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Synergetic effect of cationic surfactive ionic liquid C12mimBr and anionic surfactant SDBS on the phase behavior and solubilization of microemulsions
Fluid Phase Equilibria 314 (2012) 90–94
Contents lists available at SciVerse ScienceDirect
Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid
Synergetic effect of cationic surfactive ionic liquid C12 mimBr and anionic surfactant SDBS on the phase behavior and solubilization of microemulsions Meili Zhu, Jinling Chai ∗ , Lusheng Chen, Lei Xu, Wei Liu, Shuchuan Shang, Jianjun Lu College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, 250014 Jinan, Shandong, PR China
a r t i c l e
i n f o
Article history: Received 2 April 2011 Received in revised form 17 October 2011 Accepted 23 October 2011 Available online 3 November 2011 Keywords: ε–ˇ Fishlike phase diagram Microemulsion Solubilization Ionic liquid
a b s t r a c t Synergistic effect of the surfactant mixture of ionic liquid 1-dodecyl-3-methylimidazolium bromide (C12 mimBr) and sodium dodecyl benzene sulfonate (SDBS) on the phase behavior and the solubilization ability of the microemulsion systems was studied with ε–ˇ fishlike phase diagrams. The middle-phase microemulsion appears at the range of the mole ratios of C12 mimBr to SDBS 0–0.1 and 0.7–1.0. The solubilization ability (SP*) of the mixed surfactant based microemulsion systems increased markedly compared to the pure C12 mimBr or SDBS-based microemulsion systems. The mixture of C12 mimBr and SDBS also shows preferable synergism in the solubility of the alcohol and the composition of the interfacial layer. The effects of alcohols, alkanes and NaCl concentrations on the ε–ˇ fishlike phase diagrams of C12 mimBr/SDBS (molar ratio = 8:2)/alcohol/alkane/brine microemulsion systems were also investigated. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Most commercial and industrial surfactants involve several types or many isomers of a main surfactant [1–3]. These composite surfactants exhibit considerable synergistic advantages like solubilization, dispersion, suspension and transportation capabilities, and are used in a wide range of applications in relation to an individual surfactant [1–5]. Mixtures of anionic and cationic surfactants show extraordinary synergism that is exhibited by the ultralow critical micelle concentration (CMC) and interfacial tension (IFT) values [6], and are widely used in pharmaceutical delivery, analytical chemistry, wastewater treatment and textile detergency, and so on. Due to the strong interaction of oppositely charged head groups, the mixtures of anionic and cationic surfactants would produce a precipitate or form a liquid crystal. This has limited the use of the anionic–cationic surfactant mixtures. Fortunately, when anionic–cationic surfactants were mixed with an appropriate ratio and at the aid of a medium-chain alcohol cosurfactant, a middlephase microemulsion system would be prepared [6–8]. Synergistic solubilization of mixed surfactants was observed in previous studies [1,2,6]. Ionic liquids (ILs), consisting of a large organic cation and a quite little anion, are a sort of molten salt at or near room temperatures [9–15]. Special physical and chemical properties and
∗ Corresponding author. Fax: +86 531 82615258. E-mail address: [email protected] (J. Chai). 0378-3812/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2011.10.023
applications of ILs have been researched. In addition to “green” reagents, the long-chained imidazolium ILs, including a charged hydrophilic head group and one or more hydrophobic tails, could be used as surfactants just like the conventional cationic surfactant [12–15]. The enthalpy entropy compensation study reveals the effect of hydrocarbon chain length on micellization. That is, the longer the hydrocarbon chain, the easier it is to form aggregates [15]. 1-Alkyl-3-methylimidazolium bromide ionic liquids with at least nine carbon atoms of their hydrophobic tail can be used as cationic surfactant [14,15]. With regard to the long-chain imidazolium, the surface activities of [Cn mim] Br were in the order [C16 mim] Br > [C14 mim] Br > [C12 mim] Br [15]. Microemulsions are optically transparent and thermodynamically stable systems containing oil and water stabilized by an interfacial layer composed of surfactant and cosurfactant [16,22,24]. Winsor III type microemulsion system has a bicontinuous specific structure with oil and water microdomains separated by an interfacial layer. Experiments and mathematical models have been developed to investigate the structures and properties [18–22], such as the ultrahigh solubilization ability of the system. The effects of alkanes [21,23–25], alcohols [21,24], temperature [16,18,22] and inorganic salts [16,18,20,22–25], etc., were also investigated. Microemulsions comprising anionic and cationic surfactants together were called catanionic microemulsions. The catanionic microemulsions can be used as a reaction medium for organic and particle synthesis [17]. Microemulsions formed by ILs have been reported [10,11], but middle-phase microemulsion formations of long-chained imidazolium ionic liquids as cationic
M. Zhu et al. / Fluid Phase Equilibria 314 (2012) 90–94
0.16
surfactant mixed with other common anionic surfactants system were few reported. In this paper, the cationic surfactant 1-dodecyl3-methylimidazolium bromide (C12 mimBr) and anionic surfactant sodium dodecyl benzene sulfonate (SDBS) were mixed to form middle-phase microemulsions. The phase behavior and solubilization of the microemulsion systems with mixed surfactants were studied by ε–ˇ fishlike phase diagrams.
1 2
ε
3 0.08
The preparation of samples was weighing the equal mass of brine and alkane into Teflon-sealed glass tubes, and the surfactants C12 mimBr and SDBS with fixed mole ratios were weighed into these tubes, respectively. Then different amounts of alcohol were added into the tubes. The foregoing preparation process was repeated. In order to get phase equilibrium, the tubes with different compositions were put into a thermostatic water bath at 40 ± 0.1 ◦ C for about one week. The volume of each equilibrated phase was observed and recorded accurately. 3. Results and discussion 3.1. ε–ˇ Fishlike phase diagram of the microemulsion systems In C12 mimBr(S1 )/SDBS(S2 )/1-butanol (A)/n-octane (O)/brine (5.0% NaCl, W) microemulsion system, ˛ = mO /(mW + mO ) is the mass fraction of n-octane in brine plus n-octane, ε = mA /(mS1 + mS2 + mA + mW + mO ) is the mass fraction of alcohol in the whole system, and ˇ = (mS1 + mS2 )/(mS1 + mS2 + mA + mW + mO ) represents the mass ratio of surfactants in the whole system [23–25]. The experiments were carried out under the temperature T = 40 ± 0.1 ◦ C, the pressure P = 1.01 × 105 Pa and ˛ = 0.5. ˇ and ε were plotted as x-axis and y-axis, respectively, then an ε–ˇ fishlike phase diagram was obtained. Fig. 1 shows the ε–ˇ fishlike phase diagrams of the microemulsion systems formed by C12 mimBr and SDBS mixture with different mole ratios of C12 mimBr to SDBS (XC12 mimBr ) for C12 mimBr/SDBS/1butanol/n-octane/brine (5.0% NaCl) systems. Fig. 1 shows that the middle-phase microemulsions would form at the XC12 mimBr range of 0–0.1 and 0.7–1.0. Precipitate would appear at the XC12 mimBr range of 0.2–0.6, because of the strong interaction of oppositely charged head groups [6–8]. The middle-phase microemulsions begin to form at the “fish head” (point B) and come to an end at the “fish tail” (point E). When ˇ was held constant, increasing ε would cause a series
3 3
B B B B
0.04
0.00
1 E
3
B B
2.1. Materials and apparatus
2.2. Methods
E
0.12
2. Experimental
1-Dodecyl-3-methylimidazolium bromide (C12 mimBr) was synthesized by our research group according to Ref. [26], and characterized by infrared spectrum etc. The purity of C12 mimBr synthesized was examined by surface tension measure, and no surface tension minimum was found in the surface tension curve. Sodium dodecyl benzene sulfonate (SDBS) was purchased from National Drug Group Chemical Reagent Company, China with purity greater than 88%. After three times recrystallization with ethanol, the purity of SDBS was examined and no minimum point was found in the curve of surface tension. n-Octane, n-hexane and 1-butanol were purchased from Sinopharm Chemical Reagent Company Ltd., China, and are all of A.R. grade. Doubly distilled water was used. An FA 1104 electron balance and a 501 super thermostat were used.
91
3 3
0.00
E
2
0.04
0.08
E 1 1 E 1 1 E
0.12
0.16
β Fig. 1. The ε–ˇ fishlike phase diagrams of the microemulsions formed by C12 mimBr and SDBS mixture with different mole ratios XC12 mimBr of C12 mimBr to SDBS for C12 mimBr/SDBS/1-butanol/n-octane/brine (5.0% NaCl) systems. XC12 mimBr : , 1.0; 䊉, 0.9; , 0.8; , 0.7; , 0.1 and *, 0. Table 1 Physicochemical properties of C12 mimBr/SDBS/1-butanol/n-octane/brine (5.0% NaCl) microemulsion systems. XC12 mimBr
ˇB
εB
ˇE
εE
ˇi
εi
0 0.1 0.7 0.8 0.9 1
0.0006 0.0002 0.0003 0.0006 0.0015 0.0064
0.0314 0.0132 0.0189 0.0360 0.0570 0.0665
0.1327 0.1083 0.1334 0.1418 0.1444 0.1519
0.0553 0.0204 0.0316 0.0602 0.0921 0.1387
0.1322 0.1082 0.1332 0.1413 0.1432 0.1469
0.0290 0.0087 0.0156 0.0303 0.0458 0.0878
¯ of phase inversions Winsor I(2 - ) → III(3) → II(2) . The single-phase microemulsions (Winsor IV) appear at point E. (ˇB , εB ) and (ˇE , εE ) represent the coordinate values of the “fish head” and the “fish tail”. Moreover, ˇi and εi stand for the mass fractions of the surfactant and alcohol in the interfacial layer in the whole system, respectively. From Fig. 1, the above physicochemical properties of the ε–ˇ fishlike phase diagrams were attained in accordance with Refs. [23–25] and listed in Table 1. 3.2. The solubilities of the alcohol, the composition of the interfacial layer and the solubilization ability of the system The values of ˇB and εB reveal, respectively, the solubilities of surfactant and alcohol in the whole microemulsion system. ˇi and εi suggest the composition of the interfacial layer of the microemulsion. At the same time, ˇE and εE reflect the solubilization ability of the microemulsion systems. In Table 1, the values of ˇB are quite smaller than ˇi values. This tells that the surfactant is primarily incorporated into the interfacial layer, and little is solubilized in the aqueous or oleic phase. Fig. 2 shows the relationship between εB and XC12 mimBr of microemulsion systems C12 mimBr/SDBS/1-butanol/noctane/brine (5.0% NaCl). Fig. 2 shows that the solubility of the alcohol (εB ) in the C12 mimBr–SDBS – mixed microemulsion systems was significantly decreased compared to the pure C12 mimBr-based or pure SDBS – based microemulsion systems. The number of moles of the surfactant in the interfacial layer, S and the number of moles of alcohol in the interfacial layer, AS SM M can be obtained from εi and ˇi values, and were plotted in Fig. 3a. S S SM and especially AM were decreased in the C12 mimBr–SDBS-mixed microemulsion systems. The interfacial layer was equilibrated with the oleic or aqueous phase, so the decrease of ASM values in the
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0.08
4.5
4.0
0.06
SP*
εΒ
3.5 0.04
3.0 0.02 2.5 0.00
0.0
0.2
0.4
0.6
XC
12
0.8
AMS
SMS , AMS /10-3
2.1
SMS
1.4
0.0
0.2
0.4
0.6
0.8
1.0
Xc12 mimBr 0.8
, XA S A
S
XAS
0.4
S
0.0
0.8
1.0
Fig. 4. Plot of SP∗ vs XC12 mimBr of microemulsion systems C12 mimBr/SDBS/1butanol/n-octane/brine (5.0% NaCl).
interfacial layer would result in the decrease of the solubility of the alcohol (εB ) in the mixed microemulsion systems [25]. Fig. 3a shows that there exists synergistic effect in the mixed microemulsion systems. That is, the ability of regulating the curvature of the interfacial layer gets stronger after the two surfactants mixed, therefore less surfactant and alcohol were needed to balance the interfacial layer. The mass fraction of the alcohol in interfacial layer composed of surfactant and alcohol, AS , also can be calculated from ˇi and εi values [25] AS =
εi ˇi + εi
XAS =
0.2
0.6
(1)
The molar fraction of alcohol in the interfacial layer XAS can be S and AS values [25] obtained from SM M
0.7
0.6
0.4
12 mimBr
2.8
b
0.2
Xc
3.5
0.0
0.0
mimBr
Fig. 2. The relationship between εB and XC12 mimBr of microemulsion systems C12 mimBr/SDBS/1-butanol/n-octane/brine (5.0% NaCl).
a
2.0
1.0
A
0.2
0.4
0.6
0.8
1.0
XC
12 mimBr
S Fig. 3. Plot of SM , ASM vs XC12 mimBr (a) and AS , XAS vs XC12 mimBr (b) of microemulsion systems C12 mimBr/SDBS/1-butanol/n-octane/brine (5.0% NaCl).
S + AS SM M
(2)
Fig. 3b shows the relationship between AS , XAS and XC12 mimBr of microemulsion systems C12 mimBr/SDBS/1-butanol/noctane/brine (5.0% NaCl). Fig. 3b indicates that both the mass fraction (AS ) and the mole fraction (XAS ) of alcohol in the interfacial layer were decreased in the mixed surfactant-based microemulsion systems. This is conformity with the sharp decrease of ASM S values shown in Fig. 3a. compared to SM At the “fish tail” in the fishlike phase diagrams in Fig. 1, ˇE and εE values represent the minimum amount of the surfactant and alcohol, respectively, which were needed to turn water and oil into a single phase. In other words, point E can be used to evaluate the solubilization ability of the microemulsion systems. From ˇE and εE values, the solubilization parameter SP* of the microemulsion phase, which is equal to the mass of oil solubilized in the microemulsion phase per gram of surfactant, can be calculated as [25] SP∗ =
0.0
ASM
1 − ˇE − εE 2ˇE
(3)
The solubilization parameter SP∗ value reveals the solubilization ability of the system. Fig. 4 plots the relationship between the solubilization parameter SP∗ of the microemulsion phase and XC12 mimBr for the microemulsion systems C12 mimBr/SDBS/1-butanol/noctane/brine (5.0% NaCl).
M. Zhu et al. / Fluid Phase Equilibria 314 (2012) 90–94
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Fig. 5. Effects of alcohols (a), alkanes (b) and NaCl concentration (c) on the ε–ˇ fishlike phase diagrams for C12 mimBr/SDBS/alcohol/alkane/brine microemulsion systems. (a) C12 mimBr/SDBS (mole ratio = 8:2)/alcohol/n-octane/brine (5.0%NaCl): , n-butanol; 䊉, n-pentanol; (b) C12 mimBr/SDBS (mole ratio = 8:2)/1-butanol/oil/brine (5.0% NaCl): , n-hexane; 䊉, n-octane; , n-decane and (c) C12 mimBr/SDBS (mole ratio = 8:2)/1-butanol/n-octane/brine, NaCl concentration: , 2.5%; 䊉, 5.0% and , 7.5%.
It can be seen from Fig. 4 that the solubilization ability (SP∗ ) of the mixed surfactant C12 mimBr and SDBS based microemulsion system increased markedly compared to the pure C12 mimBrbased or SDBS-based microemulsion systems. Especially the mixed surfactant-based microemulsion system exhibits the maximum solubilization parameter SP∗ when XC12 mimBr is approximately equal to 0.1. Fig. 4 indicates that the mixture of cationic surfactive ionic liquid C12 mimBr and anionic surfactant SDBS exhibits preferable synergistic effect in terms of solubilization ability. The synergistic effect between cationic and anionic surfactants results from the strong electrostatic attraction among the positive and negative ions of the surfactants. This kind of attraction makes molecules of surfactants in the interfacial layer tend to arrange more compactly, so the solubilization ability increases.
3.3. Effects of alcohols, alkanes and NaCl concentrations on the ε–ˇ fishlike phase diagrams The effects of alcohols, the carbon chain length of the alkanes and NaCl concentrations on the ε–ˇ fishlike phase diagrams of C12 mimBr/SDBS (molar ratio = 8:2)/alcohol/alkane/brine microemulsion systems were shown in Fig. 5. The related physical–chemical parameters were listed in Table 2. Fig. 6 shows the relationship between εB , AS , SP* and the length of the carbon chain (nc ) of the alcohol and the alkane molecules and the NaCl concentration (NaCl%) of microemulsion systems C12 mimBr/SDBS/alcohol/alkane/brine. Fig. 6 indicates that as the carbon chain length of the alcohol molecules increases, the solubilization ability of the microemulsion (SP∗ ) increases. This would be explained that the alcohol molecules
Fig. 6. Plot of εB , AS , SP* vs the length of carbon chain length nc of alcohol or alkane and NaCl concentration (NaCl%) of microemulsion systems C12 mimBr/SDBS/alcohol/alkane/brine.
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M. Zhu et al. / Fluid Phase Equilibria 314 (2012) 90–94
Table 2 Effects of alcohols, alkanes and salinity on the physical-chemical parameters for C12 mimBr/SDBS (molar ratio = 8:2)/alcohol/alkane/brine systems. ˇB
εB
C12 mimBr/SDBS (=8:2)/alcohol/n-octane/brine (5.0% NaCl) 0.0360 n-Butanol 0.0006 n-Pentanol 0.0005 0.0183 C12 mimBr/SDBS (=8:2)/n-butanol/oil/brine (5.0% NaCl) 0.0244 n-Hexane 0.0005 n-Octane 0.0006 0.0360 0.0012 0.0492 n-Decane C12 mimBr/SDBS (=8:2)/n-butanol/n-octane/brine (NaCl) 0.0483 2.5% 0.0006 5.0% 0.0006 0.0360 0.0302 7.5% 0.0005
with longer carbon chain could change the hydrophilicity and bend the elastic modulus of the interfacial layer more effectively [21,24,25]. When the carbon chain length of the alkane molecules increases, the penetration ability of the alkane molecules to the interfacial layer to decrease the lipophilicity of the interfacial layer was decreased [21,23–25]. Therefore, more surfactant and alcohol molecules were needed to balance the interfacial layer and the solubilization ability of the microemulsion (SP∗ ) decreases (Fig. 6). Through the “salting-out” effect, the solubility of the alcohol (εB ) decreases with increasing NaCl concentrations (Fig. 6). Moreover, the solubilization ability (SP∗ ) of the system increases as NaCl concentrations increase, while the mass fraction of the alcohol in the interfacial layer (AS ) decreases.
ˇE
εE
ˇi
εi
0.1418 0.1162
0.0602 0.0300
0.1413 0.1158
0.0303 0.0142
0.1269 0.1418 0.1736
0.0453 0.0602 0.0980
0.1264 0.1413 0.1727
0.0247 0.0303 0.0602
0.1600 0.1418 0.1228
0.0748 0.0602 0.0495
0.1596 0.1413 0.1224
0.0360 0.0303 0.0237
SP∗ nc
the solubilization ability SP∗ = (1 − ˇE − εE )/(2ˇE ) the length of the carbon chain
of
the
system,
Super/subscripts B “fish head”, the start point of the middle-phase microemulsion “fish tail”, the end point of the middle-phase microemulE sion the interfacial layer of the microemulsion i Acknowledgment This work was supported by the Natural Science Foundation of Shandong Province (Grant ZR2009BM036).
4. Conclusions References The synergetic effect of C12 mimBr and SDBS on the solubilization ability of the microemulsion systems was investigated systematically by the ε–ˇ fishlike phase diagrams. The solubility of the alcohol in the C12 mimBr–SDBS-mixed microemulsion systems decreases significantly compared to the pure C12 mimBr or SDBS-based microemulsion systems. The amounts of surfactant and alcohol needed to balance the interfacial layer were also decreased in the mixed surfactant-based microemulsion systems. The solubilization ability (SP∗ ) of the mixed surfactant C12 mimBr–SDBS-based microemulsion system is superior to the individual C12 mimBr or SDBS-based microemulsion system. The results will be beneficial to the optimization of the microemulsion formations and further the better use of the anionic–cationic surfactant-based microemulsion systems in many fields. List of symbols
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
T P ˛ ε ˇ S SM
ASM AS XAS
temperature (◦ C) pressure (Pa) the mass fraction of n-octane in brine plus n-octane, ˛ = mO /(mW + mO ) the mass fraction of alcohol in the whole system, ε = mA /(mS1 + mS2 + mA + mW + mO ) the mass fraction of surfactants in the system, ˇ = (mS1 + mS2 )/(mS1 + mS2 + mA + mW + mO ) the number of moles of the surfactant in the interfacial layer the number of moles of alcohol in the interfacial layer the mass fraction of the alcohol in the interfacial layer composed of surfactant and alcohol, AS = εi /(ˇi + εi ) the mole fraction of alcohol in the interfacial layer, XAS = S + AS ) ASM /(SM M
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
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Contents lists available at SciVerse ScienceDirect
Fluid Phase Equilibria journal homepage: www.elsevier.com/locate/fluid
Synergetic effect of cationic surfactive ionic liquid C12 mimBr and anionic surfactant SDBS on the phase behavior and solubilization of microemulsions Meili Zhu, Jinling Chai ∗ , Lusheng Chen, Lei Xu, Wei Liu, Shuchuan Shang, Jianjun Lu College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, 250014 Jinan, Shandong, PR China
a r t i c l e
i n f o
Article history: Received 2 April 2011 Received in revised form 17 October 2011 Accepted 23 October 2011 Available online 3 November 2011 Keywords: ε–ˇ Fishlike phase diagram Microemulsion Solubilization Ionic liquid
a b s t r a c t Synergistic effect of the surfactant mixture of ionic liquid 1-dodecyl-3-methylimidazolium bromide (C12 mimBr) and sodium dodecyl benzene sulfonate (SDBS) on the phase behavior and the solubilization ability of the microemulsion systems was studied with ε–ˇ fishlike phase diagrams. The middle-phase microemulsion appears at the range of the mole ratios of C12 mimBr to SDBS 0–0.1 and 0.7–1.0. The solubilization ability (SP*) of the mixed surfactant based microemulsion systems increased markedly compared to the pure C12 mimBr or SDBS-based microemulsion systems. The mixture of C12 mimBr and SDBS also shows preferable synergism in the solubility of the alcohol and the composition of the interfacial layer. The effects of alcohols, alkanes and NaCl concentrations on the ε–ˇ fishlike phase diagrams of C12 mimBr/SDBS (molar ratio = 8:2)/alcohol/alkane/brine microemulsion systems were also investigated. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Most commercial and industrial surfactants involve several types or many isomers of a main surfactant [1–3]. These composite surfactants exhibit considerable synergistic advantages like solubilization, dispersion, suspension and transportation capabilities, and are used in a wide range of applications in relation to an individual surfactant [1–5]. Mixtures of anionic and cationic surfactants show extraordinary synergism that is exhibited by the ultralow critical micelle concentration (CMC) and interfacial tension (IFT) values [6], and are widely used in pharmaceutical delivery, analytical chemistry, wastewater treatment and textile detergency, and so on. Due to the strong interaction of oppositely charged head groups, the mixtures of anionic and cationic surfactants would produce a precipitate or form a liquid crystal. This has limited the use of the anionic–cationic surfactant mixtures. Fortunately, when anionic–cationic surfactants were mixed with an appropriate ratio and at the aid of a medium-chain alcohol cosurfactant, a middlephase microemulsion system would be prepared [6–8]. Synergistic solubilization of mixed surfactants was observed in previous studies [1,2,6]. Ionic liquids (ILs), consisting of a large organic cation and a quite little anion, are a sort of molten salt at or near room temperatures [9–15]. Special physical and chemical properties and
∗ Corresponding author. Fax: +86 531 82615258. E-mail address: [email protected] (J. Chai). 0378-3812/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2011.10.023
applications of ILs have been researched. In addition to “green” reagents, the long-chained imidazolium ILs, including a charged hydrophilic head group and one or more hydrophobic tails, could be used as surfactants just like the conventional cationic surfactant [12–15]. The enthalpy entropy compensation study reveals the effect of hydrocarbon chain length on micellization. That is, the longer the hydrocarbon chain, the easier it is to form aggregates [15]. 1-Alkyl-3-methylimidazolium bromide ionic liquids with at least nine carbon atoms of their hydrophobic tail can be used as cationic surfactant [14,15]. With regard to the long-chain imidazolium, the surface activities of [Cn mim] Br were in the order [C16 mim] Br > [C14 mim] Br > [C12 mim] Br [15]. Microemulsions are optically transparent and thermodynamically stable systems containing oil and water stabilized by an interfacial layer composed of surfactant and cosurfactant [16,22,24]. Winsor III type microemulsion system has a bicontinuous specific structure with oil and water microdomains separated by an interfacial layer. Experiments and mathematical models have been developed to investigate the structures and properties [18–22], such as the ultrahigh solubilization ability of the system. The effects of alkanes [21,23–25], alcohols [21,24], temperature [16,18,22] and inorganic salts [16,18,20,22–25], etc., were also investigated. Microemulsions comprising anionic and cationic surfactants together were called catanionic microemulsions. The catanionic microemulsions can be used as a reaction medium for organic and particle synthesis [17]. Microemulsions formed by ILs have been reported [10,11], but middle-phase microemulsion formations of long-chained imidazolium ionic liquids as cationic
M. Zhu et al. / Fluid Phase Equilibria 314 (2012) 90–94
0.16
surfactant mixed with other common anionic surfactants system were few reported. In this paper, the cationic surfactant 1-dodecyl3-methylimidazolium bromide (C12 mimBr) and anionic surfactant sodium dodecyl benzene sulfonate (SDBS) were mixed to form middle-phase microemulsions. The phase behavior and solubilization of the microemulsion systems with mixed surfactants were studied by ε–ˇ fishlike phase diagrams.
1 2
ε
3 0.08
The preparation of samples was weighing the equal mass of brine and alkane into Teflon-sealed glass tubes, and the surfactants C12 mimBr and SDBS with fixed mole ratios were weighed into these tubes, respectively. Then different amounts of alcohol were added into the tubes. The foregoing preparation process was repeated. In order to get phase equilibrium, the tubes with different compositions were put into a thermostatic water bath at 40 ± 0.1 ◦ C for about one week. The volume of each equilibrated phase was observed and recorded accurately. 3. Results and discussion 3.1. ε–ˇ Fishlike phase diagram of the microemulsion systems In C12 mimBr(S1 )/SDBS(S2 )/1-butanol (A)/n-octane (O)/brine (5.0% NaCl, W) microemulsion system, ˛ = mO /(mW + mO ) is the mass fraction of n-octane in brine plus n-octane, ε = mA /(mS1 + mS2 + mA + mW + mO ) is the mass fraction of alcohol in the whole system, and ˇ = (mS1 + mS2 )/(mS1 + mS2 + mA + mW + mO ) represents the mass ratio of surfactants in the whole system [23–25]. The experiments were carried out under the temperature T = 40 ± 0.1 ◦ C, the pressure P = 1.01 × 105 Pa and ˛ = 0.5. ˇ and ε were plotted as x-axis and y-axis, respectively, then an ε–ˇ fishlike phase diagram was obtained. Fig. 1 shows the ε–ˇ fishlike phase diagrams of the microemulsion systems formed by C12 mimBr and SDBS mixture with different mole ratios of C12 mimBr to SDBS (XC12 mimBr ) for C12 mimBr/SDBS/1butanol/n-octane/brine (5.0% NaCl) systems. Fig. 1 shows that the middle-phase microemulsions would form at the XC12 mimBr range of 0–0.1 and 0.7–1.0. Precipitate would appear at the XC12 mimBr range of 0.2–0.6, because of the strong interaction of oppositely charged head groups [6–8]. The middle-phase microemulsions begin to form at the “fish head” (point B) and come to an end at the “fish tail” (point E). When ˇ was held constant, increasing ε would cause a series
3 3
B B B B
0.04
0.00
1 E
3
B B
2.1. Materials and apparatus
2.2. Methods
E
0.12
2. Experimental
1-Dodecyl-3-methylimidazolium bromide (C12 mimBr) was synthesized by our research group according to Ref. [26], and characterized by infrared spectrum etc. The purity of C12 mimBr synthesized was examined by surface tension measure, and no surface tension minimum was found in the surface tension curve. Sodium dodecyl benzene sulfonate (SDBS) was purchased from National Drug Group Chemical Reagent Company, China with purity greater than 88%. After three times recrystallization with ethanol, the purity of SDBS was examined and no minimum point was found in the curve of surface tension. n-Octane, n-hexane and 1-butanol were purchased from Sinopharm Chemical Reagent Company Ltd., China, and are all of A.R. grade. Doubly distilled water was used. An FA 1104 electron balance and a 501 super thermostat were used.
91
3 3
0.00
E
2
0.04
0.08
E 1 1 E 1 1 E
0.12
0.16
β Fig. 1. The ε–ˇ fishlike phase diagrams of the microemulsions formed by C12 mimBr and SDBS mixture with different mole ratios XC12 mimBr of C12 mimBr to SDBS for C12 mimBr/SDBS/1-butanol/n-octane/brine (5.0% NaCl) systems. XC12 mimBr : , 1.0; 䊉, 0.9; , 0.8; , 0.7; , 0.1 and *, 0. Table 1 Physicochemical properties of C12 mimBr/SDBS/1-butanol/n-octane/brine (5.0% NaCl) microemulsion systems. XC12 mimBr
ˇB
εB
ˇE
εE
ˇi
εi
0 0.1 0.7 0.8 0.9 1
0.0006 0.0002 0.0003 0.0006 0.0015 0.0064
0.0314 0.0132 0.0189 0.0360 0.0570 0.0665
0.1327 0.1083 0.1334 0.1418 0.1444 0.1519
0.0553 0.0204 0.0316 0.0602 0.0921 0.1387
0.1322 0.1082 0.1332 0.1413 0.1432 0.1469
0.0290 0.0087 0.0156 0.0303 0.0458 0.0878
¯ of phase inversions Winsor I(2 - ) → III(3) → II(2) . The single-phase microemulsions (Winsor IV) appear at point E. (ˇB , εB ) and (ˇE , εE ) represent the coordinate values of the “fish head” and the “fish tail”. Moreover, ˇi and εi stand for the mass fractions of the surfactant and alcohol in the interfacial layer in the whole system, respectively. From Fig. 1, the above physicochemical properties of the ε–ˇ fishlike phase diagrams were attained in accordance with Refs. [23–25] and listed in Table 1. 3.2. The solubilities of the alcohol, the composition of the interfacial layer and the solubilization ability of the system The values of ˇB and εB reveal, respectively, the solubilities of surfactant and alcohol in the whole microemulsion system. ˇi and εi suggest the composition of the interfacial layer of the microemulsion. At the same time, ˇE and εE reflect the solubilization ability of the microemulsion systems. In Table 1, the values of ˇB are quite smaller than ˇi values. This tells that the surfactant is primarily incorporated into the interfacial layer, and little is solubilized in the aqueous or oleic phase. Fig. 2 shows the relationship between εB and XC12 mimBr of microemulsion systems C12 mimBr/SDBS/1-butanol/noctane/brine (5.0% NaCl). Fig. 2 shows that the solubility of the alcohol (εB ) in the C12 mimBr–SDBS – mixed microemulsion systems was significantly decreased compared to the pure C12 mimBr-based or pure SDBS – based microemulsion systems. The number of moles of the surfactant in the interfacial layer, S and the number of moles of alcohol in the interfacial layer, AS SM M can be obtained from εi and ˇi values, and were plotted in Fig. 3a. S S SM and especially AM were decreased in the C12 mimBr–SDBS-mixed microemulsion systems. The interfacial layer was equilibrated with the oleic or aqueous phase, so the decrease of ASM values in the
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0.08
4.5
4.0
0.06
SP*
εΒ
3.5 0.04
3.0 0.02 2.5 0.00
0.0
0.2
0.4
0.6
XC
12
0.8
AMS
SMS , AMS /10-3
2.1
SMS
1.4
0.0
0.2
0.4
0.6
0.8
1.0
Xc12 mimBr 0.8
, XA S A
S
XAS
0.4
S
0.0
0.8
1.0
Fig. 4. Plot of SP∗ vs XC12 mimBr of microemulsion systems C12 mimBr/SDBS/1butanol/n-octane/brine (5.0% NaCl).
interfacial layer would result in the decrease of the solubility of the alcohol (εB ) in the mixed microemulsion systems [25]. Fig. 3a shows that there exists synergistic effect in the mixed microemulsion systems. That is, the ability of regulating the curvature of the interfacial layer gets stronger after the two surfactants mixed, therefore less surfactant and alcohol were needed to balance the interfacial layer. The mass fraction of the alcohol in interfacial layer composed of surfactant and alcohol, AS , also can be calculated from ˇi and εi values [25] AS =
εi ˇi + εi
XAS =
0.2
0.6
(1)
The molar fraction of alcohol in the interfacial layer XAS can be S and AS values [25] obtained from SM M
0.7
0.6
0.4
12 mimBr
2.8
b
0.2
Xc
3.5
0.0
0.0
mimBr
Fig. 2. The relationship between εB and XC12 mimBr of microemulsion systems C12 mimBr/SDBS/1-butanol/n-octane/brine (5.0% NaCl).
a
2.0
1.0
A
0.2
0.4
0.6
0.8
1.0
XC
12 mimBr
S Fig. 3. Plot of SM , ASM vs XC12 mimBr (a) and AS , XAS vs XC12 mimBr (b) of microemulsion systems C12 mimBr/SDBS/1-butanol/n-octane/brine (5.0% NaCl).
S + AS SM M
(2)
Fig. 3b shows the relationship between AS , XAS and XC12 mimBr of microemulsion systems C12 mimBr/SDBS/1-butanol/noctane/brine (5.0% NaCl). Fig. 3b indicates that both the mass fraction (AS ) and the mole fraction (XAS ) of alcohol in the interfacial layer were decreased in the mixed surfactant-based microemulsion systems. This is conformity with the sharp decrease of ASM S values shown in Fig. 3a. compared to SM At the “fish tail” in the fishlike phase diagrams in Fig. 1, ˇE and εE values represent the minimum amount of the surfactant and alcohol, respectively, which were needed to turn water and oil into a single phase. In other words, point E can be used to evaluate the solubilization ability of the microemulsion systems. From ˇE and εE values, the solubilization parameter SP* of the microemulsion phase, which is equal to the mass of oil solubilized in the microemulsion phase per gram of surfactant, can be calculated as [25] SP∗ =
0.0
ASM
1 − ˇE − εE 2ˇE
(3)
The solubilization parameter SP∗ value reveals the solubilization ability of the system. Fig. 4 plots the relationship between the solubilization parameter SP∗ of the microemulsion phase and XC12 mimBr for the microemulsion systems C12 mimBr/SDBS/1-butanol/noctane/brine (5.0% NaCl).
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Fig. 5. Effects of alcohols (a), alkanes (b) and NaCl concentration (c) on the ε–ˇ fishlike phase diagrams for C12 mimBr/SDBS/alcohol/alkane/brine microemulsion systems. (a) C12 mimBr/SDBS (mole ratio = 8:2)/alcohol/n-octane/brine (5.0%NaCl): , n-butanol; 䊉, n-pentanol; (b) C12 mimBr/SDBS (mole ratio = 8:2)/1-butanol/oil/brine (5.0% NaCl): , n-hexane; 䊉, n-octane; , n-decane and (c) C12 mimBr/SDBS (mole ratio = 8:2)/1-butanol/n-octane/brine, NaCl concentration: , 2.5%; 䊉, 5.0% and , 7.5%.
It can be seen from Fig. 4 that the solubilization ability (SP∗ ) of the mixed surfactant C12 mimBr and SDBS based microemulsion system increased markedly compared to the pure C12 mimBrbased or SDBS-based microemulsion systems. Especially the mixed surfactant-based microemulsion system exhibits the maximum solubilization parameter SP∗ when XC12 mimBr is approximately equal to 0.1. Fig. 4 indicates that the mixture of cationic surfactive ionic liquid C12 mimBr and anionic surfactant SDBS exhibits preferable synergistic effect in terms of solubilization ability. The synergistic effect between cationic and anionic surfactants results from the strong electrostatic attraction among the positive and negative ions of the surfactants. This kind of attraction makes molecules of surfactants in the interfacial layer tend to arrange more compactly, so the solubilization ability increases.
3.3. Effects of alcohols, alkanes and NaCl concentrations on the ε–ˇ fishlike phase diagrams The effects of alcohols, the carbon chain length of the alkanes and NaCl concentrations on the ε–ˇ fishlike phase diagrams of C12 mimBr/SDBS (molar ratio = 8:2)/alcohol/alkane/brine microemulsion systems were shown in Fig. 5. The related physical–chemical parameters were listed in Table 2. Fig. 6 shows the relationship between εB , AS , SP* and the length of the carbon chain (nc ) of the alcohol and the alkane molecules and the NaCl concentration (NaCl%) of microemulsion systems C12 mimBr/SDBS/alcohol/alkane/brine. Fig. 6 indicates that as the carbon chain length of the alcohol molecules increases, the solubilization ability of the microemulsion (SP∗ ) increases. This would be explained that the alcohol molecules
Fig. 6. Plot of εB , AS , SP* vs the length of carbon chain length nc of alcohol or alkane and NaCl concentration (NaCl%) of microemulsion systems C12 mimBr/SDBS/alcohol/alkane/brine.
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Table 2 Effects of alcohols, alkanes and salinity on the physical-chemical parameters for C12 mimBr/SDBS (molar ratio = 8:2)/alcohol/alkane/brine systems. ˇB
εB
C12 mimBr/SDBS (=8:2)/alcohol/n-octane/brine (5.0% NaCl) 0.0360 n-Butanol 0.0006 n-Pentanol 0.0005 0.0183 C12 mimBr/SDBS (=8:2)/n-butanol/oil/brine (5.0% NaCl) 0.0244 n-Hexane 0.0005 n-Octane 0.0006 0.0360 0.0012 0.0492 n-Decane C12 mimBr/SDBS (=8:2)/n-butanol/n-octane/brine (NaCl) 0.0483 2.5% 0.0006 5.0% 0.0006 0.0360 0.0302 7.5% 0.0005
with longer carbon chain could change the hydrophilicity and bend the elastic modulus of the interfacial layer more effectively [21,24,25]. When the carbon chain length of the alkane molecules increases, the penetration ability of the alkane molecules to the interfacial layer to decrease the lipophilicity of the interfacial layer was decreased [21,23–25]. Therefore, more surfactant and alcohol molecules were needed to balance the interfacial layer and the solubilization ability of the microemulsion (SP∗ ) decreases (Fig. 6). Through the “salting-out” effect, the solubility of the alcohol (εB ) decreases with increasing NaCl concentrations (Fig. 6). Moreover, the solubilization ability (SP∗ ) of the system increases as NaCl concentrations increase, while the mass fraction of the alcohol in the interfacial layer (AS ) decreases.
ˇE
εE
ˇi
εi
0.1418 0.1162
0.0602 0.0300
0.1413 0.1158
0.0303 0.0142
0.1269 0.1418 0.1736
0.0453 0.0602 0.0980
0.1264 0.1413 0.1727
0.0247 0.0303 0.0602
0.1600 0.1418 0.1228
0.0748 0.0602 0.0495
0.1596 0.1413 0.1224
0.0360 0.0303 0.0237
SP∗ nc
the solubilization ability SP∗ = (1 − ˇE − εE )/(2ˇE ) the length of the carbon chain
of
the
system,
Super/subscripts B “fish head”, the start point of the middle-phase microemulsion “fish tail”, the end point of the middle-phase microemulE sion the interfacial layer of the microemulsion i Acknowledgment This work was supported by the Natural Science Foundation of Shandong Province (Grant ZR2009BM036).
4. Conclusions References The synergetic effect of C12 mimBr and SDBS on the solubilization ability of the microemulsion systems was investigated systematically by the ε–ˇ fishlike phase diagrams. The solubility of the alcohol in the C12 mimBr–SDBS-mixed microemulsion systems decreases significantly compared to the pure C12 mimBr or SDBS-based microemulsion systems. The amounts of surfactant and alcohol needed to balance the interfacial layer were also decreased in the mixed surfactant-based microemulsion systems. The solubilization ability (SP∗ ) of the mixed surfactant C12 mimBr–SDBS-based microemulsion system is superior to the individual C12 mimBr or SDBS-based microemulsion system. The results will be beneficial to the optimization of the microemulsion formations and further the better use of the anionic–cationic surfactant-based microemulsion systems in many fields. List of symbols
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
T P ˛ ε ˇ S SM
ASM AS XAS
temperature (◦ C) pressure (Pa) the mass fraction of n-octane in brine plus n-octane, ˛ = mO /(mW + mO ) the mass fraction of alcohol in the whole system, ε = mA /(mS1 + mS2 + mA + mW + mO ) the mass fraction of surfactants in the system, ˇ = (mS1 + mS2 )/(mS1 + mS2 + mA + mW + mO ) the number of moles of the surfactant in the interfacial layer the number of moles of alcohol in the interfacial layer the mass fraction of the alcohol in the interfacial layer composed of surfactant and alcohol, AS = εi /(ˇi + εi ) the mole fraction of alcohol in the interfacial layer, XAS = S + AS ) ASM /(SM M
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
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