Abstract
Equations are developed relating the thermodynamic properties of a mixture of rubber + liquid with the vapor pressure of the liquid above the mixture. Experimental methods are described for the determination of vapor pressure over the whole range of composition of the mixture. By the use of four different methods, it was possible to measure relative vapor pressure lowerings Apo/po° from 2×10−6 to 0.997. Complete vapor pressure data are given for rubber-benzene mixtures at 25° C, together with the calculated Gibbs' free energies of dilution and solution. Temperature coefficient measurements at a number of concentrations are employed to calculate heats of dilution, and these are interpplated by a modified form of an equation due to Langmuir. In this way the heats of dilution and solution are also obtained over the whole range of composition. Combining the heat and free energy data gives the entropies of dilution and solution. The entropy of dilution is approximately twice the heat of dilution over a wide concentration range and, except in dilute solutions (< 5% rubber), both are independent of the molecular weight of the rubber. The entropy of dilution is very much larger than its ideal value, and can be approximately represented by an equation of Flory, though there are significant discrepancies in the region of dilute solutions. The molar heat of solution of rubber is so large that the miscibility of rubber and benzene can be explained only by the anomalously large entropy of solution.
Determination of Optimal Vapor Pressure Data by the Second and Third Law Methods
