J . Phys. Chem. 1989, 93, 2643-2648
2643
Fluorescence Probing and Ultrasonic Absorption Study of the Self-Association of 1-Butanol in Aqueous Solution R. Zana* Institut C. Sadron. 6 rue Boussingault, 67000 Strasbourg, France
and B. Michels Laboratoire de SpectromPtrie et d’lmagerie Ultrasonores. U.L.P.. rue BIaise Pascal, 67000 Strasbourg, France (Received: June 6, 1988; In Final Form: August 29, 1988)
Fluorescence probing with pyrene as a probe and ultrasonic absorption have been used to show that 1-butanol self-associates in water at concentrations above 0.9 M (solubility of 1-butanol in water: about 1 M at 25 “ C ) . The 1-butanol aggregates appear to be small, water-penetrated, and short-lived. Additions of low concentrations of ionic surfactants appear to stabilize these aggregates through mixed 1-butanolsurfactant micelle formation. The semiquantitative interpretation of the ultrasonic relaxation data yields reasonable values of the butanol exit rate constant from butanol aggregates and of the volume change associated with this process.
Introduction Nonionic surfactants of the poly(oxyethy1ene) monoalkyl ether type (C,H2,,+l(OCH2CH2),0H, referred to as C,E,) have been recently extensively in~estigated.l-~Even short-alkyl-chain C,E, surfactants such as C6E3,C6Es,and CsE5 show in aqueous solution a well-characterized micellization.’,6 These solutions also display a critical behavior'^^ and can give rise upon addition of oil to a complex phase pattern that includes microem~lsions.~ As part of our current interest in C,E, surfactants6.8-10we have investigated I-butanol, which can be considered as the C4Eo homologue, in aqueous solution. To our knowledge the self-association of I-butanol has never been evidenced. However, some results listed in the recent paper by Kilpatrick et al.” suggested to us that a possibility existed to obtain such evidence. These results indicate that C4E1self-associates in aqueous solutions at concentrations above 0.93 M. This concentration can be referred to as the “operational cmc” (cmc,) of C4EIbecause the aggregates of C4EI molecules are probably small and have been recently shown to be short-lived.I2 Moreover, the cmc’s of C,E, surfactants are known to depend only slightly on the value of m.I On the basis of the association behavior of C4H, one would thus expect I-butanol to be characterized by a cmcOparound 0.9 M. Since the solubility of 1-butanol in water is close to 1 M at 25 OC, this leaves only a very narrow concentration range where I-butanol aggregates may be present. This range, in fact, is too narrow for many methods normally used to evidence micelles, because the fraction of aggregated material would be too small. (1) Degiorgio, V. Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; Degiorgio, V., Corti, M., Eds.; North Holland: Amsterdam, 1985 and references therein. (2) Zulauf, M.; Rosenbuch, J.-P. J . Phys. Chem. 1983,87, 856. Zulauf, M.; Hayter, J. Colloid Polym. Sci. 1982,260, 1023. Zulauf, M.; Weckstrom, K.; Hayter, J.; Degiorgio, V.; Corti, M. J . Phys. Chem. 1985, 89, 3411. (3) (a) Corti, M.; Degiorgio, V.; Zulauf, M. Phys. Reu. Lett. 1982, 48, 1617. Corti, M.; Degiorgio, V. Surfactants in Solutions; Mittal, K., Lindman, B., Eds.; Plenum: New York, 1984; p 471. (b) Corti, M.; Minero, C.; Degiorgio, V. J . Phys. Chem. 1984, 88, 309. (4) Magid, L.; Triolo, R..; Johnson, J. J . Phys. Chem. 1984, 88, 5730. (5) Kato, T.; Seimiya, T. J . Phys. Chem. 1986, 90,3159. (6) Borthakur, A.; Zana, R. J . Phys. Chem. 1987, 91, 5957. (7) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985, 24, 654 and references therein; J. Phys. Chem. 1986, 90, 5239. (8) Zana, R.; Weill, C. J. Phys. Lett. 1985, 46, L-953. (9) Dormoy, Y.; Hirsch, E.; Candau, S. J.; Zana, R. Prog. Colloid Polym. Sci. 1987, 73, 8 1. (10) Binana-Limbel6, W.; Zana, R. J . Colloid Interface Sci. 1988, 121, XI
(1 1) Kilpatrick, P.; Davis, H. T.; Scriven, L. E.; Miller, W. G.J. Colloid Interface Sci. 1987, 118, 270. (12) Bodet, J. F.; Davis, H. T.; Scriven, L. E.; Miller, W. Langmuir 1988, 4, 455.
0022-3654/89/2093-2643$01.50/0
The purpose of this paper is to report evidence for the selfassociation of 1-butanol at concentrations above 0.9 M, obtained by means of ultrasonic absorption and fluorescence probing methods. Recall that these two methods have been very successfully used to detect micellization and to study micelle^.'^ To further support our conclusions, we also report results obtained for aqueous 1-butanol in the presence of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), and for 1-pentanol, 1-hexanol, and 2-butoxyethanol in water.
Experimental Procedure 1-Butanol, I-pentanol, 1-hexanol, and 2-butoxyethanol were of the best available grade (Fluka, Aldrich) and were used as received. The samples of SDS (Touzart-Matignon) and CTAB (Aldrich) have been purified by repeated recrystallizations. The ultrasonic absorption measurements were performed by using the standard pulse technique6 in the frequency range 3.91-155 MHz and the resonator method in the range 0.49-5 MHz.14 The absorption was measured as a function of concentration, temperature, and frequency and expressed as a/y ( a = ultrasonic absorption coefficient in m-’;f = frequency in hertz). Special care was taken in the measurements as a function of frequency to have the same temperature within 0.1 OC in the two series of experiments by the resonator method and the pulse technique, performed on two different campuses, because the excess absorption of the solutions with respect to water was found to be strongly dependent on temperature (see below). However, the temperature was controlled to within 0.01 “ C in each series of measurements. The error is estimated to be ;t3% in the resonator method and &8% and 1 3 % in the frequency ranges 3.91-1 1.68 and 15-155 MHz, respectively, in the pulse method. The fluorescence measurements involved pyrene as probe.I3J5 The fluorescence emission spectra were obtained with aerated solutions at very low pyrene concentration ((2-7) X lo”), which ensured the absence of pyrene excimer, by using a Fica 55 differential spectrofluorometer at an excitation wavelength of 335 nm. The spectra were used to determine the ratio 1,/13of the intensities of the first and third vibronic peaks of monomeric pyrene and I , . Recall that 11/Z3is sensitive to the microenvironment of pyrene and can be used to detect the changes of polarity sensed by pyrene when, upon micellization or aggregate formation, pyrene ~~
~
(1 3) See for instance Surfactant Solutions. New Methods of Investigation; Zana, R., Ed.; Dekker: New York, 1987; Chapters 5 and 8. (14) Eggers, F.; Funck, T. Reu. Sci. Instrum. 1973, 44,969. Dormoy, Y.; Michels, B. Acustica 1981, 49, 119. (15) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
0 1989 American Chemical Society
2644
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 1
I
1
I
Zana and Michels
I
70
10 30
20 Figure 1. Variation of the surface tension at the air-aqueous butanol solution interface with the butanol concentration at 25 ‘ C .
0
2 .I
II
1.6
ll 0.3
CA(M/P I
I
I
I
I
1
11
I
0.L
0.6
0.8
1.0
Figure 2. Variations of 11/13(0)and I , (+) with the concentration of butanol. The pyrene concentration was 6.58 X lod M (a qualitatively similar result was obtained with [pyrene] = 2.35 X IO4 M).
becomes partly or wholly solubilized within the aggregate^.^^^'^^'^^'^ The surface tensions have been measured with a Lauda MGW tensiometer. The measurements were performed at 25 OC unless stated otherwise.
Experimental Results Figure 1 shows the variation of the surface tension y of aqueous solutions of I-butanol with the butanol concentration CA,up to 0.847 M. The linear regime corresponding to the saturated airsolution interface is clearly apparent and yields a surface area per butanol molecule of 28.6 f 1 A2, when activities are substituted for concentration^.^' This value is in good agreement with that reported for butanol” and with other values that can be calculated from data reported for several linear I-alcohols.18 However, the y vs log CA curve shows no break. This indicates no butanol self-association in the investigated CA range. Recall that the y vs CAcurve for 2-butoxyethanol shows a break at CA N 0.9 M.” Above this concentration y remains constant and equal to about 27 mN/m. This value is close to that found for the 0.847 M butanol solution in the present investigation. Additions of tetradecyltrimethylammonium bromide (TTAB) from 2.64 X to 5.6 X M to the 0.847 M butanol solution left y unchanged at 26.2 mN/m. Figure 2 shows the changes of I , and 1 1 / 1 3with CA. The measurements were performed by progressively diluting the in(16) Lianos, P.; Lang, J.; Strazielle, C.; Zana, R. J. Phys. Chem. 1982, 86, 1019. (17) Harkins, W.; Wampler, R. J. Am. Chem. SOC.1931, 53, 850. (18) Posner, A.; Anderson, J.; Alexander, A. J. Colloid Sci. 1952, 7, 263.
( 1 9) Schwarz, F. P. J . Chem. Eng. Data 1977, 22, 213. (20) Mast, R.; Haynes, L. J. Colloid Interface Sci. 1975, 53, 35. (21) Zana, R.; Weill, C.; Muto, Y . ;Kimoto, Y., unpublished results. (22) Zana, R.; Lianos, P.; Lang, J. J. Phys. Chem. 1985, 89, 41.
The Journal of Physical Chemistry, Vol. 93, No. 6,1989 2645
Self-Association of 1-Butanol
I/
100
0
0
10-5
Figure 4. Variations of 11/13(0, +) and I , ( 0 ) a t constant butanol concentration (0.963 M) and increasing concentration C, of SDS (0,0 ) and CTAB (+). [Pyrene] = 2.48 X lod M. I
1
1
1
I
I
210 200 190 180
I
I
I
I
I
I
+'
0.2
10-2
10-3
10-4
0.1
CA(M/Q)
0 1
I
0.7
0.8
0.9
I 1
Figure 6. Variation of the ultrasonic absorption of butanol solutions in water a t 25 "C, with the butanol concentration, a t 9.05 (+) and 11.65 (0)MHz. The inset shows the results for aqueous solutions of 1-pentanol (+) and 1-hexanol (0)at 25 "C and 11.68 MHz.
I
0.5
0.6 0.7 0.8 0.9 1.0 1.1 Figure 5. Variation of 1,/13 (0)and I ] ( 0 ) at constant SDS concen., [Pyrene] = tration (0.01 M) and increasing butanol concentration C 1.9 X IOd M.
noticeable at C,= 5 X M, that is, a concentration much lower and 9 X lo-" M for than the cmc of both surfactants (8.3 X SDS and CTAB, r e s p e ~ t i v e l y ~ ~ -Figure ~ ~ ) . 4 also shows that I I decreases upon increasing SDS concentration. A qualitatively similar behavior was found upon additions of CTAB. On the other hand, Figure 5 shows that in experiments at a constant SDS concentration of 0.01 M, that is, above the cmc of pure SDS, 11/13 remains constant and 1, decreases slightly when CAis increased from about 0.5 to 1 M, indicating that the microenvironment of pyrene is not affected by this change of CA. Notice that the value 0.01 M in Figure 4 and when CA ranges from of 11/13at C, 0.5 to 1 M in Figure 5 is lower than those in pure SDS or CTAB micelles, in agreement with previous findings.16-26 The decrease of 11/13, that is of polarity sensed by micelle-solubilized pyrene upon addition of alcohols to micellar solutions of SDS,I6CTAB, or tetradecyltrimethylammonium bromide,26was attributed to a partial substitution of the water in the micelle outer shell (the so-called palisade layer made of the surfactant head groups, the first one or two methylene groups, and water) by the solubilized alcohol. An alternate or complementary explanation may be that pyrene migrates from the palisade layer to a solubilization site located somewhat deeper within the micelle. As noted before, the decrease of [ ] / I 3 in Figure 4 occurs at C, well below the concentrations where SDS or CTAB start forming micelles in the presence of butanol. It must therefore be associated with the incorporation of surfactant to the butanol aggregates, which makes them more compact and less water-penetrated and corresponds to the formation of mixed butanol-surfactant micelles. Thus the results of Figures 4 and 5 confirm the existence of butanol aggregates, which can incorporate surfactants. (23) The presence of alcohol, such as butanol, may lower or increase the cmc of ionic surfactants in water. For instance the cmc of tetradecyltrimethylammonium bromide is 3.5 X low3M in water and about 8 X lo4 M in H20-0.8 M butanol,24whereas that of SDS oes from 8 X lo-' M in water M in H20-0.8 M butanol.& to about 9 X (24) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J . Colloid Interface Sci. 1981, 80, 208. (25) Singh. H.; Swarup, S. Bull. Chem. SOC.Jpn. 1978, 51, 1534. (26) Lianos, P.; Zana, R. Chem. Phys. Left. 1980.72, 171 and 1980, 76, 62.
0.8
0.9
1.0
1.1
1.2
1.3
1.4
Figure 7. Variation of the phase separation temperature of aqueous butanol solutions with the butanol concentration.
The ultrasonic absorption a/f of aqueous butanol solutions has been measured as a function of C,, at constant frequency and temperature. The results at T = 25 OC, represented in Figure 6 , show a sharp increase of a/f at CA> 0.9 M, similar to those observed with aqueous solutions of ionic2' and nonionic6 surfactants, when the surfactant concentration is increased above the cmc. At CA = 0.7 M the absorption of the butanol solution is equal to that of water (no excess absorption). Figure 6 shows that this is also the case for aqueous solutions of I-pentanol and 1hexanol up to the solubility limit (about 0.25 and 0.065 M, respectively). The a/f vs CAplot for butanol solutions at 15 OC was qualitatively similar to that at 25 OC, but the excess absorption with respect to water was much smaller. The fact that the excess absorption occurs essentially at CA > 0.9 M, that is, close to the solubility limit of butanol, suggested the possibility that this excess absorption arose owing to the proximity of a critical point for the water-butanol mixtures. Recall that ultrasonic absorption is very sensitive to the concentration fluctuations taking place in liquid mixtures close to critical con(27) Graber, E.; Lang, J.; Zana, R. Kolloid 2.Z . Polym. 1970, 238, 470.
2646
Zana and Michels
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
I
(m-'s*)
++-\
8o
11c
+
t
+\
1
+
90
7c
5C 54
1
2
3
5
10
20
50
100
150
Figure 10. Difference relaxation spectra for the 0.9467 (0) and 0.9687 (t)M butanol solutions, with respect to the 0.8808 M solution.
30
T ("0 I
I
I
15
20
25
Figure 8. Effect of temperature on the ultrasonic absorption of water and of a 0.966 M butanol solution (0),a t 11.65 MHz.
(X)
solutions are characteristic of more than a single relaxation frequency. There appear to be three relaxation ranges: (i) a first one, below 1 MHz, not reported in previous studies of alcohol and were ethoxylated alcohol solution^^^-^^ as all such studies but restricted to frequencies above a few megahertz; (ii) a second range between 5 and 50 MHz, not seen with the 0.8808 M solution, and which thus appears to arise from the self-association of butanol; (iii) a third range at f > 50 MHz. The variations of a/f with f for the three solutions are somewhat similar at frequencies below 2 and above 50 MHz. If one assumes the amplitude of the relaxation processes taking place in these ranges to be only little affected by the small increase of CAin going from 0.8808 to 0.9687 M, the difference between the absorptions at a given concentration above 0.9 M and at CA = 0.8808 M should closely amount to the contribution of self-association. Figure 10 shows the difference spectra as just defined for the 0.9467 and 0.9687 M solutions. The lines drawn through the experimental data obey the relaxation equation
fiMHz) 4
,
I
,
I
I
,
I
I
,
20 30 50 100 150 Figure 9. Ultrasonic relaxation spectra a t 25 "C for butanol solutions in water: C = 0.8808 (a),0.9467 (0),and 0.9687 (t)M. 0.L
0.8 1
2
3
5
10
d i t i o n ~ . ~ ~ - ~We O therefore determined the phase separation temperature T , of water-butanol mixtures in the CA range of interest (Figure 7) and investigated the temperature dependence of the ultrasonic absorption of a 0.966 M butanol solution that phase-separates at T, = 26 OC (Figure 8). Figure 7 shows that T, decreases monotonically from 41 to 1 "C when CAis increased from 0.86 to 1.35 M. Clearly the critical concentration and temperature of the water-butanol mixtures must be well above 1.4 M and well below 1 O C , that is, far from the experimental conditions under which most of the ultrasonic measurements were performed. Figure 8 shows that the excess absorption with respect to water increases somewhat linearly with T, above 20 O C , without showing the divergence characterizing the approach of the critical temperature. In view of the results of Figures 7 and 8, it can be concluded that the observed excess absorption is not due to critical effects. Figure 9 shows the ultrasonic absorption relaxation spectra of three butanol solutions, one of which has a concentration slightly below 0.9 M. This solution is characterized by a low excess absorption that decreases only very slightly between 1 and 50 MHz, whereas the absorption of the other two solutions decreases mainly between 5 and 50 MHz. However, the spectra for the three (28) Labowski, M. Acoust. Lett. 1978, 2, 130 and 1979, 3, 37. (29) Anantaraman, A.; Walters, A.; Edmonds, P.; Pings, C. J. Chem. Phys. 1966, 44, 265 1 . (30) D'Arrigo, G.;Sette, D. J . Chem. Phys. 1968, 48, 691.
where A and B are two constants and fR is the relaxation frequency. It can be seen that excellent fits are obtained in the whole frequency range (0.49-155 MHz) when one uses a least-square procedure with A , B, and fR as adjustable parameters. The B values (2.5 and 3.5 X m-l s2) for the 0.9467 and 09687 M solutions are rather small compared to those of the relaxation m-I s2,respectively). They may amplitude A (52.5 and 76 X very well arise from the procedure adopted in calculating the contribution Aa/f of the self-association process but may also reveal the existence of a high-frequency relaxation process. Unfortunately, the limited CArange that can be investigated does not allow a choice to be made. Nothing can be said as regards the possible CA dependence of fR in view of the very narrow concentration range where the contribution of self-association can be measured. For both spectra, the fits yieldedfR = 21 1 MHz. Figure 11 shows that the ultrasonic absorption of butanol solutions in the presence of 0.01 M SDS or 0.005 M CTAB increases with the butanol concentration particularly at CA> 0.9 M. Three
*
(31) Nishikawa, S.; Mashima, M.; Yasunaga, T. Bull. Chem. SOC.Jpn. 1976, 49, 1413 and references therein.
(32) Tamura, K.; Maekawa, M.; Yasunaga, T. J. Phys. Chem. 1977,81, 2122. (33) Nishikawa, S.; Shibata, M. Bull. Chem. SOC.Jpn. 1984, 57, 2357 and references therein. (34) Madigosky, W.; Warfield, R. J . Chem. Phys. 1987, 86, 1491 and references therein. (35) Fanning, R.; Kruus, P. Can. J . Chem. 1970, 48, 2052. (36) Nishikawa, S. Bull. Chem. SOC.Jpn. 1987, 60, 2785. J . Solution Chem. 1986, 15, 221 and references therein. (37) Rao, N.; Verrall, R. J . Phys. Chem. 1986, 86, 4777. (38) Kato, S.; Jobe, D.; Rao, N. P.; Ho, C.; Verrall, R. J. Phys. Chem. 1986, 90, 4167.
Self-Association of 1-Butanol
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
000
500
000
io0
0.5 0.6 0.7 0.8 0.9 1.0 1.1 Figure 11. Effect of the butanol concentration on the ultrasonic absorption of butanol solutions in H z 0 4 . 0 1 M SDS (X, 0 ) and Hz0-0.005 0 ) MHz at 25 "C. M CTAB (+, 0) at 3.91 ( X , +) and 11.65 (0, facts are noteworthy. First, the comparison of Figures 6 and 11 shows that the absorption measured in the presence of surfactant is much larger, by up to 1 order of magnitude, than in the absence of surfactant. Second, the experimental data points for the systems containing SDS and CTAB fall on the same curve at f = 3.91 MHz, within the experimental accuracy. At f = 11.65 MHz, the data for the SDS and CTAB systems define two different curves, but the relative difference between the two sets of data decreases rapidly as CAincreases. Recall that 11/13remained constant and low (see Figure 5 ) under the experimental conditions corresponding to Figure 11. Thus the ultrasonic absorption results in this figure appear to reveal the formation of butanol-rich micelles containing some solubilized surfactant at CA> 0.9 M. The third fact concerns the frequency dependence of the absorption measured in the presence of surfactant. Even though the data refer to restricted ranges of ultrasonic frequency (3.91-1 1.68 MHz) and alcohol concentration (0.5-1.1 M), their examination nevertheless reveals that at CA> 0.9 M the absorption increases very rapidly and that the dominant relaxation process has a relaxation frequency probably in the 5-IO-MHz range, that is, lower than that measured in the absence of surfactant (see above). This lowering may be due to the fact that the butanol aggregates now contain some long-chain surfactant ions and are thus larger than in the absence of surfactant. Larger aggregates are always characterized by lower relaxation frequencies, in surfactant system^.^' At CA ranging between 0.7 and 0.9 M the dominant relaxation process appears to have a relaxation frequency above 10 MHz. The origin of this relaxation is unclear at the present time, and its assignment would require systematic studies that are beyond the scope of this study. Indeed in this CArange the solution contains mixed SDS-butanol micelles, while our aim was to investigate the self-association of butanol.
Discussion 1 . SelfAssociation of 1-Butanol in Water. The above fluorescence and ultrasonic absorption data suggest that 1-butanol self-associates in water at concentrations above 0.9 M. This concentration can be considered as the operational cmc of l-butanol in water. Because of the limited solubility of 1-butanol in water, our studies were restricted to concentrations close to this operational cmc. This may be the reason for which butanol aggregates appear to remain fairly loose and small in the range between 0.9 M and the solubility limit of butanol in water, as indicated by the high polarity still sensed by pyrene in this range. We have unsuccessfully attempted to detect the presence of butanol
2641
aggregates by quasi-elastic light scattering at CA> 0.9 M. This negative result suggests that the butanol aggregates are short-lived, with a lifetime typically lower than 0.3 ~s (delay time of the correlator used in these measurements). A similar conclusion was reached in the case of 2-butoxyethan01.~~ Such a short lifetime raises the question as to whether these aggregates are really aggregates in the sense given to this word in the case of micellar solutions, or whether they should be considered as fluctuations of concentration taking place in the solution. Recall that the excess ultrasonic absorption of alcohol solutions has often been assigned to concentration fluctuation^.^^^^^ However, in the only quantitative study known to us, on water-propanol mixtures,34 the measurements involved very concentrated alcohol solutions where indeed concentration fluctuations can be significant. In the case of the butanol and butoxyethanol aqueous solutions investigated, the changes of fluorescence intensity ratio Z1/Z3 reported above suggest that we are indeed dealing with aggregates. Their lifetimes are short because the hydrophobic chains include only four carbon atoms, but this is expected from a direct extrapolation of known results of micellar kinetics for longer chain surfactant~'~ (see also next paragraph). This interpretation does not preclude the occurrence of an ultrasonic absorption due to concentration fluctuations, at higher CA in the case of b u t ~ x y e t h a n o l . ~ ~ 2. Semiquantitative Interpretation of Ultrasonic Spectra of Aqueous Butanol Solutions. The ultrasonic data, relaxation amplitude A and relaxation frequencyfR obtained from the spectra in Figure 10, can be used to infer some information on the thermodynamics and dynamics of the exchange of butanol between butanol aggregates and the bulk, on the basis of the Aniansson and Wall theory of micelle kinetic^.^^^^ Recall that this treatment applies only to micelles with a fairly large aggregation number. Such is clearly not the case of butanol aggregates. thus the use of this theory is expected to only yield semiquantitative data. In the framework of Aniansson and Wall theory the relaxation frequency is given by
-1
CA-cmc cmc N and
k- = k+
X
cmc
(2)
where k- and k+ are the rate constant for the exit and hxrporation of a surfactant (here butanol) from/into the micelles proper, of aggregation number N , and u is the width of the micelle size distribution curve assumed to be of Gaussian shape. As the present measurements were performed at CAclose to the cmc and since u Z / Nhas been consistently found to be between 1 and 2,13 the bracketed term in eq 1 can be set equal to 1 and
(3) On the other hand, for short-chain surfactants k+ has been observed to be close to its diffusion-controlled limit.I3 Using k+ N 4 X lo9 M-I s-l a nd cmc = cmcOp= 0.9 M, we calculated
k-
E
3.6
X
lo9 s-I
and a - 5
Recall that the rate constant for the exit of pentanol from micelles of ionic surfactants (TTAB or CTAB) has been found41 to be about lo9 s-l. This lower value is in line with the known decrease of k- by a factor of 3 per additional CH, group.I3 The value of u can be checked by comparing it to the value of the aggregation (39) Aniansson, E. A. G.;Wall, S . J . Phys. Chem. 1974, 78, 1024 and 1975, 79, 857. (40) Aniansson, E. A. G.; Wall, S.; Almgren, M.; Hoffmann, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J . Phys. Chem. 1976, 80, 905. (41) Yiv, S . ; Zana, R.; Ulbricht, W.; Hoffmann, H. J . Colloid Interface Sci. 1981, 80, 224.
2648
J. Phys. Chem. 1989, 93. 2648-2652
number N of butanol aggregates calculated on the basis of the oil drop model for the micelle core and assuming the oil core radius to be 7.4 A, that is, about 1 A longer than the length of the fully N 11 and o2 N extended chain. Under these assumptions Ncald 2NCaId.This result is consistent with tabulated values of a and N for a variety of surfactant^.^^ As a last test we have used the expression of the relaxation amplitude43
dv Avo2 k- -(C - cmc) R T fR2 N
A = 0.05-
(4)
to calculate the volume change AVOupon incorporation of one butanol molecule into butanol aggregates. In eq 4 d is the density of the solution (g ~ m - ~u) is , the velocity of ultrasound in the solution ( N 1.5 X los cm s-l), and A is expressed in m-l s2. The two spectra yield AVO = 2.2 f 0.2 cm3/mol. Note that values ranging between 1 and 4 cm3/mol have been reported for the transfer of 1-butanol from water to various micellar pseudo-
phase^.^^-^^ (42) See ref 13, p 433.
(43) Zana, R.; Yiv, S.Can. J . Chem. 1980, 58, 1780. (44) De Lisi, R.; Turco Liveri, V.; Castagnolo, M.; Inglese, A. J . Solution Chem. 1986, 15, 2 3 .
Conclusions
The results obtained in this study reveal that I-butanol selfassociates in aqueous solutions at concentrations above 0.9 M. Since its solubility in water is close to 1 M, the narrow concentration range where I-butanol aggregates are present has prevented thus far the observation of its self-assocation. The 1-butanol aggregates appear to be small, water-penetrated, and short-lived. They can co-micellize with ionic surfactants, giving rise to somewhat larger micelles. The semiquantitative interpretation of the ultrasonic relaxation data yields reasonable values of the rate constant for butanol exit from butanol aggregates and of the volume change associated with this process.
Acknowledgment. This work has benefited from the financial assistance of the PIRSEM (CNRS, France) under AIP No. 1887. We thank Drs. S. J. Candau and E. Hirsch for performing the quasi-elastic light-scattering measurements. Registry No. SDS, 151-21-3; CTAB, 57-09-0; TTAB, 11 19-97-7; I-butanol, 71-36-3; 1-pentanol, 71-41-0; I-hexanol, 11 1-27-3; 2-butoxyethanol, 11 1-76-2; pyrene, 129-00-0. (45) De Lisi, R.; Genova, C.; Testa, R.; Turco Liveri, V. J. Solution Chem. 1984, 13, 121. (46) Manabe, M.; Shirahama, K.; Koda, M. Bull. Chem. SOC.Jpn. 1976, 49, 2904.
Thermat Behavior of Several Hyperquenched Organic Glasses G. P. Johari,* Department of Materials Science and Engineering, McMaster Univeristy, Hamilton, Ontario L8S 4L7, Canada
Andreas Hallbrucker, and Erwin Mayer Institut fur Anorganische und Analytische Chemie. Universitat Innsbruck, A 6020 Innsbruck, Austria (Received: June 17, 1987; In Final Form: September 16, 1988)
Glassy states of propylene glycol and its two polymers have been prepared by hrperquenching (>lo5 K s-I) of their micrometer-size droplets from an aerosol. Their thermal behavior was studied by differential scanning calorimetry and compared with that of slow-cooled glasses. Rate heating and isothermal annealing experiments show that, although initially the hyperquenched glasses undergo a rapid enthalpy relaxation, the relaxation becomes slower at temperatures closer to their respective Tgs than of the corresponding slow-cooled glasses. The initial rapid relaxation is attributed to a high structural state of “fictive temperature” dependence for the average relaxation time in the hyperquenched glass whose distribution is comparatively narrow. The evolution of its fictive temperature (structure) on densification and its slow relaxation keep it from reaching the equilibrium state point in the same period as the slow-cooled glass. Possible reasons for the differences between the structural states of hyperquenched and slow-cooled glasses are discussed.
Introduction One of the techniques recently developed for vitrifying liquids involves rapid cooling or hyperquenching of micrometer-size droplets, obtained from an aerosol, by depositing them on a substrate held at 77 K.Is2 This technique, with an estimated cooling rate higher than lo5 K s-’, is as effective in producing a glassy solid as is melt spinning, splat quenching, and laser melting and has the further advantage of being particularly useful for vitrifying liquids of low thermal conductivity and for producing nonductile glasses. A hyperquenched glass, obtained from these techniques, has a fictive temperature, T,, much higher than the normal glass transition temperature, Tg,at which the viscosity of a liquid is lOI3 P and its calorimetrically determined structural relaxation time is -200 s. Because of their high Tf, such glasses are less dense, with relatively higher enthalpy and entropy and differ in several of their kinetic behaviors from the corresponding glasses of lower Tf. ( I ) Mayer, E. J . Appl. Phys. 1985,58,663; J . Phys. Chem. 1985.89, 3474. (2) Mayer, E. J . Microsc. 1985, 140, 3.
0022-3654/89/2093-2648$01.50/0
The effect of a large change in Tf on the relaxation features of a glass is now known. This is mainly because a comparative study of two samples of a glass, one obtained by hyperquenching using the above techniques and the second by normal supercooling ( 10 K m i d ) has not been possible. Substances usually obtained by the first procedure of vitrification crystallize on supercooling during the second procedure and it is difficult to determine their heat capacity during the very rapid quenching. Computer simu l a t i o n ~ ,which ~ , ~ used an approximate Lennard-Jones interaction potential for argon atoms, indicate that on rapid cooling (10’o-1012 K SKI) glass transition occurs over a wide temperature range or that the decrease of heat capacity at the transition is smeared out over a wide temperature range. We report an investigation of how the kinetic properties of a rapidly quenched glass differ from those of a slow-cooled glass and discuss the reasons for such differences. A low molecular weight and a high molecular weight polymer and a molecular glass of the same substance were chosen N
(3) Fox, G.;Anderson, H. C . Ann. N.Y. Acad. Sci. 1981, 37, 123. (4) Damgaard Kristensen, W. J . Non-Cryst. Solids 1976, 21, 303.
0 1989 American Chemical Society
