Fluorocarbons, contact angle

The extensive use of the Young equation (Eq. X-18) reflects its general acceptance. Curiously, however, the equation has never been verified experimentally since surface tensions of solids are rather difficult to measure. While Fowkes and Sawyer [140] claimed verification for liquids on a fluorocarbon polymer, it is not clear that their assumptions are valid. Nucleation studies indicate that the interfacial tension between a solid and its liquid is appreciable (see Section K-3) and may not be ignored. Indirect experimental tests involve comparing the variation of the contact angle with solute concentration with separate adsorption studies [173]. 

Most LB-forming amphiphiles have hydrophobic tails, leaving a very hydrophobic surface. In order to introduce polarity to the final surface, one needs to incorporate bipolar components that would not normally form LB films on their own. Berg and co-workers have partly surmounted this problem with two- and three-component mixtures of fatty acids, amines, and bipolar alcohols [175, 176]. Interestingly, the type of deposition depends on the contact angle of the substrate, and, thus, when relatively polar monolayers are formed, they are deposited as Z-type multilayers. Phase-separated LB films of hydrocarbon-fluorocarbon mixtures provide selective adsorption sites for macromolecules, due to the formation of a step site at the domain boundary [177]. 

The critical surface tension has been evaluated by means of advancing contact angle measurements for water, various hydrocarbon liquids and for plasma polymerized fluorocarbon films " " . The most comprehensive, however, is that of Yasuda... 

The susceptibility of a building exterior material to soiling is closely related to its contact angle with water. The material used on the outside wall of a building is actually more like to be soiled if it is more water-repellent. Thus, plastic is more likely to be soiled than sheet glass or tiles. A water-repellent material like a fluorocarbon plastic is the most likely to be soiled. A highly... 

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. 

Figure 9. Cosine of contact angles of hydrogen-bonding liquids on fluorocarbon deposits on polyethylene before extraction. Power, 50 W time, 1 min. (1) Hexafluoropropylene (2) hexafluoroace-tone (3) hexafluoroethane (4) glow discharge treated polyethylene without fluorocarbon.

Contact angle data, and the surface tensions of the test liquids, were taken from the work of Zisman et aL [4, 5, 8,9,14,15,22]. Data for the computation of 4>, to use in Equation 11, were from the following sources Ionization energies were from Field and Franklin [ll] and (for fluorocarbons) from Reed [28]. Dipole moments were taken from Sm3dli [29] and from the compilation of Wesson [30]. Polarizabilities were computed from refractive indices by the formula... 

Similar findings arise in contact angle studies with solid fluorocarbons or adsorbed monolayers of fluorochemicals [1,10,12,23,24]. 

Figure 3. Contact angles of liquid fluorocarbons on solid fluorocarbons...

Paraffin Wax Surfaces. Figure 3 shows the variation of water contact angle with surface roughness for paraffin wax surfaces. As with the fluorocarbon waxes, the roughness scale is a relative one the positions of the various surfaces on this scale were obtained from microscopic examination of the surfaces. Air was observed under the water drops on surfaces D and E. 

Fluorocarbon and hydrocarbon modified PDMS surfaces are compared in Table 2. The contact angle data are obtained by the Good-Girifalco-Fowkes equation. It is striking that the hydrocarbon contact angle liquid gives better agreement with the JKR result for the hydrocarbon surface whereas the fluorocarbon liquid data better fit the fluorocarbon surface... 

When the membrane is placed in liquid water, rearrangement and a phase-transition occur. This could be due to surface rearrangements wherein the fluorocarbon-rich skin of the membrane is repelled from the interface between the water and membrane. What this means is that in order to minimize the energy of the system the side chains and backbone of the polymer reorient so that the chains are now arranged at the membrane/ water interface. This hypothesis agrees with the data that show that the water contact angle on the membrane surface becomes more hydrophilic after the membrane is placed in liquid water [32]. The presence of liquid water also results in the removal of a vapor-liquid meniscus, which could also aid in the above rearrangements [28]. 

---