Journal of Colloid and Interface Science224,265–271 (2000)doi:10.1006/jcis.2000.6721, available online at http://www.idealibrary.com on

Adsorption of Gemini and Conventional Cationic Surfactants ontoMontmorillonite 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, [CnH2n+1N+

(CH3)2-CH2CH2]2 · 2Br−, where n= 10, 12, 14, and 16, on clay (Na-montmorillonite) from their aqueous solution in 0.01 M KBr andthe effect of this adsorption on the removal of 2-naphthol and 4-chlorophenol have been studied. Compared to those of conventionalcationic surfactants with similar single hydrophilic and hydropho-bic groups (CnH2n+1N+(CH3)3 ·Br−, where n= 10, 12, 14, and 16),the molar adsorptions of the gemini and conventional surfactantsare almost identical. This indicates that only one of the hydrophilicgroups in the gemini molecule is adsorbed onto the clay and thatthe second hydrophilic is presumably oriented toward the aqueousphase, 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 typesof surfactants confirm this. The slight increase in the moles of sur-factant to values above the CEC of the clay with an increase in thecarbon number of the hydrophobic chain indicates that adsorptionthrough hydrophobic group interaction occurs in addition to themajor ion exchange. Adsorption studies of the pollutants onto theclay treated by either the gemini or the conventional surfactantsshow that the former are both more efficient and more effective atremoving the pollutants from the aqueous phase. C© 2000 Academic Press

Key Words: gemini surfactants; adsorption; montmorillonite; 2-naphthol; 4-chlorophenol; pollutant removal; cationic surfactants;clay.

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INTRODUCTION

In recent years, cationic surfactants, especially quaternarymonium cationic surfactants, have been used to treat soithen remove pollutants from aqueous media (1–8). Since gini surfactants (surfactants containing two hydrophilic andhydrophobic groups in the molecule) have been shown tmuch more surface-active than conventional surfactants coning a single similar hydrophilic and hydrophobic group inmolecule (9, 10), there was interest in determining whetheformer would be more efficient and/or effective than the lain removing pollutants from aqueous media. Although thesorption of gemini surfactants at the air/solution interfacebeen much investigated (11), little attention has been paid tadsorption of gemini surfactants at the solid/solution interfa

1 To whom correspondence should be addressed. was

26

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So far as we know, only two papers (12, 13) involving the siliaqueous solution interface concern the adsorption of geminifactants at the solid/aqueous solution interface. In Ref. (the gemini surfactants used by Zanaet al.are alkanediyl-α, ω-bis(dodecyldimethylammonium bromide), with the alkane-dspacer group C2H4, C4H8, C6H12, and C10H20. They studiedmainly the influence of the spacer on the amount of surfacadsorbed and discussed the adsorption mechanism. They dstudy the effect of the gemini surfactants on the removal ofpollutant. The only reported study of the effect of gemini sfactants on the removal of organic contaminants is that of Eset al. (12), who used one gemini surfactant to treat silica astudied the effect of this gemini surfactant on the remova2-naphthol. The only gemini surfactant they studied is etha1,2-bis(dodecyldimethylammonium bromide). They found tthe molar amount of gemini surfactant adsorbed on silicalower than for the corresponding conventional surfactant, docyltrimethylammonium bromide, but that the ratio of the maimum amount of 2-naphthol adsorbed to the adsorbed amof surfactant on silica increases from conventional surfactangemini surfactant.

The adsorption of surfactant onto silica is not strong, aro5× 10−5 mol/g silica (12, 13). In this study, we chose Nmontmorillonite, which has a much larger cation exchancapacity (CEC= 7.64× 10−2 eq/100 g). Adsorption of a serieof cationic gemini surfactants, [CnH2n+1N+(CH3)2-CH2CH2]2·2Br−, wheren= 10, 12, 14, and 16, onto the montorilloniteinvestigated, and the effect of this adsorption on the removatwo pollutants, 2-naphthol and 4-chlorophenol, is noted. Thtwo pollutants are widely used in surfactant-concerned eronmental studies and can be easily analyzed by UV specphotometry (7, 8, 12). The adsorption and removal of the plutants are compared to those of conventional catiosurfactants with similar single hydrophilic and hydrophobgroups (CnH2n+1N+(CH3)3 ·Br−, wheren= 10, 12, 14, and 16)

EXPERIMENTAL PROCEDURES

Materials

Synthesis of the gemini surfactant.A 0.01 mol amountof 1,4-dibromobutane (95%) and 0.022 mol of alkylN,N-dimethylamine were added to isopropanol, and the mixture

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266 LI AND

heated to reflux for about 12 h. After the reaction, the solvwas removed and the residue was recrystallized from tetrdrofuran (THF) several times. The pure diquaternary ammondibromide was finally obtained as a colorless powder by rectallization three times from ethanol/acetate mixed solvent.gemini surfactants are referred to as C10 gemini, C12 gemini,C14 gemini, and C16 genimi, based on their hydrophobic chalengths. 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

lows: C12 gemini,N= 4.36% (calcd.), 4.32% (found); C16 gem-ini, N= 3.71% (calcd.), 3.86% (found).

The conventional surfactants used in this study are detrimethylammonium bromide (C10TMAB), dodecyltrimethyl-ammonium bromide (C12TMAB), tetradecyltrimethylam-monium bromide (C14TMAB), and hexadecyltrimethylammonium bromide (C16TMAB).

The clay [Na-montmorillonite (Wyoming)] was obtainefrom Professor Stephen Aja, Department of Geology, Brolyn College of CUNY. The surface area was 31.82± 0.22 m2/g[measured by BET(N2)]. The cation exchange capacity (CEis 76.4 meq/100 g.

Surface tension measurements.Measurements were peformed at 25± 0.1◦C on a K-12 tensiometer, by use of thWilhelmy plate technique, with a sandblasted platinum blof ca. 4 cm perimeter. The instrument was calibrated agaquartz-condensed double-distilled, previously deionized w(the last distilled stage from alkaline KMnO4 through a 1-m highVigreaux column). Values were taken until the surface tenswas constant for 0.5 h. Reproducibility of the surface tensmeasurement is less than 0.2 mN/m.

Adsorption isotherm of surfactants on clay and the adsotion of the pollutants on surfactant-treated clay.A series ofaqueous surfactant solutions in 0.01 mol/dm3 KBr aqueous so-lution was prepared in the absence and presence of eithnaphthol or 4-chlorophenol. The initial concentration of eitpollutant was 4× 10−4 mol/dm3. A 50-ml amount of the solution was added to 50 mg of clay in a capped centrifuge tuThe solution was equilibrated by shaking for at least 24 h,then the solid was removed by centrifugation. Since the mmorillonite, when stirred with the aqueous phase containingsurfactant, did not reduce the surface tension of the aquphase, the surface tension of the supernatant solution inlibrium with the clay can be used to measure the concentraof the surfactants in the solution. In addition, the concention of the surfactant in the aqueous phase was measured bmixed indicator two-phase titration method (14, 15), usingcationic or gemini solution as the titrant. The same results wobtained by either method. The concentration of either pollu

in the aqueous phase was determined by UV spectrophotom(Kontron Instruments, Unikon 9410).

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FIG. 1. Surface tension vs log of surfactant molar concentration forgemini 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 montmorilloand the amount of either pollutant adsorbed onto the surfactreated montmorillonite is given by the following equation:

ns= (C0−Ceq)×V

g, [1]

wherens is the number of moles of adsorbate (surfactanpollutant) per gram of adsorbent (mol/g),C0 is the initial con-centration of adsorbate (mol/dm3), Ceq is the equilibrium con-centration of adsorbate (mol/dm3), V is the volume of solution(dm3), andg is the weight of adsorbent (clay) (g).

FIG. 2. Surface tension vs log of surfactant molar concentration for the

etryventional surfactants at 25◦C in 0.01 M KBr: (m) C10TMAB; (r) C12TMAB;(d) C14TMAB; (w) C16TMAB.

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ADSORPTION OF SURFACT

RESULTS AND DISCUSSION

Adsorption of the Gemini and Conventional Surfactantsat the Air/Aqueous Solution and Montmontillonite/Aqueous Solution Interfaces

The surface tension (γ )–log surfactant molar concentratio(C) curves at 25◦C in 0.01 M KBr, used to determine the cocentration of surfactants in the aqueous phase in equilibwith the adsorbed surfactant, are shown in Fig. 1 for gemsurfactants and in Fig. 2 for the conventional surfactants.

The concentration of the surfactant at the air/solution in

face,0, and the minimum area,Amin, occupied by the surfactantthere can be determined from the Gibbs adsorption equation in

Avogadro’s number. The value ofn (the number of species atthe interface whose concentration at the interface changes with

◦

FIG. 3. Adsorption isotherms of the conventional surfactants and theC10 gemini; (b) C12TMAB and C12 gemini; (c) C14TMAB and C14 gemini; (d) C

NTS ON MONTMORILLONITE 267

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gemini surfactants on montmorillonite at 25C in 0.01 M KBr. (a) C10TMAB and

16TMAB and C16 gemini.

268 LI AND ROSEN

TABLE 1Maximum Surfactant Adsorption onto Montmorillonite at 25◦C from 0.01 M KBr and the Stability of the Resulting Dispersion

CMCa Amin Cs(max) Ceq(max)c Initial absorbance t1/2Surfactant (mol/dm3) (A

a2) (mol/g Clay) (mol/dm3) (A0) (min) Abs/A0

C10TMAB 6.22× 10−2 70.6 7.43× 10−4 1.82× 10−2 0.17 9.96 /C10 gemini 7.95× 10−4 101.6 7.20× 10−4 6.59× 10−3 0.73 22.1 0.41C12TMAB 9.29× 10−3 67.6 8.13× 10−4 3.61× 10−3 0.47 11.4 /C12 gemini 1.64× 10−4 89.8 7.54× 10−4 1.43× 10−3 1.56 >60 0.61C14TMAB 2.47× 10−3 63.4 8.27× 10−4 6.47× 10−4 0.60 12.6 /C14 gemini 7.82× 10−6 21.7 8.12× 10−4 1.90× 10−4 1.84 >60 0.69C16TMAB 8.20× 10−4 59.3 8.92× 10−4 1.64× 10−4 0.75 13.0 /C16 geminib 2.25× 10−6 9.3 / / / / /

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 B58, 97 (1955)]: C10TMAB, 5.9× 10−2;C12TMAB, 1.08× 10−2; C14TMAB, 2.1× 10−3.

b Maximum amount adsorbed on clay cannot be reached due to poor solubility of surfactant in water.a

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change inC) is taken as 1 for the gemini surfactants becathe surfactant concentrations used to calculateAmin are muchless than one-tenth of the ionic strength (i.e., 0.01 M). Forconventional surfactants,n is taken as 1 when the surfactaconcentrations used to calculateAmin are less than one-tenth o0.01 M, otherwise, the value ofn can be obtained by use of thfollowing equation (17):

n= 1+ Csurf

Csurf+ I S, [4]

whereIS is the ionic strength of the added electrolyte.Csurf isthe average surfactant concentration used for [∂γ /(∂ logC)]T .

The slopes of the plots in Fig. 1 indicate that the concention of surfactant at the air/aqueous solution interface increa

FIG. 4. Time dependence of the absorbance of the clay suspensiontreatment by the conventional and gemini surfactants. (1) C14 gemini; (2)

C14TMAB; (3) C12 gemini; (4) C12TMAB; (5) C10 gemini; (6) C10TMAB;(7) C16TMAB.

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as the number of carbons in the alkyl chain increases. Tashows that theAmin values decrease with increase in the lenof the alkyl chain for the gemini surfactants, the same asthe conventional surfactants studied here and for other contional surfactants (16b). The very large slopes (and thesmall Amin values) for the C14 and C16 gemini surfactants havbeen observed before and may be due to multilayer formaof the surfactants at the air/solution interface (18).

Adsorption isotherms of the conventional surfactantsthe geminis onto montmorillonite at 25◦C from aqueous 0.0M KBr solution are shown in Fig. 3a–d. Data are listedTable 1.

FIG. 5. Highly simplified representation of the expected adsorption

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) C10TMAB and C10 gemini; (b) C12TMAB andC12 gemini; (c) C14TMAB and C14 gemini; (d) C16TMAB and C16 gemini.

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The typical S-shaped isotherm (16c) is apparent onlythe case of C10TMAB. For the other surfactants, the amouadsorbed on the clay increases sharply to a maximumincrease in equilibrium surfactant concentration. For bothgemini and the conventional surfactants, the maximum amof surfactant at the clay/aqueous solution interface,Cs(max),increases with the length of hydrophobic chain. The mamum amount of surfactant absorbed onto the clay is lathan the CEC of the clay in the case of C12TAMB, C12 gemini,C14TMAB, C14 gemini, and C16TMAB. This indicates that al-though cation exchange plays a major key role in the adsorphydrophobic bonding between chains also occurs and becostronger as the hydrophobic chains become longer. The da

the surfactant concentration in the solution phaseCeq(max) atwhich this maximum adsorption of surfactant,Cs(max), occurs

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are shown on Table 1. Noteworthy is the much lower conctration of gemini, about13 that of the conventional surfactawith the same alkyl chain length, needed to achieve this mimum adsorption. This indicates that the gemini surfactanmuch more efficient than the conventional surfactant at aching this maximum adsorption on the montmorillonite. The dalso show that, in spite of the fact that the gemini havequaternary nitrogen groups that can adsorb onto negativeonto the montmorillonite, the molar adsorptions of the gemand conventional surfactants are almost identical. This indicthat only one of the hydrophilic groups in the gemini moleculadsorbed onto the clay and that the second hydrophilic gropresumably oriented towards the aqueous phase. This mo

adsorption would be in marked contact to the adsorption of theconventional surfactants, which would also have their quaternary

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270 LI AND

nitrogens oriented toward the clay, but with their hydrophogroups oriented towards the aqueous phase.

The adsorption of the gemini surfactants onto the montmrillonite would consequently make the surface of the latterdrophilic, while the adsorption of the conventional surfactawould make the surface of the latter hydrophobic. That thisindeed the case is shown by the absorbance in the visible reof aqueous suspensions of the clay with the maximum amoof surfactant adsorbed onto it (Fig. 4). After the clay suspenswas shaken for 24 hrs, its absorbance was immediately msured in the visible at 580 nm. From the absorbance dataclay suspension with the adsorbed gemini surfactant is mmore stable than that with the adsorbed conventional surfacIn the case of the clay with adsorbed conventional surfactantabsorbance rapidly decreases with time, indicating rapid floclation of the clay as a result of its hydrophobic surface. Incase of the clay with adsorbed gemini surfactant, the soluremains turbid for an extended period, due to dispersion ofclay in the aqueous phase as a result of the positive chargthe surface of the particles and the hydrophilic surface charaimparted by the second hydrophilic group oriented away frthe clay surface. Figure 5 illustrates the expected gemini adtion compared with that of conventional surfactant. The initabsorbance,A0, andt1/2 (the time when the absorbance reducto half of its initial value) of the gemini-treated clay suspesion are both larger than those of its corresponding conventisurfactant (Table 1).

As a result of the increased adsorption with increase inlength of the hydrophobic group to values above the CEC,initial absorbance,t1/2, and Abs/A0 (where Abs is the absorbancvalue at 60 min) increase from C10TMAB to C16TMAB and fromC10 gemini to C14 gemini (Table 1).

Adsorption of 2-Naphthol and 4-Chlorophenolonto Montmonrillonite with Adsorbed Layersof Gemini or Conventional Surfactant

Adsorption isotherms of 2-naphthol and 4-chlorophenol omontmorillonite with the maximum amount of the adsorbgemini or conventional surfactant on it are shown in Fig. 6aand the data are listed in Table 2. They show that the amoof 2-naphthol adsorbed per gram of clay at the point wheresurfactant is adsorbed at its maximum is 1.5 times for the gini surfactant, compared to that adsorbed onto the conventisurfactant. More than twice as much 4-chlorophenol is adsoronto the gemini-treated clay than onto the conventional-treaclay, at the point where the amount of adsorbed surfactant is mimum. Table 2 also shows the moles of pollutant adsorbedmole of surfactant adsorbed, at this point of maximum adsotion of the surfactant. Again, the ratio of the amount of pollutaadsorbed to the mole of surfactant adsorbed is 1.5–1.9 timethe gemini surfactant than on the conventional surfactant incase of 2-naphthol and 2–3 times in the case of 4-chlorophe

For both the conventional and the gemini surfactants, becaof the slight increase of the maximum adsorbed amount of s

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TABLE 2Pollutant Adsorption onto Surfactant-Treated Montmorillonite

Max. amount Pollutant/of pollutant surfactantadsorbed ratio

Pollutant Surfactant Cs (mol/g) (mol/g) (mol/mol)

2-Naphthol C10TMAB 7.43× 10−4 1.48× 10−4 0.20C10 gemini 7.20× 10−4 2.61× 10−4 0.36C12TMAB 8.13× 10−4 1.73× 10−4 0.20C12 gemini 7.54× 10−4 2.95× 10−4 0.38C14TMAB 8.27× 10−4 2.09× 10−4 0.25C14 gemini 8.12× 10−4 3.14× 10−4 0.38C16TMAB 8.92× 10−4 2.40× 10−4 0.27C16 geminia / / /

p-Chlorophenol C10TMAB 7.43× 10−4 4.33× 10−5 0.058C10 gemini 7.20× 10−4 1.68× 10−4 0.24C12TMAB 8.13× 10−4 7.01× 10−5 0.084C12 gemini 7.54× 10−4 2.08× 10−4 0.28C14TMAB 8.27× 10−4 1.10× 10−4 0.13C14 gemini 8.12× 10−4 2.42× 10−4 0.29C16TMAB 8.92× 10−4 1.39× 10−4 0.16C16 geminia / / /

a Maximum amount adsorbed on clay can not be reached due to poor soluof surfactant in water.

factants with the carbon number of their hydrophobic chain,maximum adsorbed amount and ratio of the amount of polluadsorbed to the mole of surfactant adsorbed also increasethe carbon number of the surfactant’s alkyl chain. The decreof the adsorption amount of either pollutant with increase inequilibrium surfactant above its critical micelle concentratiafter the maximum adsorption amount has been reached issumably due to solubilization of the pollutant in the micellesthe aqueous phase (12).

This increased adsorption of the pollutant onto the gemcompared to that of the conventional surfactant, shows thaformer is more effective than the latter in removing pollutafrom aqueous media. The observation that the maximumsorption of the gemini surfactant onto the clay occurs at abo1

3the concentration of the conventional surfactant in the soluphase (Table 1) shows that the gemini is also more efficienremoving the pollutant from aqueous media.

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hn

ADSORPTION OF SURFACTA

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(1996).

NTS ON MONTMORILLONITE 271

13. Chorro, C., Chorro, M., Dolladille, O., and Zana, R.,J. Colloid InterfaceSci.199,169 (1998).

14. Reid, V. W., Alston, T., and Heinerth, E.,Tenside4, 292 (1967).15. Reid, V. W., Alston, T., and Heinerth, E.,Tenside5, 90 (1967).16. Rosen, M. J.,in “Surfactants and Interfacial Phenomena,” 2nd edition. Jo

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).

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