J Surfact Deterg DOI 10.1007/s11743-015-1703-9

ORIGINAL ARTICLE

Complexation of Surfactant/β-Cyclodextrin to Inhibit Surfactant Adsorption onto Sand, Kaolin, and Shale for Applications in Enhanced Oil Recovery Processes. Part II: Dynamic Adsorption Analysis Sirinthip Kittisrisawai1 · Laura Beatriz Romero-Zerón1

Received: 13 August 2014 / Accepted: 26 May 2015 © AOCS 2015

Abstract Surfactant adsorption onto solid surfaces is problematic in some industrial processes, such as in surfactant flooding for enhanced oil recovery. In this work, it was hypothesized that the use of a surfactant delivery system could prevent surfactant adsorption onto solid surfaces. Therefore, the encapsulation of sodium dodecyl sulfate (SDS) into the hydrophobic core of β-cyclodextrin (β-CD) to generate a surfactant delivery system (SDS/ β-CD) was evaluated in this work. This complexation was characterized using optical and scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FT-IR). Dynamic adsorption evaluation was applied to determine the effectiveness of the complexation in inhibiting surfactant adsorption onto a variety of solid adsorbents including sand, and mixtures of sand–kaolin and sand–shale. Surfactant adsorption was also evaluated applying the quartz crystal microbalance technology (QCM-D). The formation and morphology of the complexation was confirmed by optical microscopy, SEM, and FT-IR. Dynamic adsorption tests demonstrated the effectiveness of the surfactant delivery approach in preventing the adsorption of surfactant (up to 74 % adsorption reduction). The QCM-D technology confirmed these observations. Several mechanisms were proposed to explain the inhibition of surfactant adsorption including steric hindrance, self-association of inclusion complexes,

hydrophilicity increase, and disruption of hemimicelles formation. Keywords Surfactant delivery system · Surfactant adsorption · Surfactant carrier system · Enhanced oil recovery (EOR) · Surfactant flooding · Chemical flooding · Surfactant adsorption inhibition Abbreviations CD CMC DW EOR FT-IR K QCM-D technology SEM SDS TOC S Sh XRD β-CD Γ

Cyclodextrins Critical micelle concentration Distilled water Enhanced oil recovery Fourier transform infrared Kaolin Quartz crystal microbalance technology Scanning electron microscopy Sodium dodecyl sulfate Total organic carbon Sand Shale X-ray diffraction Beta-cyclodextrin Surfactant adsorption (mg surfactant/g solid)

& Laura Beatriz Romero-Zero´n [email protected] Sirinthip Kittisrisawai [email protected] 1

Department of Chemical Engineering, University of New Brunswick, Head Hall, 15 Dineen Drive, Fredericton E3B 5A3, Canada

Introduction Surfactant flooding is a chemical enhanced oil recovery (EOR) process, which is considered to be highly effective in mobilizing and displacing immobile oil trapped in the

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porous rock due to capillary forces. Surfactants work by reducing the interfacial tension (IFT) between oil and water from approximately 30 mN/m to ultralow values 10−2 mN/m or less [1–6]. However, during surfactant flooding through porous media, adsorption, precipitation, and chromatographic separation of surfactant species normally occur. Unwanted surfactant loss due to these mechanisms reduces the effectiveness of this chemical EOR flooding [7–14]. Adsorption is the partitioning of the adsorbate species (i. e. surfactant) between the bulk of the solution and the solid interface that occurs if the interface is energetically favored by the adsorbate species [15]. Surfactant adsorption is affected by several factors including the type and concentration of surfactant, the morphological and mineralogical characteristics of the rock, and the bulk solution properties such as salinity, pH, temperature, and hardness, among others [3, 4]. The chemical potential of the surfactant molecules in solution and the nature of the solid are determining factors for adsorption [7]. According to Trogus et al. [16] the level of adsorption decreases with increasing molecular weight for nonionic surfactants; while the opposite is true for anionic surfactants. At low surfactant concentrations, the adsorption of surfactant occurs in the form of monomers, and as the concentration of surfactant increases, the adsorbed surfactant monomers tend to aggregate and form micelle-like structures. If the aggregated surfactant forms one layer, the micelles are called admicelles and if the aggregates form two layers it is called hemimicelles [12]. In any case, surfactant adsorption tends to level off when the critical micelle concentration (CMC) of the surfactant is reached for single surfactant systems [7]. Paria and Khilar [8] reported that the adsorption of ionic surfactants on similarly charged solid surface increases in the presence of electrolytes. The mineral composition of rock formations plays an important role in surfactant adsorption. For instance, silica tends to adsorb simple organic bases (cationic surfactants), because at a neutral pH, silica generally has a negatively charged weak acidic surface. On the contrary, carbonate (calcite) rocks have positively charged weak base surfaces at neutral pH and adsorb simple organic acids (anionic surfactant). Additional information on these effects is reported elsewhere [3, 4, 15]. Surfactant adsorption mechanisms include ion pairing, electrostatic attraction, covalent bonding, hydrogen bonding, hydrophobic bonding, adsorption by polarization of π electrons, adsorption by dispersion forces, and solvation of various species [7, 8]. Several approaches to overcome the surfactant adsorption problem have been attempted, these include the use of antiadsorption agents [17], addition of alkali [18], formulation of surfactant mixtures [7, 19] modifications of the injection scheme [3], and the use of sacrificial agents that are adsorbed

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in place of the surfactant and are applied in a preflush or as a competitive additive to the surfactant slug [16], among others. This article, which is the second of a series of three papers, evaluates a new approach to inhibit the adsorption of surfactant onto rock surfaces that is based on the use of a surfactant delivery system. This method is based on the formation of inclusion complexes between EOR surfactants and beta-cyclodextrin (β-CD). Cyclodextrins (CD) are macro cyclic oligosaccharides that are naturally composed of 6, 7, and 8 glycosidic units, which are named α-, β-, and γ-CD, respectively. CD are molecules characterized as having hydrophobic inner cavities, while the outer surface of the molecules are hydrophilic. The hydrophobic cavity allows the insertion of non-polar molecules called “guest molecules” and the resulting compound is called an inclusion complex. “Guest molecules” can be aromatics, alcohols, halides, fatty acids, esters, etc. in the solid, liquid, or gas phase [20]. Therefore, CD can be used to transport or carry non-polar molecules within a polar media. The driving force of this phenomenon is the release of enthalpy-rich water molecules from the CD cavity. Currently, CD are widely applied in several industries and processes including pharmaceutical, cosmetic, food, chemical, and biochemical processes [20, 21]. Several analytical techniques are available to establish the complexation between guest molecules and CD. These techniques include the surface tension method, which is used when the guest molecule is a surfactant [22], conductivity [23–25], UV and visible spectroscopy [26], and nuclear magnetic resonance (NMR) spectroscopy [27], among others. The use of any of these approaches depends on the type and properties of the “guest” molecule. This article summarizes the results of a proof of concept research on the effectiveness of a new surfactant delivery system for inhibiting surfactant adsorption onto solid materials (sand, kaolin, and shale) commonly found in oil rock formations through dynamic adsorption testing. As such, this paper is structured in three main sections that include first, the confirmation of the surfactant:β-CD complexation by several analytical techniques; secondly, the adsorption performance of the surfactant delivery system obtained from dynamic adsorption tests and QCM-D technology, and finally several mechanisms to explain the adsorption inhibition process via the SDS/β-CD complexation are proposed and discussed.

Materials and Experimental Methodology Materials The anionic surfactant sodium dodecyl sulfate (SDS, C12H25NaSO4, molecular weight 288.38 g/mol), was acquired

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from Sigma-Aldrich with a high grade of purity (≥99 %). βCyclodextrin (β-CD) with a purity ≥98.4 % and molecular weight of 1,135 g/mol was purchased from Cyclodextrin Technologies Development Inc. (Gainesville, FL, USA). NaCl was acquired from Windsor-The Canadian Salt Company Limited (Pointe-Claire, QC, Canada). All chemicals were used as received. Sand was purchased from Shaw Resources Company, Milford, NS, Canada. Kaolin powder was acquired from Matheson Coleman & Bell Company (Gardena, CA, USA). Oil shale samples were provided by the New Brunswick Department of Natural Resources, NB, Canada. The samples of sand, kaolin, and shale were used as received without further purification. Experimental Methods

Optical Microscopy Characterization The optical microscopy study was performed to determine the compatibility between SDS and β-CD and consequently the tendency of these components for forming inclusion complexes and/or self-associations. Two or three drops of the solutions previously prepared with various SDS concentrations (0, 5, 10, 20, and 30 wt%) relative to the concentration of β-CD were placed on the glass slides that were allowed to dry overnight inside a fume hood. Care was taken to avoid the deposit of any solid particles and/or dust on the glass slides. The dried samples were subjected to optical microscopy analysis using a compact inverted metallurgical microscope; model Olympus GX41, manufactured by Olympus (Center Valley, PA, USA). The images were taken at 2009 magnification.

Characterization of Solid Adsorbents Scanning Electron Microscopy (SEM) Characterization The mineral composition of the solid adsorbents was determined by X-ray diffraction (XRD) using a model D8 Advance spectrometer manufactured by the Bruker Corporation (Billerica, MA, USA). The surface area of the solid materials (sand, kaolin, and shale) was determined by applying the BET technique using an AUTOSORB-1 analyzer manufactured by Quantachrome Instruments (FL, USA).

The surface morphology of β-CD in free-state, SDS, and β-CD in complex-state (SDS/β-CD solution containing 30 wt% SDS and 70 wt% β-CD) was determined using a scanning electron microscope Model SU-70 manufactured by Hitachi (Tokyo, Japan) at an accelerating voltage ranging from 5 to 14 kV with 3009 magnitude. Dry samples were spread on double sided carbon coated slide mounted on the SEM stage before scanning.

SDS/β-CD Inclusion Complex Characterization The characterization of the inclusion complex SDS/β-CD was conducted following the methodology and procedures reported by Yallapu et al. [28]. Preparation of SDS/β-CD Solutions A solution of β-CD with a concentration of 0.5 wt% was prepared in distilled water (500 ml). This solution was thoroughly stirred to ensure complete dissolution of the βCD powder. After which, the β-CD solution (0.5 wt%) was poured into five different 100 ml volumetric flasks. Then, various amounts of surfactant (SDS) powder were added to the volumetric flasks to prepare several solutions of inclusion complex with different concentrations of SDS relative to the concentration of β-CD as follows: baseline with no SDS added (0 wt% SDS), 0.025 g (5 wt% SDS), 0.050 g (10 wt% SDS), 0.10 g (20 wt% SDS), and 0.15 g (30 wt% SDS). These solutions were well-stirred to ensure the complete dissolution of the chemical species. Aliquots of the solutions were dried using a Freeze Dryer manufactured by Labconco Company (Kansas City, MO, USA.) for 3 days under a vacuum at 0.064 mbar at a temperature ranging from −44 to −45 °C to avoid the risk of chemical degradation.

Fourier Transform Infrared (FT-IR) Spectroscopy Characterization The formation of the inclusion complex between SDS/βCD was validated through FT-IR using a Perkin Elmer Spectrum 100 FT-IR and FT-NIR Spectrometers (Shelton, CT, USA). The data was acquired for a wavelength ranging from 4,000 to 450 cm−1 at a scanning speed of 4 cm−1 over 32 scans. Dry samples of SDS, β-CD in free-state, and βCD in complex state (30 wt% SDS in the inclusion complex relative to the concentration of β-CD) were subjected to analysis. Surfactant Adsorption Evaluation Critical Micelle Concentration Prior to performing the dynamic adsorption tests, it was necessary to determine the surface activity behavior of the inclusion complex (SDS/β-CD) solutions, and the surfactant (SDS) in free-state in soft brine containing 2 wt% NaCl. The aim was to determine the CMC of the corresponding systems: surfactant in free- and in complex-state. For the case of the inclusion complex SDS/β-CD, the solution contained a fixed concentration of β-CD of 1.6 wt

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% while the concentration of SDS was step-wised increased from 0 to 0.5 wt%. The inclusion complex SDS/β-CD reaches a 1:1 stoichiometric molar ratio when the SDS concentration increases over 0.004 wt%; therefore is expected that the CMC of the free surfactant monomers available in the bulk of the solution after complexation occurs must be higher than 0.004 wt%. For this reason, the concentration of surfactant added during CMC determination was extended up to 0.5 wt% to cover a broad range of surfactant concentration. Surface tension measurements were performed using a tensiometer model TensioCAD manufactured by CAD Instruments (Les Essarts-le-Roi, France) using the Du Nou`¨ y ring method (EN 14370). The tensiometer was equipped with a platinum–iridium ring and a temperature controller (Cole Parmer Company). The temperature was set at 25 ± 1 °C for all measurements.

Fig. 1 Simplified schematic of the dynamic adsorption experimental set-up

Dynamic Adsorption Tests Dynamic adsorption experiments were conducted to evaluate the adsorption behavior of the SDS/β-CD complexation solution and the SDS solution in free-state during propagation through a sandpack system. These flooding experiments were carried out using different packing material including sand, kaolin, and shale that are commonly present in oil reservoir rock formations. The packing material used consisted of sand (100 wt%) and mixtures of sand (S)–kaolin (K) (97 wt% S–3 wt% K) and (95 wt% S–5 wt% K), and mixtures of sand (S)–shale (Sh) (97 wt% S–3 wt% Sh) and (95 wt% S–5 wt% Sh). Surfactant solutions were prepared in soft brine containing 2 wt% of NaCl. Figure 1 presents a simplified schematic of the experimental set-up of the dynamic adsorption equipment. The key section of this experimental set-up is the packed column (diameter 3.7 cm, height 37 cm), in which 100 g of the corresponding solid adsorbents were placed. The surfactant systems were circulated through the column at a flow rate of 1.0 cm3/min that corresponds to a superficial velocity of 0.093 cm/min at room temperature (≈25 °C). In the case of the solid adsorbent mixtures, the adsorption column was packed by adding alternate layers of the corresponding solid materials (sand, kaolin, and/or shale) to simulate the layered sedimentation process that takes place during rock formation. The initial concentration of the surfactant systems was 0.004 wt% (40 ppm, pH 7.63 ± 0.32) and the inclusion complex solution was prepared following a stoichiometric molar ratio of 1:1 of SDS–β-CD (pH 7.97 ± 0.26). The concentrations of surfactant and β-CD before and after adsorption were determined using a total organic carbon (TOC) analyzer, model TOC-VCPH manufactured by SHIMADZU

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Scientific Instruments (Shimadzu Corporation, Japan) equipped with a TNM-1 total nitrogen measuring unit. Surfactant solutions in free- and complex-state were constantly circulated through the packed column and the concentration of surfactant at the effluent was continuously monitored. The flow through the sandpack was maintained until the adsorption equilibrium was reached, after which the effluent was collected and centrifuged to separate any solid particles that might be carried out with the effluent. Initial samples and final effluent samples were subjected to TOC analysis to determine the initial and final concentration of surfactant. The adsorption of surfactant, Γ (mg surfactant/g solid), onto solid surfaces was conducted by material balance using Eq. (1). C¼

ðCi  Ce Þ  V W

ð1Þ

where Ci and Ce are the initial and equilibrium liquid phase concentrations of surfactant solutions (g/l), respectively; V the volume of the surfactant solution (l); and W is the mass of dry adsorbents (g). Ci and Ce were indirectly determined as follows. Previous research [29, 30] have demonstrated that β-CD does not show adsorption onto sand, which is the principal solid adsorbent used in this work, therefore the baseline of TOCβ-CD corresponding to the β-CD was deducted from the value of TOC obtained for the bulk solution containing surfactant and β-CD in the experiments involving the SDS/β-CD complexation. In addition to this, it was also determined that a small concentration of organic carbon components leached from the adsorbent materials (sand, kaolin, and oil shale), therefore this value of TOCSolid was also deducted from the total gross value of TOCTotal. Then, the concentration of Ci for the surfactant solutions in complex-state was calculated as indicated in Eq. (2).

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Ci ¼ TOCTotal  TOCbCD

ð2Þ

While Ce is calculated from Eq. (3) as follows. Ce ¼ TOCTotal  TOCbCD  TOCSolid

t statistic for related samples. All graphs were plotted using Microsoft Excel 2010.

ð3Þ

where TOCTotal is the TOC content in the bulk solution containing surfactant, β-CD, and carbon components leaching from the solid adsorbents; TOCβ-CD the TOC concentration corresponding to β-CD; and TOCSolid corresponds to the organic carbon concentration coming out from the solid adsorbents during the dynamic adsorption tests. Quartz Crystal Microbalance (QCM-D Technology) QCM-D technology was applied to characterize the adsorption behavior of the surfactant onto anionic and cationic surfaces. These measurements were conducted using a QCM-D model Q-Sense D300 and E4 manufactured by Biolin Scientific/Q-Sense (Va¨stra Fro¨lunda, Sweden). For anionic surfaces the following conditions were applied. Surface: silica after UV-ozone cleaning and activation in NaOH solution to develop negative charges from silanol groups. Temperature 25 °C and flow rate 100 μl/min. The surface was rinsed using MilliQ water during 10 min and then the sample was injected into the respective chambers. After 50 min of running time, water was injected again to rinse non-adsorbing material. The cationic surface was prepared by placing a layer of a dendrimeric form of polyethylenimine (PEI), which was injected on top of the silica surface at a concentration of 500 ppm at 25 °C, at a flow rate of 100 μl/min. After the silica surface was cationized, surfactant solutions (free- and in complex-state) were injected over the cationized silica to determine the adsorption behavior of the surfactant solutions. Statistical Analysis The experimental results were processed using Microsoft Excel 2010 software and expressed as mean ± SD of the mean of n separate experiments. The statistical analysis of the surfactant adsorption data was performed using the

Results and Discussion Characterization of Solid Adsorbents Table 1 presents the composition and surface area of the solid adsorbents as determined by XRD and the BET techniques, respectively. The BET analysis indicates that kaolin is the solid adsorbent with the largest surface area of 19.75 m2/g followed by shale with 6.73 m2/g, and sand with 0.24 m2/g. Therefore, kaolin is the solid adsorbent with the highest adsorption capacity in terms of area available for adsorption (adsorption sites). The mineral composition and the surface charge of the solid adsorbents play a significant role on the behavior of surfactant adsorption onto solid surfaces. The surface charge of the solid materials shows a significant dependence with the pH of the aqueous solutions surrounding the minerals. For instance, at low pH, the surface charge of silica (quartz) and calcite in aqueous solution is positive, while a negative surface charge develops at high pH. More specifically, silica shows a negative surface charge when the pH of the aqueous solution is increased from a pH [ 2 [3, 4]. At neutral pH, the surface of silica presents a weak negative charge, therefore it tends to adsorb organic bases (cationic surfactants) [3, 11, 18]. The surface charge of muscovite responds to the pH of the aqueous media in the opposite direction of silica, thus at low pH it displays a negative charge, while at high pH the surface charge becomes positive. In the case of the sylvite mineral, the surface charge is negative for any pH lower than 10.5 [32, 33]. The mineral composition (Table 1) of the sand used in this work contains 74 % of quartz, 18 % of muscovite, and 8 % of sylvite and the pH of the brine used in all the experiments is 7.43 (SD ± 0.29), which indicates that the SDS (anionic surfactant) will be exposed to positive (muscovite) and negative (quartz and sylvite) surface charges. The positive sites on the sand will induce the adsorption of surfactant onto the mineral surface by electrostatic attraction. Kaolin was composed of 100 %

Table 1 Mineral composition of the solid adsorbents Solid adsorbents

Surface area (m2/g)

Composition (XRD) wt% Quartz

Sand

0.24

Kaolin

19.75

Shale

6.73

Sylvite

Kaolinite

Albite

Dolomite

Muscovite

Clinochlore

Calcite

74.0

8.0

–

–

–

18.0

–

–

–

–

100

–

–

–

–

–

–

32.0

26.0

7.0

7.0

8.0

20.0

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J Surfact Deterg Fig. 2 Optical microscopic images: a β-CD in free-state, b SDS in free-state, and c– f SDS/β-CD complexation with increasing concentration of SDS. Magnification 9200

kaolinite clay (Table 1), which displays a negative charge on the face of the crystal and a positive charge at the edges of the crystal at neutral pH [34]. Therefore, kaolin is highly predisposed to adsorb anionic surfactants; a process that is also supported by its large surface area (19.75 m2/g). The shale material contained albite (32 %), dolomite (26 %), clinochlore (7 %), quartz (20 %), and muscovite (7 %) (Table 1). The pH dependence of the albite surface charge shows a positive charge at pH lower than 6 (acid region), a neutral surface at pH ranging from 6 to 10, and a negative surface charge at pH higher than 10 (basic region) [35]. Dolomite and clinochlore minerals show the same pH dependence as silica and calcite that is positive at low pH but negative at high pH [36]. The surface charge dependence of the minerals contained in shale indicates that at the pH of the brine (7.43 ± 0.29) used during the dynamic adsorption tests, the surface of the shale displays positive and negative charges; therefore, the anionic surfactant SDS will be attracted towards the solid interface by electrostatic forces.

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SDS/β-CD Inclusion Complex Characterization Optical Microscopy Characterization The compatibility between SDS and β-CD was evaluated through optical microscopy. Figure 2 displays the optical microscopic images of β-CD and SDS in free-state and SDS/β-CD inclusion complexes with increasing concentrations of SDS. β-CD in free-state presents a well-defined rod-shaped crystal structure (Fig. 2a); while the SDS in free-state (Fig. 2b) shows a needle-shaped crystalline structure. The SDS/β-CD complexations show a very different structure. The inclusion complexes appear as aggregates or clomps that grow as the SDS concentration increases (from 5 to 30 wt%) relative to the concentration of β-CD (Fig. 2c–f). This change in morphology supports the affinity between the “guest” and the “host” that leads to the formation of individual inclusion complexes and the further molecular aggregation of these complexes through

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Fourier Transform Infrared Spectroscopy

Fig. 3 Scanning electron microscope (SEM) images of β-CD, SDS and SDS/β-CD inclusion complexes. Scale bar on SEM images represent 50 µm

nonbonding interactions to form supramolecular clusters as shown in Fig. 2f [37]. Scanning Electron Microscopy The surface morphology of SDS and β-CD in free and in complex-state was determined via SEM as presented in Fig. 3a–c. β-CD in free-state presents a flake-like irregular crystalline structure throughout the sample (Fig. 3a); whereas SDS in free-state shows a needle-like crystalline structure (Fig. 3b). In the case of the SDS/β-CD complexation, the surface morphology of the crystals is neither flake-like nor needle-like shapes but a tighter scale-like structure (Fig. 3c). The morphological changes of the SDS and β-CD in free-state to a more tight-fitting crystal structure suggest the formation of the new entity via the encapsulation of the SDS into the β-CD cavity.

FT-IR spectroscopy was used to support the formation of the inclusion complex SDS/β-CD. The FT-IR spectrum of β-CD in free-state (solid line), SDS in free-state (dashed line), and the spectrum of SDS/β-CD inclusion complex (round dashed line) are displayed in Fig. 4. The β-CD in free-state spectrum show characteristic peaks around 3,372–3,434 and 2,942 cm−1 due to the O–H and C–H stretching vibrations. In addition, peaks at 1,659, 1,154, 1,051, and 952 cm−1 correspond to H–O–H, C–O, C–O–C glucose units and C–O–C of the CD rings, respectively [28]. The FT-IR spectrum of SDS in free-state (dashed line) presents absorption bands at 2,921 and 2,853 cm−1 corresponding to C–H stretching vibrations of asymmetric and symmetric CH3 and CH2; while, the sharp peak at 1,471 cm−1 indicates the bending vibrations of CH3 and CH2 deformation. Furthermore, the strongest band at 1,224 and 1,254 cm−1 that results from the combination of several overlapping peaks that is generally observed as a double band corresponds to the asymmetric vibrations of the SO2 head group of the surfactant [38]. The spectrum of the SDS/β-CD inclusion complex (round dashed line) displays all the representative peaks belonging to β-CD in free-state with only few distinctive peaks of the SDS such as the peaks at 2,920 and 2,849 cm−1. Nevertheless, the most intense peaks of the SDS correspond to the polar head group at 1,224 and 1,254 cm−1, which clearly appear in the spectrum of the inclusion complex as peak 1,248 cm−1. This demonstrates that the polar head group of the surfactant extends out from the hydrophobic core of the β-CD; while the hydrophobic tail is encapsulated in the cavity, which explains the shifting of the typical peaks of the hydrophobic tail of the SDS. Similarly, all the β-CD related peaks in free-state were shifted to higher and/or lower wave numbers such as 3,428–3,496, 1,659–1,655, and 868–755 cm−1; these changes confirm the complexation of SDS/β-CD. The physical and chemical characterization of the chemical species in free-state and in complex-state provided definite confirmation of the formation of the inclusion complex between SDS and β-CD. Once the complexation was unequivocally demonstrated, it was necessary to establish its adsorption behavior onto solid surfaces under dynamic conditions that resemble an oil reservoir environment. Critical Micelle Concentration The CMC of SDS in free-state and in complex-state with βCD in 2 wt% NaCl solution were determined via surface tension. Table 2 displays the CMC values obtained for both systems.

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J Surfact Deterg Fig. 4 FT-IR spectra of β-CD and SDS in free-state and SDS/ β-CD complexation

Table 2 The critical micelle concentration of SDS in free-state and the complex-state

Dynamic Adsorption Tests

System

The goal of this experimental section was to evaluate the adsorption behavior of the inclusion complex SDS/β-CD during propagation through porous media. In these experiments, the equilibrium time for surfactant adsorption was previously determined to be 5 h for both surfactant systems (data not reported here). Table 3 presents the results of the dynamic adsorption obtained for different solid adsorbent mixtures and Fig. 5 plots the adsorption data expressed in mg/g solid as a function of solid adsorbent. The adsorption data demonstrates the effectiveness of the surfactant delivery system in inhibiting the adsorption of the surfactant onto solid adsorbents. The reduction in adsorption ranged from 30 to 74 % and the maximum adsorption reduction was observed for the solid mixture containing 95 % sand and 5 % shale. The noticeable adsorption of surfactant onto this solid mixture

CMC (mM)

SD

SDS

0.798

±0.028

SDS/β-CD

3.66

±0.135

The CMC of the inclusion complex SDS/β-CD is significantly higher than the CMC for the SDS in free-state. The higher CMC showed by the inclusion complex system can be explained by the fact that no covalent bonds are formed or broken during the complexation of SDS/β-CD and surfactant molecules in the complex are in rapid equilibrium with free surfactant molecules in the solution [37]. Therefore, at any time there are less free surfactant monomers in the solution for micellization. Consequently, a higher concentration of surfactant is needed to satisfy the simultaneous processes of complexation and micellization [39].

Table 3 Results of dynamic adsorption experiments Packing material (solid adsorbent)

Adsorption of SDS in free-state

Adsorption of SDS in complex-state

mg/g solid

mg/g solid

SD (±)

SD (±)

Adsorption reduction (%)

100 % sand (baseline)

0.142

0.024

0.075

0.001

47.0

97 % sand + 3 % kaolin

0.115

0.006

0.081

0.001

30.0

95 % sand + 5 % kaolin

0.335

0.037

0.123

0.002

63.0

97 % sand + 3 % oil shale

0.125

0.018

0.055

0.001

56.0

95 % sand + 5 % oil shale

0.356

0.049

0.094

0.002

74.0

SD standard deviation

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J Surfact Deterg Fig. 5 Surfactant adsorption as a function of solid adsorbent

might be explained by the fact that the shale contained organic material (kerogen) forming a film on the surface of the shale grain. The presence of organic material might promote the flat adsorption of the tail of the surfactant onto the surface of the shale driven by hydrophobic interactions [40]. In any case, the mechanisms of surfactant adsorption onto rock surface are highly dependent on the mineral composition of the rock (Fig. 5). In the succeeding section, some mechanisms that might explain the inhibition of surfactant adsorption by the surfactant/β-CD complexation are suggested. Quartz Crystal Microbalance Technology The QCM-D measurements show that the adsorptions of SDS surfactant solutions in free- and complex states are negligible on bare silica (strongly anionic surface), while the adsorption measurements of the SDS solutions onto strongly cationic surface shows that the adsorption of SDS in free-state is very extensive and seems not to reach equilibrium (adsorption [2.2 mg/m2), while the SDS solution in complex-state with β-CD shows an insignificant adsorption of only 0.2 mg/m2, which is equivalent to an adsorption reduction of 91 % onto the cationized silica surface. The QCM-D measurements support the findings from the dynamic adsorption tests. Therefore, the encapsulation of the surfactant into the hydrophobic core of the β-CD prevents the adsorption of surfactant onto solid surfaces.

Mechanisms for Surfactant Adsorption Inhibition Sand Surfaces According to Table 1, sand is composed of 18 % of muscovite, which carries a positive surface charge, and 82 % of quartz and sylvite, which are negatively charged at neutral pH (soft brine pH 7.43 ± 0.29). Although, this composition renders the surface of the sand predominantly negative, which prevents the adsorption of anionic surfactant due to electrostatic repulsive forces (Fig. 6a) [11, 41]; the presence of muscovite, even at low concentrations, renders a positive surface charge (adsorption sites) that drives the adsorption of anionic surfactants onto the sand surface due to electrostatic attraction as shown in Fig. 6a [31, 32]. Another mechanism for the adsorption of anionic surfactants onto sand surfaces was proposed by Trogus et al. [16], in which the surfactant interacts via weak hydrogen bonding with the terminal OH groups on the sand surface at neutral pH. The aforementioned mechanisms of anionic surfactant adsorption onto sand are inhibited by the inclusion complex surfactant/β-CD by the following proposed mechanisms. The first mechanism is considered to be steric hindrance. According to Paria and Khilar [8, p. 87] “…the structure of the adsorbed layer depends on the packing of the molecules, which in turn depends on the mutual repulsion and steric constrains among adsorbate species”. As Fig. 6a shows, anionic surfactant molecules in free-state

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surfactant molecules at the solid surface due to steric hindrance, which interrupts the adsorption process. The second mechanism is the self-assembly of inclusion complexes (Fig. 6b) through nonbonding interactions (e.g., van der Waals interactions and hydrogen bonding) forming supramolecular structures that prevent the migration of surfactant molecules from the bulk of the solution towards the adsorption sites [37]. Kaolin Surfaces At neutral pH, kaolinite displays negative surface charge on the face and positive surface charge at the edges [34]. Consequently, anionic surfactant molecules are adsorbed onto the kaolin surface due to attractive electrostatic interactions. As in the previous case, the encapsulation of the hydrophobic tail of the surfactant into the β-CD cavity prevents the adsorption of surfactant onto the kaolin surface due to steric hindrance and self-association of inclusion complexes. The steric hindrance mechanism interrupts the formation of surfactant aggregates at the solid surface (Fig. 6b). Shale Surfaces

Fig. 6 a Mechanisms for anionic surfactant adsorption, b mechanisms of adsorption inhibition by the SDS/β-CD complexation

migrate towards the solid surface driven by electrostatic attractions and/or hydrogen bonding interactions. At the solid surface, surfactant molecules arrange themselves forming well-packed clusters of adsorbed surfactants at the adsorption sites. However, when the surfactant is in complex-state with β-CD (Fig. 6b), the bulky volume of the inclusion complex might prevent the orderly packing of

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The shale particulate contained organic matter (kerogen) forming a film on the grain surface. In this case, the flat adsorption of surfactant on the solid surface is driven by hydrophobic interactions between the non-polar tail of the surfactant and the organic matter on the shale surface [40]. Therefore, the hydrophobic tail of the surfactant adsorbs flat onto the solid surface. Furthermore, surfactants can form aggregates and/or hemicylindrical hemimicelles at the shale surface depending on the number of surfactant layers of the aggregates (Fig. 7a, b). Once these structures are formed, more surfactant adsorption can occur. These adsorbed surfactant monomers and surfactant aggregates exist in thermodynamic equilibrium with surfactant monomers in the bulk solution. Therefore, significant adsorption of SDS occurs towards the organic matter and clay minerals contained in the shale [11, 42, 43]. The encapsulation of the hydrophobic tail group in the β-CD cavity hinders the above-mentioned surfactant adsorption mechanisms on the shale surface because it prevents hydrophobic interactions between the surfactant tail and the organic matter on the shale, in other words, the inclusion complex makes the surfactant tail hydrophilic circumventing any hydrophobic interactions with the solid surface (Fig. 7c). The formation of inclusion complexes also creates disorder and disrupts the packing of hemimicelles structures (Fig. 7d), which prevents the adsorption of surfactants onto the shale surface. Furthermore, the self-

J Surfact Deterg Fig. 7 a Adsorption mechanism of anionic surfactants onto shale surfaces, b adsorption of surfactant via formation of hemicylindrical hemimicelles, c surfactant adsorption inhibition by hydrophilicity increase, d surfactant adsorption inhibition by hemimicelles disruption

association of inclusion complexes (Fig. 7c, d) could also preclude the migration of surfactant molecules towards the solid surfaces [37].

Conclusions This proof of concept research demonstrates the effectiveness of the surfactant delivery approach for inhibiting the adsorption of SDS onto solid surfaces; which supports

the research hypothesis. The conclusions drawn from this exploratory study are as follows. The complexation between SDS and β-CD was definitely confirmed by optical and SEM, and FT-IR spectrometry. The physical and chemical characterization of the SDS/ β-CD demonstrates the formation of a stable inclusion complex. Dynamic adsorption tests indicated the effectiveness of the surfactant delivery system in preventing the adsorption of SDS onto sand, kaolin, and shale. The adsorption

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tests demonstrated that the encapsulation of the SDS into the hydrophobic core of the β-CD decreases the adsorption of surfactant up to 74 % when compared with the adsorption of the surfactant in free-state. The QCM-D technology supports the findings from the dynamic adsorption tests. This technology demonstrated that the adsorption of surfactant in complex-state onto a strongly cationic surface is reduced by 91 %. Several mechanisms to explain the inhibition of surfactant adsorption through the surfactant delivery approach are suggested in this exploratory study. It is proposed that the main operative mechanism is steric hindrance due to the position of the β-CD cavity in the surfactant structure; which decreases the orderly packing of surfactants at the adsorption sites. Likewise, hemimicelles formation is also disrupted by steric hindrance. It is elucidated that the flat adsorption of surfactant onto hydrophobic surfaces is prevented due to encapsulation of the hydrophobic tail of the surfactant into the β-CD cavity, which establishes a barrier between the surfactant and the solid surface. It is considered that this barrier offers a hydrophilicity increase to the system. Another mechanism that might deter the adsorption of surfactant is the self-association of inclusion complexes within the aqueous media of the porous media environment. Validation of these adsorption inhibition mechanisms is necessary; therefore significantly more research is required to fully understand the adsorption prevention mechanisms. The results of this exploratory research support the potential of the new surfactant delivery system for EOR applications.

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18. Acknowledgments The authors would like to acknowledge Dr. Orlando Rojas, Professor of Forest Biomaterials and Biomolecular Engineering, North Caroline State University, for his contributions with the application of the QCM-D analysis. Financial support provided by the Chemical Engineering Department at the University of New Brunswick, the Natural Sciences and Engineering Research Council (NSERC), and the Canadian Foundation for Innovation (CFI) is acknowledged.

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Sirinthip Kittisrisawai holds a Ph.D. in chemical engineering from the University of New Brunswick, Fredericton, Canada. Dr. Kittisrisawai also holds a Master of Science degree in Petrochemical Technology from The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand and a bachelor’s degree in chemical engineering from Mahidol University, Salaya Campus, Nakhon Pathom, Thailand. Laura Romero-Zerón is a chemical engineer who holds a Ph.D. in petroleum and chemical engineering from the University of Calgary, Alberta, Canada. Currently, Romero-Zero´n is a professor in the Chemical Engineering Department at the University of New Brunswick, New Brunswick, Canada. Her research interests cover several areas related to the energy sector such as enhanced oil recovery (EOR) processes (surfactant and polymer flooding).

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