The solvation thermodynamics and in particular the solvation heat capacity of
polar and charged solutes in water is studied using atomistic molecular
dynamics simulations. As ionic solutes we consider a F− and a Na+ ion, as an
example for a polar molecule with vanishing net charge we take a SPC/E water
molecule. The partial charges of all three solutes are varied in a wide range
by a scaling factor. Using a recently introduced method for the accurate
determination of the solvation free energy of polar solutes, we determine the
free energy, entropy, enthalpy, and heat capacity of the three different
solutes as a function of temperature and partial solute charge. We find that
the sum of the solvation heat capacities of the Na+ and F− ions is negative,
in agreement with experimental observations, but our results uncover a
pronounced difference in the heat capacity between positively and negatively
charged groups. While the solvation heat capacity ΔCp stays positive and even
increases slightly upon charging the Na + ion, it decreases upon charging the
F− ion and becomes negative beyond an ion charge of q = −0.3e. On the other
hand, the heat capacity of the overall charge-neutral polar solute derived
from a SPC/E water molecule is positive for all charge scaling factors
considered by us. This means that the heat capacity of a wide class of polar
solutes with vanishing net charge is positive. The common ascription of
negative heat capacities to polar chemical groups might arise from the neglect
of non-additive interaction effects between polar and apolar groups. The
reason behind this non-additivity is suggested to be related to the second
solvation shell that significantly affects the solvation thermodynamics and
due to its large spatial extent induces quite long-ranged interactions between
solvated molecular parts and groups