- Email: [email protected]
Salt effect on the interactions between gemini surfactant and oppositely charged polyelectrolyte in aqueous solution
Journal of Colloid and Interface Science 306 (2007) 405–410 www.elsevier.com/locate/jcis
Salt effect on the interactions between gemini surfactant and oppositely charged polyelectrolyte in aqueous solution Yingying Pi a , Yazhuo Shang a , Honglai Liu a,∗ , Ying Hu a , Jianwen Jiang b a Lab for Advanced Materials and Department of Chemistry, East China University of Science and Technology, Shanghai 200237, China b Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117576 Singapore
Received 17 July 2006; accepted 10 October 2006 Available online 20 November 2006
Abstract The effect of alkali halides (NaBr, NaCl, KCl) on the interactions between the cationic gemini surfactant hexylene-1,6-bis(dodecyldimethylammonium bromide) (12-6-12) and the anionic polyelectrolyte sodium polyacrylate (NaPAA) in aqueous solution has been investigated by fluorescence emission spectroscopy, UV transmittance, zeta potential, and transmission electron microscopy (TEM). With increased addition of NaBr, a counterbalancing salt effect on the critical aggregation concentration (CAC) is observed. At low concentrations, NaBr facilitates the formation of micelle-like structures between surfactant and polyelectrolyte and results in a smaller CAC. At high concentrations, NaBr screens the electrostatic attraction between surfactant and polyelectrolyte and leads to a larger CAC. Upon the formation of micelle-like structures at high surfactant concentrations, the addition of NaBr is favorable for larger aggregates. The microstructure detected by TEM show that a global structure is generally formed in the presence of NaBr. The interactions also depend on ion species. Compared to NaBr, the addition of NaCl or KCl yields a smaller CAC. © 2006 Elsevier Inc. All rights reserved. Keywords: Gemini surfactant; Polyelectrolyte; Interactions; Salt; Alkali halide
1. Introduction Currently the interactions of ionic surfactants with polyelectrolytes are the subject of extensive investigation due to their wide applications in many industrially important processes and products, such as water treatment, detergency, and oil recovery [1]. Ionic surfactants can strongly interact with oppositely charged polyelectrolytes to form micelle-like structures at a very low critical aggregation concentration (CAC), which is usually a few orders of magnitude lower than the critical micellization concentration (CMC) of the free surfactant [2–6]. This is attributed to the electrostatic attraction between the surfactant and the polyelectrolyte, as well as to the hydrophobic interaction between the surfactant tails. In a surfactant/polyelectrolyte solution, the addition of salt is expected to have a significant effect on their interactions. * Corresponding author. Fax: +86 21 64252921.
E-mail address: [email protected] (H. Liu). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.10.020
It has been shown that the addition of salt reduces the strength of electrostatic interaction between surfactants and polyelectrolytes, which results in a larger CAC [7–12]. Moreover, the addition of a sufficiently large amount of salt can completely screen the electrostatic interaction and prevent the formation of polyelectrolyte/surfactant complexes [13–16]. Nevertheless, the effect of added salt in the surfactant/polyelectrolyte solution is rather complicated. In addition to the reduction of electrostatic interaction, stabilization of surfactant aggregates was reported [6]. As a consequence, a decrease of CAC in the surfactant/polyelectrolyte solution can be expected by adding salt, e.g., in sodium carboxymethylcellulose and dodecyltrimethylammonium bromide with the addition of NaBr [17]. While many studies have been devoted to salt effects on interactions between single-chain surfactants and oppositely charged polyelectrolytes, salt effects on interactions of gemini surfactants with polyelectrolyte are less understood. Only recently was a study reported on the effect of NaBr on complex formation between a cationic gemini surfactant and an anionic
406
Y. Pi et al. / Journal of Colloid and Interface Science 306 (2007) 405–410
polyelectrolyte, and the CAC was found to be insensitive to the added NaBr [18]. A gemini is a dimeric surfactant consisting of two identical amphiphilic moieties (twins) covalently joined by a spacer group at or close to the ionic head groups [19]. As a new family of amphiphilic molecules, gemini surfactants have stimulated extensive interest with stronger surface activity and better solubilizing, wetting, foaming, and lime–soap dispersing capability than conventional surfactants [20]. Due to the unique features of gemini surfactants, the effect of salt is expected to be different from that on traditional surfactants. More systematic investigations are needed to further understand complex formation between gemini surfactants and polyelectrolytes. We have recently examined the interactions between the gemini surfactant alkanediyl-α,ω-bis(dodecyldimethylammonium bromide) (12-n-12, n = 3, 4, 6) and the anionic polyelectrolyte sodium polyacrylate (NaPAA) [26]. In this work we further examine the effect of added alkali halides (NaBr, NaCl, KCl) on the interactions between 12-6-12 and NaPAA in aqueous solution at 25 ◦ C. Fluorescence emission spectroscopy and UV transmittance were measured. The effect of salt on the microstructure change of the 12-6-12/NaPAA complex induced by increasing surfactant concentration with zeta potential was determined. The microstructure of mixtures in the presence of NaBr was detected by negative-staining and transmission electron microscopy. The interactions for different salt concentrations and ion species were also studied. 2. Experimental materials and methods 2.1. Materials The gemini surfactant 12-6-12 was prepared in our lab by a reaction of 1,6-dibromohexane with N ,N -dodecyldimethylamine [21]. Polyelectrolyte NaPAA (M¯ w = 5100) and pyrene as the fluorescence probe were purchased from Aldrich Chemicals and used as received. Sodium bromide (NaBr), sodium chloride (NaCl), and potassium chloride (KCl) were analytical grade without further treatment before use. Deionized H2 O was treated with KMnO4 and redistilled.
2.2.2. UV–vis transmittance Transmittance of 10−4 M pure NaPAA solution, 12-6-12/ NaPAA, and 12-6-12/NaPAA/salt solution was recorded using a UV spectrophotometer (UV-2450, Shimadzu) at room temperature of about 24–26 ◦ C. The slit width is 2 nm. 2.2.3. Zeta potential Nano-ZS (MALVERN) using laser Doppler velocimetry and phase analysis light scattering was used for zeta potential measurement. The temperature of the scattering cell was controlled at 25 ◦ C. A light scattering angle of 17◦ was combined with the reference beam, and the data were analyzed with the software supplied for the instrument. 2.2.4. Transmission electron microscopy Samples were prepared by negative staining with uranyl acetate. The microstructure of the mixture was determined using a transmission electron microscope (JEM-100CX, Japan). 3. Results and discussion 3.1. Effect of NaBr The I1 /I3 in the fluorescence spectrum reflects the intensity of the micropolarity around pyrene, and a change in I1 /I3 can be used to detect the formation of micelles and aggregates. Fig. 1 shows I1 /I3 as a function of surfactant concentration cs in 12-6-12/NaPAA/NaBr solution at various NaBr concentrations. At a low cs , I1 /I3 is nearly a constant, which implies that no hydrophobic microdomain is formed in the solution. A later sharp decease in I1 /I3 reflects the CAC, the onset for the formation of micelle-like structures near the binding site of the polyion chain. Due to the strong attraction between the oppositely charged surfactant and polyion, the CAC is much lower than the CMC in the absence of polyelectrolyte, as we observed previously [26]. Apparently, in the presence of 0.002 M NaBr, the CAC becomes smaller. The decrease in the CAC is much
2.2. Methods 2.2.1. Fluorescence emission spectroscopy Samples were prepared by mixing a pyrene stock solution with 12-6-12/NaPAA in the absence or presence of salt and allowed to stand for 3 days to equilibrate. The pyrene stock solution was prepared by dissolving pyrene in hot water up to saturation, cooled to 25 ◦ C, and filtered. The concentration of pyrene was determined to be 6.53 × 10−7 M [22]. The emission spectrum (λEX = 335 nm) of the mixed solution was recorded by F4500 (HITACHI) at wavelengths between 350 and 450 nm at a room temperature of about 24–26 ◦ C. The slit width of excitation is 5 nm, and the slit width of emission is 2.5 nm. A typical emission spectrum has five peaks at 373, 379, 384, 390, and 397 nm, respectively. The ratio of the first to the third vibronic peak I1 /I3 is sensitive to the local environment of pyrene [23–25].
Fig. 1. Surfactant concentration dependence of the micropolarity in 12-6-12/ NaPAA/NaBr solution at various NaBr concentrations (cNaPAA = 10−4 M). The curves are drawn to guide the eye.
Y. Pi et al. / Journal of Colloid and Interface Science 306 (2007) 405–410
407
Fig. 2. NaBr concentration dependence of the CAC in 12-6-12/NaPAA/NaBr solution.
less pronounced at a higher NaBr concentration of 0.02 M. However, with 0.1 M NaBr, the CAC is greater and even exceeds the CAC without NaBr. Fig. 2 shows the dependence of CAC in 12-6-12/NaPAA/ NaBr solutions on NaBr concentration. The effect of salt concentration on CAC is more complicated than that on the CMC of free surfactant, which decreases more significantly in the presence of a higher salt concentration [27]. Compared to 12-612/NaPAA solution without salt, a small amount of added NaBr (0.002 or 0.02 M) results in a smaller CAC; however, CAC is larger with increased amount of NaBr. This implies that NaBr has a counterbalancing effect on 12-6-12/NaPAA solution. On one hand, the addition of NaBr compresses the diffusive electric double layers and hence reduces the repulsion between surfactant heads; on the other hand, it screens the electrostatic attraction between the ionic surfactant head and the polyion. These two factors come into play with increased addition of NaBr. At a low NaBr concentration, the former factor dominates, facilitates the formation of a micelle-like structure, and yields a smaller CAC. In contrast, at a high salt concentration, the latter factor plays a major role and results in a larger CAC. Fig. 3A shows the transmittance as a function of wavelength λ in 12-6-12/NaPAA and NaPAA/NaBr solution. The transmittance has a minimum at λ = 200 nm, which is the absorption band of NaPAA, and approaches 99.6% of the maximal magnitude as λ > 300 nm. The transmittance at λ = 450 nm (T450 ), higher than the absorption band of NaPAA, was taken from the measured spectra in order to examine the surfactant concentration effect [26,28], as shown in Fig. 3B. With increasing surfactant concentration cs , the transmittance initially decreases slowly and then rapidly because of the occurrence of microphase separation. The formation of a micelle-like structure is nearly completed when the transmittance starts to decrease. The microphase separation is caused by the formation of larger aggregates or precipitates as a consequence of the strong interactions between 12-6-12 and NaPAA. As we can see, added NaBr has a significant influence on the transmittance. The onset of transmittance decrease begins at a lower cs when the added NaBr concentration is larger. Added NaBr
Fig. 3. (A) transmittance of 12-6-12/NaPAA solution as a function of wavelength. (i) cs = 0; (ii) cs = 0 in the presence of 0.02 M NaBr; (iii) cs = 5.86 × 10−4 M; (iv) cs = 8.79 × 10−4 M without salt. (B) Surfactant concentration dependence of the transmittance at 450 nm in 12-6-12/NaPAA/NaBr solution at various NaBr concentrations (cNaPAA = 10−4 M).
compresses the diffusive electric double layers and reduces the curvature of aggregate; consequently, larger aggregates can form more easily [29,30]. Our previous research [30] reveals that at high surfactant concentrations, in which the micelle-like structure is completely formed, the added NaBr only affects the formation of aggregates. Consequently, the more NaBr is added, the stronger is the ability to form larger aggregates. Fig. 4 shows the zeta potential as a function of surfactant concentration. Zeta potential increases initially with increasing surfactant concentration; however, after passing a maximum, it decreases. The initial increase is attributed predominantly to the increased number of surfactant molecules bound to polyions; as a result, the repulsion between charged segments in polyions is reduced, which leads the extended polyion chain to wrap around discrete micelle-like structures. With further increasing surfactant concentration, the charge of the polyion chain tends to be neutralized by 12-6-12 with the formation of complexes. The variation of zeta potential depends not only on surfactant concentration but also on NaBr concentration. With 0.002 M NaBr, the surfactant concentration at which the zeta potential starts to increase is smaller than that without NaBr. Fluores-
408
Y. Pi et al. / Journal of Colloid and Interface Science 306 (2007) 405–410
cence experiments reveal that little addition of NaBr can facilitate the formation of micelle-like structures. Therefore, the binding of 12-6-12 to NaPAA is promoted by NaBr, and hence zeta potential increases earlier. At a higher NaBr concentration of 0.02 M, the lowering of the surfactant concentration at which the zeta potential starts to increase is less pronounced, and the change of zeta potential becomes less noticeable. This is because NaBr is absorbed onto the partially neutralized polyion chain and prevents polyions from curling. At 0.1 M NaBr, the change of zeta potential is different from that without NaBr or with 0.002 or 0.02 M NaBr. At low surfactant concentrations, the zeta potential with 0.1 M NaBr is much larger (more negative) than others. This is because a large amount of NaBr significantly screens the repulsion between charged segments in polyions, and polyion chains tend to adopt a random coiled conformation. With further increasing surfactant concentration, polyion chains continue to curl due to the enhanced binding with surfactant by strong electrostatic attraction. Nevertheless, NaBr is also absorbed onto the partially neutralized polyion chains and therefore prevents polyions from curling. The ten-
dency to prevent polyions from curling is enhanced at a higher NaBr concentration. Thus the change extent of zeta potential with 0.1 M NaBr is the smallest among the three solutions. TEM images of 12-6-12 with increasing surfactant concentration cs in the presence of 10−4 M NaPAA and 0.1 M NaBr were used to identify the microstructures formed in solution. Below CAC, no microstructure was observed. With increasing cs , a variety of inhomogeneous structures appeared, as demonstrated in Fig. 5. Fig. 5A shows the TEM image of 12-612/NaPAA/NaBr solution at cs = 2 × 10−4 M, well above the CAC. There are spherical micelle-like structures with diameter in the range 20–40 nm, which is larger than for those without salt [26]. At cs = 5×10−3 M, shown in Fig. 5B, the TEM image indicates larger spherical structures and vesicles with diameter about 40–100 nm. The aggregates here are obviously larger than micelle-like structures. As revealed by Fig. 3, at this surfactant concentration the formation of larger aggregates is caused by the fact that the binding sites of polyion are saturated, and excessive surfactant cannot form new micelle-like structures but forms larger aggregates inside the pre-existing micelle-like structures [26]. At cs = 7.4 × 10−2 M, shown in Fig. 5C, uniform global structures with diameter approximately 150 nm are observed in the TEM image. All the observed microstructures at various surfactant concentrations are larger than free micelles. This is attributed to the addition of polyelectrolyte and salt. On one hand, the added polyelectrolyte counteracts the electrostatic repulsion between surfactant heads and induces larger aggregates to form. On the other hand, the added salt compresses the electrical double layers and reduces the curvature of aggregates; consequently, the size of aggregates is further increased in the presence of salt. 3.2. Effect of ion species
Fig. 4. Surfactant concentration dependence of the zeta potential in 12-6-12/ NaPAA/NaBr solution at various NaBr concentrations (cNaPAA = 10−4 M).
Fig. 6 is the surfactant concentration dependence of micropolarity in the 12-6-12/NaPAA solution in the presence of different alkali halide salts. The initial I1 /I3 in the presence of NaCl or KCl is smaller than that with NaBr. The hydrated diameter of Cl− is larger than that of Br− [31]; therefore, the
Fig. 5. TEM images of 12-6-12/NaPAA/NaBr solution (cNaPAA = 10−4 M, cNaBr = 0.1 M) at various surfactant concentrations: (A) cs = 2 × 10−4 M; (B) cs = 5 × 10−3 M; (C) cs = 7.4 × 10−2 M. Prepared by negative staining with uranyl acetate.
Y. Pi et al. / Journal of Colloid and Interface Science 306 (2007) 405–410
409
fect on the formation of aggregates because the counterion of the aggregate is negative. 4. Conclusions
Fig. 6. Surfactant concentration dependence of the micropolarity in 12-6-12/ NaPAA/salt solution (cNaPAA = 10−4 M).
The effect of salt on the interactions between 12-6-12 and NaPAA is found to be governed by two controversial factors; one favors the formation of micelle-like structures and the other screens the electrostatic attraction between surfactant and polyelectrolyte. At a low salt concentration, the Stern layer is compressed and aggregation occurs at a smaller CAC. At a high salt concentration, the screening dominates and leads to a larger CAC. Upon the formation of micelle-like structures at a high surfactant concentration, the addition of NaBr is favorable for larger aggregates to form. The microstructures detected by TEM show that a global structure is generally observed in the presence of NaBr, which is different from that without salt. Therefore, NaBr can be used to monitor the microstructure of 12-6-12/NaPAA complex. Variation in ion species shows different effect on the interactions. The less hydrated anion Br− has a stronger effect than Cl− and leads to a larger CAC; however, CAC is not distinctly influenced by the type of cation. Acknowledgments This work was supported by the National Natural Science Foundation of China (Projects 20236010, 20476025), the Doctoral Research Foundation sponsored by the Ministry of Education of China (Project 20050251004), the Shanghai Municipal Science and Technology Commission of China (05DJ14002), and the National University of Singapore. References
Fig. 7. Surfactant concentration dependence of the transmittance at 450 nm in 12-6-12/NaPAA/salt solution (cNaPAA = 10−4 M).
mobility of Cl− is smaller, and its ability to increase micropolarity is weaker. The micelle-like structure is positively charged, and negative ions can be absorbed to form counterion layers. Less hydrated Br− binds more strongly to micelle-like structures; therefore, it has a stronger ability to screen the interaction between gemini surfactant and polyelectrolyte. As a result, the CAC in a solution with NaBr is larger than that with NaCl. However, the CAC is almost identical in the presence of NaCl or KCl. The insensitivity of the CAC to the type of cation is probably because the local counterions in the binding region of gemini surfactant and NaPAA are primarily negative. Fig. 7 shows the surfactant concentration dependence of T450 for 12-6-12/NaPAA/salt solutions. The decrease of transmittance in the presence of NaBr begins at a lower surfactant concentration. NaBr, with a larger diameter, has a stronger ability to suppress the Stern layer and reduces the curvature of the aggregate. Thus, the formation of larger aggregates is more favorable in the presence of NaBr. The difference between KCl and NaCl is not pronounced. At high surfactant concentrations, at which larger aggregates start to form, the cation has little ef-
[1] J.C.T. Kwak, Polymer–Surfactant Systems, Dekker, New York, 1998. [2] M. Almgren, P. Hansson, E. Mukhtar, J. Van Stam, Langmuir 8 (1992) 2405. [3] H. Arai, M. Murata, K. Shinoda, J. Colloid Interface Sci. 37 (1971) 223. [4] E.D. Goddard, R.B. Hannan, J. Colloid Interface Sci. 55 (1976) 73. [5] M. Murata, H. Arai, J. Colloid Interface Sci. 44 (1973) 475. [6] B. Lindman, K. Thalberg, in: E.D. Goddard, K.P. Ananthapadmanabhan (Eds.), Interactions of Surfactants with Polymers and Proteins, CRC Press, Boca Raton, FL, 1993, chap. 5. [7] K. Hayakawa, J.C.T. Kwak, J. Phys. Chem. 86 (1982) 3866. [8] K. Hayakawa, J.C.T. Kwak, J. Phys. Chem. 87 (1983) 506. [9] A. Malovikova, K. Hayakawa, J.C.T. Kwak, J. Phys. Chem. 88 (1984) 1930. [10] P. Hansson, M. Almgren, J. Phys. Chem. 99 (1995) 16,684. [11] C. Wang, K.C. Tam, Langmuir 18 (2002) 6484. [12] C. Wang, K.C. Tam, R.D. Jenkins, C.B. Tan, J. Phys. Chem. B 107 (2003) 4667. [13] K. Thalberg, B. Lindman, J. Phys. Chem. 93 (1989) 1478. [14] Å. Herslöf-Björling, M. Björling, L. Sundelöf, Langmuir 15 (1999) 353. [15] K. Thalberg, B. Lindman, K. Bergfeldt, Langmuir 7 (1991) 2893. [16] M.A. Villetti, R. Borsali, J.S. Crespo, V. Soldi, K. Fukada, Macromol. Chem. Phys. 205 (2004) 907. [17] X. Wang, Y. Li, J. Wang, Y. Wang, Z. Guo, H. Yan, J. Phys. Chem. B 109 (2005) 10807. [18] X. Wang, J. Wang, Y. Wang, H. Yan, Langmuir 20 (2004) 9014. [19] F.M. Menger, C.A. Littau, J. Am. Chem. Soc. 113 (1991) 1451.
410
Y. Pi et al. / Journal of Colloid and Interface Science 306 (2007) 405–410
[20] R. Zana, J.D. Xia, R. Zara (Eds.), Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications, Dekker, New York, 2003. [21] Q.B. Chen, Y.H. Wei, Y.H. Shi, H.L. Liu, Y. Hu, J. East China Univ. Sci. Technol. 29 (2003) 33. [22] M. Pisárˇcik, T. Imae, F. Devínsky, I. Lacko, Colloids Surf. A 183–185 (2001) 555. [23] P. Hansson, Langmuir 17 (2001) 4161. [24] K. Thalberg, J. Van Stam, C. Lindblad, M. Almgren, J. Phys. Chem. 95 (1991) 8975. [25] A. Hashidzume, M. Mizusaki, K. Yoda, Y. Morishima, Langmuir 15 (1999) 4276.
[26] Y. Pi, Y. Shang, C. Peng, H. Liu, Y. Hu, J. Jiang, J. Colloid Interface Sci. 301 (2006) 631. [27] G.X. Zhao, Physical Chemistry of Surfactant, Peking University Press, Beijing, 1991. [28] M. Pisárˇcik, T. Imae, F. Devínsky, I. Lacko, F. Bakoš, J. Colloid Interface Sci. 228 (2000) 207. [29] A. Svensson, L. Piculell, B. Cabane, P. Ilekti, J. Phys. Chem. B 106 (2002) 1013. [30] Y. Pi, Y. Shang, C. Peng, H. Liu, Y. Hu, J. Jiang, J. Colloid Interface Sci. 299 (2006) 410. [31] L. Liu, M.J. Rosen, J. Colloid Interface Sci. 179 (1996) 454.
Salt effect on the interactions between gemini surfactant and oppositely charged polyelectrolyte in aqueous solution Yingying Pi a , Yazhuo Shang a , Honglai Liu a,∗ , Ying Hu a , Jianwen Jiang b a Lab for Advanced Materials and Department of Chemistry, East China University of Science and Technology, Shanghai 200237, China b Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117576 Singapore
Received 17 July 2006; accepted 10 October 2006 Available online 20 November 2006
Abstract The effect of alkali halides (NaBr, NaCl, KCl) on the interactions between the cationic gemini surfactant hexylene-1,6-bis(dodecyldimethylammonium bromide) (12-6-12) and the anionic polyelectrolyte sodium polyacrylate (NaPAA) in aqueous solution has been investigated by fluorescence emission spectroscopy, UV transmittance, zeta potential, and transmission electron microscopy (TEM). With increased addition of NaBr, a counterbalancing salt effect on the critical aggregation concentration (CAC) is observed. At low concentrations, NaBr facilitates the formation of micelle-like structures between surfactant and polyelectrolyte and results in a smaller CAC. At high concentrations, NaBr screens the electrostatic attraction between surfactant and polyelectrolyte and leads to a larger CAC. Upon the formation of micelle-like structures at high surfactant concentrations, the addition of NaBr is favorable for larger aggregates. The microstructure detected by TEM show that a global structure is generally formed in the presence of NaBr. The interactions also depend on ion species. Compared to NaBr, the addition of NaCl or KCl yields a smaller CAC. © 2006 Elsevier Inc. All rights reserved. Keywords: Gemini surfactant; Polyelectrolyte; Interactions; Salt; Alkali halide
1. Introduction Currently the interactions of ionic surfactants with polyelectrolytes are the subject of extensive investigation due to their wide applications in many industrially important processes and products, such as water treatment, detergency, and oil recovery [1]. Ionic surfactants can strongly interact with oppositely charged polyelectrolytes to form micelle-like structures at a very low critical aggregation concentration (CAC), which is usually a few orders of magnitude lower than the critical micellization concentration (CMC) of the free surfactant [2–6]. This is attributed to the electrostatic attraction between the surfactant and the polyelectrolyte, as well as to the hydrophobic interaction between the surfactant tails. In a surfactant/polyelectrolyte solution, the addition of salt is expected to have a significant effect on their interactions. * Corresponding author. Fax: +86 21 64252921.
E-mail address: [email protected] (H. Liu). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.10.020
It has been shown that the addition of salt reduces the strength of electrostatic interaction between surfactants and polyelectrolytes, which results in a larger CAC [7–12]. Moreover, the addition of a sufficiently large amount of salt can completely screen the electrostatic interaction and prevent the formation of polyelectrolyte/surfactant complexes [13–16]. Nevertheless, the effect of added salt in the surfactant/polyelectrolyte solution is rather complicated. In addition to the reduction of electrostatic interaction, stabilization of surfactant aggregates was reported [6]. As a consequence, a decrease of CAC in the surfactant/polyelectrolyte solution can be expected by adding salt, e.g., in sodium carboxymethylcellulose and dodecyltrimethylammonium bromide with the addition of NaBr [17]. While many studies have been devoted to salt effects on interactions between single-chain surfactants and oppositely charged polyelectrolytes, salt effects on interactions of gemini surfactants with polyelectrolyte are less understood. Only recently was a study reported on the effect of NaBr on complex formation between a cationic gemini surfactant and an anionic
406
Y. Pi et al. / Journal of Colloid and Interface Science 306 (2007) 405–410
polyelectrolyte, and the CAC was found to be insensitive to the added NaBr [18]. A gemini is a dimeric surfactant consisting of two identical amphiphilic moieties (twins) covalently joined by a spacer group at or close to the ionic head groups [19]. As a new family of amphiphilic molecules, gemini surfactants have stimulated extensive interest with stronger surface activity and better solubilizing, wetting, foaming, and lime–soap dispersing capability than conventional surfactants [20]. Due to the unique features of gemini surfactants, the effect of salt is expected to be different from that on traditional surfactants. More systematic investigations are needed to further understand complex formation between gemini surfactants and polyelectrolytes. We have recently examined the interactions between the gemini surfactant alkanediyl-α,ω-bis(dodecyldimethylammonium bromide) (12-n-12, n = 3, 4, 6) and the anionic polyelectrolyte sodium polyacrylate (NaPAA) [26]. In this work we further examine the effect of added alkali halides (NaBr, NaCl, KCl) on the interactions between 12-6-12 and NaPAA in aqueous solution at 25 ◦ C. Fluorescence emission spectroscopy and UV transmittance were measured. The effect of salt on the microstructure change of the 12-6-12/NaPAA complex induced by increasing surfactant concentration with zeta potential was determined. The microstructure of mixtures in the presence of NaBr was detected by negative-staining and transmission electron microscopy. The interactions for different salt concentrations and ion species were also studied. 2. Experimental materials and methods 2.1. Materials The gemini surfactant 12-6-12 was prepared in our lab by a reaction of 1,6-dibromohexane with N ,N -dodecyldimethylamine [21]. Polyelectrolyte NaPAA (M¯ w = 5100) and pyrene as the fluorescence probe were purchased from Aldrich Chemicals and used as received. Sodium bromide (NaBr), sodium chloride (NaCl), and potassium chloride (KCl) were analytical grade without further treatment before use. Deionized H2 O was treated with KMnO4 and redistilled.
2.2.2. UV–vis transmittance Transmittance of 10−4 M pure NaPAA solution, 12-6-12/ NaPAA, and 12-6-12/NaPAA/salt solution was recorded using a UV spectrophotometer (UV-2450, Shimadzu) at room temperature of about 24–26 ◦ C. The slit width is 2 nm. 2.2.3. Zeta potential Nano-ZS (MALVERN) using laser Doppler velocimetry and phase analysis light scattering was used for zeta potential measurement. The temperature of the scattering cell was controlled at 25 ◦ C. A light scattering angle of 17◦ was combined with the reference beam, and the data were analyzed with the software supplied for the instrument. 2.2.4. Transmission electron microscopy Samples were prepared by negative staining with uranyl acetate. The microstructure of the mixture was determined using a transmission electron microscope (JEM-100CX, Japan). 3. Results and discussion 3.1. Effect of NaBr The I1 /I3 in the fluorescence spectrum reflects the intensity of the micropolarity around pyrene, and a change in I1 /I3 can be used to detect the formation of micelles and aggregates. Fig. 1 shows I1 /I3 as a function of surfactant concentration cs in 12-6-12/NaPAA/NaBr solution at various NaBr concentrations. At a low cs , I1 /I3 is nearly a constant, which implies that no hydrophobic microdomain is formed in the solution. A later sharp decease in I1 /I3 reflects the CAC, the onset for the formation of micelle-like structures near the binding site of the polyion chain. Due to the strong attraction between the oppositely charged surfactant and polyion, the CAC is much lower than the CMC in the absence of polyelectrolyte, as we observed previously [26]. Apparently, in the presence of 0.002 M NaBr, the CAC becomes smaller. The decrease in the CAC is much
2.2. Methods 2.2.1. Fluorescence emission spectroscopy Samples were prepared by mixing a pyrene stock solution with 12-6-12/NaPAA in the absence or presence of salt and allowed to stand for 3 days to equilibrate. The pyrene stock solution was prepared by dissolving pyrene in hot water up to saturation, cooled to 25 ◦ C, and filtered. The concentration of pyrene was determined to be 6.53 × 10−7 M [22]. The emission spectrum (λEX = 335 nm) of the mixed solution was recorded by F4500 (HITACHI) at wavelengths between 350 and 450 nm at a room temperature of about 24–26 ◦ C. The slit width of excitation is 5 nm, and the slit width of emission is 2.5 nm. A typical emission spectrum has five peaks at 373, 379, 384, 390, and 397 nm, respectively. The ratio of the first to the third vibronic peak I1 /I3 is sensitive to the local environment of pyrene [23–25].
Fig. 1. Surfactant concentration dependence of the micropolarity in 12-6-12/ NaPAA/NaBr solution at various NaBr concentrations (cNaPAA = 10−4 M). The curves are drawn to guide the eye.
Y. Pi et al. / Journal of Colloid and Interface Science 306 (2007) 405–410
407
Fig. 2. NaBr concentration dependence of the CAC in 12-6-12/NaPAA/NaBr solution.
less pronounced at a higher NaBr concentration of 0.02 M. However, with 0.1 M NaBr, the CAC is greater and even exceeds the CAC without NaBr. Fig. 2 shows the dependence of CAC in 12-6-12/NaPAA/ NaBr solutions on NaBr concentration. The effect of salt concentration on CAC is more complicated than that on the CMC of free surfactant, which decreases more significantly in the presence of a higher salt concentration [27]. Compared to 12-612/NaPAA solution without salt, a small amount of added NaBr (0.002 or 0.02 M) results in a smaller CAC; however, CAC is larger with increased amount of NaBr. This implies that NaBr has a counterbalancing effect on 12-6-12/NaPAA solution. On one hand, the addition of NaBr compresses the diffusive electric double layers and hence reduces the repulsion between surfactant heads; on the other hand, it screens the electrostatic attraction between the ionic surfactant head and the polyion. These two factors come into play with increased addition of NaBr. At a low NaBr concentration, the former factor dominates, facilitates the formation of a micelle-like structure, and yields a smaller CAC. In contrast, at a high salt concentration, the latter factor plays a major role and results in a larger CAC. Fig. 3A shows the transmittance as a function of wavelength λ in 12-6-12/NaPAA and NaPAA/NaBr solution. The transmittance has a minimum at λ = 200 nm, which is the absorption band of NaPAA, and approaches 99.6% of the maximal magnitude as λ > 300 nm. The transmittance at λ = 450 nm (T450 ), higher than the absorption band of NaPAA, was taken from the measured spectra in order to examine the surfactant concentration effect [26,28], as shown in Fig. 3B. With increasing surfactant concentration cs , the transmittance initially decreases slowly and then rapidly because of the occurrence of microphase separation. The formation of a micelle-like structure is nearly completed when the transmittance starts to decrease. The microphase separation is caused by the formation of larger aggregates or precipitates as a consequence of the strong interactions between 12-6-12 and NaPAA. As we can see, added NaBr has a significant influence on the transmittance. The onset of transmittance decrease begins at a lower cs when the added NaBr concentration is larger. Added NaBr
Fig. 3. (A) transmittance of 12-6-12/NaPAA solution as a function of wavelength. (i) cs = 0; (ii) cs = 0 in the presence of 0.02 M NaBr; (iii) cs = 5.86 × 10−4 M; (iv) cs = 8.79 × 10−4 M without salt. (B) Surfactant concentration dependence of the transmittance at 450 nm in 12-6-12/NaPAA/NaBr solution at various NaBr concentrations (cNaPAA = 10−4 M).
compresses the diffusive electric double layers and reduces the curvature of aggregate; consequently, larger aggregates can form more easily [29,30]. Our previous research [30] reveals that at high surfactant concentrations, in which the micelle-like structure is completely formed, the added NaBr only affects the formation of aggregates. Consequently, the more NaBr is added, the stronger is the ability to form larger aggregates. Fig. 4 shows the zeta potential as a function of surfactant concentration. Zeta potential increases initially with increasing surfactant concentration; however, after passing a maximum, it decreases. The initial increase is attributed predominantly to the increased number of surfactant molecules bound to polyions; as a result, the repulsion between charged segments in polyions is reduced, which leads the extended polyion chain to wrap around discrete micelle-like structures. With further increasing surfactant concentration, the charge of the polyion chain tends to be neutralized by 12-6-12 with the formation of complexes. The variation of zeta potential depends not only on surfactant concentration but also on NaBr concentration. With 0.002 M NaBr, the surfactant concentration at which the zeta potential starts to increase is smaller than that without NaBr. Fluores-
408
Y. Pi et al. / Journal of Colloid and Interface Science 306 (2007) 405–410
cence experiments reveal that little addition of NaBr can facilitate the formation of micelle-like structures. Therefore, the binding of 12-6-12 to NaPAA is promoted by NaBr, and hence zeta potential increases earlier. At a higher NaBr concentration of 0.02 M, the lowering of the surfactant concentration at which the zeta potential starts to increase is less pronounced, and the change of zeta potential becomes less noticeable. This is because NaBr is absorbed onto the partially neutralized polyion chain and prevents polyions from curling. At 0.1 M NaBr, the change of zeta potential is different from that without NaBr or with 0.002 or 0.02 M NaBr. At low surfactant concentrations, the zeta potential with 0.1 M NaBr is much larger (more negative) than others. This is because a large amount of NaBr significantly screens the repulsion between charged segments in polyions, and polyion chains tend to adopt a random coiled conformation. With further increasing surfactant concentration, polyion chains continue to curl due to the enhanced binding with surfactant by strong electrostatic attraction. Nevertheless, NaBr is also absorbed onto the partially neutralized polyion chains and therefore prevents polyions from curling. The ten-
dency to prevent polyions from curling is enhanced at a higher NaBr concentration. Thus the change extent of zeta potential with 0.1 M NaBr is the smallest among the three solutions. TEM images of 12-6-12 with increasing surfactant concentration cs in the presence of 10−4 M NaPAA and 0.1 M NaBr were used to identify the microstructures formed in solution. Below CAC, no microstructure was observed. With increasing cs , a variety of inhomogeneous structures appeared, as demonstrated in Fig. 5. Fig. 5A shows the TEM image of 12-612/NaPAA/NaBr solution at cs = 2 × 10−4 M, well above the CAC. There are spherical micelle-like structures with diameter in the range 20–40 nm, which is larger than for those without salt [26]. At cs = 5×10−3 M, shown in Fig. 5B, the TEM image indicates larger spherical structures and vesicles with diameter about 40–100 nm. The aggregates here are obviously larger than micelle-like structures. As revealed by Fig. 3, at this surfactant concentration the formation of larger aggregates is caused by the fact that the binding sites of polyion are saturated, and excessive surfactant cannot form new micelle-like structures but forms larger aggregates inside the pre-existing micelle-like structures [26]. At cs = 7.4 × 10−2 M, shown in Fig. 5C, uniform global structures with diameter approximately 150 nm are observed in the TEM image. All the observed microstructures at various surfactant concentrations are larger than free micelles. This is attributed to the addition of polyelectrolyte and salt. On one hand, the added polyelectrolyte counteracts the electrostatic repulsion between surfactant heads and induces larger aggregates to form. On the other hand, the added salt compresses the electrical double layers and reduces the curvature of aggregates; consequently, the size of aggregates is further increased in the presence of salt. 3.2. Effect of ion species
Fig. 4. Surfactant concentration dependence of the zeta potential in 12-6-12/ NaPAA/NaBr solution at various NaBr concentrations (cNaPAA = 10−4 M).
Fig. 6 is the surfactant concentration dependence of micropolarity in the 12-6-12/NaPAA solution in the presence of different alkali halide salts. The initial I1 /I3 in the presence of NaCl or KCl is smaller than that with NaBr. The hydrated diameter of Cl− is larger than that of Br− [31]; therefore, the
Fig. 5. TEM images of 12-6-12/NaPAA/NaBr solution (cNaPAA = 10−4 M, cNaBr = 0.1 M) at various surfactant concentrations: (A) cs = 2 × 10−4 M; (B) cs = 5 × 10−3 M; (C) cs = 7.4 × 10−2 M. Prepared by negative staining with uranyl acetate.
Y. Pi et al. / Journal of Colloid and Interface Science 306 (2007) 405–410
409
fect on the formation of aggregates because the counterion of the aggregate is negative. 4. Conclusions
Fig. 6. Surfactant concentration dependence of the micropolarity in 12-6-12/ NaPAA/salt solution (cNaPAA = 10−4 M).
The effect of salt on the interactions between 12-6-12 and NaPAA is found to be governed by two controversial factors; one favors the formation of micelle-like structures and the other screens the electrostatic attraction between surfactant and polyelectrolyte. At a low salt concentration, the Stern layer is compressed and aggregation occurs at a smaller CAC. At a high salt concentration, the screening dominates and leads to a larger CAC. Upon the formation of micelle-like structures at a high surfactant concentration, the addition of NaBr is favorable for larger aggregates to form. The microstructures detected by TEM show that a global structure is generally observed in the presence of NaBr, which is different from that without salt. Therefore, NaBr can be used to monitor the microstructure of 12-6-12/NaPAA complex. Variation in ion species shows different effect on the interactions. The less hydrated anion Br− has a stronger effect than Cl− and leads to a larger CAC; however, CAC is not distinctly influenced by the type of cation. Acknowledgments This work was supported by the National Natural Science Foundation of China (Projects 20236010, 20476025), the Doctoral Research Foundation sponsored by the Ministry of Education of China (Project 20050251004), the Shanghai Municipal Science and Technology Commission of China (05DJ14002), and the National University of Singapore. References
Fig. 7. Surfactant concentration dependence of the transmittance at 450 nm in 12-6-12/NaPAA/salt solution (cNaPAA = 10−4 M).
mobility of Cl− is smaller, and its ability to increase micropolarity is weaker. The micelle-like structure is positively charged, and negative ions can be absorbed to form counterion layers. Less hydrated Br− binds more strongly to micelle-like structures; therefore, it has a stronger ability to screen the interaction between gemini surfactant and polyelectrolyte. As a result, the CAC in a solution with NaBr is larger than that with NaCl. However, the CAC is almost identical in the presence of NaCl or KCl. The insensitivity of the CAC to the type of cation is probably because the local counterions in the binding region of gemini surfactant and NaPAA are primarily negative. Fig. 7 shows the surfactant concentration dependence of T450 for 12-6-12/NaPAA/salt solutions. The decrease of transmittance in the presence of NaBr begins at a lower surfactant concentration. NaBr, with a larger diameter, has a stronger ability to suppress the Stern layer and reduces the curvature of the aggregate. Thus, the formation of larger aggregates is more favorable in the presence of NaBr. The difference between KCl and NaCl is not pronounced. At high surfactant concentrations, at which larger aggregates start to form, the cation has little ef-
[1] J.C.T. Kwak, Polymer–Surfactant Systems, Dekker, New York, 1998. [2] M. Almgren, P. Hansson, E. Mukhtar, J. Van Stam, Langmuir 8 (1992) 2405. [3] H. Arai, M. Murata, K. Shinoda, J. Colloid Interface Sci. 37 (1971) 223. [4] E.D. Goddard, R.B. Hannan, J. Colloid Interface Sci. 55 (1976) 73. [5] M. Murata, H. Arai, J. Colloid Interface Sci. 44 (1973) 475. [6] B. Lindman, K. Thalberg, in: E.D. Goddard, K.P. Ananthapadmanabhan (Eds.), Interactions of Surfactants with Polymers and Proteins, CRC Press, Boca Raton, FL, 1993, chap. 5. [7] K. Hayakawa, J.C.T. Kwak, J. Phys. Chem. 86 (1982) 3866. [8] K. Hayakawa, J.C.T. Kwak, J. Phys. Chem. 87 (1983) 506. [9] A. Malovikova, K. Hayakawa, J.C.T. Kwak, J. Phys. Chem. 88 (1984) 1930. [10] P. Hansson, M. Almgren, J. Phys. Chem. 99 (1995) 16,684. [11] C. Wang, K.C. Tam, Langmuir 18 (2002) 6484. [12] C. Wang, K.C. Tam, R.D. Jenkins, C.B. Tan, J. Phys. Chem. B 107 (2003) 4667. [13] K. Thalberg, B. Lindman, J. Phys. Chem. 93 (1989) 1478. [14] Å. Herslöf-Björling, M. Björling, L. Sundelöf, Langmuir 15 (1999) 353. [15] K. Thalberg, B. Lindman, K. Bergfeldt, Langmuir 7 (1991) 2893. [16] M.A. Villetti, R. Borsali, J.S. Crespo, V. Soldi, K. Fukada, Macromol. Chem. Phys. 205 (2004) 907. [17] X. Wang, Y. Li, J. Wang, Y. Wang, Z. Guo, H. Yan, J. Phys. Chem. B 109 (2005) 10807. [18] X. Wang, J. Wang, Y. Wang, H. Yan, Langmuir 20 (2004) 9014. [19] F.M. Menger, C.A. Littau, J. Am. Chem. Soc. 113 (1991) 1451.
410
Y. Pi et al. / Journal of Colloid and Interface Science 306 (2007) 405–410
[20] R. Zana, J.D. Xia, R. Zara (Eds.), Gemini Surfactants: Synthesis, Interfacial and Solution-Phase Behavior, and Applications, Dekker, New York, 2003. [21] Q.B. Chen, Y.H. Wei, Y.H. Shi, H.L. Liu, Y. Hu, J. East China Univ. Sci. Technol. 29 (2003) 33. [22] M. Pisárˇcik, T. Imae, F. Devínsky, I. Lacko, Colloids Surf. A 183–185 (2001) 555. [23] P. Hansson, Langmuir 17 (2001) 4161. [24] K. Thalberg, J. Van Stam, C. Lindblad, M. Almgren, J. Phys. Chem. 95 (1991) 8975. [25] A. Hashidzume, M. Mizusaki, K. Yoda, Y. Morishima, Langmuir 15 (1999) 4276.
[26] Y. Pi, Y. Shang, C. Peng, H. Liu, Y. Hu, J. Jiang, J. Colloid Interface Sci. 301 (2006) 631. [27] G.X. Zhao, Physical Chemistry of Surfactant, Peking University Press, Beijing, 1991. [28] M. Pisárˇcik, T. Imae, F. Devínsky, I. Lacko, F. Bakoš, J. Colloid Interface Sci. 228 (2000) 207. [29] A. Svensson, L. Piculell, B. Cabane, P. Ilekti, J. Phys. Chem. B 106 (2002) 1013. [30] Y. Pi, Y. Shang, C. Peng, H. Liu, Y. Hu, J. Jiang, J. Colloid Interface Sci. 299 (2006) 410. [31] L. Liu, M.J. Rosen, J. Colloid Interface Sci. 179 (1996) 454.