Molecular dynamics simulation of Na+-DMP− and Na+-MP2− ion pair in aqueous solution

Molecular dynamics simulations with an umbrella potential function in solution were carried out to obtain the potential of mean force for sodium-dimethyl phosphate (Na+-DMP−) and sodium-methyl phosphate (Na+-MP2−). The Na+-DMP− showed two minima in pmf profile, a less stable minimum of ca. 3.5 Å and a more stable minimum of ca. 5.0 Å, resulting in direct and indirect contact between two ions. On the other hand, the Na+-MP2− showed one prominent minimum in pmf profile, at approximately ca. 4.5 Å. Water molecules around Na+-DMP− better formed a hydrogen-bonded network than around Na+-MP2−, showing a stronger interaction between water and the Na+-MP2−.