We discuss the interplay among theory, molecular simulations, and electric conductance experiments as an important tool for the extraction of ion-pair interaction potentials to make possible the bridging of the density gap between the lowest experimentally attainable conductance measurement and the theoretically acceptable zero-density limit of the ion-pair association constant. The density dependence of the Na(+)center dot center dot center dot Cl(-) pair association constant in ultrasupercritical (USC) steam environments is predicted by a constraint molecular dynamics simulation over state conditions relevant to the new generation of USC steam power plants. Finally, we draw attention to relevant modeling challenges associated with the behavior of these systems around the zero-density limit, discuss ways to overcome the drawbacks of traditional log K(a)-log rho models, and illustrate the case with an explicit quartic (rho)(1/2)-polynomial representation for log K(a).