Environ. Sci. Technol. 1997, 31, 2407-2412
Counterion Effects on the Sorption of Cationic Surfactant and Chromate on Natural Clinoptilolite ZHAOHUI LI* AND ROBERT S. BOWMAN Department of Earth and Environment Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801
We determined the effect of selected counterions (Cl-, Br-, and HSO4-) on the sorption of the cationic surfactant hexadecyltrimethylammonuium (HDTMA) on clinoptilolite zeolite and on the subsequent sorption of chromate by HDTMA-zeolite. The HDTMA sorption on the zeolite, as characterized by the Langmuir sorption maximum, followed the trend HDTMA-Br > HDTMA-Cl > HDTMA-HSO4 (208, 151, and 132 mmol/kg, respectively). The same counterion trend was observed for HDTMA sorption on KGA-1 kaolinite. Measurement of counterion sorption indicated that HDTMABr and HDTMA-Cl formed complete bilayers on the zeolite, whereas HDTMA-HSO4 showed less than full bilayer formation. Competitive sorption between HDTMA-Br and HDTMA-Cl on the zeolite also showed a preference for the Br- counterion. The counterion stabilization of HDTMA admicelles on the zeolite surface follows the same trends as the counterion stabilization of micelles in solution. Chromate sorption was also strongly influenced by the HDTMA-zeolite counterion, with chromate sorption maxima decreasing in the order HDTMA-HSO4 > HDTMA-Cl > HDTMA-Br (28, 16, and 11 mmol/kg, respectively). The sorption of chromate and other divalent anions on HDTMA-zeolite results from a combination of entropic, Coulombic, and hydrophobic effects, all of which are functions of the initial HDTMA counterion. In the design of surfactant-modified clays and zeolites for environmental applications, the strong influence of the surfactant counterion must be considered.
Introduction The sorption of cationic surfactants from solution onto solid surfaces has been studied extensively, primarily from the perspective of flotation in mineral separation processes. In contrast to clay minerals and zeolites, most of the solid adsorbents studied have low surface charge densities, e.g., quartz (1) and colloidal silica (2), or occur as macroscopic crystals with fixed compositions, e.g., amosite asbestos (3) and mica plates (4). The sorption of cationic surfactants on clay minerals and the potential applications of such modified clays as environmental remediation materials have been studied recently (5-7). One of the exceptional properties of the cationic surfactant-modified clays is their ability to sorb organic contaminants and thus reduce contaminant migration in the subsurface and groundwater. Studies on the sorption of benzene, toluene, and ethylbenzene on soils modified with cationic surfactants of different chain lengths indicated that the sorption coefficients of organic contami* Author to whom correspondence should be addressed. E-mail:
[email protected]; Fax: (505) 835-6436. Phone: (505) 835-5466.
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1997 American Chemical Society
nants increased with respect to the chain length of the cationic surfactant (8). Studies on the sorption of dodecylpyridinium bromide on several natural materials, including clays and soils, revealed that the sorption was adsorbent-specific and pH-independent (9). Burris and Antworth (10) conducted batch and column studies on the sorption of PCE and naphthalene on an aquifer material modified with hexadecyltrimethylammonium (HDTMA)-Cl and concluded that the sorption coefficients of PCE and naphthalene on the HDTMAmodified aquifer material increased by 2 orders of magnitude relative to that of the unmodified material. Zhang et al. (11) studied the effect of exchangeable cations on the sorption and desorption of several quaternary ammonium cations with different chain lengths on montmorillonite and found that K+ was more difficult to replace than Na+. Stapleton et al. (12), studying the sorption of pentachlorophenol to HDTMAclay as a function of ionic strength and pH, concluded that the sorption of the deprotonated (anionic) form can be represented by a Langmuir-type isotherm while that of the protonated (neutral) form is best represented by a linear isotherm. Zeolites share many of the surface properties of clays and have also been used recently as substrates to sorb cationic surfactants. Such modified zeolites not only have the ability to sorb organic contaminants such as PCE, TCE, and BTX but also retain inorganic anions such as chromate, sulfate, and selenate (13, 14). The sorption of a cationic surfactant onto a negatively charged surface involves both cation exchange and hydrophobic bonding. At low loading levels, surfactant monomers are retained by ion exchange and eventually form a monolayer. As the amount of available surfactant increases, interactions among hydrocarbon tails cause the formation of a bilayer or patchy bilayer (15). Due to the intercalation of the long chain surfactant molecules into the interlayer space of clay minerals, it is difficult to differentiate between the amount of surfactant held in interlayer positions due to hydrophobic interactions and the amount retained by strictly cation exchange processes. The ability to differentiate between internal and external cation exchange sites would enable one to better investigate changes in modified mineral properties as a function of surfactant loading. This may have important implications in determining the optimum level of surfactant treatment for a given application. Zeolites offer an advantage over clay minerals in distinguishing between electrostatic and hydrophobic effects on surfactant binding. Long chain quaternary ammonium cations are too large to enter zeolite channels or access internal cation exchange positions. Thus, the sorption of long chain quaternary ammonium cations is limited exclusively to external surfaces of zeolite particles (14). For this reason, we chose a natural zeolite as the substrate for determining the effects of counterions on cationic surfactant sorption and the subsequent sorption of the inorganic anion chromate by the modified zeolite. Sorption of inorganic anions on cationic surfactantmodified zeolite has been attributed to the formation of a surface-anion complex (14). In order to sorb anions and form a complex, the modified surface must possess positively charged exchange sites. These sites are formed when positively charged surfactant head groups are presented to the surrounding solution in the form of a bilayer or patchy bilayer. The positively charged head groups are balanced by counterions, and the sorption or exchange of other anionic constituents involves the replacement of weakly held counterions by more strongly held counterions.
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The effects of counterions on stabilizing surfactant micelle formation in solution follow the lyotropic series (16-18). The influence of counterions on sorbed surfactant has not been fully investigated, however. Chen et al. (19) studied the selfassembly of surfactant layers on mica surfaces and investigated the counterion effect on HDTMA sorption density and advancing contact angle at surfactant concentrations below the critical micelle concentration (CMC). They found that greater monolayer HDTMA sorption occurred when Cl-, rather than Br-or I-, was the counterion. This observation is consistent with the stronger interaction between HDTMA and Br- or I- compared to that with Cl-. In contrast, Xu and Boyd (15) concluded bilayer formation can be minimized when Cl- is the HDTMA counterion. Differences in surfactant counterions can result in different amounts of surfactant sorbed to yield a stable “bilayer” or “patchy bilayer”. Thus, the effect of counterions on the sorption of surfactants onto solid surfaces has theoretical importance in helping one to fully understand mechanisms behind cationic surfactant sorption. Furthermore, the subsequent sorption or exchange of other inorganic anions on surfactant-modified minerals is mainly due to replacement of less strongly sorbed counterions. Understanding counterion effects thus also has practical importance in selecting the initial counterion to achieve maximum sorption of target anions. The primary objectives of this study therefore were to (1) determine the relative effects of the counterions Br- , Cl-, and HSO4- on the sorption of HDTMA onto clinoptilolite zeolite and (2) determine the effect of the initial counterion on the sorption of chromate onto HDTMA-modified zeolite.
Materials and Methods Zeolite. The zeolite used in this study is clinoptilolite from the St. Cloud mine in Winston, NM. The material consists of 74% clinoptilolite, 12% feldspar, 12% quartz and cristobalite, and trace amounts of clay minerals (based on quantitative X-ray diffraction analysis). K+ and Ca2+ are the major exchangeable cations. The external surface area by N2 adsorption is 15.7 m2/g (20). The raw zeolite was sieved to yield particles (aggregates) in the range of 0.42-0.83 mm (20-40 mesh). The zeolite was used without pretreatment for HDTMA sorption and the subsequent chromate sorption experiments. Although the zeolite aggregates broke down somewhat during the batch sorption experiments, particularly in the presence of surfactant solution, no more than 10% of the final zeolite mass was represented by particles of less than 0.1 mm. HDTMA sorption is relatively insensitive to aggregate size; the HDTMA sorption capacity remained essentially constant when the aggregates were ground to less than 0.064 mm (data not presented). Kaolinite. The KGa-1 kaolinite is a well-crystallized kaolinite from the Source Clay Minerals Repository (University of Missouri, Columbia, MO). The mineral was used without further purification. The reported surface area of KGa-1 is 10 m2/g, and the cation exchange capacity is 2-4 mequiv/kg (21). Surfactants. The HDTMA-Cl surfactants used were AMMONYX CETAC-30 [30% active ingredient (AI) in water] from Stepan (Northfield, IL), CARSOQUAT CT-429 (29% AI in water) from Lonza (Fair Lawn, NJ), DEHYQUAT A (24% AI in water) from Henkel (Hoboken, NJ), and HDTMA-Cl (30% AI in water) from Aldrich (Milwaukee, WI). The HDTMA-Br surfactants used were BROMAT (99% AI) from Zeeland Chemical (Zeeland, MI), RHODAQUAT m-242B/99 (99% AI) from Rhone-Poulenc (Cranbury, NJ), VARISOFT CTB-40 (liquid form, 40% AI, 15% ethanol, 45% water) from Witco (Dublin, OH), and HDTMA-Br (99% AI) from Aldrich. The HDTMAHSO4 (99% AI) was from Aldrich. We used HDTMA from various sources to test the effect of different formulations on the sorption of HDTMA and the subsequent sorption of
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chromate on the HDTMA-zeolite, in order to select the best brand for future field applications. Surfactant Sorption. Batch sorption isotherms were prepared to determine the HDTMA sorption plateau (maximum amount of HDTMA sorbed) as a function of surfactant counterion. For zeolite treated with HDTMA-Br and HDTMACl, 5 g of raw zeolite and 20 mL of a HDTMA aqueous solution were put into 50 mL polyallomer centrifuge tubes, while for HDTMA-HSO4, 2.5 g of zeolite and 10 mL of a HDTMA solution were used. The initial HDTMA concentrations ranged from 50 to 200% of the sorption plateau for HDTMA-Br, which is about 200 mmol/kg (14). The initial HDTMA concentrations were all greater than the CMC regardless of the counterion. Samples were equilibrated on a shaker at 150 rpm at 25 °C for 8 h, which was sufficient to achieve sorption equilibrium (14). The mixture was then centrifuged to yield a clear supernatant solution (29100g for 15 min) and the supernatant saved for later analysis. For comparative purposes, HDTMA sorption on KGa-1 kaolinite (from the Clay Mineral Repository, University of Missouri) was also determined. The same HDTMA concentrations and solid:solution ratios as in the zeolite isotherms were used. HDTMA-zeolite for chromate sorption was treated to the HDTMA sorption plateau by mixing 60 g of zeolite and 180 mL of a 0.066 M HDTMA-Br solution, a 0.050 M HDTMA-Cl solution, or 0.045 M HDTMA-HSO4. Following equilibrium at 8 h and 25 °C, the supernatant (approximately 135 mL) was removed and the HDTMA-zeolite washed with two 180 mL portions of type I water. The treated zeolite was allowed to air-dry. Each treatment (HDTMA source and initial concentration), along with appropriate blanks, was prepared in duplicate. The pH values of the equilibrium solutions were on average 7.0 ((0.2), 6.6 ((0.2), and 2.9 ((0.4) after surface modification with HDTMA-Br, HDTMA-Cl, and HDTMA-HSO4, respectively. Chromate Sorption. Zeolite treated to the sorption plateau with HDTMA-Br, -Cl, or -HSO4 was used for chromate sorption experiments. HDTMA-zeolite (2.5 g) was put into a 50 mL centrifuge tube, and 10 mL of chromate solution (K2CrO4) was added. The initial chromate concentrations ranged from 0.04 to 4 mmol/L. The tubes were shaken for 24 h at 25 °C and 150 rpm, conditions shown previously to result in sorption equilibrium (13). The mixture was centrifuged to yield a clear supernatant solution (29100g for 15 min) and supernatant withdrawn for analysis of chromate. All the sorption experiments were done in duplicate for each surfactant brand and each chromate concentration. The pH values of the equilibrium solution following chromate sorption were on average 7.7 ((0.2), 7.5 ((0.2), and 3.7 ((0.2) for HDTMA-Br-, HDTMA-Cl-, and HDTMA-HSO4-modified samples, respectively. Chemical Analyses. The external cation exchange capacity (ECEC) of the zeolite was determined both by the method of Ming and Dixon (22) and by a modification of that method. After four washes with sodium acetate, external inorganic exchangeable cations were displaced either by tert-butylammonium as per Ming and Dixon (22) or by HDTMA. Since native Ca2+, Mg2+, and K+ were not fully displaced even after four washes with sodium acetate, all these exchangeable cations along with Na+ were analyzed by atomic absorption spectrophotometry, and the sum of the cation equivalents released was used in calculating the ECEC. Aqueous concentrations of HDTMA, Br-, Cl-, HSO4-, and chromate were analyzed via HPLC. All analyses utilized a model 510 pump, a Model 481 tunable UV-Vis detector, and a Model 717 autoinjector from Waters (Milford, MA). Injection volumes ranged from 10 to 25 µL. Chrom Perfect software from Justice (Mountain View, CA) was used for data collection and processing.
HDTMA was analyzed with a Nucleosil CN column from Sigma-Aldrich (St. Louis, MO) using an aqueous solution of 5 mM p-toluenesulfonate and methanol (45:55, v/v) as the mobile phase (23). The detection wavelength was 254 nm. At a flow rate of 1.0 mL/min, the retention time of HDTMA was 3.3 min. The detection limit was 0.01 mmol/L, and the linear response range was up to 6.6 mmol/L (peak area). Bromide was analyzed using a Vydac 302IC ion chromatography column from The Separations Group (Hesperia, CA). The mobile phase was 0.02 M KH2PO4 adjusted to a final pH of 3.80 with phosphoric acid. The detection wavelength was 200 nm. At a flow rate of 2.0 mL/min, the retention time was about 6.5 min and the linear response range was up to 3.0 mmol/L. Chloride was analyzed using a Spherex 5 SAX column from Phenomenex (Torrance, CA). The mobile phase was 0.03 M K2HPO4 (pH 2.66) containing 18% acetonitrile (v/v). The detection wavelength was 190 nm. At a flow rate of 1.3 mL/ min, the retention time was 16 min and the linear response range was up to 6 mmol/L. Chromate was analyzed using a Waters Nova-Pak C18 narrow bore column. The mobile phase was 5 mM tetrabutylammonium hydrogen sulfate in water with 10% acetonitrile (v/v). The detection wavelength was 365 nm. At a flow rate of 0.4 mL/min, the retention time was 3 min and the linear response range was up to 0.5 mmol/L. Hydrogen sulfate was analyzed as sulfate using a Hamilton PRP-X100 column (Reno, NV). The mobile phase was 2 mmol/L potassium hydrogen phthalate at pH 6.0. Detection was with a Waters model 431conductivity detector. At a flow rate of 1.2 mL/min, the retention time was 4.5 min and the linear response range was up to 2 mmol/L. All calibrations were based on four to six standards over the range of sample concentration. Coefficients of determination (r 2) for the standard curves were no less than 0.995. For each solute, the amount sorbed was calculated from the difference between the initial and equilibrium concentrations. The CMCs for HDTMA-Br, HDTMA-Cl, and HDTMA-HSO4 were determined at 25 °C from changes in the surface tension of aqueous solutions with respect to HDTMA concentration. Surface tension decreases from 72 dyn/cm at 25 °C for pure water to an almost constant value beyond the CMC (24). The surface tensions of duplicate solutions at each concentration were measured with a model 20 surface tensiometer (Fisher, Pittsburgh, PA).
Results Sorption of cationic surfactants from solution onto solid surfaces can be described by the Langmuir isotherm, even though the assumptions of Langmuir sorption may not be completely satisfied (25). The Langmuir sorption isotherm has the form
S)
KLSmC 1 + KLC
(1)
where S is the amount sorbed on solid at equilibrium (mmol/ kg), C the equilibrium liquid concentration (mmol/L), Sm the sorption capacity or sorption maximum (mmol/kg), and KL is the sorption intensity or Langmuir coefficient (L/mmol). Equation 1 can be rearranged to a linear form
C 1 C + ) S KLSm Sm
(2)
Sorption of HDTMA was similar regardless of the chemical source as long as the HDTMA counterion was the same. Unless otherwise specified, data presented in the following discussion are for Rhodaquat (HDTMA-Br), Carsoquat (HDTMA-Cl), and HDTMA-HSO4 from Aldrich.
FIGURE 1. Effect of counterions on HDTMA sorption to zeolite. Solid lines are Langmuir fits to the observed data using the parameters in Table 1.
TABLE 1. Fitted Langmuir Parameters for Sorption of HDTMA by Zeolite with Different HDTMA Counterions Sm (mmol/kg) KL (L/mmol) r2
Br-
Cl-
HSO4-
208 27 0.9996
151 30 0.9999
132 8 0.9996
The sorption isotherms for HDTMA-Br, HDTMA-Cl, and HDTMA-HSO4 on the zeolite are presented in Figure 1. Each data set was well described by the Langmuir isotherm, with coefficients of determination (r 2) exceeding 0.999 (Table 1) when the data were plotted according to eq 2. The most striking property of the isotherms in Figure 1 is the strong influence of the HDTMA counterion on the Langmuir sorption maximum. The observed maxima were 208, 152, and 133 mequiv/kg for HDTMA-Br, HDTMA-Cl, and HDTMA-HSO4, respectively (Table 1). Similarly, Xu and Boyd (15) observed greater sorption of HDTMA-Br than HDTMA-Cl on vermiculite and greater sorption of (HDTMA)2-SO4 than HDTMA-Br. When HDTMA sorbs as a monolayer or submonolayer, there should be no counterion sorption (Figure 2a). In order to sorb negatively charged chromate, the sorbed HDTMA layer must make use of its positively charged head group. This can only happen when the sorbed HDTMA molecules form bilayers or patchy bilayers and chromate has a stronger affinity for the positively charged HDTMA head group than for the counterions already sorbed (Figure 2b,c). The ratio of the amount of HDTMA sorbed to the ECEC of zeolite should be 2:1 for complete bilayer coverage (Figure 2c). The ECEC of the zeolite was 90 mequiv/kg by the method of Ming and Dixon (22) and 110 mequiv/kg by our modified method. Comparing the ECEC with the sorption plateau (Figure 1 and Table 1), it can be seen that the bilayer coverage is most complete when Br- is the counterion and least complete when HSO4- is the counterion. To test the counterion effect on the sorption of HDTMA to negatively charged surfaces in general, sorption of HDTMA on KGa-1, a well-crystallized kaolinite, was also determined (Figure 3). The HDTMA sorption maximum when Br- was the counterion was 63 mequiv/kg, compared to 48 mequiv/ kg when Cl- was the counterion (Table 2). The ratio of HDTMA-Br to HDTMA-Cl sorption maxima is equal to about 1.3 for both clinoptilolite and kaolinite. As with clinoptilolite, the HDTMA sorption sites on kaolinite are primarily external. Since the CEC value for KGa-1 ranges between 20 and 40 mequiv/kg (21), it appears that at the sorption maximum HDTMA-Br also forms a bilayer. At complete bilayer coverage, the ratio of counterion sorbed to HDTMA sorbed should be 0.5 (Figure 2c). The counterion sorption at maximum HDTMA sorption is presented in Table 3. From Table 3, it can be seen that the ratio
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and Bowman (13) and Bitting and Harwell (26), or hemimicelles, as described by Xu and Boyd (15), the counterion to HDTMA ratio should slightly exceed 0.5 due to excess counterions balancing lateral head groups. The lower counterion to surfactant ratios when Cl- and HSO4- are the counterions indicate incomplete bilayer formation, assuming monolayer coverage equivalent to the zeolites’ ECEC. The sequential decrease in the counterion to HDTMA ratio with Br- > Cl- > HSO4- suggests the packing density of the HDTMA molecules in the upper layer is different for different counterions. This counterion-dependent HDTMA sorption correlates well with the hydrated radii of the counterions (see Discussion).
FIGURE 2. Sorption of HDTMA molecules as a monolayer (a), patchy bilayer (b), and bilayer (c) on clinoptilolite surfaces. Monovalent anions are shown balancing the charge on outward-pointing surfactant head groups.
FIGURE 3. Effect of counterions on HDTMA sorption to kaolinite (KGa-1). Solid lines are Langmuir fits to the observed data using the parameters in Table 2.
TABLE 2. Fitted Langmuir Parameters for Sorption of HDTMA by Kaolinite (KGa-1) with Different HDTMA Counterions Sm (mmol/kg) KL (L/mmol) r2
Br-
Cl-
63 16 0.9999
48 12 0.9998
of Br- sorbed to HDTMA sorbed is 0.55. In contrast, the ratio of Cl- sorbed to HDTMA sorbed is 0.49. The ratio of HSO4to HDTMA is 0.33. For admicelles, as mentioned by Haggerty
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Mixtures with the same total amount of HDTMA (200 mmol/kg, about double the ECEC) but varying ratios of HDTMA-Br and HDTMA-Cl were equilibrated with zeolite. The initial Br- : (Br- + Cl-) ratios were 0.00, 0.25, 0.50, 0.75, and 1.00. Figure 4 shows the amounts of HDTMA, Br-, Cl-, and (Br- + Cl-) sorbed with respect to the initial Br- : (Br+ Cl-) ratio. It can be seen from Figure 4 that the amounts of HDTMA and (Br- + Cl-) sorbed follow the same trend and increase with respect to the initial HDTMA-Br content. When only one counterion is present (abscissa values of 0.00 and 1.00 in Figure 4), the counterion to HDTMA ratios are 0.54 and 0.43 for Br- and Cl-, respectively, in agreement with the data of Table 3. It can also be seen that there is a preferred sorption of Br- relative to Cl- (about a 3:1 molar ratio) when the initial counterion concentrations are equal (abscissa value of 0.50 on Figure 3). This observation suggests that the lyotropic series for solution micelle stabilization (Table 3) also applies to formation of the HDTMA bilayer on the zeolite surface. Similarly, studies of dodecyldimethylammonium sorption indicate that at low concentrations Br- preferentially displaces Cl- from the surface but at high concentrations Brand Cl- are sorbed nonpreferentially (27). In our study, conducted at an initial HDTMA-counterion concentration of 0.033 M, the preference of sorbed HDTMA for Br- is clear (Figure 4). The effect of the initial counterion (Br-, Cl-, or HSO4-) on chromate sorption by HDTMA-treated zeolite was examined. In each case, the zeolite was treated with HDTMA to the sorption maximum corresponding to each counterion (Table 1). The resulting chromate sorption isotherms are shown in Figure 5. All the curves in Figure 5 were well described by the Langmuir equation; the Langmuir coefficients derived from a fit of eq 2 to the data are shown in Table 4. The chromate sorption maxima for HDTMA-Br-, -Cl-, and -HSO4treated zeolite were 11, 16, and 28 mmol/kg, respectively. The results indicate that the ability of chromate to replace HSO4- is greater than that of chromate to replace Cl-, which in turn is greater than that of chromate to replace Br-. The more stable the counterion-HDTMA pair, the lower the ability of chromate to displace the counterion. Thus, the exchangeability of the counterion is more important than the total HDTMA loading in controlling the amount of chromate adsorbed by HDTMA-treated zeolite. Further evidence for the preference of chromate for HDTMA-Cl compared to HDTMA-Br is presented in Figure 6. In this case, the zeolite was treated with a 50:50 mixture of HDTMA-Br-/HDTMA-Cl-, resulting in a 3:1 ratio of Cl- to Br- on the surface (abscissa value of 0.50 on Figure 4), and then equilibrated with a series of chromate solutions. Some Br- (2.5 mmol/kg) and Cl- (4.7 mmol/kg) was desorbed even in the absence of chromate; the points plotted in Figure 6 have been corrected for Br- and Cl- release in these control samples. The ratio of Cl- to Br- desorbed upon chromate sorption was close to 1:1 at low initial chromate concentrations, increasing to about 3:1 at higher chromate levels (Figure 6).
TABLE 3. Effect of Counterions on HDTMA Sorption to Zeolite and on Chromate Sorption to HDTMA-Treated Zeolite
X-
HDTMA sorption capacity (mmol/kg)
CrO42sorption capacity (mmol/kg)
CrO42sorption intensity (L/kg)
counterion: HDTMA ratio
micelle radius (Rh) nm
micellar aggregation numbers
hydration numbers of anions
BrClHSO4-
208 151 132
11 16 28
31 38 42
0.55 0.49 0.33
3.22a 2.90a 2.72a
120c 103c -
1.5c 2c 8a
a
From ref 30.
b
From ref 31. c From ref 29.
d
CMC (mM) from this study (25 °C)
-2.19d -0.55d -
0.9b 1.3b -
0.9 1.25 0.5
From ref 18.
FIGURE 4. Sorption of HDTMA and counterions as a function of initial Br-: (Br- + Cl-) ratio. The lines are trend fits of observed data. The initial HDTMA-counterion concentration was 0.033 M.
FIGURE 5. Effect of counterions on chromate sorption by HDTMAtreated zeolite. Solid lines are Langmuir fits to the observed data using the parameters in Table 4.
TABLE 4. Fitted Langmuir Parameters for Sorption of Chromate by HDTMA-Treated Zeolite with Different HDTMA Counterions Sm (mmol/kg) KL (L/mmol) r2
∆Hm° (kcal/mol)
CMC (mM) from the literature (25 °C)
Br-
Cl-
HSO4-
11 31 0.994
16 38 0.99
28 42 0.8
Discussion The roles that the counterions play in stabilizing a sorbed surfactant bilayer can be compared to the roles they play in stabilizing micelles in solution. First, counterions affect the aggregation number and thus the size of micelles. If HDTMA sorbs as admicelles, the size and shape of the admicelles may reflect the size and shape of the micelles in solution. Apparent admicelle sorption has been observed using atomic force microscopy (20, 28) and neutron light scattering (1). The size of the micelles for HDTMA-X, as characterized by the aggregation number N, follow the lyotropic series, i.e., IO3- < HCO3- < BrO3- < F< Cl- < NO3- < Br- < ClO3- < SCN- (17). The affinity of positively charged surfaces for common anions follows: I-
FIGURE 6. Counterion (Br- or Cl-) release as a function of chromate sorption on HDTMA-treated zeolite. Zeolite was initially treated with a 50:50 mixture of HDTMA-Br/HDTMA-Cl (abscissa value of 0.50 on Figure 3). All the data points were corrected for Br- and Cldesorption at zero chromate concentration. > ClO4- and NO3- > Br- > Cl- > OH-, F-, SO42- (24). Since hydrated Cl- is larger than Br-, it should be less able to penetrate deeply into the Stern layer to effectively neutralize the cationic micelle head group (29). Hence, Cl- should be less closely bound to the HDTMA micelle than Br-, resulting in a more highly charged head group (i.e., larger fractional micellar ionization R), which must increase the average head group separation to minimize electrostatic repulsion, thereby resulting in a smaller aggregation number N and a smaller micellar hydrodynamic radius Rh. Similarly, the larger the separation, the lower the sorption density of the second layer when HDTMA is sorbed to a solid surface, resulting in less bilayer coverage when Cl- is the counterion. Although the conclusions of Dorshow et al. (29) were based on stabilization of micelles in solution, the results from this research indicate that the counterion stabilization of bilayers also follows the lyotropic series, i.e., Br- > Cl- > HSO4- (Figure 1, Table 1). A similar correlation between solution micelle size and sorbed surfactant aggregates was observed for anionic surfactant sorption to alumina, where the dodecyl sulfate sorption capacity decreased with counterions Cs+ > Na+ > Li+ (26). Bitting and Harwell (26) also concluded that the strength of counterion binding on sorbed surfactant aggregates appeared to be significantly higher than binding on micelles in solution. Second, the counterions affect the CMC. The values of CMC for cationic surfactants follow the lyotropic series F- > formate- > IO3- > Cl- > BrO3- > Br- > NO3- (17, Table 3). For HDTMA, both the enthalpy and free energy of micellization decrease in the counterion sequence OH-, F-, Cl-, NO3-, and Br- (18) (Table 3). The higher the CMC, the larger the ratio between the monomers and the micelles at a given solution concentration and the lower the likelihood of monomers associating to form micelles in solution or admicelles on the surface. Therefore, it may be concluded that, when Cl- is the counterion, the ratio of bilayer to monolayer will be smaller than the ratio when Br- is the counterion. This again confirms that the Br- stabilizes the bilayer HDTMA more than Cl-.
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Third, the charge of the counterion affects the Coulombic interaction with the surfactant head group. There is an entropic advantage in binding a single divalent ion rather than two univalent ions (30). Divalent anions also have a stronger Coulombic interaction with the head group of cationic surfactants and preferentially replace monovalent counterions. This preference for divalent cations is confirmed by the observed strong sorption of chromate in this study as well as by the sorption of chromate, selenate, and sulfate on HDTMA-treated zeolite observed by Haggerty and Bowman (13). Cation exchange is responsible for retaining the lower surfactant layer on a charged surface while hydrophobic bonding causes formation of the upper surfactant layer. Without counterions to balance the head group repulsion, surfactant molecules in the second layer would tend to diffuse back to the bulk solution. Clearly, the attraction between the head group and the counterion offsets the head group repulsion, reduces surfactant diffusion out of the bilayer, and stabilizes the hydrophobic interaction. Thus, the force and interaction balance for the surfactant bilayer sorption is the summation of (1) the electrostatic attraction between the negatively charged zeolite surfaces and the positively charged head groups, (2) the tail-tail hydrophobic interaction between the first layer and the second layer, which is of the Londonvan der Waals type, (3) the electrostatic attraction between the positively charged head groups of the second layer and the negatively charged counterions, (4) the electrostatic repulsion between the head groups of the first layer and the second layer, and (5) the electrostatic repulsion between the negatively charged zeolite surface and the negatively charged counterions in solution. Therefore, the effect of the counterion in stabilizing HDTMA bilayer sorption cannot be neglected. Understanding the contributions of the counterions to the sorption and stabilization of cationic surfactants enables selection of a surfactant with a suitable counterion for surface treatment. For example, to achieve the maximum sorption intensity and capacity of inorganic anion contaminants on HDTMA-treated zeolite, HDTMA-Cl will be more effective than HDTMA-Br. In addition, less HDTMA is required to achieve the same chromate sorption when Cl- instead of Bris the counterion. Extraction of sorbed chromate from the HDTMA-treated zeolite with different reagents also indicates that less chromate can be extracted from zeolite prepared with HDTMA-Cl than from zeolite prepared with HDTMA-Br (unpublished data). HDTMA-HSO4-treated zeolite showed the greatest chromate sorption and the lowest HDTMA surface coverage. Industrial-grade HDTMA-HSO4 is not currently available, however; thus, it may not be economical to prepare HDTMA-HSO4-zeolite for environmental applications. Table 3 summarizes the effects of counterions on HDTMA and chromate sorption by zeolite. The HDTMA sorption capacity on zeolite follows Br- > Cl- > HSO4-. The ratio of counterion sorbed to HDTMA sorbed follows Br- > Cl-. The chromate sorption capacity and intensity on HDTMA-zeolite follow HSO4- > Br- > Cl-. The micelle size follows Br- > Cl> HSO4-. The micelle aggregation number follows Br- > Cl-. All these observations strongly indicate that counterions have significant effects on the HDTMA sorption on zeolite and chromate sorption on HDTMA-treated zeolite, and that the effect of counterions in stabilizing the sorbed HDTMA bilayer follows the lyotropic series Br- > Cl- > HSO4-.
Acknowledgments This research was supported by the U.S. Department of Energy under Contract DE-AR21-95-MC32108 from the Morgantown Energy Technology Center. We thank William Carey and David Bish of Los Alamos National Laboratory for performing the clinoptilolite mineralogical analysis and Barry Allred, Enid
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Sullivan, and Stephen Roy of New Mexico Tech for reviewing the draft manuscript.
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Received for review December 30, 1996. Revised manuscript received April 7, 1997. Accepted April 14, 1997.X ES9610693 X
Abstract published in Advance ACS Abstracts, June 15, 1997.
