Calorimetric investigation of the aggregation of lithium perfluorooctanoate on poly(ethyleneglycol) oligomers in water

The aim of this work is to investigate the effect of the nature of the counterion on the thermodynamics of aggregation of an ionic perfluorosurfactant on poly(ethyleneglycol) (PEG) of varying molecular weight in aqueous solution. The physicochemical properties of micellization and aggregation on PEG of lithium perfluorooctanoate (LiPFO) were investigated at 298.15 K by microcalorimetry, electrical conductivity, and viscosity. They were then compared with the same properties measured for the CsPFO/PEG system. As in the latter system, when wrapping around LiPFO micellar clusters the PEG polymers undergo a conformational change and form a compact complex. This complex reaches a constant thermodynamic behaviour for molecular weights larger than ∼2400 Da, which nicely matches the value ∼2600 Da found for CsPFO-PEG aggregates. The electrostatic screen provided by the polymer chain practically cancels the differences in the thermodynamic properties of the free micelles. This leads to thermodynamic parameters of aggregation of the two salts on the polymers, which are practically independent of the nature of the counterion.