2520
J. Phys. Chem. 1989, 93, 2520-2526
Structure of the Intermediate Phase and Its Transformation to Lamellar Phase in the Lithium Perfluorooctanoate/Water System P. Kekichefft LURE, Laboratoire CNRS- CEA-MEN, Britiment 2090, UniversitC Paris Sud Orsay, F-91405 Orsay, France
and G. J. T. Tiddy* Unilever Research Port Sunlight Laboratory, Quarry Road East, Bebington Wirral, Merseyside L63 3JW, United Kingdom (Received: March 3, 1988; In Final Form: August 1 1 , 1988)
The structures of the intermediate and lamellar lyotropic liquid crystalline phases of the surfactant lithium perfluorooctanoate have been studied by using high-intensity, high-resolution X-rays from a synchrotron source. The intermediate phase has a repeated layer structure which is closely related to the L, structure. The surfactant layers consist of a tetragonal array of rods joined by fours, further ordered in three dimensions in a body-centered lattice. Thus the intermediate phase is a lamellar structure where the layers contain a regular array of holes through which water and ions can diffuse rapidly. The correlations between hole positions in the intermediate phase persist to some extent in the L, phase, indicating that “holes” in the bilayer are present here also. The proposed tetragonal (T,) structure is fully supported by previous order parameter and self-diffusion data.
1. Introduction
For many years, amphiphilic compounds have been attracting the attention of investigators to obtain information relevant to biological Recently, attention has been focused on the phase transition phenomenon. Indeed one of the most important unsolved structural problems of surfactant liquid crystals is the nature of the hexagonal/lamellar transition region, particularly in binary systems.s Two main types of behavior are observed: either q “bicontinuous” cubic phase (V,) occurs as with nonionic surfactants6v7and short-chain ionic surfactants,1° or birefringent intermediate phases form5,8-11as for long-chain ionic surfactants. Occassionally, for a particular chain length derivative, both VI and intermediate phases can occur. This has been demonstrated both by polarizing microscopy10and separately by a detailed X-ray study of.one surfactant system (sodium dodecyl sulfate, SDS).I2 The reasons why intermediate rather than cubic phases occur are not fully understood. The shapes of such apolar/polar interfaces are governed by a set of constraints at the molecular level which involve both the segregation of aqueous and hydrocarbon regions, and a balance between inter- and intramicellar forces. These have been analyzed in terms of “frustrations” by Sadoc and Charv01in.l~ Here, the occurrence of various mesophases are topologically described; nevertheless a prediction of their appearance in accordance to the surfactant chemical nature has not yet been elucidated. In order to address this problem, the phase behavior of a range of surfactants has been s ~ r v e y e d . ~ , ’ ~InJ ~ previous reports of intermediate-phase formation the surfactants were always ionic (usually anionic). Now nonionic surfactants that form these structures have been dis~0vered.l~ This demonstrates that intermicellar electrostatic repulsions are not essential for the occurrence of the intermediate phases. The studies give firm support to the hypothesisI4 that the major factor which determines whether cubic or intermediate phases occur is the balance between the type of polar head (which determines mainly the interface curvature) and restrictions on alkyl chain conformational freedom/packing (which increase with alkyl chain length). An increase in head-group size/micellar curvature favors V I formation while increased hydrocarbon chain length favors intermediate-phase formation. In order to test this idea, surfactants with perfluoroalkyl chains instead of hydrocarbon surfactants were examined.14 Fluorocarbon chains have a much lower flexibility because of the larger energy ‘Present address: Laboratoire de Physique des Solides, associe au CNRS (LA 2). Batiment 510, Universitt Paris Sud, 91405 Orsay, France.
0022-3654/89/2093-2520$01 .50/0
difference between gauche and trans c ~ n f o r m e r s ; ’ ~hence J ~ intermediate phases are expected to occur rather than V I . The results obtained to date have confirmed this predi~tion.’~J~J* A survey of the behavior of a range of fluorocarbon surfactants demonstrated14that only intermediate phases are formed even for the smallest chain size surfactant examined (C7). Indeed, an intermediate phase is present in one of the earliest of the fluorocarbon surfactants examined for mesophase formation (lithium perfluorooctanoate, C7F15C02Li)but it was not recognized as s ~ c h . ~The ~ . ’exact ~ structure of this phase has not to date been elucidated. In this paper we reexamine the structure of the intermediate mesophase and the transition to the lamellar mesophase in the lithium perfluorooctanoate (LiPFO)/water system. New X-ray diffraction studies have been carried out. The good resolution (AQ A-’) combined with the high-flux source used (synchrotron radiation) yields a large number of diffraction lines, which remove any uncertainty as to the symmetry of the phase.
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(1) Luzzati, V. In Biological Membranes; Chapman, D., Ed.; Academic Press: London, 1968; Chapter 3. (2) Winsor, P. A. Chem. Rev. 1968, 68, 1. (3) Ekwall, P. In Advances in Liquid Crystals; Brown, G.H., Ed.; Academic Press: London, 1971; Vol. 1, Chapter 1, p 1. (4) Tiddy, G. J. T. Phys. Rep. 1980, 57, 1. ( 5 ) Luzzati, V.; Mustacchi, H.; Skoulios, A.; Husson, F. Acta Crystallogr. 1960, 13,660. Husson, F.; Mustacchi, H.; Luzzati, V. Acta Crystallogr. 1960, 13, 668. (6) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J . Chem. SOC.,Faraday Trans. 1 1983, 79,975. (7) Rancon, Y.; Charvolin, J. J . Phys. (Paris) 1987, 48, 1067. (8) Leigh, I. D.; McDonald, M. P.; W d , R. M.; Tiddy, G. J. T.; Trevethan, M. A. J . Chem. Soc., Faraday Trans. 1 1982, 77,2867. (9) Hendrikx, Y.; Charvolin, J. J . Phys. 1981, 42, 1427. (10) Rendall, K.; Tiddy, G. J. T.; Trevethan, M. A. J . Chem. SOC.,Faraday Trans. 1 1983, 79,637. (1 1) Alperine, S.; Hendrikx, Y . ;Charvolin, J. J . Phys. Left. 1985, 46, L27. (12j Kekicheff, P.; Cabane, B. J . Phys. (Paris), in press. (13) Sadoc, J. F.; Charvolin, J. J. Phys. 1986, 47,683. (14) Hall, C.; Tiddy, G. J. T. Paper presented to the 6th International Symposium on Surfactants in Solution, New Delhi, Aug 1986; Conference proceedings in press. (1 5 ) Tiddy, G. J. T. In Modern Trends of Colloid Science in Chemistry and Biology; Eicke, H. F., Ed.; Birkhauser Verlag: Basel, Switzerland, 1985; p 158. (16) Flory, P. J. In Statistical Mechanisms of Chain Molecules; Interscience: New York, 1969. (17) Everiss, E.; Tiddy, G. J. T.; Wheeler, B. A. J . Chem. Sac., Faraday Trans. I 1976, 7 2 , 1147. (18) Tiddy, G.J. T. J . Chem. Soc., Faraday Trans. 1 1977, 73, 1731.
0 1989 American Chemical Society
LiPFO/Water Intermediate Phase
1rhe Journal of Physical Chemistry. Vol. 93, No. 6. 1989
II T L.
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Figure 1. Powder diffraction pattern from a sample in the intermediate mesophase (LiPFO 72.04%.T = 25 "C) recorded in a Guinier camera (linear collimation; sample to film distance 125 mm). Thc granular texture of the pattern indicates that the sample is made of many large. disoriented crystallites. The diffuseband at large angles corresponds to the distance (5.6 A) between neighboring fluarmarbon chains in a liquid like state within one aggregate. ' The structure resembles that of the T"phase previously reported for anhydrous long-chain calcium soapsI9 and which appears in the sequence of SDS/water intermediate mesophases." Moreover, the transition to the lamellar phase shows the very close structural relationship between these two phases, as observed in the SDS/water system." Most importantly, all the published N M R data (self-diffusion coefficients" and order are accounted for by this structure. 2. Methods 2.1. Materials. Materials and sample homogenization were the same as those described previou~ly.~'Then the sample tubes were opened a t room temperature and their contents were used to fill thin X-ray scattering cells made of mica windows separated by Teflon spacers. The windows did not induce an orientation of the sample. Weighing sample holders before and after each exposure indicated that loss of water from the sample was insignificant. 2.2. Data Collection. The diffraction patterns come out as Debye-Scherrer rings produced by all domains in the irradiated volume. As detectors we used photographic films, which have the advantages of covering a much larger area of reciprocal space and at the same time offering a better resolution than linear position sensitive detectors. Moreover, because they give a twodimensional image, they still give information when the diffraction pattern consists of spots instead of smooth rings, as may happen with the LiPFO intermediate mesophase which easily grow large crystallites (Figure I). In order to identify the phases present in a sample, and to locate the temperatures of phase transitions, we first used a classical apparatus: the X-rays produced by a Cu tube were focused on the sample with a crystal monochromator and vekical slits (linear collimation); the scattered rays were collected in a Guinier camera (sample to film distance I25 mm). There are, however, significant drawbacks associated with the linear collimation: first, some features in the scattering which would appear at scattering vectors Q (with the scattering vector Q defined as (4r/A) sin 0,where 20 is the scattering angle) slightly below that of an intense line tend to be obscured; second, it may be difficult to assign diffraction spots produced by samples which contain large oriented domains (Figure 1). For these reasons, we also used a point collimation and long sample-t&lm distances (from 400 to 1200 mm) in order to resolve overlapping rings. The intensity loss due to the point collimation and to the large solid angles used in these scattering geometries is compensated by the high flux of the synchrotron radiation (X-ray beam of the line D24 at LURE, A = 1.608 A). In this and the Q range configuration the resolution was AQ = 0.01 < Q < 0.55 was adequate, since the first diffraction line and its higher of the intermediate phase starts a t Q 0.17 orders become too weak to be observed beyond Q = 0.6 A-I.
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3. Results Surfactants with perfluoroalkyl chains are similar to hydrocarbon surfactants in that they form micelles and lyotropic liquid (19) Luzzali. V . ; T a r d i c u . A : G " l i k - K r ~ w ~ ~ kN~o. Tt u. r ~ 1 % 8 . 2 / 7 .1028 (20) Kckichcff, P.;C&bane. R : Rawso. M.J . Phls. lac! 1W6.45.L813. (21) Mornr.P.G.; Manslield.P:Tjdd).G. J . T Forodo) Sbmp chem. Sm. 1919. 13. 37.
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TABLE I: X-ray Diffraction Data for the Body-Centered Tetragonal h t t l e e of the Intermediate Mesophase' hkP Q-M. A-' error. % intensity obsd Qow,A-' 101
ll0 002 200 I12 21 I 202 103 220 301
0.1732 -0.1940 0.2145 0.2718 -0.2810 0.3210 0.3495
0.1731 0.1922 0.2145 0.2718 0.2879 0.3222 (0.3462 (0.3491 0.3842 0.4216
0.06 0.93 0.00 0.00 2.45 0.37 0.94) 0.11)
ws "W YS
s W
m m
'Qow and Q?,. are the scattering vectors (Q = (4r/A) sin O j of the observed reflections (see Figure 3a). and calculated for the three-dimensional body-centered tetragonal phase with a = 46.2 A and c = 58.6 A (A = 1.608 A). The observed intensities were visually estimated (vvs, extremely strong; vs. very strong; s. strong; m, medium; w. w e a k vw, very weak w w , extremely weak). The lines observed close to the beam stop are modified by a subharmonic of the wavelength (mainly A/3). The are not reported in the table. bh, k permutable; h + k + I
= 2".
crystals. The LiPFO/water phase diagram was determined previously, mainly by optical mi~roscopy.~~J'It shows three mesophase regions (Figure 2a); the hexagonal phase (HI),where cylinders of amphiphile are arranged on a two-dimensional hexagonal lattice, the lamellar phase (La), where bilayers of amphiphilic molecules are separated by water layers, and the intermediate phase (int), which has boundaries with the L. phase both at high and low concentrations above 30". These two phases coexist in a very narrow two-phase region (at LiPFO 72 wt % the transition from the intermediate to the lamellar phase is completed over only 2 "C). The phase boundary is easily detected by polarizing microscopy, since the two phases have very different optical textures (Figure 2b.c) and viscosities [s(lam)
