Effect of Temperature on the Critical
Micelle Concentration and Micellization
Thermodynamic of Nonionic Surfactants:
Polyoxyethylene Sorbitan Fatty Acid Esters
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Abstract
In this study, non-ionic surfactants, polyoxyethylene sorbitan fatty acid
esters (polysorbate) are chosen to examine the temperature effect on the CMC over a
wide temperature range. The enthalpy and entropy of micelle formation are evaluated
according to the phase separation model. The surface tension of solutions was
determined by means of Du Nöuys ring. The CMC values were taken from the sharp
breaks in the surface tension vs. logarithms of surfactant concentration plots. As the
surfactants’ chain length increases the CMC at a constant temperature decreases,
which is directly related to the decrease of hydrophilicity of the molecules. For each
surfactant, as the system temperature increases, the CMC initially decreases and then
increases, owing to the smaller probability of hydrogen bond formation at higher
temperatures. The onset of micellization tends to occur at higher concentrations as the
temperature increases. To evaluate the enthalpy of micellization, the CMCs are first
correlated by a polynomial equation. It is found that ΔG°m decreases monotonically
as the temperature increases over the whole temperature range. Both ΔH°m and ΔS°m
appear to be decrease monotonically with an increase in temperature. The
compensation temperature was found to be 42 oC by linear regression over the whole
temperature range and for all three surfactant systems together