  Experimental realizations of self-propelled colloidal Janus particles exploit
the conversion of free energy into directed motion. One route are phoretic
mechanisms that can be modeled schematically as the interconversion of two
chemical species. Here we consider the situation when the difference of
chemical potential between the two species (the driving affinity) can be
assumed to be constant, and we derive the thermodynamically consistent
equations of motion. In contrast to the standard model of active Brownian
particles parametrized by a constant self-propulsion speed, this yields a
non-constant speed that depends on the potential energy of the suspension. This
approach allows to consistently model the breaking of detailed balance and the
accompanying entropy production without non-conservative forces.
