Langmuir 1991, 7, 1874-1879
1874
Structure and Elastic Properties of Lamellar Mesophases from Direct Force Measurementst P. KBkicheff,*P. Richetti,*J and H. K. Christenson Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, G.P.O. Box 4, Canberra ACT 2601, Australia Submitted t o Symposium Chairman March 28, 1990. Received November 15, 1990. In Final Form: February 25, 1991 We present results of direct force measurements between mica surfaces immersed in two different lamellar mesophases: one oil-swollen phase that is stabilized by undulation forces and a water-swollen one stabilized by electrostatic forces. These mesophases align with the layers parallel to the solid mica surfaces and the measurements can be used to yield structural information on confined mesophases as well as bulk elastic properties. In particular, the modulus of compressibility obtained for the lamellar pentanol, and water (bulk interlayer spacing d = 8.8 nm) is in good phase of sodium dodecyl sulfate (SDS), agreement with values from methods such as X-ray scattering. In the oil-swollen system of sodium octylbenzenesulfonate-water-pentanol-decane (d = 28 nm), the spacing of the aligned lamellar mesophase is significantly reduced at small mica-mica separations. We discuss this in terms of a reduction in the thermal undulations of the individual membranes imposed by the proximity of the solid surfaces.
Introduction Amphiphilic molecules may aggregate to form an astonishing range of association structures in certain solvents, particularly in the presence of both oil and water. Unfavorable contacts between polar and nonpolar moieties are avoided a t the expense of increased order by assembly of the amphiphiles into monolayers or bilayers. These may take up a variety of three-dimensional configurations of different curvatures, leading to well-known structures such as micelles, rods, lamellae, and the more intricate networks found in cubic mes0phases.l The phase behavior of these lyotropic systems is often remarkably sensitive to factors such as temperature, salinity, or the nature of the nonpolar liquid. The structurally simplest of these lyotropic phases are the lamellar liquid crystals. The basic unit is a bilayer of infinite extent and these are stacked in a regular array with one-dimensional periodicity. The separation between the lamellae may change by solvent uptake in the aqueous and/or oil phase. Stability of the stacking at a certain interlamellar spacing requires the existence of some repulsive, interlamellar interaction that prevents collapse due to attractive van der Waals forces and consequent drainage of the swelling liquid (i.e., equilibrium with excess solvent). In view of this, the existence of lamellar phases that swell to interlayer spacings of hundreds, even up to one thousand nanometers, is an a t first sight intriguing p h e n ~ m e n o n . ~In - ~oil or brine this is far beyond the range of electrostatic or structural forces and some other mechanism must be invoked to explain the stability of these stacked membranes. I t is now commonly accepted that out-of-plane thermal undulations of sufficiently flexible membranes are able to provide the necessary stabilization in many such systems. + This work was presented at the 198thAmerican Chemical Society Meeting held in Miami, FL, September 10-15, 1989. Permanent address: CRPP-CNRS, Chateau Brivazac, Av. Schweitzer. 33600 Pessac. France. (1) Luzzati, V. In Bioiogicalkembranes;Chapman, D., Ed.;Academic Press: New York, 1968; pp 71-123. (2) Palmer, K. J.; Schmitt, F. 0. J. Cell. Comp. Physiol. 1941, 17, RA5-RqA - -- -- - . (3) Rand, R. P. Annu. Reu. Biophys. Bioeng. 1984, 10, 277-314. (4) Larch6, F. C.;Appel, J.;Porte, G.;Bassereau, P.;Marignan, J. Phys. Reu. Lett. 1986, 56, 1700-1703; Appell, J.; Bassereau, P.; Marignan, J.; Porte, G. Colloid Polym. Sci. 1989, 267, 600-606.
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Such forces, originally proposed by Helfrich, are able to balance attractive van der Waals forces even a t very large interlayer separations? This entropic repulsion is ultimately due to steric hindrance of the thermal fluctuations of neighboring membranes. Various scattering techniques have recently been employed to obtain experimental information on the magnitude of the elastic constants of these layered systems, and the results confirm the Helfrich interpretation.G8 The surface force apparatus has proven itself as a powerful technique for studying the interactions between surfaces in simple liquids and solutions?JO A wide range of experiments with surfactants adsorbed to mica surfaces from solution11or lipids deposited as Langmuir-Blodgett films12has been carried out. The forces measured between monolayers or bilayers held to the mica surfaces may provide information on the interlamellar forces in liquid crystalline systems with the same constituent bilayers. For example, the results of direct force measurements between phospholipid bilayers have been compared to osmotic stress measurements on bulk lamellar phases.13 The agreement is striking, and the slight reduction in the range of the interaction for the deposited bilayers is undoubtedly related to the expected absence or suppression of membrane undulations and/or lipid head group mobility when compared to the free bilayers. In cases where the interaction may be dominated by undulation forces, the application of the surface force technique is not as straightforward as usual. Simple (5) Helfrich, W. Z. Naturforsch., A.: Phys., Phys. Chem., Kosmophys. 1978,33A, 305-315. ( 6 ) Sdinya, C. R.; Roux, D.; Smith, G. S.; Sinha, S. K.; Dimon, P.; Clark, N. A.; Bellocq, A. M. Phys. Rev. Lett. 1986,57,2718-2721. Row, D.; Sdinya, C. R. J. Phys. Fr. 1988,49,307-318. Safinya, C. R.; Sirota, E. B.; ROUX,D.; Smith, G. S. Phys. Rev. Lett. 1989,62,1134-1137. (7) Meunier, J. J. Phys. (Paris) 1985, 46, L1005-Ll007. (8) Nallet, F.; Roux, D.; Prost, J. Phys. Reu. Lett. 1989,62,276-278. Nallet, F.; ROUX,D.; Prost, J. J.Phys. (Paris) 1989,50, 3147-3166. (9) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975-1001. (10) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Reo. Sei. Znstrum. 1989,60, 3135-3138. (11) Pashley, R. M.; McGuiggan, P. M.; Ninham, B. W.; Brady, J.; Evans, D. F. J. Phys. Chem. 1986,90,1637-1642. KBkicheff, P.; Christenson, H. K.; Ninham, B. W. Colloids Surf. 1989,40,31-41. (12) Marra, J.; Israelachvili, J. N. Biochemistry 1985,24,4608-4618. Marra, J. J. Phys. Chem. 1986,90, 2145-2150. (13) Horn,R. G.;Israelachvili,J.N.;Marra, J.;Parsegian,V.A.;Rand, R. P. Biophys. J. 1988,54, 1185-1186.
0743-7463/91/2407-1874$02.50/0 0 1991 American Chemical Society
Direct Force Measurements of Lamellar Mesophases
attachment of bilayers to the mica surfaces, by whatever method, is obviously not useful. We must resort to some more indirect way of studying the interlamellar interactions. At the simplest level, force measurements between solid surfaces in a lamellar mesophase yield direct information on the interlayer spacing, as shown by Ter-MinassianSaraga and Perez for the lamellar phase of the egg lecithin system.14 A related study of structure in a thermotropic liquid crystal has been carried out by Horn et aL15 As we shall see, there is a wealth of additional information to be gained by application of the surface force technique to lamellar systems. We can extract the modulus of compressibility of the layered structure; we obtain insight into the behavior of the mesophases in confined geometries and thereby their stabilization mechanism. We here present some recent results on forces between mica surfaces immersed in two different lamellar mesophases, one oil-swollen and one water-swollen. The former is believed to be stabilized mainly by undulation forces, whereas the latter has more rigid membranes and a much smaller solvent uptake and is largely stabilized by electrostatic and/or hydration forces.
Experimental Section The oil-swollen lamellar phase was a system of sodium n-octylbenzenesulfonate (SOBS),1-pentanol,and water swollenwith a n-decanell-pentanol mixture.16 The SOBSwas kindly supplied by F. C. Larchb. The water-swollen lamellar phase was sodium dodecyl sulfate (SDS), 1-pentanol, and water.s The SDS was from BDH (extra pure) and was reneutralized with excess NaOH and then recrystallized from ethanol in order to prevent hydr01ysis.l~ The pentanol and decane were from Sigma and distilled once before use. The water was deionized, charcoalfiltered, and doubly distilled. The samples were prepared by weighing the components and mixing them in a sealed tube by repeated centrifugation. The compositions by weight were as follows: oil-swollenphase 9.23% SOBS, 10.04% water, 12.78% pentanol, and 67.94% decane; water-swollen phase 7.17% SDS, 17.48% pentanol, and 75.35% water. The lamellar texture was checked by observation between crossed polarizers in a microscope. Homeotropic alignment was obtained in 1or 2 h with the SDS mesophase, whereas more than 24 h was required for the SOBSsample. The interlamellarspacing was checked with X-ray diffraction before and after the surface force measurements. The water-swollen system was checked on a small angle apparatus with a rotating anode generator (A.N.U., Canberra) and gave a reticular distance, d (bulk layer spacing), of 8.8 f 0.1 nm. The oil-swollen system was irradiated under the synchrotron radiation of the beam line D22 of LURE (France). As the Bragg diffraction peak is dominated by the diffuse scattering, the mean layer spacing was estimated to be 28 f 1 nm, according to the procedure of Porte et ala,’*who obtained very similar patterns in the same system. The interaction between two mica surfaces immersed in the lamellar phases was measured with two types of surface force a p p a r a t u ~ . ~These J ~ instruments are capable of measuring the separation between two surfaces mounted in a crossed-cylinder configuration with an accuracy of f0.1-0.2 nm and the force with a resolution of 10-7 N (when normalized by the radius of curvature of the surfaces (R= 1-2 cm) this is equivalent to 0.01 mN/m). As the surface separation is measured interferometrically,lS extensive scattering of light from the mesophases is the main experimental problem. In the old surface force apparatuss (14) Ter-Minassian-Saraga, L.; Perez, E. Colloids Surf. 1984,12,213225. (15) Horn, R. G.; Israelachvili, J. N.; Perez, E. J. Phys. (Paris) 1981, 42, 39-52. (16) LarchB, F. C.; El Qebbaj, S.; Marignan, J. J. Phys. Chem. 1986, -.w. - , i.n- .w .i -n-.. (17) KBkicheff, P.; Grabielle-Madelmont, C.; Ollivon, M. J. Colloid Interface Sci. 1989, 131,112-132.
(18) Porte, G.;Marignan, J.; Bassereau, P.; May, R. Europhys. Lett.
1988, 7,713-717.
(19) Israelachvili, J. N. J. Colloid Interface Sci. 1973,44, 259-272.
Langmuir, Vol. 7, No. 9,1991 1875 this necessitated the use of a bath containing the mesophase, in order to shorten the light path in the liquid as much as possible. Simple injection of a droplet between the surfaces leads to difficulties with evaporation of the components and consequent drifts and risk of phase separation. Experiments with the SDS mesophase were performed with a new, improved version of the surface force apparatus.1° This instrument permits the attachment of chambers of varying volume and the light path in the medium can be minimized, without problems due to evaporation or creep of the liquid from the bath. Drifts of the surfaces are hence kept to a minimum (
