pubs.acs.org/Langmuir © 2010 American Chemical Society
Dilational Properties of Anionic Gemini Surfactants with Polyoxyethylene Spacers at Water-Air and Water-Decane Interfaces Jie Feng,†,‡ Xue-Peng Liu,†,‡ Lu Zhang,*,† Sui Zhao,*,† and Jia-Yong Yu† †
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, No. 2 North 1 Alley, Zhong Guan Cun, P.O. Box 2711, 61Section, Beijing 100190, People’s Republic of China, and ‡ Graduate School, Chinese Academy of Sciences, Beijing 100039, People’s Republic of China Received March 22, 2010. Revised Manuscript Received May 2, 2010
The dilational properties of anionic gemini surfactants (oligooxa)-R,ω-bis(m-octylbenzene sulfonate) (C8ExC8) with polyoxyethylene spacers at the water-air and water-decane interfaces were investigated via the oscillating barriers method. The influences of oscillating frequency and bulk concentration on dilational properties were explored. The interfacial tension relaxation method was employed to obtain dilational parameters in a reasonably broad frequency range. The experimental results show that the number of ethylene oxide groups is one of the principal factors to control the nature of the interfacial film. With an increase of ethylene oxide groups, the dilational modulus of C8E8C8 shows two maxima with the increasing concentration. Furthermore, the dilational moduli at the water-decane interface are remarkably lower than those at the water-air interface for C8E1C8 and C8E4C8, while the dilational modulus at the water-decane interface is close to that at the water-air interface for C8E8C8, which indicates that the structure of the adsorption sublayer plays a more important role. Possible schematic diagrams of adsorbed molecules with different polyoxyethylene spacers at the water-air and water-decane interfaces are proposed. The results of relaxation experiments and Cole-Cole plots can support our provided mechanism strongly.
1. Introduction A gemini surfactant is made up of two aliphatic chains and two polar head groups linked by a rigid or flexible spacer.1 Compared with conventional monochain surfactants, gemini surfactants demonstrate unique features in critical micelle concentration, interfacial properties, and solubility in water.2-5 They are used as promising surfactants in many industrial, biological, and daily processes. Taking into account the wide applications of this kind of surfactant, these studies are important not only from the theoretical but also from the practical point of view. For gemini surfactants, in the last two decades, the investigations of the relationship between spacer group structures and functionalities were mainly focused on two typical cationic geminis called alkanediyl-R,ω-bis-(alkyldimethylammonium bromide)6-12 and (oligooxa)-alkanediyl-R,ω-bis(dimethyldodecyl ammonium *To whom correspondence should be addressed. E-mail: luyiqiao@ hotmail.com (L.Z.);
[email protected] (S.Z.). Telephone: 86-10-82543587. Fax: 86-10-62554670. (1) Kirby, A. J.; Camilleri, P.; Engberts, J. B. F. N.; et al. Angew. Chem., Int. Ed. 2003, 42, 1448. (2) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451. (3) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (4) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906. (5) Zana, R. In Geminal Surfactants, Gemini (Dimeric) Surfactants in Water: Solubility, cmc, Thermodynamics of Micellization, and Interaction with WaterSoluble Polymers; Zana, R., Xia, J., Eds.; Marcel Dekker: New York, 2003. (6) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (7) Alami, E.; Beinert, G.; Marie, P. Langmuir 1993, 9, 1465. (8) Wettig, S. D.; Verrall, R. E. J. Colloid Interface Sci. 2001, 235, 310. (9) Wang, X.; Wang, J.; Wang, Y. Langmuir 2004, 20, 53. (10) Rodrı´ guez, A.; del Mar Graciani, M.; Mu~noz, M. Langmuir 2006, 22, 9519. (11) Burrows, H. D.; Tapia, M. J.; Silva, C. L. J. Phys. Chem. B 2007, 111, 4401. (12) Grosmaire, L.; Chorro, M.; Chorro, C.; Partyka, S.; Zana, R. J. Colloid Interface Sci. 2002, 246, 175. (13) Wettig, S. D.; Li, X.; Verrall, R. E. Langmuir 2003, 19, 3666. (14) Zheng, O.; Zhao, J. X.; Yan, H.; Gao, S. K. J. Colloid Interface Sci. 2007, 310, 331. (15) Bendjeriou-Sedjerari, A.; Derrien, G.; Charnay, C.; Zajac, J.; De Menorval, L. C.; Lindheimer, M. J. Colloid Interface Sci. 2009, 331, 281.
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bromide).13-15 These studies pointed out that long flexible hydrophobic spacers (>10 methylene groups) penetrated into the micelle core or adopted a special conformation with a loop at the air side of the water-air interface.15 In contrast, the long hydrophilic polyoxyethylene spacers (>7 ethylene oxide groups) penetrate into the water phase, in a manner analogous to the proposed folding of the hydrophobic polymethylene spacers. However, there are few papers to investigate the influence of the spacer group structures involving anionic geminis.16,17 Recently, we designed and synthesized a series of anionic gemini C8ExC8 (x=1, 2, 3, 4, 8) called (oligooxa)-R,ω-bis(m-octylbenzene sulfonate), and preliminary results showed that the spacer group of the C8ExC8 surfactants changes location at the water-air interface as x increases, going from the interface for x < 3 to the water side for x > 3.18 When the number of ethylene oxide groups reaches 8, the spacer may also adopt a conformation with a loop at the water side of the interface.18 The dilational viscoelasticity plays a significant role in many fields, such as foam and emulsion stability. Its microcosmic basis is relaxation processes at the interface and near the interface. It is beneficial to better understand the microcosmic properties of interfacial film through the research of dilational viscoelasticity. The rheological interfacial properties of surfactant solution can provide more accurate information about the structure of adsorption layers.19 To detect the nature of adsorption films formed by gemini surfactants with ethylene oxide groups as spacers, in this paper the dilational viscoelastic properties of C8E1C8, C8E4C8, and C8E8C8 (16) Zhu, S.; Cheng, F.; Wang, J.; Yu, J. G. Colloids Surf., A 2006, 281, 35. (17) Du, X. G.; Lu, Y.; Li, L.; Wang, J. B.; Yang, Z. Y. Colloids Surf., A 2006, 290, 132. (18) Liu, X. P.; Feng, J.; Zhang, L.; Zhao, S.; Yu, J. Y. Colloids Surf., A 2010, 362, 39-46. (19) Huang, Y. P.; Zhang, L.; Zhang, L.; Luo, L.; Zhao, S.; Yu, J. Y. J. Phys. Chem. B 2007, 111, 5640.
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at the water-air and water-decane interfaces have been investigated by an oscillating barriers method at low frequency (0.0050.1 Hz). It may be useful for us to understand the influence of long polyoxyethylene spacers on the interfacial behavior of gemini surfactants.
2. Theory The dilational rheology gives a measure of the interfacial resistance to changes in area. When the interfacial area is subjected to a periodic compression and expansion, there is a change in the interfacial tension. The interfacial dilational modulus ε is defined as the increase in interfacial tension γ for a small relative increase of interfacial area at constant temperature, that is, ε ¼
dγ d ln A
ð1Þ
where ε is the dilational modulus, γ is the interfacial tension, and A is the area of the interface. When a relaxation process takes place in or near the interface as a result of a disturbance, the interface will exhibit viscoelastic rather than pure elastic behavior. The viscoelastic modulus can be presented as a complex number, ε ¼ εd þ iωηd
ð2Þ
In such a case, dilational modulus exhibits two contributions: an elastic component, εd, accounting for the elastic energy stored in the interface, and a loss modulus, εη = ωηd, representing the energy dissipated in the relaxation process. The phase angle θ can be obtained by phase comparison between interfacial tension and area variations by computer. θ is calculated according to tan θ ¼
εη εd
ð3Þ
Interfacial tension relaxation experiments are a reliable way to obtain interface dilational parameters, which use small but fast area expansion or compression to slightly disturb the monolayer equilibrium. This causes an interfacial tension jump, and then the interfacial tension will decay to the equilibrium again. For an instantaneous area change rising from ΔA(t)=0 for t=0 to ΔA(t) = ΔA for t > 0, the values of ε are obtained as a function of the frequency by Fourier transformation (FT) of the interfacial tension decay obtained from the experiment by the following relationship:20-22 R¥ ΔγðtÞ expð- iωtÞ dt FTΔγðtÞ ¼ R¥ 0 ð4Þ εðωÞ ¼ FTðΔA=AÞðtÞ ½ΔAðtÞ=A expð- iωtÞ dt 0 where ω is the angular frequency. In an ideal system that is not diffusion controlled and in which only one relaxation mechanism occurs, and the decay curve of γ versus t can be represented by an exponential equation. For a real system, a number of relaxation processes may occur and the decay curve would be expressed by the summation of a number of exponential functions:23,24 n X Δγ ¼ Δγi expð- τi tÞ ð5Þ i¼1
(20) Miller, R.; Loglio, G.; Tesei, U.; Schano, K. H. Adv. Colloid Interface Sci. 1991, 37, 73. (21) Loglio, G.; Tesei, U.; Miller, R.; Cini, R. Colloids Surf. 1991, 61, 219. (22) Loglio, G.; Tesei, U.; Miller, R.; Pandolfini, P.; Cini, R. Colloids Surf., A 1996, 114, 23. (23) Cardenas-Valere, A. E.; Bailey, A. I. Colloids Surf., A 1993, 79, 115. (24) Murry, B. S.; Ventura, A.; Lallemant, C. Colloids Surf., A 1998, 143, 211.
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where τi is the characteristic frequency of the ith process; Δγi is the fractional contribution that relaxation process makes to restore the equilibrium; n is the total number of the relaxation processes. For an instantaneous change in area, Z ¥ ΔA ΔA=A expð- iωtÞ dt ¼ ð6Þ A iω 0 With this, eq 4 becomes Z ¥ iω ε ¼ ΔγðtÞ½cos ωt - i sin ωt dt ΔA=A 0
ð7Þ
The real part of eq 7 is the dilational elasticity εd, and the imaginary part is the dilatonal viscosity component ωηd: that is, Z ¥ ω εd ðωÞ ¼ ΔγðtÞ sinðωtÞ dt ð8Þ ΔA=A 0 ωηd ðωÞ ¼
ω ΔA=A
Z
¥
ΔγðtÞ cosðωtÞ dt
ð9Þ
0
Other parameters such as the tangent of phase angle, dilational modulus, and so on can all be obtained from these two parameters.
3. Experimental Section 3.1. Materials. The anionic gemini surfactants, oligooxa-
R,ω-bis(m-octylbenzene sulfonate), with different numbers of ethylene oxide groups were synthesized from phenol through several processes, including Friedel-Crafts acylation, esterification, Fries rearrangement, Pd-catalyzed hydrogenation, sulfonation, and neutralization, and the structures of the products were characterized by 1H NMR, ESI-MS, and elemental analysis methods.18 The series of anionic gemini surfactants have the same structure except for the spacer length of the polyoxyethylene chain. The structures of the surfactants (C8ExC8, x = 1, 4, 8) are shown in Figure 1. The solutions were prepared with ultrapure water (resistivity>18.2 MΩ 3 cm). n-Decane was obtained from Beijing Xingjin Chemical Co., Ltd., China and used as oil phase, which was purified by further distillation. 3.2. Measurements. In this study, the interfacial dilational viscoelasticity meter JMP2000A (Powereach Ltd., Shanghai, China) was employed.25,26 The working principle is similar to that of Lucassen and Giles.27 It includes a Langmuir trough with two symmetrically oscillating barriers for changing the interfacial area and a Wilhelmy plate for measuring the interfacial tension. When we measure the dilational viscoelasticity of adsorption film at the water-air interface, the water phase (about 90 mL) is poured into the trough to make the low edge of the slide barriers and the Wilhelmy plate just on the top of the water surface. If we probe the dilational viscoelasticity of film at the water-decane interface, the water phase (90 mL) and oil phase (50 mL) are poured into the trough successively, and the plate should be completely submerged under the surface of the oil. The dilational viscoelasticity experiment began after 8 h of pre-equilibrium. Then the barriers were expanded and compressed at a chosen amplitude (ΔA/A, 10%) in sine oscillation mode. The oscillating frequency can be varied between 0.005 and 0.1 Hz. In the interfacial tension relaxation measurements, the film was expanded about 15% in area by a sudden expansion in 2 s. All experiments were performed at the temperature 30 ( 0.1 C. (25) Sun, T. L.; Zhang, L.; Wang, Y. Y.; Zhao, S.; Peng, B.; Li, M. Y.; Yu, J. Y. J. Colloid Interface Sci. 2002, 255, 241. (26) Wang, Y. Y.; Zhang, L.; Sun, T. L.; Zhao, S.; Yu, J. Y. J. Colloid Interface Sci. 2004, 270, 163. (27) Lucassen, J.; Giles, D. J. Chem. Soc., Faraday Trans. I 1975, 71, 217.
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Figure 1. Structures of the studied anionic gemini surfactants with polyoxyethylene spacers.
4. Results and Discussion 4.1. Dilational Properties of C8ExC8 at the Water-Air Interface. 4.1.1. Influence of Oscillating Frequency on the Dilational Modulus of C8ExC8 at the Water-Air Interface. The variation of dilational viscoelastic parameters with oscillating frequency could reflect the properties of the interfacial film. The influence of oscillating frequency on dilational modulus of C8ExC8 at the water-air interface is summarized in Figure 2. The curves of log|ε|-log ω are quasi-linear for the solutions of C8ExC8, which indicate that the characteristic frequency of the relaxation process at the interfacial layer could be higher than the highest oscillating frequency used in this experiment (0.1 Hz).28 The dilational modulus of C8ExC8 at the water-air interface increases gradually with increasing oscillating frequency. At much lower oscillating frequency, surfactant molecules have enough time to diminish the interfacial tension gradient resulting from the deformation of interface. As the oscillating frequency increases, the restoration of interfacial tension becomes slower as compared with the quick change of interface area, leading to a higher interfacial tension gradient. Therefore, dilational modulus increases with increasing oscillating frequency.29 4.1.2. Influence of Bulk Concentration on the Dilational Modulus of C8ExC8 at the Water-Air Interface. The effect of bulk concentration on the dilational modulus is complex. Generally speaking, an increase of surfactant concentration has two different effects. On the one hand, the molecular exchange between bulk and interface increases, which decreases the interfacial tension gradients and induces a decrease in dilational modulus. On the other hand, the increase of interfacial concentration would cause stronger intermolecular interaction, which results in an increase of dilational modulus.29-31 Figure 3 shows the variations of dilational modulus versus bulk concentration for the surfactants C8ExC8 at the water-air interface. We can see from Figure 3 that the dilational moduli at the water-air interface decrease monotonously for C8E1C8 and C8E4C8 in the studied bulk concentration. One could speculate that there may be a maximum in concentration lower than the experimental range, which embodies a general regulation of concentration dependence on the interfacial dilational modulus. An increase of the ethylene oxide chain length makes the molecule size larger and the diffusion process from the bulk to the interface slower. Therefore, the corresponding concentration of the maximum dilational modulus shifts to a larger concentration and can be observed in our experiments for C8E8C8. It is very interesting to point out that the rheological behavior of C8E8C8 solution is much more different from those of C8E1C8 and C8E4C8 solutions, as shown in Figure 3. It can be seen clearly that the dilational modulus of C8E8C8 shows two maxima with the increasing concentration, passing through the first maximum (28) Wang, H. R.; Gong, Y.; Lu, W. C.; Chen, B. L. Colloids Surf., A 2008, 254, 3380. (29) Zhang, H. X.; Xu, G. Y.; Wu, D.; Wang, S. W. Colloids Surf., A 2008, 317, 289. (30) Ravera, F.; Ferrari, M.; Santini, E.; Liggieri, L. Adv. Colloid Interface Sci. 2005, 117, 75. (31) Zhang, L.; Wang, X. C.; Gong, Q. T.; Zhang, L.; Luo, L.; Zhao, S.; Yu, J. Y. J. Colloid Interface Sci. 2008, 327, 451.
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Figure 2. Influence of oscillating frequency on the dilational modulus of C8ExC8 at the water-air interface.
value at 110-6 mol 3 L-1 and the second one at 510-5 mol 3 L-1. The dilational modulus at the first maximum is about 115 mN 3 m-1 (0.1 Hz), which is much higher than the maximum dilational parameter of normal surfactants.32,33 (32) Fainerman, V. B.; Aksenenko, E. V.; Lylyk, S. V.; Makievski, A. V.; Ravera, F.; Petkov, J. T.; Yorke, J.; Miller, R. Colloids Surf., A 2009, 334, 16. (33) Zhang, C. R.; Li, Z. Q.; Luo, L.; Zhang, L.; Song, X. W.; Cao, X. L.; Zhao, S.; Yu, J. Y. Acta Phys.-Chim. Sin. 2007, 23, 247.
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Figure 4. Influence of oscillating frequency on the dilational modulus of C8ExC8 at the water-decane interface. Figure 3. Influence of bulk concentration on the dilational modulus of C8ExC8 at the water-air interface.
It is also very important to point out that the surface tension monotonically decreases with increasing bulk concentration until reaching a plateau value around the critical micelle concentration, which indicates that the surface tension can be insensitive to the state of the surface layer.18,19 The two maxima in the concentration dependence of the modulus were also found for Triton surfactants, and these peculiarities in the rheological behavior are much more 11910 DOI: 10.1021/la101131v
pronounced with increasing ethylene oxide chain length.32 According to ref 32, the first maximum in the concentration dependence of the modulus is caused by the transition of the expanded state of the adsorbed Triton molecules in the surface layer (ethylene oxide groups are adsorbed at the interface) to a more compact state (increase in surface pressure leads to a desorption of the ethylene oxide groups from the interface). The existence of the second maximum is caused by the internal compressibility of Triton molecules at higher surface pressure.32 Therefore, these dilational Langmuir 2010, 26(14), 11907–11914
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Figure 6. Dependence of the dilational modulus on the bulk concentration at a fixed oscillating frequency of 0.1 Hz.
Figure 5. Influence of bulk concentration on the dilational modulus of C8ExC8 at the water-decane interface.
modulus maxima of C8E8C8 may be caused by the reorientation and compression of the C8E8C8 molecules in the interfacial layer. 4.2. Dilational Properties of C8ExC8 at the Water-Decane Interface. 4.2.1. Influence of Oscillating Frequency on the Dilational Modulus of C8ExC8 at the Water-Decane Interface. Figure 4 shows the influence of oscillating frequency on the Langmuir 2010, 26(14), 11907–11914
dilational modulus of C8ExC8 at the water-decane interface. We can see clearly from Figure 4 that the dilational modulus of C8ExC8 increases with increasing oscillation frequency at the water-decane interface, which shows a similar tendency as that obtained at the water-air interface. 4.2.2. Influence of Bulk Concentration on the Dilational Modulus of C8ExC8 at the Water-Decane Interface. Figure 5 illustrates the influence of bulk concentration on the dilational modulus of C8ExC8 at the water-decane interface. In the narrow experimental concentration range, the dilational modulus of C8E1C8 at the water-decane interface decreases monotonously with increasing concentration, and that of C8E4C8 shows a maximum with increasing concentration at higher frequency (0.05 and 0.1 Hz). The dilational modulus of C8E8C8 at the water-decane interface has a similar tendency as that obtained at the waterair interface, which appears two maxima with the increasing concentration. 4.3. Comparison of Dilational Modulus for C8ExC8 between the Water-Air Interface and Water-Decane Interface. For C8ExC8 solutions, the dependence of the dilational modulus on the bulk concentration at a fixed oscillating frequency of 0.1 Hz is shown in Figure 6. Dilational moduli at the water-decane interface are remarkably lower than those at the water-air interface for C8E1C8 and C8E4C8. One can assume that the oil phase will be a better solvent than “air” for hydrophobic chains of the adsorbed surfactant.34 The hydrophobic chains incline to orient toward the oil interface as a result of intervening of oil molecules, which leads to the decrease of hydrophobic interactions of the alkyl chains and the faster diffusion exchange between the bulk and the interface. Therefore, dilational moduli at the waterdecane interface decrease sharply. It is worthwhile to note that similar dilational viscoelastic behavior can be observed for C8E8C8 at water-air and water-decane interfaces. It comes forth the first maximum value at 1.0 10-6 mol 3 L-1 and the second maximum value around 5.0 10-5 mol 3 L-1, which indicates that the adsorption films formed at water-air and water-decane interfaces have similar structure. We try to propose the possible schematic of adsorbed C8ExC8 molecules with different numbers of ethylene oxide groups at the water-air and water-decane interfaces from the dilational properties and surface active properties below. (34) Murray, B. S.; Nelson, P. V. Langmuir 1996, 12, 5973.
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Figure 7. Schematic of adsorbed C8ExC8 molecules with different ethylene oxide groups at the water-air interface (a-c) and water-decane interface (d-f).
Amin is the minimum adsorption area per molecule at the interface, which is calculated from the surface tension measurements as a function of bulk concentration. As the number of ethylene oxide groups increases, Amin decreases continually from C8E1C8 to C8E4C8 (2.17 and 1.38 nm2 for C8E1C8 and C8E4C8, respectively). When the number of ethylene oxide group reaches 8, the value of Amin tends to a changeless value of approximately 1.32 nm2.18 Therefore, we speculate that, for C8E8C8, there are enough ethylene oxide groups to make the spacer enter into the water side of the interface to adopt a conformation with a loop. This way, the hydrophilic spacers can cross-link each other to form an interfacial sublayer. In the case of C8E1C8 and C8E4C8, the dilational properties are mainly controlled by the interaction between hydrophobic alkyl chains of adsorbed molecules. The insertion of oil molecules will weaken intermolecular interaction dramatically and result in the obvious decrease of the modulus. However, the nature of the sublayer formed by ethylene oxide groups plays a crucial role in dilational properties.35 The oil molecules have little effect on the sublayer structure at the water side, which results in the similar dilational viscoelastic behavior at both the water-air interface and water-decane interface for C8E8C8. A schematic of the adsorbed C8ExC8 molecules is shown in Figure 7. It is also indicated by the data of interfacial tension relaxation, which we will discuss in the next part. 4.4. Cole-Cole Plots of C8ExC8. Experiments in a reasonably broad frequency range would allow an analysis of the relaxation mechanism in detail. Interfacial pressure relaxation experiments are a reliable way to obtain interfacial dilational parameters. These could detect the microscopic relaxation processes in the interface at equilibrium by fitting the decay curve. In this paper, the fitting of the decay could be achieved using the sum of two (or three) exponential functions, and the dilational (35) Noskov, B. A.; Akentiev, A. V.; Bilibin, A. Y.; Zorin, I. M.; Miller, R. Adv. Colloid Interface Sci. 2003, 104, 245.
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elasticity and viscosity component are calculated by eqs 8 and 9 in a reasonably broad frequency range. ε0 is the static elasticity or Gibbs elasticity, which corresponds to the high frequency limit value. In order to detect the relationship between experimental results and relaxation processes, a form of Cole-Cole plots, εi/ε0 is plotted against εr/ε0. The shape of the Cole-Cole plots can provide the information about relaxation processes.23 Every semicircle in the plot stands for a relaxation process. For a single reorientation process at the interface, a plot of εi/ε0 versus εr/ε0 is a semicircle centered at (1/2, 0) of radius 1/2, whereas if the relaxation mechanism is diffusion controlled, the corresponding ColeCole √ plot describes an arc of a circle centered at (1/2, -1/2) of radius 2/2. Single reorientation and diffusion processes are therefore easily distinguished from each other.24,36,37 The Cole-Cole plots of the water-air and water-decane interfaces for C8ExC8 are listed in Figures 8-10. It can be seen that the low-frequency process appears at the left-hand side of the horizontal axis, and the high-frequency process appears at the right-hand side from the Cole-Cole plot. The dotted line indicates the theoretical curve expected for a diffusion-controlled relaxation, and the solid curve indicates the theoretical behavior for a reorientation process. From Figure 8, it is found that the interfacial behaviors of C8E1C8 are connected with a single reorientation process at low concentration, and the number of the relaxation process increases from one to two as the concentration increase. When the concentration of C8E1C8 is low, there are few molecules at the water-air interface, and the reorientation process occurs by the form of angle change of hydrophobic alkyl chains. With increasing concentration, the reorientation turns to difficult and the diffusion relaxation process becomes important. As a result, the interfacial behaviors are in part conformance with a diffusion (36) Van Hunsel, J.; Joos, P. Colloids Surf. 1987, 25, 251. (37) Kitching, S.; Johnson, G. D. W.; Midmore, B. R.; Herrington, T. M. J. Colloid Interface Sci. 1996, 177, 58.
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Figure 8. Cole-Cole plots of C8E1C8 for the water-air (A) and water-decane (B) interfaces. Surfactant concentrations (mol 3 L-1): (9) 5.00 10-7; (0) 1.00 10-6; (2) 5.00 10-6; (4) 1.00 10-5; (1) 5.00 10-5; (3) 1.00 10-4; ([) 5.94 10-4.
Figure 9. Cole-Cole plots of C8E4C8 for the water-air (A) and water-decane (B) interfaces. Surfactant concentrations (mol 3 L-1): (9) 5.00 10-7; (0) 1.00 10-6; (2) 5.00 10-6; (4) 1.00 10-5; (1) 5.00 10-5; (3) 1.10 10-4.
Figure 10. Cole-Cole plots of C8E8C8 for the water-air (A) and water-decane (B) interfaces. Surfactant concentrations (mol 3 L-1): (9) 5.00 10-7; (0) 1.00 10-6; (2) 5.00 10-6; (4) 1.00 10-5; (1) 5.00 10-5; (3) 1.00 10-4; ([) 5.00 10-4. Langmuir 2010, 26(14), 11907–11914
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controlled process and part conformance with a reorientation controlled process at concentrations higher than 5 10-6 mol 3 L-1. At the water-decane interface, the interaction between longer alkyl chains decreases due to the intervention of oil molecules and the change of alkyl chain reorientation will be enhanced, which results in the predominance of molecular reorientation processes at higher frequency in Cole-Cole plots at all experimental concentration ranges. It can be seen from Figure 9 that there is no pure molecular reorientation process at both the water-air and water-decane interfaces, which indicates the stronger interaction between alkyl chains of C8E4C8 due to the increasing interfacial concentration.18 The reorientation process plays a more predominant role for the water-decane interface than for the water-air interface, which is similar to the case of C8E1C8 and indicates strongly that the nature of the adsorption film is determined mainly by the structure of the hydrophobic part. From Figure 10, it can be seen clearly that the characteristics of Cole-Cole plots of C8E8C8 are quite different from those of C8E1C8 and C8E4C8. There are three relaxation processes at both the water-air and water-decane interfaces for C8E8C8. Moreover, the relaxation processes at the water-decane interface are similar to those at the water-air interface. It can be deduced that the existence of the sublayer formed by the polyoxyethylene loop at the water side, which is independent of the insertion of oil molecules, plays a crucial role in determining the nature of the C8E8C8 film.35 The experimental results obtained by interfacial tension relaxation measurements are in accord with the proposed
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mechanism responsible for dilational characteristics by the oscillating barriers method.
5. Conclusions The dilational properties of gemini surfactants C8ExC8 with polyoxyethylene spacers at the water-air and water-decane interfaces were investigated in the present work. The experimental results show that the number of ethylene oxide groups is one of the principal factors to control the nature of the interfacial film. The dilational properties of C8E1C8 and C8E4C8 are similar to those of common surfactants, and the dilational moduli at the waterdecane interface are remarkably lower than those at the waterair interface, due to intervening of oil molecules, which indicates that the nature of the adsorption film is determined mainly by the structure of the hydrophobic part. Meanwhile, the dilational modulus of C8E8C8 shows two maxima with the increasing concentration, which has the same trend as that of Triton molecules. Moreover, the dilational modulus at the water-decane interface is close to that at the water-air interface. It cannot be attributed only to the dominant diffusion process, and the structure of the adsorption sublayer at the water side also plays a more important role. The results of relaxation experiments and ColeCole plots can support our provided mechanism strongly. Acknowledgment. The authors are thankful for financial support from the National Science & Technology Major Project (2008ZX05011) and National High Technology Research and Development Program (2008AA092801) of China.
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