Langmuir 1989, 5, 276-278
276
A. A pore diameter distribution obtained from the SAW device adsorption isotherm data by using the Digisorb 2600 calculation routine (based on the BJH method25) was found to be unimodal, with a median diameter of 31 A.26 The adsorption isotherm recorded for the A2 film-coated SAW device is shown in Figure 3. The shape of the isotherm is type 11, typical for nonporous samples.' This is consistent with the low calculated value for percent porosity (2%) and with the surface area, 0.93 cm2/cm2of film, calculated from the BET plot of the data (Figure 3, inset). This result, along with other measurements made on bare SAW device s u r f a c e ~ , 9suggests ' ~ ~ ~ ~ ~that the mass sensitivity coefficient value used in eq 3 is reliable. In contrast to the thin A2 film, the bulk A2 sample has a high measured surface area (730 m2/g) and exhibits a type I isotherm,13typical of samples containing micropores (diameters < 20 &.l Heating the bulk sample as high as 600 "C did not cause a significant decrease in surface area (although a 900 "C anneal did reduce the surface area to 40 m2/g), indicating that the thin film's lack of porosity is not a result of the 400 "C anneal it experienced. These differences show that the A2 film formation process produces a denser silicate matrix, with few micropores, than does the bulk process, in which solution gelation is allowed (25) Barrett, E. P.; Joyner, L. G.; Halenda, P. P. J. Am. Chem. SOC. 1951, 73, 373. (26) Frye, G. C.; Brinker, C. J.; Ricco, A. J.; Martin, S. J., manuscript in preparation.
to occur before drying. Thus, analysis of bulk samples alone would lead to the erroneous conclusion that a hightemperature anneal is necessary to obtain a nonporous A2 film.
Conclusions Use of SAW devices to obtain Nzadsorption isotherms provides a powerful new technique for the direct measurement of thin-film surface areas. The results with a nonporous A2 film indicate that the nominal film area probed by the SAW, 0.15 cm2, is sufficient for accurate measurement. Comparison of these results to bulk A2 samples demonstrates the danger of estimating thin-film surface areas from measurements on bulk samples. The surface area of any thin-film material which can be deposited on a SAW device substrate and survive cooling to 77 K should now be readily measurable. Experiments are underway to characterize the surface area and pore size distribution of thin f i b s prepared from a variety of sol-gel solutions. Acknowledgment. We gratefully acknowledge the work of C. S. Ashley and C. J. Brinker of Sandia National Labs for sample preparation and H. L. Tardy of Sandia National Labs for ellipsometric analysis. This work was performed at Sandia National Laboratories, supported by the U.S. Department of Energy under contract no. DEAC04-76DP00789. Registry No. N2, 7727-31-9.
Ultrathin Monolayers and Vesicular Membranes from Calix[ Glarenes' Michael A. Markowitz, Roman Bielski,2 and Steven L. Regen* Department of Chemistry and Zettlemoyer Center for Surface Studies, Lehigh University, Bethlehem, Pennsylvania 18015 Received June 23, 1988 p-tert-Butylcalix[G]areneproduces stable monolayers at the air-water interface having a limiting area of 260 A2/moleculeand a film thickness of ca. 10 A. Complete removal of the p-tert-butyl groups affords a unique vesicle-forming surfactant, calix[6]arene, which forms membranes having an apparent thickness of ca. 20 5 A.
*
Phospholipid bilayer membranes have attracted considerable attention over the past 25 years as models for biological membranes, microcapsules, and devices for solar energy c o n v e r ~ i o n . ~In~ a series of pioneering papers published in the late 19709 and early 198Os, Kunitake and co-workers extended the range of vesicle-formingsurfac-
tants to include double-chain cationic, anionic, and zwitterionic amphiphiles as well as certain single-chain amA popular theory that has since been used to phiphile~.~ judge whether or not a surfactant will form a lamellar phase is based on the concept of a surfactant number, ~ / a 1 , 8where, .~ u is the volume of the hydrocarbon chain, 1 is its length, and a is the head group area. This surfactant
(1) Supported by the Division of Basic Energy Sciences of the Department of Energy (DE-FGO2-85ER-13403). (2) On leave from the Institute of General Chemistry, Aricultural University of Warsaw, Poland. (3) Bangham, A. D.; Hill, M. W.; Miller, N. G. A. In Methods in Membrane Biology; Korn, E. D., Ed.; Plenum Press: New York, 1974;
(7) Kunitake, T.; Okhata, Y. J.Am. Chem. SOC.1977,99,3860. Okahata, Y.; Kunitake, T. Ibid. 1979,101,5231. Kunitake, T.; Okhata, Y. Ibid. 1980, 102, 549. Kunitake, T.; Okahata, Y.; Shimomura, M.; Yasunumi, S.; Takarabe, K. Ibid. 1981,103, 5401. (8) Tartar, H. V. J. Phys. Chem. 1955,59, 1195. Israelachvili, J. N.; Jarcelja, S.; Horn, R. G.Q. Reu. Biophys. 1980, 13,121. Mitchell, D. J.; Ninham, B. W. J . Chem. SOC.,Faraday Tram. 2 1981, 77, 609. BenShaul, A.; Szleifer, J.; Gelbart, W. M. Proc. Natl. Acad. Sci. U.S.A. 1984, 81, 4601. (9) Reviews: Evans, D. F. Langmuir 1988,4,3. Evans, D. F.; Ninham, B. W. J. Phys. Chem. 1986, 90,226.
VOl. 1. cD 1. --7
(4) Papahadjopoulos, D. Ann. N. Y. Acad. Sci. 1978, 308, 1-462. (5) Calvin, M. Acc Chem. Res. 1978, I, 369. (6) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982.
0743-7463/89/2405-0276$01.50/0
0 1989 American Chemical Society
Langmuir, Vol. 5, No. I , 1989 277
Letters
P
*REA (A~moleculc)
Figure 2. Surface pressurearea isotherm for 2. I
- .
- -
Figure 1. CPK spacefilling models of ealix[6]arene(left) and p-tert-butylcalix[6]arene (right).
number, which is a measure of the local curvature at the aggregatesolvent interface, predicts bilayer formation when u/al approaches 1.0,i.e., when the surfactant has a near-cylindrical geometry. Calix[G]arene (1) is a macrocyclic hexamer derived from phenol and formaldehyde; in simplest terms, it can be viewed as a "thin aromatic cylinder" rimmed with hydroryl groups a t ita tapered end (Figure l).lo Based on ita geometry, water insolubility, and polar face, l would appear to meet the above criteria for lamellar phase formation. Unlike conventional surfactants, which possess flexible aliphatic chains, 1 contains a relatively rigid, hydrophobic framework. An intriguing question that one may then ask is as follows: Can calix[6]areneproduce stable monolayers and vesicular membranes?" Also, is lamellar phase formation sensitive to the precise geometry of the calixarene? If the calixarene is splayed, for example, due to the presence of bulky pendant groups (e.g., p-tert-butylcalix[6]arene, Figure l),is vesicle formation prevented? This paper addregses h t b of these questions by examining the surfactant properties of 1 and 2.
1,X-H
2, X = C(CHda
A surface pressurearea isotherm of 2 (the synthetic precursor of 1) readily establishes ita surfactant behavior. Stable monolayers were prepared by spreading a hexanechloroform (411, v/v) solution onto a pure water subphase (25 'C, pH 6) of an MGW Lauda film balance and compressing the film at a rate of 60 cm2/min. The limiting area observed for 2 was ca. 260 A2/molecule (Figure 2).12 Stable monolayers were also obtained from 1; in this case, (lo) Review Gutrhe, C. D.h Syntheaia o/Macm&es: The h i m of Seleetiuo Complexing &en& Izatt, R. M.. Christensen. J. J.. Eds.; Wiley-Intemience: New York, 1987; p 93. (l!) ReQntly, a water-soluble cali.r[4]arsnemlfonatah.s been found to exist in the form of bilavers in the ervstolline state: Bott. 5. G.:' Coleman, A. w.; Atwoad, J.L. J. ~ mC. h k SW. 1988,110,6io. (12) Concentrations of caliiarsnes (typically 1.35 mg/mL) were d e tarmined by IH NMR analysis using cycloheme a an internal standard.
Figure 3. Transmiwion electron micrographs of (a, stained 2% uran I acetate and 1 1 1 ~unstained dispersions of 1. Rar represenls 500
X.
the limiting area was ca. 70 A2. Transfer of compressed monolayers to glass microscope slides, by single passage from water into air, resulted in transfer ratio8 of 0.95 and 0.75, for 2 and I , respecti~ely.'~The advancing contact angle for water on each of these supported films was 0'. (13) The transfer ratio is defined a the decrenea in mof tha monolayer at ths ga-teer interface divided by the total geometrid surfam area of the subatrate passing through the interface. Transfern were wried out at B rate ofO.8 emlmin, using 18 dymlem and 10.5 dynlcm for 2 and I. respectively.
Langmuir 1989,5, 278-280
278
If it is assumed that 2 lies at the gas-water interface, with each of the hydroxylic groups in contact with water (parallel alignment of a 10-A-thick monolayer), and that the assembly is hexagonally packed, the limiting area which is predicted from CPK models is ca. 260 A2; for a perpendicular alignment, the predicted area is ca. 180 A2. With 1, the limiting areas that are expected for parallel and perpendicular orientations are 150 and 90 A2, respectively. While the above experimental data do not firmly establish the orientation of these surfactants at the gas-water interface, the high transferability of 2, together with its observed limiting area, infers a parallel orientation. The relatively poor transferability of 1, along with its observed limiting area, suggests a loss of parallel orientation and f or monolayer organization upon compression. Rapid injection of 50 pL of a 20 mM THF solution of 1 into 1 mL of water produced a translucent vesicle dispersion. Dynamic light scattering (Nicomp 200,632.8 nm, 90" scattering angle) revealed particles having diameters
ranging between 500 and lo00 A, which waii confirmed by transmission electron microscopy (Figure 3a). Unstained samples showed particles of similar size, with discrete membranes having an apparent thickness of ca. 20 f 5 A (Figure 3b). The resulting dispersion was stable for more than 1week without any apparent change in particle size (light scattering). In contrast to 1, injection of a THF solution of 2 into water resulted in immediate precipitation. We propose that the failure of 2 to form a stable vesicle dispersion is due to the splay of the molecule, which precludes the formation of a lamellar phase. The feasibility of preparing stable monolayers and vesicle membranes from di[6]arenes of the type reported herein shows that a much wider range of lamellar-forming molecules is possible and also provides additional support for the notion that surfactant geometry plays a key role in defining aggregate s t r u ~ t u r e . ~ Registry No. 1, 96627-08-6; 2, 78092-53-2.
An Ellipsometric Study of a Diblock Copolymer: A Test of Microscopic Theory Bryan B. Sauer and Hyuk Yu Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
Mahn Won Kim* Exxon Research and Engineering Co., Annandale, N e w Jersey 08801 Received J u l y 7, 1988. I n Final Form: September 19, 1988 An ellipsometry study was performed on monolayers of a polystyrene-poly(ethy1ene oxide) diblock copolymer at the air/water interface over a range of surface fractional coverage 0.007 I 6 I 1,where we assume that the instability onset of the static surface pressure is the full coverage point, 6 = 1, and the surface mass density I' is directly proportional to 6. The sample was found to form a macroscopically homogeneous monolayer over the entire range of 6. Because of the large refractive index ( n 1.5) and high segment-segment cohesion of the polystyrene segments in the block copolymers, the phase angle difference 8A was found to be very sensitive to F. Upon taking advantage of this fact, we were able to show that 8A is directly proportional to 8 over a wide range, for the first time in our view, in agreement with microscopic theories based on independent molecular polarizability. Further, by estimating the film refractive index at the highly packed state, macroscopic models were used to evaluate the film thickness.
-
Introduction In the past, ellipsometry has been used to deduce film thicknesses and refractive indices of partially covered surfaces including films on solid substrate^,'-^ metallic liquids? and ~ a t e r . " ~One of the important outstanding (1)Archer, R. J. J. Opt. SOC.Am. 1962, 52, 970. (2)Archer, R. J. In Ellipsometry in the Measurement of Surfaces and Thin Films; Passaglia, E., Stromberg, R. R., Kruger, J., Eds.; Natl. Bur.
Std. Misc. Publ. 256,U S . Government Printing Office: Washington, D.C., 1964;p 255. (3)Bootsma, G. A.; Meyer, F. Surf. Sci. 1969, 14, 52. (4)Smith, T. J. Opt. SOC.Am. 1968, 58, 1069. (5) den Engelsen, D.; de Koning, B. J. Chem. SOC.,Faraday Trans. 1 1974, 70, 1603. (6)den Engelsen, D.; de Koning, B. J. Chem. SOC.,Faraday Trans. 1 1974, 70,2100. (7)Rasing, Th.; Hsiung, H.; Shen, Y. R.; Kim, M. W. Phys. Reu. A , Rapid Commun. 1988, 37, 2732.
issues in ellipsometry, in our view, is the relationship of the ellipsometric phase angle difference 6A to the surface mass density r. The phase angle differencelo is 6A = A' - A, where A' and A are the ellipsometric phase angles for the monolayer covered surface and the clean water surface, respectively. With use of a sensitive phase-modulated ellipsometerlo together with a novel amphiphile which is an oligomeric diblock copolymer,l' we set out to examine how 6A depends on F. Further, if the monolayer is shown (8) Kawaguchi, M.; Tohyama, M.; Mutoh, Y.; Takahashi, A. Langmuir 1988, 4 , 407. Kawaguchi, M.; Tohyama, M.; Takahashi, A. Langmuir 1988, 4 , 411.
(9) Sauer, B. B.; Yu, H.; Yazdanian, M.; Zogrdi, G.; Kim, M. W., submitted for publication in Macromolecules. (10)Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North Holland: New York, 1977. (11)Sauer, B.B.;Yu, H.; Tien, C.-F.; Hager, D. F. Macromolecules 1986, 20, 683.
0743-7463/89/2405-0278$01.50/0 0 1989 American Chemical Society
