Density and sound velocity measurements were performed on water solutions of the nonionic surfactant belonging to the family of the oligooxyethylene glycol (CiEj), with i = 12 and j ranging from 5 to 8 EO units. Up to a threshold concentration, the density (rho) and the compressibility coefficient (beta = rho(-1)c(-2)) measured as a function of the temperature cross the density and the compressibility coefficient of water at the temperatures (T-rho and T-beta) which depend on the surfactant. Such conditions are proper of ideal solutions; thus, our water-surfactant solutions can be analyzed in this context as ideal mixtures of the pure solvent and effective solutes. We microscopically model such effective solutes as hydrated monomers assembled in micellar aggregates. Under some assumptions on the volumetric properties of the hydrophobic micellar core, we experimentally determine the molar volume of the EO terminations at the micellar interface. By comparing these values with those obtained for solutions of water and PEG (a polymer composed by j EO units), we estimated the degree of hydration of the interfacial EO-water mixture. The compressibility K = betaV obtained from our model embodies a contribution due to the derivative of the number of the hydrated water molecules with respect to the pressure. Such derivative is characteristic of a solution in osmotic equilibrium with its solvent and, in the case of the micelles, can be rationalized on the bases of the "small system thermodynamic" theory. By comparing the compressibility of the micellar interface with that of an equivalent PEG-water solutions, we estimate the osmotic contributions for each of the investigated surfactant. The threshold concentrations, below which these solutions mix ideally, reflect an effect due to the different sizes of the solute aggregates and the solvent molecules.