Ionic surfactant aggregates in saline solutions: Sodium dodecyl sulphate (SDS) in the presence of excess sodium chloride (NaCl) or calcium chloride (CaCl₂)

The properties of sodium dodecyl sulfate (SDS) aggregates in saline solutions of excess sodium chloride (NaCl) or calcium chloride (CaCl2) ions were studied through extensive molecular dynamics simulations with explicit solvent. We find that the ionic strength of the solution affects not only the aggregate size of the resulting anionic micelles but also their structure. Specifically, the presence of CaCl2 induces more compact and densely packed micelles with a significant reduction in gauche defects in the SDS hydrocarbon chains in comparison with NaCl. Furthermore, we observe significantly more stable salt bridges between the charged SDS head groups mediated by Ca2+ than Na+. The presence of these salt bridges helps stabilize the more densely packed micelles.