CORRESPONDENCE
Measurement o f .
..
Thermal Conductivity of n-Octadecane Does molecular orientation affect the data? SIR: I n an article by Sutherland, Davis, and Seyer (3) the conclusion is reached that, “Under quiescent conditions the orientating forces on the copper surface can extend several millimeters deep into liquid octadecane, and this depth appears to be dependent upon the size and shape of the molecule.” Sutherland and coworkers reached this conclusion as a result of measurements of thermal conductivity made in an unguarded-plate apparatus for samples of varying thickness, ranging from 0.01 cm. to 0.69 cm., and, in conscquence, they state that, “The most reliable method for determining the heat conductivity of a liquid is the thick film method.”
Values of thermal conductivity at 30’ C., as derived from their Figure 4, decrease from 0.000365 to 0.000046 cal. cm. per sq. cm. sec. O C. as the sample thickness is decreased from 0.69 to 0.01 cq., The range of thermal conductivity values covered by all liquids is only about 10 to 1, decreasing from a value of about 0.00145 cal. cm. pzr sq. cm. sec. O C. for water to about 0.00014 for chlorotrifluoromethane. This suggestion that molecular orientation of a long-chain molecule can affect the thermal conductivity of a single liquid to almost the same extent is rather staggering. The increase in order associated with the orientation might well be
expected to lead to an increase in tbe thermal conductivity, yet the results presented are in the opposite direction, and, for thicknesses of 2 mm. and less, the values are lower than any known liquid thermal conductivity. I n fact, for the thinnest films, the values decrease into the region appropriate to gases or vapors, and a n alternative and more likely explanation of the results would appear to be the presence of air or vapor films. At the National Physical Laboratory, thermal conductivity determinations on liquids have been made (7, 2) in a guarded-plate apparatus using liquid film thicknesses of 2 or 3 mm. As the work and conclusions of Su therland and
+SAMPLE
x
THICKNESS 3 mm.
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RINGED NUMBERS INDICATE 0.0004E
SEQUENCE OF OBSERVATlONS
E V
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SOLID
LIQUID
Q
($
I
I
THERMAL CONDUCTIVITY OF
n
- OCTADLCANE
Results show the variation of thermal conductivity of n-octadecane with temperature VOL. 53, NO. 7
JULY 1961
581
others imply that this mrthod would not be applicable to liquids such as n-octadecane, it seemed essential that check measurements should be made in our apparatus.
Procedure
A sample of n-octadecane, prepared by the Eastman Kodak Co., was obtained from Kodak Limited, and its thermal conductivity was determined at thicknesses of 3 and 2 mm. by the method normally used. The interior of the guarded-plate apparatus had been nickel-plated and lapped, so in these experiments the test sample was in contact with solid nickel and not with copper as in the work at the University of California. The apparatus was assembled with the sample well above the melting point of about 26” C., so that freedom from entrapped air could be ensured. Determinations were first made in the liquid phase over the range 32’ to 60’ C. using 3-mm. distance pieces and then were made in the solid phase and a repeat point in the liquid phase. The apparatus was then opened, the distance pieces changed to give a 2-mm. thickness, and the thermal conductivity determinations made for the liquid and solid phases.
Results The results obtained are given in the figure where the sequence of observations is indicated by ringed numbers. The determinations made for the two thicknesses of liquid are in good agreement with each other and with the scanty existing data mentioned by Sutherland. ’They tend to confirm the data for the maximum film thickness used by these workers, but the negative temperature coefficient is smaller. At 40’ C. the thermal conductivity is 0.000364 cal. cm. per sq. cm. sec. ’ C., which is about 2.4 and 2.8 times the values obtained by Sutherland for thicknesses of 3 and 2 mm., respectively. It can only be concluded that their results for these thicknesses were subject to considerable error.
No evidence of extensive molecular orientation was found in our experiments, and there seems to be no reason to doubt the suitability of o u r apparatus a n d methods for n-octadecane a n d presumably, for all liquid thermal conductivity determinations. Regarding the solid phase, the two thicknesses again give results in fairly good agreement, but the scatter is greater and, for the narrow temperature range covered, the temperature coefficient cannot be deduced with any cer-
tainty. A possible explanation for the high points first obtained after solidification-those marked 5 and 9-could be the existence of rhombic and monoclinic forms, as stated by Sutherland. When these values are ignored the line drawn in the figure is obtained, which indicates the thermal conductivity at 23’ C. to be about 10% greater than the highest value of Sutherland (3).
Acknowledgment The authors are indebted to their colleagues D. J. Hamblin and R. P. Tye for assistance with the experiments, and to E. F. G. Herington of the National Chemical Laboratory for helpful discussions. The work has formcd part of the General Research Program of the Basic Physics Division of the Sational Physical Laboratory and is published by permission of the Director. Literature Cited (1) Challoner: A. R., Powell, R. W., Proc. Roy. Soc. (London) Ser. A . 238, 90 (1956). (2) Powell, K. W.,Challoner, A . I
