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o microemulsion with different amphiphiles and oils
Colloids and Surfaces A: Physicochem. Eng. Aspects 247 (2004) 99–103
Interfacial composition and formation of w/o microemulsion with different amphiphiles and oils Yuksel Bayrak∗ Department of Chemistry, Trakya University, 22030 Edirne, Turkey Received 29 April 2004; accepted 21 August 2004 Available online 28 September 2004
Abstract Microemulsions were produced by mixing different combinations of the non-ionic surfactants Triton X-100 and Triton X-405, n-alkanes (C6, C7, C8 and C10) and benzene as oils, n-alcohols (C5 and C7) as cosurfactants with water. The influence of chainlength of the alkanes and alcohols on water solubilization behavior of these systems has been investigated. The solubilization of water in a particular microemulsion is governed by the partitioning of alcohols among oil, water and interfacial phases, depending on the chainlength and nature of oil and alcohol, and their interaction with the surfactant. The molar ratio of alcohol to surfactant at the droplet interface increased with the length of the oil chain. The free energy changes accompanying cosurfactant adsorption at the interface have also been computed. © 2004 Elsevier B.V. All rights reserved. Keywords: Cosurfactant; Interface; Microemulsion; Solubilization; Surfactant
1. Introduction Microemulsions are isotropic, clear or translucent, thermodynamically stable dispersions of oil, water, emulsifier, usually salt, and often a small amphiphilic molecule called a cosurfactant [1–5]. The droplet diameters in microemul˚ The droplets are solubilized sions range from 100 to 1000 A. by a mixed interfacial film of surfactant and cosurfactant. A number of studies have been carried out on microemulsions with alcohols as cosurfactant [6–9]. Recently, monohexyl and monobutyl [10] ethers of ethylene or diethylene glycol have been used as cosurfactants. The alkyl chainlengths of oil and cosurfactant strongly influence the interfacial composition and distribution of cosurfactant in the oil and water phases [11,12,16]. The formation and various physicochemical properties of the microemulsions are influenced by the alkyl chainlength of alcohol and hydrocarbons [10–12]. The purpose of this study was formation and characterization of microemulsions composed of non-ionic surfactants ∗
Corresponding author. Tel.: +90 284 212 0923; fax: +90 284 213 7053. E-mail address: [email protected].
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.08.019
Triton X-100 and Triton X-405, water, n-alkanes (C6, C7, C8 and C10)/benzene and pentanol, hexanol and heptanol.
2. Experimental 2.1. Materials Triton X-100 and Triton X-405 were obtained from Merck (Darmstadt, Germany). Other chemicals were also purchased from Merck. Distille water, redistilled from alkaline potassium permanganate was used. 2.2. Methods Microemulsions were produced by titrating a coarse emulsion of oil–water-surfactant with cosurfactant, until the mixture became clear. Addition of oil in small amounts turned the mixture turbid. The entire contents were mixed thoroughly with a vortex and titrated with cosurfactant until the mixture was transparent again. For water solubilization studies, maximum water solubilization in the microemulsion was obtained
100
Y. Bayrak / Colloids and Surfaces A: Physicochem. Eng. Aspects 247 (2004) 99–103
Table 1 Water solubilization capacities of n-pentanol, n-hexanol and n-heptanol microemulsion systems composed of 1 g surfactant (fixed), 10 mL n-alkane or benzene (fixed) 5 mL alcohol (fixed) and water at 25 ◦ C Oil
Water solubilized (mL) Triton X-100
Hexane Heptane Octane Decane Benzene
Triton X-405
n-Pentanol
n-Hexanol
n-Heptanol
n-Pentanol
n-Hexanol
n-Heptanol
9.99 10.54 11.21 13.34 7.64
11.80 12.59 13.69 14.84 6.21
13.89 15.17 16.12 19.23 4.89
11.18 11.37 11.49 13.82 8.21
10.52 10.89 11.62 12.95 6.49
9.83 10.08 10.27 10.89 5.24
from the titration of initially formed microemulsion with water until turbidity commenced. At the endpoint, the systems were initially turbid, but after a few minutes standing, two clear phases were formed which, on further shaking, resulted in a turbid solution. All titration experiments were carried out at 25 ± 0.1 ◦ C.
K=
noa no
(2)
where no is the number of moles of oil. By combining Eqs. (1) and (2) and dividing by moles of surfactant, ns , we get
3. Results and discussion Table 1 also shows clearly that the water solubility in benzene microemulsions is much lower than in n-alkanecontaining systems. This is possibly due to a difference in the interaction based on the structural difference of oils, such as chainlength and polarity, with the head-group of the surfactant, which influences the partitioning of alcohol in the continuous and interfacial phases. Similar interaction effects have been reported for polyoxyethylene microemulsions, and it was found that the cosurfactant preferentially partitioned at the interface for alkanes than for benzene [13]. Table 1 also shows that water solubilization is higher for both n-hexanol and n-heptanol/Triton X-100 microemulsions, while it is lower in a Triton X-405 microemulsion. The opposite behavior was observed with n-pentanol systems. Because the head-group size of the surfactant is one of the factors that decide the packing of molecules at the droplet interface (I), would expect a difference of packing in the case of Triton X-100 and Triton X-405. Of course, with Triton X-405 there would be delocalization of charge as well as charge-shielding in comparison to Triton X-100. This is the reason for the opposite behavior of these two surfactants. The interrelated factors undoubtedly affect interactions between alcohols and water and, therefore, the water-solubilizing capacity. The partitioning of alcohol at the interface was determined by the titration method [11]. The present microemulsion system may be considered to be made up of three phases, namely continuous oil phase, dispersed water phase and the interfacial phase. Assuming that the alcohol is distributed in these phases, the total concentration of alcohol, na , can be written as [14,15] na = noa + nda + nia
where noa , nda and nia are the number of moles of alcohol in the oil, dispersed (water) and interfacial phases, respectively. Further, because the solubility of alcohol is constant in the continuous (oil) phase, it can be written as:
(1)
na na nd + nia =K + a ns ns ns
(3)
Eq. (3) suggests that a plot of na /ns versus no /ns would give a straight line with slope (K) and intercept (I), where I = (nad + nia )/ns . Assuming that the solubility of these alcohols in water is negligible (nda = 0), then I simply gives the number of moles of alcohol per mole of surfactant at the interface (nia /ns ). Plots of na /ns versus no /ns for different oil, n-pentanol and surfactant combinations are shown in Fig. 1
Fig. 1. Plots of na /ns vs. no /ns for microemulsion systems composed of 1 g Triton X-100 (fixed), 1 g water (fixed), oil and n-pentanol at 25 ◦ C. (): Hexane, (䊉): heptane, (): octane, (): decane, (): benzene.
Y. Bayrak / Colloids and Surfaces A: Physicochem. Eng. Aspects 247 (2004) 99–103
Fig. 2. Plots of na /ns vs. no /ns for microemulsion systems composed of 1 g Triton X-405 (fixed), 1 g water (fixed), oil and n-pentanol at 25 ◦ C. (): Hexane, (䊉): heptane, (): octane, (): decane, (): benzene.
and Fig. 2. Similar plots were obtained for n-hexanol and nheptanol microemulsions but are not shown here. From the slope, K and intercept I of these straight-line plots, the mole fraction of alcohol at the interface, Xai , and in the continuous oil phase, Xao , were calculated [15] Xai =
I (I + 1)
(4)
Xao =
K (K + 1)
(5)
Values of I, K, mole fraction of alcohol at the interface (Xai ) and mole fraction of alcohol in the continuous phase (Xao ) for Triton X-100 and Triton X-405 microemulsion systems are given in Tables 2 and 3, respectively. The experimental results presented in Tables 2 and 3 show many interesting features of microemulsion formation. It is clear that the mole fraction of alcohol, Xai , and number of moles of alcohol per mole of surfactant, I, at the interface increase with an increase in the oil chainlength. It may be
101
concluded that the partitioning of alcohols at the interface is favored for higher-chainlength oils. A similar type of behavior was reported for sodium stearate and cetyltrimethylammonium bromide (CTAB) microemulsions with mediumchainlength alcohols as cosurfactants [15]. I (nia /ns ) values for medium-chainlength alcohols [15] and alcohols for Triton X-100 microemulsions are compared in Table 4. Water solubilization limits in n-pentanol-containing microemulsion systems prepared with the same quantity of CTAB and n-alkanes (as used for alcohol microemulsion) were reported to be 4.45, 4.55 and 5.10 mL for n-pentane, n-hexane and n-heptane systems, respectively. From Table 1, water solubilization in Triton X-100 and Triton X-405 microemulsion systems seems to be very high in comparison with CTAB systems [15]. Tables 2 and 3 show that Xai values increase with an increase in chainlength of the alcohol and oil for both Triton X-100 and Triton X-405 microemulsions. The standard free energy change, Gos , for transfer of alcohol from the continuous oil phase to the interfacial region was calculated from the relation [15] Xai o Gs = −RT ln (6) Xao where, T is the experimental temperature. The calculated values of Gos from Eq. (6) are tabulated in Table 5. The negative value of Gos suggests that microemulsions form spontaneously. Table 5 shows that the free energy change increases with an increase in chainlength of the alcohol for both Triton X-100 and Triton X-405 microemulsions, whereas reverse behavior is observed with an increase in chainlength of the alcohols. This suggests that the association between emulsifier and alcohol at the interface becomes more favored in higher-chainlength oils. Microemulsion structures are not completely described in the literature at the molecular level. Therefore, no satisfactory theory is available to explain the Gos values. However, in all surface chemical systems where amphiphilic molecules are used, it has always been the practice to measure the effect of alkyl chainlength on the free energy of the system [15]. The standard free energy, Gos , for transfer of alcohol is found to increase linearly with the number of carbon atoms,
Table 2 Intercept (I = nia /ns ) and slope (K = noa /no ) of plots of na /ns vs. no /ns and mole fraction of alcohol at the interface (Xai ) and in continuous oil phase (Xao ) for the microemulsion system composed of 1 g Triton X-100 (fixed), oil and alcohol at 25 ◦ C Oil
Hexane Heptane Octane Decane Benzene
n-Pentanol
n-Hexanol
I
K
Xai
3.26 3.58 3.88 4.48 2.32
0.1055 0.1148 0.1246 0.1457 0.1051
0.7653 0.7817 0.7951 0.8175 0.6988
Xao
I
0.0969 0.1046 0.1125 0.1292 0.0951
4.02 4.39 4.81 5.54 1.97
n-Heptanol
K
Xai
Xao
I
K
Xai
Xao
0.0878 0.0951 0.1031 0.1206 0.1011
0.8008 0.8145 0.8279 0.8471 0.6633
0.0807 0.0868 0.0934 0.1076 0.0918
4.70 5.12 5.58 6.46 1.61
0.0724 0.0790 0.0861 0.1021 0.0951
0.8246 0.8366 0.8480 0.8660 0.6169
0.0675 0.0732 0.0793 0.0926 0.0868
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Y. Bayrak / Colloids and Surfaces A: Physicochem. Eng. Aspects 247 (2004) 99–103
Table 3 Intercept (I = nia /ns ) and slope (K = noa /no ) of plots of na /ns vs. no /ns and mole fraction of alcohol at the interface (Xai ) and in continuous oil phase (Xao ) for the microemulsion system composed of 1 g Triton X-405 (fixed), oil and alcohol at 25 ◦ C Oil
n-Pentanol
Hexane Heptane Octane Decane Benzene
n-Hexanol
n-Heptanol
I
K
Xai
Xao
I
K
Xai
Xao
I
K
Xai
Xao
3.39 3.73 4.05 4.69 2.45
0.1073 0.1168 0.1267 0.1484 0.1025
0.7722 0.7886 0.8020 0.8243 0.7102
0.0969 0.1046 0.1125 0.1292 0.0930
4.58 5.04 5.46 6.33 2.28
0.1031 0.1111 0.1210 0.1414 0.1069
0.8208 0.8344 0.8452 0.8636 0.6951
0.0935 0.1000 0.1079 0.1239 0.0966
5.15 5.59 6.56 7.97 1.84
0.0983 0.1062 0.1167 0.1373 0.1088
0.8374 0.8483 0.8677 0.8885 0.6479
0.0895 0.0960 0.1045 0.1207 0.0981
Table 4 Comparison of the values for moles of cosurfactant per mole of surfactant, nicos /ns for different medium-chainlength alcohols in CTAB microemulsionsa and alcohols in the, microemulsion system composed of 1 g Triton X-100, 1 g water, oil and cosurfactant Oil
n-Pentanol
n-Hexanol
n-Heptanol
n-Pentanola
n-Hexanola
n-Heptanola
Hexane Heptane Octane Decane Benzene
3.26 3.58 3.88 4.48 1.61
4.02 4.39 4.81 5.54 1.97
4.70 5.12 5.58 6.46 2.32
1.64 1.97 2.11 2.49 1.52
1.45 2.42 2.44 2.59 1.00
1.38 2.21 2.47 2.63 0.50
a
Reference [15].
Table 5 The standard free energy of transfer, Gos , for the microemulsion system composed of 1 g surfactant (fixed), 1 g water (fixed), oil and alcohol at 25 ◦ C Oil
−Gos (kJ mol−1 ) Triton X-100
Hexane Heptane Octane Decane Benzene
Triton X-405
n-Pentanol
n-Hexanol
n-Heptanol
n-Pentanol
n-Hexanol
n-Heptanol
5.1608 5.0235 4.8863 4.6117 4.8612
5.6875 5.5504 5.4025 5.1145 4.9056
6.2030 6.0381 5.8731 5.5431 4.9439
5.1450 5.0008 4.8688 4.5937 4.6784
5.3861 5.2584 5.1022 4.8137 4.8918
5.5432 5.4012 5.2468 4.9493 5.0406
nc , in the alkyl chain of the oil phase (Fig. 3). In Triton X100 microemulsions, when hexane was used as a oil, the free energy per methylene group, Gs /CH2 of the oil phase (for n-alkanes), has been estimated to be −516.08, −568.75 and −620 J/mol for n-pentanol, n-hexanol and n-heptanol, respectively. In Triton X-405 microemulsions, these values are −128.62, −134.65 and −138.58 J/mol for same cosurfactants, respectively. These results clearly indicate that Gs /CH2 values depend much more on the chainlength of alkane than on the alkanol chain length. Table 5 also shows that Gos values are less negative for Triton X-405 systems than for Triton X-100 systems, indicating that the alcohols are preferentially associated with Triton X-405 than with Triton X-100 surfactants. The partitioning of cosurfactant at the microemulsion droplet interface is responsible for higher water solubilization.
Fig. 3. Variation of Gos with the number of carbon atoms, nc , in the alkyl chain of the n-alkane (in Triton X-100 microemulsions (): n-pentanol, (䊉): n-hexanol, (): n-heptanol); (in Triton X-405 microemulsions (): npentanol, (): n-hexanol, () : n-heptanol).
References [1] R. Aveyard, B.P. Binks, S. Clark, P.D.I. Fletcher, J. Chem. Soc. Faraday Trans. 86 (1990) 311–315. [2] W.H. Lim, Langmuir 15 (1999) 7176–7179.
Y. Bayrak / Colloids and Surfaces A: Physicochem. Eng. Aspects 247 (2004) 99–103 [3] [4] [5] [6] [7] [8] [9] [10]
M. Kahlweit, R. Strey, D. Haase, J. Phys. Chem. 89 (1985) 163–171. H. Kleinert, J. Phys. Chem. 84 (1986) 964–967. M. Gradzielski, H. Hoffmann, J. Phys. Chem. 98 (1994) 2613–2623. D. Donescu, L. Fusulan, M. Vasilescu, J. Dispersion Sci. Technol. 22 (2001) 231–244. A.S. Muller, J. Gagnaire, Y. Queneau, J. Colloid Surface A 203 (2002) 55–66. L. Foret, A. Wurger, J. Phys. Rev. Lett. 86 (2001) 5930–5933. T.Q. Liu, A.H. Zou, R. Guo, Acta Phys. Chim. Sin. 16 (2000) 899–905. C.H. Chew, L.M. Gan, L. Koh, M.K. Wong, J. Dispersion Sci. Technol. 9 (1988) 17–31.
103
[11] L. Digout, K. Bren, R. Palepu, J. Colloid Polym. Sci. 279 (2001) 655–663. [12] C.S. Chern, L.J. Wu, J. Polym. Sci. Polym. Chem. 39 (2001) 898–912. [13] H. Arai, K. Shinoda, J. Colloid Interf. Sci. 25 (1967) 396–400. [14] S.P. Moulik, L. Digout, W.M. Aylward, R. Palepu, Langmuir 16 (2000) 3101–3106. [15] H.N. Singh, Ch.D. Prasad, S. Kumar, J. Am. Oil Chem. Soc. 70 (1993) 69–73. [16] S.K. Hait, S.P. Moulik, Langmuir 18 (2002) 6736–6744.
Interfacial composition and formation of w/o microemulsion with different amphiphiles and oils Yuksel Bayrak∗ Department of Chemistry, Trakya University, 22030 Edirne, Turkey Received 29 April 2004; accepted 21 August 2004 Available online 28 September 2004
Abstract Microemulsions were produced by mixing different combinations of the non-ionic surfactants Triton X-100 and Triton X-405, n-alkanes (C6, C7, C8 and C10) and benzene as oils, n-alcohols (C5 and C7) as cosurfactants with water. The influence of chainlength of the alkanes and alcohols on water solubilization behavior of these systems has been investigated. The solubilization of water in a particular microemulsion is governed by the partitioning of alcohols among oil, water and interfacial phases, depending on the chainlength and nature of oil and alcohol, and their interaction with the surfactant. The molar ratio of alcohol to surfactant at the droplet interface increased with the length of the oil chain. The free energy changes accompanying cosurfactant adsorption at the interface have also been computed. © 2004 Elsevier B.V. All rights reserved. Keywords: Cosurfactant; Interface; Microemulsion; Solubilization; Surfactant
1. Introduction Microemulsions are isotropic, clear or translucent, thermodynamically stable dispersions of oil, water, emulsifier, usually salt, and often a small amphiphilic molecule called a cosurfactant [1–5]. The droplet diameters in microemul˚ The droplets are solubilized sions range from 100 to 1000 A. by a mixed interfacial film of surfactant and cosurfactant. A number of studies have been carried out on microemulsions with alcohols as cosurfactant [6–9]. Recently, monohexyl and monobutyl [10] ethers of ethylene or diethylene glycol have been used as cosurfactants. The alkyl chainlengths of oil and cosurfactant strongly influence the interfacial composition and distribution of cosurfactant in the oil and water phases [11,12,16]. The formation and various physicochemical properties of the microemulsions are influenced by the alkyl chainlength of alcohol and hydrocarbons [10–12]. The purpose of this study was formation and characterization of microemulsions composed of non-ionic surfactants ∗
Corresponding author. Tel.: +90 284 212 0923; fax: +90 284 213 7053. E-mail address: [email protected].
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.08.019
Triton X-100 and Triton X-405, water, n-alkanes (C6, C7, C8 and C10)/benzene and pentanol, hexanol and heptanol.
2. Experimental 2.1. Materials Triton X-100 and Triton X-405 were obtained from Merck (Darmstadt, Germany). Other chemicals were also purchased from Merck. Distille water, redistilled from alkaline potassium permanganate was used. 2.2. Methods Microemulsions were produced by titrating a coarse emulsion of oil–water-surfactant with cosurfactant, until the mixture became clear. Addition of oil in small amounts turned the mixture turbid. The entire contents were mixed thoroughly with a vortex and titrated with cosurfactant until the mixture was transparent again. For water solubilization studies, maximum water solubilization in the microemulsion was obtained
100
Y. Bayrak / Colloids and Surfaces A: Physicochem. Eng. Aspects 247 (2004) 99–103
Table 1 Water solubilization capacities of n-pentanol, n-hexanol and n-heptanol microemulsion systems composed of 1 g surfactant (fixed), 10 mL n-alkane or benzene (fixed) 5 mL alcohol (fixed) and water at 25 ◦ C Oil
Water solubilized (mL) Triton X-100
Hexane Heptane Octane Decane Benzene
Triton X-405
n-Pentanol
n-Hexanol
n-Heptanol
n-Pentanol
n-Hexanol
n-Heptanol
9.99 10.54 11.21 13.34 7.64
11.80 12.59 13.69 14.84 6.21
13.89 15.17 16.12 19.23 4.89
11.18 11.37 11.49 13.82 8.21
10.52 10.89 11.62 12.95 6.49
9.83 10.08 10.27 10.89 5.24
from the titration of initially formed microemulsion with water until turbidity commenced. At the endpoint, the systems were initially turbid, but after a few minutes standing, two clear phases were formed which, on further shaking, resulted in a turbid solution. All titration experiments were carried out at 25 ± 0.1 ◦ C.
K=
noa no
(2)
where no is the number of moles of oil. By combining Eqs. (1) and (2) and dividing by moles of surfactant, ns , we get
3. Results and discussion Table 1 also shows clearly that the water solubility in benzene microemulsions is much lower than in n-alkanecontaining systems. This is possibly due to a difference in the interaction based on the structural difference of oils, such as chainlength and polarity, with the head-group of the surfactant, which influences the partitioning of alcohol in the continuous and interfacial phases. Similar interaction effects have been reported for polyoxyethylene microemulsions, and it was found that the cosurfactant preferentially partitioned at the interface for alkanes than for benzene [13]. Table 1 also shows that water solubilization is higher for both n-hexanol and n-heptanol/Triton X-100 microemulsions, while it is lower in a Triton X-405 microemulsion. The opposite behavior was observed with n-pentanol systems. Because the head-group size of the surfactant is one of the factors that decide the packing of molecules at the droplet interface (I), would expect a difference of packing in the case of Triton X-100 and Triton X-405. Of course, with Triton X-405 there would be delocalization of charge as well as charge-shielding in comparison to Triton X-100. This is the reason for the opposite behavior of these two surfactants. The interrelated factors undoubtedly affect interactions between alcohols and water and, therefore, the water-solubilizing capacity. The partitioning of alcohol at the interface was determined by the titration method [11]. The present microemulsion system may be considered to be made up of three phases, namely continuous oil phase, dispersed water phase and the interfacial phase. Assuming that the alcohol is distributed in these phases, the total concentration of alcohol, na , can be written as [14,15] na = noa + nda + nia
where noa , nda and nia are the number of moles of alcohol in the oil, dispersed (water) and interfacial phases, respectively. Further, because the solubility of alcohol is constant in the continuous (oil) phase, it can be written as:
(1)
na na nd + nia =K + a ns ns ns
(3)
Eq. (3) suggests that a plot of na /ns versus no /ns would give a straight line with slope (K) and intercept (I), where I = (nad + nia )/ns . Assuming that the solubility of these alcohols in water is negligible (nda = 0), then I simply gives the number of moles of alcohol per mole of surfactant at the interface (nia /ns ). Plots of na /ns versus no /ns for different oil, n-pentanol and surfactant combinations are shown in Fig. 1
Fig. 1. Plots of na /ns vs. no /ns for microemulsion systems composed of 1 g Triton X-100 (fixed), 1 g water (fixed), oil and n-pentanol at 25 ◦ C. (): Hexane, (䊉): heptane, (): octane, (): decane, (): benzene.
Y. Bayrak / Colloids and Surfaces A: Physicochem. Eng. Aspects 247 (2004) 99–103
Fig. 2. Plots of na /ns vs. no /ns for microemulsion systems composed of 1 g Triton X-405 (fixed), 1 g water (fixed), oil and n-pentanol at 25 ◦ C. (): Hexane, (䊉): heptane, (): octane, (): decane, (): benzene.
and Fig. 2. Similar plots were obtained for n-hexanol and nheptanol microemulsions but are not shown here. From the slope, K and intercept I of these straight-line plots, the mole fraction of alcohol at the interface, Xai , and in the continuous oil phase, Xao , were calculated [15] Xai =
I (I + 1)
(4)
Xao =
K (K + 1)
(5)
Values of I, K, mole fraction of alcohol at the interface (Xai ) and mole fraction of alcohol in the continuous phase (Xao ) for Triton X-100 and Triton X-405 microemulsion systems are given in Tables 2 and 3, respectively. The experimental results presented in Tables 2 and 3 show many interesting features of microemulsion formation. It is clear that the mole fraction of alcohol, Xai , and number of moles of alcohol per mole of surfactant, I, at the interface increase with an increase in the oil chainlength. It may be
101
concluded that the partitioning of alcohols at the interface is favored for higher-chainlength oils. A similar type of behavior was reported for sodium stearate and cetyltrimethylammonium bromide (CTAB) microemulsions with mediumchainlength alcohols as cosurfactants [15]. I (nia /ns ) values for medium-chainlength alcohols [15] and alcohols for Triton X-100 microemulsions are compared in Table 4. Water solubilization limits in n-pentanol-containing microemulsion systems prepared with the same quantity of CTAB and n-alkanes (as used for alcohol microemulsion) were reported to be 4.45, 4.55 and 5.10 mL for n-pentane, n-hexane and n-heptane systems, respectively. From Table 1, water solubilization in Triton X-100 and Triton X-405 microemulsion systems seems to be very high in comparison with CTAB systems [15]. Tables 2 and 3 show that Xai values increase with an increase in chainlength of the alcohol and oil for both Triton X-100 and Triton X-405 microemulsions. The standard free energy change, Gos , for transfer of alcohol from the continuous oil phase to the interfacial region was calculated from the relation [15] Xai o Gs = −RT ln (6) Xao where, T is the experimental temperature. The calculated values of Gos from Eq. (6) are tabulated in Table 5. The negative value of Gos suggests that microemulsions form spontaneously. Table 5 shows that the free energy change increases with an increase in chainlength of the alcohol for both Triton X-100 and Triton X-405 microemulsions, whereas reverse behavior is observed with an increase in chainlength of the alcohols. This suggests that the association between emulsifier and alcohol at the interface becomes more favored in higher-chainlength oils. Microemulsion structures are not completely described in the literature at the molecular level. Therefore, no satisfactory theory is available to explain the Gos values. However, in all surface chemical systems where amphiphilic molecules are used, it has always been the practice to measure the effect of alkyl chainlength on the free energy of the system [15]. The standard free energy, Gos , for transfer of alcohol is found to increase linearly with the number of carbon atoms,
Table 2 Intercept (I = nia /ns ) and slope (K = noa /no ) of plots of na /ns vs. no /ns and mole fraction of alcohol at the interface (Xai ) and in continuous oil phase (Xao ) for the microemulsion system composed of 1 g Triton X-100 (fixed), oil and alcohol at 25 ◦ C Oil
Hexane Heptane Octane Decane Benzene
n-Pentanol
n-Hexanol
I
K
Xai
3.26 3.58 3.88 4.48 2.32
0.1055 0.1148 0.1246 0.1457 0.1051
0.7653 0.7817 0.7951 0.8175 0.6988
Xao
I
0.0969 0.1046 0.1125 0.1292 0.0951
4.02 4.39 4.81 5.54 1.97
n-Heptanol
K
Xai
Xao
I
K
Xai
Xao
0.0878 0.0951 0.1031 0.1206 0.1011
0.8008 0.8145 0.8279 0.8471 0.6633
0.0807 0.0868 0.0934 0.1076 0.0918
4.70 5.12 5.58 6.46 1.61
0.0724 0.0790 0.0861 0.1021 0.0951
0.8246 0.8366 0.8480 0.8660 0.6169
0.0675 0.0732 0.0793 0.0926 0.0868
102
Y. Bayrak / Colloids and Surfaces A: Physicochem. Eng. Aspects 247 (2004) 99–103
Table 3 Intercept (I = nia /ns ) and slope (K = noa /no ) of plots of na /ns vs. no /ns and mole fraction of alcohol at the interface (Xai ) and in continuous oil phase (Xao ) for the microemulsion system composed of 1 g Triton X-405 (fixed), oil and alcohol at 25 ◦ C Oil
n-Pentanol
Hexane Heptane Octane Decane Benzene
n-Hexanol
n-Heptanol
I
K
Xai
Xao
I
K
Xai
Xao
I
K
Xai
Xao
3.39 3.73 4.05 4.69 2.45
0.1073 0.1168 0.1267 0.1484 0.1025
0.7722 0.7886 0.8020 0.8243 0.7102
0.0969 0.1046 0.1125 0.1292 0.0930
4.58 5.04 5.46 6.33 2.28
0.1031 0.1111 0.1210 0.1414 0.1069
0.8208 0.8344 0.8452 0.8636 0.6951
0.0935 0.1000 0.1079 0.1239 0.0966
5.15 5.59 6.56 7.97 1.84
0.0983 0.1062 0.1167 0.1373 0.1088
0.8374 0.8483 0.8677 0.8885 0.6479
0.0895 0.0960 0.1045 0.1207 0.0981
Table 4 Comparison of the values for moles of cosurfactant per mole of surfactant, nicos /ns for different medium-chainlength alcohols in CTAB microemulsionsa and alcohols in the, microemulsion system composed of 1 g Triton X-100, 1 g water, oil and cosurfactant Oil
n-Pentanol
n-Hexanol
n-Heptanol
n-Pentanola
n-Hexanola
n-Heptanola
Hexane Heptane Octane Decane Benzene
3.26 3.58 3.88 4.48 1.61
4.02 4.39 4.81 5.54 1.97
4.70 5.12 5.58 6.46 2.32
1.64 1.97 2.11 2.49 1.52
1.45 2.42 2.44 2.59 1.00
1.38 2.21 2.47 2.63 0.50
a
Reference [15].
Table 5 The standard free energy of transfer, Gos , for the microemulsion system composed of 1 g surfactant (fixed), 1 g water (fixed), oil and alcohol at 25 ◦ C Oil
−Gos (kJ mol−1 ) Triton X-100
Hexane Heptane Octane Decane Benzene
Triton X-405
n-Pentanol
n-Hexanol
n-Heptanol
n-Pentanol
n-Hexanol
n-Heptanol
5.1608 5.0235 4.8863 4.6117 4.8612
5.6875 5.5504 5.4025 5.1145 4.9056
6.2030 6.0381 5.8731 5.5431 4.9439
5.1450 5.0008 4.8688 4.5937 4.6784
5.3861 5.2584 5.1022 4.8137 4.8918
5.5432 5.4012 5.2468 4.9493 5.0406
nc , in the alkyl chain of the oil phase (Fig. 3). In Triton X100 microemulsions, when hexane was used as a oil, the free energy per methylene group, Gs /CH2 of the oil phase (for n-alkanes), has been estimated to be −516.08, −568.75 and −620 J/mol for n-pentanol, n-hexanol and n-heptanol, respectively. In Triton X-405 microemulsions, these values are −128.62, −134.65 and −138.58 J/mol for same cosurfactants, respectively. These results clearly indicate that Gs /CH2 values depend much more on the chainlength of alkane than on the alkanol chain length. Table 5 also shows that Gos values are less negative for Triton X-405 systems than for Triton X-100 systems, indicating that the alcohols are preferentially associated with Triton X-405 than with Triton X-100 surfactants. The partitioning of cosurfactant at the microemulsion droplet interface is responsible for higher water solubilization.
Fig. 3. Variation of Gos with the number of carbon atoms, nc , in the alkyl chain of the n-alkane (in Triton X-100 microemulsions (): n-pentanol, (䊉): n-hexanol, (): n-heptanol); (in Triton X-405 microemulsions (): npentanol, (): n-hexanol, () : n-heptanol).
References [1] R. Aveyard, B.P. Binks, S. Clark, P.D.I. Fletcher, J. Chem. Soc. Faraday Trans. 86 (1990) 311–315. [2] W.H. Lim, Langmuir 15 (1999) 7176–7179.
Y. Bayrak / Colloids and Surfaces A: Physicochem. Eng. Aspects 247 (2004) 99–103 [3] [4] [5] [6] [7] [8] [9] [10]
M. Kahlweit, R. Strey, D. Haase, J. Phys. Chem. 89 (1985) 163–171. H. Kleinert, J. Phys. Chem. 84 (1986) 964–967. M. Gradzielski, H. Hoffmann, J. Phys. Chem. 98 (1994) 2613–2623. D. Donescu, L. Fusulan, M. Vasilescu, J. Dispersion Sci. Technol. 22 (2001) 231–244. A.S. Muller, J. Gagnaire, Y. Queneau, J. Colloid Surface A 203 (2002) 55–66. L. Foret, A. Wurger, J. Phys. Rev. Lett. 86 (2001) 5930–5933. T.Q. Liu, A.H. Zou, R. Guo, Acta Phys. Chim. Sin. 16 (2000) 899–905. C.H. Chew, L.M. Gan, L. Koh, M.K. Wong, J. Dispersion Sci. Technol. 9 (1988) 17–31.
103
[11] L. Digout, K. Bren, R. Palepu, J. Colloid Polym. Sci. 279 (2001) 655–663. [12] C.S. Chern, L.J. Wu, J. Polym. Sci. Polym. Chem. 39 (2001) 898–912. [13] H. Arai, K. Shinoda, J. Colloid Interf. Sci. 25 (1967) 396–400. [14] S.P. Moulik, L. Digout, W.M. Aylward, R. Palepu, Langmuir 16 (2000) 3101–3106. [15] H.N. Singh, Ch.D. Prasad, S. Kumar, J. Am. Oil Chem. Soc. 70 (1993) 69–73. [16] S.K. Hait, S.P. Moulik, Langmuir 18 (2002) 6736–6744.