The Journal of Physical Chemistty, Vol. 83, No. 10, 1979
Communications to the Editor
Nickel foil about 2.5 pm thick (from Reactor Experiments, Inc., San Carlos, Calif.) was cut into 10 cm X 0.6 cm strips, onto one side of which ca. 0.08 pm of gold was vacuum evaporated to give a relatively inert surface. Three of these were joined end to end by spot welding with 1-2 mm overlap. The resulting ribbon, about 30 cm long? was suspended from one end in a metal vacuum system, which was shock mounted on tennis balls and connected to vacuum and gas with rubber tubing. We measured the position of the lower end of the ribbon to a precision of 2-3 pm against a glass scale just below it, using the filar micrometer eyepiece of a telemicroscope to interpolate between the graduations of the scale. Flowing helium was used instead of vacuum to remove active gases because vibration of the ribbon was too great to permit reading in the absence of gas damping. The ribbon could be heated by a coaxial aluminum tube furnace that surrounded all but about the bottom 1 cm of the ribbon. Corrections for the reversible thermal deflection of the bimetallic strip were avoided by using a fixed temperature (just above ambient) for measurement. Figures 1 and 2 show that measurable deflections are produced by the gases used. With H2, the nickel side of the ribbon becomes concave. The deflection reached a t room temperature is small, but at higher temperatures it becomes larger, a behavior suggesting activated adsorption. Removal of Hz by He partially removes the deflection, the more so the higher the temperature. The largest deflection observed in H2 (after exposure at 150 "C) was about 2 mm. Water and NH3 each produces a deflection opposite to that of H2. In each case the rate is greater than with H, and there is a maximum in the deflection-time curve. This last observation points to the existence of more than one process with different orientations or intrinsic forces. Part
(12) W. Jessen, H. G. Elusse, and B. Havsteen, Angew Chem., Int. Ed. Engl., 15, 689 (1976). Edwln J. Heilweil Irving R. Epstein"
Department of Chemistry Brandeis University Waltham, Massachusetts 02 154
1361
Received October 27, 1978: Revised Manuscript Received February 22, 1979
Measurement of Adsorptive Forces Publication costs assisted by the U.S. Deparfment of Energy
Sir: Most nzeasurements of the strengths of molecular interactions are measurements of energy rather than of force. For adsorption, information is lost if energy is the measured quantity, since the surface defines directions to which forces can be related. Some time ago it occurred to one of us that it might be possible to measure adsorptive forces if adsurption could be confined to one side of a thin ribbon. Them, any component of the adsorptive force that was parallel to the surface would bend the ribbon. A rough calculation suggested that an effect should be observable with a ribbon a few microns thick. We have performed preliminary experiments that demonstrate such an effect. This is not the first observation of mechanical effects accompanying adsorption, but the earliest ones,l length changes in porous rods, were limited to physical adsorption and did not relate forces to directions. An observation on single crystals2came to our attention while this report was being written. Thin crystals of GaAs, etc., in which opposite [ill] faces have different atomic arrangements, were bent in opposite senses by NH3 and by H2S. 1.0
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0022-3654/79/2083-1361$0~.00/0 0 1979 American Chemical Society
1362
The Journal of Physical Chemistry, Vo/. 83, No. 10, 1979
Communications to the Editor
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Figure 2. Deflection of the same nickel-gold ribbon in (a) H,O vapor (He carrier) or He and (b) NH, or He. Treatments and Mn2+ were at a fixed temperature just above ambient, and at ambient pressure, the water vapor being saturated at ambient temperature in He. The measurements were too closely spaced to show as individual points; the shapes of the curves are real. Other notes as for Figure 1.
of the deflectioi for HzO and NH3 is removed rather quickly by He, but in each case there is a residual deflection that increases with successive exposures. With NH3 this is in the same direction as the fast, reversible component, but with HzO it is in the opposite direction. The maximum deflections observed for HzO (at about 5 torr in He) and for NH3 (at about 1atm) were 0.2-0.3 mm at room temperature. A ribbon prepared by evaporating gold and then nickel onto a glass slide and then stripping it showed in Hz a deflection opposite to that of our commercial specimen. We showed that this arose from a real difference between the surfaces of the two nickels by preparing another ribbon from the commercial material, this time from a thinner sheet and only 10 cm long, again with an evaporated gold backing. In Hz this sample deflected with the nickel concave as with the original sample. This ribbon was then flushed with He, demounted, and coated with evaporated nickel on the nickel side. In H2 this ribbon now deflected with the nickel convex as with the all-evaporated sample. We conclude that adsorption from a gas produces measurable changes in the tangential forces on a solid surface, and that these changes show interesting chemical differences. Our tentative picture attributes the changes to oblique forces between the adsorbate and two points on the adsorbent, but we do not know the extent of changes in the intrinsic surface tension of the solid resulting from polarization. The preliminary experiments included no provisions for monitoring the cleanliness of the surface or the extent of adsorption. This makes any detailed chemical deductions impossible at present. If
absolute measurements of reasonable accuracy can be made on well-characterized systems, then quantitative comparisons of measured forces with those calculated from theory should provide important checks on the latter. At an intermediate level, where only relative values of forces are available but on well-characterized surfaces, measurements of rates of deflection should help to untangle complex adsorption behavior. The unique feature of the technique at whatever level of refinement is that it provides a vector measure of the adsorbate-adsorbent interaction. References and Notes (1) F. T. Meehan, Proc. R . SOC. London, Ser. A , 115, 199 (1927); D. H, Bangham and N. Fakhoury, ibid., 130, 81 (1930); J. W. McBain, J. L. Porter, and R. F. Session, J. Am. Chem. Soc., 55, 2294 (1933); R. McIntosh and R. S. Haines, J . Chem. Phys., 15, 28 (1947); D. J. C. Yates, Proc. R . SOC.London, Ser. A , 224, 526 (1954); E. A. Flood and R. D. Heyding, Can. J . Chem., 32, 660 (1954). (2) M. C. Finn and H. C. Gatos, Surface Sci., 1, 361 (1964). (3) A numerical solution of the equation for the shape of a thin elastic ribbon under the influence of a one-sided tension and gravity shows that only about the bottom 20% of a 30-cm ribbon contributes to the deflection. We have made satisfactory measurements with 5-cm ribbons. (4) Research sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, US. Department of Energy under Contact W-7405-eng-26 with the Union Carbide Corporation.
Chemistry Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37830 Received February 6, 1979
Elllson H. Taylor" W. Cole Waggener
