Forces of very long range

A further curious force operates when charged polymeric molecules are in a stretched out state, or indeed with any thin cylindrical structure. Fluctuations of the ions in the associated double-layer can lead to forces of extremely long range. Their existence is not in dispute, and they probably play a luge role in the organisation of DNA in cell biology [3-10]. 

A schematic representation of the evolution of concepts of molecular forces is given in Appendices 3A,3B. 

At one level, the litany above presents us with an appalling situation. The reader will observe that except for hydration forces, for which there is no real theory, we have treated water as if it is a continuum, and the ions of electrolytes as hydrated hard spheres. That approximation is not always valid [60], and one simply has to be aware of the limitations of present physical theories. Again the hydrophobic surfaces , as discussed in section 3.5.7, the forces, and their salt dependence, depend critically on the amount of dissolved gas, and nature of the charged, or polar species. 

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