Determination of Solid Surface Tension by Contact Angle

Determination of Solid Surface Tension by Contact Angle 

Keywords Solid surface tension Solid surface energy Contact angle Work of adhesion Zisman method Surface tension component mefliod Fowkes method Owais-Wendt-Rabel-Kaelble mefliod Extended Fowkes mefliod Equation of state 

---

Example 15.1. Effect of contact angle measurements on the determination of solid surface tensions using the Owens-Wendt method. Based on data by Owens and Wendt (1969). 

In practice, the experimental procedure is quite long and fastidious referring to the successive determinations of the Gibbs energy of adhesion Aadh G ibetween a given solid and an apolar or polar liquid, which requires, in accordance with Eq. 6.17, measurement of the surface tension ylg, contact angle , and vapour adsorption isotherm (to calculate for each solid-liquid couple. To make matters worse, the very precise measurement of is possible only for atomically smooth surfaces. Finally, the additive approximation expressed by Eqs.6.9a, 6.9b and 6.12 is better suited to calculation of the enthalpy term than to that of the free energy, since the interactions may have both mechanical and entropic contributions [41]. 

Contact Angle Measurements. Axisymmetric drop shape analysis - profile (ADSA-P) The hydrophobicity/ hydrophilicity of a solid surface is usually expressed in terms of wettability, which can be quantified by contact angle measurements. ADSA-P is a technique to determine liquid-fluid interfacial tensions and contact angles... 

The contact angles of water and suitable solvents at the solid/liquid/gas interface allow the determination of the surface tension of solids as well as the dispersive (y ) and polar (yP) components. A semiquantitative prediction of the hy-drophilicity and hydrophobicity of polymer surfaces has already been achieved by contact angle determination with water [74,75]. 

TABLE 9 Surface Tension Components (in 10 N/m) of Various Solid Surfaces at 20C Determined by Contact Angle Measurements... 

Surface tension studies of the most common fluorosilicone, poly(3,3,3-trifluoropropylmethylsiloxane) (PTFPMS), give unexpected results. Compared with (PDMS), PTFPMS has a higher liquid surface tension, a similar critical surface tension of wetting, and a considerably lower solid surface tension, as determined by water and methylene iodide contact angles and the method of Owens and Wendt (67). These results are summarized in Table X (7, 67, 72-74, 76, 77), in which PTFPMS is compared with two other fluorocarbon polymers, poly(tetrafluoroethylene) (PTFE) and poly(chlorotrifluoroethylene) (PCTFE). PTFE behaves like PTFPMS, whereas PCTFE behaves like PDMS. 

The surface area expansion process in Figure 3.5 must obey the basic thermodynamic reversibility rules so that the movement from equilibrium to both directions should be so slow that the system can be continually relaxed. For most low-viscosity liquids, their surfaces relax very rapidly, and this reversibility criterion is usually met. However, if the viscosity of the liquid is too high, the equilibrium cannot take place and the thermodynamical equilibrium equations cannot be used in these conditions. For solids, it is impossible to expand a solid surface reversibly under normal experimental conditions because it will break or crack rather than flow under pressure. However, this fact should not confuse us surface tension of solids exists but we cannot apply a reversible area expansion method to solids because it cannot happen. Thus, solid surface tension determination can only be made by indirect methods such as liquid drop contact angle determination, or by applying various assumptions to some mechanical tests (see Chapters 8 and 9). 

---