Calorimetric and physico-chemical study of sulfobetaines micellization in the presence and the absence of polystyrene nanoparticles.

This work investigates thermodynamic and physico-chemical properties of two zwitterionic surfactants (n-alkyl-N, N-dimethyl-۳-ammoniopropanesulfonate, SB۳-۱۲ and SB۳-۱۴) in the presence and absence of polystyrene nanoparticles PS-NPs. The micellization process in water was studied using isothermal titration calorimetry (ITC) and dynamic light scattering (DLS) in the temperature range from ۲۹۳.۱۵ K to ۳۲۳.۱۵ K as well as by tensiometry at ۲۹۸.۱۵ K. For pure surfactant solutions, the increase in both temperature and the alkyl chain length leads to more negative values of Gibbs free energy of micellization (ΔG°mic), favoring the micellization. From the temperature dependence of micellization enthalpy (ΔH°mic), the values of micellization heat capacity changes (ΔC°p, mic) were found negative for the two surfactants relating to the removal of water accessible non-polar surfaces. DLS measurements showed that micelles aggregates size Dh are around ۵.۵ nm and ۶.۵ nm for SB۳-۱۲ and SB۳-۱۴ respectively with a pronounced hydrophobic effect as confirmed from tensiometry experiments. Furthermore, the aggregation numbers Nagg of SB۳-۱۲ and SB۳-۱۴ micelles were determined from isothermal titration calorimetry for the first time. The deduced values were in agreement with (DLS) results at high temperatures. In the presence of polystyrene nanoparticles (PS-NPs), the formation of micellar aggregates occurs by association between PS-NPs and surfactants at low concentration of surfactants as revealed from ITC and DLS analysis. These results suggest that the PS-NPs/surfactant association is based on hydrophobic interactions.

کلیدواژه ها:

Zwitterionic Surfactants ، aggregation number ، Hydrodynamic diameter ، critical aggregation concentration ، DLS ، ITC