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  Since the pioneering works of Debye, H¨uckel, and 
  Onsager, electrolyte solutions have been commonly 
  described by continuous solvent models, 
  for which 
  the McMillan-Mayer theory [1] provides a rigorous 
  statistical-mechanical foundation. Within that level of 
  description, simple phenomenological models such as the 
  primitive model (PM), for which the ions are assimi- 
  lated to charged hard spheres [2], can lead to explicit 
  formulas for the thermodynamic and structural proper- 
  ties (e.g., with the help of the mean spherical approxima- 
  tion (MSA) [3] or the binding MSA (BIMSA) [4]). These 
  models are the most practical to use [5], since they allow 
  for a direct link between the experimental measurements 
  and the microscopic parameters of the system. Never- 
  theless, they ignore the molecular structure of the sol- 
  vent. Consequently, they cannot properly account for 
  the complex speciﬁc eﬀects of the ions, which appear in 
  numerous biological, chemical, and physical interfacial 
  phenomena [6, 7], without further developments. 