Binary Mixed Micelles of Polyoxyethylene (10) Stearyl Ether with Polysorbate 20 and Polysorbate 60: Thermodynamic Description

Comprehension of the nature of the micellisation process is of great importance for the characterisation of micelles and the assessment of their applications. In this research, the influences of temperature and the length of the hydrophobic chain on the micellisation process of polyoxyethylene (10) stearyl ether (PE10) with polysorbate 20 and polysorbate 60 were studied. Values of critical micelle concentrations (CMC) of the individual surfactants and of their binary mixtures were acquired by spectrofluorimetric measurements, in the temperature range 273.15–313.15 K, using pyrene as a probe molecule. Obtained CMC values served for the calculation of interaction parameters and excess Gibbs energies of the mixed micelles. It was found that in PE10—polysorbate 20 mixed micelles (due to a shorter hydrophobic segment of polysorbate 20 molecule) PE10 alkyl chains were more likely to assume globular conformations in the interior of micelles, in comparison to PE10—polysorbate 60 mixed micelles (in which hydrophobic segments had the same lengths). For each analysed binary system, interaction parameter decreased with the rise of temperature. It could be assumed that on higher temperatures the hydrophobic chains of studied surfactants were more likely to take globular conformations, having higher values of conformational freedom. This observation implied that the process of micellisation was followed by an increase in entropy. Accordingly, the excess Gibbs energy of micellisation was determined not only by entropic contributions, but also by enthalpic input.