INTERFACIAL COMPOSITION AS A FUNCTION OF TEMPERATURE IN C12E6 AND C12E8 MICELLAR SOLUTIONS

In [CH3-[CH2]((i-1))-[O-CH2-CH2](j)-O-H] (CiEj) non-ionic surfactant s olutions, at different temperatures depending on the different i,j mon omer compositions, the micellar aggregates undergo a change in shape f rom sphere to rod. At higher temperatures a phase separation occurs. R ecently we have developed a method to investigate the interfacial comp osition of non-ionic micellar solutions by means of dielectric spectro scopy. We applied this methodology to C12E6 water solutions and in thi s work we compare the results with those obtained from C12E8 water sol utions. Differences are observed in the polyethyleneoxide (PEO) water composition at the interface for the two kinds of micellar aggregates. On increasing temperature to 40 degrees C, C12E8 dehydrates with a fa ster rate than C12E6, but in this temperature range C12E8 retains a sp herical shape, whereas C12E6, between 15 degrees C and 20 degrees C, u ndergoes the sphere-rod transition. In C12E8, at 40 degrees C the PEO composition at the interface is close to the one expected for a comple te dehydration. Above 40 degrees C the degree of hydration increases w ith temperature and micellar growth occurs. The phase separation seems to require a specific interfacial composition, despite the number of PEO units per monomer of the suspended surfactant. (C) 1998 Elsevier S cience B.V. All rights reserved.