Exploring the free energy landscape to predict surfactant adsorption isotherm at nanoparticle-water interface

The long-lasting stability of nanoparticle (NP) suspensions in aqueous solutions is commonly reached by the addition of surfactants. However, a tailored prediction of surfactant concentration enabling a well dispersion of NPs is still an ambitious objective (Cardellini et al., 2019). Here, by coupling Steered Molecular Dynamics (SMD) with the Langmuir theory, we predict the adsorption isotherm of Sodium-Dodecyl-Sulphate (SDS) on α-alumina NP. Beyond the design, our results unveil the role of NP curvature and morphology in the adsorption phenomena at solid-liquid interface. First, the Free Energy Landscapes (FELs) obtained via SMD show the dominance of the entropic contribution by increasing the SDS molecules on target bare NP. Second, a thorough analysis on the FELs, clarifies how the enhanced adsorption deviates from a Markovian dynamics in a curved interface. In fact, both NP curvature and morphology promote modification of the thermodynamics state of adsorption with consequent free energy profiles splitting and the identification of specific sites of adsorption. The proposed modelling framework provides physical insights in the surfactant adsorption onto spherical NPs and suggests guidelines to design NP suspensions in aqueous solutions.
(De Angelis et. al, accepted on ACScs).