The Present State of Molecular Forces

摘要

The DLVO theory of colloidal particle interactions has been at the core of colloid science for a long time. Quantitatively, agreement between experiment and theory was illusory except at salt concentrations less than about 10-2 molar. The same problem with theory exists for pH measurements, buffers, electrochemistry, zeta potentials, electrolyte activities, interfacial tension of salt solutions and a host of phenomena that depend on so called specific ion effects, This is so, most dramatically in biology, but also in colloid, polymer and surface science generally. The problems date back to Hofmeister whose work stands in the scheme of things as Mendel's did to genetics. Where problems occurred we have tended to argue them away, capturing specificity in unquantifiable terms embodied in words like cosmotropes, chaotropes, hydrophilicity, hydrophobicity, soft and hard ions, pi-cation interactions, hydration and hydrophobic forces, water structure. To complicate the puzzle further the role of dissolved atmospheric gas or other sparingly soluble (hydrophobic) solutes is sometimes major, and has been completely ignored in theories or simulations. Some progress in unravelling these difficulties has been made. It turns out that theories have been seriously flawed. They depend on an ansatz that separates electrostatic forces from the totality of non electrostatic (NES) quantum mechanical electrodynamic fluctuation (Lifshitz or dispersion) forces. These NES forces are ignored, as for the Born self energy of an ion, or its decorations. Or else the electrostatic forces are treated in a non linear theory (e.g. Poisson Boltzman), and the quantum forces via Lifshitz theory as for DLVO. Even for the continuum solvent approximation this violates both the Gibbs adsorption equation, and the gauge condition on the electromagnetic field. These problems are highly non trivial and occur equally in quantum field theories and biophysical problems that couple electron and photon transfer. When the faults are repaired, the revised theory does seem to account for ion specificity and a veritable zoo of postulated new forces begin to fall into place quantitatively. An account will be given of the emerging situation, the role of dissolved gas and "hydrophobic" forces. This leads to new insights into the necessary cooperativity that occurs with water in biological and other systems.