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Polymer–surfactant interaction: differences between alkyl sulfate and alkyl sulfonate
Colloids and Surfaces A: Physicochem. Eng. Aspects 244 (2004) 39–44
Polymer–surfactant interaction: differences between alkyl sulfate and alkyl sulfonate Peng Yan, Jin-Xin Xiao∗ Institute of Physical Chemistry, Peking University, Beijing 100871, PR China Received 15 January 2004; accepted 24 June 2004 Available online 17 August 2004
Abstract The interactions between sodium alkyl sulfates (Cn SO4 )/sodium alkyl sulfonates (Cn SO3 ) and polyethylene oxide (PEO) were studied by surface tension, fluorescence and dynamic light scattering measurements, assisted with quantum-chemical calculations. It was found that, although many properties of Cn SO4 and Cn SO3 were similar, there were striking differences in their interactions with PEO. Sodium alkyl sulfonates were seen to interact more weakly than their sulfate analogs. Such differences were especially distinct when compared C10 SO4 -PEO with C10 SO3 -PEO, the former exhibits obvious interactions, but no observable interaction in the latter. With the help of ab initio quantum-chemical calculation, the results were explained. © 2004 Elsevier B.V. All rights reserved. Keywords: Sodium alkyl sulfate; Sodium alkyl sulfonate; Polyethylene oxide; Polymer–surfactant interaction; Quantum-chemical calculation
1. Introduction Polymer and surfactant are two typical kinds of soft matter, and their respective properties have been widely studied [1]. It has been shown that the mixtures of polymer and surfactant [2,3], which is another special kind of soft matter, have many characteristics compared with their single components, such as surface activity, viscosity, wetting, foaming, solubilization, etc. [4]. Therefore, this kind of soft matter has been extensively used in a variety of industrial processes. From the scientific viewpoint, the interest in polymer–surfactant systems is the different intermolecular interactions in the multiplicity of possible combinations. Interaction between water-soluble neutral polymer and surfactant is generally attributed to the surfactant bind noncovalently to the polymer. With the addition of polymer, surfactant aggregates form on polymer chains when surfactant concentration reaches to a critical level (critical aggregation concentration, cac), which is usually lower than ∗ Corresponding author. Tel.: +86 10 62764973; fax: +86 10 62751708. E-mail address: [email protected] (J.-X. Xiao).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.06.023
the critical micellar concentration (cmc) of polymer-free surfactant solution. Although the detail mechanism of the polymer–surfactant interaction has been still unknown, previous researches indicated that the interaction mainly depended on the surfactant tail length, surfactant headgroup, polymer hydrophobicity and flexibility [5]. Among all the mixed polymer–surfactant systems, sodium alkyl sulfates (Cn SO4 ), especially sodium dodecyl sulfate (C12 SO4 ), are the most used anionic surfactants [6–20]. However, only a few investigations involved sodium alkyl sulfonate (Cn SO3 ) [21–24]. It has been known that Cn SO3 is quite similar to Cn SO4 in many physicochemical properties. Very recently, we found that Cn SO4 and Cn SO3 exhibit remarkable differences when mixed with cationic surfactants [25,26]. In this work, we observed that there were also distinct differences between Cn SO4 and Cn SO3 (n = 10, 12) with respect to the interaction with polyethylene oxide (PEO), based on the measurements of surface tension, fluorescence and dynamic light scattering. With the results of ab initio quantum-chemical calculation, we proposed a possible explanation.
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2. Experimental
tions and molecular charge distribution calculations were performed at the HF level using the 6-31G* basic set [30].
2.1. Materials Sodium decyl sulfate (C10 SO4 ) was synthesized from chlorosulfonic acid and decanol, and then the solution was neutralized. The crude product was recrystallized five times from ethanol. Sodium dodecyl sulfate (C12 SO4 ), 99% pure from Sigma, was used as received. Sodium decyl sulfonate (C10 SO3 ), from Tokyo Kasei, A.R. grade, was used as received. Sodium dodecyl sulfonate (C12 SO3 ), a product from Beijing Chemical Co., C.P. grade, 97%, was recrystallized from water and ether. Polyethylene oxide (PEO, Mw = 20,000 Da, Sigma) was used as received. The purity of all the surfactants was examined, and no minima were observed in the surface tension curves. All the solutions were prepared using deionized water. 2.2. Surface tension The surface tension of surfactant solutions was measured by the drop-volume method [27]. Because the Krafft Points of C10 SO3 and C12 SO3 are 24 and 38 ◦ C, respectively, surface tension measurements of systems containing C10 SO3 or C10 SO4 were performed at 25 ◦ C; while, systems containing C12 SO3 or C12 SO4 were performed at 40 ◦ C. The choice of the temperatures was also followed in the fluorescence and dynamic light scattering experiments. 2.3. Fluorescence The steady state pyrene fluorescence measurements were performed on Hitachi 4500 fluorescence spectrophotometer. The concentration of pyrene used in all the measurements was approximately 5 × 10−7 M. The excitation wavelength was 337 nm. The ratio of the intensities of the first vibrational peaks (I1 , 374 nm) to the third vibrational peaks (I3 , 384 nm) of the emission spectrum of pyrene was used in evaluating the polarity of the local microenvironment [28]. 2.4. Dynamic light scattering Dynamic light scattering (DLS) measurements were made using a spectrometer of standard design (ALV-5000/E/WIN Multiple Tau Digital Correlator) and a Spectra-Physics 2017 200 mW Ar laser (514.5 nm wavelength). All measurements were made at a scattering angle of 90◦ , and the intensity autocorrelation functions were analyzed using the methods of Contin [29]. 2.5. Quantum-chemical calculation All the calculations were performed using HyperChem Release 7.1 for Windows (Hypercube, Inc., Gainesville, FL). Both the three-dimensional molecular geometry optimiza-
3. Results and discussion 3.1. Differences between Cn SO4 and Cn SO3 : calculation of charge distribution The molecular structures of alkyl sulfate and alkyl sulfonate are very alike, except that Cn SO4 have one more oxygen atom than Cn SO3 in the headgroup. Because of the similar molecular structure, physicochemical properties of Cn SO4 and Cn SO3 , such as critical micellar concentration, surface excess concentration, free energy of micellar formation and average surface area per surfactant molecule at water surface [31–32], are quite similar. The most distinct difference between Cn SO4 and Cn SO3 is the Krafft Point, and the Krafft Point of Cn SO3 is much higher than that of Cn SO4 [33]. For ionic surfactants, the entire charge is generally considered to be a point charge at the headgroup. Recently, however, Huibers calculated the charge distribution of common ionic surfactants using semiempirical quantum-chemical methods, and found that in reality the headgroup charge was partially distributed to the rest of the molecule [34]. We used ab initio methods, which are more accurate than semiempirical ones, to calculate the charge distribution of Cn SO4 and Cn SO3 (Table 1). It was shown that, owing to the highly polar C S bond, the ␣-methylene group (␣-CH2 , the first CH2 group of the alkyl tail attached to the headgroup) of Cn SO3 held partial negative charge, while the ␣-CH2 of Cn SO4 held partial positive charge because it attached to the headgroup with the highly electronegative oxygen atom. Meanwhile, headgroup of Cn SO4 held much more negative charge than that of Cn SO3 . Therefore, it is the absence of an oxygen atom in the headgroup that results in the remarkable differences in the charge distribution. 3.2. Differences between Cn SO4 and Cn SO3 in interacting with PEO One of the most significant characteristics of polymer–surfactant interaction is there are two turning points in the surface tension (γ) versus surfactant concenTable 1 Grouped atomic partial charges for alkyl sulfonate and alkyl sulfatea C10 [SO3 − ] C10 [SO4 − ] C12 [SO3 − ] C12 [SO4 − ] Charge on headgroup −0.750 Charge on ␣-CH2 −0.229 Charge on alkyl tail −0.021
−1.319 +0.374 −0.055
−0.750 −0.214 −0.036
−1.319 +0.374 −0.055
a Groups consist of headgroup atoms (noted by square brackets), ␣methylene, and the remaining alkyl tail. Molecular charge distribution calculations and geometry optimizations were made using HyperChem 7.1 at HF/6-31G* level.
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points in the γ–c curve of C10 SO4 -PEO mixture, but only one turning point in that of C10 SO3 -PEO mixture at which the concentration equals to the cmc of C10 SO3 . Although the surface tension of C10 SO3 in PEO solution is a little lower than that in pure water when C10 SO3 concentrations are far below the cac, the lowering of surface tension might be caused by PEO itself, not by the interaction of C10 SO3 with PEO, since PEO itself is a little surface active [35] (surface tension of PEO (Mw = 20,000, 2.5 wt.%) was 60 mN/m (T = 25 ◦ C)/58 mN/m (T = 40 ◦ C)). Therefore, it could be thought that there is no observable interaction in sense of surface tension between C10 SO3 and PEO under the experimental conditions. The fluorescence probe method is also often used to study the polymer–surfactant interaction [36]. Pyrene is one of the most used fluorescence probes. The value of I1 /I3 is very sensitive to the polarity of the microenviron-
Fig. 1. Surface tension (γ) of surfactants vs. surfactant concentrations (c) in the presence and absence of PEO (Mw = 20,000 Da, 2.5 wt.%). (A) Solutions contain sodium alkyl sulfate (Cn SO4 ); (B) solutions contain sodium alkyl sulfonate (Cn SO3 ). Black arrows show the two turning points in the γ–c curve: (a) T = 25 ◦ C; (b) T = 40 ◦ C.
tration (c) curve (γ–c curve). The surfactant concentration at the first turning point (T1 ) represents critical aggregation concentration (cac), and the concentration at the second turning point (T2 ) means at which the polymer becomes saturated with the surfactant and free micelles form [6]. Fig. 1 shows the γ–c curves of Cn SO4 and Cn SO3 in the presence and absence of PEO. In both C12 SO4 -PEO and C12 SO3 -PEO systems, it is observed that their γ–c curves have two turning points in the present of PEO (Mw = 20,000, 2.5 wt.%), which indicates that interactions do exist between the polymer and the surfactants. However, the cac of C12 SO4 -PEO mixture is obviously lower than that of C12 SO3 -PEO mixture. It is interesting to compare C10 SO4 -PEO mixture and C10 SO3 -PEO mixture. Although the γ–c curves of C10 SO4 and C10 SO3 are very similar, their γ–c curves differ greatly in presence of PEO. Fig. 1 shows that there are two turning
Fig. 2. Plot of pyrene fluorescence intensity (I1 /I3 ) as a function of surfactants concentration (c) in the presence and absence of PEO (Mw = 20,000 Da, 2.5 wt.%). (A) Solutions contain sodium alkyl sulfate; (B) solutions contain sodium alkyl sulfonate: (a) T = 25 ◦ C; (b) T = 40 ◦ C.
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ment. When surfactant molecules aggregate, pyrene is solubilized in a hydrophobic environment and the I1 /I3 decreases [36]. Fig. 2 shows the dependence of the I1 /I3 on the surfactant concentration in the presence and absence of PEO (Mw = 20,000, 2.5 wt.%). It is shown that all these results support the conclusions drawing from surface tension measurements. Similar to the surface tension as already mentioned, at very low surfactant concentrations, far below the cac, the I1 /I3 value in PEO solution is lower than that in water, suggesting that the microenvironment of pyrene is changed [37,38]. The possible mechanisms might be that pyrene, as a strongly hydrophobic molecule, is preferentially solubilized inside the PEO coil, which considers being a little hydrophobic (I1 /I3 value in surfactant-free PEO (Mw = 20,000, 2.5 wt.%) solution is 1.7 (T = 25 ◦ C)/1.5 (T = 40 ◦ C)). The strength of the interaction between a polymer and a surfactant can be characterized by the cac/cmc ratio semiquantitatively [9]. The stronger the interaction between the polymer and the surfactant is, the smaller the value of the cac/cmc is. Table 2 lists the values of the cmc, cac and cac/cmc of the investigated systems. Consequently, we might conclude that the interactions of C10 SO4 -PEO and C12 SO4 -PEO are stronger than that of C10 SO3 -PEO and C12 SO3 -PEO at the corresponding temperature, respectively. Dynamic light scattering techniques are highly sensitive and noninvasive methods for investigating polymer–surfactant complex. DLS results were shown in Fig. 3. The measured autocorrelation functions were well resolved with two decay modes except for the C10 SO3 -PEO system. The fast diffusion modes corresponded to diffusivities identical to those obtained in surfactant-free polymer solution (apparent hydrodynamic radius Rh of PEO 2.5 wt.% were 2–3 nm) and could be identified with PEO itself. The slow diffusion modes exhibiting larger Rh should be polymer–surfactant complex. DLS measurements confirmed that there was no complex formed in C10 SO3 -PEO system. Fig. 3 shows that Rh of C12 SO4 -PEO complex was nearly the same as that of C12 SO3 -PEO complex. However, previous researches performed by NMR showed that the structures of C12 SO4 -PEO complex and C12 SO3 -PEO complex Table 2 cmc/cac of the investigated surfactants in the absence/presence of PEO (Mw = 20,000 Da) Surfactant
Polymer (wt.% PEO)
T (◦ C)
cmc or cac (mM)
cac/cmc
C10 SO3 C10 SO3 C10 SO4 C10 SO4 C12 SO3 C12 SO3 C12 SO4 C12 SO4
0 2.5 0 2.5 0 2.5 0 2.5
25 25 25 25 40 40 40 40
30a /30b 30a /30b 30a /30b 20a /20b 11.5a /10b 8.0a /7.0b 8.5a /8.0b 4.0a /4.0b
1.0a /1.0b
a b
Determined by surface tension. Determined by fluorescence.
0.67a /0.67b 0.70a /0.70b 0.47a /0.50b
were quite different. It has been indicated that there are two possible structures of polymer–surfactant complex [39]: (a) the polymer wraps around the micelles (“necklace” structures); (b) the micelles nucleate on the polymer hydrophobic sites (“bead” structures). Studies of Cabane [10] and Hou et al. [22] found that the C12 SO4 -PEO and C12 SO3 -PEO complexes were “necklace” and “bead” structures, respectively. 3.3. Mechanism of the differences Why there are such striking differences between Cn SO4 PEO interaction and Cn SO4 -PEO interaction? We tried to figure out the answers from the headgroup structure and charge distribution of these two kinds of surfactants. Because of the absence of an oxygen atom in the headgroup of Cn SO3 respect to Cn SO4 , the headgroup size of Cn SO3 is smaller than that of Cn SO4 . Furthermore, the results of the charge distribution in Table 1 show that the charge on the Cn SO4 headgroup and ␣-CH2 are opposite, while the Cn SO3 headgroup and ␣-CH2 have the same charge. This makes the hydration cosphere of the hydrophobic tail of Cn SO4 very different from the corresponding tail of Cn SO3 . As a result, the hydration effect of Cn SO3 is stronger than that of Cn SO4 , which means the hydration shell of the headgroup may extend over a larger part of the alkyl chain in Cn SO3 Consequently, this effect “shortens” the efficient hydrophobic tail with respect to that of Cn SO4 [24]. It has been well known that the hydrophobic effect is the most important driving force in the interaction between nonionic polymer and surfactant [40]. Therefore, the difference of the hydrophobic nature between Cn SO3 and Cn SO4 induces that the interaction of Cn SO3 -PEO is weaker than that of Cn SO4 -PEO. The differences in total charge between headgroups of Cn SO4 and Cn SO3 might be another important factor. With the quantum-chemical calculation, we found that the headgroup of Cn SO4 approximately held 0.5 more negative charges than that of Cn SO3 . It has been known that, because of the electronegative potential on the PEO chain, the electrostatic interaction between the ether oxygen group ( CH2 CH2 O ) and the electropositive counterions, such as Na+ and H3 O+ , results in a partially positive charge transferring to the ether oxygen group [8]. Therefore, the electrostatic interaction (including ion–dipole interaction) between the headgroup of Cn SO4 and the PEO chain is stronger than that of Cn SO3 and the PEO chain. Consequently, the interaction of Cn SO4 with PEO is stronger than that of Cn SO4 with PEO. The differences in the complex structures might also be explained in terms of the differences in charge distributions. As already mentioned, the headgroup of C12 SO4 has more negative charges than that of C12 SO3 , which makes that the headgroup of C12 SO4 has stronger electrostatic interaction with the PEO chain than that of C12 SO3 with PEO chain. Therefore, C12 SO4 and PEO trend to form “necklace-structure” complexes. The less negative charge on the headgroup and the stronger hydration effect greatly reduce the electrostatic
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Fig. 3. Distribution of apparent hydrodynamic radius (Rh ) from DLS for Cn SO4 /Cn SO3 and PEO (Mw = 20,000 Da, 2.5 wt.%). f(Rh ) is the scattered intensity averaged distribution of Rh : (a) T = 25 ◦ C; (b) T = 40 ◦ C.
interaction between C12 SO3 headgroup and PEO chain; thus, C12 SO3 and PEO trend to form “bead-structure” complexes. 4. Conclusion There are striking differences between C12 SO3 and C12 SO4 in interacting with PEO. These differences show us that both of the hydrophobic interaction and the electrostatic interaction play important roles in the Cn SO4 PEO and Cn SO3 -PEO interactions. Further studies in mixed Cn SO4 /Cn SO3 -polyelectrolyte systems are under way.
Acknowledgements We greatly appreciate Referee’s insightful discussion. This project was financially supported by National Natural Science Foundation of China (No. 20273006).
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[22] Z. Hou, Z. Li, H. Wang, Colloid Polym. Sci. 277 (1999) 1011. [23] O. Ortona, G. D’Errico, L. Paduano, R. Sartorio, Phys. Chem. Chem. Phys. 4 (2002) 2604. [24] P. Roscigno, G. D’Errico, O. Ortona, R. Sartorio, L. Paduano, Prog. Colloid Polym. Sci. 122 (2003) 113. [25] L. Chen, J.-X. Xiao, K. Ruan, J. Ma, Langmuir 18 (2002) 7250. [26] L. Chen, J.-X. Xiao, J. Ma, Colloid Polym. Sci. 282 (2004) 524. [27] B.-Y. Zhu, G.-X. Zhao, Principles of Surfactant Action, China Light Industry Press, Beijing, 2003 (Chapter 3). [28] K. Kalyanasundaram, J.K. Thomas, J. Am. Chem. Soc. 99 (1977) 2039. [29] S.W. Provencher, Makromol. Chem. 180 (1979) 201. [30] M.J. Frisch, J.A. Pople, J.S. Binkley, J. Chem. Phys. 80 (1984) 3265.
[31] M. Dahanayake, A.W. Cohen, M.J. Rosen, J. Phys. Chem. B 90 (1986) 2413. [32] G. Broze (Ed.), Handbook of Detergents. Part A: Properties, Marcel Dekker, New York, 1999. [33] P.D.T. Huibers, Ph.D. Thesis, MIT, 1996. [34] P.D.T. Huibers, Langmuir 15 (1999) 7546. [35] J. Israelachvili, Proc. Natl. Acad. Sci U.S.A. 94 (1997) 8378. [36] F.M. Winnik, S.T.A. Regismond, Coll. Surf. A 118 (1996) 1. [37] J.K. Kim, M.M. Domach, R.D. Tilton, J. Phys. Chem. B 103 (1999) 10582. [38] M. Vasilescu, D.F. Anghel, M. Almgren, P. Hansson, S. Saito, Langmuir 13 (1997) 6951. [39] N.J. Turro, X.G. Lei, K.P. Ananthapadmanabhan, M. Aronson, Langmuir 11 (1995) 2525. [40] B. Lindman, K. Thalberg, Chapter 5 in [2].
Polymer–surfactant interaction: differences between alkyl sulfate and alkyl sulfonate Peng Yan, Jin-Xin Xiao∗ Institute of Physical Chemistry, Peking University, Beijing 100871, PR China Received 15 January 2004; accepted 24 June 2004 Available online 17 August 2004
Abstract The interactions between sodium alkyl sulfates (Cn SO4 )/sodium alkyl sulfonates (Cn SO3 ) and polyethylene oxide (PEO) were studied by surface tension, fluorescence and dynamic light scattering measurements, assisted with quantum-chemical calculations. It was found that, although many properties of Cn SO4 and Cn SO3 were similar, there were striking differences in their interactions with PEO. Sodium alkyl sulfonates were seen to interact more weakly than their sulfate analogs. Such differences were especially distinct when compared C10 SO4 -PEO with C10 SO3 -PEO, the former exhibits obvious interactions, but no observable interaction in the latter. With the help of ab initio quantum-chemical calculation, the results were explained. © 2004 Elsevier B.V. All rights reserved. Keywords: Sodium alkyl sulfate; Sodium alkyl sulfonate; Polyethylene oxide; Polymer–surfactant interaction; Quantum-chemical calculation
1. Introduction Polymer and surfactant are two typical kinds of soft matter, and their respective properties have been widely studied [1]. It has been shown that the mixtures of polymer and surfactant [2,3], which is another special kind of soft matter, have many characteristics compared with their single components, such as surface activity, viscosity, wetting, foaming, solubilization, etc. [4]. Therefore, this kind of soft matter has been extensively used in a variety of industrial processes. From the scientific viewpoint, the interest in polymer–surfactant systems is the different intermolecular interactions in the multiplicity of possible combinations. Interaction between water-soluble neutral polymer and surfactant is generally attributed to the surfactant bind noncovalently to the polymer. With the addition of polymer, surfactant aggregates form on polymer chains when surfactant concentration reaches to a critical level (critical aggregation concentration, cac), which is usually lower than ∗ Corresponding author. Tel.: +86 10 62764973; fax: +86 10 62751708. E-mail address: [email protected] (J.-X. Xiao).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.06.023
the critical micellar concentration (cmc) of polymer-free surfactant solution. Although the detail mechanism of the polymer–surfactant interaction has been still unknown, previous researches indicated that the interaction mainly depended on the surfactant tail length, surfactant headgroup, polymer hydrophobicity and flexibility [5]. Among all the mixed polymer–surfactant systems, sodium alkyl sulfates (Cn SO4 ), especially sodium dodecyl sulfate (C12 SO4 ), are the most used anionic surfactants [6–20]. However, only a few investigations involved sodium alkyl sulfonate (Cn SO3 ) [21–24]. It has been known that Cn SO3 is quite similar to Cn SO4 in many physicochemical properties. Very recently, we found that Cn SO4 and Cn SO3 exhibit remarkable differences when mixed with cationic surfactants [25,26]. In this work, we observed that there were also distinct differences between Cn SO4 and Cn SO3 (n = 10, 12) with respect to the interaction with polyethylene oxide (PEO), based on the measurements of surface tension, fluorescence and dynamic light scattering. With the results of ab initio quantum-chemical calculation, we proposed a possible explanation.
40
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2. Experimental
tions and molecular charge distribution calculations were performed at the HF level using the 6-31G* basic set [30].
2.1. Materials Sodium decyl sulfate (C10 SO4 ) was synthesized from chlorosulfonic acid and decanol, and then the solution was neutralized. The crude product was recrystallized five times from ethanol. Sodium dodecyl sulfate (C12 SO4 ), 99% pure from Sigma, was used as received. Sodium decyl sulfonate (C10 SO3 ), from Tokyo Kasei, A.R. grade, was used as received. Sodium dodecyl sulfonate (C12 SO3 ), a product from Beijing Chemical Co., C.P. grade, 97%, was recrystallized from water and ether. Polyethylene oxide (PEO, Mw = 20,000 Da, Sigma) was used as received. The purity of all the surfactants was examined, and no minima were observed in the surface tension curves. All the solutions were prepared using deionized water. 2.2. Surface tension The surface tension of surfactant solutions was measured by the drop-volume method [27]. Because the Krafft Points of C10 SO3 and C12 SO3 are 24 and 38 ◦ C, respectively, surface tension measurements of systems containing C10 SO3 or C10 SO4 were performed at 25 ◦ C; while, systems containing C12 SO3 or C12 SO4 were performed at 40 ◦ C. The choice of the temperatures was also followed in the fluorescence and dynamic light scattering experiments. 2.3. Fluorescence The steady state pyrene fluorescence measurements were performed on Hitachi 4500 fluorescence spectrophotometer. The concentration of pyrene used in all the measurements was approximately 5 × 10−7 M. The excitation wavelength was 337 nm. The ratio of the intensities of the first vibrational peaks (I1 , 374 nm) to the third vibrational peaks (I3 , 384 nm) of the emission spectrum of pyrene was used in evaluating the polarity of the local microenvironment [28]. 2.4. Dynamic light scattering Dynamic light scattering (DLS) measurements were made using a spectrometer of standard design (ALV-5000/E/WIN Multiple Tau Digital Correlator) and a Spectra-Physics 2017 200 mW Ar laser (514.5 nm wavelength). All measurements were made at a scattering angle of 90◦ , and the intensity autocorrelation functions were analyzed using the methods of Contin [29]. 2.5. Quantum-chemical calculation All the calculations were performed using HyperChem Release 7.1 for Windows (Hypercube, Inc., Gainesville, FL). Both the three-dimensional molecular geometry optimiza-
3. Results and discussion 3.1. Differences between Cn SO4 and Cn SO3 : calculation of charge distribution The molecular structures of alkyl sulfate and alkyl sulfonate are very alike, except that Cn SO4 have one more oxygen atom than Cn SO3 in the headgroup. Because of the similar molecular structure, physicochemical properties of Cn SO4 and Cn SO3 , such as critical micellar concentration, surface excess concentration, free energy of micellar formation and average surface area per surfactant molecule at water surface [31–32], are quite similar. The most distinct difference between Cn SO4 and Cn SO3 is the Krafft Point, and the Krafft Point of Cn SO3 is much higher than that of Cn SO4 [33]. For ionic surfactants, the entire charge is generally considered to be a point charge at the headgroup. Recently, however, Huibers calculated the charge distribution of common ionic surfactants using semiempirical quantum-chemical methods, and found that in reality the headgroup charge was partially distributed to the rest of the molecule [34]. We used ab initio methods, which are more accurate than semiempirical ones, to calculate the charge distribution of Cn SO4 and Cn SO3 (Table 1). It was shown that, owing to the highly polar C S bond, the ␣-methylene group (␣-CH2 , the first CH2 group of the alkyl tail attached to the headgroup) of Cn SO3 held partial negative charge, while the ␣-CH2 of Cn SO4 held partial positive charge because it attached to the headgroup with the highly electronegative oxygen atom. Meanwhile, headgroup of Cn SO4 held much more negative charge than that of Cn SO3 . Therefore, it is the absence of an oxygen atom in the headgroup that results in the remarkable differences in the charge distribution. 3.2. Differences between Cn SO4 and Cn SO3 in interacting with PEO One of the most significant characteristics of polymer–surfactant interaction is there are two turning points in the surface tension (γ) versus surfactant concenTable 1 Grouped atomic partial charges for alkyl sulfonate and alkyl sulfatea C10 [SO3 − ] C10 [SO4 − ] C12 [SO3 − ] C12 [SO4 − ] Charge on headgroup −0.750 Charge on ␣-CH2 −0.229 Charge on alkyl tail −0.021
−1.319 +0.374 −0.055
−0.750 −0.214 −0.036
−1.319 +0.374 −0.055
a Groups consist of headgroup atoms (noted by square brackets), ␣methylene, and the remaining alkyl tail. Molecular charge distribution calculations and geometry optimizations were made using HyperChem 7.1 at HF/6-31G* level.
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points in the γ–c curve of C10 SO4 -PEO mixture, but only one turning point in that of C10 SO3 -PEO mixture at which the concentration equals to the cmc of C10 SO3 . Although the surface tension of C10 SO3 in PEO solution is a little lower than that in pure water when C10 SO3 concentrations are far below the cac, the lowering of surface tension might be caused by PEO itself, not by the interaction of C10 SO3 with PEO, since PEO itself is a little surface active [35] (surface tension of PEO (Mw = 20,000, 2.5 wt.%) was 60 mN/m (T = 25 ◦ C)/58 mN/m (T = 40 ◦ C)). Therefore, it could be thought that there is no observable interaction in sense of surface tension between C10 SO3 and PEO under the experimental conditions. The fluorescence probe method is also often used to study the polymer–surfactant interaction [36]. Pyrene is one of the most used fluorescence probes. The value of I1 /I3 is very sensitive to the polarity of the microenviron-
Fig. 1. Surface tension (γ) of surfactants vs. surfactant concentrations (c) in the presence and absence of PEO (Mw = 20,000 Da, 2.5 wt.%). (A) Solutions contain sodium alkyl sulfate (Cn SO4 ); (B) solutions contain sodium alkyl sulfonate (Cn SO3 ). Black arrows show the two turning points in the γ–c curve: (a) T = 25 ◦ C; (b) T = 40 ◦ C.
tration (c) curve (γ–c curve). The surfactant concentration at the first turning point (T1 ) represents critical aggregation concentration (cac), and the concentration at the second turning point (T2 ) means at which the polymer becomes saturated with the surfactant and free micelles form [6]. Fig. 1 shows the γ–c curves of Cn SO4 and Cn SO3 in the presence and absence of PEO. In both C12 SO4 -PEO and C12 SO3 -PEO systems, it is observed that their γ–c curves have two turning points in the present of PEO (Mw = 20,000, 2.5 wt.%), which indicates that interactions do exist between the polymer and the surfactants. However, the cac of C12 SO4 -PEO mixture is obviously lower than that of C12 SO3 -PEO mixture. It is interesting to compare C10 SO4 -PEO mixture and C10 SO3 -PEO mixture. Although the γ–c curves of C10 SO4 and C10 SO3 are very similar, their γ–c curves differ greatly in presence of PEO. Fig. 1 shows that there are two turning
Fig. 2. Plot of pyrene fluorescence intensity (I1 /I3 ) as a function of surfactants concentration (c) in the presence and absence of PEO (Mw = 20,000 Da, 2.5 wt.%). (A) Solutions contain sodium alkyl sulfate; (B) solutions contain sodium alkyl sulfonate: (a) T = 25 ◦ C; (b) T = 40 ◦ C.
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ment. When surfactant molecules aggregate, pyrene is solubilized in a hydrophobic environment and the I1 /I3 decreases [36]. Fig. 2 shows the dependence of the I1 /I3 on the surfactant concentration in the presence and absence of PEO (Mw = 20,000, 2.5 wt.%). It is shown that all these results support the conclusions drawing from surface tension measurements. Similar to the surface tension as already mentioned, at very low surfactant concentrations, far below the cac, the I1 /I3 value in PEO solution is lower than that in water, suggesting that the microenvironment of pyrene is changed [37,38]. The possible mechanisms might be that pyrene, as a strongly hydrophobic molecule, is preferentially solubilized inside the PEO coil, which considers being a little hydrophobic (I1 /I3 value in surfactant-free PEO (Mw = 20,000, 2.5 wt.%) solution is 1.7 (T = 25 ◦ C)/1.5 (T = 40 ◦ C)). The strength of the interaction between a polymer and a surfactant can be characterized by the cac/cmc ratio semiquantitatively [9]. The stronger the interaction between the polymer and the surfactant is, the smaller the value of the cac/cmc is. Table 2 lists the values of the cmc, cac and cac/cmc of the investigated systems. Consequently, we might conclude that the interactions of C10 SO4 -PEO and C12 SO4 -PEO are stronger than that of C10 SO3 -PEO and C12 SO3 -PEO at the corresponding temperature, respectively. Dynamic light scattering techniques are highly sensitive and noninvasive methods for investigating polymer–surfactant complex. DLS results were shown in Fig. 3. The measured autocorrelation functions were well resolved with two decay modes except for the C10 SO3 -PEO system. The fast diffusion modes corresponded to diffusivities identical to those obtained in surfactant-free polymer solution (apparent hydrodynamic radius Rh of PEO 2.5 wt.% were 2–3 nm) and could be identified with PEO itself. The slow diffusion modes exhibiting larger Rh should be polymer–surfactant complex. DLS measurements confirmed that there was no complex formed in C10 SO3 -PEO system. Fig. 3 shows that Rh of C12 SO4 -PEO complex was nearly the same as that of C12 SO3 -PEO complex. However, previous researches performed by NMR showed that the structures of C12 SO4 -PEO complex and C12 SO3 -PEO complex Table 2 cmc/cac of the investigated surfactants in the absence/presence of PEO (Mw = 20,000 Da) Surfactant
Polymer (wt.% PEO)
T (◦ C)
cmc or cac (mM)
cac/cmc
C10 SO3 C10 SO3 C10 SO4 C10 SO4 C12 SO3 C12 SO3 C12 SO4 C12 SO4
0 2.5 0 2.5 0 2.5 0 2.5
25 25 25 25 40 40 40 40
30a /30b 30a /30b 30a /30b 20a /20b 11.5a /10b 8.0a /7.0b 8.5a /8.0b 4.0a /4.0b
1.0a /1.0b
a b
Determined by surface tension. Determined by fluorescence.
0.67a /0.67b 0.70a /0.70b 0.47a /0.50b
were quite different. It has been indicated that there are two possible structures of polymer–surfactant complex [39]: (a) the polymer wraps around the micelles (“necklace” structures); (b) the micelles nucleate on the polymer hydrophobic sites (“bead” structures). Studies of Cabane [10] and Hou et al. [22] found that the C12 SO4 -PEO and C12 SO3 -PEO complexes were “necklace” and “bead” structures, respectively. 3.3. Mechanism of the differences Why there are such striking differences between Cn SO4 PEO interaction and Cn SO4 -PEO interaction? We tried to figure out the answers from the headgroup structure and charge distribution of these two kinds of surfactants. Because of the absence of an oxygen atom in the headgroup of Cn SO3 respect to Cn SO4 , the headgroup size of Cn SO3 is smaller than that of Cn SO4 . Furthermore, the results of the charge distribution in Table 1 show that the charge on the Cn SO4 headgroup and ␣-CH2 are opposite, while the Cn SO3 headgroup and ␣-CH2 have the same charge. This makes the hydration cosphere of the hydrophobic tail of Cn SO4 very different from the corresponding tail of Cn SO3 . As a result, the hydration effect of Cn SO3 is stronger than that of Cn SO4 , which means the hydration shell of the headgroup may extend over a larger part of the alkyl chain in Cn SO3 Consequently, this effect “shortens” the efficient hydrophobic tail with respect to that of Cn SO4 [24]. It has been well known that the hydrophobic effect is the most important driving force in the interaction between nonionic polymer and surfactant [40]. Therefore, the difference of the hydrophobic nature between Cn SO3 and Cn SO4 induces that the interaction of Cn SO3 -PEO is weaker than that of Cn SO4 -PEO. The differences in total charge between headgroups of Cn SO4 and Cn SO3 might be another important factor. With the quantum-chemical calculation, we found that the headgroup of Cn SO4 approximately held 0.5 more negative charges than that of Cn SO3 . It has been known that, because of the electronegative potential on the PEO chain, the electrostatic interaction between the ether oxygen group ( CH2 CH2 O ) and the electropositive counterions, such as Na+ and H3 O+ , results in a partially positive charge transferring to the ether oxygen group [8]. Therefore, the electrostatic interaction (including ion–dipole interaction) between the headgroup of Cn SO4 and the PEO chain is stronger than that of Cn SO3 and the PEO chain. Consequently, the interaction of Cn SO4 with PEO is stronger than that of Cn SO4 with PEO. The differences in the complex structures might also be explained in terms of the differences in charge distributions. As already mentioned, the headgroup of C12 SO4 has more negative charges than that of C12 SO3 , which makes that the headgroup of C12 SO4 has stronger electrostatic interaction with the PEO chain than that of C12 SO3 with PEO chain. Therefore, C12 SO4 and PEO trend to form “necklace-structure” complexes. The less negative charge on the headgroup and the stronger hydration effect greatly reduce the electrostatic
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Fig. 3. Distribution of apparent hydrodynamic radius (Rh ) from DLS for Cn SO4 /Cn SO3 and PEO (Mw = 20,000 Da, 2.5 wt.%). f(Rh ) is the scattered intensity averaged distribution of Rh : (a) T = 25 ◦ C; (b) T = 40 ◦ C.
interaction between C12 SO3 headgroup and PEO chain; thus, C12 SO3 and PEO trend to form “bead-structure” complexes. 4. Conclusion There are striking differences between C12 SO3 and C12 SO4 in interacting with PEO. These differences show us that both of the hydrophobic interaction and the electrostatic interaction play important roles in the Cn SO4 PEO and Cn SO3 -PEO interactions. Further studies in mixed Cn SO4 /Cn SO3 -polyelectrolyte systems are under way.
Acknowledgements We greatly appreciate Referee’s insightful discussion. This project was financially supported by National Natural Science Foundation of China (No. 20273006).
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