You should define a homogeneous binary mixture (see Material Details View: Homogeneous Binary Mixture in the CFX-Pre User's Guide) that links the primary particle and gas phase materials. Primary materials are the materials that characterize the mass transfer between the particle and the gas phase. For example, when modeling decomposition of raw coal into gaseous volatiles and char, the homogeneous binary mixture should be set up for 'Raw Coal' and 'Volatiles'. This allows the particle solver to calculate characteristic numbers for the particle (for example, Reynolds, Sherwood, and Nusselt numbers) based on properties in the particle boundary layer. These properties are calculated using the Antoine equation (vapor pressure equation) that is specified in the homogeneous binary mixture.