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Spin probe study of concentrated aqueous solutions of nonionic surfactant
Spin Probe Study of Concentrated Aqueous Solutions of Nonionic Surfactant HISASHI YOSHIOKA Sh&uoka College o f Pharmacy, Oshika, Shizuoka-shi, 422, Japan R e c e i v e d M a y 25, 1977; a c c e p t e d J u n e 10, 1977
Concentrated aqueous solutions of a nonionic surfactant, C12H25(OCH2CH2)21OH (2lED), were studied using a spin probe method. The correlation time (%) of the probe, 2,4-dinitrophenylhydrazone of 2,2,6,6-tetramethyl-4-piperidone N-oxide, in the micelle was - 1 . 2 × 10-a sec, suggesting that the probe was in the polyoxyethylene group. The value did not change greatly in the region of 25-90% water content, in contrast to the macroscopic properties. The hydration of the group of the surfactant in the micelle was discussed on the basis of %, the hyperfine coupling constant, and the hmax of UV absorption. It was concluded that the polyoxyethylene group was hydrated by the ratio of about one H20 molecule per oxygen atom of the ether linkage.
of the hydration increased gradually toward the terminal hydroxyl group. However, the extent of hydration did not become apparent quantitatively. In the present paper, the properties of 21ED-H20 system are examined using the spin probe method and the hydration of the hydrophilic polyoxyethylene group is discussed.
INTRODUCTION
Spin label or probe methods have been applied in various fields since they were proposed by McConnell et al. (1, 2). Micelle chemistry is a notably important field of applications and many papers have been published with respect to the structure, CMC, hydration, solubilization, etc. (3). However, all of them are concerned with ionic surfactants and a paper on a nonionic one has never been reported. The structure and the hydration of nonionic surfactant molecules in micelles have been studied using light scattering (4, 5) or charge transfer interactions (6, 7). The hydrophilic parts of these surfactants are predominantly polyoxyethylene groups. Pado et al. (8) used high-resolution nuclear magnetic resonance spectroscopy to examine the hydration of each part of the surfactant molecule forming a micelle (8). They proved that the lipophilic alkyl group existing in the inner part of the micelle was not hydrated to a significant extent. On the contrary, the polyoxyethylene group was hydrated even at the first ethoxy group adjacent to the alkyl group and the extent
EXPERIMENTAL
The probe used for the experiments is 2,4-dinitrophenylhydrazone of 2,2,6,6tetramethyl-4-piperidone N-oxide, with the following structure.
CH371-~ NH~N02 o-N CH3
CH3~ CHa
This material was synthesized by Rozantsev's method (10) from 2,4-dinitrophenylhydrazine (Wako, guaranteed) and 2,2,6,6tetramethyl-4-piperidone N - o x i d e described in Ref. (9). Dodecane (Wako), 378
002/-9797/78/0632-0378502.00/0 Copyright © 1978by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloidand Interface Science, Vol. 63, No. 2, February 1978
379
SPIN PROBE STUDY OF MICELLES
)
21ED
~f H20 #
/
j
capillary tube with an o.d. of 1 mm and sealed in the presence of air for ESR measurement. The UV spectra were measured by a Hitachi EPS-3T Recording Spectrometer. ESR spectra were recorded on a Japan E l e c t r o n O p t i c s L a b o r a t o r y Model JES-3BS.X Spectrometer (X-band) with 100-kHz field modulation. The width of the modulation was set up as about half a peakto-peak width of each absorption line in order to avoid overmodulation and to improve the S/N ratio. RESULTS AND DISCUSSION
The correlation time (%) of the molecular tumbling of the probe was calculated according to the formula (11) re = A
, - - 5G--~
FIG. 1. ESR spectra of the probe in 2lED, 21EDH20(40%), and H20 at room temperature.
ethylene glycol (Koso), tetraethylene glycol (Tokyo Kasei), ethylene glycol dimethylether (Tokyo Kasei), and tetraethylene glycol dimethylether (Tokyo Kasei) are guaranteed or chromatographic pure reagents and were used as model compounds of each part of the surfactant molecule. PEG 1000 (Wako) is a mixture of polyethylene glycols having various degrees of polymerization; the average molecular weight is 1000. The probe was dissolved in a concentration of about 10-~-10 -4 M in each sample. Some of them, for example, pure PEG 1000 or pure 2lED, are solids at room temperature. Therefore, the dissolution of the probe or dilution with water was carried out after the solids were melted at 60-70°C and the solutions were cooled to room temperature, but the aggregation of the probe was not discerned. The samples were drawn into a
" A n(m=+l)[(I(m=+l)/I(m=_l))
1/2 - -
1].
Here, H(m-+l) is a peak-to-peak width in gauss of the low-field absorption line and /(,,=+1) and I(m=_l) are peak-to-peak heights for the low- and high-field lines, respectively. The value of A was assumed as 6.6 x 10-1° according to Ref. (12). Figure 1 shows the ESR spectra of the probe in 100% 2lED and 21ED-H20(40%). The three absorption lines of each spectrum are highly symmetric, suggesting that it is unnecessary to consider the overlapping of the two types of absorption lines due to different circumstances of the probe. This observation is also true in the case of 21E D H20(90%). However, the lines become asymmetric as a result of the overlapping of the two lines due to the fact that the probe dissolves in the micelle and in water, if the concentration of the surfactant is near the CMC (13). Thus it is thought that most of the probes dissolve in the micelle in such concentrated solutions beyond 10g. Figure 2a shows the values of rc plotted against the concentration of the surfactant. The value increases abruptly with a decrease in water content in the region of 0-20% water content, but hardly changes Journal of Colloid and Interface Science, Vol. 63, No. 2, F e b r u a r y 1978
380
HISASHI YOSHIOKA
in the region of 2 0 - 9 0 % . Taking into account that ~'c is generally dependent on the microscopic viscosity, we are able to discern an extreme difference between the concentration dependence of ~-c and the m a c r o s c o p i c property. That is to say, 21ED-H~O(30%) is a viscous solution, but f r o m 40 to 70% water content, 2 1 E D - H 2 0 loses fluidity and b e c o m e s a transparent gel. H o w e v e r , the mixture turns once again into a viscous solution with a further increase in w a t e r content. This change is reflected in the viscoelastic p r o p e r t y (14). N e x t , P E G 1000 was used as a model compound of the hydrophilic part of the surfactant. The results of the P E G 1 0 0 0 - H 2 0 system are shown in Fig. 2b. N o flat part was found, in contrast to the case of the 2 1 E D H20 system, suggesting that micelle formation did not occur in this case. H o w e v e r , a discontinuous point of the slope was found on the curve near 30% w a t e r content. This percentage corresponds to a ratio of one oxygen a t o m of a P E G molecule hydrated with one w a t e r molecule, assuming that the molecular formula of P E G 1000 is H(OCH2CH2)22OH. H o m e p r o p o s e d a model for the hydration of P E G in which the adjacent O atoms in a P E G molecule were combined with one H 2 0 molecule through a hydrogen bond using two hydroxyl groups of the H 2 0 molecule (15). In this case, the ratio of H20 molecules vs O atoms is exactly 1:1. Accordingly, the curve in Fig. 2b suggests that each O a t o m of P E G 1000 is successively hydrated through a hydrogen bond by the addition of w a t e r up to 30%. Further added w a t e r m a y enter in p o l y m e r chains and form a different hydration core. In order to examine the position of the probe in the micelle, ~-~ was m e a s u r e d in pure dodecane and in pure H20. The values obtained, 6 × 10 -11 and 6 x 10 -11 sec, for two solvents are far smaller than those in the 2 1 E D - H 2 0 system. Therefore, the probe must be in the p o l y o x y e t h y l e n e groups of the surfactant molecules forming the micelle. Since the probe is considered to Journal o f Colloid and Interface Science, Vol. 63, N o . 2, F e b r u a r y 1978
E ° Uz. c)
i
\
: '8.,
', -.. , * " a" o o-T~.~_..0 _ . - o -
a o o
o...8-.~ o
_J rio u
"i
,,
",u
~.. b 2O
40
60
WATER CONTENT
80
100
(%)
FIG. 2. Concentration dependency of the correlation time in 21ED-H20 and PEG ]000-H20 systems. Each point was measured twice. monitor the environment, a decrease of % with an increase in water content up to about 25% must be the result of hydration of the group. Assuming that the hydration of the dodecyl group is negligible (8), the percentage of water is 26% when every O atom of the group is hydrated with one H20 molecule. A discontinuous point of the slope is not distinctly observed on the curve; however, the values of ~'e in the flat region are nearly equal to or somewhat smaller than those of 21ED-H20(26%). Therefore, it was concluded that the polyoxyethy]ene group of the suffactant is hydrated in the micelle by a ratio of about one H 2 0 molecule per O a t o m independent of the concentration of the surfactant, even if the micelle is formed in the presence of enough water. In order to p r o v e this concept, hyperfine coupling c o n s t a n t s were m e a s u r e d in various solvents. In the cases of the P E G 1000 and 2 l E D systems, the h y d r o x y l group or the dodecyl group attaching to the polyoxyethylene group at the ends of these molecules m a y have an effect on the constants, so ethylene glycol dimethylether (EGdiMe) was used as a model c o m p o u n d
381
SPIN PROBE STUDY OF MICELLES TABLE I Isotropic Hyperfine Coupling Constants in Various Solvents Solvent
Constant (gauss)
HeO Dodecane 2lED 21ED-H20(90%) EGdiMe EGdiMe-H20(28.5%) PEG 1000-H20(20%) PEG 1000-H20(30%)
16.3 14.6 15.1 15.6 14.9 15.5 15.3 15.6
to exclude the effect. Furthermore, an EGdiMe-HzO(28.5%) solution was prepared in which the ratio of the O atoms of the ether to the H20 molecules was 1:1. The results are shown in Table I. Again it was proved that the probe does not dissolve in the dodecyl group on the micelle. The values in the 2lED system agree fairly well with those of the EGdiMe system, showing that EGdiMe is a good model compound, and the 1:1 ratio of the hydrated water molecules to the O atoms of the ether linkage is confirmed. The electronic spectra of the probe in each solution were measured in order to determine the position of the probe. The probe has many absorption bands from the ultraviolet to the visible region. The band at hma x = 343 nm in dodecane is mainly solvent sensitive and the values of the band are shown in Table II. The values of 343 and 372 nm in dodecane and in water agree with those measured by Oakes (13). In the micelles of 2 1 E D - H 2 0 ( 3 0 % ) and 21ED-H20(90), the hmax are 364 nm, suggesting that the microscopic circumstances of the probe are identical in these two solutions. This is the same conclusion obtained from the %. The )tm~xis 361 nm in EGdiMe and 364 nm in EGdiMe-H20(28.5%), equal to that in the micelle in accord with expectation. The same value was obtained in tetraethylene glycol dimethylether, so the chain length did not affect the ~-max-
The data in the homologous series of ethylene glycol (EG) are not in accord with expectation. That is to say, they are 368 nm in EG, 366 nm in tetraethylene glycol, and 366 nm in PEG 600. The values of 369 nm in PEG 1000-H20(20%) and 370 nm in PEG 1000-H20(30%) are considerably shifted to longer wavelengths compared with those in the micelle. Therefore, PEG 1000 is not considered to be a good model compound for the UV absorption spectra. Next, it became necessary to determine whether or not there is a discrepancy between the Xmax in pure 2lED and that in pure PEG 1000 in the absence of water. These samples are solids at room temperature, so measurement was made at 70°C in the molten state of the melt. The values are 359 and 363 nm, respectively, showing the discrepancy even in the absence of hydration. This discrepancy and the results in EG homologous series suggest that the chromophore of the probe derived from the phenylhydrazine is affected by the terminal hydroxyl group. That is to say, the chromophore exists near the dodecyl group in the polyoxyethylene group and is not affected by the terminal hydroxyl group in 2 1 E D H20 system. Accordingly, EGdiMe is a TABLE II The Values of the hmax of the Most Solvent-Sensitive Absorption Band in Various Solvents Solvent
hmax (nm)
H20 Dodecane 21ED-H20(30%) 21 ED-H20(90%) EGdiMe EGdiMe- H20(28.5%) Tetraethylene glycol dimethyl ether PEG 1000- H20(20%) PEG 1000-H20(30%) Ethylene glycol Tetraethylene glycol PEG 600 2lED (70°C) PEG 1000 (70°C)
372 343 364 364 361 364 361 369 370 368 366 366 359 363
Journal o f Colloid and Interface Science, Vol. 63, No. 2, February 1978
382
HISASHI YOSHIOKA
good model compound even for the UV spectra. On the other hand, the group effect on the Xmax in EG homologous series including PEG 1000 probably acts through a hydrogen bond with the nitro or the imino group, so considerable red shift was observed even in the absence of the hydrated water. However, the effect may not be concerned with the hyperfine coupling constant because the N - O group is electronically independent of the chromophore. The fact that the values of the ~-c in the PEG 1000H20 system below 30% water content are larger than those in the 21ED-H20 system may be explained in terms of the immobilization of the probe by this interaction. From these results and discussion, it was concluded that the probe is in the polyoxyethylene group in the micelle near the dodecyl group and the ~'c are not affected by the terminal hydroxyl group. The oxygen atom of this part of the surfactant molecule is hydrated with water in the ratio 1:1. Even if entanglement occurs, resulting in geration of the solution, the ratio does not change, which is proved by the existence of the fiat region. The hydration examined by light scattering is 5.2-10.5 molecules of water per ethoxy group (5). However, the number is not definitive (for example, see Ref. (4)) depends on the method of measurement. It is calculated from the overall volume of the hydrated micelle by methods such as light scattering or viscosity. Therefore, the water
Journal o f Colloid and Interface Science, Vol. 63, No. 2, February 1978
adsorbed on the surface of the micelle may seriously affect the value. On the contrary, the spin probe method derives more precise information about the hydration of the inner part of the micelle. REFERENCES l. Stone, T. J., Buckman, T., Nordio, P. L., and McConnell, H. M., Proc. Nat. Acad. Sci. USA 54, 1010 (1965). 2. Griffith, O. H., and McConnell, H. M., Proc. Nat. Acad. Sci. USA 55, 8 (1966). 3. Fendler, J. H., and Fendler, E. J., "Catalysis in Micellar and Macromolecular Systems," Chap. 3. Academic Press, New York, 1975. 4. Becher, P., and Arai, H., J. Colloid Interface Sci. 27, 634 (1968). 5. E1Eini, D. I. D., Barry, B. W., and Rhodes, C. T., J. Colloid Interface Sci. 54, 348 (1976). 6. Muto, S., Deguchi, K., Kobayashi, K., Kaneko, E., and Meguro, K., J. Colloid Interface Sci. 33, 475 (1970). 7. Meguro, K., Akasu, H., and Ueno, M. J. Am. Oil Chem. Soc., 53, 145 (1976). 8. Pado, F., Ray, A., and Nemethy, G., J. Amer. Chem. Soc. 95, 6164 (1973). 9. Yoshioka, H., Uno, S., and Higashide, F., J. Appl. Polym. Sci. 20, 1425 (1976). 10. Rozantsev, E. G., and Neiman, M. B., Tetrahedron 20, 131 (1964). 11. Martinie, J., Michon, J., and Rassat, A., J. Amer. Chem. Soc. 97, 1818 (1975). 12. Griffith, O. H., Cornell, D. W., and McConneU, H. M., J. Chem. Phys. 43, 2909 (1965). 13. Oakes, J., J. Chem. Soc., Faraday Trans. H 68, 1464 (1972). 14. Kuroiwa, S., Kogyo Kagaku Zasshi 63, 106 (1960). 15. Horne, R. A., Almeida, J. P., Day, A. F., and Yu, N. T., J. Colloid Interface Sci. 35, 77 (1971).
Concentrated aqueous solutions of a nonionic surfactant, C12H25(OCH2CH2)21OH (2lED), were studied using a spin probe method. The correlation time (%) of the probe, 2,4-dinitrophenylhydrazone of 2,2,6,6-tetramethyl-4-piperidone N-oxide, in the micelle was - 1 . 2 × 10-a sec, suggesting that the probe was in the polyoxyethylene group. The value did not change greatly in the region of 25-90% water content, in contrast to the macroscopic properties. The hydration of the group of the surfactant in the micelle was discussed on the basis of %, the hyperfine coupling constant, and the hmax of UV absorption. It was concluded that the polyoxyethylene group was hydrated by the ratio of about one H20 molecule per oxygen atom of the ether linkage.
of the hydration increased gradually toward the terminal hydroxyl group. However, the extent of hydration did not become apparent quantitatively. In the present paper, the properties of 21ED-H20 system are examined using the spin probe method and the hydration of the hydrophilic polyoxyethylene group is discussed.
INTRODUCTION
Spin label or probe methods have been applied in various fields since they were proposed by McConnell et al. (1, 2). Micelle chemistry is a notably important field of applications and many papers have been published with respect to the structure, CMC, hydration, solubilization, etc. (3). However, all of them are concerned with ionic surfactants and a paper on a nonionic one has never been reported. The structure and the hydration of nonionic surfactant molecules in micelles have been studied using light scattering (4, 5) or charge transfer interactions (6, 7). The hydrophilic parts of these surfactants are predominantly polyoxyethylene groups. Pado et al. (8) used high-resolution nuclear magnetic resonance spectroscopy to examine the hydration of each part of the surfactant molecule forming a micelle (8). They proved that the lipophilic alkyl group existing in the inner part of the micelle was not hydrated to a significant extent. On the contrary, the polyoxyethylene group was hydrated even at the first ethoxy group adjacent to the alkyl group and the extent
EXPERIMENTAL
The probe used for the experiments is 2,4-dinitrophenylhydrazone of 2,2,6,6tetramethyl-4-piperidone N-oxide, with the following structure.
CH371-~ NH~N02 o-N CH3
CH3~ CHa
This material was synthesized by Rozantsev's method (10) from 2,4-dinitrophenylhydrazine (Wako, guaranteed) and 2,2,6,6tetramethyl-4-piperidone N - o x i d e described in Ref. (9). Dodecane (Wako), 378
002/-9797/78/0632-0378502.00/0 Copyright © 1978by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloidand Interface Science, Vol. 63, No. 2, February 1978
379
SPIN PROBE STUDY OF MICELLES
)
21ED
~f H20 #
/
j
capillary tube with an o.d. of 1 mm and sealed in the presence of air for ESR measurement. The UV spectra were measured by a Hitachi EPS-3T Recording Spectrometer. ESR spectra were recorded on a Japan E l e c t r o n O p t i c s L a b o r a t o r y Model JES-3BS.X Spectrometer (X-band) with 100-kHz field modulation. The width of the modulation was set up as about half a peakto-peak width of each absorption line in order to avoid overmodulation and to improve the S/N ratio. RESULTS AND DISCUSSION
The correlation time (%) of the molecular tumbling of the probe was calculated according to the formula (11) re = A
, - - 5G--~
FIG. 1. ESR spectra of the probe in 2lED, 21EDH20(40%), and H20 at room temperature.
ethylene glycol (Koso), tetraethylene glycol (Tokyo Kasei), ethylene glycol dimethylether (Tokyo Kasei), and tetraethylene glycol dimethylether (Tokyo Kasei) are guaranteed or chromatographic pure reagents and were used as model compounds of each part of the surfactant molecule. PEG 1000 (Wako) is a mixture of polyethylene glycols having various degrees of polymerization; the average molecular weight is 1000. The probe was dissolved in a concentration of about 10-~-10 -4 M in each sample. Some of them, for example, pure PEG 1000 or pure 2lED, are solids at room temperature. Therefore, the dissolution of the probe or dilution with water was carried out after the solids were melted at 60-70°C and the solutions were cooled to room temperature, but the aggregation of the probe was not discerned. The samples were drawn into a
" A n(m=+l)[(I(m=+l)/I(m=_l))
1/2 - -
1].
Here, H(m-+l) is a peak-to-peak width in gauss of the low-field absorption line and /(,,=+1) and I(m=_l) are peak-to-peak heights for the low- and high-field lines, respectively. The value of A was assumed as 6.6 x 10-1° according to Ref. (12). Figure 1 shows the ESR spectra of the probe in 100% 2lED and 21ED-H20(40%). The three absorption lines of each spectrum are highly symmetric, suggesting that it is unnecessary to consider the overlapping of the two types of absorption lines due to different circumstances of the probe. This observation is also true in the case of 21E D H20(90%). However, the lines become asymmetric as a result of the overlapping of the two lines due to the fact that the probe dissolves in the micelle and in water, if the concentration of the surfactant is near the CMC (13). Thus it is thought that most of the probes dissolve in the micelle in such concentrated solutions beyond 10g. Figure 2a shows the values of rc plotted against the concentration of the surfactant. The value increases abruptly with a decrease in water content in the region of 0-20% water content, but hardly changes Journal of Colloid and Interface Science, Vol. 63, No. 2, F e b r u a r y 1978
380
HISASHI YOSHIOKA
in the region of 2 0 - 9 0 % . Taking into account that ~'c is generally dependent on the microscopic viscosity, we are able to discern an extreme difference between the concentration dependence of ~-c and the m a c r o s c o p i c property. That is to say, 21ED-H~O(30%) is a viscous solution, but f r o m 40 to 70% water content, 2 1 E D - H 2 0 loses fluidity and b e c o m e s a transparent gel. H o w e v e r , the mixture turns once again into a viscous solution with a further increase in w a t e r content. This change is reflected in the viscoelastic p r o p e r t y (14). N e x t , P E G 1000 was used as a model compound of the hydrophilic part of the surfactant. The results of the P E G 1 0 0 0 - H 2 0 system are shown in Fig. 2b. N o flat part was found, in contrast to the case of the 2 1 E D H20 system, suggesting that micelle formation did not occur in this case. H o w e v e r , a discontinuous point of the slope was found on the curve near 30% w a t e r content. This percentage corresponds to a ratio of one oxygen a t o m of a P E G molecule hydrated with one w a t e r molecule, assuming that the molecular formula of P E G 1000 is H(OCH2CH2)22OH. H o m e p r o p o s e d a model for the hydration of P E G in which the adjacent O atoms in a P E G molecule were combined with one H 2 0 molecule through a hydrogen bond using two hydroxyl groups of the H 2 0 molecule (15). In this case, the ratio of H20 molecules vs O atoms is exactly 1:1. Accordingly, the curve in Fig. 2b suggests that each O a t o m of P E G 1000 is successively hydrated through a hydrogen bond by the addition of w a t e r up to 30%. Further added w a t e r m a y enter in p o l y m e r chains and form a different hydration core. In order to examine the position of the probe in the micelle, ~-~ was m e a s u r e d in pure dodecane and in pure H20. The values obtained, 6 × 10 -11 and 6 x 10 -11 sec, for two solvents are far smaller than those in the 2 1 E D - H 2 0 system. Therefore, the probe must be in the p o l y o x y e t h y l e n e groups of the surfactant molecules forming the micelle. Since the probe is considered to Journal o f Colloid and Interface Science, Vol. 63, N o . 2, F e b r u a r y 1978
E ° Uz. c)
i
\
: '8.,
', -.. , * " a" o o-T~.~_..0 _ . - o -
a o o
o...8-.~ o
_J rio u
"i
,,
",u
~.. b 2O
40
60
WATER CONTENT
80
100
(%)
FIG. 2. Concentration dependency of the correlation time in 21ED-H20 and PEG ]000-H20 systems. Each point was measured twice. monitor the environment, a decrease of % with an increase in water content up to about 25% must be the result of hydration of the group. Assuming that the hydration of the dodecyl group is negligible (8), the percentage of water is 26% when every O atom of the group is hydrated with one H20 molecule. A discontinuous point of the slope is not distinctly observed on the curve; however, the values of ~'e in the flat region are nearly equal to or somewhat smaller than those of 21ED-H20(26%). Therefore, it was concluded that the polyoxyethy]ene group of the suffactant is hydrated in the micelle by a ratio of about one H 2 0 molecule per O a t o m independent of the concentration of the surfactant, even if the micelle is formed in the presence of enough water. In order to p r o v e this concept, hyperfine coupling c o n s t a n t s were m e a s u r e d in various solvents. In the cases of the P E G 1000 and 2 l E D systems, the h y d r o x y l group or the dodecyl group attaching to the polyoxyethylene group at the ends of these molecules m a y have an effect on the constants, so ethylene glycol dimethylether (EGdiMe) was used as a model c o m p o u n d
381
SPIN PROBE STUDY OF MICELLES TABLE I Isotropic Hyperfine Coupling Constants in Various Solvents Solvent
Constant (gauss)
HeO Dodecane 2lED 21ED-H20(90%) EGdiMe EGdiMe-H20(28.5%) PEG 1000-H20(20%) PEG 1000-H20(30%)
16.3 14.6 15.1 15.6 14.9 15.5 15.3 15.6
to exclude the effect. Furthermore, an EGdiMe-HzO(28.5%) solution was prepared in which the ratio of the O atoms of the ether to the H20 molecules was 1:1. The results are shown in Table I. Again it was proved that the probe does not dissolve in the dodecyl group on the micelle. The values in the 2lED system agree fairly well with those of the EGdiMe system, showing that EGdiMe is a good model compound, and the 1:1 ratio of the hydrated water molecules to the O atoms of the ether linkage is confirmed. The electronic spectra of the probe in each solution were measured in order to determine the position of the probe. The probe has many absorption bands from the ultraviolet to the visible region. The band at hma x = 343 nm in dodecane is mainly solvent sensitive and the values of the band are shown in Table II. The values of 343 and 372 nm in dodecane and in water agree with those measured by Oakes (13). In the micelles of 2 1 E D - H 2 0 ( 3 0 % ) and 21ED-H20(90), the hmax are 364 nm, suggesting that the microscopic circumstances of the probe are identical in these two solutions. This is the same conclusion obtained from the %. The )tm~xis 361 nm in EGdiMe and 364 nm in EGdiMe-H20(28.5%), equal to that in the micelle in accord with expectation. The same value was obtained in tetraethylene glycol dimethylether, so the chain length did not affect the ~-max-
The data in the homologous series of ethylene glycol (EG) are not in accord with expectation. That is to say, they are 368 nm in EG, 366 nm in tetraethylene glycol, and 366 nm in PEG 600. The values of 369 nm in PEG 1000-H20(20%) and 370 nm in PEG 1000-H20(30%) are considerably shifted to longer wavelengths compared with those in the micelle. Therefore, PEG 1000 is not considered to be a good model compound for the UV absorption spectra. Next, it became necessary to determine whether or not there is a discrepancy between the Xmax in pure 2lED and that in pure PEG 1000 in the absence of water. These samples are solids at room temperature, so measurement was made at 70°C in the molten state of the melt. The values are 359 and 363 nm, respectively, showing the discrepancy even in the absence of hydration. This discrepancy and the results in EG homologous series suggest that the chromophore of the probe derived from the phenylhydrazine is affected by the terminal hydroxyl group. That is to say, the chromophore exists near the dodecyl group in the polyoxyethylene group and is not affected by the terminal hydroxyl group in 2 1 E D H20 system. Accordingly, EGdiMe is a TABLE II The Values of the hmax of the Most Solvent-Sensitive Absorption Band in Various Solvents Solvent
hmax (nm)
H20 Dodecane 21ED-H20(30%) 21 ED-H20(90%) EGdiMe EGdiMe- H20(28.5%) Tetraethylene glycol dimethyl ether PEG 1000- H20(20%) PEG 1000-H20(30%) Ethylene glycol Tetraethylene glycol PEG 600 2lED (70°C) PEG 1000 (70°C)
372 343 364 364 361 364 361 369 370 368 366 366 359 363
Journal o f Colloid and Interface Science, Vol. 63, No. 2, February 1978
382
HISASHI YOSHIOKA
good model compound even for the UV spectra. On the other hand, the group effect on the Xmax in EG homologous series including PEG 1000 probably acts through a hydrogen bond with the nitro or the imino group, so considerable red shift was observed even in the absence of the hydrated water. However, the effect may not be concerned with the hyperfine coupling constant because the N - O group is electronically independent of the chromophore. The fact that the values of the ~-c in the PEG 1000H20 system below 30% water content are larger than those in the 21ED-H20 system may be explained in terms of the immobilization of the probe by this interaction. From these results and discussion, it was concluded that the probe is in the polyoxyethylene group in the micelle near the dodecyl group and the ~'c are not affected by the terminal hydroxyl group. The oxygen atom of this part of the surfactant molecule is hydrated with water in the ratio 1:1. Even if entanglement occurs, resulting in geration of the solution, the ratio does not change, which is proved by the existence of the fiat region. The hydration examined by light scattering is 5.2-10.5 molecules of water per ethoxy group (5). However, the number is not definitive (for example, see Ref. (4)) depends on the method of measurement. It is calculated from the overall volume of the hydrated micelle by methods such as light scattering or viscosity. Therefore, the water
Journal o f Colloid and Interface Science, Vol. 63, No. 2, February 1978
adsorbed on the surface of the micelle may seriously affect the value. On the contrary, the spin probe method derives more precise information about the hydration of the inner part of the micelle. REFERENCES l. Stone, T. J., Buckman, T., Nordio, P. L., and McConnell, H. M., Proc. Nat. Acad. Sci. USA 54, 1010 (1965). 2. Griffith, O. H., and McConnell, H. M., Proc. Nat. Acad. Sci. USA 55, 8 (1966). 3. Fendler, J. H., and Fendler, E. J., "Catalysis in Micellar and Macromolecular Systems," Chap. 3. Academic Press, New York, 1975. 4. Becher, P., and Arai, H., J. Colloid Interface Sci. 27, 634 (1968). 5. E1Eini, D. I. D., Barry, B. W., and Rhodes, C. T., J. Colloid Interface Sci. 54, 348 (1976). 6. Muto, S., Deguchi, K., Kobayashi, K., Kaneko, E., and Meguro, K., J. Colloid Interface Sci. 33, 475 (1970). 7. Meguro, K., Akasu, H., and Ueno, M. J. Am. Oil Chem. Soc., 53, 145 (1976). 8. Pado, F., Ray, A., and Nemethy, G., J. Amer. Chem. Soc. 95, 6164 (1973). 9. Yoshioka, H., Uno, S., and Higashide, F., J. Appl. Polym. Sci. 20, 1425 (1976). 10. Rozantsev, E. G., and Neiman, M. B., Tetrahedron 20, 131 (1964). 11. Martinie, J., Michon, J., and Rassat, A., J. Amer. Chem. Soc. 97, 1818 (1975). 12. Griffith, O. H., Cornell, D. W., and McConneU, H. M., J. Chem. Phys. 43, 2909 (1965). 13. Oakes, J., J. Chem. Soc., Faraday Trans. H 68, 1464 (1972). 14. Kuroiwa, S., Kogyo Kagaku Zasshi 63, 106 (1960). 15. Horne, R. A., Almeida, J. P., Day, A. F., and Yu, N. T., J. Colloid Interface Sci. 35, 77 (1971).