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Synergism in mixed zwitterionic–anionic surfactant solutions and the aggregation numbers of the mixed micelles
Colloids and Surfaces A: Physicochemical and Engineering Aspects 145 (1998) 167–174
Synergism in mixed zwitterionic–anionic surfactant solutions and the aggregation numbers of the mixed micelles Fang Li *, Gan-Zuo Li, Jian-Bo Chen Institute of Colloid and Interface Chemistry, Shandong University, Jinan 250100, People’s Republic of China Received 25 February 1998; accepted 16 April 1998
Abstract Synergistic interaction between sodium dodecyl sulfonate (C AS) and three zwitterionic surfactants was measured 12 at 40.0°C in aqueous 0.1 M NaCl solution. The three zwitterionic surfactants were alkyldimethylammoniopropanesulfonates with different alkyl chain length — 12, 10 and 8 (abbreviated as C DPAS, C DPAS and C DPAS, respectively). 12 10 8 Surface tension results showed that a certain efficiency exists both in mixed micelle formation and surface tension reduction owing to the electrostatic attraction between the anionic and zwitterionic surfactants. The strength of the interaction between the zwitterionic surfactant and C AS in the three mixed systems obeys the following order: 12 C AS/C DPAS>C AS/C DPAS>C AS/C DAPS. The synergism is greatest when the molar ratio of anionic 12 12 12 10 12 8 surfactant to zwitterionic surfactant in the solution is around 7:3. Fluorescence studies on the C AS/C DPAS system 12 12 showed that the mixed micellar aggregation number becomes smaller than that of the zwitterionic surfactant with the addition of a small amount of anionic surfactant to the zwitterionic surfactant. However, after a small amount of C DPAS was added to C AS, the mixed C AS/C DPAS micellar aggregation number is much larger than that of 12 12 12 12 C AS. The mixed micellar aggregation numbers become largest at the composition point where the efficiency of 12 mixed micelle formation is greatest. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Synergism; Zwitterionic surfactant; Anionic surfactant; Fluorescence; Aggregation number
1. Introduction Zwitterionic surfactants are becoming increasingly attractive because of their good solubility in water and their reduced sensitivity to salts and temperature [1], but their wider application in industry is limited by their higher production cost compared with anionic surfactants. In general, the interfacial and colloid properties of a solution of two or more surfactants can be quite different from those of solutions of the individual compo* Corresponding author. Present address: Chemistry Department, Clarkson University, Potsdam, NY 13699-5810, USA. E-mail: [email protected]
nents. When a solution of mixed surfactants shows greater surface activity (e.g., lower surface tension or critical micelle concentration) than that attainable with any of the individual surfactants of the mixture at the same concentration, then the mixture is said to exhibit synergism. Many reports [2– 6 ] have revealed that zwitterionic surfactants can have a strong interaction with anionic surfactants in aqueous solution. So it is very interesting and useful to design some mixed anionic/zwitterionic surfactant systems in which the advantages of either surfactant can be exploited. Most of the previous studies have focused on alkyl betaine [7– 10], which is pH-sensitive zwitterionic surfactant. In this sense, the betaines, which are capable of
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accepting a proton, can certainly interact much more strongly with anionic surfactants than with cationic surfactants. The zwitterionic surfactants investigated in the present work — alkyldimethylammoniopropanesulfonates — are not sensitive to the pH value of solutions, remaining as real zwitterionic surfactants at any pH after dissolution in water. By means of surface tension measurements, we have studied the strength of the interaction of these zwitterionic surfactants with dodecylsulfonate. The results showed that both electrostatic attraction and van der Waals’ attraction play important roles in the synergism. With the fluorescence probe method, we also investigated the relationship between the mixed micelle aggregation number and the surfactant composition of the solution.
The surface tensions were obtained until no significant changes occurred. The critical micelle concentration of the mixed systems was determined by plotting surface tension values against the logarithm of the total surfactant concentration. 2.3. Fluorescence measurement The fluorescence measurements were recorded on an RF-540 spectrofluorimeter at 40±0.02°C. The excitation of the probe [Ru(C H N ) · 10 8 2 3 6H O] was performed at 476 nm and the emission 2 was monitored at 620 nm. The concentration of probe was kept constant (7.0×10−5 M ) in all solutions. The method of preparing solutions, which contain probe and quencher (9-MeA), was the same as that described in a preceding paper [11].
2. Experimental 3. Results and discussion 2.1. Materials Sodium dodecyl sulfonate (originally chemical pure) was purified as described elsewhere [11]. Ndodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (C DPAS ), N-decyl-N,N-dimethyl-3-amm12 onio-1-propane sulfonate (C DPAS ) and N-octyl10 N,N-dimethyl-3-ammonio-1-propane sulfonate (C DPAS ) were all purchased from Sigma 8 Company (purity >99.5%) and used without further purification. The structure of the zwitterionic surfactants is:
3.1. Zwitterionic surfactant–anionic surfactant interaction The surface tension–log C (surfactant concentration) plots for C AS/C DPAS, C AS/ 12 12 12 C DPAS and C AS/C DPAS are shown in 10 12 8 Figs. 1–3, respectively. The nature and strength of the interaction
CH –(CH ) –N+(CH ) –CH –CH –CH –SO− 3 2n 32 2 2 2 3 where n=11, 9 and 7 for C DPAS, C DPAS 12 10 and C DPAS, respectively. 8 The fluorescence probe [Ru(C H N ) · 6H O] 10 8 2 3 2 and quencher [9-methylanthracene (abbreviated as 9-MeA)] were also purchased from Sigma Chemical Company and used directly. 2.2. Surface tension measurement Surface tension was measured at constant ionic strength (0.1 M NaCl ) and at 40±0.02°C by the Wihelmy plate technique with a CVBP-A model 3 tensionmeter ( Kyowa Kanmenkagaku Co., Ltd ).
Fig. 1. Variation of the surface tension versus total surfactant concentration for different mole ratios of anionic to zwitterionic surfactant in the C AS/C DPAS system. Molar ratio of 12 12 C AS to C DPAS: % 8:2; # 7:3; × 1:1; 6 3:7; $ 2:8. 12 12
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larger the negative value of b, the stronger the attractive interaction between the two different surfactants and the greater the synergism between them. According to Rubingh’s non-ideal solution theory [12], after measuring the critical micelle concentration (CMC ) of the aqueous solution of mixed surfactants and the CMC of the individual surfactants, the value of the parameter bm — representing the interaction in the mixed micelle in the aqueous phase — can be calculated from the equation: X2 ln [(CMC · a)]/(CMC · X ) 1(m) 1 1 1(m) =1 [1−X ]2[CMC · (1−a)]/[CMC · (1−X )] 1(m) 1,2 2 1(m) (1) Fig. 2. Variation of the surface tension versus total surfactant concentration for different mole ratios of anionic to zwitterionic surfactant in the C AS/C DPAS system. Molar ratio of 12 10 C AS to C DPAS: % 8:2; # 7:3; × 1:1; 6 3:7; $ 2:8. 12 10
where X is the mole fraction of surfactant 1 in 1(m) the total surfactant in the mixed micelle, a is the mole fraction of surfactant 1 in the total surfactant in solution. CMC , CMC and CMC are the 1 2 1,2 critical micelle concentrations in the aqueous phase of surfactant 1, surfactant 2 and their mixture at a mole fraction of a, respectively. After determining X from Eq. (1), we can obtain the inter1(m) action parameter bm according to: ln [(CMC · a)]/(CMC · X ) 1,2 1 1(m) (2) (1−X )2 1(m) Similarly, the treatment can be used in the mixed monolayer at the aqueous solution/air interface and the parameter for the interaction in the mixed monolayer at the solution/air interface ( bS) can be obtained [13]. All of the results calculated for the three mixed anionic/zwitterionic systems are listed in Table 1. The existence of synergism in mixtures containing two surfactants depends not only on the strength of the interaction between them (measured by the values of b), but also on relevant properties of the individual surfactant components of the mixture [13]. Thus the conditions for synergism to exist in mixed micelle formation are: (1) bm must be negative; and (2) |bm|>|ln(CMC / 1 CMC )|. Similarly, the synergism in surface tension 2 reduction efficiency exists when bS is negative and the absolute value of bS is larger than that of ln(C0 /C0 ). The results for the C AS/C DPAS 1 2 12 12 bm=
Fig. 3. Variation of the surface tension versus total surfactant concentration for different mole ratios of anionic to zwitterionic surfactant in the C AS/C DPAS system. Molar ratio of 12 8 C AS to C DPAS: % 8:2; # 7:3; × 1:1; 6 3:7; $ 2:8. 12 8
between two surfactants can be measured by values of the so-called b parameter, which show the degree of the non-ideality of the interaction in the mixed monolayer at the air/solution interface or in the mixed micelle in the solution phase. The
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Table 1 Molar interaction and synergism parameters in the three mixed systems System
C AS/C DPAS 12 12
C AS/C DPAS 12 10
C AS/C DPAS 12 8
Molar ratio of 1 to 2a
0:1 2:8 3:7 1:1 7:3 8:2 1:0 0:1 2:8 3:7 1:1 7:3 8:2 1:0 0:1 2:8 3:7 1:1 7:3 8:2 1:0
In the mixed micelles
In the solution/air monolayer
CMC (M )
X 1m
X 2m
Ln(CMC / 1 CMC ) 2
bm
C (40 mN/m)b 0
Ln(C0 /C0 ) 1 2
bS b
˚ 2) A (A 2
1.0×10−3 5.6×10−4 4.5×10−4 3.2×10−4 2.5×10−4 3.5×10−4 1.6×10−3 1.0×10−2 2.4×10−3 2.1×10−3 1.5×10−3 1.3×10−3 1.6×10−3 1.6×10−3 3.6×10−2 5.6×10−3 3.8×10−3 2.6×10−3 2.5×10−3 2.6×10−3 1.6×10−3
0 0.33 0.39 0.46 0.52 0.56 1 0 0.55 0.60 0.67 0.73 0.84 1 0 0.75 0.78 0.85 / / 1
1 0.67 0.61 0.54 0.48 0.44 0 1 0.45 0.40 0.33 0.27 0.16 0 1 0.25 0.22 0.15 / / 0
/ 0.53 0.53 0.53 0.53 0.53 / / −1.77 −1.77 −1.77 −1.77 −1.77 / / −3.05 −3.05 −3.05 −3.05 −3.05 /
/ −3.53 −4.20 −5.36 −6.81 −5.92 / / −2.89 −2.72 −3.29 −3.57 −2.21 / / −1.20 −1.82 −1.96 / / /
7.5×10−4 2.1×10−4 1.8×10−4 1.3×10−4 1.5×10−4 1.7×10−4 8.4×10−4 7.5×10−4 / / / / / / 7.5×10−4 / / / / / /
/ 0.11 0.11 0.11 0.11 0.11 / / / / / / / / / / / / / / /
/ −4.88 −5.15 −6.08 −6.10 −5.86 / / / / / / / / / / / / / / /
46.4 46.7 40.0 37.0 28.0 34.0 48.1 48.9 45.5 42.6 40.4 35.0 45.2 48.1 53.0 53.7 50.5 50.7 54.0 58.2 48.1
/ means no reasonable values or no reasonable calculated results. a1 refers to anionic surfactant (C AS) and 2 refers to zwitterionic surfactant. 12 bC is the molar concentration of surfactant 1, 2 or the mixture, respectively, required to yield the same value of the surface tension 0 (here, 40 mN/m). bS is the parameter for the surfactant–surfactant interaction in the mixed monolayer at the aqueous solution/air interface.
mixed system ( Table 1) satisfied the conditions for the existence of synergism in both mixed micelle formation and the surface tension reduction, meaning that C AS has a synergistic effect with 12 C DPAS both in surface tension reduction effi12 ciency and in mixed micelle formation. The bS values are more negative than the bm values in this system, which may be due to the difficulty of incorporating tightly packed chains into a micelle. For the C AS/C DPAS system only synergism in 12 10 mixed micelle formation exists; there is no synergism in the surface tension reduction. No synergism exists at all in the mixed C AS/C DPAS 12 8 system, although its bm value is negative. Both the difference between bS and bm in the C AS/C DPAS system, and the absolute values 12 12 of bS and bm in all three systems, are all much less
than those in mixed anionic/cationic surfactant systems [11,14], indicating the interaction between C AS and alkyldimethylammoniopropanesulfo12 nates is much weaker than that with some cationic surfactants such as alkyltrimethylammonium bromide. For the two mixed systems, C AS/C DPAS 12 12 and C AS/C DPAS, the change of CMC values 12 10 versus molar fraction of the anionic surfactant (C AS ) is shown in Fig. 4. It can be seen that the 12 CMC values of the mixed system decrease with increasing mole fraction of C AS before showing 12 a minimum when the mole fraction of C AS is 12 around 0.7. If the thermodynamics of the micellization process for the two systems obey the ideal solution theory, when surfactant monomer and mixed
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the mole fraction of C AS in the mixed 12 surfactants. The theoretical CMC values for each C AS 12 molar ratio of the two mixed systems are also shown in Fig. 4. The experimental CMC values of the two mixed systems are substantially lower than those predicted by Eq. (3), indicating that the mixed micelle formation shows a negative deviation with respect to ideal behavior. The general Gibbs adsorption equation in our anionic (NaR)/zwitterionic (Z ) mixed surfactants can be written as: dc
=∑ C d ln a =C d ln a i i Na+ Na+ i +C d ln a +C d ln a (4) R− R− Z Z where C , C and C are the surface excess Na+ R− Z concentrations for Na+, R− and the zwitterionic surfactant (Z ), respectively. a , a and a are Na+ R− Z their activities. With constant ionic strength (0.2 M NaCl ) and the lower concentration of the surfactants, the following equation holds: −
(a)
−
(b)
Fig. 4. The relationship between CMC values and different mole fractions of C AS in the C AS/zwitterionic mixed surfactant 12 12 systems: (a) C AS/C DPAS system; (b) C AS/C DPAS 12 12 12 10 system.
1
=
X
+
1−X
(3) CMC CMC CMC a/z a z where CMC , CMC and CMC are the critical a/z a z micelle concentrations of the anionic/zwitterionic mixed surfactant, the anionic surfactant (C AS ) 12 and the zwitterionic surfactant, respectively. X is
dc RT
=C
R−
d ln a +C d ln a =C R− Z Z R−
×d ln C +C d ln C (5) R− Z Z where C and C are the concentrations of R− R− Z and the zwitterionic surfactant, respectively. When the mole ratio of anionic surfactant to zwitterionic surfactant is kept constant, then: −
micelles are in equilibrium in the mixed system, then the CMC values of the mixed micelles should fall on the line predicted by the following relationship [15]:
RT
dc RT
=C d ln C +C d ln C R− T Z T =(C
+C ) d ln C =C d ln C (6) R− Z T T T C and C being the total surface excess concenT T tration and the total surfactant concentration in the solution, respectively. So, from the c–ln C curve, we also get the T largest surface excess concentration (C ) for the 2 average area (A ) occupied per surfactant molecule 2 at the air/solution surface when saturated absorption is reached. For the two mixed systems (C AS/C DPAS and C AS/C DPAS ), the area 12 12 12 10 per molecule at the air/solution surface is smallest
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at the point where the mole fraction of C AS is 12 0.7 and the interaction parameters ( bm and bS) are largest. When the mole fraction of C AS is not 12 0.7, the value of A is also smaller than that of 2 every individual surfactant, meaning there is obvious synergism between C AS and C DPAS, 12 12 C DPAS. But for C AS/C DPAS, the values of 10 12 8 A for the mixed surfactant are almost the same 2 as or even larger than that of any individual surfactant, suggesting that no synergism exists between C AS and C DPAS. 12 8 The interaction between anionic surfactant and cationic surfactant is mostly due to the strong electrostatic attraction between the negatively charged surfactants. For some anionic/zwitterionic mixed surfactant systems, such as anionic/betaine systems [4,5,10], the betaines are capable of accepting a proton and act as a cationic surfactant. So the betaines can interact more strongly with anionic surfactants than with cationic surfactants. The zwitterionic surfactants we studied here, alkyldimethylammoniopropanesulfonates, are not sensitive to pH in the solution. They cannot easily accept a proton from the solution and they can never function as anionic surfactants. But the head group of these zwitterionic surfactants, –N+(CH ) –CH –CH –CH –SO− , is as long as 2 2 3 ˚ [16 ].3 2Their2 negative 6A charge center cannot overlap the center of their positive charge. So, although there is no net charge, many physical properties of the alkyldimethylammoniopropanesulfonates are quite different from those of nonionic surfactants and very similar to those of ionic surfactants [17], and they can have a synergistic effect with C AS by means of electrostatic attrac12 tion between the N+(CH ) group in the zwitter32 ionic surfactant and the SO− group in C AS. 3 12 In addition to the electrostatic attraction effect, van der Waals’ attraction between the hydrocarbon chains of the surfactants involved also plays an important role in their synergism effect [18]. Although the electrostatic attraction between C DAPS, C DPAS and C DPAS with C AS is 12 10 8 12 almost same, the van der Waals’ interaction between the different zwitterionic surfactants and C AS should have the following order because of 12 the difference in alkyl chains between C DPAS n and C AS: C AS/C DPAS>C AS/C DPAS> 12 12 12 12 10
C AS/C DPAS. So the interaction between 12 8 C DPAS and C AS is the greatest amongst the 12 12 three mixed systems and there is no obvious synergism between C DPAS and C AS. When the chain 8 12 length of both surfactants is equal, the molecular packing at the air/water interface is closer. However, when the chain length is different, the portion of the molecules above the height of the adjacent molecules will exhibit thermal motion [19], causing larger values of A . Thus, at a certain 2 molar ratio of C AS to C DPAS, the values of 12 n A in C AS/C DPAS are the largest and in 2 12 12 C DPAS/C DPAS are the smallest ( Table 1). 12 8 3.2. Mixed micellar aggregation numbers in the C AS/C DPAS system 12 12 A fluorescence method based on fluorescence quenching was used to determine the values of the aggregation number in the three mixed systems. With some assumptions [20], the aggregation number N in the mixed systems can be determined by measuring the total intensity of the fluorescence with and without quencher, I and I , respectively. t 0 The following equation holds:
A
[Q] I =I exp − t 0 [M ]
B
(7)
where [Q] is the concentration of the quencher (9-MeA). [M ] is the concentration of mixed micelles or individual micelles, which is related to the micellar aggregation number by: [M ]=
[S ]−[S ] t f N
(8)
where [S ] is the total concentration of surfactants t in the individual or mixed system. [S ] is the f concentration of monomer surfactant which, to a good approximation, can be taken as the CMC values in the individual or mixed system. So:
AB
N([S ]−CMC ) t = t (9) I [Q] 0 Keeping the total surfactant concentration S as t 0.018 M, we recorded the fluorescence probe emission spectra at different quencher (9-MeA) concenln
I
F. Li et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 145 (1998) 167–174
trations with various mole ratios of C AS to 12 C DPAS ( Fig. 5). Compared with S , the CMC 12 t values are small enough to be neglected. When a different amount of NaCl is added to the mixed system, the CMC values will decrease and can also be neglected. From each straight line with a linear relationship between ln(I /I ) and [Q], we obtained t 0 the N values of C AS/C DPAS at different mole 12 12 ratios of C AS to C DPAS with different NaCl 12 12 concentrations (shown in Fig. 6). From the results in Fig. 6, we can see that C AS/C DPAS is much different from the 12 12 C AS/CTAB system [11]. In the anionic/cationic 12 mixed system, both a small addition of CTAB to C AS and a small addition C AS to CTAB can 12 12 result in a great increase of the aggregation number of C AS or CTAB. Moreover, the value of N 12 becomes larger and larger as the mole fraction of the anionic surfactant approaches that of the cationic surfactant. Clearly, the great increase of N and the largest N value at equal mole fractions of
(a)
(b)
Fig. 5. The fluorescence emission spectra of the probe in the C AS/C DPAS mixed micelle system at different quencher 12 12 concentrations without NaCl. ( From top to bottom, the concentration of quencher is 0.0, 1.0×10−4, 2.0×10−4, 3.0×10−4, 4.0×10−4 and 5.0×10−4 M, respectively.) (a) 7:3 Molar ratio of C AS to C DPAS; (b) 8:2 molar ratio of 12 12 C AS to C DPAS. 12 12
173
Fig. 6. The micellar aggregation numbers (N ) in the C AS/C DPAS mixed system at different NaCl concentrations 12 12 and different C AS/C DPAS molar ratios. 12 12
C AS and CTAB are due to the strong electro12 static attraction between the oppositely charge surfactants and, as a result, the surfactants tend to arrange more closely with one another in the mixed micelles. However, there is no net charge in the zwitterionic surfactant (C DPAS ) micelle. A 12 small addition of C AS can result in a great 12 increase in total charge and generally make the surfactant arrange more loosely, resulting in a decrease of the micellar aggregation number. On the other hand, when a small amount of zwitterionic surfactant is added to C AS, it will locate 12 between C AS in the mixed micelles and can 12 partly screen the electrostatic repulsion between the –SO− groups. As a result, the surfactants 3 arrange more closely in the micelles and the micelle aggregation number increases. The biggest N value appears at the composition point where the synergism is greatest. There is still a net charge on the mixed anionic/zwitterionic surfactant system, and so the addition of NaCl can compress their double electrical layers and increase the N values, as it does in ionic surfactant systems.
Acknowledgment This work was supported by the Natural Scientific Research Foundation of China.
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References [1] B.R. Bluestein, C.L. Hilton (Eds.), Amphoteric Surfactants, Surfactants Series 12, Dekker, New York, 1982. [2] D.G. Kol, R.G. Laughlin, F.P. Krause, R.E. Zimmerer, J. Phys. Chem. 67 (1963) 51. [3] M.J. Rosen, D. Fredman, M. Gross, J. Phys. Chem. 68 (1964) 3219. [4] M.J. Rosen, Langmiur 7 (1991) 885. [5] T. Iwasaki, M. Ogawa, K. Esumi, K. Meguro, Langmuir 7 (1991) 30. [6 ] Y. Konodo, H. Uchiyama, N. Yoshino, K. Nishiyama, M. Abe, Langmuir 11 (1995) 2380. [7] J. Swarbick, J. Daruwala, J. Phys. Chem. 74 (1970) 3022. [8] J. Marignan, F. Gauthier-Fournier, J. Appell, J. Phys. Chem. 92 (1988) 440. [9] A. Bhatia, S. Qutubuddin, Colloids Surf. 69 (1992) 277.
[10] A. de la Maza, J.L. Para, Colloids Surf. 90 (1994) 79. [11] G.Z. Li, F. Li, L.Q. Zheng, H.L. Wang, Colloids Surf. 76 (1993) 257. [12] D.N. Rubingh, in: K.L. Mittal (Ed.), Solution Chemistry of Surfactants, vol. 1, Plenum Press, New York, 1979, p. 337. [13] X.Y. Hua, M.J. Rosen, J. Colloid Interf. Sci. 90 (1982) 212. [14] L.Q. Zheng, F. Li, J.C. Hao, G.Z. Li, Colloids Surf. A 98 (1995) 11. [15] M.F. Cox, N.F. Borys, T.P. Matson, J. Am. Oil. Chem. Soc. 62 (1985) 1139. [16 ] R. Nagarajan, E. Ruckenstien, J. Colloid Interf. Sci 71 (1979) 80. [17] P. Liaos, R. Zana, J. Colloid Interf. Sci. 84 (1981) 100. [18] G.Z. Li, F. Li, H.J. Yuan, J. Dispersion Sci. Technol. 16 (1995) 440. [19] D.O. Shah, J.H. Schulman, J. Lipid Res. 8 (1967) 227. [20] M. Almgren, J.E. Iofroth, J. Colloid Interf. Sci. 81 (1981) 486.
Synergism in mixed zwitterionic–anionic surfactant solutions and the aggregation numbers of the mixed micelles Fang Li *, Gan-Zuo Li, Jian-Bo Chen Institute of Colloid and Interface Chemistry, Shandong University, Jinan 250100, People’s Republic of China Received 25 February 1998; accepted 16 April 1998
Abstract Synergistic interaction between sodium dodecyl sulfonate (C AS) and three zwitterionic surfactants was measured 12 at 40.0°C in aqueous 0.1 M NaCl solution. The three zwitterionic surfactants were alkyldimethylammoniopropanesulfonates with different alkyl chain length — 12, 10 and 8 (abbreviated as C DPAS, C DPAS and C DPAS, respectively). 12 10 8 Surface tension results showed that a certain efficiency exists both in mixed micelle formation and surface tension reduction owing to the electrostatic attraction between the anionic and zwitterionic surfactants. The strength of the interaction between the zwitterionic surfactant and C AS in the three mixed systems obeys the following order: 12 C AS/C DPAS>C AS/C DPAS>C AS/C DAPS. The synergism is greatest when the molar ratio of anionic 12 12 12 10 12 8 surfactant to zwitterionic surfactant in the solution is around 7:3. Fluorescence studies on the C AS/C DPAS system 12 12 showed that the mixed micellar aggregation number becomes smaller than that of the zwitterionic surfactant with the addition of a small amount of anionic surfactant to the zwitterionic surfactant. However, after a small amount of C DPAS was added to C AS, the mixed C AS/C DPAS micellar aggregation number is much larger than that of 12 12 12 12 C AS. The mixed micellar aggregation numbers become largest at the composition point where the efficiency of 12 mixed micelle formation is greatest. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Synergism; Zwitterionic surfactant; Anionic surfactant; Fluorescence; Aggregation number
1. Introduction Zwitterionic surfactants are becoming increasingly attractive because of their good solubility in water and their reduced sensitivity to salts and temperature [1], but their wider application in industry is limited by their higher production cost compared with anionic surfactants. In general, the interfacial and colloid properties of a solution of two or more surfactants can be quite different from those of solutions of the individual compo* Corresponding author. Present address: Chemistry Department, Clarkson University, Potsdam, NY 13699-5810, USA. E-mail: [email protected]
nents. When a solution of mixed surfactants shows greater surface activity (e.g., lower surface tension or critical micelle concentration) than that attainable with any of the individual surfactants of the mixture at the same concentration, then the mixture is said to exhibit synergism. Many reports [2– 6 ] have revealed that zwitterionic surfactants can have a strong interaction with anionic surfactants in aqueous solution. So it is very interesting and useful to design some mixed anionic/zwitterionic surfactant systems in which the advantages of either surfactant can be exploited. Most of the previous studies have focused on alkyl betaine [7– 10], which is pH-sensitive zwitterionic surfactant. In this sense, the betaines, which are capable of
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F. Li et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 145 (1998) 167–174
accepting a proton, can certainly interact much more strongly with anionic surfactants than with cationic surfactants. The zwitterionic surfactants investigated in the present work — alkyldimethylammoniopropanesulfonates — are not sensitive to the pH value of solutions, remaining as real zwitterionic surfactants at any pH after dissolution in water. By means of surface tension measurements, we have studied the strength of the interaction of these zwitterionic surfactants with dodecylsulfonate. The results showed that both electrostatic attraction and van der Waals’ attraction play important roles in the synergism. With the fluorescence probe method, we also investigated the relationship between the mixed micelle aggregation number and the surfactant composition of the solution.
The surface tensions were obtained until no significant changes occurred. The critical micelle concentration of the mixed systems was determined by plotting surface tension values against the logarithm of the total surfactant concentration. 2.3. Fluorescence measurement The fluorescence measurements were recorded on an RF-540 spectrofluorimeter at 40±0.02°C. The excitation of the probe [Ru(C H N ) · 10 8 2 3 6H O] was performed at 476 nm and the emission 2 was monitored at 620 nm. The concentration of probe was kept constant (7.0×10−5 M ) in all solutions. The method of preparing solutions, which contain probe and quencher (9-MeA), was the same as that described in a preceding paper [11].
2. Experimental 3. Results and discussion 2.1. Materials Sodium dodecyl sulfonate (originally chemical pure) was purified as described elsewhere [11]. Ndodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (C DPAS ), N-decyl-N,N-dimethyl-3-amm12 onio-1-propane sulfonate (C DPAS ) and N-octyl10 N,N-dimethyl-3-ammonio-1-propane sulfonate (C DPAS ) were all purchased from Sigma 8 Company (purity >99.5%) and used without further purification. The structure of the zwitterionic surfactants is:
3.1. Zwitterionic surfactant–anionic surfactant interaction The surface tension–log C (surfactant concentration) plots for C AS/C DPAS, C AS/ 12 12 12 C DPAS and C AS/C DPAS are shown in 10 12 8 Figs. 1–3, respectively. The nature and strength of the interaction
CH –(CH ) –N+(CH ) –CH –CH –CH –SO− 3 2n 32 2 2 2 3 where n=11, 9 and 7 for C DPAS, C DPAS 12 10 and C DPAS, respectively. 8 The fluorescence probe [Ru(C H N ) · 6H O] 10 8 2 3 2 and quencher [9-methylanthracene (abbreviated as 9-MeA)] were also purchased from Sigma Chemical Company and used directly. 2.2. Surface tension measurement Surface tension was measured at constant ionic strength (0.1 M NaCl ) and at 40±0.02°C by the Wihelmy plate technique with a CVBP-A model 3 tensionmeter ( Kyowa Kanmenkagaku Co., Ltd ).
Fig. 1. Variation of the surface tension versus total surfactant concentration for different mole ratios of anionic to zwitterionic surfactant in the C AS/C DPAS system. Molar ratio of 12 12 C AS to C DPAS: % 8:2; # 7:3; × 1:1; 6 3:7; $ 2:8. 12 12
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larger the negative value of b, the stronger the attractive interaction between the two different surfactants and the greater the synergism between them. According to Rubingh’s non-ideal solution theory [12], after measuring the critical micelle concentration (CMC ) of the aqueous solution of mixed surfactants and the CMC of the individual surfactants, the value of the parameter bm — representing the interaction in the mixed micelle in the aqueous phase — can be calculated from the equation: X2 ln [(CMC · a)]/(CMC · X ) 1(m) 1 1 1(m) =1 [1−X ]2[CMC · (1−a)]/[CMC · (1−X )] 1(m) 1,2 2 1(m) (1) Fig. 2. Variation of the surface tension versus total surfactant concentration for different mole ratios of anionic to zwitterionic surfactant in the C AS/C DPAS system. Molar ratio of 12 10 C AS to C DPAS: % 8:2; # 7:3; × 1:1; 6 3:7; $ 2:8. 12 10
where X is the mole fraction of surfactant 1 in 1(m) the total surfactant in the mixed micelle, a is the mole fraction of surfactant 1 in the total surfactant in solution. CMC , CMC and CMC are the 1 2 1,2 critical micelle concentrations in the aqueous phase of surfactant 1, surfactant 2 and their mixture at a mole fraction of a, respectively. After determining X from Eq. (1), we can obtain the inter1(m) action parameter bm according to: ln [(CMC · a)]/(CMC · X ) 1,2 1 1(m) (2) (1−X )2 1(m) Similarly, the treatment can be used in the mixed monolayer at the aqueous solution/air interface and the parameter for the interaction in the mixed monolayer at the solution/air interface ( bS) can be obtained [13]. All of the results calculated for the three mixed anionic/zwitterionic systems are listed in Table 1. The existence of synergism in mixtures containing two surfactants depends not only on the strength of the interaction between them (measured by the values of b), but also on relevant properties of the individual surfactant components of the mixture [13]. Thus the conditions for synergism to exist in mixed micelle formation are: (1) bm must be negative; and (2) |bm|>|ln(CMC / 1 CMC )|. Similarly, the synergism in surface tension 2 reduction efficiency exists when bS is negative and the absolute value of bS is larger than that of ln(C0 /C0 ). The results for the C AS/C DPAS 1 2 12 12 bm=
Fig. 3. Variation of the surface tension versus total surfactant concentration for different mole ratios of anionic to zwitterionic surfactant in the C AS/C DPAS system. Molar ratio of 12 8 C AS to C DPAS: % 8:2; # 7:3; × 1:1; 6 3:7; $ 2:8. 12 8
between two surfactants can be measured by values of the so-called b parameter, which show the degree of the non-ideality of the interaction in the mixed monolayer at the air/solution interface or in the mixed micelle in the solution phase. The
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Table 1 Molar interaction and synergism parameters in the three mixed systems System
C AS/C DPAS 12 12
C AS/C DPAS 12 10
C AS/C DPAS 12 8
Molar ratio of 1 to 2a
0:1 2:8 3:7 1:1 7:3 8:2 1:0 0:1 2:8 3:7 1:1 7:3 8:2 1:0 0:1 2:8 3:7 1:1 7:3 8:2 1:0
In the mixed micelles
In the solution/air monolayer
CMC (M )
X 1m
X 2m
Ln(CMC / 1 CMC ) 2
bm
C (40 mN/m)b 0
Ln(C0 /C0 ) 1 2
bS b
˚ 2) A (A 2
1.0×10−3 5.6×10−4 4.5×10−4 3.2×10−4 2.5×10−4 3.5×10−4 1.6×10−3 1.0×10−2 2.4×10−3 2.1×10−3 1.5×10−3 1.3×10−3 1.6×10−3 1.6×10−3 3.6×10−2 5.6×10−3 3.8×10−3 2.6×10−3 2.5×10−3 2.6×10−3 1.6×10−3
0 0.33 0.39 0.46 0.52 0.56 1 0 0.55 0.60 0.67 0.73 0.84 1 0 0.75 0.78 0.85 / / 1
1 0.67 0.61 0.54 0.48 0.44 0 1 0.45 0.40 0.33 0.27 0.16 0 1 0.25 0.22 0.15 / / 0
/ 0.53 0.53 0.53 0.53 0.53 / / −1.77 −1.77 −1.77 −1.77 −1.77 / / −3.05 −3.05 −3.05 −3.05 −3.05 /
/ −3.53 −4.20 −5.36 −6.81 −5.92 / / −2.89 −2.72 −3.29 −3.57 −2.21 / / −1.20 −1.82 −1.96 / / /
7.5×10−4 2.1×10−4 1.8×10−4 1.3×10−4 1.5×10−4 1.7×10−4 8.4×10−4 7.5×10−4 / / / / / / 7.5×10−4 / / / / / /
/ 0.11 0.11 0.11 0.11 0.11 / / / / / / / / / / / / / / /
/ −4.88 −5.15 −6.08 −6.10 −5.86 / / / / / / / / / / / / / / /
46.4 46.7 40.0 37.0 28.0 34.0 48.1 48.9 45.5 42.6 40.4 35.0 45.2 48.1 53.0 53.7 50.5 50.7 54.0 58.2 48.1
/ means no reasonable values or no reasonable calculated results. a1 refers to anionic surfactant (C AS) and 2 refers to zwitterionic surfactant. 12 bC is the molar concentration of surfactant 1, 2 or the mixture, respectively, required to yield the same value of the surface tension 0 (here, 40 mN/m). bS is the parameter for the surfactant–surfactant interaction in the mixed monolayer at the aqueous solution/air interface.
mixed system ( Table 1) satisfied the conditions for the existence of synergism in both mixed micelle formation and the surface tension reduction, meaning that C AS has a synergistic effect with 12 C DPAS both in surface tension reduction effi12 ciency and in mixed micelle formation. The bS values are more negative than the bm values in this system, which may be due to the difficulty of incorporating tightly packed chains into a micelle. For the C AS/C DPAS system only synergism in 12 10 mixed micelle formation exists; there is no synergism in the surface tension reduction. No synergism exists at all in the mixed C AS/C DPAS 12 8 system, although its bm value is negative. Both the difference between bS and bm in the C AS/C DPAS system, and the absolute values 12 12 of bS and bm in all three systems, are all much less
than those in mixed anionic/cationic surfactant systems [11,14], indicating the interaction between C AS and alkyldimethylammoniopropanesulfo12 nates is much weaker than that with some cationic surfactants such as alkyltrimethylammonium bromide. For the two mixed systems, C AS/C DPAS 12 12 and C AS/C DPAS, the change of CMC values 12 10 versus molar fraction of the anionic surfactant (C AS ) is shown in Fig. 4. It can be seen that the 12 CMC values of the mixed system decrease with increasing mole fraction of C AS before showing 12 a minimum when the mole fraction of C AS is 12 around 0.7. If the thermodynamics of the micellization process for the two systems obey the ideal solution theory, when surfactant monomer and mixed
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the mole fraction of C AS in the mixed 12 surfactants. The theoretical CMC values for each C AS 12 molar ratio of the two mixed systems are also shown in Fig. 4. The experimental CMC values of the two mixed systems are substantially lower than those predicted by Eq. (3), indicating that the mixed micelle formation shows a negative deviation with respect to ideal behavior. The general Gibbs adsorption equation in our anionic (NaR)/zwitterionic (Z ) mixed surfactants can be written as: dc
=∑ C d ln a =C d ln a i i Na+ Na+ i +C d ln a +C d ln a (4) R− R− Z Z where C , C and C are the surface excess Na+ R− Z concentrations for Na+, R− and the zwitterionic surfactant (Z ), respectively. a , a and a are Na+ R− Z their activities. With constant ionic strength (0.2 M NaCl ) and the lower concentration of the surfactants, the following equation holds: −
(a)
−
(b)
Fig. 4. The relationship between CMC values and different mole fractions of C AS in the C AS/zwitterionic mixed surfactant 12 12 systems: (a) C AS/C DPAS system; (b) C AS/C DPAS 12 12 12 10 system.
1
=
X
+
1−X
(3) CMC CMC CMC a/z a z where CMC , CMC and CMC are the critical a/z a z micelle concentrations of the anionic/zwitterionic mixed surfactant, the anionic surfactant (C AS ) 12 and the zwitterionic surfactant, respectively. X is
dc RT
=C
R−
d ln a +C d ln a =C R− Z Z R−
×d ln C +C d ln C (5) R− Z Z where C and C are the concentrations of R− R− Z and the zwitterionic surfactant, respectively. When the mole ratio of anionic surfactant to zwitterionic surfactant is kept constant, then: −
micelles are in equilibrium in the mixed system, then the CMC values of the mixed micelles should fall on the line predicted by the following relationship [15]:
RT
dc RT
=C d ln C +C d ln C R− T Z T =(C
+C ) d ln C =C d ln C (6) R− Z T T T C and C being the total surface excess concenT T tration and the total surfactant concentration in the solution, respectively. So, from the c–ln C curve, we also get the T largest surface excess concentration (C ) for the 2 average area (A ) occupied per surfactant molecule 2 at the air/solution surface when saturated absorption is reached. For the two mixed systems (C AS/C DPAS and C AS/C DPAS ), the area 12 12 12 10 per molecule at the air/solution surface is smallest
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at the point where the mole fraction of C AS is 12 0.7 and the interaction parameters ( bm and bS) are largest. When the mole fraction of C AS is not 12 0.7, the value of A is also smaller than that of 2 every individual surfactant, meaning there is obvious synergism between C AS and C DPAS, 12 12 C DPAS. But for C AS/C DPAS, the values of 10 12 8 A for the mixed surfactant are almost the same 2 as or even larger than that of any individual surfactant, suggesting that no synergism exists between C AS and C DPAS. 12 8 The interaction between anionic surfactant and cationic surfactant is mostly due to the strong electrostatic attraction between the negatively charged surfactants. For some anionic/zwitterionic mixed surfactant systems, such as anionic/betaine systems [4,5,10], the betaines are capable of accepting a proton and act as a cationic surfactant. So the betaines can interact more strongly with anionic surfactants than with cationic surfactants. The zwitterionic surfactants we studied here, alkyldimethylammoniopropanesulfonates, are not sensitive to pH in the solution. They cannot easily accept a proton from the solution and they can never function as anionic surfactants. But the head group of these zwitterionic surfactants, –N+(CH ) –CH –CH –CH –SO− , is as long as 2 2 3 ˚ [16 ].3 2Their2 negative 6A charge center cannot overlap the center of their positive charge. So, although there is no net charge, many physical properties of the alkyldimethylammoniopropanesulfonates are quite different from those of nonionic surfactants and very similar to those of ionic surfactants [17], and they can have a synergistic effect with C AS by means of electrostatic attrac12 tion between the N+(CH ) group in the zwitter32 ionic surfactant and the SO− group in C AS. 3 12 In addition to the electrostatic attraction effect, van der Waals’ attraction between the hydrocarbon chains of the surfactants involved also plays an important role in their synergism effect [18]. Although the electrostatic attraction between C DAPS, C DPAS and C DPAS with C AS is 12 10 8 12 almost same, the van der Waals’ interaction between the different zwitterionic surfactants and C AS should have the following order because of 12 the difference in alkyl chains between C DPAS n and C AS: C AS/C DPAS>C AS/C DPAS> 12 12 12 12 10
C AS/C DPAS. So the interaction between 12 8 C DPAS and C AS is the greatest amongst the 12 12 three mixed systems and there is no obvious synergism between C DPAS and C AS. When the chain 8 12 length of both surfactants is equal, the molecular packing at the air/water interface is closer. However, when the chain length is different, the portion of the molecules above the height of the adjacent molecules will exhibit thermal motion [19], causing larger values of A . Thus, at a certain 2 molar ratio of C AS to C DPAS, the values of 12 n A in C AS/C DPAS are the largest and in 2 12 12 C DPAS/C DPAS are the smallest ( Table 1). 12 8 3.2. Mixed micellar aggregation numbers in the C AS/C DPAS system 12 12 A fluorescence method based on fluorescence quenching was used to determine the values of the aggregation number in the three mixed systems. With some assumptions [20], the aggregation number N in the mixed systems can be determined by measuring the total intensity of the fluorescence with and without quencher, I and I , respectively. t 0 The following equation holds:
A
[Q] I =I exp − t 0 [M ]
B
(7)
where [Q] is the concentration of the quencher (9-MeA). [M ] is the concentration of mixed micelles or individual micelles, which is related to the micellar aggregation number by: [M ]=
[S ]−[S ] t f N
(8)
where [S ] is the total concentration of surfactants t in the individual or mixed system. [S ] is the f concentration of monomer surfactant which, to a good approximation, can be taken as the CMC values in the individual or mixed system. So:
AB
N([S ]−CMC ) t = t (9) I [Q] 0 Keeping the total surfactant concentration S as t 0.018 M, we recorded the fluorescence probe emission spectra at different quencher (9-MeA) concenln
I
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trations with various mole ratios of C AS to 12 C DPAS ( Fig. 5). Compared with S , the CMC 12 t values are small enough to be neglected. When a different amount of NaCl is added to the mixed system, the CMC values will decrease and can also be neglected. From each straight line with a linear relationship between ln(I /I ) and [Q], we obtained t 0 the N values of C AS/C DPAS at different mole 12 12 ratios of C AS to C DPAS with different NaCl 12 12 concentrations (shown in Fig. 6). From the results in Fig. 6, we can see that C AS/C DPAS is much different from the 12 12 C AS/CTAB system [11]. In the anionic/cationic 12 mixed system, both a small addition of CTAB to C AS and a small addition C AS to CTAB can 12 12 result in a great increase of the aggregation number of C AS or CTAB. Moreover, the value of N 12 becomes larger and larger as the mole fraction of the anionic surfactant approaches that of the cationic surfactant. Clearly, the great increase of N and the largest N value at equal mole fractions of
(a)
(b)
Fig. 5. The fluorescence emission spectra of the probe in the C AS/C DPAS mixed micelle system at different quencher 12 12 concentrations without NaCl. ( From top to bottom, the concentration of quencher is 0.0, 1.0×10−4, 2.0×10−4, 3.0×10−4, 4.0×10−4 and 5.0×10−4 M, respectively.) (a) 7:3 Molar ratio of C AS to C DPAS; (b) 8:2 molar ratio of 12 12 C AS to C DPAS. 12 12
173
Fig. 6. The micellar aggregation numbers (N ) in the C AS/C DPAS mixed system at different NaCl concentrations 12 12 and different C AS/C DPAS molar ratios. 12 12
C AS and CTAB are due to the strong electro12 static attraction between the oppositely charge surfactants and, as a result, the surfactants tend to arrange more closely with one another in the mixed micelles. However, there is no net charge in the zwitterionic surfactant (C DPAS ) micelle. A 12 small addition of C AS can result in a great 12 increase in total charge and generally make the surfactant arrange more loosely, resulting in a decrease of the micellar aggregation number. On the other hand, when a small amount of zwitterionic surfactant is added to C AS, it will locate 12 between C AS in the mixed micelles and can 12 partly screen the electrostatic repulsion between the –SO− groups. As a result, the surfactants 3 arrange more closely in the micelles and the micelle aggregation number increases. The biggest N value appears at the composition point where the synergism is greatest. There is still a net charge on the mixed anionic/zwitterionic surfactant system, and so the addition of NaCl can compress their double electrical layers and increase the N values, as it does in ionic surfactant systems.
Acknowledgment This work was supported by the Natural Scientific Research Foundation of China.
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