The percolation transition of hydration water: from planar hydrophilic   surfaces to proteins
  The formation of a spanning hydrogen-bonded network of hydration water is found to occur via a 2D percolation transition in various systems: smooth hydrophilic surfaces, the surface of a single protein molecule, protein powder and diluted peptide solution. The average number of water-water hydrogen bonds nH at the percolation threshold varies from 2.0 to 2.3, depending on temperature, system size and surface properties. Calculation of nH allows an easy estimation of the percolation threshold of hydration water in various systems, including biomolecules. 
Alla Oleinikova, Ivan Brovchenko, Nikolai Smolin, Aliaksei Krukau,   Alfons Geiger and Roland Winter