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CORRELATIONS Correlation and Prediction of Environmental Properties of Alcohol Ethoxylate Surfactants Using the UNIFAC Method Hongyuan Cheng,† Georgios M. Kontogeorgis,* and Erling H. Stenby Centre for Phase Equilibria and Separation Processes (IVC-SEP), Department of Chemical Engineering, Building 229, Technical University of Denmark, DK-2800 Lyngby, Denmark
Environmental properties of one type of nonionic surfactants, the alcohol ethoxylates (polyoxyethylene alcohols), are predicted using the UNIFAC (universal quasi-chemical functional group activity coefficient) method. Various properties are considered; the octanol-water partition coefficient (Kow), the bioconcentration factor (BCF), and the toxicity. Kow values of alcohol ethoxylates are difficult to measure. Existing methods such as those in commercial software like ACD,13 ClogP,14 and KowWin15 have not been applied to surfactants, and they fail for heavy alcohol ethoxylates (alkyl carbon numbers above 12). Thus, the Kow values are predicted here via UNIFAC and compared to the few available experimental data. Based on the predicted Kow values, a correlation between Kow and hydrophilic-lipophilic balance (HLB) is established because HLB is a widely used parameter in surfactant applications. Finally, BCF and toxicity of alcohol ethoxylates are correlated with their Kow. The proposed approach can be extended to other families of nonionic surfactants. Introduction The octanol-water partition coefficient (Kow), the toxicity, and the bioconcentration factor (BCF) are important properties in the assessment of environmental and health effects of chemicals. To represent these properties, quantitative structure-activity relationships (QSARs) are widely employed.1,2 Toxicity and BCF are parameters that can describe effects and distributions of chemicals between organism and surroundings because enrichment of chemicals in an organism is considered as a distribution process from surroundings to the organism. Among the various physicochemical properties, the octanol-water partition coefficient (Kow) is one of the most often used to set up a QSAR for toxicity or BCF since Kow is a quantitative measure of the hydrophobic and hydrophilic nature of organic chemicals. Kow of chemicals has also been successfully used for QSARs in many fields,3 such as drug and pesticide design, pharmaceuticals, anaesthesiology, environmental transport and soil binding, toxicology, bioaccumulation, protein folding, enzyme binding, enzymatic reactions in nonaqueous solvents, and host-guest complexation. However, Kow of surfactants are seldom studied due to experimental difficulties. Some commercial tools such as ACD,13 ClogP,14 and KowWin15 have been widely used to estimate Kow. However, surfactant molecules have not been considered in these studies. Moreover, these commercial tools fail to esti* Corresponding author. Fax: +45-45882258. Tel: +45 45252859. E-mail:
[email protected]. † Present address: Food Biotechnology & Engineering, BioCentrum-DTU, Technical University of Denmark, Building 221, DK-2800, Lyngby, Denmark.
mate the Kow values when the alkyl carbon number of a hydrocarbon compound exceeds 12. Thus, an estimation method for the Kow of surfactants is needed, and this is one of the targets of this work. The UNIFAC method4 is selected and evaluated for Kow estimation and prediction for a sample surfactant-alcohol ethoxylate as UNIFAC has a molecular thermodynamic background and has been widely accepted by industrial and research applications. Madsen et al.5 investigated the toxicity and BCF of surfactants and household detergents. In the work of Madsen et al., various experimental data and environmental effects of surfactants are collected or measured. On the basis of a large amount of experimental data, environmental and human hazard assessments have been given for these household detergents. However, quantitative structure-activity relationships (QSARs) for the environmental-related properties were not developed in the work of Madsen et al.5 In this work, QSARs of toxicity and BCF are developed for alcohol ethoxylates based on Kow values estimated using UNIFAC. Correlations between Kow and HLB (hydrophilic-lipophilic balance) are also developed. Kow Calculation and Prediction Octanol-Water Partition Coefficient. The partition coefficient of a single compound i between the octanol (O) and water (W) phases can be defined for dilute solutions at 25 °C as
Kow )
COγiW,∞ CWγiO,∞
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where C O and CW represent the total molar concentrations of the octanol-rich and water-rich phases at 25 °C, respectively. Because only a small amount of solute is added to the octanol-water mixture, the solute concentration is very small in both phases. Thus the activity coefficients at infinite dilution (γiW,∞, γiO,∞) are used. Using the solubility data of water and octanol in waterrich phase and octanol-rich phases from the Handbook of Chemical Property Estimation Methods,6 the Kow expression in eq 1 becomes
Kow ) 0.151
γiW,∞ γiO,∞
(2)
The solubilities of water and octanol in the waterrich phase and octanol-rich phase at 25 °C are, respectively, 55.3 mol/L for water and 4.5 × 10-3 mol/L for octanol in the water-rich phase and 2.30 mol/L for water and 6.07mol/L for octanol in the octanol-rich phase. Experimental Data and Calculation Methods. Many experimental methods have been developed to measure Kow for different chemicals.7 Experimental Kow data for more than 20 000 organic compounds have been collected.8 However, only few Kow data for surfactants9 are available due to experimental difficulties. Surfactants are amphiphilic molecules that include both a hydrophilic and a hydrophobic part. The surfactant molecules are adsorbed at the interface between the two different fluids (e.g., in the water-air and the water-octanol interfaces). Thus, they can significantly change the surface or interfacial tension. This property of surfactant molecules is useful in numerous industrial products (e.g., detergents, washing powders, etc.). However, exactly because of this surface adsorption, the Kow measurements are difficult for surfactants. While performing Kow measurements, the surfactant molecules accumulate at the octanol-water interface, which results in lower surfactant concentration in the bulk solvent. At the same time, the surfactants may also form emulsions and enhance the mutual solubility of octanol and water by strong interaction with octanol even below their critical micelle concentration.10 Thus, it is useful to develop a predictive method to evaluate the Kow of surfactants since the experimental data are often not available. This work is an effort toward this direction. Various Kow calculation methods have been systematically compiled and carefully compared by several researchers7,11,12. A number of commercial tools are also available for Kow prediction, such as ACD/LogP, 13 KowWin,14 and ClogP.15 Surfactant molecules have not been considered with these models. In eq 1, the activity coefficients at infinite dilution can be calculated from activity coefficient models, such as UNIFAC4sa group contribution method based on the local composition concept. This method has been successfully used to predict many physical properties especially in chemical engineering applications (e.g., vapor-liquid equilibrium,4 liquid-liquid equilibrium,16 etc.). Kow has also been studied with UNIFAC by several researchers.17,18,12 UNIFAC has been proven in some cases to be a good model for predicting Kow for various chemicals, although surfactants have not been so far studied. In this study the UNIFAC method is selected for estimating Kow of surfactants. Based on the comprehensive review of different UNIFAC methods for Kow calculations,12,19 the UNIFAC LLE1 (one temperature-
Figure 1. Experimental and predicted Kow values for CnE1. Table 1. Methods Used for Kow Calculations method
abbreviation
ref
original UNIFAC VLE 1 UNIFAC LLE 1 UNIFAC VLE 2 modified UNIFAC VLE 3 water-UNIFAC UNIFAC for surfactants ClogP ACD/LogP KowWin solvation model
VLE1 LLE1 VLE2 VLE3 H2O Micelle ClogP ACD Kowwin GCS
26 16 27 28 17 20 15 13 14 21
independent parameter) and water-UNIFAC were recommended for predicting the Kow. A new UNIFAC model (Micelle) has been also recently developed for predicting the critical micelle concentration for nonionic surfactants.20 In the “Micelle UNIFAC”, a new functional group (CH2CH2O) was introduced into UNIFAC VLE1 (one temperature-independent parameter); the interaction parameters were obtained from vaporliquid equilibrium data. These different UNIFAC methods are used to calculate Kow in this work. Alcohol ethoxylates (polyoxyethylene alcohol, R(CH2CH2O)nOH, R is alkyl chain) are widely used as surfactants in industrial applications and represent the nonionic surfactants studied in this work. Alcohol ethoxylates are often abbreviated as CiEj, where i is the number of alkyl carbon (CH3-, CH2-) in the R group and j is the number of oxyethylene groups (CH2CH2O-) in the molecule. Because of the lack of experimental data for surfactants, the available Kow data from the same alcohol ethoxylate family, such as 2-methoxyethanol (C1E1), 2-ethoxyethanol (C2E1), etc., are very useful to evaluate the calculation methods even though these are not strictly speaking surfactant molecules. The Kow values of alcohol ethoxylates calculated by the UNIFAC model are also compared to three commercial tools, namely, ClogP, ACD, KowWin, and a recently developed group contribution solvation model.21 All methods are summarized in Table 1. The prediction results of the various models are compared to the limited experimental Kow data for alcohol ethoxylates, taken from the LOGKOW databank.8 Results and Discussion. Kow prediction results for alcohol ethoxylates with the various methods and comparison to experimental data are presented in Figures 1-5 and in Table 2. Based on these results, the following points summarize our observations: Kow Prediction for Alcohol Ethoxylates. It is useful to identify which alcohol ethoxylates (see Table 3) can be classified as surfactant molecules. D’Arrigo et al.22 suggested that the C4E1 molecule is the shortest alcohol ethoxylate surfactant. Following D’Arrigo et al.,
Ind. Eng. Chem. Res., Vol. 44, No. 18, 2005 7257 Table 2. Mean Deviation in Kow Prediction for Alcohol Ethoxylates with Different Methodsa method
mean deviation %
method
mean deviation %
ClogP Kowwin ACD GCS Micelle
33 39 22 311 199
VLE1 LLE1 H2O VLE2 VLE3
36 287 80 48 135
a Abbreviations as in Table 1. Experimental data used in the prediction are listed in Table 3. The mean deviation is defined as follows with n as the total number of data:
Figure 2. Experimental and predicted Kow values for C4En.
mean deviation % )
1 n
n
∑ 1
cal | log Kexp ow - log Kow | % | log Kexp ow |
Table 3. Log Kow Calculation Results for Alcohol Ethoxylates Using Equations 3 and 4
Figure 3. Experimental and predicted Kow values for C6En.
Figure 4. Predicted Kow values for CnE6.
Figure 5. Predicted Kow values for C12En.
four compounds in Table 3 (C4E1, C4E2, C6E1, and C6E2) can be classified as surfactants. It is of interest to investigate whether UNIFAC can provide reasonably correct Kow values for both nonsurfactants (C1E1C3E1) and surfactant molecules (C4E1-C6E1) including both short chain and longer chain surfactants (C4E1, C4E2-C6E1, C6E2).
compound
expa
VLE1b
eq 3
eq 4
2-methoxethanol (C1E1) 2-ethoxyethanol (C2E1) 3,6-dioxa-1-octanol (C2E2) iso-propoxyethanol (C3E1) 2-butoxyethanol (C4E1) 3,6-dioxadecanol (C4E2) 2-(hexyloxy) ethanol (C6E1) 3,6-dioxa-1-dodecanol (C6E2) mean deviation %
-0.77 -0.28 -0.54 0.05 0.8 0.56 1.86 1.7
-0.83 -0.38 -0.75 0.07 0.51 0.15 1.41 1.04 36
-0.82 -0.37 -0.74 0.07 0.52 0.15 1.41 1.04 36
-0.83 -0.39 -0.75 0.06 0.51 0.14 1.40 1.03 35
a Exp, experimental data from Sangster.8 UNIFAC VLE 1.25
b
VLE1, original
The Kow values can assist in interpreting the distribution trend of chemicals. Hydrophilic chemicals with low Kow values will be largely partitioned into the waterrich phase. Thus, the longer hydrophilic group chain a chemical has, the lower Kow value is expected. This means that the C4E2 should have lower Kow value than C4E1 because the C4E2 has two hydrophilic oxyethylene groups (two CH2CH2O groups). Similarly, longer hydrophobic chains of a chemical indicate a larger Kow value (i.e., Kow must increase from C1E1 to C6E1). All experimental Kow data (see Table 3) agree with these general considerations. As shown in Table 2, the three commercial tools (ACD, ClogP, KowWin) and UNIFAC VLE 1 (VLE1) perform similarly in predicting Kow for the investigated compounds with ACD showing the lowest deviation. UNIFAC VLE 1 is the best among the various UNIFAC methods for Kow calculations. The GCS method for mono-functional chemicals and the UNIFAC micelle method yield larger deviations. Plots of log Kow against the chain length are useful for verifying the model prediction results following the physical characteristics of chemicals (i.e., the distribution trend). Such plots are shown in Figures 1-3 for three types of alcohol ethoxylate surfactants. Figure 1 shows that all methods predict the correct trend with increasing alkyl group numbers n in CnE1. However, Figures 2 and 3 indicate that the ACD and ClogP methods exhibit different trends from the VLE1, KowWin, and GCS models. It seems that only VLE1, KowWin, and GCS follow the experimental trend (i.e., decreasing Kow with increasing oxyethylene group number n in C4En and C6En based on the few data available). Based on these comparisons, it can be concluded that UNIFAC VLE 1 provides very good qualitative (often
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quantitative) Kow predictions for the investigated systems. UNIFAC VLE 1 is a method that only uses group parameters based on vapor-liquid equilibrium (VLE) data. These VLE systems do not contain any surfactant molecules. Thus, the results can be considered to be straight predictions. Kow Prediction for Alcohol Ethoxylates in Case No Experimental Data Are Available. The Kow values of alcohol ethoxylates, for which no reported experimental Kow data are available, are calculated using UNIFAC VLE 1 (VLE1), commercial tools and the GCS solvation model. Some typical results are shown in Figures 4 and 5. The Kow calculations from ACD, ClogP, and KowWin produce unreliable results when the alkyl carbon number of alcohol ethoxylates exceeds 12 (indicated by the error messages in the software).
where r2 is the correlation coefficient; nC is the number of alkyl group (CH2), nC ) 4-16; and nEO is the number of oxyethylene group (OCH2CH2), nEO ) 1-30. Table 3 presents a comparison between the calculation results from different methods and experimental data. The log Kow values calculated from eqs 3 and 4 with UNIFAC VLE1 method (VLE1) are very close to each other. It can be seen that these simple correlations (eqs 3 or 4) can be used to estimate log Kow value instead of UNIFAC VLE1 method for alcohol ethoxylates. In Table 3, the mean deviation is defined as in Table 2. Equations 3 and 4 are then used to estimate Kow in environmental property correlations in the following sections. Correlations between Kow and Environmentally Related Properties
Correlation of Kow with HLB As discussed above, Kow data of alcohol ethoxylates can be estimated through UNIFAC in the lack of experimental Kow data. Such Kow data can be then used in the development of correlations for the bioconcentration factor, toxicity, or HLB (hydrophilic-lipopholic balance)sa parameter commonly used in surfactant science. HLB is a parameter (in a scale from 1 to 40) that characterizes the effectiveness of polyoxyethylene surfactants as emulsifiers based on their molecular structure.23 HLB is nowadays widely used in comparison of surfactants for various applications. Lower numbers indicate high solubility in oil (hydrophobic) while the higher numbers indicate high solubility in water (hydrophilic). HLB value is often reported for surfactant products. Davies and Rodeal24 suggested a simple groupcontribution method for estimating HLB of surfactants when their structure is known. In many cases, such estimated HLB numbers are in satisfactory agreement to experimental data. However, this method is based on the assumption that each individual group has the same hydrophilic or hydrophobic contribution in all situations, which is not always true. Similarly to Kow, the empirically based HLB parameter also represents the distribution of surfactants in hydrophilic and lipopholic substances. However, in environmental studies, use of Kow is preferred. Some correlations between HLB and partition coefficients can be found in the literature. Davies and Rideal24 proposed a linear correlation for HLB and the partition coefficients. Birdi25 suggested that this partition coefficient could be replaced by the octanol-water partition coefficient (Kow). However, no correlation between Kow and HLB is reported for alcohol ethoxylates. In this work, HLB of alcohol ethoxylates (R(CH2CH2O)nOH, CiEj) are directly calculated by Davies’ method. Kow values are obtained from UNIFAC VLE1. Based on these, two different linear correlations between Kow and HLB can be obtained as following:
log Kow ) 9.05947 - 0.08492nC - 1.11858HLB
Surfactants as parts of commercial cleaning products are widely used in our daily life. Every year, millions of tons of surfactants are consumed and then discarded into the environment. The environmental effect and health hazard assessment for the surfactants are therefore of high importance. One of the important aspects for the assessment of the environmental effect is to study the relationships between the physical properties of chemicals (e.g., water solubility or Kow and environmental effects, for example, toxicity, biodegradation, etc.). In this work an attempt is made to develop relations between Kow and environmental properties for surfactants. Madsen et al.5 reported the environmental and health hazard assessment of substances in household detergents and cosmetic detergent products. In the work of Madsen et al.,5 a large amount of experimental data have been collected and classified according to the type of surfactants. These data have been used to give environmental effects and health hazard assessment of household detergents to a governmental organization. Based on the data collection of Madsen et al.,5 correlations between Kow and environmental properties are developed in this work for alcohol ethoxylates. Correlation between Kow and Bioconcentration Factor (BCF). The BCF is a measure of the bioaccumulation of a chemical in aquatic organisms, such as fish. It is defined as:
BCF )
mean measured concentration in fish mean measured concentration in water
BCF represents the accumulation of chemicals in aquatic organisms and indicates the amount of chemical concentrated in organisms. For example, the BCF value for C14E7 (26 °C) is 799,5 which means that the concentration of C14E7 in aquatic organisms is around 799 times higher than in water. Most chemicals are indeed highly hydrophobic. It has been observed29 that the most common method for estimating BCF is based on its correlation to log Kow. A linear equation is often used:
log BCF ) a + b log Kow
(5)
2
r ) 0.999 (3) log Kow ) 7.46141 - 0.05685nEO - 0.94101HLB r2 ) 0.997 (4)
BCF QSAR equations derived from log Kow are widely available in the literature.30-32 However, no such correlations have been reported for alcohol ethoxylates.
Ind. Eng. Chem. Res., Vol. 44, No. 18, 2005 7259 Table 4. Experimental and Model Calculation Results for Whole Body BCF of Alcohol Ethoxylates in Fish log BCF surfactant/species
expa
cal
log Kow
C12E4, carp (Cyrinus carpio) C12E8, carp C12E16, carp C13E4, fathead minnow C13E8, fathead minnow C14E4, fathead minnow C14E7, bluegill subfish C14E8, fathead minnow C14E11, fathead minnow C14E14, fathead minnow C16E8, fathead minnow
2.49 2.35 0.63 2.37 1.62 2.37 2.90 1.98 1.20 0.70 2.59
2.30 1.65 0.35 2.50 1.85 2.69 2.21 2.04 1.56 1.07 2.44
2.98 1.51 -1.441 3.43 1.96 3.877 2.77 2.402 1.296 0.19 3.296
a
Experimental BCF values are from Madsen et al.5
Figure 6. Log BCF of alcohol ethoxylates in fish against log Kow, calculated from eq 6. Experimental BCF values are from Madsen et al.5
Figure 8. Correlation results of Kow and toxicity EC50 of alcohol ethoxylates to invertebrates. Experimental EC50 data are from Madsen et al.5
Figure 7. Correlation results of Kow and toxicity (EC50) of alcohol ethoxylates to algae. Experimental EC50 data are from Madsen et al.5
On the basis of the data collection of Madsen et al.,5 a correlation between BCF and Kow can be obtained from linear regression for alcohol ethoxylates:
log BCF ) 0.98638 + 0.44038 log Kow r2 ) 0.85 (6) where r2 is the correlation coefficient. Table 4 and Figure 6 give the correlation results. In this correlation, the log Kow values of alcohol ethoxylates from eq 4 are used. Similar results can be obtained with Kow values based on eq 3. As can be seen from Table 4 (shown fish species) and Figure 6, eq 6 is in reasonably good agreement with the experimental BCF values and gives the correct BCF trend (i.e., the BCF value in fish decreases with increasing hydrophilic group (OCH2CH2) number of alcohol ethoxylates). Correlations between Kow with Toxicity. Toxicity is a very important property in environmental studies and is often used to establish regulations for the management of chemicals. Using Kow to correlate and predict toxicity is an accepted approach1. Kow is one of the standard parameters employed by the U.S. EPA (Environmental Protection Agency) to evaluate the
Figure 9. Correlation results of Kow and toxicity EC50 of alcohol ethoxylates to fish. Experimental EC50 data are from Madsen et al.5
toxicity of chemicals. In this work, a new correlation between toxicity and Kow is developed for alcohol ethoxylates. The toxicity of alcohol ethoxylates to algae, invertebrates, fish, and rat data are taken from the data collection of Madsen et al.,5 which includes various CiEj, i ) 9-18 and j ) 1-30. The Kow values are obtained from eq 4. Based on the toxicity and Kow values, linear
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Table 5. Correlations of Kow and Toxicitya species
correlation equation
Figure
r2
algae invertebrates fish rat
log 1/EC50 ) -1.13599 + 0.4507 log Kow log 1/EC50 ) -0.71905 + 0.18504 log Kow
7 8
0.67 0.52
log 1/EC50 ) -0.55571 + 0.1776 log Kow log LD50 ) 0.04356 + 0.18631 log Kow
9 10
0.76 0.88
a
EC50, mg/L; LD50, g/kg.
alternative Kow estimation method when there is no experimental Kow available. Subsequently, the estimated Kow values are used to correlate with HLB for surfactants, which is useful as HLB represents some of the same characteristics as the Kow parameter and is a very commonly used concept in applied surfactant science. Kow estimation results from UNIFAC are then transferred to simple equations and are easier to use in different applications. It is demonstrated that use of the Kow values obtained from the HLB-Kow correlations, QSAR correlations can be established for environmental properties (e.g., BCF and toxicity). Although applied to a specific family of nonionic surfactants, the methodology followed in this work can be applied to other families of nonionic surfactants as well. Acknowledgment The authors are grateful to the Danish Environmental Research Programme for financial support of this work. Literature Cited
Figure 10. Correlation results of Kow and toxicity LD50 of alcohol ethoxylates to rat. Experimental EC50 data are from Madsen et al.5
correlations are developed, and these are presented in Table 5 and graphically in Figures 7-10. In the equations shown in Table 5, EC50 is the concentration of a chemical causing a defined effect to 50% of a group of test organisms (e.g., immobilization or growth inhibition), and LD50 is the dosage causing death to 50% of the exposed animals after a single administration. As can be seen in Figures 7-10, the experimental toxicity data are rather scattered when plotted against Kow. The agreement is not quite as good as it was in the case of BCF-Kow plots. Still, Kow does capture qualitatively the trend of the experimental toxicity data. The scattered plots may indicate that toxicity is a more complex property and cannot be captured alone with Kow. Conclusions In this work, an approach is proposed to predict octanol-water partition coefficients (Kow) of surfactants using the UNIFAC method because experimental measurements of octanol-water partition coefficients (Kow) are very difficult for surfactants and such data are very scarce. Widely used Kow estimation methods (ACD, ClogP, and KowWin) do not contain enough Kow information of surfactants because of lacking experimental Kow data. The extrapolation of the Kow values of surfactants through ACD, ClogP, and KowWin methods can give unreliable results for longer chain surfactants. UNIFAC is a predictive group contribution model derived from molecular thermodynamics (local composition principles); it has thus a semi-theoretical background. It is shown here that it can provide reasonably accurate estimation for the Kow of the nonionic surfactants considered here. The UNIFAC method is a good
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Received for review January 24, 2005 Revised manuscript received June 12, 2005 Accepted July 8, 2005 IE050096B
