A three-scale problem involving swelling media is considered. The structure of the system (e.g. clay, polymers, etc.) is assumed to be characterized by particles composed of more than one phase (usually solid and liquid) which may swell or shrink. The swelling (or shrinkage) is the result of mass transfer from (to) a multicomponent solution which is in contact with the particles. Mesoscopic equations for the particles themselves have been obtained elsewhere. The macroscopic equations for the combination of swelling particles and bulk solution as well as their interfaces, considered as a mixture, are obtained here. Constitutive theory for the system is the subject of a subsequent manuscript.