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Thermodynamics of polytetrafluoroethylene/aqueous electrolyte solution interface

Polytetrafluoroethylene (PTFE) is an example of a highly hydrophobic material which does not bear surface functional groups and therefore should neither chemically react with water molecules nor with ions from an aqueous electrolyte solution. However, due to the hydrophobicity water dipoles are repelled from the surfaces and orientation of water molecules is less random than in the bulk of the solution. This ordering and distribution of water molecules and ions in the vicinity of the surface cause the formation of an electrical interfacial layer (EIL) [1]. It was found for all hydrophobic materials that the electrophoretic mobility, and thus the surface charge and surface potentials, are pH dependent [2]. The value of isoelectric point was found to be in the acidic region, between pH 2 and 4. The origin of electrical charge at a hydrophobic material/aqueous water solution interface, and pH dependency have been a subject of numerous debates in the recent years and is still not solved [3]. The purpose of this research is to establish a model to evaluate the thermodynamic parameters for surface charging of hydrophobic materials. In this respect, the enthalpy of surface reactions was directly determined by means of calorimetric experiments and indirectly by measuring the temperature dependency of the electroneutrality point by potentiometric mass titration and streaming current measurements.

Polytetrafluoroethylene, ion distribution, electrical interfacial layer, enthalpy of surface reactions, point of zero charge, isoelectric point

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