Interface Adsorption versus Bulk Micellization of Surfactants: Insights from Molecular Simulations

Surfactants play essential roles in many commonplaceapplicationsand industrial processes. Although significant progress has been madeover the past decades with regard to model-based predictions of thebehavior of surfactants, important challenges have remained. Notably,the characteristic time scales of surfactant exchange among micelles,interfaces, and the bulk solution typically exceed the time scalescurrently accessible with atomistic molecular dynamics (MD) simulations.Here, we circumvent this problem by introducing a framework that combinesthe general thermodynamic principles of self-assembly and interfacialadsorption with atomistic MD simulations. This approach provides afull thermodynamic description based on equal chemical potentialsand connects the surfactant bulk concentration, the experimental controlparameter, with the surfactant surface density, the suitable controlparameter in MD simulations. Self-consistency is demonstrated forthe nonionic surfactant C12EO6 (hexaethyleneglycol monododecyl ether) at an alkane/water interface, for whichthe adsorption and pressure isotherms are computed. The agreementbetween the simulation results and experiments is semiquantitative.A detailed analysis reveals that the used atomistic model captureswell the interactions between surfactants at the interface but lessso their adsorption affinities to the interface and incorporationinto micelles. Based on a comparison with other recent studies thatpursued similar modeling challenges, we conclude that the currentatomistic models systematically overestimate the surfactant affinitiesto aggregates, which calls for improved models in the future.