Kirkwood–Shumaker versus van der Waals–Lifshitz interactions: Charge and polarization fluctuations

Salient aspects of the Kirkwood–Shumaker (KS) theory of thermal monopolar fluctuation interactions will be put in perspective and compared with the thermal van der Waals–Lifshitz (WL) interactions due to dipolar fluctuations. The origin of monopolar charge fluctuations is in the charge-dissociation equilibria of dissociable molecular moieties described by the capacitance response function. The KS forces are of much longer range and stronger then the WL interactions, but are not universal and depend on the charge dissociation mechanism. A generalized theory of fluctuation interactions with nonlinear charge dissociation free energy term will be outlined, and shown how it naturally leads to the long-range KS interactions. The formalism also allows for other free energy terms, enabling a complete description of fluctuation interaction with monopolar and dipolar fluctuations included, leading to the KS and WL theories as limiting forms.