Anomalous Ion Effects on Rupture and Lifetime of Aqueous Foam

In the case of NaCl and LiCl, the foam films prepared from the salt solutions below 0.1 .... acetate, and sodium chlorate, all of which were provided ...
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Langmuir 2008, 24, 11587-11591

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Anomalous Ion Effects on Rupture and Lifetime of Aqueous Foam Films Formed from Monovalent Salt Solutions up to Saturation Concentration Stoyan I. Karakashev, Phong T. Nguyen, Roumen Tsekov, Marc A. Hampton, and Anh V. Nguyen* DiVision of Chemical Engineering, The UniVersity of Queensland, Brisbane, QLD 4072, Australia ReceiVed May 11, 2008. ReVised Manuscript ReceiVed August 2, 2008 We report the effects of ions on rupture and lifetime of aqueous foam films formed from sodium chloride (NaCl), lithium chloride (LiCl), sodium acetate (NaAc), and sodium chlorate (NaClO3) using microinterferometry. In the case of NaCl and LiCl, the foam films prepared from the salt solutions below 0.1 M were unstablesthey thinned until rupturing. The film lifetime measured from the first interferogram (appearing at a film thickness on the order of 500 nm) until the film rupture was only a second or so. However, relatively long lasting and nondraining films prepared from salt solutions above 0.1 M were observed. The film lifetime was significantly longer by 1 to 2 orders of magnitude, i.e., from 10 to 100 s. Importantly, both the film lifetime and the (average) thickness of the nondraining films increased with increasing salt concentration. This effect has not been observed with foam films stabilized by surfactants. The film lifetime and thickness also increased with increasing film radius. The films exhibited significant surface corrugations. The films with large radii often contained standing dimples. There was a critical film radius below which the films thinned until rupturing. In the cases of NaAc and NaClO3, the films were unstable at all radii and salt concentrationssthey thinned until rupturing, ruling out the effect of solution viscosity on stabilizing the films.

* Email: [email protected]; Phone: +61 7 336 53665; Fax: +61 7 336 54199.

rate of bubble coalescence13 depend significantly on the type of electrolyte added into the system. The satisfactory explanation of these phenomena requires a profound understanding of the behavior of ions at the phase boundaries of the colloidal systems. The simplest example showing the ion-specific effect is bubble coalescence in salt solutions. Most inorganic salts have a stabilizing effect on bubbles in contact.13-22 This effect appears above a certain minimum salt concentration (named the critical concentration), specific for each salt.13,15,18,21 The critical concentration also depends on the bubble sizes.20,23 In addition, some electrolytes do not influence the bubble coalescence. Therefore, different inorganic salts have different abilities to inhibit the bubble coalescence. An empirical classification of anions and cations, based upon their ability to inhibit the bubble coalescence, was established.13,15,18,21 According to this classification, the ions are divided into two groups, namely, the R and β ions. The combining rule postulates that the ion-specific effect is due to the proper combination of the salt ions, viz., the salts consisting of the same R- or β-type ions inhibit bubble coalescence, whereas the salts consisting of mixed R and β ions do not affect bubble coalescence.

(1) Kunz, W.; Henle, J.; Ninham, B. W. Curr. Opin. Colloid Interface Sci. 2004, 9(1,2), 19–37. (2) Kunz, W.; Lo Nostro, P.; Ninham, B. W. Curr. Opin. Colloid Interface Sci. 2004, 9, 1–18. (3) Kunz, W.; Belloni, L.; Bernard, O.; Ninham, B. W. J. Phys. Chem. B 2004, 108(7), 2398–2404. (4) Pitzer, K. S.; Mayorga, G. J. Phys. Chem. 1973, 77(19), 2300–8. (5) Pitzer, K. S.; Kim, J. J. J. Am. Chem. Soc. 1974, 96(18), 5701–7. (6) Kim, H. T.; Frederick, W. J., Jr. J. Chem. Eng. Data 1988, 33(2), 177–84. (7) Weissenborn, P. K.; Pugh, R. J. J. Colloid Interface Sci. 1996, 184(2), 550–563. (8) Maheshwari, R.; Sreeram, K. J.; Dhathathreyan, A. Chem. Phys. Lett. 2003, 375(1,2), 157–161. (9) Bostroem, M.; Kunz, W.; Ninham, B. W. Langmuir 2005, 21(6), 2619– 2623. (10) Iwanagaa, T.; Kuniedab, H. J. Colloid Interface Sci. 2000, 227(2), 349– 55. (11) Ninham, B. W.; Yaminsky, V. Langmuir 1997, 13(7), 2097–2108. (12) Karakashev, S. I.; Manev, E. D. J. Colloid Interface Sci. 2001, 235(1), 194–196.

(13) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Nature (London) 1993, 364(6435), 317–19. (14) Kim, J. W.; Chang, J. H.; Lee, W. K. Kor. J. Chem. Eng. 1990, 7(2), 100–8. (15) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. J. Phys. Chem. 1993, 97(39), 10192–7. (16) Christenson, H. K.; Yaminsky, V. V. J. Phys. Chem. 1995, 99(25), 10420. (17) Weissenborn, P. K.; Pugh, R. J. Proc. Int. Miner. Process. Congr., 19th 1995, 3, 125–128. (18) Pashley, R. M.; Craig, V. S. J. Langmuir 1997, 13(17), 4772–4774. (19) Deschenes, L. A.; Barrett, J.; Muller, L. J.; Fourkas, J. T.; Mohanty, U. J. Phys. Chem. B 1998, 102(26), 5115–5119. (20) Tsang, Y. H.; Koh, Y.-H.; Koch, D. L. J. Colloid Interface Sci. 2004, 275(1), 290–297. (21) Craig, V. S. J. Curr. Opin. Colloid Interface Sci. 2004, 9(1,2), 178–184. (22) Henry, C. L.; Dalton, C. N.; Scruton, L.; Craig, V. S. J. J. Phys. Chem. C 2007, 111(2), 1015–1023. (23) Chan, B. S.; Tsang, Y. H. J. Colloid Interface Sci. 2005, 286(1), 410–413.

1. Introduction A significant effect of inorganic salts on the stability of colloidal-dispersed systems has been established by experiments for over a century but still remains poorly understood.1 This ion-specific effect was first discovered by Hofmeister, who observed that different inorganic salts had different capacities to precipitate proteins. Further studies showed that this effect was also valid for various systems, e.g., colloidal mineral particles and soap solutions. It is now known that ion specificity results from the different effects of anions and cations on the properties of the systems. For example, the solubility of salts,2 the osmotic coefficients and ion activities,3-6 the surface tension,7-9 and the zeta potential8 are all influenced by the ion-specific effect. In addition, the cloud point of nonionic amphyphilic substances,10 the CMC and the shape of micelles and the properties of hydrophobic suspensions,11 the stability of foams containing nonionic surfactants,12 and the

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It was believed that the Marangoni effect impedes the bubble coalescence by the Gibbs elasticity of the adsorption layer of the inorganic salts.21,23-25 However, this effect was recently found unreliable.22 An alternative way to explain the slower rate of bubble coalescence suggested recently26 is the electrostatic repulsion between the bubbles. The surface potential is generated by the specific spatial distribution of anions and cations at the air-water interface. However, according to the DLVO theory, this interaction must be suppressed at high salt concentrations. Another way to explain the inhibition of bubble coalescence22 is that some combinations of ions can decrease the surface mobility of the gas-liquid interface, thus immobilizing the bubble surfaces during coalescence. It is noted that all of the experiments on bubble coalescence were performed with bubble columns, where the multiple gas bubbles were generated by blowing purified air or nitrogen through an orifice or through a porous surface such as a sintered glass disk. Therefore, the experimentally established rate for bubble coalescence is a statistical value based on the observation of multiple bubble collisions. There are no literature data showing the behavior of single foam films between two bubbles, prepared from highly concentrated salt solutions. In this regard, one can pose the question whether or not inorganic salts can stabilize foam films and the ion-specific effect can be obtained in foam films. If the answer is yes, how does the stable film thickness depend on the salt concentration? It is a challenge to study the ion-specific behavior of such foam films. As already mentioned, at high salt concentration the electrostatic interaction between the film surfaces is negligible.27 The Hamaker constant is positive, corresponding to attraction between the film surfaces.28 Therefore, according to the DLVO theory such films are unstable and should thin until rupturing. This work aims to examine the salt effects on foam films formed between two air bubbles in monovalent salt solutions up to saturation concentration. Microinterferometry is used in conjunction with the Scheludko thin film balance to investigate the film drainage, the film rupture, and the film lifetime.

2. Experimental Section Shown in Figure 1 is the experimental microinterferometric setup consisting of the following major units: (1) a Scheludko cell for producing the foam films with a gastight microsyringe pump; (2) a metallurgical inverted microscope (Nikon, Japan) for illuminating and observing the film and the interference fringes (Newton rings) in reflected light; (3) a CCD video camera (Kodak, Japan) for registering transient interferometric images; (4) a computer for recording the transient interferometric images and storing them for further off-line processing and analysis. The film solutions were stirred intensively before preparing the foam films. The film holder of the Scheludko cell was flushed preliminarily with the appropriate solution of the inorganic salt before every experiment. A droplet of the investigated salt solution was formed inside the film holder and left for some time to establish thermal equilibrium. The film holder with an inner diameter of 4 mm was connected with a capillary tube with a gastight ultramicrosyringe pump (WPI Inc., USA) for accurately regulating the liquid amount in the drop. The microscopic film was formed between the surfaces of the double concave meniscus by pumping out the liquid from the drop with a velocity between 10 and 100 nL/s. The radius of the film was dependent on the amount of liquid withdrawn. The (24) Marrucci, G. Chem. Eng. Sci. 1969, 24(6), 975–85. (25) Prince, M. J.; Blanch, H. W. AIChE J. 1990, 36(9), 1425–9. (26) Marcelja, S. J. Phys. Chem. B 2006, 110(26), 13062–13067. (27) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1992; p 291 (28) Nguyen, A. V.; Schulze, H. J. Colloidal science of flotation; Marcel Dekker: New York, 2004; p 840

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Figure 1. Experimental microinterferometric setup with a Scheludko cell, a metallurgical inverted microscope, and a CCD video camera.

film was illuminated with a white coherent light generated by a metal halide bulb (150 W, Nikon, Japan). As a result of the interference of the light reflected from the two film surfaces, a set of fringes with different colors (Newton rings) was observed and recorded on the computer using the CCD video camera. The transient film images were saved at 100 ms time intervals. To determine the film thickness, the interferometric images were processed by a digital filtration procedure29 using software for digital processing (Optimas 6.1, Optimas, USA) and a digital green filter with wavelength λ ) 546 nm. This procedure converted the polychromatic into monochromatic interferograms suitable for calculating the film thickness. A narrow strip passing through the center of the monochromatic interferograms was selected and the digital signals were converted into photocurrent versus radial distance using a user-defined macro developed in Optimas 6.1. The film thickness, h, was calculated using the interferometric equation:28,30

h)

[



λ lπ ( arcsin 2πn

∆(1 + r2) (1 - r2) + 4r2∆

]

(1)

In eq 1, the Fresnel reflection coefficient, r ) (n - 1)2/(n + 1)2, for the air-solution interface is a function of the refractive index, n, of the film solution; l ) 0, 1, 2, · · · , is the order of interference; and ∆ ) (I - Imin)/(Imax - Imin), where I is the instantaneous intensity of the photocurrent and Imin and Imax are its minimum and maximum values. The foam films were produced from aqueous solutions of ultrahigh purity sodium chloride, lithium chloride, sodium acetate, and sodium chlorate, all of which were provided by Sigma-Aldrich (Australia). The salts were additionally purified to remove organic trace contaminants by heating in a furnace for 5 h at 500 °C for sodium and lithium chlorides and at 300 °C for sodium acetate. Sodium chlorate was purified by a repeated recrystallization process. A freshly prepared alkaline cleaning solution was used to clean the cell and the glassware. The solution was prepared from potassium hydroxide, water, and ethanol in the mass ratio 12.5:16:84. This cleaning solution is a very strong solvent which dissolves organic contaminants. The cell was soaked in the cleaning solution for a couple of minutes. After this, it was flushed with ultrahigh purity water produced by a new UltraPure Milli-Q system and then soaked for a minute in diluted hydrochloric acid. The final cleaning step involved flushing the cell intensively with ultrahigh purity water. Before each experiment, the cell was tested for purity via forming foam films from ultrapure water. If the cell was clean, no stable foam films could be formed. This final test indicating the purity of the system was crucial for the experiments because even a very small amount of organic contaminants could dramatically change the behavior of the foam films. The experiments were conducted at (29) Karakashev, S. I.; Nguyen, A. V.; Manev, E. D. J. Colloid Interface Sci. 2007, 306, 449–453. (30) Sheludko, A. AdV. Colloid Interface Sci. 1967, 1(4), 391–464.

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a constant room temperature (20 °C). The foam films were produced from aqueous solutions of each salt in the concentration range from very dilute (10-4 M) to saturation concentrations, which are about 5 M for sodium chloride, 10 M for lithium chloride, 9 M for sodium acetate, and 9.5 M for sodium chlorate. The evolutions of at least 20 foam films with various radii, produced from each of the solutions, were recorded and analyzed.

3. Results and Discussion 3.1. Unstable Foam Films of NaAc and NaClO3. All of the foam films produced from sodium acetate and sodium chlorate were unstable at all film radii and at all salt concentrations (up to saturation). At low concentration, the films ruptured instantaneously without appearance of any interferometric fringes recordable by the camera system used. At high concentration, the captured interferometric fringes showed significant dynamic corrugations of the film surfaces but the dynamic surface processes could not stop the instant film rupture. The film lifetime measured from the first interferogram (appearing at a thickness on the order of 500 nm) until the film rupture was only a few seconds. Generally, this microscopic observation is in line with the results of the bubble coalescence obtained with the bubble column experiments,13,15,18,21 in that both NaAc and NaClO3 do not inhibit bubble coalescence at all concentrations investigated. 3.2. Foam Films of NaCl and LiCl. Our experimental observation showed that foam films prepared from dilute solutions of NaCl and LiCl in the concentration range of 10-4 M to 0.1 M were unstable. The films drained until rupture. The films ruptured instantaneously without appearance of any interferometric fringes recordable by the camera system. According to the DLVO theory, the salt suppresses the electrostatic repulsion between the film surfaces. Therefore, only the van der Waals attraction between the film surfaces causes the film rupture. Again, this microscopic observation supports the bubble column experiments on the ion-specific effect of bubble coalescence.13-22 However, when the salt concentration was above 0.1 M, the foam films were relatively stable. Three interesting features of these foam films were observed, namely: (1) A dynamic process of corrugations of the film surfaces evidently occurred during and after the film drainage, which normally lasted for a few minutes after the film stopped thinning. (2) The nondraining foam films were formed after the dynamic corrugations completed. The film thickness profile and lifetime significantly depended on the film radius. It was also observed that very small films (with a radius below 0.03 mm) thinned until rupturing. The lifetime for these small films was shorter than 1 s. (3) The average thickness for the nondraining films increased with increasing salt concentration. These results have not been observed with foam films stabilized by surfactants and cannot be explained by the DLVO theory. The details about the three characteristic features observed by the experiments are now delineated. 3.3. Dynamic Surface Corrugations of Foam Films during and after Film Drainage. The foam films prepared from aqueous solutions of all the inorganic salts exhibited dynamic corrugations. For NaCl and LiCl salt solutions, corrugations were observed during and after the film drainage, whereas the films thinned with dynamic corrugations until rupture in the case of NaAc and NaClO3. Figure 2 shows a typical example of dynamic corrugations occurring in a foam film of 4 M NaCl during and after drainage. The image frequency chosen for this particular film was 5 s. This film drained for about 8 s, while the dynamic corrugations lasted for about 105 s, after which a nondraining film was formed and lasted for 12 s. The film thickness at the thinnest part of the

Figure 2. Evolution with significant dynamic surface corrugations of a 4 M NaCl foam film, shown at a time interval of 5 s (from the top left to bottom right corner).

Figure 3. Film thickness at the thinnest spot of the nondraing foam film shown in Figure 2 (in the last 12 s).

nondraining film versus time is shown in Figure 3. It is important to note that the nondraining film contained dimples, which could not be observed for nondraining foam films stabilized by surfactants. Significant flux exchange of liquid between the film and the outside solution was also observed during the dynamic surface corrugations for both the draining and nondraining films. In addition to the dimples, multiple internal channels in which the liquid was flowing were formed. Vortexes and circulation were observed at the end of the channels, indicating an additional internal flow of liquid within the film. 3.4. Dependence of the Lifetime and Film Thickness Profile on the Radius of the Foam Film. As discussed previously, only quasi-static foam films produced from aqueous solutions of NaCl and LiCl were formed. The film thickness and lifetime were significantly affected by the film radius. Figure 4 presents an example of the dependence of the film thickness profile on the film radius. After formation, the foam film was slowly shrunk to different radii and left to drain. The shrinking made the nondraining film significantly thinner until a critical film radius (ca. 0.03 mm) below which films ruptured and no nondraining films existed. For the nondraining films, the film stability was measured by the lifetime, which was strongly dependent on the film radius. Figure 5 shows an example of the dependence of the film lifetime on the film radius. The film lifetime decreases upon the

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Figure 4. Typical thickness profile for a 0.1 M NaCl film versus film radius, R. The last (smallest) film ruptured before it stopped thinning.

Figure 5. Lifetime of foam films prepared from 0.1 M NaCl solutions versus film radii.

decrease of the film radius. A critical film radius of about 0.03 mm was established by the experiments. Films with radii smaller than the critical radius ruptured before they stopped thinning and the film lifetime was always shorter than 1 s. The film lifetime correlates with literature data20,23 which indicate that larger bubbles coalesce more slowely. The film lifetimes for small and large films are different by 2 orders of magnitude and are the key factor controlling the bubble coalescence in the bubble column experiments. Typically, the bubbles used in these experiments are significantly large to produce large films during the bubble collision, which have a film lifetime longer than the contact time between the bubbles, and therefore, coalescence does not occur.

In addition to the critical film radius, our microscopic experiments also reveal important information about the critical salt concentration above which nondraining foam films with long lifetimes (>10 s) can be produced. For NaCl or LiCl salts, the critical concentration is about 0.1 M, which is consistent with the bubble column experiments on bubble coalescence.13,15,18,21 3.5. Dependence of Film Thickness on Salt Concentration. Some of the nondraining foam films, in particular, those produced from highly concentrated solutions, lasted longer than 10 min. The film stability and equilibrium thickness depended on the film radius and the salt concentration. The dependence of the nondraining film thickness on the salt concentration was examined for films with the same radius. Shown in Figure 6 are the typical data for the nondraining film thickness versus the NaCl or LiCl salt concentration. The foam films with a radius of about 0.1 mm were chosen for this analysis because the films were considerably plane-parallel. The larger films contained larger standing dimples, while the smaller films were less stable. It should be noted that the difference in film stability and thickness for the four salts examined cannot be explained by a change in viscosity. For example, the viscosity for sodium acetate solutions increases with increasing concentration from 1 (for dilute solutions) to about 6 cP (for saturated solution at 9 M), but the foam films from sodium acetate thinned until rupture within the whole concentration range. The change in viscosity for both sodium and lithium chloride solutions with increasing

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Figure 7. Thickness versus drainage time of a 4 M NaCl film in saturated environment.

Figure 6. Film thickness versus salt concentration for sodium chloride and lithium chloride. The radii of all films analyzed were approximately 0.1 mm. The minimum film lifetime was about 20 s.

salt concentration is insignificant, but the films of these two salts are significantly stable. 3.6. General Discussion. Our experimental observation of the anomalous effect of salts on the foam film drainage and stability is difficult to explain at present. Recently, theoretical results obtained by molecular modeling26,31 have been used to infer the salt inhibition of bubble coalescence.22 The molecular modeling shows that, for the Rβ or βR salts such as sodium chlorate, the salt ions equally concentrate at the interface, but for RR or ββ salts such as sodium chloride or lithium chloride, the salt ions unequally concentrate at the interface. Different stabilities of foam films consisting of the differently combined ions can be related to the ion distributions at the air-water interface. However, we do not know why and how this small difference in the ion distributions at the air-water interface can be so powerful in making the nondraining films so thick (∼200 nm). It is possible that the different ion distributions at the air-water interface can result in different interfacial properties important to the surface motion as observed by the surface corrugations. Indeed, the recent studies by Craig et al.22 show that the surface tension gradient is not the mechanism for inhibiting bubble coalescence in salt solutions, but the surface mobility of the salt solutions can be the key factor for explaining the salt inhibition of bubble coalescence. Furthermore, if the cell is (31) Jungwirth, P.; Tobias, D. J. Chem. ReV. 2006, 106(4), 1259–1281.

saturated by the film solution, no surface corrugation is present and the film drainage follows typical observation reported in the literature, i.e., the film thins rapidly until rupture. The salts show similar drainage curves. Shown in Figure 7 is a typical drainage curve for a 4 M NaCl film in a saturated environment. The experimental data presented in this paper call for further theoretical developments of the anomalous effects of ions in liquid films of concentrated salt solutions.

4. Conclusions In this paper, the ion-specific effect of inorganic salts in foam films has been demonstrated with sodium chloride (NaCl), lithium chloride (LiCl), sodium acetate (NaAc), and sodium chlorate (NaClO3). The foam films prepared from the NaAc and NaClO3 solutions were unstable at all film radii and salt concentrationssall of the films thinned until rupture. The foam films prepared from dilute (below 0.1 M) NaCl and LiCl salt solutions were also unstable at all film radii and salt concentrations. However, the foam films prepared from concentrated NaCl and LiCl salt solutions were relatively stable. The film lifetime and stable thickness increased significantly with increasing film radius and salt concentration. The film surfaces exhibited dynamic corrugations during and after film drainage before establishing a nondraining condition. The films with large radius often contained standing dimples. There was a critical film radius below which the films thinned until rupture. Acknowledgment. A.V.N. gratefully acknowledges the Australian Research Council for financial support through a Discovery grant. LA801456J