- Email: [email protected]
Dynamics of counterion binding during acoustic nebulisation of surfactant solutions
Ultrasonics Sonochemistry 18 (2011) 958–962
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Dynamics of counterion binding during acoustic nebulisation of surfactant solutions Beenamma Jimmy, Sandra Kentish, Muthupandian Ashokkumar ⇑ School of Chemistry and Department of Chemical and Biomolecular Engineering, University of Melbourne, Melbourne, VIC 3010, Australia
a r t i c l e
i n f o
Article history: Received 18 October 2010 Received in revised form 12 January 2011 Accepted 18 January 2011 Available online 25 January 2011 Keywords: Acoustic nebulisation Dynamic condition Interface Counterion binding ability
a b s t r a c t A metal ion (Cu2+) and a complex copper species, copper (II) bis-bipyridine, were used as alternate counterions in an aqueous surfactant solution of sodium dodecylbenzenesulfonate (SDBS) to investigate the dynamics of counterion interactions in an acoustic field. Sonoluminescence spectral studies showed that such counterions were able to replace sodium ions at the interface, even when the interface was rapidly oscillating under the acoustic field. Ultrasound induced nebulisation was then used to probe the interfacial profile of surfactant and bound counterions in a dynamic environment. At low bulk concentrations, the copper (II) bis-bipyridine cation was more effective at enhancing the loading of the dodecylbenzenesulfonate anion on the interface, due to its documented greater binding ability. However, at higher bulk concentrations, the movement of this cation is limited by its larger size and the smaller Cu2+ cation is more effective in enhancing the loading of the dodecylbenzenesulfonate anion. The results show that under dynamic conditions, the surface concentrations are governed by mass transfer kinetics rather than equilibrium thermodynamics. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The behaviour of a charged interface is of both fundamental and practical importance for a broad range of phenomena in cell biology, colloid and polymer science [1–3]. When ionic surfactant molecules are adsorbed at the interface, an electrical double layer develops with a diffuse layer of counterions neutralising the charge of surfactant head groups. Counterions from a background electrolyte also play a significant role in this interfacial structure. An ion selective mechanism has been found to influence the binding of counterions to the surfactant ion in micelles [4,5] and in separation techniques such as ion flotation [6–8]. Experimental work has shown that the nature of the counterions has an important influence on surfactant adsorption. For example, Koelsch and Motschmann [2] showed that the use of different halide counterions led to ion specific effects that were not readily categorized. Similarly, Cross and Jayson show that calcium ions can increase the adsorption of sodium dodecylsulfate, with an increased concentration of calcium ions observed near the interface [9], even though they could observe these effects only at higher surface concentrations. While there is extensive literature available on such surfactant/counter ion interactions at air/water interface under equilibrium (steady-state) conditions [10–12], there is a lack of information on this type of interaction in more dynamic systems. For example, when cavitation bubbles are generated in an aqueous solution containing surface active solutes, the ⇑ Corresponding author. Tel.: +61 383447090; fax: +61 393475180. E-mail address: [email protected] (M. Ashokkumar). 1350-4177/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2011.01.009
bubble/solution interface is subjected to constant changes. Under this dynamic environment, the kinetics of adsorption/desorption of surfactant molecules to the bubble/solution interface can dominate and the interface may not reach thermodynamic equilibrium [12–15]. The rate of mass transfer of the surfactant towards the interface becomes the dominant factor in interfacial behaviour. In our recent work [16], the air/water interface of aqueous surfactant systems was analysed under such dynamic conditions using a nebulisation technique. The investigation was able to isolate the composition of the oscillating interface through the analysis of the composition of nebulised droplets. Droplets evolved during nebulisation are surfactant rich, reflecting their greater surface area relative to the bulk solution. We were able to show that surface concentrations of between 15% and 30% of the equilibrium value were observed under dynamic conditions, indicating mass transfer as the rate limiting step rather than the diffusion coefficient of the surfactant molecules. The rate of mass transfer was dependent on the bulk concentration and the surface activity of the surfactant. In this paper, we use the same technique for the investigation of the distribution of counterions at the interface. The major factors that control the interfacial dynamics in such a system are both the electrostatic attraction between surfactant ions and the counterions (similar to that observed under steady-state conditions) and the mass transfer rate of the counterions to the surface. The work focuses specifically on the behaviour of the dodecylbenzenesulfonate surfactant anion (DBS) in the presence of two different counterions. The first cation studied is a simple copper (II) ion, while the second is a much bulkier species, a bis-bipyridyl complex of copper (II) ðCuðbpyÞ2þ 2 Þ, Fig. 1). The larger size of this species
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may lead to a reduced mass transfer rate. Conversely, this complex metal ion has been reported to have a higher binding ability to an anion than Cu (II) alone. Thermodynamic studies show that the ligand, bis-bipyridyl enhances the ability of Cu (II) to bind to anionic ligands [17]. The ligands, which are aromatic in nature, receive p electron density from the metal ion. This enhances the formal charge on Cu and thereby its coulombic interaction, resulting in the formation of stable complexes with the anions. In addition, it has been shown that the ðCuðbpyÞ2þ 2 Þ complex can be incorporated into polyanionic electrode films in amounts that far exceed the counter-cations required for the anionic groups present [18]. It is also well known for its ability to bind to and cause cleavage of DNA [19,20]. 2. Materials and methods Sodium dodecylbenzenesulfonate (SDBS) was obtained from Sigma Aldrich, Australia and copper (II) chloride from J.T. Baker, Australia. The copper (II) bis-bipyridinetetrafluoborate (Cu(C5H4N– C5H4N)2(BF4)2) was synthesised in-house. About 1 g of copper carbonate in ethanol (7.0 cm3) was added to a 40% aqueous HBF4 solution (1.5 cm3) and gently simmered for 10 min. 8 cm3 of hot acetone was added to the above mixture and the hot solution filtered. The filtrate was added to a solution of 1.2 g bipyridine in 30 cm3 acetone. The crystals of Cu(NC5H4–C5H4N)2(BF4)2 formed were purified through re-crystallization and dried in an oven. MilliQ water was used to prepare all solutions. The equilibrium surface tension of aqueous solutions of background electrolytes Cu(bpy)2(BF4)2 and CuCl2 in the presence and absence of SDBS was measured using the Wilhelmy plate technique. Ultrasound of 1062 kHz was generated by an ELAC RF generator and delivered through an Allied Signal transducer. A water jacketed Pyrex cell with 220 cm3 volume capacity, mounted on the stainless steel top of the transducer was used as the sonication cell. Surfactant solutions containing different concentrations of each background electrolyte were prepared. A volume of 170 cm3 was used for nebulisation. The temperature was maintained at 20 ± 5 °C. The electric power delivered from the generator was 100 W. About 3–4 cm3 of the nebulised aerosol droplets was collected to determine the surfactant and counterion concentrations. Quantitative analysis of DBS- was carried out using a Varian 2+ UV–visible spectrophotometer and that of CuðbipyÞ2þ by 2 and Cu a Shimadzu atomic absorption spectrophotometer. The size distribution of droplets produced by the ultrasound was measured using a laser light scattering technique (Malvern Instruments, SparaytecSTP2000v300). A Hitachi fluorimeter was used to record the sonoluminescence emission spectra from SDBS solutions. The solutions were argon saturated and irradiated with an ultrasound frequency of 358 kHz to produce the excited state sodium emission. 3. Results and discussion
3.2. Sonoluminescence spectral analysis of counterion at the interface 2+ The association of ðCuðbpyÞ2þ with the adsorbed surfac2 Þ or Cu tant layer requires that these ions exchange with Na+ to act as counterions within this layer due to their stronger binding ability. This exchange of sodium ions for Cu2+ in the SDBS system was further analysed under the dynamic conditions of acoustic cavitation by measuring the intensity of sodium emissions (Fig. 3) observed with sonoluminescence. The intensity of sodium emission, indicated by a peak at around 590 nm, is found to be dependent upon the concentration of sodium ions near the bubble interface during ultrasonic irradiation of aqueous solutions containing sodium ions [23]. As sonoluminescence emission intensity is reported to be
Fig. 2. Equilibrium surface tension data as a function of cation concentrations in the absence and presence of SDBS. Note: The surface tensions of 0.02 mM and 0.04 mM SDBS without any counterions were about 72.6 and 71.2 mN/m, respectively. Note that the surface tensions at zero concentration of Cu2+ or ðCuðbpyÞ2þ 2 Þ are not included in the figure. They both have similar values as shown above.
1.4
Relative intensity of sodium emission
Fig. 1. Structure of Copper bis-bipyridine ðCuðbpyÞ2þ 2 Þ.
the surface tension indicating that these counterions are not surface active at the low concentration range used in this study. However, when used as background electrolytes in a surfactant (SDBS) solution, they caused significant reduction in the surface tension compared to that observed without SDBS. The effect of a background electrolyte on the interfacial adsorption of ionic surfactants under equilibrium conditions has been reported in many studies [21–22] and is attributed to the electrolyte reducing the repulsion between the headgroup charges of the surfactant oriented at the interface. This allows more surfactant molecules to approach and to occupy the interface. Of importance in the present case is that the ðCuðbpyÞ2þ 2 Þ system has a much greater effect upon the surface tension than Cu2+ even though the ionic strength is comparable. This reflects the greater binding ability of ðCuðbpyÞ2þ 2 Þ to the surfactant anion than Cu (II) alone, as discussed above.
1mM SDBS 1 mM SDBS + .250 2+ mM Cu
1.2
1 mM SDBS + 0.50 2+ mM Cu
1
1 mM SDBS + 0.625 2+ mM Cu 0.8
3.1. Equilibrium surface tension results The equilibrium surface tension data (Fig. 2) showed that the addition of either Cu(bpy)2(BF4)2 or CuCl2 to water did not reduce
350
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500
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600
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700
Wavelength (nm)
Fig. 3. Intensity of sodium emission from an SDBS solution in the presence of Cu2+ at various concentrations.
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0.02 mM SDBS
Aerosol Concentration (mM)
Aerosol Concentration (mM)
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Bulk Cu(bpy)22+ Concentration (mM) 0.04 mM SDBS
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Bulk Cu(bpy)2
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Concentration (mM)
Fig. 4. Concentration of surfactant anion, DBS (closed symbols) and counterions, Cu2+ or ðCuðbpyÞ2þ 2 Þ (open symbols) in the aerosol as a function of the counterion concentration in the bulk.
3.3. Effect of counterion on the interfacial composition The behaviour of both Cu2+ and ðCuðbpyÞ2þ 2 Þ counterions were then examined using acoustic nebulisation. The efficiency of nebulisation (the formation of maximum amount of aerosol droplets) is the highest at frequencies greater than 1 MHz, hence these experiments were conducted at 1062 kHz. Fig. 4 shows the concentrations of the surfactant and the counterions in the nebulised droplets. In the absence of any other counterions, nebulisation of 0.02 and 0.04 mM SDBS solutions produced aerosols containing 0.033 and 0.058 mM concentrations of SDBS, respectively. Such enrichment of a surface active solute in an aerosol has been reported in our prior work [16] and that of other workers [25,26]. The extent of enrichment can be related to the rate of mass transfer of surfactant to the air–liquid interface. 2+ The addition of small amounts of ðCuðbpyÞ2þ counteri2 Þ or Cu ons enhances the DBS concentration in the aerosol significantly, with more pronounced effects for higher SDBS concentration. In all cases, the concentration of both Cu2+ and ðCuðbpyÞ2þ 2 Þ are higher in the aerosol than in the bulk solution. The cations themselves are not surface active as shown in Fig. 2. Thus the observed increase in 2+ concentration indicates that ðCuðbpyÞ2þ ions directly 2 Þ and Cu associate with the adsorbed surfactant layer.
3.4. Size distribution of nebulised droplets To clarify the differences between the two copper counterions, the surface excess concentrations (C) for DBS, ðCuðbpyÞ2þ 2 Þ and Cu2+ were estimated based on the total surface area available in the aerosol [25]:
P ðC A C B Þ i ni 43 pr 3i P C¼ 2 i ni 4pr i
ð1Þ
where CA is the measured concentration of solute in the aerosol (Fig. 4) and CB that in the bulk while ni represent the number fraction of droplets of radius ri. This calculation requires knowledge of the size distribution of aerosol droplets which is given in Fig. 5. Most droplets are less than 10 lm in diameter with a relatively narrow size distribution that is independent of surfactant concentration.
18 15
Number Percentage
greatest at 200–400 kHz [24], these measurements were carried out at 358 kHz. It can be seen in Fig. 3 that the Na⁄ emission at 590 nm is a maximum in the absence of Cu2+. As the Cu2+ concentration is increased, the Na⁄ emission intensity decreases indicating the replacement of the interfacial Na+ ions by the Cu2+. It can also been seen in this figure that Na⁄ emission is not completely eliminated in the presence of Cu2+ indicating the presence of small amount of Na+ ions at the bubble/solution interface.
12 9 6 3 0 0
5
10
15
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Diameter µm Fig. 5. Distribution of droplet diameters based on their number percentage.
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Surface Excess Concentration x 10 2 (mol/m )
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Concentration (mM)
Fig. 6. Comparison of surface excess concentrations of surfactant ion (DBS, shown as closed symbols) and counterions (Cu2+ and ðCuðbpyÞ2þ 2 Þ, shown as open symbols).
3.6. Surface concentration of counterions and surfactant ion The calculated surface excess concentration of DBS and respective counterions are shown in Fig. 6. In the absence of any copper cations, the surface excess of the DBS anion is 0.2 107 mol/m2 at 0.02 mM bulk concentration and 0.4 107 at 0.04 mM bulk concentration. This surface excess is considerably lower than the steady state equilibrium values of 1.4 107 and 2.8 107 mol/ m2, reflecting the mass transfer limitations under the dynamic environment of the acoustic field. The DBS surface concentration increase significantly in the 2+ presence of the ðCuðbpyÞ2þ counterions. This shows that 2 Þ and Cu these cations are both effective in increasing the adsorption of DBS to the interface and this increase the driving force for mass transfer to the interface. However, there are significant differences in the behaviour between the two cations. At the 0.02 mM DBS bulk concentration, the presence of ðCuðbpyÞ2þ 2 Þ results in greater surface enrichment of both cation and anion, with values 30–40% above that with the Cu2+ cation. This strong adsorption reflects the greater binding ability of the ðCuðbpyÞ2þ 2 Þ to anions as has already been discussed. However, there is also a much more prominent plateau in the surface concentrations for this cation, with the concentration seemingly limited to about 0.6 and 1.4 107 mol/ m2 for the DBS and ðCuðbpyÞ2þ 2 Þ ions, respectively, in spite of increasing bulk cation concentrations. A plateau is also evident in the Cu2+ data at 0.02 mM SDBS, but it is less pronounced. This leveling off in surface excess concentrations is representative of the limits in the ability of the species to transfer to the interface while it is rapidly oscillating and nebulising. The plateau values correspond to 0.36 mols of Cu2+ and 0.39 mols of ðCuðbpyÞ2þ 2 Þ to each mol of DBS. Hence in both cases, there are still some sodium ions within the electrical double layer, representing 28% and 22% of the total counterbalancing charge. The data for ðCuðbpyÞ2þ 2 Þ in the presence of a bulk concentration of 0.04 mM DBS-, changes only marginally. The peak values are little changed, perhaps increasing marginally to 0.7 and
1.7 107 mol/m2 respectively. Conversely, at these greater anion concentrations, surface concentrations for the Cu2+ system are notably increased, with peak values doubling, in line with the doubling of bulk concentrations. Thus at this higher bulk anion concentration, the Cu2+ concentration is higher at the interface than ðCuðbpyÞ2þ 2 Þ. The differences between the results at 0.02 mM and 0.04 mM probably relate to mass transfer limitations associated with the counter ion. That is, while ðCuðbpyÞ2þ 2 Þ has a greater affinity for the interface, it is much larger and bulkier and hence cannot move quickly enough to adhere to the rapidly oscillating interface. The ratios of counterion to anion are little changed from the 0.02 mM case, correspond to 0.33 mols of Cu2+ and 0.42 mols of ðCuðbpyÞ2þ 2 Þ to each mol of DBS . Further, the peak concentrations of DBS are still well below that observed under steady-state conditions (2.2 and 3.4 107 mol/m2 for 0.02 and 0.04 mM SDBS respectively). These results again support the hypothesis that the changes are related to mass transfer limitations of the surfactant and counterions rather than surface chemistry.
4. Conclusion These results show that copper cations are effective in increasing the surface concentrations of a surfactant anion even under the mass transfer limiting conditions of acoustic nebulisation. In addition to the mobility of the cations, the difference in ability of surface adsorption may also play some role in the overall nebulisation process. The SL spectra showing the exited state sodium atoms provide direct evidence of the replacement of sodium cations with such species. Under the conditions of acoustic nebulisation, the greater binding ability of ðCuðbpyÞ2þ 2 Þ means this is most effective at low bulk anion and cation concentrations. However, the impact is limited as bulk concentrations increase. In particular, when the bulk anion concentration is increased from 0.02 mM to 0.04 mM, the Cu2+ cation becomes more effective than ðCuðbpyÞ2þ 2 Þ due to the smaller size and hence greater mobility of this species. These
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results show that under such dynamic conditions mass transfer effects cannot be ignored. Acknowledgement We thank the Australian Research Council for the funding through ARC Discovery Grant DP0771094. References [1] W. Bu, D. Vaknin, A. Travesset, Monovalent counterion distributions at highly charged water interfaces: proton-transfer and Poisson-Boltzmann theory, Phys. Rev. E 72 (2005) 060501. [2] P. Koelsch, H. Motschmann, Varying the counterions at a charged interface, Langmuir 21 (2005) 3436–3442. [3] J. Pittler, W. Bu, D. Vaknin, A. Travesset, D.J. McGillivray, M. Losche, Charge inversion at minute electrolyte concentrations, Phys. Rev. Lett. 97 (2006) 046102. [4] C.J. Drummond, F. Grieser, A study of competitive counterion binding to micelles using the acid-catalyzed reaction of hydrogen peroxide with iodide ions, J. Coll. Interface Sci. 127 (1989) 281–291. [5] P. Paton-Morales, F.I. Talens-Alesson, Effect of competitive adsorption of Zn2+ on the flocculation of lauryl sulfate micelles by Al3+, Langmuir 18 (2002) 8295– 8301. [6] F.M. Doyle, Ion flotation – its potential for hydrometallurgical operations, Int. J. Miner. Process 72 (2003) 387–399. [7] R.B. Grieves, R.N. Kyle, Models for Interaction between ionic surfactants and nonsurface-active ions in foam fractionation processes, Separ. Sci. Technol. 17 (1982) 465–483. [8] J.C. Schulz, G.G. Warr, Selective adsorption of metal cations onto AOT and dodecyl sulfate films at the air/solution interface, J. Chem. Soc. Faraday Trans. 94 (1998) 253–257. [9] A.W. Cross, G.G. Jayson, The effect of small quantities of calcium on the adsorption of sodium dodecyl sulfate and calcium at the gas-liquid interface, J. Coll. Interface Sci. 162 (1994) 45–51. [10] S.G. Oh, D.O. Shah, Effect of counterions on the interfacial tension and emulsion droplet size in the oil/water/dodecyl sulfate system, J. Phys. Chem. 97 (1993) 284–286.
[11] I. Weil, Surface concentration and the Gibbs adsorption law. The effect of the alkali metal cations onsurface behaviour, J. Phys. Chem. 70 (1966) 133–140. [12] P.A. Kralchevsky, K.D. Danov, G. Broze, A. Mehreteab, Thermodynamics of ionic surfactant adsorption with account for the counterion binding: effect of salts of various valency, Langmuir 15 (1999) 2351–2365. [13] J. Lee, S.E. Kentish, M. Ashokkumar, The effect of surface-active solutes on bubble coalescence in the presence of ultrasound, J. Phys. Chem. B 109 (2005) 5095–5099. [14] J.Z. Sostaric, P. Riesz, Sonochemistry of surfactants in aqueous solutions: an EPR spin-trapping study, J. Am. Chem. Soc. 123 (2001) 11010–11019. [15] J.Z. Sostaric, P. Riesz, Adsorption of surfactants at the gas/solution interface of cavitation Bubbles: an ultrasound intensity-independent frequency effect in sonochemistry, J. Phys. Chem. B 106 (2002) 12537–12548. [16] B. Jimmy, S. Kentish, F. Grieser, M. Ashokkumar, Ultrasonic nebulization in aqueous solutions and the role of interfacial adsorption dynamics in surfactant enrichment, Langmuir 24 (2008) 10133–10137. [17] M.S. Mohan, D. Bancroft, E.H. Abbott, Thermodynamic study of the formation of some mixed-ligand complexes of copper (II), Inorg. Chem. 18 (1979) 344– 346. [18] D. Rong, F.C. Anson, Severe inhibition of the diffusion and electroactivity of Ò ðCuðbpyÞ2þ coatings on electrodes, J. 2 Þ complexes incorporated in Nafion Electroanal. Chem. 404 (1996) 171–177. [19] J. Aronovitch, D. Godinger, A. Samuni, Ascorbic acid oxidation and DNA scission catalyzed by iron and copper chelates, Free Rad. Res. Com. 2 (1987) 241–258. [20] Z.S. Yang, Y.L. Wang, Y.Z. Zhang, Electrochemically induced DNA cleavage by copper-bipyridyl complex, Electrochem. Commun. 6 (2004) 158–163. [21] D. Myers, Surfactant Science and Technology, second ed., VCH Publishers, New York, 1992. [22] D. J. Shaw, Colloid & surface Chemistry, fourth ed., Elsevier Science Ltd, 1992. [23] D. Sunartio, K. Yasui, T. Tuziuti, T. Kozuka, Y. Iida, M. Ashokkumar, F. Grieser, Correlation between Na⁄ emission, and chemically active acoustic cavitation bubbles, Chem. Phys. Chem 8 (2007) 2331–2335. [24] P. Kanthale, M. Ashokkumar, F. Grieser, Sonoluminescence, sonochemistry (H2O2 yield) and bubble dynamics: frequency and power effects, Ultrason. Sonochem. 15 (2008) 143–150. [25] D.N. Rassokhin, J. Phys, Chem. B 102 (1998) 4337–4341. [26] H. Takaya, S. Nii, F. Kawaizumi, K. Takahashi, Enrichment of surfactant from its aqueous solution using ultrasonic atomization, Ultrason. Sonochem. 12 (2005) 483–487.
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Dynamics of counterion binding during acoustic nebulisation of surfactant solutions Beenamma Jimmy, Sandra Kentish, Muthupandian Ashokkumar ⇑ School of Chemistry and Department of Chemical and Biomolecular Engineering, University of Melbourne, Melbourne, VIC 3010, Australia
a r t i c l e
i n f o
Article history: Received 18 October 2010 Received in revised form 12 January 2011 Accepted 18 January 2011 Available online 25 January 2011 Keywords: Acoustic nebulisation Dynamic condition Interface Counterion binding ability
a b s t r a c t A metal ion (Cu2+) and a complex copper species, copper (II) bis-bipyridine, were used as alternate counterions in an aqueous surfactant solution of sodium dodecylbenzenesulfonate (SDBS) to investigate the dynamics of counterion interactions in an acoustic field. Sonoluminescence spectral studies showed that such counterions were able to replace sodium ions at the interface, even when the interface was rapidly oscillating under the acoustic field. Ultrasound induced nebulisation was then used to probe the interfacial profile of surfactant and bound counterions in a dynamic environment. At low bulk concentrations, the copper (II) bis-bipyridine cation was more effective at enhancing the loading of the dodecylbenzenesulfonate anion on the interface, due to its documented greater binding ability. However, at higher bulk concentrations, the movement of this cation is limited by its larger size and the smaller Cu2+ cation is more effective in enhancing the loading of the dodecylbenzenesulfonate anion. The results show that under dynamic conditions, the surface concentrations are governed by mass transfer kinetics rather than equilibrium thermodynamics. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The behaviour of a charged interface is of both fundamental and practical importance for a broad range of phenomena in cell biology, colloid and polymer science [1–3]. When ionic surfactant molecules are adsorbed at the interface, an electrical double layer develops with a diffuse layer of counterions neutralising the charge of surfactant head groups. Counterions from a background electrolyte also play a significant role in this interfacial structure. An ion selective mechanism has been found to influence the binding of counterions to the surfactant ion in micelles [4,5] and in separation techniques such as ion flotation [6–8]. Experimental work has shown that the nature of the counterions has an important influence on surfactant adsorption. For example, Koelsch and Motschmann [2] showed that the use of different halide counterions led to ion specific effects that were not readily categorized. Similarly, Cross and Jayson show that calcium ions can increase the adsorption of sodium dodecylsulfate, with an increased concentration of calcium ions observed near the interface [9], even though they could observe these effects only at higher surface concentrations. While there is extensive literature available on such surfactant/counter ion interactions at air/water interface under equilibrium (steady-state) conditions [10–12], there is a lack of information on this type of interaction in more dynamic systems. For example, when cavitation bubbles are generated in an aqueous solution containing surface active solutes, the ⇑ Corresponding author. Tel.: +61 383447090; fax: +61 393475180. E-mail address: [email protected] (M. Ashokkumar). 1350-4177/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2011.01.009
bubble/solution interface is subjected to constant changes. Under this dynamic environment, the kinetics of adsorption/desorption of surfactant molecules to the bubble/solution interface can dominate and the interface may not reach thermodynamic equilibrium [12–15]. The rate of mass transfer of the surfactant towards the interface becomes the dominant factor in interfacial behaviour. In our recent work [16], the air/water interface of aqueous surfactant systems was analysed under such dynamic conditions using a nebulisation technique. The investigation was able to isolate the composition of the oscillating interface through the analysis of the composition of nebulised droplets. Droplets evolved during nebulisation are surfactant rich, reflecting their greater surface area relative to the bulk solution. We were able to show that surface concentrations of between 15% and 30% of the equilibrium value were observed under dynamic conditions, indicating mass transfer as the rate limiting step rather than the diffusion coefficient of the surfactant molecules. The rate of mass transfer was dependent on the bulk concentration and the surface activity of the surfactant. In this paper, we use the same technique for the investigation of the distribution of counterions at the interface. The major factors that control the interfacial dynamics in such a system are both the electrostatic attraction between surfactant ions and the counterions (similar to that observed under steady-state conditions) and the mass transfer rate of the counterions to the surface. The work focuses specifically on the behaviour of the dodecylbenzenesulfonate surfactant anion (DBS) in the presence of two different counterions. The first cation studied is a simple copper (II) ion, while the second is a much bulkier species, a bis-bipyridyl complex of copper (II) ðCuðbpyÞ2þ 2 Þ, Fig. 1). The larger size of this species
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may lead to a reduced mass transfer rate. Conversely, this complex metal ion has been reported to have a higher binding ability to an anion than Cu (II) alone. Thermodynamic studies show that the ligand, bis-bipyridyl enhances the ability of Cu (II) to bind to anionic ligands [17]. The ligands, which are aromatic in nature, receive p electron density from the metal ion. This enhances the formal charge on Cu and thereby its coulombic interaction, resulting in the formation of stable complexes with the anions. In addition, it has been shown that the ðCuðbpyÞ2þ 2 Þ complex can be incorporated into polyanionic electrode films in amounts that far exceed the counter-cations required for the anionic groups present [18]. It is also well known for its ability to bind to and cause cleavage of DNA [19,20]. 2. Materials and methods Sodium dodecylbenzenesulfonate (SDBS) was obtained from Sigma Aldrich, Australia and copper (II) chloride from J.T. Baker, Australia. The copper (II) bis-bipyridinetetrafluoborate (Cu(C5H4N– C5H4N)2(BF4)2) was synthesised in-house. About 1 g of copper carbonate in ethanol (7.0 cm3) was added to a 40% aqueous HBF4 solution (1.5 cm3) and gently simmered for 10 min. 8 cm3 of hot acetone was added to the above mixture and the hot solution filtered. The filtrate was added to a solution of 1.2 g bipyridine in 30 cm3 acetone. The crystals of Cu(NC5H4–C5H4N)2(BF4)2 formed were purified through re-crystallization and dried in an oven. MilliQ water was used to prepare all solutions. The equilibrium surface tension of aqueous solutions of background electrolytes Cu(bpy)2(BF4)2 and CuCl2 in the presence and absence of SDBS was measured using the Wilhelmy plate technique. Ultrasound of 1062 kHz was generated by an ELAC RF generator and delivered through an Allied Signal transducer. A water jacketed Pyrex cell with 220 cm3 volume capacity, mounted on the stainless steel top of the transducer was used as the sonication cell. Surfactant solutions containing different concentrations of each background electrolyte were prepared. A volume of 170 cm3 was used for nebulisation. The temperature was maintained at 20 ± 5 °C. The electric power delivered from the generator was 100 W. About 3–4 cm3 of the nebulised aerosol droplets was collected to determine the surfactant and counterion concentrations. Quantitative analysis of DBS- was carried out using a Varian 2+ UV–visible spectrophotometer and that of CuðbipyÞ2þ by 2 and Cu a Shimadzu atomic absorption spectrophotometer. The size distribution of droplets produced by the ultrasound was measured using a laser light scattering technique (Malvern Instruments, SparaytecSTP2000v300). A Hitachi fluorimeter was used to record the sonoluminescence emission spectra from SDBS solutions. The solutions were argon saturated and irradiated with an ultrasound frequency of 358 kHz to produce the excited state sodium emission. 3. Results and discussion
3.2. Sonoluminescence spectral analysis of counterion at the interface 2+ The association of ðCuðbpyÞ2þ with the adsorbed surfac2 Þ or Cu tant layer requires that these ions exchange with Na+ to act as counterions within this layer due to their stronger binding ability. This exchange of sodium ions for Cu2+ in the SDBS system was further analysed under the dynamic conditions of acoustic cavitation by measuring the intensity of sodium emissions (Fig. 3) observed with sonoluminescence. The intensity of sodium emission, indicated by a peak at around 590 nm, is found to be dependent upon the concentration of sodium ions near the bubble interface during ultrasonic irradiation of aqueous solutions containing sodium ions [23]. As sonoluminescence emission intensity is reported to be
Fig. 2. Equilibrium surface tension data as a function of cation concentrations in the absence and presence of SDBS. Note: The surface tensions of 0.02 mM and 0.04 mM SDBS without any counterions were about 72.6 and 71.2 mN/m, respectively. Note that the surface tensions at zero concentration of Cu2+ or ðCuðbpyÞ2þ 2 Þ are not included in the figure. They both have similar values as shown above.
1.4
Relative intensity of sodium emission
Fig. 1. Structure of Copper bis-bipyridine ðCuðbpyÞ2þ 2 Þ.
the surface tension indicating that these counterions are not surface active at the low concentration range used in this study. However, when used as background electrolytes in a surfactant (SDBS) solution, they caused significant reduction in the surface tension compared to that observed without SDBS. The effect of a background electrolyte on the interfacial adsorption of ionic surfactants under equilibrium conditions has been reported in many studies [21–22] and is attributed to the electrolyte reducing the repulsion between the headgroup charges of the surfactant oriented at the interface. This allows more surfactant molecules to approach and to occupy the interface. Of importance in the present case is that the ðCuðbpyÞ2þ 2 Þ system has a much greater effect upon the surface tension than Cu2+ even though the ionic strength is comparable. This reflects the greater binding ability of ðCuðbpyÞ2þ 2 Þ to the surfactant anion than Cu (II) alone, as discussed above.
1mM SDBS 1 mM SDBS + .250 2+ mM Cu
1.2
1 mM SDBS + 0.50 2+ mM Cu
1
1 mM SDBS + 0.625 2+ mM Cu 0.8
3.1. Equilibrium surface tension results The equilibrium surface tension data (Fig. 2) showed that the addition of either Cu(bpy)2(BF4)2 or CuCl2 to water did not reduce
350
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 3. Intensity of sodium emission from an SDBS solution in the presence of Cu2+ at various concentrations.
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0.02 mM SDBS
Aerosol Concentration (mM)
Aerosol Concentration (mM)
0.25
0.2
0.15
0.1
0.05 DBSCu2+
0.02 mM SDBS
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0.05 DBSCu(bpy)22+
0
0 0
0.02
0.04
0.06
0.08
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0.14
0
0.02
Bulk Cu2+ Concentration (mM)
0.06
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0.04 mM SDBS
Aerosol Concentration (mM)
Aerosol Concentration (mM)
0.25
0.04
0.20
0.15
0.10
0.05
0.08
0.1
0.12
0.14
Bulk Cu(bpy)22+ Concentration (mM) 0.04 mM SDBS
0.2
0.15
0.1
0.05
DBSCu2+
DBSCu(bpy)22+
0.00
0
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.02
0
Bulk Cu2+ Concentration (mM)
0.04
0.06 2+
Bulk Cu(bpy)2
0.08
0.1
0.12
0.14
Concentration (mM)
Fig. 4. Concentration of surfactant anion, DBS (closed symbols) and counterions, Cu2+ or ðCuðbpyÞ2þ 2 Þ (open symbols) in the aerosol as a function of the counterion concentration in the bulk.
3.3. Effect of counterion on the interfacial composition The behaviour of both Cu2+ and ðCuðbpyÞ2þ 2 Þ counterions were then examined using acoustic nebulisation. The efficiency of nebulisation (the formation of maximum amount of aerosol droplets) is the highest at frequencies greater than 1 MHz, hence these experiments were conducted at 1062 kHz. Fig. 4 shows the concentrations of the surfactant and the counterions in the nebulised droplets. In the absence of any other counterions, nebulisation of 0.02 and 0.04 mM SDBS solutions produced aerosols containing 0.033 and 0.058 mM concentrations of SDBS, respectively. Such enrichment of a surface active solute in an aerosol has been reported in our prior work [16] and that of other workers [25,26]. The extent of enrichment can be related to the rate of mass transfer of surfactant to the air–liquid interface. 2+ The addition of small amounts of ðCuðbpyÞ2þ counteri2 Þ or Cu ons enhances the DBS concentration in the aerosol significantly, with more pronounced effects for higher SDBS concentration. In all cases, the concentration of both Cu2+ and ðCuðbpyÞ2þ 2 Þ are higher in the aerosol than in the bulk solution. The cations themselves are not surface active as shown in Fig. 2. Thus the observed increase in 2+ concentration indicates that ðCuðbpyÞ2þ ions directly 2 Þ and Cu associate with the adsorbed surfactant layer.
3.4. Size distribution of nebulised droplets To clarify the differences between the two copper counterions, the surface excess concentrations (C) for DBS, ðCuðbpyÞ2þ 2 Þ and Cu2+ were estimated based on the total surface area available in the aerosol [25]:
P ðC A C B Þ i ni 43 pr 3i P C¼ 2 i ni 4pr i
ð1Þ
where CA is the measured concentration of solute in the aerosol (Fig. 4) and CB that in the bulk while ni represent the number fraction of droplets of radius ri. This calculation requires knowledge of the size distribution of aerosol droplets which is given in Fig. 5. Most droplets are less than 10 lm in diameter with a relatively narrow size distribution that is independent of surfactant concentration.
18 15
Number Percentage
greatest at 200–400 kHz [24], these measurements were carried out at 358 kHz. It can be seen in Fig. 3 that the Na⁄ emission at 590 nm is a maximum in the absence of Cu2+. As the Cu2+ concentration is increased, the Na⁄ emission intensity decreases indicating the replacement of the interfacial Na+ ions by the Cu2+. It can also been seen in this figure that Na⁄ emission is not completely eliminated in the presence of Cu2+ indicating the presence of small amount of Na+ ions at the bubble/solution interface.
12 9 6 3 0 0
5
10
15
20
Diameter µm Fig. 5. Distribution of droplet diameters based on their number percentage.
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Surface Excess Concentration x 10 2 (mol/m )
7
0.02 mM SDBS
2.50 2.00 1.50 1.00 0.50
DBSCu2+
Surface Excess Concentration x 107 (mol/m2)
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3.00
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Bulk Cu
3.0
0.08
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7
Surface Excess Concentration x 10 2 (mol/m )
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Bulk Cu
0.5 DBSCu(bpy)22+
0
0.02
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2.0
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2.5
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0.04 mM SDBS
0
2.0
Concentration (mM)
0.12
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Surface Excess Concentration x 107 (mol/m2)
0.02
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0.04 mM SDBS
2.5 2.0 1.5 1.0 0.5 DBSCu(bpy)22+
0.0 0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
Bulk Cu(bpy)22+ Concentration (mM)
Concentration (mM)
Fig. 6. Comparison of surface excess concentrations of surfactant ion (DBS, shown as closed symbols) and counterions (Cu2+ and ðCuðbpyÞ2þ 2 Þ, shown as open symbols).
3.6. Surface concentration of counterions and surfactant ion The calculated surface excess concentration of DBS and respective counterions are shown in Fig. 6. In the absence of any copper cations, the surface excess of the DBS anion is 0.2 107 mol/m2 at 0.02 mM bulk concentration and 0.4 107 at 0.04 mM bulk concentration. This surface excess is considerably lower than the steady state equilibrium values of 1.4 107 and 2.8 107 mol/ m2, reflecting the mass transfer limitations under the dynamic environment of the acoustic field. The DBS surface concentration increase significantly in the 2+ presence of the ðCuðbpyÞ2þ counterions. This shows that 2 Þ and Cu these cations are both effective in increasing the adsorption of DBS to the interface and this increase the driving force for mass transfer to the interface. However, there are significant differences in the behaviour between the two cations. At the 0.02 mM DBS bulk concentration, the presence of ðCuðbpyÞ2þ 2 Þ results in greater surface enrichment of both cation and anion, with values 30–40% above that with the Cu2+ cation. This strong adsorption reflects the greater binding ability of the ðCuðbpyÞ2þ 2 Þ to anions as has already been discussed. However, there is also a much more prominent plateau in the surface concentrations for this cation, with the concentration seemingly limited to about 0.6 and 1.4 107 mol/ m2 for the DBS and ðCuðbpyÞ2þ 2 Þ ions, respectively, in spite of increasing bulk cation concentrations. A plateau is also evident in the Cu2+ data at 0.02 mM SDBS, but it is less pronounced. This leveling off in surface excess concentrations is representative of the limits in the ability of the species to transfer to the interface while it is rapidly oscillating and nebulising. The plateau values correspond to 0.36 mols of Cu2+ and 0.39 mols of ðCuðbpyÞ2þ 2 Þ to each mol of DBS. Hence in both cases, there are still some sodium ions within the electrical double layer, representing 28% and 22% of the total counterbalancing charge. The data for ðCuðbpyÞ2þ 2 Þ in the presence of a bulk concentration of 0.04 mM DBS-, changes only marginally. The peak values are little changed, perhaps increasing marginally to 0.7 and
1.7 107 mol/m2 respectively. Conversely, at these greater anion concentrations, surface concentrations for the Cu2+ system are notably increased, with peak values doubling, in line with the doubling of bulk concentrations. Thus at this higher bulk anion concentration, the Cu2+ concentration is higher at the interface than ðCuðbpyÞ2þ 2 Þ. The differences between the results at 0.02 mM and 0.04 mM probably relate to mass transfer limitations associated with the counter ion. That is, while ðCuðbpyÞ2þ 2 Þ has a greater affinity for the interface, it is much larger and bulkier and hence cannot move quickly enough to adhere to the rapidly oscillating interface. The ratios of counterion to anion are little changed from the 0.02 mM case, correspond to 0.33 mols of Cu2+ and 0.42 mols of ðCuðbpyÞ2þ 2 Þ to each mol of DBS . Further, the peak concentrations of DBS are still well below that observed under steady-state conditions (2.2 and 3.4 107 mol/m2 for 0.02 and 0.04 mM SDBS respectively). These results again support the hypothesis that the changes are related to mass transfer limitations of the surfactant and counterions rather than surface chemistry.
4. Conclusion These results show that copper cations are effective in increasing the surface concentrations of a surfactant anion even under the mass transfer limiting conditions of acoustic nebulisation. In addition to the mobility of the cations, the difference in ability of surface adsorption may also play some role in the overall nebulisation process. The SL spectra showing the exited state sodium atoms provide direct evidence of the replacement of sodium cations with such species. Under the conditions of acoustic nebulisation, the greater binding ability of ðCuðbpyÞ2þ 2 Þ means this is most effective at low bulk anion and cation concentrations. However, the impact is limited as bulk concentrations increase. In particular, when the bulk anion concentration is increased from 0.02 mM to 0.04 mM, the Cu2+ cation becomes more effective than ðCuðbpyÞ2þ 2 Þ due to the smaller size and hence greater mobility of this species. These
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results show that under such dynamic conditions mass transfer effects cannot be ignored. Acknowledgement We thank the Australian Research Council for the funding through ARC Discovery Grant DP0771094. References [1] W. Bu, D. Vaknin, A. Travesset, Monovalent counterion distributions at highly charged water interfaces: proton-transfer and Poisson-Boltzmann theory, Phys. Rev. E 72 (2005) 060501. [2] P. Koelsch, H. Motschmann, Varying the counterions at a charged interface, Langmuir 21 (2005) 3436–3442. [3] J. Pittler, W. Bu, D. Vaknin, A. Travesset, D.J. McGillivray, M. Losche, Charge inversion at minute electrolyte concentrations, Phys. Rev. Lett. 97 (2006) 046102. [4] C.J. Drummond, F. Grieser, A study of competitive counterion binding to micelles using the acid-catalyzed reaction of hydrogen peroxide with iodide ions, J. Coll. Interface Sci. 127 (1989) 281–291. [5] P. Paton-Morales, F.I. Talens-Alesson, Effect of competitive adsorption of Zn2+ on the flocculation of lauryl sulfate micelles by Al3+, Langmuir 18 (2002) 8295– 8301. [6] F.M. Doyle, Ion flotation – its potential for hydrometallurgical operations, Int. J. Miner. Process 72 (2003) 387–399. [7] R.B. Grieves, R.N. Kyle, Models for Interaction between ionic surfactants and nonsurface-active ions in foam fractionation processes, Separ. Sci. Technol. 17 (1982) 465–483. [8] J.C. Schulz, G.G. Warr, Selective adsorption of metal cations onto AOT and dodecyl sulfate films at the air/solution interface, J. Chem. Soc. Faraday Trans. 94 (1998) 253–257. [9] A.W. Cross, G.G. Jayson, The effect of small quantities of calcium on the adsorption of sodium dodecyl sulfate and calcium at the gas-liquid interface, J. Coll. Interface Sci. 162 (1994) 45–51. [10] S.G. Oh, D.O. Shah, Effect of counterions on the interfacial tension and emulsion droplet size in the oil/water/dodecyl sulfate system, J. Phys. Chem. 97 (1993) 284–286.
[11] I. Weil, Surface concentration and the Gibbs adsorption law. The effect of the alkali metal cations onsurface behaviour, J. Phys. Chem. 70 (1966) 133–140. [12] P.A. Kralchevsky, K.D. Danov, G. Broze, A. Mehreteab, Thermodynamics of ionic surfactant adsorption with account for the counterion binding: effect of salts of various valency, Langmuir 15 (1999) 2351–2365. [13] J. Lee, S.E. Kentish, M. Ashokkumar, The effect of surface-active solutes on bubble coalescence in the presence of ultrasound, J. Phys. Chem. B 109 (2005) 5095–5099. [14] J.Z. Sostaric, P. Riesz, Sonochemistry of surfactants in aqueous solutions: an EPR spin-trapping study, J. Am. Chem. Soc. 123 (2001) 11010–11019. [15] J.Z. Sostaric, P. Riesz, Adsorption of surfactants at the gas/solution interface of cavitation Bubbles: an ultrasound intensity-independent frequency effect in sonochemistry, J. Phys. Chem. B 106 (2002) 12537–12548. [16] B. Jimmy, S. Kentish, F. Grieser, M. Ashokkumar, Ultrasonic nebulization in aqueous solutions and the role of interfacial adsorption dynamics in surfactant enrichment, Langmuir 24 (2008) 10133–10137. [17] M.S. Mohan, D. Bancroft, E.H. Abbott, Thermodynamic study of the formation of some mixed-ligand complexes of copper (II), Inorg. Chem. 18 (1979) 344– 346. [18] D. Rong, F.C. Anson, Severe inhibition of the diffusion and electroactivity of Ò ðCuðbpyÞ2þ coatings on electrodes, J. 2 Þ complexes incorporated in Nafion Electroanal. Chem. 404 (1996) 171–177. [19] J. Aronovitch, D. Godinger, A. Samuni, Ascorbic acid oxidation and DNA scission catalyzed by iron and copper chelates, Free Rad. Res. Com. 2 (1987) 241–258. [20] Z.S. Yang, Y.L. Wang, Y.Z. Zhang, Electrochemically induced DNA cleavage by copper-bipyridyl complex, Electrochem. Commun. 6 (2004) 158–163. [21] D. Myers, Surfactant Science and Technology, second ed., VCH Publishers, New York, 1992. [22] D. J. Shaw, Colloid & surface Chemistry, fourth ed., Elsevier Science Ltd, 1992. [23] D. Sunartio, K. Yasui, T. Tuziuti, T. Kozuka, Y. Iida, M. Ashokkumar, F. Grieser, Correlation between Na⁄ emission, and chemically active acoustic cavitation bubbles, Chem. Phys. Chem 8 (2007) 2331–2335. [24] P. Kanthale, M. Ashokkumar, F. Grieser, Sonoluminescence, sonochemistry (H2O2 yield) and bubble dynamics: frequency and power effects, Ultrason. Sonochem. 15 (2008) 143–150. [25] D.N. Rassokhin, J. Phys, Chem. B 102 (1998) 4337–4341. [26] H. Takaya, S. Nii, F. Kawaizumi, K. Takahashi, Enrichment of surfactant from its aqueous solution using ultrasonic atomization, Ultrason. Sonochem. 12 (2005) 483–487.