  By means of density, dielectric spectroscopy and sound velocity measurements
we perform a systematic study on the polyoxyethylene $C_{12}E_{6}$ nonionic
surfactant solutions as a function of temperature and concentration. Both
density and sound velocity data, at about $34^{\circ}C$, coincide with the
value obtained for pure water. Above this temperature the density is lower than
the water density whereas below it is greater, the opposite happens for the
compressibility. Combining results from these different techniques we tempt a
very detailed description of the evolution of the micellar interfacial
properties with temperature. It is well known that nonionic surfactant
solutions dehydrate, growing temperature. Our results indicate that this
process is associated with a continuous change in the polymer conformation and
in the local density of the micellar interface.
