Mayer -function and excluded volume

Consider the energy cost U(r) of bringing two monomers from oo to within distance r of each other in a solvent. A typical profile of this function is sketched in Fig. 3.1. It contains a repulsive hard-core barrier that corres- 

Effective interaction potential between two monomers in a solution of other molecules. 

On the other hand, if monomers like each other less than surrounding solvent (for example, similarly charged monomers) there is no attractive well in U(r) but instead, extra repulsion. 

The probability of finding two monomers separated by a distance r in a solvent at temperature T is proportional to the Boltzmann factor 

The hard-core potential prevents monomers from overlapping. 

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