A New Look at Surface Films The measurement of surface films has been described in various laboratory manuals' as a suitable experiment for the introductory chemistry course. As usually performed, a very dilute solution of stearie or oleie acid in toluene is'floated on a water surface: after the solvent has evaoorated the areaof the monolaver of acid is measured and the film thickness Le.. molecule lenL?hl is calculated. TOdetpriine the film area, it is asumed that the monolnyer rs m the shape of s circular diri and itsdiameter ir thecrucial meauurement. Twureriuuvernrs art. mherent in the pn,cedure: 11m reality the f h n is very irregular. far irum circular, and 21 its hundary is not clearly defined. h s t i n g lightly with lycupodium powder aids in the definition somewhat, but this treatment raises a question about the validity of the monolayer assumption. A modification of the experiment used by the author retains the useful applications hut improves the precision considerablv. This is done hv enclosing the film between harriers t o define a rectangle whose area is readily determined. The procedure is as follows: 1) Prepare a dozen or so barriers by cutting strips of paper '12 X 11in. and dipping them in molten paraffin; allow excess t o drain off. 21 Scrupoluuuly clean a ylarsdi3h with an even rim (acrystallirmg dish works well) and till heaping full with water. Clean thr iurfarr hy sliding a barrier arrusa it (,trip3 must be diwardrd after "ring just wrt. and muat not bv cuntarninated wrrhuil tmm the fingrrsi.'l'hesurfare isclran when twu barrier+will lie parallel without puthingapart.'rhestripsfl~,at on the surface without difficulty. 3) Place a single drop of 0.005 M stearic acid in toluene between two barriers on the water surface. After the toluene has evaporated (allow 2 min) carefully nudge one strip toward the other; almost mystically the second one begins to move as the acid monolayer is confined. Measure the length and width of the rectangle, using an average length if a circular dish is used, and calculate the film area. 4) Find the number of moles of acid fromconcentration and volumeof a drop (pipet calibration), which together withmolecular weight and density of stearic acid determines the volume of the film. From this and the film area find the film thickness, i.e., molecule length. ~~
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The experiment report provides a useful review of geometry, concentration, and density plus useful application of scientific notation. 'Bailey, P. S., and Bailey, C. A. "Experimental Chemistry for Contemporary Times," Allyn and Bacon, Boston, 1915. Allen L. Hanson St. Olaf College Northfield, MN 55057
Volume 59
Number 5
May 1982
379
