A small-angle neutron scattering study of biologically relevant mixed surfactant micelles comprising 1,2-diheptanoyl-sn-phosphatidylcholine and sodium dodecyl sulfate or dodecyltrimethylammonium bromide

Small-angle neutron scattering (SANS) has been employed to characterize mixed micelles comprising the surfactants 1,2-diheptanoyl--phosphatidylcholine (DHPC)/sodium dodecyl sulfate (SDS) and 1,2-diheptanoyl--phosphatidylcholine (DHPC)/dodecyltrimethylammonium bromide (DTAB) using the classical model of a micelle consisting of a hydrocarbon core surrounded by a polar shell. Various constraints are applied to the data fitting, specifically the volume of the hydrocarbon core using the aggregation number determined from time-resolved fluorescence quenching (TRFQ) and the degree of micelle hydration as determined from an electron paramagnetic resonance (EPR) experiment. The morphologies of the DHPC/SDS and DHPC/DTAB mixed micelles are largely invariant with composition-as might be expected given the similarity in the tail volumes of the respective surfactants, and the observed behaviour of the aggregation number-with a radius comparable to the fully extended length of a dodecyl chain, the longer of the two hydrophobic moieties. The shell thickness is also largely invariant with composition, indicating that the phosphatidylcholine headgroup lies flat at the interface. For both cases, the degree of counter-ion dissociation extracted from the fitting of the SANS data increases significantly on addition of DHPC, in a manner very similar to that observed for related binary surfactant mixtures, provided there is only a small change in micelle aggregation number (micelle curvature). The coincidence of the degrees of counter-ion dissociation for these quite different systems suggests a common underlying behaviour in which the nonionic species merely dilutes the surface charge associated with the ionic headgroups.