J . Phys. Chem. 1990, 94,4635-4639
A Raman Investigation of a Nonionic Surfactant (C,,E,)-Water with the C,E,-Water System
4635
System. Comparison
Robert Gaufr&,* Serge Sportouch, Jamal eddine Ammour, Laboratoire de Spectroscopie MolZculaire, UniversitP de Montpellier 11, Place EugBne Bataillon, 34095 Montpellier Cedex 2, France
and Jacques Maillols Laboratoire de Physicochimie des SystBmes PolyphasPs. URA 330, UniversitP de Montpellier II, Place E u g h e Bataillon, 34095 Montpellier Cedex 2, France (Received: October 26, 1989) The Raman spectra of aqueous mixtures of n-dodecyl octa(oxyethy1ene) monoether (CI2E8)have been recorded for various temperatures (0-60 "C) and compositions (10-35.5% w / w CI2E8)corresponding to 32 points of the phase diagram, in the LI and l1 phases. The spectrum of the neat liquid has also been recorded for various temperatures, in the range 35-65 OC, and the spectrum of an aqueous mixture of n-octyl penta(oxyethy1ene)monoether (C8ES),56% w/w C8Es,in the range 10-65 OC was also recorded for comparison. The experimental results are treated by extensive use of difference spectra, according to a method which allows the characterization of the spectral changes by a measurable quantity, the reduced absolute difference area. The conformational changes as a function of temperature are found to be progressive over the whole range of temperature and composition under study, without any discontinuity at the transition between the I, and L, phases. The best characterized conformational changes concern the oligo(oxyethy1ene) part of the molecules of surfactant and are easily interpreted as an increase of population of trans conformations around the C-C bond, as temperature increases. They are more important in the aqueous mixtures than for the neat liquid, but of the same nature. The spectral changes in the L, phase of C8E5are identical with those found for C12E8.On the other hand, no significant spectral changes were found as a function of the composition.
Introduction Raman spectroscopy has widely been used to study systems with a supramolecular structure, such as lipidic bilayers,, and, in some cases, it is a quite good technique as phase transition monitor. It must be emphasized that the Raman spectrum does not give direct information about the supramolecular structure, but it often happens that a change of this structure is accompanied by a conformational alteration of the molecules, which has a Raman signature, so that it becomes possible to follow a change in the supramolecular structure. The micellar systems have also been investigated by Raman spectroscopy. For surfactants of rather high cmc (critical micellar concentration), spectral changes between the "monomer" and the molecule brought into a micelle, inside the isotropic phase of surfactant-water systems, have been observed, especially by Okabayashi and co-workers," and also by some other Another field of possible application of Raman spectroscopy is the study of phase transitions, and, in a few cases, the Raman spectra of a micellar isotropic phase and of a mesophase have been But in this case, some caution must be exercized for the interpretation of the spectral alteration at phase transition in terms of conformational changes. As a matter of fact, the first question to be asked is whether the phases, between which the transition occurs, are both isotropic or not. In the field of nonionic (1) Carey, P. R. Biochemical Applications of Raman and Resonance Raman Specfroscopies; Academic Press: New York, 1982; Chapter 8. (2) Okabayashi, H.; Okuyama, M.; Kitagawa, T.; Miyazawa, T. Bull. Chem. SOC.Jpn. 1974, 47, 1075. (3) Okabayashi, H.; Okuyama, M.; Kitagawa, T. Bull. Chem. SOC.Jpn. 1975, 48, 2264. (4) Okabayashi, H.; Abe, M. J . Phys. Chem. 1980.84, 999. (5) Okabayashi, H.; Taga, K.; Tsukamoto, K.; Tamaoki, H.; Yoshida, T.; Matsuura, H. Chem. Scr. 1985, 25, 153. (6) Tsukamoto, K.; Ohshima, K.; Taga, K.; Okabayashi, H.; Matsuura, H . J. Chem. Soc., Faraday Trans. I 1987, 83, 189. (7) Kalyanasundaram, K.; Thomas, J. K. J . Phys. Chem. 1976,80, 1462. (8) Rosenholm, J. B.; Stenius, P.; Danielsson, 1. J . Colloid Interface Sci. 1976, 57, S51. (9) Amorin da Costa, A. M.; Geraldes, C. F. G. C.; Texeiras-Dias, J. J. C. J. Colloid Interface Sci. 1982, 86. 254. (IO) Brooker. M. H.; Jobe, D. J.; Reinsborough, V. C. J . Chem. SOC., Faraday Trans. I 1984, 80, 13. ( 1 I ) Faiman, R.; Long, D. A. J. Raman Spectrosc. 1975, 3, 371. (12) Picquart, M. J. Phys. Chem. 1986, 90, 243. ( I 3) Bribes, J.-L.; GauMs, R.; Sportouch, S.;Maillols, J. J. Mol. Struct. 1986, 146, 267.
0022-3654/90/2094-4635$02.50/0
surfactant-water systems, we had observed important spectral changes at the hexagonal (HI)-micellar isotropic (L,)phase transition, especially in the v(CH) region, for the n-octyl penta(oxyethylene) glycol monoether (C8E5)-water system, but we later realized that these changes were to be accounted for on the ground of polarization changes of the incident and scattered beams in the anisotropic hexagonal phase. As the depolarization ratio in the Raman spectra of nonionic surfactants is medium in most regions, except for the u(CH) bands, the most important changes were observed in this range. In order to overcome this problem, a new method has been proposed.14 Anisotropic phases display reproducible Raman spectra insofar as the oriented domains are of very little size, so that the incident and scattered beams are completely depolarized, and the number of scattering "crystals" seen by the spectrometer is big enough so that the observed spectrum originates from randomly oriented domains. In such experimental conditions, the Raman spectrum of an anisotropic mesophase depends neither on the direction of polarization of the incident beam and of the component admitted in the spectrometer nor on the part of the sample seen by the spectrometer and, thus, may be compared to the spectrum defined as (1/411,+ 3/41,) for an adjacent isotropic phase. Using this method, we realized that the spectra of the molecules of C8Es remain identical on both sides of the boundary between the isotropic and hexagonal phases, a result which might be interpreted as fotlows: assuming that a change of micellar shape must involve some conformational modifications of the molecules of surfactant, the transition between the H I and L, phases of the C8E5-water systems is an order-disorder one. Nevertheless, such a result was obtained by means of an original method, involving a thermal processing of the sample, and we attempted to give further evidence of the invariance of the vibrational spectrum of the surfactant at a phase transition in a case for which no polarization effects are possible. Such a case occurs for a transition between a cubic mesophase and the adjacent micellar phase, and such a transition is found in the n-dodecyl octa(oxyethy1ene) glycol monoether (C12E8)-water system.Is Furthermore, because of the optical heterogeneity of a micro(14) Gaufrts, R.; Bribes, J.-L.; Sportouch, s.;Ammour, J.; Maillols, J. J. Raman Spectrosc. 1988, 19, 149. (IS) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. SOC.,Faraday Trans. I 1983, 79, 975.
0 1990 American Chemical Societv
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The Journal of Physical Chemistry, Vol. 94, No. 1I , 1990
r
T
Gaufrss et al. Raman Measurements. The excitation of the Raman spectra was obtained by the line at 514.5 nm of an argon ion laser, Spectra Physics 2020-03, operated at about 200 mW. The spectra were recorded with an OMARS 89 multichannel spectrometer of Dilor (France). The Raman cell was placed inside a temperatureregulated box, and its temperature was measured by a thermocouple set as close as close as possible by it. All along this work, we have dealt with the polarized I,, component. Data Treatment
weight
fraction CI2E8
Figure 1. Pertinent part of the phase diagram of the C,&-water system (after ref 15). The filled circles correspond to the compositions and temperatures investigated and the dotted lines to the paths along which the evolution of the spectra has been considered.
crystalline sample, the laser beam cannot be focused in the bulk of the sample, and the spectra of the microcrystalline anisotropic phase in our previous paperI4 displayed a poor signal/noise ratio and that was prejudicial to the experimental evidence of the identity of the spectra of both phases. Such a problem is not encountered with an isotropic mesophase. We got new evidence of the identity of the vibrational spectra on both sides of a phase boundary in such systems, and this result led us to consider the spectral behavior of the liquid phase Ll at various temperatures and for various compositions, in order to search spectral evidence of pretransitional phenomena, if any. What is of importance, especially as Jennings et al. have reported in a recent paper,16 is that electric birefringence gives evidence of three different relaxing species in this phase. All the points investigated in the diagram (L, and 1, phases) are reported in Figure I , with the paths (at constant composition or temperature) along which the spectra have been compared. We found differences between the spectra of the various points of the diagram investigated in the L, phase, especially a temperature dependence, and we have compared this behavior to that of the neat liquid. Finally, we have also investigated the temperature effect on the Raman spectra of the LI phase of the n-octyl penta(oxyethy1ene) glycol monoether (CBE,)-water system from IO to 65 OC, starting from the boundary of the hexagonal phase and approaching the consolution curve. As a matter of fact, there is an analogy between the L,phases of both systems (Cl,E8 and C8ES)as they are limited by a hexagonal phase and a consolution curve, and we have searched whether there is an analogy between the temperature dependence of the spectra. All along this work, we have extensively used difference spectra in a rather novel manner which will be discussed in this paper. Experimental Section
Materials. Both samples (CI2E8and CsE5) were of Nikko Chemicals (Japan) origin. They were used without any further purification. The composition and homogeneity of the binary mixtures were checked, in every case where it was possible, by the observation of a transition at the temperature in agreement with the data of the literature. For the 35.5% w/w ClzE8-water mixture, the transition was monitored by viscosimetry. When an anisotropic phase exists for the due composition, the temperature of transition was directly observed by a sudden change of the Raman spectrum in the v(C-H) region, varying temperature from the domain of the mesophases toward that of the liquid phase, the change being due, of course, to polarization effects. (16) Jennings, 6.R.; Nash, M. E.;Tiddy, G.J. T.J . Colloid Interface Sci. 1989.127, 537
The Raman spectra of CI2E8-water mixtures do not vary conspicuously with temperature or concentration. The features of the neat phases (liquid or solid) only are different from those of the aqueous mixtures. However, it is possible to follow slight modifications of the spectra as a function of temperature, for instance, from difference spectra. The process involved has already been 0ut1ined.I~ The spectra were divided into five ranges, chosen in such a manner that a nearly horizontal base line could be drawn between the boundaries, and each of them was included in one multichannel acquisition. These boundaries were 661-982, 994-1 172, 1199-1348, 1408-1564, and 2756-3037 cm-l for C8E5 and 660-975,976-1197, 1197-1346, 1378-1540, and 2752-301 1 cm-I for C12ES,respectively, referred, in what follows, as the 800-, 1075-, 1275-, 1450-, and 2900-cm-' ranges. In a set of spectra (or spectral ranges) to be compared, a reference spectrum was chosen, all the spectra of the set were normalized with respect to the area onto the reference spectrum, and the reference spectrum was subtracted. The difference spectrum represents the alteration of the intensity distribution within a spectral range at constant overall intensity. Its algebraic area vanishes, and its absolute area J", '
in which ul and u2 are the boundaries of the spectral range, gives a numerical estimate of the alteration of the spectrum. In order to compare the alteration of the intensity distribution either from one set of experiments to another, or between different ranges, the quantity AA must be divided by the area of the reference spectrum, and this results a "reduced absolute difference area"
which represents a relative variation of the intensity distribution. If now the increment of Aa is divided by the increment of the experimental variable, i.e., temperature or composition, one obtains the rate of variation of the intensity distribution, d(Aa)/d(AT) or d(Aa)/d(Ac). All along this work, we found that the graphs of the quantity Aa vs AT result in straight lines, the slopes being d(Aa)/d(AT). The intercept must be positive, as it represents the contribution of the noise to the difference spectrum. This method offers the following advantages: (i) In Raman spectra, such as those of the nonionic surfactants, there are only very smooth bunches of bands among which it is impossible to identify one specific band, but the difference spectrum may display more sharp peaks or valleys with measurable extremums. (ii) It allows comparisons between spectra recorded under quite different experimental conditions, as is the case for different phases, and, in any case, it allows freedom from the difficulties associated with the reproducibility of the optical adjustment. (iii) It results in a coherent numerical estimate of the variation of the intensity distribution, useful for trend studies and compari sons. The major disadvantage of the method lies in the fact that, due to the area normalization, a maximum (or minimum) in the difference spectrum denotes an increase (or decrease) of the local intensity referred to the overall intensity in the spectral range, but it may also correspond to a band of constant or decreasing (or increasing) absolute intensity. Moreover, an artificial intensity is induced by a spectral calibration shift, but that results in typical patterns in the difference spectrum (at wavenumbers corresponding
The Journal of Physical Chemistry, Vol. 94, No. 1 1 , 1990 4637
Raman Spectra of the CIzEs-Water System
Aa
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.
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.
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0 15 45 AT Figure 3. Plots of Aa vs AT for a CI2E8-watermixture (35.5% w/w C,,E8) from 0 to 60 "C and for the various spectral ranges, showing the linearity of Aa as a function of AT, and the continuity of d(Aa)/d(AT) across a phase transition. The temperatures between which the transition I, L, occurs are represented by empty circles.
-
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TABLE I: Rate of Variation X lo4 (K-I) of Aa as a Function of AT for the LI Phase and the Neat Liquid of Cll& spectral range composn (% w / w 800 1075 1275 1450 2900 CI2E8) cm-' cm-' cm-l cm-l cm-' 8.4 5.5 IO 15 14 5 4.8 9.3 5.7 20 14.3 12.4 9.4 4.2 14.6 12.3 6 30 7.3 4.5 35.5 16.6 13.2 5.9 8.1 4.6 4.5 6.7 neat liquid 5.9 With this composition, the transition was observed by viscosimetry between 12.5 and 15 OC. The difference between the spectrum of the cubic phase, recorded at 11.5 OC, and the spectrum of the micellar phase at 15 OC displays only noise (Figure 2, a-e). As for an isotropic mesophase there is no optical problem with the size of the crystals, it was possible to vary gradually temperature from 0 OC, in the midst of the cubic phase, up to 60 O C , in the L, phase, toward the consolution curve and to analyze the spectral changes in the whole temperature range. The plots of Aa vs AT, the reference temperature being 0 OC,are given in Figure 3, for the various spectral ranges. All the points for a given spectral range are disposed on a straight line, with a good correlation coefficient. This shows that the spectral change is progressive throughout the temperature range, without any discontinuity of Aa or d(Aa)/d(AT) at the transition. ( b ) Temperature Effects in the Spectrum of the L,Phase. The spectrum of the L1 phase for lo%, 20%, and 30% w/w CI2Es mixtures have been recorded at various temperatures in the ranges 15-70, 15-75, and 15-60 OC, respectively. A joint analysis of the results may be done with the 35.5% w/w mixture, used for the study of the transition I, L,. For all the compositions and all the spectral ranges, the graphs of Aa vs AT result in straight lines (an example is given in Figure 3) and the values of d(Aa)/d(AT) are given in Table I. The difference spectra for the various compositions are not significantly different. We give, for example, the difference spectrum between the maximum and minimum temperatures of the range of our study (60 and 15 "C) for the 30% w/w CI2Escomposition (Figure 4). This spectrum essentially displays smooth peaks and valleys with ill-defined maximum or minimum. However, the most striking features appear in the spectral ranges for which Aa/AT has the highest values, that is the 1075-cm-I and especially the 8 0 0 - ~ m -ranges. ~ The wavenumbers of the best-defined extremums are reported in Table 11. The same features are met at each composition under study. ( c ) Concentration Effects in the Spectrum of the L I Phase. Having at our disposal a lot of spectra of the LIphase recorded at the same temperatures for four different compositions, namely
-
I
2800
10
Figure 2. Raman spectrum ( I , component) of the CI2E8molecules in the I , phase ( T = 1 I .5 O C ; composition, 35.5% C12E8w/w) and difference spectrum between the former one and the spectrum ( I f i )of the L,phase (T = 15 OC,same composition) for the various spectral ranges. The trace of the difference spectrum remains close to the zero intensity line. (a) 800-cd range; (b) 1075-cm-' range; (c) 1275-cm-' range; (d) 1450-cm-' range; (e) 2900-cm-' range. to the steepest parts of the original spectra) and the shift may be corrected by computer methods. Results C12EB-WaterBinary System. ( a ) The Transition I l - L I . The transition has been studied with a 35.5% w/w CIZE8mixture.
Gaufrcs et al.
4638 The Journal of Physical Chemistry, Vol. 94, No. 1 1 , 1990 TABLE 11: Wavenumbers of the Best Defined Extremums of the Difference of Spectra Recorded at Different Temperatures' B, cm-' C, cm-l A , cm-l 810 (+. w) 810 (+, w) 810 (+. w ) 832 (-, S) 834 (-, W ) 840 (-. S) 851 (+, w ) 855 (+. w) 851 (+, w) 877 (-, W ) 879 (-, S) 894 (+, w) 890 (+. s) 894 (+, s) 1057 (-. W , d) 1056 (-. S) 1090 (+, w, d) 1131 (-, S, d ) I125 (-, W , d) 1125 (-, S) 1430 (-W, d,) 1432 (-, S) 1431 (-, W ) 1450 (+, s, d) 1470 (-, W, d) 1478 (-, W ) 1472 (-, S) 2842 (-, W ) 2840 (-. W ) 2858 (+, s, d) 2887 (-, W , d) 2882 (-, W ) 'A: CI2E8-water 30% w/w. Spectrum recorded at 60 O C - spectrum recorded at 15 OC. B: Neat CI2E8. Spectrum recorded at 65 OC - spectrum recorded at 35 "C. C: C8E,-water 56% w/w. Spectrum recorded at 65 OC - spectrum recorded at 10 OC. + indicates a maximum, - a minimum. s = strong, w = weak, d = diffuse.
TABLE 111: Rate of Variation X lo4 (K-') of A8 as a Function of AT, for the Ll Phase 56%w/w C B E ~ spectral range 1450 2900 1075 1275 800 cm-' cm-' cm-' cm-' cm" 5.2 3.8 8.9 2.3 d(Au)/d(An 9.7
IO%, 20%, 30%, and 35.5% w/w Ci2Es,we seized the opportunity to investigate the evolution of the spectra as a function of the composition along isotherms, though the composition range was rather narrow and we had at disposal four points only along an isotherm. We have also recorded the Raman spectrum of the L, phase for a point on the diagram just above the top of the H i boundary that is at 60 OC for a mixture of 50% w/w Ci2Es. The difference spectra we obtained are still less intense and sharp than those due to temperature effect, so that it is quite impossible to give reliable wavenumber of the extremums. However, the little differences we observed can in no way be related to the differences due to temperature effects. Neat Ci2Es.Temperature Effects. For the sake of comparison, the neat liquid has been investigated in the temperature range of 35-65 OC according to the process used for the binary mixtures. The rate of variation of Aa as a function of AT is also given in Table I. For the 800- and 1075-cm-' ranges, this rate is considerably lower (the half or even lower) than those found in the L, phase. On the other hand, they are quite similar for the other spectral ranges. Nevertheless, the most striking features of the difference spectra are found again in the 8 0 0 - ~ m -range. ~ The results are given in Table 11. CsE5-Water Binary Mixture. Temperature Effects. The spectra of a 56% w/w CsE5 mixture have also been recorded at various temperatures between I O and 65 OC. This composition is the same as that used in our previous work concerning the phase transitioni4 and corresponds to the top of the hexagonal phase, as can be seen on the phase diagram.,' The path we chose crosses over the micellar phase from the hexagonal phase to the consolution curve. The spectral ranges chosen for the treatment of the spectra are closely similar to those determined for CI2EB,the spectra themselves being also similar. The rates of variation of An as a function of AT for the various spectral ranges are given in Table 111. These rates are, for each spectral range, lower than for the L1 phase of C,,Es, but the trend over all the domains remains the same for both surfactants. The best defined extremums observed in the difference spectra are listed in Table I1 and the striking analogies with the difference spectra observed with Cl,E, (aqueous mixtures or neat liquid) may be seen at first glance. (17) Zuiauf, M.; Weckstrom, K.;Hayter, J. B.; J . Phys. Chem. 1985. 89, 341 1 ,
Degiorgio, V.; Corti, M.
1
.
0
cm-' 1325
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I -r
-
--
A
p
I
I
p
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cm-' 3000 2800 Figure 4. Raman spectrum (I,,component) of the CI2E8molecules in the L, phase ( T = 60 "C; composition, 30% CI2E8w/w) and difference
spectrum between the former one and the spectrum ( I , component) of the same mixture at T = 15 OC. The trace of the difference spectrum runs on both sides of the zero intensity line. (a) 800-cm-l range; (b) 1075-cm-I range; (c) 1275-cm-' range; (d) 1450-cm-I range; (e) 2900cm-l range.
Discussion All these experimental results, for both systems (C,,E8-water and CsE5-water), may be summarized as follows: (i) The Raman spectrum varies far more by temperature changes than by composition changes, at least in the composition range we have studied.
Raman Spectra of the C12E6-Water System (ii) The weak spectral changes we have observed as a function of composition are different from those observed as a function of temperature. They have indeed a different origin. (iii) The Raman spectrum does not change at a phase transition, and the spectral evolution as a function of temperature is the same on both sides of a phase boundary. (iv) The features of the difference spectra obtained by temperature variation are the same for the micellar phases and the neat liquids, but the rate of variation is significantly higher for the aqueous mixture than for the neat liquid. We shall discuss the weak spectral changes we have observed as a function of the composition. They may be assigned to changes of shape and/or size of the micelles. These changes would affect rather the hydrophobic heart than the poly(oxyethy1ene) outer layer. As a matter of fact, the conformer populations of the alkyl chains could be varied as the so-called "packing constraint"I5 is changed. In any case, Raman spectroscopy cannot give direct information about a change of a size or shape of the micelles. A correlation must be established with results obtained from other techniques such as pulsed field gradient NMR, light scattering (elastic and quasi-elastic), neutron scattering, and so on. Some authors have studied the C12E6-water system by these techn i q u e ~ , " - within ~~ composition and temperature ranges more or less overlapping the region we have studied, and, according to their conclusions, there are no dramatic changes of size and shape of the micelles in this region of the L, phase. The study of the evolution of the spectra as a function of the concentration might be restarted with experimental improvements in order to obtain difference spectra with a better signal/noise ratio. The evolution of the spectra as a function of temperature is far more important and better characterized. The most affected spectral regions are the 800- and 1075-cm-' ranges, at least for the hydrated surfactants (see Tables I and HI),but the difference spectra are better defined in the 800-cm-I range. The difference spectra in this most sensitive region may be interpreted in terms of conformational changes of the poly(oxyethy1ene) moiety of the molecules of surfactant. It seems now well established that the most stable conformation of a poly(oxyethy1ene) chain is, for a segment 0-CH2-CH2-0, the T-G-T sequence, as well as in oligomers (1,2-dimethoxyethane and its parent oligomers), in poly(oxyethy1ene) and in non-ionic surfactants of the n-alkyl oligo(oxyethy1ene) monoether type. This is supported by theoretical calculations2' and by vibrational spectroscopy, thanks to the extensive work of Matsuura and Fukuhara on n-alkyl oligo(oxyethylene) monoethers in the solid ~ t a t e , on ~ ~glyme . ~ ~ and oligoglymes as model molecules, and on poly(oxyethy1ene) in aqueous solution.2626 In the liquid state and in aqueous solutions, a partial conformational disorder is observed and the predominant sequence in poly(oxyethy1ene) in aqueous solution is T-G-T-G(18) Corti, M.; Degiorgio, V.; Hayter, J. B.; Zulauf, M. Chem. Phys. Len. 1984, 109, 579. (19) Degiorgio, V.; Corti, M.; Cantii, L. Chem. Phys. Lett. 1988, 151, 349. (20) Brown, W.; Pu, 2.;Rymden, R. J . Phys. Chem. 1988, 92, 6086. (21) Andersson, M.; KarlstrGm, G. J . Phys. Chem. 1985, 89, 4957. (22) Matsuura, H.; Fukuhara, K. J . Phys. Chem. 1986, 90, 3057. (23) Matsuura, H.; Fukuhara, K. J . Phys. Chem. 1987, 91, 6139. (24) Matsuura, H.; Fukuhara, K. J . Mol. Struct. 1985, 126, 251. (25) Matsuura, H.; Fukuhara, K. J . Polym. Sci. B 1986, 24, 1383. (26) Matsuura, H.; Fukuhara, K.; Tamaoki, H. J . Mol. Struct. 1987, J56,
293.
The Journal of Physical Chemistry, Vol. 94, No. 11, 1990 4639 G-T-T-G-T for the segment (0-CH2-CH2)3-0.24 The most intense extremum in the difference spectra of aqueous CI2E8and CsEs are found at 840 and 832 cm-I, respectively, and at 894 and 890 cm-I. The first signals are minimums and they correspond to a decrease of intensity as temperature is increased. This band may be assigned, on the ground of the normal coordinate analysis25 to the conformations G-x-x-G, for a CH2-CH2-0-CH2-CH2 segment, where x denotes either G or T. The other intense signals, which are maximums, correspond to the band at 890 cm-l in the Raman spectrum of poly(oxyethy1ene) chains which is assigned to the conformations T-x-x-G, defined as above. These two observations are coherent and denote the conformational change G-x-x-G T-x-x-G as temperature is increased. In liquid CI2EB, these extremum are also observed, but they are not the most striking features of the difference spectrum. A weak maximum at 810 cm-' is observed for neat and aqueous CI2E6,as for aqueous C&. It is assigned to the T-x conformations for a CH2-CH2-0 sequence, and this result is quite consistent with the previous ones. More difficult is the interpretation of a weak maximum at 855-851 cm-I, which would correspond to the band at 850 cm-' given by Matsuura and Fukahara for poly(oxyethy1ene) and assigned to G-x conformations. However, keeping in mind the fact that it is a weak signal, it may be interpreted as follows: this wavenumber is found as well in the G-x-x-G sequence as in the T-x-x-G one, the intensity may be stronger for the last one, and the sum of the intensities of all the conformers would increase as the population of T-x-x-G sequences increases. There is yet another extremum in the 800-cm-' range, which is a minimum appearing only in the spectra of C&, at 877 cm-' (weak) for aqueous mixture, and at 879 cm-I (strong) in the spectrum of the neat liquid. As no characteristic band of the poly(oxyethy1ene) appears at this wavenumber, and as it is not observed in the diffference spectrum of &E5, it may be assigned to the alkyl chain. At this stage of the discussion, it appears that the outer part of the micelles, formed by the oligo(oxyethy1ene) moieties of the molecules of surfactant, undergoes, as temperature increases, conformational changes which are progressive and without any break of continuity at phase transitions, at least between a I, and a L1 phase. These conformational changes are more important in an aqueous mixture than in the neat liquid, but are of the same nature. This can be seen from the intensity of the signals assigned to this part of the molecules in the difference spectra and is corroborated by the values of d(Aa)/d(AT) (Table 11). We previously interpreted the identity of the Raman spectra on both sides of a phase boundary as evidence of the fact that the shape of the micelles are not changed during the tran~iti0n.l~ In light of this more extensive work, in which it is found that the effect of temperature upon the conformations and the vibrational spectra is the most conspicuous phenomenon, at least within the concentration range studied, and as far as there is no experimental evidence of spectral changes related to changes of micellar shape, the weight of our previous statement must be attenuated. Finally, this work points out that the most direct information given by Raman spectroscopy in its application to nonionic surfactant water systems concerns the conformational changes of the hydrophilic moiety which, as recently ~uggested,~' might play an important role in many phenomena observed in micellar systems.
-
(27) Lindman, B.; Karlstrom, G.Z. Phys. Chem. 1987, 155, 199.
