Liquid Oil That Flows in Spaces of Aqueous Foam without Defoaming

Article pubs.acs.org/JPCB

Liquid Oil That Flows in Spaces of Aqueous Foam without Defoaming Junko Sonoda,† Takaya Sakai,*,‡ and Yukio Inomata§ †

Health Beauty Research Laboratory and §Skin Care Research Laboratory, Kao Corporation, 2-1-3 Bunka, Sumida-ku, Tokyo, Japan ‡ Eco Innovation Laboratory, Kao Corporation, 1334 Minato, Wakayama-City, Wakayama 640-8580, Japan S Supporting Information *

ABSTRACT: A very interesting phenomenon has been observed in which foam formed from an aqueous fatty acid potassium salt solution spontaneously absorbs liquid oil immediately upon contact without defoaming. Although this phenomenon initially appeared to be based on capillary action, it was clarified that the liquid oil that flows in foam film did not wet the air/water interface. In this study, it is discussed why aqueous foam can spontaneously soak up liquid oil without defoaming using equilibrium surface tension, dynamic oil/water interfacial tension, and image analysis techniques. The penetration of oil was attributed both to the dynamic decrease in the surface tension at the oil/water interface and to Laplace pressure, depending on the curvature of the plateau border. Therefore, the foam does not absorb the oil, but the oil spontaneously penetrates the foam. This interesting behavior can be expected to be applied to aqueous detergents for liquid oil removal.

■

their properties and stability.14−19 Because foam bubbles will gradually rupture with time, it is difficult to predict and control the formation and lifespan of foams. When additional materials are added to the foam systems being studied, it becomes even more difficult to model these systems. The mixing of an oil with a foam, for example, usually results in rupture of the foam, and this phenomenon has been applied to produce antifoaming agents.20−23 As soon as the oil makes contact with the foam, it is believed that the oil penetrates the foam film and promotes rupture by removing surfactants at the air/water interfaces.20−22 In addition, it is understood that the foam film is broken when it is partially replaced by the oil.23 These studies were performed at the slightly higher concentration than the critical micelle concentration (CMC), and oil acts as an antifoaming agent effectively. While various studies have proposed a simple model in which the oil moves outward from the foam film or air/water interface, the mechanism by which the foam and oil come into contact with one another has not been so examined in detail. The scenario in which oil comes into contact with foam should not be considered identical to the interactions between bulk surfactant solutions and oil because there are significant differences between these two situations. Surfactant solution forms capillary filled with itself in a foam, meaning that the surfactant molecules become adsorbed with a defined orientation at the air/water interface prior to contact.24 In

INTRODUCTION When air enters a surfactant solution, surfactant molecules become adsorbed at the air/water interface, and if the resulting surfactant monolayer can sufficiently stabilize the air pocket, a bubble is formed. In cases where many such bubbles accumulate, the result is known as foam. Foam can be considered a colloidal solution in which a large quantity of air is dispersed in water containing a small amount of surfactant. Each air bubble is surrounded solely by a thin liquid film, while foams themselves contain both a liquid film and plateau borders at the contact points between three or more bubbles. It is difficult to study foams due to these complex three-dimensional structures. In addition, there are numerous curved air/water interfaces in foams, and these may lead to differences in pressure along the individual bubble films. It is known that these differences induce water flow in the liquid films and promote the degradation of the foam via water drainage.1−4 There have been numerous studies on bubbles,5−9 focusing especially on observations of bubble rupture. On the other hand, concerning foam, a lot of studies about its structure have been performed from the viewpoint of physics.10−13 However, the physicochemical studies about foaming properties and the function of foam have not been investigated very much. This is unfortunate because foams have a variety of valuable functions in industrial applications. There are, for example, foam thermal insulations that make use of the lower thermal conductivity of air and foam fire extinguishers that operate by a suffocation effect. In addition, foams are present in beers and soft drinks to enhance the taste and in cosmetics such as shampoos. Thus, although we commonly encounter foams in daily life, the study of foam has not yet elucidated many of the factors governing © 2014 American Chemical Society

Received: February 13, 2014 Revised: July 7, 2014 Published: July 14, 2014 9438

dx.doi.org/10.1021/jp501599v | J. Phys. Chem. B 2014, 118, 9438−9444

The Journal of Physical Chemistry B

Article

addition, liquid flow, or water drainage, occurs within the capillary pathways in the foam. Fatty acid salt, which is called “soap”, is one of the oldest surfactants and has been playing an important role as the main surfactant of detergents in the world. Especially, the fatty acid salts that have C12−C18 alkyl chains have been used to design the washing products that make comfortable foam for personal care detergents. In this way, it is important in industry to understand the action between foam made by the fatty acid salt, which is a general-purpose surfactant, and the oil. In the study reported herein, we observed a very interesting phenomenon when foam composed of the potassium salt of two fatty acids came into contact with oil. The oil was observed to spontaneously penetrate into the liquid foam without degrading it. This phenomenon seems to result from capillary action.25−28 Although it is generally considered necessary for the oil to wet the air/water interface prior to the occurrence of capillary action, the hydrophilic groups of surfactant molecules are facing the exterior side of foam film when surfactant molecules are adsorbed to the interface of foam. Therefore, it is unlikely that capillary action of oil appears between the foam. We discuss alternate reasons why this phenomenon might occur, focusing on the behavior seen between liquid oil and an air/water interface in the presence of a surfactant.

Figure 1. Technique used to observe contact between oil and foam: (A) top view and (B) side view.

Figure 1B. As illustrated in Figure 1B, a 50 μL oil sample, to which an oil-soluble colorant had been added (Oil Orange SS, Tokyo Chemical Industry Co., Ltd.), was carefully injected between the slides using a PIPETMAN instrument, and the dynamics resulting from contact between the oil and the foam were observed with a digital microscope (VHX-1000, Keyence, Osaka, Japan) equipped with a VH-Z20 zoom lens (200×). Measurement of the Penetration Distance of the Oil into the Foam. The dynamics of oil penetration into the foam following initial contact were observed by digital microscopy, and still images were obtained from the resulting videos at the point in time 5 s after the moment of initial contact. The distance over which the oil moved during observations was measured using image analysis software (WinROOF V7.0), as shown in Figure 2.

■

EXPERIMENTAL METHODS Materials. A stock surfactant solution was prepared by neutralizing a solution of myristic acid (Wako Pure Chemical Industries, Ltd.) and palmitic acid (Wako Pure Chemical Industries, Ltd.) with potassium hydroxide (Asahi Glass Co., Ltd.) while stirring at 70 °C, such that a 1:1 mass ratio of the potassium salt of myristic acid to that of palmitic acid was obtained and both were present at 5 wt %. Sample surfactant solutions with a variety of concentrations were prepared by diluting this stock solution with deionized water. Samples were stored for 1 week at room temperature prior to analysis to allow further equilibration. CREASIL ID CG (2,2,4,6,6-pentamethylheptane, Cosmetics Innovations and Technologies Sarl), PARLEAM 4 (2,2,4,4,6,6,8-heptamethylnonane, NOF Corp.), PARLEAM EX (2,2,4,4,6,6,8,8,10-nonamethylundecane, NOF Corp.), liquid paraffin (Wako Pure Chemical Industries, Ltd.), CETIOL OE (octanoxy octane, BASF Personal Care and Nutrition, GmbH), EXCEPARL ML-85 (methyl dodecanoate, Kao Corp.), EXCEPARL IPM (2-methylethyl tetradecanoate, Kao Corp.), EXCEPARL OD-M (2-octyldodecyl myristate, Kao Corp.), COCONAD MT (tri(decanoate/octanoate) triol, Kao Corp.), and cottonseed oil (tri(cis,cis-9,12-octadecadienate/hexadecanoate/cis-9-octadecenoate) triol, Wako Pure Chemical Industries, Ltd.) were all employed as oils. These were used as received without further purification. Contact between Foam and Oil. In order to prepare the foam, 10 g aliquots of each surfactant solution were placed in 500 mL beakers and stirred with a mixer (7000 rpm, blade size: 1 × 2 cm, Pencil Mixer DX, ASONE, Osaka, Japan) for 120 s. The resulting foam was composed of relatively uniform bubbles with a volume-weighted average diameter of 300 μm. An 8 mm diameter drop of each foam was subsequently placed on a glass slide (76 mm × 26 mm, thickness 1.0 mm, 2926WSLID-P-N, Asahi Glass Co., Ltd.), and four spacers were positioned around the sample (Maitakku Label, ML-120, 8 mm diameter, thickness 0.11 mm, Nichiban Co., Ltd.), as shown in Figure 1A. A second glass slide was then placed over the sample, as in

Figure 2. Measurement of the penetration distance of oil relative to the foam surface. The distance was measured along the path of the bubbles indicated by the dotted line. The material shown here in yellow is oil that has spontaneously penetrated the foam. The white bar represents 200 μm.

Oil Contact at the Air/Water Interface. Solutions containing from 0 to 10 wt % of the potassium salts of the fatty acids were added to a plastic container, and 10 μL portions of each oil were introduced into these surfactant solutions using a U-shaped syringe. (Figure 3A) The state and shape of the oil droplet near the surface was captured using a digital camera D 100 equipped with a Medical-NIKKOR lens (1×) (Nikon, Tokyo, Japan). Measurement of Equilibrium Surface Tension. The equilibrium surface tension of the fatty acid potassium salt solutions was measured by the Wilhelmy technique with a Pt plate at 25 °C, using a K100MK2 apparatus (KRÜ SS GmbH, Hamburg).29 The sample solutions were allowed to stand at 25 °C for more than 24 h prior to measurements. Surface tension measurements thus took place in the equilibrium state, after it was ascertained that tension fluctuations were less than 0.1 mN/m for 1.0 h. 9439



Article

potassium salt solutions that were used in this study had concentrations significantly above the CMC, and therefore, the influence of acid-soap dimerization need not be considered. Observation of the Dynamics of Contact between the Oil and Foam. The dynamics associated with the contact of oil with foam made from a 10 wt % solution of fatty acid potassium salts were observed in detail. Figure 5 shows a series of pictures Figure 3. Method of observing the state and shape of an oil droplet near the air/water interface: (A) adding oil to a surfactant solution using a U-shaped syringe and (B) the point at which the oil droplet at the air/water interface was photographed.

Measurement of Dynamic Oil/Water Interface Tension Values. Dynamic interface tension values were acquired using the drop volume technique employing a drop volume tensiometer (DVT50, KRÜ SS GmbH, Hamburg). The measurement cell was filled with surfactant solution (myristic acid K salt/palmitic acid K salt = 0.5/0.5 wt %), and a drop of oil was injected via a syringe from the bottom of the cell. The instrument parameters were a flow rate of 0.00833 mL/min, capillary diameter of 0.254 mm, and syringe total volume of 0.5 mL. Measurements were repeated until the standard deviation of three results was less than 0.1 mN/m. We repeated these trials five times and used the average value.

Figure 5. Photographs of the penetration of EXCEPARL ML-85 oil into fatty acid potassium salt foam prior to contact, at contact, and at 0.5 s intervals up to 2 s. The white bar represents 200 μm.

■

RESULTS Measurement of Surface Tension. Figure 4 shows a plot of the surface tension of the fatty acid potassium salt solutions

taken every 0.5 s after the initial contact. EXCEPARL ML-85 was used as the model oil in these trials. Refer to the Supporting Information for a video of the experimental observations. Surprisingly, even after significant contact with the oil, the foam did not rupture. Furthermore, the oil was observed to spontaneously penetrate into the foam. This is a very interesting phenomenon because the surfactant foam appears to absorb a significant quantity of oil without rupturing. Typically, except some examples36,37 of stabilization by the emulsification, a foam will immediately rupture as soon as it comes into contact with an oil, and many examples of bubble rupture have been reported during the applications of antifoaming agents following contact of the oil with foam bubbles.20−23 However, the phenomenon reported here is completely the opposite. After penetration of the oil, the foam film appeared to thicken, and the bubble morphology changed from a distorted polygon to almost round. To the best of our knowledge, oil will typically not be stable within a narrow space with yielding walls, such as in a foam film. Furthermore, considering the water flow in foam films, this phenomenon is rather unique. Generally, water flow proceeds from the interior to the exterior of a foam because the driving force for water drainage is the intramembrane pressure difference.1−4 However, the direction in which the oil proceeds in Figure 5 is again completely the opposite. In the subsequent Discussion section, we describe the proposed driving force behind this phenomenon. As a means of better understanding this uncharacteristic observation, the behavior of the oil phase near the air/water interface was also investigated, as detailed in the next section. Oil at the Air/Water Interface. We next examined the behavior of the oil when it comes into contact with the foam of the fatty acid potassium salt solution by studying the air/water/ oil interfaces. A drop of oil was inserted into fatty acid potassium salt solutions of varying concentrations, and the contact state of the oil as it floated at the air/water interface was

Figure 4. Equilibrium surface tension versus log C (concentration) of fatty acid potassium salt aqueous solutions (myristic acid K salt/ palmitic acid K salt = 1/1). The CMC is estimated from the concentration indicated by the dotted line.

(myristic acid K salt/palmitic acid K salt = 1/1), as measured using the Wilhelmy technique. Unlike the typical surface tension curve, which has only one break point at the CMC, the plots obtained in this study exhibit two break points with a plateau following the lower concentration break point. It is thought that this is caused by the formation of a dimer of the neutralized fatty acid and the fatty acid potassium salt.30 This dimer has been termed an acid-soap and is known to deposit as a solid precipitate from aqueous solutions due to its waterinsolubility.31−34 When the fatty acid soap micelles begin to form at the CMC, it is well-known that any acid-soap precipitates will once again dissolve, and a completely homogeneous solution will result. Therefore, the CMC for this fatty acid potassium salt solution is considered to be the concentration at which the break point appears in the higherconcentration region. The resulting CMC, at a concentration of approximately 0.1 wt %, agrees with a value reported in the literature.35 On the basis of this result, the 10 wt % fatty acid 9440



Article

observed. Figure 6 presents a series of photographs showing COCONAD MT oil drops floating at the air/water interface.

Figure 6. Photographic images of the state of COCONAD MT oil drops at the air/water interface in fatty acid potassium salt solutions (concentrations 0−10 wt % at 25 °C). The oil has been colored with an oil-soluble dye (Oil Orange SS).

The CMC for the fatty acid potassium salt solution was previously found to be 0.1 wt % (3.6 mmol/L). Below the CMC, the oil drop forms a lens shape and floats at the air/ water interface. Conversely, above the CMC, the drop floats under the water without contacting the air/water interface and has a spherical shape. This same behavior was observed with all of the oils tested, although the state of expansion differed between different oils. These results demonstrated that the manner of contact between the oil and the air/water interface changes at the CMC of the surfactant solution. Effect of Oil Type. The penetration distance 5 s after initial contact was measured for several of the oils, and the data are summarized in Figure 7. Although most of the oils were able to penetrate the foam film, the degree of penetration was evidently dependent on the particular oil that was applied.

Figure 8. Penetration distance versus dynamic oil/water interfacial tension for each oil in Figure 7. Fatty acid potassium salt solutions with concentrations of 1.0 wt % were used to acquire the data.

higher than the CMC of the soap, and the relationship between the oil drop in the solution and the air/water interface is maintained above the concentration, as shown in Figure 6. Therefore, the difference of the concentrations between both axes is not thought to affect this result. We therefore believe that the ready oil penetration into the foam film is related to the dynamic affinity of the oil with the surfactant solution at the moment of contact between the the oil and water phases.

■

DISCUSSION Behavior such as that noted above, when observed during experiments in which oil comes into contact with foam, seems to be some form of capillary action.25−28 However, if the phenomenon that we have observed is due to capillary forces, the oil must wet the inner surfaces of the capillaries, meaning the water side of the air/water interface of the foam films. Considering the adsorption orientation of the fatty acid potassium salt molecules at the air/water interface, whereby the hydrophilic head groups must face the liquid,24 it is difficult to picture the manner in which the penetrating oil wets the interface. The photographic image in Figure 9 shows that the

Figure 7. Penetration distances of oils 5 s after initial contact with the foam film.

Measurement of Dynamic Oil/Water Interfacial Tension. In Figure 5, we can see that the oil/water interface spreads into the foam film as soon as the oil phase comes into contact with the outermost surface of the foam, meaning that the area of the oil/water interface rapidly increases just after contact. This should not happen unless the oil/water interfacial tension declines instantaneously upon contact. Therefore, a dynamic interfacial tension measurement was performed with the oil samples shown in Figure 7. In Figure 8, we do indeed see a very high correlation between the penetration distance of the oil and the dynamic oil/water interfacial tension (R = 0.96). The soap solutions (1 wt %) used for these measurements are more dilute than those for the oil penetration into foam measurements (10 wt %) because the tensiometer cannot be applied to such high-concentration solutions. However, the concentration, 1 wt %, is enough

Figure 9. Image showing that the tip of a penetrating drop of EXCEPARL IPM oil does not wet the air/water interface as it enters the foam. The black bar represents 100 μm.

oil invades the foam. If the oil wets the air/water interface, the tip of oil will exhibit a negative curvature; however, the actual tip shown in this image has a spherical shape. This indicates that the oil/water interface at the tip of the oil droplet has a positive curvature, which is opposite from the morphology expected from capillary action. Therefore, the phenomenon observed in this study cannot be explained by capillary action, and we propose an alternate mechanism in this section for this novel observation. 9441



Article

oil. This closely resembles the myelin phenomenon,38,39 which occurs when oil spreads underwater. Moreover, Piroird and Lorenceau have provided the discussion by which this uncommon phenomenon that oil spontaneously flows in foam film could be explained.40 They have proposed that the elongation of an oil drop in the Plateau border without bubble rupture, which results from both the viscous dissipation in the oil and the capillary suction due to differences between atmospheric pressure and the Laplace pressure that foams generate, could occur using the simple model system. Their physical consideration could support our studies strongly. In this study, we showed that a significant reason for the penetration of the oil is the immediate stabilization of the oil/water interface by the fast adsorption of the surfactant from the results about oil/water interfacial tension measurements. In addition, it is quite natural that the capillary suction due to Laplace pressure depending on the curvature of the Plateau border might be also one of the reasons. Overall Mechanism of Oil Absorption by the Aqueous Foam. Figure 11 shows our proposed mechanism for the

Reasons Why the Foam Is Not Ruptured by Penetration of the Oil. The contact state between a drop of oil and the air/water interface at different surfactant solution concentrations is shown in Figure 6, while Figure 10 portrays the manner in which the drop behaves in solutions below and above the CMC.

Figure 10. Contact state model showing a drop of oil inserted into a surfactant solution at the air/water interface. Below the CMC (left), the oil forms a lens shape and floats at the interface, while above the CMC (right), the fatty acid potassium salts form stable monolayers at both the air/water and oil/water interfaces. The ensuing electrostatic repulsion may prevent the approach of the oil to the air/water interface, resulting in the oil remaining underwater with a spherical shape.

Below the CMC, the oil droplet floats on the surface of the solution and forms a lens shape, presumably because the quantity of surfactant is insufficient to saturate both the air/ water and oil/water interfaces. Above the CMC, however, the oil maintains a spherical shape and positions itself under the air/water interface. The experimental trials examining oil penetration into the foam shown in Figure 5 were performed using a 10 wt % fatty acid potassium salt solution, which is approximately 100 times the CMC. Because enough surfactant molecules would have been present to saturate the air/water and oil/water interfaces in this system, the drop of oil should have been covered with a surfactant monolayer, and the rightmost photograph in Figure 6 indeed clearly shows that the surface of the oil is highly stabilized and a spherical drop shape is maintained. Of course, the air/water interface in the foam is also saturated with the surfactant molecules, and as a result, it is expected that strong electrostatic repulsion will exist between the air/water and oil/water interfaces due to the positioning of anionic head groups at both interfaces. Therefore, the oil phase in the foam film exists under the air/water interface. We propose that the foam is not destroyed by the penetrating oil because the oil never replaces the surfactant monolayer of the interface. Reasons Why the Foam Is Able to Absorb the Liquid Oil. In our observations of oil penetration, there are only two phases around the oil drops, either air or the foam of the fatty acid potassium salt solution. Figure 8 shows that the oil/water interfacial tension decreases to less than 6 mN/m as soon as the oil comes into contact with the foam film. Because the air/oil interfacial tension is normally 20−30 mN/m,35 the free energy is evidently reduced when the oil phase makes contact with the fatty acid solution. Consequently, the oil may start penetrating the foam film in order to reduce the unfavorable air/oil interface and increase the favored water/oil interface. Although a new oil/water interface is surely generated as the oil proceeds into the foam film, sufficient surfactant molecules exist in the foam film to adsorb to the new oil/water interface and decrease the interfacial tension. This allows the oil to continue penetrating such that the foam appears to soak up the liquid

Figure 11. Proposed mechanism by which aqueous foam of fatty acid potassium salt solution absorbs oil.

penetration of the oil into the foam film. When the oil initially comes into contact with the foam, the fatty acid potassium salts immediately adsorb to the oil/water interface from micelles in the solution, and this momentarily reduces the oil/water interfacial tension. Because the oil/water interfacial tension is much lower than that for the oil/air interface, the oil spontaneously moves into the foam film in order to increase the oil/water interfacial area. The oil subsequently undergoes spontaneous spreading into the foam film. Furthermore, the rapid adsorption of the fatty acid potassium salt to the oil/water interface forms a stable surfactant monolayer at the oil/water interface. The repulsive force between the air/water and oil/ water interfacial monolayers prevents contact between the oil and the air/water interface. Because the oil can then proceed without contacting the air/water interface, the oil does not degrade the foam as it penetrates.

■

CONCLUSION A novel phenomenon has been observed in which the aqueous foam of a fatty acid potassium salt solution spontaneously absorbs oil without defoaming. This initially appeared to occur via capillary action, but it is clear that the observations cannot be explained in this manner because the oil does not wet the air/water interface. The true mechanism was investigated using equilibrium surface tension measurements, image analysis of the contact state of oil at the air/water interface, and dynamic oil/water interfacial tension measurements. On the basis of the results, the rapid adsorption of the surfactant at the oil/water interface and the concurrent rapid decline of the interfacial 9442



Article

(16) Mittal, K. L.; Kumar, P. Emulsions, Foams, and Thin Films; Marcel Dekker: New York, 2000; pp 131−150. (17) Djuve, J.; Pugh, R. D.; Sjoblom, J. Foaming and Dynamic Surface Tension of Aqueous Polymer/Surfactants Solutions 1: Ethyl(hydroxyethyl) Cellulose and Sodium Dodecyl Sulphate. Colloids Surf. 2001, 186, 189−202. (18) Lngevin, D. Aqueous Foams: A Field of Investigation at the Frontier between Chemistry and Physics. ChemPhysChem 2008, 9, 510−522. (19) Stubenraunch, C.; Shrestha, L. K.; Varade, D.; Johansson, I.; Olanya, G.; Aramaki, K.; Claesson, P. Aqueous Foams Stabilized by nDodecyl-β-D-maltoside, Hexaethyleneglycol Monododecyl Ether, and Their 1:1 Mixture. Soft Matter 2009, 5, 3070−3080. (20) Princen, H. M.; Goddard, E. D. The Effect of Mineral Oil on the Surface Properties of Binary Surfactant Systems. J. Colloid Interface Sci. 1972, 38, 523−534. (21) Kulkarni, R. D.; Goddard, E. D.; Kanner, B. Mechanism of Antifoaming: Role of the Filler Particle. Ind. Eng. Chem. Fundam. 1977, 16, 472−474. (22) Kulkarni, R. D.; Goddard, E. D.; Rosen, M. R. Antifoams. J. Soc. Cosmet. Chem. 1979, 30, 105−125. (23) Denkov, N. D. Mechanisms of Foam Destruction by Oil-Based Antifoams. Langmuir 2004, 20, 9463−9505. (24) Sebba, F. Foams and Biliquid Foams  Aphrons; John Wiley & Sons: New York, 1987; pp 48−62. (25) Tsujii, K. Surface Activity Principles, Phenomena, and Applications; Academic Press: San Diego, CA, 1998; pp 49−52. (26) Adam, N. K. The Physics and Chemistry of Surface; Oxford University Press: London, 1941; pp 1−16. (27) Ross, S. Chemistry and Physics of Interfaces; American Chemical Society: Washington, DC, 1971; pp 1−13. (28) Adanson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: New York, 2000; pp 10−13. (29) Sakai, T.; Ikoshi, R.; Toshida, N.; Kagaya, M. Thermodynamically Stable Vesicle Formation and Vesicle-to-Micelle Transition of Single-Tailed Anionic Surfactant in Water. J. Phys. Chem. B 2013, 117, 5081−5089. (30) Verhagen, J.; Vliegenthart, F. G. J.; Boldingh, J. Micelle and Acid-Soap Formation of Linoleic Acid and 13-L-Hydro-Peroxylinoleic Acid being Substrates of Lipoxygenase-1. Chem. Phys. Lipids 1978, 22, 255−259. (31) Shinoda, K. The Effect of Alcohols on the Critical Micelle Concentrations of Fatty Acid Soaps and the Critical Micelle Concentration of Soap Mixture. J. Phys. Chem. 1954, 58, 1136−1141. (32) Lynch, M. L.; Pan, Y.; Langhlin, R. G. Spectroscopic and Thermal Characterization of 1:2 Sodium Soap/Fatty Acid Acid-Soap. J. Phys. Chem. 1996, 100, 357−361. (33) Lynch, M. L. Acid-Soaps. Curr. Opin. Colloidal Interfacial Sci. 1997, 2, 495−500. (34) Lynch, M. L.; Wireko, F.; Tarek, M.; Klein, M. Intermolecular Interactions and the Structure of Fatty Acid-Soap Crystals. J. Phys. Chem. B 2001, 105, 552−561. (35) Matijević, E. Surface and Colloid Science; John Wiley & Sons, Inc.: New York, 1969; Vol. 1, pp 91−91. (36) Koczo, K.; Lobo, L. A.; Wasan, D. T. Effect of Oil on Foam Stability: Aqueous Foams Stabilized by Emulsions. J. Colloid Interface Sci. 1991, 150, 492−506. (37) Salonen, A.; Lhermerout, R.; Rio, E.; Langevin, D.; Saint-James, A. Dual Gas and Oil Dispersions in Water: Production and Stability of Foamulsion. Soft Matter 2012, 8, 699−706. (38) Ravoo, B. J.; Engberts, J. B. F. N. Single-Tail Phosphates Containing Branched Alkyl Chains. Synthesis and Aggregation in Water of a Novel Class of Vesicle-Forming Surfactants. Langmuir 1994, 10, 1735−1740. (39) Shulz, E. P.; Piñeiro, A.; Rodriguez, J. L.; Minardi, R. M.; Frechero, M.; Schulz, P. C. Intermediate Structures for Higher Level Arrangements: Catching Disk-Like Micelles in Decane Phosphonic Acid Aqueous Solutions. J. Phys. Chem. B 2013, 117, 6231−6240.

tension are thought to cause the spontaneous penetration of the oil into the aqueous foam. Moreover, the electrostatic repulsion between surfactant monolayers at the air/water and oil/water interfaces prevents rupture of the foam. This interesting feature of the aqueous foam can be expected to have future applications to a variety of aqueous detergents intended for the removal of liquid oil contaminants.

■

ASSOCIATED CONTENT

S Supporting Information *

A video of the dynamics associated with the contact of oil with foam made from a 10 wt % solution of fatty acid potassium salts. EXCEPARL ML-85 was used as the model oil in this trial. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-73-426-5065. Fax: +81-73-426-5067. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS I would like to thank Prof. Kaoru Tsujii, who offered continuing support and constant encouragement. REFERENCES

(1) Prud’homme, R. K.; Khan, S. A. Foams, Theory, Measurement, and Applications; Marcel Dekker: New York, 1996; pp 106−124. (2) Porter, M. R. Handbook of Surfactants; Blackie and Son: Glasgow, Scotland, 1991; pp 38−39. (3) Koehler, S. A.; Hilgenfeldt, S.; Weeks, E. R.; Stone, H. A. Drainage of Single Plateau Borders: Direct Observation of Rigid and Mobile Interfaces. Phys. Rev. E 2002, 66, 04061/1−040601/4. (4) Lngevin, D. Aqueous Foams: A Field of Investigation at the Frontier between Chemistry and Physics. ChemPhysChem 2008, 9, 510−522. (5) Knelman, F.; Dombrowski, N.; Newitt, D. M. Mechanism of the Bursting of Bubbles. Nature 1954, 173, 261−261. (6) Blanchard, D. C. Bursting of Bubbles at an Air−Water Interface. Nature 1954, 173, 1048−1048. (7) Debrégeas, G.; Gennes, P. G.; Brochard-Wyart, F. The Life and Death of ‘Bare’ Viscous Bubbles. Science 1998, 279, 1704−1707. (8) Silveira, R.; Chaïeb, S.; Mahadevan, L. Rippling Instability of a Collapsing Bubble. Science 2000, 287, 1468−1471. (9) Bird, J. C.; Ruiter, R.; Courbin, L.; Stone, H. A. Daughter Bubble Cascades Produced by Folding of Ruptured Thin Films. Nat. Lett. 2010, 465, 759−762. (10) Pittet, N.; River, N.; Weaire, D. Cylindrical Packing of Foam Cells. Forma 1995, 10, 65−73. (11) Durian, D. J. Bubble-Scale Model of Foam Mechanics: Melting, Nonlinear Behavior, and Avalanches. Phys. Rev. E 1997, 55, 1739− 1751. (12) Monnereau, C.; Vignes-Alder, M. Optical Tomography of Real Three-Dimensional Foams. J. Colloid Interface Sci. 1998, 202, 45−53. (13) Thomas, P. D.; Darton, R. C.; Whalley, P. B. Resolving the Structure of Cellular Foams by the Use of Optical Tomography. Ind. Eng. Chem. Res. 1998, 37, 710−717. (14) Engels, T.; Ryninski, W. V.; Schmiedel, P. Structure and Dynamics of Surfactant-Based Foams. Prog. Colloid Polym. Sci. 1998, 111, 117−126. (15) Patist, A.; Axelberd, T.; Shah, D. O. Effect of Long Chain Alcohols on Micellar Relaxation Time and Foaming Properties of Sodium Dodecyl Sulfate Solutions. J. Colloid Interface Sci. 1998, 208, 259−265. 9443



Article

(40) Piroird, K.; Lorenceau, É. Capillary Flow of Oil in a Single Foam Microchannel. Phys. Rev. Lett. 2013, 111, 234503−1−234503−5.

9444


Liquid Oil That Flows in Spaces of Aqueous Foam without Defoaming

Recommend Documents