1231
Changes in reflectance caused by a nioiiolaye~film of fatt.y acid on a water surface have been measured. Measurements were made a t near normal iiicicle?ce and 4000 4.wive length with several fatty acids. Lleasurements were also made 011 stearic and palmitic acid a t 2537 A. wave length and a t 4000 A. with polarized light anr! a high angle of incidence. The fatty acid monolayers r e r e compressed with oleic acid to about 30 dynes/cm. pressure for all the measurements. The reflectance change is caused by interference between light reflected at the air-film and film-xatcr interfaces. For st.earic,and palmit,ic acid, if th,e index of refraction of the monolayer is assumed t'o be the same as that for solid acid,Jhe observed reflectance changes at 4000 A. agree with the calculations, provided the film thickness is taken to be about 3 A. less than the molecular chain lcngth of the solid. Redwed film thickness may be due t3 immersion into t,he water of the hydrophilic group a t the end of :ieh chain, or t,o tilt of eazh chain away from an axis perpendicular t,o the water surface. Measurements on monolayers of t y acids a t 4000 9. shox-ed that the reflectance changes varied as the square of the number of carbon atoms in the i' cha,in, and t,hus approximately as the square of the film thickness, as expected theoretically. The effects of change i n wave !ength and angle of incidence are consistent with the interference t,heory. The reflectance change is sensitive to t,he purity of the iratrr used as the film substrate, probably because impurities in the water change the film thickness.
Theory R h e n :L wiooth Furface is coated with a thin film of a transparent substance, the reflectivity of the surface is changed due to interference between the radiation reflected at the air-film and filmsubstrate interfaces. Expressions for the reflectivity of such a system have been derived and summarized by Heavens.' The film is assumed to be homogeneous, isotropic and transparent. The reflectivity R at any angle of inridenre is gireii in Heavens' notation as (1)
where r1 aiid r2 are the Fremel coefficients. A subscript s or p is added to R, rl and r2 to deitote the polarization of the incident radiatioii. The thickness parameter & is given by
where dl is the film thickness, arid X is the ware length of the radiation. At normal incidence R, = R,, and in the special case of very thin films ( ~ 3Ian AR/R, %
1':hnitic acid AR/R, :5 Za
x,
A.
1.89 rt 23 4000 0.05 Quartz distilled water. Tap
the scatter aiid total weight for the observations on a given acid, and are given in Table I. Discussion The scatter betm-een repeated ohserr.a t ioiis ' on one acid iii a given run, given in Table I, is uncomfortably small compared t'o the differences between average 1,alues for different runs. It is possible that the chemical composition of the films was different in diff erent runs. Dist,illed water was always used in the trays in the experiments iucluded i l l Table I. The n-nt.er was changed several times a d the pans were washed with detergent, and rew-asect i n thc, c w m e of all the observations. Termiiiatioii of' the hydrophilic crids of the molecules in the 51111by metal ions, such as Zn, Ca, Ea, et,(;., might inotlify tlie film thickness. Owing to t.he lack of rlefiiiite knowledge on this point however, the results for each acid in different rims may hai-e been aw-aged toget'her and further discussion will be concerned with the average values of the reflectivity changes. Solid stearic acid is a monoclinic cryst'a.1. The chain length of the rnolecule in the cryst3+llinesolid as measured by X-ray diffraction is 2-1.4 A. and the index of refraction n, = l.53.4 When laid down as (3) Unit mrigl.t was amigned t o an obberration on a film in one pan only. Weight two mns asbigned t o the average of two observations n a d e in succession (firat on one pan and then on tlie other) il the i n i t i d and final balm eiidings differed b y more t h a n 1057, of t h e change of balance. 1.T !it four W-RS assigned if t h e initisl and final balance rradinls d i i r c r e d b y less than 10Y0because this indicated tllat probably tiit! filnis were free of fluws, t,lie watei way adequately cleaned brfore eitlicr film was l a i d r1oii.n. and t h a t little dust fell into the field. (1) .4. N. Winchell, "Opticd Properties of Organic Conlpoundu," Academic Press, Surs York, S. Y.,105.1, PIA 23-2,1.
a compressed monolayer on wat,er, the chain is nearly perpendicular to t,he water surface. Equation 3 may be used with $he ohserved values AR/R = 0.72, X = 4000 A. and n 1.53 ,to deduce the, film thickness. The result, 21.5 A., is about 3 A. less than the rnoleculnr &:tin length of the crystalline so1id.j A similar calculatioii for palmitic acid, with AR/R = 0.55, n o = 1.53, X = 400 A. gives d = 18.8 A., about 4 A. jess than the X-ray diffraction chain length of 22.9 These differences may be due to the fact that the hydrophilic, (COOH), ends of t,he molecules in the film are immersed in the water to some depth, thus reducing the thickness of that part of the film having a refractive index markedly diff went from water. Another possibility is that even when compressed by oleic acid the molecules on the surface are tilted, which reduces the average film thickness. Lyons and Ridea16 have suggested that the molecules in compressed fa,t,ty acid monolnyers are all tilt'ed to an angle of 26l/," to the surface normal. It is interesting t'hat this tilt angle account's very well for the thickness difference noted above. il third possibility is that the iiidex of refract
