Ethanol-Based Foam Stability As Probed by Foam Lamella Thinning

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Ethanol-Based Foam Stability As Probed by Foam Lamella Thinning Gopi N. Sethumadhavan, Alex D. Nikolov, and Darsh T. Wasan* Department of Chemical and Environmental Engineering, Illinois Institute of Technology, Chicago, Illinois 60616

Vipul J. Srivastava, John J. Kilbane II, and Thomas D. Hayes Environmental Science and Technology Center, Gas Technology Institute, Des Plaines, Illinois 60018

In this study, we investigate the stability of the bulk-foam and single-foam lamellae using a mixture of two nonionic surfactants in ethanol-based aqueous solutions. The second virial coefficient, which is indicative of the intermicellar interactions in solutions containing both surfactants and ethanol, was measured. We found a good correlation between the foam stability and the second virial coefficient. Surfactant formulations with a more positive virial coefficient lead to a more stable foam. The foam lamella stability was probed by using the capillary force balance. It was observed that the foam lamella thinned in a stepwise manner (stratified). It was found that, at an optimum surfactant formulation, foam stability was at its maximum when the foam lamella was stable and remained at a high equilibrium thickness with a low rate of thickness transition. Introduction Surfactant solutions and foams have been used in the petroleum industry for decades to aid in the recovery of crude oil.1-3 These approaches may also be employed in environmental remediation applications to remove petroleum hydrocarbons or other contaminants from the soil.4-13 Surfactant solution flooding is a promising remediation technology that removes contaminants in situ,12 but it has several major drawbacks. Precise containment of the treatment zone is not possible with surfactant flooding. Contaminants are mobilized (solubilized/desorbed) by surfactant solutions and pose an increased risk of contaminating groundwater and previously uncontaminated soil.5,12,14 Surfactant flooding is also expensive. Foams are better suited for contaminant remediation than a surfactant solution flush; they are far more amenable to containment than surfactant solutions. Air injection and vacuum extraction used in conjunction with foams could avoid concerns about spreading contamination.5 Colloidal gas aphrons, or monodispersed air bubbles in water, have been used in the past to stimulate in situ biodegradation by enhancing the delivery of oxygen to the subsurface.10,15,16 Foam by its inherent structure can carry air and aid in biodegradation. This will greatly assist in bioremediating any contaminant that has not been removed by the foam. The cost of chemicals is also expected to be lower because a significant percentage of the foam volume is occupied by air. Thus, foams are better suited for in situ contaminant remediation from soil. Aqueous ethanol-based surfactant (EBS) solutions have shown superior soil remediation performance over aqueous surfactant solutions. The recent work of Kilbane et al.17 showed that while aqueous surfactant solutions were unsuccessful in removing polyaromatic hydrocarbons from soil, EBS solutions could remove * To whom correspondence should be addressed. Tel.: 312567 3001. Fax: 312-567 3003. E-mail: [email protected].

close to 50%. Another key consideration for in situ remediation is maintaining a relatively high soil hydraulic conductivity during the process so low-pressure drops can move the foam/solution in the soil. Hydraulic conductivity refers to the relative ease with which a solution can flow through a medium (proportional to Q/∆P). Previous investigations into surfactant solution flooding demonstrated that most surfactants absorb into the soil and/or alter the soil structure, resulting in a dramatic decrease in hydraulic conductivity.18 EBS solutions showed only a modest decrease in hydraulic conductivity, remaining close to that of water.17 Thus, EBS solutions remove pollutants efficiently and maintain a high hydraulic conductivity in the soil.19 Foam drainage and its stability can be attributed to a single-foam lamella drainage and stability. The combined effects of capillary forces, the surface tension gradient (Marangoni-Gibbs effect), and hydrostatic forces (i.e., gravity) govern foam lamella drainage. The Marangoni-Gibbs effect is the dominant factor in foam drainage at surfactant concentrations below the critical micelle concentration (cmc). However, the micellar interactions and the adsorbed layer interactions govern the foam lamella stability (and the foam stability) at high surfactant concentrations much greater than the cmc. A more comprehensive review of the role of these interactions is available elsewhere.20-23 In this study, the role of the surfactant composition at concentrations considerably greater than the cmc of the surfactants in foam and single-foam lamella stability is analyzed. The intermicellar interactions are quantified by the second virial coefficient measured by the static light scattering method and correlated with the foam stability. The role of the confinement-induced micellar-layering phenomenon in the single-foam lamella thickness stability is discussed. Materials Our earlier work17,19 demonstrated that the use of an amide surfactant was necessary to produce ethanol-

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Figure 1. Surface force balance for measuring the film thickness and thin film drainage of the curved foam film. Table 1. Surfactants Used in the Study

based foams of even moderate stability. In the absence of amide surfactants, interfacial turbulence caused by ethanol evaporation rendered the foam extremely unstable. The proprietary surfactants used previously have been replaced by commercially available surfactants in this work. A mixture of two nonionic surfactants manufactured by Witco Corp., Houston, TX (now a subsidiary of Akzo-Nobel), was used: (a) Witconol 2722 sorbate [abbreviated to W2722; cmc in 15% EOH of 0.008% (w/w) or 0.06 mM]; (b) Witcamide 5085 [abbreviated to W85; cmc in 15% EOH of 0.0008% (w/w) or 0.02 mM]. The structures of these surfactants are shown in Table 1. Several solutions of various ratios of the two nonionic surfactants were prepared and equilibrated for 2 days before use in all of the experiments. The final solution contained 4.78% (w/w) surfactants, 15% (w/w) ethanol, and water. The total surfactant concentration (W85 + W2722) was kept constant, and the ratio of W85/W2722 was varied in all of the experiments. Five surfactant compositions were studied: (i) W85 ) 0% (0 mM), W2722 ) 4.78% (37.2 mM); (ii) W85 ) 1.2% (31.2 mM), W2722 ) 3.6% (28.1 mM); (iii) W85 ) 2.4% (62.5 mM), W2722 ) 2.4% (18.7 mM); (iv) W85 ) 3.6% (93.8 mM), W2722 ) 1.2% (9.3 mM); (v) W85 ) 4.78% (124.5 mM), W2722 ) 0% (0 mM).

in height) in a graduated cylinder in a standardized manner for 15 s to produce foam. We recorded the foam height after 8 h. The data reported are the average of three runs. Runs retaining a greater percentage of their initial height are classified as more stable. All experiments were carried out at 24 °C. Foam Lamella Thinning. We used a capillary force balance (Figure 1) similar to the one developed recently24 to investigate the thinning of a curved foam lamella. An air bubble is formed from a tiny capillary (of radius 0.05 cm). In the reflected light interference microscopic technique, white light incident on the foam lamella surfaces reflects and produces interference patterns. A microscope was used in conjunction with a CCD camera, a video recorder, and an image analyzer to monitor the interference patterns as a function of time. The foam lamella thickness as a function of time was evaluated from the color of the interference patterns, and the rate of lamella thinning was calculated. The system was sealed to prevent any ethanol evaporation. All experiments were carried out at 24 °C. Micellar Interactions Probed by Light Scattering. These studies were carried out to investigate the effects of the change in surfactant composition on intermicellar interactions. The standard technique of static light scattering introduced by Debye 25 was adopted. Light from a monochromatic source (green light, 541 nm) was passed through a sample and its intensity measured at 90° to the incident light. The turbidity of the sample was obtained from this measured intensity. This was done directly using the Hach model 2100A turbidimeter. To get the absolute turbidities of the different samples, the instrument was calibrated using the known turbidity values of pure solutions of benzene, carbon tetrachloride,26 and water.27 This technique was also used to locate the cmc of the surfactants. Debye showed that for reasonably dilute solutions25

H(c-cmc) 1 ) + 2B(c-cmc) τ - τsolv M

(1)

where Experiments Foam Stability Test. Foam stability was determined by agitating 10 mL of the surfactant formulation (5 cm

H)

32π3n2 ∂n 2 3Navλ4 ∂c

( )

(2)

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Table 2. Summary of Light Scattering Experiments sample

second virial coefficient

intermicellar interaction

foam and film stability (from Figure 2 and Table 3)

W2722, 4.78%; W5780, 0% W2722, 2.4%; W5780, 2.4% W2722, 0%; W5780, 4.78%

2.3 × 10-4 14 × 10-4 -4.3 × 10-4

intermediate repulsion most repulsion most attraction

intermediate stability most stable least stable

Figure 2. Foam stability as a function of the surfactant composition.

Here, n is the refractive index of the medium, Nav is the Avogadro number, c is the concentration of the surfactant (in g/cm3), λ is the wavelength of light, M is the average molecular weight of the scattering unit (g/ mol), and B is the second virial coefficient. Different formulations have different values of H. We obtain B from the slope of the curve by substituting H in eq 2 and plotting the left-hand side of the equation as a function of c-cmc (i.e., the Debye plot). The second virial coefficient is an indicator of intermicellar interactions (positive values signify repulsion). Results and Discussion The results for foam stability from the Bartsch test, which measures the decrease in the foam height over time, are shown in percent of initial height in Figure 2. The foam stability reaches a maximum when an equal weight ratio of the surfactants W85 (amide) and W2722 (sorbate) was used (i.e., optimal formulation). We find that the optimum surfactant formulation corresponding to a maximum in foam stability had the highest positive

Figure 3. Debye plot from turbidity measurements.

value of the second virial coefficient (Table 2). The second virial coefficient was calculated from the slopes of the Debye plot (Figure 3). Our data show that more repulsive intermicellar interactions (i.e., more positive virial coefficients) lead to more stable foams. These observations confirm our previously reported findings on the correlation between the foam stability and the second virial coefficient.23 Foam lamella thinning was monitored using the capillary force balance technique in order to reveal the role of the micellar interactions on foam lamella stability. When the foam lamella thinned to a thickness of a few micelle diameters, further thinning occurred in a stepwise manner as the micelles left the film layer by layer. The process of foam lamella thinning is sketched in Figure 4. The photomicrographs shown in this figure depict the manner in which the stepwise thinning process occurs. The height of the thickness transition is approximately constant for all transitions. We found in our previous studies with well-characterized surfactant systems that the height of the thickness transition is equal to the effective diameter of the surfactant micelle.21 Such stepwise thinning of the foam lamella is due to the formation of micellar layers induced by the confinement effect of the foam lamella surfaces.28,29 The micelles trapped inside the foam lamella as it thins self-organize into a layered structure. The number of thickness transitions corresponds to the number of micellar layers. Micelles leave the film area layer by layer during foam lamella thinning. For example, a film with five micellar layers during its thinning process passed through five thickness transitions before it reached the final film thickness without any micelles. Earlier, we had studied the micellar-layering phenomenon using the Monte Carlo simulation.30 These results revealed that the foam lamella with five micellar layers have a micellar volume fraction of 0.43. In reality, nonionic micelles are immersed in the solvent and

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Figure 4. Schematic of the change of film thickness with time showing stepwise film thinning in the presence of micelles. Table 3. Foam Lamella Stability, Rate of Stratification, and Second Virial Coefficient no. of step transitions

surfactant composition

lamella radius 170 µm

lamella radius 268 µm

time between occurrence of the penultimate and the last transition(s)

W85, 0%; W2722, 4.78% W85, 1.2%; W2722, 3.6% W85, 2.4%; W2722, 2.4% W85, 3.6%; W2722, 1.2% W85, 4.78%; W2722, 0%

5 5 3 5 5

5 5 5 5 5

11.4 21.3 23.9 11.0 7.7

interact with it, and a solvent hydration layer is formed around the micelles. In the presence of the solvent environment, the micellar interactions are characterized by the effective volume, which includes the hydration layer around the micelle. All of the formulations studied having a lamella size larger than 280 µm (radius) exhibited five thickness transitions. Hence, all of the formulations have nearly the same effective micellar volume fraction of about 0.43. However, the foam lamella stability (and foam stability) in the presence of micelles depends not only on the effective volume fraction but also on the micellar interaction.23 The micelle-micelle interactions were quantified by the value of the second virial coefficient. We can conclude that the micellar solution with the most positive virial coefficient is expected to have a high foam lamella stability based on the calculated values of the second virial coefficient determined by the light scattering data. The number of micelle layers that leave the film (i.e., the film thickness transitions) depends on the film size. Table 3 lists the number of film thickness transitions for two different film sizes (170 and 286 µm). For the larger film size, all of the five layers leave the film and the film reached a state with no micelles. However, the smaller size film stays stable at an equilibrium thickness corresponding to two micelle layers. The film has three thickness transitions at an optimum surfactant formulation, whereas it has five thickness transitions and the film is less stable at all other concentrations. An important factor that greatly affects the foam lamella stability, and thereby foam stability, is the

second virial coefficient 2.3 × 10-4 14 × 10-4 -4.3 × 10-4

Figure 5. Time between the occurrence of the penultimate and ultimate transition for various surfactant formulations.

polydispersity in the micelle size.31,32 For example, for a pure amide formulation, the film is much less stable because the polydisperse micelles have a higher tendency to leave the foam lamella as compared to the monodispersed system. The foam lamella thickness stability was characterized by the time between the penultimate transition and the last transition (the lifetime of the last intermediate state). Table 3 and Figure 5 show these data. These observations clearly show that the time for the last transition is at its maximum (i.e., 23.9 s) at an optimum formulation, while it is 2.7 and 11.4 s for the pure amide and pure sorbate systems, respectively. These data indicate that, at an optimum formulation, micelles show a higher tendency to stay inside the lamella. This is typical of surfactant

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formulations that display good micellar in-layer structure in the film.20,29,30 Previous studies have shown that the structural disjoining pressure (i.e., the pressure exerted by the micelles on the film surfaces) is high for a monodisperse surfactant formulation, whereas a 30% polydispersity in the micelle size can greatly decrease the structural barrier, thereby reducing the foam lamella stability. In conclusion, we have established a good correlation between the foam stability and the second virial coefficient that characterizes the intermicellar interactions. Surfactant formulations with a more positive virial coefficient lead to more stable foams. The role of micellar interactions in affecting foam lamella was further analyzed in terms of the film thickness transitions that are indicative of the formation of the micelle layering inside the film. These results revealed that an optimum surfactant formulation corresponds to a maximum in foam stability, and the foam lamella is much more stable because it remains at a high thickness and has a low rate of thickness transition. Both the lamella size (i.e., the bubble size) and the polydispersity in the micelle size greatly affect the lamella thickness stability and foam stability. Literature Cited (1) Ross, S. Foams. Kirk-Othmer Encyclopedia of Chemical Technology; John Wiley & Sons: New York, 1980. (2) Rai, C.; Bernard, G. G. Method of Secondary Recovery Employing Successive Foam Drives of Different Ionic Characteristics. U.S. Patent 3,323,588, 1967. (3) Nutt, C. W.; Burley, R. W. The Influence of Foam Rheology in Enhanced Oil Recovery Operations. Foams: Physics, Chemistry and Structure; Springer-Verlag: New York, 1989; pp 105-147. (4) Ellis, W. D.; Payne, J. R.; McNabb, G. D. Treatment of Contaminated Soils with Aqueous Surfactants; EPA/600/S2-85/129; U.S. EPA: Washington, DC, 1985. (5) Peters, R. W.; Enzien, M. V.; Bouillard, J. X.; Frank, J. R.; Srivastava, V. B.; Kilbane, J.; Hayes, T. Nonaqueous-phaseliquids-contaminated Soil/Groundwater Remediation Using Foams. In In situ Remediation: Scientific Basis for Current and Future Technologies; Gee, G. W., Wing, N. R., Eds.; Battelle Press: Columbus, OH, 1994. (6) Raghavan, R.; Coles, E.; Dietz, D. Cleaning Excavated Soils Using Extraction Agents: a State of the Art Review; EPA Project Summary EPA/600/S2-89/034; U.S. EPA: Washington, DC, 1990. (7) Rouse, J. D.; Sabatini, D. A.; Suflita, J. M.; Harwell, J. H. Influence of Surfactants on the Microbial Degradation of Organic Compounds. Crit. Rev. Environ. Sci. Technol. 1994, 24, 325. (8) Seagren, E. A.; Rittmann, B. E.; Valocchi, A. J. Quantitative Evaluation of the Enhancement of NAPL-pool Dissolution by Flushing and Biodegradation. Environ. Sci. Technol. 1994, 28, 833. (9) Simms, J. L. In Situ Bioremediation of Contaminated Unsaturated Subsurface Soils. USEPA Engineering News; Issue no. EPA/540/S-93/501; Robert S. Kerr Environmental Research Laboratory: Ada, OK, 1993. (10) Michelsen, D. L.; Smith, J. W.; Suggs, J. A. Use of Colloidal Gas Aphrons for In-situ Biodegradation of Contaminated Ground Water. U.S. Environ. Prot. Agency, Res. Dev., [Rep.] EPA-600/988/0211988, 203. (11) Valine, S. B.; Chilcote, D. D.; Zambrano, A. R. Development of Soil Washing System. Proc. Purdue Ind. Waste Conf. 1989, 49, 83. (12) Vignon, B. W.; Rubin, A. J. Practical Considerations in Surfactant-aided Mobilization of Contaminants in Aquifers. J.s Water Pollut. Control Fed. 1989, 61, 1233.

(13) Wunderlich, R. W. In Situ Remediation of Aquifers Contaminated by Dense Nonaqueous Phase Liquids by Chemically Enhanced Solubilization. J. Soil Contam. 1992, 1 (4), 361. (14) Luthy, R. G.; Dzombak, D. A.; Peters, C. A.; Roy, S. B.; Ramaswami, A.; Nakles, D. V.; Nott, B. R. Remediating Tarcontaminated Soils at Manufactured Gas Plant Sites. Environ. Sci. Technol. 1994, 28, 266. (15) Jenkins, K. B.; Michelsen, D. L.; Non, J. T. Application of Oxygen Microbubbles for In Situ Biodegradation of p-Xylene Contaminated Ground Water in a Soil Column. Biotechnol. Prog. 1993, 9, 394. (16) Michelsen, D. L. Feasibility Study of Use of Predispersed Solvent Extraction/Floatation Techniques for Removal of Organics from Waste Waters. Chem. Eng. Commun. 1986, 49, 155. (17) Kilbane, J. J.; Chowdiah, P.; Kayser, K.; Misra, B.; Jackowski, K. A.; Srivastava, V. J.; Sethu, G. N.; Nikolov, A. D.; Wasan, D. T.; Hayes, T. D. Remediation of Contaminated Soils Using Foams. Land Contam. Reclam. 1997, 5 (1), 41. (18) Allred, B.; Brown, G. O. Surfactant Induced Reductions in Soil Hydraulic Conductivity. Ground Water Monit. Rem. 1994, Spring, 174. (19) Sethu, G.; Nikolov, A.; Wasan, D. Foam Mediated Remediation; Project reports, Contract No. 5092-253-2336; Gas Research Institute: Mount Prospect, IL, 1997. (20) Nikolov, A. D.; Wasan, D. T. Dispersion Stability Due to Structural Contributions to the Particle Interaction as Probed by Thin Liquid Film Dynamics. Langmuir 1992, 8 (1), 2985. (21) Wasan, D. T.; Nikolov, A. D.; Lobo, L. A.; Koczo, K. A.; Edwards, D. A. Foams, Thin Films and Surface Rheological Properties. Prog. Surf. Sci. 1992, 39, 119. (22) Chu, X. L.; Nikolov, A. D.; Wasan, D. T. Thin Liquid Film Structure and Stability: The Role of Depletion and Surface Induced Structural Forces. J. Chem. Phys. 1995, 103, 6653; 1996, 105, 4892. (23) Lobo, L.; Nikolov, A. D.; Wasan, D. T. Foam Stability in the Presence of Oil: On the Importance of the Second Virial Coefficient. J. Dispersion Sci. Technol. 1989, 10, 143. (24) Nikolov, A. D.; Wasan, D. T. A Novel Method for Studying Dynamic Behavior of Both Plane Parallel and Curved Thin Films. Colloids Surf. A 1997, 123-124, 375. (25) Debye, P. Molecular-weight Determination by Light Scattering. J. Phys. Colloid Chem. 1947, 51, 18. (26) Stamm, R. F.; Button, P. A. Rayleigh’s Ratio (Absolute Turbidity Levels) For Benzene and Carbon Tetrachloride. II. Corrections. J. Chem. Phys. 1955, 23, 2456. (27) Fessenden, R. W.; Stein, R. S. The Absolute Turbidity of Water. J. Chem. Phys. 1954, 22 (1), 1778. (28) Nikolov, A. D.; Wasan, D. T. Ordered Micelle Structuring in Thin Films Formed from Anionic Surfactant Solutions: I. Experimental. J. Colloid Interface Sci. 1989, 133 (1), 1. (29) Nikolov, A. D.; Kralchevsky, P. A.; Ivanov, I. B.; Wasan, D. T. Ordered Micelle Structuring in Thin Films Formed from Anionic Surfactant Solutions: II. Model Development. J. Colloid Interface Sci. 1989, 133 (1), 13. (30) Chu, X. L.; Nikolov, A. D.; Wasan, D. T. Monte Carlo Simulation of In-Layer Structure Formation in Thin Liquid Films. Langmuir 1994, 10, 4403. (31) Sethumadhavan, G. N.; Nikolov, A. D.; Wasan, D. T. Stability of Thin Liquid Films Containing Polydisperse Particles. Colloids Surf. A 2002, 204 (1-3), 51. (32) Chu, X. L.; Nikolov, A. D.; Wasan, D. T. Effect of Particle Size and Polydispersity on Depletion and Structural Forces in Colloidal Dispersions. Langmuir 1996, 12 (21), 5004.

Received for review August 6, 2002 Revised manuscript received March 20, 2003 Accepted March 20, 2003 IE0205896