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Binding of a Cationic Surfactant by Polyacrylate, Poly(vinyl sulfate), and Montmorillonite. Tensammetric Measurements in Very Dilute Aqueous Solution Jean-Marie Se´quaris Institute of Applied Physical Chemistry (ICG 7), Research Centre Ju¨ lich, P.O. Box 1913, D-52425 Ju¨ lich, Germany Received May 28, 1996. In Final Form: November 7, 1996X Out-of-phase alternating current polarography, a tensammetric method, was used to directly measure the concentration of hexadecylbenzyldimethylammonium (HBDA), a cationic surfactant, in equilibrium with very dilute solutions of anionic polymers such as poly(vinyl sulfate) (PVS) and polyacrylate (PA), as well as montmorillonite, a negatively charged clay mineral. The sensitive response of the time-controlled mercury drop electrode allows binding isotherms to be calculated in a µM range of HBDA. In the case of montmorillonite, a double-step binding isotherm is observed which is equal to twice the cation exchange capacity (CEC) of the clay mineral. A cation exchange reaction for the first step and hydrophobic interactions between adsorbed surfactant molecules are commonly considered in the adsorption process. Electrophoretic measurements along the binding isotherm allow the formation of a surfactant molecule bilayer to be observed. Up to a 1.5-1.6 CEC equivalent of adsorbed HBDA, the formation of montmorillonite plate aggregates can also be proposed by considering a demixing of Na and HBDA cations between external and interplate sites. In the case of PA and PVS, a quantitative binding of HBDA to the carboxylate and sulfate functional groups respectively is found. Two domains of cooperativity for the binding process are discriminated along the isotherms. Considering a specific charge neutralization with HBDA, it can be proposed, in a first step, that the Na counterion condensation controls the surfactant fixation at both vinyl polyelectrolyte, PVS and PA, surfaces at an ionic strength of 10-2 M. This can be satisfactorily modeled by a one-dimensional nearest neighbor lattice model. In a further step, a higher cooperative character for the binding of surfactant aggregates is observed, which can be related to the breakdown of the Na counterion condensation. At high HBDA solution concentrations, the building of micelles in solution competes with this binding mechanism.
Introduction Mixtures of surfactants with minerals or polymers are continually finding new industrial applications1 in improving, for example, flotation or solubilization methods. In the same way, under environmental aqueous conditions,2,3 interactions between amphiphilic molecules with organic or mineral surfaces modify their colloidal behavior. The understanding of their interactions requires specific methods which allow the activity of surfactant molecules to be measured and the extent of their bindings in complex systems to be observed. Relatively few analytical methods are able to quantitatively detect the equilibrium concentration of surfactant in situ. Thus, the potentiometric method using ion sensitive electrodes4-15 has been used X Abstract published in Advance ACS Abstracts, January 15, 1997.
(1) Myers, D. Surfactant Science and Technology; VCH Publishers, Inc.: New York, 1988. (2) Stumm, W. Chemistry of the Solid-Water Interface; John Wiley & Sons, Inc.: New York, 1992; p 87. (3) Klumpp, E.; Schwuger, M. J. In Detergents in the Environment; Surfactant Science Series; Schwuger, M. J., Ed.; Marcel Dekker, Inc.: New York, 1997; Vol. 65, Chapter 2. (4) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1982, 86, 3866. (5) Hayakawa, K.; Ayub, L.; Kwak, J. C. T. Colloids Surf. 1982, 4, 389. (6) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Biophys. Chem. 1983, 17, 175. (7) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules 1983, 16, 1642. (8) Hayakawa, K.; Kwak, J. C. T. In Cationic Surfactants; Rubingh, D. N., Holland, P. M., Eds.; Marcel Dekker: New York, 1991; p 189. (9) Shirahama, K.; Tashiro, M. Bull. Chem. Soc. Jpn. 1984, 57, 377. (10) Kiefer, J. J.; Somasundaran, P.; Ananthapadmanabhan, K. P. Langmuir 1993, 9, 1187. (11) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115. (12) Hansson, P.; Almgren, M. J. Phys. Chem. 1995, 99, 16684 (13) Hansson, P.; Almgren, M. J. Phys. Chem. 1995, 99, 16694 (14) Hansson, P.; Almgren, M. J. Phys. Chem. 1996, 100, 9038 (15) Anthony, O.; Zana, R. Langmuir 1996, 12, 1967
S0743-7463(96)00513-6 CCC: $14.00
to determine surfactants in µM-mM concentration ranges. A long equilibration time is required to obtain sensitive reponses in a low concentration range of surfactants leading to a time-dependent parameter which is not negligible in the evaluation of the results.6 Such a sensitive time-controlled response based on an adsorption process can be of benefit in the utilization of a mercury drop electrode for surfactant detection.16,17 Thus, the recent improvement of the voltammetric instrumentation has renewed interest in the application of phase-sensitive alternating current polarography as a tensammetric method for the detection of surface active compounds in aqueous media.18-20 In the present work, this method was used to study the binding of a cationic surfactant, hexadecylbenzyldimethylammonium (HBDA), with very low-concentration solutions of polycarboxylate and a clay mineral, montmorillonite. These compounds carrying a high density of negative charges respectively were chosen as models for organic and inorganic colloidal matter encountered in environmental conditions.2,21 Indeed, a high content of carboxylate groups as in the case of poly(acrylic acid) at alkaline pH or polyacrylate (PA) characterizes the polyanionic character of dissolved high molecular weight organic matter like the humic acids. For comparison, another vinyl polyelectrolyte, poly(vinyl sulfate) (PVS), carrying a strong acid function, was also studied. The permanent negatively charged montmoril(16) Jehring, H. Elektrosorptionsanalyse mit der Wechselstrompolarographie; Akademie-Verlag: Berlin, 1974. (17) Jehring, H. J. Electroanal. Chem. 1969, 20, 33. (18) Bersier, P. M.; Bersier, J. Analyst 1988, 113, 3. (19) Lukaszewski, Z. Electroanalysis 1993, 5, 375. (20) Bos, M. In Cationic Surfactant; Surfactant Science Series; Cross, J., Singer, E. J. Eds.; Marcel Dekker, Inc.: New York, 1994; Vol. 53, Chapter 7. (21) Se´quaris, J.-M. In Detergents in the Environment; Surfactant Science Series; Schwuger, M. J., Ed.; Marcel Dekker, Inc.: New York, 1997; Vol. 65, Chapter 7.
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lonite clay mineral is among the most active ionic exchangers in soil or sediment materials. This swelling clay has a 2:1 layer type where an octahedral aluminate sheet is sandwiched between two tetrahedral silicate sheets. The permanent negative charge results from a cation isomorphous substitution in the crystal and is at the origin of the binding of inorganic and organic cations such as the cationic surfactants.22,23 Materials and Methods Materials. Hexadecylbenzyldimethylammonium chloride (purum) (HBDA) was obtained from Fluka. Poly(acrylic acid) (PAA) with a molecular weight of 250 000 g/mol and potassium PVS with a molecular weight of 245 000 g/mol were supplied by Aldrich and Serva, respectively. Calculated maximal charge densities (milliequivalents per gram of polyelectrolyte) for PAA and PVS were 13.9 and 6.2 mequiv/g, respectively. Na-montmorillonite was prepared from Ca-bentonite (Su¨dChemie, FRG): 100 g of Ca-bentonite is treated with 5 L of 30% H2O2 at 70 °C for 30 min to remove organic matter. The clay is thus saturated with Na with 1 L of 0.5 M NaCl solution (twice) and 1 L of 1 M NaCl (5 times in 24 h). The clay mineral is thus washed 3 times with distilled water. Each washing cycle consists of suspension, agitation, and centrifugation steps. The 16 µM). For a long-time detection (8 s), a transient plateau can be observed for Ic at the cmc characteristic of an equilibrium concentration of free HBDA in the presence of more slowly diffusing micelles. For higher HBDA concentrations, a further decrease of Ic can be discussed in terms of HBDA aggregates forming during longer adsorption times at the mercury electrode surface.24 It is found that only the first part of the curve obtained with a drop time of 8 s is usable for a sensitive detection of free HBDA in a concentration range up to 12 µM. These obtained curves are fitted by a polynomial procedure and used in the binding studies as calibration curves for the direct analytical determination of surfactant equilibrium concentrations. A standard error of estimations lower than 0.5 and 1 µM can be reported for mercury drop times of 8 and 2 s, respectively. In order to illustrate the sensitivity of this method, the titration curves of very dilute solutions of PA (2.5 mg/L) and montmorillonite (20 mg/L) by HBDA are illustrated in Figure 2. They are recorded with mercury electrode drop times of 2 and 8 s, respectively. For comparison, the variations of Ic against the total added concentration of HBDA are also reported in the absence of PA and Namontmorillonite. An observed slight variation of Ic in the presence of both adsorbants indicates that the activity of surfactant molecules is largely decreased by the interactions with PA and Na-montmorillonite. A larger decrease of Ic due to the free surfactant molecules in solution is only detected after the exhaustion of the cation binding capacity of their anionic sites. It can also be shown that under these experimental conditions no variation of Ic is detected before adding HBDA, i.e., in the presence of montmorillonite and polymers alone. Indeed, their much lower diffusion coefficient, their hydrophilic character, as well as electrostatically unfavorable conditions render their adsorption negligible at the negatively charged and hydrophobic mercury electrode surface during the drop time. (25) Cosovic, B. In Aquatic Chemical Kinetics; Stumm, W., Ed.; Wiley-Interscience: New York, 1990; p 291. (26) Haegel, F-H.; Ko¨nig, M.; Schwuger, M. J. Analyst 1993, 118, 703.
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Figure 2. Variation of the out of phase ac current intensity (Ic) at a mercury electrode potential of -798 mV vs (Ag/AgCl) as a function of the total HBDA concentration in 0.01 M NaNO3, pH 9.5-9.7, and at T 20 °C: (A) 9, HBDA; +, HBDA with added 2.5 mg/L PA; mercury electrode drop time, 2 s; (B) 9, HBDA; +, HBDA with added 20 mg/L montmorillonite; mercury electrode drop time, 8 s.
Figure 3. Binding isotherm of HBDA by montmorillonite in 0.01 M NaNO3, pH 9.5-9.7, T 20 °C; montmorillonite, 20 mg/L, i.e., 20 µequiv/L of anionic binding sites.
Binding Isotherm of HBDA by Montmorillonite. The adsorption of cationic surfactants at the clay mineral surface involves two mechanisms: a cation-exchange reaction and hydrophobic interactions.3,5,22,23,26-29 In a general way, the binding isotherm in Figure 3 of a cationic surfactant with a rather long alkyl chain can firstly be described at low free HBDA equilibrium concentration by an electrostatic interaction between the organic cations and the permanently negatively charged clay mineral surface measured with the cation exchange capacity (CEC). This electrostatic binding up to a value close to 1 for the normalized HBDA adsorption against the CEC or CEC equivalent is cooperatively enhanced by the hydrophobic interaction between alkyl chains from bound surfactants. Secondly, a further weaker binding of (27) Xu, S.; Boyd, S. A. Langmuir 1995, 11, 2508. (28) Xu, S.; Boyd, S. A. Soil Sci. Am. J. 1994, 58, 1382. (29) Xu, S.; Boyd, S. A. Environ. Sci. Technol. 1995, 29, 312.
surfactants up to twice the CEC, along the binding isotherm at higher free HBDA equilibrium concentrations, assumes the formation of a bilayer of surfactant molecules at the clay mineral surface. This can be achieved at the expense of the formation of surfactant micelles in the equilibrium solution (cmc ) 16 µM). In order to obtain information on the the surfactant adsorbed layer, the electrophoretic mobility of clay montmorillonite dispersion was plotted against the adsorbed HBDA expressed in CEC equivalent in Figure 4A. A typical profile of the electrophoretic mobilities curve going from negative values to positive values can be observed. This electrophoretic behavior is generally accepted in the case of the adsorption of ionic surfactants at oppositely charged colloids.30 The negative electrophoretic mobility first increases up to the zero value region at an adsorbed amount of 1 CEC due to the specific interaction of positively charged surfactant heads with the negatively charged montmorillonite surface by forming a surfactant monolayer. It must be noted that the constancy of the electrophoretic mobility up to an adsorbed amount of 0.50.7 CEC equivalent would imply a cation exchange mechanism with Na ions condensed on the platelike particles without affecting the electrical properties of the colloids. However, for higher adsorbed amounts, the hydrophobic interaction between alkyl chains greatly modifies the electrical properties of the particles. Thus, the formation of a second surfactant layer with the postively charged surfactant heads oriented toward the solution would develop a positive charge at the colloid surface probed by the positive electrophoretic mobility of the particles. However, considering the swelling nature of Namontmorillonite, an adsorption mechanism can be proposed where hydrophobic aggregation phenomena induced by surfactant-modified surfaces play an important role. Indeed, the X-ray diffraction method measures a crystal(30) Fuerstenau, D. W. Pure Appl. Chem. 1970, 24, 135.
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Figure 5. Binding isotherm of HBDA by PVS in 0.01 M NaNO3, pH 9.5-9.7, T 20 °C; PVS, 5 mg/L, i.e., 31 µequiv/L of anionic binding sites. The dashed curve is calculated with eq 2 and corresponding parameter values in Table 1.
Figure 4. Electrophoretic mobility of montmorillonite particles (A) and optical transmission (λ ) 670 nm) of montmorillonite solutions (B) as a function of the adsorbed amount of HBDA expressed in CEC equivalent (see other conditions in Figure 3).
line-like structure for the dry state of montmorillonite plates stabilized by the intercalation of bilayers or pseudotrimolecular layers of cationic surfactants.22,23,27 Taking these peculiar properties of Na-montmorillonite into account, only the electrical properties of the aggregates external surface are considered with the microelectrophoretic method. Thus, in Figure 4A, the constancy of the electrophoretic results up to 0.7 CEC equivalent, at low free surfactant equilibrium concentrations, could be now related to a demixing of cations where Na occupies the external sites and HBDA resides in the interlayer of the formed aggregate.28,29 From 0.5-0.7 up to 1.5-1.6 adsorbed CEC equivalent, the variation of the electrophoretic mobility would reflect the formation of a surfactant bilayer at the outside surface of the aggregate. For higher HBDA loadings, of more than to 1.5-1.6 adsorbed CEC equivalent, the adsorption would further proceed on the adsorbed surfactant layer inside the aggregate. However, the formation of an adsorbed surfactant bilayer with a net positive charge induces a dispersion of the aggregate due to electrostatic repulsion forces. The constancy of the electrophoretic mobility would reflect this monotonous disaggregation into single montmorillonite plates stabilized by adsorbed surfactants bilayer on both sides. In the same way, a lowering of the optical transmission or increase of the turbidity for the HBDA/montmorillonite solutions in Figure 4B shows a high dispersity with a high loading level of HBDA. Binding Isotherms of HBDA by Polyacrylate and Poly(vinyl sulfate). In Figures 5 and 6, the binding
Figure 6. Binding isotherm of HBDA by PA in 0.01 M NaNO3, pH 9.5-9.7, T 20 °C; PA, 2.5 mg/L, i.e., 35 µequiv/L of anionic binding sites. The dashed curve is calculated with eq 2 and corresponding parameter values in Table 1. The dotted curve is calculated with eq 4 and corresponding parameter values in text.
isotherms of HBDA by the PVS and the PA are expressed by the degree of binding β of the carboxylate or the sulfate groups by the cationic surfactant. β equals the bound surfactant concentration divided by the polyanion equivalent concentration. At pH 9.5-9.7 and an ionic strength of 10-2 M, it can be considered that the PAA is totally ionized into the PA form.31 A saturation value of β equal or close to 1 for PVS and PA clearly indicates a 1/1 stochiometric complexation between the monovalent cat(31) Sastry, N. V.; Se´quaris, J.-M.; Schwuger, M. J. J. Colloid Interface Sci. 1995, 171, 224.
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ionic surfactant and the anionic group of polymers. A detailed examination of the binding isotherms shows a monotonous fixation of surfactants up to β values of about 0.6, while at higher β values a more complex stepwise binding occurs. Considering a decrease of the ionization of polyelectrolyte due to a neutralization by HBDA, this critical value of β ≈ 0.67,9-11 can be related to the breakdown of the Na counterion condensation as predicted by Manning’s model for polyelectrolyte solutions.32 Thus, a linear charge density parameter can be defined for the polyelectrolyte
ξ ) (1 - β)e2/b0kT
(1)
where e is the elementary charge, is the permittivity of the medium, k the Boltzmann constant, T the temperature, b0 the average linear charge separation, and (1 - β) the apparent degree of ionization. Physical observations and theoretical considerations have shown that this counterion condensation only occurs for ξ > 1 by keeping a charge fraction of polyelectrolyte equal to 1/ξ. Taking a b0 value of 0.28 nm for the two substituted vinyl polyelectrolytes PVS and PA, it can be calculated with β > 0.61 that the Na+ counterion condensation vanishes, i.e., 1 - 1/ξ ) 0. Indeed, for a structural charge density parameter ξ < 1, a counterion condensation controlled by the presence of Na+ in excess is no longer required for the thermodynamic stability of the HBDA surfactant-modified polymers. Thus, depending on the extent of the charge neutralization of PA and PVS by HBDA, an ion exchange mechanism with the condensed Na+ would take place at β < 0.61. This can be satisfactorily modeled by a one-dimensional nearestneighbor lattice model33,34 where the reduced elecrostatic potential for ξ > 1 is kept constant4,6,7,9,10 through the Na+ counterion condensation and the HBDA site-binding fixation. According to this model, the degree of binding is given by
β ) (1/2){1 + (s - 1)/[(1 - s)2 + 4s/u]1/2}
(2)
where s ) Ku[HBDA]. It can be calculated in (2) with s ) 1
Ku ) ([HBDA]β)0.5)-1
(3)
where [HBDA]β)0.5 is the free surfactant concentration at β ) 0.5. The overall binding constant Ku includes K, the binding constant of a surfactant with an isolated site, and u, a cooperative parameter. Thus, Ku is the binding constant of a surfactant with a site adjacent to an occupied site. In Table 1, the best fitting parameters from the nonlinear regression analysis of the results up to β < 0.7 are reported. Thus, the cooperative parameter, u, between bound surfactant molecules is of the same order for PVS and PA, which is expected for the substituted vinyl polyelectrolyte with similar b0. However, the binding constant K is an order of magnitude higher in the case of PVS. In the case of an ion exchange reaction, this difference in K values can be related to a variation in the dehydration degree on the binding of cation35 to carboxylate or sulfate groups of PA and PVS, respectively. Dashed curves, in Figures 5 and 6, calculated with eq 2 and the parameters in Table 1 illustrate the relatively good (32) Manning, G. S. Q. Rev. Biophys. 1978, 11, 179. (33) Satake, I.; Yang, J. T. Biopolymers 1976, 15, 2263. (34) Applequist, J. J. Chem. Educ. 1977, 7, 417. (35) Satoh, M.; Kawashima, T.; Komiyama, J. Polymer 1991, 32, 892.
Table 1. Thermodynamic Parameters (β < 0.7) for the Binding of HBDA to PVS and PA at 20 °C in 0.01 M NaNO3, pH 9.5-9.7 K/M-1 PA PVS
(1.1 ( 0.1) × (1.2 ( 0.4) × 105 104
u
Ku/M-1
11 ( 1 5.5 ( 1.8
(1.2 ( 0.2) × 105 (6.6 ( 4.4) × 105
agreement for β < 0.7 between the binding results and the lattice model. For β g 0.6-0.7, a sharp rise in binding isotherms would imply a favorable binding to anionic sites in the absence of a condensed Na+ ion screening effect. The stepwise binding isotherm, in the case of PA (Figure 6), clearly shows a very high cooperative character for the further binding process occurring in the very narrow free surfactant concentration range around 9.5 µM, which is lower than the cmc value (16 µM). In the case of PVS, a similar effect is observed for a free surfactant concentration of 2 µM. The present tensammetric results thus allow an extreme case of cooperativity to be discriminated at higher β, which can be compared to the values in Table 1. Thus, in the case of PA, when the free surfactant concentration for the half-binding isotherm is put equal to 9.5 µM and from (3) an overall binding constant Ku ) 1.04 × 105 M-1 is calculated, K and u values of (1-3) × 101 M-1 and (0.351) × 104 can be respectively estimated with the lattice model. These results assume that the decrease of the electrostatic potential for ξ < 1, expressed into K, is largely compensated by hydrophobic interactions, expressed into u. Large aggregates of surfactant molecules at the polyelectrolyte surface similar to micelles can thus be formed.11-15 A concomitant polymer conformational change from an extended chain, stabilized by internal electrostatic repulsions, to a more coiled structure of nonionic character is also to be taken into consideration.10,36 The size of surfactant molecule aggregates stabilized by the polyelectrolyte acting as a couterion has been measured in the case of PAA12 and PVS13 interacting with dodecyltrimethylammonium bromide (C12TAB). At β ) 0.5, aggregation numbers Ns of 60 and 120 have been found respectively from time-resolved fluorescence quenching experiments as compared to 62 for free micelles. A mass action model for a cooperative binding has thus been developed which includes a simple exchange ion-exchange process and a simultaneous binding of Ns surfactant molecules to the polymer with Ns binding sites. While in the former one-dimensional nearest-neighbor lattice model, Ns is predicted to increase monotonously with β,33,34 the latter model implies a constant Ns for a cooperativity binding of aggregates. Thus, the stepped part of the HBDA binding curve with PA in Figure 6 can be phenomenologically described according to Hill’s equation37,38
β)
KNs[HBDA]Ns 1 + KNs[HBDA]Ns
(4)
where an average value of 90 for the Ns found with solution micelles of C16 cationic surfactants has been chosen.14,39 In this model, taking a K value of 1.04 × 105 M-1 for the binding of one surfactant molecule, the fitting curve (dotted curve in Figure 6) at β g 0.6. gives satisfactory results. Notwithstanding the size of the aggregates, the results indicate that a critical β value limits the cooperative character of the binding. In the same way recent (36) Chandar, P.; Somasundaran, P.; Turro, N. J. Macromolecules 1988, 21, 950. (37) Hill, A. V. J. Physiol. (London) 1910, 40, 4. (38) Shirahama, K. Colloid Polym. Sci. 1974, 252, 978. (39) Roelants, E.; De Schrijver, F. C. Langmuir 1987, 3, 209.
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potentiometric results10 on the ionization degree effect in the binding of tetradecyltrimethylammonium bromide (TTAB) by PAA at an ionic strength of 10-2 M have shown that the onset of the cooperative binding shifts to lower free TTAB concentrations at low pH. The hydrophobic properties of the backbone from the coiled PAA structure at low ionization degrees would influence the cooperative binding. This result also implies that the breakdown of the Na counterion condensation (β ) 0.61) by the specific neutralization of ionized carboxyl groups with protons and/or cationic surfactants would favor the onset of a strong cooperative binding. For the highest concentrations of surfactant in solution, in Figures 5 and 6, the formation of micelles in solution (cmc ) 16 µM) competes with the binding process. It results in a stabilization or a decrease of β values for PVS and PA before reaching the saturation value β ) 1 at the highest total surfactant concentrations. It must be noted that the interaction of the resulting less soluble HBDApolyanion neutral complex with the sensitive membrane of an iodide ion selective electrode can be used for the polyanion determination.40 (40) Se´quaris, J.-M.; Kalabokas, P. Anal. Chim. Acta 1993, 281, 341.
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Conclusion To conclude, a comparison of HBDA binding strengths with PA, PVS, and montmorillonite at very low concentrations can be made from the inverse equilibrium concentrations obtained at the half-saturation of the anionic binding sites. In the case of PVS and PA, the overall binding constants Ku are reported in Table 1. In the same way, a rough binding constant Kmont. of (1-2) × 106 M-1 is calculated for the half-saturation of the montmorillonite cation exchange capacity or 0.5 CEC equivalent in Figure 3. It follows that a decrease of the binding strength can be observed in the order Kmont > KPVS> KPA at an ionic strength of 0.01 M and pH 9.5-9.7. Although averaged distances between anionic sites are 0.65 and 0.28 nm for montmorillonite and PVS or PA, respectively, it clearly appears that a two-dimensional array of binding sites at the clay mineral surface promotes an efficient cationic surfactant immobilization. Acknowledgment. Thanks are due to C. Walraf for technical assistance. LA960513C
