Sodium Atom Emission from Aqueous Surfactant Solutions Exposed to

Article pubs.acs.org/Langmuir

Sodium Atom Emission from Aqueous Surfactant Solutions Exposed to Ultrasound Joe Z. Sostaric, Muthupandian Ashokkumar, and Franz Grieser* Particulate Fluids Processing Centre, School of Chemistry, University of Melbourne, Parkville, 3010 Victoria, Australia S Supporting Information *

ABSTRACT: Emission from electronically excited sodium atoms (Na*) was observed when argon saturated aqueous solutions of the anionic surfactants, sodium dodecyl sulfate, sodium octyl sulfate, sodium 1-pentanesulfonate, and sodium 1-octanesulfonate were sonicated using 358 kHz ultrasound. The same emission band, centered at about 590 nm, was also obtained in aqueous NaCl solutions, although a ∼100-fold higher concentration than that used for the surfactant solutions was required to obtain an emission of comparable intensity. The results have been interpreted in terms of the surfactant adsorbing at the gas−solution interface of the bubbles generated by the ultrasound, generating an electrostatic surface potential, and attracting Na+ counterions to the bubble surface. It is reasoned that Na+ ions are simultaneously reduced and electronically excited at the bubble−solution interface during the final stages of the collapse phase of the acoustically driven bubble. It is proposed that sodium ion bound water molecules reduce interfacial Na+ under the extreme, perhaps supercritical, conditions the interface experiences on bubble implosion.

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INTRODUCTION It is now well established that sonochemistry comes about from the collapse of microbubbles in a liquid when it is exposed to ultrasound. Ultrasound can induce cavitation bubbles to oscillate in size and to inertially collapse, thereby generating localized hot-spots with temperatures, in water for example, in excess of 4000 °C and internal pressures of several hundred atmospheres.1−3 These are extreme conditions, albeit shortlived, being on a time scale of less than a microsecond. Sonochemical reactions, in general, can occur within a cavitation bubble, at the cavitation bubble−solution interface, or in bulk solution following the formation of reactive species within a cavitation “hot-spot”.4 Of course, depending on the particular reaction being considered, more than one domain may be involved. It is also possible to initiate chemical reactions from the dynamic shear forces generated by violently collapsing bubbles; this can be particularly significant in polymer systems. However, it is accurate to say in the vast majority of sonicated systems the latter process is more conducive for convective stirring of the solution than for initiating sonochemical reactions. It has long been known that metal atom emission can be produced during the sonication of aqueous and nonaqueous alkali metal salt solutions, and there has been considerable speculation over the mechanism for the process.5−8 However, a convincing assignment of the actual reaction steps involved has yet to be made. The location of where the excited metal atom is produced in a sonochemical system has also not been clearly resolved, © XXXX American Chemical Society

although its formation in bulk solution can be considered highly unlikely for several reasons. Experimental results on excited metal atom emission have been interpreted so as to locate the production site as being either in a hot-shell at the bubble− solution interface, or within the hot-spot core of a collapsed bubble as a consequence of the incorporation of nanodroplets of solution prior to or at the onset of the bubble’s collapse.6−9 Although never raised, it could well be that both sites provide the conditions that are required for excited state metal atom production. The present study examines the emission from electronically excited sodium atoms (Na*) in aqueous anionic surfactant solutions. Charged surfactants are able to adsorb to the bubbles generated by ultrasound and in doing so influence the concentration of the counterions of the surfactants, and also of the ions of added salts, around the bubble−solution interface. Our aim was to make use of this feature of anionic surfactants to help in developing an understanding of the mechanism involved in the sonochemical formation of Na*.

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EXPERIMENTAL SECTION

Materials. The surfactants were of the purest grade that could be purchased and were used as supplied. Sodium 1-pentanesulfonate Special Issue: Tribute to Toyoki Kunitake, Pioneer in Molecular Assembly Received: April 29, 2016 Revised: June 7, 2016

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Langmuir (SPSo) was supplied by Tokyo Kasei. Sodium 1-octanesulfonate (SOSo, 98%) was supplied by Aldrich. Sodium octyl sulfate (SOS) was obtained from Merck and sodium dodecyl sulfate (SDS, especially pure, > 99%) was obtained from BDH chemicals. The nonionic surfactant, octa-ethylene glycol mono n-decyl ether (C10E8) was supplied by Nikko Chemicals. Dodecyltrimethylammonium chloride (DTAC), a cationic surfactant, was obtained from Kodak chemicals. The zwitterionic surfactant, n-dodecyl-N,N-dimethyl-3-ammonio-1propanesulfonate (C12APS) had a purity of ≥99% and was purchased from Calbiochem. Lithium chloride (99%) was obtained from the Aldrich Chemical Co. Inc. Zinc sulfate (ZnSO4.7H2O) and sodium chloride were obtained from BDH Chemicals and both were of A.R. grade. Ultra high purity argon was obtained from BOC gases. Water was acquired from a three stage ’Milli-Q’ purification system with a conductivity of less than 10−6 S cm−1 and a surface tension of 72.0 mN m−1 at 25 °C. All the glassware used in the experiments was thoroughly cleaned prior to use. It was first soaked in an alkaline detergent for at least 2 h, rinsed with hot tap water and then with Milli-Q filtered water. The glassware was then soaked in hot nitric acid (70% v/v) for an hour and was then rinsed thoroughly with Milli-Q filtered water. All of the solutions were prepared using Milli-Q filtered water, as described below. Salt and surfactant solutions were prepared simply by adding the required amount of the chemical in Milli-Q water to produce a 200 mL solution. If required, the salts were dried prior to weighing to ensure that any adsorbed water was at a minimum and the surfactants were always kept in a dry environment. Sonication procedure. The sonication of 200 mL aqueous solutions of varying surfactant and salt compositions was conducted using an ultrasonic transducer (model USW 51-52, flat plate diameter = 5.45 cm) operating at a frequency of 358 kHz and an intensity of 1.3 W/cm2 calculated calorimetrically as previously described.10 The transducer was powered by an RF generator (model LVG 60 A) both of which were obtained from Allied Signal (ELAC Nautik). The sonication vessel was a quartz cylinder that had an internal diameter of 5.5 cm and an internal height of 10 cm. The 200 mL solutions were bubbled with argon gas for a total time of 60 min prior to sonication and the gas was passed over the solution during sonication, at atmospheric pressure. The solutions were held at a constant temperature (16.9 ± 0.3 °C) by passing a steady stream of cooled water through a water jacket that surrounded approximately 70% of the vessel, thereby leaving a quartz window through which emission spectra could be gathered. A Hitachi fluorescence spectrophotometer (model F-4500), with the slit width set at 20 nm, was used to record the sonoluminescence spectrum generated during sonication. A reference spectrum was taken prior to each run and subtracted from the emission spectra in order to remove any background effects.

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Figure 1. Sonoluminescence spectrum obtained when argon saturated water was sonicated at a frequency of 358 kHz and at a temperature of 16.9 ± 0.3 °C. The intensity is shown in arbitrary units (a.u.).

Figure 2. Sonoluminescence spectra of argon saturated aqueous solutions of C10E8 at concentrations of 0, 0.001, 0.005, 0.01, 0.02, 0.05, 0.1, and 0.5 mM, sonicated at a frequency of 358 kHz and at a temperature of 16.9 ± 0.3 °C. The intensity is shown in arbitrary units (a.u.).

emission from excited OH* species and the broad continuum is either blackbody radiation or bremsstrahlung from the plasma environment created in the bubble on collapse. It is also likely that parts of the continuum are composed of emission from chemically excited species produced by the acoustic hotspot.11,12 The drop off in emission intensity toward the ultraviolet is due to decreasing instrument detection sensitivity for which no correction has been made. The small band at ≈620 is due to the secondary overtone of the emission band at 310 nm. [If a 400 nm cut-off filter was placed in front of the detection slit of the spectrophotometer, the band at 620 nm was eliminated; however, the overall intensity of the emission detected was reduced and it was therefore decided not to use a filter. None of these factors have a bearing on the primary purpose of the study, which was to examine the effect of surfactants on the overall emission and, in particular, the emission from Na*.] The spectra of argon saturated aqueous solutions of NaCl in the concentration range of 0 to 1 M are shown in Figure 3, and the spectra of argon saturated aqueous solutions of sodium dodecyl sulfate in the concentration range of 0 to 10 mM are shown in Figure 4. It can be readily seen, on comparing Figures 3 and 4, that the intensity of the Na* (D bandthis is a doublet but is unresolved with our experimental wavelength resolution conditions) emission centered at ∼590 nm in the SDS

RESULTS

As the main purpose of our study was to examine the formation of Na* in sodium ion (Na+) containing aqueous surfactant solutions, several control experiments were undertaken. For relative comparison purposes, the SL spectra of Milli-Q filtered water saturated with argon, as well as argon saturated aqueous solutions containing various concentrations of a nonionic surfactant (C10E8), at a temperature of 16.9 ± 0.3 °C and sonicated at a frequency of 358 kHz, are shown in Figures 1 and 2, respectively. [SL spectra of cationic (DTAC) and zwitterionic (C12APS) surfactant systems are provide as Supporting Informationthe spectral trends are similar to those in Figure 2.] In both systems, there can be seen a broad wavelength continuum emission that extends from the ultraviolet to the infrared region of the spectrum, onto which is a superimposed band with a maximum at ≈310 nm. Similar observations have been previously made in a number of studies.8,11−13 It is generally accepted that the band at ≈310 nm is due to the B

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relatively strong emission is seen in 10 mM SDS (Figure 4). [SL spectra of cationic and zwitterionic surfactant systems in the presence of NaCl are provided as Supporting Informationthe spectra are similar to that seen in Figure 5. The emission spectra for SPSo, SOSo, and SOS as a function of concentration are also shown in the Supporting Information, and they are similar to the SDS results, but notably over different concentration ranges. This is explained in the Supporting Information, as it is not of direct relevance here.] By correcting for the broad wavelength SL background, the relative change in the Na* emission intensity with changing surfactant concentration can be extracted. This is shown in Figure 6 for SDS and in the Supporting Information for the other surfactants. The observation that a maximum intensity is reached around 5 mM will be discussed in the next section.

Figure 3. Sonoluminescence spectra of argon saturated aqueous solutions of NaCl at concentrations of 0, 2.0, 10, 50, 100, 300, 500, and 1000 mM, sonicated at a frequency of 358 kHz and at a temperature of 16.9 ± 0.3 °C. The intensity is shown in arbitrary units (a.u.).

Figure 6. Effect of SDS concentration on the Na* emission intensity at 590 nm. Argon saturated solutions sonicated at a frequency of 358 kHz and at a temperature of 16.9 ± 0.3 °C.

Figure 4. Sonoluminescence spectra of argon saturated aqueous solutions of SDS at concentrations of 0 (---), 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 8.0, 10, 15, 20, and 50 mM, sonicated at a frequency of 358 kHz and at a temperature of 16.9 ± 0.3 °C. The intensity is shown in arbitrary units (a.u.).

In Figures 7 and 8 are shown the effects of changing Na+ for Li+ and Zn2+, respectively. These experiments were conducted to explore the role of the surfactant counterion, particularly in relation to the surfactant adsorbing to the bubble-solution

solutions is comparable to the NaCl solutions for [Na+] that are as much as 30 to 50 times lower than in the SDS solutions. Also for comparison purposes, the spectra of 0.02 mM C10E8 with 10 mM NaCl present are shown in Figure 5. It can be seen that there is no discernible Na* emission in this latter system, yet a

Figure 7. Sonoluminescence spectra of argon saturated aqueous solutions of SDS (2.0 mM) in the presence of 10 mM added electrolyte. The decrease in the electrolyte ratio of NaCl:LiCl occurs in the order 10:0; 8:2, 6:4; 4:6; 2:8; and 0:10 mM, in the direction of the arrows shown in the figure. The solutions were sonicated at a frequency of 358 kHz and at a temperature of 16.9 ± 0.3 °C. The intensity is shown in arbitrary units (a.u.). For comparison, the sonoluminescence spectra of water (---) and that of (a) 2 mM SDS with no added electrolyte, are also shown.

Figure 5. Sonoluminescence spectra of argon saturated aqueous solutions of C10E8 (0.02 mM) in the absence and presence of 10 mM NaCl, sonicated at a frequency of 358 kHz and at a temperature of 16.9 ± 0.3 °C. The intensity is shown in arbitrary units (a.u.). C

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can be attracted to the bubble interface reach limiting values. This plateau value in the maximum attainable Na* emission intensity occurred in the surfactant concentration range of 10− 20 mM for SPSo and 1−3 mM for SOSo, SOS, and SDS solutions. [The same behavior as seen for the anionic surfactants is observed with sodium alkyl carboxylatesdata shown in the Supporting Information.] In contrast to the above, there is no observable emission from Na* in the presence of the nonionic C10E8 with 10 mM NaCl (Figure 5). (Nor is there any detectable emission in the comparable, cationic DTAC and zwitterionic systems: see Supporting Information.) In addition, it can be seen from Figure 3 that there is no discernible Na* emission from sonicated 10 mM NaCl solutions. This all adds to the already strong conclusion that the Na+ ions that are reduced and excited, stem from ions at the bubble solution interface, and require a relatively high local interfacial concentration of >10 mM. Furthermore, based on the results of Figures 7 and 8, it can be said that Na+ must be directly in contact with the interface. This can be deduced from the observation that the addition of 10 mM Li+ ions to a 2 mM SDS reduces the Na* emission from about 0.5 a.u. to less than 0.1 a.u.. Likewise, the addition of 10 mM of the divalent Zn2+ ion completely eliminates the Na* emission in sonicated 2 mM SDS solution. [Specifically this refers to the data for the NaCl:LiCl ratio of 0:10 and the NaCl:ZnSO4 ratio of 0:10 in Figures 7 and 8, respectively, and the Na* emission spectrum from 2 mM SDS in Figure 4 and Figure S5(a)]. The Li+ ion has about the same binding affinity18 to a surfactant sulfate headgroup at an air− water interface as the Na+ ion, and the Zn2+ ion, because of its higher formal charge, has a much greater electrostatic affinity of binding to surface adsorbed anionic surfactants.18 The above information clearly indicates that the Na+ ions that are reduced to produce excited sodium atoms originate from the interface between the bulk solution and the bubble core. It also suggests that the vast majority, if not all, of the Na+ ions that are involved in producing Na* are those that are bound to the surfactant headgroup at the bubble−solution surface. There are essentially two ways the interface can experience the hot-spot temperature produced on bubble collapse. The first is shown in the stylized representation of the hot-shell model in Figure 9. The second possible pathway is where some of the interface is “pinched-off” at early stages of nonsymmetric bubble collapse to form nanodroplets, which are then heated within the later stages of the collapsing bubble. This mechanism, as already mentioned earlier, has been suggested by a number of groups. However, the results of Figures 7 and 8 would seem to rule out the second possibility. We have previously calculated that the adsorption of SDS at the acoustic bubble−aqueous solution interface gives rise to an electrostatic surface potential (ψsurf) on these bubbles of about −100 mV.20 Based on this potential and considering that the bulk [Na+] in Figures 7 and 8 is 2 mM, for the specific conditions mentioned earlier, the concentration of free Na+ ions in solution immediately in the vicinity of the charged solution−bubble interface can be calculated using the Boltzmann equation ([Na+]surf = [Na+]bulkexp(−ψsurf/25 mV)) to be ∼0.1 M. That is, even though the Li+ and Zn2+ ions, at their highest concentrations, have essentially replaced all surface bound Na+ ions, the solution concentration of free Na+ ions immediately near the interface, that would be expected to make up the nanodroplet produced at the aqueous

Figure 8. Sonoluminescence spectra of argon saturated aqueous solutions of SDS (2.0 mM) in the presence of 10 mM added electrolyte. The decrease in the electrolyte ratio of NaCl:ZnSO4 occurs in the order 10:0; 9:1; 8:2, 6:4; 4:6; 2:8; and 0:10 mM, in the direction of the arrows shown in the figure. The solutions were sonicated at a frequency of 358 kHz and at a temperature of 16.9 ± 0.3 °C. The intensity is shown in arbitrary units (a.u.).

interface, under sonication conditions. This will be developed in the Discussion section later.

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DISCUSSION There are several points that need to be considered concerning the data presented before engaging in a discussion of the actual process, and location, for the formation of electronically excited sodium atoms. It is well-known that surfactants adsorb at the gas−solution interface, and this will also be the case for any bubbles created in a surfactant solution, including acoustic bubbles.14 The presence of these surfactants under hot-spot conditions leads to their decomposition and the incorporation of volatile products within acoustic bubbles, resulting in SL quenching. This is seen in the decrease in the broad wavelength SL emission in all figures with surfactant present. The phenomenon has been observed previously, and details can be found elsewhere.15−17 Also, the preferential accumulation of ionic surfactant molecules at the gas−solution interface of the bubble results in the creation of an electrostatically charged interface, by virtue of the fact that ionic surfactants possess formally charged head groups. The consequence of this surface charge is that the distribution of ions in the solution near the interface will be different from that present in the bulk solution. The observation of an increase in the Na* emission band with increasing anionic surfactant concentration can be explained in terms of surfactant adsorption at the gas−solution interface of the bubble. Since the surfactants in question contain negatively charged sulfate or sulfonate head groups, a negative surface potential will be created at the bubble interface due to the interfacial adsorption of the surfactant. This results in the attraction of the positively charged sodium ions from the bulk solution to the bubble interface. As the interfacial concentration of surfactant is increased, the surface potential also increases and a greater amount of sodium ions will be attracted to the gas−solution interface of the bubble. Eventually a point is reached where the further addition of surfactant to solution no longer influences the Na* emission band. At this point, the amount of surfactant that can be adsorbed at the oscillating bubble interface reaches a maximum. As such, the maximum surface potential and the amount of sodium ions that D

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based on the line width of the D line emission using a high wavelength resolution spectrophotometer.6 This can be compared to the natural lifetime of the D lines of isolated Na* of about 16 ns.28 If it is assumed that the short lifetime determined is a consequence of collisional quenching, then the pseudo first order quenching rate constant k (s−1) (=1/τ) can be equated to k[Q] (s−1). In the later term, [Q] is the quencher concentration and k (M−1 s−1) is the bimolecular quenching constant. The value of k under the conditions of the hot-spot is an unknown; however, it is instructive to take an upper limit for such a bimolecular quenching constant of 1 × 1011 M−1 s−1, which can then be used to calculate a [Q] = 200 M. This value is, of course, not possible, but it does imply a liquid state condition for the quencher. Interestingly, Flint and Suslick using quite a different approach, estimated that the environment from where K* emitted, produced on sonicating alcohol solutions containing K+, was in a condensed phase.6 The identification of the location of where the Na+ is converted into Na* provides an important clue in developing a mechanism for the process. The standard mechanism for forming alkali metal atom emission in a flame photometer is given in Scheme I.29

Figure 9. (Left) Diagrammatic representation of the collapse of a cavitation bubble in aqueous anionic surfactant solutions. (Right) Hypothesized situation at the bubble interface just prior to collapse and following the formation of a hot-spot at the end of collapse. Ion binding to interfacial surfactant head groups and free counterions are not quantitatively depicted. The dimensions of the interfacial hot-shell depicted are, of course, arbitrary, but as a guide, Yasui19 has calculated that a 10 nm thick shell would have a temperature of >500 K for 10 ns for a bubble in water if the maximum core temperature on bubble collapse were ∼3000 K.

Scheme I. Aerosol Dehydration, Vaporization of Salts, and Electronic Activation of Metal Atoms under High Temperature Conditions (>2000 K)

solution−bubble interface, could be expected to give relatively strong Na* emission, when considering the data in Figure 3. Further to this, several research groups21−24 have reported that broad wavelength SL emission comes from a different set of cavitation bubbles than those that produce sonochemistry, such as reactions that yield Na*, and has been interpreted as evidence for the incorporation of nanodroplets into the latter type of cavitation bubbles. The argument is that the nonsonoluminescing bubbles collapse somewhat nonsymmetrically, and this provides the conditions for nanodroplet pinch-off from the interface. However, this need not necessarily be the case, and it is more likely that the collapse of slightly asymmetric bubbles simply produces lower core temperatures, and no SL, than highly symmetric bubble collapse, which produces a relatively higher core temperature and SL. For example, it has been shown for a stably oscillating single bubble that a nonsonoluminescing bubble produces sonochemistry.25 Such single bubbles are unlikely to remain stable if producing nanodroplet injections into the bubble core, but they are still generating core temperatures hot enough to produce sonochemistry. There are also other data in the literature that argue against the droplet injection model in being involved in producing Na*.6 Even though the surface tension of the solution bubble interface can be expected to approach zero as the temperature rises to above 100 °C on bubble collapse, and this would be conducive to droplet formation, the internal pressure concomitantly rises rapidly to several hundred atmospheres. The latter condition can be expected to act against droplets entering the bubble core. Although it was originally reported that no Na* was generated in single bubble SL experiments,26 and this was used to infer that the highly spherical nature of single bubble collapse prevented droplet pinch-off, more recent work has shown that Na* can be produced in single bubble experiments in aqueous NaCl solution.13,27 There is also the observation that the lifetime (τ) of Na* is around 0.05 ps,

Scheme I depicts aerosol droplets containing electrolyte undergoing water evaporation to form the dry salt, NaX, followed by reduction of the Na+ by the anion (X−), and dissociation. Collisional excitation of the metal atom by sufficiently translational energetic species at 2000 K and above produces the electronically excited alkali metal atom (Na*). Since we have ruled out a gas phase process for producing Na* in a collapsing bubble, we propose an alternative set of reactions below: Na +(H 2O)x (bubble interface) → Na +(H 2O)x (hot‐spot interface)

(1)

Na +(H 2O)x (hot‐spot interface) → (Na*(H 2O)x − 1 + H 2O+)

(2)

Na*(H 2O)x − 1 → hv + Na(H 2O)x − 1 E

(3)

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H 2O+ → OH + H+

(4)

Na(H 2O)x − 1 + H 2Obulk → Na + + OH− + H

(5)

H + OH → H 2O

(6)

showing Na* emission. It has been argued that the presence of this band is evidence that Na* forms in the gas phase by collision of gas phase Na* with argon in the gas core of the collapsing bubble. However, on bubble collapse, Yasui31 has calculated that the density of argon reaches around 0.7 g/mL (about half the density of liquid argon). This very high concentration near the hot-spot interface would mean that it is plausible that argon could, by collision, react with Na* at the site where it is formed. Notably, the solubility of argon in supercritical water is higher than that in water, so there is no thermodynamic barrier for argon to exist in the hot-spot shell as it forms.32 In summary, our results have been collectively interpreted to suggest that Na* formation occurs in a condensed phase directly at the hot-spot solution interface and its production is by the reduction of a Na+ ion by water molecules in the ion’s hydration shell. The implication is also that the environment, where the reduction occurs, may well be supercritical water, or an environment having similar properties. We have previously found other systems where this type of environment appears to exist around a collapsing bubble.33 We also note that, even though our study has focused on Na+, the mechanism is, in general, applicable to other alkali metal ions.

The key step to the proposed mechanism above lies with the electron transfer (reaction 2). Here the water of hydration of the Na+ ion partakes in reducing the ion and, in the process, directly creates an electronically excited sodium atom. It identifies the process occurring at the bubble−aqueous solution interface during the hot-spot condition. From the work of Giri and Arakeri,27 the period over which the conditions prevail for the reaction to take place is of the order of 10 to 70 ns, depending on the acoustic power used. The driving force for the reaction is the extreme thermal conditions of the interface at the latter stages of bubble collapse, in principle conditions that are supercritical for water. Under these extreme conditions, the static dielectric constant (εo) of water reduces from 78 at ambient conditions to 2000 K and hundreds of atmospheres30environmental conditions that are thermodynamically unstable for formally charged Na+ ions. Some support for our proposed mechanism comes from the observation that the sonication of aqueous solutions of NaCl, NaBr, and NaI all yield the same amount of Na* emission (see Figure 10). If electron transfer were to occur from the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01633. Emission spectra from sonicated solutions of SPSo, SOSo, and SOS as a function of concentration, and Na* emission spectra from alkyl carboxylic acids at pH = 9 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 10. Sonoluminescence spectra from aqueous solutions of 1.0 M NaCl, NaBr, and NaI. Solutions were argon saturated and sonicated using a frequency of 363 kHz at 21 ± 3 °C. [Note, these spectra were run under different conditions to the other data and the emission intensities are not directly comparable to the other systems.]

ACKNOWLEDGMENTS We thank Thanh Vu for his work associated with the data presented in Figures 10 and S13. This work was supported by the Australian Research Council, through a funding grant to MA and FG.

counterion in these systems to reduce Na+ (see Scheme I), then yields of the Na* emission could be expected to vary based on the electron affinity of the halide ions. [We also obtained the same Na* emission yields when the counterions were NO3− and SO42−; data not shown.] Other mechanisms have been considered for the formation of Na* involving the reduction of Na+ by radicals formed in the hot spot, and also by 3 body collisions between radicals and already formed Na atoms (via Scheme I), all in the gas phase.6,7 As our results strongly suggest that the site for the formation of Na* occurs directly at the bubble−solution interface, in a condensed phase, these other reactions are unlikely to be responsible for the production of excited sodium atoms in acoustic bubbles. An additional comment that can be made concerns the formation of the exciplex (Na.Ar)*, for which emission is seen as a relatively small band at about 560 nm in all the spectra

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REFERENCES

(1) Didenko, Y. T.; McNamara, W. W.B., III; Suslick, K. S. Hot spot conditions during cavitation in water. J. Am. Chem. Soc. 1999, 121, 5817−5818. (2) Rae, J.; Ashokkumar, M.; Eulaerts, O.; von Sonntag, C.; Reisse, J.; Grieser, F. Estimation of ultrasound induced cavitation bubble temperatures in aqueous solutions. Ultrason. Sonochem. 2005, 12, 325−329. (3) Okitsu, K.; Suzuki, T.; Takenaka, N.; Bandow, H.; Nishimura, R.; Maeda, Y. Acoustic Multibubble Cavitation in Water. J. Phys. Chem. B 2006, 110, 20081−20084. (4) Suslick, K. S.; Hammerton, D. A.; Cline, R. E. The sonochemical hot spot. J. Am. Chem. Soc. 1986, 108, 5641−5642. (5) Sehgal, C.; Steer, R. P.; Sutherland, R. G.; Verrall, R. E. Sonoluminescence of argon saturated alkali metal salt soultions as a probe of acoustic cavitation. J. Chem. Phys. 1979, 70, 2242−2248.

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DOI: 10.1021/acs.langmuir.6b01633 Langmuir XXXX, XXX, XXX−XXX

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Langmuir (6) Flint, E. B.; Suslick, K. S. Sonoluminescence from alkali-metal salt solutions. J. Phys. Chem. 1991, 95, 1484−1488. (7) Lepoint-Mullie, F.; Voglet, N.; Lepoint, T.; Avni, R. Evidence for the emission of ‘alkali-metal-noble-gas’ van der Waals molecules from cavitation bubbles. Ultrason. Sonochem. 2001, 8, 151−158. (8) Choi, P.-K.; Abe, S.; Hayashi, Y. Sonoluminescence of Na atom from NaCl solutions doped with ethanol. J. Phys. Chem. B 2008, 112, 918−922. (9) Flannigan, D. J.; Suslick, K. S. Emission from electronically excited metal atoms during single-bubble sonoluminescence. Phys. Rev. Lett. 2007, 99, 134301−4. (10) Sostaric, J. Z.; Mulvaney, P.; Grieser, F. Sonochemical dissolution of MnO2 colloids. J. Chem. Soc., Faraday Trans. 1995, 91, 2843−2846. (11) Sehgal, C.; Sutherland, R. G.; Verrall, R. E. Optical spectra of sonoluminescence from transient and stable cavitation in water with various gases. J. Phys. Chem. 1980, 84, 388−395. (12) Didenko, Y. T.; Pugach, S. P. Optical spectra of water sonoluminescence. Ultrason. Sonochem. 1994, 1, S9−S12. (13) Schneider, J.; Pflieger, R.; Nikitenko, S. I.; Shchukin, D.; Möhwald, H. Line emission of sodium and hydroxyl radicals in singlebubble sonoluminescence. J. Phys. Chem. A 2011, 115, 136−140. (14) Sunartio, D.; Ashokkumar, M.; Grieser, F. Study of the coalescence of acoustic bubbles as a function of frequency, power, and water-soluble additives. J. Am. Chem. Soc. 2007, 129, 6031−6036. (15) Barbour, K.; Ashokkumar, M.; Caruso, R. A.; Grieser, F. Sonochemistry and sonoluminescence in aqueous AuCl4 − solutions in the presence of surface-active solutes. J. Phys. Chem. B 1999, 103, 9231−9236. (16) Choi, P.-K.; Funayama, K. Multibubble sonoluminescence and Na atom emission in sodium dodecyl sulfate surfactant solutions. Jpn. J. Appl. Phys. 2007, 46, 4768−4770. (17) Sostaric, J. Z.; Riesz, P. Sonochemistry of Surfactants in Aqueous Solutions: An EPR Spin-Trapping Study. J. Am. Chem. Soc. 2001, 123, 11010−11019. (18) Schulz, J. C.; Warr, G. G. Selective adsorption of metal cations onto AOT and dodecyl sulfate films at the air/solution interface. J. Chem. Soc., Faraday Trans. 1998, 94, 253−257. (19) Yasui, K. Variation of liquid temperature at bubble wall near the sonoluminescence threshold. J. Phys. Soc. Jpn. 1996, 65, 2830−2840. (20) Grieser, F.; Ashokkumar, M. The effect of surface active solutes on bubbles exposed to ultrasound. Adv. Colloid Interface Sci. 2001, 89− 90, 423−428. (21) Sunartio, D.; Yasui, K.; Tuziuti, T.; Kozuka, T.; Iida, Y.; Ashokkumar, M.; Grieser, F. Correlation between Na* emission and “chemically active” acoustic cavitation bubbles. ChemPhysChem 2007, 8, 2331−2335. (22) Nakajima, R.; Hayashi, Y.; Choi, P.-K. Mechanism of two types of Na emission observed in sonoluminescence. Jpn. J. Appl. Phys. 2015, 54, 07HE02. (23) Hayashi, Y.; Choi, P.-K. Two components of Na emission in sonoluminescence spectrum from surfactant aqueous solutions. Ultrason. Sonochem. 2015, 23, 333−338. (24) Xu, H.; Eddingsaas, N. C.; Suslick, K. S. Spatial separation of cavitating bubble populations: The nanodroplet injection model. J. Am. Chem. Soc. 2009, 131, 6060−6061. (25) Lepoint, T.; Lepoint-Mullie, F.; Henglein, A. Single bubble sonochemistry. In Crum, L. A., Mason, T. J., Reisse, J. L., Suslick, K. S., Eds.; Sonoluminescence and Sonochemistry; NATO ASI Series, 524; Kluwer: Dordrecht, 1999; pp 285−290. (26) Matula, T. J.; Roy, R. A.; Mourad, P. D.; McNamara, W. B., III; Suslick, K. S. Comparison of multibubble and single-bubble and sonoluminescence spectra. Phys. Rev. Lett. 1995, 75, 2602−2605. (27) Giri, A.; Arakeri, V. H. Measured pulse width of sonoluminescence flashes in the form of resonance radiation. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1998, 58, R2713−R2716.

(28) Volz, U.; Majerus, M.; Liebel, H.; Schmitt, A.; Schmoranzer, H. Precision lifetime measurements on NaI 3p 2P1/2 and 3p 2P3/2 by beam-gas-laser spectroscopy. Phys. Rev. Lett. 1996, 76, 2862−2865. (29) Ramírez-Muñoz, J. Atomic-absorption spectroscopy; Elsevier: Amsterdam, 1968. (30) Pan, D.; Spanu, L.; Harrison, B.; Sverjensky, D. A.; Galli, G. Dielectric properties of water under extreme conditions and transport of carbonates in the deep Earth. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 6646−6650. (31) Yasui, K. Alternative model of single-bubble sonoluminescence. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1997, 56, 6750−6760. (32) Guillot, B.; Guissani, Y. A computer simulation study of the temperature dependence of the hydrophobic hydration. J. Chem. Phys. 1993, 99, 8075−8094. (33) Ashokkumar, M.; Grieser, F. Proton transfer between organic acids and bases at the acoustic bubble-aqueous solution interface. J. Phys. Chem. B 2005, 109, 19356−19359.

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DOI: 10.1021/acs.langmuir.6b01633 Langmuir XXXX, XXX, XXX−XXX