Thermodynamics in micellar solutions: Confirmation of complex formation between sodium dodecyl sulfate and polyethylene glycol

The interactions between sodium dodecyl sulfate (SDS) and polyethylene glycol (PEG 10000) were investigated through density and heat capacity measurements at 298.15 K. From experimental data, apparent and transfer properties of solutes were deduced. The changes of these properties with the concentration of each solute are characteristic of the existence of preponderant hydrophobic interactions between SDS and PEG which lead to the formation of a rather stable complex corresponding to a ratio [EO]/[SDS] of about 2.3, [EO] being the concentration in repeat units of the polymer. When saturation of the polymer is attained, the properties of small adsorbed micelles are no more affected by the polymer in excess. While SDS is in excess, beyond polymer saturation, free normal micelles are identified in solution. This latter consideration is also verified when SDS concentrations are exceeding 0.2 mol . kg(-1) while the structure of micelles is changed in water-surfactant solutions. Our results show that until its complete saturation the polymer inhibits this transition, whereas at larger SDS concentrations, the free micelles in excess are able to undergo the postmicellar transition as observed in the absence of polymer.