Langmuir 1992,8, 48-50
48
Investigation of the Microenvironment in Triton X- 100 Reverse Micelles in Cyclohexane, Using Methyl Orange as a Probe De-min Zhul and Z. A.
Schelly*
Center for Colloidal and Interfacial Dynamics, Department of Chemistry, The University of Texas a t Arlington, Arlington, Texas 76019-0065 Received April 12, 1991.I n Final Form: September 3, 1991 The microenvironments in Triton X-1001cyclohexane reverse micelles are investigated through the UV-vis absorption spectra of solubilized methyl orange as a probe. At 25 f 0.1 "C, we find that in 'dry" reverse micelles with no added water present (when the natural R value of the solution is R = [HzOlI [Triton X-1001 = O.l), cyclohexane penetrates the polar core of the aggregates to an extent that the molar ratio r = [cyclohexane]/[Triton X-1001 in the core is about 4.5. Upon an increase of the water content of the solution to R = 2.5, part of the cyclohexane is driven out of the micellar interior, whereby r is reduced to about 3.0.
Introduction In some of the previous studies of our laboratory, optical probes were used to detect the aggregation of surfactants in nonpolar solvents,2 to establish the equilibrium description of reverse micellar solution^,^ to determine the relaxation amplitudes: and to monitor the relaxation rates5 in concentration-jumped reverse micellar systems. We also introduced 1-methyl-8-oxyquinoliniumbetain (QB) as a highly sensitive absorption probe of the micropolarity of the interior of reverse micellar aggregates.6 In the present paper, we report the results of investigations of the ternary system Triton X-lOO/H~O/cyclohexane, by the use of methyl orange as an absorption probe. Triton X-100 (or TX-100) is the trade name of the liquid, nonionic surfactant poly(oxyethy1ene)(tetramethylbuty1)phenyl ether, with the formula of
CH,C(CH,),CH,C(CH,),C,H,(OCH2CH2)~,~OH Unlike typical ionic surfactants, the hydrophilic part of nonionic surfactants of the class of poly(oxyethy1ene) alkyl or alkylphenyl ethers may be a chain longer than the hydrophobic part of the molecule, as is actually the case in TX-100. Thus, the polar interior of reverse micellar aggregates formed of such surfactants may structurally resemble more the interior of normal micelles in aqueous solution than that of reverse micelles formed of ionic surfactants. Compared to ionic surfactants, systems of nonionic surfactants in apolar solvents exhibit some characteristic properties such as greater sensitivity totemperature7 and solvent8of their phase behaviors and greater ability to accommodate brinesgJOand to serve as compartmentalized media for chemical reactions.1° Their aggregation mainly depends on a delicately balanced (1)R. A. Welch postdoctoral fellow. On leave of absence from Qingdao Institute of Chemical Technology, Qingdao, People's Republic of China. (2) Herrmann, U.; Schelly, Z. A. J. Am. Chem. SOC. 1979, 101, 2665. (3) Tamura, K.; Schelly, Z. A. J. A m . Chem. Soc. 1981, 103, 1013. (4) Schelly, Z. A.; Chao, D. Y.; Sumdani, G. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum: New York, 1979; Vol. 1, p 323. (5) Tamura, K.; Schelly, Z. A. J . Am. Chem. SOC.1981, 103, 1018. (6) Ueda, M.; Schelly, Z. A. Langmuir 1989, 5, 1005. (7) Ravey, J. C.; Buzier, M. In Macro- and Microemulsions Theory and Applications; Shah, D. O., Ed.; American Chemical Society: Washington, DC, 1985; p 253. (8)Ravey, J. C.; Buzier, M.; Picot, C. J. Colloid Interface Sei. 1984, 97, 9. (9) Bourrel, M.; Salager, J. L.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1980, 75, 451. (10) Zhu, D.-M.; Schelly, Z. A. Work in preparation.
interaction between the polar chain of the surfactant and the solvent.1l For instance, we found that TX-100 does not form reverse micelle either in benzene or in n-hexane. It is too soluble in the highly polarizable benzene, and its solubility is too low in n-hexane. But it does form reverse micelle and w/o microemulsion in cyclohexane, which we have extensively used as compartmentalized media for chemical reactions.1° The purpose of the present study was to obtain information about the microenvironment in TX-lOO/cyclohexane reverse micelle and to establish if, and to what extent, cyclohexane penetrates the interior of the aggregates. To ascertain that the probe is anchored to the polar core of the aggregates, as well as to satisfy the procedural requirement of being soluble in a.wide variety of media, we selected methyl orange (MO) (CH,)2N-C,H4-N=N-C6H4-So~Na
as the probe. The absorption spectrum of MO is sensitive to the polarity of its environment and, because of its structure and ionic character, it is insoluble in cyclohexane. In the following, as reported by the probe, the microenvironment in the aggregates is discussed as a function of composition, where the composition will be characterized by the R (molar ratio of added water to surfactant) and r (molar ratio of cyclohexane to surfactant) values of the solutions. Experimental Section Materials. TX-100 (a Rohm and Haas product) was purchased from Eastman Kodak, and cyclohexane (>99.5%) was purchased from Fluka. Their water content was found through Karl-Fischertitration (Aquastar V1B Titrator) as 0.28% (w/w) and 0.014% (wiw),respectively. Methyl orange (MO, >95%) was purchased from Eastman Kodak and recrystallized 4 times from water. Both triethylene glycol monomethyl ether [TGME, CH3(0CH2CH2)30H, >97%, water content 0.22% (w/w)] and triethyleneglycol dimethylether [TGDE,CH3(OCH&H2)30CH3, >98%,water content 0.094% (w/w)lwere purchased from Fluka. Methanol was absolute,Baker Analyzed Reagent, and water was double-deionized and distilled. Absorption Spectra. The UV-vis absorption spectra were taken on a Gilford Response I1 computerized spectrophotometer at 25 f 0.1 O C , in 1cm path length, stoppered quartz cells, at0.2 nm increment setting. In addition to displaying the spectra, the instrument also computes the wavelengths of absorption (11)Friberg, S. E. In Interfacial Phenomena in Apolar Media; Eicke, H.-F., Parfitt, G. D., Eds.; Marcel Dekker: New York, 1987; p 93.
0743-746319212408-0048$03.00/0 0 1992 American Chemical Society
Microenvironments in Reverse Micelles
3*50
t
I
I
I
I
a I-\
I I i
I
Langmuir, Vol. 8, No. 1, 1992 49
\ ‘
t
I I
W
u
z a R
0: 0 0)
m
a
3ee.e
650.0
Figure 1. Absorption spectra of 6.6 X M MO in different Solvents: acetic acid (a),water (b), methyl sulfoxide (c), TGME (d), and TGDE (e). A, (nm): (a) 511.5, (b) 464.0, (c) 429.0, (d) 422.0, (e) 417.0. maxima, A,. The spectrum of each solution was recorded 6 times, and we found that A, could be determined with a precision of *0.2 nm (maximum deviation from the mean). To introduce the probe into the solutions, 0.05 g of MO was dissolved in 10 mL of methanol, the appropriate amount (4-5 pL) of which was transferred into a small vial, and the methanol was evaporated. Upon addition of a surfactant solution to the solid residue, the MO was solubilized. The molar ratio of TX100 to MO was always greater than 2.6 X lo3. Since under the experimental conditions used the mean aggregation number of TX-100 is in the range of 20-40,12 only one of no less than 70 micelles contained a probe molecule. Thus the perturbation of the system by the presence of the probe may be assumed negligible. In the concentration range of MO used, the absorbance of the solutions obeys the Bouguer-Lambert-Beer law, and the extinction coefficient c can be calculated. For instance, in a 0.466 M TX-100 solution (R = l.O), at 419.5 nm, c is 2.65 X lo4 M-1 cm-l.
0.01 250.0
(12)Zhu, D.-M.; Feng,K.-I.; Schelly, Z. A. J.Phys. Chem., submitted for publication.
(NANOMETERS]
550.0
Figure 2. Absorption spectra of 7.6 X M MO in dry TX100/cyclohexanereverse micelles as a function of the surfactant concentration. Concentration of TX-100: (a) 0.21 M, (b) 0.27 M, (c)0.375 M, (d)0.466 M, (e)pure TX-100. Reference: solution a but without MO. 3.00
U 0
a
m
0:
0 VI
m a
250.0
Results a n d Discussion 1. Calibration. The solvatochromism of MO is shown in Figure 1, where its absorption maximum is red-shifted with increasing polarity of the pure solvents used. TGME and TGDE were also included as calibrating solvents, to mimic the environment the probe would find in the polar interior of TX-100 reverse micelles, if it is surrounded by the oxyethylene segments of the surfactant. Indeed, the wavelengths of the absorption maxima in TGME (422.0 nm) and TGDE (417.0 nm) are almost identical to those found in pure, liquid TX-100 as solvent (A, = 422.5 nm; see spectrum e in Figure 2, and Tables I and 111) and in a “dry” solution of cyclohexane in TX-100 with r = 6.0 (A, = 417.0 nm; see spectrum e in Figure 3, and Table I), respectively. “Dry” means that no water is added to the system beyond the small amounts that are naturally present in the components; Le., 0.28% (w/w) in TX-100 and 0.014% (w/w) in cyclohexane. Clearly, the probe reports a more polar environment in neat TX-100 because of the relatively greater amount of water present than in the cyclohexane/TX-100 (r = 6.0) solution. Similarly, MO senses a greater polarity in TGME than in TGDE, because of an OH group and higher water content present in the former. These results suggest that the spectrally relevant association of the probe is with the oxyethylene segments of the surfactant in the interior of the reverse micelles. 2. Microenvironment i n D r y ReverseMicelles. The absorption spectra of dry TX-100/cyclohexane reverse mi-
A
A
[NANOMETERS]
55
.0
Figure3. Absorption spectra of7.6 X 10+ M MO in cyclohexane/ TX-100 solutions, with TX-100 as solvent, as a function of cyclohexane content. Molar ratio of cyclohexaneto TX-100, r: (a) 0.5, (b) 1.5, (c) 2.5, (d) 4.5, (e) 6.0. Reference: 0.466 M TX-100 in cyclohexane, without MO. celles as a function of the surfactant concentration, with and without MO present, are shown in Figures 2 and 4, respectively. Comparison of the two figures indicate that, because of the small concentration of MO, the absorption band of TX-100 (at 300 nm) is unaffected by the presence of the probe. The intensity of the MO band (Am = 417.0 nm) increases with, but its location is independent of, the surfactant concentration (Figure 2). Hence, whatever the surfactant concentration of the solutions investigated, the micropolarity of the dry reverse micellar interior is constant and is lower than that of pure TX-100 (A, = 422.5 nm; see spectrum e in Figure 2). This suggests that cyclohexane penetrates the interior of the micellar aggregates. As a test of this assumption, one should be able to simulate the microenvironment in the micellar core by adding cyclohexane to pure TX-100 as solvent until MO reports the same polarity as in dry reverse micelles. Indeed, experiments show (Figure 3 and Table I) that with increasing cyclohexane concentration Am is blue-shifted, and a t an r value of 4.5 A, reaches the value of 417.5 nm, very close to the value of 417.0 nm found in dry reverse micelles. Further increase of the cyclohexane concentration to r = 6.0 results in a solution that scatters light, indicating the onset of aggregation or phase transition. Thus, we conclude that in dry TX-100 reverse micelles in cyclohexane, the solvent penetrates the polar core of the
Zhu and Schelly
50 Langmuir, Vol. 8, No. 1, 1992 Table 11. A.,
c
R
1.0 419.5
1.5 420.5
2.5 421.5
3.0 421.6
4.0 422.0
A, in pure water is 464.0 nm.
R Xm,nm
0 422.5
Am
of M O in HzO/TX-100 Solutions
1.0 423.0
2.0 424.0
2.5 424.1
3.0 424.4
4.0 426.0
7.0 426.2
Table IV. A, of MO in Cyclohexane/HzO/TX-100 Solutions with Constant R = 2.5
a
.
.
I
-=----.-. -
_ _ * - . .c - - . i
-0.02
I
200.0
+
-
\. _ ’.* ~
-1
I
I
A
r Am, nm
----- --- _ -- - - -
I
I
[NONOMETERS]
400.0
Figure 4. Absorption spectraof dry TX-l00/cyclohexane reverse micelles without MO, as a function of the surfactant concentration. Concentration of TX-100: (a) 0.27 M, (b) 0.375 M, (c) 0.466 M, (d) pure TX-100. Reference: 0.21 M TX-100 in cyclohexane. 4.00
0.5 418.5
Table 111.
I
i l
0 417.0
X,,nm a
of MO in 0.466 M TX-100 as a Function of the Water Content of the Solution
0.5 423.9
1.5 422.6
2.5 421.7
4.0 421.5
micelles. If water is dissolved in pure TX-100, resulting in R values similar to those of Table 11,MO reports higher polarities (Table 111) than those found in the micelles (Table 11). However, if cyclohexane is successively added to one of the binary HzO/TX-100 solutions (e.g. to one with R = 2.5), MO reports environments (Table IV) similar to those in the micelles. Notice (in Table IV) that in the range of r = 2.5-4.0, Am changes very little, indicating that the microenvironment of MO in the bulk solution is approaching saturation by cyclohexane. Also, a t r = 4.0, Am = 421.5 nm is the same as in a wet reverse micelle of R = 2.5 (Table 11). Thus, in the polar core of a TX-100 reverse micelle in cyclohexane (R = 2.51, MO reports the same microenvironment as in a solution of cyclohexane ( r = 2.5-4.0) and water (R = 2.5) in TX-100 as solvent. Finally, a review of the data in Tables I1 and I11provides comparison of the effects of increasing the water content by an equal amount (from R = 0 to R = 4.0) in reverse micelles and in pure TX-100. In reverse micelles (Table 111,Am increases by 5 nm (=422.0 nm-417.0 nm), whereas in TX-100 (Table 111)Am increases only by 3.5 nm (=426.0 nm-422.5 nm). In other words, addition of the same amount of water to the solutions increases the polarity of the micellar cores more than that of pure TX-100. This suggests that added HzO does not simply share the micellar interior with the cyclohexanemolecules but displaces some of them.
I
...* --=\
!
-0.01 250.0
I
I
I
A
I
[NONOMETERS]
I
550.0
Figure 5. Absorption spectra of MO in TX-l00/HzO/cyclohexane wet reverse micelles, as a function of the water content, but with constant concentration of TX-100 (0.466 M). Molar ratio of water to TX-100, R: (a) 0, (b) 0.5, (c) 1.5, (d) 2.5, (e) 4.0. To aid the distinguishability of the spectra, different probe concentrations were used for the solutions from (a) to (e): 6.5 X M, and 7.6 X M, 6.7 X 10-5 M, 7.0 X 10-5 M, 7.3 X M, respectively. Reference: 0.466 M TX-100 in cyclohexane,without MO. Table I. X, of MO in Cyclohexane/TX-100 Solutions r Xm,nm
0 422.5
0.5 422.0
1.5 421.0
2.5 419.5
4.5 417.5
6.P
417.0
Dynamic light scattering measurements indicate weak scattering.
aggregates where the molar ratio of cyclohexane to TX100 is about 4.5. 2. Microenvironment in Wet Reverse Micelles. The effect of successive addition of water to TX-l00/cyclohexane is demonstrated in the MO spectra of Figure 5. With an increase in the R value from 0.5 to 4.0 of a 0.466 M TX-100 solution, Am of MO is red-shifted but stays far below that in pure water (Table 11). Further addition of water (up to R = 5.5) leads to a turbid system. Evidently, the polarity of the wet micellar interior is far less than that of HzO. Hence, water does not form a pool in the core of the aggregates, rather it seems to be dispersed between the poly(oxyethy1ene) chains. However, the question remains as to the relative amounts of cyclohexane and water in the wet micellar core. Again, TX-100 being a liquid, one can use it as solvent for solutions simulating the polarity found in the interior of the reverse
Summary The microenvironment of the polar core of reverse micelles of the nonionic surfactant Triton x-100 in cyclohexane was investigated by utilizing the solvatochromic behavior of methyl orange as an absorption probe. The ionic MO is soluble in water and in pure, liquid TX-100 but insoluble in cyclohexane. Thus in reverse micelles, it is solubilized in the polar interior of the aggregates. The wavelength of the absorption maximum Am of MO is redshifted with increasing polarity of the medium. The observed Am and its shift as a function of composition were used as a measure of relative polarity. We found that in dry TX-100 reverse micelle, the nonpolar solvent penetrates the polar interior of the aggregates, where the molar ratio r of cyclohexane/TX-100 is about 4.5. Upon addition of water to the solution, where the molar ratio R of added water/TX-100 becomes 2.5, the polarity of the micellar interior increases. Concurrently, some of the cyclohexane molecules are displaced from the micellar core, leading to a reduction of its r value to about 3. Acknowledgment. This material is based in part upon work supported by the National Science Foundation (Grant No. CHE-87063451, the R. A. Welch Foundation, the Texas Advanced Research Program under Grant No. 1766, and Alcon Laboratories, Inc. Their support is gratefully acknowledged.
