D. CHURCHILL . ~ S D I,. S. BARTELL
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\-o1. 67
IY)LL1l-lI&lETRICDETERMINATION O F ARSORPTIOS SPECTIIA OF THIN FILRIS ON RIETAL. 11. SURFACE CHERIICAL L-1PI’LICATIOSS’2 BY D. CHUMXIILLAND I,. S. BARTELL Institide for ‘1 tomic Research and Lhpartment of Chentistry, Iowa State Tnicersily, IZeceived August 19, 1963
A\
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h new spectroscopic method for surfacte films is applied to several systems of c~licmicalinterest, including dyes, clielates, and adsorbed Lewis bases. Films containing tlie equivalent of less than a nionolayer of rhodamine B dye dispersed in collodion solids were found to exhibit a strikingly rapid change in molar extinction coefficient with cwncentration changes. 1)inietliylglyoxinie adsorbed on nickel shoned a spectrum cliaracateristic not of physically adsorbed dirriethylglyoxime b u t rather of rnicrocrystalline nickel dirnet hvlgl\-ouirne. Evidence was obtained which is not cornpletely conclusive that octadc m i n e adsort)ctl on chrorriirini gives it band not possessed by the amine or its simple complexes. The possibility that this is an cxiniple of a previously postulated charge-transfer spectrum is briefly discusscd, along with other potcritial app1ic:itions.
In the first paper of this series3 a method was described for determining electronic absorption spectra of films of molecular thickness using polarimetric reflection data. The method also yields the refractive index and the mean thickness of films to a fraction of an Angstrom unit. In the present paper are presented the results obtained for several quite different systems. These results, while far from comprehensive, serve to illustrate the applicability of the new spectral approach in problems of chemical interest. Experimental Polarization Spectrometer.-The pol‘arixation spectrometer used in this research was described in detail in paper I. I t is an instrument for characterizing with great delicacy the change of polarization sufferedby light at various wave lengths on reflection from a polished ~ u r f a c e . ~This change is measured in terms of the phase difference, A , between components of the light vibrating in the plane of incidence and perpendicular to the plane of in&dence, and the amplitude ratio, tan $, of the two components. In the following, unprirned symbols (e.g., A) will refer to measurements of surface plus film and primed symbols (e.g., A‘) will refer to measurements made on the refcrenve surface without film. Preparation of Films.--?lletal surfaces were polished using standard metallographic procedures. Rhodamine B films were prepared from a biological stain obtained from the Hartman1,eddon Company. The dye w m not further purified before use. hlixed films of dye and collodion solids were formed in the manner described in paper I. Films of Lewis bases were adsorbed either from solution in purified caetane by the method of Zisman, el a1.,6 or by adsorption from the molten compound. Adsorption of dimethylglyosime took place overnight from nearly saturated solutions of the solute in amyl acetate.
Results The results discussed in the following section are presented more as examples of problems susceptible to attack by the new polarimetric approach than as dctailed solutions to specific problems. It was shown in paper I that the “phase dispersion” curve, ie., the variation of ( A - A’) with wave length, exhibits anomalous features similar to those found for the refractive index in the vicinity of absorption maxima, provided the absorption coefficient is not too large. This can be seen in Fig. 1-3. Accordingly, the (1) Contribution No. 1373. Work was performed in the Arnes Laboratory of the U . S. Atomic Energy Commission. (2) Basrd on a dissertation by L). Churchill to the Graduate School, Iowa State University, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. 1960. (3) L. S. Bartell and D . Churchill, J. Phgs. Chem.. 66, 2242 (1961); note also the corrections, ibid.. 66, 2719 (1962), and a useful transformation b y H. A. Roth and R. D. Schile. ibad., 67, 530 (1963). (4) I t is a variant of the instrument which has become popularly known as the “ellipsonietrr.” (5) W. C. Bigelow, E. Glass, and W. A. Zisrnan, J. Collord Sci., 2, 563 (1947).
search for absorption bands in this investigation was made principally by determining phase dispersion curvcs. I n the case of rhodamine 13 films the ahsorption coefficients, li, and refractive indices, n, were also dctermined from the optical data as outlined in paper I. Rhodamine B.-The wsults obtained for films of niolccular t hickncss of rhodamiiie I3 on chi~on~ liiim ‘ are illustrated by the solid curws in Fig. 1-3. The dye exhibited a large incrcasc in molar extinction coefficient as it was diluted by collodion solids. While it is known that the molar extinction cocfIicicrit of a dilute solution of dye is about five times as great as that of pure solid dye,Git was unexpected to find this increase vir%ually attained upon a mere fivefold dilution of pure dye in the present films. Surprisingly, theii, for a film of fixcd total thickness the maximum absorption reinaind more or less constant, dwpite a dilutioii from 100 to 17% dye! The spectra obtaiiied for the thin films are compai-ed in Fig. 1 and 3 with the spectra of their bulk counterparts. The thin films in both figures contain an aniouiit of dye equivalent to a thickness of only about 6 A. The bulk results are calculated from Wiegl’s experimental data for pure solid dyeGand frgm data of the present investikgxtion for a 10-5 M aqueous solution. Results are converted into the absorption coefficient, li, using the relation In (I,]’I) = 4 x k L ’X =
2.303ecL
where L is the thickness of the film, and c is its molarity. The agreement between the samples of molecular and macro dimensions is quite good, except for a blue shift in the thin film spectra. Absorption coefficients are remarkably close, in fact, considering the extreme difference in thicknesses of material transvcrsed and, in the case of Fig. 3, the enormous difference in concentrations involved. As discussed above, it was not expected that Beer’s law mould hold for solutions as concentrated as 17% dye ( 4 . 5 A I ) . Dimethylglyoxime on Nickel.-Aidsorbcd films of dimethylglyoximc (molecular dimensions about 3 X 7 X 8 A.) were found to vary considerably in thickness depending on conditions of their formation. Several films formed on freshly cleaned surfaces7were 8 to 10 ‘4. (6) J. W. Weigl, J. Chem. Phys.. 24, 3G4 (1950). (7) The freehness IS only relative, as the slldps suffered an e\iiosurP of several hours to laboratory air during thr slirctroinrter rrfrrencc IiirlisilrPmcnts s h i c h preceded adsorption of the dimrthylgl) o u m e .
Dec., 1963
2519
hBSOllPT1OS 8PECTIlA O F T H I S FILMS ON 1IETAL
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WAVELENGTH (MILLIMICRONS)
Fig. 2-Experiinent:Ll optical constmts for a 38-.I. film on chromium. The film contained rhodamine H in a nmtrix of collodion.
in thickness. il. film formed on a slide exposed to laboratory air for a week before absorption was only about 3 A.thick. A film formed on a surface which had accidentally been touched by a water droplet just before immersion in the dimethylglyoxime solution was nonuniform, considerably exceeding 100 A. in spots. The absorption spectra were all similar, nevertheless. The “phasc dispcrsion” curve for an 8-A. film is shown in Fig. 4. Absorption bands with estimated molar extinction coefficients of lo3.*and 103.6were found a t about 405 and 540 nip, respectively, even for the 3-A. film. The spectrum is not a t all similar to the solution spectra of dimethylglyoxime or nickel dimethylglyoxime. It agrees strikingly, however, with that determined by Sone8 for aqueous suspensions of solid, colloidal nickel dimethylglyoxime. Since the strong band a t 340 nip is characteristic of crystalline nickel dimethylglyoxime and not of its monomeric units, a rather firm conclusion can be drawn about the nature of the film formed in these experiments. It is certainly not physically adsorbed dimethylglyoxime. The dimethylglyoxime apparently reacted with oxidized nickel atoms at the surface to give the well known chelate molecules, and these aggregated into niicrocrystallites. Whether the extent of reaction is governed by (8) I
