Environ. Sci. Techno/. 1995, 29,3022-3028
Alternative Model for Cationic Surfactant Adsorption by Layer S H I H E X U A N D STEPHEN A . B O Y D * Department of Crop and Soil Sciences, Michigan State University, East Lansing, Michigan 48824-1325
Cationic surfactants are used in a wide range of household and industrial activities and have potential utility in in-situ remediation of contaminated soils and aquifers. Although many models have been proposed to describe the adsorption of cationic surfactants by soils and sediments, none can quantitatively account for the adsorption of quaternary ammonium compounds (QAC) by swelling clays or soils. In this study, w e developed an alternative model for adsorption of hexadecyltrimethylammonium, a model QAC, by major clay types common to subsoils. The model uses a randomness parameter to account for the variation of cation exchange selectivity coefficients arising from differences in the distribution of inorganic and surfactant cations on the surfaces or in the interlayers of clays. In addition, the model employs an empirical relationship to predict the amount of QAC adsorption by hydrophobic bonding. Experimental data demonstrate that the model quantitatively describes the major characteristics of QAC-clay interactions for both swelling and nonswelling clays and provides a quantitative linkage between QAC adsorption and the nature of the QAC and clay minerals as well as solution conditions.
Introduction Cationic surfactants are used in a wide range of industrial and household activities (1, 2). The adsorption and desorption of these compounds in soils and sediments strongly influences their fate and transport in environment (1, 3). Several studies have examined the adsorption of a variety of organic cations on mineral surfaces (4- 11). More recently, the potential utility of a certain type of cationic surfactant, namely, quaternary ammonium compounds (QACs), for in-situ remediation of organic contaminated subsoils and aquifer materials has stimulated interest in understanding their chemical interactions with soil components (8, 12-15). The in-situremediation technique under study involves direct injection of QAC solutions into the subsurface to create sorptive zones for immobilizing organic groundwater contaminants. Previous studies have demonstrated that * Correspondingauthortelephone: (517)353-3993;fax: (517)3550270.
3022 1 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 12, 1995
replacing native exchangeable cations of soil clays with long-chain QACs (e.g.,hexadecyltrimethylaonium, HDTMA) results in dramatic increases in the sorption of hydrophobic organic compounds (HOCs)by the resultant modified soil or subsoil (16-19). If properly placed, such sorbent zones could intercept and immobilize contaminant plumes containing HOCs. This, coupled with subsequent biodegradation of the immobilized contaminants may provide a comprehensive remediation approach for contaminated aquifers (20). The feasibility of in-situ soil modification and contaminant plume immobilization has been demonstrated recently using an aquifer box model (21, 22). One major need in applying the soil modification technique to environmental restoration is the ability to predict QAC adsorption by soil components because this determines the efficiency of soil modification. Once surfactant solutions are delivered into the subsurface, it is important to know the surfactant distribution among the liquid and solid phases. Obviously from an environmental safety prespective, the goal is to maximize surfactant binding; this also has the advantage of increasing the effectiveness of the sorbent zone. Furthermore, dissolved QAC (e.g., HDTMA) may be toxic to pollutant degrading bacteria (201,thwarting efforts to bioremediate immobilized contaminants. It is also important how much surfactant is bound via cation exchange versus the nonexchange mechanism at a given surfactant loading. Surfactant adsorbed via the latter mechanism (e.g., hydrophobic bonding) is more susceptible to desorption than that adsorbed directly on exchange sites (14). QAC adsorption by clays depends on the clay type, the nature of exchangeable cations initially saturating the clay, and the ionic strength of the aqueous solution (15). For nonswelling clays ( e g , kaolinite),the adsorption isotherm is generally monotonic (15). For swelling clays (e.g., montmorillonite), in dilute salt solutions (e.g.,2.5 mM CaC12 or 5 mM NaCl), monotonic HDTMA adsorption isotherms have been observed for Ca-montmorillonites,but s-shaped non-monotonic isotherms were observed for Na-montmorillonites. The s-shaped HDTMA adsorption isotherms changed to monotonic as the ionic strength of the NaCl solution increased to greater than 0.01 M (15). The s-shaped HDTMA adsorption isotherm and the dependence of isotherm shape on solution conditions are unique characteristics of HDTMA adsorption by swelling clays. Previous surfactant adsorption models (9-1 1 , 23) were developed using data obtained from nonswelling clays and cannot account for these phenomena (15). Based on X-ray diffraction patterns, electrophoretic mobility of clays withvarious HDTMAloadings,Xu and Boyd (15) attributed the s-shaped adsorption isotherms of swelling clays to a random distribution of inorganic and HDTMA cations in the interlayer region and attributed the monotonic isotherms to layer segregation of HDTMA and inorganic cations. Swelling clays also behave differently from nonswelling clays with regard to the hydrophobic adsorption of surfactants. For example, Xu and Boyd (14, 1 5 ) tested several ionic surfactant adsorption models using HDTMA adsorption and cation release data, X-ray diffraction, electro-
0013-936X/95/0929-3022$09 OO/O
P 1995 Arner,can Chemical Society
phoretic mobility, and clay suspension turbidity measurements and found that the existing models could not describe HDTMA adsorption via hydrophobic bonding for swelling clays. In this study, we have developed and tested a new model for cationic surfactant adsorption using HDTMA as a model compound. The model consists of two submodels describing cation exchange and hydrophobic bonding. The cation exchange submodel is based on the hypothesis that variations in the strength of the HDTMA-claybinding result from the interlayer distribution of inorganiclorganic cations and the structure of the HDTMA adsorption layer. The hydrophobic bonding submodel is based on an empirical observation that HDTMA adsorption at high loadings is linearly related to square root of aqueous concentration of HDTMA. The model is conceptually simple and accounts for most of the observed variations in HDTMA adsorption under different experimental conditions.
Model Description Cationic surfactants may be adsorbed by two mechanisms, viz., cation exchange and hydrophobic bonding. Therefore =~
+
(1)
C E ~ H B
where 41,~ C E and , ~ H denote B the total surfactant adsorbed (e.g.,in mol kg-'), surfactant adsorbed by cation exchange, and surfactant adsorbed by hydrophobic bonding, respectively. In the following paragraphs, we will describe how to obtain these two components for monovalent hydrophobic cationic surfactant (e.g., HDTMA) adsorption by layer silicates. Cation Exchange ( ~ c E ) . Binary exchange reactions between inorganic cations (A"+) and cationic surfactant molecules (Q+)on clay exchange sites (x-) can be described as
The conditional exchange constant K, (Vanselowselectivity coefficient) for the entire exchanger is given by
and then
For divalent inorganic cations, eq 6 has a quadratic form with the solution
(11)
Vanselow Selectivity Coefficients, (Kv and KVJ. Clay surfaces may have various types of exchange sites of different energies. K, is defined on the basis of the mole fraction of exchange complexes over the entire exchanger and, hence, is a weighted average value of individual Vanselow selectivity coefficients for different classes of exchange sites (24). Let Pi be the fraction of the ith class of exchange sites with aVanselow selectivity coefficient Kvi defined in the same fashion as in eq 3. Then
where EQX~ is the fraction of the ith class of exchange sites occupied by organic cation, and EQxi= [QXi]/CECi
(13)
where CECi is the contribution to the total CEC of the clay from the ith class of exchange sites (Le., CCECi = CEC). For clay minerals tested in this study, only two types of exchange sites are assumed (internal and external), and if the inorganic cation is monovalent, eq 12 becomes:
& = (PiEQxi+ P~EQ,)/(P~EQx,/&~ f P~EQx~/I;(V~) (14) If the inorganic cation is divalent, eq 12 becomes
K , = (aAlaQ"l( N Q g / N ~ J
(3)
where a refers to the activity of cations in solution and N refers to the cation mole fraction on the exchange sites. Furthermore NQ, = [Qxl/{[QXI +
[&,I
}
(4)
and
(5) where [ ] denotes the amount of QAC adsorbed via cation exchange ( e g ,in mol kg-'1. Combining eqs 3-5, we obtain a general expression: Nag
+ (K,a$/aA)NQx- &aQL'/aA= 0
(6)
By definition, ~ C isE related to NQXand the cation exchange capacity (CEC) of clay by
qcE = [QXI = NQxCEC/[NQ,(1-
+ VI
4 = (PlEQX1+ p2EQX2)2(1+ PlEQXl f P&QU)-'/ [plEQX12(1 + EQX1)-l/&l f P2EQZ2 (1 + EQ=)-l/&zl (15) One important differencebetween organic and inorganic cation adsorption is that for organic cations the adsorption energy arises from hydrophobic bondings between organic cations and surfaces and from lateral interactions between the adsorbed organic cations themselves. We defined Kvi as
where KO is the combined contribution of the electrostatic interaction and the hydrophobic bonding of the organic cation to the selectivity coefficient; wi is the free energy of hydrophobic lateral interactions and depends mainly on length of alkyl chain, surfactant organization, and loading on the clay surfaces. The term wi can be defined as
(7)
For monovalent inorganic cations, eqs 6 and 7 can be reduced to
where wo is the free energy of lateral interactions at 100% VOL. 29. NO. 1 2 , 1 9 9 5 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
3023
surfactant-saturation of the exchange sites; is a parameter accounting for how randomly the surfactant is distributed on surfaces comprising the interlayers. For a random organization, e > 0, and K,i increases with surfactant loading. For complete segregation, e = 0, w , = a",and Kvi remains constant as loading increases. Hydrophobic Bonding (~HB).The submodel for preB amount of surfactant adsorbed by hydrodicting ~ H (the phobic bonding) is an empirical equation based on experimental data for HDTMA adsorption on layer silicates and soils (14, 1.5):
u
1'75
g
4
0.50
0.oc
3024 1 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 12, 1995
I
i
0.75
k?
9
where ~ H B is, the ~ adsorption plateau, and C, and C, are equilibrium aqueous phase monomeric surfactant concentrations corresponding to ~ H and B ~ H B ,respectively. ~,
Cation Exchange. Although there are many reports showing that adsorption isotherms of ionic surfactants on nonswellingclays (e.g.,kaolinite in Figure 1)are monotonic, only recently have non-monotonic s-shaped adsorption isotherms (e.g.,HDTMA adsorption in Na-SWy-1in Figure 1)been observed in swelling clays (15) and soils containing
"la-Kaoiinite
.^ c
0.25
Results and Discussion
0
0
F0
Experimental Section
Montmorillonite
*
(18)
Adsorption isotherms of HDTMA (Cl-, Br-, S042-) were obtained for montmorillonite, vermiculite, illite, and kaolinite, allfrom Clay Mineral Society. The detailed procedure for clay preparation and HDTMA adsorption determination are given elsewhere (14, 1.5). Briefly, the homoionic clays were prepared by washing 10 g of clay with 250 mL of 1 M MC1 (M = Li, Na, K, Cs) or 0.1 M DC12 (D = Ca and Mg) solutions three times with at least 8 h equilibrium for each washing. The clays were then stored as suspensions in 0.01 M MCl or 0.005 M DC12 solutions. For HDTMA adsorption, a volume of clay containing 90 pmol (+) exchangeable cations was pipetted into a 25-mL Corex tube, and the ionic strengths of initial solutions were adjusted to adesignatedvalue. Various volumes (from 2.25 to 450 pmol) of HDTMA solutions (all of the HDTMA salts were obtained from Fluka) containing known amounts of '*C-labeled HDTMA (1abeled:nonlabeled = 1:5000) were pipetted into clay suspensions and mixed gently. The samples were allowed to equilibriate for 5 days with occasional gentle mixing and then centrifuged at 9860gfor 25 min. The supernatant was analyzed for 14Cactivity by liquid scintillation counting and for cation release by atomic absorptionlflame emission spectrophotometry. To test the proposed cation exchange submodel, Vanselow selectivity coefficients for the entire exchanger were calculated from HDTMAadsorption data and cation release isotherms obtained in this and a previous study (15) using eq 3 . It was assumed that HDTMA adsorbed by cation exchange resulted in an equivalent amount of inorganic cation release. The calculated selectivity coefficients were then fitted to eqs 6,12,16, and 17 to calculate KO, wo,e, and PI using an iteration method. To test the proposed hydrophobic bonding submodel, ~ H was B calculated as total HDTMA adsorption less the equivalent release of inorganic exchangeable cations. The q ~ was, i ~ by , ~definition, the difference between the HDTMA adsorption plateau and the maximal release of inorganic exchangeable cations.
1
log
C"DTH*
FIGURE 1. Characteristic HDTMA adsorption isotherms for (a) a swelling clay (montmorillonite) initially saturated with either Naor CaZCand (b) a nonswelling clay (kaolinite). &,r~a is in mol L-'. Data from Xu and Boyd (15).
0
I
M=L: o M=No A M=Cs DO
02
04
05
08
10
- *0 0
02
0 4
36
G8
1 3
Mole Fraction of Exchange Sites Sotorated by I-D'MP
FIGURE 2. Comparison of cation selectivity coefficients of HDTMA predicted by the proposed model and those measured with varioustypes of (a) monovalent and (b)divalent inorganic cations initially saturating the exchange sites of Wyoming montmorillonite. (&,.HD~A)
swelling clays (14). The difference between these two types of adsorption isotherms reflected two different trends in cation exchange selectivity coefficients as a function of HDTMA loading on the exchange sites. The monotonic isotherm corresponds to constant K, or K, decreasing with HDTMA loading, whereas, the non-monotonic s-shaped isotherm corresponds to K, increasing with HDTMA loading (1.5).
The dependence of cation selectivity coefficients on HDTMA loading on a montmorillonite differed above and below about 75% saturation of the CEC (0.75 CEC). When HDTMA loading was smaller than 0.75 CEC, the shape of cation selectivity coefficient versus HDTMA loading curves depended strongly on inorganic cation type and ionic strength (Figures 2 and 3 ) . When HDTMA loading was greater than 0.75 CEC, the cation selectivity coefficients however always decreased with HDTMA loading, regardless of inorganic cation type and ionic strength (Figures 2 and 3 ) . The two-type-site cation exchange submodel proposed here successfully accounted for the dependence of K,.'s on HDTMA loading for various types of clays and at different ionic strengths as indicated by the correspondence of the measured data with model predictions (Figures 2-41, The deviation of data from the prediction was somewhat higher
i
5-
4 -
9
b 43Y
2 -
0
3 I
Measured, 5 mM
- Predicted
1 /
0.2
0.0
0.4
0.6
0.8
1 .o
Mole Fraction of Exchange Sites Saturated by HDTMA
FIGURE 3. Comparison of cation selectivity coeffients of HDTMA
(KN~.HDTMA) predicted by the proposed model and those measured for Wyoming montmorillonite at different NaCl concentrations. Wyoming Montmorillonite Kaolinite Illite - Predicted 0
o v
4 -
0.0
0.2
0.4
0
0.6
0.8
1 .o
Mole Fraction of Exchange Sites Saturated by HDTMA
FIGURE 4. Comparison of cation selectivity coefficients of HDTMA (KN~.HDTMA) predicted by the proposed model and those measured for both swelling and nonswelling clays.
at HDTMA loadings 20.8 CEC (Figures 2-4) due to the high susceptibility of K, values to errors in measuring the amount of HDTMA and Na on the surface at high HDTMA loading levels. Clay minerals are generally recognized as nonideal exchangers. The coexistence of multitypes of sites with various adsorption energies is one major factor responsible for such nonideality, and this is manifested by changes in adsorption energy with cation loading. Based on charge origin, three types of exchange sites have been identified for swelling 2:l clays: octahedral, tetrahedral, and edge sites. In reality, charge density may differ from one 2:l sheet to another or from one domain to another on a single 2:l sheet (25). The correlation of cation selectivity coefficient with charge density (26) may therefore result in a heterogeneous adsorption energy distribution among exchange sites of the same origin. For HDTMA adsorption, however, the most important difference in adsorption energy exists between interlayer and external sites, not between sites of different charge origin or density. This is because interactions between adsorbed HDTMA on the surfaces of opposing clay sheets and a reduction in the contact of the hydrophobic alkyl chains with H20 as a result of such interactions both stabilize the HDTMA-clay exchange complex in the clay interlayers. The absence of these interactions on external surfaces of the clays manifest weaker HDTMA-clay complexes as indicated by the fact
that little HDTMA was distributed on external surfaces at HDTMA loading levels less than 0.75 CEC (13). This important differenceis the primary rationale for considering the interlayer sites and external surface sites separately in the proposed model. It may be that more than one K, value exists for external sites; however, these differences, if existent, are too small to discern with current data precision. Although Z@, oo,P1 (or P2),Kv2 and e are all fitting parameters, only e is freely variable (Table 1); the other parameters are subject to certain constraints. Although oo may vary for different minerals, it was fixed for a given mineral. For SWy-1 for instance, roo was fixed at -8vRT (where Y = valency of inorganic exchangeable cation) regardless of ionic strength (Table 1). P1, designated as fraction of exchange sites whose selectivitycoefficients may change with HDTMAloading,varied from 0.74 for Na-SWy-1 to 0.83 for Mg-SWy-1, close to the estimated fraction of interlayer sites in flocculated montmorillonite (0.8-0.85). For a given cation (e.g., Na+) on a given surface (e.g., SWy-I),KO and Kv2 were all fixed regardless of ionic strength (Table 1). The single most important characteristic of the exchange submodel is the randomness factor e, which accounts for the different organiclinorganic cation distributions in the interlayers. The influence of cation type and ionic strength on e (Table 1) as revealed by the model fits was consistent with our understanding of the structure of the HDTMAclay complexes. As Xu and Boyd (15) reported, the initial degree of clay dispersion strongly influences the inorganic/ HDTMA distribution in clay interlayersat low loading levels. In dilute solutions of LiCl and NaCl, montmorillonite exists largely as separate elementary platelets ( 2 7 ,and therefore HDTMA can readily access all of the exchange sites. The adsorption of HDTMA on the exchange sites is followed by rapid formation of face-to-face aggregates. This results in a random distribution of HDTMA and Na on the clay surfaces and a loose structure of adsorbed HDTMA in the interlayers (1.3, consistent with the large e values. In contrast, the extensive face-to-face aggregation in Ca- and Mg-clays (27)or Cs-montmorillonitein 5 mM CsCl solution, restricts access of HDTMA to those interlayer exchange sites proximal to the edges of the clay particles. This results in aless random (more segregated) distribution of HDTMA and Na and a compact HDTMA adsorption layer in interlayers (13-15) and thus low values of 8. Similarly, lower ionic strength resulted in a higher degree of dispersion of Na-montmorillonite and higher e values (Table 1). The influence of e on the K, values can be understood by examining how the distribution of cations in the interlayers influences lateral hydrophobic (“tail-tail”) bonding of adsorbed HDTMAs. When organic cations are randomly distributed in the interlayers of swelling clays (large e), the distance between neighboring adsorbed HDTMA ions at low loading levels is too large to allow any significant lateral interactions and thus lol/ will be very small. The probability of lateral interactions between absorbed surfactants increases as surfactant loading increases, resulting in increased Iwil. The increase of Iwil, and hence the increase in Kv,i,with HDTMAloading is solely responsible for non-monotonic HDTMA adsorption isotherms. In contrast, approaches zero when organic cations are segregated in the interlayers of flocculated clay, e.g., in Cs-, Ca-SWy-1,or Na-SWy-1 in concentrated NaCl solutions (Table 1). In these cases, lateral interactions will VOL. 29. NO. 12. 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
3025
TABLE 1
Parameters Used To Predict Cation Selectivity Coefficients Shown in Figures 2-4 minerals
bulk solution
Wyoming Montmorillonite (SWy-1) Li-SWy-I Na-SWy- 1 Na-SWy-I Na-SWy-I cs-swy-I Ca-SWy-I Mg-SWY-1 Na-kaolinite Na-illite
5 m M LiCl 5 m M NaCl 12 m M NaCl 44 m M NaCl 5 m M CsCl 2.5 m M CaC12 2.5 m M MgC12 5 m M NaCl 5 m M NaCl
KO
45 a 90 a 90 a 90 a 1.58 a ai0 16 1000 16 20 58 125 3.38 For these samples, K, does n o t change w i t h EaX!because Q = 0, a n d t h u s KO a n d ( ( l oare not separable. The
b y a s s u m i n g -rvo/RT= 8, a n d t h e values for - d / R T w e r e calculated by a s s u m i n g
-
be strong even at low loading levels ( E Q X I ~ ~1,' and loil = !di)due to the close proximity of the alkyl chains of adsorbed HDTMA in the segregated arrangement. The strong lateral interactions at all loading levels result in a large and rather constant K,,, and consequently monotonic adsorption isotherms. The linkage between the degree of randomness of organic cation distribution in the interlayers (e value) and the degree of clay dispersion prior to HDTMA addition is also a key to understanding the effect of ionic strength on the shape of HDTMA adsorption isotherms at low loading levels. Xu and Boyd ( 1 3 observed that the non-monotonic s-shaped isotherms describing HDTMA adsorption by Namontmorillonite at low NaCl concentrations changed to monotonic curves as the NaCl concentration increased to 20.01 M. This transition of non-monotonic to monotonic reflects the influence of ionic strength on the dependence of cation selectivity coefficients on HDTMA loading as shown in Figure 3 and is not predicted by any existing surfactant adsorption models. We argue that this transition results from a decrease in the randomness (e)of organic cation distribution in the interlayers due to a decrease in the degree of dispersion of Na-montmorillonite as ionic strength increased, similar to the effect of cation type discussed above. For Na-kaolinite and Na-illite,the selectivitycoefficients for Na to HDTMA exchange appeared as a step function of HDTMA loading (Figure 4), indicating the existence of two types of exchange sites with a constant K, value for each. The independence of K , on HDTMA loading, however, suggested that e was equal to zero for both types of sites, implying that Na and HDTMA are segregated on the surfaces of nonswelling clays, which is a more stable arrangement than that resulting from a random distribution. The random distribution of HDTMA and Na in the interlayers of swelling clays is a metastable state formed due to the fast kinetics of HDTMA adsorption (15). The conversion of such a metastable state to a m o r e stable state (Le.,segregation) is impeded by the extremely low mobility of the adsorbed HDTMA in the interlayers (15). On the surfaces of nonswelling clays or the external surfaces of swelling clays, this conversion is not impeded because the mobility of adsorbed HDTMA is not limited, and hence a segregated HDTMA distribution likely occurs. It is not clear exactly what these two types of exchange sites are. HDTMA adsorption on exchange sites causes flocculation of illite and kaolinite. Some adsorbed HDTMA from one clay particle may interact with the HDTMA adsorbed on other clay particles; some other HDTMA may 3026
ENVIRONMENTAL SCIENCE & TECHNOLOGY VOL 29. NO 12. 1995
P
1 .o
0.5
-
1.5
Pl
1.5 0.9 0.32 0.12 0 0.04 0.05 0 0
20 15 15 15 1.5 225 225 20 125
0.80 0.74 0.74 0.74 0.74 0.82 0.83 0.51 0.45
KO value for Cs-SWy-I was calculated
= Kv2.
,
no
,
,
,
,
,
I
0 42 rnM NaCl A 12 mM NaCl 0
8
Ku2
.
2.0 I
1.5
e
-&RT
4 rnM NaCl
I
,
I
0.04
0.06
1.5
1 .o
0.5
nn
0.00
0.02
.-5
p 1.0
B
2 < 1 I
o.81; 1 A
8 rnM NaCl
0.5
0.5 1.2 ,
0.0 0.000 0.005 0.010 0.015 0.020
.-
0.6 0.4
0.2
00
o; ;n
0
Square Root of Aqueous HOTMA Concentration
Na-Vermiculite 0 42 rnM NaCl 0 4 mM NaCl
0.0 0.00 0.02 0.04 0.06 Square Root of Aqueous HDTMA Concentration
FIGURE 5. Dependence of HDTMA adsorption on square root of aqueous concentration of HDTMA for layer silicates and a subsoil (Oshtemo Bt (14)).
simply be adsorbed on the external surfaces. It seems reasonable to speculate that the adsorbed HDTMA involved in interparticle interactions would be more stable than HDTMA adsorbed on surfaces in a direct contact with water. Hydrophobic Bonding. A striking characteristic of HDTMA-clay interaction is that HDTMA adsorption (prior to reaching its plateau) is linearly related to the square root of the aqueous HDTMA concentration once hydrophobic bonding commences (Figure 5). Interestingly, this simple relationship is applicable to soils as well as swelling and nonswelling clays. Furthermore, if we plot the HDTMA adsorption by hydrophobic bonding normalized to its adsorption plateau value (Le.,9~~/913~,..) against the square root of the surfactant concentration ratio [Le., (ClC,)] 2 , where C is the total surfactant concentration in aqueous solution (monomeric plus micellar surfactant concentration)], all of adsorption isotherms merge into a single isotherm with a slope of 1 at (C/C,)'12 4 1 and into a flat line (slope = 0) at (C/C,)1'2 2 1 (Figure 6). The observed
TABLE 2
Response of Hydrophobic Adsorption Plateau
( Q H B , ~to )
surf. chg. density kmol kg-’1
minerals swy-1 SAz-1 kaolinite Illite vermiculite (S.Carolina)
Variation in Ionic Strength and Anion Type
me,M response to ionic strength increase change from CI- to Br-
1.28 1.7 2.0 2.5
no change increase increase increase decrease
data sources 14, this study 15, this study 15, this study this study 13
no change increase increase increase increase
a Surface charge density w a s calculated based o n the f o l l o w i n g surface area: 750 m 2 g-’ for m o n t m o r i l l o n i t e s , 20 m 2 g-’ f o r kaolinite, and EO rn2kg-’ f o r illite (30).
1.2
-2.5
I
v Wyoming Montmorillonite
0
0 Arizona Montmorillonite
1 .o -3.0
0.8
A
Vermiculite
0
Kaolinite
8.
= “8
0.6
m
T
T
0.4
0
0.2
0.0
-
5.3 m M NaCl A 7.6 m M NaCl 0 2.7 mM CaCI,
0.5
1.0
1.5
2.0
-3.5
2.1 m M MgCI,
2.5
3.0
3.5
-4
.-n I
-3.0
( C / c y
FIGURE 6. Relation between HDTMA adsorption via hydrophobic bonding (me) normalized to the adsorption plateau ( me,J and the aqueous concentration of surfactant (C)normalized to the surfactant monomer concentration at the adsorption plateau (CJ
1:lrelationship betweenqHB/qHB,,and (C/C,)”* at (C/C,)1/2 5 1 completely verifies the hydrophobic adsorption submodel that we have proposed. Because C, is about identical to the cmc of HDTMA, the flat line at (C/C,)1/2 2 1 reflects
the fact that micellar HDTMA is not adsorbed and that the amount of adsorbed HDTMA is determined only by the concentration of surfactant monomers, which is constant above the cmc, thereby justifymg the use of C, in eq 18. Obviously, the two most important parameters for predicting the adsorption of HDTMA by hydrophobic bonding are the adsorption plateau (~HB,,) and the surfactant concentration corresponding to the plateau (C,), As pointed out, the critical concentration C, is approximated by the cmc of HDTMA. In addition, C, depends on ionic strength and anion type in the same way as the cmc does. For example, it has been reported that counterion type has a substantial influence on the cmc of ionic surfactants (27), and for a given counterion,log cmc is linearly related to the logarithm of the salt concentration (28). Similarly,we found that in C1- salt solutions, log C, of HDTMA = -4.93-0.79 log C,,~,withR2= 0.989 (the degree of freedom = 18) (Figure 7). At the same anion concentration, C, for HDTMA-Br or HDTMA-SO4 were lower than for HDTMA-C1 (Figure 7). The change of qHB,.. with anion type and ionic strength is strongly dependent on clay type. We do not yet have a model that quantitatively links ~ H B , , with anion type, ionic strength, and clay type. We have found however that the effect of anion type and ionic strength on ~HB,.. can be put into three categories (Table 2). For HDTMA adsorption on low-charge montmorillonite (e.g., SWy-l), ~ H B , , does not change with anion type or ionic strength. This is a
\ -2.5
-2.0 log
-1.5
-1.0
Cti.,
FIGURE 7. Dependence of the aqueous phase surfactant monomer concentration at the hydrophobic bonding adsorption plateau (C,) on salt concentration in solution (in mol L-l).
manifestation of the strong HDTMA-surface interactions resultingfrom the flat-lying bilayer or pseudo-trimolecular layer arrangements of HDTMAin the interlayers (15). These arrangements prevent the alkyl chains of HDTMA adsorbed on exchange sites from adopting a vertical orientation, which imposes a limit on the HDTMA adsorption capacity, ~ H B , - (15). For high charge montmorillonite (e.g., SAz-1) or nonswelling clay-like kaolinite (15) and illite (Xu and Boyd, unpublished data), near-saturation or saturation of exchange sites with HDTMA resulted in paraffin type structure of adsorbed HDTMA (15). In this vertical orientation the HDTMA tail-surface interaction is weak and thus does not impose a limitation on further packing of HDTMA into those layers via hydrophobic (“tail-tail”) interactions in response to higher ionic strength or changing fromHDTMA-C1 to HDTMA-Br or HDTMA-S04. Finally, in high charge swelling clay such as vermiculite, qHB,.. decreased with ionic strength (13). This occurred because high ionic strength promoted inorganic cation entrapment in clay interlayers by adsorbed HDTMA. This results in a lower density of interlayer HDTMA, which may ultimately participate in hydrophobic bonding, and therefore the aggregates are not dismantled due to the buildup of excessive charge. This manifests a decrease in the surface area that would otherwise be available for HDTMA adsorption via hydrophobic bonding ( 1 3 .
Conclusions Interlayer swelling of clays has tremendous influence on cationic surfactant adsorption via cation exchange and hydrophobic bonding mechanisms. This influence can be accounted for by a new model comprised of cation exchange and hydrophobic bonding submodels. The cation subVOL. 29, NO. 12, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
3027
model uses a randomness parameter, e, to account for the variation of cation selectivity coefficient arising from differences in the distribution of inorganic and surfactant cations. A random distribution of inorganic and surfactant cations (Q > 0) is observed in well-dispersed clays resulting in a s-shaped adsorption isotherm at surfactant loadings of
