Molecular dynamics investigation of oil/water microemulsions with telechelic polymer: Interaction properties and sol/gel transitions

This study investigates the interaction and structural properties of an oil/water microemulsion system as a function of three parameters: the volume fraction ϕ, the number of polymers grafted onto the microemulsion particles $n(D-PE{O}_{227}-D)$, and the temperature T. These microemulsions are stabilized by an ionic surfactant, cetylpyridinium chloride (CpCl), and an octanol-based co-surfactant, all dispersed in a saltwater solution. Using molecular dynamics simulations, we aim to explore changes in the structural properties and interactions during the sol/gel transition of the microemulsion system. The potential used to model the interactions between microemulsion particles accounts for both Van der Waals and electrostatic interactions. When the dodecyl-poly(ethylene oxide)₂₂₇-dodecyl (D-PEO₂₂₇-D) polymer is added to the microemulsions, it introduces two distinct contributions: a Yukawa-type repulsive interaction and an attractive interaction. Comparison of small-angle neutron scattering results with molecular dynamics simulation outcomes confirms the validity of the potential parameters, as both sets of results show good agreement. The scattered intensity $I_{MD}\left(q\right)$, calculated from molecular dynamics simulations, reveals that the microemulsions become increasingly correlated and their structure more ordered with rising volume fraction or decreasing temperature. At a certain high volume fraction or low temperature, a sol/gel transition is observed. Furthermore, the results demonstrate that the addition of the D-PEO₂₂₇-D polymer facilitates gel formation even at medium volume fractions or higher temperatures. Controlling the sol/gel transition of drug delivery systems, such as microemulsion systems, enables the preparation of formulations tailored to specific routes of administration, thereby maximizing therapeutic efficacy.