Effect of Ca2+ and Mg2+ ions on surfactant solutions investigated by molecular dynamics simulation

The effect of Ca(2+) and Mg(2+) on the H-bonding structure around the headgroup of the surfactants sodium dodecyl sulfate (SDS) and sodium dodecyl sulfonate (SDSn) in solution has been studied by molecular dynamics simulation. Our results show that binding between the headgroup of the surfactant and Ca(2+) or Mg(2+) is prevented by a stabilizing solvent-separated minimum formed in the potential of mean force (PMF) between the interacting ion-pair. Among the contributions to the PMF, the major repulsive interaction is due to the rearrangement of the hydration shell after the ions enter into the original H-bonding structure of water around the headgroup, leading to a decrease in the number of H-bonds and an increase in their lifetimes. In the second hydration shell around the headgroup, additional water molecules are bound to the headgroup oxygen atoms either directly or bridged by Ca(2+) and Mg(2+). The PMF shows that the energy barriers to ion-pairing between the headgroup and Ca(2+) and Mg(2+) in the SDSn system are higher than those in the SDS system, and the water coordination numbers for Ca(2+) or Mg(2+) in SDS solution are lower. This result indicates that SDS binds the ions easily compared with SDSn, and the ions have a strong effect on the original hydration structure. That is why sulfonate surfactants such as SDSn have better efficiency in salt solution with Ca(2+) and Mg(2+) for enhanced oil recovery.