Salt Effects on the Growth of Mixed Anionic and Zwitterionic Surfactant Micelles

We investigate the changes in 3:1 mixed sodium dodecyl sulfate (SDS) and cocamidopropyl betaine (CAPB) surfactant solutions (at the 400 mM scale) upon addition of alkali/halide salts up to a concentration of 2.5 M using coarse-grained molecular dynamics simulations coupled with a Hamiltonian replica exchange scheme and accounting for long-range electrostatic effects. Upon addition of salt, micelle properties (size, morphology, degree of neutralization, equilibrium between surfactant chains within 'end-cap' and 'cylindrical' micellar subdomains) as well as surfactant solution shear viscosities all obey a linear regime until a salt concentration of about 1 M. Then, they obey a new regime in which they all rapidly and nonlinearly diverge, in line with available experimental evidence regarding similar mixed anionic/zwitterionic surfactant solutions. Interestingly, the transition between these two regimes can be rationalized by considering an equilibrium between only SDS chains in 'end-cap' and 'cylindrical' micelle domains (according to the pseudophase thermodynamic framework), with CAPB chains taken into account implicitly. Although there is room for improvement, our simulation protocol appears to be a promising tool to investigate the effects of cosurfactants and/or hydrophobic (fragrance) molecules on surfactant solutions.