A small-angle neutron scattering study of the ethyl (n-octyl) phosphate micelles in water

Mixed-double chain anionic surfactants, barium- and lithium-salts of ethyl(n-octyl) phosphate (EOP), which are asymmetric in the molecular shape, and a series of identical chain di-n-alkyl phosphate lithium salts have been synthesized. The limiting partial molar volume of a PO₄^- group (23.43 ± 0.41 cm³mol^-1) for use in small-angle neutron scattering analysis was determined by density measurements of a series of identical chain di-n-alkyl phosphate lithium salts. For lithium EOP-D₂O system, a critical micellar concentration (2.3 wt%) was determined by ³¹P NMR spectra. The micellar shape and size in the EOP-water binary system has been investigated by using small-angle neutron scattering (SANS) spectra. It has been found that the micelles of barium EOP in water have the shape of a prolate spheroid and aggregation numbers (n) equal to 48 at 23° C and 52 at 50° C. For the lithium EOP-micellar system, it has been found that the minimum micelle with an aggregation number n = 21 is spherical and micellar growth and variation from the spherical to the prolate shape might occur with an increase in concentration above the CMC.