The adsorption of cetyltrimethylammonium bromide and propanol mixtures with regard to wettability of polytetrafluoroethylene: II. Adsorption at polytetrafluoroethylene–aqueous solution interface and wettability

Abstract

Measurements of contact angles ( θ) of aqueous solutions of cetyltrimethylammonium bromide (CTAB) and propanol mixtures at constant CTAB concentration equal to 1 × 10 −5 , 1 × 10 −4 , 6 × 10 −4 and 1 × 10 −3 M on polytetrafluoroethylene (PTFE) were carried out. The obtained results indicate that the wettability of PTFE by aqueous solutions of these mixtures depends on their composition and concentration. They also indicate that, contrary to Zisman, there is no linear relationship between cos θ and the surface tension ( γ LV ), but a linear relationship exists between the adhesional ( γ LV cos θ ) and surface tension of aqueous solutions of CTAB and propanol mixtures. Curve γ LV cos θ vs γ LV has a slope equal −1 suggesting that adsorption of CTAB and propanol mixtures and the orientation of their molecules at aqueous solution–air and PTFE–aqueous solution interfaces is the same. Extrapolating this curve to the value of γ LV cos θ corresponding to θ = 0 , the value of the critical tension of PTFE wetting equal 23.4 mN/m was determined. This value was higher than that obtained from contact angles of n-alkanes on PTFE surface (20.24 mN/m). The difference between the critical surface tension values of wetting probably resulted from the fact that at cos θ = 1 the PTFE–aqueous solution of CTAB and propanol mixture interface tension was not equal to zero. This tension was determined on the basis of the measured contact angles and Young equation. It appeared that the values of PTFE–aqueous solution of the CTAB and propanol mixtures interface tension can be satisfactorily determined by modified Szyszkowski equation only for solutions in which probably CTAB and propanol molecules are present in monomeric form. However, it appeared that using the equation of Miller et al., in which the possibility of aggregation of propanol molecules in the interface layer is taken into account, it is possible to describe the PTFE–solution interfacial tension for all systems studied in the same way as by the Young equation. On the basis of linear dependence between the adhesional and surface tension it was established that the work of adhesion of aqueous solution of CTAB and propanol mixtures does not depend on its composition and concentration, and the average value of this work was equal to 46.85 mJ/m 2, which was similar to that obtained for adhesion of aqueous solutions of two cationic surfactants mixtures to PTFE surface.