Final answer:
For charged species, the free energy of transfer between phases often increases due to the energy required to overcome electrostatic interactions associated with separating the charged species from its solvating shell. The correct option is C.
Step-by-step explanation:
The free energy of transfer between phases when using charged species is particularly significant because it includes not only the usual enthalpic and entropic components but also an electrostatic component due to the charge. When a charged species is transferred between phases, such as from a polar to a nonpolar phase, there is an energy cost associated with separating the charged species from its solvating shell of opposite charges or dipoles. This process usually increases free energy because work must be done to overcome the electrostatic interactions. Therefore, the correct answer to the question is that the free energy of transfer for charged species increases.