The influence of temperature on the charging of Polytetrafluoroethylene surfaces in electrolyte solutions

The surface properties of Polytetrafluoroethylene (PTFE) depend on the pH and the ionic strength of the electrolyte solution. The point of zero charge (pHpzc) and the isoelectric point (pHiep) of PTFE were found to be 2.9 and 3.2 at 25 °C, respectively. The electrophoretic mobility at pH > pHiep indicates that the PTFE particles are negatively charged in the neutral pH region. In absence of surface functional groups, this observation can be explained by the distribution of OH− and H+ ions between the PTFE interface and the remaining solution. A thermodynamic model of ion distribution and the temperature dependency of the electroneutrality points which enables evaluation of the thermodynamic parameters is proposed. The enthalpies of interfacial reactions were indirectly determined by measuring the temperature dependency of the electroneutrality points by potentiometric mass titration and streaming current measurements. It was found that the exchange of the H+ and OH− ions between the interfacial region and bulk of the solution is an endothermic reaction. The specific enthalpy of dilution of PTFE dispersions was found to be pH dependent, with the lowest value at pH ≈ 3, i.e. in the zero surface charge region.