Langmuir 1995,11, 356-365
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Nematic and Disrupted Lamellar Phases in Cesium PentadecafluorooctanoatePH20: A Small Angle Scattering Study Michael C. Holmes,* Marc S. Leaver, and Andrew M. Smith Department of Physics and Astronomy, University of Central Lancashire, Preston, Lancashire, PRl 2HE,United Kingdom Received September 5, 1994. In Final Form: October 27, 1994@ The cesium pentadecafluorooctanoate (CsPFO)PH20 system has been studied by small angle X-ray and neutron scattering. The lamellar phase has previously been shown to consist of lamellae pierced by waterfilled defects. sen: (Leaver, M. s.;Holmes, M. C. J Phys. II 1993,3,105. Holmes, M.C.;Smith, A. M.; Leaver, M. S. J.Phys. II 1993,3,1357.)Here we present further evidence for this structure together with experimental evidence that profound changes in structure take place in the nematic phase. In the isotropic micellar phase, the surfactant aggregatesform disk-shaped micelles. As the nematic phase is crossed from the isotropic micellar phase to the lamellar phase, there is an increase in orientational and positional order of the aggregates. There is also evidence for a structural rearrangement from disk-shaped micelles to elongated, flattened micelles which act as the precursor to the defected lamellar structure. Addition of electrolyte or cosurfactant accentuates this process,
1. Introduction Surfactant molecules which self-assemble in aqueous mixtures to form, in some cases, complex one-, two-, and three-dimensional structures have been the subject of much recent One area of particular interest has been those systems which form lyotropic nematic p h a ~ e s ~between -~l isotropic micellar phases and either lamellar or hexagonal phases to lower temperature or higher surfactant concentration. In nematic phases the inter- and intra-aggregate interactions are finely balanced and they have the added advantage that they are readily ordered by externally applied magnetic fields. It would seem to be well-established in the literature that the nematic phase associated with a lower temperature hexagonal phase is composed of rod-shaped micelles and that associated with a lower temperature lamellar phase
* Abstract published in Advance A C S Abstracts, December 15, 1994. (1)Leaver, M. S.; Holmes, M. C. J. Phys. II 1993,3,105. (2)Holmes, M. C.; Smith, A. M.; Leaver, M. S. J . Phys. II 1993,3, 1357. (3)Seddon, J. M.Biochem. Biophys. Acta 1990,1031,1. (4)Lawson, K.D.; Flautt, T. J. J.Am. Chem. SOC.1967,89,5489. (5)Amaral, L. Q.; Pimentel, C. A.; Tavares, M. R.; Vanin, J. A. J. Chem. Phys. 1979,71,2940. (6)Boden, N.;Jackson, P. H.; McMullen, K.; Holmes, M. C. Chem. Phys. Lett. 1979,65, 476. (7)Charvolin,J.;Levelut, A. M.; Samulski, E. T. J . Phys. Lett. 1979, 40,L587. (8)Amaral, L. Q. Mol. Cryst. Liq. Cryst. 1980,56,203. (9)Sdderman, 0.; Lindblom, G.; Johansson, L. B.-A.; Fontell, K. Mol. Cryst. Liq. Cryst. 1980,59, 121. (10)Yu,L. J.; Saupe, A. Phys. Rev. Lett. 1980,45,1000. (11)Forrest, B. J.; Reeves, L. W. Chem. Rev. 1981,81,1. (12)Hendrikx, Y.;Charvolin, J. J . Phys. 1981,42,1427. (13)Photinos, P.; Saupe, A. Mol. Cryst. Liq. Cryst. 1981,67,277. (14)Holmes, M. C.; Boden, N.; Radley, K. Mol. Cryst. Liq. Cryst. 1983,100,93. (15)Holmes, M. C.; Charvolin, J. J. Phys. Chem. 1984,88, 810. (16)Boden, N.; Corne, S. A.; Holmes, M. C.; Jackson, P. H.; Parker, D.; Jolley, K. W. J. Phys. 1986,47,2135. (17)Amaral, L. Q.; Marcondes Helene, M. E.; Bittencourt, D. R. S.; Itri, R. J.Phys. Chem. 1987,91,5949. (18) Holmes, M. C.; Reynolds, D. J.; Boden, N. J . Phys. Chem. 1987, 91,5257. (19)Amaral, L. Q.; Marcondes Helene, M. E. J . Phys. Chem. 1988, 92,6094. (20)Holmes, M. C.; Charvolin, J.; Reynolds, D. J. Liq. Cryst. 1988, 3,1147. (21)Quist, P.; Halle, B.; Furd, I. J. Chem. Phys. 1992,96,3875.
is composed of disk-shaped mi~e1les.l~ These are referred to as Nc and ND (sometimes as NL),respectively. A number of surfadantiwater systems exhibiting nematics have been studied. In some cases the binary system requires the addition ofcosurfactant or salt for the nematic phase to form. Perhaps the most studied systems are those of sodium decyl sulfate/decanollwate13~~~J~J~~~~ and decyl ammonium chloride/ammonium chloride/ ~ a t e r ; ~ ~ Jthe ~ vformer ~ ~ , exhibits ~ ~ - ~ both ~ Nc and ND phases separated by a narrow biaxial nematic phase, and the latter only shows an NDphase. Biaxial nematic phases occur in other systems, notably in the potassium lauratel decanollwaters y ~ t e m . ' ~ aBiaxial ~ , ~ nematic are believed to consist of micelles which are neither rods nor disks but which have all three axes of different length. One system which exhibits a nematic phase of disk micelles is the cesium pentadecafluorooctanoate/water system, which is unusual in that the it occurs in the binary system. Additives such as electrolyte (CsC1) and cosurfactant cause the progressive loss of the nematic phase. Considerable work has been done on this system with a variety of techniques such as optical microscopy,6 small angle X-ray and neutron ~ c a t t e r i n g , ~ ~2H ~;~~~~~,~ NMR,6Jas32-34ionic c o n d u ~ t i v i t y , l ~magnetic ~ ~ ~ - ~ ~bire(22)Hendrikx, Y.;Charvolin, J.; Rawiso, M.; Liebert, L.; Holmes, M. C. J . Phys. Chem. 1983,87,3991. (23)Sammon, M. J.; Zasadzinski, J. A. N.; Kuzma, M. R. Phys. Reu. Lett. 1986,57,2834. (24)Rizzatti, M. R.; Gault, J. D. J . Colloid Interface Sci. 1986,110, 258. (25)Gault, J. D.;Kavanagh, E.; Rodrigues, L. A.; Gallardo, H. J. Phys. Chem. l966,90,1860. (26)Haven, T.;Radley, K.; Saupe, A. Mol. Cryst. Liq. C y s t . 1981,
"..
76 .-, R 7
(27)Doane, J. W.; Chidichimo, G.; Golemme,A. Mol. Cryst.Liq. Cryst. 1984,113,25. (28)Formoso, V.; Galerne, Y . ;Nicoletta, F. P.; Pepy, G.; Picci, N.; Bartolino, R. J. Phys. ZV 1993,C1,3,271. (29)Figueiredo Neb, A. M.; Galerne, Y.; Levelut, A. M.; Liebert, L. J . Phvs. Lett. 1986.46.L499. (36)Boden,N.;-Holmes, M. C. Chem. Phys. Letts. 1984,109,76. (31)Holmes, M. C.; Reynolds, D. J.;Boden, N. Mol. Cryst.Liq. Cryst. 1987.146.377. ---(32)Hoffmann, H. Ber. Bunsen, a s . Phys. Chem. 1984,88, 1078. (33)Boden, N.;Corne, S. A.; Jolley, K W. J.Phys. Chem. 1987,91, 4nw (34)Boden, N. J . Mol. Liq. 1992,54,215. (35)Boden, N.;Clements, J.; Dawson, K. A.; Jolley, K. W.; Parker, D.Phys. Rev. Lett. 1991, 66,2883.
. ---.
0743-7463/95/2411-0356$09.00100 1995 American Chemical Society
Langmuir, Vol. 11, No. 1, 1995 357
Nematic and Disrupted Lamellar Phases in CsPFO 12H20 fringen~e,~O-~~ and water s e l f - d i f f ~ s i o nThe . ~ ~existence ~~~ of disk-shaped micelles in both the isotropic phase and in the nematic phase together with the evidence that the phase change from nematic phase to lamellar phase was almost imperceptible with many techniques reflecting microscopic structure lead to the conclusion that the lamellar phase might also be composed of disk-shaped micelles which were positionally ordered in planes. This idea was attractive because it made the lyotropic liquid crystal directly analogous to thermotropic phases, the mesogenic unit being the disk micelle and the lamellar phase being a smectic phase of disks. The transitions from isotropic to nematic and nematic to lamellar were then simply disorder-order transitions in which the orientational and then positional order of the micelles were established. This model required the micelles to decrease in size (aggregation number) with increasing surfactant concentrationfor volume fractions greater than 0.318>46 and, consequently, an attractive intermicelle interaction of the type proposed by S ~ g a mbut i ~for ~ which there is no other parallel evidence in similar surfactant systems. Recently, we have used small angle neutron scattering to study the lamellar phase of this system in the presence of electrolyte1 and cosurfactant.2 Both additives cause the progressive loss of the water-filled defects in the lamellar phase. The advantage of small angle scattering techniques over other experimental techniques is the simplicityof the interpretation of the experimental results. Using simple assumptions, the results can be related directly to the aggregate structure. In refs 1and 2 it was shown that by assuming that the thickness of the water layer around the aggregate was uniform, the most consistent interpretation of the results was that the lamellae consisted of surfactant continuous layers broken by elongated water-filled holes, referred to by the symbol LaH. Increasing the surfactant concentrationin the binary system causes the defects to shorten in length toward circular pores although their number density remains constant. Adding both CsCl and W,W-perfluoroheptan1-01 causes the number density of defects to decrease although their shape remains constant. In both cases the bilayers are approaching classical bilayers. These different types of behavior can be attributed to the differences in the modifications to the intra-aggregated interactions. It is interesting to compare the addition of l H , W perfluoroheptan-1-01and CsCl. The former has the effect of reducing the head group surface charge density. The addition of CsCl also reduces surface charge density since more counterions are associated with the head groups, but in addition, solvated counterions screen the interfaces on adjacent bilayers. This leads to changes in the shape of the interlamella Bragg r e f l e ~ t i o n . ~ ~ 16~18930946
(36) Boden, N.; Hedwig, G. R.; Holmes, M. C.; Jolley, K. W.; Parker, D. Liq. Cryst. 1992, 11, 311. (37) Boden, N.; Jolley, K. W. Phys. Rev. A 1992,45, 8751. (38) Boden, N.; Clements, J.; Dawson, K. A.; Jolley, K. W.; Parker, D. Structure and dynamics of strongly interacting colloids and supramolecular aggregates in solution; Chen, S. H., Ed.; Kluver Academic Publishers: Netherlands, 1992; p 831. (39) Barnes, C.; Frank, C.; Leybold, B.; Photinos, P. Phys. Rev. E 1993,48,2792. (40) Rosenblatt, C.; Kumar, S.; Litster, J. D. Phys. Rev. A 1984,29, 1010. (41) Rosenblatt, C.; Zolty, N. J . Phys. Lett. 1985,46, L1191. (42) Rosenblatt, C. J . Phys. Chem. 1987, 91, 3830. (43) Rosenblatt, C. J . Phys. Chem. 1988, 92, 5770. (44) Ukleja, P.; Chidichims, G.; Photinos, P. Liq. Cryst. 1991,9,359. (45) Holmes, M. C.; Sotta, P.; Hendrikx, Y.; Deloche, B. J . Phys. II 1993, 3, 1735. (46) Holmes, M. C.; Boden, N. Mol. Cryst. Liq. Cryst. 1985,124,131. (47) Sogami, I. J . Chem. Phys. 1984,81,6320.
c-.
X
sample
'Y"
Y
/ X-rays dir'ector
Figure 1. The geometry of t h e SAXS a n d SANS experiments a n d definition of t h e coordinate axes.
There remains a problem; namely, at what point does the system change from disk micelles to surfactant continuous lamellae? In this paper evidence from small angle X-ray and neutron scattering is presented for the structures of lamellar, nematic, and isotropic phases in both the binary system and in the ternary systems. Some of the results have already been published previously but are presented again together with new results so that a complete picture is 0btaine.d. It is shown that the radical change from disks to ribbon-shaped aggregates takes place in the nematic phase.
2. Experimental Section 2.1. Sample Preparation. The CsPFO was prepared as in ref 1with a development from the method of N a k a ~ a m a The .~~ cosurfactant lH,lH-perfluoroheptan-l-ol (Fluorochem Ltd., purity > 99%)was chosen because it was readily available and has a similar chain length to CsPFO. It is not possible to have a fully fluorinated alcohol since a CF2 bond next to an OH group is readily hydrolyzed, so this alcohol molecule has one CH2 group with the alcohol functional group attached to it. It has been assumed that the presence of this CH2 group will not affect the chain packing within the micelle. Cesium chloride was obtained from Fluka Chemicals with a purity of better than 99.9%. The samples were prepared by weighing out the surfactant and other components into constricted tubes together with 2H20 from Fluka Chemicals (purity > 99.8%). The tubes were then flame sealed. Samples were mixed by repeated centrifugation through the tube constriction and storing in a n oven in the isotropic phase until they were optically uniform. Each series of samples (one containing CsC1, the other lH,lH-perfluoroheptan-1-01) was prepared with the mole ratio of surfactant to 2Hz0 held constant at 1:27.26, equivalent to a binary sample with a weight fraction of CsPFO of X , = 0.50. Particular care was required in mixing samples with salt greater than 8% by weight because ofthe loss ofthe nematic phase and their tendency to phase separate. 2.2. X-ray Scattering. Nickel-filtered Cu K, X-rays were used with pinhole cameras with sample to film distances of 113 and 260 mm. Samples were contained in 0.5 mm Lindeman capillary tubes and were held in a copper block, the axis of the tube being perpendicular to the direction of the beam, and the temperature was controlled by a Haake F3 water bath and circulator, with an accuracy of f0.5 "C. Small angle X-ray scattering experiments were also conducted at the SERC Synchrotron Radiation source at Daresbury Laboratory, Warrington, UK, using experimental station 7.2 with film as the detector. The sample cell and temperature control were as outlined above for the laboratory-based experiments. Samples were aligned using a magnetic field with the geometry shown in Figure 1 and after the method of ref 29. 2.3. Neutron Scattering. Neutron scattering experiments, at the small angle scattering facility, LOQ at ISIS, the SERC's neutron spallation source, gave better contrast, particularly for those samples containing CsC1. The samples were held in 1mm (48) Holmes, M. C.; Smith, A. M.; Leaver, M. S. J . Phys. N 1993,3, C8, 177. (49) Nakayama, H. Bull. Chem. SOC.Jpn. 1967,40, 1592.
358 Langmuir, Vol. 11, No. 1, 1995
Holmes et al.
Figure 2. A sequence of small angle X-ray scattering patterns taken from a W,= 0.5 sample at the followingtemperatures: (a) 48 "C, isotropic phase; (b) 47 "C; nematic phase; ( c ) 46 "C; (d) 45 "C; (e) 44 "C; (0 43 "C; (g) 42 "C, nematic phase; and (h) 41 "C, lamellar phase. Note the evolution of the scattering pattern across the nematic phase (b-g), the almost identical appearance of g and f, and the early appearance of the second order reflection at b.
path length quartz glass Helma cells with electricaltemperature regulation (fO.l "C). Samples were aligned using a magnetic field in the same way as for the X-ray scattering experiments (Figure l).29 3. The Scattering Pattern 3.1. Observations. Figure 2 shows a temperature sequence of synchrotron X-ray scattering patterns from a magnetically aligned binary sample CsPFOPH20 with W, = 0.50. In the isotropic phase, L1, a single diffuse ring is observed characteristic of an isotropic micellar solution, with average separation between the micelles, do. The sequence of pictures (b-g) shows the evolution of the scattering pattern across the nematic phase. The pattern consists of the intense Bragg reflectionsfrom the dll spacing along they axis, the aggregate separation in the direction parallel to the nematic director, and the aligning magnetic field. Along the x axis there is a much more diffuse reflection from the dl spacing, the aggregate separation in the direction perpendicular to the nematic director, and the aligning magnetic field. It is within the narrow nematic phase that the most profound changes in the scattering pattern take place. When the nematic phase is first entered, the pattern is elliptical in shape, characteristic of the classical pattern for a nematic phase of disk micelle^.^ The pattern rapidly evolves with decreasing temperature. The Bragg reflections sharpen in both directions (Figure 3a), and a second-order reflection appears in the ratio 1:2,indicating a pseudolamellar type order. The diffuse reflection in thex direction also changes, becoming less curved and more parallel to they axis. The
transition to the lamellar phase is imperceptible from the X-ray scattering. Within the lamellar phase, the scattering pattern (Figure 2e) is unchanged down to low temperatures, where the system phase separates. The Bragg reflections are sharp and well-defined in both directions and have a second and sometimes a third-order reflection in the ratio 1:2:3,confirming the lamellar phase identification. The diffuse reflections in the x direction remain clearly visible and form a diffuse line running parallel to the y axis, along y'-y" in Figure 1. This behavior is generic, occurring at all concentrations across the phase diagram where the sequenceof phases isotropic, nematic, and lamellar are seen. 3.2. Analysis. Figure 3a shows the line width of the Bragg reflection measured in the y direction to decrease with decreasing temperature across the nematic phase. This decrease in line width and the appearance of a second order to the dll reflection is indicative of an increase in positional order between the aggregates. Reference 50 shows that the line width, AS, from a one-dimensional lattice of planes of average separation dll is given by
where N is the number of lattice parameters making up a single domain which therefore has an extension Ndll (50) Guinier, A. X-ray diffraction in crystals, imperfect crystals and amorphous bodies; W .H. Freeman and Co.: San Francisco and London, 1963.
Nematic and Disrupted Lamellar Phases in CsPFO I2H20
Langmuir, Vol. 11, No. I , 1995 359
3.0
(a:
1.0-
4
-
0.9 A
I
I
I
I
-1
0
I
2
Q I nm-l
.
Figure 4. The profile of the lateral reflection along the y‘-y“
Q 0.8-
-8
“8
’ 0.7-
a”
-16
I
-2
A
ND
-14
-12
-8
-10
-6
-4
-2
-
T T, / OC
Figure 3. (a)A plot of the Bragg peak width at half-height for a W,= 0.55 CsPFO sample plotted as a function of reduced temperature T - TNIPC. Note that the width decreases across the nematic phase, ND,reaching an approximately constant value in the lamellar, LuH,phase. (b)The orientationalorder parameter, (Ppz(cos 13))plotted as a function of reduced temperature for the W,= 0.40 (0),0.50 (A), and 0.60 (0)CsPFO samples, Notice that within experimental error the three curves overlap and that in the LuHphase the order parameter is close to unity. and 6 is the half-width of an assumed Gaussian distribution in the nearest neighbor separation dll. It is likely that as the nematic phase is cooled there is a narrowing of the Gaussian distribution in dll. In ref 48, we showed that this distribution is sensitive to the counterion distribution between the lamellae which is expected to be a function of temperature. It is also likely that the longer range correlation between aggregates increases so that N is increasing as well. The combination of both scattering line width decrease and the appearance of a second order reflection are indicative of a growth of positional order as the nematic phase is crossed. It is also possible to extract information about the aggregates orientational order.16 Delord et al.51,52has shown for thermotropic nematic liquid crystals that analyzing the distribution of scattering intensity along an arc passing through the Bragg reflection enables an orientational order parameter to be obtained. The order parameter calculated in this way is shown in Figure 3b and is seen to be rather high both in the nematic and the lamellar phase. In ref 16 it was argued that this was because the X-ray-measured order parameter reflected long-range director fluctuations rather than short range fluctuations. In other words, provided the micelles were not locally, orientationally correlated, scattering techniques would not be sensitive to their local orientational order but only to the long-range fluctuations in director orientation. From the evidence of increasing local positional order given above, it would seem unlikely that while the aggregate centers of mass were becoming increasingly (51) Delord, P.; Malet, G. Mol. Cryst. Liq. Cryst. 1974,28, 223. (52) Delord, P.; Malet, G. Mol. Cryst. Liq. Cryst. 1974, 27, 231.
direction together with the fit obtained from the Fourier transform of the assumed bilayer electron density shown in the inset. The parameter a is the semithickness of the bilayer with a best fit value of 1.6 nm.
positionally ordered, at least on a local scale, they would not be orientationally ordered as well. Finally, in the lamellar phase, taking a section along y‘-y“ through the lateral reflection allows the degree of correlation between the water-filled defects in adjacent lamellae to be analyzed. A similar procedure has already been carried out for the lamellar phase of the decylammonium chloride/a”onium chloriddwater system.20The shape of the reflection along the y’-y” axis arises from the product of two terms, the electron density profile of a single bilayer and the interference between a regularly repeating series of defect features. If, for example, the water-filled defects in the lamellar phase were correlated from lamella to lamella, forming a “crystal lattice” of defects, the interference function would generate several peak features along y‘-y” and would have the effect of lowering the intensity in the center of the pattern ( Q = 0 nm-’1. Such correlations have been seen in the sodium dodecyl sulfate/water system.53 The lack of such features in the system considered here implies that there is very little correlation between defects in adjacent lamellae. It should be possible to model the profile of the lateral reflection along y’-y” with only the Fourier transform of the electron density of a single lamella. For simplicity we have chosen to model the electron density profile of the bilayer by a simple step function, shown in the insert of Figure 4, with a bilayer half-width o f a . This simplification allows the experimental profile to be fitted with a single adjustable parameter to the Fourier transform ofthe step function:
where a is the adjustable parameter and I(Q) is the intensity of scattering at Q . The fitting procedure gives a = 1.6 nm, which is rather larger than the expected value of 1.1nm; see section 5. Refining the fit further would require the introduction of two further parameters for the width ofthe head group region and its electron density relative to the perfluoroalkyl chain region. There is insufficient information to justify this refinement. The method does clearly show that it is possible to fit the lateral reflection profile using only the electron density of a single, isolated bilayer, and therefore that there is no correlation between the water-filled defects in adjacent lamellae. (53) K6kicheff, P.; Cabane, B. Acta Crystallogr. 1988, B44, 395.
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360 Langmuir, Vol. 11,No. 1, 1995
4.2. Analysis of the Phases. 4.2.1. The Lamellar Phase. The pioneering work of Luzzatti et al.54showed that for a classical bilayer the relationship between the X-ray-measuredlayer repeat distance dll and the thickness of, in this case, the fluorocarbon layer is given by
-
7.5
-
7.0
-
6.5
dfc = @ad11
.-
(1)
6.0-
and the surface area per molecule measured at the interface between hydrophobic and hydrophilic regions, Sa, is given by
-0
5.5
20
30
40
50
60
70
80
TIoC
Figure 5. The d spacings plotted as a function of temperature for the Wa= 0.50 CsPFO sample: (B) dll, (0) d l , and ( x in a box) do. The three phases, isotropic micellar, L1;nematic, ND,and defected lamellar, LaH are marked. There are narrow twophase regions at each ofthe two phase boundaries which cannot be distinguished with these measurements. 7 ,
a
0
a
0.1
0.2
0.3
0.4
015
I
0, Figure 6. The d spacings as a function of volume fraction, $a, of CsPFO at 40 "C. Symbols are as defined in the legend t o Figure 5.
To summarize this section, the nematic phase not only exhibits long-range orientational order but also increasing local positional order. Both increase with decreasing temperature, so when the lamellar phase is entered, the orientational order is rather high. The water-filled defects in the lamellae show no correlation between adjacent lamellae. 4. Scattering Peak Positions 4.1. Observations. Figure 5 shows the variation of do, dll,and dl as a function of temperature in the Wa = 0.50 sample. This behavior is general to all the sample concentrations studied and there are a number of interesting features. In the isotropic phase, do rises slowly as TNIis approached. In the nematic phase, dlland dl behave rather differently from each other. The spacing dll hardly changes at all with decreasing temperature while dl increases rapidly. In the lamellar phase both dll and dl are increasing with decreasing temperature but neither with the rapidity seen for dl in the nematic phase; indeed, d l becomes nearly temperature independent at low temperature. Figure 6 shows a plot of the variation of do, dll, and dl as a function of volume fraction a t a fixed temperature of 40 "C. It is interesting to note that dlland dl appear to vary in the same way with volume fraction.
where V, = 0.360 nm3 is the volume of the surfactant molecule. Using these equations to determine dfcand Sa yield values that are too low and high, respectively, to be physically realistic when compared to the dimensions of the fluorocarbon chain. For example at 40 "Cin the W, = 0.50 sample, i.e. just below TNL,the calculated values ofda and Saare 1.41 nm and 0.51 nm2,while at 26 "C the respective values are 1.65nm and 0.44 nm2. These values should be contrasted with twice the all-trans extended chain length of 2.45 nm calculated and measured experimentally in the anhydrous soap and to the values obtained by Fontell and Lindman55in aqueous solutions of the salts of heptadecafluorononanoic acid. In the lamellar phases of the diethylammonium, dimethylammonium, and the acid itself, bilayer thicknesses of ca. 2.4 nm and surface areas per molecule of between 0.30 and 0.46 nm2 were found.55 ks has been previously reported,lJ* the explanation of the apparent thinness of the lamellae is because of the presence of water-filled defects. This is now wellestablished since these defects constitute intralamella structure which scatters X-rays and neutrons, giving rise to the d l reported here and in previous p a p e r ~ . ' J ~In, ~ ~ the case of a lamellar phase in which the lamellae are pierced by water-filled defects of whatever shape, eqs 1 and 2 become
(3)
(4) respectively where f is the fractional lamella area occupied by the water-filled defects. In a previous paper,45 we showed a direct correlation between f and the water obstruction factor measured from the water self-diffusion parallel to the lamellar normal, confirming the identification of the intralamella structure with water-filled defects. There remains the problem of identifying the form that these water-filled defectstake. Currently there is no direct method of unequivocally distinguishing between possible structures. In ref 2 we argued in favor of surfactant continuous lamellae containing elongated water-filled defects using SANS results from binary and ternary systems and making use of the assumption that the water width of a n intralamella defect would be comparable to the water layer thickness between lamellae. An alternative approach is to compare the variation ofthe dimensions ofthe system at k e d temperature with surfactant volume fraction and to contrast this with ideal behavior. In (54) Luzzati, V. Biological Membranes; Chapman, D., Ed.; Academic Press: London and New York, 1968; p 71. ( 5 5 ) Fontell, K.; Lindman, B. J.Phys. Chem. 1983,87, 3289.
Langmuir, Vol. 11, No. 1, 1995 361
Nematic and Disrupted Lamellar Phases in CsPFO I2H20 2.0
,
Table 1. Values Obtained from Equation 5 at Four Temperaturesa TPC
dfdnm
n
T/"C
dfdnm
n
50 40
2.3 2.46
-0.6 -0.52
30 20
2.5 2.9
-0.6 -0.5
a At temperatures of 50, 30 and 20 "C fewer data points were available and therefore the results are less reliable than at 40 "C.
0.8 -2.5
I
I
1
-2.0
-1.5
I
I
-1.o
I
-0.5
I
0.0
In oa Figure 7. A plot of In d against In at 40 "C. Symbols are as defined for Figure 5 . The lines are the best fit straight lines and danhydrous marks the layer spacing in the anhydrous soap (2.45 nm).
systems where changing the surfactant water ratio has no other effect than changing the distance between aggregates of fixed size, the dimensions of the system will vary with surfactant volume fraction, 4a9 in a way which depends upon the dimensionalityof the structure. In other words, if d is a dimension of the phase structure, d = #avn where n = 1for an infinite defect-free lamellar phase with one-dimensionalpositional order, n = 1/2for an hexagonal or rectangular phase with two-dimensional positional order, and n = 113 for a micellar phase with threedimensional positional order. A number of real systems, both ionic and nonionic, exhibit this ideal behavior. For example, in some of the salts of the heptadecafluorononanoic acid/water systems55where there is no strong association of the counterionswith the head group surface, the lamellar phase exhibits ideal behavior. The corollary of ideal behavior is that dimensions of the aggregate are independent of surfactant concentration. In the CsPFO/water system there clearly is a strong association of the counterions with the head groups; however, there is evidence that the thickness of the lamellae are nearly constant. The lamellae contain waterfilled defects which are separated by distances equivalent to, typically, 5 times the all-trans-perfluoroalkyl chain length.2 The radius of curvature of these edges is likely to be close to the all-trans-perfluoroalkyl chain length, i.e. about 1.23 nm. Because of the relative inflexibility of the chains, it is unlikely that there will be substantial thinning of the center of the aggregates to give "dumbbell" shaped cross sections. Moreover, when substantial amounts of electrolyte are added,l the thickness of the defect-free classical lamellae is found to be ca. 2.2 nm. Given the small range of variation in df, that is possible, we treat it as a constant and look a t the variation of phase dimensions with volume fraction for clues to the aggregate structure. Figure 7 is a plot of the equation
In di = In k
- n In @a
(5)
for di = do, dll, and d l at 40 "C. Both plots of dll and d l are nearly parallel to each other and give n = -0.52 and -0.47, respectively. The intercept for In dll at +a = 1corresponds to dll= 2.46 nm, very close to the (001) spacing reflection in the anhydrous soap crystals (2.45 nm). Repeating the procedure a t other temperatures gives a set of values for n and df, which vary only slightly with temperature (Table 1). The system appears to show ideal behavior with both dll and d l varying as 4a-0.5. The small difference between
- L -
r\
I I
A
Figure 8. (a) An impression of the structure of the defected
lamellar phase, LaH. Note that the defects are elongated, irregular, and not correlated between layers. Parts b and c illustrate models of the nematic phase lookingalongthe director (i.e. along they axis) at TNI(b) and at TLN(c). This diagram also defines the dimensions used in equations 6-8 and presented in Table 2. n for dll and d l (dll= 2.464a-0.52and d l = 3.374a-0.47)may arise from a slight difference in expansion in the two directions; the product of dll and d l gives an exact power law dependence of -1 (dldl = 8.304a-0.99). The ideal behavior with n = 0.5 leads to the conclusion that the structure in the lamellar phase has a two-dimensional structure and, given the lamellar symmetry, that the lamellae consist of ribbonlike aggregates. The previous section showed that there was no correlation of water-filled defects between lamellar planes. Thus the proposed ribbonlike structures are uncorrelated between layers and within layers run in no particular direction. There are also likely to be bridges between ribbons within planes. A representation of the structure is shown in Figure 8. The lack of correlation and its interconnectedness within the layers means that it would not show any biaxiality in NMR experiments. 4.2.2. The Isotropic Phase. For the isotropic phase, the relationship between do and the volume fraction is found to be
do=
Holmes et al.
362 Langmuir, Vol.11,No. 1, 1995
T N ,at which point they become orientationally ordered. No significant change in the size of the micelles has been detected across the isotropic-nematic phase transition. + f Figure 9 shows a decrease in micellar volume as the * surfactant volume fraction and hence the phase transition Y temperature, TNI,increases. If one dimension of the + X micelles is fmed by the length of two all-trans-perfluoX + roalkyl chains, then the decrease in V, corresponds to a decrease in micelle diameter and eccentricity with increasing concentration. What is significant is that the + ratio dlldll at TNIis independent of concentration and has X X the value of 1.17 f 0.06. This ratio is a measure of the 30 anisotropy of the system. Once this critical value is R + reached and exceeded, the intermicellar interaction is sufficiently anisotropic for long-range orientational order 20 0.1 0.2 0.3 0.4 0.5 to be established. 4.2.3. The Nematic Phase. It is pertinent to ask, at what point do the disk micelles aggregate to form the Figure 9. Micellar volumes calculated along the nematicsurfactant continuous lamellar phase? Such an aggregaisotropic phase boundary: ( x ) from the isotropic phase using tion process must result in profound changes in the small equation V, = (3/4)&C$,d,3 and (+) from the nematic phase angle scattering of neutrons and X-rays. Experimentally using equation V, = (2/&)C$,dI$~. the largest and most rapid changes occur across the nematic phase. In the nematic phase, dllis nearly constant Since -0.5 < n < -0.167, the aggregates have a threebut the associated reflection shows an increase in both dimensionalmicellar structure but one where the micelles positional and orientational order of the micellar agare increasing in size (i.e.increasing aggregation number) gregates. In addition, across the nematic phase, dL with increasing surfactant concentration. This would be increases rapidly with decreasing temperature. Assuming expected from the theoretical models of mi~ellization~~ the aggregates to be located in a layerlike structure, the and can only occur if the micelles become nonspherical, constancy of dll indicates that (1- f , is also constant since being either rod- or disk-shaped. df, does not change significantly. The rapid change in d l The anisometry of the micelles in the isotropic phase must result from a reorganization of the structure in the is confirmed by X-ray scattering, since taking do from the plane perpendicular to the director. isotropic scattering peak and assuming a hexagonal If the system consists of disk micelles at TNIthen it arrangement of micelles gives the equation might be imagined that as the temperature is lowered the micelles aggregate together to form elongated, flatten "ruler-shaped))aggregates at TLNacting as a precursor to the connected ribbon structure of the lamellar phase. This would explain the observed increases of positional and This equation is only approximate50and at best gives an orientational order. Can such a model also explain the estimate of V, for a concentrated micellar system to within behavior of dl and dll with temperature and provide a &lo%. However, even the lowest estimate of V, is 50% physical explanation for why it is happening? larger than the volume of a spherical micelle calculated Models of the aggregate structures in a plane perpenfrom a radius equal to the all-trans-perfluoroalkylchain dicular to the director (the aggregation plane) at TN and length, leading to the conclusion that the micelles are T m are shown in Figure 8b,c. At TNIit has been assumed anisometric. Since the micelles are anisometric and the that the micelles are monodispersed disks and that they scattering in the lower temperature nematic and lamellar are arranged on a hexagonal lattice in the aggregation phases is consistent with layered structures, it seems plane. The assumption of monodispersity is quite good reasonable to assume that the micelles in the isotropic for disks since the highly curved hemicylindrical edge phase just above TNIare discoidal in shape. controls disk growth. The assumption of hexagonal Just below TNIthe X-ray and neutron scattering is packing is not so realistic, but this assumption has been precisely that expected for a nematic phase of disk-shaped shown to yield realistic values for micelle v01ume.'~ At micelle^.^ Making the assumption that disk-shaped T w they are assumed to form infinite ribbons (since there micelles are present in both isotropic and nematic phases is no information about their length)again lying in planes. above and below TNIallows V, to be calculated on either Ifdl at TNIand T m are denoted bydil and d k , respectively, side of the transition from equations16 then the radius of the disk micelles at TNIare given by
-7
x
I
401
3
V, = -p%#Jad,3
for the isotropic phase
and
V, =
2
d$
for the nematic phase
Both sets of values are plotted as a function of qjain Figure 9. They agree within the limits of the experimental error and the crudity of the equations. It is concluded that, in the binary system, disk micelles exist in the isotropic phase. These grow as the temperature is lowered toward (56)Mukerjee, P. J.Phys. Chem. 1972, 76, 565.
from which their average circumference can be calculated. It is now assumed that the fractional area of waterfilled defects in the aggregation plane, f , remains unchanged across the nematic phase. Thus at Tm
(7) and
Nematic and Disrupted Lamellar Phases in CsPFO I2H20
Langmuir, Vol. 11, No. 1, 1995 363
Table 2. Structural Parameters, V,, r, and !kr Calculated from Experimental Results for Disk Micelles at TM and L and 2A Calculated for Ribbons at TLN
Wa
4a
dlhm
dlLlnm
ddnm
Vm/nms
rlnm
Llnm
2mlnm
2Nnm
0.30 0.35 0.40 0.42 0.45 0.50 0.55 0.62
0.156 0.189 0.224 0.239 0.262 0.302 0.346 0.414
7.80 7.31 6.03 6.50 6.27 5.82 4.66 3.90
8.16 7.99 7.60 7.98 7.32 6.81 5.28 4.68
6.75 6.05 5.47 5.57 5.24 4.61 4.21 3.95
74.0 70.5 51.4 64.9 62.3 54.4 36.5 28.7
3.3 3.2 2.7 3.1 3.0 2.8 2.3 2.0
3.9 4.2 4.2 4.8 4.6 4.3 3.5 3.5
20.6 20.1 17.1 19.3 18.9 17.6 14.4 12.8
17.2 15.4 11.1 12.2 12.4 11.5 9.5 7.5
80
where th length of the hemispherical curved edge associated with the two sides of the ribbon is 2A. Using eqs 6-8 and dll and dlL tabulated in Table 2, values of r, L, and the lengths of the perimeters of both disk and ribbon are calculated. The value of df, = 2.2 nm has been taken from experiments on electrolyte and alcohol addition, section 5, and refs 1and 2 which show that the bilayer thickness reaches this as a limiting value as the waterfilled defects are removed. Clearly the model makes several extreme assumptions about the structures at TNIand TLN;however, there are some clear trends summarized in Table 2. The ribbon width, L, is less than 2r; the disk diameter and the ratio 2rlL is ca. 1.4. The perimeter of the ribbon, 211, representing the length of hemicylindrical curved edge, is shorter by ca. 50%than the perimeter of the disk micelle enclosing the same area of surfactant. If the system is being driven to reduce surface curvature, then the aggregation of disk micelles into elongated, flatten aggregates provides a mechanism by which it can occur and explains the experimental observations. How far this aggregation mechanism goes is not clear. To perform the above calculations we have assumed infinite ribbons at TLN; we believe that in reality they would be quite short, perhaps 3 or 4 times the diskmicelle diameter before steric effects lead to coalescence of the aggregates into the surfactant continuous sheets of the lamellar phase.
5. The Effect of Additives 5.1. Observations. In two previous papers,l,awe have explored the effects of adding electrolyte (CsC1) and a nonionic alcohol (VI,VI-perfluoroheptan-1-01)on the lamellar phase of this system. Figures 10a and l l a show the effect on the phase diagrams1S2and Figures 10b and l l b show the effect on dll measured at 40 "C. The increase in dllis directly correlated with the loss of the lateral reflection from dL. These two effects have been interpreted as the loss of the water-filled defects in the lamellae. At the highest concentrations of the third component, the bilayer reaches a limiting bilayer thickness of 2.2 nm. A concomitant effect of the loss of water-filled defects in the lamellar phase is that the higher temperature nematic phase is replaced by an isotropic-lamellar twophase coexistence region (Figures 10a and l l a ) . However, in both ternary systems, the nematic phase persists t o quite high concentrations of the third component. The small angle scattering from the nematic phase is significantly different from its binary counterpart. Figure 12 shows a sequence of SANS patterns from the nematic phase as CsCl is added. The diffise lateral reflections which come from the large dimension of the micelles is progressively lost. Adding CsCl also has the effect of broadening the Bragg reflection from the interlamella reflection in the lamellar phasea which has been explained
-,
c 60
50
T
4 0.00
0.02
0.04
0.06
0.08
1
0
%weight CsCl
Figure 10. (a)The phase diagram for CsPFOPH20 in the mole ratio of 1:27.3 (equivalentt o a Wa= 0.50 binary sample) with the addition of electrolyte (CsC1). Note the loss of the nematic phase and its replacement by a two-phase region. The phase diagram was obtained by optical microscopy.' (b)The lamellar spacing, dll, plotted as a function of CsCl concentration. by the screening of the interlamella electrostatic interaction by the added CsCl and leading to an increased variability in the interlamella separation. A second feature in the CsCl addition is the change in the temperature dependence of dll illustrated in Figure 13. In the binary system, dll is virtually independent of temperature in the nematic region and only starts to increase as the lamellar phase is entered. As CsCl is added, dll becomes temperature dependent. Indeed dlland do become continuous across the isotropic-nematic phase transition and across nematic and lamellar phases. 5.2. Analysis. The effect of these additives on the lamellar phase has already been extensively discussed,1,2,48 so here we shall focus on the nematic phase and on the transition from the isotropic phase. Addition of electrolyte and cosurfactant, lH,lH-perfluoroheptan-1-01, have the same effect, namely to encourage the loss of surface curvature and the growth oflarger, flatter interfaces. Both additives will also affect the intermicellar interaction; the electrolyte addition significantly screens the electrostatic intermicellar interaction while the addition of cosurfactant has the effect of diluting the interfacial charge. Micelles will grow larger than in the binary system before experiencing a sufficiently strong enough anisotropic intermicellar interaction t o cause the formation of the orientationally ordered nematic phase. The growth in
Holmes et al.
364 Langmuir, Vol. 11, No. 1, 1995
T-79
8
i
z I I
3
(b 0.0
0.1
I
I
0.2
0.3
Molarity of cosurfactant Figure 11. (a)The phase diagram for CsPFOPH20in the mole ratio of 1:27.3(equivalent to a W,= 0.50 binary sample) with the addition of lH,lH-perfluoroheptan-1-01.Note the loss of the nematic phase and its replacement by a two-phase region. The phase diagram was obtained by optical microscopy.2 (b) The lamellar spacing, dll (m), and intralamella spacing, d~ (U), plotted as a function of lH, W-perfluoroheptan-1-01concentration. micelle size with additives may not only produce larger disk-shaped micelles but may also preempt the formation of more elongated micelles producing micelles polydispersed in both shape and size in the isotropic phase. Scattering from the isotropic phase tends, therefore, to show only the cZ,, dimension of the aggregates. Their polydispersity would also explain the progressive loss of the lateral reflection as CsCl is added. 6. Conclusion
In previous papers we have suggested that the lamellar phase consists of surfactant continuous lamellae containing elongated water-filled defects. Here, we have argued that the thickness of the perfluoroalkyl chain region is nearly constant. The variation of dll and dl with #a can be treated as ideal, reflecting the underlying structure of the aggregates in the lamellar phase. These phase structures expand with water addition in two dimensions, implying ribbonlike aggregates in the lamellar planes. It has been shown that there is no correlation between defects in adjacent lamellae. We conclude that the lamellar structure is best represented by the type of structure shown in Figure 8a in which surfactant continuous lamellae are broken by irregular water-filled defects with no correlations between layers. In keeping with previous findings in this and similar systems, the isotropic phase was found to be consistent with being composed of disk-shaped micelles. An important question which this paper has addressed is the mechanism by which the aggregates make the transition from discrete disks to continuous lamellae. Section 4 presented evidence that profound changes in aggregate order take place as the nematic phase is crossed. Both positional and orientational order increase as temperature decreases. The absence of any change in dll and therefore
(1- f , across the nematic phase while d i changes rapidly suggests that aggregate rearrangement takes place in the nematic phase. Dimensions for the disk micelles at TNI and ribbon-shaped aggregates at TLNare consistent with such a rearrangement. Although this structural transition cannot, at the moment, be proven equivocally, the arguments in its favor are, we feel, rather strong and we are currently seeking confirmation from other techniques. The change, from isotropic to defected lamellar phase, is driven by a decrease in surface curvature. As the temperature is lowered,the equilibrium between hydrated and surface-bound counterions will be shifted toward more Cs+ions being associatedwiththe surface. This facilitates a decrease in the Coulombic repulsion between surfactant head groups and an increase in flatter interface. If the diskmicelle shape was retained, this would require larger, flatter micelles that would quickly become sterically hindered. Aggregation to form elongated flat micelles and then irregular, ribbon-type structures provides a geometry where flat interfaces can grow, and while some curvature remains, the hemicylindrical curved edges can be limited and reduced. Section 5 discussed the effects of the addition of electrolyte and cosurfactant, lH,W-perfluoroheptan-l01. Both have the effect of encouraging the growth of larger, flatter interfaces and ultimately driving the lamellar phase toward a classical bilayer. They also modify the intermicellar interactions, allowing the formation of larger micelles with more polydispersity in shape and size in the isotropic phase. The phase structures seen in this simple system are critically determined by two factors. First, intra-aggregate forces in the form of competing perfluoroalkyl chain interactions and the head grouplcounterion electrostatic interactions, which together determine a mean surface area per molecule. This is achieved by forming nonuniformly curved interfaces, either disk micelles, elongated flat micelles, or defected lamellar phase. Second, interaggregate interactions play some part in determining aggregate shape, but more importantly they determine the point at whichliquid crystal phases form (section4.2.2). It is interesting to compare this system and its phase structures with that of lithium pentadecafluorooctanoate (LiPFOPH20, which has been studied by Kbkicheff and T i d d ~ The . ~ ~strongly hydrated lithium ion is larger than the cesium ion and is less able to bind to the surfactant head groups. The electrostatic head group repulsions are therefore stronger, giving rise to more highly curved interfaces. The LiPFO system forms a hexagonal phase at intermediate surfactant concentrations and a tetragonal mesh phase at high surfactant concentration and low temperatures. The latter phase is also characterized by strong correlations between the mesh layers; the hole in one layer correlating with the nodes in the two adjacent layers. Changing the counterion from lithium to cesium means that more counterions bind to the head group region, allowing larger, flatter aggregate structures. The tetragonal mesh phase is replaced by a defected lamellar phase, or perhaps more strictly it should be referred to as a random mesh intermediate phase. Although there are water-filled holes in the layers, they are not correlated between layers. The interaggregate electrostatic interaction is weaker because more head group charge is balanced by bound counterions. Adding further counterions screens the interaggregate interaction fiwther and the interlamella repeat distance becomes less ~ e l l - d e f i n e d . ~ ~ Not only should the defected lamellar phase in this (57)K&icheff, P.; Tiddy, G. J. T. J . Phys. Chem. 1989,93,2520.
Nematic and Disrupted Lamellar Phases in CsPFO I 2Hz0
Langmuir, Vol. 11, No. 1, 1995 365
Y
t I
X
000 0 0
Figure 12. A sequence of small angle neutron scatteringpatterns taken at approximatelyT - TNI= 4 “C in a CsPFOPHzO sample at a mole ratio of 127.3 with the addition of CsCl: (a) 1%,(b) 2%, (c) 3%,and (d) 4% by weight CsC1. The lowest contour on the scattering patterns has been set to the same level in each picture and shows the progressive loss of the lateral reflection with CsCl concentration. I
E
5.4
-
5.2
-
I
I
5.0-
. K
-0 4.8-
4.6
-
4.4
1
-30
-20
.
1
-10
.
1
0
10
.
1
20
,
liquid crystal nematic phases. The latter phases are characterized by a “schlieren” texture under the optical polarizing microscope and on a microscopic level by the existence of long-range orientational order of the molecules but with only limited short-range positional order. In this (and o t h e r 9 lyotropic nematic phaseb), although the optical texture is a schlieren texture, there are qualitative changes in the texture as the nematic phase is crossed. The structural units are not single molecules but micelles which change in shape and size and which acquire both orientational and positional order. I t would therefore appear dangerous to draw too many analogies between thermotropic nematic phases and their lyotropic counterparts simply on the basis of schlieren optical textures and the ability to be affected by applied magnetic fields.
30
T - T, I ‘C
Figure 13. do and da plotted as functions of reduced temperature for different amounts of added CsC1: (W) 0%,(0)1.26%, (0) 2.72%, and (A) 5.37% by weight CsCl.
system (and probably in other similar systems15) be regarded more correctly as a random mesh intermediate phase but also the name nematic phase is perhaps misleading because of its association with thermotropic
Acknowledgment. We would like to thank Gordon Tiddy for helpful discussions and SERC for granting beam time on LOQ at ISIS and 7.2 at SRS, Daresbury. We would particularly like to thank Steve King and Richard Heenan for assistance with the SANS measurements and Martin Bellwood for his invaluable help in making both SANS and SAXS measurements. M.S.L. and A.M.S. would like to thank the University for studentships. LA940710G
