First Evaluation of the Thermodynamic Properties for Spheres to Elongated Micelles Transition of Some Propanediyl-α,ω-bis(dimethylalkylammonium bromide) Surfactants in Aqueous Solution

The apparent and partial molar enthalpies, apparent molar volumes, and adiabatic compressibilities at 298 K of the aqueous solutions of the cationic gemini surfactants propanediyl-α,ω-bis(octyldimethylammonium bromide) (8-3-8) and propanediyl-α,ω-bis(dodecyldimethylammonium bromide) (12-3-12) have been measured as a function of concentration. The trends of the partial molar enthalpies versus concentration are the first well documented thermodynamic evidence of sphere to rod transition in the micellar phase, involving a detectable quantity of heat, and allow the determination of the change in enthalpy associated with this transition. The changes in enthalpies upon micellization and for the sphere to elongated micelles transition, ΔH s → r, have been obtained from the experimental data by using a pseudo-phase transition approach: −1.5 kJ mol-1 for 8-3-8 and −3.9 kJ mol-1 for 12-3-12. No evidence of the above transition is found in the trends of volumetric properties versus m. The apparent adiabatic molar compressibilities for the compounds under investigation are also reported here for the first time: a negative group contribution for the methylene group is evaluated, when the surfactants are present in solution as a single molecule, reflecting its solvation structure. In the micellar phase, the −CH2− group contribution becomes positive. A value of 1.17 × 10-3 cm3 bar-1 mol-1 for the change in adiabatic molar compressibility upon micellization is obtained. The lower values of the methylene group contributions to the volumetric properties for the monomers support the hypothesis of partial association of the chains before the cmc.