Multistimuli-Responsive Foams Using an Anionic Surfactant

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Multi-Stimuli Responsive Foams Using an Anionic Surfactant Robin Singh, Krishna Panthi, Upali P. Weerasooriya, and Kishore K. Mohanty Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01796 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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Multi-Stimuli Responsive Foams Using an Anionic Surfactant Robin Singh, Krishna Panthi, Upali Weerasooriya, Kishore K. Mohanty* Department of Petroleum and Geosystems Engineering, The University of Texas at Austin, Austin, Texas 78712, United States

ABSTRACT In this work, we report a novel class of commercially-available surfactant which shows multistimuli responsive behavior toward foam stability. It comprises of three components—a hydrophobe (tristyrylphenol), a temperature-sensitive block (polypropylene oxide, PO) and a pHsensitive moiety (carboxyl group). The hydrophobicity-hydrophilicity balance of the surfactant can be tuned by either changing the pH or temperature of the system. At or below pH 4, the carboxyl functional group is dominantly protonated resulting in zero foamability. At higher pH, the surfactant exhibits good foamability and foam stability marked with a fine bubble texture (~ 200 µm). Foam destabilization could be achieved rapidly either by lowering the pH or bubbling CO2 gas. At a fixed pH in the presence of salt, increasing the temperature to 65 ˚C resulted in rapid defoaming due to the increased hydrophobicity of the PO chain. This stimuli-induced stabilization and destabilization of foam were found to be reversible. We envisage the use of such multi-responsive foaming system in diverse applications such as foam enhanced oil recovery and environmental remediation where spatial and temporal control over foam stability are desirable. The low-cost commercial availability of the surfactant further makes it lucrative.

* To whom correspondences should be addressed

ACS Paragon Plus Environment

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Keywords: pH-responsive, temperature-responsive, CO2-responsive, foamability, responsive foams

INTRODUCTION Foams are thermodynamically metastable systems characterized by high interfacial energy. They spontaneously evolve into a low energy state by foam collapse and eventual phase separation. Amphiphiles/particles such as surfactants1, macromolecules2 or surface-functionalized nanoparticles3 are typically employed to stabilize the air-water interface. Based on the stability of the foams, they are usually categorized into different groups. These include unstable or transient foams, metastable4 foams and ultrastable1,5,6 foams with lifetimes of the order of few seconds, minutes to hours, months to years, respectively. There is a growing interest to develop formulations whose foam stability can be controlled using external stimulus. In the last five decades, surfactant-stabilized foams have been extensively studied for subsurface applications such as foam enhanced oil recovery7–10 and environmental remediation11,12 with several successful field-scale implementations

13–15

. Foams have an exceptional property to

reduce the gas mobility by several orders of magnitude by reducing its relative permeability16,17 and increasing the fraction of trapped gas in porous media. They can also divert the injection fluid from high permeability regions to low permeability regions making them a robust conformance control agents18,19. Despite these advantages, there are several operational challenges of injecting foams in the subsurface. Notably, it suffers from injectivity issues near wellbore regions due to the viscous nature of the foams20. One potential solution to this problem is to design stimuli-responsive foaming formulations which do not foam during injection leading to better injectivity but create a stable foam later away from wellbore region. Also, since these


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applications require large quantities of the surfactant solution, it is critical to develop responsive foams using surfactants which could be economically synthesized in the industrial scale. Stimuli-responsive foams are an emerging field of research with numerous industrial applications where on-demand control over foam stability is required. Contrary to the field of responsive emulsions, scant literature exists in this area. The use of stimuli such as pH21–23, CO2/N224,25, temperature26,27, ionic strength22, light28–30, redox reactions31, UV irradiation32,33 and magnetic fields34,35 has been reported in the literature. There are two strategies which are commonly adopted to achieve responsive aqueous foams36. The first strategy involves modifying the bulk properties of the aqueous phase in the foam liquid channels using external stimuli. Fameau et al. (2011) employed this approach to obtain ultrastable foams using 12-hydroxy stearic acid at 20 ˚C which collapses quickly upon heating above 60 ˚C. At 20 ˚C, the fatty acid self-assembles to micron-sized tubes forming a gel-like structure in the foam lamellae ensuing enhanced foam stability. Upon heating, the tubes reassemble to form spherical micelles destroying the gel-network resulting in foam collapse. The second strategy involves modulating the interfacial properties of the adsorbed species at the air-water interface which could be achieved using different techniques. The external stimuli could be used to tune the self-assembly or aggregate structure37, vary the solubility24, trigger crystallization of surfactant38, modify foam film thickness22, trigger conformational changes30 or perform in-situ surface modification17,39. Herein, we report a novel, commercially available surfactant which exhibits multi-stimuli responsive behavior toward foam stabilization. The aforementioned second strategy was employed in this work where the foam stability was switched from a stable to an unstable state by tuning the interfacial behavior of self-assembled structures of surfactant. The surfactant molecules comprise of three main components- a hydrocarbon hydrophobe, a temperature-


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sensitive block, and a pH-sensitive terminal functional group. The raw materials of the surfactant are derived from petrochemicals which are economically available in bulk quantities. The reversible foaming-defoaming cycle was achieved by either invasive triggers such as a change in pH by acid/base addition or CO2/N2 bubbling, or by a non-invasive trigger such as a change in the system temperature. Such multi-responsive foaming system can find potential applications in subsurface applications such as foam enhanced oil recovery, foam environmental remediation to mitigate injectivity issues or perform selective conformance control in heterogeneous reservoirs.

EXPERIMENTAL SECTION Materials. The surfactant, Tristyrylphenol propoxy carboxylate (TSP-PO45-COOH) with an activity of 10 w/w% in water was prepared from carboxylation of Tristyrylphenol propoxylate supplied by Harcros Chemical Company (MW = 3064 g mol-1). The average number of PO units were 45 as per the vendor. Its structure is shown in Figure 1. This surfactant is referred to as “TPC” in this paper. Hydrochloric acid (HCl, 37 wt%), and sodium hydroxide (pellets, anhydrous) were obtained from Sigma-Aldrich company. Nitrogen gas (research grade) and carbon dioxide gas (99.998%) were supplied by Praxair Inc. All solutions were prepared with ultrapure water with a resistivity greater than 18.2 MΩ-cm obtained using B-Pure™ Water Purification System (Barnstead). The pH of the various surfactant solutions was measured using pHTestr® 20 (Oakton Instruments) which has the precision of ± 0.01. The pH electrode was calibrated with standard pH buffer solutions of pH 4, 7, and 10. Sample Preparation. A stock solution of 5 wt% surfactant was used to prepare samples. Surfactants with different concentrations were prepared by diluting the stock solution with DI water. The samples were thoroughly mixed using magnetic agitation for 24 hrs. The pH of the


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samples was regulated by dropwise adding 1M HCl/ NaOH. All the samples were prepared at 25 ˚C.

Figure 1. Structure of tristyrylphenol propoxy carboxylate (TSP-PO45-COOH) Transmittance. 2-ml surfactant solutions were taken in quartz cuvettes with 10 mm optical path lengths. The transmittance at different pH was measured using Varian Cary® 50 UV-Vis Spectrophotometer at 600 nm. Dynamic Light Scattering. The intensity-averaged hydrodynamic diameters of the surfactant micelles were characterized by dynamic light scattering (DLS) technique using the Delsa™ Nano analyzer. It was equipped with Delsa™ Nano UI Software (2.21). This software uses the CONTIN method to resolve particle size distribution from the measured autocorrelation functions. The surfactant solutions of different pH were prepared using 1M NaOH, and 1M HCl and agitated vigorously on a MaxiMix™ Vortex Mixer (Thermo Scientific™) for 2 minutes and


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then stirred for 24 hrs using magnetic stir bars. Finally, 2 ml samples were taken in glass cuvettes for DLS measurement at ambient temperature. Three consecutive runs were performed, and the results were averaged. It was ensured that no foam bubble was trapped in any sample before every measurement. Transmission Electron Microscopy (TEM). The self-assembled surfactant aggregates under different pH conditions were visualized using an FEI Tecnai transmission electron microscope operating at 80 kV. The TEM samples were prepared using negative-staining method40 with phosphotungstic acid, PTA (1 wt%) as the staining agent. This method is a powerful tool and has been widely used in literature to study surfactant aggregation41,42 and biological macromolecules43. Stained sample was prepared by taking 5µL of the surfactant solution on the formvar-coated copper grid (400-mesh, EMS, PA). After 5 minutes, the excess solution was removed by blotting using a filter paper. Subsequently, one drop of PTA solution was placed on the grid for 1 minute. The excess liquid was then removed by blotting and sample was dried at room temperature before TEM examination. Rheological Measurement. The viscosity of surfactant solutions was measured using a rotating disk-type rheometer with a cone-plate geometry, AR-G2 Magnetic Bearing Rheometer (TA Instruments Ltd, RH) at 25 ˚C. A cone geometry with a cone angle of 2˚ and radius of 40 mm was used for the measurements. A steady-state flow method was used where the shear rate was varied from 10 to 100 s−1, with 6 data points acquisition per decade. Foamability and Foam Stability. 4-ml samples were taken in glass vials and were sealed using a PTFE-lined cap. The vials were hand shaken vigorously for 30 seconds to generate foam. All experiments were conducted by the same operator. The foamability of the sample as measured


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by the initial foam height above the liquid phase was recorded. The foamability of different samples was compared by normalizing the initial foam heights with respect to the maximum foam height observed among the samples. For each case, after forming the static foams, the decay of foam height (above the drained liquid phase) was recorded as a function of time. The foam stability of the samples was determined by the measuring the half-life of the foams which is the time it took for the foam to reach half of its original height. Optical Micrographs. The morphology of the foam bubbles was characterized using optical microscopy. 2-ml surfactant solutions were placed in glass cuvettes (optical path length: 10 mm) covered with coverslips and were agitated vigorously for 30 seconds to generate aqueous foams. The flat and thin walls of the cuvettes ensured proper focus adjustment. The micrographs of the foam texture were taken using an optical microscope (Nikon) with a 4X objective. It was equipped with a high-resolution color camera (Digital Sight DS-Fi1) to capture the bubble morphology. The image analysis was performed using ImageJ software (version 1.51w). The inbuilt plugins “Threshold” and “Analyse Particles” were used to measure the average size and size distribution. The Sauter mean diameter, DSM was calculated using Equation 1. =

∑ [1] ∑

The dimensionless polydispersity index, PDI was calculated using Equation 2.

PDI = 100 x

−

[2]

The Di is the diameter of the ith bubble, and the overbar symbol represents the average value.


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CO2/N2 Treatment. A custom-made bubbling device was used to study the reversibility of aqueous foam from stable to no-foam state. It comprises of a rectangular glass cell (FD Glass, NJ) (dimension: 10 mm X 24 mm x 11.5 inches) fitted with a porous, sintered frit (pore size: 7 µm) at the bottom using rubber septa and HPLC fittings (Valco) for foam generation. 5 ml of the surfactant solution (0.25 wt%) was taken in the cell and nitrogen, or carbon dioxide gas was bubbled at a fixed pressure (5 psi) using a gas regulator. Surface Tension. The surface tensions between air-surfactant solutions were measured via the pendant drop method using a Ramé-Hart goniometer. An aqueous drop (with varying surfactant concentration) was held in air from a hypodermic needle for a sufficient time (>5 minutes) to allow it to equilibrate with the air phase. The goniometer acquires a grayscale image of the suspended drop. The axisymmetric shape analysis of the drop was performed using DROPimage Advanced software. It measures the surface tension by fitting the drop profile to the YoungLaplace equation using a contour-fitting algorithm.

RESULTS AND DISCUSSION The goal of this work is to develop a pH-sensitive surfactant system whose foamability can be tuned from a stable foam state to a no-foam state. The change in pH could be triggered using different stimuli such as the addition of acid/bases or CO2/N2 bubbling. The surfactant used in the present study, TSP-PO45-COOH comprises of three main parts: a hydrocarbon hydrophobe, a polypropylene (PO) block, and a terminal carboxylic acid. The carboxylic acid functional group was chosen as it can act as a pH-sensitive moiety whose degree of protonation depends strongly on the pH of the solution. The motivation behind choosing a surfactant with a TSP (tristyrylphenol) hydrophobe was multi-fold. First, TSP is commercially available and could be


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produced on an industrial scale economically using common petrochemicals. It is synthesized by a one-step reaction of acid-catalyzed alkylation of phenol with styrene44. TSP-derivatives are already being used in various agrochemical formulations45,46. Second, the presence of large aromatic functional groups in TSP has unique advantages in certain subsurface applications such as enhanced oil recovery and surfactant-enhanced NAPL remediation. It has been shown that for crude oil with large equivalent alkane carbon number (EACN)47, a hydrophobe such as TSP is required to achieve low interfacial tensions (IFT) between oil and surfactant solutions48. In such cases, the aromatic groups in TSP can interact with crude oil resulting in ultra-low IFT between surfactant and oil which are often desirable. Third and most importantly, PO groups can easily be added to the TSP molecule by one-step alkoxylation reaction44. Thus, the hydrophobicity of the surfactant or hydrophile-lipophile balance (HLB) can be tuned by changing the number of hydrophobic PO groups attached to surfactant49. The presence of large hydrophobic groups ensures complete foam destabilization when the acid is protonated (-COOH form) under low pH. In the absence of such large hydrophobic groups, the foamability is only expected to reduce but not collapse to zero at low pH. It is to be noted that surfactant showed low solubility when it is under protonated form due to the presence of the PO and hydrocarbon groups. One of the key desired qualities of a pH-sensitive surfactant system for it to be adaptable in different applications is the long-term aqueous stability of the solution under different pH and temperature conditions. The initial pH of the 2-wt% surfactant solution (6.52 mM) was 8.9. Figure 2a shows the digital images of the TPC surfactant solution at different pH after 30 days of preparation. No phase separation was observed indicating the aqueous stability of the dispersions. The appearance of the solution changed from optically clear (pH 12, 10, 8.9) to translucent (pH 7) to opaque (pH 4, 2). This change in turbidity, which is a macro-indicator of


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the degree of aggregation, was quantified by measuring the transmittance of the solution using a UV-Vis spectrophotometer. The transmittance of the solution drops from nearly 100 % to 0% when pH is progressively reduced from 12 to 2 with a steep change near the pH of 7 (Figure 2b). An increase in turbidity of the solution indicates the formation of aggregates of surfactant molecules due to lower solubility owing to a high degree of protonation. However, interestingly, despite the formation of these aggregates no phase separation was observed at any pH. Moreover, the system was found to be highly reversible (at least six times) where the solution can be easily switched from opaque to clear and vice-versa by changing the pH. The morphology of these aggregates was characterized using dynamic light scattering (DLS) and transmission electron microscopy (TEM). It is important to note that DLS measurement yields hydrodynamic diameter while TEM captures the actual diameter of the dispersed particles/aggregates. The intensity-averaged hydrodynamic diameter profiles are shown in Supplementary Information (Figure S1) which clearly illustrates the strong response of the pH of the solution. At pH 7, the average aggregate size was 193 ± 156 nm. Two different peaks of aggregates were observed indicating the onset of surfactant aggregation due to protonation. At lower pH, the average aggregate size increases with a decrease in pH. For example, at pH 4, the DLS hydrodynamic diameter was 354 ± 206 nm. The formation of large aggregates was confirmed by the TEM analysis as shown in Figure 3. The average diameter of the aggregates was measured to be 76 ± 32 nm. At intermediate pH, the average aggregate size was reduced. At pH 6, the DLS size was 251 ± 141 nm. This reduction in size was confirmed by TEM micrograph which yielded a size of 51 ± 18 nm. Above pH 9, the average aggregate size was less than 19 nm, and only a single peak was observed in DLS measurements. Figure 3c shows the TEM image for the case of pH 10. Since the aggregate size was too small at this pH; it was


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difficult to obtain a sharper image owing to the resolution limit of the instrument. Nevertheless, it shows that surfactant aggregates (marked with arrows) were less than 25 nm. Thus, a combined analysis using DLS, TEM, and transmissivity shows that the surfactant aggregation behavior was significantly influenced by the system pH. In the aforementioned DLS analysis, the surfactant concentration was 2 wt% or 6.52 mM. It is important to note that DLS analysis relies on the assumption that the system is dilute enough to neglect the interparticle interactions. The high particle concentration can lead to multiple scattering which makes the analysis of the correlation function challenging50. In order to verify that the concentration used in the study is not high enough to cause interparticle interactions, we systematically investigated the effect of surfactant concentration on the hydrodynamic diameter of surfactant aggregates. The surfactant concentration was varied from 2 wt% (6.52 mM) to 0.125 wt% (0.4 mM). Two different pH cases (pH 4 and 9) were chosen. Supplementary Information (Figures S2 and S3) shows the results of hydrodynamic diameters of varying concentration of surfactant aggregates for the cases of pH 4 and 9, respectively. No significant shift in the peaks was observed as a function of surfactant concentration and the average hydrodynamic diameters were within the error range (Supplementary Information, Figure S4). It shows that interparticle interactions were not dominating factor when the surfactant concentration is less than or equal to 2 wt% (6.5 mM).


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Figure 2. (a) Surfactant solutions (2 wt %) with varying pH at 25 °C after 30 days of preparation; (b) percentage transmittance (primary y-axis) and average hydrodynamic diameter as measured by DLS in nm (secondary y-axis) for the surfactant solutions with varying pH values


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Figure 3. TEM image of the surfactant aggregates at three different pH obtained using the negative-staining method

pH-Responsive Surfactant solutions of different pH were prepared, and foaming behavior was studied by foam shake test. Figure 4a shows the surfactant solutions at different pH in glass vials mixed vigorously for 30 seconds. The image was taken after 5 minutes of mixing. Surfactants solutions at pH 2 and 4 show no foaming tendency with zero foam height. Conversely, solutions at higher pH (>4) form foams with a fine texture. Supplementary Information (Figure S5) shows the decay of relative foam height (foam height normalized wrt maximum foam volume) with time for the different pH cases. Foamability of the sample typically depends on the ease of the surface-active species to mobilize from the bulk solution to air-water interface. As the surfactant becomes protonated at lower pH, it becomes increasingly hydrophobic (evident from the turbid aqueous phase) and thus loses its affinity for the interface. Therefore, foamability of the samples decreased from 0.96 (pH 12) to 0.46 (pH 6) to zero (pH 2 and 4), as shown in Figure 4b.


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The pKa of the TPC surfactant was measured using acid-base titration (Supplementary Information, Figure S6) via half-equivalence point method and was found to be 7.27 which is higher than a typical pKa value of short-chain carboxylic acids 51. For example, the reported pKa of acetic acid and propionic acid (CH3CH2COOH) are 4.74 and 4.87, respectively. Kanicky et al. (2000) studied the effect of chain length on the pKa of the fatty acid. They reported that pKa values remain close to 4.8 when fatty acid chain length less than 6. A linear increase in pKa values was observed with increase in chain-length above 6. For example, the pKa of lauric acid (CH3(CH2)10COOH) is close to 7.5. This increase in pKa was attributed to the cooperativity among molecules induced by the van der Waals interactions between chains and the concomitant polar group interactions52. The degree of protonation, Θ of the carboxyl group in the TPC surfactant, can be calculated using the Henderson–Hasselbalch equation53, Θ =

1

10

+1

[3]

Figure 4b shows the plot of Θ and initial normalized foam height as a function of pH. When the pHpKa, the Θ is close to 0 and the carboxyl functional group in dominantly deprotonated leading to enhanced foam stability as seen in case of pH 10 and 12. When pH was close to pKa, a transition from stable foam to unstable foam was observed as shown in Figure 4b. The viscosity of the solutions can also significantly affect the foamability and foam stability of the system. To understand its effect, viscosity measurement was performed for the different pH cases. The viscosity of the samples at


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different pH was found to be similar to DI water indicating the minimal effect of pH modification. Thus, it was confirmed that foam stability was not governed by viscosity variation. The foam half-life (the time it takes the foam to achieve half its original foam height), which is a measure of foam stability increased monotonically from 0 (pH 2 and 4) to 20 minutes (pH 6) to 62 mins (pH 8.9) (Figure 4c). The decay of foam height above pH 6 was found to be quite similar, as shown in Supplementary Information (Figure S5). It is interesting to note that for pH> pKa the foamability (initial foam height) was almost constant. This was in contrast with the behavior of some reported pH-sensitive surfactant in the literature which shows a maximum foam stability at pH close to pKa. For example, Kanicky et al. (2000) demonstrated that sodium laurate shows a maximum foam height and foam stability when pH was close to its pKa value of 7.5. They proposed that at pKa (when 50% of the surfactant is in ionized form and 50% is in unionized form) a strong ion-dipole interaction occurs between ionized carboxylate (RCOO-), unionized fatty acid (RCOOH),which result in the formation of dimers. These ion-dipole interactions along with van der Waals interaction results in a reduction of intermolecular distance which causes optimal foaming conditions at pH close to pKa52. Recently, Stubenrauch et al. (2017) discussed that these ion-dipole interactions are essentially hydrogen bonds (H-bonds). The presence of these intersurfactant H-bonds results in a reduction of the distance between the surfactant molecules at the interface. It causes a dense packing of molecules which results in optimal foaming conditions. They systematically studied the foaming behavior of two types of surfactants in which one was capable of forming H-bonds and other was not. They demonstrated that the formation of H-bonds results in enhanced foam stability close to pKa of surfactants. It is important to note these studied surfactants were linear alkyl chain type-surfactant. Similarly, Kim and Kinsella (1985) observed maximum foam stability using bovine serum albumin ( a pH-


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sensitive surface-active protein) when the pH was close to the isoelectric pH (pI). They attribute this behavior to extensive protein:protein interaction via hydrophobic interactions and H-bonding when pH is close to pI54. Counter-examples are also reported in the literature. Recently, Micheau et al. (2018) reported the foaming behavior of a pH-sensitive surfactant, nonaoxyethylene oleylether carboxylic acid, RO-(CH2CH2O)9-CH2COOH, R = C16/C18. This polyethoxylated carboxylate surfactant is similar to the surfactant studied in this work which comprises of EO groups instead of PO groups such as in TPC surfactant. They showed that the surfactant does not show a local maxima of foam stability/height at pH equal to pKa55. The calculated surface area per polar head for this surfactant was also not found to be minimum at pH equal to pKa (in contrast with classical carboxylate surfactants such as sodium laurate)22. They attributed this behavior to the presence of large EO groups which prevents the ion-dipole interaction between the ionized and unionized forms of surfactants. Similarly, in the present study, we believe that the presence of large PO groups along with a large hydrocarbon moiety prevents the effective ion-dipole interaction or formation of H-bonds. Therefore, a local maxima of foam stability and foamability was not found close to pH equal to pKa for the TPC surfactant.


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Figure 4. (a) Digital images of vials containing surfactant solutions at different pH which are vigorously mixed; (b) Plot of initial normalized foam height for surfactant at different pH (primary y-axis) and degree of protonation of carboxyl groups (secondary y-axis); (c) The halflives of foam as a function of pH The bubble morphology, a measure of foam stability, was characterized using optical microscopy. Figure 5 shows the micrographs of the foams for different pH cases showing the fine texture of the bubbles. Note that only hand mixing was performed to generate this finetextured foam and not a high rpm mixing in a blender. The Sauter mean diameter, DSM, and the polydispersity index are reported in the Supplementary Information (Table S1). The Dsm ranged


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from 271.2 µm to 197.9 µm for the case of pH 6 and pH 12, respectively. The PDI was >>5% in all cases indicating a polydisperse bubble size distribution.

Figure 5. Optical micrographs of foam stabilized by surfactant solutions at pH of (a) 6, (b) 7, (c) 8.9, (d) 10, (e) 12. Scale bars are 500 µm. To verify the switchability of foaming behavior from a stable to no-foam state, the pH of the sample (initial pH: 8.9) was decreased and increased cyclically by addition of HCl/NaOH. Foam shake test was performed to monitor the initial foamability or foam height. Figure 6 shows the images of the same sample (mixed vigorously) at the end of each pH cycle. Two observations can be made. First, the relative foam height changed from 1 to 0 when pH was changed from 8.9 to 4. The same foam height was restored when pH was increased even after multiple cycles (Figure S7). Second, as expected, the dispersion changed from clear to turbid with pH change from 8.9 to 4 (at least 6-times) as surfactant switches between deprotonated (-COO-) and protonated (-COOH) forms. This shows the outstanding pH-responsive behavior of the system


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enabling a robust control on the foam stability. It is important to note that in static bulk foams (such as in the present case), foam regeneration is not possible due to the absence of continuous mixing and only foam decay (coalescence, drainage, and collapse) occurs. Conversely, the foam in subsurface applications undergo dynamic mixing with continuous generation and coalescence of in-situ foam. However, a foaming formulation which shows zero foamability in bulk (after vigorous mixing) is expected to show no in-situ foaming tendency even under continuous mixing. Similarly, a surfactant formulation which shows good foamability in bulk is expected to form in-situ foam in porous media under dynamic conditions.

Figure 6. Digital images of surfactant solutions after different pH cycle indicating the reversible foaming-defoaming capability Surface tension is strongly governed by the molecular packing parameter of the self-assembled surfactant molecules at the interface which in turn is dependent on the complex interaction between the protonated and deprotonated form of surfactant molecules56. To better understand this, surface tension measurements were performed at varying pH conditions to quantify the


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surface activity of the surfactant (Figure 7). The surface tension ranged from ~71 mN/m (0.004 wt% or 0.013 mM) to ~35 mN/m ( 1 wt% or 3.26 mM). The critical micelle concentration (CMC) was calculated from the plot, and the results are tabulated in Supplementary Information (Table S2). For the cases of pH>> pKa such as pH 9, 10, and 12, the CMC was found to be similar and equal to 0.0309 wt% or 0.101 mM. An increase in CMC value was observed with a decrease in pH below 7. The CMC value increased by 33.3, 56.3, and 98.0 % for the case of pH 7, 4 and 2, respectively. This large variation of surface tension with a change in pH shows the different adsorption affinity of surfactant molecules to the air-water interface for varying pH conditions which critically influence their foaming tendency. It is to be noted that for pH < 7, the surfactant exhibits low solubility (as evident from transmissivity and DLS data) due to an increased hydrophobicity of the surfactant which may explain the significant increment in the CMC value with a reduction in system pH.


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Figure 7. The plot of surface tension as a function of surfactant concentration (mM) under different pH conditions The surfactant surface concentration (Γ) at the air-water interface can be estimated from the slope of γ-ln(C) plot just before CMC at constant temperature using the Gibbs adsorption equation as shown below Γ=−

1 %γ $ * [4] !"# %'!() +

where Γ is the surface excess concentration in mol/m2, γ is the surface tension in mN/m, R is the gas constant (8.314 J / mol. K) and T is the temperature (298 K). The factor n is a constant that


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depends on the type of surfactant and the presence/absence of electrolyte57. For the present case of pH-sensitive anionic surfactant in the absence of salt, the effect of ionization degree of the surfactant (α) has to be considered which is equal to 1- Θ (degree of protonation)22. In such case, the parameter n = 1 + α. The average area occupied by each molecule, A at the interface is given by the following equation - =

1 [5] Γ./

where NA is the Avogadro number. Supplementary Information (Table S3) lists the value of the parameter n and the calculated value of Γ (mol/m2), A in Angstrom2 (Å2). As the system pH increases the surfactant becomes increasingly deprotonated resulting in an increase in hydrophilicity. Therefore, the lateral interaction between negatively charged polar groups is expected to increase due to electrostatic repulsion. As expected, an increase in average area, A occupied by each molecule with an increase in pH was observed. CO2/N2 Responsive Clearly, pH-induced stabilization-destabilization of aqueous foam is possible using the TPC surfactant. It is to be noted that acid-base addition for pH changes will result in the accumulation of salts due to neutralization reactions after several pH cycles which could adversely affect the foam stability58. In this context, carbon dioxide/ nitrogen (CO2/N2) triggered pH-change offers unique advantages. The bubbling of CO2 results in the formation of carbonic acid which can lower the system pH. The dissolved CO2 can be removed by bubbling inert gas such as argon or nitrogen to restore the original pH. Additionally, this process is environment-friendly and


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relatively inexpensive. Moreover, CO2 and N2 are the most common gases used in subsurface applications. First, the effect of CO2/N2 bubbling on the solution pH was studied. The initial pH of the surfactant solution (0.25 wt %) was 7.5 and the appearance of the solution was translucent. The pH of the solution was reduced to 4.75 within 5 minutes of CO2 bubbling (Figure 8), and the solution becomes completely white and opaque. After bubbling nitrogen for 30 minutes, the initial pH of the solution was restored, and the solution again becomes completely clear. It shows that the process is highly reversible and the pH of the solution can be mediated precisely by adjusting the bubbling time of the gases. To study the switchable performance of foamability by alternatingly bubbling of CO2/N2 gas, a custom glass cell was designed. It was equipped with a porous frit at the bottom to generate static foams. Surfactant solution (0.25 wt%) of pH 7.5 was placed in the cell, and nitrogen gas was injected at a constant pressure. It resulted in the formation of a strong, stable foam (Figure 9a). Then, carbon dioxide was bubbled at same pressure conditions. It resulted in complete destruction of foam volume and solution become turbid and white as observed during pH measurements (Figure 9b). Nitrogen gas was again bubbled through the system, and the solution became clear again and regained the foaming tendency. Figure 9 shows the digital images of the foam volume during different cycles of N2 and CO2 bubbling. Figure 9 (Inset) shows the micrograph of the foam stabilized during the nitrogen cycle. The Sauter mean diameter of the bubbles was 321 µm. The reversibility of the process was confirmed by repeating these steps at least five times. The same foam height was achieved at the end of each nitrogen cycles.


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Figure 8. The variation of pH of the solution upon bubbling of CO2/ N2 as a function of time


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Figure 9. Digital images of the foam obtained by bubbling nitrogen and carbon dioxide cyclically; the inset the shows the micrographs of the foam for the N2 case. Scale bars are 5000 µm (black) and 500 µm (blue) Temperature-Responsive Temperature is a non-invasive trigger which could be used to modulate the interfacial properties of the surface-active components in a colloidal system. Unlike, pH modification using acid/base or CO2 bubbling, the chemical composition of the system remains unchanged during temperature adjustment. Surfactants are often employed in subsurface applications such as chemical/foam enhanced oil recovery (EOR) to recover hydrocarbon from reservoirs. They can lower interfacial tensions between oil-water, alter the reservoir wettability from oil-wet to water-wet59, and improve volumetric sweep efficiency18,60. Since different reservoirs have different temperature


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and salinity conditions, surfactants need to be custom-engineered for each reservoir. The addition of propylene oxide (PO) and ethylene oxide (EO) groups in the surfactant to tune the HLB of the surfactant is a commonly used technique to achieve desired aqueous stability and interfacial properties61. The PO groups are relatively more hydrophobic than EO groups. The effect of parameters such as PO/EO ratio, temperature, and ionic strength has been widely studied for polymeric surfactants in the literature62. In such systems, the hydrophobicity of the surfactant increases with an increase in PO groups, temperature and salt concentration. In this work, we utilized this concept to tune the hydrophobicity of the TPC surfactant which has long chain propylene oxide (PO) groups by varying the temperature (at a fixed salt concentration) making the surfactant thermo-responsive. We were also interested in understanding the effect of salt concentration on foaming behavior. Most reservoirs contain clays which are naturally-occurring minerals such as smectite, illite, chlorite etc. These clays are typically sensitive to water which result in their macroscopic swelling resulting in several operational issues such as shale instability in oil well drilling operation or plugging of pores in a reservoir63. The addition of electrolytes such as sodium chloride, potassium chloride (along with polymeric species such as partially-hydrolyzed polyacrylamide) in the injection fluids is often performed to inhibit clay swelling. In this work, we used sodium chloride which is the most common salt present in a typical injection fluid. Surfactant solutions (2 wt%) with varying salt concentration were prepared and mixed for 24 hours on LabQuake shaker (Barnstead Thermolyne). The samples were then kept undisturbed and were monitored for aqueous stability at 25 ˚C. Supplementary Information (Figure S8) shows the glass vials containing these mixtures after 1 day and 7 days of preparation. The surfactant aggregation was seen in the case of 2 and 3 wt% NaCl during day 1. Complete


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precipitation was observed for the cases of 3 wt% salt after 7 days. However, no visual phase separation was observed for cases with salt less than 2 wt%. Sodium ions react with carboxylate groups and decrease the electrostatic repulsion between surfactants as the salinity increases which leads to increasing surfactant aggregation. The samples were again mixed vigorously for 24 hours, and the morphology of the surfactant aggregates was characterized using the dynamic light scattering (DLS). Supplementary Information (Figure S9) shows the DLS results at room temperature. Several observations can be made from the plot. First, as the salt concentration increases the hydrodynamic particle size becomes larger due to the increased surfactant aggregation. For salt concentration greater than 1 wt%, a multimodal distribution was observed indicating the formation of large aggregates. Note that since the sample with 3 wt% NaCl has a tendency to precipitate quickly, the DLS results could underestimate the particle size as bigger aggregates could settle down during the measurement. A similar aqueous stability trend was observed at 75 ˚C also. To demonstrate that surfactant hydrophobicity increases with an increase in salt concentration, the foamability of samples were studied at 25 ˚C. Supplementary Information (Figure S10) shows the foam height with varying salt concentrations. Clearly, the foamability of the samples was critically influenced by the salt concentration. The initial normalized foam height was found to be 1, 0.87, 0.59, 0.39, and 0.32 for salt concentration of 0, 0.5, 1, 2, and 3 wt%, respectively. Since the surfactant solutions above 1 wt% salt were not aqueous stable at 75 ˚C, subsequent experiments at the high temperature were done using salt concentration below or at 1 wt%. Note that the salt tolerance of the surfactant could be increased by the addition of EO groups64 to the surfactants and this concept would be explored in the future work.


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Surfactant solutions (2 wt%) with varying salt concentrations (0, 0.5, 1 wt%) were taken, and foam shake test was performed to measure the initial foam height at different temperatures. The solution temperature was progressively increased from 25 ˚C to 75 ˚C. Supplementary Information (Figure S11) shows the result of initial foamability for the different cases. Two observations can be made from the plot. First, for the cases of salt concentration of 0 and 0.5 wt%, an increase in temperature has a minor effect on the initial foam height. The normalized foam height was 0.87 ± 0.09 and 0.72 ± 0.07 for 0 and 0.5 wt% case, respectively. Second, surfactant solution with 1 wt% salt concentration was found to be highly temperature sensitive. The foam height reduced monotonically with an increase in temperature with zero foam height for temperature ≥ 65 ˚C. To better understand this result, dynamic light scattering analysis of the solution was performed at varying temperatures, as shown in Supplementary Information (Figure S12). It can be seen that with an increase in the temperature the DLS peak shifts towards right indicating an increase in surfactant aggregate size due to an increase in the hydrophobicity of the surfactant. The average hydrodynamic diameter increased from 76 ± 34 nm at 25 ˚C to 179 ± 6 nm at 75 ˚C (Figure S13). The reversibility of the foaming behavior was then studied. Surfactant solution (2 wt%) with 1 wt% NaCl concentration was cyclically heated to 65 ˚C and then cooled to 25 ˚C. The initial foam height was recorded using foam shake test after each temperature step. Figure 10 shows the results of the normalized foam height for different cycles. The same foam height was restored at 25 ˚C, and complete foam collapse was observed at 65 ˚C, even after multiple heating-cooling cycles (Figure S14). The solution appearance also interconverts between translucent to opaque (Figure 10, inset) as surfactant molecules reversibly deaggregates and aggregates, as evident from the DLS measurement (Figure 10, inset). The average size changed between 76 ± 34 nm at


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25 ˚C to 122 ± 8 nm at 65 ˚C. This shows the robust thermo-responsive foaming behavior of the surfactant. Such multi-stimuli responsive foaming system can be exploited in a broad range of industrial applications.

Figure 10. The initial normalized foam height after different temperature cycles; the inset shows the solution appearance and the hydrodynamic diameters of the surfactant aggregates at 25˚C and 65˚ C

CONCLUSIONS We report a novel commercially available surfactant that can undergo a reversible transition between surface-active to inactive forms in the presence of external stimuli such as pH variation (via acid/base addition or CO2/N2 bubbling) and temperature modification. The following conclusions can be drawn from this work:


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a) The surfactant exhibits robust pH-responsive behavior in stabilizing aqueous foams due to the presence of pH-sensitive, terminal carboxyl functional group. The system demonstrated excellent reversibility (at least six times) where the foamability can be switched from stable foam to no-foam state by adjusting the pH cyclically from a high (pH 8.9-12) to low (pH 2-4) value. b) A decrease in pH value below pKa of the carboxyl group results in the formation of reversible, large surfactant aggregates due to the hydrophobic nature of protonated acid as evident from the dynamic light scattering, transmissivity data, and TEM micrographs. c) Reversible foaming and defoaming can be achieved by alternatingly bubbling nitrogen and carbon dioxide gas at the room temperature. Precise control over the solution pH could be achieved by adjusting the bubbling time. d) Due to the presence of propylene (PO) groups, the surfactant solution (with 1 wt% salt concentration) demonstrated thermo-responsive behavior where foamability can be varied from stable-foam to no-foam state by changing the temperature from 25 to 65 ˚C. The thermo-responsive behavior was not seen at lower salt concentrations (0 and 0.5 wt%). Such stimuli-responsive foams have potential in diverse subsurface processes such as foam enhanced oil recovery, and environmental remediation where on-demand destruction/formation of foam is desirable.

ASSOCIATED CONTENT Supporting Information Available The Supporting Information is available free of charge on the ACS Publications website.


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Hydrodynamic diameter of the surfactant aggregates under different pH, salt, concentration and temperature conditions, decay of relative foam height with time, pKa determination using titration curve, Sauter mean diameter and polydispersity index of foam bubbles, critical micelle concentration, surface excess concentration, area per head group of surfactant under different pH, appearance of surfactant solution under different salt conditions, foamability of surfactant under different salt conditions, initial foam height as a function of temperature and salt concentration, average hydrodynamic diameters of surfactant at different temperatures, reversible foaming-defoaming of surfactant solution induced by cooling-heating cycle.

AUTHOR INFORMATION Corresponding Author *Kishore K. Mohanty Telephone: 512-471-3077, Fax: 512-471-9605, E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank the Gas EOR IAP at the Center for Petroleum & Geosystems Engineering for partial funding of this research. The authors thank Dr. Dwight Romanovicz at Institute for Cellular and Molecular Biology (ICMB), UT Austin for assistance with transmission electron microscopy.

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(63) Anderson, R. L.; Ratcliffe, I.; Greenwell, H. C.; Williams, P. A.; Cliffe, S.; Coveney, P. V. Clay Swelling — A Challenge in the Oilfield. Earth-Sci. Rev. 2010, 98 (3), 201–216. (64) Micheau, C.; Schneider, A.; Girard, L.; Bauduin, P. Evaluation of Ion Separation Coefficients by Foam Flotation Using a Carboxylate Surfactant. Colloids Surf. Physicochem. Eng. Asp. 2015, 470, 52–59.


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Table of Content Graphic


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1146x748mm (120 x 120 DPI)


Multistimuli-Responsive Foams Using an Anionic Surfactant

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