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 Angstrom and a more stable minimum of ca. 5,0 Angstrom, 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 Angstrom. 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-. (C) 2002 Published by Elsevier Science B.V.