Accurate Molecular Dimensions from Stearic Acid Monolayers Charles A. Lane, D. Edward Burton, and Charles C. Crabb University of Tennessee at Knoxville, Knoxville, TN 37996 General chemistry students performing the fatty acid monolayer experiment usually obtain molecular data which are inaccurate by a factor of two (1-4). These errors are due to impurities in commercial fatty acids which cause the monolayers to occupy large areas under the small pressures present in student experiments. Pure stearic acid monolayers comN m-'; an press to solid films a t pressures less than 1X equimolar mixture of stearic and oleic acid requires a pressure of 2.6 X N m-I to compress to the same area per molecule (51. We added commercial stenricacid (mp6fi-6Y°CJ in hexane solution to water on a watch glass until solid w a s visible on the water surface. Thc caIcuIa1~:dcrnss-~ectionalarea per molecule was 36 f I I\', which compares poorly with the value of 20.7 4' from X-rav diffraction methods ifil. . , After two recrvstallizations, one from 95% ethanol and one from acetone, the stearic acid (mp 69-70°C) gave a cross-sectional area of 20.4 f 0.2 A2. The assumption that the density of the film was the same as solid stearic acid allowed us to calculate the length of the acid as 24.7 f 0.3 A; the X-ray length is 24.7 A. Forty honors freshman students followed the same procedure in their first lahoratory. Averaging the student data gave a cross-sectional area ner molecule of 20.7 A2 and a leneth of 24.2 A. These dimenhis were calculated from the volumeof 6.50 x lo-'' M stearic acid solution reauired to estahlish a monomolecular film on a watch glass wiihan area of 314.2 cm2.The average volume from their 81 determinations was 0.387 mL, the range of volumes was 0.218 to 0.465 mL, and the standard deviation was 0.042 mL. In addition to recrystallizing the commercial stearic acid, we made several other modifications to the techniques usually described for this experiment. Counting drops from a micropipet is not a very satisfactory method for delivering stearic acid solution to the water surface. We found that a disposable, ~
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l-mL serological pipet would fit into the upper hole of the plastic stopcock assembly from a 50-mL buret. The stopcock gave the students excellent control of the delivered solution, and they could estimate volumes to 0.001 mL. Another advantage is that evaporation in the pipet is minimal and there is no correction for evaporation in the tip. The assembly must be clam~edbv the buret ti^ or else the students mav break the conneciion h;taeen the pipet and stopcock. We ;leaned the watch rlnsses wilh detereent followed with water and dilute HC1 since detergent is known t o penetrate some monolayers (7).Clean. d w Pasteur ~ i ~ ewere t s keot in a box near the hekane sol'utick, and the' students were;equired to place the used w i ~ e t in s a "used" box. Thev were also reauired to recao the hkiane solutions after eachtransfer. he-students prepared the water surface by dusting it with talcum powder and sweeping the sbrface with a glass rod until no more talc was visible. Finally, the endpoint was not lens formation but the first visible scum left after evaporation of the hexane. Incidentally, the film will'spread if kept in hexane vapor (8). Copies of our experimental procedure are available on request. A Pascal program to work up the data is also available. Literature Cited (11 Koke, R.J., Dorfman, M. K., and Mathia,T., J. CHEM. EDUC.,39.18 (19621. (21 King, L. C., and Nielsen, E. K., J. CHeM. EDuc.,35,198 (19581. (3) Wells. N., Bwchmann, E., Fife, W., Cobsuer. P.,"Chemistry in Action, Novel and Classical Approaches:
2nd ed., Science Enterprises.
he.. Indianapolis, 1980, p.
