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The originalwork of Gay(4)with eyelohexane and the recent determinations ofBernstein(1)with the same solvent have indicated that an oil-eyelohexanemixture may approach this ideal behavior and for this reason should beapplicable for number average or mean molecular weight determinations.
Cryoscopic determinations witheyelohexane have been further investigated and extended in our laboratory forthermally polymerized and mechanically oxidized raw linseed oil and forpurposes of comparison determinations have also been made in benzene.
Experimental
The freezing point determinationswere carried out in the commonly used Beckmann apparatus using ice and water asthe cooling mixture. In all instances 25 ml. of solvent, pipetted at 25℃ were used. Therespective weights for this volume at the given temperature were 1.9.36 gramsfor cyclohexane and 21.81 grams for benzene. The temperature depressions wereread to 0.01℃ andb y means of a magnifying device estimated to 0.001℃
The solvents were purified by thefollowing procedure: Commercial eyelohexane and thiophene free, reagent gradebenzene were each passed through an adsorption apparatus 100 era. in length and35'ram.in diameter, containing approximately 400 grams of silica gel accordingto the method of Graft, O'Connor, and Skau (5).The purified materials weredistilled through a Widmer column, and the eyclohexane fraction boiling at 80.8~andthe benzene fraction boiling at 80-80.1℃ were collected.
Molecularweights of all samples were determined at a concentration of approximately4.5-5%.At least three samples of each run were checked over a concentrationrange from 2-10% in cyelohexane and benzene. It should be mentioned at thispoint that all experiments were run in duplicate, but since results checkedwithin narrow limits the analytical or molecular weight data of the cheek runswere not included in this paper.
Results and Discussion
In Figure 1 the molecular weightsof the polymerized and oxidized samples determined in cyclohexane and benzeneare plotted arithmetically against viscosity. A comparison of these curvesshows a wide variation of the numerical values obtained in cyclohexane andbenzene. This applies to the original oil as well as to the oil samplespolymerized or oxidized to higher viscosities. The molecular weight value of theraw oil in cyclohexane agrees well with the calculated theoretical value of878.This confirms the findings of Gay and Bernstein. Moreover, the abnormally lowvalue(700)found in benzene also has been reported in the literature on manyoccasions.
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发表于 2012-12-12 15:59
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