Micellar Parameters of Aqueous Solutions of Tween 20 and 60 at Different Temperatures: Volumetric and Viscometric Study

Density, viscosity and speed of sound of aqueous solutions of nonionic surfactants such as polyoxyethylene (20) sorbitan monolaurate (Tween 20) and polyoxyethylene (20) sorbitan monostearate (Tween 60) at T = 293, 303 and 313 K are reported. From these measured values different parameters such as, for example, isentropic compressibility, molecular free length, acoustic impedance, primary hydration numbers and internal pressure have been calculated and employed to discuss molecular packing, structural alteration and molecular interactions. The variation in these parameters with temperature indicates that the mobility of surfactant molecules increases the disordered state of the liquid (surfactant + water) due to irregular packing of the molecules. Also, for Tween 20 solutions, more conversion to bulk water of the structured water molecules was observed, obtaining lower compressibilities and higher values of hydration numbers as well as internal pressure for a given T.