A small-angle neutron scattering study of nonionic surfactant molecules at the water-oil interface: Area per molecule, microemulsion domain size, and rigidity

The small-angle neutron scattering (SANS) of bicontinuous microemulsions of 19 different water-n-alkane-C(i)E(j) (n-alkylpolyglycolether) systems has been measured. All scattering curves exhibit a broad scattering peak which permits determining the characteristic length scale xi for bicontinueus structures at symmetric water and oil volume fractions, i.e., phi=0.5. Various random models predict xi=a delta phi(1-phi)/phi(c). We find that xi is indeed inversely proportional to the surfactant volume fraction phi(c). Approximating the effective surfactant chain length delta by delta = v(c)/a(c), where a(c) and v are the area and the volume of the surfactant molecule, the numerical value for a is determined c to be a = 7.16, which is close to, but significantly different from those used in theoretical models. The head group area a(c) at the water-oil interface is obtained from the large q part of the scattering curves. It is found to be independent of i and k, the carbon numbers of the alkyl chain of the surfactant and of the alkane, respectively. However, it depends strongly, and nearly linearly, on the head group size j of the surfactant. Within experimental error it is described by a(c),= 29.3+6.20j (Angstrom(2)). (C) 1997 American Institute of Physics.