J . Phys. Chem. 1984,88, 3415-3418
3415
Highly Curved Defects in Lyotropic (Nonionic) Lamellar Phases. Origin and Role in Hydration Process L. Paz, J. M. Di Meglio, M. Dvolaitzky, R. Ober, and C. Taupin* Laboratoire de Physique de la MatiPre Condensee,? CollPge de France, 75231 Paris Cedex 05, France (Received: August 9, 1983; In Final Form: February 23, 1984) Lamellar phases of several concentrated nonionic surfactant [C12H25(0CH2CH2)nOH]-water systems are investigated. In disagreement with structural techniques, which show a classical behavior of these lamellar phases, the local technique of spin-labels reveals the existence of highly curved defects which are the precursors of the isotropic high-temperature phase. The number of such defects is increased either by the presence of added low-molecular-weight poly(oxyethy1ene glycols) or by polydispersity in the number of ethylene oxide groups in the molecule. Hydration-rate measurements indicate that these defects play a role in the kinetics of water diffusion through the lamellae.
Introduction In the past few years, new efforts have been devoted to the characterization of films of amphiphilic molecules, due to the intensive study of new puzzling phases: microemulsions’ and lyotropic nematics.2 Among the amphiphiles, nonionic surfact a n t ~in~particular were investigated for theoretical and practical reasons. Their aqueous micellar solutions exhibit a lower consolute point the nature of which is not completely They are able to give rise to microemulsions without the help of a cosurfactant,8 in contrast to ionic surfactants. This capability seems to be correlated with the extension of the lamellar phase in the temperature-composition diagram. In this paper, we describe studies of local perturbations on the state of lamellar phases of several nonionic surfactants, produced by temperature changes and the presence of foreign molecules. Experimental Section The pure nonionic surfactants, pentaethylene glycol and hexaethylene glycol dodecyl ethers (C12(EO), and CI2(EO),), were purchased from Nikko Chemicals (Tokyo). Their phase diagrams have been published by T i d d ~ . Figure ~ 1 shows schematically the lamellar domains in a temperature-composition representation. The samples (73% w/w of the surfactant in distilled water) were observed under the microscope between crossed polarizers and verified to be lamellar and to exhibit a transition toward a nonbirefringent isotropic phase at 73 OC for C12(E0)5and 66 OC for C12(EO),, in good agreement with Tiddy’s diagrams. The industrial nonionic surfactant UK 36 and the poly(ethy1ene glycols) (PEG) added to the samples were a gift of Mr. Tilquin (Centre de Recherche PCUK, Levallois). A givenweight of PEG was added to the lamellar phase and the mixture was then heated in the nonviscous L2 phase in order to increase the solubilization rate. The spin-label measurements were performed on a Varian E-9 EPR spectrometer. The phases were contained in different types of glass tubes which have been described elsewhere.’O Either an isotropic or a parallel distribution of lamellae could be obtained by using the orienting effect of glass walls. The spin-labeled amphiphile 1, which resides exclusively in the interfacial film,”
is used at a molar concentration of 0.1% with respect to the nonionic surfactant. This well-known techniqueI2 gives information about the local state of the labeled carbon of the chain-the order Equipe de Recherche Associte au C.N.R.S. (E.R.A. no. 542).
0022-3654/84/2088-3415$01.50/0
TABLE I: Lamellar Repeat Distances and Molecular Areas Measured by X-ray Diffraction for 73%w/w Various Nonionic Surfactant-Water Systems at 20 O C product d, A A , A2 C12(W5 C12(E0)6
C12(E0)5 + 1.3% PEG 600
44.6 42 46.5
42.9 50
parameter and the correlation times of the molecular motions. The hydration rate of the lamellar phase (73% w/w in surfactant) was determined as follows. A 0.4-mL sample of the lamellar phase was carefully deposited on 1.1 mL of water in a graduated tube. The equilibrium state of the final system is located in the L, domain (20% w/w in surfactant). Because of the birefringence of the lamellar phase, the interface between the two phases can be easily observed between crossed polarizers. The samples were then thermostated in a water bath at 25 OC, well below the corresponding cloud points ( T , 3 40 “C), and the decrease of the birefringent volume was recorded as a function of time. The X-ray experiments were performed on the scattering apparatus equipped with a rotating copper anode generator, located at the Centre de G&n&tiqueMoldculaire in Gif-sur-Yvette. The wavelength was 1.54 A and the distance between the sample and the counter (of the position-sensitive proportional type) was varied from 25 to 60 cm in order to observe either diffraction peaks or small-angle scattering.
Results X-ray Diffraction Study. The X-ray diffraction patterns of the different samples were recorded at several temperatures. Figure 2 shows a typical spectrum. Table I gives the lamellar repeat distance values, in excellent agreement with ref 13. Several points should be noticed. The repeat distances, and thus the molecular areas, are almost insensitive to the temperature increase (2%for 30 “C, to be compared to 6% for classical soaps14). (1) L. M. Prince, “Microemulsion Theory and Practice”, Academic Press, New York, 1977. (2) J. Charvolin, A. M. Levelut, and E. T. Samulski, J . Phys. Lett., 40, L-587 (1979). (3) (a) M. J. Schick, “Nonionic Surfactants”, Marcel Dekker, New York, 1967; (b) ibid., p 86. (4) M. Corti, V. Degiorgio, and M. Zulauf, Phys. Rev. Lett., 48, 1617 (1982). ( 5 ) M. Zulauf and J. P. Rosenbusch, J . Phys. Chem., 87, 856 (1983). (6) P. G. Nilsson, H. Wennerstrom, and B. Lindman, J . Phys. Chem., 87, 1377 (1983). (7) J. C. Ravey, J . Colloid Interface Sci., 94, 289 (1983). (8) K Shinoda and H. Kunieda, J . Colloid Interface Sci., 42, 381 (1973). (9) D. J. Mitchell, G. J. T. Tiddy, L. Waring, T. Bostock, and M. P. McDonald, J . Chem. Soc., Faraday Trans. I , 79, 975 (1983). (10) J. M. Di Meglio, M. Dvolaitzky, R. Ober, and C. Taupin, J . Phys. Lett., 44, L-229 (1983). (11) M. Dvolaitzky and C. Taupin, Nouu. J . Chim., 1, 355 (1977). (12) L. J. Berliner, “Spin-Labeling: Theory and Applications”, Academic Press, New York, 1976. (13) J. S . Clunie, J. F. Goodman, and P. C. Symons, Trans. Faraday Soc., 65, 287 (1969).
0 1984 American Chemical Society
3416 The Journal of Physical Chemistry, Vol. 88, No. 16, 1984
Paz et al.
I
M3
25
50
75 C12E06(%W&)
Figure 1. Schematic representation of the domain of existence of the lamellar phase La for the Clz(E0)5-Hz0and C12(E0)6-Hz0 systems. L, and L2 refer to the direct and high-temperature micellar phases,
respectively. I
500
I
I
I
Figure 2. X-ray diffraction pattern of the C12(E0)5lamellar phase at room temperature. Two orders of diffraction are clearly visible.
Addition of 1.3% PEG 600 slightly increases the repeat distance (4%). The addition of an ethylene oxide group increases the molecular area (Table I). This increase is associated with a decrease of the order parameter as observed by Melyl5 in soap systems. At room temperature the samples exhibit no small-angle scattering and the diffraction line widths are equal to the width of the incident beam. This is the classical behavior of well-organized lyotropic lamellar phases.16 Local State of the Lamellar Film as Seen by Spin-Labels. Figure 3 shows a typical spectrum obtained with an unoriented sample. This is in fact the superposition of two spectra: the well-known powder spectrum of the lamellar phases (L)I7 and a small three-line spectrum (M). The order parameter S3 can be calculated from the splitting values Til' and TI’(Figure 3) of the powder spectrum s3 = TII’ - T,’ -a Tzz - Txx a’ with a‘ = v3(Til’ + 2TL’)
a=
1/3(Txx
+
Tyy
+
Tzz)
(14) B. Gallot and A. Skoulios, Kolloid Z . 2.Polym., 208, 37 (1966). (15) B. Mely, J. Charvolin, and P. Keller, Chem. Phys. Lipids, 15, 161 11975) \ - - .-,.
(16) P. Ekwall in ‘Advances in Liquid Crystals”, Vol. 1, G. Brown, Ed., Academic Press, New York, 1971, p 1. (17) W. L. Hubbel and A. M. McConnell, J . Am. Chem. Soc., 93, 314 (1971).
(18) J. Seelig and H. Limacher, Mol. Cryst. Liq. Cryst., 25, 1051 (1974). (19) J. Seelig, J . Am. Chem. SOC.,93, 5017 (1971).
Highly Curved Defects in Lyotropic Lamellar Phases
Figure 5. Plot of the proportion N as a function of temperature for a 73% w/w surfactant-water system: ( 0 )C12(E0)5,(A) C12(E0)6,and (e) equimolecular mixture of C12(E0)5and CIZ(E0)6.
(2) The relative intensity of the M spectrum increases. It is difficult to make a quantitative evaluation of M intensity in the unoriented sample spectrum of Figure 3 because of the overlap of two types of spectra. It is easier to do so for samples that are oriented by the walls of flat glass cells, which give well-separated spectra as shown in Figure 4. It is then possible to evaluate the label proportion N which gives rise to an M-type spectrum. Figure 5 represents the variation of N with temperature for various systems. N first increases slowly, and then very rapidly a few degrees before the transition from the lamellar La to the high-temperature micellar phase. Interpretation of the M Spectrum. The characteristics of the M spectrum give some insight into its origin: (1) If the translational diffusion constant D and the correlation time r of the spectrum are known, the radius of curvature can be calculated with the relation a2 = 407, which is valid for two-dimensional diffusion. The lateral diffusion coefficient was measured by using the method of Trauble,20 which uses the broadening of the lines due to spin exchange when the label concentration increases. A value of the order of (5 f 1) X lo-’ cm2/s was found in agreement with the value of D in cubic soap systems.21 The correlation time was determined by solving the Bloch equations modified by an extra term for rotational Brownian diffusion as described in ref 22. The rigid limit was assumed to be the lamellar phase (order parameter 0.45)itself. We found r = (1 f 0.3)X s leading to a radius of the order of 15 f
5 A.
(2) A question arises about the nature of these curved objects or defects. Are they intrinsic to the lamellar phase or the result of a phase that has separated as a consequence of the presence of the spin-labels? In order to prove that the label concentration is the same in the two environments, lamellar and curved, we varied the label concentration from 0.1 to 0.6 mol %. It is known from the diffusion coefficient determination that in these systems at 0.7 mol %, the width of the lines begins to increase due to spin exchange between the nitroxide radicals. We observed no variation of the line widths with the label concentration. This fact proves that the curved regions are not specially concentrated in labeled surfactant; as a consequence the N proportion of labels which exhibit a M spectrum effectively reflects the proportion of curved regions. Furthermore, the relative intensity of the M spectrum is independent of the label concentration, which gives the first indication that no demixing has been induced in the sample. (20) H. Triuble and E. Sackman, J . Am. Chem. Soc., 94, 4482 (1972). (21) J. Charvolin and A. Tardieu in “Liquid Crystals”, Solid State Physics, Suppl. 14, L. LiBbert, Ed., Academic Press, New York, 1978. (22) R. C. McCalley, E. J. Shimshick, and H. M. McConnell, Chem. Phys. Lert., 13, 115 (1972). (23) M. Dvolaitzky, R. Ober, and C. Taupin, C. R. Hebd. Seances Acad. Sci., Ser. B, 293, 27 (1982).
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3417
Figure 6. Plot of the percentage of defects N vs. temperature for a 73% w/w surfactant-water system: ( 0 ) C1Z(EO)5,(0)C12(E0)5+ 0.85%
+
PEG ( M , = 600), (A) C12(E0)5 1.3% W / W PEG ( M , 600), (V) CIZ(E0)5 1.3% W / W PEG (M,= 2100), ( 0 ) C12(E0)5 + 1.3% w/w PEG ( M , = 4000), (+) C,,(EO):, + 3.8% w/w PEG ( M , = 600), W/W
(X)
+
UKANIL 36.
Moreover, both the La-L2 transition temperature and the cloud point are unchanged by the presence of 0.1 mol % of the label. Finally, a microscopic observation under phase contrast confirms the phase homogeneity even when N is around 40%. A demixing of droplets is visible only in the immediate vicinity (1 “C)of the transition. We conclude then that the curved regions are intrinsic to the lamellar phase. (3) The activation energy for creation of defects as calculated from the low-temperature slope of Figure 5 is 12 kcal/mol, to be compared to 1 kcal/mol for the trans-gauche motion of a methylene group. This low value is associated with the exact reversibility of the thermal curve of Figure 5, on a time scale of a few minutes. Note that these nonionic systems differ from microemulsion systemsI0 where the defects, which are due to a transitory perturbation of the structure by pipetting, are not present in oriented samples. In the nonionic systems two types of defects can exist: unstable mechanically induced ones and stable thermally created ones. (4)Another interesting feature of such motion-averaged spectra is the polarity, which is measured by the splitting of the lines and gives information about the local environment of the labeled carbon. First, one can remark that the spectrum of the hightemperature isotropic phase, which is characterized by a short correlation time due to curvature and a lower polarity than La (14.4G instead of 15 G), appears exactly at the same place as the M spectrum a few degrees below the transition. From this result, one can infer that the defects might be the high-temperature micellar phase precursors. Finally, it is interesting to consider the change in polarity with temperature. At low temperature, the polarity is high (16 G), revealing an aqueous environment. The polarity drops very sharply a few degrees below the transition, corresponding to the increase in the number of defects. These facts suggest that at low temperature the defects behave like layer edges or pores in the lamellae24and that they undergo an important change just before the transition. At that point, there are two possibilities. The change in polarity is either due to a curvature inversion of the interfacial film or due to the dehydration of the ethylene oxide part of the molecule. In a previous study on a microemulsion system it was shown that the structural inversion of the system (w/o o/w) is associated with polarity changes. Such an interpretation would indicate the presence of an inverted micellar phase (of the L2 type) at high temperature. Effect of Various Additives on the Number of Defects. Two different kinds of additives were used: low-molecular-weight
-
(24) W. Helfrich in “Physics pf Defects”, R. Balian, Amsterdam, 1981, p 713.
Ed., North-Holland,
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The Journal of Physical Chemistry, Vol. 88, No. 16, 1984
TABLE II: L,-Lz Transition Temperatures for 73% w/w C1z(EO)5-Hz0Systems as Functions of Concentration and Molecular Weight of Added PEG
Paz et al.
d
concn, product PEG 600
PEG 2100 PEG 4000
7% (w/w) 0 0.85 1.3
3.8 1.3 1.3
T, “C 74 65 58.5 45 53.8 44
0.15
0.50
poly(ethy1ene glycols) and molecules with different numbers of ethylene oxide groups. These additives are normally present in ethoxylated alcohols, as impurities that result from the chemical preparation process.3b In both cases, the number of defects N increases. Figure 5 shows the thermal variation of N for an equimolecular mixture of C12(E0)5and as compared to the variation in the pure substances. The number of defects outside the L,-L2 transition region is several times larger than that in the pure compounds. Similar results for various concentrations of PEG 600 added to the phase are given in Figure 6. One can calculate that one molecule of PEG 600 modifies the state of 12 surfactant molecules. Apart from the increase of the number of defects at low temperature, an addition of PEG decreases the L,-L2 transition temperature, reducing the domain of existence of the lamellar phase (Table 11). On the contrary, at room temperature the limit between L, and L1 phase (56% in amphiphile) is unchanged by the addition of 1% of PEG 4000 or 4%of PEG 600. The industrial product UK 36 which, as regards the cloud point, is equivalent to the C12(E0)*compound, but contains two types of impurities, was also studied. As expected, the number of defects at low temperature, in this case, is almost 10 times larger than it is in the pure substances. Role of Defects in the Hydration Rate of the Lamellar Phase. Figure 7 shows the displacement of the L,-L1 phase boundary with time during the hydration process. As the L,-Ll limit is independent of the presence of the additives, these curves represent the rate of hydration of the L, phase. One observes that the hydration rate increases with N . At low temperature, the plot of the half-time of hydration t l I 2vs. N shows a clear correlation between the two phenomena. Nevertheless, the effect of the polymer molecular weight is also a very important factor since the three different P E G S which were used induce approximately the same number of defects but lead to very different kinetics of hydration.
Conclusion The lamellar phase of the aqueous C12(E0)5and C12(E0)6 nonionic surfactants, which has a large domain of existence in temperature (70 “C), behaves quite classically as regards X-ray diffraction diagrams, mean areas per molecule, and order parameters. Spin-label measurements, on the other hand, demonstrate the presence of highly curved regions which have the following features: They are thermally nucleated and their number increases enormously a few degrees below the L,-L, transition temperature. Their activation energy is remarkably low (around 12 kcal/mol). Their number is larger for the C12(E0)6,which has a smaller lamellar domain. Chemical inhomogeneity or additives increase the number of highly curved regions and promote the L,-L2 transition, thus reducing the extension in temperature of the L, phase. The presence of such defects is clearly correlated with the hydration rate of the lamellar phase.
0.25
0
Figure 7. Variation with time (in hours) of the relative decrease of the volume d equal to (uo - u,)/uo of the birefringent lamellar phase in contact with water for a 73% w/w surfactant-water system: ( 0 )C12(E0)5,(v) CIZ(E0)S + 1.3% W/W PEG 600,(0)CIZ(E0)5 + 1.3% W / W PEG 2100, ( X ) UKANIL 36, (0) C12(EO)S+ 1.3% w/w PEG 4000. Inset: plot of the half-time of hydration fl,2 (in hours) vs. N .
These highly curved regions can be considered as defects of the lamellar structure; we attempted to detect an increase with temperature of the intensity of the small-angle scattering. Although the observed effect was very small, it may perhaps indicate the presence of scattering objects of a few tens of angstroms. The very efficient role of these defects in the hydration process proves that their formation induces the breaking of the layers. Beyond a screw di~location?~ one can think of more localized defects such as pores which could explain the increased effect of the highest-molecular-weight PEG. Finally, these results raise a problem. Such an effect has never been noticed before in the numerous EPRI7-” studies of lyotropic lamellar phases. In order to observe the defect spectrum, two conditions must be fulfilled: there must be a sufficient number of defects (a few percent) and the lateral diffusion must be fast. The lateral diffusion constant in phospholipidic systems is more than 10 times smaller than that in the systems that we have studied,20thereby making it impossible to observe such an effect. The same argument is not a priori valid for ionic surfactant systems. Therefore, our experimental observations may be a step in understanding the well-known different behavior of the ionic and nonionic films.
Acknowledgment. We thank M. Hermant and M. Tilquin (Centre de Recherche PCUK, Levallois), who drew our attention to the properties of nonionic systems. We have benefited from the hospitality of Vittorio Luzzati and from many discussions about X-ray diagrams with Annette Tardieu at the Centre de G6nnBtique Mol6culaire (Gif-sur-Yvette). We thank one of the reviewers for several suggestions and Professor C. Knobler for correcting the English. Registry No. C12(E0)5,3055-95-6; C12(E0)6,3055-96-7; PEG monH20, 7732-18-5. ododecyl ether, 9002-92-0; (25) M. Kleman, in “Springer Series in Chemical Physics”, Vol. 11,
Springer-Verlag, West Berlin, 1980, p 97.
