Determination of the structure of complexes formed by a cationic polymer and mixed anionic surfactants by small-angle neutron scattering

Aqueous solutions of sodium alkanoates (C7H15COONa, NaOct; C9H19COONa, NaDe ; C11H23COONa NaDod; C15H31COONa, NaPal) and mixtures of NaPal with NaDod, NaDe, and NaOct, as well as complexes formed by these surfactants with cati onic potato starch (CS), were investigated by small-angle neutron scatterin g (SANS). The results indicate that mixed NaPal/NaDod micelles (without add ed CS) are prolate ellipsoids with a degree of dissociation of 10-20%. NaPa l and NaOct do not form mixed micelles, the effect of NaOct an micelle form ation being akin to that of a simple electrolyte. In the NaPal/NaDe systems , which were studied at concentrations between the cmc's of NaPal and NaDe, it can be indirectly inferred from SANS measurements that some NaDe is inc orporated in the micelles. NaPal/NaDod systems were investigated at concent rations well above the cmc of the mixture. At such concentrations, the comp osition of the micelles is equal to the total composition of the surfactant in solution. SANS indicates that the CS molecules have helical conformatio n. When they associate with surfactants, more compact cylindrical aggregate s are formed, in which a core of surfactant is Surrounded by the helical ch ains of CS. In dilute solution, the surfactant monomers do not form micelle like aggregates. The structure of the CS/surfactant complexes is more simil ar to the inclusion complexes found in amylose-surfactant systems. In mixed systems, the longer chain surfactant is enriched into the surfactant/CS co mplex.