Small angle neutron scattering from micellar solutions of triton X-100

Abstract

Micellar solutions of non-ionic surfactant triton X-100 (8% by weight) show phase separation at cloud pointT _cp ~ 335 K. This paper reports results of small angle neutron scattering (SANS) experiments from this solution as a function of temperature between 298 and 332 K. The range of wave-vector transferQ, covered in these experiments is from 0.02 to 0.15 Å^-1. It is seen that as one approachesT cp, the neutron scattering cross section diverges in the region of lowQ (<Å^-1) while it is independent of temperature in region of largeQ(>0.06 Å^-1). We believe that the divergence of scattering at lowQ with an increase in temperature is because of changes in the structure factorS(Q) of the solution. The measured distributions have been analyzed using four different models for inter-micellar potential. The models used to calculate the structure factorS(Q) are (1) mean spherical approximation (MSA) with Yukawa tail for attractive potential, (2) MSA with an attractive square well potential, (3) random phase approximation (RPA) with an attractive square-well potential and (4) Sticky hard sphere model (attractive square-well potential with Percus-Yevick approximation). The strengths of the attractive potential required to fit the SANS data are (-6.6 to - 14.4)/kt for model (1), (- 6.6 to - 15.0)/kt for model (2), (- 3.8 to - 7.3)/k B T for model (3) and (-2 to -2.7)/kt for model (4). On the basis of reasonableness of the derived strength of the potential near the phase separation temperature and its relative temperature dependence, it is concluded that present data favour the Sticky hard sphere model.