Calorimetric Investigation of the Interaction between Lithium Perfluorononanoate and Poly(ethylene glycol) Oligomers in Water

Abstract

The interaction of lithium perfluorononanoate (LiPFN) with poly(ethylene glycol) (PEG) molecules of different molecular weights (300 < MW < 20000 Da) has been investigated in water at 298.15 and 308.15 K by isothermal titration calorimetry (ITC). Density, viscosity, and conductivity measurements were also performed at 298.15 K. The aggregation process of this surfactant on the PEG polymeric chain was found to be very similar to that exhibited by cesium perfluorooctanoate (CsPFO) and appears to be consistent with the necklace model. ITC titrations indicated that a fully formed LiPFN micellar cluster can be wrapped by a PEG chain having a molecular weight (MW) of approximately 3200 Da, longer than that required by the shorter perfluorooctanoate (MW approximately 2600 Da), and also suggested a stepwise mechanism for the aggregation of successive micelles. Viscosity data indicate that the formation of polymer-surfactant complexes between PEG and LiPFN involves a conformational change of the polymer. The aggregation of preformed micelles of LiPFN or CsPFO or SDS on the PEG polymeric chain always gives rise to further stabilization.

Physicochemical Studies on the Interfacial and Micellization Behavior of CTAB in Aqueous Polyethylene Glycol Media

“…6. Similar observation was also reported by Gianni et al [70]. We have already mentioned in the introduction section that the lower molecular weight PEGs behave as cosolvents and higher PEG homologs behave like polymers in addition to their solvation behavior.…”

“…In our present study we aimed to investigate the micellization and interfacial behavior of CTAB in the presence of PEGs of different molar mass and so, in the studied concentration range of CTAB, we observed no such surfactant-polymer complexation as reported by others. For the previously reported studies by Gianni et al [44] the concentration range studied was as high as 150 mmol dm -3 , however, in the present study the concentration of CTAB was confined to within the range of 0-2.0 mmol dm -3 .…”

“…However, our present results show that PEGs could become accommodated around the CTAB micelles. Although, no direct or indirect associative interactions are involved between PEGs and CTAB micelles, our results suggest that PEG could wrap around the micelles as it happens in the case of perfluoroalkanoate-PEG systems [70]. However, further studies such as electron microscopy, small angle neutron scattering, etc., are warranted to confirm it.…”

“…The standard state is a hypothetical state where mole fraction of CTAB was considered to be unity at its CMC. Change in the standard free energy of interfacial adsorption (DG ad 0 ) for CTAB at its CMC can be obtained using following expression: [70] …”

“…EG oligomers are not expected to be involved in any other associative interaction with surfactants. However, the cationic micelles can be further stabilized by higher EG homologs as they could wrap around micellar aggregate [36,[44][45][46]. To the best of our knowledge, no systematic study on micellization of CTAB in the presence of EG oligomers of varying molecular weight has been reported.…”