Environ. Sci. Technol. 1994, 28, 1874-1881
Surfactant Adsolubilization and Modified Admicellar Sorption of Nonpolar, Polar, and Ionizable Organic Contaminants Sandeep P. Nayyar, David A. Sabatinl,’ and Jeffrey H. Harwell School of Civil Engineering and Environmental Science and School of Chemical Engineering and Material Science, The Institute for Applied Surfactant Research, The University of Oklahoma, Norman, Oklahoma 73019
* Corresponding author. Telephone: (405) 325-4273; Fax: (405) 325-4217; e-mail address:
[email protected] Solubilization and adsolubilization (contaminant partitioning into admicelles) have been investigated; similarities observed between the processes lend a greater understanding of the mechanisms involved. Lee et al. (21)showed the solubilization of alkanes to be very similar to their adsolubilization. For alcohols,however, anomalies were found: high ratios of alcohol to surfactant adsorption at low surfactant coverage; the progressive increase in the adsorption of alcohol up to the CMC; and a slight decrease in the plateau adsorption of the surfactant. A two-site model was presented to explain these observations. The model assumed that sorption of polar molecules (e.g., alcohols) took place both in the palisade region of admicelles (polar/ionic exterior) and at the perimeter of patchwise admicelles (conceptually, disc-shaped admicelles which occur at low surfactant coverage). As surfactant coverage increased, the availability of this hydrophobic surface decreased and the admicellar partition coefficient decreased, thereby approaching the micellar partition coefficient. Additional studies have provided insights into the fundamentals of the adsolubilization phenomena (22-27) and evaluated the environmental applications and implications of adsolubilization (10-16). Adsolubilization is the basis for a separation process known as admicellar chromatography (AC). This process has been suggested as a viable alternative to carbon adsorption and as a separation process in biotechnology (28-30). Grout (30),in his study, obtained encouraging results for the removal of tert-butyl phenol with a SDSalumina admicellar chromatographic system (concentrations at regeneration of over 200% of the feed concentration and minimal effluent levels were reported). An inherent drawback of the system was the need for continual surfactant feed to maintain the bilayer; this surfactant was wasted since it was leaving the column in a dilute solution which was uneconomical to recover. An attempt to alleviate this very problem was the original impetus for the present study, which evaluated admicellar solubilization below the Krafft temperature of the surfactant. At room temperature (RT), surfactants generally show decreasing solubility in water with increasing molecular weight. Very large ionic surfactant molecules are practically insoluble in water at RT, but go into solution and exhibit surface activity a t elevated temperatures (specifically, above the Krafft temperature-Tk). Weilet al. (31) studied the detergency of long-chain surfactants at high temperatures; they found that sodium octadecyl sulfate (SODS) is an excellent detergent above its Tk of 56 “c. However, if cooled below the T k , SODS precipitated onto the medium and in the solution. Below its T k , it is hypothesized that no surfactant feed (monomers) would be necessary to maintain such a “modified bilayer” in a packed column. Assuming that this “modified admicellar” system retained its adsolubilization properties, this system could mitigate the surfactant bleed problem (since continual surfactant injection would not be necessary to
1874 Environ. Scl. Technol., Vol. 28, No. 11, 1994
0013-936X/94/0928-1874$04.50/0
Adsolubilization of contaminants by media-sorbed surfactants is an important phenomenon for surfactant-based environmental technologies. The present research evaluates the impacts of contaminant properties on adsolubilization (e.g., nonpolar, polar, and ionizable organic compounds). In addition, adsolubilization by modified admicelles is investigated (operatingbelow the surfactant’s Krafft temperature). The medium and surfactant investigated were alumina and sodium dodecyl sulfate, respectively. Naphthalene, naphthol, and 4-amino-1-naphthalenesulfonic acid were investigated as nonpolar, polar, and ionizable organic compounds, respectively. Variations in adsolubilization results for these compounds are explained based on surfactant fundamentals and contaminant properties. Modified admicelles effectively adsolubilized organic molecules without requiring the presence of surfactant monomers. Implications of these results to surfactant-based environmental technologies are discussed . ~~
~~
~
~~
~
Introduction The adsorption of surfactants on media was first studied by mineralogists trying to maximize the recovery of ores. Ore flotation was enhanced by surfactants that adsorbed onto the media and rendered it hydrophobic, thus allowing mineral fines to adhere to bubbles sparged into a slurry (I). Petrogeologists have investigated the possible use of surfactants to improve the secondary and tertiary removal of oil through surfactant flooding of reservoirs (2). Recently, surfactant-based environmental applications have been evaluated, including enhanced contaminant extraction via micellar solubilization, in-situ retention of contaminants, separation processes, etc. (3-1 7). In all of these applications, surfactant sorption had to be addressed; a phenomenon desirable in ore flotation, undesirable in oil recovery, and both desirable and undesirable in environmental remediation technologies. Fundamental studies have evaluated the mechanisms of surfactant adsorption including the microenvironment of the adsorbed surfactant aggregates. Adsorption of ammonium acetates on quartz has been studied by measuring the { potential of quartz, confirming that the adsorption behavior of surfactants on mineral surfaces is primarily ion exchange a t low concentrations and aggregation at higher concentrations (18). Fuerstenau (18) also showed that the hemimicelle concentration (HMC, the concentration at which surfactant aggregates first begin to accumulate at the solid-liquid interface, sometimes also referred to as the critical admicelle concentration or CAC) is an inverse function of hydrocarbon chain length. These and other studies (19,20)have improved our understanding of surfactant/media interactions.
0 1994 American Chemical Society
were reverse phase, with c18 Nucleosil packing (Alltech Associates, Inc., Deerfield, IL). Methanol (Fisher Scientific, HPLC grade), most commonly a t 80% strength, was chemical solubility Merck used as the mobile phase for HPLC assays; all dilutions compound formula MW (mg/L) Index No. were done with HPLC-grade water. The solutes showed 30-33b 6289 128.16 naphthalene CmHs clean chromatographic peaks and were used as received. 311 6323 CloHeNOaS 233.26 ANSA SDS was easily detected by a conductivity detector; The 6303 CloHeOH 144.17 1000c 2-naphthol the lack of chromatographic evidence of any decomposition Ref 34 (unless noted otherwise). b Ref 35. Measured in this products confirmed that the alumina was not degrading research. the SDS (36). The organic solutes (all aromatic) showed good UV absorbance; naphthalene and 4-amino-1-naphmaintain the admicellar phase, the continual loss of thalenesulfonic acid (ANSA) were analyzed a t 225 nm, and naphthol was analyzed at 240 nm. All calibration surfactant monomers from the column would be mitiruns resulted in linear fits with good correlations (r2 > gated). The objectives of this research were to evaluate the 0.99). solubilization and adsolubilization of organic contaminants The reactors for studies a t room temperature (RT) were with varying properties (nonpolar, polar, and ionizable) equilibrated in a wrist-action shaker. Studies below RT and to determine if adsolubilization is still feasible below were done either in a refrigerator or a Sherer (Marshall, the Krafft point of the surfactant. The experiments MI) controlled environmental chamber; the reactors were conducted include adsorption studies for sodium dodecyl periodically shaken by hand. The adsorption and adsulfate (SDS) on alumina; adsolubilization studies for solubilization studies were conducted in duplicate 40-mL aromatic molecules of polar, nonpolar, and ionizable nature vials; the vial caps had Teflon-lined septa that allowed with SDS admicelles above and below its Krafft point; sample withdrawal for analysis. Sorption of the organic solubilization studies for the aromatics into SDS micelles; solutes onto the media by itself was tested using control and simulated core studies for the aromatics using a vials without surfactant. Kinetic studies determined that surrogate solvent. The hypotheses directing the funda6 h of equilibration were sufficient throughout the range mental aspects of this research are that variations in of useful surfactant solutions (37). Solubilization studies adsolubilization for the contaminants evaluated can be were done in duplicate vials using the maximum additivity explained from fundamental surfactant chemistry and method (6). contaminant properties and that modified admicellar For SDS, with a Tk of 16 "C, a conventional bilayer systems will retain their ability to adsolubilize contamicould be established a t RT. Once a bilayer had been nants while mitigating the need for monomer concentraestablished, the reactors were cooled to 6 "C, well below tions of surfactant to maintain the admicelle. the Tk. After modification had been achieved (assumed to be complete once the precipitate was discerned in the Materials and Methods stock solutions), the organic solute was introduced into the vial a t the cold conditions. After equilibration, the Sodium dodecyl sulfate (SDS), the anionic surfactant supernatant was decanted and analyzed for surfactant and used in this research, was purchased from Fisher Scientific solute concentrations. Co. (Pittsburgh, PA) and had a manufacturer-reported Adsolubilization studies were conducted in a series of purity of 98%. It was recrystallized three times from a batch reactors each with the same initial contaminant and 50x50 mixture of water and reagent alcohol, filtered, and concentration but with varying surfactant concen, dried as described by Shiau (32). SDS ( C I Z H ~ ~ O S O ~ N ~media trations (and thus varying admicellar coverage on the Merck Index No. 8587) has a critical micelle concentration (CMC) of 8.20 mM a t 25 "C and a Krafft temperature of alumina); no excess contaminant was present in the 16 "C (33). The A1203 medium, mesh size 150,was obtained adsolubilization studies. This method worked well for from Aldrich Co. (Milwaukee, WI) and had a point of zero naphthalene and ANSA. However, due to the high chargeof9,landasurfaceareaof 155m2/g. Thealuminasolubility of naphthol, only small widths of the solubility SDS system has been widely evaluated and was thus ideal range could be covered for a given initial naphthol for the fundamental studies in this research. The results concentration. Two methods were utilized to address this are equally applicable to other surfactant-media systems problem. First, batch reactor series were evaluated three of opposite charge. The organic solutes studied were times, each time with a differing initial naphthol connaphthalene (Fisher Scientific, 99% pure), 4-amino-lcentration. Second, a titration method was utilized naphthalenesulfonic acid-ANSA (Aldrich Co., 97 % pure), whereby the naphthol concentration was increased in a and naphthol (Aldrich Co., 99% pure). Relevant propersingle reactor. By maintaining a constant admicellar ties of the organic solutes are listed in Table 1. The coverage and varying the naphthol concentration in the dodecane used for the admicellar core simulation studies titration method, it was possible tomore thoroughly cover was obtained from Fisher Scientific. Studies were done the entire solubility range for naphthol. Between suca t natural pH (ca. 6.0-6.5); properties of the medium and cessive titrations, 8 h was allowed to ensure equilibration the ionizable organic (ANSA) will be relatively unaffected (confirmed by analyzing the first data point, the kinetically by variations in this pH range. "slowest" data point, initially and after 24 h). The flask High-performance liquid chromatography (HPLC) was was stirred magnetically for the RT study. For analysis, used to analyze the surfactants and the organic solutes. about 40 mL of the supernatant was decanted, centrifuged, The system consisted of a Beckman (San Ramon, CA) and analyzed for surfactant and solute. Only 400 ~ Lwas L solvent delivery module, a Beckman UV absorbance used in the actual analysis, amounting to a total decrease module or an Alltech conductivity detector, and a controlin the original volume of less than 1% over the entire ling NEC PC. The 150 X 4.6 mm chromatographic columns titration method study. Table 1. Properties of Organic Solutes Used in Present Researcha
0
Envlron. Scl. Technol., Vol. 28, No. 11, 1994
1875
looT-
1-01
7
I I
0.1
I
I
I I I I I '
I
I
I
10
1
O
.
I
4 0 . 0 0 1 100
[SDSIeq, mM Flgure 1. Naphthaleneadsolubilization isotherm Dlotted with CorresDonding SDS adsorption isotherm; initial aqueous naphthalene concentration =-32.9mglLf volume of solution = 40 mL; mass of alumina = 1 g.
The aromatics required energy input before they went into solution; even in surfactant solutions, the compounds needed to be magnetically stirred for a t least 8 h unless the organic was introduced as a spike in methanol. Naphthol, which served as the polar compound for the research, is light sensitive and was shielded from light using aluminum foil. Being also air-sensitive (volatile), naphthol was always stored under nitrogen and sparged with the gas after every use.
Results and Discussion Adsorption of SDS. The adsorption of SDS on alumina has been extensively studied, and four regions have been defined for its adsorption isotherm (18). In this present study, the regions of interest on the adsorption isotherm were regions I1 and I11 of bilayer formation. A typical isotherm obtained showing these regions is seen in Figure 1. The figure also has a superimposed adsolubilization isotherm describing the equilibrium concentration of an organic (in this case naphthalene) in the presence of different bilayer coverage. In Figure 1, naphthalene removal from solution is clearly seen to be most significant in regions I1 and 111, concomitant with the greatest surfactant sorption. Relative to regions I1 and 111,the monolayer region (region I) does not show significant surfactant sorption; this is further evidenced by relatively minor naphthalene removal in this region. Others have observed more significant sorption for organoclays in region I; however, higher surfactant coverage (higher percent organic carbon upon surfactant sorption) and more hydrophobic contaminants were utilized (10-13,15-16). Low concentrations of surfactant coverage (region I) were not studied in greater detail in this research. Likewise, concentrations of surfactant greater than the critical micelle concentration (CMC) were not evaluated since the presence of micelles complicates 1876
Envlron. Scl. Technol., Vol. 28, No. 11, 1994
the testing of adsolubilization, which is the focus of this research. Also, admicellar chromatography (the original focus of this research) will be operated below the CMC to optimize the efficiency of the system. The maximum surfactant coverage obtained (without pH control) was just under 70 pg of SDS/mg of alumina realized at a liter per gram ratio of 0.04. There was no indication that adsorption was enhanced a t the Krafft boundary. Both 21 "C and 17 "C isotherms virtually coincided, as do isotherms at 4 "C via two paths [data not shown here (391. Thus, surfactant sorption was not observed to be dramatically impacted by variations in temperature in the vicinity of the Krafft temperature. Adsolubilization Studies. Though SDS-alumina sorption has been studied a great deal, partitioning of organics into this adsorbed bilayer has received less attention. This study looked at the adsolubilization of organics of polar, nonpolar, and ionogenic nature. Parameters similar to those used in solubilization research (38) were developed for adsolubilization. The molar fraction of the organic in the admicelle, X a d m , is defined similar to X m i c , the molar fraction of the organic in the micelle
CO- C e q (1) (C, - Ceq) + (So - SeJ where COand Ceqare the initial and equilibrium concentrations of the organic solute, and SO and S q are concentrations of the surfactant added and present as monomers, respectively. The admicellar partitioning coefficient, K a d m ( = X a d m / X a q ) , is defined analogous to Kmic (the micellar partitioning coefficient) where the aqueous molar fraction of the organic, X a q , is given by Xadm
=
xaq
= c,,
eq
+ 55.55
where 55.55 represents limolar volume for water.
Table 2. Synopsis of Partitioning Values Obtained during the Researche compound
type
log Kadm *
1% Kadm’
naphthalene ANSA naphthol
nonpolar ionizable polar
4.66 f 0.15 3.45 f 0.10 3.56 f 0.10
4.27 f 0.19
log Kmic 4.09 f 0.03 2.95 f 0.03 3.50 f 0.02
-e
3.58 f 0.01
log Kow
log Kc,,
3.36 0.17 f 0.02 2.69 f 0.03
4.46 1.56 4.72
Kadm= partitioning coefficient into SDS admicelle; Kadm’= partitioning coefficient into modified SDS admicelle; Kmio= partitioning coefficient into SDS micelle; K , = octanol-water partitioning coefficient; Kc,* = partitioning coefficient for organic solute into dodecane. b Obtained from slope of corresponding isotherm. KO,for ANSA and naphthol obtained (based on KO, for naphthalene, ref 35) using a method outlined in ref 40. d Obtained from reactor studies. e Not conducted due to strong sorption of ANSA onto alumina. (I
Naphthalene shows very distinct regions in its adsolubilization isotherm, as seen in Figure 1. At low surfactant concentrations (monolayer coverage), minimal naphthalene loss from solution occurs; i.e., minimal adsorption onto the medium and accumulation at the SDS monolayer hydrophobic surface is realized. However, as bilayer (admicelle) formation proceeds, there is increased partitioning of the naphthalene into the increasing bilayer. This phenomenon is demonstrated in the two regions described as regions I1 and 111on the sorption isotherm. In region IV, the equilibrium aqueous concentration of naphthalene once again begins to increase due to the presence of micelles in solution, There is now a distribution of naphthalene between the micellar and admicellar pseudophases as the two phases compete for the organic. Analysis in this region would yield information on the relative affinity of the organic compound for the micellar and admicellar environments. However, this would require an assay of the surfactant in the monomeric and micellar forms [possibly using semiequilibrium dialysis methods (39)l;such a study was beyond the scope of this research. Regions I1 and I11describe the partitioning of the organic between the admicellar and aqueous phases. A plot of Xadm versus X,, resulted in a good h e a r correlation (Figure 2) indicating classical partitioning behavior. This was as expected for core adsolubilization (i.e., partitioning into the hydrophobic interior or core of admicelles). Since naphthalene is nonpolar, it would be expected to favor the core of the micelle or admicelle formed by the aliphatic alkyl chains of the surfactant molecules. The value of log Kadm for the adsolubilization of naphthalene into a SDS bilayer was determined to be 4.66 f 0.15 (expressed on a mol/mol basis and 95 % confidence interval throughout this study) for a liter per gram ratio of 0.04 (see Table 2). The adsolubilization of naphthol into varying levels of SDS bilayer coverage for three different initial concentrations of naphthol is shown in Figure 3 (resulting in three isotherms). Since the naphthol has high solubility in water, the first isotherm (series of batch reactors with Co = 500 mg/L) only scanned a small portion of the solubility range of naphthol. Two more batch studies (at CO= 200 mg/L and at CO= 800 mg/L) were conducted to more thoroughly cover the solubility range of naphthol. The data for each value of Co (each isotherm) is linear, as previously observed for naphthalene. However, unlike naphthalene, these plots do not go through the origin, preventing the definition of a linear partitioning coefficient for the naphthol’s distribution into the SDS bilayer. More importantly, a study of the graph reveals two interesting observations. Each “isotherm” (each set of batch reactors with a common value of CO)shows an increasing partition coefficient with increasing Xaq for that isotherm (as seen from the increasing slope; Le., Kadm; given a zero intercept for each isotherm). It should be noted that increasing Xa, for a
E
0 14l
x
0.121
P
g Y
2
~
l
I 0.1-
P
L/g = 0 04
pH = 7
temp = 20 deg C
I
0 02-
:
00
05
1 15 2 Naphthalene Aqueous Mole Fraction, Xaq (Times 10E-6)
25
3
Figure 2. Computation of naphthalene’sKsrm(linear partitioningconstant into the admicelle) using data corresponding to regions I1 and I11 in naphthalene’s adsolubilization isotherm (see Figure 1).
given value of COcorresponds to decreasing surfactant coverage. The decreasingKadmwith increasing surfactant coverage of the medium can be explained by the two-site model for adsolubilization as discussed by Lee et al. (21). Polar organic solutes (e.g., alcohols) adsolubilize in the palisade region of admicelles (the polar/ionic exterior of the admicelle). In addition, at low surfactant coverage the polar compounds will also adsolubilize at the perimeter of the admicelle (sites a t the oil-water interface amenable to accumulation of naphthol molecules) due to the patchwise nature of the admicelles (envisioned as admicellar discs). At higher surfactant coverage, the perimeter areas decrease and thus K a d m decreases. Conversely, alkanes partition only into the core of the admicelle, irrespective of the bilayer coverage. Thus, this effect was not observed for naphthalene. With naphthol (an alcohol), the two-site behavior would be expected a t the low coverage occurring in the present system 12 molecule/nm2 as against total bilayer coverage of 8 molecule/nm2 (22)l. It is also obvious that the slope of the isotherm decreases with increasing CO (between isotherms as opposed to within a given isotherm as discussed above). This indicates that at higher concentrations of naphthol the perimeter of the admicelle is being saturated and the partitioning decreases, resulting in an overall Langmuirian tendency. To confirm this Langmuirian behavior, another adsolubilization study was done with a variation in technique. This time, the surfactant concentration was kept constant, and the alumina-SDS suspension was titrated with naphthol (the suspension volume was altered by less than 1%during the titration). In this manner, the solubility range of naphthol could be investigated more thoroughly. Environ. Sci. Technol., Vol. 28, No. 11, 1994
1877
0,3r--
,II
3E~
0.251
I
Langmuir isotherm coefficients K1 = 18202moleimole; qmax = 0.36 moleimole
0
,
-e
X
I
-LI
E 4
nix4
Co = 500 mdL 1
/
log slope = 4.49 Co = 850 mg/L
m
z
A 0 0
---__ 7 2 3 4
1
Titrimetric Datal I
5
6
___.
7
8
9
10
Naphthol Aqueous Mole Fraction, Xaq (Times 10E-5)
Flgure 3. Adsolubilization isotherms for naphthol across its solubility range. The isotherm obtained from the titrimetric study (constant surfactant coverage, increasing naphthol concentration) is superimposed on the three isotherms found with different initial naphthol concentrations (Co, as shown) and increasingsurfactant coverage (decreasing X,, for a given value of Co-a given isotherm). The solid line corresponds to a Langmuirian fit of the continuous data.
Co = 250 mg/L
250
* - .c-
-/+-+-+-
-1
4
aoo-
3
-+ + -
‘
E
,
50
O+0
_~_ 2
_
4
6 [SDSI-eq
a
_
~ 10
~ 4 _ _I 12
14
mM
Flgure 4. Adsolubilizationisothermfor ANSA in SDS-alumina system; volume of solution = 40 mL; mass of alumina = 1 g.
The Langmuirian trend (K1= 1.82 X lo4 mol/mol, qmax= 0.36 mol/mol) is apparent in Figure 3. For comparison, the log Kadmreported in Table 2 for naphthol was reported from the initial linear portion of the Langmuir adsolubilization isotherm. ANSA adsorbed significantly onto the positively charged alumina even in the absence of surfactant (note the y intercept in Figure 4). With an increasing addition of surfactant, and therefore increased surfactant at the media-water interface, the ANSA concentration in solution increased gradually. It was deduced that a t increasing surfactant coverage, the surfactant displaced the ANSA and caused it to reappear in solution. However, the competition of admicelles for the released ANSA was evidenced by the fact that each mole of surfactant adsorbed did not result in an equal mole of ANSA in solution (as ion exchange alone would mandate). This logic was used to mathematically infer a Xadm value and resulted in an 1878 Environ. Sci. Technol., Vol. 28, No. 11, 1994
average log Kadmvalue of 3.45 (37). Ionogenic species like ANSA can thus be removed from solution by virtue of their strong adsorptive attraction for oppositely charged mineral surfaces or, in the absence of charged surfaces, by their adsolubilization tendency with admicelles. Modified Adsolubilization Studies. The (admicelle) bilayer was modified by cooling below the Krafft point to study the nature of the partitioning of naphthalene into the admicelle under these conditions. The resulting partitioning coefficient, log Kadm’,was found to be 4.27 f 0.19 (see Table 2). This showed clearly that the bilayer cooled below its Tk was still effective in removing naphthalene from solution. For naphthol, comparison of modified adsolubilization and adsolubilization was based on the titrimetric method. For the modified admicelle, the palisade effect (if it exists at all for the system at this temperature) is not evidenced, likely due to the fact that naphthol solubility quickly becomes limiting. For the solubility range investigated, the modified adsolubilization is linear with a partitioning coefficient,lOg&dm’, of 3.58 f 0.01. For the same solubility range, conventional adsolubilization gives a similar log Kadm of 3.56 f 0.1. The difference is not statistically significant and demonstrates that the modified surfactant/ media system is still very much involved in the adsolubilization process. Because of the low adsolubilization seen for ANSA and its sorptiononto the medium, modified adsolubilization for ANSA was not studied. The partitioning constants summarized in Table 2 provide encouragement that the modified admicelle provides a favorable environment for nonpolar and polar molecules. Statistically, the partitioning coefficients of the organics into the conventional and modified admicelles do not significantly differ (for a 95% confidence test). It should be noted that supernatant removal and replacement with surfactant-free DI water did not result in surfactant bleed; Le., surfactant monomer concentrations were not
0
naphthol
ANSA I
I
naphthalene I
I
I
Figure 5. Linear correlation between the admicellar partitioning constant (Kh,LIKE)),modified admicellar partitioning constant ( K , ,L/Kg), and the octanol-water coefficient (/Cow). Data obtained from literature and present research.
necessary to maintain the media-associated surfactant structure (admicellar or otherwise). Also, the process was observed to be stable with time and reversible with temperature. The realization of good adsolubilization without surfactant bleed is very encouraging and suggests further evaluation of modified adsolubilization for use in environmental technologies. Future studies should carefully analyze the nature of this modified admicellar phase. KO, Correlations. The octanol-water partitioning constant (KO,)has been identified as an effective parameter for normalizing micelle-water partitioning coefficients (41); similar behavior is expected for admicelle-water partitioning (42). For the organic solutes in the present research, partitioning coefficients similar to those used by previous researchers were determined [Kh, mass of organic adsolubilized per mass of surfactant sorbed normalized by the aqueous organic concentration (291. Figure 5 shows that for naphthalene the linear correlation between the admicellar partitioning constant and the octanol-water partitioning is consistent with trends of other researchers studying micellar systems. Naphthol partitioning is represented on Figure 5 by two extreme values of log Kh. At low surfactant coverage, greater partitioning of naphthol is observed due to the above-discussed two-site effects. A t higher surfactant coverage, the resulting lower log Kh value is observed to be more consistent with the log KO, trends from solubilization studies. For ANSA, it is clear that much greater partitioning is evidenced than would be predicted by its hydrophobicity (KO,). ANSA adsolubilizes due to its electrostatic attraction for the admicellar surface and is only marginally affected by hydrophobic forces. Thus, it is observed that, when properly applied, KO, based estimates can fairly accurately describe solubilization and adsolubilization. However, it is also noted that when applied inappropriately KO,estimates may be grossly in error (e.g., for polar and ionizable organic compounds). Solubilization Studies. Solubilization isotherms for the organics studied (with SDS) are not available and
,.A7
0
2
4
6
8
IO
12
14
16
18
20
[SDS], m M
Figure 6. Solubilization isotherm for naphthalene in SDS. No alumina is present in this system.
therefore had to be evaluated. The micellar partitioning coefficient, Kmic (=Xmic/Xaq), was found to describe the partitioning of the various organics into the micellar environment, where X m i c is (3)
Nonpolar naphthalene is expected to partition into the core of the SDS micelle. At the CMC (around 8 mM), solubilization of naphthalene is first evidenced (Figure 6). The partitioning is described by the slope of this region of the isotherm which gives the molar solubilization ratio (MSR), indicating the moles of the organic solute (solubilizate) per mole of the micellar surfactant (for naphthalene the MSR was 0.064). The molar fraction of the solubilizate in the micelle (XmiJ is related to the MSR by the simple relation: Xmic= MSR/[l + MSRI (27). This gives a Xmic for naphthalene of 0.06. Using a X,, of 5.04 X 10-6 gives a Kmicbetween the micellar and aqueous phases Environ. Sci. Technol., Vol. 28. No. 11. 1994
1879
of 1.19 x lo5(lOgKmj, = 4.09 0.03) (Table 2). The micellar environment thus has more than 3 orders of magnitude greater solubilization capacity than water. For the solubilization of naphthol by SDS, the two regions are clearly seen but the boundary is not as distinct as with naphthalene (presumably due to the surface activity of naphthol and associated lowering of the CMC of SDS). The MSR for naphthol is computed to be 1.145, Xmic to be 0.534, and log Kmjc to be 3.50 f 0.02. The MSR and Xmic values are much higher than values for naphthalene, which would suggest that the polarity of the molecule allows it the possibility of partitioning into the palisade region of the micelle, besides dissolving in the nonpolar core of the micelle. Swollen micelles would cause an increase in the palisade region available for the partitioning of these polar molecules. The higher water solubility of naphthol (higher Xaq) causes it to have a Kmjc lower than for naphthalene; thus, while it is higher on an absolute basis, it is lower on a relative basis. ANSA, with its ionic amine and sulfonate groups, is 10 times more soluble in water than naphthalene. Its solubilization into micelles was therefore not expected to be as high as that for a nonpolar molecule like naphthalene; the resulting log Kmicvalue was 2.95 f 0.03. The little solubilization evidenced probably takes place at the ionic surface of the micelle. It is thus observed that Kmicvalues for naphthalene, naphthol, and ANSA are of the same order as corresponding Kadm values, but consistently lower. A recent study by Park and Jaffe (43) reports nearly identical micellar and admicellar partitioning for small organic molecules and more efficient (but of the same order) uptake by the micelle as the molecular size of the solute increased. The study found micellar/admicellar partitioning constants from solutions containing both these pseudophases and found the amount of solute adsolubilized as a function of the organic matter content. The point to be stressed here is that admicellar partitioning can be as attractive as micellar uptake and that this phenomena can be exploited in environmental technologies. SimulatedCore Studies. Dodecane (a ‘2x2alkane) was selected to simulate the core of the SDS aggregate (SDS is a Clz surfactant). Partitioning studies were performed with naphthalene, ANSA, and naphthol using standard techniques ( 4 4 ) . The contaminant-dodecane partitioning coefficient (KcI2)was determined for these compounds (where Kclz is defined as the ratio of the molar fraction of the organic solute in dodecane Xc12,to the molar fraction of the organic in water, Xaq). XclZis given by (4) where COand C,, are expressed in moles per liter, r is the ratio of water:oil used, and 4.395 represents the moles per liter of dodecane. From Table 2, it is seen that the log Kmic and log Kadm for naphthalene are similar to the log Kclzvalue obtained for naphthalene, corroborating the suggestion that naphthalene would partition into the core of the micelle/ admicelle. Partitioning of the ANSA into dodecane was found to be relatively very low (log Kclz = 1.56; Table 2), which is a strong indication that ANSA tends to partition only slightly into the core of the SDS and favors the ionic surface of the micelle. Most of the ANSA apparently 1880
Envlron. Sci. Technol., Vol. 28, No. 11. 1994
accumulates at the charged surface of the micelle/admicelle with the admicelle providing a more effective surface than dodecane for accumulation. In the case of naphthol, attempts to assess the partitioning into the dodecane were not altogether successful. Due to its surface activity (evident during solubilization experiments), naphthol formed an emulsion at most oil/water (o/w) ratios. Thus, observed values of log KclZ(as reported in Table 2j are most likely higher than that for pure partitioning of naphthol between dodecane and water (loss of surfactant to the interface is misinterpreted as micellar partitioning). This observation (accumulation at the interface) does indirectly confirm the theory that naphthol would locate itself in the palisade region of micelles or admicelles, as against the relative affinity of the ANSA for water and the affinity of naphthalene for the oil.
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Abstract published in Advance ACS Abstracts, September 1, 1994.
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