Self-assembly of mixed dodecylamine-dodecanol molecules at the air/water interface based on large-scale molecular dynamics

Dodecylamine (DDA)-alcohol mixtures have motivated great interest recently due to their superior performance in industrial processes compared with pure DDA. Self-assembly of surfactants at the air/water interface plays an important role in their surface properties. However, the self-assembly of mixed DDA-alcohol at an atomic level is still unknown. In this study, large-scale atomistic dynamic simulations of DDA-dodecanol (DOD) mixtures with different ratios were performed to clarify their molecular arrangement and explain the synergistic effect. The structural properties, such as molecular aggregation, orientation, angle distributions, and H bond distributions, of DDA-DOD mixtures were characterized in detail. Simulation results show considerable differences in the behavior of mixed DDA-DOD molecules with different ratios. The self-assembly of pure DDA molecules is not significant due to the strong electrostatic repulsion between head groups. The surface activity of pure DOD molecules is not strong due to their poor water solubility based on their fewer amounts of H-bonds with water. For mixed DDA-DOD, the addition of DOD does not directly affect the arrangement of DDA molecules; rather, DOD molecules insert between DDA molecules and form stable clusters at the air/water interface due to the van der Waals interactions between surfactant chains. Two main interaction patterns exist between DDA and DOD, including van der Waals interactions between the surfactant chains and H bonds between their head groups. DOD and DDA play an important role in forming the cluster and solvent action, respectively. The ratio of these two surfactants can affect the two above properties. This study provides a good strategy to investigate the self-assembly cluster of mixed surfactants and supports the experimental theory at the atomic level. (C) 2018 Elsevier B.V. All rights reserved.