Alkylphenol Ethoxylate Surfactants Research Articles

Specific anion effects on the interfacial properties and aggregation of alkylphenol ethoxylate surfactants

The effect of Hofmeister anions on aggregation of nonionic surfactants and the formation of microemulsion systems was evaluated. Added salt effects on the interfacial properties and aggregation of ethylene oxide condensates of nonylphenol were characterized by cloud point measurements, surface tension, fluorescence spectroscopy, light scattering, and formulation scans. The specific effects of anions on the cloud point followed the trend predicted by the Hofmeister series: H2PO4− < Cl− < Br− < NO3− < I− < SCN−, with the H2PO4− being the most kosmotropic anion and generating the most significant cloud point reduction. The specific effects of anions on the critical micelle concentration (CMC) also followed the Hofmeister series, with kosmotropic anions causing a decrease and chaotropic anions generating an increase in the CMC. Chaotropic anions decreased the size of the aggregates, while kosmotropic anions increased it, as evaluated by dynamic light scattering. The effect of Hofmeister anions on microemulsion formation in nonionic surfactant-oil–water (SOW) systems was assessed by formulation scans. Although the kosmotropicity of the anion did not influence the optimum formulation value, it did affect the phase behavior around the optimum formulation, pointing to the possibility of tuning microemulsion formulations via the choice of added electrolyte.

Nonionic surfactants and their effects on asymmetric reduction of 2-octanone with Saccharomyces cerevisiae

In an aqueous buffer system, serious reverse and side reactions were found in the asymmetric reduction of 2-octanone with Saccharomyces cerevisiae. However, some nonionic surfactants added to the aqueous buffer system improved the bioreduction process by decreasing the reverse and side reaction rates in addition to effectively increasing the average positive reaction rate. Further, a shorter carbon chain length of hydrophilic or hydrophobic moieties in surfactants resulted in a higher yield of (S)-2-octanol. The alkylphenol ethoxylate surfactants had a less influence than polyoxyethylenesorbitan trialiphatic surfactants on the product e.e. It suggested that the product e.e. resulting from the change of carbon chain length of the hydrophobic moieties varied markedly compared with the change of carbon chain length of the hydrophilic moiety. Emulsifier OP-10 and Tween 20 markedly enhanced the yield and product e.e. at the concentration of 0.4 mmol L−1 with a yield of 73.3 and 93.2%, and the product e.e. of 99.2 and 99.3%, respectively, at the reaction time of 96 h.

The UNIFAC model and the partition of alkyl and alkylphenol ethoxylate surfactants in the excess phases of middle phase microemulsions

The partition of non-ionic surfactants between oil and water is an important parameter that influences the performance of surfactant-based separations, the fate of surfactants in the environment, and the absorption by surfactants in living organisms. From a thermodynamics point of view, the partition of surfactants between oil and water can be used as a method to quantify the hydrophilic–lipophilic nature of surfactants and oil. In this work, a group contribution model, the universal functional activity coefficient (UNIFAC) framework, has been modified to fit and predict the partition of alkylphenol, and alkyl ethoxylated surfactants between aqueous and oil phases. To perform such calculations the interaction parameters of an ethoxylate functional group were optimized, including the temperature dependence for functional groups known to form hydrogen bonds (CH2CH2OH2O, OHH2O). The optimization was performed by fitting this modified UNIFAC model to experimental partition coefficient of octylphenol ethoxylated surfactants in the excess phases (water and n-heptane) of middle phase microemulsions at 25 and 55°C and to experimental critical micelle concentration (CMC) data. The modified UNIFAC is capable of predicting the partition of alkylphenol ethoxylates and alkyl ethoxylates with varying degrees of ethoxylation, tail length and temperature and different oils. The predicted partition values correlate with experimental bioconcentration factors (BCFs) reported for alkyl ethoxylates. The model also predicts properly the critical micelle concentration (CMC) of alkyl and alkylphenol ethoxylate surfactants. The predicted partitions also helped estimate the equivalent alkane carbon numbers (EACNs) of various oils.

The HLD–NAC equation of state for microemulsions formulated with nonionic alcohol ethoxylate and alkylphenol ethoxylate surfactants

The net–average curvature (NAC) model ( Langmuir, 19, 186, 2003) is a simple, yet powerful, equation of state to fit and predict the phase behavior of microemulsions (μEs) formulated with ionic surfactants. However, its use for nonionic surfactants systems has not been evaluated until now. The objective of this work is to use the NAC equation of state to model the phase behavior of μEs formulated with pure alcohol ethoxylate (CiEj) and commercial nonylphenol ethoxylate (NPEj) surfactants with a range of alkanes (Cn). The NAC model requires three basic parameters: the characteristic curvature of the nonionic surfactant (Ccn), the characteristic length ( ξ), and the scaling length parameter ( L). The first part of this contribution presents a brief review of the NAC model and other thermodynamic models of μEs. Later, five example applications of the NAC model are presented and discussed: (I) fit/prediction of phase inversion temperature (PIT) values, (II) prediction of phase transition temperatures and characteristic lengths, (III) fit/prediction of interfacial tensions, (IV) prediction of “fish” phase diagrams, and (V) prediction of the composition of bicontinuous systems. The NAC model was able to fit the phase behavior observed in all the experimental scenarios, and predict within 30% the value of phase volumes and transition temperatures for CiEj systems. The largest deviations can be attributed to a poor estimation of ξ obtained through empirical correlations. It was also noted that for commercial NPEj surfactants, the NAC model can also fit and predict the phase behavior of these systems, only that more complex empirical correlations are needed for ξ and Ccn due to the partition of different surfactant n-mers into the excess oil and aqueous phases. In all cases, a good agreement was found when the length parameter ( L) was estimated as 1.4 times the extended length of the surfactant tail.

Effects of Enhanced APG Surfactant on Leaching and Wettability of Six Bark Substrates

Alkyl phenol ethoxylate (APE) surfactants are used in horticultural substrates but are considered nonbiodegradable, whereas others such as alkyl polyglucoside (APG) are derived from biodegradable sugar compounds. APE reduced total porosity (TP), container capacity (CC), and aeration porosity (AP) whereas APG with polyalkylene oxide block copolymer surfactant (APG/BLK) increased TP, CC, and AP for some substrates but other substrates remained unaffected by either surfactant. We determined substrate leaching fraction (LF) and wettability rating (WR) after drenches of 0.003 to 0.2 mL·L−1 APG/BLK for six bark substrates for three wetting cycles. After the third wetting cycle, five substrates had reduced LF and increased WR. Drenches of 0.2 mL·L−1 APE or APG/BLK for three wetting cycles indicated that APE was more efficacious than APG/BLK for reducing LF or increasing WR. APE was determined to be an effective surfactant for difficult-to-wet substrates, but drenching sometimes reduced TP. No reductions in TP were noted when using APG/BLK. Drenching rates of 0.003–0.2 mL·L−1 APG/BLK for three successive wetting cycles reduced LF and increased WR for most substrates, indicating potential usage for some horticultural applications.

Separation of Neutral Red and Methylene Blue from Wastewater using Two Aqueous Phase Extraction Methods

The aim of this work was the two‐aqueous phase extraction of neutral red and methylene blue from wastewater. This has been achieved on the basis of phase separation properties of non‐ionic surfactants above the so‐called cloud point curve and the solubilization phenomena of coacervate micelles (surfactant rich phase). Three commercial surfactants were used in this work; the Oxo‐C15E7 which is an ethoxylate fatty alcohol (Dehydol LT 7), Triton X‐114 (iso‐C8H17‐C6H4‐(OCH2CH2)7,5OH), and Triton X‐100 (iso‐C8H17‐C6H4‐(OCH2CH2)9,5OH) which are alkyl phenol ethoxylate surfactants. The phase diagrams of binary water/surfactants systems were drawn and the effect of sodium sulphate on water‐surfactant systems was therefore studied. Oxo‐C15E7 and Triton X‐114 were used for the cloud point extraction of neutral red and methylene blue from their aqueous solutions at 100 mg/L and 7 mg/L, respectively. The experimental extraction results were expressed by the following three responses: percentage of the extracted dye (E), residual concentrations of dye in the dilute phase (Xs,w), and the volume fraction of coacervate (φC) at the equilibrium. The results obtained for each parameter were also represented on three‐dimensional diagrams using an empirical smoothing method. The empirical modelling data were in agreement with the experimental ones. The main advantage was sought between E, which has reached 99% in the case of neutral red, whereas in the case of mehtylene blue 92% was obtained with respect to (φC), which should have a minimum value. At the optimal conditions, neutral red and methylene blue concentrations in the effluent were reduced to about 500 times and 7 times, respectively.

Adsorption of aqueous alkylphenol ethoxylate surfactants by mesoporous carbon CMK-3

Mesoporous carbon, CMK-3, was prepared using hexagonal SBA-15 mesoporous silica as the template and the adsorption of nonylphenol ethoxylates (NPE) onto CMK-3 was investigated. The adsorption process was well described using pseudo-second-order kinetics. At initial NPE concentrations of 107 and 530 mg l −1, the adsorption rate constants were found to be 5.6 × 10 −3 and 8.7 × 10 −4 g mg −1 min −1 , indicating that a higher initial concentration or adsorption amount resulted in a lower adsorption rate. NPE adsorption onto CMK-3 fitted a Langmuir–Freundlich model and the maximum amounts of NPE absorbed at 15, 25, and 35 °C were 923, 720, and 463 mg g −1, suggesting an elevated adsorption capacity of CMK-3 for NPE with decreased adsorption temperature. In addition, increasing adsorption temperature led to the change of the adsorption model from the Langmuir–Freundlich to the Langmuir model. N 2 adsorption results showed that the adsorption of NPE led to a decrease in the mesopore volume of CMK-3. However, the pore width of NPE-loaded CMK-3 was found to be identical to that of CMK-3.

The Removal of Alkylphenol Ethoxylate Surfactants in Activated Sludge Systems

The Removal of Alkylphenol Ethoxylate Surfactants in Activated Sludge Systems

Estuarine and coastal zone marine pollution by the nonionic alkylphenol ethoxylates endocrine disrupters: Is there a potential ecotoxicological problem?

The nonionic biodegradation-resistant (“hard”) alkylphenol ethoxylate (APEO) surfactants and their degradation products are known endocrine disrupting chemicals (EDCs). We report here the findings concerning the APEOs concentrations and homologic distribution profiles in Israel's estuarine and coastal zone seawater to serve as a case study. The concentrations in sewage-containing rivers, estuaries and 50–60-m offshore sea (Mediterranean) water were found to be 12.5–75.1, 4.2–25.0 and 0.9–2.6 μg/L, respectively. The corresponding homologic distribution profiles were found to be within the range of 1–10% each, somewhat skewing, as expected, towards the more toxic shorter-chain ethoxylates. Egg production by zebrafish, exposed to 75, 25 and 10 μg/L of a typical industrial APEOs was reduced up to 89.6%, 84.7% and 76.9%, respectively, between the 8th and 28th days of exposure. Apparently, there is a potential APEOs-related ecotoxicological health risk problem.

The case of the nonionic alkylphenol ethoxylates in the Mediterranean Sea region: is there a problem?

The concentration profiles of the potential endocrine disrupting nonionic alkylphenol ethoxylate (APEO) surfactants in Israel's rivers, groundwaters and coastal water of the eastern Mediterranean Sea, were found to be within the range of 12.5-74.6, trace - 20.2 and 4.2-25.0 microg/L respectively. Determination of the APEO's homologic distribution revealed "skewing" towards the more toxic shorter-chain ethoxylates. Egg production of zebrafish, Danio rerio, exposed to these actually found the environmental concentrations range of the APEOs decreased, after 20 days, to 89.6+/-2.1, 84.7+/-3.9 and 76.9+/-2.2% of the baseline levels, compared with control, in concentrations of 10, 25 and 75 microg/L respectively. These results suggest that, (a) there is a potential health problem, particularly in countries in which the "hard"/environmentally persistent APEOs are still in use; and (b) the related health-risk is seasonally-dependent, particularly in semi-arid regions where the fluctuations in the water quantities in surface- and groundwater are substantial.

Multi-component analysis of polar water pollutants using sequential solid-phase extraction followed by LC-ESI-MS.

A multi-component screening analysis method for polar to medium-polar water pollutants was developed. Sample clean-up and group separation are performed by sequential solid-phase extraction (SSPE) using automated SPE with C18 and polymeric sorbent materials. Analyses are performed by liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS) using a single-quadrupole instrument. More than 90 priority compounds of environmental interest--comprising the most important chemical and substance classes: phenols, carboxylic acids, aromatic sulfonates, aromatic amines, pharmaceuticals, surfactants, dyes, and pesticides--have been chosen for the experiments. The compounds are divided by the SSPE procedure into 3 different polarity classes. The extraction recoveries were determined in the 3 fractions for every single substance, and were for most of the analytes in the range of 50-100%. A mixture of hexane-dichloromethane was used for the elution of nonpolar compounds like alkylphenols from C18. Methanol and acetone are well suited for the elution of more polar substances. The limits of detection (LODs) were determined for all compounds. Effluents from municipal and industrial wastewater treatment plants (WWTPs) treating waste water from textile industries; and the corresponding receiving waters (rivers and lakes) have been analysed with the developed method. Urban and industrial pollution was observed in rivers and streams in the area north of Milan, Italy. In the water samples different phenols (nitrophenols, bisphenol A, nonylphenol), alkylphenol ethoxylate surfactants, their metabolites with endocrine disrupting potential, aromatic sulfonates, linear alkylbenzenesulfonate surfactants, dyes, pesticides, pharmaceuticals, and a dichlorobenzidine compound were identified.

A flow immunoassay for alkylphenol ethoxylate surfactants and their metabolites—questions associated with cross-reactivity, matrix effects, and validation by chromatographic techniques

This paper describes the application and evaluation of a competitive enzyme flow injection immunoassay (EFIIA) for screening of alkylphenol ethoxylate (APEO) surfactants in different water samples based on a generic immunoassay system previously developed (see E. Burestedt, C Nistor, U. Schagerlof and J. Emneus, Anal. Chem., 2000, 72, 4171–4177). The detection limits for octylphenol ethoxylates (OPEOs), nonylphenol ethoxylates (NPEOs), and nonylphenol (NP) were 0.5 µg l−1, between 2 and 3 µg l−1, and 50 µg l−1, respectively, with a sample throughput of 6 h−1 (i.e., for triplicate analysis of each sample). Different OPEOs and NPEOs were highly cross-reactive within the assay, with sensitivities in the same order of magnitude for all the ethoxylates tested, thus the result obtained by the EFIIA method could be used as an “alkylphenol ethoxylate index”. No or minor matrix effects with recoveries between 70–120% for the reference analyte NPEO10 in tap, and surface water, and acceptable for rainwater, were observed. Influent and effluent surfactant containing wastewater samples were analysed by EFIIA, LC-MS, LC-Fluoresence (LC-FL), and a commercial microplate ELISA. High recoveries for different concentrations of APEO10 spiked into a 200 times diluted raw influent and effluent wastewater were achieved with the EFIIA method, however, the found APEO content of the same diluted wastewater samples, before spiking, could not be correlated directly to the chromatographic result by any of the immunoassays, and the possible reasons for this are discussed. The same trend of decreasing APEO content from influent to effluent wastewater could, however, be followed for all methods employed.

Ultimate biodegradation of alkylphenol ethoxylate surfactants and their biodegradation intermediates

The ultimate biodegradation of the commercial products nonylphenol ethoxylates (NPE9), octylphenol ethoxylates (OPE9), and their biodegradation intermediates, NPE1.5, OPE1.5, NP, and OP, was measured using conventional methods. The extent of biodegradation was also determined by gas chromatography/mass spectrometry (GC/MS) and high-performance liquid chromatography (HPLC) analysis of metabolites and measurement of dissolved organic carbon. Optimized pseudo-first order degradation rate constants and half-lives were calculated from these data and from previously published data, using Larson's equation for surfactant degradation kinetics. Ultimate aerobic biodegradation of alkylphenol ethoxylates and their biodegradation intermediates has now been examined using acclimated wastewater treatment plant sludge as the microbial seed, household wastewater treatment systems, and unacclimated fresh water, seawater, and soil. Ultimate biodegradation half-lives ranged from about one to four weeks and provide definitive evidence for the extensive biodegradability of alkyphenol ethoxylates (APE) surfactants and their metabolites and their lack of persistence in the environment.

Measures for Protecting Water Quality: Current Approaches and Future Developments

An important component in protecting the ecological quality of watercourses is the regulation of point source discharges. These are usually expressed in terms of “end of pipe” limits on certain physical parameters or on the concentrations of specific chemicals. Sometimes discharge licences take the form of Emission Limit Values (ELVs) which are based on Best Available Technology with respect to emissions and apply to all discharges (usually for a specified industry sector) irrespective of the dilution capacity of the receiving watercourse. Alternatively, they may be based on water quality standards (e.g., EQSs) for specific chemicals which describe thresholds below which no adverse impact on the receiving water is predicted and which take explicit account of available dilution at different discharge locations. The strengths and limitations of these approaches are reviewed, along with a consideration of new approaches that address some of the limitations associated with control measures based on ELVs or EQSs. These include approaches for controlling well-defined mixtures by a “Toxic Equivalent” approach, exemplified here for discharges containing alkylphenol ethoxylate surfactants, and a fundamentally different approach for regulating complex effluents based on Direct Toxicity Assessment of whole effluents. Circumstances under which these different approaches to controlling point source emissions to surface waters might be appropriate are discussed.

Distribution and fate of neutral alkylphenol ethoxylate metabolites in a sewage-impacted urban estuary.

The distribution and fate of neutral metabolites of the alkylphenol ethoxylate (APEO) surfactants in an urbanized estuarine environment were examined utilizing a recently developed, highly sensitive LC-MS method. Results indicated that short ethoxyl-chain APEOs and alkylphenols (APs) were present in surficial sediments throughout the estuary at concentrations roughly correlated to the organic carbon content of the sediment and that the APEO mixture was dominated by nonylphenol ethoxylate (NPEOs) metabolites (0.05-30 microg/g), with lesser amounts of octylphenol ethoxylate metabolites (OPEOs)(<0.005-0.09 microg/ g) and halogenated nonylphenols (<0.001-0.03 microg/g). NPEO metabolites in surface water (0.22-1.05 microg/L) were also present at higher concentrations than OPEO metabolites (0.007-0.040 microg/L). APEO metabolite concentrations in both sediment and water showed a strong correlation with conventional sewage tracers, affirming a wastewater source of these contaminants. APEO distributions in surface waters within the estuary could be explained by a combination of post-discharge degradation and mixing with a seawater end-member enriched in OPEO metabolites. Measured in situ Koc values of APEO metabolites were comparable to previously reported values derived from field experiments but higher than Kow and Koc values derived from laboratory experiments. Results from the present work indicate that the fate of APEO metabolites entering the estuarine environment through discharge of wastewater is directed primarily by scavenging onto particles and subsequent burial in sediments, degradation during residence in the water column, and transport out of the estuary through advective and dispersive processes.

Estrogenic potency of chemicals detected in sewage treatment plant effluents as determined by in vivo assays with Japanese medaka (Oryzias latipes)

Gonadal intersex and high prevalences of the female phenotype have been observed in fish populations in urbanized areas. Environmental estrogens discharged in sewage treatment plant effluents may be responsible for feminization of fish but many compounds with the potential to induce these responses occur in effluents, including natural and synthetic estrogen hormones, degradation products of alkylphenol ethoxylate surfactants, and plasticizers. In this study, the estrogen hormones 17 alpha-ethinylestradiol, 17 beta-estradiol, estrone, and estriol induced intersex (i.e., testis-ova) and altered sex in Japanese medaka (Oryzias latipes) when these fish were exposed to nanogram per liter concentrations of test compounds from hatch to approximately 100 d after hatch. A mix of nonylphenol mono- and diethoxylate induced a weak response and a mix of nonylphenol mono- and diethoxycarboxylate did not give a response in this assay at microgram per liter concentrations, indicating that these degradation products of nonylphenol ethoxylates have little or no estrogenic activity in fish. Bisphenol A induced testis-ova in medaka exposed to a concentration of 10 micrograms/L, but diethylhexyl phthalate did not induce a response. Results with the medaka assay were consistent with estrogenic responses in the yeast estrogen screening assay. Analyses of monitoring data reported in the literature indicate that concentrations of estrogen hormones detected in the final effluents of sewage treatment plants are generally greater than the lowest-observed-effect levels for alterations to gonadal development in medaka.

Analysis of alkylphenol ethoxylate metabolites in the aquatic environment using liquid chromatography-electrospray mass spectrometry.

A quantitative method is described for the analysis of the metabolites of alkylphenol ethoxylate (APEO) surfactants in estuarine water and sediment samples using reversed-phase high-performance liquid chromatography with electrospray mass spectrometry detection. Nonyl- and octylphenols, nonyl- and octylphenol mono-, di-, and triethoxylates, halogenated nonylphenols, and nonylphenol ethoxycarboxylates were concentrated from water samples using a C18 solid-phase extraction procedure. A novel, continuous-flow, high-temperature, sonicated extraction system was developed to isolate APEO metabolites from sediment samples. Quantitative LC-MS was performed in the negative ion mode for nonylphenols, octylphenols, and halogenated nonylphenols and in the positive ion mode for nonyl- and octylphenol ethoxylates using selected ion monitoring with isotopically labeled surrogate standards. Recoveries for sediment and water analyses ranged between 78 and 94%, and detection limits for APEO metabolites were between 1 and 20 pg injected on column. This is a significant improvement over previously reported methods. Suppression of analyte response was encountered in the presence of matrix components in sediment samples, but this effect was eliminated by careful selection of surrogate and internal standards. Individual APEO metabolite concentrations of 1-320 ng/L and 5-2000 ng/g are reported for water and sediment samples, respectively, from Jamaica Bay, NY.

Effects of colloidal stabilizer on vinyl acetate-ethylene copolymer emulsions and films

For vinyl acetate-ethylene (VAE) emulsion polymerization, a comparison of stabilization mechanisms and their effect upon latex and film properties was made in which the different stabilizers were poly(vinyl alcohol) (PVOH), alkylphenol ethoxylate (APE) surfactants, and a urethane linked poly(ethylene glycol) (PEG) based polymer. PVOH stabilized VAE emulsions possessed high viscosity and no freeze-thaw stability while the films were actually continuous in PVOH, much of which was due to the high content of PVOH in the continuous aqueous phase. APE stabilized VAE latexes were also of higher viscosity and the surfactants were miscible in the VAE continuous films. PEG-based stabilization resulted in emulsions and films which were largely affected by the hydrophobicity of the latex (amount of ethylene incorporated into the copolymer).

Distribution of degradation products of alkylphenol ethoxylates near sewage treatment plants in the lower Great Lakes, North America

Degradation of alkylphenol ethoxylate (APEO) surfactants in the environment leads to the formation of relatively hydrophobic compounds such as nonylphenol (NP), octylphenol (OP), nonylphenol monoethoxylate (NP1EO), and nonylphenol diethoxylate (NP2EO) that have been shown to have estrogenic activity. Previous studies have shown that sewage treatment plants (STPs) are point sources for these compounds in the aquatic environment. We collected sediment samples at several sites in the vicinity of STPs in Hamilton Harbour and in the Detroit River to determine the spatial distribution of the degradation products of APEOs. In addition, we deployed semipermeable membrane devices (SPMDs) and caged freshwater mussels (Elliptio complanata) at these locations to determine the distribution of these compounds in the dissolved phase and their potential to bioaccumulate in aquatic organisms. The NP, OP, NP1EO, and NP2EO were found at μg/g (dry wt.) concentrations in sediments and accumulated to ng/g (wet wt.) concentrations in caged mussels near the STPs. However, in the Detroit River, the concentrations of these compounds declined to near background levels in the sediments, water column (i.e., SPMDs), and biota at stations about 1 km downstream from STPs. At stations in Hamilton Harbour, concentrations of APEO degradation products also declined markedly in sediments and SPMDs located a few hundred meters from the STP. These data indicate that degradation products of APEOs can be accumulated by biota near STPs. However, the environmental distribution of these compounds is localized to areas close to the point of discharge.

Degradation of Alkylphenol Ethoxylate Surfactants in Water with Ultrasonic Irradiation

During the last years, many efforts have been devoted to the elimination of alkylphenol ethoxylate surfactants from aqueous systems. In this paper, the sonochemical degradation of aqueous solutions of Triton X-100 was performed at an ultrasonic frequency of 358 kHz and an applied power of 50 W. Analysis of the reaction products by HPLC-ES−MS suggests that the hydrophobic alkyl chain is the preferential site for oxidation. Alkylphenol, or short-chain ethoxylated phenols, were not generated as byproducts. To verify this hypothesis, the sonochemical degradation of the corresponding alkylphenols (e.g., tert-octylphenol) was performed under the same conditions; in these cases, similar rate constants and products were observed. These results differ from those reported for the biodegradation of alkylphenol ethoxylates. A substantial increase in the rate constant was observed for the degradation of Triton X-100 below its critical micelle concentration. This observation indicates that micelle formation serves to effectively isolate the free surfactant monomers from the water−air interface of the oscillating cavitation bubbles, thus decreasing the overall efficiency of the sonochemical process. The hydrophobic tail of the molecule is no longer exposed directly to the bubble “hot spot” when it is pointed into the core of the micelles.