3815
J. Phys. Chem. 1991.95, 3815-3819 band of a clathrate hydrate, has also been observed. The greater breadth and structure of the type-I large-cage wj band of W 0 2 , compared to that of the dilute 13C02isotopomer, together with a contrasting response to warming, provides strong evidence that the ' T O 2molecules in the large cages are subject to significant transition dipole interactions. The ease of epitaxial formation of the nonpolar molecule clathrate hydrates, which does not occur on other surfaces regardless of temperature, is believed to relate directly to the un-
usually great activity of Bjerrum L defects in the ether hydrate substrates. This relationship will be examined in detail in a forthcoming paper. Acknowledgment. The funding of this research by the National Science Foundation under Grant CHE-87 19998 is gratefully acknowledged. Registry No. H20:C02,30486-16-9;H20:THF, 18879-05-5;H20:
EO,57266-52-1 I
,H NMR Study of Molecular Dynamics and Organization In the System C,,E,-Water Ulf Henriksson, Department of Physical Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden
Mikael Jonstromer, Ulf Olsson, Olle Merman,* Division of Physical Chemistry 1, University of Lund, P.O.Box 124, S-221 00 Lund, Sweden
and Gotthard Klose Sektion Physik, Karl Marx University of Leipzig, GDR-7010 Leipzig, DDR (Received: March 7 , 1990)
A sample containing 20 wt % of the nonionic surfactant tetraethylene glycol dodecyl ether (CI2E4), specifically deuterated in the a-position, was investigated with 2HNMR relaxation in H20. From the frequency dependence of the longitudinal relaxation rate in the Larmor frequency range 2-55 MHz, it was concluded that the solution contains rodlike micelles. A slow motion in the microsecond time scale, as determined from the transverse relaxation rate, was interpreted taking the
flexibility of the rodlike micelles explicitly into account.
Introduction The aggregation behavior and phase equilibria in binary systems of water-oligo (ethylene oxide) alkyl ethers (C,E,) depend strongly on the length of the alkyl chain and the number of EO units in the polar group.lJ For the CI2surfactants it is only when the number of EO groups is 14 that an aqueous micellar solution phase L,appears in the phase diagram. The phase diagram for the system C12E4-water is shown in Figure 1. The LIphase is stable from the freezing point to the cloud point (16 OC for a 20 wt % solution).. For surfactants with a higher number of EO groups the cloud point is raised and the L1phase is stable in a wider temperature range. For C,E, surfactants the micellar size and especially its temperature dependence have been the topic of several st~dies.~-'l However, it seems that there is little agreement among the results obtained by different experimental methods. Recently, multifield 2H nuclear magnetic relaxation studies have (1) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J . Chem. Soc.. Faraday Trans. I 1983, 79,975. (2) SjBblom, J.; Stenius, P.; Danielsson, I. In Nonionic Surfactants;Schick, M. J., Ed.; Marcel Decker: New York. 1987;p 369. (3) Brown, W.; Johnsen, R.; Stilbs, P.; Lindman, B. J . fhys. Chem. 1983, 87, 4548. (4)Brown, W.;Rymddn, R. J . fhys. Chem. 1987,91, 3565. (5) Zulauf, M.; WeckstrBm, K.; Hayter, J. B.; Degiorgio, V.; Corti, M. J . Phys. Chem. 1985.89, 341 1. (6) Ldfroth, J. E.; Almgren, M. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984;Vol. 1, p 627. (7)Binana-Limbeld. W.; Zana, R. J. 1. Colloid Interface Sci. 1988, 121, 81. ( 8 ) Herrington, T.; Sahi, S. S. J . Colloid Interface Sci. 1988, 121, 107. (9)Nash, M. E.;Jennings, B. R.; Tiddy, G. J. T. J . Colloid Interface Sci. 1987. 120, 342. (IO)Nilsson, P. G.; WennerstrBm, H.; Lindman. B. J. Phys. Chem. 1983, 87, 1377. (11) Magid, L. J. In Nonionic Surfactants; Schick, M. J., Ed.;Marcel Decker: New York, 1987;p 677.
0022-3654/91/2095-3815.$02.50/0
given information about the molecular dynamics in ionic surfactant systems. In micellar solutions and in cubic liquid crystals containing finite size aggregates of surfactant molecules, it has been possible to infer information about the aggregate shape and size from the spectral density function for the overall aggregate m ~ t i o n . l ~ - 'In ~ this paper we report the results from a 2H relaxation study of a micellar aqueous solution of C12E4 with the purpose of exploring the use of multifield nuclear magnetic relaxation to get information about aggregate size and growth in a nonionic surfactant system. Experimental Section Tetraethylene glycol dodecyl ether, specifically labeled in the a-methylene group of the dodecyl chain, was synthesized according to the reaction H(OCH2CH2)40H CH,ONa H(OCH2CH2)40Na+ C H 3 0 H
+
-
and
-
H3C(CH2)loC2H2Br+ NaO(CH2CH20)4H H3C(CH2)loC2H20(CH2CH20)4H + NaBr Attention must be paid to the condition that the second reaction takes place under total exclusion of oxygen; otherwise simultaneously different ketones are formed which cannot be separated afterward from the desired product. The obtained CI2E4was (12)SMerman, 0.; Walderhaug, H.; Henriksson, U.; Stilbs, P. J . fhys. Chem. 1985,89,3693. (13) SMerman, 0.;Henriksson, U.; Olsson, U. J . fhys. Chem. 1987,91, 116. (14) Sbderman, 0.Henriksson, ; U. J. Chem. Soc.,Faraday Trans. I 1987, 83, 1515. (IS) Ndry, H; SMerman, 0.;Walderhaug, H.; Lindman, B. J . fhys. Chem. 1986,90,5802.
0 1991 American Chemical Society
3816 The Journal of Physical Chemistry, Vol. 95, No.9, 1991 901
'
'
'
'
W+L,
v
60
'
'
'
Henriksson et al.
I
1
250
200
. >
150 100
\
b
30
50 0
6
6.5
7
0
7.5
8
1% ( v / Hz)
20
60 80 100 wt% C12E, Figure 1. Phase diagram for the system C12E,-H20 taken from ref 1. L,and L2 denote the micellar and reverse micellar phase, respectively, L3denotes the isotropic so-called "anomalous phase", L, the lamellar phase, and W a very diluted surfactant solution. 0
40
separated from the mixture which also contains C12E4C12by vacuum distillation (p = 6.6 Pa, T = 190 OC).'4'' The tetraethylene glycol (for synthesis) was purchased from Merck and used without further purification, and the partially deuterated dodecyl bromide was obtained from Isocommerz, Leipzig (about 95 atom 96 2Hin the a-methylene group). Samples were prepared by weighing the components directly in the NMR tubes. Since we only had access to a limited amount of the deuterated surfactant, samples were prepared that where mixed with commercial C12E4, obtained from Nikko Chemicals, Tokyo, Japan. Four samples were prepared. The total surfactant concentration and the fraction of deuterated compound (given in parentheses) were 5 (1/5), 10 (1/1), 20 (1/2), and 40 wt 96 (1/4), respectively. The cloud point of the deuterated compound was slightly lower than for the commercial compound. The clouding temperatures were about 8.5, 3.5, and 13.5 "C for the samples with $10, and 15 wt 96 while the sample with 40 wt 96 had a L1 lamellar phase transition around 15 OC. The main investigation was carried out on the sample with 20 wt 96 surfactant for which the longitudinal relaxation rate was measured in an extensive frequency range 2-55 MHz. For the other samples, relaxation experiments were only performed at the two highest field strengths (39 and 55 MHz). The 2H longitudinal relaxation rates, Rl, were measured with the inversion-recovery method, and the transverse relaxation rates, R2, were obtained from the line widths, corrected for the magnetic field inhomogeneity. The measurements at 55.3 and 39.1 MHz were performed on a Nicolet 360 spectrometer and a home-built spectrometer with an Oxford wide-bore magnet, respectively. For all measurements below 15 MHz a Bruker MSL spectrometer equipped with an iron-core magnet and a field variable flux stabilizer HS 90var was used. The temperature was controlled to within f l OC of the desired temperature. The 2H quadrupole splittings in the lamellar phase were measured at 55.3 MHz on the Nicolet 360 spectrometer. Results and Discussion The 2H longitudinal relaxation rates RI at 8 OC for a 20 wt % ' aqueous solution of C12E4, specifically deuterium labeled in the a-methylene group in the dodecyl chain, were measured in the frequency range 2-55 MHz, and the results are shown in Figure 2. The R2values for the two highest frequencies are presented in Table I. As seen, there is a clear frequency dependence in the observed Ri values in the whole studied frequency range, and R2 a t the highest frequencies are considerably higher than R i at 2 MHz. It should be mentioned here that all 2H N M R band(16) Gerhardt, W. Monatsberichre; Deutsche Akademie: Berlin, 1967; Vol. 9, p 922. (17) Schrlring, S.;Ziegenbein, W. Tenride 1967, 4, 161.
i
.
L 150 1 &-
100
f
50
-
\\-:
"
0
'
"
"
"
8 8
39.9 55.3 39.9 55.3
2 2
'
"
~
~
"
'
800 730 2370 2190
'
'
740 730
shapes observed in the present study are Lorentzian, to within the experimental uncertainty. This immediately permits us to draw the conclusion that reorientational motions with correlation times of the order 10 ns are present and that still slower motions (correlation times 1 1 0 0 ns) also must occur in order to account for the very rapid transverse relaxation. The frequency dependence in the low-frequency range (2-10 MHz) resembles that found in studies of spherical micelles. In the case of spherical micelles the quadrupolar interaction is averaged to zero by the isotropic micellar tumbling and surfactant diffusion over the curved micellar surface. Consequently, extreme narrowing conditions are obtained a t low frequencies (-2 M H z ) . ' ~ ' ~In the present case,however, R2 >> R1even at 2 MHz, and we can therefore rule out the presence of spherical micelles. The micelles must be extended in one (rodlike micelles) or two (disklike micelles) dimensions. Of these two possibilities, disklike micelles can be ruled out on account of the R i dispersion below 10 MHz. In rodlike micelles the surfactant diffusion around the long axis of the rod is expected to give an R1 dispersion in this particular frequency range, while in an oblate micelle this intermediate dynamical process must be assigned to surfactant surface diffusion. However, in order to predict the dispersion below 10 MHz, unreasonably large surfactant lateral diffusion coefficients have to be used.'* Note
-
(18) Halle,
B. Mol. Phys. 1987, 61, 963.
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3817
Molecular Dynamics of the System Ci2E4-Water further that a hexagonal liquid crystalline phase forms in the present C12E4 system at lower temperatures (-2 OC)l and is therefore not indicated in the phase diagram in Figure 1. Thus, we may conclude that the solution of 20 wt 7%Ci2E4in H 2 0most likely contains rodlike micelles. However, from the present set of data we cannot rule out the presence of aggregate shapes that slightly differ from cylindrical geometry-for instance, elongated micelles with a slightly ellipsoid cross-sectional area. The 2H relaxation rates contain information about the spectral density function J ( w ) and thus about the molecular dynamics in the system according to the equation^'^ Ri = 1/Tl = ( 3 ~ ~ / 4 o ) x ~ [ w + ( ~8oJ ()2 ~ o ) ] R2 = 1/T2 = (3r2/4O)x2[3J(0) + sJ(W0)
+ 2J(24]
TABLE II:
Panmeter Values Obtained from the Best Fit of the Four-SteD Model to the Experimental R I rod R* Data' zL' 0.9 i 0.1 ps 7p 40 f 20 pa 711' 23 2 ns S 0.139 f 0.006 TP' 1.9 0.7 ns SI 0.26 0.06
*
*
"The uncertainties correspond to a 80% level of confidence, taking only random errors into account.
(1)
(2)
where wo is the 2H Larmor frequency and x is the 2Hquadrupole coupling constant. Slightly different values for x have been proposed (167-181 kHz).Zo In this work we will use the value x = 181 kHz.2' For spherical micelles it has been shown that a spectral density function of the form J ( w ) = S2P(w)
+ (1 - S2)P(w)
(3)
very well accounts for the relaxation dispersion in the 1-50-MHz regime.22*23 The time scale separation of the spectral density function corresponds physicall to a separation of fast local molecular motions, described by (a), and slower motions associated with the micellar aggregate, Le., aggregate tumbling and surfactant diffusion within the aggregate, described by P(o). S is denoted the order parameter and is the time average
1
s = f/2(3 cos2 e ( t ) - 1 )
(4)
where e(?) is the time-dependent angle between the C-D bond and a local normal to the aggregate surface and the time average is to be taken over the fast local molecular motions. For a stiff rodlike micelle with an aspect ratio >>1, P(w) splits further into one part JIIB(w) associated with the dynamics around the long axis and a second part J l s ( w ) associated with the "end-over-end tumbling". J(w) =
f/4JlSb)
+ 3/4J11"4
(5)
The full spectral density can then be written J ( w ) = f/4S2J,'(0)
+ 3/J2Jl16(w) + (1 - S2)P(w)
(6)
JHs(w)includes rotational diffusion of the rod and surfactant
diffusion around the long axis and is expected to be Lorentzian with a correlation time 711-l
= (Tllrot)-'
+
(T1ldifq-l
(7)
assuming the surfactant diffusion and the rotation of the rod to be statistically independent. Since R2 >> R I , J L s ( w ) 0 in the megahertz regime. We have therefore fitted the Rl data using a spectral density of the form J ( w ) = (3/4)S2Jlls(w)+ ( 1 S 2 ) f ( w ) .This fit is shown in Figure 2a. The fit is reasonably good a t lower frequencies; however, it is clear that the fast local motions are not in extreme narrowing (Le., there are motions with that contribute to the relaxation) correlation times 7c < at 35-50 MHz, a fact which has also been shown to be the case in several other systems.24 This effect is more visible in situations where, as in the present case, the aggregate dynamics are shifted toward lower frequencies. The simplest way to account for the (19) Abragam, A. The Principles of Nuclear Magnetism; Clarendon: Oxford. 1961. (20)'Greenfield, M. S.;Vold, R. L.; Vold, R. R. J. Chem. fhys. 1985,83, 1440. (21) Sideman, 0. J . Magn. Reson. 1986, 75, 296. (22) WennerstrBm, H.; Lindman, B.; Merman, 0.; Drakenberg, T.; Rw senholm, J. 9. J. Am. Chem. Soc. 1979, 101, 6860. (23) Halle, B.; WennerstrBm. H. J . Chem. fhys. 1981, 84, 4475. (24) Olsson. U. Thesis, University of Lund, Sweden, 1988.
20
30
50
40
TI
60
O C
Figure 3. Order parameters Pbdirectly measured in the lamellar phase at three different C12E4concentrations which are given in the figure. The order parameters have been calculated from the observed quadrupole splittings using the quadrupole coupling constant x = 181 kHz.
frequency dependence seen at higher frequencies is to separate J'(w) into two Lorentzians, viz.
where SIis a parameter, describing the fraction of the quadrupolar interaction which is averaged by the most rapid motion. Again setting J I s ( w ) = 0, we have fitted the RI data using the extended expression (eq 8) for the spectral density. As can be seen from Figure 2b and in Table I, where predicted R2values are presented, the quality of the fit is satisfactory. The parameters obtained from the fit are given in Table 11. The parameters S and 711' are obtained with good accuracy since they describe the observed dispersion around 5 MHz, where there are quite a few data points. ?', and 7Twhich describe the faster motions The parameters SI, have higher uncertainties on account of the fewer number of measurements at higher frequencies. In reality, eq 8 represents a four-step model and we must address the question of whether it is physically meaningful to use such a complicated spectral density function and whether the parameter values obtained from the fit are reasonable. For the fast local motions we obtain the two correlation times 7p = 40 ps and 7"' = 1.9 ns. The most rapid local motion with a correlation time 7T = 40 ps can be assigned to local motions within the alkyl chain like trans-gauche isomerizations and torsional oscillations. The correlation time T~'= 1.9 ns is too short to be associated with any overall motion of micellar aggregates formed by the C I 2 surfactant.'* This motion, which is responsible for the observed frequency dependence above 10 MHz, must therefore be a local motion, most probably the rotational diffusion of the whole surfactant molecule around its long axis and/or wobbling motion of this axis within a cone.25 Since our data set at frequencies above 10 MHz is rather limited, we feel that a more detailed modeling of the local surfactant molecular motions, for instance along the lines of ref 25, is not warranted. Turning to the value of the local order parameter S, viz. S = 0.139, this value can be compared with the order parameter SUI, directly determined from the 2H quadrupolar splittings id the
-
(25) Pastor, R.; Venable, R. M.; Karplus, M.; Szabo, A. J. Chem. fhys. 1988.89, 1128.
Henriksson et al.
3818 The Journal of Physical Chemistry, Vol. 95, No. 9, 1991
lamellar phase as shown in Figure 3 for the same compositions and a t some different temperatures. In agreement with Ward et a1.?6 who have determined the order parameter profiles for both the alkyl chain and the polar group for a 60:40CI2E4-H20 lavalues are considerably smaller mellar phase, we find that the Phm than the corresponding directly measured order parameters in lamellar phases formed by ionic surfactants?' They are also much smaller than the local order parameter S in the rodlike micelles obtained from the fitting procedure. The anchoring of the surfactants at the hydrophopic/hydrophilic interface is expected to be weaker in nonionic micelles as compared to micelles formed by ionic surfactants. This would explain the lower order parameter found in the former systems. In addition, on account of the weaker anchoring, the values of the order parameters are expected to be rather sensitive to parameters such as aggregate geometry, which would explain the difference in the order parameter found between the rodlike micelles and the lamellar phase. In this context it is interesting to note the X-ray study by Clunie et of the lamellar and hexagonal liquid crystalline phases in the system C12E,-water, where it is suggested that there is a considerable difference in the chain packing between hexagonal and lamellar aggregates formed by C,E, surfactants. It is also possible that the quadrupole interaction in the lamellar phase is averaged not only by the local molecular motions but also by low-frequency collective deformations that might be ~resent.2~ If such deformations have correlation times that are longer than T ~ they ~ do , not affect the relaxation in the micelles even if they are present since the tumbling of the aggregates averages the quadrupole interaction to zero and still slower motions have no relaxation effect. A third possibility for the small order parameter found for the lamellar phase is the presence of defects (water-filled holes) within the surfactant layers. The presence of such irregularities within the bilayer structure has been suggested by Charvolin et al. for related surfactant systems.M However, in a recent scattering study of the C12ES-watersystem, Strey et al. argue against such defects." Introducing the R2 values measured at the two highest frequencies, whose main contribution arises from the zero-frequency spectral density, we get T~~ = (1/2)JLs(0) = 0.9 ps (cf. Table 11) where we have assumed that JIs((w) is given by a Lorentzian spectral density. For comparison of order of magnitudes, this correlation time equals the correlation time for the end-over-end tumbling of an infinite diluted 300-A-long, stiff rod with a cross-section radius of 25 A. However, in a 20 wt % solution the rods are entangled. Moreover, we expect rodlike micelles made of nonionic surfactants to be rather flexible. In a flexible rod, the long axis is curved in space and the surfactant diffusion along the curved long axis will constitute a mechanism for reorientation and can therefore contribute to the relaxation. The correlation function for such a motion is not exponential but has a long-time tail which has to be cut off by some isotropic motion in order to account for the experimentally observed Lorentzian bandshapes. This cutoff may be due to rotational aggregate diffusion or the exchange of surfactant molecules between aggregates. The combined motion of lateral diffusion and an isotropic motion has a complex correlation function, but the zero-frequency spectral density can be written as32
(26) Ward, A. J. I.; Ku, H.; Phillippi, M. A.; Marie, C. Mol. Crysr. Liq. Crysl. 1988, 154, 5 5 . (27) Klason, T.; Hcnriksson, U. In Solurion Eehuuiour of Surfactants; Mittal, K. L., Fendlcr, E. J., Eds.; Plenum: New York, 1982; Vol. 1. p 417. (28) Clunic, J. S.;Goodman, J. F.; Symons, P. C. Trans. Furuduy Soc. 1969, 65, 287. (29) Rommel, E.; Noack, F.; Meicr, P.; Kothe, G. J . Phys. Chem. 1988, 92, 298 1. (30) Charvolin, J. Conremp. Phys. 1990, 31, 1 . (31) Strey, R.; Schomlcker, R.; Roux, D.; Nallet, F.; Olsson, U. J . Chem. soc., Furuduy Trans. 1990,86, 2253. (32) Halle, B.; WennerstrBm, H.; Piculcll, L. J . Phys. Chem. 1984, 88, 2482.
,
.
. . . . . . . .,
. . . . . . .,
10-5
10-6
107
Tiso
.
'
'
""'I
104
1s
Figure 4. T,N as a function of T for various values of 4,assuming 4, = 1 x 10-l' m2S-I: (a) 4 = 5 0 1 , (b) rp = 100 A, (c) 4 = 200 A, (d) = 400 A, (e) L~ =
-.
qatand T~~ are correlation times associated with the lateral diffusion and some isotropic motion, respectively. To interpret T,,,~ further, we need to specify the configuration of the flexible rod. Assuming that it follows the statistics of the wormlike chain model (WCM),33*34T~~~is given by35 T1at
= L:/~DI,,
(10)
where L,,is the persistence length and Dla,is the lateral diffusion coefficient. Dlatcan be estimated from the surfactant diffusion constant in bicontinuous cubic phases. Data for CI2E4do not exist. However, there are data for C12E8and C12ESin the bicontinuous cubic phases, V,,that form in their respective binary systems with water. For C&8 diffusion constants of 2.5 X m2 s-I and 7.5 X m2 s-l have been measured at 5 and 25 OC, respectively.1° For C12Es,D = 1.6 X lo-" m2 s-' was measured at 20 0C.36 Taking into account the dependence on volume fraction, temperature, and molecular weight, a reasonable estimate of Dlat for CI2E4a t 8 "C is 1 X lo-" m2 s-l. F~ and T~~ cannot be determined independently. However, for a given htand & we can investigate their individual contributions to the effective correlation time T,P To do so, we fix Dbt to lo-" m2 and plot T~~ as a function of Tso for different values of L,,. This is done in Figure 4, where is varied between 50 and 400 A. A line with L,, = m, correspon ing to a stiff rod, is also shown in Figure 4. Note that, according to eq 10, T~~~depends more strongly on L,, than on Dlat. For Teff = 0.9 ps it is seen in Figure 4 that pL0has to be about 5 ps for a L,, of 50 8, (of the same order as the rod diameter), while for rp = 400 A, F~ = ~ ~ f This l . value of Teff is shorter by 1-2 orders of magnitude than the value found for concentrated solutions of ionic surfactants containing very long micelles." We now have to interpret ~hwhich, according to the discussion above, is of the order of a few microseconds. For a stiff rod at infinite dilution this would correspond to a length of several thousand angstroms. However, as pointed out above, entanglement is expected at the present surfactant concentration of 20 wt %. Such an entanglement slows down the reorientational dynamics, and consequently much smaller lengths have to be invoked in order to give reorientational dynamics in the microsecond regime. A second candidate is the exchange of surfactant monomers. Due to the very low cmc and the high aggregation number, the exchange with free monomers in the bulk is expected to be very slow. On the other hand, exchange can occur through collisions of two aggregates, Le., through a fission-fusion process. Such a process operates in the closely related system C12Es-water'o and
3
(33) Kratky, 0.;Porod, G. R e d . Truu. Chim. Pays-Bus 1949,68, 1106. (34) Saito, N.; Takahashi. K.; Yonoki,Y. J. Phys. Soc. Jpn. 1%7, 22,219. (35) Hallc, B. Personal communication. (36) Olsson, U.;SMerman, 0. Unpublished work. (37) Olsson, U.; Werman, 0.; GuEring, P. J. Phys. Chem. 1986,90,5223.
J. Phys. Chem. 1991,95, 3819-3823
also operates when the micelles are swollen with 0 i 1 . ~ ~ In ~ ' ~both the binary and ternary systems, a maximum in R2 was found as a function of temperature while a minimum in the surfactant diffusion constant, coinciding with the R2 maximum, was observed. It was concluded that at temperatures above the R2 maximum exchange through fission-fusion processes was more rapid than aggregate reorientation. In the present system R2 was also measured at 2 OC and was found to be much larger than at 8 OC (cf. Table I). This difference is too large to be due to the temperature dependence of thermal motions only, assuming structure, length scales, and interactions to remain unchanged. For a 20 wt % solution in the CI2E5-water system, the fission-fusion processes appear to contribute to the spin relaxation even at lower OC)'O far below the clouding temperature ( e 4 0 temperatures (4 "C).Since the clouding temperature for the present sample is about 16 OC,it is reasonable to assume that this is also the case in this sample at the two temperatures 2 and 8 "C. Another possible contribution to the increase in R2 with decreasing temperature is a growth of the micellar length. If the fission-fusion processes provide the dominating contribution to R2,then the large difference in R2 at the two temperatures reflects a strong temperature dependence in the intermicellar interactions. This is not inconsistent with the upper consolute boundary, found in these types of systems, indicating that interactions are becoming progressively attractive with increasing temperature. Measurements of Rl and R2at the highest field strengths (39 and 55 MHz) were also performed at three other compositions ( 5 , 10, and 40 wt %) in the Ll phase. For all samples, R 1was found, within the experimental uncertainty, to be the same as for the 20 wt % ' sample, indicating that the local molecular motions
3819
are similar at all concentrations. Moreover, R2 was found to be much larger than R I at these concentrations as well, and we can therefore conclude that large aggregates, presumably rodlike micelles, are formed in the whole concentration range studied. In the present system of rod-shaped micelles the isotropic surfactant dynamics are much more rapid (1-2 orders of magnitude) than what is found in concentrated systems with ionic surfactant containing very long rod-shaped micelle^.^' Also, the zero shear viscosity appears to be significantly lower and no low-frequency viscoelasticity could be detected. (This can be qualitatively detected by observing the recoil of trapped air bubbles in the solution after swirling the sample.) However, to obtain visually detectable shear birefringence and viscoelasticity in systems of the present kind, the lifetime of the aggregates has to be in the millisecond range (or longer). As argued above, the lifetime of the CI2E4micelles is in the microsecond regime; therefore, no such effects are observed. Finally, in solutions of rodlike micelles an exponential stress relaxation is often observed@ which cannot be explained by the reptation mechanism.4I It has recently been suggested that breaking of rodlike micelles may play an important role for the relaxation of the transient netw0rk.4~.~~ In this context, it is possible that spin relaxation of surfactant nuclei can be an important technique for estimating the time scale for breaking and recombining of rodlike micelles in solution. Acknowledgment. This work was supported by the Swedish Natural Science Research Council. We also want to thank Peter Stilbs for the use of his curve-fitting programs. Registry NO. C12E4,5 2 1 4 4 8 4 .
(38) Olsson, U.; Nagai, K.; Wcnncrstrijm, H. J. Phys. Chem. 1988, 92, 6675. (39) Olsson, U.; JonstriSmer, M.; Nagai, K.; SBdcrman, 0.; Wennerstrijm, H.; Klose, G. Prog. Colloid Sei. 1988, 76, 75.
(40) (41) (42) (43)
Rehage, H.; Hoffmann, H.J. Phys. Chem. 1988, 92, 4712. de Gennes, P.-G. J . Chem. Phys. 1971,55, 572. Cates, M. E. Mueromolecules 1987, 20, 2289. Cates, M. E. J . Phys. (Les Ulis, Fr.) 1988, 49, 1593.
Fluorescence and Phosphorescence Study of AOT/H,O/Alkane Systems in the L2 Reversed Mlcellar Phase R.J6hannsson, M. Almgren,* and J. Alsins The Institute of Physical Chemistry, University of Uppsala, S-751 21 Uppsala, Sweden (Received: May 14, 1990; In Final Form: September 28, 1990)
Time-resolved fluorescenceand phosphorescence quenching measurements were made to determine the structure and dynamical behavior of micelles in oil-continuous microemulsions stabilized by aerosol OT, (AOT). In particular water/AOT/dodecane and water/AOT/isooctane systems were studied. It was found that reversed micelles, or water droplets stabilized by the surfactant, formed clusters in the AOT/alkane/water systems. The average cluster size was determined in the L2phase. The clusters were polydisperse in size whereas the micelles were not. Cluster formation increased with the concentration of micelles and with the chain length of the alkane solvent. It was not possible to determine whether the processes of exchange within a cluster was due to fusion-fission or some other process. Exchange between different clusters, however, seemed to be very slow.
Introduction Aerosol OT is well-known to form reversed micelles or water-in-oil microemulsions. The reversed micelles in the isotropic solution phase, L2, have been studied by dynamic and quasi-elastic light scattering (QELS),'I small-angle neutron scattering (SANS),C6 ~ltracentrifugation,'~J'and static and time-resolved fluorescence quenching e~periments.'J*'~ All these methods show that the micellar size increases with the molar concentration ratio R = ~ a t e r / A O T , ' - ~ *but ~ ~ ~the ' ' increase in size is almost independent of the volume fraction 4 of the dispersed phase (Le., AOT To whom correspondence should be addressed.
+
H2O),I3at least outside the critical region beyond which phase separation into two reversed phases occurs. Different experimental (1) Day, R.; Robinson, B. H. J. Chem. Soe., Faraday Tram. I 1979,75, 132. (2) Zulauf, M.; Eike, H. F. J . Phys. Chem. 1979, 83, 480. (3) Huang, J. S.;Kim, M. W. Phys. Rev. Le??.1982, 47, 1446. (4) Cabos, P. C.; Delord, P. J. Appl. Crystallogr. 1979, 12, 502. (5) Kotlarchyk, M.;Chen, S. H. J . Phys. Chem. 1982, 86, 3273. (6) Robinson, B.;Toprakciogiu, C.; Dore, J. J. Chem. Soc., Faraday Tram. I 1985, 80, 13, 431. (7) Atik, S. S.; Thomas, J. K . J . Am. Chem. Soc. 1981, 103, 3543. (8) Eicke, H. F.;Borkovec, M.; Das-Gupta, B. J. Phys. Chem. 1989, 93. 314.
0022-3654/91/2095-38 19$02.50/0 0 1991 American Chemical Society
