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 microemulsio ns of 19 different water-n-alkane-C(i)E(j) (n-alkylpolyglycolether) sy stems has been measured. All scattering curves exhibit a broad scatter ing peak which permits determining the characteristic length scale xi for bicontinueus structures at symmetric water and oil volume fraction s, 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 surfac tant volume fraction phi(c). Approximating the effective surfactant ch ain length delta by delta = v(c)/a(c), where a(c) and v are the area a nd 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 dif ferent 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 s cattering 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, respe ctively. However, it depends strongly, and nearly linearly, on the hea d group size j of the surfactant. Within experimental error it is desc ribed by a(c),= 29.3+6.20j (Angstrom(2)). (C) 1997 American Institute of Physics.