b) With n-‐dodecyl β-‐D-‐maltoside

Figure 11. Thin sections of rock samples after spontaneous imbibition tests (a) without surfactant and (b) with n-‐dodecyl β-‐D-‐maltoside surfactant



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Figure 12. Schematic of NAPL removal from contaminated porous rocks. Surfactants can mobilize LNAPL, especially in the presence of branched-‐chain alkyl tails. Micellar solubilization, on the other hand, is promoted by surfactants with large hydrogen-‐bonding heads and straight-‐chain alkyl tails, which are able to arrange at interfaces and favor DNAPL desorption from rock surfaces.


Impact of Surfactant Structure on NAPL Mobilization and Solubilization

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