Effect of temperature on the volumetric and thermoacoustic properties of polyoxyethylene (10) cetyl ether: C16E10 and polyoxyethylene (20) cetyl ether: C16E20 in aqueous solution

Density and speed of sound of aqueous solutions of the nonionic environmentally friendly surfactants polyoxyethylene (10) cetyl ether, Brij 56, C16E10 in the temperature range from 309.15 to 325.15 K at intervals of 4 K and polyoxyethylene (20) cetyl ether, C16E20, Brij 58 in the temperature range from 293.15 to 323.15 K were measured at intervals of 5 K using the vibrating tube technique. The apparent molar volumes and the apparent molar compressibilities were determined as a function of molality at each temperature from the experimental density and speed of sound data. The partial molar properties (volume and adiabatic compressibility) at infinitesimal concentration of the surfactants in aqueous solution were determined by extrapolation at each temperature. The hydration numbers of the surfactants in water were calculated from the acoustic properties, using the method proposed by Pasynski. The influence of temperature on the behavior of volumetric and acoustic properties is discussed in terms of solute-solute and solute-solvent interactions and structural effects according to Hepler's criteria. As expected, C16E20 was found to have a more hydrophilic character than C16E10. A qualitative explanation of the speed of sound behavior is proposed using the van der Waals equation, showing a close relation with the behavior of Pasynski's hydration numbers.