Molecular Dynamics Insights into the Origins of Interfacial Tension Reduction by Nonionic Surfactants at the Water-Alkane Interface

<div> <i>Hypothesis:</i> Accurate Gibbs adsorption isotherms for surfactants in water–alkane systems are required to understand how surfactants reduce interfacial tension. We hypothesize that such isotherms must be constructed from independent analysis of the interfaces and determination of the true interfacial area. We further hypothesize that pressure anisotropy profiles can decompose interfacial tension into structure-dependent contributions. </div> <div> <span><i><br></i></span> </div> <div> <span><i>Simulations</i></span><span>: Coarse-grained molecular dynamics simulations with the Martini 3.0 force field were performed for water–dodecane interfaces containing five homologous nonionic CnEm surfactants. C4E8, C7E7, C10E6, C13E5, and C16E4. The total number of beads was kept constant while HLB was systematically varied. Systems were analyzed using local stress tensor calculations and a surface-reconstruction algorithm based on DBSCAN clustering and Delaunay triangulation. This approach enabled the independent calculation of interfacial tensions and adsorption values.</span> </div> <div> <span><i><br></i></span> </div> <div> <span><i>Findings</i></span><span>: The proposed analysis reveals a non-monotonic dependence of the minimum achievable interfacial tension,&nbsp;</span><span><i>γ</i></span><span>min</span><span>, on HLB, with the lowest&nbsp;</span><span><i>γ</i></span><span>min obtained for intermediate HLB values. This trend harmonizes experimental data from five independent studies on related surfactants. For saturated monolayers, pressure anisotropy profiles identify three contributions to interfacial tension: positive oil–surfactant and water–surfactant contributions and a negative surfactant–surfactant contribution. Their HLB-dependent redistribution provides a roadmap for further surfactant design. The same profiles also explain monolayer collapse pathways: hydrophilic surfactants form multilayers, intermediate-HLB surfactants develop curved monolayers, and hydrophobic surfactants transfer into the oil phase as reverse micelles. This work advances the molecular dynamics simulation of water–alkane interfaces at high surfactant adsorption and ultralow interfacial tension.</span> </div>