J. Phys. Chem. B 2001, 105, 8563-8568
8563
Self-Assembly of Crystalline Films of Interdigitated Long-Chain Cholesteryl Esters at the Air-Water Interface C. Alonso,† I. Kuzmenko,† T. R. Jensen,‡ K. Kjaer,‡ M. Lahav,*,† and L. Leiserowitz*,† Department of Materials and Interfaces, The Weizmann Institute of Science, 76100 RehoVot, Israel, and Materials Research Department, Risø National Laboratory, DK 4000 Roskilde, Denmark ReceiVed: February 19, 2001; In Final Form: June 1, 2001
Cholesterol and some cholesterol-like derivatives such as stigma-sterol and cholesterol acetate self-assemble into semicrystalline monolayers, bilayers, or trilayers on the surface of water, as has been reported previously. Here we have extended our thin film studies toward cholesteryl esters that, in their bulk crystals, pack as interdigitated bilayers due to the mismatch between the cross-sectional area of the rigid cholesterol moiety (40 Å2) and the attached hydrocarbon chain (20 Å2). As shown by grazing incidence X-ray diffraction (GIXD), cholesteryl esters spontaneously self-assemble on the surface of water in order to form interdigitated bilayer films. The unit cell parameters of such bilayers at the water surface almost match those of their three-dimensional counterparts. Other experiments such as surface pressure vs area per molecule isotherms and ellipsometry measurements corroborate this result. To control the growth of the interdigitated films, we have used “tailor-made” additives (long-chain alcohols or acids) that fill the voids in the chain region of the cholesteryl ester layer caused by the structural mismatch, thereby inhibiting interdigitation. Using this strategy, we were able to form a mixed monolayer composed of the ester and the additive molecules in a 1:1 ratio, where the hydrocarbon chain of additive molecule intercalates between the ester chains and effectively inhibits the growth of the interdigitated bilayer.
1. Introduction
SCHEME 1
Interdigitation between hydrocarbon chain moieties of certain classes of molecules is an effective way to ensure cohesive contact between the molecules, forming ordered, layerlike systems provided certain structural conditions are met.1 To enhance molecular contacts in natural membranes, which are composed of fluid bilayers, some phospholipids demonstrate partial interdigitation by virtue of a difference in the length of the two chains constituting the phospholipid molecule.2 Chain interdigitation can also be achieved in crystalline thin films at the air-solution interface, making use of molecules A and B exhibiting an acid-base complementarity. When A is a waterinsoluble amphiphile bearing a long hydrocarbon chain substituent and B is water-soluble without a chain, they spontaneously form -A-B-A-B-A-B- two-dimensional arrays at the water surface. Upon compression of the monolayer beyond its collapse point, the film is transformed into a multilayer incorporating interdigitated chains, according to grazing incidence X-ray diffraction (GIXD) studies.3,4 Here we demonstrate the spontaneous formation of a crystalline interdigitated bilayer composed of a molecule of a single kind at the air-water interface. We make use of molecules composed of two parts: a hydrocarbon chain and another moiety with twice the crosssectional area of the chain (18-20 Å2). The long-chain esters of cholesterol fulfill this condition; indeed, such molecules form interdigitated assemblies in three-dimensional crystals.5,6 Pure cholesterol itself, which has a cross-sectional area of 38 Å2, assembles on water into a monolayer film of trigonal symmetry
and is crystalline but with a low lateral order. Upon compression, this film transforms into a highly crystalline bilayer, incorporating a partial interdigitation between the exocyclic sterol octyl moieties.7 We therefore envisaged that such molecules could spontaneously self-assemble into crystalline interdigitated bilayers at the air-water interface. The formation of such crystalline bilayers led us also to investigate the effect of tailormade amphiphilic molecules on film thickness and molecular arrangement. Here we present the GIXD studies of thin crystalline films of three cholesteryl esters: cholesteryl tridecanoate (CE-13), cholesteryl palmitate (CE-16), and cholesteryl stearate (CE-18), depicted in Scheme 1. Binary mixtures of CE-13 and CE-18 have also been studied to examine the effect of the hydrocarbon chain mismatch of the two constituents on the degree of crystallinity and interdigitation in the resulting thin films. Finally, focus is placed on mixtures of cholesteryl esters and long-hydrocarbon-chain additives, alcohols, acids, and alkanes.
* To whom correspondence should be addressed. Fax: 972 8 934 41 38. E-mail:
[email protected]. † The Weizmann Institute of Science. ‡ Risø National Laboratory.
2. Experimental Section Materials. All the cholesteryl ester compounds (Scheme 1) and the tailor-made additives (alkanes, primary alcohols, and
10.1021/jp010658e CCC: $20.00 © 2001 American Chemical Society Published on Web 08/21/2001
8564 J. Phys. Chem. B, Vol. 105, No. 36, 2001
Alonso et al. TABLE 1: Ellipsometric Measurements on Cholesteryl Ester Films at the Water Surfaceb CE-13 CE-16 CE-18
∆a (0.2 (deg)
Lellip (1 (Å)
L3D (Å)
9.0 10.5 11.7
49.7 58.0 64.7
50.9 52.8 57.5
a Ellipsometric angle (∆). b Thickness of films (Lellip) are deduced from ∆ assuming a refractive index n ) 1.445 and values (L3D) for interdigitated bilayers as estimated from the 3D crystal structure counterpart.
Figure 1. Surface pressure vs molecular area (π-A) isotherms recorded at 5 °C for cholesteryl tridecanoate (CE-13), palmitate (CE-16), and stearate (CE-18) on water. Arrows indicate points at which GIXD patterns were measured.
acids) were used as purchased from Aldrich. Spreading solutions were prepared by dissolving compounds in chloroform in the concentration range of 10-7 M. Films were spread on ultrapure water (Millipore, resistivity of 18 MΩ/cm) at room temperature; however, all the GIXD measurements were performed at a subphase temperature as low as 5 °C in order to enhance the sample crystallinity. Surface Pressure-Molecular Area (π-A) Isotherms. Experiments were performed on a KSV MiniTrough apparatus. The barrier speed was set at 20 mN m-1 min-1. The surface pressure was measured with a platinum Wilhelmy plate. Ellipsometry Measurements. Measurements were carried out on a homemade ellipsometer.8 The angle of incidence of the He-Ne laser beam (λ ) 633 nm) was set at 54.2°, 1° larger than the Brewster angle of water (53.16°). The reflected beam passes trough a λ/4 plate and an analyzer before being collected in a photomultiplier. The zero intensity was monitored by small variations of the polarizer located before the sample and the analyzer. In this configuration, the phase difference ∆ between the two orthogonal polarizations is twice the analyzer rotation angle.9 Grazing Incidence X-ray Diffraction. GIXD experiments were conducted on the liquid surface diffractometer at the undulator beamline BW1 in HASYLAB at DESY (Hamburg, Germany). A beam with a wavelength λ ) 1.304 Å was selected by a Be(002) monochromator crystal. The incident grazing angle was adjusted to Rf ) 0.85Rc, where Rc ≈ 0.14° is the critical angle for total reflection. The vertical dispersion of the diffracted beam was analyzed by a position sensitive detector (PSD), while the horizontal angle was analyzed by scanning Soller slits. A more detailed explanation of the method can be found in the literature.10 Structure Determination. Structure determination of the crystalline films were carried out with the use of computer programs: Cerius2 for molecular modeling,11 and SHELXL-97 for the X-ray structure-factor least-squares refinement.12 3. Results and Discussion 3.1. Pure Cholesteryl Esters. The surface pressure-area (πA) isotherms (Figure 1) for all the cholesteryl ester compounds
are similar. For large areas per molecule, the surface pressure is almost zero; for areas of
