Small-Angle Neutron Scattering Study of the Structural Effects of Substitution of Tetramethylammonium for Sodium as the Counterion in Dodecyl Sulfate Micelles

Article abstract of DOI:10.1021/j100282a017

Small-angle neutron scattering (SANS) data have been obtained at 303 K for aqueous micellar solutions of CH3(CH2)11SO4Na (sodium dodecyl sulfate (SDS)) and CH3(CH2)11SO4N(CH3)4 (tetramethylammonium dodecyl sulfate (TMADS)) and their deuterated analogues CH3(CH2)10CD2SO4Na (D2-SDS) and CH3(CH2)11SO4N(CD3)4 (D12-TMADS).Results have been obtained for 0.4 mol ddm^-3 surfactant in various H2O/D2O mixtures and for TMADS as a function of surfactant concentration in D2O.The SANS data are well described by charged, monodisperse hard spheres interacting through a screened Coulomb potential.The asphericity and the polydispersity of the systems were estimated to be small.The deuterium for the deuterated surfactants lies mainly in the Stern region of the micelle and increases contrast between the hydrocarbon core and the water-saturated Stern layer.Therefore, these deuterated micellar systems were used to determine the radius of the dry core, which was then used in the determination of the structural parameters for the micellar systems.The degree of hydration was determined without resorting to the assignment of the values for the water of hydration for the individual ions present.It was found that TMADS micelles are smaller and have a higher charge, smaller degree of aggregation, and less but more deeply penetrating water than do SDS micelles.Both SDS and TMADS micelles had substantial amounts of hydrocarbon residing in the aqueous Stern layer.