Calcium-Regulated Self-Assembly of Zwitterionic/Cationic Surfactant-Counterion Complexes: Molecular Dynamics Insights...

Wormlike micelles provide the structural basis for the viscoelasticity of clean fracturing fluids, yet the molecular mechanism by which calcium-containing brine regulates their growth remains insufficiently understood. In this work, all-atom molecular dynamics simulations were performed to investigate the calcium-regulated self-assembly of an EAHSB/EHAC/NaPts surfactant-counterion system. In the absence of Ca²⁺, dispersed surfactant and counterion molecules first formed small spherical aggregates, which subsequently fused into larger micelles, rod-like intermediates, and finally wormlike micelles. Increasing the local surfactant/counterion loading further promoted one-dimensional micellar growth and network-like association. When Ca²⁺ was introduced, the assembly process became slower and the final aggregate morphology shifted from elongated wormlike micelles to smaller spherical aggregates. Radial distribution functions and coordination-number analysis show that Ca²⁺ redistributes charged-group correlations within the mixed surfactant/counterion assemblies. Hydration analysis indicates enlarged polar hydration shells, whereas hydrophobic-tail hydration and gauche-defect probabilities reveal enhanced tail exposure and looser chain packing. These results demonstrate that Ca²⁺ suppresses wormlike micelle growth through coupled electrostatic redistribution, interfacial hydration, and hydrophobic-chain disorder, thereby favoring high-curvature spherical aggregates over extended wormlike networks.