Environ. Sci. Technol. 1994, 28, 1829-1837
Solubilization of Nonaqueous Phase Liquid Hydrocarbons in Micellar Solutions of Dodecyl Alcohol Ethoxylates Mamadou S. Diallo, Linda M. Abrlola,’ and Walter J. Weber, Jr.
Environmental and Water Resources Engineering, Department of Civil and Environmental Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2125 Results of an experimental investigation of hydrocarbon solubilization in 0.01 M micellar solutions of dodecyl alcohol ethoxylates at 25 “C are presented. The effects of surfactant hydrophile-lipophile balance (HLB) and hydrocarbon molar volume and polarity on the molar solubilization ratios (MSRs) and micelle-water partition coefficients of 11 nonaqueous phase liquid (NAPL) hydrocarbons are examined. The MSRs of the alkanes (n-dodecane,n-decane, n-hexane, and cyclohexane), which sharply decrease and asymptotically approach zero with increasing HLB, are shown to depend on micellar core volume and hydrocarbon molar volume and affinity for the micellar core. In contrast, the MSRs of the aromatic hydrocarbons (benzene, toluene, chlorobenzene, o-xylene, and o-dichlorobenzene) and the chlorinated alkenes (trichloroethylene and tetrachloroethylene) exhibit a maximum within the same HLB range. This behavior appears to be the result of two opposing effects: (i) loss of solubilization capacity with increasing HLB and (ii) enhancement of solubilization capacity by electron donoracceptor (EDA) complexation. The logarithms of the micelle-water partition coefficients for all of the hydrocarbons follow the trends of the MSRs and increase with hydrocarbon hydrophobicity. Introduction Enhancement of the solubility of hydrophobic organic pollutants in aqueous solutions of ethoxylated nonionic surfactants is increasingly becoming the basis of a variety of new water treatment and aquifer/soil remediation processes. Scamehorn and Harwell (1)have described a number of water treatment processes based on the solubilization of hydrocarbon pollutants by micellar solutions of ionic and ethoxylated nonionic surfactants. These include micellar-enhanced ultrafiltration, admicellar chromatography, and surfactant-enhanced carbon regeneration. General Electric ( 2 ) ,Ellis et al. (3),Abdul and Gibson (4), Fountain et al. ( 5 ) ,and Pennell et al. (6) have reported the successful utilization of micellar solutions of ethoxylated nonionic surfactants to remove pollutants such as dodecane, tetrachloroethylene, and anthracene from contaminated soils. Chawla et al. (7) and West and Harwell (8) have discussed the utilization of surfactant solutions to enhance the remediation of aquifers and subsurface soils contaminated by residual and sorbed hydrophobic organic pollutants. Aronstein et al. (9) have observed an increase in the extent of biodegradation of sorbed phenanthrene in micellar solutions of alcohol ethoxylates. An enhancement of the rates of biodegradation of n-decane and n-tetradecane in aqueous solutions of linear alcohol ethoxylates has been reported by Burry and Miller (10). The efficiency of surfactant-based water treatment and aquifer/soil remediation processes will depend, to a large extent, on the 0013-936X/94/0928-1829$04.50/0
0 1994 American Chemical Society
solubilizing capacity of micellar solutions. Thus, an understanding of the effects of surfactant and hydrocarbon molecular properties and solution composition and temperature on solublization capacity is critical to the design and evaluation of these processes. The solubilization of hydrocarbons in micellar solutions of ethoxylated nonionic surfactants has been the subject of several experimental and theoretical investigations (1122). Research efforts have focused on the effects of hydrocarbon and surfactant molecular structure and solution composition and temperature on solubilization capacity. In dilute micellar solutions of ethoxylated nonionic surfactants, i.e., those with surfactant concentrations ranging from the criticalmicelle concentration (CMC) to approximately 0.2 M ( I 7), hydrocarbon solubility has been shown to be a linear function of surfactant concentration, to increase with temperature and surfactant lipophile chain length, and to be unaffected by the presence of small amounts of electrolytes. At a given temperature, the solubility has been shown to depend on surfactant hydrophile-lipophile balance (HLB) and hydrocarbon molecular structure. Kile and Chiou (15) reported that the micelle-water partition coefficient of 1,2,3-trichlorobenzene in aqueous solutions of octylphenol ethoxylates depends on the “nonpolar group content” of the surfactants. Edwards et al. (16) found a linear relationship between the logarithms of the micelle-water and octanolwater partition coefficients of naphthalene, phenanthrene, and pyrene in aqueous solutions of alkylphenol ethoxylates. UV, NMR, and ESR spectroscopy (14,17)have established that nonpolar and polar hydrophobic organic compounds (HOCs) are predominantly solubilized in the hydrophobic core and polyoxyethylene (POE) shell, respectively, as shown in Figure 1. Despite previous investigations, hydrocarbon solubilization in micellar solutions of ethoxylated nonionic surfactants is still not adequately understood. Experimental data on the solubility of chlorinated nonaqueous phase liquid (NAPL) hydrocarbons in micellar solutions of biodegradable and nontoxic ethoxylated nonionic surfactants of interest to surfactant-enhanced aquifer remediation (SEAR) are scarce. Virtually, all solubilization data for chlorinated NAPLs reported to date have been measured in aqueous solutions of alkylphenol ethoxylates (5, 15, 19). Because these surfactants have been shown to be resistant to microbial degradation (23), their utilization in surfactant-based environmental remediation processes such as SEAR could raise public and regulatory concern. Thus, there is a need for data on the solubilization of chlorinated NAPLs in micellar solutions of biodegradable and nontoxic ethoxylated nonionic surfactants of interest to SEAR. From a more fundamental point of view, the effect of surfactant HLB on the solubility of polar HOCs in these solutions is not well understood. It is not known, for example, why the solubility of some polar NAPLs in Environ. Sci. Technoi., Voi. 28,
No. 11, 1994 1829
\
/
\
/
Approximate solubilization loci of hydrophobic organic compounds in ethoxylated nonionic Surfactant micelles.
Figure 1.
aqueous solutions of ethoxylated nonionic surfactants increases to a maximum value and then subsequently decreases as surfactant HLB increases (5, 11). An understanding of this behavior will be needed to assess the effect of surfactant HLB on solubilization capacity and hydrocarbon activity and reactivity in micellar solutions of ethoxylated nonionic surfactants.
Objectives and Approach This paper describes an experimental investigation of thesolubilization ofnonpolar and polar NAPLs inmicellar solutionsofdodecylalcoholethoxylates. Thesesurfactants are nontoxic and biodegradable (23) and have been investigated for uses in aquifer and soil remediation (4, 10). They are widely employedin the formulation of drugs and detergents and have been the subject of several fundamental investigations (24). The primary objectives of this paper are to (i) gain insight into the mechanisms of NAPL solubilization in dilute solutions of dodecyl alcohol ethoxylates and (ii) determine the extent to which the solubilization capacity for NAPLs in these solutions can he correlated with surfactant and hydrocarbon molecular properties. To achieve these objectives, literature data relating to the effects of surfactant chain length on micellar size and spectroscopic information on hydrocarbonlethylene oxide (EO) interactions are combined with measured hydrocarbon solubilities in dilute solutions of dodecyl alcohol ethoxylates to assess the relationships between solubilization capacity and hydrocarbon and surfactant molecular properties. Specifically, the effects of surfactant HLB and hydrocarbon molar volume and polarity on solubilization capacity are examined. The HLB number of a surfactant is one of the most widely used indicators of its suitability for a given application (24, 25). It is a measure of a surfactant partitioning tendency between oil and water. The higher the HLB of a surfactant, the more it tends to partition into water. For an ethoxylated nonionic surfactant, the HLB may be expressed as (25)
HLB = mass%EO 5
(1)
where mass % EO is the percentage of EO in the surfactant on a mass basis. The HLB scale defined by eq 1ranges from 0 to 20. A hydrophobic group has an HLB of 0, 1820 Environ. Sd. Technol.. VoI. 28. No. 11. 1994
whereas an EO head group has an HLB of 20. For the most part, the HLBs of ethoxylated nonionic surfactants used in industrial applications range from 10 to 20 (24). The HLB number was introduced by Griffin to correlate the performanceof a nonionic emulsifier with its molecular structure (24). Use of the HLB number as a predictor of emulsion stability is predicated upon the fact that, for an homologous series of ethoxylated nonionic surfactants, there is an optimal surfactant HLB for emulsification of a given oil. Because the stability of an emulsion is a function of solution composition and temperature, the optimal HLB for emulsification of an oil has been shown to depend on solution temperature and background electrolyte composition, e.&, salinity and hardness (24). The HLB number defined in Eq 1,however, is not affected by solution physicochemical properties because it depends solely on surfactant hydrophile chain length. Two measures of solubilization capacity are employed in this study: the molar solubilization ratio and the micelle-water partition coefficient. The molar solubilization ratio (MSR), the number of moles of hydrocarbon solubilized per number of moles of surfactant micellized, can be calculated as follows (16):
where C-1 is the concentration of solubilized hydrocarbon, C., is the hydrocarbon aqueous solubility, C., is the surfactant concentration, and CMC is the critical micelle concentration. The micelle-water partition coefficient (K&, an important parameter for assessing the effects of surfactants on the mobility and reactivity of hydrophobic organic pollutants in natural water systems, may he expressed as follows (16): (3) where X, and X. are the mole fractions of solubilizate in the micellar and aqueous phases, respectively. For saturated systems, Le., those in which an excess organic phase is in equilibrium with a micellar solution, there is a simple relationship between X, and MSR (16):
X,
=
MSR 1+ MSR
(4)
For dilute solution solutions, X . is given by x a =C V ,W
(5)
where V , is the molar volume of water (Llmol), and all concentrations are expressed in mole per liter.
Experimental Section
Materials. Three n-alkanes (dodecane, decane, and hexane), a cycloalkane (cyclohexane), two chlorinated alkenes (trichloroethylene and tetrachloroethylene), and five aromatic hydrocarbons (benzene, toluene, o-xylene, chlorobenzene,and o-dichlorobenzene)were evaluated in this study. The alkanes were selected to model the nonpolar saturated hydrocarbons found in sites contaminated by light nonaqueous phase liquids (LNAPLs) such asgasoline, kerosene,and jet fuels. Benzene, toluene, and o-xylene were chosen to model the so-called BTX pollutants, whereas trichloroethylene, tetrachloroethylene,
were used in all CMC determinations. The aggregation numbers were estimated by regression using weight average aggregation numbers taken from Rosen (25). solubility Methods and Procedures. For each hydrocarbon and solubility parameterd molar void log surfactant pair, experiments were conducted to determine hydrocarbon (mol/L) (J1/2/cm3/2) (cm3/mol) KO,e the concentration of hydrocarbon solubilized. All experi16.0 228.6 6.51f n-dodecane 2.1 X 104 a ments were carried out at constant surfactant concentra15.8 195.9 5.62f 3.6 X n-decane tion (0.01 M) and in a temperature-controlled room at 25 14.8 131.6 4.11 1.4 X loAa n-hexane OC. A constant water/hydrocarbon volumetric ratio of 40 16.8 108.7 3.44 cyclohexane 6.5 X loAa 18.6 89.4 2.13 2.2 X lo-' was used in all experiments. This relatively large ratio benzene 18.2 106.9 2.69 toluene 5.6 x 10-3* was chosen to minimize surfactant partitioning into the 18.0 121.2 3.12 1.6 x 10-3 b o-xylene organicphase. For dodecane,decane, hexane, cyclohexane, 19.6 102 2.92 chlorobenzene 4.3 X 10-3 benzene, toluene, and o-xylene, the solubilization experi20.5 112.8 3.38 o-dichlorobenzene 1.1 X 19 90.2 2.42 ments were carried out in 15-mL Pierce borosilicate glass trichloroethylene 8.4 X 2.88 20.3 103 tetrachloroethylene 9.0 X lo-' vials. To each vial, 10 mL of surfactant solution and 0.25 mL of hydrocarbon (14C-labeledfor dodecane and decane) a Ref 26. * Ref 27. Ref 28. Ref 29. e Ref 30. f Estimated using correlation given on pp 2-15 of ref 31. were added. The vials were then sealed with open-top screw caps and Teflon-backed septa and gently agitated for 48 h at approximately 22 cycles/min on a T415-110 Table 2. Selected Physicochemical Properties of Dodecyl (Labindustries) shaker. After being shaken,the vials were Alcohol Ethoxylates (C12H2a(OCH&H2),,0H) Investigatede inverted and allowed to settle until complete separation of the aqueous micellar and excess organicphases occurred. NOH* 121.1 101.0 87.5 77.0 74.5 71.3 60.8 51.0 35.9 Complete separation between the two phases was deemed 555 641 729 753 787 923 1100 1563 MWc 463 to have occurred when no emulsified hydrocarbon was 20.8 31.3 13.6 16.7 12.9 10.3 12.3 nd 6.3 8.4 present in the aqueous micellar phase. The times needed HLBe 11.9 13.3 14.2 14.9 15.1 15.3 16.0 16.6 17.6 CMCf 1.4h 1.P 1.2h 1.2g 1.3h 1.4h legh 2.D 3.68 for complete phase separation ranged from 24 h for 52 41 29 68 154 105 79 74 NA' 285 hydrocarbons mixed with surfactant solutions of high HLB a Surfactant lipophile solubility parameter (61): 16.7 J1/2/cm3/2 to 7 wk for those mixed with solutions of lower HLB. from ref 27, p 446. NOH,hydroxylnumber (mg of KOH/g) measured Because the vials were sealed and inverted, there was no by Witco (32). M,, average molecular weight (g/mol). n, average contact between the micellar phase and the atmosphere. number of EO units. e HLB = 20 X 44n/(186 + 44n). f CMC ()(lo4) Thus, mass losses by volatilization from the micellar phase at room temperature (mol/L). 8 Measured by Witco (32). Deterwere insignificant. mined from measured values by polynomial interpolation. CMC = -65.886 + 15.673HLB - 1.209HLB' + 3.086 X lo-' HLB', R = 1.0. For the remaining hydrocarbons, the solubilization N A ,aggregation number estimated by regression of weight average experiments were conducted in 25-mL Corex (PGC aggregation numbers taken from Rosen (25); N A = 6.0082 X los Scientifics) centrifuge tubes. Experimental procedures HLB-6.M6, R = 0.97. were similar to those described above; however, in this case the tubes were centrifuged at 7500 rpm on a Sorvall chlorobenzene, and o-dichlorobenzene were chosen as RC-5B (Dupont Instruments) at 25 "C centrifuge to representative dense nonaqueous phase liquids (DNAPLs). separate the micellar and organic phases. This modificaSelected physicochemical properties of the hydrocarbons tion was introduced to reduce the times needed for phase are given in Table 1. Anhydrous dodecane; reagent-grade separation. With centrifugation, phase separation times decane and hexane; spectrophotometricgradecyclohexane, were reduced to 45 min for hydrocarbons mixed with trichloroethylene, and o-dichlorobenzene;and HPLC grade surfactant solutions of higher HLB and 90 min for those benzene, toluene, tetrachloroethylene, and chlorobenzene mixed with solutions of lower HLB. were purchased from Aldrich. o-Xylene was purchased Liquid scintillation counting (LSC) was used to assay from Fluka. All hydrocarbons were 99 % + pure and used the concentration of solubilized dodecane and decane, The as received. 98%+ pure I4C-labeled dodecane (5.3 mCi/ specific activity of each hydrocarbon was determined by mmol) and decane (4.6 mCi/mmol) were purchased from counting the decay events per minute (DPM) of three Sigma. 0.25-mL aliquots. Each sample was mixed with 15 mL of Dodecyl alcohol ethoxylates, C I ~ H ~ ~ ( O C H ~ C H ~ ) , O H , scintillation cocktail Ecolume (ICN) in a 20-mL vial and were selected as model ethoxylated nonionic surfactants. counted for 3 min on a 1219Rackbeta (LKB Wallac) liquid A homologous series of polydisperse dodecyl alcohol scintillation counter. To determine the concentration of ethoxylates synthesized, characterized, and donated by solubilized dodecane and decane, a 0.8-2-mL sample was the Witco Corp. were evaluated in this study. The withdrawn from each vial by a disposable plastic syringe, hydroxyl number ( N O H )the , average molecular weight mixed with 15 mL of Ecolume in a scintillation vial, and (AIw),the average number of EO monomers ( n ) the , HLB, counted for 3 min. All measured DPM were corrected by the CMC, and the aggregation number (NA)in dilute subtraction of background activity (52 DPM). The solutions at 26 "C for each surfactant are given in Table concentration of solubilized hydrocarbon, Csol(mol/L),was 2. The hydroxyl numbers were determined by Witco (32) calculated as follows: using the standard ASTM acetic anhydride acetylation procedure (33). These were subsequently employed to HA calculate the average molecular weight and number of EO csO1 = (HSA)MwV monomers of each surfactant. The HLBs were computed using equation 1. The CMCs of the surfactants were also measured by Witco (32). Surface tension measurements where HA is the hydrocarbon activity (DPM), HSA is the on a Kruss K12 process tensiometer at room temperature hydrocarbon specific activity (DPM/mg) in the micellar
Table 1. Selected Physicochemical Properties of Hydrocarbons Investigated
Envlron. Sci. Technol., Vol. 28, No. 11, 1994 1831
phase, M, is the hydrocarbon molar mass (mg/mol), and V is the sample volume (L). The micellar concentrations of the remaining hydrocarbons were assayed by GC with a flame ionization detector (FID) on a H P 5880 A gas chromatograph with a H P 7672 A automatic sampler and a H P 5880 A plotting integrator. A 0.7-mL sample of surfactant solution was withdrawn from each solubilization vial and mixed with 0.7 mL of HPLC-grade methyl alcohol from Mallinckrodt in a GC vial. A 1-mL aliquot of the mixture was then injected into the GC column under the following conditions: helium flow rate, 5 mL/min; injection temperature, 180 "C; oven temperature, 60 "C for hexane, 90 "C for cyclohexane, benzene, and trichloroethylene, and 120 "C for toluene, o-xylene, tetrachloroethylene, chlorobenzene, and o-dichlorobenzene; detector temperature, 300 "C. A 30-m DB624 Megabore (J&W Scientifics) column with a split/splitless inlet and an inlet liner guard packed with a 1-cm bed of 80/100 Porapak P (Alltech) was employed. The response factor, Rf [counts/(mg/L)I,of each hydrocarbon was determined using external standards diluted in HPLC-grade methyl alcohol. The concentration of solubilized hydrocarbon was calculated as follows: A CSOl= 2"
where A, (counts) is the area of the hydrocarbon peak. To ensure the collection of accurate and precise data, all solubilization experiments were conducted in triplicate, and all LSC and GC analyses were duplicated. Any difference of more than 10% between two duplicate analyses was attributed to instrument malfunction. Samples were reanalyzed after correction of the malfunction. All MSRs reported in this study are averages of at least six measurements. A propagation of error analysis was carried out to determine the uncertainty in the MSRs of the hydrocarbons. Because the uncertainty in surfactant concentration was estimated to be less than 1% , only the random errors in Csol, (AC,,]) were considered. The standard error in MSR (AMSR) was expressed as
2
+@ X2
(8)
where X is the hydrocarbon-specific activity/response factor and AX is the standard error in the hydrocarbonspecific activity/response factor. To achieve a 95 % confidence limit, the absolute value of the standard error in the MSR was taken as 1.96 AMSR. The errors in the MSRs reported in this study are relatively small, less than 10% in most cases. Results and Discussion
Effect of Surfactant HLB on Solubilization Capacity for Alkanes. The effects of HLB on the MSRs of dodecane, decane, hexane, and cyclohexane in 0.01 M micellar solutions of dodecyl alcohol ethoxylates at 25 "C are shown in Figure 2. The symbols represent experimental data and error bars; the dashed and solid lines are best fit curves. Figure 2 indicates that the MSRs of the alkanes decrease rapidly and then gradually go to zero as 1832
I
9
2
I
1.5 1
0.5
1
i
0
10
12
Environ. Sci. Technol., Vol. 28, No. 11, 1994
14 16 Surfactant HLB
20
18
Figure 2. Effect of surfactant HLB on molar solubilization ratio for alkanes in 0.01 M micellar solutions of dodecyl alcohol ethoxylates at 25 'C. The symbols represent experimental data and error bars; the lines are nonlinear regresslon curves. I
-
560
--
(7)
Rf
AMSR/MSR =
-1
12
,-------lj
4
- - Micellar Volume iV,)
-Core
10
,
"
Volume (V,)
14 16 Surfactant HLB
18
EO
Figure 3. Effect of surfactant HLB on total and core volumes of dodecyl alcohol ethoxylate micelles in dilute solutions at 25 OC. The symbols are estimated volumes; the dashed line is a smooth curve connecting the data points; the solid line is a nonlinear regression curve.
surfactant HLB increases. Because nonpolar HOCs are preferentially solubilized in the core of ethoxylated nonionic surfactant micelles (Figure l),the core volumes of the micelles are expected to control solubilization capacity. The effect of HLB on the core volume of dodecyl alcohol ethoxylate micelles in dilute solutions at 25 "C is shown in Figure 3. Here, the symbols are computed core volumes, and the solid line represents a best fit curve. Following Tanford (34),the core volume of a micelle, V , (A3), was estimated as
V , = NA(27.4+ 26.9(N, - 1))
(9)
where NAis the aggregation number and NCis the number of carbon atoms of the surfactant lipophile. Micellar reorganizations and subsequent increases of aggregation numbers, which have been observed to occur during the solubilization of hydrocarbons such as decane and ndodecanol (13),were not taken into account in these calculations. Despite these assumptions, the trend of core volume with HLB is similar to that for MSR with HLB shown in Figure 2. This similarity suggests that micellar core volume determines, to a large extent, the solubilization
Table 3. MSR Ratios and Inverse Molar Volume Ratios of Alkanes Investigated to Dodecane
hydrocarbon decane
hexane
cyclohexane
inverse molar vol ratio
HLB
MSR ratio
1.16
11.9 13.3 14.2 14.9 15.1 15.3 16.0 16.6 17.6 11.9 13.3 14.2 14.9 15.1 15.3 16.0 16.6 17.6 11.9 13.3 14.2 14.9 15.1 15.3 16.0 16.6 17.6
1.09 1.33 0.88 0.99 0.98 0.97 0.86 0.78 0.67 1.67 1.42 1.29 1.37 1.33 1.52 1.40 1.21 0.97 4.24 2.85 2.94 2.90 3.10 3.13 2.92 2.30 2.11
1.73
2.10
capacity of dilute solutions of ethoxylated nonionic surfactants for nonpolar NAPLs. Figure 2 also indicates that the higher the molar volume (Vm)of a hydrocarbon, the lower its MSR. Although no simple relationship between MSR and molar volume can be inferred from the data, the ratios of the MSRs of decane and hexane to that of dodecane are approximately equal to the inverse ratios of the molar volumes of these compounds for the lower HLB surfactant, as shown in Table 3. This inverse relationship between MSR and molar volume, which has also been observed by Klevens (35)for alkanes solubilized in 0.63 M solutions of an anionic surfactant with a dodecyl lipophilicgroup (potassium dodecanoate) at 25 O C, suggests that molar volume significantly affects the MSRs of alkanes in dilute solutions of ethoxylated nonionic surfactants. While hydrocarbon molecular size as measured by the molar volume significantly affects the extent of solubilization of alkanes, differences in molar volume alone cannot account for the large difference observed between the MSRs for cyclohexaneand hexane. A possible explanation for this difference may be that cyclohexane has more affinity for the micellar core than does hexane. Because the core of an ethoxylated nonionic surfactant micelle is similar to a bulk hydrocarbon phase of surfactant lipophiles (36,37),dispersion forces between hydrocarbon molecules and surfactant lipophiles are expected to control their interactions in the micellar core. These forces arise from the interactions between temporary dipoles induced in molecules by other nearby molecules. Because dispersion forces control molecular interactions in mixtures of nonpolar liquids, only moderate and positive deviations from ideality are expected to occur when alkane molecules mix with the lipophilic groups of ethoxylated nonionic surfactants (29). Thus, the Flory-Huggins enthalpic interaction parameter between an alkane molecule and a surfactant lipophile, x h l , can be used as a measure of the affinity of an alkane for the micellar core. Foll-
Table 4. Alkane-Surfactant Lipophile Interaction Parameters
dodecane XH
4.5 X 10-'
decane 6.4 X
hexane
cyclohexane
1.7 X lo-'
4.4X lo4
owing Nagarajan and Ruckenstein (381, x h l is expressed as (10)
where v h is the molar volume of the hydrocarbons and 6h and 61 are the hydrocarbon and surfactant lipophile Hildebrand solubility parameters, respectively. The solubility parameter of a liquid is a measure of its cohesive energy density; that is, the strength of the interactions between its molecules per unit volume (29). Because the difference in cohesive energy is usually the main cause of unfavorable interactions between two nonpolar liquids (29),the lower the xhl, the more favorable the interactions between alkane molecules and surfactant lipophilic groups. Calculated interaction parameters for the alkanes evaluated in this study are given in Table 4. The solubility parameters of the hydrocarbons (Table 1)and surfactant lipophile (Table 2) were taken from Barton (29). These calculations show that the x h l of cyclohexane is approximately400 times lower than that of hexane, indicating a greater affinity for the micellar core. This affinity could cause the formation of a separate cyclohexane core inside the micelle, a process referred to as type I1 solubilization by Nagarajan and Ruckenstein (38). Because the presence of such a core would provide a more favorable microenvironment for cyclohexane partitioning into the micellar core, a type I1 solubilization mechanism would result in an MSR for cyclohexane that is higher than that which would be expected based on its molar volume alone. However,no corroborating spectroscopic evidence for this hypothesis has been found in the literature; thus, a type I1 solubilization mechanism for cyclohexane is purely speculative at present. The effects of surfactant HLB on the micelle-water partition coefficients of the alkanes are shown in Figure 4. The symbols represent log K m w , and the dashed and solid lines are best fit curves. Not surprisingly, values of log K,, for the alkanes decrease when HLB increases. Figure 4 also shows that the more hydrophobic alkanes, Le., those with the highest octanol-water partition coefficients (logKO,), have the highest micelle-water partition coefficients. This trend is consistent with the fact that the hydrophobic effect is the driving force of alkane partitioning between the micellar and aqueous phases of surfactant solutions. Effect of Surfactant HLB on Solubilization Capacity for Aromatics. The effects of surfactant HLB on the MSRs of benzene, toluene, o-xylene,chlorobenzene, and o-dichlorobenzene in 0.01 M micellar solutions of dodecyl alcohol ethoxylates are shown in Figures 5 and 6. Here, the symbols represent experimental data and error bars; the dashed and solid lines represent smooth curves connecting the data. In contrast to Figure 2, an absolute maximum value for MSR is observed in all cases. Although absolute maxima in the MSRs of benzene and toluene at 25 "C in 0.1 M micellar solutions of a homologous series of dodecyl alcohol ethoxylates were reported as early as Environ. Scl. Technol., Vol. 28, No. 11, 1994 1833
t
J
J
i I
2k/ 3
'
'
,
1
I
12
10
/
,
,
1
,
16
14
,
,
I
,
/
/
20
18
Surfactant HLB
Flgure 4. Effect of surfactant HLBon micelle-water partition coefficient for alkanes in 0.01 M micellar solutions of dodecyi alcohol ethoxylates at 25 O C . The symbols represent log Kmwvalues estimated from measured solubilization data using eqs 3-5. The lines are linear regression curves. 25
'p, C$
o-xylene d
/
:E: 1 15 1: *
0'5 0
Toluene
I1
1
9
1
1
iBenleneB;--:,/" 10
p.
\ \ ?bO'
3
12
,
I
14
,
1
,
,
16
I
18
,
,
#I 20
Surfactant HLB
Figure 5. Effect of surfactant HLB on molar solubilization ratios for benzene and alkylbenzenes In 0.01 M micellar solutions of dodecyl alcohol ethoxylates at 25 O C . The symbols represent experimental data and error bars; the lines are smooth curves connecting the data points.
1949 by Cohen (11);no explanations have been given for these observations. Because aromatic hydrocarbons can be solubilized in both the micellar core and POE shell (37), micellar volumes are expected to control the MSRs of aromatic hydrocarbons. The effect of HLB on the volumes of dodecyl alcohol ethoxylate micelles in dilute solutions at 25 "C is shown in Figure 3. The symbols are computed micellar volumes, and the dashed line is a smooth connecting curve. Assuming that the total volume of a micelle is equal to the sum of its core and hydrated POE shell volumes, the micellar volume (V,) for each surfactant in a dilute solution was computed as:
V , = NA(V,+ 4nV,)
(11)
where N Ais the aggregation number, V , is the surfactant molecular volume, n is the number of EO monomers, and V , is the molecular volume of water. Density data were used to evaluate V , and V,. Surfactant densities were taken from Schott (39). I t was also assumed that (i) the micellar aggregation number is not affected by solubilization and (ii) an average of 4 water molecules are bound 1834
Environ. Sci. Technol., Vol. 28, No. 11, 1994
E
I 10
/
12
14
16
18
20
Surfactant HLB
Flgure 8. Effect of surfactant HLB on molar solubilization ratio for benzene and chlorobenzenes in 0.01 M micellar solutions of dodecyl alcohol ethoxylates at 25 OC. The symbols represent experimental data and error bars; the lines are smooth curves connecting the data points.
to each EO monomer. The first assumption is generally valid for hydrocarbons which are solubilized predominantly in the POE shell of ethoxylated nonionic surfactant micelles (36). Because water/EO mole ratios ranging from 2 to 4 are often reported for ethoxylated nonionic surfactants (17), the second assumption is also expected to hold in most cases. Figure 3 indicates that the micellar volumes of dodecyl alcohol ethoxylates in dilute solutions at 25 O C decrease when HLB increases. Because a decrease of solubilization capacity is generally expected with a decrease of micellar size (17, 37), the trends shown in Figures 5 and 6 may, thus, be the net result of two opposing effects: (i) loss of solubilization capacity with increasing HLB and (ii) enhancement of solubilization capacity by specific interactions between EO head groups and aromatic hydrocarbons. Two possible mechanisms of interaction are (i) hydrogen bonding between the aromatic hydrocarbons and water molecules bound to the EO chains and (ii) dehydration of the EO chains followed by the formation of electron donor-acceptor (EDA) complexesbetween the aromatic hydrocarbons and the EO head groups. The formation of hydrogen bonds between water and benzene has been the subject of speculation for several years (40). Recently, low temperature-high resolution gas phase spectroscopicinvestigations have provided evidence of such bonding (40-42). Hydration of EO chains of nonionic surfactant micelles is known to be quite extensive (17,37), and one might thus envision hydrogen bonding between water molecules bound to the EO head groups and aromatic hydrocarbons as an alternative mechanism of solubilization for these compounds. Because hydrogen bonding between water and benzene has to date been observed only in a gas phase at low temperature, this alternative mechanism of solubilization is purely speculative at the present time. A more plausible mechanism may be the dehydration of the EO chains and the subsequent formation of EDA complexes between aromatic hydrocarbons and EO head groups. Several studies have indicated that aromatic hydrocarbons depress the cloud points of nonionic surfactant solutions (1417). The cloud point of an ethoxylated nonionic surfactant is the temperature at which its aqueous solution separates into two phases: a micellar phase and an aqueous phase saturated with surfactant
18
5
-HLB,,,
= 1J 96 + 4
.....HLB,,
= 14 674
'
"
~
'
'
'
~
'
"
~
'
'
'
i
'
"
68560 R = 0 99
+ 2 88290
R = 0 86
Chlorobenzene
4 : b
d
3.5
M
L
s 1
t
3 : 2.5
1 t
10
-0.5
-0.25
0.25
0 Hammett Constant
0.5
(0)
Figure 7. Surfactant HLB for maximum molar solubilization ratios of aromatic hydrocarbons in 0.01 M micellar solutions of dodecyl alcohol ethoxylates at 25 O C as a function of Hammett constant. The symbols represent experimental data: the lines are linear regression curves.
monomers (24). Because the cloud point increases with the extent of EO chain hydration (19,any compound which depresses the cloud point is expected to cause dehydration of EO chains. Thus, it is conceivable that the solubilization of aromatic hydrocarbons in micellar solutions of ethoxylated nonionic surfactants could involve dehydration of EO chains followed by EDA complexation between the hydrocarbons and the EO head groups. NMR, IR, and UV spectroscopic investigations of poly(ethy1ene glycol) alkyl ether/benzene/water mixtures (43) and thermodynamic studies of polyethylene oxidelbenzene mixtures (44) have provided evidence of specific interactions between benzene and polyethylene oxide. Such interactions are likely to involve nn EDA complexes; these are formed when an electron is transferred from the highest occupied molecular orbital (HOMO) of an n-donor to the lowest unoccupied molecular orbital (LUMO) of a 'IT acceptor (45). n-Donors are compounds of the types R20, R2S, R3N, etc. 'IT-Acceptors, on the other hand, are aromatic and unsaturated hydrocarbons with electronacceptor properties (45). Figures 5 and 6 also show that the HLB at which the MSR of an aromatic hydrocarbon reaches a maximum, henceforth referred to as HLB,,,, depends on the nature of the substituents. Because the transfer of electron density is the driving force of EDA complexation, the extent to which a substituent enhances or hinders this transfer could be the reason why HLB,, depends on the nature of the substituents. Further evidence for this dependence is provided by Figure 7. The linear relationship between HLBmax and substituent constant shown in this figure is strong indirect evidence of EDA complexation between the aromatic hydrocarbons and the EO head groups. The substituent constant (a) measures the electronic effect of a substituent on unsubstituted benzene and depends only on its position on the ring (46);electron withdrawingldonating compounds have positivelnegative substituent constants. Substituent constants were first introduced in 1937 by Hammett to quantify the effect of substitution on the dissociation of paralmeta-substituted benzoic acids (46). Since then, these constants, commonly referred to as Hammett constants, have found applications in various fields of chemistry including spectroscopy (UV, IR, and NMR) and quantitative structure-activity relationships (QSAR). The Hammett constants used in Figure 7 were taken from Shorter (46). Taft's procedure for
10
12
14
16
ia
20
Surfactant HLB
Flgure 8. Effect of surfactant HLB on micelle-water partition coefficient for aromatic hydrocarbons in 0.01 M micellar solutions of dodecyl alcohol ethoxylates at 25 O C . The symbols represent log Kmwvalues estimated from measured solubilization data using eqs 3-5. The lines are smooth curves connecting the data polnts.
separation of the polar, steric, and resonance effects in ortho-substituted aromatic systems was employed to estimate the Hammet constants of the hydrocarbons (46). Because this procedure is predicated upon the fact that the electronic effects of substituents operate equally from the ortho- and the para-position (45),para-constants (ap) for the methyl/chloro substituent were used for toluene1 chlorobenzene. For o-xylene and o-dichlorobenzene, the substituent constants were determined by adding methyl and chloro ortho-substituent constants (a,) to the substituents constants of toluene and chlorobenzene, respectively. To the best of our knowledge, the linear relation shown in Figure 7 is the first reported correlation between HLB,,, and substituent constant. However, when odichlorobenzene is included in the plot HLBm, vs Hammett constant, the correlation coefficient decreases from 0.99 to 0.86, indicating a less satisfactory linear relationship. At present, we have no explanations for this behavior. Two additional interesting features may be observed in Figures 5 and 6. The MSRs of the alkybenzenes are higher a t lower HLBs, whereas those of the chlorobenzenes are higher a t higher HLBs. Although no quantitative explanations for these observations are available at present, qualitatively, they can be rationalized within the framework of the two-site model of solubilization (14): partitioning into the micellar core and dissolution andlor binding into the POE shell. Since partitioning into the micellar core has been shown to be an important mechanism of solubilization for alkylbenzenes (17, 37), their MSRs are expected to be higher at low HLB. Conversely, the MSRs of chlorobenzenes, which have greater affinity for the POE shell, are expected to be larger a t higher HLB. The effects of surfactant HLB on the micelle-water partition coefficients of the aromatic hydrocarbons are shown in Figure 8. The symbols represent log K,, computed from MSR data, and the dashed and solid lines are smooth connecting lines. A maximum in log K,, is observed in all cases. These maxima are, however, less pronounced than those shown in Figures 5 and 6 because of the logarithmic representation of K,,. Except for chlorobenzene and o-dichlorobenzene a t low HLB, values of log Kmw increase with the hydrophobicity of the hydrocarbons. This is again not surprising and is conEnviron. Sci. Technol., Vol. 28, No. 11, 1994
1835
I
"
'
through a maximum. The more hydrophobic compound has again the largest micelle-water partition coefficient.
]
Summary and Conclusions
12
14
16
18
20
Surfactant HLB
Flgure 9. Effect of surfactant HLB on molar solubilization ratio for chlorinated alkenes in 0.01 M micellar solutions of dodecyl alcohol ethoxylates at 25 OC.The symbols represent experimental data and error bars; the lines are smooth curves connecting the data points.
4.5
1 t
f4
/
I
Tetrachlomth lene Log KOw = 2 . h
I I
3.5
3
?
i1
I I
r6 Tnchloroeth lene LogK,,,=J42
0
1 10
12
14
16
18
20
Surfactant HLB
Figure 10. Effect of surfactant HLB on micelle-water partition coefficient for chlorinatedalkenes in 0.01 M micellar solutionsof dodecyl alcohol ethoxylates at 25 O C . The symbols represent log Kmwvalues estimated from measured solubilization data using eqs 3-5. The lines are smooth curves connecting the data points.
sistent with the fact that the hydrophobic effect is the major driving force of hydrocarbon partitioning between micellar and aqueous phases of surfactant solutions. Effect of Surfactant HLB on Solubilization Capacity for Chlorinated Alkenes. The effects of surfactant HLB on the MSRs for trichloroethylene and tetrachloroethylene in 0.01 M micellar solutions of dodecyl alcohol ethoxylates at 25 OC are shown in Figure 9. The symbols and error bars represent experimental measurements; the solid and dashed lines are smooth curves connecting the data. The MSRs of trichloroethylene and tetrachloroethylene go through a maximum when HLB increases. Fountain et al. (5)have also reported that the solubility of trichloroethylene and tetrachloroethylene in aqueous solutions of nonylphenol ethoxylates (5 % by weight) a t room temperature go through a maximum when HLB increases. We believe that this is also the result of the two opposing effects previously described. The effects of HLB on the micelle-water partition coefficients for trichloroethylene and tetrachloroethylene are shown in Figure 10. When HLB increases, Log K,, for trichloroethylene and tetrachloroethylene also go 1836
Environ. Sci. Technol., Vol. 28, No. 11, 1994
The effects of surfactant hydrophile-lipophile balance (HLB) on the molar solubilization ratios (MSRs) and micelle-water partition coefficients (Kmw)of 11nonaqueous phase liquid (NAPL) hydrocarbons in 0.01 M micellar solutions of dodecyl alcohol ethoxylates, of average number of EO monomers ranging from 6 to 31, were investigated at 25 "C. Three n-alkanes (dodecane,decane, and hexane), a cycloalkane (cyclohexane), five aromatics (benzene, toluene, o-xylene, chlorobenzene, and o-dichlorobenzene), and two chlorinated alkenes (trichloroethylene and tetrachloroethylene) were evaluated. The MSRs of the alkanes were found to decrease sharply and then gradually approach zero as HLB increases. The lower the molar volume of an alkane, the higher its MSR. The results suggest that micellar core and hydrocarbon molar volume determine, to a large extent, the MSRs of alkanes in micellar solutions of ethoxylated nonionic surfactants. Because differences in molar volume alone could not account for the relatively large difference between MSR values observed for cyclohexane and hexane, the Flory-Huggins enthalpic interaction parameter between the alkane molecules and surfactant lipophiles, Xhl, was used as a measure of alkane affinity for the micellar core. Calculations show that xhl for cyclohexane is approximately 400 times lower than that for hexane, indicating greater affinity for the micellar core. This could explain why the ratio of the MSR of cyclohexane to that of hexane is much higher than the inverse ratio of their molar volumes. The MSRs of the aromatic hydrocarbons, on the other hand, go through a maximum as HLB increases. Because the HLB at which this maximum occurs is correlated with the Hammett constants of the hydrocarbons, we conclude that this behavior may be the net result of two opposing effects: (i) loss of solubilization capacity with increasing HLB and (ii) enhancement of solubilization capacity by specific interactions between EO head groups and aromatic hydrocarbons. Such interactions are likely to involve dehydration of the EO chains followed by formation of electron donor-acceptor (EDA) complexes between the aromatic hydrocarbons and the ether oxygen atoms of the EO head groups. The MSRs for trichloroethylene and tetrachloroethylene were also observed to go through a maximum as surfactant HLB increases. This is also attributed to the two opposing effects described above. The micelle-water partition coefficients of all the hydrocarbons follow the trends of the MSRs and increase with hydrocarbon hydrophobicity. This is consistent with the hypothesis that hydrophobic effect is the main driving force for hydrocarbon partitioning between the micellar and water phases of surfactant solutions. Acknowledgments The surfactants used in this study were synthesized, characterized, and donated by the Witco Corp. We express our appreciation to Ms. Joanne Geils and Mr. Dennis Anderson of Witco for making this possible. We thank Ms. Kinsley Binard for her assistance with the experiments, and Dr. Kurt Pennell and Mr. Tom Yavaraski for
their assistance in development of the experimental procedures. Funding for the research was provided by the Great Lakes and Mid-Atlantic Hazardous Substance Research Center under Grant R-819605 from the Office of Research and Development, US.Environmental Protection Agency. Partial funding of the research activities of the Center is also provided by the State of Michigan Department of Natural Resources. The content of this publication does not necessarily represent the views of either agency. Literature Cited (1) Scamehorn, J. F.; Harwell, J. F. Surfactants in Emerging Technologies;Rosen, M. J., Ed.; Surfactant Science Series, Vol. 26; Marcel Dekker: New York, 1987; p 169. (2) General Electric Co. Research and Development Program for the Destruction of PCB's; Third Progress Report; June 1984. (3) Ellis, W. D.;Payne, J. R.; Mc Nabb, G. D. Treatment of Contaminated Soils with Aqueous Surfactants; NTIS PB86-122561; U.S. Government Printing Office: Washington, DC, 1986. (4) Abdul, A. S.;Gibson, T. L. Environ. Sci. Technol. 1991,25, 665. ( 5 ) Fountain, J. C.; Klimek, A,; Beikirch, M.; Middleton, T.; Hodge, D. S. Proceedings of Aquifer Reclamation and Source Conference; New Jersey Institue of Technology, Division of Continuing Education: Newark, NJ, 1990. (6) Pennell, K. D.; Abriola, L. M.; Weber, W. J., Jr. Environ. Sci. Technol. 1993, 27, 2332. (7) Chawla, R. C.; Diallo, M. S.; Cannon, J. N.; Johnson, J. H., Jr.; Porczucek, C. In Proceedings of the International Symposium on Solid-Liquid Separations: Waste Management and Productivity Enhancement;Muralidhara, H., Ed.; Battelle Press: Columbus, OH, 1989; p 355. (8) West, C.; Harwell, J. H. Environ. Sci. Technol. 1992, 26, 2324. (9) Aronstein, B. N.; Calvillo, Y. M.; Alexander, M. Environ. Sci. Technol. 1991, 25, 1728. (10) Burry, S. J.; Miller, C. A. Environ. Sci. Technol. 1993,27, 104. (11) Cohen, M.M. C. R. Hebd. Seances Acad. Sci. 1949,229, 1074. (12) Saito, H.; Shinoda, K. J.Colloid Interface Sci. 1967,24,10. (13) Nakagawa, T. Nonionic Surfactants;Schick, M. J., Ed.; Surfactant Science Series, Vol. 1;Marcel Dekker: New York, 1967; p 559. (14) Mukerjee, P. Solution Chemistry of Surfactants; Mittal, K. L.; Ed.; Plenum: New York, 1979; Vol. 1, p 153. (15) Kile, D. E.; Chiou, C. T. Enuiron. Sci. Technol. 1989, 23, 832. (16) Edwards, D.A.; Luthy, R. G.; Liu, Z. Environ. Sci. Technol. 1991, 25, 127. (17) Mackay, R. A. Nonionic Surfactants: Physical Chemistry; Schick, M. tJ., Ed.; Surfactant Science Series, Vol. 23; Marcel Dekker: New York. 1987: D 297. (18) Barry, M. M.S. Thesis, Penisylvania State University, 1986. (19) Amos, D. A. M.S. Thesis, University of California, Berkeley, 1992. (20) Jafvert, C. T.; Van Hoof, P. L.; Heath, J. K. Water. Res. 1994. 28,1009.
(21) Mukerjee, P. J.Pharm. Sci. 1971, 10, 1528. (22) Mukerjee, P. J.Pharm. Sci. 1971, 10, 1531. (23) Swisher, R. D. Surfactant Biodegradation;1987,Surfactant Science Series, Vol. 18; Marcel Dekker: New York, 1987; p 415. (24) Myers, D. Surfactant Science and Technology; VCH Publishers, Inc.: New York, 1988. (25) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Plenum Press: New York, 1989. (26) Bunger, W. B.; Sakano, T. K. Organic Solvents: Physical PropertiesandMethodsofPurification,4thEd.; John Wiley & Sons: New York, 1986. (27) Howard, P. E. Handbook of Environmental Fate and Exposure Data for Organic Chemicals;Lewis Publishers: Chelsea, MI, 1989. (28) Gilham, R. W.; Rao, P. S. C. Significance and Treatment of Volatile Organic Compounds;Ram, R. M., Russell, F. C., Contor, K. P., Eds.; Lewis Publishers: Chelsea, MI, 1990. (29) Barton, A. F. M. CRC Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press, Inc.: Boca Raton, FL, 1983. (30) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environmental Organic Chemistry; John Wiley & Sons: New York, 1993. (31) Lyman, W. J.; Reehl, W. F.; Rosenblatt, D. H. Handbook of Cheemical Property Estimation Methods; ACS: Washington, DC, 1990; pp 2-15. (32) Meadows, W. Witco Corp., Organics Division, Houston,TX, personal communication, 1992. (33) American Society for Testing and Materials. Standard Test Methods for Hydroxyl Groups by Acetic Anhydride Acetylation; E222-88; ASTM: Philadelphia, PA, 1988. (34) Tanford, C. The Hydrophobic Effect;John Wiley & Sons: New York, 1980. (35) Klevens, H. D. Chem. Rev. 1950,47, 1. (36) Podo, F.; Ray, A.; Nemethy, G. J. Chem. SOC.1973, 95, 6164. (37) Atwood, D.; Florence, A. T. Surfactant Systems: Their Chemistry, Pharmacy and Biology; Chapman and Hall: New York, 1983. (38) Nagarajan, R.; Ruckenstein, E. Langmuir, 1991, 7, 2934. (39) Schott, H. J. Pharm. Sci. 1984, 73, 790. (40) Suzuki, S.;Green, P. G.; Bumgarner, R. E.; Dasgupta, S.; Goddard, W. A.; Blake, G. Science 1992, 257, 942. (41) Gotch, A. W.; Zwier, T. S. J. Chem. Phys. 1992,96, 3388. (42) Garrett, A. W.; Zwier, T. S. J. Chem. Phys. 1992,96,3402. (43) Christenson, H.; Friberg, S. E. J. Colloid Interface Sci. 1980, 75, 276. (44) Booth, C.; Devoy, C. J. Polymer 1971, 12, 320. (45) Gur'yanova, E. N.; Gol'dshtein, I. P.; Romm, I. P. DonorAcceptor Bond; John Wiley & Sons: New York, 1975. (46) Shorter, J. CorrelationAnalysis in Organic Chemistry: An Introduction to Linear Free Energy Relationships; Clarendon Press: Oxford, 1973.
Received for review November 23, 1993. Revised manuscript received May 9, 1994. Accepted July 25, 1994.' ~~
e Abstract
published in Advance ACS Abstracts, September 1,
1994.
Environ. Scl. Technol., Vol. 28, No. 11, 1994
1837
