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Langmuir 1999, 15, 3134-3142
Aggregation Behavior in Water of Monomeric and Gemini Cationic Surfactants Derived from Arginine Aurora Pinazo,†,‡ Xinyun Wen,† Lourdes Pe´rez,‡ Maria-Rosa Infante,‡ and Elias I. Franses*,† School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907-1283, and Department of Surfactant Technology, CID (CSIC), J. Girona 18-26, 08034 Barcelona, Spain Received September 22, 1998. In Final Form: February 23, 1999 A new class of gemini surfactants, bis(Args) (Cn(LA)2, where n ) 3, 6, and 9), has been synthesized. Their solution and tension behaviors were studied and compared to those of the corresponding monomeric surfactant, LAM (NR-lauroylarginine methyl ester) and of a common cationic surfactant, CPC (cetylpyridinium chloride). Bis(Args) are made up of two symmetrical long chain NR-acyl-L-arginine residues of 12 carbon atoms linked by amide covalent bonds to an R,ω-alkylidenediamine spacer chain of varying length (n). By being produced from amino acid sources (arginine), these surfactants are biocompatible and less toxic to the environment. The solution behavior is also important for potential applications in foaming, agrichemical spreading aids, and cleaning processes, and in understanding the interfacial behavior. Strong evidence of two cmc’s with different characters of aggregates was obtained from different techniques for the gemini surfactants but not for the monomeric surfactants. Surface tensiometry indicates that the geminis form aggregates of substantial size at 0.001-0.01 mM (at 25 °C) or at concentrations about 3 orders of magnitude lower than that of LAM. Fluorescence results and lower chloride counterion binding than that for LAM suggest that the aggregates are nonglobular. These methods reveal also a second cmc for larger globular aggregates at 0.09-0.5 mM. Conductivity measurements and calculations are consistent with the above inferences and were used to estimate the aggregation number N and the counterion binding parameter β. The nonglobular aggregates have lower β and smaller N values than the globular aggregates (micelles), and unlike conventional micelles, they tend to increase the molar conductivity compared to that of the pre-cmc solution.
1. Introduction In recent years, new classes of amphiphilic molecules have emerged and have attracted the attention of various industrial and academic research groups. One of these classes is the “gemini” or “dimeric” surfactants, which have two hydrophilic head groups and two hydrophobic groups per molecule, separated by a covalently bonded spacer.1 These surfactants appear to be better in certain important properties than the corresponding, and more conventional, monomeric surfactants, which are made up of one headgroup and one hydrophobic group which may include one or more alkyl chains. They tend to have much lower cmc’s, can produce lower tensions than monomeric surfactants at the same molar or mass concentrations, and have better wetting properties.2 In the past decade, new gemini cationic surfactants, bis quaternary ammonium salts with halides, or “bis-quats”, have been synthesized and studied extensively.1-4 Due to their extraordinary surface activity, they have excellent properties of dispersion stabilization and soil cleanup. However, because these molecules have poor chemical and biological degradability, due to their chemical stability, posing a risk of toxicity to aquatic organisms, they could become ecologically unacceptable. New classes of gemini cationic surfactants have been synthesized by Infante and collaborators who produced them from amino acid sources * To whom correspondence should be addressed. Telephone: (765)494-4078. Fax: (765)494-0805. † Purdue University. ‡ CID (CSIC). (1) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (2) Zana, R. Curr. Opin. Colloid Interface Sci. 1996, 1, 566. (3) Rosen, M. J.Chemtech. 1993, 23, 30. (4) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448.
(arginine).5 They are NR, Nω-bis(NR-lauroylarginine)-R, ω-alkylidenediamide or bis(Args), which consist of two symmetrical long chain NR-lauroyl-L-arginine residues linked by amide covalent bonds to an R, ω-alkylidenediamine spacer chain of various lengths (n) (Figure 1). These gemini surfactants are much less toxic or ecologically hazardous. Their decomposition products after long-term exposure to the environment are less toxic or perhaps nontoxic. These surfactants have potential applications in foaming, agrichemical spreading aids, and cleaning processes. Results on the synthesis procedures, phase behavior, and equilibrium surface tension have been published.5-7 In this paper, we report on the aggregation behavior and unusual cmc behavior, as probed with surface tension, conductimetry, ion activity, and fluorescence techniques. For interpreting the conductivity results, we have used novel micellization and conductivity models which were first reported in refs 8-10 for monovalent ionic surfactants. In section 2, we review briefly these models and also extend them to bivalent surfactants. 2. Micellization and Conductivity Models 2.1. Mass Action Micellization Model. The “direct” problem of predicting solution properties from the concentrations of surfactant species, counterions, and salts (5) Pe´rez, L.; Torres, J. L.; Manresa, A.; Solans, C.; Infante, M. R. Langmuir 1996, 12, 5296. (6) Pe´rez, L.; Pinazo, A.; Rosen, M.; Infante, M. R. Langmuir 1998, 14, 2307. (7) Infante, M. R.; Garcia Dominguez, J. J.; Erra, P.; Julia´, M. R.; Prats, M. Int. J. Cosmet. Sci. 1984, 6, 275. (8) Kamrath, R. F.; Franses, E. I. J. Phys. Chem. 1984, 88, 1642. (9) Kamrath, R. F. M.S. Thesis, Purdue University, West Lafayette, IN, 1981. (10) Shanks, P. C.; Franses, E. I. J. Phys. Chem. 1992, 96, 1794.
10.1021/la981295l CCC: $18.00 © 1999 American Chemical Society Published on Web 04/02/1999
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Langmuir, Vol. 15, No. 9, 1999 3135
KN )
c*[1-N(1+β)] N(1 - )N(1 - β)Nβ
(3)
For the calculation, we have taken to be 0.02. If ) 0.01 or 0.03, or close to these small numbers, the results change little, by less than the usual uncertainty of 5-10% in the measurement of the cmc. The mass balances lead to a system of two highly nonlinear algebraic equations, which could be solved by Newton-Raphson iteration.8 Because this method becomes quite unstable for large values of N, we used an alternative solution method. The method relies on the transformation of the mass balances into a system of coupled nonlinear ordinary differential equations, with the dependent variables being c1 and cM- and the independent variable being the total concentration ct:
dcM- 2 dc1 Nβ-1 (1 + N2KNcN-1 cNβ (N βKNcN 1 M-) + 1 cM- ) ) 1 (4) dct dct
Figure 1. Molecular structures of the single-chain surfactant LAM and gemini surfactants bis(Args).
dcMdc1 2 Nβ-1 (N βKNcN-1 cNβ (1 + N2β2KNcN 1 M-) + 1 cM- ) ) 1 dct dct (5) Solving for dc1/dct and dcM-/dct, we get
requires a realistic micellization equilibria model. Since the pseudo-phase separation (PSM) model, described in section 3, cannot account for the micelle size and their contribution to the conductivity, it is not used here. The mass action micellization (MAM) model for monodisperse micelle size is the simplest model that can provide a reasonable quantitative description of micellization of real systems. In this model, micelles of one size are treated as separate chemical species, and the dependence of the average aggregation number on concentration is ignored. The micellization equilibrium for an ionic surfactant R+M- is KN
+
N(1-β) NR+ 1 + NβM 798 (RNMNβ)
(1)
The equilibrium constant KN is
KN )
γN
Nβ-1 1 + N2βKNcN dc1 1 cM- (β - 1) ) 2 dct 1 + N2K cN-1cNβ-1 N 1 M- (cM- + β c1)
(6)
cNβ 1 + N2KNcN-1 dcM1 M-(1 - β) ) 2 N-1 Nβ-1 dct 1 + N KNc1 cM- (cM- + β2c1)
(7)
The initial conditions, at zero concentration (ct ) 0), are
c1 ) cM- ) ct ) 0
Equations 6-8 are solved numerically with the fourthorder Runge-Kutta method. Similarly, we extend the MAM model to a 1:2 surfactant R2+(M-)2 with the micellization equilibrium being now KN
cN
Nβ N Nβ γN 1 γM- c1 cM-
(2)
where c1 is the monomer concentration, cM- is the counterion concentration, cN is the molar concentration of micelles of aggregation number N, and β is the counterion binding parameter (0 e β e 1); γ’s are the activity coefficients. For dilute solutions it is often assumed that the activity coefficients are equal to 1. If one wishes to account for thermodynamic activity coefficients to the first order, the Debye-Hu¨ckel theory can be used. Because this use may complicate the calculation, and because the activity coefficients’ ratio above may be closer to 1 than the individual activity coefficients, we chose to take this ratio equal to 1. The mass balances for the monomers and counterions are used in a straightforward fashion and are not shown for brevity. A numerical definition for the cmc has been given previously.8 Briefly, the cmc is defined as the concentration at which a “small” mole fraction of the surfactant is in micellar form. The equilibrium constant KN in eq 2 can be expressed in terms of the experimental cmc (c*), N, and β, by combining the definition of the cmc with the mass balances for monomers and counterions:
(8)
+
2N(1-β) NR2+ 1 + 2NβM 798 (RNM2Nβ)
(9)
The mass balances are 2Nβ c1 + NcN ) c1 + NKNcN 1 cΜ- ) ct
(10)
2Nβ cM- + 2NβcN ) cM- + 2NβKNcN 1 cΜ- ) ct
(11)
Equations 10 and 11 are again solved after being transformed into a system of two coupled nonlinear ordinary differential equations, as in the previous 1:1 case, as follows: 2Nβ-1 (β - 1) 1 + 4N2βKNcN dc1 1 cM) dct 1 + N2K cN-1c2Nβ-1 (cM- + 4β2c1) N 1 M-
(12)
2 + 2N2KNcN-1 c2Nβ dcM1 M- (1 - β) ) N-1 2 2Nβ-1 dct 1 + N KNc1 cM- (cM- + 4β2c1)
(13)
The initial conditions are the same as those for the R+Msurfactant:
c1 ) cM- ) ct ) 0
(14)
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Pinazo et al.
The micellization equilibrium constant in terms of the cmc, N, and β becomes
c*[1-N(1+2β)] KN ) N(1 - )N(2 - 2β)2Nβ
(15)
One determines the effective value of KN from measurements of c* and β, from an estimate of N, and from the value of ) 0.02. Then one obtains c1(ct), cM-(ct), and cN(ct) and uses these in estimating conductivities, as detailed below. 2.2. Conductivity Model of Micellar Solutions. In the conductivity equations used here for micellar systems, one treats the aqueous monomers and micelles as a solution of mixed electrolytes. The equivalent conductivities of the monomers plus counterions and the micelles plus counterions, with a common ionic strength, are calculated independently. The limiting conductivity λ°i of each ion is based on a Stokes-Einstein expression for the ionic mobility in a solvent of viscosity η0 without interionic interactions (c f 0).11 In this model, ions are treated as point charges for the electrostatic calculations, or as spheres of effective radius ri for the hydrodynamic calculations. Interionic interactions are an important factor for nearly all concentrations at which aggregation occurs, and they cannot be ignored, as we have shown previously.10 They reduce the molar conductivities by an amount which depends on the size, charge, and limiting conductivity of each individual ion, as well as the effective ionic strength. For dilute solutions, interionic interactions are accounted for to the first order by the Debye-Hu¨ckelOnsager theory. The molar conductivity Λ for a monomeric surfactant solution has been given previously.10 The equations are described briefly here:
(
Λ ) Λ°1 -
) (
)
A1I1/2 c1 ANI1/2 (1 - β)NcN + Λ°N 1 + κa1 ct 1 + κaN ct (16)
where ai (i ) 1 or N) is the mean effective diameter of the ions, given by
ai ) 2xr+r-
(17)
I is the effective ionic strength (see below), and κ is the Debye inverse length. Λ°1 and Λ°N are given, respectively, by the Stokes-Einstein expressions
(
)
eF 1 1 Λ°1 ) λ°1 + λ°M- ) + 6πη0 r1 rM-
(18)
and
Λ°N )
(
)
2/3 eF N (1 - β) 1 + 6πη0 r1 rM-
(19)
The interionic interactions are manifested via the constants A1 and AN for monomers and micelles. On inserting values of physical constants, and with Λ in S‚cm2/mol and I in mol/L, A is found as
A)
2.801 × 106|z+z-|qΛ° (T) (1 + xq) 3/2
+
41.25(|z+| + |z-|) η0(T)1/2
(20)
(11) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions, 2nd ed.; Academic Press: New York, 1959.
where q is the “mobility” function given by
q≡
(
)(
)
|z+z-| λ°+ + λ°|z+| + |z-| |z+|λ°- + |z-|λ°+
(21)
The first term in A arises from the “ion atmosphere relaxation effect”, and the second correction term results from the “electrophoretic effect”. These constants are calculated independently for monomers and micelles and are functions of the valences zi and the limiting ionic conductivities λ°i. In this model, the micelles are assumed to contribute to the conductivity according to their charge and size. This contribution is comparable to that of the counterions and cannot be ignored, as we explained previously.10 Otherwise, the available conductivity data for aqueous SDS and other surfactants cannot be accounted for.10 For aqueous NaCl solutions at 25 °C (T ) 298 K), with ) 78.54 and η0 ) 0.00894 P, it is wellknown that Λ° ) 126 S‚cm2/mol. For I in mol/L, the value of A is 89.2.11 An important issue which arises at this point is what is the effective ionic strength I which affects the conductivity. There are several possible formulas to express the ionic strength of a micellar solution, depending upon which ions make contribution to it and to what extent they do so.10 Two expressions for ionic strength have been used, each accounting for different individual ionic contributions. In model A (or model 3C in ref 10), the ionic strength is determined by the monomer ions and all counterions, regardless of their origin, but not by the micelles. The expression for a monomeric (1:1) surfactant solution is
1 I ) (c1 + cM-) 2
(22)
In model B (or model 3D in ref 10) one assumes that all of the charged species in solution, regardless of their sizes and charges, including the micelles, contribute to the ionic strength. In general, micelles are expected to contribute to the effective ionic strength primarily when the ion size ai (i ) 1 or N) is small compared to the Debye length κ-1, that is, when κai , 1, because then they have to behave as the smaller ions. Then the effective ionic strength is taken to be
1 I ) [c1 + cM- + N2(1 - β)2cN] 2
(23)
The ionic strength of model B increases more rapidly than that of model A with increasing micellar concentration, because of the high micellar charge N(1 - β). The rapid increase in the effective ionic strength above the cmc can cause the predicted conductivity to drop drastically at the higher concentrations in model B, which results in much lower conductivities than those experimentally observed.10 Therefore, model B may not be realistic at high concentrations, if the micelle size is comparable or larger than the Debye length. Previous approaches of interpreting conductivity data of micellar solutions either ignore the contribution of micelles to Λ or use the ratio of the slopes above and below the cmc to infer β.12,13 The first approach is clearly erroneous.10 In the second approach, nonideality (hydrodynamic and electrostatic) effects on Λ are ignored, and only a decrease in Λ above the cmc due to micelle formation is accounted for. (12) Hoffmann, H.; Tagesson, B. Z. Phys. Chem. N. F. 1978, 110, 113. (13) Zana, R. J. Colloid Interface Sci. 1980, 78, 330.
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The conductivity model described above for 1:1 (monomeric) ionic surfactants can be easily extended to gemini (1:2) cationic surfactants, as shown below. If there are no interionic interactions, the molar conductivity of the micellar solution can be expressed by:
c1 NcN + [λN + 2(1 - β)λM-] (24) ct ct
Λ ) (λ1 + 2λM-)
where λM- is the limiting molar conductivity of the counterion at infinite dilution and
4eF Φ 6πη0r1
(25)
2/3 2 eF N 4(1 - β) Φ 6πη0 r1
(26)
λ1 ) λN )
Φ is a correction factor (of order 1/2) used to account for the effect of the two hydrophobic groups on the mobility. To a first approximation, we use
( )
M1 Φ) M2
1/3
(27)
where M1 and M2 are the molecular weights for the single and double hydrophobic chains. Then to account for the interionic interactions, one uses eq 16 with the following modifications for Λ°1 and Λ°N:
Λ°1 ) λ1 + 2λM- ) Λ°N )
(
(
)
eF 4 2 Φ+ 6πη0 r1 rM-
(28)
)
(29)
2/3 eF 4 N (1 - β) 2 Φ+ 6πη0 r1 rM-
The effective ionic strengths for models A and B are, respectively,
1 I ) (4c1 + cM-) 2
(30)
1 I ) (4c1 + cM- + 4N2(1 - β)2cN] 2
(31)
and
The above two models are solved, and the predictions are compared to the data for several cationic surfactants in section 4. 3. Materials and Experimental Methods 3.1. Materials. NR-Lauroyl-arginine methyl ester (LAM) and the bis(Args) surfactants (Cn(LA)2, where n ) 3, 6, and 9) (Figure 1) were synthesized following the methods described previously.5,7 The purity of the products was analyzed with high-performance liquid chromatography (HPLC), capillary electrophoresis, 1H NMR, 13C NMR, mass spectroscopy, and elemental analysis. All results indicate that the purity is higher than 99%. Cetylpyridinium chloride (CPC) was purchased from Sigma Chemical (St. Louis, MO). NaCl and KCl were analytical reagent grade from Mallinckrodt Specialty Chemicals Co. (Paris, KY). All materials were used as received. The surfactant solutions were prepared on a weight basis. The pure water used for all samples was first distilled and then passed through a Millipore four-stage cartridge system, which has an organic adsorption column, two mixed ionexchange columns, and an ultrafiltration unit, resulting in a water resistivity of 18 MΩ‚cm at the exit port.
3.2. Surface Tension Measurements. A Kru¨ss KA12 tensiometer with a roughened platinum plate attached to a precision torsion balance was used for measuring equilibrium surface tensions. The measurements were made after the solutions were “aged” for 24 h (while covered with Parafilm to prevent water evaporation) for the most reproducible results. 3.3. Fluorescence Measurements. The fluorescence spectrum of micelle-bound pyrene is sensitive to the polarity of the microenvironment at the site of solubilization of the fluorophore.14 The cmc’s of the surfactant solutions were determined from the plots of the intensity ratio II/IIII of the first and the third vibronic peaks in the fluorescence emission spectrum of pyrene solubilized at 9 × 10-6 M in surfactant solutions, versus concentration. The fluorescence spectra were recorded using a Shimatzu RF 540 spectrofluorometer at the excitation wavelength 335 nm and a bandwidth of 1.5 nm. 3.4. Conductivity Measurements. Conductances G were measured using Jones cells (Metrohm Models EA-655-5 and EA655-50) in conjunction with a Radiometer CDM3 conductivity meter. The cell constants
θ≡
K G
(32)
were calibrated with NaCl solutions of known conductivities K and were used for calculating the conductivity of the surfactant solution. The conductivity of water was subtracted from the measured conductivity of each sample. Measurements were made at increasing concentrations to minimize errors from possible contamination from the electrode. The precision was significantly lower for c < 0.1 mM when measurements were made at decreasing concentrations.15 These measurements were discarded. 3.5. Ion Activity Measurements. The chloride ion activities were measured with a chloride combination electrode (Orion, model 94-17B) and a pH/ion/conductivity meter (Fisher Scientific, model 50). For aqueous salt solutions (NaCl and KCl) the plots of the emf versus log(c) were straight lines from 0.1 to 40 mM. The nonlinear response at concentrations lower than 0.1 mM may be partly due to OH- interference and the low sensitivity of the electrode.16,17 Moreover, the two curves overlap in the linear region, and the calculated electrode slope was 58 mV, which was close to the Nernst factor of 2.303RT/F ) 59.16 mV at 25 °C. For surfactant solutions, due to micellization and counterion binding above the cmc, a break can be observed in the emf versus log(c) plot. Therefore, this method can be used for detecting the onset of micellization or cmc, and the counterion binding parameter. The latter can be estimated from free ion concentrations by using a micellization model, either the pseudo-phase separation (PSM) model or the mass action micellization (MAM) model. Predictions of the MAM model approach those of the PSM model when the aggregation number N of the micelle is large. Here, because N may be large and for simplicity, the PSM model will be used. In this model, the total surfactant concentration ct, the monomer concentration c1, the counterion concentration cM-, the concentration of the surfactant in micellar form cm, and the counterion binding parameter β are related by the following three equations for ct g cmc (c*):
c 1 + c m ) ct
(33)
cM- + βcm ) ct
(34)
c1(cM-)β ) c*(1+β)
(35)
The last equation was derived from the MAM model as N f ∞.8,9 (14) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (15) Wen, X. M.S. Thesis, Purdue University, West Lafayette, IN, 1998. (16) Bailey, P. L. Analysis with Ion-Selective Electrodes, 2nd ed.; Heyden & Son Ltd.: London, 1980. (17) Eisenman, G. Glass Electrodes for Hydrogen and Other Cations; Marcel Dekker: New York, 1967.
3138 Langmuir, Vol. 15, No. 9, 1999
Figure 2. Potentials of a chloride selective electrode in contact with aqueous solutions of NaCl (b), KCl ([), CPC (cetylpyridinium chloride, 9), and LAM (NR-lauroyl-arginine methyl ester, 2), at 25 °C. Arrows indicate the approximate cmc’s. NaCl and KCl solutions were used for calibration. Below 0.1 mM, the electrode response is nonlinear, and no special significance can be given to small variations.
Pinazo et al.
Figure 3. Concentrations of free chloride ions (9) in aqueous solutions of CPC at 25 °C. Solid lines are species inventories from the best fit of Model A with cmc ) 0.9 mM, β ) 0.65, determined from ion selective electrode measurements and the pseudo-phase separation model, and N ) 40; c( ) (cCP+cCl-)1/2. The model-predicted free chloride concentrations fit the data (9) well. The arrow indicates the approximate cmc by this method.
From ct and the measurements of cM-, the quantities c1, cm, and β can be calculated.
4. Results and Discussion 4.1. Cationic Single-Chain Surfactants CPC and LAM. Chloride Activity and Conductivity Results. The electrode emf’s of a chloride selective electrode were measured for aqueous cetylpyridinium chloride (CPC) solutions with concentrations between 0.1 and 10 mM and for aqueous LAM solutions with concentrations between 0.004 and 60 mM. Their emf values are compared to those of two standard electrolyte solutions in Figure 2. For aqueous NaCl or KCl, the measured emf’s decreased linearly with increasing log(c), as expected from the Nernst equation. An exception is the behavior of concentrations lower than 0.1 mM, where the nonlinear response is probably due to OH- ion interference.16 The measured emf’s of aqueous CPC solutions decreased linearly with increasing log(c) up to about 0.8 mM, where a break occurred. The emf curve of LAM has the same pattern as that of CPC, except that the break occurred at about 6 mM. Both curves overlap with those of NaCl and KCl in the linear region, indicating that the performance of the electrode was quite good (electrode slope ) 56 mV for CPC and 58 mV for LAM) and that the results are reliable. The observed breaks are due to counterion binding to the micelles above a well-defined cmc. The free chloride ions are responsible for the emf developed, and their concentrations can be calculated by using the NaCl and KCl data as calibration. The plot of the free chloride ion concentration versus the total surfactant concentration of CPC in the range 0.1-10 mM shows a clear break at 0.9 ( 0.1 mM, which defines the cmc by this method (Figure 3). For LAM, ideal behavior is exhibited from 0.004 mM to the cmc, which is 6.0 ( 0.5 mM (Figure 4). The results also indicate that LAM is a strong electrolyte and the chloride ions are completely dissociated below the cmc. On the basis of the pseudo-phase separation model for micellization (section 3) the estimated counterion binding parameters are 0.65 ( 0.05 for CPC and 0.7 ( 0.1 for LAM. The conductivities of aqueous CPC at 25 °C were measured for solutions in the concentration range from
Figure 4. Concentrations of free chloride ions (2) in aqueous solutions of LAM at 25 °C. Solid lines are species inventories from the best fit of model A with cmc ) 6 mM, β ) 0.7, determined from ion selective electrode measurements and the pseudophase separation model, and N ) 60; c( ) (c+cCl-)1/2. The modelpredicted free chloride concentrations fit the data (2) fairly well. The arrow indicates the approximate cmc by this method.
0.001 to 10 mM. At about 0.9 mM, a “break” in the molar conductivity versus concentration curve is observed (Figure 5), due to micellization and counterion binding. Therefore, the cmc determined from conductivity measurements is consistent with the ion-activity-based cmc. The error bar is quite high for c e 0.01 mM because of the low sensitivity of the measurements at such low concentrations. The pre-cmc data (from about 0.01 to 0.8 mM), extrapolated to zero concentration, yield a limiting molar conductivity of Λ° ) 96 ( 1 S‚cm2/mol. This value was used in conductivity model calculations and corresponds to the sum of the literature values of 76 S‚cm2/mol for Cland 20 S‚cm2/mol for CP+, estimated from similar molecular weight surfactants.10,11 Figure 5 shows a comparison of the data and the conductivity predictions from model A with cmc ) 0.9 mM, aggregation number N ) 40,
Cationic Surfactants Derived from Arginine
Figure 5. Comparison of CPC molar conductivity data (9), obtained at increasing concentrations, with model A predictions, for cmc ) 0.9 mM, β ) 0.65, and N ) 40.
Figure 6. Comparison of LAM molar conductivity data (2) at increasing concentrations with model A-predicted results for cmc ) 6 mM, β ) 0.7, and N ) 60. Because the error bars are large at c < 0.1 mM, extrapolation to c f 0 is done from c ) 0.1 to 0.02 mM.
and β ) 0.65 (section 2). The value of 40 for N gave the best fit of the data in the study of the effects of N on conductivity profiles. With the values of cmc and β, and with N as estimated from conductimetry and the MAM model, the concentrations of surfactant ion cCP+ ) c+ and counterion cCl- ) c- were calculated and plotted in Figure 3, along with the effective surfactant concentration c( ) (c+c-)1/2. As expected, the agreement between the calculated values of cCl- and the data from Cl- activity is quite good, giving some confidence about the predicted values of cCP+ and c(. The conductivities of aqueous LAM solutions, measured at 25 °C, increased from 0.12 µS/cm at 0.001 mM to 2300 µS/cm at 60 mM. There is a visual cmc or “break” at about 6 mM (Figure 6), which agrees with the ion activity measurement results (Table 1). The limiting molar conductivity Λ° obtained by extrapolating the pre-cmc data (from about 0.02 to 0.1 mM) to zero concentration is 96 ( 1 S‚cm2/mol, where Λ° ) ΛCl- + ΛR+ from which ΛR+ ) 20 S‚cm2/mol. Model A was also used for calculating conductivity profiles and fitting the data (Figure 6). The best fit of the data was obtained with cmc ) 6 mM, N )
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60, and β ) 0.7. Below the cmc the agreement between the predictions and the data is fairly good, except for data with large error bars at concentrations less than 0.01 mM, probably due to the same reasons discussed above. Above the cmc the predicted profiles match the data quite well, indicating that model A is a good conductivity model for estimating not only the cmc but also β and N for aqueous LAM solutions. The concentrations of surfactant ion and counterion, and the effective surfactant concentration were calculated with the micellization parameters estimated from conductimetry. The predictions of cCl- agree with the Cl- activity data well below the cmc and fairly well above the cmc (Figure 4). Surface Tension and Fluorescence Results. The equilibrium surface tensions of aqueous LAM solutions decreased gradually with increasing concentration from 66 mN/m at 0.001 mM to about 30 mN/m at 6 mM (Figure 7). Above the break, indicating the cmc, the tension curve is flat. The cmc agrees with those from conductivity and ion activity data (Table 1). The cmc of a surfactant solution can also be estimated from the analysis of the fluorescence spectra of solutions of pyrene solubilized in micelles.14,18,19 The ratio of the fluorescence intensity of the highest energy vibration band (II) to the fluorescence intensity of the third highest energy vibrational band (IIII) has been shown to decrease when pyrene is solubilized in or near the interior hydrophobic region of the surfactant micelles. Figure 8 shows the changes of II/IIII for various concentrations of LAM. A fairly abrupt step change starts at about 2 mM and is completed at about 8 mM. The midpoint of this change, due to surfactant micellization, is about 5 mM, which is close to the cmc obtained with other methods (Table 1). This procedure will be used for the gemini surfactants, which show nonstandard aggregation behavior. 4.2. Cationic Gemini Surfactants. Overview. The solution and tension behavior of bis(Args) gemini surfactants, a novel class of gemini surfactants, was studied by different techniques. It was found that these surfactants have two cmc’s, or breaks in properties, as determined by different methods. The tension-derived cmc’s are quite small, and the tension-concentration curves are flat above the cmc’s, indicating that the surfactants form aggregates of substantial size at extremely low concentrations.5,6 Fluorescence results and lower chloride counterion binding than that for LAM suggest that these gemini aggregates are nonglobular. These methods reveal also that there is a second cmc leading to globular and perhaps larger aggregates at much higher concentrations. The length n of the spacer chain has a small effect on the surfactant properties and the cmc’s. Surface Tension and Fluorescence Results. Figure 7 shows the surface tension versus logarithmic concentration plots for three members of the Cn(LA)2 class of gemini surfactants (see structures in Figure 1). The tension of each surfactant decreased gradually with increasing surfactant concentration and then showed a break point, which was taken as the cmc. We will call such a break the first cmc or cmc1, as is further documented later in this section. The first cmc’s of C3(LA)2, C6(LA)2, and C9(LA)2 are about 0.005, 0.002, and 0.003 mM, respectively (consistently with Pe´rez et al.6), or about 3 orders of magnitude lower than that of the corresponding singlechain surfactant LAM (section 4.1). The results indicate (18) Gratzel, M.; Thomas, J. K. In Modern Fluorescence Spectroscopy; Wehry, E. L., Eds.; Plenum Press: New York, 1976; Vol. 2, p 169. (19) Turro, N. J.; Baretz, B. H.; Kuo, P.-L. Macromolecules 1984, 17, 1321.
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Table 1. Summary of cmc’s of Aqueous LAM and Cn(LA)2 Surfactant Solutions with Different Techniques C3(LA)2
C6(LA)2
C9(LA)2
method
LAM cmc, mM
cmc1, mM
cmc2, mM
cmc1, mM
cmc2, mM
cmc1, mM
cmc2, mM
surface rension fluorescence ion activity conductivity
6 5 6 6
0.005 NDa ND ND
ND 0.4 0.4 0.5
0.002 ND ND ND
ND 0.4 0.6 0.4
0.003 ND ND ND
ND 0.3 0.3 0.3
a
ND: not detected.
Figure 7. Comparison of equilibrium surface tensions at 25 °C of aqueous solutions of LAM (a) and Cn(LA)2 (b: b, n ) 3; 2, n ) 6; 9, n ) 9). The arrow in part a indicates the cmc; arrows in part b indicate the approximate first cmc’s, or cmc1’s.
that these gemini surfactants at extremely low concentrations form certain aggregates of sizes substantial enough to produce fairly constant monomer activity above the cmc1. Forming small aggregates, such as dimers or trimers, would not be sufficient for maintaining the tension constant. Formation of mostly larger aggregates, with average aggregation number perhaps larger than 2 or 3, would be consistent with a flat post-cmc tension curve.8,9 The minima in the curves are hard to interpret, as they could be due to complex aggregation behavior or small amounts of impurities. A direct method is of course needed to establish the aggregates sizes. The cmc decrease from the monomeric to the dimeric surfactant has been accounted for certain systems from the free energy changes upon micellization.2 In this theory, and in our data, the spacer chain length affects the critical micelle concentration to some degree. The lowest cmc, or perhaps the maximum hydrophobic interaction between the alkyl chains, was estimated to occur for n ) 6. In our data, the lowest cmc is at n ) 6, although the differences are not large. Moreover, the minimum surface tensions of bis(Args) surfactants are about the same: 32 ( 2 mN/m for C3(LA)2, 31 ( 2 mN/m for C6(LA)2, and 33 ( 2 mN/m for C9(LA)2.
Figure 8. Comparison of the fluorescence ratio II/IIII of pyrene with concentrations of aqueous solutions of LAM (a) and Cn(LA)2 (b: b, n ) 3; 2, n ) 6; 9, n ) 9). The abrupt step change of II/IIII is due to solubilization of pyrene in surfactant micelles. In part a, arrow 1 indicates the approximate cmc of LAM, as determined from the chloride ion activity, conductivity, and surface tension data (Figures 4, 6, and 7). Arrow 2 is the midpoint of the ratio change, usually defined as the fluorescencebased cmc. In part b, the first set of arrows indicates the first cmc’s as determined from surface tension data (Figure 7). The second set of arrows indicates the fluorescence-based cmc2’s.
Figure 8 shows that no change in fluorescence intensity ratio II/IIII occurs at the cmc1, suggesting no change in the solubilization behavior of pyrene and no strongly hydrophobic environment. Nonetheless, an abrupt change, apparently due to regular surfactant micellization, does occur at a much higher concentration for all three surfactants, starting at 0.1 mM and possibly being completed at approximately 0.8 mM. If the midpoint is taken as the cmc, then the cmc values for C3(LA)2, C6(LA)2, and C9(LA)2 are 0.4, 0.4, and 0.3 mM, respectively (see Table 1). This cmc is termed the second cmc or cmc2. The surface-tension-based cmc may indicate that if aggregates form between 0.003 and 0.1 mM, they are probably nonglobular and do not affect the fluorescence ratio by not solubilizing the fluorophore pyrene. This behavior (two cmc’s) has also been reported for certain other gemini surfactants.20 In summary, the cmc1 may be attributed to nonglobular aggregates which do not affect (20) Frindi, M.; Michels, B.; Le´vy, H.; Zana, R. Langmuir 1994, 10, 1140.
Cationic Surfactants Derived from Arginine
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Figure 10. Concentrations of free chloride ions in aqueous gemini surfactant Cn(LA)2 (O, n ) 3; 4, n ) 6; 0, n ) 9) solutions; see Figure 9. The C3 and C6 surfactants exhibit more sharply defined “breaks” (second cmc; see text) than those for the C9 surfactant. Inset: data at low concentrations.
Figure 9. Potentials of a chloride selective electrode in contact with aqueous solutions of C3 (a, b), C6 (b, 2), and C9 (b, 9), at 25 °C. Arrows indicate the approximate cmc’s by this method. The cmc’s are close to the fluorescence-based cmc2’s. NaCl (0) and KCl (+) solutions were used for calibration. For C9, some slope change occurs at about c ) 0.03 mM, which is above the tension-based cmc1. Aggregate formation is indicated at c > 0.03 mM, but only a slight change in slope is observed at c = 0.1 mM.
the pyrene fluorescence behavior and cannot be detected by fluorescence spectral measurements but have an overall effect of producing constant surfactant monomer concentration. By contrast, the cmc2 seems to lead to globular aggregates, analogous in their solubilization behavior to globular micelles of monomeric surfactants. Chloride Activity and Conductivity Results. For investigating this unusual aggregation behavior further, the electrode emf’s of a chloride selective electrode were measured for aqueous solutions of the three gemini surfactants, C3(LA)2, C6(LA)2, and C9(LA)2 (Figure 9). The emf curves of C3(LA)2 and C6(LA)2 are similar and overlap with the curves of NaCl and KCl in the linear region up to the “break” point. For concentrations lower than about 0.1 mM, where measurements were less reliable due to low sensitivity and OH- ion interference, the emf’s are somewhat lower than those of NaCl and KCl. Since the curves are nonlinear in this range, we cannot place much significance in the data at these concentrations. The break points are attributed to substantial aggregation and counterion binding, and occur at concentrations higher than the cmc1, or about 0.4 mM for C3(LA)2 and 0.6 mM for C6(LA)2. The emf curve of C9(LA)2 overlaps little with those of NaCl and KCl. No clear break point was observed, either because of different aggregation behavior or probably because in the emf measurements the cationic surfactant molecules interact with the reference solution quite differently from those of Na+ and K+ ions, affecting the reference junction potential. The estimated free chloride ion concentrations, found by using NaCl and KCl as calibration standards, are shown in Figure 10. The observed breaks, which are consistent with the fluorescence-based cmc2’s, are 0.4 ( 0.1 mM for C3(LA)2, 0.6 ( 0.1 mM for C6(LA)2, and 0.3 ( 0.1 mM for C9(LA)2 (Table 1). Above the cmc2, the counterion binding parameters β2,
Figure 11. Conductivities of aqueous gemini surfactant Cn(LA)2 (O, n ) 3; 4, n ) 6; 0, n ) 9) solutions at 25 °C. Arrows suggest the onset of aggregation formation, consistently with the second cmc’s.
estimated with the pseudo-phase separation model, are 0.62 ( 0.05, 0.55 ( 0.05, and 0.56 ( 0.05 for n ) 3, 6, and 9, respectively. These values are of the same order as those for LAM or CPC (section 4.1), which presumably form globular aggregates. At the cmc1, where nonglobular, and perhaps smaller, aggregates are suggested by tensiometry, the counterion binding parameters were estimated to be nonzero and about 0.2 ( 0.1 for all three surfactants. These values are more imprecise due to the low sensitivity of the electrode at concentrations lower than 0.1 mM. The conductivities of the aqueous gemini surfactants increased linearly with increasing concentration up to the following break points: 0.50 ( 0.05 mM for C3(LA)2, 0.40 ( 0.05 mM for C6(LA)2, and 0.30 ( 0.05 mM for C9(LA)2 (Figure 11). For showing the low-concentration data in better detail, the molar conductivities of C9(LA)2 are plotted versus log(c) in Figure 12, where one may detect more than one “break”. We caution that these “breaks” may not necessarily signify changes in aggregation patterns. The first “break” at about 0.004 mM is close to cmc1 determined from surface tension data. A plateau of
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Figure 12. Comparison of molar conductivity data for C9(LA)2 (O) with model B-predicted results with cmc ) 0.003 mM, β ) 0.2, and N ) 5, 10, and 20 (solid lines). The cmc value was obtained from tensiometry, and the value of β was inferred from chloride activity data. Possible N values consistent with a sharp change in γ(c) were used. Filled arrows indicate the cmc1 obtained from tensiometry (Figure 7) and the cmc2 inferred from fluorescence data (Figure 8). Open arrows indicate “breaks” in Λ(c); see text.
about 216 S‚cm2/mol is observed between 0.006 and 0.05 mM. At 0.06 mM the molar conductivity decreases slightly to about 195 S‚cm2/mol, and then a second plateau occurs, ending at 0.2-0.3 mM. The fourth break occurs close to cmc2 determined from the fluorescence data. Due to the lack of the range of constant Λ’s at low concentrations, it is difficult to estimate the limiting molar conductivity of C9(LA)2. The value of the first plateau, 216 S‚cm2/mol, was used in certain conductivity model calculations because it is very close to the value 214 S‚cm2/ mol, estimated by using the Stokes-Einstein equation (section 2.2). In Figure 12 data are compared to predictions by using the values cmc ) 0.003 mM and of β ) 0.2 and several different values of the possible aggregation number N. Model B was used in the first calculation, because micelles are expected to contribute to the ionic strength at such low concentrations if their size is much smaller than the Debye length (section 2.2). With this model and the parameters used, the molar conductivity versus concentration curves are predicted to have a maximum above the cmc1. The maximum increases with increasing N, because of the large charge of the aggregates (section 2.2). However, at high concentrations, conductivities decrease more rapidly with the larger value of N, due to the larger increases in ionic strength or stronger ionic interactions. Figure 13 shows the predictions from model A with β ) 0.6 and cmc ) 0.3 mM. The predicted molar conductivities decrease with increasing concentration and increase with increasing N above the cmc. With cmc ) 0.3 mM, the predictions fit the data well for concentrations between 0.006 and 0.05 mM. From 0.06 to 0.1 mM the predictions are a little higher than the data. More data and more detailed calculations are needed for testing the model at concentrations higher than 0.3 mM. Overall, the solution properties of these dimeric (gemini) surfactants can be quite different from those of the corresponding conventional monomeric surfactants and can be tuned slightly by modifying the length of the spacer group. Bis(Args) surfactants form aggregates at concentrations well below that of LAM, indicating that they are more efficient in lowering the surface tension of water.
Pinazo et al.
Figure 13. Comparison of molar conductivity data for C9(LA)2 (O) with model A-predicted results with cmc ) 0.3 mM, β ) 0.6, and N ) 10, 20, and 40 (solid lines, from bottom to top).
5. Conclusions Cationic gemini surfactants, bis(Args) (Cn(LA) 2, n ) 3, 6, and 9), form aggregates at a much lower concentration, cmc1, than that for the corresponding monomeric surfactant, LAM, and their minimum tensions at the cmc are comparable. Therefore, gemini surfactants can produce lower tensions than monomeric surfactants at the same molar or mass concentrations. Because of the hydrophobic interactions between the two alkyl chains, the length of the spacer chain affects the concentration at which surfactant molecules form aggregates. Unconventional aggregation behavior and two cmc’s were inferred from several techniques. The first cmc’s, or cmc1, inferred primarily from tension data and indirectly supported by chloride ion activity and conductivity data, are between 0.002 and 0.005 mM. Fluorescence and ion activity data also indicate clearly a conventional cmc, or cmc2, at about 0.3-0.6 mM (for these surfactants), which is about 2 orders of magnitude higher than that from the tension results. Conductivity data are generally consistent with these inferences. The low-concentration cmc may be attributed to nonglobular aggregates with small aggregation numbers, but perhaps larger than 2 or 3, to account for the nearly constant monomer activity and the nearly constant tension above the cmc1, and to small counterion binding parameters. At concentrations higher than cmc2, strong and diverse evidence shows that surfactants form micellelike globular micelles, that their degree of counterion binding is higher, that their molar conductivity is smaller, and that the fluorophore solubilization behavior is similar to those of conventional monomeric ionic surfactants. Future experiments, possibly by light scattering or cryotransmission electron microscopy techniques, may shed additional light onto their unusual aggregation behavior and obtain direct quantitative information on the microstructures and dimensions of the two (or more) types of aggregates present at the various concentrations above the first cmc. Acknowledgment. We are grateful to the National Science Foundation for partial support of this work through Grants Nos. CTS 93-04328 and CTS 96-15649. LA981295L
