  We consider modified theories of gravity with a direct coupling between
matter and geometry, denoted by an arbitrary function in terms of the Ricci
scalar. Due to such a coupling, the matter stress tensor is no longer conserved
and there is an energy transfer between the two components. By solving the
conservation equation, we argue that the matter system should gain energy in
this interaction, as demanded by the second law of thermodynamics. In a
cosmological setting, we show that although this kind of interaction may
account for cosmic acceleration, this latter together with direction of the
energy transfer constrain the coupling function.
