title: Effects of bulk colloidal stability on adsorption layers of poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate at the air-water interface studied by neutron reflectometry.
abstract: We show for the oppositely charged system poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate that the cliff edge peak in its surface tension isotherm results from the comprehensive precipitation of bulk complexes into sediment, leaving a supernatant that is virtually transparent and a depleted adsorption layer at the air/water interface. The aggregation and settling processes take about 3 days to reach completion and occur at bulk compositions around charge neutrality of the complexes which lack long-term colloidal stability. We demonstrate excellent quantitative agreement between the measured surface tension values and a peak calculated from the surface excess of surfactant in the precipitation region measured by neutron reflectometry, using the approximation that there is no polymer left in the liquid phase. The nonequilibrium nature of the system is emphasized by the production of very different interfacial properties from equivalent aged samples that are handled differently. We go on to outline our perspective on the "true equilibrium" state of this intriguing system and conclude with a comment on its practical relevance given that the interfacial properties can be so readily influenced by the handling of kinetically trapped bulk aggregates.
authors.affiliation: Institut Laue-Langevin , 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France. campbell@ill.eu
authors.forename: Richard A
authors.initials: RA
authors.lastname: Campbell
authors.forename: Marianna
authors.initials: M
authors.lastname: Yanez Arteta
authors.forename: Anna
authors.initials: A
authors.lastname: Angus-Smyth
authors.forename: Tommy
authors.initials: T
authors.lastname: Nylander
authors.forename: Imre
authors.initials: I
authors.lastname: Varga
mesh_terms: D000327:Adsorption; D000388:Air; D003102:Colloids / Q000737:chemistry*; D009502:Neutrons; D011095:Polyethylenes / Q000737:chemistry*; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D012967:Sodium Dodecyl Sulfate / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D013500:Surface Tension; D014867:Water / Q000737:chemistry*
chemical_list: D003102:Colloids; D011095:Polyethylenes; D000644:Quaternary Ammonium Compounds; D014867:Water; C041004:poly-N,N-dimethyl-N,N-diallylammonium chloride; D012967:Sodium Dodecyl Sulfate