Skin Barrier Modulation by Oleyl-Based Co-Formulants: Mechanistic Insights from Molecular Simulations

Understanding and predicting dermal absorption is important for many applications including transdermal drug delivery, personal care product development, and chemical risk assessment. Real-world dermal exposures typically involve chemically complex formulations that contain not only the active ingredient but also coformulants such as solvents, thickeners, humectants and emulsifiers, which can significantly alter skin permeability and dermal absorption. In the case of personal care and pharmaceutical formulations, penetration enhancers are often added to improve delivery of active ingredients, however the interaction of these ingredients with the skin and how they modulate skin barrier function is poorly understood. In this work, we have used molecular dynamics simulations to investigate how a series of oleyl homologues with varying polyoxyethylene (POE) headgroups, partition into model stratum corneum lipid bilayers and alter their structural organization and barrier properties. These molecules are widely used as coformulants in the chemical and pharmaceutical industries. The simulations show that all of the compounds spontaneously partition into the bilayer within hundreds of nanoseconds. Once partitioned, they disrupt the interfacial hydrogen bonding network and lipid order, reducing the free energy barrier to water permeation across the bilayer. The extent of disruption correlates with POE headgroup length, with longer POE chains causing more pronounced disruption of the interface and enhanced permeability. These simulations provide the first atomistic insights into how methyl oleate and POE oleyl ethers affect skin barrier properties. Our findings highlight the critical role of coformulants in modulating dermal absorption and provide a molecular basis to guide formulation design and support robust safety assessment of dermal exposure.