Thermodynamic study of anionic surfactant Sodium Dodecyl Sulphate by ultrasonic method

Surfactants are larger volume chemicals used in detergents, household cleaning and personal care
products and to lesser extent in pesticides, herbicides, paints and plastics. Sodium dodecyl sulphate
selected for the present study is an anionic surfactant whose head group is having a negative charge. It is
highly effective surfactant and is used in any task requiring the removal of oily stains and residues.
Ultrasonic velocity measurements of SDS have been carried out at different temperatures (308°K to
323°K) to determine the CMC, soap-solvent interaction and various thermodynamical parameters. The
density and viscosity of aqueous solution of SDS from 2mM to 14mM concentration are measured using
10 ml specific gravity bottle and Ostwald viscometer. Thermodynamic parameters such as adiabatic
compressibility, Rao’s constant, Wada’s constant, van der Waal’s constant, internal pressure, free
volume, molar cohesive energy and Gibb’s free energy were computed. The internal pressure variation in
aqueous solution of SDS is sensitive in ultrasonic parameters of molecular interactions either due to
temperature variation or due to concentration variations. Various molecular interactions in these
solutions have been analyzed on the basis of variation of above parameters and eventually emphasizing
the possible molecular interactions in terms of structure making and structure breaking effects of SDS in
water. The results show that the aqueous solution of SDS behaves like an ionic substance and acts as a
structure maker at lower temperatures and becomes a structure breaker at higher temperatures. The
critical micellar concentration (CMC) of SDS is estimated to be 10mM. SDS can be interpreted as an
effective stain remover at higher temperatures and above CMC.