Surface charge at Teflon/aqueous solution of potassium chloride interfaces

The effect of potassium chloride and pH on the interfacial water layer around Teflon particles and flat Teflon surfaces was investigated. Electrokinetic measurements and potentiometric titrations were carried out as well as contact angle measurements for some selected pH values. The electrokinetic measurements at flat Teflon plates in contact with potassium chloride aqueous solution shows that the isoelectric point is at pHiep ≈ 3.7. The electrokinetic potential of Teflon depends on ionic strength. Potentiometric mass titrations of Teflon suspensions in different concentrations of potassium chloride show that the point of zero charge lies in the same pH region (pHpzc ≈ 3.5) as the pHiep. A minimum of the contact angle was observed in the acidic region, at pH ≈ 3, close to the pHpzc and pHiep. Interestingly, the calculated surface charge densities were found to be independent of the electrolyte concentration.

As a possible and widely brought forward explanation it is suggested that the concentrations of OH− and H+ ions within the interfacial water layer depend on pH and are not equal. All findings together support the hypothesis that the interfacial water layer around Teflon is the origin of the interfacial charge.