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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 T H F 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 T H F 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, New Jersey 08801 Received July 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.
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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.
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Letters
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/
0.4
0
20
40
60
-0
rXlo~imgicm2
to be macroscopically homogeneous and a t the fully covered state is known (designated as Pfd), then we can relate r to the fractional surface coverage B by setting B a r. Here we assume implicitly that the high-contrast segment of the amphiphile maintains the same conformation throughout 0 Ir Irfull. The diblock copolymer sample has a high ellipsometric contrast because of its polystyrene segment in addition to forming macroscopically homogeneous monolayers. There have been other high contrast vertically oriented amphiphiles;"' however, in general they exhibit phase separations a t certain mass densities whereby the monolayers no longer become homogeneous macroscopically. The inhomogeneity is easily detected by visual inspection upon illuminating the monolayer surface with a laser beam and observing the patchy contrast of the illuminated area by reflection a t oblique angles. The details of the block copolymer and some of its monolayer characteristics at the &/water and oil/water interfaces have appeared elsewhere;" hence it will suffice here to note only that the degrees of polymerization of the two segments are 34 and 25 for poly(ethy1ene oxide) (PEO) and polystyrene (PS),respectively. Relative to rfd,the onset of instability for the surface pressure measurements is taken to be this point. From the earlier study," we are fairly certain that the instability point is well corraborated by surface light-scattering measurements. Thus, we set r/rfull
4
6
8
10
r x i 05imgicm2
Figure 1. Surface pressure If and the change in ellipsometric phase angle bA vs surface concentration r for PS-PEO at 22 O C . The thick curve and thin line are predictions of macroscopic and microscopic ellipsometric theories, respectively. The dashed box at the lower left corner is amplified in Figure 2 without open squares but with 6A for the PEO homopolymer.
0=
2
A
(1)
and our range covers 0.007 I0 I1,over which 6A has been examined. For the sake of comparison we also examined a PEO homopolymer, and such a parallel study is definitely called for in dealing with these kinds of amphiphiles.
Results The experimental results for the copolymer are plotted along with II in Figure 1. The solid curves and lines are theoretical predictions, to which we will return. While there are several inflections in the II-r isotherm, 6A is seen to increase rather monotonically. Even when the surface pressure is essentially zero for a range 0 5 r I8 X mg/cm2, 6A increases continuously, which we take as an indication that it is mainly dependent on 0 and not on any changes in monolayer conformation. Figure 2 is an expanded plot of the low-concentration region of Figure 1,and results for PEOg (open triangles) are also included for comparison. Not only does 6A for PEO reach a plateau of 0.13O at = 6 X loa mg/cm2, but this maximum change is 25 times less than that for the
Figure 2. Change in ellipsometric phase angle 6A for the lower concentration region for PS-PEO. Values of 66 for the PEO homopolymer (open triangles) are also included. The thick curve and thin line are predictions of macroscopic and microscopic ellipsometric theories, respectively.
copolymer. Because the copolymer is almost two-thirds by weight PS, the contribution of PEO to the copolymer signal is even less than would be indicated by Figure 2, and any changes in PEO conformation with surface density should have a negligible effect with respect to 6A. Returning to Figure 1, the plateau a t about II = 8 dyn/cm has been ascribed to the collapse of PEO segments into the water," and the values of 6A continue to increase for PS-PEO well past this collapse. The macroscopic collapse of the monolayer occurs at about II 21 dyn/dm depending on the compression rate. The values of 6A deviate from the linear dependence upon collapse a t II = 21 dyn/cm or r = 44 X loe5mg/cm2. II is not reported for concentrations above r = 44 X mg/cm2 because of the onset of instability, drifting from about 50 to 15 dyn/cm in a matter of minutes. Small patches of multilayer material also were detected by visual observation in the collapse region. The gradual leveling of 6A after I? = 44 X loa mg/cm2 in Figure 1is attributed to the formation of inhomogeneities, whereas for PEO in Figure 2, 6A reaches a plateau at high r because chains are desorbing from the interface into solution, reducing the refractive index c ~ n t r a s t . ~
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Discussion We have chosen two routes to analyze the data. The f i s t is to appeal to various microscopic t h e ~ r i e s ~ *which ~JJ~ take into account the molecular polarizability and predict a linear dependence of 6A on 0 but without making any inference to the film thickness. The second is to make use of the Drude equations with two macroscopic parameters, namely, the refractive index nl and the thickness dl of the film, to predict 6A. Relative to the first route, Hall13 has questioned whether B is directly proportional to the number of scattering indices in these microscopic theories and has predicted that 6A should be proportional to O.llz The direct proportionality, however, has been confirmed by e~periment.~.~ The comparison of microscopic theories (thin lines in Figures 1 and 2) with the experimental data for PS-PEO show that 6A varies linearly with r. This is the central observation of the paper. If we accept eq 1, then we have proven 6A a 0. While we cannot comment on the thickness of the monolayer, the demonstrated linear dependence is clear-cut. (12) Strachan, C. S.Proc. Cambridge Philos. SOC. 1933, 29,116. (13) Hall, A. C.J. Phys. Chem. 1966, 70, 1702.
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As for the second route, one recipe for determining the mean film refractive index nl as a function of 8 is to average the polymer (n,) and water (n2)refractive indices by using the Lorentz-Lorenz equation:14 (n12- 1)/(nl2 + 2) = 8(n,2 - l ) / ( n , 2 + 2) + (1 - 8)(n22 - 1 ) / ( n z 2+ 2) (2) Alternatively, the values of nl could also be calculated by averaging n, with that of air instead of water. In this case the calculated values of nl would be less than 1.332 at low coverages. Solving the Drude equations gave negative values of 6A.4 We did not observe such values of 6A; hence nl seems to start at 1.332. This is the basis of our postulate that PS-PEO starts to form a monolayer by immersion into the water subphase from the very beginning. The thick solid curves in Figures 1and 2 are predictions of the macroscopic theory using eq 2 to estimate the film refractive index, assuming constant thickness, and using the Drude equations to calculate 6A. The thickness at the fully packed state is calculated upon choosing 8 = 1 a t 6A = 2.55'. Then the Drude equations were solved for d l assuming that nl = nP = 1.589.15 These assumptions are equivalent to equating n1to the refractive index of bulk PS a t rfd. The thickness so calculated is dl = 28 f 0.5 8,at rfuu. The r dependence of 8A shown in Figures 1and 2 was calculated assuming that the thickness remains at 28 8, and then solving eq 2 for n, a t various values of 8 between 0 and 1. The macroscopic theory (the Drude equation with the Lorentz-Lorenz equation) gives a reasonable agreement with the experimental data. However, the microscopic prediction shows better agreement with the experimental data. The disparity of the macroscopic prediction may be contributed by a failure of the Lorentz-Lorenz equation or the assumption in Drude equations for inhomogeneous monolayers. (14)H u g h , M. B. (Ed.) In Light Scattering from Polymer Solutions; New York: Academic, 1972; p 35. (15) Polymer Handbook; Brandrup, J., Immergut, E. H., Eds.; Wiley: New York, 1975; p V-60.
Even though the 8 dependence of the macroscopic theory is not quite correct, we can check whether the assumption of 8 = 1 at rfull = 44 X mg/cm2 is reasonable. The surface concentration divided by the mass ratio of P S to the total mass per copolymer molecules gives the polystyrene mass per unit area, i.e., 44 x (mg/cm2)/1.572 (grams of PS/(grams of PS - PEO)) = 28.0 X mg of PS/cm2. Dividing this by the density of bulk PS, p = 1.04 g/cm3, gives dl = 26.9 X lo4 cm = 26.9 8,,which is in good agreement with d l = 28 f 0.5 8, calculated by using the experimental value of 6A = 2.65', the Drude equations, and n, = 1.589.
Summary We have shown for the first time a t the air/water interface that 6A is directly proportional to the fractional coverage 8, which is predicted by microscopic theories for partially covered thin films. We cannot extract much information about the PS conformation based on the microscopic theories, but with the macroscopic theories the picture we have of the monolayer at different 0 is one of a PS chain for each molecule which does not spread out but remains in some sort of condensed blob. Upon an increase in the surface concentration, the PS blobs are pushed together. The film refractive index used in the Drude equations was determined by ignoring the contribution of the PEO segments and averaging PS and water refractive indices via the Lorentz-Lorenz equation at different coverages 0.007 I6 I1. A comparison with PEO homopolymer data indicated that the PEO segments do not contribute significantly to 6A because of their low refractive index contrast. Acknowledgment. This work was in part supported by the Research Laboratories of Eastman Kodak Co. and the Research Committee of the University of WisconsinMadison. We thank Meheran Yazdanian for help with the ellipsometry experiments. Registry No. (Polyoxyethylene)(styrene) (block copolymer), 108548-52-3.
Determination of Hg(1) Adsorption Accompanying the Coulostatic Underpotential Deposition of Mercury on Gold Using the Quartz Crystal Microbalance Michael Shay and Stanley Bruckenstein* Chemistry Department, State University of New York at Buffalo, Buffalo, New York 14214 Received June 10, 1988. In Final Form: November 3, 1988 Emersion experiments were carried out from 2 X M Hg(1) solutions of 0.2 M H2SO4 at the gold electrode of a quartz crystal microbalance. Prior to emersion, the gold electrode was open-circuited in this solution and one monolayer, 1.7 X lo* g-atomcm-2, of underpotential Hg(0) deposited by a coulostatic process. Also, 0.29 X g-mol cm-2 of Hg(1) bisulfate was adsorbed.
Introduction Underpotential deposition of mercury occurs by both a coulostatic and disproportionation reaction when a reduced gold electrode is open-circuited in Hg(1) so1ution.l
The coulostatic underpotential deposition (UPD) of Hg(0) initially shifts the potential in a positive direction, and then the disporportionation reaction
(1) Sherwood, W. G.; Untereker, D. F.; Bruckenstein, S. J. Electrochem. Sac. 1978, 125, 384.
shifts the potential in a negative direction. The potential relaxation ceases upon reaching the equilibrium Nernst
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0 1989 American Chemical Society
+ Hg(I1)
(1)
