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anionic surfactants
Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 28–33
Influence of salt and polymer on the critical vesicle concentration in aqueous mixture of zwitterionic/anionic surfactants Limin Zhai a,∗ , Xiaojun Tan a , Tao Li a , Yanjing Chen a , Xirong Huang b,∗∗ a
b
College of Chemistry and Chemical Engineering, Jinan University, Jinan 250022, PR China Key Lab for Colloid and Interface Chemistry of State Education Ministry, Shandong University, Jinan 250100, PR China Received 15 August 2004; received in revised form 28 September 2005; accepted 30 September 2005 Available online 17 November 2005
Abstract Vesicles can be formed spontaneously in aqueous solution of surfactant mixture of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and lauryl sulfonate betaine (LSB). Different from the catanionic vesicles, the formation is dependent on surfactant concentration, mixing ratio and salinity due to the weak attraction between the two kinds of surfactant molecules, which provides possibility to determine the critical vesicle concentration (CVC), just as the critical micelle concentration (CMC). In this paper, a simple but sensitive conductivity titration was employed to determine the critical concentration since it is concentration-continuous and the structural transition of the ionic surfactant relates directly to their conductivity. Especially, after vesicles are formed, some of the surfactant molecules form the inner layer of the vesicle and do not conduct any more, which will lead to a much evident change in conductivity. As a result, the turning points appear in both the conductivity and molar conductivity titration curve at the transition from monomers to micelles and micelles to vesicles with increasing concentration and/or salinity, with assistant proof from surface tension measurement and TEM. The CVC drops markedly after the addition of salt due to the compression of salt on the double electric layer of the surfactant polar head. But the addition of nonionic polymer PVP (K30) delays the vesicle formation due to the adsorption of surfactant molecules to the polymer. The results may make much contribution to the understanding of the mechanism of vesicle formation. © 2005 Elsevier B.V. All rights reserved. Keywords: Zwitterionic/anionic vesicles; Salt-promoted; PVP; Critical vesicle concentration; Conductivity titration
1. Introduction Spontaneously formed vesicles have been obtained from a great many surfactant systems, which may be sorted into single surfactant solution [1] or surfactants mixtures (including catanionic [2–6], cationic/cationic [7], zwitterionic/anionic [8], nonionic/ionic mixtures [9], or addition of cosurfactants in ionic mixtures [10,11]). However, there is still no well-defined critical vesicle concentration (CVC) reported. The only understanding comes from the formation of catanionic vesicles since they are the direct result of the strong interaction between oppositely charged head groups of surfactant, and such aggregates may form in aqueous solution at concentration far below that of the CMC of either pure surfactant [12–14]. Besides, the study on
∗ ∗∗
Corresponding author. Co-corresponding author. E-mail address: [email protected] (L. Zhai).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.09.043
the transition from micelles to vesicles were mainly carried out through controlling the relative proportion of the two surfactants, instead of controlling concentration, which hampers the understanding of the vesicle formation [15–17]. In a previous paper, we have found spontaneous vesicle formation in a surfactant mixture of dodecyl sulfonate betaine (DSB) (here nominated as lauryl sulfonte betaine (LSB)) and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) [8], which can be classified as zwitterionic/anionic vesicles. Different to the catanionic vesicles, the attraction between the two kinds of molecules is rather weak. The formation should proceed at high surfactant concentration and the growth of vesicles needs the promotion of salinity. It reminds us that we can control the formation of vesicle through varying the above two variables, which provides possibility to determine the critical aggregate concentration in such mixture. In this paper, conductivity titration [18] was employed to monitor the formation of vesicle since it could monitor the continuous variation of associates in solution with increasing
L. Zhai et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 28–33
concentration, and surface tension [19], TEM were used as auxiliary proof in the determination. Conductivity titration has been well accepted in determining the critical micelle concentration of ionic surfactant. For the transition of micelles to vesicles, it can be predicted that there will be a much evident change of conductivity because some of the surfactant molecules will form an inner layer of the vesicle and do not conduct any more. Meanwhile, molar conductivity is also introduced in the paper, since it relates directly to the concentration, which is defined as K c where Λm an K are the molar conductivity and the conductivity, respectively. c is the apparent concentration of the surfactant solution, but usually not the actual concentration (or effective concentration) due to the strong interactions between surfactant molecules. With increasing concentration, the effective concentration is usually little than apparent concentration. In this way, the actual conductivity could not reach the theoretical value resulting from apparent concentration. The molar conductivity generally decreases with increasing concentration. When there is structural transition, there will be change in both conductivity and concentration, which can also be reflected by the molar conductivity, sometimes maybe more sensitive than conductivity. The influences of salt and nonionic polymer on the structural transition were studied. Some interesting and instructive conclusions may make any contribution to the understanding of the formation mechanism of vesicles. Λm =
2. Experimental
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2.2. Conductivity titration The electrical conductivity was measured at 25 ± 0.1 using a DDSJ-308 conductometer from REX Instrument Co., Shanghai, China, and the cell constant, 0.95 cm−1 , was obtained by calibration with KCl solutions of known concentration. Because the correct determination by this method requires a large number of experimental data, a conductivity titration was employed. During the titration, solutions must be equilibrated for a few minutes after dilution until a stable value was obtained. 2.3. Surface tension The surface tension was measured at 25 ◦ C using a tensiometer JK99B, Shanghai Zhongchen Digital Instrument Co., China with a platinum plate. Equilibrium was confirmed by multiple measurements of a constant surface tension, and at least three measurements were taken for each point. 2.4. potential The potential of the micelles and vesicles were measured at 25 ◦ C on a Zeta potentiometer from Brookhaven Co., using a Laser-Doppler electrophoresis light scattering method. 3. Results and discussion The critical aggregate concentration of single surfactant was measured firstly, followed by that of their mixture and the influences of salt and polymer on the formation.
2.1. Materials and preparation of vesicles Lauryl sulfonate betaine (3-dodecyldimethylammoniopropanesulfonate inner salt, LSB) and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) were from Sigma, and used as received; NaCl from Shanghai Chemical Co. was A.R. grade, water was doubly distilled. Polyvinylpyrrolidone (PVP, K30), molecular weight 38,000 was from Fluka. Stock solutions of both zwitterionic and anionic surfactant at desired concentration in distilled water were equilibrated at room temperature and filtered through a 0.2 m filter prior to preparing samples. Samples were prepared by mixing stock solutions at the desired molar ratio. After brief vortex mixing, the solutions were not subject to any type of mechanical agitation. All samples were equilibrated at 25 ± 0.1 ◦ C. 2.1.1. Negative staining TEM Vesicles were observed with a transmission electron microscope by using the negative-staining method. As soon as the surfactant mixture solution and an aqueous solution of 2% phosphotungstic acid (PTA, pH 7) were mixed volumetrically at the ratio of 2:1, the resultant solution was then added dropwise to a 150-mesh copper grid coated with colloidin sprayed with a carbon film. Extra droplet was instantly removed by using a filter paper, and then the grid was dried in a vacuum desiccator for 6 h as a TEM sample.
3.1. CMC of single surfactant solution Since zwitterionic surfactant LSB does not conduct in aqueous solution, the critical micelle concentration was determined by surface tension method, which is shown in Fig. 1. The CMC is about 0.0025 M, which coincides with the literature value [19].
Fig. 1. The surface tension in function of LSB concentration.
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L. Zhai et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 28–33
The phase behavior of AOT aqueous solution is much complicated [20–23]. Double-tailed AOT molecules may easily form bilayer in aqueous solution, and then form vesicles spontaneously at high concentration or in salt solution. But the vesicle stability varies with the nature and concentration of salt. Vesicles will evolves into “floppy” bilayers. And the polydispersity of the vesicles strongly increases with time and the system evolves toward equilibrium between bilayer segments and micelles [24,25]. At high concentration, the solution becomes gray turbid and opaque. Therefore, there may exist several kinds of aggregates in AOT aqueous solution, such as monomers, micelles bilayer segments or rod-like micelles, vesicles, and of course the mixed structures of two or three of the them. The messy state leads to an argument on the association structure of Aerosol OT in aqueous solution. It is generally believed that without salt, only spherical micelles exist below the AOT concentration of 1% (0.0225 M) [25]. But there was also report that the vesicles has formed in AOT solution at a critical vesicle concentration, CVC, of 0.0061 M at 29.9 ◦ C [26]. In this paper, the variation of conductivity and molar conductivity with the concentration of AOT was recorded in Fig. 2. The titration was separated to two concentration ranges, 00.01 M (Fig. 2a) and 0.01 M–0.05 M (Fig. 2b). In Fig. 2a, it is hard to
distinguish the turning point from the curve of conductivity, but from that of molar conductivity, one can clearly see the turning points at concentration of 0.0025 M, which could be regarded as the CMC of AOT and well coincident to the literature value [21]. And there is another turning point at concentration of about 0.006 M in the latter section, which is similar to the above said critical vesicle concentration. However, considering no evident change in appearance was observed, we would rather regard it as the transition from micelles to rod-like micelles or bilayer segments. For the latter concentration range, in Fig. 2b, both the curves of conductivity and molar conductivity begins to bend obviously at high concentration, the turning point of which is at the concentration of about 0.02 M. The bending could be interpreted as the formation of closed bilayers, where the inner layer does not conduct any more. It is coincident with another literature value, which regarded it as the formation concentration of vesicles. Due to the messy state of different kinds of aggregates in AOT solution and the formation of multibilayer vesicles, the curve turns to be flat thereafter. 3.2. CMC and CVC of surfactant mixture general behavior of LSB/AOT mixtures The messy state in pure AOT solution could be improved by the incorporation of another surfactant LSB [8]. The incorporation of the zwitterionic surfactant LSB into AOT bilayer may not only stabilize the AOT vesicles, but also effectively reduce the polydispersity of the vesicles, which could be easily perceived by the appearance change from bluish turbid to translucent blue with increasing the proportion of LSB. Since the mixture containing AOT conducts in aqueous solution, it is feasible to find out the critical concentration through conductivity titration. Mixture of the two surfactants was titrated with water. Titration was also performed in two concentration ranges (0–0.003 and 0.003–0.02 M) in order to distinguish the CMC and CVC. Fig. 3a and b show the variation of conductivity with concentration at mixing ratio of AOT/LSB = 7/3. One can clearly see break point in molar conductivity curve versus concentration in Fig. 3a, which implies that there must exist structural transitions at that concentration. The turning point must be the CMC of mixed micelle at about 0.0012 M, because mixed surfactants usually has a lower CMC than that of the pure surfactant. The value is also proved by surface tension measurement, which is shown in Fig. 3c. The latter turning point at 0.009 M indicated by both conductivity and molar conductivity should then correspond to another transition from micelle to vesicle. The TEM image proves the existence of vesicle at high concentration (Fig. 4). Further titration performed on the mixtures of different mixing ratios also obtains evident break points in the curves, which shows the critical micelle concentration (CMC) and critical vesicle concentration (CVC), respectively.
Fig. 2. Conductivity of AOT aqueous solution at different concentrations: (a) 0–0.01 M conductivity (blank square), molar conductivity (filled square) and (b) 0.01–0.05 M.
3.2.1. Influence of salt on critical aggregate concentration Considering the addition of salt reduces the CMC of ionic surfactant, it can be assumed that it will have the similar effect
L. Zhai et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 28–33
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Fig. 4. Image of vesicles using negative-staining TEM. Bar = 100 nm, AOT/LSB = 7/3, Ctotal = 0.02 M.
ity was fixed in each titration by accurately weighing salt into surfactant mixture beforehand, and then the solution was titrated with brine of the same salinity to the initial solution. From Fig. 5, one can see the turning in conductivity curve is much more evident because there are more vesicles formed under the promotion of salt. But the molar conductivity does not give a distinct turning as before. Comparing the two values of CVC, it can be seen that a slight addition of NaCl can reduce it markedly from 0.09 M (without salt) to 0.06 M (at salinity of 0.005 M). Further increasing salinity, the CVC quickly reaches a stable value of 0.005 M and seems not to decrease any more as shown in Fig. 6. The variation of the curve in Fig. 6 may be interpreted by the variation of ζ potentials of the mixture at different salinities as shown in Fig. 7. The shape of the ζ potentials curve is just opposite to that of the variation of CVC with salinity. The addition of salt reduces the absolute value of the potential of vesicle at first but turns to be flat at higher salinity. That means vesicles with a
Fig. 3. Variation of conductivity and surface tension with concentration at mixing ratio of AOT/LSB = 7/3: (a and b) conductivity (blank square) and molar conductivity (filled circle); and (c) surface tension.
on the CVC of ionic vesicles. So conductivity titration on the mixture at mixing ratio of AOT/LSB = 7/3 was continued at different salinity. Because salt conducts in aqueous solution and compresses the double electric layer of the association structures, the salin-
Fig. 5. Influence of salt on the critical vesicle concentration at mixing ratio of AOT/LSB = 7/3. CNaCl = 0.005 M, AOT/LSB = 7/3, conductivity (blank square), molar conductivity (filled square).
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L. Zhai et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 28–33
Fig. 6. The CVC at different salinity (AOT/LSB = 7/3).
high potential correspond to a high CVC, vice versa. That is, vesicles with low ζ potentials could be formed more easily. 3.2.2. Influence of PVP on critical aggregate concentration Nonionic polymer PVP (K30) interacts with the negative micelles and vesicles due to the electric attraction but does not produce precipitates as ionic polymer does, which exhibits a “necklace” structure with micelle and an insertion form with vesicles [27]. So it can be predicted that PVP will also influence the formation concentration of negatively charged vesicles, which was examined by conductivity titration. Similar to the brine titration, PVP was added into the mixture at the mixing ratio of AOT/LSB = 7/3 prior to the titration, and solution was titrated with polymer solution of the same concentration. Fig. 8 shows the variation of conductivity and molar conductivity with the surfactant concentration. It is found that there also exist two turning points in the titration range. The first turning point is at about 0.005 M, which is about 4 times that of mixed CMC and little than that of CVC without polymer. Considering the adsorption of surfactants on the polymer and especially, there
Fig. 8. Influence of PVP on the critical vesicle concentration: (A) CPVP = 2 × 10−4 M and (B) CPVP = 5 × 10−4 M. AOT/LSB = 7/3.
is no evident turning in the conductivity curve, the first turning point should correspond to the formation of bilayer segment containing PVP molecules, not the vesicles. Until after the concentration of 0.009 M, another turning point appears and may be regarded as the delayed CVC because the surfactant molecules absorbed on polymer are delayed to form aggregates. When the concentration reaches the original critical value, the adsorbed surfactants could not aggregate freely and timely. Fig. 8 shows that the CVC is 0.0136 M at PVP concentration of 2 × 10−4 M, and 0.0145 M at PVP concentration of 5 × 10−4 M, which are both greater than the value of 0.009 M without PVP. 4. Conclusion
Fig. 7. Variation of the ζ potential with salinity in mixture of LSB and AOT (AOT/LSB = 7/3).
By combining conductivity and molar conductivity titration, assisted by surface tension and TEM or visual observation, critical vesicle concentration can be determined in aqueous solution of zwitterionic/anionic surfactant mixture. The determination is based on the different conductivity behavior afore and after the formation of vesicle. In the conductivity curve of vesicle, evident break point was obtained due to the formation of the inner layer of vesicle. The CVC of the solution is different for the different mixing ratio. The influences of NaCl and PVP on the formation were also studied by this method. The addition
L. Zhai et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 28–33
of salt compresses the double electric layer of the surfactant polar head, and reduces the critical formation concentration of vesicles. But the influence of nonionic polymer PVP (K30) is different, which delays the vesicle formation due to the adsorption of surfactant molecules to the polymer. The results provide a simple and effective method for the study on the aggregation behavior of surfactant molecules in aqueous solution. References [1] J.E. Brady, D.R. Evabs, R. Kacharr, B.W. Ninham, J. Am. Chem. Soc. 106 (1984) 4279. [2] E.W. Kaler, A.K. Murthy, J.A.N. Zasadzinski, Science 245 (1989) 1371. [3] C.A. McKelvey, E.W. Kaler, J.A.N. Zasadzinski, B. Coldren, H.-T. Jung, Langmuir 16 (2000) 8285. [4] E.F. Marques, O. Regev, A. Khan, M.G. Miguel, B. Lindman, J. Phys. Chem. B. 102 (1998) 6746. [5] K.L. Herrington, E.W. Kaler, A.K. Murthy, J.A.N. Zasadzinski, S. Chirucolus, J. Phys. Chem. B 97 (1993) 13792. [6] M.T. Yatcilla, K.L. Herrington, L.L. Brasher, E.W. Kaler, J. Phys. Chem. B 100 (1996) 5874. [7] I.V. Maria, E. Katarina, S.C. Claudia, M.B.C. Silva, Langmuir 16 (2000) 2105. [8] L. Zhai, G. Li, Z. Sun, Colloids Surf. A 190 (3) (2001) 275–283. [9] K. Edwards, M. Almgren, Langmuir 8 (1992) 824. [10] M. Berstrom, J.C. Eriksson, Langmuir 12 (1996) 624.
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[11] M. Gradzielski, M. Muller, M. Bergmeier, H. Hoffmann, E. Hoinkis, J. Phys. Chem. B 103 (1999) 1416–1424. [12] E.W. Kaler, K.L. Herriton, A.K. Murthy, J.A.N. Zasadzinski, J. Phys. Chem. 96 (1992) 6698. [13] K.L. Herriton, E.W. Kaler, D.D. Miller, J.A. Zasadzinski, S. Chirulvolu, J. Phys. Chem. 97 (1993) 13792. [14] P.M. Holland, D.N. Rubingh, J. Phys. Chem. 87 (1984). [15] K. Nichole, E.A. Mahacine, A. Jacqueline, L. Marc, J. Colloid Interface Sci. 143 (1991) 463. [16] A.J. O’Connor, T. Alan Hatton, J. Phys. Chem. 13 (1997) 6931. [17] O. Soderman, K.L. Herriton, E.W. Kaler, J. Phys. Chem. 13 (1997) 5531. [18] J.I. Briz, M.M. Velazquez, J. Colloid Interface Sci. 247 (2002) 437–446. [19] Weers, Langmuir 67 (1991) 854. [20] M.I. Viseu, M.M. Velazquez, C.S. Campos, et al., Langmuir 16 (2000) 4882–4889. [21] K. Fontell, J. Colloid Interface Sci. 44 (1973), 156–164 and 318–329. [22] M. Skouri, J. Marignan, R. May, Colloid. Polym. Sci. 269 (1991) 929–937. [23] B. Balinov, U. Olsson, O. Soderman, J. Phys. Chem. 95 (1991) 5931–5936. [24] J. Bergebholtz, N.J. Wagner, Langmuir 12 (1996) 3122–3126. [25] I. Grillo, E.I. Kats, A.R. Muratov, Langmuir 19 (2003) 4573–4581. [26] Mukerjee, P., Mysels, K.J. Critical Micelle concentration of aqueous surfactant systems, National Standard Reference Data Series, p.136. National Bureau of Standards (U.S.), Washington, DC, 1971. [27] L.M. Zhai, X.H. Lu, W.J. Chen, C.B. Hu, L. Zheng, Colloids Surf. A 236 (2004) 1–5.
Influence of salt and polymer on the critical vesicle concentration in aqueous mixture of zwitterionic/anionic surfactants Limin Zhai a,∗ , Xiaojun Tan a , Tao Li a , Yanjing Chen a , Xirong Huang b,∗∗ a
b
College of Chemistry and Chemical Engineering, Jinan University, Jinan 250022, PR China Key Lab for Colloid and Interface Chemistry of State Education Ministry, Shandong University, Jinan 250100, PR China Received 15 August 2004; received in revised form 28 September 2005; accepted 30 September 2005 Available online 17 November 2005
Abstract Vesicles can be formed spontaneously in aqueous solution of surfactant mixture of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and lauryl sulfonate betaine (LSB). Different from the catanionic vesicles, the formation is dependent on surfactant concentration, mixing ratio and salinity due to the weak attraction between the two kinds of surfactant molecules, which provides possibility to determine the critical vesicle concentration (CVC), just as the critical micelle concentration (CMC). In this paper, a simple but sensitive conductivity titration was employed to determine the critical concentration since it is concentration-continuous and the structural transition of the ionic surfactant relates directly to their conductivity. Especially, after vesicles are formed, some of the surfactant molecules form the inner layer of the vesicle and do not conduct any more, which will lead to a much evident change in conductivity. As a result, the turning points appear in both the conductivity and molar conductivity titration curve at the transition from monomers to micelles and micelles to vesicles with increasing concentration and/or salinity, with assistant proof from surface tension measurement and TEM. The CVC drops markedly after the addition of salt due to the compression of salt on the double electric layer of the surfactant polar head. But the addition of nonionic polymer PVP (K30) delays the vesicle formation due to the adsorption of surfactant molecules to the polymer. The results may make much contribution to the understanding of the mechanism of vesicle formation. © 2005 Elsevier B.V. All rights reserved. Keywords: Zwitterionic/anionic vesicles; Salt-promoted; PVP; Critical vesicle concentration; Conductivity titration
1. Introduction Spontaneously formed vesicles have been obtained from a great many surfactant systems, which may be sorted into single surfactant solution [1] or surfactants mixtures (including catanionic [2–6], cationic/cationic [7], zwitterionic/anionic [8], nonionic/ionic mixtures [9], or addition of cosurfactants in ionic mixtures [10,11]). However, there is still no well-defined critical vesicle concentration (CVC) reported. The only understanding comes from the formation of catanionic vesicles since they are the direct result of the strong interaction between oppositely charged head groups of surfactant, and such aggregates may form in aqueous solution at concentration far below that of the CMC of either pure surfactant [12–14]. Besides, the study on
∗ ∗∗
Corresponding author. Co-corresponding author. E-mail address: [email protected] (L. Zhai).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.09.043
the transition from micelles to vesicles were mainly carried out through controlling the relative proportion of the two surfactants, instead of controlling concentration, which hampers the understanding of the vesicle formation [15–17]. In a previous paper, we have found spontaneous vesicle formation in a surfactant mixture of dodecyl sulfonate betaine (DSB) (here nominated as lauryl sulfonte betaine (LSB)) and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) [8], which can be classified as zwitterionic/anionic vesicles. Different to the catanionic vesicles, the attraction between the two kinds of molecules is rather weak. The formation should proceed at high surfactant concentration and the growth of vesicles needs the promotion of salinity. It reminds us that we can control the formation of vesicle through varying the above two variables, which provides possibility to determine the critical aggregate concentration in such mixture. In this paper, conductivity titration [18] was employed to monitor the formation of vesicle since it could monitor the continuous variation of associates in solution with increasing
L. Zhai et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 28–33
concentration, and surface tension [19], TEM were used as auxiliary proof in the determination. Conductivity titration has been well accepted in determining the critical micelle concentration of ionic surfactant. For the transition of micelles to vesicles, it can be predicted that there will be a much evident change of conductivity because some of the surfactant molecules will form an inner layer of the vesicle and do not conduct any more. Meanwhile, molar conductivity is also introduced in the paper, since it relates directly to the concentration, which is defined as K c where Λm an K are the molar conductivity and the conductivity, respectively. c is the apparent concentration of the surfactant solution, but usually not the actual concentration (or effective concentration) due to the strong interactions between surfactant molecules. With increasing concentration, the effective concentration is usually little than apparent concentration. In this way, the actual conductivity could not reach the theoretical value resulting from apparent concentration. The molar conductivity generally decreases with increasing concentration. When there is structural transition, there will be change in both conductivity and concentration, which can also be reflected by the molar conductivity, sometimes maybe more sensitive than conductivity. The influences of salt and nonionic polymer on the structural transition were studied. Some interesting and instructive conclusions may make any contribution to the understanding of the formation mechanism of vesicles. Λm =
2. Experimental
29
2.2. Conductivity titration The electrical conductivity was measured at 25 ± 0.1 using a DDSJ-308 conductometer from REX Instrument Co., Shanghai, China, and the cell constant, 0.95 cm−1 , was obtained by calibration with KCl solutions of known concentration. Because the correct determination by this method requires a large number of experimental data, a conductivity titration was employed. During the titration, solutions must be equilibrated for a few minutes after dilution until a stable value was obtained. 2.3. Surface tension The surface tension was measured at 25 ◦ C using a tensiometer JK99B, Shanghai Zhongchen Digital Instrument Co., China with a platinum plate. Equilibrium was confirmed by multiple measurements of a constant surface tension, and at least three measurements were taken for each point. 2.4. potential The potential of the micelles and vesicles were measured at 25 ◦ C on a Zeta potentiometer from Brookhaven Co., using a Laser-Doppler electrophoresis light scattering method. 3. Results and discussion The critical aggregate concentration of single surfactant was measured firstly, followed by that of their mixture and the influences of salt and polymer on the formation.
2.1. Materials and preparation of vesicles Lauryl sulfonate betaine (3-dodecyldimethylammoniopropanesulfonate inner salt, LSB) and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) were from Sigma, and used as received; NaCl from Shanghai Chemical Co. was A.R. grade, water was doubly distilled. Polyvinylpyrrolidone (PVP, K30), molecular weight 38,000 was from Fluka. Stock solutions of both zwitterionic and anionic surfactant at desired concentration in distilled water were equilibrated at room temperature and filtered through a 0.2 m filter prior to preparing samples. Samples were prepared by mixing stock solutions at the desired molar ratio. After brief vortex mixing, the solutions were not subject to any type of mechanical agitation. All samples were equilibrated at 25 ± 0.1 ◦ C. 2.1.1. Negative staining TEM Vesicles were observed with a transmission electron microscope by using the negative-staining method. As soon as the surfactant mixture solution and an aqueous solution of 2% phosphotungstic acid (PTA, pH 7) were mixed volumetrically at the ratio of 2:1, the resultant solution was then added dropwise to a 150-mesh copper grid coated with colloidin sprayed with a carbon film. Extra droplet was instantly removed by using a filter paper, and then the grid was dried in a vacuum desiccator for 6 h as a TEM sample.
3.1. CMC of single surfactant solution Since zwitterionic surfactant LSB does not conduct in aqueous solution, the critical micelle concentration was determined by surface tension method, which is shown in Fig. 1. The CMC is about 0.0025 M, which coincides with the literature value [19].
Fig. 1. The surface tension in function of LSB concentration.
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L. Zhai et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 276 (2006) 28–33
The phase behavior of AOT aqueous solution is much complicated [20–23]. Double-tailed AOT molecules may easily form bilayer in aqueous solution, and then form vesicles spontaneously at high concentration or in salt solution. But the vesicle stability varies with the nature and concentration of salt. Vesicles will evolves into “floppy” bilayers. And the polydispersity of the vesicles strongly increases with time and the system evolves toward equilibrium between bilayer segments and micelles [24,25]. At high concentration, the solution becomes gray turbid and opaque. Therefore, there may exist several kinds of aggregates in AOT aqueous solution, such as monomers, micelles bilayer segments or rod-like micelles, vesicles, and of course the mixed structures of two or three of the them. The messy state leads to an argument on the association structure of Aerosol OT in aqueous solution. It is generally believed that without salt, only spherical micelles exist below the AOT concentration of 1% (0.0225 M) [25]. But there was also report that the vesicles has formed in AOT solution at a critical vesicle concentration, CVC, of 0.0061 M at 29.9 ◦ C [26]. In this paper, the variation of conductivity and molar conductivity with the concentration of AOT was recorded in Fig. 2. The titration was separated to two concentration ranges, 00.01 M (Fig. 2a) and 0.01 M–0.05 M (Fig. 2b). In Fig. 2a, it is hard to
distinguish the turning point from the curve of conductivity, but from that of molar conductivity, one can clearly see the turning points at concentration of 0.0025 M, which could be regarded as the CMC of AOT and well coincident to the literature value [21]. And there is another turning point at concentration of about 0.006 M in the latter section, which is similar to the above said critical vesicle concentration. However, considering no evident change in appearance was observed, we would rather regard it as the transition from micelles to rod-like micelles or bilayer segments. For the latter concentration range, in Fig. 2b, both the curves of conductivity and molar conductivity begins to bend obviously at high concentration, the turning point of which is at the concentration of about 0.02 M. The bending could be interpreted as the formation of closed bilayers, where the inner layer does not conduct any more. It is coincident with another literature value, which regarded it as the formation concentration of vesicles. Due to the messy state of different kinds of aggregates in AOT solution and the formation of multibilayer vesicles, the curve turns to be flat thereafter. 3.2. CMC and CVC of surfactant mixture general behavior of LSB/AOT mixtures The messy state in pure AOT solution could be improved by the incorporation of another surfactant LSB [8]. The incorporation of the zwitterionic surfactant LSB into AOT bilayer may not only stabilize the AOT vesicles, but also effectively reduce the polydispersity of the vesicles, which could be easily perceived by the appearance change from bluish turbid to translucent blue with increasing the proportion of LSB. Since the mixture containing AOT conducts in aqueous solution, it is feasible to find out the critical concentration through conductivity titration. Mixture of the two surfactants was titrated with water. Titration was also performed in two concentration ranges (0–0.003 and 0.003–0.02 M) in order to distinguish the CMC and CVC. Fig. 3a and b show the variation of conductivity with concentration at mixing ratio of AOT/LSB = 7/3. One can clearly see break point in molar conductivity curve versus concentration in Fig. 3a, which implies that there must exist structural transitions at that concentration. The turning point must be the CMC of mixed micelle at about 0.0012 M, because mixed surfactants usually has a lower CMC than that of the pure surfactant. The value is also proved by surface tension measurement, which is shown in Fig. 3c. The latter turning point at 0.009 M indicated by both conductivity and molar conductivity should then correspond to another transition from micelle to vesicle. The TEM image proves the existence of vesicle at high concentration (Fig. 4). Further titration performed on the mixtures of different mixing ratios also obtains evident break points in the curves, which shows the critical micelle concentration (CMC) and critical vesicle concentration (CVC), respectively.
Fig. 2. Conductivity of AOT aqueous solution at different concentrations: (a) 0–0.01 M conductivity (blank square), molar conductivity (filled square) and (b) 0.01–0.05 M.
3.2.1. Influence of salt on critical aggregate concentration Considering the addition of salt reduces the CMC of ionic surfactant, it can be assumed that it will have the similar effect
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Fig. 4. Image of vesicles using negative-staining TEM. Bar = 100 nm, AOT/LSB = 7/3, Ctotal = 0.02 M.
ity was fixed in each titration by accurately weighing salt into surfactant mixture beforehand, and then the solution was titrated with brine of the same salinity to the initial solution. From Fig. 5, one can see the turning in conductivity curve is much more evident because there are more vesicles formed under the promotion of salt. But the molar conductivity does not give a distinct turning as before. Comparing the two values of CVC, it can be seen that a slight addition of NaCl can reduce it markedly from 0.09 M (without salt) to 0.06 M (at salinity of 0.005 M). Further increasing salinity, the CVC quickly reaches a stable value of 0.005 M and seems not to decrease any more as shown in Fig. 6. The variation of the curve in Fig. 6 may be interpreted by the variation of ζ potentials of the mixture at different salinities as shown in Fig. 7. The shape of the ζ potentials curve is just opposite to that of the variation of CVC with salinity. The addition of salt reduces the absolute value of the potential of vesicle at first but turns to be flat at higher salinity. That means vesicles with a
Fig. 3. Variation of conductivity and surface tension with concentration at mixing ratio of AOT/LSB = 7/3: (a and b) conductivity (blank square) and molar conductivity (filled circle); and (c) surface tension.
on the CVC of ionic vesicles. So conductivity titration on the mixture at mixing ratio of AOT/LSB = 7/3 was continued at different salinity. Because salt conducts in aqueous solution and compresses the double electric layer of the association structures, the salin-
Fig. 5. Influence of salt on the critical vesicle concentration at mixing ratio of AOT/LSB = 7/3. CNaCl = 0.005 M, AOT/LSB = 7/3, conductivity (blank square), molar conductivity (filled square).
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Fig. 6. The CVC at different salinity (AOT/LSB = 7/3).
high potential correspond to a high CVC, vice versa. That is, vesicles with low ζ potentials could be formed more easily. 3.2.2. Influence of PVP on critical aggregate concentration Nonionic polymer PVP (K30) interacts with the negative micelles and vesicles due to the electric attraction but does not produce precipitates as ionic polymer does, which exhibits a “necklace” structure with micelle and an insertion form with vesicles [27]. So it can be predicted that PVP will also influence the formation concentration of negatively charged vesicles, which was examined by conductivity titration. Similar to the brine titration, PVP was added into the mixture at the mixing ratio of AOT/LSB = 7/3 prior to the titration, and solution was titrated with polymer solution of the same concentration. Fig. 8 shows the variation of conductivity and molar conductivity with the surfactant concentration. It is found that there also exist two turning points in the titration range. The first turning point is at about 0.005 M, which is about 4 times that of mixed CMC and little than that of CVC without polymer. Considering the adsorption of surfactants on the polymer and especially, there
Fig. 8. Influence of PVP on the critical vesicle concentration: (A) CPVP = 2 × 10−4 M and (B) CPVP = 5 × 10−4 M. AOT/LSB = 7/3.
is no evident turning in the conductivity curve, the first turning point should correspond to the formation of bilayer segment containing PVP molecules, not the vesicles. Until after the concentration of 0.009 M, another turning point appears and may be regarded as the delayed CVC because the surfactant molecules absorbed on polymer are delayed to form aggregates. When the concentration reaches the original critical value, the adsorbed surfactants could not aggregate freely and timely. Fig. 8 shows that the CVC is 0.0136 M at PVP concentration of 2 × 10−4 M, and 0.0145 M at PVP concentration of 5 × 10−4 M, which are both greater than the value of 0.009 M without PVP. 4. Conclusion
Fig. 7. Variation of the ζ potential with salinity in mixture of LSB and AOT (AOT/LSB = 7/3).
By combining conductivity and molar conductivity titration, assisted by surface tension and TEM or visual observation, critical vesicle concentration can be determined in aqueous solution of zwitterionic/anionic surfactant mixture. The determination is based on the different conductivity behavior afore and after the formation of vesicle. In the conductivity curve of vesicle, evident break point was obtained due to the formation of the inner layer of vesicle. The CVC of the solution is different for the different mixing ratio. The influences of NaCl and PVP on the formation were also studied by this method. The addition
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of salt compresses the double electric layer of the surfactant polar head, and reduces the critical formation concentration of vesicles. But the influence of nonionic polymer PVP (K30) is different, which delays the vesicle formation due to the adsorption of surfactant molecules to the polymer. The results provide a simple and effective method for the study on the aggregation behavior of surfactant molecules in aqueous solution. References [1] J.E. Brady, D.R. Evabs, R. Kacharr, B.W. Ninham, J. Am. Chem. Soc. 106 (1984) 4279. [2] E.W. Kaler, A.K. Murthy, J.A.N. Zasadzinski, Science 245 (1989) 1371. [3] C.A. McKelvey, E.W. Kaler, J.A.N. Zasadzinski, B. Coldren, H.-T. Jung, Langmuir 16 (2000) 8285. [4] E.F. Marques, O. Regev, A. Khan, M.G. Miguel, B. Lindman, J. Phys. Chem. B. 102 (1998) 6746. [5] K.L. Herrington, E.W. Kaler, A.K. Murthy, J.A.N. Zasadzinski, S. Chirucolus, J. Phys. Chem. B 97 (1993) 13792. [6] M.T. Yatcilla, K.L. Herrington, L.L. Brasher, E.W. Kaler, J. Phys. Chem. B 100 (1996) 5874. [7] I.V. Maria, E. Katarina, S.C. Claudia, M.B.C. Silva, Langmuir 16 (2000) 2105. [8] L. Zhai, G. Li, Z. Sun, Colloids Surf. A 190 (3) (2001) 275–283. [9] K. Edwards, M. Almgren, Langmuir 8 (1992) 824. [10] M. Berstrom, J.C. Eriksson, Langmuir 12 (1996) 624.
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