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J. Phys. Chem. C 2007, 111, 5236-5243
Characteristic Aspects of Bubble Coalescence during Electrolysis of Ammonium Salt Solutions J. L. Trompette* and H. Vergnes Laboratoire de Ge´ nie Chimique UMR 5503, UniVersite´ Paul Sabatier, 118 route de Narbonne-31062 Toulouse Cedex, France
G. He´ brard and P. Bamrungsri Laboratoire d’Inge´ nierie des Proce´ de´ s de l’EnVironnement EA 833, INSA-31077, Departement G.P.I., 135 aVenue de Rangueil-31077 Toulouse Cedex 4, France ReceiVed: December 8, 2006; In Final Form: January 22, 2007
Comparative electrolysis experiments were performed at platinum electrodes in the presence of some aqueous salt solutions at a concentration of 0.3 M and at the same imposed electrical potential, i.e., -1500 mV/SCE. A characteristic coalescence behavior, which is called the “train of coalescence”, was observed when ammonia gas was either electrogenerated from ammonium salt solutions or directly dissolved in the presence of some specific electrolytes.
1. Introduction The electrogeneration of gases and bubbling phenomena at electrode surfaces have long been studied in the field of electrochemistry.1-8 The knowledge of the bubble coverage fraction is crucial in several applications in the electrochemistry industry.9-11 Adhering bubbles may significantly alter the yield of electrochemical reactions, and the prevention of the sudden formation of a gas film represents a challenging topic. The way the bubbles form and grow at electrode surfaces has benefited from the use of modern high-speed cameras, which allow individual events on the sub-millisecond time scale to be observed.12,13 Some characteristic and sometimes unexpected motions of bubbles generated at or near electrodes have been described, such as the “bubble jump-off and return”, the “specific radial coalescence”, and the “oscillating tracks”.14 Recently, bubbling aspects at electrodes have gained a renewed interest since electrogenerated hydrogen nanobubbles were evidenced using atomic force microscopy (AFM).15,16 Some times earlier, many experimental results without gas electrogeneration at surfaces were shown to support the presence of nanobubbles on hydrophobic surfaces17-20 and some hydrophilic surfaces21 through various techniques (notably, direct imaging by AFM). Although various arguments were invoked to question the existence of stable submicroscopic bubbles, which have a high internal Laplace pressure, the presence of such nanobubbles is now well-confirmed.22 However, the importance of sample preparation procedures has been noted to obtain good reproducibility.23 The presence of nanobubbles has been already assumed from surface force measurements, based on the observation of steps in force data.24-26 Nanobubbles have been expected to have a significant role in various phenomena encountered in surface science, because they could favor the establishment of gas bridges between the surfaces * Author to whom correspondence should be addressed. Tel.: (33) 05 62 88 58 59. Fax: (33) 05 61 55 61 39. E-mail: JeanLuc.Trompette@ ensiacet.fr.
through hydrophobic attraction at separation distances up to hundreds of molecular water diameters in some cases.22,24,27 As such, their presence was invoked to explain the origin of the long-range attraction between hydrophobic colloidal particles dispersed in water.28,29 This hydrophobic attraction was determined to be more short range when the experiments were performed in degassed water or solution, thereby indicating that nanobubble formation was intimately related to the amount of dissolved gas. The removal of dissolved gases, through an appropriate degassing procedure, was observed to increase the stability of free-surfactant emulsion of oil droplets dispersed in water for the same reasons.30 Moreover, it has been argued that the presence of dissolved gas, in correlation with the amount and type of electrolytes, may exert a significant influence on bubble coalescence in the aqueous bulk phase.27,31,32 The ability of electrolytes to inhibit bubble coalescence has been clearly demonstrated; the extent of this ability is dependent on the nature of salt and even on specific ion pairs.32-39 However, despite the many assumptions that have been proposed, the exact mechanism that reflects this behavior still remains an open question.38,39 In a preceding study devoted to iron electrodeposition onto hydrophobic silicon wafers, the release of hydrogen bubbles, coming from the hydrogen evolution reaction that occurred concomitantly with the incipient hydrophilic iron phase on the surface electrode, was determined to differ according to the background electrolyte that was used.40 In particular, the bubbling behavior (size and intensity) was apparently related to the specific nature of the salt. To investigate such observations more deeply, classical electrolysis experiments were performed in this study on hydrophilic platinum electrodes in the presence of varying electrolytes at a concentration of 0.3 M and at the same electrical potential that was used previously, for example, -1500 mV/ SCE. The aspects of bubbling, correlated to electrochemical measurements, were filmed and analyzed with a high-speed camera and a video camera.
10.1021/jp0684426 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/09/2007
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J. Phys. Chem. C, Vol. 111, No. 13, 2007 5237
Figure 1. Variation of the ratio intensity/conductivity as a function of time at -1500 mV/SCE for sodium perchlorate (light curve) and ammonium perchlorate (thick curve).
Figure 2. Photograph of the white strip of paper wetted by the solution of Nessler’s reagent when exposed above the solution interface during the electrolysis of the ammonium salt solution.
2. Experimental Section All the electrolytessammonium perchlorate (NH4ClO4), sodium perchlorate (NaClO4), ammonium acetate (NH4CH3CO2), sodium acetate (NaCH3CO2), tetramethylammonium acetate (N(CH3)4CH3CO2), sodium thiocyanate (NaSCN), sodium fluoride (NaF), ammonium fluoride (NH4F), sodium sulfate (Na2SO4), and the Nessler’s reagentswere obtained from Sigma-Aldrich (France) and were used as-received. Bottled ammonia (NH3) gas was purchased from Air-Liquide (France). The electrolytic solutions (50 mL) were prepared by dissolving the required amount of studied salt in distilled and deionized water. Prior to each experiment, electrolytic baths were subjected to a bubbling of nitrogen gas for 10 min. The pH and conductivity of the solutions were measured, respectively, with an electronic pH-meter (Hanna Instrument) and a Tacussel
electrode. Surface tension measurements were performed with an automatic tensiometer (GBX, France). The electrochemical setup was composed of a rectangular piece (2.5 cm × 1 cm) of a platinum plate (with a lateral edge 0.2 cm thick) as the working electrode, a platinum foil as the counter electrode, and a saturated calomel reference electrode (SCE). The electrolysis experiments were performed using either a homemade rectangular cell in Plexiglas for image acquisitions or a glass vase for simple observation of the bubbling behavior. Before each measurement, the working electrode was thoroughly rinsed with distilled water and dried in a stream of nitrogen gas before it was immersed (to a depth of 1 cm) in an electrolytic bath. The electrochemical experiments were executed with a potentioscan (Radiometer Analytical S.A., Copenhagen, Tacussel DEA 332) coupled with a digital converter (Radiometer Analytical, IMT 102) and controlled by a personal computer (PC) that was running the electrochemical software (Radiometer Analytical, VoltaMaster 2). A PCO high-speed camera (type 1200 HS), with a variable acquisition frequency of 625-1350 pictures per second and a resolution of 1280 × 1024 (with an exposure time from 1 µs to 100 ms), was used to record bubble formation. The size of the bubbles and their rising speed were evaluated using a software image treatment (Visilog 5). A 3CCD digital video camera (Panasonic model NV-GS 120, from Crystal Engine) was used to film the bubbling behavior at the electrode/solution interface during electrolysis. 3. Results and Discussion 3.1. Comparative Electrolysis Experiments. To confirm the trends obtained in a previous study,40 first, electrolysis experiments were performed with sodium and ammonium perchlorate at a concentration of 0.3 M. The pH values of both solutions were, respectively, 5.6 and 5.2. For sufficiently high electrical potential, hydrogen evolution reaction occurs through the reaction of water reduction at the platinum cathode surface:
1 H2O + e- f H2 + OH2
(1)
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Figure 3. Variation of the current intensity as a function of the electrical potential (scan rate of 5 mV/s) for sodium perchlorate (light curve) and ammonium perchlorate (thick curve).
There is neither convection (unstirred solution) nor the presence of an appreciable concentration gradient (no diffusion flux) close to the electrode/solution interface, but the conductivities of the electrolyte solutions are different: 29.6 mS/cm for NH4ClO4 and 23.9 mS/cm for NaClO4. Normalized curves that represent the variation of the ratio intensity/conductivity, as a function of time at -1500 mV/SCE, are shown in Figure 1. Although the two curves present the same qualitative behavior, the values in the case of ammonium are greater (absolute value) from the beginning. When the electrical potential is applied, the current intensity slightly decreases and a flow of rising hydrogen bubbles is observed spontaneously. After a few seconds, the intensity then becomes constant, because of the establishment of a steady state, resulting from the alternated screening and release of bubbles at the cathode surface with an apparent constant surface coverage ratio. However, the weak but perceptible smell of ammonia gas (NH3) was detected during the electrolysis of the ammonium salt. The presence of ammonia gas can be evidenced through the Nessler’s test, where ammonia reacts with the Nessler’s reagent to form a red-orange complex, according to the following reaction:
2K2[HgI4] + 2NH3 f NH2Hg2I3 + 4KI + NH4I
(2)
For that purpose, a white strip of paper was primarily wetted with the colorless solution of Nessler’s reagent and it was approached above the solution interface near the electrode plane, where the electrolysis of the ammonium salt was occurring. As shown in Figure 2, a characteristic orange color was observed to develop, thereby confirming the release of ammonia gas. This indicates that a reaction that allows ammonia to be produced during electrolysis should exist. Comparative linear voltammograms of both solutions were performed (using a scan rate of 5 mV/s) (see Figure 3). In the case of ammonium, the current intensity decreases at a lower value of the electrical potential, for example, approximately -740 mV/SCE, thereby indicating the presence of an other electrochemical reaction. Thus, the current intensity is determined to be always greater (absolute value) than that with sodium at any electrical potential value. This explains the previous results obtained in Figure 1. These observations are not surprising and suggest that, in the investigated range of
electrical potential, the reaction of proton reduction coupled to the acid-base equilibrium of ammonia (pKa value of 9.2) must be considered:
NH4+ S NH3 + H+
(3)
2H+ + 2e- f H2
(4)
Therefore, the supplementary electrochemical reaction, relative to proton reduction, should correspond to
2NH4+ + 2e- f 2NH3(aq) + H2
(5)
According to the Nernst relation, the associated electrical potential is given by
(
[NH4+]2 RT ENH4+/H2 ) E°NH4+/H2 + ln 2F [NH ]2p 3 H2
)
(6)
where pH2 is the partial pressure of hydrogen gas and E°NH4+/H2 is the standard potential of the redox couple NH4+/H2. Its value can be calculated through the equation
∆G°(5) ) -2FE°NH4+/H2 ) 2∆G°(3) + ∆G°(4) ) -2RT ln Ka - 2FE°H+/H2 (7) If, for example, by convention, E°H+/H2 ) 0 mV/NHE (normal hydrogen electrode):
E°NH4+/H2 )
RT ln Ka ) -0.06pKa ) -0.55V/NHE F
which corresponds to -794 mV/SCE. The short difference with the experimental value (-740 mV/SCE) may be ascribed to overpotential effects that are due to the platinum electrode surface. 3.2. Comparative Bubbling Aspect. During electrolysis, the size of H2 bubbles leaving the cathode surface and their behavior when they reach the solution interface were determined to be quite distinct. In the case of sodium, the bubbling aspect corresponds to that which is often observed during electrolysis experiments. The average size and rising speed of hydrogen
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Figure 5. Bubbling aspect at the cathode surface during the electrolysis of the ammonium perchlorate solution at -1500 mV/SCE: (A) front view and (B) top view.
Figure 4. Bubbling aspect at the cathode surface during the electrolysis of the sodium perchlorate solution at -1500 mV/SCE: (A) front view, (B) top view, and (C) side view.
bubbles are, respectively, 0.29 ( 0.03 mm and 20.0 ( 0.1 mm/s (see Figure 4A). The generated bubbles become collected at the interface with no coalescence between them (see Figure 4B), and they disappear after a few seconds, thereby creating an effervescent layer at the electrode/solution interface (see Figure 4C). In the case of ammonium, the average size and rising speed are greater: 0.38 ( 0.03 mm and 29.0 ( 0.1 mm/s (see Figure 5A). Bubble coalescence is clearly observed at the interface (see Figure 5B), and a characteristic coalescence phenomenon is occurring (see images A-F in Figure 6) (the time between each image is 160 ms). Contrary to the case with sodium, the bubbles reaching the interface from both the extreme sides of the electrode plane seem attracted with their closer neighbor and they coalesce rapidly, instead of creating a froth layer. As a consequence, a characteristic motion, which is called the “train
of coalescence”, is initiated along the interface toward the center of the electrode plane, where the size of the advancing bubbles continues to grow as more rising bubbles, arriving vertically, are incorporated (see image A in Figure 6). When the two approaching trains of coalescence (coming from both sides) meet approximately at the middle of the electrode plane, the formation of a big coalesced bubble occurs (see image B in Figure 6), whose size increases (see image C in Figure 6) to reach a diameter of ∼4 mm, and that explodes few moments later (see image D in Figure 6). The trains of growing bubbles never stops (image E in Figure 6) as rising bubbles continuously reach the interface and coalesce. After the trains meet again, the central coalesced bubble forms and grows (see image F in Figure 6) before exploding, and so on. This characteristic motion is observed to occur indefinitely with an apparent constant frequency, as long as the potential is applied (e.g., bubbles are electrogenerated). The surface tension of ammonium and sodium perchlorate solutions at a concentration of 0.3 M was, respectively, 69.5 and 68.9 mN/m (72.2 mN/m for water), and these values were determined to be practically unchanged when measured just after electrolysis. This suggests that surface tension effects are not predominant in the observed behavior. The characteristic motion was not altered in the investigated temperature range of 298-323 K. Its magnitude was observed to increase (higher bubbles speed and greater size of the central coalesced bubble) with the ammonium perchlorate concentration and with the imposed electrical potential (lower negative values), because the number of generated hydrogen bubbles increases. It was not observed anymore for electrical poten-
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Figure 6. Sequence of images (A-F) relative to the characteristic coalescence behavior at the cathode surface during the electrolysis of the ammonium perchlorate solution at -1500 mV/SCE.
tials greater than -950 mV/SCE. When the ammonium salt concentration was significantly decreased, for example, to ∼0.015 M, the phenomenon became hardly observable, because of a very low solution conductivity. However, the train of coalescence was obsevred to occur again, at such a low concentration, when the electrical potential was appreciably decreased (e.g., -3500 mV/SCE) to allow sufficient electrogenerated hydrogen bubbles. Nevertheless, the coalescence behavior at the cathode was effectively enhanced when the electrolysis of sodium perchlorate
at a concentration of 0.3 M was performed at an electrical potential lower than -2000 mV/SCE, because of a greater amount of generated bubbles, without preventing, however, the formation of a thick froth layer. Moreover, the train of coalescence and the formation of the central coalesced bubble were also observed at the anode, at -1500 mV/SCE, when ammonium perchlorate was used (see image A in Figure 7), but not with sodium perchlorate (see image B in Figure 7). However, the bubbling intensity was weaker than that at the cathode in both cases. At the anode
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J. Phys. Chem. C, Vol. 111, No. 13, 2007 5241 TABLE 1: Values of pH, Conductivity, and Surface Tension of Acetate Salts at 0.3 M Concentration and 298 K
Figure 7. Bubbling aspect at the anode surface during electrolysis of (A) ammonium perchlorate solution and (B) sodium perchlorate solution at -1500 mV/SCE.
surface, the bubbling of oxygen gas occurs through the reaction of water oxidation:
1 H2O f 2H+ + O2 + 2e2
(8)
These results suggest that the specific influence of ammonium should be investigated. 3.3. Specific Influence of the Ammonium Ion. To extend the comparative study to other electrolytes, electrolysis experiments were performed within the same conditions in the presence of acetate salts with varying cations. The case of sodium and ammonium acetate was compared with that of the tetramethylammonium ion (TMA), which presents a stronger chaotropic character than ammonium.41 The pH, conductivity, and surface tension of these solutions, at a concentration of 0.3
acetate salt
pH
conductivity (mS/cm)
surface tension (mN/m)
sodium, Na ammonium, NH4 tertamethylammonium, TMA
7.8 6.7 8.0
10.8 21.5 16.0
72.2 72.4 72.1
M, measured before and just after electrolysis, were determined to be practically constant. The representative values are listed in Table 1. The normalized chronoamperometric (intensity/ conductivity) curves are shown respectively in Figure 8. The qualitative behavior is the same and, although the curves for sodium and TMA ions are closer, as expected, the curve that corresponds to ammonium still remains distinct, because of the supplementary electrochemical reduction of protons coupled with the acid-base equilibrium of the ammonium ion. The same observation was also made with chloride salts (results not shown). In regard to the bubbling aspects, the train of coalescence was only observed with ammonium acetate at the cathode and anode surfaces. These results are in accordance with those relative to perchlorate solutions, thereby indicating a weak influence of the type of ammonium salt. The coalescence behavior is not dependent on the chaotropic character of the cation; however, it is due to the presence of ammonium ions. Interestingly, the train of coalescence started again when a substantial amount of ammonium salt was added to the solution of tetramethylammonium acetate during electrolysis. Although the coalescence behavior was determined to be enhanced when the amount of generated bubbles was increased (with lower negative values of the electrical potential), it may also be intimately related to the presence of ammonia gas. Ammonia is continuously produced at the proximity of the cathode surface; the reduction reaction of protons, coming from the acid-base equilibrium of ammonium, is acting as an “ammonia pump”. 3.4. Effect of Dissolved Ammonia Gas. To shed more light on the influence of dissolved gas, ammonia (bottled gas) was bubbled moderately in NaClO4 at a concentration of 0.3 M for 5 min before electrolysis was performed some minutes later at -1500 mV/SCE (experiments were realized under an extractor hood, for the sake of hygienic convenience). The obtained pH was ∼12, thereby indicating that the amount of ammonium ions present was ∼0.01 M. The train of coalescence with the resulting big central coalesced bubble were determined to occur during the entire electrolysis time (see Figure 9). This reflects the influence of dissolved ammonia gas, because the phenomenon was not observed when nitrogen was bubbled 10 min in NaClO4 before electrolysis (as indicated in the Experimental Section). Moreover, as an indication, the same behavior (presence of the train of coalescence) was observed when carbon dioxide (CO2) was bubbled initially in a sodium perchlorate solution. However, the intensity of bubbling was significantly less than that observed in the case with ammonia gas, probably because of the lesser solubility of CO2. Indeed, ammonia at ambient temperature and atmospheric pressure is a gas that is known to be exceptionally soluble in water. The aqueous solubility of ammonia gas (NH3) at 293 K and normal atmospheric pressure (1 atm) is ∼700 L/(kg of water), whereas it is 810 mL/kg for carbon dioxide (CO2), 31 mL/kg for oxygen (O2), 16 mL/kg for nitrogen (N2), and 19 mL/kg for hydrogen (H2).42 At constant temperature and pressure, the solubility of gases, in the form of gaseous nanobubbles, in aqueous salt solutions decreases as the concentration of electrolyte increases.31
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Figure 8. Variation of the ratio intensity/conductivity, as a function of time, at -1500 mV/SCE for acetate salts: sodium (Na), ammonium (NH4), and tetramethylammonium (TMA).
and sodium sulfate (Na2SO4) at a concentration of 0.3 M, the bubble coalescence behavior was effectively not observed and only a froth layer of small bubbles was present, even for very low investigated electrical potential values, e.g., up to -3500 mV/SCE. These results emphasize the influence of anion specificity in the case of sodium salt solutions. Because the mechanism behind electrolyte inhibition is still not understood,39 some caution is necessary to interpret the obtained results. However, it may be suggested that, at relatively high electrolyte concentration, the combination of anions such as fluoride and sulfate with sodium cations decreases the ability of dissolved ammonia gas to enhance bubble coalescence more strongly. 4. Conclusion
Figure 9. Bubbling aspect at the cathode surface during the electrolysis of the sodium perchlorate solution at -1500 mV/SCE after ammonia bubbling.
Whatever the electrolyte, the amount of dissolved ammonia gas in these aqueous salt solutions is expected to be still significant. After small hydrogen bubbles have been electrogenerated (when the solubility limit has been attained in the bulk), ammonia gas can be incorporated into the bubbles and bubble coalescence is promoted to give rising bubbles of greater size. The diffusion of ammonia gas in the entire volume allows the coalescence of rising hydrogen bubbles at the cathode and oxygen bubbles at the anode. This may explain the reason why the characteristic motion is only observed when ammonia is present at the studied electrical potential (-1500 mV/SCE) and even for higher (less-negative) values, despite the fact that the amount of electrogenerated bubbles is significantly reduced. 3.5. Effect of Electrolyte Nature. The train of coalescence was also observed to occur during electrolysis at -1500 mV/ SCE when ammonia gas was previously bubbled for 5 min in sodium thiocyanate (NaSCN) and sodium acetate (NaCH3CO2) solutions with a concentration of 0.3 M, as in the case with NaClO4 (see Figure 9). Nevertheless, when the same experiments were conducted in the presence of sodium fluoride (NaF)
A characteristic coalescence behavior, which is called the “train of coalescence”, that occurs during the electrolysis of aqueous salt solutions at platinum electrodes has been reported. It was observed when ammonia gas was present (electrogenerated or dissolved), together with specific electrolytes. The obtained results indicate the influence of dissolved gas on bubble coalescence. On a practical level, these bubbling aspects may offer the opportunity to modulate the efficiency of some electrochemical processes in which bubble coalescence must be hindered or enhanced according to the profitable use, or nonuse, of ammonium electrolytes. Acknowledgment. The authors are grateful to P. Destrac (LGC), A. Muller (LGC), and S. Mouysset (LGC) for technical assistance, and they are also grateful to T. Tzedakis (LGC) for valuable discussions. References and Notes (1) Vogt, H. J. Appl. Electrochem. 1983, 13, 87. (2) Kreysa, G.; Hakansson, B.; Ekdunge, P. Electrochim. Acta 1988, 33, 1351. (3) Huot, J. Y. J. Electrochem. Soc. 1989, 136, 1933. (4) Vogt, H. Electrochim. Acta 1989, 34, 1429. (5) Ekdunge, P.; Juttner, K.; Kreysa, G.; Kessler, T.; Ebert, M.; Lorenz, W. J. J. Electrochem. Soc. 1991, 138, 2660. (6) Iwasaki, A.; Kaneko, H.; Abe, Y.; Kamimoto, M. Electrochim. Acta 1997, 43, 509. (7) Jerkiewicz, G. Prog. Surf. Sci. 1998, 57, 137. (8) Vogt, H.; Balzer, R. J. Electrochim. Acta 2005, 50, 2073. (9) Wu¨thrich, R.; Comninellis, C.; Bleuler, H. Electrochim. Acta 2002, 50, 5242.
Bubble Coalescence during Electrolysis of NH4 Salts (10) Matsushima, H.; Nishida, T.; Konishi, Y.; Fukunaka, Y.; Ito, Y.; Kuribayashi, K. Electrochim. Acta 2003, 48, 4119. (11) Kikuchi, K.; Tanaka, Y.; Saiharu, Y.; Maeda, M.; Kawamura, M.; Ogumi, Z. J. Colloid Interface Sci. 2006, 298, 914. (12) Guelcher, S. A.; Solomentsev, Y. E.; Sides, P. J.; Anderson, J. L. J. Electrochem. Soc. 1988, 145, 1848. (13) Penner, R. M. J. Phys. Chem. B 2002, 106, 3339. (14) Lubetkin, S. Electrochim. Acta 2002, 48, 357. (15) Zhang, L.; Yi, Z.; Zhang, X.; Li, Z.; Shen, G.; Ye, M.; Fan, C.; Fang, H.; Hu, J. Langmuir 2006, 22, 8109. (16) Kikuchi, K.; Tanaka, Y.; Saihara, Y.; Ogumi, Z. Electrochim. Acta 2006, 52, 904. (17) Carambassis, A.; Jonker, L. C.; Attard, P.; Rutland, M. Phys. ReV. Lett. 1998, 80, 5357. (18) Lou, S.; Ouyang, Z.; Zhang, Y.; Li, X.; Hu, J.; Li, M.; Yang, F. J. Vac. Sci. Technol. B 2000, 18, 2573. (19) Ishida, N.; Inoue, T.; Miyahara, M.; Higashitani, K. Langmuir 2000, 16, 6377. (20) Tyrell, J. W. G.; Attard, P. Phys. ReV. Lett. 2001, 87, 176104. (21) Zhang, X. H.; Zhang, X. D.; Lou, S. T.; Zhang, Z. X.; Sun, J. L.; Hu, J. Langmuir 2004, 20, 3813. (22) Attard, P. AdV. Colloid Interface Sci. 2003, 104, 75. (23) Zhang, X. H.; Maeda, N.; Craig, V. S. J. Langmuir 2006, 22, 5025. (24) Israelachvili, J. N. Intermolecular and Surface Forces; Second Edition; Academic Press: London, 1992. (25) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468.
J. Phys. Chem. C, Vol. 111, No. 13, 2007 5243 (26) Attard, P. Langmuir 1996, 12, 1693. (27) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. J. Phys. Chem. 1993, 97, 10192. (28) Meagher, L.; Craig, V. S. J. Langmuir 1994, 10, 2736. (29) Considine, R. F.; Hayes, R. A.; Horn, R. G. Langmuir 1999, 15, 1657. (30) Pashley, R. M. J. Phys. Chem. B 2003, 107, 1714. (31) Weissenborn, P. K.; Pugh, R. J. J. Colloid Interface Sci. 1996, 184, 550. (32) Craig, V. S. J. Curr. Opin. Colloid Interface Sci. 2004, 9, 178. (33) Marrucci, G.; Nicodemo, L. Chem. Eng. Sci. 1967, 22, 1257. (34) Lessard, R. R.; Zieminski, S. A. Ind. Eng. Chem. Fundam. 1971, 10, 260. (35) Hofmeier, U.; Yaminsky, W.; Christensson, H. K. J. Colloid Interface Sci. 1995, 174, 199. (36) Deschenes, L. A.; Barrett, J.; Muller, L. J.; Fourkas, J. T.; Mohanty, U. J. Phys. Chem. B 1998, 102, 5115. (37) Tsang, Y. H.; Koh, Y. H.; Koch, D. L. J. Colloid Interface Sci. 2004, 275, 290. (38) Marcelja, S. Curr. Opin. Colloid Interface Sci. 2004, 9, 165. (39) Henry, C. L.; Dalton, C. N.; Scruton, L.; Craig, V. S. J. J. Phys. Chem. C 2007, 111, 1015. (40) Trompette, J. L.; Vergnes, H. J. Phys. Chem. B 2006, 110, 14779. (41) Leontidis, E. Curr. Opin. Colloid Interface Sci. 2002, 7, 81. (42) Alberty, R. A. Physical Chemistry; 7th Edition; Wiley: New York, 1987.
