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J. Phys. Chem. B 2008, 112, 852-858
Effect of Ultrasound Frequency on Pulsed Sonolytic Degradation of Octylbenzene Sulfonic Acid Limei Yang, Joe Z. Sostaric,*,† James F. Rathman,‡ and Linda K. Weavers* Department of CiVil and EnVironmental Engineering and Geodetic Science, The Ohio State UniVersity, Columbus, Ohio 43210 ReceiVed: September 17, 2007; In Final Form: October 19, 2007
It has been shown that pulsed ultrasound can influence the amount of surfactant that can adsorb to and decompose at the surface of cavitation bubbles. However, the effect of ultrasound frequency on this process has not been considered. The current study investigates the effect of ultrasound frequency on the pulsed sonolytic degradation of octyl benzenesulfonate (OBS). Furthermore, the effect of pulsing and ultrasound frequency on the rate of •OH radical formation was determined. OBS degradation rates were compared to the rates of •OH radical formation. In this way, conclusions were made regarding the relative importance of accumulation of OBS at cavitation bubble surfaces versus sonochemical activity to the sonochemical decomposition of OBS under different conditions of sonolysis. Comparisons of the data in this way indicate that sonolytic degradation of OBS depends on both the sonochemical activity (i.e., •OH yield) and the accumulation of OBS on cavitation bubble surfaces. However, under a certain set of pulsing and ultrasound frequency exposure conditions, enhanced accumulation of OBS at the gas/solution interface of cavitation bubbles is the sole mechanism of enhanced degradation due to pulsing. On the basis of this finding, conclusions on how pulsing at various ultrasound frequencies affects cavitation bubbles were made.
Introduction Changes in certain parameters during sonolysis, such as the physicochemical properties of the solution, the ambient temperature and pressure, the type of saturating gas, and the frequency and intensity of ultrasound, result in significant changes in the cavitation phenomenon.1,2 Over the past decade or so, there has been growing interest in the effect of ultrasound frequency on acoustic cavitation and the associated sonochemistry. The effects of ultrasound frequency on sonochemical systems has been difficult to elucidate because other parameters, such as the ultrasound intensity, inherently affect acoustic cavitation and sonochemistry to different extents when the frequency is changed.3,4 Ultrasound frequency effects are further complicated by other factors, for example, the physicochemical properties of target compounds and participation of the saturating gas in chemical reactions. In order to address the quantitative problems associated with varying the ultrasound frequency in sonochemical systems, it has become commonplace to conduct sonochemical research at a constant calorimetric power input to the solution at different frequencies. On the basis of constant calorimetric power input to the system, sonochemical degradation of nonvolatile hydrophilic compounds has been shown to occur more rapidly at higher frequencies (487 kHz,5 500 kHz,6 900 kHz7) than at the lower frequency of 20 kHz. Beckett and Hua found that the degradation of a polar volatile solute, 1,4-dioxane, was faster at 358 kHz than at 205, 618, or 1071 kHz and attributed this to the optimal •OH radical yield and bubble lifetimes at 358 kHz * Corresponding authors. E-mail:
[email protected] (L.K.W.) and
[email protected] (J.Z.S.). Phone: 614-292-4061. Fax: 614-292-3780. † Also at Center for Biomedical EPR Spectroscopy and Imaging, Davis Heart and Lung Research Institute, College of Medicine. ‡ Also at Department of Chemical and Biomolecular Engineering.
allowing mass transfer of reactive species in to and out of the bubble.8 However, Hung and Hoffmann investigated the degradation of carbon tetrachloride over a similar frequency range and observed the highest degradation rate at 500 kHz.9 Finally, Pe´trier and Francony compared the reaction rates of phenol and carbon tetrachloride in aqueous solutions at 20, 200, 500, and 800 kHz.10 The results showed that phenol degradation reached a maximum at 200 kHz, while the degradation rate of carbon tetrachloride, which decomposes inside the bubble, increased with increasing frequency. These results indicate that frequency effects are dependent on the nature of the molecules and their localization in the interior of the cavitation bubbles or on their surface. Nonvolatile surfactants have been shown to accumulate at the gas/solution interface of cavitation bubbles. The main pathway of degradation of nonvolatile surfactants is through oxidation by •H atoms and •OH radicals.11 Pyrolysis products due to decomposition of surfactant in the hot interfacial region of collapsing bubbles12 are also observed, the extent of which depends on the nature of the surfactant11 and the saturating gas type.13 It has been shown that the radical scavenging ability of the homologous series of n-alkyl alcohols (methanol to npentanol) depends on the equilibrium surface excess of the alcohols.14 However, it has been shown that surfactant effects in sonoluminescence,15a sonochemistry,4,11,16,17 and most recently in acoustic cavitation18 studies can also depend on the dynamic ability of surfactants to accumulate at the gas/solution interface of cavitation bubbles. Currently, there is a limited understanding of the fundamental mechanism of frequency effects on the accumulation of surfactants on cavitation bubble surfaces and subsequent degradation rates of surfactants, especially when changing the sonication mode (continuous wave or pulsing). On the basis of constant calorimetric power input to the system, our previous work showed that sonolysis at a frequency
10.1021/jp077482m CCC: $40.75 © 2008 American Chemical Society Published on Web 12/18/2007
Degradation of Octylbenzene Sulfonic Acid of 354 kHz resulted in faster degradation of surfactants than sonolysis at a lower frequency of 20 kHz under continuous wave (CW) exposure.19 Under CW, spin-trapping and electron paramagnetic resonance (EPR) spectroscopic studies have shown that, at higher ultrasound frequencies, the rate of change of surface area of the gas/solution interface of cavitation bubbles decreases because of smaller bubble sizes and oscillation amplitudes.4 Therefore, the thermodynamic properties of the surfactant become more important for adsorption at higher frequencies than at lower frequencies where kinetic adsorption ability prevails.4 This effect arises because the time required for equilibrium adsorption of surfactants under CW is significantly longer than the lifetime of the gas/solution interface of the bubbles and/or the bubble lifetime, resulting in nonequilibrium adsorption and insufficient accumulation of surfactants on cavitation bubble surfaces.4,11 For the same power input to the system, it was shown that pulsed ultrasound enhanced the degradation of surfactants at 354 kHz compared to CW, leading us to conclude that pulse intervals allow more time for surfactants to adsorb to cavitation bubble surfaces.16,20 The frequency of ultrasound is known to control various characteristics of cavitation bubbles, including the resonance and maximum radius of bubbles.21a Therefore, it would be expected that the pulsing effects observed previously in relation to the accumulation of surfactants at the gas/solution surface of cavitation bubbles17 would also depend on the ultrasound frequency. For example, smaller bubbles would be more prone to dissolution through a higher Laplace pressure during pulse intervals.4,17 The sonochemical decomposition of millimolar concentrations of surfactants in aqueous solution occurs at the surface of cavitation bubbles.22,23 Therefore, it would be expected that the degradation rate of OBS will depend on the interfacial concentration of OBS at the bubble surface and on the sonochemical activity of the system under different pulsing and ultrasound frequency conditions. The degradation rates of OBS and the rate of •OH radical formation (a measure of the sonochemical activity) are determined herein under different sonication modes and as a function of ultrasound frequency. With this information the effect of ultrasound frequency and sonication mode on the mechanism of OBS degradation is elucidated under identical exposure conditions. By using this comparative method,3,4,6 the effect of ultrasound frequency and sonication mode on surfactant accumulation at the gas/solution surface of bubbles are determined, even though a number of parameters, including the amount of ultrasound energy converted into chemistry by cavitation bubbles cannot be kept constant when the frequency is changed.3,4 Experimental Details Materials. The surfactant octylbenzene sulfonic acid (OBS) was purchased from Aldrich with a purity of 97%. Water used in all experiments was obtained from a Milli-Q water purification system operating at R ) 18.2 MΩ cm. Terephthalic acid (99%) was purchased from Fluka. Hydroxyterephthalic acid (HTA) was synthesized according to the method of Field and Engelhardt.24 Sonochemical Experiments. All sonochemical experiments were conducted using geometrically equivalent flat plate-type ultrasonic reactors (model USW 51-52) supplied by ELAC Nautik, Incorporated, Kiel, Germany, operating at various ultrasound frequencies. Continuous or pulsed signals were generated from a SM-1020 function/pulse generator (Signametrics Corporation, Seattle, WA) and amplified by an AG 1021
J. Phys. Chem. B, Vol. 112, No. 3, 2008 853 linear amplifier (T & C Power Conversion, Inc., Rochester, NY). A 100 MHz 54501A digitizing oscilloscope (Hewlett-Packard) was used to detect the pulse signals received by the transducer. The 500 mL solutions were sonicated, and samples were taken at specific times during sonication. The total volume withdrawn during a single run did not exceed 5% of the total sonication volume. The power applied for both CW and pulsed ultrasonic mode was 33 W, as measured by calorimetry. The actual sonication times were the same for continuous and pulsed ultrasound in order to compare degradation of OBS and •OH radical production rates under constant power input at the different exposure modes.16 During sonication under all conditions, a solution temperature of 20 ( 1 °C was maintained using a 1006S Isotemp cooling system (Fisher Scientific, Pittsburgh, PA). Analysis. The concentration of OBS following sonolysis was determined by a Hewlett-Packard 1100 high performance liquid chromatograph (HPLC).16 Hydroxyl radical formation during sonication of aqueous solutions was determined by reaction with terephthalic acid (1 mM). This reaction produced hydroxyterephthalate (HTA), which was detected using a fluorimeter (Shimadzu RF-5301 PC spectrofluorophotometer).25,26 The •OH radical yield was calculated from a standard fluorescence curve for aqueous HTA solutions and on the assumption that TA reacts with •OH radicals in a 1:1 molar ratio. Results Adsorption of surfactants at the surface of cavitation bubbles is limited because of the short lifetime of the gas/solution interface of cavitation bubbles during CW exposure.11,15a,16 This effect has been shown to depend on the frequency of ultrasound.4 We have also shown that, under certain conditions, pulsed ultrasound can result in greater adsorption of surfactants at the surface of cavitation bubbles compared with CW exposure.16,17,20 On the basis of this, the degradation of OBS should depend on the effect of pulsing and ultrasound frequency on sonochemical yields (i.e., •OH radical formation) and on the adsorption of OBS to the surface of cavitation bubbles, as described below. •OH Radical Yield and Degradation of OBS under CW. •OH radical formation (i.e., HTA formation) followed a pseudozero-order rate during sonolysis at all ultrasound frequencies, as shown in Figure 1a. The degradation of OBS followed pseudo-first-order kinetics at all frequencies, as shown in Figure 1b. Figures 2 and 3 show the pseudo-zero-order rate constants of •OH radical formation and the initial degradation rates of OBS (1 mM), respectively, as a function of ultrasound frequency and under various sonication modes, either continuous or pulsed. Note that the initial degradation rate of OBS is defined as kC0, where C0 is the initial concentration of OBS and k is the pseudofirst-order rate constant. Under CW exposure, the •OH radical yield at different frequencies followed the order 354 > 620 > 803 > 206 > 1062 kHz (Figure 2a). On the basis of constant calorimetric power at all frequencies, our results are consistent with those of Beckett and Hua8 which showed that the production of H2O2 resulting from the recombination of •OH radicals is highest at 354 kHz. The initial degradation rates of OBS (Figure 3a) followed a similar trend to that of the rate of •OH radical formation (Figure 2a) under CW exposure, although there was a noticeable difference from 354 to 620 kHz (comparing Figure 2a to Figure 3a). This data suggests that there is not always a correlation between OBS degradation and •OH radical formation rates when the frequency of ultrasound is varied during CW exposure. The effect may be due to changes in the amount of OBS that can
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Figure 1. (a) •OH concentrations measured by HTA formation as a function of sonication time under CW at various frequencies; (b) OBS (1 mM) degradation as a function of sonolysis time under pulse ultrasound (354 kHz) with a pulse interval of 100 ms and a pulse length of 100 ms.
accumulate at the gas/solution surface of cavitation bubbles, since this is known to be affected by the frequency of ultrasound.4 •OH Radical Yield and Degradation of OBS under Pulsed Ultrasound. For pulsed ultrasound with a specified frequency, there is a minimum pulse length (T) that must be exceeded for chemical reactions to occur.27 Our previous work showed that a pulse length of 100 ms gave the maximum pulse enhancement for OBS degradation at 354 kHz.16 Shorter pulse intervals (T0) may limit the amount of surfactant that can adsorb at the gas/ solution surface of bubbles resulting in lower degradation rates of surfactants, whereas cavitation bubbles may dissolve during longer pulse intervals. A pulse interval (T0) of 100 ms was found to provide sufficient time for the most efficient adsorption of OBS to bubble surfaces at 354 kHz.16 Therefore, initially a pulse length of 100 ms and a pulse interval of 100 ms was used, and the effect of ultrasound frequency during pulsing was investigated (Figures 2b and 3b). Subsequently, an equal number of acoustic cycles per pulse was used (Figures 2c and 3c). T ) 100 ms; T0 ) 100 ms. Figures 2b and 3b show the pseudo-zero-order rate constant for •OH radical formation and the initial degradation rates of OBS (1 mM), respectively, as a function of ultrasound frequency and pulsing with T ) 100 ms, T0 ) 100 ms. The trend for •OH radical production under pulsed ultrasound (Figure 2b) was similar to that of CW (Figure 2a), that is, an increase in formation rate from 206 kHz to 354 kHz followed by a decreasing rate in the order 354 > 620 > 803 >
Figure 2. Pseudo zero-order rate constant of •OH radical formation as a function of frequency sonicated under (a) CW, (b) pulsed ultrasound with T ) 100 ms, T0 ) 100 ms, and (c) pulsed ultrasound with T ) (3.54 × 104 cycles/f) ms, T0 ) 100 ms (error bars represent 95% confidence intervals).
1062 kHz. However, unlike the results of CW exposure (Figure 3a), the initial rate of degradation of OBS decreased as the frequency was increased from 206 to 1062 kHz (Figure 3b). Figures 2b and 3b show that pulsing has affected the overall trend for OBS degradation rates as a function of ultrasound frequency but that the •OH radical formation rate follows the same trend with ultrasound frequency, regardless of whether sonolysis is conducted in CW or pulsed mode. To further investigate this observation, the pulsing conditions were changed, as described in the following section. T ) 3.54 × 104 Cycles per Pulse; T0 ) 100 ms. In order to further explore the effects of pulsed ultrasound on the degradation of OBS, a pulse interval of 100 ms was fixed, but pulse lengths (shown in Table 1) were adjusted for each frequency so that each pulse consisted of 3.54 × 104 acoustic cycles. We did this because the work of Henglein and co-workers indicates that approximately 104 acoustic cycles are required for bubbles to become active.28 The pseudo-zero-order rate constants for •OH radical formation and the initial degradation rates of OBS
Degradation of Octylbenzene Sulfonic Acid
J. Phys. Chem. B, Vol. 112, No. 3, 2008 855 those under CW (Figure 3a) or with pulsing at T ) 100 ms, T0 ) 100 ms (Figure 3b). Discussion
Figure 3. Initial degradation rates of 1 mM OBS as a function of frequency sonicated under (a) CW, (b) pulsed ultrasound with T ) 100 ms, T0 ) 100 ms, and (c) pulsed ultrasound with T ) (3.54 × 104 cycles/f) ms, T0 ) 100 ms, respectively (error bars represent 95% confidence intervals).
TABLE 1: Comparison of Parameters at Various Frequencies frequency (kHz)
acoustic cycles (T ) 100 ms)
T (ms) for 3.54 × 104 acoustic cycles
resonance radiusa (µm)
206 354 620 803 1062
2.06 × 104 3.54 × 104 6.20 × 104 8.03 × 104 1.062 × 105
172 100 57 44 33
17.4 10.0 5.8 4.5 3.4
a
Values were calculated as described elsewhere.4
(1 mM) are shown in Figures 2c and 3c. It is clear from Figure 2 that the rate of •OH radical formation continues to follow the same trend as a function of ultrasound frequency regardless of whether the exposure conditions are CW (Figure 2a) or pulsed (Figures 2b and 2c). However, the trend with ultrasound frequency for the initial rate of OBS degradation under the specific pulsing conditions of Figure 3c are not consistent with
The results of the current study clearly show that the trends in •OH radical formation rates observed as a function of ultrasound frequency are similar regardless of the sonication modes investigated, as shown in Figure 2. However, this is not the case for the sonolysis of OBS solutions, where very apparent differences in the trends of OBS degradation rates were observed as a function of ultrasound frequency at the different exposure modes (Figure 3). The results presented in Figures 2 and 3 suggest that the ultrasound frequency has an important effect on the rate of OBS degradation under various pulsing conditions that is not entirely dependent on the sonochemical activity in the system. The effect of ultrasound frequency on •OH formation (Figure 2) and OBS degradation (Figure 3) rates could be occurring for a number of different reasons for the two systems. The rates of •OH radical formation (Figure 2) would be dependent on a number of factors1,2 including the number of bubbles undergoing inertial collapse, the intensity of this collapse, the amount of water vapor inside the bubble just prior to collapse, and the amount of •OH radicals that can escape the interfacial region of the bubble28 to react with TA in the bulk solution. All of these factors depend on the frequency of ultrasound and on the sonication mode. For example, at lower frequencies, fewer cavitation bubbles are present in a given volume, but the collapse of cavitation bubbles would be more violent compared with higher frequencies because of greater resonant bubble sizes and more rapid growth resulting in higher collapse temperatures in the bubble.29 When the ultrasound frequency is increased to the megahertz range, the nucleation of cavitation bubbles decreases because rarefaction and compression cycles are too short to permit substantial expansion of nucleation sites to generate a bubble. In addition, bubbles that grow via rectified diffusion at higher ultrasound frequencies are expected to collapse with lower temperatures because of lower compression ratios and a larger fraction of vaporous bubbles compared with lower frequencies resulting in a less adiabatic collapse.1 The lowest •OH radical yield at the highest frequency of 1062 kHz in Figure 2 supports the ideas discussed above. Thus, there is an optimal frequency for •OH radical yield under CW which is 354 kHz in our system, on the basis of constant calorimetric power under all sonication conditions shown in Figure 2. The rate of degradation of OBS would similarly be affected by the same parameters as those discussed above for •OH radical formation rates, aside from the ability of •OH radicals to escape into the bulk since all •OH radical scavenging reactions would be occurring at the gas/solution interface of the cavitation bubbles at an OBS concentration of 1 mM.16 However, additional parameters may come into play during sonolysis of aqueous OBS solutions. These include the thermodynamic and kinetic adsorption properties of the surfactant at the gas/solution interface of cavitation bubbles4,11 and the effect of adsorption at the gas/solution interface on bubble coalescence.30 Since OBS is an anionic surfactant, there is also a potential for the enhancement of sonochemical degradation due to the minimization of the impedance shielding effect.15b,21b Impedance shielding arises because of the clustering of bubbles during the sonolysis of water. Therefore, bubbles in the middle of the cluster are shielded from the ultrasonic wave and experience an attenuated intensity. The adsorption of ionic surfactants to the surface of cavitation bubbles creates an electrostatic potential at the bubble
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TABLE 2: Initial Degradation Rates of OBS, Pulse Enhancements, and Zero-Order Rate Constants of •OH Formation at Various Frequencies of Sonolysis frequency (kHz) 206
354
620 (103
CW T ) 100 ms, T0 ) 100 ms Ta ) (3.54 × 104 cycles/f ) ms, T0 ) 100 ms CW T ) 100 ms, T0 ) 100 ms Ta ) (3.54 × 104 cycles/f ) ms, T0 ) 100 ms
Initial Degradation Rate of OBS 1.1 ( 0.1 1.75 ( 0.07 4.4 ( 0.1 3.4 ( 0.2 1.6 ( 0.1 (3.4 ( 0.2)b
mM
min-1)
3.0 ( 0.4 2.7 ( 0.2 2.8 ( 0.1
Zero-Order Rate of •OH Formation (µM min-1) 0.43 ( 0.01 0.68 ( 0.01 0.57 ( 0.02 0.52 ( 0.02 0.70 ( 0.03 0.48 ( 0.01 0.43 ( 0.02 (0.70 ( 0.03)b 0.52 ( 0.01
803
1062
1.8 ( 0.1 2.2 ( 0.2 1.5 ( 0.2
1.50 ( 0.04 0.70 ( 0.02 0.20 ( 0.03
0.49 ( 0.02 0.41 ( 0.01 0.41 ( 0.02
0.26 ( 0.01 0.100 ( 0.005 0.080 ( 0.004
T ) 100 ms, T0 ) 100 ms Ta ) (3.54 × 104 cycles/f ) ms, T0 ) 100 ms
Pulse Enhancement OBS (%) (set 1) (set 1) 300 ( 23 94 ( 28 45 ( 15 (94 ( 28)b
(set 2) -10 ( 17 -7 ( 8
(set 2) 22 ( 28 -17 ( 21
(set 3) -53 ( 3 -87 ( 5
T ) 100 ms, T0 ) 100 ms Ta ) (3.54 × 104 cycles/f ) ms, T0 ) 100 ms
Pulse Enhancement •OH (%) 21 ( 5 3(4 0(5 (3 ( 4)b
-16 ( 2 -9 ( 2
-16 ( 2 -16 ( 4
-62 ( 2 -69 ( 2
a
Where f is the frequency of ultrasound in units of kHz. b Equivalent to T ) 100 ms; T0 ) 100 ms.
surface, causing bubbles to repel each other and resulting in a greater number of bubbles being exposed to ultrasound.15b Therefore, it is difficult to gauge what aspects of cavitation had changed in order to produce the results shown in Figures 2 and 3. This is further complicated by the fact that such an analysis of the results depends on the essentially unknown relationship between “constant calorimetric power input” to the system and cavitation activity in relation to sonochemistry.3,4 There is an alternative way of analyzing the results shown in Figures 2 and 3 which would be more in line with the concept of “comparative sonochemistry”, which has been described in detail by a number of researchers.3,4,6 Rather than considering the absolute effect of ultrasound frequency on either •OH radical formation rates or OBS degradation rates at various pulsing conditions, it is of greater value to compare the effect of pulsing on •OH radical formation rates to the OBS degradation rates at a given frequency. For example, at 206 kHz, pulsing with T ) 100 ms and T0 ) 100 ms (Figure 2b) has very little effect on •OH formation rates compared with CW exposure (Figure 2a), yet a dramatic increase is observed in the OBS degradation rates under pulsing with T ) 100 ms and T0 ) 100 ms compared with CW (compare Figure 3a,b at 206 kHz). To more adequately evaluate these relative changes under different frequency and pulsing conditions, pulse enhancements for OBS degradation and •OH radical formation were calculated and compared:16
pulse enhancement (%) )
Rpulsed - RCW × 100 RCW
(1)
where, Rpulsed and RCW are the degradation rate of OBS or formation rate of •OH radicals under pulsed conditions or continuous exposure mode, respectively. The results of these calculations are shown in Table 2. By comparing the pulse enhancement percent for OBS degradation to that of •OH radical formation at a given ultrasound frequency, it is possible to make conclusions regarding the effect of pulsing on OBS degradation at any given frequency. With using this comparative method, a number of variables need not be considered when comparing the pulse enhancement of OBS degradation to •OH radical formation as a function of frequency. Since the actual comparisons are made at any given frequency, the effect of ultrasound frequency on the comparisons need not be considered. For example, ultrasound frequency affects the quantity of •OH radicals that escape from the bubble
interface and migrate into the bulk solution to react with TA.31 However, at any given frequency, comparisons of pulse enhancements for OBS degradation to that of •OH radical formation would not depend on how many •OH radicals can enter the bulk solution, since inevitably bubble collapse will be determined by the resonance and maximum radii which are dependent on the frequency of ultrasound and not the pulsing conditions. Essentially, the comparative method, based on the assumptions made above, isolates the effects of ultrasound frequency as a variable in any given OBS degradation to •OH formation pulse enhancement comparison, since these comparisons are only made within a given ultrasound frequency. Therefore, the effect of ultrasound frequency on whether pulsing is likely to enhance OBS degradation relative to •OH radical formation can be considered, without actually considering the parameters that typically vary as a function of ultrasound frequency. There are three main sets of data that will be described. The first set of pulse enhancement data (Table 2, set 1) relates to conditions under which a significant pulse enhancement percent was observed for OBS degradation, but not for hydroxyl radical formation rates (i.e., compare the pulse enhancement percent in the 206 kHz and 354 kHz columns for OBS degradation to that of •OH radical formation at each pulsing condition). The second set of data relates to conditions under which there was effectively no pulse enhancement observed for either OBS degradation or •OH radical formation within the error of the experiment (i.e., the 620 and 803 kHz columns labeled set 2 in Table 2). The third set of data describes conditions under which pulsing was less effective for both OBS degradation and •OH radical formation, regardless of the pulsing conditions (i.e., the 1062 kHz column labeled set 3 in Table 2). Data set 1 (Table 2) is of most interest from a practical perspective. Clearly, pulsing under either of the pulsing modes has resulted in considerable increases in the OBS degradation rate at both 206 kHz and 354 kHz, whereas little or no increase in the •OH radical formation rate was observed. It can be concluded that pulsing under the conditions described in Table 2 would be very advantageous to the degradation of contaminants possessing surfactant properties, at ultrasound frequencies of 206 and 354 kHz. In addition, because pulsing resulted in virtually no enhancement of the •OH radical formation rate (and based on the assumptions discussed earlier for this comparative method), some conclusions can be made regarding the effect
Degradation of Octylbenzene Sulfonic Acid of pulsing on acoustic cavitation and OBS degradation as a function of frequency. First, it is clear that the dramatic pulse enhancement percentages observed for OBS degradation in the data set 1 (Table 2) cannot be due to an increase in the sonochemical activity at either 206 kHz or 354 kHz under pulsed ultrasound exposure. There are other possibilities for enhanced OBS degradation under pulsed ultrasound which are supported by earlier studies in the literature, as described below. It is possible that under CW ultrasound exposure at 206 and 354 kHz sonolysis, OBS does not reach equilibrium adsorption at the gas/solution surface of cavitation bubbles. Such an effect has been described previously for relatively long n-alkyl chain possessing surfactants.4,11 However, when pulsed ultrasound is applied to the system, an increased amount of surfactant can adsorb to bubble interfaces possibly because of increased bubble lifetimes and an increase in the lifetime of the gas/solution surface of cavitation bubbles (i.e., the bubble wall would be expected to slow down during a pulse interval). This has been shown to occur for surfactants adsorbing at the gas/solution interface of cavitation bubbles at 354 kHz under pulsed conditions and is consistent with the earlier study.17 The bubbles present in solution during a pulse interval are inherently unstable and either tend to dissolve away because of the Laplace pressure in the bubble32 or, alternatively, may coalesce32 and become too large to ever undergo inertial collapse. However, our observations of such a dramatic pulse enhancement for OBS degradation in the data set 1 (Table 2) is consistent with findings related to the effect of surfactants on the stabilization of bubbles to prevent coalescence during pulse intervals.32 Either or both of these two parameters, that is, enhanced accumulation of surfactants at the bubble interface and prevention of coalescence, readily explain the large pulse enhancements in OBS degradation observed at relatively low ultrasound frequencies. From the data sets 2 and 3 in Table 2, the following conclusions can be made. In the case where no pulse enhancement was observed for either OBS degradation or •OH radical formation (Table 2, set 2) it can be concluded that the pulsing conditions used were not conducive to enhanced sonochemical activity nor surfactant adsorption at the gas/solution interface of cavitation bubbles. Pulsing at these two frequencies (i.e., 620 kHz and 803 kHz) did little to affect the active bubble population. The possible reasons for this could be that bubbles formed during sonolysis at these two frequencies are at a size where neither dissolution effects nor coalescence play a large role during the pulse intervals. With regard to the data set 3 in Table 2, clearly CW ultrasound was more effective at both OBS degradation and •OH radical formation compared with the pulsed modes used. At the higher ultrasound frequency (i.e., 1 MHz), bubbles will have a smaller resonance radius21c (Table 1), so the possibility of bubble dissolution due to a relatively large Laplace pressure during the pulse interval would have an effect on both sonochemical systems. This appeared to occur for the pulsing conditions that we chose in the current study. A summary of the results of the current study is provided in Figure 4, which is a graph of the initial rate of OBS degradation and •OH radical formation under the conditions studied. Information contained in this figure should be viewed with a certain degree of caution. Comparisons should only be made between CW or pulsed ultrasound within a particular ultrasound exposure frequency. For example, relative improvements or decreases in the rate of OBS decomposition or •OH radical formation due to pulsing within a particular ultrasound frequency can be assessed, but such a comparison should not be made
J. Phys. Chem. B, Vol. 112, No. 3, 2008 857
Figure 4. Ratio of initial degradation of OBS to •OH radical production rate under CW and pulsed ultrasound with a pulse interval of 100 ms and pulse lengths of T ) 100 ms or T ) (3.54 × 104 cycles/f) ms.
between ultrasound frequencies since a number of variables cannot be kept constant when the ultrasound frequency is changed. It can clearly be seen that, at 206 kHz, pulsing (T ) 100 ms; T0 ) 100 ms) has had a large effect on the rate of OBS degradation compared with •OH radical formation and a similar effect is observed at 354 kHz. Within the error of the experiment, pulsing had no effect on the rate of OBS degradation compared with •OH radical formation at 620 kHz and 803 kHz. At a frequency of 1 MHz, pulsing either had no effect or resulted in a significant decrease in the rate of OBS degradation relative to the effect of pulsing on •OH radical formation. Conclusions The accumulation and subsequent degradation of the nonvolatile surfactant OBS are strongly dependent on the frequency and sonication mode applied. The effect is understood in terms of both cavitation bubble dynamics and surfactant adsorption kinetics. The accumulation of surfactant at the cavitation bubble interface plays an important role in the rate of OBS degradation during pulsing at lower frequencies (i.e., 206 and 354 kHz). At a pulse length and pulse interval of 100 ms:100 ms, a 300% enhancement in OBS degradation occurred over that observed under CW exposure, for the same total time of sonolysis. This large increase in efficiency of surfactant degradation needs to be investigated further at other frequencies and under different pulsing conditions. In using the comparative method, we have shown in this study that the method of analysis of results is critical if conclusions are to be made regarding the effects of ultrasound frequency on pulsing effects in sonochemical systems and regarding how these relate to changes in acoustic cavitation activity. Acknowledgment. Financial support from the Office of Naval Research (ONR) is gratefully acknowledged. References and Notes (1) Ultrasound: Its Chemical, Physical and Biological Effects; Suslick, K. S., Ed.; VCH: New York, 1988. (2) Mason, T. J.; Lorimer, J. P. Sonochemistry: Theory, applications and uses of Ultrasound in Chemistry; Ellis Horwood Limited: West Sussex, 1988. (3) Reisse, J.; Caulier, T.; Deckerkheer, C.; Fabre, O.; Vandercammen, J.; Delplancke, J. L.; Winand, R. Ultrason. Sonochem. 1996, 3, S147S151. (4) Sostaric, J. Z.; Riesz, P. J. Phys. Chem. B 2002, 106, 12537-12548.
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