The adsorption behavior at the air/water interface of saturated cardanol nonionic surfactants through molecular dynamic simulations

Context

Cardanol surfactants exhibit significant development potential owing to their advantages of abundant availability, low cost, and environmental sustainability. In this study, a series of saturated cardanol nonionic surfactants were designed. The structure–activity relationships of these surfactants with varying lengths and positions of PO and EO chains were investigated from three perspectives: surface activity, adsorption morphology, and molecular bonding forces. The results indicated that the chain length ratio and position of PO and EO significantly influenced the performance of cardanol nonionic surfactants at the air/water interface. The PO chains can significantly mitigate the solvation effect at the terminus of surfactants, thereby enhancing their aggregation at the air/water interface. Additionally, the ratio of PO to EO chains influences both the radius of gyration and tilt angle of hydrophilic and hydrophobic segments within surfactant molecules. Notably, when both PO and EO chain lengths are set to 8, optimal adsorption of surfactant molecules occurs at the interface. This phenomenon is primarily attributed to hydrogen bonding interactions that lead water molecules to exhibit varying degrees of aggregation around PO or EO chains; these effects, in conjunction with adsorption morphology, ultimately influence the interfacial properties of surfactants. This study provides a theoretical foundation and reference for the structural design, synthesis, and interfacial properties of cardanol surfactants.

Method

In this study, Packmol was employed for model construction, Gromacs for molecular dynamics simulations, and all simulations were conducted using the GAFF force field. The simulation process primarily involved the steepest descent method, followed by NPT ensemble simulations for 1 ns and 10 ns, respectively. The Berendsen and Parrinello-Rahman methods are employed to maintain system pressure. The LINCS algorithm and Lennard–Jones potential are utilized to effectively constrain molecular bond lengths and cutoff radius. The long-range electrostatic interactions are treated using the Particle-Mesh Ewald (PME) summation method.