Langmuir 1997, 13, 4251-4255
4251
Comparative Phase Behavior About the L2 Phase of Ternary and Quaternary Systems of Triton X-100 and Its Separated p-tert-OPEn (n ) 5, 7, and 9) Components in Cyclohexane Jingyuan Gu and Z. A. Schelly* Center for Colloidal and Interfacial Dynamics, Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019-0065 Received November 29, 1996. In Final Form: May 18, 1997X The nonionic surfactant Triton X-100 (TX-100) is separated by preparative column chromatography into its poly(oxyethylene tert-octylphenyl ether) (OPEn) components. The phase behavior in the neighborhood of the L2 phase is investigated for ternary (cyclohexane/H2O) and quaternary (cyclohexane/H2O/n-hexanol) systems of TX-100 and OPEn (n ) 5, 7 and 9). The extent of the L2 domain and the maximum amount of water wo,max that can be solubilized by isotropic solutions strongly depend on the concentration and the length (n) of the polar chain of the surfactant, temperature, and the amount of n-hexanol present. Depending on these variables, with increasing wo (molar ratio of water to surfactant) some of the following phase transitions occur: L2 f LC f L f liquid/gel f gel. Except for the liquid crystalline LC phase, all phases involved are optically isotropic, and the liquid L phase is most likely bicontinuous. Since n-hexanol is a better solvent than cyclohexane for both the surfactant and water, it destabilizes the reverse micellar and LC assemblies. Hence, its effect on the systems investigated is more prominently that of a “cosolvent” than a cosurfactant.
Introduction The commercially available nonionic surfactant Triton X-100 (TX-100) is a mixture of polyoxyethylene tertoctylphenyl ethers (OPEn), with an average of n ) 9.5 oxyethylene (OE) units in the molecules
CH3C(CH3)2CH2C(CH3)2C6H4(OCH2CH2-)n)9.5OH and with an approximately Poisson distribution of molecular weights.1 Although homogeneous (single species) OPEs can be synthesized,2 in practical use (in aqueous solution) the normal distribution homologs offer certain advantages such as greater foam and emulsion stability and greater detersive power.3 In previous studies of TX100 reverse micellar systems, we found through mapping the micropolarity of the aggregates that nonpolar solvents such as cyclohexane4 and benzene/n-hexane5 can penetrate into the polar core of the reverse micelles. The extent of penetration depends, among other factors, on the water content wo (molar ratio of water to surfactant) of the solution. Controlled partial pressure-vapor pressure osmometry (CPP-VPO)6,7 and quasi-elastic light scattering measurements revealed that TX-100 reverse micelles in cyclohexane are nonspherical and that their size and mean aggregation number N are functions of wo, temperature, and the presence of an electrolyte.8 Although TX-100 does not aggregate either in pure benzene or in n-hexane, it does form reveres micelles in mixtures of the two solvents in the composition range 20-50% (v/v) benzene.9 X
Abstract published in Advance ACS Abstracts, July 15, 1997.
(1) Kelly, J.; Greenwald, H. N. J. Phys. Chem. 1958, 62, 1096. (2) Crook, E. H.; Fordyce, D. B.; Trebbi, G. F. J. Phys. Chem. 1963, 67, 1987 and references therein. (3) Crook, E. H.; Fordyce, D. B.; Trebbi, G. F. J. Am. Oil Chem. Soc. 1964, 20, 231. (4) Zhu, D.-M.; Schelly, Z. A. Langmuir 1992, 8, 48. (5) Zhu, D.-M.; Wu, X.; Schelly, Z. A. J. Phys. Chem. 1992, 96, 7121. (6) Ueda, M.; Schelly, Z. A. J. Colloid Interface Sci. 1988, 124, 673. (7) Ueda, M.; Schelly, Z. A. Langmuir 1988, 4, 653. (8) Zhu, D.-M.; Feng, K.-I; Schelly, Z. A. J. Phys. Chem. 1992, 96, 2382. (9) Zhu, D.-M.; Wu, X.; Schelly, Z. A. Langmuir 1992, 8, 1538.
S0743-7463(96)02051-3 CCC: $14.00
In contrast to the structural information available on TX-100 assemblies in the L2 phase (reverse micelle and water-in-oil microemulsion), less is known about the phase behavior of TX-100, or that of poly(oxyethylene alkylphenols) in general,10 in ternary systems. Since much of the phase behavior and the structural characteristics of the assemblies present are expected to depend on the length of the hydrophilic OE chain of the surfactant and its interaction with the solvent,11 in the present study the different OPEn components of TX-100 are separated by preparative column chromatography, and the phase behavior of the pure components are investigated. Specifically, the phase behavior in the vicinity of the L2 phase is reported for ternary and quaternary (with n-hexanol as the fourth component) systems of TX-100 as well as its OPEn (n) 5, 7, and 9) components in cyclohexane. The choice of the particular OPEn investigated was limited by the low solubility in cyclohexane of the oligomers with n g 10, and the insufficient quantities obtained of the compounds with n < 5. The main purpose of the study was to establish the extent of the L2 phase as a function of water content wo and temperature, which is crucial for the characterization of the corresponding reverse micelles and w/o microemulsions. Namely, in the proximity of a phase boundary, usually anomalies in micellar size and in the characteristic time of dynamic processes are observed. Investigations of the reverse micellar aggregates by vapor pressure osmometry, dynamic light scattering, and electric birefringence are presented in the companion paper.12 Experimental Section Materials. TX-100 (density 1.059 g cm-3) was obtained from Kodak, and cyclohexane (HPLC grade) and n-hexanol (density 0.8153 g cm-3, purified) from Baker. Silica gel (Merck, grade 60, 60 Å pore size, 230-400 mesh) was from Aldrich, and ethyl acetate (10) Sjo¨blom, J.; Stenius, P.; Danielsson, I. In Nonionic Surfactants; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; p 369 and references therein. (11) Friberg, S. E. In Interfacial Phenomena in Apolar Media; Eicke, H.-F., Parfitt, G. D., Eds.; Marcel Dekker: New York, 1987. (12) Gu, J.; Schelly, Z. A. Langmuir 1997, 13, 4256.
© 1997 American Chemical Society
4252 Langmuir, Vol. 13, No. 16, 1997 (HPLC grade) was from EM Science. Acetic acid (Du Pont, reagent) was distilled before use. Acetone (HPLC grade) was from Fisher Scientific, and the water used was double deionized and distilled. Separation of TX-100 into Its OPEn Components. Since the individual OPEn components of TX-100 are not available commercially, column chromatography was used for separation on a preparative scale. A 120 cm × 5 cm column (Radnoti Glass Technology) was filled with silica gel to a height of 90 cm. Specific mixtures of ethyl acetate, acetic acid and water were used for gradient elution. Similar to Allen and Rice’s HPLC method,13 different polarity gradients were achieved by adjusting the composition of each eluent. A Model Response UV-vis spectrophotometer (Gilford) was used to monitor (at 277.5 nm, 1 cm pathlength) the OPEn peaks in the eluate. Both the primary and secondary (see below) chromatographic separations were carried out at room temperature and under a pressure of 4-5 psi over atmospheric pressure, corresponding to a ca. 17 mL min-1 flow rate. The stock solution (S) ethyl acetate/acetic acid/ water (100/32/30 by volume) was used as the highest polarity eluent. Eluents with successively lower polarity were prepared by mixing ethyl acetate (E) with the stock solution at various volume ratios E/S. For the peaks 1 (n ) 0) through 7 (n ) 6), a solution with E/S ) 3/1 was used; for peaks 8-9, E/S ) 2/1; for peaks 10-12, E/S ) 1/1; and for peaks 13-15, E/S ) 0.5/1. All the oligomers OPEn with n g 15 were eluted together using pure S. This choice of eluents resulted in a column resolution14 around 1. The lowest resolution (0.7) was obtained between fractions 5 and 6, and the highest resolution (1.7) between fractions 7 and 8. The corresponding fractions collected from 10 separations (each taking 3 days) were combined and individually rechromatographed in secondary runs for further purification. After evaporation of the solvent under reduced pressure, the pure, dry OPEn fractions (ca. 2 g obtained for n ) 5, 3 g for n ) 7, and 5 g for n ) 9) were individually dissolved in cyclohexane. Residual fine particulates of the silica gel were removed through ultracentrifugation for an hour at 4 × 104 rpm (1.5 × 105g). After the solvent of the supernatant was evaporated off, acetone solutions of the pure oligomers were filtered through 0.45 µm pore size membrane. The n values of the pure OPEn oligomers were determined through their UV absorption and confirmed by NMR, MS, and CH elemental analysis. No cross-contamination between the separated OPEn components could be detected above the sensitivity of the these analytical methods used. The OE chain length distribution of the original TX-100 is shown in Figure 1. The number average molecular weight of TX-100 was found to be 620.30, and the average n ) 9.4 similar to that (9.5) obtained previously.13 The water content of dried TX-100 and its pure OPEn (n ) 5, 7, and 9) components was found to be 2 ( 0.5% (w/w) by Karl Fischer titration and CH elemental analysis, respectively. Phase Studies. Solutions of the desired composition were equilibrated and thermostated to (0.1 °C. Occasionally, the use of an ultrasonic bath was necessary to expedite equilibration. The temperature range investigated (20-45 °C) followed that of earlier studies on TX-100/cyclohexane.4,8,15 The choice of surfactant concentrations used was dictated by the minimum concentration necessary for meaningful evaluation of data obtained in other related experiments (such as CPP-VPO, transient electric birefringence, and light scattering).12 Information about the phase behavior of the systems were obtained by visual inspection for turbidity (indicating phase separation) and by optical testing of each system between crossed polarizers for optical anisotropy (pointing to a liquid crystalline phase).
Results and Discussion Following an approach commonly used in studies of reverse micelles and w/o microemulsions, the phase behavior was examined upon successive addition of water to the solution of a particular surfactant (and cosurfactant, if present) in oil. In such a procedure, the molar ratios (13) Allen, C. F; Rice, L. I. J. Chromatogr. 1975, 110, 151. (14) Skoog, D. A.; Leary, J. J. Principles of Instrumental Analysis, 4th ed.; Saunders: Austin, TX, 1992. (15) Kumar, C.; Balasubramanian, D. J. Colloid Interface Sci. 1979, 69, 271.
Gu and Schelly
Figure 1. Distribution of OPEn components in TX-100. (Note: the bar for n ) 15 represents the sum of mole fractions of all components with n g15.)
surfactant/oil, cosurfactant/oil and cosurfactant/surfactant stay constant, and so do the molalities (m) of the surfactant and cosurfactant. Thus, with increasing amount of water added (wo), the successive compositions of the system fall on a straight line pointing toward the water corner of a ternary (or pseudoternary) diagram. An illustration is given in Figure 2. However, due to the limited range of surfactant concentration examined, the data obtained are not suitable for presentation in ternary phase diagrams. Instead, the results are reported in tables, which reflect the irregular, stepwise variation of the experimental parameters and, at the same time, provide accurate information of the conditions for the identity of the corresponding phase(s) observed. In the tables, increasing wo is equivalent to a course along a straight line pointing toward the water corner if plotted in a ternary diagram (e.g., Figure 2). The approximate location of a temperature-dependent phase boundary can easily be perceived from the transition between adjoining symbol fields in the tables. The phase behavior in the vicinity of the L2 phase of TX-100, OPE5, OPE7, and OPE9 under several different conditions is summarized in Tables 1-14. In the tables the dash (-) denotes a clear, optically isotropic solution (L2 or occasionally a bicontinuous L phase), and “x” represents a turbid but optically isotropic solution that ultimately undergoes macroscopic phase separation. Based partly on some of the patterns observed for the analogous poly(oxyethylene isononylphenyl ethers) in cyclohexane,16 a tentative identification of the separating phases (in x) is given in the remarks to the tables. The identity symbol (≡) stands for a single, stable, optically anisotropic phase (liquid crystalline phase, LC), and the designation “liq/gel” indicates a 2-phase system where optically isotropic oil-rich liquid and gel (opaque and dense) phases are in equilibrium. The entries wo ) 0 mean that no water was added; i.e., the actual water content of the
Phase Behavior of Triton X-100 and p-tert-OPEn
Langmuir, Vol. 13, No. 16, 1997 4253 Table 3. Phase Behavior of the OPE7 Ternary System, Cn)7 ) 0.60 ma composition (% by weight)
temperature (°C)
H2O
OPE7
oil
wo
20
25
30
35
0.00 0.82 1.62 2.42 3.20 3.97 4.72 5.47 6.20 6.92 7.63
23.60 23.40 23.21 23.02 22.84 22.66 22.48 22.31 22.13 21.96 21.80
76.40 75.78 75.16 74.56 73.96 73.37 72.80 72.23 71.67 71.12 70.58
0 1 2 3 4 5 6 7 8 9 10
x x x x
x x x
x x x
x x
a
x f L2 + aqueous phase.
Table 4. Phase Behavior of the OPE9 Ternary System, Cn)9 ) 0.60 ma composition (% by weight)
Figure 2. Pseudoternary diagram of the TX-100 quaternary system (CTX-100 ) 0.60 m, Chex/CTX-100 ) 1.00), plotted for the data at 20 °C given in Table 6. Scale: weight fraction. Surfactant (S) ) TX-100 + cosurfactant. To enlarge the relevant portion of the diagram, the lower part of the axes-triangle is not shown. The approximate locations of phase boundaries are marked by line segments parallel to the oil axis. With increasing wo, the sequence of phases observed are L2 f LC f L f X (two-phase) f liq/gel (two-phase). Or, with the symbols used in the table: - f ≡ f -a f x f liq/gel. Table 1. Phase Behavior of the TX-100 Ternary System, CTX-100 ) 0.60 ma composition (% by weight)
temperature (°C)
OPE9
oil
wo
20
25
30
35
0.00 0.79 1.56 2.33 3.08 3.82 4.55 5.26
26.56 26.35 26.15 25.94 25.74 25.55 25.35 25.16
73.44 72.86 72.29 71.73 71.18 70.64 70.10 69.57
0 1 2 3 4 5 6 7
x x x
x x x
x x
x x
a
x f L2 + aqueous phase.
Table 5. Phase Behavior of the OPE5 Ternary System, wo ) 12a composition (% by weight)
H2O
TX-100
oil
wo
20
25
30
35
40
45
0.00 0.78 1.93 3.05 3.79 4.51 5.23 5.93
27.12 26.91 26.60 26.29 26.10 25.90 25.71 25.52
72.88 72.31 71.47 70.65 70.11 69.59 69.07 68.56
0 1 2.5 4 5 6 7 8
x x x x x x x
x x x x x x x
x x
x
x
x
a
temperature (°C)
H2O
x f L2 + aqueous phase.
temperature (°C)
H2O
OPE5
oil
Cn)5 (m)
20
25
30
35
6.88 9.36 11.42 13.16
13.57 18.47 22.54 25.97
79.55 72.17 66.04 60.87
0.40 0.60 0.80 1.00
-
x -
x x x x
x x x x
a
x f L2 + aqueous phase.
Table 6. Phase Behavior of the TX-100 Quaternary System, CTX-100 ) 0.60 m, and Chex/CTX-100 ) 1.00 composition (% by weight)
Table 2. Phase Behavior of the OPE5 Ternary System, Cn)5 ) 0.40 ma composition (% by weight)
temperature (°C)
H2O
OPE5
oil
wo
20
25
30
35
0.00 0.61 1.22 1.81 2.40 2.99 3.56 4.13 4.69 5.25 5.80 6.34 6.88
14.58 14.49 14.40 14.31 14.23 14.14 14.06 13.97 13.89 13.81 13.73 13.65 13.57
85.42 84.90 84.38 83.87 83.37 82.87 82.38 81.89 81.41 80.94 80.47 80.01 79.55
0 1 2 3 4 5 6 7 8 9 10 11 12
-
-
x x x
x x x x x x x
a
x f L2 + aqueous phase.
corresponding solutions is only 2 ( 0.5% of the weight of the surfactant. Ternary Systems. The results for the three-component systems investigated are listed in Tables 1-5 (mainly as a function of added water content wo and temperature), (16) Shinoda K.; Ogawa T. J. Colloid Interface Sci. 1967, 24, 56.
temperature (°C)
H 2O
TX-100
hex
oil
wo
20
25
30
35
0.00 0.75 1.49 2.21 2.93 3.63 5.69 7.01 8.30 10.16 10.77 11.36 11.95 12.53 13.10 14.23 15.86 18.45 20.88 23.17 25.33
25.96 25.77 25.58 25.39 25.20 25.02 24.49 24.14 23.81 23.33 23.17 23.01 22.86 22.71 22.56 22.27 21.85 21.17 20.54 19.95 19.39
4.28 4.24 4.21 4.18 4.15 4.12 4.03 3.98 3.92 3.84 3.81 3.79 3.77 3.74 3.72 3.67 3.60 3.49 3.38 3.29 3.19
69.76 69.24 68.72 68.22 67.72 67.23 65.79 64.87 63.97 62.67 62.25 61.83 61.42 61.02 60.62 59.83 58.70 56.89 55.19 53.60 52.09
0 1 2 3 4 5 8 10 12 15 16 17 18 19 20 22 25 30 35 40 45
≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ -a xb liq/gel liq/gel
≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ -a xb liq/gel
≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ -a xb liq/gel
≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ xb liq/gel
a
Probably a bicontinuous L phase.
b
x f ? + ??.
which reveal some general trends. The domain of the L2 phase depends on the length of the hydrophilic chain of OPEn: We find that the shorter the chain, the larger the reverse micellar domain. Apparently, the greater solubil-
4254 Langmuir, Vol. 13, No. 16, 1997
Gu and Schelly
Table 7. Phase Behavior of the TX-100 Quaternary System, CTX-100 ) 0.60 m and Chex/CTX-100 ) 1.56 composition (% by weight)
Table 10. Phase Behavior of the OPE9 Quaternary System, Cn)9 ) 1.00 m and Chex/Cn)9 ) 1.55
temperature (°C)
composition (% by weight)
H2O
TX-100
hex
oil
wo
20
25
30
35
0.00 6.86 8.12 9.95 10.54 11.13 11.70 12.27 12.84 13.94 15.55 18.10 20.49 22.75 24.89 26.91
25.36 23.62 23.30 22.83 22.68 22.54 22.39 22.24 22.10 21.82 21.41 20.77 20.16 19.59 19.05 18.53
6.51 6.06 5.98 5.86 5.83 5.79 5.75 5.71 5.68 5.60 5.50 5.33 5.18 5.03 4.89 4.76
68.13 63.46 62.60 61.35 60.95 60.55 60.16 59.77 59.39 58.63 57.54 55.80 54.17 52.63 51.17 49.80
0 10 12 15 16 17 18 19 20 22 25 30 35 40 45 50
≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ -a xb gel
≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ -a xb
≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ -a
≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡
a
Probably a bicontinuous L phase.
b
x f ? + ??.
Table 8. Phase Behavior of the OPE9 Quaternary System, Cn)9 ) 0.60 m and Chex/Cn)9 ) 1.00 composition (% by weight)
temperature (°C)
H2O
OPE9
hex
oil
wo
20
25
30
35
5.05 7.06 7.71 8.35 8.99 9.61 10.23 10.84 11.44 13.19 15.96 18.56 21.00 23.30 25.47 27.53
24.13 23.62 23.46 23.29 23.13 22.97 22.82 22.66 22.51 22.07 21.36 20.70 20.08 19.49 18.94 18.42
4.09 4.00 3.98 3.95 3.92 3.89 3.87 3.84 3.82 3.74 3.62 3.51 3.40 3.30 3.21 3.12
66.73 65.31 64.86 64.40 63.96 63.52 63.09 62.66 62.24 61.01 59.06 57.23 55.52 53.90 52.37 50.93
7 10 11 12 13 14 15 16 17 20 25 30 35 40 45 50
xa ≡ ≡ ≡ ≡ ≡ liq/gel liq/gel liq/gel liq/gel liq/gel liq/gel gel
≡ ≡ ≡ ≡ liq/gel liq/gel liq/gel liq/gel liq/gel liq/gel liq/gel
≡ ≡ ≡ liq/gel liq/gel liq/gel liq/gel liq/gel liq/gel liq/gel
≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡
a
The turbidity of the solution is probably due to the nucleation of the LC phase. Table 9. Phase Behavior of the OPE9 Quaternary System, Cn)9 ) 0.60 m and Chex/Cn)9 ) 1.55 composition (% by weight)
temperature (°C)
H2O
OPE9
hex
oil
wo
20
25
30
35
4.26 6.91 8.80 10.02 11.20 11.78 12.36 12.92 13.48 14.03 15.65 18.21 20.62 22.89 25.03 27.06 28.98 30.81 32.54
23.77 23.11 22.64 22.34 22.05 21.90 21.76 21.62 21.48 21.34 20.94 20.31 19.71 19.15 18.61 18.11 17.63 17.18 16.75
6.24 6.07 5.95 5.87 5.79 5.75 5.72 5.68 5.64 5.61 5.50 5.34 5.18 5.03 4.89 4.76 4.63 4.51 4.40
65.72 63.91 62.61 61.77 60.96 60.56 60.17 59.78 59.39 59.01 57.91 56.15 54.50 52.94 51.46 50.07 48.75 47.50 46.31
6 10 13 15 17 18 19 20 21 22 25 30 35 40 45 50 55 60 65
≡ ≡ ≡ ≡ ≡ ≡ ≡ liq/gel liq/gel liq/gel liq/gel liq/gel liq/gel liq/gel gel
≡ ≡ ≡ ≡ ≡ ≡ ≡ liq/gel ≡ ≡ ≡ ≡ ≡ ≡
≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡
≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡
ity in oils of OPEn with the shorter OE chains2 is overcompensated by the easier packing of their chains in the interior of the reverse micellar aggregates. In a similar vein, notice that although the average OE chain length of TX-100 (n ) 9.4) is similar to that of OPE9, at lower
temperature (°C)
H2O
OPE9
hex
oil
wo
20
25
30
35
10.93 11.74 13.30 14.81 16.27 17.68 19.05 20.36 23.48
30.49 30.21 29.67 29.16 28.66 28.18 27.71 27.26 26.19
8.01 7.94 7.80 7.66 7.53 7.40 7.28 7.16 6.88
50.57 50.12 49.23 48.37 47.54 46.74 45.97 45.22 43.45
12 13 15 17 19 21 23 25 30
≡ ≡ ≡ ≡ ≡
≡
-
-
Table 11. Phase Behavior of the OPE7 Quaternary System, Cn)7 ) 0.60 m and Chex/Cn)7 ) 1.00 composition (% by weight)
temperature (°C)
H2O
OPE7
hex
oil
wo
20
25
30
35
6.63 7.31 13.63 14.21 14.79 15.36 15.92 16.47 17.02 17.56 19.14 20.66 21.64 23.99 26.20 28.29 30.26
21.05 20.89 19.47 19.34 19.21 19.08 18.95 18.83 18.70 18.58 18.23 17.88 17.66 17.13 16.63 16.16 15.72
4.18 4.15 3.86 3.84 3.81 3.79 3.76 3.74 3.71 3.69 3.62 3.55 3.51 3.40 3.30 3.21 3.12
68.15 67.65 63.04 62.61 62.19 61.78 61.37 60.96 60.56 60.17 59.02 57.91 57.19 55.48 53.86 52.34 50.90
9 10 20 21 22 23 24 25 26 27 30 33 35 40 45 50 55
≡ ≡ ≡ ≡ ≡ ≡ ≡ ≡ -a -a -a -a -a -a
x
x x x x x
x x x x x x x x x x
a
Probably a bicontinuous L phase. x f L2 + aqueous phase. Table 12. Phase Behavior of the OPE7 Quaternary System, Cn)7 ) 0.60 m and Chex/Cn)7 ) 1.58a composition (% by weight)
temperature (°C)
H2O
OPE7
hex
oil
wo
20
25
30
35
7.14 10.34 10.96 13.33 14.47 16.12 17.19 18.74 21.21 23.53 25.71
20.40 19.70 19.56 19.04 18.79 18.43 18.19 17.85 17.31 16.80 16.32
6.40 6.18 6.14 5.97 5.89 5.78 5.71 5.60 5.43 5.27 5.12
66.06 63.78 63.35 61.66 60.85 59.67 58.91 57.81 56.05 54.41 52.85
10 15 16 20 22 25 27 30 35 40 45
x
x x x x
x x x x x x x
x x x x x x x x x
a
x f L2 + aqueous phase. Table 13. Phase Behavior of the OPE5 Quaternary System, Cn)5 ) 0.40 m and wo ) 12a
composition (% by weight)
temperature (°C)
H2O
OPE5
hex
oil
Chex/Cn)5
6.88 6.83 6.79 6.76 6.73
13.57 13.49 13.41 13.34 13.27
0.00 0.65 1.22 1.76 2.23
79.55 79.03 78.58 78.15 77.78
0.00 0.20 0.38 0.55 0.70
a
20
25
30
35
x
x x x
x x x x x
x x x x x
x f L2 + aqueous phase.
temperatures the TX-100 solution phase-separates very early (at wo ) 1, Table 1) compared to OPE9 (at wo ) 5, Table 4). Evidently, the presence of OPEn components with n > 9 in TX-100 strongly reduces its capacity to form reverse micelles and thus to solubilize water. Similar to what was found for poly(oxyethylene isononylphenyl ethers) in cyclohexane,16 the phase separation of TX-100
Phase Behavior of Triton X-100 and p-tert-OPEn
Langmuir, Vol. 13, No. 16, 1997 4255
Table 14. Phase Behavior of the OPE9 Quaternary System, wo ) 17 and Chex/Cn)9 ) 1.55 composition (% by weight)
temperature (°C)
H2O
OPE9
hex
oil
Cn)9 (m)
8.58 11.20 14.81
16.90 22.05 29.16
4.44 5.79 7.66
70.08 60.96 48.37
0.40 0.60 1.00
20
25
30
35
≡ ≡ -
≡ -
≡ -
-
solutions results in a w/o phase and a surfactant-rich phase at the upper consolute boundary17 (UCB). For instance, at a TX-100 concentration of CTX-100 ) 0.60 m and wo ) 6, phase separation occurs between 25 and 30 °C (Table 1). At 25 °C, the upper (w/o) phase amounts to ca. 30% of the total volume. The L2 domain extends from 30 to 70 °C (not shown in Table 1). Between 70 and 75 °C the system reaches the solubilization limit of water, and separates into w/o and o/w phases. Increasing temperature augments the L2 domain for the surfactants with the longer OE chains (TX-100, OPE7, and OPE9; Tables 1, 3, and 4, respectively), whereas for OPE5 the effect is opposite (Table 2). This is due to its lower UCB, which is associated with the higher solubility of OPE5. Within our experimental temperature range, the rising temperature moves the longer chain systems further above the UCB (into the L2 domain), whereas it takes OPE5 toward and across the solubilization limit. The lower the OPE5 concentration, the sooner the solubilization limit is reached (Table 5). Quaternary Systems. Generally, in the systems investigated, the addition of n-hexanol as cosurfactant leads to a more complex phase behavior, including the appearance of additional LC, L, and gel phases. It is noteworthy to mention that in the analogous C12E5/ tetradecane/H2O system under similar conditions,18 no cosurfactant is needed for a comparable phase behavior. Our results are compiled in Tables 6-14, where the amount of cosurfactant present is specified as the ratio of molal concentrations of hexanol and the particular surfactant, Chex/Csurfactant. For the surfactants with the longer hydrophilic chains (TX-100, OPE7, and OPE9), the presence of hexanol greatly increases the L2 domain and the maximum amount of water wo,max that can be solubilized in the solution (Tables 6-12). For TX-100 (Tables 6 and 7) and OPE9 (Tables 8 and 9), the higher the fraction of the cosurfactant, the greater its effects (Tables 6 and 7). In contrast, even a small amount of cosurfactant (Chex/ Cn)5 ) 0.38) reduces the L2 domain for the surfactant with the shortest OE chain (OPE5) at constant wo ) 12 (compare Tables 2 and 13). In TX-100 (Tables 6 and 7) and OPE9 (Tables 8 and 9), with increasing amount of water added to the solution, typically the following sequence of phase transitions occurs: L2 f LC f L f liquid/gel f gel. The isotropic liquid phase L subsequent to LC, which appears only in TX-100 (Tables 6 and 7) and OPE7 (at 20 °C, Table 11) at high water content, is most likely a bicontinuous phase. For OPE9, with increasing surfactant concentration Cn)9, the LC phase precedes L2 in the above sequence (Table 14). In OPE7, the cosurfactant greatly increases wo,max which, however, falls if the temperature is raised (Tables 11 and 12). This is in contrast with the temperature dependence of its ternary system (Table 3). Interestingly, increasing (17) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: New York, 1994. (18) Kunieda, H.; Shinoda, K. J. Dispers. Sci. Technol. 1982, 3, 233.
the amount of n-hexanol from Chex/Cn)7 ) 1 (Table 11) to 1.58 (Table 12) reduces wo,max. Thus, among all the systems within the ranges investigated, the highest amount of water (wo ) 55) solubilized is found for OPE7 at the lowest temperature (20 °C) with Chex/Cn)7 ) 1 (Table 11). The introduction of n-hexanol as a fourth component has several effects on the structure of the assemblies and thus the general phase behavior. With increasing amounts present, although it reduces the size and aggregation number of reverse micelles in the L2 phase,12 it enlarges the L2 domain at the expense of the adjoining LC region (compare Tables 6 and 7, 8 and 9, and 11 and 12). It also diminishes the liq/gel domain. The destabilizing effect of n-hexanol for the organized assemblies involved is due to the fact that it is a better solvent than cyclohexane for both the surfactants investigated and water. Thus, for the combination of components examined, n-hexanol behaves as a “cosolvent” with respect to phase behavior. Summary Investigations of TX-100 and its separated p-tert-OPEn components (with n ) 5, 7, and 9) in ternary (cyclohexane/ H2O) and quaternary (cyclohexane/H2O/n-hexanol) systems in the vicinity of the L2 domain reveal complex phase behavior. TX-100 (n ) 9.4), and OPE9 exhibit similar trends with the variation of temperature and water content wo, while the oligomer with the shortest hydrophilic chain (OPE5) displays an opposite temperature dependence. OPE7 occupies an intermediate position: In ternary systems its behavior resembles that of the longer chain analogues, whereas in quaternary systems its behavior resembles that of OPE5. Typically, in ternary systems, the L2 domain is adjacent to a 2-phase region (x f L2 + aqueous surfactant phase), whereas in quaternary systems it is adjacent to a liquid crystalline phase. The extent of the L2 region depends on the length (n) of the hydrophilic OE chain of the homologous surfactants, temperature, surfactant concentration, and the amount of cosurfactant present. In fact, as far as phase behavior is concerned, the role of n-hexanol may be characterized as that of a “cosolvent” since it destabilizes the reverse micellar12 and liquid crystalline assembliessas it is a better solvent than cyclohexane for both the surfactant and water. This attribute of n-hexanol, however, leads to an increase in the maximum amount of water (wo,max) that can be solubilized in the isotropic solutions with a concomitant reduction of micellar size12 and to the expansion of the L2 region at the expense of the adjoining LC domain. With increasing wo in quaternary systems, TX-100 and OPE9 exhibit the greatest number of different phases: L2 f LC f L f liquid/gel f gel. All these phases, except LC, are optically isotropic, and the L phase is probably a bicontinuous phase. Acknowledgment. This work was supported in part by the National Science Foundation, the Welch Foundation, the donors of Petroleum Research Fund, administered by the American Chemical Society, and the Texas Advanced Research Program. The authors thank Messrs. W. Cook, T. Leeds, and M. W. Lutes for technical assistance. LA9620519
