The mixtures were prepared gravimetrically by adding each component to 2 mL vials with septa in the caps. ECO is a solid at room temperature, but it dissolved readily in the other components. The mass of each component was measured with a Mettler Toledo NewClassic MF analytical balance (0.1 mg precision), mass fractions were calculated from these masses, and then mole fractions were calculated from the known molecular mass of each component. SI B lists the final mass fractions and mole fractions. The uncertainties in these values are estimated to be ±0.001.