Size-dependent structural, interaction, and thermodynamic properties of cationic microemulsions: A molecular dynamics study

M Khatouri and A Arbia and L Talha and R Ahfir and Z Basbassi and R Elhajjam and S El Khaoui and M Filali, COLLOIDS AND SURFACES A-PHYSICOCHEMICAL AND ENGINEERING ASPECTS, 726, 137812 (2025).

DOI: 10.1016/j.colsurfa.2025.137812

Microemulsions are droplet systems widely used in the field of drug delivery due to their unique properties. Among the various physical parameters, the size of microemulsions has played a key role in enhancing their performance in pharmaceutical applications. In this study, we investigate the influence of increasing droplet size on the structural properties and interactions within a microemulsion system, using molecular dynamics simulations at different reduced number densities. The simulated system consists of oil-in-water microemulsions stabilised by the cationic surfactant cetylpyridinium chloride (CpCl) and octanol as a co-surfactant, with saline water as the solvent. These droplets are hereafter referred to as cationic microemulsions. The interactions between them are modelled by a total potential comprising a hard-sphere potential, a van der Waals potential, and a Coulomb electrostatic potential. Results from the molecular dynamics simulations show that both increasing the reduced number density and enlarging the radius of the cationic microemulsions enhance the interparticle correlations within the system. Structural properties such as the radial distribution function g(r), the structure factor S(q), and the scattering intensity I(q) reveal that the cationic microemulsion droplets become more ordered either at high concentrations or when their size is significantly increased. Furthermore, the effective interaction potential Ue f f(r) demonstrates that increasing the droplet radius strengthens both attractive and repulsive interactions. For a concentrated system with a reduced number density p & lowast; = 0.03342, composed of cationic microemulsions with a large radius R = 82 & Aring;, the droplets tend to aggregate, and the system transitions into a gel state. In addition, the thermodynamic properties indicate that increasing the radius of the cationic microemulsions leads to a much stronger enhancement of repulsive interactions compared to attractive ones.

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