Energitics of micellizaion of sodium n-dodecyl sulfate at physiological conditions using isothermal titration calorimetry

The micellization characteristics of sodium n-dodecyl sulfate (SDS) have been investigated by microcalorimetric technique at conditions close to the physiological ones. The thermodynamics of micellization were studied at 20, 25, 30, 35 and 40 °C in 50 mM HEPES buffer, pH 7.4 and 160 mM NaCl using isothermal titration calorimetric (ITC) technique. The calorimeter can operate in a stepwise addition mode, providing an excellent method of determination of critical micelle concentration (CMC) and enthalpy of demicellization (and hence micellization). It can as well distinguish between aggregating and non-aggregating amphiphiles (solutes) in solution. The dilution enthalpy (∆Hdil) was calculated and graphed versus concentration in order to determine the micellization enthalpy (∆Hmic) and CMC. In addition to the CMC and ∆Hmic, the effective micellar charge fraction (α) of the ionic surfactant micellization process can also be determined from ITC curves. The Gibbs free energy of the micellization (∆Gmic), entropy of the micellization (∆Smic), and specific heat capacity of the micellization (∆CP,mic) process have been evaluated by the direct calorimetric method (mass-action model) as well as by the indirect method of van’t Hoff by processing the CMC and α results of microcalorimetry at different temperatures. The differences of the results obtained by these two procedures have been discussed. The presence of NaCl (160 mM) in the solutions decreased the CMC of SDS. The enthalpy changes associated with micelle dissociation were temperature-dependent, indicating the importance of hydrophobic interactions. The ∆Gmic was found to be negative, implying, as expected, that micellization occurs spontaneously once the CMC has been reached. The values of ∆Gmic were found to become more negative with increasing temperature and the ∆Smic was found to decrease with increasing temperature in both models.