The affinities of a series of biologically relevant ions for a hydrated phospholipid 
membrane were investigated using molecular dynamics simulation. Interactions of molecular 
ions, such as guanidinium, tetramethylammonium, and thiocyanate with the bilayer were 
computationally characterized for the first time. Simulations reveal strong ion specificity.
On one hand, ions like guanidinium and thiocyanate adsorb relatively strongly to the 
headgroup region of the membrane. On the other hand, potassium or chloride interact 
very weakly with the phospholipids and merely act as neutralizing counterions. 
Calculations also show that these ions affect differently biophysical properties of 
the membrane, such as lipid diffusion, headgroup hydration and tilt angle.