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Adsorption of Gemini and Conventional Cationic Surfactants onto Montmorillonite and the Removal of Some Pollutants by the Clay
Journal of Colloid and Interface Science 224, 265–271 (2000) doi:10.1006/jcis.2000.6721, available online at http://www.idealibrary.com on
Adsorption of Gemini and Conventional Cationic Surfactants onto Montmorillonite and the Removal of Some Pollutants by the Clay Fang Li and M. J. Rosen1 Surfactant Research Institute, Brooklyn College of the City University of New York, Brooklyn, New York 11210 Received August 2, 1999; accepted January 6, 2000
The adsorption of a series of gemini surfactants, [Cn H2n+1 N+ (CH3 )2 -CH2 CH2 ]2 · 2Br− , where n = 10, 12, 14, and 16, on clay (Namontmorillonite) from their aqueous solution in 0.01 M KBr and the effect of this adsorption on the removal of 2-naphthol and 4chlorophenol have been studied. Compared to those of conventional cationic surfactants with similar single hydrophilic and hydrophobic groups (Cn H2n+1 N+ (CH3 )3 · Br− , where n = 10, 12, 14, and 16), the molar adsorptions of the gemini and conventional surfactants are almost identical. This indicates that only one of the hydrophilic groups in the gemini molecule is adsorbed onto the clay and that the second hydrophilic is presumably oriented toward the aqueous phase, in contrast to the adsorption of the conventional surfactants, where the hydrophobic group is oriented toward the aqueous phase. Stability studies on dispersions of clay treated with the two types of surfactants confirm this. The slight increase in the moles of surfactant to values above the CEC of the clay with an increase in the carbon number of the hydrophobic chain indicates that adsorption through hydrophobic group interaction occurs in addition to the major ion exchange. Adsorption studies of the pollutants onto the clay treated by either the gemini or the conventional surfactants show that the former are both more efficient and more effective at removing the pollutants from the aqueous phase. °C 2000 Academic Press Key Words: gemini surfactants; adsorption; montmorillonite; 2naphthol; 4-chlorophenol; pollutant removal; cationic surfactants; clay.
INTRODUCTION
In recent years, cationic surfactants, especially quaternary ammonium cationic surfactants, have been used to treat soil and then remove pollutants from aqueous media (1–8). Since gemini surfactants (surfactants containing two hydrophilic and two hydrophobic groups in the molecule) have been shown to be much more surface-active than conventional surfactants containing a single similar hydrophilic and hydrophobic group in the molecule (9, 10), there was interest in determining whether the former would be more efficient and/or effective than the latter in removing pollutants from aqueous media. Although the adsorption of gemini surfactants at the air/solution interface has been much investigated (11), little attention has been paid to the adsorption of gemini surfactants at the solid/solution interface. 1
To whom correspondence should be addressed.
So far as we know, only two papers (12, 13) involving the silica/ aqueous solution interface concern the adsorption of gemini surfactants at the solid/aqueous solution interface. In Ref. (13), the gemini surfactants used by Zana et al. are alkanediyl-α, ωbis(dodecyldimethylammonium bromide), with the alkane-diyl spacer group C2 H4 , C4 H8 , C6 H12 , and C10 H20 . They studied mainly the influence of the spacer on the amount of surfactant adsorbed and discussed the adsorption mechanism. They did not study the effect of the gemini surfactants on the removal of any pollutant. The only reported study of the effect of gemini surfactants on the removal of organic contaminants is that of Esumi et al. (12), who used one gemini surfactant to treat silica and studied the effect of this gemini surfactant on the removal of 2-naphthol. The only gemini surfactant they studied is ethane1,2-bis(dodecyldimethylammonium bromide). They found that the molar amount of gemini surfactant adsorbed on silica was lower than for the corresponding conventional surfactant, dodecyltrimethylammonium bromide, but that the ratio of the maximum amount of 2-naphthol adsorbed to the adsorbed amount of surfactant on silica increases from conventional surfactant to gemini surfactant. The adsorption of surfactant onto silica is not strong, around 5 × 10−5 mol/g silica (12, 13). In this study, we chose Namontmorillonite, which has a much larger cation exchange capacity (CEC = 7.64 × 10−2 eq/100 g). Adsorption of a series of cationic gemini surfactants, [Cn H2n+1 N+ (CH3 )2 -CH2 CH2 ]2 · 2Br− , where n = 10, 12, 14, and 16, onto the montorillonite is investigated, and the effect of this adsorption on the removal of two pollutants, 2-naphthol and 4-chlorophenol, is noted. These two pollutants are widely used in surfactant-concerned environmental studies and can be easily analyzed by UV spectrophotometry (7, 8, 12). The adsorption and removal of the pollutants are compared to those of conventional cationic surfactants with similar single hydrophilic and hydrophobic groups (Cn H2n+1 N+ (CH3 )3 · Br− , where n = 10, 12, 14, and 16). EXPERIMENTAL PROCEDURES
Materials Synthesis of the gemini surfactant. A 0.01 mol amount of 1,4-dibromobutane (95%) and 0.022 mol of alkyl N,Ndimethylamine were added to isopropanol, and the mixture was
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C 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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heated to reflux for about 12 h. After the reaction, the solvent was removed and the residue was recrystallized from tetrahydrofuran (THF) several times. The pure diquaternary ammonium dibromide was finally obtained as a colorless powder by recrystallization three times from ethanol/acetate mixed solvent. The gemini surfactants are referred to as C10 gemini, C12 gemini, C14 gemini, and C16 genimi, based on their hydrophobic chain lengths. The structure of the gemini surfactants is [R–N+ (CH3 )2 –CH2 –CH2 –CH2 –CH2 –N+ (CH3 )2 –R] · 2Br− , where R is the alkyl chain. Nitrogen analysis data for two gemini surfactant are as follows: C12 gemini, N = 4.36% (calcd.), 4.32% (found); C16 gemini, N = 3.71% (calcd.), 3.86% (found). The conventional surfactants used in this study are decyltrimethylammonium bromide (C10 TMAB), dodecyltrimethylammonium bromide (C12 TMAB), tetradecyltrimethylammonium bromide (C14 TMAB), and hexadecyltrimethylammonium bromide (C16 TMAB). The clay [Na-montmorillonite (Wyoming)] was obtained from Professor Stephen Aja, Department of Geology, Brooklyn College of CUNY. The surface area was 31.82 ± 0.22 m2 /g [measured by BET(N2 )]. The cation exchange capacity (CEC) is 76.4 meq/100 g. Surface tension measurements. Measurements were performed at 25 ± 0.1◦ C on a K-12 tensiometer, by use of the Wilhelmy plate technique, with a sandblasted platinum blade of ca. 4 cm perimeter. The instrument was calibrated against quartz-condensed double-distilled, previously deionized water (the last distilled stage from alkaline KMnO4 through a 1-m high Vigreaux column). Values were taken until the surface tension was constant for 0.5 h. Reproducibility of the surface tension measurement is less than 0.2 mN/m. Adsorption isotherm of surfactants on clay and the adsorption of the pollutants on surfactant-treated clay. A series of aqueous surfactant solutions in 0.01 mol/dm3 KBr aqueous solution was prepared in the absence and presence of either 2naphthol or 4-chlorophenol. The initial concentration of either pollutant was 4 × 10−4 mol/dm3 . A 50-ml amount of the solution was added to 50 mg of clay in a capped centrifuge tube. The solution was equilibrated by shaking for at least 24 h, and then the solid was removed by centrifugation. Since the montmorillonite, when stirred with the aqueous phase containing no surfactant, did not reduce the surface tension of the aqueous phase, the surface tension of the supernatant solution in equilibrium with the clay can be used to measure the concentration of the surfactants in the solution. In addition, the concentration of the surfactant in the aqueous phase was measured by the mixed indicator two-phase titration method (14, 15), using the cationic or gemini solution as the titrant. The same results were obtained by either method. The concentration of either pollutant in the aqueous phase was determined by UV spectrophotometry (Kontron Instruments, Unikon 9410).
FIG. 1. Surface tension vs log of surfactant molar concentration for the gemini surfactants at 25◦ C in 0.01 M KBr: (m) C10 gemini; (r) C12 gemini; (d) C14 gemini; (w) C16 gemini.
The amount of surfactant adsorbed onto the montmorillonite and the amount of either pollutant adsorbed onto the surfactanttreated montmorillonite is given by the following equation: ns =
(C 0 − Ceq ) × V , g
[1]
where n s is the number of moles of adsorbate (surfactant or pollutant) per gram of adsorbent (mol/g), C 0 is the initial concentration of adsorbate (mol/dm3 ), Ceq is the equilibrium concentration of adsorbate (mol/dm3 ), V is the volume of solution (dm3 ), and g is the weight of adsorbent (clay) (g).
FIG. 2. Surface tension vs log of surfactant molar concentration for the conventional surfactants at 25◦ C in 0.01 M KBr: (m) C10 TMAB; (r) C12 TMAB; (d) C14 TMAB; (w) C16 TMAB.
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ADSORPTION OF SURFACTANTS ON MONTMORILLONITE
RESULTS AND DISCUSSION
Adsorption of the Gemini and Conventional Surfactants at the Air/Aqueous Solution and Montmontillonite/ Aqueous Solution Interfaces The surface tension (γ )–log surfactant molar concentration (C) curves at 25◦ C in 0.01 M KBr, used to determine the concentration of surfactants in the aqueous phase in equilibrium with the adsorbed surfactant, are shown in Fig. 1 for gemini surfactants and in Fig. 2 for the conventional surfactants. The concentration of the surfactant at the air/solution interface, 0, and the minimum area, Amin , occupied by the surfactant there can be determined from the Gibbs adsorption equation in
the form (16a): 1 × 10−3 0=− 2.303n RT
µ
∂γ ∂ log C
¶ [2] T
and Amin =
1020 N 0max
2
˚ ), (in A
[3]
where γ is the surface tension in mN/m; 0 is the absorption amount in mol/m2 ; [∂γ /(∂ log C)]T is the slope in each case; T is absolute temperature, R = 8.314 J mol−1 K−1 ; and N is Avogadro’s number. The value of n (the number of species at the interface whose concentration at the interface changes with
FIG. 3. Adsorption isotherms of the conventional surfactants and the gemini surfactants on montmorillonite at 25◦ C in 0.01 M KBr. (a) C10 TMAB and C10 gemini; (b) C12 TMAB and C12 gemini; (c) C14 TMAB and C14 gemini; (d) C16 TMAB and C16 gemini.
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TABLE 1 Maximum Surfactant Adsorption onto Montmorillonite at 25◦ C from 0.01 M KBr and the Stability of the Resulting Dispersion Surfactant
CMCa (mol/dm3 )
Amin a (A2 )
Cs (max) (mol/g Clay)
Ceq (max)c (mol/dm3 )
Initial absorbance (A0 )
t1/2 (min)
Abs/A0
C10 TMAB C10 gemini C12 TMAB C12 gemini C14 TMAB C14 gemini C16 TMAB C16 geminib
6.22 × 10−2 7.95 × 10−4 9.29 × 10−3 1.64 × 10−4 2.47 × 10−3 7.82 × 10−6 8.20 × 10−4 2.25 × 10−6
70.6 101.6 67.6 89.8 63.4 21.7 59.3 9.3
7.43 × 10−4 7.20 × 10−4 8.13 × 10−4 7.54 × 10−4 8.27 × 10−4 8.12 × 10−4 8.92 × 10−4 /
1.82 × 10−2 6.59 × 10−3 3.61 × 10−3 1.43 × 10−3 6.47 × 10−4 1.90 × 10−4 1.64 × 10−4 /
0.17 0.73 0.47 1.56 0.60 1.84 0.75 /
9.96 22.1 11.4 >60 12.6 >60 13.0 /
/ 0.41 / 0.61 / 0.69 / /
a Literature values at 30◦ C in 1.25 × 10−2 M KBr [Trap, H. J. L., and Hermans, J. J., Koninki Ned Akad Weten. Proc. Ser B 58, 97 (1955)]: C TMAB, 5.9 × 10−2 ; 10 C12 TMAB, 1.08 × 10−2 ; C14 TMAB, 2.1 × 10−3 . b Maximum amount adsorbed on clay cannot be reached due to poor solubility of surfactant in water. c C (max) is the equilibrium surfactant concentration in solution phase at maximum amount adsorbed. eq
change in C) is taken as 1 for the gemini surfactants because the surfactant concentrations used to calculate Amin are much less than one-tenth of the ionic strength (i.e., 0.01 M). For the conventional surfactants, n is taken as 1 when the surfactant concentrations used to calculate Amin are less than one-tenth of 0.01 M, otherwise, the value of n can be obtained by use of the following equation (17): n=1+
Csurf , Csurf + I S
[4]
where IS is the ionic strength of the added electrolyte. Csurf is the average surfactant concentration used for [∂γ /(∂ log C)]T . The slopes of the plots in Fig. 1 indicate that the concentration of surfactant at the air/aqueous solution interface increases
FIG. 4. Time dependence of the absorbance of the clay suspension after treatment by the conventional and gemini surfactants. (1) C14 gemini; (2) C14 TMAB; (3) C12 gemini; (4) C12 TMAB; (5) C10 gemini; (6) C10 TMAB; (7) C16 TMAB.
as the number of carbons in the alkyl chain increases. Table 1 shows that the Amin values decrease with increase in the length of the alkyl chain for the gemini surfactants, the same as for the conventional surfactants studied here and for other conventional surfactants (16b). The very large slopes (and the very small Amin values) for the C14 and C16 gemini surfactants have been observed before and may be due to multilayer formation of the surfactants at the air/solution interface (18). Adsorption isotherms of the conventional surfactants and the geminis onto montmorillonite at 25◦ C from aqueous 0.01 M KBr solution are shown in Fig. 3a–d. Data are listed in Table 1.
FIG. 5. Highly simplified representation of the expected adsorption for both cationic gemini surfactants and conventional surfactants on the clay surface. (A) Conventional surfactant; (B) gemini surfactant.
ADSORPTION OF SURFACTANTS ON MONTMORILLONITE
269
FIG. 6. Adsorption of 2-naphthol and 4-chlorophenol onto surfactant-treated clay at 25◦ C in 0.01 M KBr. (a) C10 TMAB and C10 gemini; (b) C12 TMAB and C12 gemini; (c) C14 TMAB and C14 gemini; (d) C16 TMAB and C16 gemini.
The typical S-shaped isotherm (16c) is apparent only in the case of C10 TMAB. For the other surfactants, the amount adsorbed on the clay increases sharply to a maximum with increase in equilibrium surfactant concentration. For both the gemini and the conventional surfactants, the maximum amount of surfactant at the clay/aqueous solution interface, Cs (max), increases with the length of hydrophobic chain. The maximum amount of surfactant absorbed onto the clay is larger than the CEC of the clay in the case of C12 TAMB, C12 gemini, C14 TMAB, C14 gemini, and C16 TMAB. This indicates that although cation exchange plays a major key role in the adsorption, hydrophobic bonding between chains also occurs and becomes stronger as the hydrophobic chains become longer. The data on the surfactant concentration in the solution phase Ceq (max) at which this maximum adsorption of surfactant, Cs (max), occurs
are shown on Table 1. Noteworthy is the much lower concentration of gemini, about 13 that of the conventional surfactant with the same alkyl chain length, needed to achieve this maximum adsorption. This indicates that the gemini surfactant is much more efficient than the conventional surfactant at achieving this maximum adsorption on the montmorillonite. The data also show that, in spite of the fact that the gemini have two quaternary nitrogen groups that can adsorb onto negative sites onto the montmorillonite, the molar adsorptions of the gemini and conventional surfactants are almost identical. This indicates that only one of the hydrophilic groups in the gemini molecule is adsorbed onto the clay and that the second hydrophilic group is presumably oriented towards the aqueous phase. This mode of adsorption would be in marked contact to the adsorption of the conventional surfactants, which would also have their quaternary
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nitrogens oriented toward the clay, but with their hydrophobic groups oriented towards the aqueous phase. The adsorption of the gemini surfactants onto the montmorillonite would consequently make the surface of the latter hydrophilic, while the adsorption of the conventional surfactants would make the surface of the latter hydrophobic. That this is indeed the case is shown by the absorbance in the visible region of aqueous suspensions of the clay with the maximum amount of surfactant adsorbed onto it (Fig. 4). After the clay suspension was shaken for 24 hrs, its absorbance was immediately measured in the visible at 580 nm. From the absorbance data, the clay suspension with the adsorbed gemini surfactant is much more stable than that with the adsorbed conventional surfactant. In the case of the clay with adsorbed conventional surfactant, the absorbance rapidly decreases with time, indicating rapid flocculation of the clay as a result of its hydrophobic surface. In the case of the clay with adsorbed gemini surfactant, the solution remains turbid for an extended period, due to dispersion of the clay in the aqueous phase as a result of the positive charge on the surface of the particles and the hydrophilic surface character imparted by the second hydrophilic group oriented away from the clay surface. Figure 5 illustrates the expected gemini adsoption compared with that of conventional surfactant. The initial absorbance, A0 , and t1/2 (the time when the absorbance reduces to half of its initial value) of the gemini-treated clay suspension are both larger than those of its corresponding conventional surfactant (Table 1). As a result of the increased adsorption with increase in the length of the hydrophobic group to values above the CEC, the initial absorbance, t1/2 , and Abs/A0 (where Abs is the absorbance value at 60 min) increase from C10 TMAB to C16 TMAB and from C10 gemini to C14 gemini (Table 1). Adsorption of 2-Naphthol and 4-Chlorophenol onto Montmonrillonite with Adsorbed Layers of Gemini or Conventional Surfactant Adsorption isotherms of 2-naphthol and 4-chlorophenol onto montmorillonite with the maximum amount of the adsorbed gemini or conventional surfactant on it are shown in Fig. 6a–d, and the data are listed in Table 2. They show that the amount of 2-naphthol adsorbed per gram of clay at the point where the surfactant is adsorbed at its maximum is 1.5 times for the gemini surfactant, compared to that adsorbed onto the conventional surfactant. More than twice as much 4-chlorophenol is adsorbed onto the gemini-treated clay than onto the conventional-treated clay, at the point where the amount of adsorbed surfactant is maximum. Table 2 also shows the moles of pollutant adsorbed per mole of surfactant adsorbed, at this point of maximum adsorption of the surfactant. Again, the ratio of the amount of pollutant adsorbed to the mole of surfactant adsorbed is 1.5–1.9 times on the gemini surfactant than on the conventional surfactant in the case of 2-naphthol and 2–3 times in the case of 4-chlorophenol. For both the conventional and the gemini surfactants, because of the slight increase of the maximum adsorbed amount of sur-
TABLE 2 Pollutant Adsorption onto Surfactant-Treated Montmorillonite
Surfactant
Cs (mol/g)
Max. amount of pollutant adsorbed (mol/g)
2-Naphthol
C10 TMAB C10 gemini C12 TMAB C12 gemini C14 TMAB C14 gemini C16 TMAB C16 geminia
7.43 × 10−4 7.20 × 10−4 8.13 × 10−4 7.54 × 10−4 8.27 × 10−4 8.12 × 10−4 8.92 × 10−4 /
1.48 × 10−4 2.61 × 10−4 1.73 × 10−4 2.95 × 10−4 2.09 × 10−4 3.14 × 10−4 2.40 × 10−4 /
0.20 0.36 0.20 0.38 0.25 0.38 0.27 /
p-Chlorophenol
C10 TMAB C10 gemini C12 TMAB C12 gemini C14 TMAB C14 gemini C16 TMAB C16 geminia
7.43 × 10−4 7.20 × 10−4 8.13 × 10−4 7.54 × 10−4 8.27 × 10−4 8.12 × 10−4 8.92 × 10−4 /
4.33 × 10−5 1.68 × 10−4 7.01 × 10−5 2.08 × 10−4 1.10 × 10−4 2.42 × 10−4 1.39 × 10−4 /
0.058 0.24 0.084 0.28 0.13 0.29 0.16 /
Pollutant
Pollutant/ surfactant ratio (mol/mol)
a
Maximum amount adsorbed on clay can not be reached due to poor solubility of surfactant in water.
factants with the carbon number of their hydrophobic chain, the maximum adsorbed amount and ratio of the amount of pollutant adsorbed to the mole of surfactant adsorbed also increase with the carbon number of the surfactant’s alkyl chain. The decrease of the adsorption amount of either pollutant with increase in the equilibrium surfactant above its critical micelle concentration after the maximum adsorption amount has been reached is presumably due to solubilization of the pollutant in the micelles in the aqueous phase (12). This increased adsorption of the pollutant onto the gemini, compared to that of the conventional surfactant, shows that the former is more effective than the latter in removing pollutants from aqueous media. The observation that the maximum adsorption of the gemini surfactant onto the clay occurs at about 13 the concentration of the conventional surfactant in the solution phase (Table 1) shows that the gemini is also more efficient at removing the pollutant from aqueous media. REFERENCES 1. Boyd, S. A., Lee, J.-F., and Mortland, M. M., Nature 333, 345 (1988). 2. Xu, S.-H., and Boyd, S. A., Environ, Sci. Technol. 29, 312 (1995). 3. Lee, J.-F., Crum, J. R., and Boyd, S. A., Environ. Sci. Technol. 23, 1365 (1989). 4. Nye, J. V., Guerlin, W. F., and Boyd, S. A., Environ. Sci. Technol. 28, 944 (1994). 5. Smith, J. A., Jaffe, P. R., and Chiou, C. T., Environ. Sci. Technol. 24, 1167 (1990). 6. Zhang, Z.-Z., Sparks, D. L., and Scrivner, N. C., Environ. Sci. Technol. 27, 1625 (1993).
ADSORPTION OF SURFACTANTS ON MONTMORILLONITE 7. Lumpp, E. K., Heitmann, H., and Schwuger, M. J., Colloids Surf. A 78, 93 (1993). 8. Schieder, D., Dobias, B., Klumpp, E., and Schwuger, M. J., Colloids Surf. A 88, 103 (1994). 9. Rosen, M. J., Chem. Technol. 30 (1993). 10. Zana, R., Curr. Opin. Colloid Interface Sci. 1, 566 (1996). 11. Rosen, M. J., and Tracy, D. J., J. Surfactants Detergents 1, 547 (1998). 12. Esumi, K., Goino, M., and Koide, Y., J. Colloid Interface Sci. 183, 539 (1996).
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13. Chorro, C., Chorro, M., Dolladille, O., and Zana, R., J. Colloid Interface Sci. 199, 169 (1998). 14. Reid, V. W., Alston, T., and Heinerth, E., Tenside 4, 292 (1967). 15. Reid, V. W., Alston, T., and Heinerth, E., Tenside 5, 90 (1967). 16. Rosen, M. J., in “Surfactants and Interfacial Phenomena,” 2nd edition. John Wiley, New York, 1989: (a) pp. 65–68; (b) pp. 70–80; (c) p. 48. 17. Matijevic, E., and Pethica, B. A., Trans. Faraday Soc. 54, 1382 (1958). 18. Rosen, M. J., Mathias, J. H., and Davenport, L., Langmuir 15, 7340 (1999).
Adsorption of Gemini and Conventional Cationic Surfactants onto Montmorillonite and the Removal of Some Pollutants by the Clay Fang Li and M. J. Rosen1 Surfactant Research Institute, Brooklyn College of the City University of New York, Brooklyn, New York 11210 Received August 2, 1999; accepted January 6, 2000
The adsorption of a series of gemini surfactants, [Cn H2n+1 N+ (CH3 )2 -CH2 CH2 ]2 · 2Br− , where n = 10, 12, 14, and 16, on clay (Namontmorillonite) from their aqueous solution in 0.01 M KBr and the effect of this adsorption on the removal of 2-naphthol and 4chlorophenol have been studied. Compared to those of conventional cationic surfactants with similar single hydrophilic and hydrophobic groups (Cn H2n+1 N+ (CH3 )3 · Br− , where n = 10, 12, 14, and 16), the molar adsorptions of the gemini and conventional surfactants are almost identical. This indicates that only one of the hydrophilic groups in the gemini molecule is adsorbed onto the clay and that the second hydrophilic is presumably oriented toward the aqueous phase, in contrast to the adsorption of the conventional surfactants, where the hydrophobic group is oriented toward the aqueous phase. Stability studies on dispersions of clay treated with the two types of surfactants confirm this. The slight increase in the moles of surfactant to values above the CEC of the clay with an increase in the carbon number of the hydrophobic chain indicates that adsorption through hydrophobic group interaction occurs in addition to the major ion exchange. Adsorption studies of the pollutants onto the clay treated by either the gemini or the conventional surfactants show that the former are both more efficient and more effective at removing the pollutants from the aqueous phase. °C 2000 Academic Press Key Words: gemini surfactants; adsorption; montmorillonite; 2naphthol; 4-chlorophenol; pollutant removal; cationic surfactants; clay.
INTRODUCTION
In recent years, cationic surfactants, especially quaternary ammonium cationic surfactants, have been used to treat soil and then remove pollutants from aqueous media (1–8). Since gemini surfactants (surfactants containing two hydrophilic and two hydrophobic groups in the molecule) have been shown to be much more surface-active than conventional surfactants containing a single similar hydrophilic and hydrophobic group in the molecule (9, 10), there was interest in determining whether the former would be more efficient and/or effective than the latter in removing pollutants from aqueous media. Although the adsorption of gemini surfactants at the air/solution interface has been much investigated (11), little attention has been paid to the adsorption of gemini surfactants at the solid/solution interface. 1
To whom correspondence should be addressed.
So far as we know, only two papers (12, 13) involving the silica/ aqueous solution interface concern the adsorption of gemini surfactants at the solid/aqueous solution interface. In Ref. (13), the gemini surfactants used by Zana et al. are alkanediyl-α, ωbis(dodecyldimethylammonium bromide), with the alkane-diyl spacer group C2 H4 , C4 H8 , C6 H12 , and C10 H20 . They studied mainly the influence of the spacer on the amount of surfactant adsorbed and discussed the adsorption mechanism. They did not study the effect of the gemini surfactants on the removal of any pollutant. The only reported study of the effect of gemini surfactants on the removal of organic contaminants is that of Esumi et al. (12), who used one gemini surfactant to treat silica and studied the effect of this gemini surfactant on the removal of 2-naphthol. The only gemini surfactant they studied is ethane1,2-bis(dodecyldimethylammonium bromide). They found that the molar amount of gemini surfactant adsorbed on silica was lower than for the corresponding conventional surfactant, dodecyltrimethylammonium bromide, but that the ratio of the maximum amount of 2-naphthol adsorbed to the adsorbed amount of surfactant on silica increases from conventional surfactant to gemini surfactant. The adsorption of surfactant onto silica is not strong, around 5 × 10−5 mol/g silica (12, 13). In this study, we chose Namontmorillonite, which has a much larger cation exchange capacity (CEC = 7.64 × 10−2 eq/100 g). Adsorption of a series of cationic gemini surfactants, [Cn H2n+1 N+ (CH3 )2 -CH2 CH2 ]2 · 2Br− , where n = 10, 12, 14, and 16, onto the montorillonite is investigated, and the effect of this adsorption on the removal of two pollutants, 2-naphthol and 4-chlorophenol, is noted. These two pollutants are widely used in surfactant-concerned environmental studies and can be easily analyzed by UV spectrophotometry (7, 8, 12). The adsorption and removal of the pollutants are compared to those of conventional cationic surfactants with similar single hydrophilic and hydrophobic groups (Cn H2n+1 N+ (CH3 )3 · Br− , where n = 10, 12, 14, and 16). EXPERIMENTAL PROCEDURES
Materials Synthesis of the gemini surfactant. A 0.01 mol amount of 1,4-dibromobutane (95%) and 0.022 mol of alkyl N,Ndimethylamine were added to isopropanol, and the mixture was
265
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C 2000 by Academic Press Copyright ° All rights of reproduction in any form reserved.
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heated to reflux for about 12 h. After the reaction, the solvent was removed and the residue was recrystallized from tetrahydrofuran (THF) several times. The pure diquaternary ammonium dibromide was finally obtained as a colorless powder by recrystallization three times from ethanol/acetate mixed solvent. The gemini surfactants are referred to as C10 gemini, C12 gemini, C14 gemini, and C16 genimi, based on their hydrophobic chain lengths. The structure of the gemini surfactants is [R–N+ (CH3 )2 –CH2 –CH2 –CH2 –CH2 –N+ (CH3 )2 –R] · 2Br− , where R is the alkyl chain. Nitrogen analysis data for two gemini surfactant are as follows: C12 gemini, N = 4.36% (calcd.), 4.32% (found); C16 gemini, N = 3.71% (calcd.), 3.86% (found). The conventional surfactants used in this study are decyltrimethylammonium bromide (C10 TMAB), dodecyltrimethylammonium bromide (C12 TMAB), tetradecyltrimethylammonium bromide (C14 TMAB), and hexadecyltrimethylammonium bromide (C16 TMAB). The clay [Na-montmorillonite (Wyoming)] was obtained from Professor Stephen Aja, Department of Geology, Brooklyn College of CUNY. The surface area was 31.82 ± 0.22 m2 /g [measured by BET(N2 )]. The cation exchange capacity (CEC) is 76.4 meq/100 g. Surface tension measurements. Measurements were performed at 25 ± 0.1◦ C on a K-12 tensiometer, by use of the Wilhelmy plate technique, with a sandblasted platinum blade of ca. 4 cm perimeter. The instrument was calibrated against quartz-condensed double-distilled, previously deionized water (the last distilled stage from alkaline KMnO4 through a 1-m high Vigreaux column). Values were taken until the surface tension was constant for 0.5 h. Reproducibility of the surface tension measurement is less than 0.2 mN/m. Adsorption isotherm of surfactants on clay and the adsorption of the pollutants on surfactant-treated clay. A series of aqueous surfactant solutions in 0.01 mol/dm3 KBr aqueous solution was prepared in the absence and presence of either 2naphthol or 4-chlorophenol. The initial concentration of either pollutant was 4 × 10−4 mol/dm3 . A 50-ml amount of the solution was added to 50 mg of clay in a capped centrifuge tube. The solution was equilibrated by shaking for at least 24 h, and then the solid was removed by centrifugation. Since the montmorillonite, when stirred with the aqueous phase containing no surfactant, did not reduce the surface tension of the aqueous phase, the surface tension of the supernatant solution in equilibrium with the clay can be used to measure the concentration of the surfactants in the solution. In addition, the concentration of the surfactant in the aqueous phase was measured by the mixed indicator two-phase titration method (14, 15), using the cationic or gemini solution as the titrant. The same results were obtained by either method. The concentration of either pollutant in the aqueous phase was determined by UV spectrophotometry (Kontron Instruments, Unikon 9410).
FIG. 1. Surface tension vs log of surfactant molar concentration for the gemini surfactants at 25◦ C in 0.01 M KBr: (m) C10 gemini; (r) C12 gemini; (d) C14 gemini; (w) C16 gemini.
The amount of surfactant adsorbed onto the montmorillonite and the amount of either pollutant adsorbed onto the surfactanttreated montmorillonite is given by the following equation: ns =
(C 0 − Ceq ) × V , g
[1]
where n s is the number of moles of adsorbate (surfactant or pollutant) per gram of adsorbent (mol/g), C 0 is the initial concentration of adsorbate (mol/dm3 ), Ceq is the equilibrium concentration of adsorbate (mol/dm3 ), V is the volume of solution (dm3 ), and g is the weight of adsorbent (clay) (g).
FIG. 2. Surface tension vs log of surfactant molar concentration for the conventional surfactants at 25◦ C in 0.01 M KBr: (m) C10 TMAB; (r) C12 TMAB; (d) C14 TMAB; (w) C16 TMAB.
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ADSORPTION OF SURFACTANTS ON MONTMORILLONITE
RESULTS AND DISCUSSION
Adsorption of the Gemini and Conventional Surfactants at the Air/Aqueous Solution and Montmontillonite/ Aqueous Solution Interfaces The surface tension (γ )–log surfactant molar concentration (C) curves at 25◦ C in 0.01 M KBr, used to determine the concentration of surfactants in the aqueous phase in equilibrium with the adsorbed surfactant, are shown in Fig. 1 for gemini surfactants and in Fig. 2 for the conventional surfactants. The concentration of the surfactant at the air/solution interface, 0, and the minimum area, Amin , occupied by the surfactant there can be determined from the Gibbs adsorption equation in
the form (16a): 1 × 10−3 0=− 2.303n RT
µ
∂γ ∂ log C
¶ [2] T
and Amin =
1020 N 0max
2
˚ ), (in A
[3]
where γ is the surface tension in mN/m; 0 is the absorption amount in mol/m2 ; [∂γ /(∂ log C)]T is the slope in each case; T is absolute temperature, R = 8.314 J mol−1 K−1 ; and N is Avogadro’s number. The value of n (the number of species at the interface whose concentration at the interface changes with
FIG. 3. Adsorption isotherms of the conventional surfactants and the gemini surfactants on montmorillonite at 25◦ C in 0.01 M KBr. (a) C10 TMAB and C10 gemini; (b) C12 TMAB and C12 gemini; (c) C14 TMAB and C14 gemini; (d) C16 TMAB and C16 gemini.
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TABLE 1 Maximum Surfactant Adsorption onto Montmorillonite at 25◦ C from 0.01 M KBr and the Stability of the Resulting Dispersion Surfactant
CMCa (mol/dm3 )
Amin a (A2 )
Cs (max) (mol/g Clay)
Ceq (max)c (mol/dm3 )
Initial absorbance (A0 )
t1/2 (min)
Abs/A0
C10 TMAB C10 gemini C12 TMAB C12 gemini C14 TMAB C14 gemini C16 TMAB C16 geminib
6.22 × 10−2 7.95 × 10−4 9.29 × 10−3 1.64 × 10−4 2.47 × 10−3 7.82 × 10−6 8.20 × 10−4 2.25 × 10−6
70.6 101.6 67.6 89.8 63.4 21.7 59.3 9.3
7.43 × 10−4 7.20 × 10−4 8.13 × 10−4 7.54 × 10−4 8.27 × 10−4 8.12 × 10−4 8.92 × 10−4 /
1.82 × 10−2 6.59 × 10−3 3.61 × 10−3 1.43 × 10−3 6.47 × 10−4 1.90 × 10−4 1.64 × 10−4 /
0.17 0.73 0.47 1.56 0.60 1.84 0.75 /
9.96 22.1 11.4 >60 12.6 >60 13.0 /
/ 0.41 / 0.61 / 0.69 / /
a Literature values at 30◦ C in 1.25 × 10−2 M KBr [Trap, H. J. L., and Hermans, J. J., Koninki Ned Akad Weten. Proc. Ser B 58, 97 (1955)]: C TMAB, 5.9 × 10−2 ; 10 C12 TMAB, 1.08 × 10−2 ; C14 TMAB, 2.1 × 10−3 . b Maximum amount adsorbed on clay cannot be reached due to poor solubility of surfactant in water. c C (max) is the equilibrium surfactant concentration in solution phase at maximum amount adsorbed. eq
change in C) is taken as 1 for the gemini surfactants because the surfactant concentrations used to calculate Amin are much less than one-tenth of the ionic strength (i.e., 0.01 M). For the conventional surfactants, n is taken as 1 when the surfactant concentrations used to calculate Amin are less than one-tenth of 0.01 M, otherwise, the value of n can be obtained by use of the following equation (17): n=1+
Csurf , Csurf + I S
[4]
where IS is the ionic strength of the added electrolyte. Csurf is the average surfactant concentration used for [∂γ /(∂ log C)]T . The slopes of the plots in Fig. 1 indicate that the concentration of surfactant at the air/aqueous solution interface increases
FIG. 4. Time dependence of the absorbance of the clay suspension after treatment by the conventional and gemini surfactants. (1) C14 gemini; (2) C14 TMAB; (3) C12 gemini; (4) C12 TMAB; (5) C10 gemini; (6) C10 TMAB; (7) C16 TMAB.
as the number of carbons in the alkyl chain increases. Table 1 shows that the Amin values decrease with increase in the length of the alkyl chain for the gemini surfactants, the same as for the conventional surfactants studied here and for other conventional surfactants (16b). The very large slopes (and the very small Amin values) for the C14 and C16 gemini surfactants have been observed before and may be due to multilayer formation of the surfactants at the air/solution interface (18). Adsorption isotherms of the conventional surfactants and the geminis onto montmorillonite at 25◦ C from aqueous 0.01 M KBr solution are shown in Fig. 3a–d. Data are listed in Table 1.
FIG. 5. Highly simplified representation of the expected adsorption for both cationic gemini surfactants and conventional surfactants on the clay surface. (A) Conventional surfactant; (B) gemini surfactant.
ADSORPTION OF SURFACTANTS ON MONTMORILLONITE
269
FIG. 6. Adsorption of 2-naphthol and 4-chlorophenol onto surfactant-treated clay at 25◦ C in 0.01 M KBr. (a) C10 TMAB and C10 gemini; (b) C12 TMAB and C12 gemini; (c) C14 TMAB and C14 gemini; (d) C16 TMAB and C16 gemini.
The typical S-shaped isotherm (16c) is apparent only in the case of C10 TMAB. For the other surfactants, the amount adsorbed on the clay increases sharply to a maximum with increase in equilibrium surfactant concentration. For both the gemini and the conventional surfactants, the maximum amount of surfactant at the clay/aqueous solution interface, Cs (max), increases with the length of hydrophobic chain. The maximum amount of surfactant absorbed onto the clay is larger than the CEC of the clay in the case of C12 TAMB, C12 gemini, C14 TMAB, C14 gemini, and C16 TMAB. This indicates that although cation exchange plays a major key role in the adsorption, hydrophobic bonding between chains also occurs and becomes stronger as the hydrophobic chains become longer. The data on the surfactant concentration in the solution phase Ceq (max) at which this maximum adsorption of surfactant, Cs (max), occurs
are shown on Table 1. Noteworthy is the much lower concentration of gemini, about 13 that of the conventional surfactant with the same alkyl chain length, needed to achieve this maximum adsorption. This indicates that the gemini surfactant is much more efficient than the conventional surfactant at achieving this maximum adsorption on the montmorillonite. The data also show that, in spite of the fact that the gemini have two quaternary nitrogen groups that can adsorb onto negative sites onto the montmorillonite, the molar adsorptions of the gemini and conventional surfactants are almost identical. This indicates that only one of the hydrophilic groups in the gemini molecule is adsorbed onto the clay and that the second hydrophilic group is presumably oriented towards the aqueous phase. This mode of adsorption would be in marked contact to the adsorption of the conventional surfactants, which would also have their quaternary
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nitrogens oriented toward the clay, but with their hydrophobic groups oriented towards the aqueous phase. The adsorption of the gemini surfactants onto the montmorillonite would consequently make the surface of the latter hydrophilic, while the adsorption of the conventional surfactants would make the surface of the latter hydrophobic. That this is indeed the case is shown by the absorbance in the visible region of aqueous suspensions of the clay with the maximum amount of surfactant adsorbed onto it (Fig. 4). After the clay suspension was shaken for 24 hrs, its absorbance was immediately measured in the visible at 580 nm. From the absorbance data, the clay suspension with the adsorbed gemini surfactant is much more stable than that with the adsorbed conventional surfactant. In the case of the clay with adsorbed conventional surfactant, the absorbance rapidly decreases with time, indicating rapid flocculation of the clay as a result of its hydrophobic surface. In the case of the clay with adsorbed gemini surfactant, the solution remains turbid for an extended period, due to dispersion of the clay in the aqueous phase as a result of the positive charge on the surface of the particles and the hydrophilic surface character imparted by the second hydrophilic group oriented away from the clay surface. Figure 5 illustrates the expected gemini adsoption compared with that of conventional surfactant. The initial absorbance, A0 , and t1/2 (the time when the absorbance reduces to half of its initial value) of the gemini-treated clay suspension are both larger than those of its corresponding conventional surfactant (Table 1). As a result of the increased adsorption with increase in the length of the hydrophobic group to values above the CEC, the initial absorbance, t1/2 , and Abs/A0 (where Abs is the absorbance value at 60 min) increase from C10 TMAB to C16 TMAB and from C10 gemini to C14 gemini (Table 1). Adsorption of 2-Naphthol and 4-Chlorophenol onto Montmonrillonite with Adsorbed Layers of Gemini or Conventional Surfactant Adsorption isotherms of 2-naphthol and 4-chlorophenol onto montmorillonite with the maximum amount of the adsorbed gemini or conventional surfactant on it are shown in Fig. 6a–d, and the data are listed in Table 2. They show that the amount of 2-naphthol adsorbed per gram of clay at the point where the surfactant is adsorbed at its maximum is 1.5 times for the gemini surfactant, compared to that adsorbed onto the conventional surfactant. More than twice as much 4-chlorophenol is adsorbed onto the gemini-treated clay than onto the conventional-treated clay, at the point where the amount of adsorbed surfactant is maximum. Table 2 also shows the moles of pollutant adsorbed per mole of surfactant adsorbed, at this point of maximum adsorption of the surfactant. Again, the ratio of the amount of pollutant adsorbed to the mole of surfactant adsorbed is 1.5–1.9 times on the gemini surfactant than on the conventional surfactant in the case of 2-naphthol and 2–3 times in the case of 4-chlorophenol. For both the conventional and the gemini surfactants, because of the slight increase of the maximum adsorbed amount of sur-
TABLE 2 Pollutant Adsorption onto Surfactant-Treated Montmorillonite
Surfactant
Cs (mol/g)
Max. amount of pollutant adsorbed (mol/g)
2-Naphthol
C10 TMAB C10 gemini C12 TMAB C12 gemini C14 TMAB C14 gemini C16 TMAB C16 geminia
7.43 × 10−4 7.20 × 10−4 8.13 × 10−4 7.54 × 10−4 8.27 × 10−4 8.12 × 10−4 8.92 × 10−4 /
1.48 × 10−4 2.61 × 10−4 1.73 × 10−4 2.95 × 10−4 2.09 × 10−4 3.14 × 10−4 2.40 × 10−4 /
0.20 0.36 0.20 0.38 0.25 0.38 0.27 /
p-Chlorophenol
C10 TMAB C10 gemini C12 TMAB C12 gemini C14 TMAB C14 gemini C16 TMAB C16 geminia
7.43 × 10−4 7.20 × 10−4 8.13 × 10−4 7.54 × 10−4 8.27 × 10−4 8.12 × 10−4 8.92 × 10−4 /
4.33 × 10−5 1.68 × 10−4 7.01 × 10−5 2.08 × 10−4 1.10 × 10−4 2.42 × 10−4 1.39 × 10−4 /
0.058 0.24 0.084 0.28 0.13 0.29 0.16 /
Pollutant
Pollutant/ surfactant ratio (mol/mol)
a
Maximum amount adsorbed on clay can not be reached due to poor solubility of surfactant in water.
factants with the carbon number of their hydrophobic chain, the maximum adsorbed amount and ratio of the amount of pollutant adsorbed to the mole of surfactant adsorbed also increase with the carbon number of the surfactant’s alkyl chain. The decrease of the adsorption amount of either pollutant with increase in the equilibrium surfactant above its critical micelle concentration after the maximum adsorption amount has been reached is presumably due to solubilization of the pollutant in the micelles in the aqueous phase (12). This increased adsorption of the pollutant onto the gemini, compared to that of the conventional surfactant, shows that the former is more effective than the latter in removing pollutants from aqueous media. The observation that the maximum adsorption of the gemini surfactant onto the clay occurs at about 13 the concentration of the conventional surfactant in the solution phase (Table 1) shows that the gemini is also more efficient at removing the pollutant from aqueous media. REFERENCES 1. Boyd, S. A., Lee, J.-F., and Mortland, M. M., Nature 333, 345 (1988). 2. Xu, S.-H., and Boyd, S. A., Environ, Sci. Technol. 29, 312 (1995). 3. Lee, J.-F., Crum, J. R., and Boyd, S. A., Environ. Sci. Technol. 23, 1365 (1989). 4. Nye, J. V., Guerlin, W. F., and Boyd, S. A., Environ. Sci. Technol. 28, 944 (1994). 5. Smith, J. A., Jaffe, P. R., and Chiou, C. T., Environ. Sci. Technol. 24, 1167 (1990). 6. Zhang, Z.-Z., Sparks, D. L., and Scrivner, N. C., Environ. Sci. Technol. 27, 1625 (1993).
ADSORPTION OF SURFACTANTS ON MONTMORILLONITE 7. Lumpp, E. K., Heitmann, H., and Schwuger, M. J., Colloids Surf. A 78, 93 (1993). 8. Schieder, D., Dobias, B., Klumpp, E., and Schwuger, M. J., Colloids Surf. A 88, 103 (1994). 9. Rosen, M. J., Chem. Technol. 30 (1993). 10. Zana, R., Curr. Opin. Colloid Interface Sci. 1, 566 (1996). 11. Rosen, M. J., and Tracy, D. J., J. Surfactants Detergents 1, 547 (1998). 12. Esumi, K., Goino, M., and Koide, Y., J. Colloid Interface Sci. 183, 539 (1996).
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13. Chorro, C., Chorro, M., Dolladille, O., and Zana, R., J. Colloid Interface Sci. 199, 169 (1998). 14. Reid, V. W., Alston, T., and Heinerth, E., Tenside 4, 292 (1967). 15. Reid, V. W., Alston, T., and Heinerth, E., Tenside 5, 90 (1967). 16. Rosen, M. J., in “Surfactants and Interfacial Phenomena,” 2nd edition. John Wiley, New York, 1989: (a) pp. 65–68; (b) pp. 70–80; (c) p. 48. 17. Matijevic, E., and Pethica, B. A., Trans. Faraday Soc. 54, 1382 (1958). 18. Rosen, M. J., Mathias, J. H., and Davenport, L., Langmuir 15, 7340 (1999).