Degradation of Alkylbenzene Sulfonate Surfactants by Pulsed

J. Phys. Chem. B 2005, 109, 16203-16209

16203

Degradation of Alkylbenzene Sulfonate Surfactants by Pulsed Ultrasound Limei Yang,† James F. Rathman,‡ and Linda K. Weavers*,† Department of CiVil and EnVironmental Engineering and Geodetic Science, and Department of Chemical and Biomolecular Engineering, The Ohio State UniVersity, Columbus, Ohio 43210 ReceiVed: May 4, 2005; In Final Form: June 29, 2005

The application of pulsed ultrasound for the degradation of the nonvolatile surfactants sodium 4-octylbenzene sulfonate (OBS) and sodium dodecylbenzenesulfonate (DBS) was investigated at a frequency of 354 kHz. By comparing the degradation rate constants with those of continuous wave (CW) ultrasound, observed pulse enhancements were found to be dependent on the pulse length, pulse ratio, initial concentration, and surface activity of the surfactants. For a pulse length of 100 ms and a pulse ratio of 1:1 (equal on/off times), the degradation rate constant of 1 mM OBS was nearly twice the value for CW. Furthermore, the degradation rate constant for 1 mM DBS increased significantly when sonicated under a pulse length of 100 ms and a pulse on/off ratio of 1:50. However, the degradation rate of 0.1 mM OBS increased by only 30% with a 100 ms pulse length and pulse ratio of 1:1 as compared to CW, indicating concentration dependence. The enhanced degradation of surfactants by pulsed ultrasound was attributed to the accumulation of surfactants on cavitation bubble surfaces. In addition, as compared to shorter pulse intervals, longer pulse intervals enhanced DBS degradation, indicating that DBS, a more surface active compound, accumulated and equilibrated with the bubble interface more slowly.

Introduction Widely used surfactants such as alkylbenzene sulfonates and alkyphenol ethoxylates accumulate in the environment and constitute a well-recognized pollution problem.1,2 These compounds are biodegradable to some extent; however, some products of biodegradation (e.g., alkylphenols) are more problematic than the parent compound, exhibiting higher toxicity, estrogenic activity, persistence, and a tendency toward bioaccumulation.3 Several oxidative methods including ozonation and photochemical reactions have been used for the treatment of alkylphenol ethoxylate (APE) surfactants. Those processes have been shown to produce more toxic byproducts than APE itself because the initial reactions degrade the hydrophilic groups of the surfactants and consequently generate alkylphenols.4 In addition, reactions by these oxidation methods typically occur in bulk solution. The chemical effects of ultrasound result from the almost adiabatic collapse of cavitation bubbles. Cavitation bubble collapse produces extremely high local temperatures and pressures resulting in high-energy chemical reactions.5,6 Thermolytic reactions occur in the hot cavitation bubble itself or at the interfacial region between the gaseous bubble and the surrounding liquid. Reactions involving hydroxyl radicals generated by thermolysis of water molecules occur within the cavitation bubble, at the gas-liquid interface, and in the surrounding liquid.7,8 Reaction mechanisms are dependent on the physicochemical properties of particular compounds.9-11 Volatile compounds are degraded preferentially by thermolysis in the vapor phase of the cavitation bubble.12,13 Nonvolatile hydrophobic and surface active compounds are thought to accumulate * Corresponding author. Phone: (614) 292-4061. Fax: (614) 292-3780. E-mail: [email protected]. † Department of Civil and Environmental Engineering and Geodetic Science. ‡ Department of Chemical and Biomolecular Engineering.

and react in the interface of the cavitation bubble,14-16 a region that is characterized by temperatures on the order of 2000 K17 along with high hydroxyl radical concentrations. Nonvolatile hydrophilic compounds do not partition into the vapor phase and do not significantly accumulate at the bubble interface. Therefore, they are slowly degraded in bulk solution by reaction with OH radicals or H2O2 that have diffused from cavitation bubbles.18 Thus, sonication has been shown to be effective at degrading volatile and hydrophobic compounds,19-21 which tend to partition to the cavitation bubble or its interface. During the past several years, ultrasound has been found to be a promising tool for the degradation of surfactants.4,22,23 Previous studies proposed that nonvolatile surfactants preferentially localize at the interface of the cavitation bubble.23-25 However, diffusion of reactants to reaction sites limits degradation rates (i.e., mass transfer limited kinetics) in ultrasonic reactors with short bubble lifetimes.23 Bubble growth during continuous sonication occurs over tens of microseconds, is dependent on the applied frequency and power,26-28 and is followed by the sudden collapse of the bubble on a nanosecond time scale.29 The rate at which surfactant molecules partition to the interface is time dependent; this time may vary from milliseconds to hundreds of seconds30-32 Because of this, there is little time during the bubble lifetime for significant mass transfer to occur between the bulk solution and cavitation bubble. Increased bubble lifetime would therefore be expected to improve the adsorption and degradation of surfactants. However, the effects of surfactants are complex; for example, the adsorption of surfactants at the bubble/solution interface may lower the surface tension and affect bubble growth and dissolution dynamics.33,34 An attractive feature of pulsed sonic treatment is the retardation of bubble growth during the time interval between successive pulses,35 allowing for increased accumulation of surfactants at cavitation bubble surfaces. The purpose of this

10.1021/jp0523221 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/04/2005

16204 J. Phys. Chem. B, Vol. 109, No. 33, 2005 study was to investigate the application of pulsed ultrasound to the degradation of surfactants as compared to nonsurfactants at 354 kHz. The pulse lengths and intervals were varied to explore the effects of pulse lengths and pulse ratios on the accumulation and sonolysis of surfactants on cavitation bubbles. In addition, the sonolysis of surfactants, with different alkyl chain lengths and at various concentrations, was studied to further explore the effects of surfactant properties on degradation. Experimental Methods Material and Reagents. Two anionic surfactants, sodium 4-octylbenzene sulfonate (OBS, Aldrich, 97%) and sodium dodecylbenzenesulfonate (DBS, Wako, 99%), were chosen as model surfactants. 4-Ethylbenzene sulfonic acid (EBS, Aldrich, 95%) was chosen as a model nonsurfactant. These compounds have the same hydrophilic headgroups and similar hydrophobic tail groups but different alkyl chain lengths. All chemicals were of high purity and used as received. Solutions were prepared with water purified by a Milli-Q water system operating at resistivity R ) 18.2 MΩ cm. Sonochemical Experiments. A USW 51-52 Ultrasonic transducer (ELAC Nautik, Inc., Kiel, Germany) operating at 354 kHz was used to perform reactions in a batch reactor equipped with a cooling water jacket for temperature control. A SM-1020 Function/Pulse generator (Signametrics Corp., Seattle, WA) was used to deliver pulses ranging from 100 ns to 100 s. The generator output was connected to an AG 1021 linear amplifier (T & C Power Conversion, Inc., Rochester, NY) so that the pulse signal was amplified to drive the transducer. The amplifier was capable of generating electrical output in either continuous wave (CW) or pulsed mode. A HewlettPackard 54501A 100 MHz digitizing oscilloscope was used to detect the pulse signal received by the transducer. The power input into solution for both CW and pulsed ultrasonic modes was 33 W, as measured by calorimetry,36 indicating that an equivalent amount of acoustical energy was input in each case. A sonication area of 23.4 cm2 was used to produce an ultrasonic intensity of 1.41 W cm-2. The temperature of the solution was maintained at 23 ( 3 °C with a Fisher Scientific 1006S Isotemp cooling system. First, 500-mL solutions were sonicated, and samples were taken from the reactor at designated times during sonication. The total volume withdrawn during a single run did not exceed 5% of the total sonicating volume. All samples were filtered before analysis with 0.2 µm PTFE Millipore filters. Control experiments indicated that the target compounds did not adsorb onto the filters. Initial solutions containing either surfactant or nonsurfactant were adjusted to pH 2.8 using 33 mM phosphate buffer to ensure the nonsurfactant, EBS (pKa ) 7.0), was neutrally charged (the protonated form). Phosphate was chosen as a buffer because it has a slow rate of reaction with OH• (kH2PO4- ) 2 × 104 M-1 s-1)37. Analysis. A Hewlett-Packard 1100 high performance liquid chromatograph (HPLC) with a 100 × 2.1 mm C18 Hypersil ODS column was used to analyze the disappearance of OBS, DBS, and EBS following sonolysis. An eluent gradient of acetonitrile and phosphate buffer at pH 2.2 from 80% phosphate buffer to 65% phosphate buffer with OBS and EBS retention times of 35 and 10 min, respectively, was used.38 An isocratic eluent of 45% 0.10 M sodium perchlorate and 55% acetonitrile with a retention time of 30 min was used for DBS.39 The surface tension of the surfactants was determined using a Sensadyne Tensiometer (model PC 500, Chem-Dyne Research Corp., Mesa, AZ). Hydrogen peroxide was determined spectrophotometrically

Yang et al. TABLE 1: Apparent Pseudo First-Order Degradation Rate Constants for EBS, OBS, and DBS under CW Conditions concentration (mM) 0.1 0.2 0.5 1 2 5

k × 103 (min-1) EBS

1.86 ( 0.10

OBS

DBS

17.7 ( 0.4 9.7 ( 0.3 3.5 ( 0.1 1.75 ( 0.07 0.80 ( 0.05 0.40 ( 0.02

16.9 ( 0.3 10.1 ( 0.4 3.3 ( 0.2 1.83 ( 0.05

by molybdate-activated iodide.40,41 Relative reaction rate constants of OBS and DBS with OH• were measured using Fenton’s reagent23 and the method of relative rate determinations.42 Results and Discussion To compare degradation rates in pulsed and continuous experiments, the same amount of acoustic energy was applied in continuous and pulsed systems. The actual sonication times were 2 h for both continuous and pulsed ultrasound. As a result, the total time required for a pulsed ultrasound experiment was longer than that for a continuous mode experiment. The actual sonication time of pulsed ultrasound is related to total time by:

/(

tsonication ) ttotal 1 +

1 R

)

(1)

where tsonication is the actual sonication time (2 h for each experiment in this study); ttotal is the total time of an experimental run; and R ) T/T0 is the pulse ratio of the pulsed mode (T ) length of a pulse; T0 ) length of the interval between each pulse). Selected experiments were run in duplicate or triplicate to verify the reproducibility of the results; reported errors were determined statistically by 95% confidence intervals. The degradation of surfactants and nonsurfactants has been shown to follow pseudo first-order kinetics.7,25 Our results showed that degradation rates under both CW and pulsed ultrasound modes follow an apparent pseudo first-order rate. To compare the initial degradation rate of compounds under pulsed and CW ultrasound (C0kpulsed and C0kCW, respectively), the enhancement of compounds degrading under pulsed conditions as compared to CW at the same initial concentration was defined as

pulse enhancement (%) )

C0 kpulsed - C0 kCW × 100 (2) C0 kCW

where C0 is the initial concentration of the compound, kpulsed is the pseudo first-order rate constant of the compound under pulse mode, and kCW is the pseudo first-order rate constant of the same compound under the same concentration and conditions but with CW mode. Table 1 gives the baseline CW degradation rate constants from which pulse enhancements were calculated. To obtain the surface excess and the critical micelle concentration (CMC) for the surfactants used in our experiments, the surface tension at the air/solution interface was measured as a function of surfactant concentration (example data shown in Figure 1). The CMC was 6.3 mM for OBS and 1.2 mM for DBS. The surface tension (γ) gradually decreased with increasing concentration as the surfactant began to partition to the gas/ liquid interface. The saturated equilibrium surface excesses (Γeq) were calculated as:43

Γeq ) -

dγ 1 RT d(ln Ci)

(3)

Degradation of Alkylbenzene Sulfonate Surfactants

Figure 1. Surface tension as a function of initial surfactant concentration below CMCs of OBS and DBS (error bars represent 95% confidence intervals).

Figure 2. Pulse enhancements of 0.1 mM individual compounds of OBS and DBS degradation by pulse ultrasound with a pulse ratio of 1:1 and various pulse lengths, T (error bars represent 95% confidence intervals).

where Ci is the bulk concentration of surfactant, R is the universal gas constant, and T is temperature. The values of Γeq were 1.9 × 1014 molecules cm-2 for OBS and 3.7 × 1014 molecules cm-2 for DBS at 20 °C and pH 2.8. The surface tension of EBS as a function of concentration was also measured, and no significant changes were observed in the concentration range from 0.1 to 5 mM. Dependence of Degradation of Surfactants on Pulse Lengths. The effects of OBS and DBS on the surface tension of water as a function of bulk surfactant concentration below the CMC are shown in Figure 1. At low surfactant concentrations, for example, 0.1 mM, the surface tension in the presence of OBS and DBS was 71.0 and 69.2 mN m-1, respectively, that is, similar to that of water (72.8 mN m-1 at 20 °C).44 Thus, surface tension effects on the cavitation bubble can be ignored at this concentration, and air/solution interfaces are not saturated with surfactant at equilibrium. However, the surface tensions of 1 mM solutions were significantly reduced, 60.7 mN m-1 for OBS and 43.5 mN m-1 for DBS. (i) Low Surfactant Concentrations (Unsaturated Surface at Equilibrium). Initially, pulse mode experiments at 0.1 mM, a pulse ratio (T/T0) of 1:1 (equal on and off time), and different pulse lengths were investigated. The results in Figure 2 show that there was a statistically significant increase in the degradation rate constants when 0.1 mM OBS was sonicated under pulse lengths (T) of 50, 100, and 200 ms as compared to CW.

J. Phys. Chem. B, Vol. 109, No. 33, 2005 16205 Moreover, an enhanced rate as compared to CW was observed when 0.1 mM DBS was sonicated under a pulse length of 100 ms. Shown by an increase in the degradation rates by 31% and 7% as compared to CW, the results indicate that a pulse length equal to 100 ms provided the highest efficiency for degradation of 0.1 mM OBS and 0.1 mM DBS, respectively. At both longer and shorter pulse lengths, that is, T < 50 ms, T > 200 ms (T/T0 ) 1), for both OBS and DBS, no significant enhancements were observed as compared to continuous sonication. These results are consistent with other studies suggesting that cavitation bubbles are maximally efficient after 104-105 cycles.45 Henglein et al. explained this phenomena by taking into account the time required to “activate” the system at the start of the pulses (i.e., the time to produce chemical activity of the bubble), as well as the time required to “deactivate” the system after the end of each pulse (i.e., the time for the bubbles to dissolve completely).46 In this study, maximal degradation occurred with 50, 100, and 200 ms pulse lengths which correspond to 1.77 × 104, 3.54 × 104, and 7.08 × 104 cycles for each pulse. Lack of time to “activate” the system may explain why a negative pulse enhancement was observed at a pulse length of 0.5 ms. Longer pulse lengths and continuous ultrasound were also found to be less efficient. One possibility for explaining lower efficiencies of longer pulses is that some bubbles may dissolve during the longer pulse intervals. Under experimental conditions presented here, with T ) T0 ) 100 ms, the deactivation time appears to be less than the onset of the next pulse, giving maximal pulse enhancements for OBS and DBS degradation at T ) 100 ms. Note that deactivation time will be dependent on solution conditions and is expected to be different at low and high surfactant concentrations. At lower concentrations of surfactants, diffusion to the bubble surface is the controlling mechanism of transport.30,47 However, the diffusion coefficient (Diw) varies depending on the physical properties of surfactants. Diw of the solute i in water can be estimated using48

Diw )

8.621 × 10-14 µw1.14Vi0.589

(4)

where µw is the viscosity of water in Pa s and Vi is the molar volume of the solute i, at the solute normal boiling point, resulting in a diffusion coefficient in m2 s-1. The relative diffusion coefficients, Diw/DEBSw, were calculated as 0.69 and 0.59 for OBS and DBS, respectively. Therefore, OBS diffuses to bubble surfaces somewhat faster than does DBS. Thus, when diffusion controls the transport of surfactants to the bubble interface, greater pulse enhancements are expected with OBS than DBS due to increased time for localization of the surfactants at the site of the highest reactivity. Our results are consistent with this expectation showing that the pulse enhancements at 0.1 mM were 31% and 7% for OBS and DBS, respectively. In addition to pulsed ultrasound at T ) 100 ms and T/T0 ) 1 allowing more OBS and DBS to diffuse to the bubble interface as compared to CW, pulsing may produce bubbles with greater activities than those of the CW system. In this case, increased H2O2 production would be expected. However, H2O2 production in the absence of OBS and DBS by CW and pulsed ultrasound (T ) 100 ms; T/T0 ) 1) revealed no significant changes in bubble activity between pulsed and CW conditions. Therefore, the pulse enhancements observed are attributed to increased surface excess of surfactants on cavitation bubble surfaces. (ii) High Surfactant Concentration (Saturated Surface at Equilibrium). Similar pulse length experiments were conducted

16206 J. Phys. Chem. B, Vol. 109, No. 33, 2005

Yang et al. OBS to reach equilibrium concentrations at the bubble interface (T ) 100 ms; T/T0 ) 1) as demonstrated by its faster rate at T ) 100 ms and T/T0 ) 1. Surfactants are able to stabilize microbubbles and lengthen their lifetime. As shown in Figure 1, the surface tension of the solutions is significantly reduced as the concentration increases from 0.1 to 1 mM OBS or DBS. The reduced surface tension with increased concentration lowers the cavitation threshold and facilitates the generation of additional cavitation bubbles.51-53 Another positive effect of increased surfactant accumulation at the bubble interface is that it retards bubble dissolution.54 The approximate time required for a bubble with radius r to completely dissolve in clean water is given by54

Figure 3. Pulse enhancements of 1 mM individual compounds of OBS, DBS, and EBS degradation by pulse ultrasound with a pulse ratio of 1:1 and various pulse lengths, T (error bars represent 95% confidence intervals).

at 1 mM concentrations of OBS, DBS, and EBS with T/T0 ) 1 and various pulse lengths as shown in Figure 3. Under a pulse length of 100 ms, the first-order rate of 1 mM OBS degradation increased by 94% as compared to CW. A negative enhancement was observed when a pulse length of 0.5 ms was applied, indicating the degradation rate under this pulse condition was lower than that of CW. Under pulse lengths other than these two conditions, pulse enhancements for 1 mM OBS were not statistically significant and the degradation rates were comparable to that of CW. The degradation rate of 1 mM DBS under a pulse length of 0.5 ms was reduced by 51% as compared to CW. At longer millisecond pulse lengths, no statistically significant enhancements in degradation were observed. However, degradation rates increased by 80% and 78% when the ultrasound was pulsed with long pulse lengths (5 and 50 s, respectively). The degradation rates of 1 mM EBS did not increase significantly under pulsed ultrasound with various pulse lengths when compared to CW. In addition, the degradation rates were smaller than those of CW when pulse lengths were 0.5 and 10 ms. EBS does not have a tendency to accumulate at the bubble interface, so reactions are expected to occur primarily in bulk solution. Furthermore, the degradation of 1 mM EBS did not significantly change as compared to CW under longer pulse lengths, indicating that the concentrations of OH radical in bulk solution were not significantly changed under those pulse conditions. However, reduced degradation rates of EBS at short pulse lengths indicate a reduction of OH radicals as compared to that of CW. Clearly, the relative effectiveness of various pulsing conditions depends not only on the pulse length and interval, but also on the compounds present in the system. We suspect that with shorter pulse lengths (T), less effective chemical activity was developed in the system at each pulse.14 However, the differences observed between OBS and DBS at longer T0 suggest that differences in surfactant accumulation on the bubble surfaces also play a role. DBS is more surface active than OBS but needs more time to reach and equilibrate at the bubble interface at higher concentration.49 The time required to reach the equilibrium surface excess (Γeq) depends on the structure of the surfactants and can vary drastically, ranging from milliseconds to hundreds of seconds for these anionic surfactants.50 During shorter pulse intervals, DBS would not have enough time to accumulate and equilibrate at the bubble surface, the region where most chemical reaction is expected to occur. The short n-alkyl chain length of OBS allows this surfactant to be more dynamic than DBS, possibly even allowing

td )

P0 r3(1 + 2γ/P0 r) 6Dγd

(5)

where td is the bubble dissolution time, P0 is the ambient pressure, γ is the surface tension, D is the diffusion constant, and d is a dimensionless constant dependent upon thermodynamic factors. By rearranging eq 4 to:

td )

r2(2 + P0 r/γ) 6Dd

(6)

we see that for a given compound, the bubble dissolution time increases when the surface tension is lower. Chan et al.55 determined that the lifetime of a 20 µm bubble in a saline solution could be increased from 6 s to 5-10 min by the addition of sodium laurate (C12H23NaO2). Thus, it becomes clear why the degradation rates of 1 mM DBS were significantly enhanced under the longer pulse intervals as shown in Figures 2 and 3. First, with lower surface tension, more bubbles will be produced when the pulse is on due to the lower cavitation threshold. Second, when the pulse is off, the longer pulse interval allows DBS to accumulate at the bubble interface, significantly reducing surface tension at the bubble interface and retarding bubble dissolution while increasing degradation rates. Surprisingly, no significant enhancement was observed with OBS at long pulse lengths. This lack of enhancement suggests that either an increased pulse length or an interval is not optimal for OBS. Several studies have investigated sonochemical effects of pulsed ultrasound on nonsurfactants.46,56-60 However, the yields of pulsed sonication were either equal to or smaller than those found for CW. The degradation of EBS is consistent with those studies. The probable reason for this is that nonsurfactants do not accumulate at the cavitation bubble surface, and the degradation rates are only dependent on the change in cavitation condition under different pulse conditions. In the surfactant system, surfactant molecules need time to diffuse to and equilibrate with the bubble interface. Due to a very short lifetime of the bubbles in continuous ultrasound, there may be only limited accumulation of surfactants at the bubble interface, and, therefore, the amount of surfactants at the bubble interface when the bubble collapses is significantly smaller than Γeq.23,49,61 During pulsed sonication, bubbles grow during a larger number of compression and rarefaction cycles as compared to CW before cavitational collapse. Therefore, more time elapses, allowing increased amounts of surfactants to reach the bubble interface.35 At specific pulse lengths and intervals and surfactant concentrations, the interface of an oscillating bubble would have a greater amount of surfactant than in CW, resulting in more surfactants undergoing reaction.

Degradation of Alkylbenzene Sulfonate Surfactants

Figure 4. Pulse enhancements of 1 mM individual compounds of OBS, DBS, and EBS degradation by pulse ultrasound with a pulse length T ) 100 ms and various pulse intervals, T0 (error bars represent 95% confidence intervals).

Figure 5. Pulse enhancements of 1 mM DBS degradation by pulse ultrasound of a pulse length T ) 5 s and various intervals, T0 (error bars represent 95% confidence interval).

Dependence of Degradation of Surfactants on Pulse Intervals. To observe the effect of pulse interval on degradation, a series of experiments were conducted at a pulse length of 100 ms and different pulse intervals (100 ms, 500 ms, 1 s, 2 s, and 5 s). Figure 4 shows that 1 mM OBS degradation rates were twice as fast as compared to continuous sonication, regardless of the pulse interval. Earlier results indicated that T ) 100 ms is effective for OBS degradation, and it appears that significant amounts of OBS accumulate and approach equilibrium with the bubble surface at T0 ) 100 ms. In the presence of 1 mM DBS and pulse intervals of 2 and 5 s, pulse enhancements were 102% and 119%, respectively. No pulse enhancements were observed at shorter pulse intervals. In addition, pulse enhancements of 1 mM DBS sonicated under pulse conditions of T ) 5 s and pulse intervals from 5 to 95 s as shown in Figure 5 indicate that a 5 s interval is adequate to enhance degradation of 1 mM DBS. The degradation rates of 1 mM EBS did not change during pulse conditions as compared to those of CW at the pulse intervals tested. Shorter pulse intervals may not provide enough time for OBS to accumulate at the bubble interface. Long pulse intervals, on one hand, may allow more surfactants to adsorb to the bubble interface; on the other hand, some activity may be lost between the pulses (deactivation time) because the bubbles dissolve during the off period. Again, dissolution is a function of the solution conditions. Thus, during the pulse interval, a careful balance of allowing time for compounds to adsorb to the surface while minimizing the dissolution of small bubbles is needed.

J. Phys. Chem. B, Vol. 109, No. 33, 2005 16207

Figure 6. Initial degradation rates of OBS and DBS with [OBS] ranging from 0.1 to 5 mM and [DBS] ranging from 0.05 to 1 mM (error bars represent 95% confidence intervals).

The results indicate that the deactivation time of bubbles in 1 mM OBS solution (surface tension of 60.7 mN m-1) under a pulse length of 100 ms is longer than 5 s. Thus, the lack of enhancement of OBS at long pulse lengths with T/T0 ) 1 is not due to the long pulse interval (T0) causing deactivation but rather due to the long pulse length T eliminating the benefit of pulsing. Under longer intervals, DBS has time to adsorb to the bubble interface, and the bubbles do not appear to dissolve to a significant extent. However, at the same pulse interval of 5 s, the pulse enhancements were 119% and 80% at pulse lengths of 100 ms and 5 s, respectively. The greater enhancement at a shorter pulse length of 100 ms appears to be due to the optimal pulse length allowing maximal surfactant degradation. Longer bubble lifetime allows for accumulation of surfactants on cavitation bubble surfaces. The sonication pulses produce high temperature and OH radicals that degrade the surfactant that has accumulated on the bubble surface. Any extra high temperature and OH radicals produced after the accumulated surfactants have been depleted are inefficient, and if the pulse interval is sufficiently long, it results in a system that resembles CW. Thus, if the optimal T and T0 are determined, the interface of an oscillating bubble will have a greater amount of surfactant that will undergo sonication in an efficiently activated pulse system.62 The results with EBS are directly related to the activity of bubbles in the pulsed ultrasound system because EBS is not surface active and does not accumulate on the bubble surfaces. With EBS, some bubbles may dissolve during the longer pulse interval; therefore pulsing was detrimental to degradation at a pulse interval of 5 s. To further investigate the pulse interval effects, experiments were performed on 1 mM DBS under a pulse length of 5 s and pulse intervals of 5, 50, and 95 s. Results from Figure 5 show that the pulse enhancements were very similar for these intervals. Thus, the bubble lifetime is expected to be longer than 95 s because the pulse enhancements did not change as the interval increased, indicating that the bubbles behaved similarly regardless of the interval. Pulse enhancements of 80-86% at a given pulse length of 5 s and various intervals further verify that bubbles are less efficient at longer pulse lengths as compared to 102-119% enhancements at a pulse length of 100 ms and various intervals. Effects of Concentration and Surface Activity of Surfactants. The degradation rates of OBS and DBS by pulsed ultrasound were compared at different initial concentrations below their CMC in Figure 6. The behavior observed is dependent on the concentration and the n-alkyl chain length of

16208 J. Phys. Chem. B, Vol. 109, No. 33, 2005 the surfactants. The results verify more accumulation of surfactants on cavitation bubbles when sufficient pulse intervals are given, as described in detail below. Low Surfactant Concentrations (C/CMC e 0.2). In Figure 6, the initial rates increased almost linearly as the initial concentrations increased at low concentrations with both compounds under both pulse conditions. When a new bubble interface is formed in a surfactant solution, Γeq is not instantly reached. Surfactant molecules first diffuse to the surface from the bulk and then adsorb and orient themselves at the interface.63 The number of solute molecules that adsorb at the interface is equal to the number of solute molecules that, having diffused from the bulk to the subsurface, cross the adsorption barrier. The diffusion process is predominant at low concentrations when the interface is not saturated.30,47 Equilibrium is more rapidly attained at low concentrations because adsorption is a fast diffusion-controlled mechanism when the surface is not saturated. The linear increase in degradation rates with concentration is due to a continuous increase in surface excess. High Surfactant Concentrations (C/CMC g 0.2). In this concentration range, the discussion is divided into two cases. (i) T ) 100 ms; T0 ) 100 ms. The initial degradation rates of both OBS and DBS decreased when C/CMC values were above 0.2 and approached a constant initial rate when the concentrations approached the CMC. With increasing surfactant concentrations, surfactant molecules have to overcome increased barriers to adsorption including fewer vacant sites at the interface available for adsorption and electrostatic repulsion between the surfactant headgroup ions. A switch-over in the adsorption mechanism, from diffusion-controlled to mixed kinetic-diffusion controlled, occurs as the bulk concentration increases.30,47 Thus, surfactants require more time to reach and equilibrate with bubble surfaces at higher concentrations as compared to lower concentrations. This longer time for equilibrium at higher concentrations reduces the initial rates as compared to those observed at lower concentrations. Although pulsed ultrasound may lengthen the bubble lifetime, the time required for OBS and DBS to accumulate and equilibrate with the interface at higher concentrations is much longer than the lifetime of bubbles under 100 ms pulse conditions. As a result, equilibrium adsorption is not attained during 100 ms pulse intervals. (ii) T ) 100 ms; T0 ) 5 s. The degradation rates of both surfactants at T0 ) 5 s show different trends from those of the same pulse length and shorter pulse interval as shown in Figure 6. The degradation rates increased with increasing concentration for the longer pulse interval of 5 s. For DBS, the degradation shows a linearly increasing trend as the concentration increased and the initial rate is similar to that of OBS at a concentration near the CMC. Theoretical studies by Ferri et al.64 considered the rates at which a homologous series of surfactants could attain equilibrium between the bulk solution and the air/solution interface by comparing dynamic properties of surfactants with their equilibrium properties. They concluded that surfactants that have a higher Γeq at a particular bulk concentration would reduce the surface tension to a greater degree at equilibrium; however, these surfactants require longer times to reach equilibrium. Spintrapping studies by Sostaric et al.27,49 showed that the anionic surfactant possessing the shorter n-alkyl chain length was more efficient at accumulating at the interface of cavitation bubbles under 354 kHz continuous sonication, as compared to the more surface-active molecules. A longer pulse interval allows more time for the surfactant to adsorb to the bubble surface. Thus, eventually the system is likely to approach equilibrium and

Yang et al. significantly reduces the surface tension on the bubble surface. The saturated Γeq values were calculated to be 1.9 × 1014 molecules cm-2 for OBS and 3.7 × 1014 molecules cm-2 for DBS, respectively. The increased initial rates of DBS at higher concentrations suggest that surface excess and surface tension play a role in dynamics of bubble growth, collapse, and dissolution. Increased surface excess at high bulk concentrations results in more molecules on the cavitation bubble surfaces available to react with OH• and high temperature than at low bulk concentrations. Lower surface tension may result in more bubbles collapsing in a given time and delayed bubble dissolution. DBS, which has a longer n-alkyl chain length than OBS, is more surface active and takes longer to accumulate at the bubble interface. Once equilibrium is attained, more molecules are near the site of high temperature and OH• and surface tension is reduced changing bubble dynamics, both of which increase degradation. Therefore, we expect that the degradation of DBS would be greater than OBS at higher concentrations when equilibrium is attained due to its greater surface excess and lower surface tension. However, Figure 6 shows similar initial degradation rates of both OBS and DBS at C/CMC ) 0.8. One explanation is that at these higher concentrations, T0 ) 5 s is not long enough for DBS to equilibrate with the bubble surface due to the change in the adsorption mechanism at higher concentrations. Another possibility is that the relative reaction rate constants with OH• ki/kEBS are 1.8 and 0.7 for OBS and DBS, respectively. Lower reactivity of OH• with DBS as compared to OBS reduces the initial degradation rate trends expected and suggests that reactivities of surfactants with OH• also play a role in degradation. Conclusions The degradation of nonvolatile surfactants by pulsed ultrasound is advantageous as compared to continuous wave ultrasound due to increased surfactant accumulation at bubble interfaces. The degradation rates are strongly dependent on the pulse length, pulse interval, initial concentration, and surface activity of the surfactants. Reduced degradation rates as compared to CW were observed at shorter pulse lengths, indicating insufficient activation of bubbles during each pulse. At higher concentrations of surfactants, mixed kinetic-diffusion adsorption must be considered to explain the accumulation of surfactants on a bubble surface. At these higher concentrations, longer pulse intervals allow slower growth of bubbles and favor the accumulation of surfactants as compared to both CW and short pulse intervals. In addition, reactivities of surfactants with OH• also play a role in degradation. Results demonstrate the potential for enhancing degradation rates of surfactants by pulsed ultrasound but also reveal that optimal pulse intervals are a complex function of surfactant characteristics and concentration. Acknowledgment. We would like to thank Lindsey Saylor and Jared Archer for laboratory assistance and Joe Sostaric for his valuable comments and suggestions. We are also grateful for the helpful discussions with Gim-Yang Pee and Clayton Drees. Financial support provided by the Office of Naval Research (ONR) is gratefully acknowledged. References and Notes (1) Renner, R. European bans on surfactant trigger transatlantic debate. EnViron. Sci. Technol. 1997, 31, 316A-320A. (2) Scott, M. J.; Jones, M. N. Review: The biodegradation of surfactants in the environment. Biochim. Biophys. Acta 2000, 1508, 235251.

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