Environ. Sci. Techno/. 1995, 29, 2484-2489
Micellar Solubilization of Unsaturated Hydrocarlron Semiequilibrium Dialysis JOSEPH D. ROUSE,* DAVID A. SABATINI, NEIL E. D E E D S , A N D R. ERIC BROWN School of Civil Engineering and Environmental Science, University of Oklahoma, 202 West Boyd, Norman, Oklahoma 73019
JEFFREY H. HARWELL School of Chemical Engineering and Material Science, Uniuersity of Oklahoma, Norman, Oklahoma 73019
Prior research on surfactant solubilization has emphasized steady-state conditions in the presence of excess hydrocarbon for single-component systems. These conditions do not accurately emulate the natural subsurface environment. Evaluation of s o h bilization potential for unsaturated hydrocarbon concentrations revealed influences of micellar core and palisade layer effects on hydrocarbons of varying polarity. Naphthane, naphthalene, and 1-naphthol were utilized to probe for core and palisade effects. Surfactants evaluated were sodium dodecyl benzenesulfonate and C16-alkyl diphenyl oxide disulfonate. Use of semiequilibrium dialysis cells provided quantitative solubilization information for unsaturated hydrocarbon concentrations. Palisade and core solubilization effects were observed as follows: as the more fraction increased, Km increased for naphthane, decreased for naphthol, and remained relatively constant for naphthalene.
Introduction The use of surface active agents (surfactants) to enhance subsurface remediation of hydrocarbon contamination has been of increasing interest in recent years (1-11).Micelleforming surfactants can enhance the solubility of highly hydrophobic compounds via the hydrocarbon pseudophase of the micellar core. Moderately soluble polar and ionic compounds can also be solubilized into the outer palisade layer of amicelle, which consists of the polar moiety of the amphipathic surfactant molecule (12). Both of these phenomena result in concentrations of hydrocarbons in solution beyond their expected solubility limits and thus contribute to enhanced product recoveries. ~
* Corresponding author e-mail address:
[email protected]; fax: (505) 835-5252. Present address: Department of Mineral and Environmental Engineering, New Mexico Institute of Technology, Socorro, NM 87801.
2484 a ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29,NO. 10, 1995
Most solubilization assays, however, are conducted under steady-state conditions in the presence of an excess hydrocarbon phase. These controlledlaboratoryconditions promote equilibrium and maximum aqueous and micellar concentrations. However, contaminant concentrations in groundwater flowing past a nonaqueous excess hydrocarbon phase may not reach their solubilitylimits due to slow dissolution rates, retardation effects, and dilution (13-19). Even when enhancements in apparent solubility are expected due to micellar solubilization, equilibrium concentrations may not be observed (20, 21). Furthermore, when a multi-component residual is considered, a realistic scenario (221, even at equilibrium individual components will only appear at some fraction of their water solubility due to Raoult’s law effects (16, 23, 24). Solubilizationpotential (the ability of surfactant micelles to increase the total hydrocarbon concentration in solution beyond the aqueous solubility component) is quantified by the micelle-water partition coefficient,K, [partitioning of hydrocarbons between the micellar and the aqueous pseudo-phases (2511, To determine solubilizationpotentials for aqueous hydrocarbon concentrations below the solubility limit, semiequilibrium dialysis (SED) cells have been utilized (26-31). While ordinary batch assays can be conducted for unsaturated systems, the distinction between the micellar and aqueous components of the hydrocarbon can only be determined with some form of separation [e.g., aqueous/micelle separation such as filtration (as in this study) or vapor pressure analysis (32-3611. With SED cells, membranes are used that retain surfactant micelles and their solubilized contents, allowing the surfactant monomers and unsolubilized hydrocarbons to come to equilibrium on both sides in relativelyshort time periods (hours). However, since micelles are dynamic, given an extended period of time (weeksto months), theywill approach equal concentrations in the retenate and permeate-thus the “semi”-equilibriumqualification. Theoretically, as the mole fraction of hydrocarbon increases in the micellar core, the core will assume a greater resemblance to the contaminant of interest, and thus the solubilization potential can increase (32,331. Conversely, as moderately or slightly polar hydrocarbon molecules fill palisade layers sites, analogous to surface adsorption, competition for remaining sites will increase, and thus the solubilization potential can decrease in a Langmuirian manner (27,28,31-34) (for Langmuirian effect see ref 37). To evaluate potential micellar core and palisade layer effects on the solubilization of hydrocarbons of varying polarity, essentially nonpolar naphthane and naphthalene and naphthalene’s polar oxidation product naphthol were used. Surfactants studied include anionic surfactants with different alkyl tail lengths and polar head group compositions. The objectives of this research are to evaluate solubilization potentials under conditions simulating nonequilibrium or mixed contaminant scenarios. Hydrocarbon contaminents with distinctly different polarities and two surfactants with differing hydrophobic and hydrophilic moieties were used. These objectives were met by developing K, values under varying unsaturated hydrocarbon concentrations with the use of SED cells.
0013-936X/95/0929-244$09.00/0
0 1995 American Chemical Societv
TABLE 1
Hydrocarbon Contaminants Used in This Research hydrocarbon
molecular weight
molecular formula
naphthalene I-naphthol naphthane
128 144 138
ClOH8 CioH70H C10H18
a
log rr,
water solubility (mM) 0.23-0.27’ 9.7d 0.00644‘
3.336 2.66e 4.008
dipole moment (0) OC
1.6c OC
25 “C (39).Average of 11 values ranging from 3.20 to 3.59 (39).Estimated ( 4 0 ) . This work. a Estimated by Leo‘s fragment constant method
( 4 7 ) . ‘At 25 “C (42). Ref 42.
TABLE 2
Anionic Surfactants Used in This Research
a
surfactant
av molecular weight
molecular formula
cmc‘sa(mM)
SDBS DPDS (DOWFAX 8390)
348.48b 642
Ci2H25Ceh (S03Na) C16H33C12H70(S03Nah
1.2 0.3
Estimated by semiequilibrium dialysis (see text). Before ultrafiltration (see text).
TABLE 3
Solubilization Parameters by Excess Additive Method hydrocarbon and surfactant
MSR
xn
naphthalene and SDBSb naphthalene and DPDSb naphthol and SDBSC naphthol and DPDSC naphthane and DPDSd
0.052 0.132 0.762 f 0.01 4 1.217 f 0.057 0.277e
0.049 0.117 0.432 0.549 0.217
x.
log Kn
a
4.5 x 10-6 4.5 x 10-6 1.75 x 10-4 1.75 x 10-4 1.16 x IO-’
4.04 4.41 3.39 3.50 6.27
a Calculated by eq 3; see Table 1 for water solubilities. Ref 11: no electrolyte added, SDBS has different molecular weight distribution than in this work (see text). This work: electrolyte added ( I O m M NaCI), surfactant concentrationsca. 5-55 mM, P > 0.99 for MSRs. dThiswork: electrolyte added (10 m M NaCI), DPDS = 20.0 mM. eCalculated by X, = MSR/(I MSR) (253.
+
Materials and Methods The hydrocarbon “contaminants”evaluated in this research are decahydronaphthalene (also known as naphthane: purified grade;Fisher Scientific, St. Louis, MO), naphthalene (99% purity; Aldrich Chemical Co., Milwaukee, WI), and 1-naphthol (99%purity; Aldrich Chemical Co.). Common characteristics of these hydrocarbons are reported in Table 1. The anionic surfactants evaluated consist of sodium dodecyl benzenesulfonate (SDBS), which was purchased in dry flake form (ca. 96% active; Aldrich Chemical Co.), and hexadecyl diphenyl oxide disulfonate (DPDS, tradename DOWFAX 8390), which was received in liquid form from Dow Chemical Co., Midland, MI. Both surfactants have straight alkane chain nonpolar moieties. DPDS was a high-purity product (Le., low salts--0.1-0.3% NaCl in concentrated stock solution of ca. 36% activity) and was about 20% by weight double-tailed. SDBS, being a heterogeneous mixture of varying alkyl tail lengths, was further purified via micellar-enhanced ultrafiltration (38)to eliminate shorter tailed components. Characteristics of these surfactants are listed in Table 2. Precipitation, sorption, and hydrocarbon solubilization behavior of these surfactants have recently been investigated (11)-a summary of solubilization parameters is provided in Table 3. SED cells were purchased from Bel-Art Products (Pequannock, NJ) via Fisher Scientific (Springfield, NJ) and also manufactured in-house. A cell (Figure 1) consists of plexiglass halves, each with a void space of approximately 5 mL, that are clamped together over a cellulose dialysis membrane filter (average molecular mass cutoff of 6000 Da, Bel-art via Fisher). Samples were loaded and extracted by syringe via threaded sampling ports that were sealed
0 hours
(a)
0 hours
(b)
24 hours
24 hours
Cer
permeate
FIGURE 1. SED cell descriptions: (a)sample components; (b) sample nomenclature.
during use with stainless steel screws. Terms used to identify retentate and permeate concentrations of surfactant and hydrocarbon at different times during SED analyses are depicted in Figure 1 and defined as follows: So, is the original hydrocarbon surfactant VOL. 29, NO. 10.1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
2485
concentration in retentate; Coris the original hydrocarbon concentration in retentate; Ser is the semiequilibrium surfactant in retentate; C, is the semiequilibrum hydrocarbon in retentate; S,, is the semiequilibrium surfactant in permeate; Cepis the semiequilibrium hydrocarbon in permeate. Semiequilibrium concentrations are based on 24-h samples (based on preliminary studies as discussed in Results). Original permeate concentrations of surfactant and hydrocarbon were always zero during this study, and all concentrations are in millimolar units. Reagent-grade sodium chloride was used as an electrolyte at a concentration of 10mM in all test solutions (includinginitially "clean" original permeate solutions), as discussed below. All reagents were mixed in deionized water, and all assays were conducted at room temperature (ca. 22 "C). Dialysis membranes were soaked in deionized water prior to use. HPLC with a Wdetector (225nm wavelength)was used to determine the concentrations of naphthalene and naphthol and both surfactants. A flow rate of 1.0 mL/min was used, and except as noted below, the mobile phase consisted of 90% methanol in HPLC-grade water. HPLC analyses were conducted with a Beckman System Gold chromatograph (Beckman Instruments, Inc., San Ramon, CA) with a 150 x 4.6 mm Nucleosil C18 reverse-phase column (AlltechAssociates, Inc., Deerfield, IL). Under the above HPLC conditions, naphthalene had a peak retention time of approximately 3.2 min, and the SDBS response consisted of peaks ranging from 1.6 to 2.0 min and the DPDS response ranged from 1.8 to 2.2 min. Due to naphthol's higher water solubility, a mobile phase of 80% methanol was necessary to enhance separation from surfactant peaks which resulted in a naphthol retention time of about 2.9 min. To quantify naphthane, aVarian 3300 (Varian Associates, Inc., Walnut Creek, CA) gas chromatograph (GC)was used with a Supelco (Bellefonte,PA) VOCOL phase glass capillary column (60 m x 0.75 mm x 1.5pm). The GC was equipped with a purge-and-trap sampler (Tekmar, Cincinnati, OH) and a flame ionization detector (FID). A sample volume of 1.00 mL was heated for 2 h on the purge-and-trap unit at 73 "C in a water bath before purging (shown to be necessary by preliminary kinetic studies). Helium was used at a flow rate of 30 mL/min with fuel gases of hydrogen (30 mL/min) and compressed air (300 mL/min). A column temperature range of 32-150 "C with an increase rate of 8.0 "C/min was used with an injector temperature of 225 "C and a detector temperature of 250 "C. Naphthane standard solutions respondedwith two major peaks of nearequal magnitude at retention times of ca. 11.5 and 12.5 min and a tertiary peak accounting for only about 3% of the combined magnitude at ca. 14.5 min.
Results and Discussion Preliminary Studies. Initial runs with naphthalene solutions (no surfactant)indicated that about 15h was necessary to achieve equilibrium on both sides of the dialysis membrane. SDBS purification via ultrafiltration (38) resulted in a more homogeneous product with a more precise critical micelle concentration (cmc). In a 10 mM NaCl solution, SDBS permeate concentrations approached a plateau value at about 24 h (data not shown), which is commonly used as a time limit for SED assays (26-30). DPDS, being a more homogeneous product, behaved well 2486
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 10, 1995
in SED analysis and also approached a plateau permeate concentration at about 24 h of operation in an electrolyte solution. Both surfactants demonstrated a gradual but consistent increase in 24-h permeate surfactant concentrations (Sep) as a function of increasing initial surfactant concentration in the retentate (So,). The magnitude of So, appeared to influence the rate of surfactant flux through the dialysis membrane. However,from about 5 to 40 mM So,, S,, values varied only by about 1 mM (1.2-2.4 mM for SDBS and 0.3-1.7 mM for DPDS). The semiequilibrium S,, is, theoretically, the surfactant's cmc, and as such, variations are potential grounds for erroneous interpretation of results. The limited fluctuation of Sep only becomes significant at S,, values below about 10 mM for both surfactants. Consequently,all SED parameter determinations are based on S,, concentrations greater than about 10 mM. In considering the above, it was deemed expedient to select the lower values of S,, (generated with lower So, concentrations-ca. 13- 15 mM) to represent surfactant cmc's to be used for all calculations. Once established, analyses for surfactant concentrations did not need to be performed with every assay, thus greatly reducing analysis time while retaining sufficient accuracy. The S,, (or cmc) values so determined were: 1.2 mM for SDBS and 0.3 mM for DPDS (see Table 2). Solubilization of Hydrocarbons. The micelle-water partition coefficient, K,, is defined as
K, = X,,,/X,
(1)
where Xm is the mole fraction of the hydrocarbon in the micellar pseudo-phase and X, is the mole fraction of the hydrocarbon in the aqueous phase (25). They are calculated as
or
and
X, = Ce,*(mole volume of water, 0.00001805 Llmmol) (3)
Plots of X, versus X, for both surfactant systems with naphthalene are shown in Figure 2. K, is defined as the slope of this trends (X,/XJ. The SDBS and DPDS data both showed no significant deviation from a straight line as determined by a Freundlich analysis ( N = 1.0,indicating constant K,, with a 95% confidence level (two-tailed statistical test assuming a normal distribution, see Table 4). For nonpolar naphthalene, core solubilization was expected to be the controllingmechanism; this would result in increasing values of K, with increasing X, or X, (N > 1.0). It is hypothesized, however, that the influence of the polar surfactant moieties may also contribute to the solubilization of naphthalene (whichis inducibly polar due to conjugated bonding). If this were the case, a decreasing trend of K, versus increasing X,, as expected for palisade solubilization,could hypotheticallyoffset the expected core effect. Plots of naphtholx, versus X,values for SED surfactant systems at select S,, concentrations are shown in Figure 3.
-_______
0081
0 0 6 k -
-
- -
-~ -
I I
--
I
P
0
1
0.5
H
1.5 Xa (Times 10E-6) SDBS A
2
I
2.5
0
3
0.05
I
X
=
0.1
0.15
I =
2
0.2
0.25
0.3
0.35
Xm
I
x
SDBS=19.6mM
DPDS=22.4mM
I
DPDS=5.4mM
DPDS
FlGURE2. 4,versusX.for surfactantsystems containingnaphthalene (SDBS, So,= 10.8-81.8 mM; DPDS, S,, = 14.4-36.6 mM). See Table 4 for slope analyses.
FIGURE 4. L versus X,,, for surfactant systems containing naphthol (concentrations in legend are Smr values). Excess additive values: SDBS, Km=2450, X,,, =0.432; DPDS, K, = 3160,& = 0.549 (see Table 3).
Taw 4
Taw 5
Freundlich Analysis of Solubilization Data
Parameters for SED Surfactant Systems Containing Naphthol
hydrocarbon and surfactant system
Freundlich IW (X,,, VI X,)
naphthalene and SDBSb naphthalene and DPDSC naphthol and SDBSd naphthol and DPDSe naphthane and DPDS‘
1.0190 0.9964 0.8849 0.6868 1.3002
f 0.07068 f 0.01758 f 0.023gh f 0.0254h f 0.053gh
Results *1 SD. S,,= 10.8-81.8 mM. S,, = 15.4-39.2 mM. S., = 19.6 mM. e S., = 22.4 mM. S,,= 19.7 mM. Not different from 1.0 at a 95% confidence level. Different from 1.0 at a 95% confidence level. a
*
0.35 A
03-
.
A
0.25A 5:
E
X
0.15-
1
v
0.1 0.05-
0
z
-
A
A
i
x
1
2
3
5
6
DPDS=224mM
1
4
7
Xa (Times 1OE-5)
1
z
SDBS=19.6mM
A
FIGURE 3. X, versus X. for surfactant systems containing naphthol (concentrations in legend are S,, values). For excess additive conditions, X, = 0.000175 (see Table 3). See Table 4 for slope analyses.
As confirmed by Freundlich Nvalues (Table 41, these plots show a decreasing slope and thus decreasing Km for increasing values X, and &. Figure 4 shaws naphthol K, versus X, values for both surfactant systems. For DPDS, a sharp decrease in Km occurred at low Xm values, which was not obvious by inspection of the X, versus X, plot (Figure3). This initial trend for DPDS systems was followed by a more gradual decline in K, for increasing X, approaching the excess additivevalue (Figure4). The same gradual trend at higher X, values was observed for SDBS systems (Figure4); the sharp change in K, values, however, was not evidenced even at extremely lowX, values. Results of both surfactant systems were suggestive of palisade
surfactant system
log Kmo
B
log KL
SDBSd DPDSe
3.76 4.05
1.24 0.95
4.16 4.27
a Determined by nonlinear regression of K, = K,Jl - BX,)*(adapted from refs. 28 and 31). bTheoretical log K, at X, = 0. cLangmuirian coefficient determined by nonlinear regression (dimensionless-based on X,versus X. molefractions). All SDBS parameters based on higher naphthol mole fraction range, X , = 0.055-0.205. e All DPDS parameters based on higher naphthol mole fraction range, X, = 0.093-0.306.
solubilization effects,as anticipated for the moderately polar naphthol. The sharp response at low X, values for the DPDS assays may have been due to the comparativelylarger and more influential palisade layer of DPDS as compared to SDBS. The structural similarities between naphthol and diphenyl in the DPDS surfactant’s polar moiety may be the cause for an initially strong attraction, which could subsequently be met with conflicting ionic or steric constraints. Lee et al. (28,311developed a simple quadratic expression that accurately represented solubilization data for various polar aromatic solutes in hexadecylpyridinium chloride (CPC)solutions. An adapted form of this equation is
(4) where Kmois the limiting K, solubilization constant as X, approaches zero, and B is a parameter such that 2B equals the number of surfactant molecules that constitute a “site” for binding a polar organic molecule on the surface of the micelle (28). Both parameters are indicators of the interactions between polar solutes and ionic micelles (28, 31). Kmois an indicator of the affinity of a polar molecule for the surface of a micelle (28). As shown in Table 5, the naphthol B values for both surfactants are similar and approximately 1.0, suggesting that two surfactant polar head groups form a micellar surface adsorption site for naphthol. Phenol with CPC surfactant has a similar B value of 1.09 (28,311. K,, values (Table 5) follow the same trend as the excess additive K, values (Table 3), with DPDS demonstrating a higher VOL. 29, NO. 10, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
2487
DPDS, Ser = 19.7 to 23 4 mM 14
3 ,;;;;y, 02
0
01
1 FIGURE 5. rZ, and M, versus X for systems containing DPDS and naphthane. For excess additive conditions, X. = 0.000000116 and & = 0.127 (see Table 3). See Table 4 for slope analysis.
naphthol solubilizationpotential than SDBS. The Langmuir coefficients, KL (Table 51, for both surfactant systems also demonstrated a higher potential-or affinity-for the DPDSnaphthol combination. Solubilization assays for naphthane were conducted using the DPDS surfactant at a single concentration for which excess additive values are shown in Table 3. While the propensity for naphthane to partition into DPDS micelles lies between that of naphthalene and naphthol (see MSR and X, values in Table 31, the K, value for naphthane is 2-3 orders of magnitude higher. This is a result of naphthane's comparatively lower water solubility. A plot of X, versus X, for SED assays with a DPDS S, of 19.7 mM and varying mole fractions of naphthane is shown in Figure 5 . As confirmed by the Freundlich Nvalue (Table 41, an increasing slope is exhibited. Thus, increasing values of Km are realized for increasing X, and &. Figure 5 also shows the increasing trend of Km, approaching that of the excess additive value (1 862 000, see Table 3) versus increasing X,. The lowest Km value shown on Figure 5 (reproduced by duplicate SED cells) suggests a different solubilization trend at low X m values as was also observed for SED studies with naphthol (see previous discussion). To demonstrate the results of DPDS systems at a common surfactant concentration (ca. 20 mM) for all three hydrocarbons tested, plots ofK, versusX,values are shown in Figure 6. In order to show the results on a common scale, X, values are normalized to excess additive results and K , values are normalized to theoretical K, values at X, = 0 (Le.,K,,). As clearly shown in Figure 6, unsaturated concentrations of hydrophobic or polar contaminants demonstrate core and palisade solubilization effects (increasing and decreasing K, with increasing X,, respectively). Changes in K, as a function of X, (&) by a factor of 2 or more (asmuch as an order of magnitude) were observed. Excess additive results were observed to overpredict K, values for the nonpolar naphthane and to underpredict values for the polar naphthol versus unsaturated hydrocarbon concentration results. Obviously, these differences can significantlyaffect estimates of remediation times, etc. Thus, if the aqueous concentration (&) is expected to be below solubility limits, appropriate determination of K, values may be critical in predicting or describing the solubilization process. Semiequilibrium dialysis has been 2488
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 10, 1995
02
03
* naphthol
0.4 Xm
0 5 06 (normalized)
= naphthalene
07 x
-1
,
,
08
09
naphthane
1
1
FlGURE6. M, vetsus &for DPDS systems for all three hydrocarbons.
X,, values are normalized to excess additive results (Table 3). and C values are normalized to theoretical M, values at X , = 0 (&,, values, see text and Table 5).
demonstrated as a useful technique for evaluatingK, values under these conditions.
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Received for review September 7, 1994. Revised manuscript received M a y 26, 1995.Accepted June 13, 1995.@
ES9405639 @Abstractpublished in Advance ACS Abstracts, August 1, 1995.
VOL. 29, NO. 10, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 12489
