The spreading of liquids on low energy surfaces. I. polytetrafluoroethylene

1. Methods of preparing surfaces of polytetrafluoroethylene (TFE) are described which permit obtaining reproducible and reliable measurements of contact angles with liquids. Polytetrafluoroethylene is found to be an ideal low energy surface for the study of the wetting relations of a solid with a wide variety of organic and inorganic liquids. 2. It is shown that all but the lowest-boiling liquids studied do not adsorb on TFE sufficiently from the vapor to produce a significant spreading pressure. Hence the πE term in the expression for the work of adhesion (WA) is negligible and WA = γLV(1 + cos θ). The calculated values of WA are found to be consistent and reasonable. 3. It is found that for each homologous series of liquids cos θ is a linear function of γLV. The constants of the resulting relations are given. From this WA − Wc can be expressed as a function of γLV alone. 4. The observations show that there is a critical surface tension (17.5–20.5 dynes/cm.) below which liquids wet TFE. 5. It is concluded that the corrected Young-Dupré relation coupled with the spreading coefficient permits a rational description of the results. The Doss and Rao relation for the surface condensation leads to such high values for high-boiling liquids as to make it unlikely that the theory is correct.