Let me try to do a suggestion.

As you describe, the activity of ethanol (defined as partial pressure of ethanol divided by vapour pressure of ethanol) in this system is smaller than the propellant. For solubility of ethanol in Ethylene Propylene Diene Monomer (EPDM), not the actual concentration is of interest, this may be a 100 times smaller, but the activity of ethanol in this system. As stated the activity depends on the partial pressure of ethanol relative to the vapour pressure. When the mixture is far non ideal, the partial pressure may be equal to the vapour pressure even at very low concentrations, as such giving an activity of 1 with regard to the EPDM (maximum solubility). This would give a high permeability for ethanol, even at low concentrations, relative to the propellant when the solubility of ethanol in Ethylene propylene diene monomer (EPDM) is higher than the propellant. So we should assess whether the mixture is ideal or non ideal.

Looking forward to your comments!

Keep up the good work,
Kate
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