Microscopic Dynamic Mechanisms of Surfactant-Induced Skin Irritation

Surfactants are essential components of detergents and personal care products due to their unique amphiphilic structure. However, the mechanism underlying surfactant-induced skin irritation remains unclear. This study systematically combined experiments with molecular dynamics (MD) simulations to investigate this mechanism, focusing on three surfactant systems including pure disodium cocoyl glutamate (DCG), a DCG/sodium lauroyl aspartate/rhamnolipid mixture (DCG/SLA/GL), and a DCG/SLA/GL/cocamidopropyl betaine mixture (DCG/SLA/GL/CAPB). Experimental results show that, among these systems, those with lower critical micelle concentration (CMC) form larger surfactant aggregates in the bulk phase and exhibit lower irritation in the HET-CAM assay. MD simulations reveal that free surfactants alter the conformation of ceramides (CER) and free fatty acids (FFA) by inserting into the lipid layer model, particularly affecting the longer tails of CER, and this insertion disrupts lipid packing and impairs the lipid layer's hydration capacity. More stable surfactant aggregates suppress the dissociation of individual surfactants, thereby reducing the insertion of free surfactants into the lipid layer. Surfactant aggregates tend to linger near the lipid-water interface and indirectly modulate the conformation of CER and FFA by interfacial perturbation, an effect substantially weaker than that of free surfactants at equivalent concentrations. For the studied systems, both free surfactants and aggregates influence lipid layer structure, but free surfactant insertion appears to be the dominant contributor to lipid layer disruption within the present model. More stable and compact aggregates, such as those formed in the DCG/SLA/GL/CAPB system, exert a significantly smaller impact on the lipid layer among the three systems studied. This study proposes a plausible microscopic mechanism linking surfactant aggregation behavior to irritation for the tested formulations, providing theoretical insights for the rational design of lower-irritation surfactant systems.