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Kirkwood-Shumaker interactions and general thermal fluctuation forces

We present an extension of the Kirkwood-Shumaker (KS) theory of proton-fluctuation interactions to situations where the perturbation theory, usually invoked to derive these interactions, fails. In order to do that we formulate a generalized theory of fluctuation interactions with non-linear macroion (surface) free energy term that naturally leads to the long-range KS interactions, but can also be straightforwardly generalized to situations where the perturbation expansion ceases to be valid. It also allows for other surface free energy terms to be treated within the same formalism, enabling us to derive the complete fluctuation interaction with monopolar and dipolar fluctuations included, leading to the KS theory as a limiting form.