title: The effects of alkylammonium counterions on the aggregation of fluorinated surfactants and surfactant ionic liquids.
abstract: The effects of organic counterions with varying carbon number on surfactant aggregation have been analysed by coupling perfluorooctanoate surfactant anions with various alkylammonium counterions. Both the degree of substitution (primary to tertiary) and alkyl chain length (0-3 carbons) of the counterions were varied to provide a comprehensive matrix of geometries and lipophilicities. Surface activity was measured using pendant drop tensiometry, while temperature-controlled small-angle neutron scattering was used to probe changes in aggregation morphology. It was found that the use of such alkylammonium counterions resulted in a strong preference for bilayer formation even at low surfactant concentration (<2wt%), when compared to simple inorganic counterions such as sodium which favour near-spherical micelles. At increased temperatures, some counterions led to unique phase behaviour wherein a transition between two structurally different lamellar phases is seen, rationalised as a transition into a microscopic phase separation wherein a surfactant-rich lamellar phase coexists with a dilute micellar phase. The results indicate that aggregation is controlled by a delicate balance of counterion size, hydrophilicity and diffuseness of charge, providing new methods for the subtle control of surfactant solutions.
authors.affiliation: School of Chemistry, Monash University, Clayton, VIC 3800, Australia.
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Pottage
authors.affiliation: School of Science, RMIT University, Melbourne, VIC 3000, Australia.
authors.forename: Tamar L
authors.initials: TL
authors.lastname: Greaves
authors.affiliation: Bragg Institute, Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Garvey
authors.affiliation: School of Chemistry, Monash University, Clayton, VIC 3800, Australia. Electronic address: rico.tabor@monash.edu.
authors.forename: Rico F
authors.initials: RF
authors.lastname: Tabor
keywords: Aggregation; Fluorosurfactant; SANS; Surfactant ionic liquid