J . Phys. Chem. 1986, 90, 707-710 The optical absorption and emission spectra of the radiation products were compared with those of unirradiated samples. In the case of PEG and PB, the polymer bound Ru complex exhibited the same absorption and excitation spectra as the nonbound complex (400-550-nm region). On the other hand, the emission spectrum was shifted from 612 (nonbound) to 627 (PEG bound) and 621 nm (PB bound). In the case of PSS solutions the spectra of the Ru complex were slightly broader but with no apparent shifts. These results clearly indicate that at least in the case of PEG and PB the radiation has induced binding of Ru(bpy)32+to the polymer and thus caused the observed shifts in the emission spectrum. Since these shifts are relatively small, and the absorption spectra were not shifted at all ( f 2 nm), it may be suggested that the polymer-bound Ru complex is fully aromatic like nonbound Ru(bpy)32+,Le. that the O H adduct reacts with the polymer radical to form a new covalent bond and then rearomatize byloss of water, as suggested above. The small shifts in the emission spectrum observed here parallel those observed previously upon addition of sodium dodecyl sulfate (SDS micelles) or
707
long-chain aliphatic alcohols to aqueous solutions of R ~ ( b p y ) , ~ + . ~ ~ In conclusion, the present results suggest radiation as a promising tool for the covalent binding of Ru(bpy)32+to a variety of polymers. The products may be used advantageously in solar energy conversion systems where the undesired back-reactions are suppressed. Acknowledgment. This research was supported by the Balfour and Schreiber Foundations, Israel, and by the Office of Basic Energy Sciences of the U S . Department of Energy. We thank Dr. R. Sassoon for helpful discussions. Supplementary Material Available: Table 11, containing the decay data for the Ru(bpy)?+-OH adduct in the presence of PB, and Table 111, containing the decay data for the R~(bpy),~+--oH adduct in the presence of PSS radicals (2 pages). Ordering information is available on any current masthead page. (26) Meisel, D.; Matheson, M. S.; Rabani, J. J . A m . Chem. SOC.1978, 100, 117.
The Swelling of a Lamellar Lyotropic Liquid Crystal by an Alkane F. C. Larche,* S. El Qebbaj, and J. Marignan Groupe de Dynamique des Phases Condensdes (LA 233), Universitd de Montpellier 2- 34060 Montpellier Cedex, France (Received: June 25, 1985; In Final Form: September 19, 1985) The swelling of the lamellar phase of the sodium p-octylbenzenesulfonate (OBS)/pentanol/water system by decane has been studied by X-ray diffraction. A model that quantitatively predicts the evolution of the interlamellar distance as a function of composition is presented. Consequences on the existence of highly diluted phases are verified and the stability of bilayer arrangements is discussed.
Introduction Lyotropic lamellar liquid crystals made up of an ionic surfactant, a long-chain alcohol, and water can often be swollen by water or brine. The extensive studies of Ekwall and co-workers' indicate that one can analyze these structures in terms of two entities. One is a diluting medium (in that case, water or brine) and the other a lamella. This elementary object, in principle an infinite plane, is composed of two layers of surfactant and alcohol (Figure la). The attractive van der Waals forces between the aliphatic tails maintain the cohesion of the object. These bilayers are charged and interact with the neighboring bilayers mostly via their electrostatic double layers. There is thus a repulsive interaction between them. The lamellar structures can also be swollen with an aliphatic hydrocarbon.2 The same analysis can be applied, but in this case we are in the presence of inverse bilayers. The stability of the overall structure could be considered at two levels: the stability of the elementary objects, Le., the bilayers, and the stability of the stacking. Helfrich, has analyzed such multilayered systems. The out-of-plane fluctuations of each membrane is restricted by the presence of its neighbors. This excluded volume effect produces a repulsive interaction and it may compete with the van der Waals attraction. The same order of magnitude has been estimated for the two effects. Since at large spacing the repulsion is expected to become dominant, two situations are possible. We can have a minimum, at a spacing d,. For all dilutions corresponding to a larger spacing, we expect a lamellar phase characterized by d,, in equilibrium with the diluent. (1) Ekwall, P. In "Advances in Liquid Crystals"; Brown, G . H., Ed.; Academic Press: New York, NY, 1975. (2) Hirsch, E.; Wittmann, J. C.; Candau, F. J . Dispersion Sci. Technol. 1982, 3, 351. (3) Helfrich, W. Z . Nururforsch, A 1978, 3 3 4 305.
0022-3654/86/2090-0707$01.50/0
TABLE I: Densities, at 25 'C, Used in the Treatment of X-ray Data (g Cm-3)
substance OBS pentanol decane water
density
ref
1.25 0.8110 0.7265 0.997
10 14 14 14
But if the steric interaction is large enough, the lamellar stacking is stable at all spacings. This result implicitly assumes, of course, that no other arrangement of the film has a lower free energy. Vesicles are a distinct possibility. A quantitative study of the dilution of a lyotropic liquid crystal would shed light on the importance of the entropic effects discussed by Helfrich. Because of the possibility of wide regions of predominant electrostatic repulsion extending up to 100 r ~ m the ,~ dilution by water would not give clear cut results. The addition of salt drastically reduces the Debye length and thus its range of interaction. Dilution by brine is known to produce large swellings5 and quantitative work is in progress along this path.6 We present the results of a dilution of inverse bilayers by an alkane. Because of the extensive studies of Marignan et al.,' the system studied was sodium n-octylbenzenesulfonate (OBS)/water/ 1pentanol. The systematic investigation of the dilution of its lamellar phase by decane is a first step toward the understanding of the forces at work in these structures. We shall show that pure decane is not the proper diluent in that its addition changes the composition, and hence the elastic properties, of the bilayer. A (4) Cluny, J. S.; Goodman, J. F.; Ingram, B. T. S u r j Colloid Sci. 1971, 3, 167. (5) Benton, W. J.; Miller, C. A. J . Phys. Chem. 1983, 87, 4981.
(6) Porte, G. et al., private communication. (7) Marignan, J.; Delichere, A,; Larche, F. C. J . Phys. Lerf. 1983, 44, L609.
0 1986 American Chemical Society
708 The Journal of Physical Chemistry, Vol. 90, No, 4, 1986
Larche et al. 140
0
WATER
120
0
z
100
X
N
t
E
DECANE
+
* *
P E N TA N 0 L
60
I
I
I
1
I
Figure 1. Schematic representation of the swelling of a lamellar lyotropic crystal. On the left side, the objects with an open circle represent the surfactant molecules and the objects with a full circle the pentanol molecules. Only molecules in the interfacial film have been represented. (a) In the ternary OBS/pentanol/water, the phase swells as water is added. We can think of this process as a dilution of the direct bilayers Bi by water. (b) When decane is added to the above ternary, the structure also swells. The process can be described by a dilution of inverse bilayers Bi, composed of water and two opposite monolayers of pentanol and surfactant. During this process, pentanol is transferred
from the bilayer to the decane. model is presented that gives the composition of the diluent, and it is tested by the obtention of highly swollen phases.
Experimental Section The OBS was synthetized according to Gray et a1.* from 1phenyloctane (Fluka 99% pure). It was purified by several recrystallizations in water and ethanol. The water was doubly distilled, and the 1-pentanol (Merck p.a.) and decane (Fluka, purum) were used as received. The X-ray measurements were taken on a low-angle GDPA camera9 equipped with a INEL position sensitive detector. The sample was drawn in a 0.3-mm-diameter Lindeman glass capillary, sealed to avoid evaporation and maintained at 25 f 0.1 "C. Although the interlamellar distance d is the directly measured quantity, it is instructive to look first at the area per polar head. It is obtained from the Bragg spacing assuming simple additivity of the molecular volumes. The density used for OBS has been deduced from the densities of solutions in water below the critical micellar concentration,10and the other densities were literature values (Table I). The results appear in Figure 2. Each addition of decane produces a decrease of this area. In the absence of oil, this quantity is controlled by the pentanol/OBS ratio. It is reasonable to think that this behavior is not influenced by the (8) Gray, F. W.; Gerecht, J. F.; Krems, I. J. J . Org. Chem. 1955, 20, 5 11. (9) Assih, T.; Larche, F. C.; Delord, P. J . Colloid Interface Sci. 1982,89, 35. (10) Marignan, J.; Bassereau, P.; Delord, P. J . Phys. Chem., following article in this issue.
where the 4's are volume fractions and the y's activity coefficients. Since we assume yAFto be constant, it is incorporated with kE into an apparent equilibrium constant
We have not found activity data for the pentanol-decane system at 25 "C. The relative vapor pressures have been measured at 20 "C by Sjoblom and Henrickssonlz and give directly the activity, but a temperature correction has to be applied. We choose the analytical representation proposed by Hanks et aI.l3 In
aAo
= In ( 4 B / 4 B * ) + K(4A04B - 4B*) + (1 - r)(l - @Ao) + P h ( 1 - 4A0)2/RT (4)
(1 1) Biais, J.; Bothorel, P.; Clin, B.; Lalanne, P. J . Dispersion Sci. Technol. 1981, 2, 67. (12) Sjoblom, E.; Henriksson, U. In "Surfactants in Solution"; Mittal, K. L., Lindman, B. Eds.; Plenum Press: New York, 1984; Vol. 3, p 1867.
(13) Hanks, R.W.;Kelly ONeill, T.; Christensen, J. J., Ind. Eng. Chem. Process Des. Deo. (1979), 18 408.
Swelling of a Lamellar Lyotropic Liquid Crystal
The Journal of Physical Chemistry, Vol. 90, No. 4, 1986 709
3 4
6
5 d
7
5
d
nm
9
( theory)
nm ( t h e o r y )
6
7 d nm
5
7
6 d nm
(theory)
(theory)
Figure 3. The measured interlamellar distance as a function of the calculated distance: a, molar ratio pentanol/OBS = 1.52; b, molar ratio pentanol/OBS = 2.52; c, molar ratio pentanol/OBS = 4.46; d, molar ratio pentanol/OBS = 5.84.
where r = VA/Vois the ratio of the molecular volumes of alcohol and hydrocarbon and 1 '#'B
=
+
- ( 1 -k 4K'#'~')l/~
2@'#'A0
$B* is obtained by replacing
d = da(1 - +AF)/4s
by 1 in the above expression. The value of B has practically no influence in the range of composition of interest here. We set it equal to zero. A value of K = 107 gives a good fit to the excess heat of mixing, as given by Hanks et al.I3 and is in agreement with the vapor pressure data. The numerical value of the activity is not very sensitive to the parameter K . A change from 107 to 150 does not produce more than a 1% change in the range of interest of this study. One can estimate at less than 10% the maximum error on the activity obtained with these expressions and numerical values. The interlamellar distance is related to the surface per polar head by '#'Ao
l / d = S'#'s/2Vs
(5)
where & is the volume fraction of surfactant in the sample and V, its molecular volume. S is related to the composition of the film by = 2[vS +
vA(NA/NS)l/da
(6)
where VAis the alcohol molecular volume, NA/Ns the molar ratio of alcohol to surfactant in the film, and da the thickness of the (14) Beilstein. "Handbook der Organischen Chemie"; 1941
surfactant and alcohol bilayer in the ternary liquid crystal. Combining Marignan's' and our own data gives a value of 1.983 nm for d,. Elimination of S between ( 5 ) and (6) yields, after an obvious change of variable (7)
This expression relates the film alcohol content '#'AF and the surfactant volume fraction in the sample '#'oto the interlamellar distance d. The alcohol balance yields
where '#'ois the oil volume fraction in the sample. Equations 2-4, I , and 8 can be solved numerically to give the theoretical value of d , the composition of the film, and the composition of the oil phase. It contains only one adjustable parameter, kA. The best fit has been obtained for a value kA = 1.03. The plots of the measured d vs. the calculated d appear in Figure 3a-d. Since the estimated precision on the interlamellar distance is 2%. the agreement between theory and experiment is quite good.
Discussion As we have seen, it is not crucial to have a very precise value of the parameter K. The results are more sensitive to the other parameter that enters the equations, Le., dA. The value obtained with the ternary data through eq 5 and 6 is such that d, = 1.983 f 0.019 nm. A calculation of dlhwith the value 1.96 nm for d, gives a change of 1 %. Thus it is clear that precise values of d in the ternary system
Larche et al.
710 The Journal of Physical Chemistry, Vol. 90, No. 4, 1986
are necessary to ensure a reasonably good prediction in the quaternary case. For the moment, kA is just an adjustable parameter. A study with different oils than decane would be interesting to test if it depends only on the nature of the film, as our model would suggest. A phase equilibrium is independent of the quantities of the phases present. As such, the model implies an unlimited swelling, as long as the alcohol in the added oil/alcohol mixture is in equilibrium with the alcohol in the film. The eq 2-4, 7, and 8 can be used to compute the composition of such a mixture, given the composition of the ternary liquid crystal. To test the limit of such a prediction, we have incorporated a mixture containing 8 wt % pentanol to a solution containing 42.1%OBS and 36.2% water. We were able to add up to 98% with no apparent phase change. The birefringence decreases gradually and is only visible with a light shearing of the solution at the highest dilutions. If the structure is still lamellar, one can estimate the interlamellar distance at 250 nm. At higher dilutions, there is precipitation of solid OBS.This can be. easily rationalized within the framework of the pseudophase model. We have neglected the water that is present in small quantity in the decane-pentanol mixture. If the quantity of mixture added is not too large, the transfer of water from the water lamellae to the oil pseudophase remain negligible. But at very high dilution, the remaining water might be in too small a quantity for hydration of the polar head, the counterions, and the alcohol in the film. It is then energetically favorable to precipitate solid OBS. Such a conclusion is supported by the return to the birefringent phase upon addition of a very small amount of water. Thus it seems in principle possible to swell even more this liquid crystal. It is not certain that such solutions still retain a lamellar structure, and investigation of this point as well as a complete phase diagram determination at high dilution near the
decane-pentanol line are in progress.
Conclusion At the end of this investigation, we can give the following conclusions, valid for the system OBS/pentanol/decane/water. The lamellar phase can absorb rather large quantities of oil. A phenomenological description of the swelling of this structure has been obtained by considering that it is composed of three phases: water, oil, and a film. At dilutions that are not too large, it is sufficient to consider the equilibrium of alcohol between the film and the oil. Use of an apparent equilibrium constant for lack of knowledge on the thermodynamics of the film gives nevertheless good quantitative agreement between theory and experiment. At very high dilution, water equilibrium between the water-rich and the decane-pentanol lamellae has to be taken into account. The degree of swelling that can be obtained when the appropriate mixture of decane and pentanol is used, as given by the above mentioned theory, is such that van der Waals interactions are very small, compared to entropic repulsions. As a consequence no minimum is expected in the free energy vs. interlamellar distance. The only limitation to swelling would be the appearance of a new phase, vesicles, or a nematic phase for instance. At the moment no theory is available for the stability of this system with respect to other topological arrangements of the films. The fact that structures as different as globules or lamellar arrangements exist at very high dilution, Le., without direct interactions of the objects, shows the importance of entropy in such problems, and should indicate the route for further theoretical research on the subject. Acknowledgment. This research has received partial financial support from PIRSEM (CNRS) under AIP 2004.
