Langmuir 1988,4, 233-234
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Letters Does a Supported Monolayer Induce Structure in the Supporting Liquid: A Synchrotron X-ray Study of Stearic Acid on Mercury Scott W. Barton, Britt N. Thomas, Erik B. Flom, Frank Novak, and Stuart A. Rice* The Department of Chemistry and The James Franck Institute, The Uniuersity of Chicago, Chicago, Illinois 60637 Received June 30, 1987. In Final Form: August 31, 1987 Using grazing incidence X-ray diffraction,we find that an ordered stearic acid monolayer induces a liquid Hg substrate to order in a commensurate manner. The (10) Fourier coefficient of the stearic acid-Hg potential is estimated to be 0.6k~T.Comment is made on the implications our findings have for insoluble monolayers at the air/water interface. The macroscopic properties of insoluble monolayers supported on liquids have been studied extensively for 70 years;' a typical system is stearic acid spread on liquid Hg.2,3 It is generally believed, and there is now direct evidence for two systems, that the close-packed two-dimensional phase is ~ r d e r e d . ~ J This letter contains a preliminary report of the results of experiments designed to determine the influence of the ordered field of a supported stearic acid film on the structure of the surface of liquid Hg. The stearic acid-Hg system was chosen for this study for several reasons: (1)We have, in earlier experiments,Bv7 determined the transverse structure factor for the transition region a t the liquid-vapor interface of pure Hg and can use those data as reference when searching for changes induced by the field of the supported monolayer. (2) The ?rAbehavior of stearic acid on Hg at room temperature is k n o ~ n . (3) ~ ~Recent ~ dark-field electron microscopic studies of stearic acid films transferred from water to a carbon substrate show that a t 36 A2/molecule the film consists of -500-A diameter ordered aggregates in a dendritic net.* (4)The system is relatively easy to handle. We assume that the qualitative features of the morphologies of stearic acid spread on water and spread on Hg are similar. In ref 8 it is established that, for stearic acid on water with inverse surface density equal to 36 A2/molecule,the ordered domains in the dendritic net have hexagonal packing with unit-cell length 4.81 A. In ref 3 (1) For a rather complete overview to 1966, see: Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966. (2) Smith, T. Adu. Colloid Interface Sci. 1972, 3, 161. (3) Kinzig, B. J. Ph.D. Thesis, Case Western Reserve University, Cleveland, OH, 1980. Available from University Microfilms, Inc., Ann Arbor, MI. (4) Dutta, P.; Peng, J. B.; Lin, B.; Ketterson, J. B.; Prakash, M.; Georgopoulos, P.; Ehrlich, S. Phys. Rev. Lett. 1987, 58, 2228. (5) Kjaer, K.; Als-Nielsen, J.; Helm, C. A,; Laxhuber, L. A.; Mohwald, H. Phys. Rev. Lett. 1987,58, 2224. (6) Barton, S.W.;Thomas, B. N.; Novak, F.; Weber, P. M.; Harris,J.: Dolmer, P.; Bloch, J. M.; Rice, S. A. Nature (London) 1986, 321, 685. (7) Thomas, B. N.; Barton, S. W.; Novak, F.; Rice, S. A. J. Chem. Phys. 1987, 86, 1036. (8) Uyeda, N.; Takenaka, T.; Aoyama, K.; Mataumoto, M.; Fujiyoshi, Y. Nature (London) 1987, 327, 319.
0743-7463/88/2404-0233$01.50/0
it is established that, for stearic acid on Hg with inverse surface density equal to 40 A2/molecule,the rate of change of pressure with area is characteristic of a condensed phase. We assume that stearic acid on Hg with inverse surface density 40 A2/molecule has the morphology of a dendritic net of ordered aggregates of molecules. A pertinent test of this assumption is the existence of diffraction corresponding to pseudohexagonal packing with a unit-cell length of expected size. We show below that the system studied passes this test. Grazing incidence X-ray diffraction experiments were carried out in the C-2 station a t the Cornel1 High Energy Synchrotron Source (CHESS). A Si(ll1) double-crystal monochromator selected 13.3-keV radiation, which was deflected down to the sample a t 1.5 mrad by a Pt-coated float glass mirror. Scattered radiation was detected a t a constant vertical pickoff angle of 1.5 mrad after passing through Soller slits with an angular resolution of f0.15'. The Hg fluorescence excited by the incident X-rays was used as a beam monitor. This fluorescence is separated from the elastically scattered X-rays with an energy dispersive detector. The apparatus used is essentially the same as described previ~usly.~ Commercial triply distilled Hg was further purified by hot oxidation of impurities and their subsequent removal by filtration through long glass capillaries. The stearic acid and cyclohexane spreading solvent were both Aldrich gold label purity and were used without further purification. The Hg was dripped into a 4.5-in. diameter 304 stainless steel sample cell in a sealed 304 stainless steel cylindrical chamber with kapton windows. The chamber was extensively purged with dry Ar before admission of the Hg. Stearic acid was spread, under a blanket of dry Ar, to a surface coverage of 40 A2/molecule, using a Hamilton microsyringe to introduce the appropriate amount of 1.547 mM solution of stearic acid in cyclohexane. The chamber was then flushed with a steady flow of dry He for the remainder of the experiment. The system was considered to be in equilibrium when the X-ray scattering intensity a t k = 0.50 A-l became constant. After the raw data are stripped of background, corrected for the variation of the atomic scattering factor and polarization of the incident radiation, and then scaled with 0 1988 American Chemical Society
Langmuir 1988, 4, 234-236
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(9) Rieter, G.; Moss,S. C.; Phys. Rev. E Condens. Matter 1986, B33, 7209. (10)Bunde, A.; Dieterich, W. In Physics of Intercalation Compounds; Pietronero, L., Tosatti, E., Eds.;Springer: Berlin, 1981.
generated by the monolayer we expect an image of the first peak of the liquid structure factor at 0.8 A-l. We identify the peak in the experimental structure factor between 0.7 and 0.8 A-' with this image. Although the intensity of scattered X-rays is very small in this region of k-space, the existence of the peak between 0.7 and 0.8 A-1 is established since the signal is well above the noise level. It does not take too much imagination to infer that the increase in S , , ( k )near k = 0.68 A-' is rapid, mirroring the increase in the transverse structure function of pure Hg near k = 2.0 A-1. Also, the asymmetry of the transverse structure function, particularly the relatively slow decrease for wave vectors larger than that at the peak, is mirrored in the slow falloff of Sll(k)from k = 0.68 to k = 0.88 %I-'. The data displayed in Figure 1, when interpreted as implied in the last paragraph, permit an estimate to be made of the strength of the periodic field generated by the stearic acid monolayer. Rieter and Mossg show that the ratio of the amplitudes of the image and the pure liquid transverse structure factors is related to the Fourier coefficients of the periodic field by (VHk/kBT)2 = S ( k f kHK)/(S(k). To estimate the Vlo coefficient we first must calculate the contribution the first Hg layer makes to the observed transverse structure factor. For the conditions of our experiment the e-folding depth is 7.5 AI, and we find the renormalized first peak amplitude to be SObed(k)[St73exp(-z/7.5) dz][S;exp(-z/7.5) dz1-l = 0.76 which, taking into account the fractional surface coverage, implies that Vlo = 0.6kBT. At this time we cannot say whether the periodic field of the stearic acid leads to corrugation of the Hg surface or not. We also note that the single Fourier coefficient of the periodic field which we have been able to estimate is not very large in magnitude. This result appears plausible since even if the proton of the stearic acid dissolves in the Hg the resultant charges will be shielded by the electrons, and the shielding length will be of the order of the Fermi wavelength. On the other hand, an ordered monolayer of, say, a long-chain alcohol on water will generate a periodic array of hydrogen bonding sites, and, since the hydrogen bond between water and an alcohol is relatively strong, the ordering of the water generated by the supported film may be very much greater than for the case of stearic acid on Hg.
Acknowledgment. This research has been supported by a grant from the National Science Foundation. We are grateful to CHESS for making available time and facilities to carry out these experiments. Registry No. Hg, 7439-97-6; stearic acid, 57-11-4.
Precursor Films and Corrosion: Wetting and Hydrodynamic Aspects 0. Teschke and M. A. Tenan* Instituto de Flsica, Universidade Estadual de Campinas, Campinas 13081, SP, Brazil Received June 18, 1987. I n Final Form: October 27, 1987
It is shown that precursor films form spontaneously on passivated iron electrodes partly immersed in sulfuric acid solutions. These f i b s are responsible for metal corrosion at the liquid-ail-electrode interface, just above the macroscopic meniscus. The model proposed in this work agrees qualitatively with experiment and shows that wetting and hydrodynamic effects play a fundamental role on corrosion phenomena. Bascorn et al.l following earlier microscopic observations2 have shown by interference microscopy and ellipsometry 0743-7463/88/2404-0234$01.50/0
that the spontaneous spreading of nonpolar liquids on smooth clean metal surfaces is characterized by the ad0 1988 American Chemical Society
