A thermodynamic framework to predict thermophysical properties that control pMDI aerosol generation

Activity coefficient models are introduced to provide a thermodynamic framework for simultaneously predicting multiple thermophysical properties of relevance to pressurized metered dose inhaler (pMDI) aerosol formation. The UNIFAC and UNIQUAC models are discussed in the context of calculation of saturated vapor pressure, surface tension and liquid viscosity using molecule and functional group interaction parameters. New interaction parameters are generated and presented for HFA134a/ethanol mixtures using experimental data for saturated vapor pressure, surface tension and viscosity. The UNIFAC model is shown to give adequate predictivity and can be used when no experimental data is available. Better predictions were obtained with the UNIQUAC model, which is most useful when high-quality measurement data are obtained. The use of these models for flexible thermophysical property prediction of low-global warming potential (GWP) formulations is discussed, with potential developments to improve model fits and better utilize the experimental data.