Langmuir 1992,8, 1538-1540
1538
Reverse Micelles and Water in Oil Microemulsions of Triton X-100 in Mixed Solvents of Benzene and n-Hexane. Dynamic Light Scattering and Turbidity Studies D.-M. Zhu, X. Wu, and Z. A. Schelly* Center for Colloidal and Interfacial Dynamics, Department of Chemistry, The University of Texas at Arlington, Arlington, Texas 76019-0065 Received December 19,1991.I n Final Form: March 25, 1992 The formation of Triton X-100 reverse micelles and water in oil microemulsions in mixed solvents of benzeneln-hexane was observed and the aggregates were investigated by dynamic light scattering and turbidity measurements. The apparent hydrodynamic diameter Dh of the aggregates is a function of the benzeneln-hexane ratio, surfactant concentration, water content, and the concentration of salt (CaClz), if present. At low water content, CaCl2 is distributed in the polar core of the aggregates. The presence of CaClz enhances the capacity of the aggregates to solubilize water, and decreases Dh. With increasing water content, once aqueous pools are formed, the solubility of CaClz is greatly enhanced and additional amounts of the salt are solubilized in the pools. Introduction The formation of reverse micelles of nonionic surfactants shows a strong solvent dependence.l-3 For instance, Triton X-100 aggregates in cyclohexane,4+ but it does neither in pure benzene nor in n-hexane. Triton X-100 (or TX-100) is the trade name of the liquid, nonionic surfactant poly(oxyethy1ene) (tetramethylbuty1)phenylether with the formula of
The highly polarizable benzene molecules have, through charge transfer:*' such a strong affinity for the poly(oxyethylene) chain that TX-100 cannot aggregate in benzene. In contrast, the interaction between n-hexane and TX100 is so weak that the surfactant is essentially insoluble in n-hexane. Thus benzene and n-hexene represent two extreme solvents with respect to solubility for TX-100, neither individually promoting aggregation. The natural question that arises is whether formation of reverse micelles would occur in mixed solvents of benzene and nhexane, where a balance of interactions may allow for aggregation? To answer this question was the aim of the work reported in the present paper. We found that TX-100 does form reverse micelles in a range of benzeneln-hexane mixed solvents. The work represents an extension of some of our recent studies on TX-100 in cyclohexane, where the micropolarity of the micellar interior6 and the effects of temperature, water content, and salinity on the aggregation behaviorS were investigated. In the present communication we report the results of dynamic light scattering and turbidity measurements on reverse micelles of TX-100 in (1) Sheih, P. S.; Fendler, J. H. J.Chem. Soc., Faraday Tram. 1 1977, 73, 1480. (2) Ravey, J. C.; Buier, M.; Picot, C. J.Colloid Interface Sci. 1984,97, 9. (3) Friberg, S. E. In Interfacial Phenomena in Apolar Media; Eicke, H.-F., Parfitt, G. D., Eds.; Marcel Dekker: New York, 1987; p 93. (4) Kumar, C.; Balasubramanian,D. J.Colloid Interface Sci. 1979,69, 271. ( 5 ) Kumar, C.; Balasubramanian,D. J.Colloid Interface Sci. 1980,74, 64. ( 6 ) Zhu, D.-M.; Schelly, 2. A. Langmuir 1992,8, 48. (7) Christenson, H.; Friberg, S. E.; Larsen, D. W. J.Phys. Chem. 1980, 84, 3633. (8) Zhu, D.-M.; Feng, K.-I.; Schelly, 2. A. J . Phys. Chem. 1992, 96, 2382.
mixed solvents of benzeneln-hexane, as a function of solvent composition, water content, surfactant concentration, and salinity. Experimental Section Materials. TX-100 was obtained from Fluka (nm = 1.491). Its water content was found through Karl Fischer titration (Aquastar V1B Titrator) as 0.11% (w/w). Benzene (Fisher Scientific,Spectranalyzed grade) had boiling range 79.5-80.3 OC and water content 0.039% (wlw). n-Hexane (Fluka, for UV spectroscopy) had water content 0.0156% (w/w). In 'dry" surfactant stock solutions (with no water added),the molar ratio of water toTX-100 (i.e.,R E [HzOlI[TX-lW])was always lese than 0.1. CaC12.6H20 (Fluka) was BioChemika Microselect grade. Water was double-deionized and distilled. Light Scattering Measurements. Quasi-elastic light scattering measurements were performed on a Brookhaven Model BI-200SM instrument with an argon ion laser light source (A = 514.5 nm) in the power range of 50-150 mW, at 25 0.1 "C. Since the angle of detection (in the range of 30" to 150") had no effect on the sizes computed, the measurements were done at 60". Possible dust was removed from the sample solutions by centrifugation at (1.4 X 103)g.The viscosityof the mixed solvents was measured with a Cannon-Ubbelohdeviscometer at 25 f 0.1 "C. The QELSdata were analyzed by four different methods: cumulants, exponentialsampling,nonnegatively constrainedleastsquares (NNLS)multiple pass, and NNLS regularized. They all yielded consistent, reproducible results, and the last three methods gave symmetric, monomodal size distributions. The translational diffusion coefficients were obtained from the intensity autocorrelation function and the apparent hydrodynamic diameter (Dh) of the aggregates was calculated through the Stokes-Einstein equation. The Dh data presented resulted from the quadratic cumulants analysis. Turbidity Measurements. The light transmittance of the turbid systems in a 1-cm path length cell was measured with a Gilford Response I1 computerized spectrometer, at X = 400 nm, at 25 f 1"C. The transmittancesof equilibrated solutions are reported.
*
Results a n d Discussion 1. Effect of Solvent Composition on &. Intense light scattering signal indicated that TX-100 does form reverse micelle in appropriate mixed solvents of benzene and n-hexane. The apparent hydrodynamic diameters Dh of the reverse micelles at a fixed concentration of TX100 (CTX= 0.21 M) but at various composition of the 0 1992 American Chemical Society
Langmuir, Vol. 8, No. 6,1992 1539
Microemulsions of Triton X-100
a
Table I. of TX-100 Reverse Micelles ( a x = 0.21 M) in Benzeneln-Hexane Mixed Solvents at 26 "C vol 5% benzene
Dh, nm
50 3.2
40 3.7
30 5.7
25 7.8
20 16.7
r
10 twophases
Table 11. of TX-100 Reverse Micelles at Different Surfactant Concentrations, &, in 30% (v/v) Benzene and 70% (v/v) n-Hexane at 26 'C
Cm,M 0.077 Dh,nm 3.3
0.146 0.210 4.5 5.7
0.270 7.2
0.324 0.375 8.8 9.7
0.466 12.3
mixed solvent, in which the benzene volume fraction varies from 20 % to 50%, are listed in Table I. When the solvent contains more than 50% benzene, the light scattering intensity is below the detection limit of the instrument. With decreasing benzene content (below 50%), the apparent Dh showsan increasing trend first slowly, then more rapidly (at 25%). Ultimately, the system reaches phase separation in a n-hexane-rich solvent (at 10% benzene). The polydispersity of the aggregates was found to decrease monotonically with the benzene content of the medium from 0.39 (in 50% benzene) to 0.055 (in 10% benzene), indicating a transition from a relatively broad to a narrow size-distribution. 2. Effect of TX-100Concentrationon The effect of surfactant concentrationon the hydrodynamic diameter of the reverse micelles is demonstrated on the Dh values obtained at the fixed composition of 30% (v/v) benzene and 70% (v/v) n-hexane of the mixed solvent (Table 11). In the rest of the paper, this solvent composition will be consistently used. At CXT< 0.077 M, the scattered light intensity was too weak to be analyzed reliably. Above 0.077 M, the size of the micelles clearly increases with the surfactant concentration. (This is in sharp contrast with the aggregation behavior of TX-100 in cyclohexane6where, in the same concentration range, we previously found Dh = 21.3 nm to be constant.) The polydispersity of the aggregates is around 0.24, indicating relatively broad size-distributions. In another previous study we also found that cyclohexane molecules penetrate into the polar core of TX-100 reverse micelles.6 Benzene and n-hexane do ~imilarly.~ However,the penetration ability of benzene is greater than that of n-hexane, as benzene molecules have a much stronger affinity for TX-100 than n-hexane. With increasing CTX (and thus micelle concentration), an increasing amount of benzene is selectively trapped inside the aggregatesand hence the bulk solvent mixture becomes depleted of benzene. Consequently, increasing surfactant concentration has a similar effect on Dh as decreasing volume fraction of benzene in the solvent (Table I), namely, an enlargement of the reverse micelles. 3. Effect of Water Content (R Value) on The presence of reverse micelles in our mixed solvents is indicated also by the significant amount of water that can be solubilized by the aggregates. For example, in a solution of 0.27 M TX-100 in 30% (v/v) benzene and 70% (v/v) n-hexane, the R value ( ~ [ H ~ 0 I / [ T X - 1 0 0can 1 ) reach 9.5, corresponding to an overall water concentration of about 2.6 M. For comparison, in a similar solution in pure benzene, emulsion will be formed already at R < 1.0, indicating that no reverse micelles of TX-100 exist in benzene. In the mixed solvents, the apparent hydrodynamic diameter Dh of the reverse micelles is a function of the water content R, as shown on an example in Figure 1(open circles). The Dh vs R curve can be divided into three parts.
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(9) Zhu,D.-M.; Wu,X.; Schelly, Z.A.
J.Phys. Chem., in press.
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R Figure 1. Apparent hydrodynamic diameter Dh of TX-100 reverse micelleswith Cm = 0.27 M,as a functionof water content R in mixed solvent of 30% (v/v) benzene and 70% (v/v) n-hexane, at 25 OC: salt-free systems (0);saline systems, C,,p = 1.0 M CaClp (0). The wet, salt-free systems are characterized by narrow (polydispersity < 0.08) size-distribution, whereas the saline systems are characterized by a broader (polydispersity between 0.09 and 0.2) size-distribution.
In the range of R = 0 to 1.5, Dh increases linearly with R. For R = 1.5 to 5,Dh is approximately constant (about 17.3 , increases, until nm). And finally, for R > 5, & steeply phase separation occurs a t R = 9.5. 4. Effect of Salt (CaC12) on We have previously used TX-100 reverse micelles as compartmentalizedmedia for the preparation of submicrometer size particles of CaC03 and CaCz04. The present examination of saline systems has been motivated, at least in part, by the need to obtain information on the effects of Ca2+ion solubilized in the micelles. To investigate the effect of salinity on aggregate size, appropriate amounts of aqueous CaClz solution of concentration Cs,aq= 1.0 M (instead of pure water) were added to dry 0.27 M surfactant solutions. Thus, the CaClz concentration of the resulting solutions, [CaClzI, approximately equaled R X Cs,aqX 0.27/55.6 M. (To a good approximation, the water concentration of the aqueous salt solution added is taken to be 55.6 M.) E.g., at R = 10, [CaClzI = 0.048 M. When only small amounta of aqueous CaClz were added resulting in an R value below 2.1, some salt precipitated yielding turbid solutions. However, upon further addition of aqueous CaClz, the solid particles dissolved. The turbid systems will be discussed in more detail in section 5. The Dh values of the clear saline systems are also shown in Figure 1(solid circles). The presence of CaClz enhances the capacity of the reverse micelles to solubilize water. This is evidenced by phase separation occurring first a t R = 13.5, compared to that taking place already at R = 9.5 in the salt-free systems. The presence of salt also reduces the size of the micelles, and the higher the concentration of CaClz in the system, the greater effect it exerts. (For instance, at R = 8.0, Dh is 16.2 nm, compared to Dh = 34.0 nm when no salt is present.) As a consequence of this, the R range of approximately constant aggregatesize is wider in the saline systems. Generally, the salt-induced size reduction may be due to complexation or ion-dipole interaction between the Ca2+ion and the poly(oxyethy1ene)chains of the surfactant.
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Zhu et al.
1540 Langmuir, Vol. 8, No. 6, 1992
Table IV. Weight Percentage (Pw)of Residual Water in Wet, Saline TX-100 Reverse Micellar Solutions after Removal of Salt Crystals as a Function of C,,.q Cs,aq,
M
P w , 76 (wiw)
1.6
0.8
0
2.4
3.2
R Figure 2. Transmittance as a function of water content R of saline (CaC12)TX-100 reverse micelle solutions with CTX= 0.27 M in mixed solvent of 30% (v/v) benzene and 70% (v/v) n-hexane, at 25 "C: C,. = 0.5 M (O), 1.0 M (A),1.5 M ( 0 ) . Table 111. & and &I. Values of the Solutions Described by Figure 2 cs,aq,
R, RCL
M
0.4
0.5 1.1
1.7
1.0 0.9 2.1
1.5 0.8 2.6
2.0 0.7 3.0
0.5 0.66
1.o
0.61
2.0 0.56
The onset of pool formation was also investigated by the use of methyl orange as a probe of micr~polarity.~ In saline TX-100 reverse micellar solution, using Cs,aq = 0.4 M, we found that pool formation begins in the range of R = 1.5 to 2, in good agreement with the R = 1.1for the higher C , a q = 0.5 M of the present study. In the turbid systems, most of the water molecules are solubilized by the micelles but some are associated with the CaClz microcrystal precipitate, probably as water of crystallization. The distribution of water can be determined by removing the precipitate through filtration and determining the residual water in the filtrate by Karl Fischer titration. The amount of water removed with the crystals depends on the Cs,aqused for the preparation of the saline micellar solution. For different C,, values but at constant surfactant concentration (CTX= 0.27 M) and R = 1, the weight percentages of water P, remaining in the filtrates are listed in Table IV. Compared with the original weight percentage of water (P, = 0.665 a t R = l),with increasing salt concentration, a decreasing amount of water remains for the micellar aggregates since an increasing amount of water is needed to hydrate the CaC12, the crystals of which are removed. On the basis of the accumulated findings, one may conclude that the solubilization of salt by TX-100 w/o microemulsion involves, in addition to complexation," also pool formation. The latter seems to be more important for solubilizing salt at higher concentration.
5. Distribution of CaC12 and the Formation of Water Pools in Wet Reverse Micelles. As mentioned in the preceding section, the saline systems with R < 2.1 were turbid, caused by the formation of CaC12 microcrystals (which can be separated by centrifugation or filtration). Turbid systems were prepared from clear, saline surfactant solutions of high R value by mixing them with dry surfactant stock solutions. The magnitude of the ensuing turbidity depends on the concentration of CaC12 in the aqueous solution (Cs,aq) used for the p r e p aration of the saline systems and the R value of the resulting micellar solution. When salt solutions of Cs,aq I0.4 M are used, the wet reverse micellar solutions are always clear, until separation of two liquid phases occurs at a certain R value. With the use of Cs,aq> 0.4 M, turbid solutions were obtained which became clear upon further addition of aqueous CaC12. The transmittance vs R plots of such solutions are shown in Figure 2. With increasing R value and [CaC121 of the micellar solutions, the transmittance goes through a minimum (turbidity maximum) at R = R,. Then, the transmittance gradually increases until clear solutions (minimum turbidity) are obtained at R L Rclr. Some of the R m and Rclrvalues of the transmittance vs R curves (as in Figure 2) are listed in Table 111. When Cs,aq > 0.4 M is used, the precipitation of microcrystals indicates that only a part of CaC12 added is solubilized by the reverse micelles. As to the question of why the solubility of the salt increases at R > R, as indicated by the decrease of turbidity (Figure 2), we propose that at R, the formation of aqueous pools1oin the core of the reverse micelles begins in which the salt has a greater solubility. With increasing R and thus pool size, an increasing amount of salt can be solubilized until at R = Rclrno salt crystals are left and the solution becomes clear.
Acknowledgment. This material is based in part upon work supported by the National Science Foundation (Grant No. CHE-8706345),the R. A. Welch Foundation, the Texas Advanced Research Program under Grant No. 1766, and Alcon Laboratories, Inc. Their support is gratefully acknowledged.
(10)Menger, F. M.; Donohue, J. A.; Williams, R. F. J.Am. Chem. SOC. 1973,95,286.
(11) Ravey, J. C. In Microemulsions: Structure and Dynamics; Friberg, S . E., Bothorel, P., Eds.;CRC Press: Boca Raton, FL,1987; p 93.
Summary Triton X-100 does not aggregate either in pure benzene or in n-hexane, but it does form reverse micelles in benzene1 n-hexane mixtures in the composition range of 2 0 4 0 % (v/v) benzene. The wet aggregates are characterized by a narrow, and the dry ones by a broader, size-distribution. The apparent hydrodynamic diameter Dh of the reverse micelles increases with increasing surfactant concentration [TX-lOOl and water content R. The presence of salt (CaC12) enhances the capacity of the aggregates to solubilize water, but decreases D h . Prior to pool formation (i.e. at low water content), the salt is solubilized in the polar core of the reverse micelles. If the amount of salt added exceeds its solubility, hydrated microcrystal precipitates, resulting in turbid solutions. However, once aqueous pools are formed in the interior of the aggregates at higher R values, additional CaClz is dissolved in the water pools.
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