Langmuir 1996,11, 2508-2514
Cationic Surfactant Adsorption by Swelling and Nonswelling Layer Silicates Shihe Xu and Stephen A. Boyd* Department of Crop and Soil Sciences, Michigan State University, East Lansing, Michigan 48824 Received September 1, 1994. I n Final Form: April 24, 1995@ The adsorption of hexadecyltrimethylammonium (HDTMA) in both swelling (montmorillonite) and nonswelling (kaolinite) layer silicates was studied by adsorption isotherms, X-ray diffraction and electrophoreticmobility. The objectiveswere to determine the effects of clay type and solution composition on the adsorption of cationic surfactants and to reveal the relation between the structure of the adsorbed surfactant layer and the stability of surfactant-clay complexes. We found that HDTMA adsorption by montmorillonites was more complex than by a nonswelling clay such as kaolinite. The major complexity arose from the fact that the structure of adsorbed HDTMA layer in the interlayers of montmorillonites depended strongly on the initial degree of clay dispersion.Highly dispersed clays (e.g.,Na-montmorillonite in dilute NaCl solutions)lead to a random HDTMA distribution on the surfaces, larger distances between silicate layers, and hence reduced lateral interactions between the alkyl chains of adsorbed HDTMA. Accordingly,cation selectivity coefficients were low at low HDTMA loadings and increased dramatically as HDTMA loading increased, resulting in a unique S-shaped adsorption isotherm, which has not been observed for nonswelling clays. Initially flocculatedclays (e.g., Na-montmorillonite in concentrated NaCl solutions or Ca- or Cs-montmorillonite)lead to HDTMA segregation and more compact adsorption layers, which enhanced the lateral interactions between alkyl groups. As a result, the cation selectivitycoefficients were high even at low HDTMAloadings and changed little as loadingincreased,similarto that for nonswelling clays. Both cation exchange and hydrophobicbonding were involvedin HDTMA adsorption at high loadings (e.g, '0.75 CEC). Charge density of clays, ionic strength of the bulk solution, and the type of companion anions all affect the adsorption of HDTMA via hydrophobic bonding.
Introduction Understanding cationic surfactant adsorption by layer silicates is of great importance due to the wide spread use of these compounds in both household and industrial activities,l and to the occurrence of layer silicates in soils, subsoils, and sediments. Furthermore, recent studies suggest the potential usage of cationic surfactants in the remediation of contaminated subsoils and aquifers.2-7The remediation approach is based on the fact that the inorganic exchangeable cations of naturally occurring layer silicate clays can be readily replaced by cationic surfactants such as quaternary ammonium compounds (QAC). This results in organo-clays with greatly increased capabilities to remove hydrophobic organic contaminants (HOCs) from aqueous solution.2-6 Increasing HOC sorption in this fashion has been demonstrated for both QAC modified clays (smectite,vermiculite, and illite) and soils, offering a means to substantially reduce the transport potential of contaminants through engineered clay barriers, soil profiles, and aquifer^.^ The successful implementation of the proposed environmental remediation technology based on clay modification depends in part on the completeness of the exchange of inorganic cations by QACs and on the chemical stability of cationic surfactant-clay c o m p l e x e ~ .The ~~~ nature of both the QAC and the clay, and the composition Abstract published in Advance A C S Abstracts, J u l y 1, 1995. (1)Linfield, W. M. In Cationic surfactants;Jungermann, Ed.; Marcel Dekker, Inc.: New York, 1970. (2)Boyd, S. A.;Lee, J. L.; Mortland, M. M. Nature 1988,33,345. (3)Boyd, S.A.;Mortland, M. M.; Chiou, C. T. Soil Sci. SOC.Am. J . 1988,52,652. (4) Boyd, S.A.; Sun, S.;Lee, J. F.;Mortland, M. M. Clays Clay Miner. 1988,36,125. ( 5 ) Jaynes, W. F.; Boyd, S. A. Soil Sci. SOC.Am. J . 1991,55,43. (6)Lee, J. F.; Crum, J.;Boyd, S. A. Enuiron. Sci. Tchnol. 1989,23, 1365. (7) Burris, D. R.; Antworth, C. P. J. Contam. Hydrol. 1992,10,325. (8) Xu, S.;Boyd, S. A. Soil Sci. Sci. Am. J . 1994,58,1382. (9)Xu, S.;Boyd, S. A. Environ. Sci. Technol. 1995,29,312.
of aqueous solution, may all affect the stability of QACclay complexes. On montmorillonites, the Gibbs free energy change for the exchange of inorganic cations by alkylammonium cations increases linearly with the molecular weight of the organic cations.1°-12 Small alkylammonium compounds (e.g.,n-butylammonium) are only adsorbed by cation exchange mechanism,13whereas QACs with chain lengths greater than a critical value (e.g., eight carbons) are adsorbed via both cation exchange and hydrophobic bonding.14-16 In addition to the QAC size, the composition of electrolyte solutions may also affect the chemical stability of QAC-clay complexes by influencing the degree of QAC adsorption via hydrophobic bonding. Our previous studies with soils have demonstrated that higher ionic strength resulted in increased QAC adsorption by hydrophobic b ~ n d i n g .Changing ~ the companion anion from C1- to Br- or S042-also increased QAC adsorption via hydrophobic b ~ n d i n g .Finally, ~ clay type also affects organic cation adsorption by determining the arrangement of adsorbed organic cations in the clay interlayers and on the clay surface^.^^^ Several models have been proposed to account for surfactant adsorption on solid surfaces. These models are distinguishable by their assumptions of the structures of the adsorbed surfactant layers. The hemimicelle model has been used successfully for the adsorption of anionic surfactants onto strongly hydrated oxide s u r f a ~ e s . l ~ - ~ ~ According to this model, the initial anionic surfactant adsorption is by simple ion exchange. Then, at a particular (10)Theng, B. K. G.; Greenland, D. J.; Quirk, J . P. clay Minerals 1967,7,1. (11)Vansant, E. F.; Peeters, J. B. Clays Clay Miner. 1978,26,279. (12)Vansant, E. F.; Uytterhoeven, J. B. Clays Clay Miner. 1972,20, 47. (13)Grim, R.E.; Allaway, W. H.; Cuthbert, F. L. J.Am. Chem. SOC. 1947,30, 137. (14)Cowan, C. T.;White, D. Trans. Faraday SOC.1958,54,691. (16)Kurlenko, 0. D.; Mikhalyuk, R. V. Kolloidn. Zh. 1959,21,181. (16)Rosen, M. J.Surfactants and Interfacial Phenomena John Wiley & Sons: New York. (17)Fuerstenau, D. W. Pure Appl. Chem. 1970,24, 135.
0743-746319512411-2508$09.00/00 1995 American Chemical Society
Cationic Surfactant Adsorption by Silicates concentration (before saturating the anion exchange capacity of oxides) known as the critical hemimicelle concentration, adsorption increases drastically due to formation of hemimicelles.lB According to Cases,20,21 surfactant-solid systems can be divided into two categories designated as “weaknormal adsorbate-adsorbent bond systems” and “strong normal adsorbate-absorbent bond systems”. The former refers to systems in which surfactants have low affinities for mineral surfaces. Adsorption is driven by lateral interactions leading to the formation of a structure called “ a d m i c e l l e ~ ” ~where, ~ - ~ ~ prior to the formation of a complete surfactant monolayer,adsorbed surfactant molecules interact laterally with additional surfactant molecules not contacting the surface. Surfactant adsorption on layer silicates such as mica is of the “strong normal bond type” indicating strong interactions between the surfactant and the clay surfaces. Accordingly, twodimensional aggregation occurs in which a complete vertical monolayer forms followed by the formation of a bilayer.21 On homogeneous surfaces, two-dimensional condensation results in stepwise adsorption isotherms, but for heterogeneous surfaces, the individual steps may not be distinguishable.20,21 Recently, Bohmer and Koopa126,26 described surfactant adsorption on nonswelling clay surfaces using the selfconsistent field lattice model. They calculate, rather than assume, the structure of the adsorbed layers. On the surface of nonswelling clays, the calculation reveals no apparent condensation loops as predicted by the twodimensional condensation model. Instead, it predicts a gradual increase in surfactant a d s o r p t i ~ n ,similar ~ ~ , ~ ~to that observed in clays27-29and a s u b ~ o i l . ~ Most experimental data supporting the above models have been obtained with nonswelling solids. Swelling layer silicates will almost certainly manifest different surfactant adsorption characteristics because the structure of the adsorbed cationic surfactants on swelling clays is quite different from that on nonswelling clays. On all of the tested nonswelling clay surfaces, surfactants always adopt a vertical (paraffin type) arrangement. The adsorbed organic cations in swelling clays such as montmorillonites and vermiculites may adopt (after drying) several configurations in the clay interlayers commonly referred to as flat-lying monolayer, bilayer, pseudotrimolecular layer, as well as the vertical paraffin type, depending on layer charge density of the clay and the size of the QAC.30-33The exact structures of the adsorption layers of organic cations a t water saturation have not yet been reported for swelling clays. The various types of arrangements determine the van der Waals contact (18) Somasundaran, P.; Fuerstenau, D. W. In Adsorption from Aqueous Solution. Adv. Chem. Series No. 79; American Chemical Society: Washington, DC, 1968. (19) Wakamastsu, T.; Fuerstenau, D. W. J. Phys. Chem. 1966,70, sn ””.
(20) Cases, J. M.; Mutaftschiev, B. Surf.Sci. 1968,9, 57. (21) Cases, J. M.; Villieras, F. Langmuir 1992,8,1251. (22)Yeskie, M. A.; Harwell, J. H. J. Phys. Chem. 1988,92,2346. (23) Harwell, J. H.; Hoskins, J. C.; Schechter, R. S.; Wade, W. H. Langmuir 1985,1, 251. (24) Bitting, D.; Harwell, J. H. Langmuir 1987,3, 500. (25) Bohmer, M. R.; Koopal, L. K. Langmuir 1992,8,1594. (26) Bohmer, M. R.; Koopal, L. K. Langmuir 1992,8,2649. (27)Brahimi, B.; Labbe,P.;Reverdy, G . Langmuir 1992,8 , 1908. (28) Zhang, Z. 2.; Sparks,D. L.;Scrivner,N. C.Enuiron. Sci. Technol. l99S,27,1625. (29) Brownawell,B. J.;Chen,H.;Collier,J. M.;Westall,J. C.Enuiron. Sci. Tchnol. 1990,24, 1234. (30)Jordan, J. W. J. Phys. Colloid Chem. 1949,53,294. (31) Lagaly, G. Clays Clay Miner. 1982,30, 215. (32) Lagaly, G.;Weiss, A. In Proceedings of the International Clay Conference, Tokyo, 1969; Heller, L., Ed.; Isreal University Press, Jerusalem, 1969; Vol. 1, pp 61-80. (33) Lagaly, G.;Weiss, A. Kolloid. 2.2.Polym. 1971,243,48.
Langmuir, Vol. 21, No. 7, 1995 2509 between the mineral surfaces and the alkyl chains of the QACs and between the alkyl chains themselves, both causingvariation in stability of the QAC-clay complexes. The present study has two objectives. First, to evaluate the influence of clay type (swelling versus nonswelling) and solution composition (especially, the type and concentration of electrolytes) on the adsorption of a model QAC, hexadecyltrimethylammonium (HDTMA). Secondly, to determine the arrangement of HDTMA adsorbed in montmorillonite interlayers at water saturation, and its influence on the stability of QAC-clay complexes. Ultimately, this knowledge is needed to understand the environmental chemistry of this important class of compounds.
Experimental Section Chemicalsand Clays. Hexadecyltrimethylaonium chloride (cmc = 1.3m M 9 and bromide (cmc = 0.9 m M 1 6 ) from Fluke were used without further purification. Hexadecyltrimethylammonium sulfate was made by mixing equimolar HDTMA hydrogen sulfate and HDTMA hydroxide, both from Fluke. The 14C-labeledhexadecyltrimethylammonium iodide was obtained from American Radiolabelled Chemicals (St. Louis, MO). Wyoming montmorillonite (SWy-1, CEC = 0.9 mol, kg-l), Arizona montmorillonite (SAz-1, CEC = 1.25 mol, kg-l), and a kaolinite (CEC = 0.04 mol, kg-1 a t pH 6.5) were obtained from the Source Clays Repository, Dept. of Geology, University of Missouri, Columbia, MO. The clays were treated with 30%H202 for 18h to remove organic matter. The clays were then saturated with Na by washing 10 gof clay with 250 mL of 1M NaCl solution 3 times. The equilibrium time was 8 h for each washing.9 Different size fractions of Na-clay were separated by wet sedimentation. The ‘0.2 pm size fraction was collected and stored in 0.01 M NaCl solution. Other homoionic clays (Cs- and Ca-clays)were made by washing Na-clays with the corresponding salt solutions (CsCl or CaC12). Adsorption Isotherms, Electrophoretic Mobility, and Clay Dispersion. For HDTMA adsorption, a volume of clay suspension containing -90 pmol, exchangeable cations (Na+, Cs+, Ca2+,and Mg2+)(e.g., 0.1 g of Na-SWy-1) was pipeted into a 25 mL Corex tube and the ionic strength of the initial solution was adjusted to a designated value using standardized NaC1, NaBr, Na2S04, CsC1, CaC12, or MgC12. Various amounts of HDTMA solutions containing 2.25-450 pmol of HDTMA with 14C-labeledHDTMA (1abeled:nonlabeled= 1:5000)were pipeted into the clay suspensions and mixed gently. After one day, a calculated volume of salt solution (NaCl, NaBr, NazS04, CsCl, CaC12, or MgC12) was added into tubes with HDTMA loading < 1 CEC so that all samples had the same final concentration of inorganic cations. The samples were allowed to equilibrate for 5 days with occasional gentle mixing using a rotating shaker. The time required to reach equilibrium was previously determined experimentally. At the end of the equilibrium time, the clay was gently mixed and then allowed to stand without disturbance for 2 h before light absorption a t 375 nm by the upper 5 mL of suspension was measured using a UV-vis spectrophotometer (8452 A diode array spectrophotometer, Hewlett Packard) to determine the optical density of clay suspension. The clay-water suspension was then centrifuged a t 9680g for 25 min. An aliquot of supernatant was analyzed for 14C-activityby liquid scintillation counting (Tri-Carb 1500Liquid Scintillation Analyzer, Packard) and for inorganic cation release by atomic absorptiodflame emission (Perkin-Elmer, 1100B).Most of the remaining supernatant was transferred to a Teflon electrophoretic cell. The clay pellet was redispersed in the residual (-3 mL) supernatant and 0.1 mL of this suspension was added to the cell which contained supernatant from the same sample to make a dilute suspension (-0.01%). The electrophoretic mobility of 25-50 particles was recorded at 50 V for each sample using a ZetaMeter (ZetaMeter 3.1from Zeta Meter, Inc., New York). All of the above procedures were conducted at pH -6.0 and 25 “C. X-ray Diffraction. X-ray diffraction patterns of clays with various HDTMA loadings were determined at three water contents using a X-ray diffractometer (Philips APD3720, Philips Electronic Instrument Inc., Mahwak, NJ). The water-saturated clay pellet was smeared onto a glass slide and X-ray diffraction
2510 Langmuir, Vol. 11, No, 7, 1995
Xu and Boyd 1 .o
"1
0.8 0
'z x
0.6
0 0
-D
0.4
.-
0"
0.2
0.0 0.5 1.0 1.5 HOTMA Adsorbed (CEC)
0.0 -6 log,,,,C
-5
-4
-3
(C in W
-2
1.0 1.5 2.0 HOTMA Adsorbed (CEC)
0.0 0.5
3 4 5 6 7 0 9 20
Figure 1. (a) Adsorption of HDTMA by Na-SWy-1 in 5 mM NaCl solution and the concomitant Na release. (b) Electrophoretic mobility and the degree of dispersion of HDTMA-(Na)SWy-1 as HDTMA loading increased (normalized to CEC).
patterns were recorded under 100%relative humidity (RH).The glass slide then was transferred into a desiccator containing saturated NaZHP04 solution (95%RH at 25 "C) and allowed to equilibratefor 3 days. Ten grams of Na-SWy-1wrapped in filter paper was also placed in desiccator,and later was placed in the sample chamber ofX-ray diffractometer to buffer humidity.After X-ray diffraction peaks of clays were recorded at 95% RH, the glass slide was then placed over dry CaClz in the desiccator (< 1% RH) for 3 days, after which the X-ray diffraction pattern was recorded. Results Low Charge Montmorillonite: SWy-1. 1. Basic Characteristicsof HDTMA Adsorption by Na-SWy1. Adsorption Isotherm. Adsorption isotherms of HDTMA by Na-SWy-1 can be divided into four distinct regions. Region 1ofthe isotherm (0-0.75 CEC) was nonmonotonic (Figure l),characterized by a n equivalence of Na release and HDTMA adsorption, resulting in superimposible HDTMA adsorption and cation release curves (Figure la). In regions 2 and 3, the adsorption isotherm was monotonic and the amount of HDTMA adsorption was greater than the corresponding Na release (Figure la). The slope of the HDTMA adsorption isotherm in region 2 was much smaller than in region 3 (Figure la). The HDTMA adsorption curve plateaued in region 4 (Figure l a ) at a point corresponding to the formation of HDTMA micelles in solution (Le., the critical micelle concentration). Electrophoretic mobility. As the surface loading of HDTMA increased in region 1,the electrophoretic mobility of clay particles remained the same a s for homoionic NaSWy-1(Figure lb). The electrophoretic mobility increased rapidly to a maximum value in region 2 and then decreased slightly in region 3; it did not change in region 4. The Degree of Clay Dispension. The degree of clay dispersion, a s indicated by the optical density of the clay suspension, was high in the absence of HDTMA (i.e., for Na-SWy-1) (Figure lb). Adsorption of HDTMA caused clay flocculation and decreased the optical density of the clay suspension. The degree of clay dispersion was reduced to zero a t the end ofregion 1. The degree of clay dispersion remained minimal in region 2, but increased with HDTMA loading in region 3. No changes in optical density were observed in region 4 (Figure lb). X-ray Diffraction. The d-spacings of wet (water saturated),partially air-dried (dried a t 95%relative humidity), and dry (dried a t 5 1%relative humidity) clays were nearly identical a t high HDTMA loadings, but completely different a t low HDTMA loadings ( 4 1CEC) (Figure 2). No X-ray diffraction peaks were found for wet Na-SWy-1 or when the amount of adsorbed HDTMA was 50.05 CEC.
,
24 .
-
22
--------
w
E
e
c
5
-
20
&
"0
18-
0.
p!
0..
Ln YI
A
.-
16/A
l4
'AAA'
A
_ _ _ _ _ _ A-A
:
~
CO- H OTMA
A
0
v
U
A
12 i. n_
0.00
100% R.H. 1 CEC. Completely drying the clays (51% RH) resulted in further decreases in the d-spacing to 5 13.6 A a t HDTMA loading 50.3 CEC and eliminated all the double peaks resulting in a three-step curve (Figure 2). The d-spacings of the mixed HDTMA-Ca-SWy-1were different from those of the HDTMA-Na-SWy-1 (Figure 3). For example, the d-spacings ofwet HDTMA-Ca-SWy-1 a t 0.2-0.95 CEC (17.6-18 A) were several angstroms less than that of HDTMA-Na-SWy-1 a t the corresponding loadings. In addition, the d-spacing change of dry HDTMA-Ca-SWy-1was more gradual than that of HDTMA-Na-SWy-1. 2. Cation Effects. Exchangeable cations on SWy-1 greatly influenced the shape of the HDTMA adsorption
Cationic Surfactant Adsorption by Silicates
Langmuir, Vol. 11, No. 7, 1995 2511 1
2.00,
0
P
._ -Q 0
E z
1.50
-
1.50 '0
.-
N
1.25 1.00
i
-
.-*
0 ._
0.75
.
< 0.50
fn
'A
0.25
A
A A
1.00
0
L
I
1.25
1 -
$ YI
0.75 -
2 u E
0.50
-
0.25
-
0
A
log
-2.5
42 m H NoCl 12 m H NoCl 7 mM NaCl v 4 mM NoCl
0
I
A
n v n v a v
A
0
A
-3.5 -3.0
xA
A*
A. O A
90 A
0.00 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0
v v
@ V
0
a a'o
i
',"
V
-
V v
-2.0 log CHrJTMA
'HDTMA
Figure 4. HDTMA adsorption by Na-, Cs-, and Ca-SWy-1in 4 mM C1- solutions.
Figure 6. Influence of NaCl concentration on HDTMA adsorption by Na-SWy-1.
4.0 3.5
2.5
3.0
-
i
v 0
v
v
CI Br
B
V
.
V
.
-Y
3.0 2.5
V
v
c 2.0
.-c0
n
8
1.5
B
3
1.0
L I 0.5
-7
-6
-5
-4 log
-3
-2
0.0
-7 -6 -5 -4 -3 -2 -7 -6 -5 -4 -3 - 2
cHDTMA
Figure 5. Influence of companion anions on HDTMA adsorption by Na-SWy-1. isotherms (Figure 4). At a constant C1- solution concentration, Na manifested nonmonotonic adsorption isotherms with low HDTMA affinity a t low loading levels (e.g., at the beginning of region 1). Clay saturated with Cs and Ca resulted in monotonic adsorption isotherms and higher HDTMA affinities in this region. Exchangeable cation types, however, had no discernible effect on HDTMA adsorption in regions 3 and 4 (Figure 4). 3. Anion Type Effects. The effects of companion anions on HDTMA retention were complex and generally manifested lower aqueous HDTMA concentrations at constant HDTMA loading levels (a leftward shift of the adsorption isotherms) as anions changed from C1 to Br and SO4(Figure 5). In addition, HDTMA-SO4 resulted in a higher adsorption plateau than HDTMA-Br; X-ray diffraction demonstrated that crystalline HDTMA-Br formed in samples with total HDTMA retention greater than 1.76 CEC, which disappeared after extracting the sample with water once at 30 "C. X-ray diffraction of clays in HDTMA-S04 solution did not show crystalline phases other than HDTMA-SWy-1. 4. Ionic Strength Effects. At low HDTMA loadings, the shape of HDTMA adsorption isotherms in Na-SWy-1 depended on ionic strength (Figure 6). At low ionic strength, nonmonotonic isotherms were observed, and as the ionic strength increased, the isotherms became monotonic. At high loading levels, HDTMA adsorption increased with ionic strength, however, the adsorption plateau did not change. Ionic strength has little influence on HDTMA adsorption by Ca-SWy-1 at low loading but
'0g
cH DTMA
40
'
20 0.0
,
I
1
1.0 1.5 2.0 HDTMA Adsorbed (CEC) 0.5
I 2.5
Figure 7. Influence of (a) NaCl concentrations and (b) companion anion type on HDTMA adsorption by Na-SAz-1, and (c) the d-spacings of water-saturated HDTMA-Na-SAz-1 as influenced by HDTMA loadings (normalized to CEC). increased HDTMA adsorption via hydrophobic bonding a t high H D T W loading levels (data not shown). High Charge Montmorillonite: SAz-1. HDTMA adsorption on SAz-1 was similar to that on SWy-1 in several respects. First, the isotherms for Na-clay are nonmonotonic at low ionic strength and changed to monotonic as ionic strength increased (Figure 7a) or as the exchangeable cation changed from Na to Ca (data not shown). Second, adsorption isotherms for both can be divided into four similar regions (Figure 7a,b). Thir$, the d-spacing of wet samples of SAz-1 started a t 21.6 A and increased linearly with HDTMA loading (Figure 7c). Finally, the change in electrophoretic mobility with HDTMA loading level occurred as a step function for both clays (data for SAz-1 not shown).
Xu and Boyd
2512 Langnuir, Vol. 11, No. 7, 1995 3 . 0 , .
$
.
,
.
,
.
,
,
,
.
2.5 i
v
mM
42
'f
V V
V
**
V
a c
4 4
V
1.01
.*
V ~
0.01
-a
' * ' -7
'
' -6
V '
' -5
'
' -4
*e
**
'
' -3
'
I
-2
log CHDTMA
Figure 8. Adsorption of HDTMA by Na-kaoliniteat two NaCl concentrations.The CEC of this kaolinite is 0.04 molc kg-l at the pH 6.5.
There are also some differences in HDTMA sorption characteristics on the two clays. First, the adsorption plateau of HDTMA on SAz-1 varied with ionic strength (Figure 7a) and anion type (Figure 7b), whereas no change was observed for SWy-1 (Figures 5 and 6). Second, the $-spacings increased with HDTMA loadiag to a point (36 A) much beyond the upper limit (22.1 A) observed for SWy-1 (Figure 2). Nonswelling Clay: Kaolinite. The only similarity in HDTMA adsorption by kaolinite and montmorillonite was the shape of the isotherm in regions 3 and 4 (Figure 8). The most distinct differences between montmorillonite and kaolinite was the nonmonotonic HDTMA adsorption isotherms observed for Na-montmorillonite versus the monotonic isotherm observed for Na-kaolinite even a t very low NaCl concentrations. In addition, the surface area normalized adsorption of HDTMA on kaolinite was much higher than that on montmorillonites. For example, the HDTMA adsorption plateau in kaolinite is 0.068-0.082 mol kg-l, only ca. one-twentieth of that in SWy-1. However, the adsorption density for kaolinite (ca. 4.55.4 pmol m-2) was 2.3-2.7 times that of SWy-1, if we assume the surface area of kaolinite is around 20 mz 8-l. Finally, a n increase in inorganic cation concentrations resulted in decreased total HDTMA adsorption a t low loading levels and, hence, a crossoverof isotherms obtained a t different ionic strengths (Figure 8).
Discussion Adsorption Isotherm Shape. In region 1, HDTMA was adsorbed strictly by cation exchange as indicated by the release of a n equivalent number of inorganic cations (Figure la), similar to region 1 of SDS isotherm on o ~ i d e s . 'The ~ ~ decrease ~~ in optical density (Figure lb) suggested the formation of clay aggregates as HDTMA replaced Na, consistent with other investigations.27 The fact that increase in HDTMA adsorption caused no change in electrophoretic mobility in this region (Figure lb) suggested that little adsorbed HDTMA was distributed on the external surfaces of the aggregates a t the equilibrium, electrophoretic mobility being sensitive only to the composition of the external surfaces.34 The driving force of this nonuniform cation distribution of HDTMA in the clay aggregates is to maximize the extent of hydrophobic interactions of adsorbed HDTMA molecules and to decrease the contact of hydrophobic C-16 moieties of adsorbed HDTMA with the aqueous phase, both of which (34)Bar-On, P.; Shainberg, I.; Michaeli, I. J. Colloid Znterface Sci. 1970,73,471.
stabilize the HDTMA-clay complexes in the interlayers. The unique nonmonotonic isotherm observed in region 1 for SWy-1has not been observed for any nonswelling clays (e.g., kaolinite and mica). The cause of this phenomenon is related to the swelling of clay and will be discussed in detail later. In region 2, HDTMA adsorption occurred via cation exchange and hydrophobic bonding (Figure la). The steep increase in electrophoretic mobility (Figure lb) with a small increment of HDTMA loading implied a dramatic change in the composition of the external surfaces of the aggregates due to HDTMA adsorption. The development of positive electrophoretic mobility in region 2 implied that a t least some HDTMA adsorbed via hydrophobic bonding was distributed on the external surfaces of aggregates. The positive charge, if sufficiently large in the interlayers, can dismantle the aggregates and increase the degree ofclay disper~ion.~ The constant and low degree of clay dispersion (Figure lb) indicates that any positive charge developed in the interlayers by hydrophobic bonding in this region was not sufficiently large to break the aggregates. Region 3 was characterized by extensive HDTMA adsorption via hydrophobic bonding (Figure la). The gradual increase in the degree of clay dispersion, with very little change in electrophoretic mobility as HDTMA loading increased, suggested the dismantlement of faceto-face association caused by HDTMA adsorption via hydrophobic bonding in the interlayers. The density of HDTMA held via hydrophobic bonding on the resultant surfaces is similar to that on the original aggregate surface, and hence the electrophoretic mobility remains constant. As the aqueous HDTMA concentration reached the cmc (region 41, the adsorption of HDTMA plateaued, indicating the absence of micelle adsorption on the clay surfaces.35 Structure of the Adsorbed HDTMA Layer. The stepwise change in the d-spacings for dry SWy-1 as HDTMA loading increased demonstrated different specific arrangements of HDTMA in the interalyers of montm 0 r i l l o n i t e 3 ~ -(Figure ~ ~ , ~ ~ 2). The thickness of a montmorillonite platelet is 9.6 A33and the gallery height of a flat-lying alkyl chain in the clay interlayers can be 4 or 4.5 A, depending on its ~ r i e n t a t i o n .The ~ ~ three steps in Figure 2 suggested a monolayer arrangement (13.6 or 14.1 A) at HDTMA loadings 50.47 mol kg-l (0.54 CEC). Increasing HDTMA loading beyond 0.55 CEC caused a stepwise d-spacing change cqrresponding to a bilayer arrangement (17.6 apd 18.1 A). A pseudotrimolecular layer (21.6 and 22.1 A) resulted from further increase in HDTMAloading beyond 0.92 mol kg-l(1.05 CEC). Jordan et aL3*observed a similar stepwise d-spacing change as a function of octadecylammonium loading on oriented dry montmorillonite film. Treating the HDTMA as a tetrahedral head group plus a linear C-15 alkyl chain, the total surface area occupied by each flat-lying HDTMA will be 120 Az assuming a vertical zigzag planez7or 129.2 Azwith a parallel zigzag plane. If we assume total area of water-solid interface is 750 mz kg-', a densely-packed HDTMA monolayer will account for 0.52 (vertical zigzag plane) or 0.48 mol (parallel zigzag plane) of HDTMA per kg of Na-SWy-1. The latter value is very close to the observed HDTMA loading a t the monolayer to bilayer transition (0.47mol kg-l), suggesting (35)Hough, D.B.; Rendall, H. M. InAdsorptionfrom solution at the solidlliquid interface; Parafitt, G . D., Rochester, C. H., Eds; Academic Press: New York, 1983;p 247. (36)Lagaly, G.InLayer Charge Characteristics of Clays; Pre-meeting workshop; CMS-SSSA, 1992. (37)Brindley, G.W.;Hoffmann, R. W. Clays Clay Miner. 1962,9, 546. (38)Jordan, J. W.;Hook, B. J.; Finlayson, C. M. J. Phys. Colloid Chem. 1950,54, 1196.
Langmuir, Vol. 11, No. 7, 1995 2513
Cationic Surfactant Adsorption by Silicates the formation of a densely packed HDTMA monolayer withaparallelzigzagplane(i.e., 14.1A)inthedrysamples. We must emphasize that the arrangement of organic cations in the interlayers discussed above has been deduced from X-ray diffraction data of “dry” or “driedrewetted” clay samples. Comparing the d-spacings of dry clays with partially dry or water-saturated samples revealed that drying altered the interlayer arrangement of HDTMA a t loadings < 1.2 CEC (Figure 2). Therefore, structures deduced from “dried” clay samples do not accurately represent those in aqueous solution and have little relevancy to the stability of HDTMA-clay complexes under ambient environmental conditions. Although a monolayer was indicated in the dried samples, it did not form in solution even at very low HDTMAloadings. The d-spaciags ofwet HDTMA-SWy-1 were neyer smaller than 17.6 A and increased from 17.6 to 22.1 A in region 1. These d-spacings are greater than expected for a densely packed monolayer or bilayer, indicating very loose and probably less ordered arrangements in the wet state. Partially drying of organoclay at 95% RH resulted in the formation of compact bilayers and monolayers a t HDTMA loading 50.55 CEC which were evident from the double peaks at loadings between 0.3 and 0.55 CEC. It is not clear whether this heterogeneous distribution of HDTMA existed in original wet samples or resulted from drying. Despite the confounding effect of moisture on the organoclays studied, these data did provide critical information on structure of the HDTMA adsorption layer which has been the subject of some previous disagreement in the literature.21,25 On the basis of the shapes of the HDTMA adsorption isotherms, montmorillonites represent clays having strong normal interactions with surfactants according to Cases’ classification.21 Our observation of stoichiometric Na release and a lack of change in electrophoretic mobility demonstrate that HDTMA adsorption via hydrophobic bonding does not play a significant role in region 1,a phenomenon contradicting the concept of a d m i c e l l e ~ . Although ~~ the adsorption involved both cation exchange and hydrophobic bonding in regions 2 and 3, no vertical configuration (i.e., paraffin complex)was observed for the low charge montmorillonite (SWy-1) even a t the highest HDTMA loading. The flatlying configuration of HDTMA in montmorillonites is inconsistent with the hemimicelle concept17or the twodimensional condensation mode1.20,21 Cation ”ype and Ionic StrengthEffects on Cation Exchange. The shapes of HDTMA adsorption isotherms in region 1can be either nonmonotonic or almost linear, depending on cation type and ionic strength. Because cation exchange is the major mechanism for HDTMA adsorption in this region, the shape of the isotherm can be better understood in terms of the cation selectivity coefficients, which define the stabilities of HDTMA-clay exchange complexes. The monotonic isotherms are characterized by a high Vanselow selectivity coefficients (e.g., for Ca replaced by HDTMA) a t HDTMA loadings < 0.75 CEC which decreased rapidly a t higher HDTMA loadings (Figure 9). This behavior can be described by assuming two types of sites, one (interlayer sites) comprising 80% of the total sites and having a log ‘K close to 9.3, the remaining 20% sites (external sites)having a log ‘Kof 1.5. The nonmonotonic isotherms had a low Vanselow selectivity coefficient initially but increased dramatically to a maximum at the end of region 1 (Figure 9). The selectivity coefficientthen decreased with HDTMA loading >0.75 CEC. This behavior can also be described by assumingtwo types of exchange sites. The nonmonotonic isotherms result from a n increase in log cKfor interlayer sites with increasing HDTMA loadings.
I’
Ca-HDTMA
o
0.0
0.2 0.4 0.6 0.8 1.0
0.0
Measured Predicted
0.2 0.4 0.6
0.8 1.0
Mole Fraction of Exchange Sites Saturated by HDTMA
Figure 9. Vanselow selectivity coefficients of HDTMA on SWy-1as influenced by HDTMA loadings and the type of
inorganic cations initially saturating the clay.
The nonmonotonic isotherms observed in region 1were first reported by us for HDTMA adsorption in a subsoil containing vermiculite? This phenomenon was attributed to the effects of exchange cation type and ionic strength on the degree of lateral interactions of adsorbed HDTMA regulated by the structure of HDTMA adsorption layers. For example in dilute NaCl solution (4.4 mM), Na-SWy-1 was fully dispersed, providing full access of HDTMA to all exchange sites, resulting in random distribution and loose organization of HDTMA in the interlayers which minimized the degree of lateral interactions among the C-16 alkyl groups of neighboring adsorbed HDTMA cations. The decrease in the equilibrium concentration of HDTMA with increased loading is due to the increased lateral interaction of adjacent HDTMA a s the density of, and hence contact between, adsorbed HDTMA increased. In contrast to Na-SWy-1, Cs-SWy-1 and Ca-SWy-1 (at a constant C1- concentration) were flocculated prior to HDTMA addition. The face-to-face aggregation of clay particles limited the access of HDTMA to the interlayer sites. As a result, the replacement of interlayer Na by HDTMA occurs primarily along the edges of the flocks. This caused segregation of adsorbed HDTMA manifesting a d-spacing change completely different from HDTMANa-SWy-1 (Figure 2). The close contact of segregated HDTMA in the interlayers ensures substantial lateral interactions even a t low HDTMA loadings. Therefore a t these low loading levels, HDTMA had a higher affinity for Cs- and Ca-saturated clays than for Na-SWy-1. The segregation of HDTMA and inorganic cations in Cs- and Ca-SWy-1 maximized the binding strength of HDTMA by increasing the degree of the lateral interactions. This represents the most stable state for adsorbed HDTMA and thus manifests high selectivity coefficients and low aqueous phase HDTMA concentrations. The random distribution of HDTMA in the interlayers, as observed for Na-SWy-1, may be viewed as a metastable state resulting from initial reaction conditions that lead to a high degree of dispersion of the clay particles. The redistribution of mixed HDTMA/Na, and thus conversion to the most stable HDTMA-clay complexes may be hampered by extremely slow diffusion of QACs in the interlayers of m o n t m ~ r i l l o n i t e . ~ ~ The relation between the stability of HDTMA-clay exchange complexes and accessibility of exchange sites, and ultimately the structure of the HDTMA adsorption layer, is further supported by the effect of ionic strength (39) Gast, R. G.; Mortland, M. M. J. Colloid Interface Sci. 1971,37, 80.
2514 Langmuir, Vol. 11, No. 7, 1995
Xu and Boyd
layer as influenced by the charge density of the clay on HDTMA adsorption. Increasing NaCl concentration surface. As mentioned earlier, strong HDTMA-surface decreased the degree of dispersion of Na-SWy-1. As a interactions resulted in a tendency of HDTMA to form a result, the nonmonotonic HDTMA adsorption isotherms flat-lying configuration in swelling clays. The conversion changed to monotonic curves, mainly through increasing of flat-lying adsorption layers to a vertical configuration the lateral interactions between the adsorbed HDTMAs (e.g., paraffin types) requires net energy input to overcome and thus increasing the affinity of HDTMA for surfaces the favorable surface-alkyl chain interactions. That at low loadings. Increasing the CaClzconcentrations had HDTMA adsorption never increased beyond pseudotrilittle effect on HDTMA adsorption because Ca-montmormolecular layers in SWy-1 suggests a lack of sufficient illonite forms face-to-face association even a t very low adsorption energy to reorient the alkyl chains in a vertical CaC12 concentration^.^^ position. This is due to the high density of head groups Finally, the nonmonotonic isotherms observed in mont(of HDTMA adsorbed via hydrophobicbonding) comprising morillonites were not observed in kaolinite. Similarly, the pseudotrimolecular layer. The repulsive forces arising we have observed nonmonotonic adsorption isotherms of from any further HDTMA adsorption via hydrophobic HDTMA in Na-vermiculite, but not in Na-illite (Xu and bonding are greater than the attractive forces arising from Boyd, unpublished data). The fact that nonmonotonic alkyl chain interactions. For surfaces of higher charge isotherms are only associated with interlayer adsorption density, e.g., SAz-1 or kaolinite, the transition from a strongly supports our interpretation. pseudotrimolecular layer to a paraffin type occurred at HDTMA Adsorption via Hydrophobic Bonding. low HDTniIA loadings when HDTMA was mainly adsorbed Although the detailed mechanism of nonelectrostatic mechanism, and repulsion of the head adsorption of ionic surfactants is not fully r e ~ e a l e d , ~ via ~ ~cation ~ ~ ~exchange ~ ~ groups does not play a role in regulating HDTMA it is generally accepted that this process is driven by the adsorption. A s a result, the adsorption plateau ofHDTMA mutual attraction between the hydrophobic alkyl chains in SAz-1 and in kaolinite corresponds to a paraffin type and the tendency to minimize the contact of the hydroconfiguration in which the loading capacity can be more phobic moieties with water.16 The limiting factor is the readily changed by altering the inclination angle. repulsion arising from the charged head groups of the nonelectrostatically sorbed surfactant molecules.25 Any factor that increases the contact of the hydrophobic Conclusions moieties of HDTMA or decreases the repulsion between the head groups will enhance nonelectrostatic adsorption, The characteristics of cationic surfactant adsorption by a s shown by the data presented here. swelling layer silicates are much more complex than those by nonswelling clays. The complexity arises mainly from Different anions affected HDTMA adsorption because the structure of the adsorption layer of cationic surfactants of their different ability to screen the charge of the head in the interlayers of swelling clays, which in turn depends groups.16 Relative to C1-, Br- is more effective for charge on the reaction conditions. Initial conditions leading to screening and, therefore, at the same concentration, more a highly dispersed swelling clay (e.g., Na-saturated clay HDTMA was adsorbed with Br- as counterion than with and low ionic strength) resulted in random H D T W C1- before the amount of HDTMA adsorption reached the inorganic cation distribution in the interlayers and a loose adsorption plateau. Relative to monovalent anions, structure of the adsorption layer at low HDTMAloadings. divalent anions are more effective in charge screening Consequently, lateral interactions between alkyl C-16 and, therefore, more HDTMA was adsorbed from S 0 2 groups of HDTMA were minimal manifesting relatively solutions than from monovalent anion solutions. It should low cation selectivity coefficients which subsequently be pointed out that the apparent retention of HDTMA by increased with HDTMA loadings as the contact between SWy-1 in Br- solution beyond 1.76 CEC was caused by adsorbed HDTMA molecules increased. This led to a precipitation of HDTMA bromide. Similar to the anion unique S-shape HDTMA adsorption isotherm. Conditions type effect, increasing counterion concentrations decreases leading to flocculation of montmorillonites before HDTMA the thickness of the double layer surrounding the head was added (e.g., Cs- or Ca-saturation, or high ionic groups and hence increases HDTMA adsorption by strength) resulted in a segregated cation distribution and hydrophobic bonding. a compact HDTMA adsorption layer. Restricted access of HDTMA adsorption via hydrophobic bonding in montHDTMA to the edges of clay flocks resulted in significant morillonites was not affected by cation type (e.g., Naversus lateral interactions among alkyl groups even at low Ca) because exchangeable cations have little effect on the HDTMA loadings, and this varied little with HDTMA repulsive or attractive forces discussed above. However, loading. Accordingly, the cation selectivity coefficients in some circumstances cation type may influence hydrodid not change substantially with HDTMA loading, and phobic bonding by changing the accessible surface area the HDTMA adsorption isotherm was non-S-shaped, of swelling clays. For example, in a subsoil containing similar to those found for nonswelling clays. vermiculite, less HDTMA was adsorbed via hydrophobic At high HDTMA loadings (>CEC) on swelling clays, bonding in Ca-saturated soil than in Na-saturated soil HDTMA adsorption via cation exchange approached its due to more limited accessibility of HDTMA to the maximum and HDTMA adsorption via hydrophobic interlayers of Ca-vermiculite than to those of Nabonding occurred, similar to that by nonswelling clays. In vermi~ulite.~ both types of clays, the charge density of the clay, ionic Clay properties may also influence the hydrophobic strength of the bulk solution, and the type of companion adsorption of HDTMA. For SWy-1, although the degree anions are three major factors regulating the adsorption of HDTMA adsorption via hydrophobic bonding changed of HDTMA via hydrophobic bonding. with ionic strength, the adsorption plateau was always 1.76 CEC, corresponding to a densely-packed pseudotrimolecular layer. For SAz-1and kaolinite, the adsorption Acknowledgment. This work was supported in part plateau increased with increasing ionic strength. These by a grant from the National Institute of Environmental differences further demonstrate the relation between Health Sciences (No. l-P42-ES0491101)and by the MichiHDTMA adsorption and the structure of the adsorption gan Agricultural Experiment Station and the Michigan State University Institute for Environmental Toxicology. (40)Schramm,L.L.;Kwak,J. C. T. Clays Clay Miner.1982,30,40. (41)Narkiewicz-Michalek,J. Languir 1992,8,7. LA940708P
