Dual Role of Telechelic Polymers in Neutral Microemulsions: A Molecular Dynamics Simulation Study on Decoration and Bridging Effects

This work investigates the structural and dynamic properties of a mixed system composed of spherical oil droplets dispersed in water, stabilized by Triton X₁₀₀ surfactant and Triton X₃₅ cosurfactant (referred to as a neutral microemulsion). The system is modified by adding a C₁₂H₂₅-poly(ethylene oxide)-C₁₂H₂₅ telechelic polymer (PEO-2m) at various volume fractions φ, namely low (φ = 2.8 and φ = 6.8%) and high (φ = 13.3 and φ = 18%). The number of polymer chains associated with each droplet is denoted by n(PEO-2m). The interactions between droplets are modeled using a combined potential including hard-sphere exclusion, van der Waals attraction, a Yukawa-type repulsion induced by surfactants, and additional Yukawa contributions to account for polymer-induced steric and bridging interactions. The validity of the model and interaction parameters is first tested by comparing small-angle neutron scattering data with molecular dynamics (MD) simulations. The scattering intensity obtained from simulations, <i>I</i>_MD(<i>q</i>), shows good agreement with the experimental curve, <i>I</i>_exp(<i>q</i>). To evaluate the effect of PEO-2m on structural properties, we compute the radial distribution function <i>g</i>(<i>r</i>) and the structure factor <i>S</i>(<i>q</i>) for different polymer coverage levels. The results indicate that PEO-2m induces attractive interactions at low volume fractions, while repulsive behavior dominates at higher concentrations. To assess the polymer's influence on dynamics, we calculate the mean square displacement, velocity autocorrelation function, and diffusion coefficient (<i>D</i>). The findings show that increasing the amount of PEO-2m on droplet surfaces leads to reduced diffusivity, higher viscosity, and more complex dynamic behavior, especially at high polymer volume fractions.