# Heat capacity of mixtures is higher than their pure contituents?

I'm currently studying the effects of how changing mixed-refrigerant compositions effects their heat capacity. Surprisingly, my data suggests that some combinations of mixed-refrigerants have heat capacities larger than their pure constituents. This seems to violate Kopp's Law which states:

The molecular heat capacity of a solid compound is the sum of the atomic heat capacities of the elements composing it; the elements having atomic heat capacities lower than those required by the Dulong–Petit law retain these lower values in their compounds.

The mixed refrigerant compositions include: methane, ethane, propane and nitrogen. The graph below shows average, minumum and maximum heat capacity values over the temperature range from $\pu{130 K}$ to $\pu{280 K}$ and pressure range from $\pu{1 bar}$ to $\pu{100 bar}$.

Is there another possible explanation as for why a combination of the above components would have a higher capacity than their pure constituents? I was thinking along the lines of molecular interactions and deviations from the ideal gas law.

Here is a graph of my findings:

Note: I am using a gas physical property software for these calculations.

link: https://www.dnvgl.com/services/gasvle-8331

• This is probably not so relevant here, but ideally you might want to provide a reference or a link to the github repo for the software you are using. – andselisk Jul 25 '17 at 10:14
• Are these mixtures of liquids or of vapors? – Chet Miller Jul 25 '17 at 20:13
• Liquid, vapour and liquid/vapour over the specified temperature and pressure range. Boiling points differ between each component quite drastically. – Joey Jul 26 '17 at 8:26
• So you are measuring "apparent" heat capacity values over phase changes, correct? – Chet Miller Jul 26 '17 at 13:53
• Yes, this is totally theoretical. Though, I am assuming the software I am using is fairly accurate for these predictions. I am calcualting the heat capacity at each temperature over the full range of pressures (1bar-100bar in 1 bar steps). – Joey Jul 26 '17 at 20:54