title: Modelling the interfacial behaviour of dilute light-switching surfactant solutions.
abstract: The direct molecular modelling of an aqueous surfactant system at concentrations below the critical micelle concentration (pre-cmc) conditions is unviable in terms of the presently available computational power. Here, we present an alternative that combines experimental information with tractable simulations to interrogate the surface tension changes with composition and the structural behaviour of surfactants at the water-air interface. The methodology is based on the expression of the surface tension as a function of the surfactant surface excess, both in the experiments and in the simulations, allowing direct comparisons to be made. As a proof-of-concept a coarse-grained model of a light switching non-ionic surfactant bearing a photosensitive azobenzene group is considered at the air-water interface at 298 K. Coarse-grained molecular dynamic simulations are detailed based on the use of the SAFT force field with parameters tuned specifically for this purpose. An excellent agreement is obtained between the simulation predictions and experimental observations; furthermore, the molecular model allows the rationalization of the macroscopic behaviour in terms of the different conformations of the cis and trans surfactants at the surface.
authors.affiliation: Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK.
authors.forename: Carmelo
authors.initials: C
authors.lastname: Herdes
authors.affiliation: Department of Chemical and Biomolecular Engineering, North Carolina State University, NC 27695-7905, USA.
authors.forename: Erik E
authors.initials: EE
authors.lastname: Santiso
authors.affiliation: Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan.
authors.forename: Craig
authors.initials: C
authors.lastname: James
authors.affiliation: School of Chemistry, University of Bristol, Bristol BS8 1TS, UK.
authors.forename: Julian
authors.initials: J
authors.lastname: Eastoe
authors.affiliation: Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK. Electronic address: e.muller@imperial.ac.uk.
authors.forename: Erich A
authors.initials: EA
authors.lastname: Müller
mesh_terms: D001391:Azo Compounds / Q000737:chemistry*; D008027:Light; D008823:Micelles; D008956:Models, Chemical; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D012996:Solutions / Q000737:chemistry; D013500:Surface Tension; D013501:Surface-Active Agents / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D001391:Azo Compounds; D008823:Micelles; D012996:Solutions; D013501:Surface-Active Agents; D014867:Water; C009850:azobenzene
keywords: Amphiphiles; CMC; Coarse graining; Complex fluids; Molecular dynamics; Molecular simulation; SAFT; Surface tension