Wetting nonpolar surfaces

In simple systems such as nonpolar films wetting on surfaces, the van der Waals forces play a dominant role, but in more complex systems long-range electrostatic forces are also involved. 

The heat values are markedly higher for the polar solid immersed in polar liquids they also vary considerably with the functional group of the liquid. For Graphon, however, the heats are almost unaffected by the structural features of the wetting liquid. This nonpolar solid, despite the presence of a small amount of hydrophilic sites on its surface 0), interacts with the liquids primarily through London dispersion forces. Because of the additive nature of these forces, each adsorbed molecule tends to lie flat on such a surface 40). In the case of a polar molecule the functional group is oriented somewhat away from the nonpolar surface toward the liquid. 

Healy et al. (134) studied experimentally the heats of adsorption of many polar and nonpolar gases on polar and nonpolar surfaces by means of their heats of immersion. It was found that the heat of immersion of rutile on a series of straight-chain compounds was a linear function of the dipole moment of the wetting liquid. In a later article (135)- this work was extended and it is shown that nearly the entire heat effect on immersion of the clean solid surface is due to adsorption of molecules in the first layer. From the slope of the line, giving the values found for the net heat of adsorption as a function of the dipole moments, the average field strength, F, of rutile can be found by means of Eq. (22). The experimental value found by these investigators is... 

Modified Surfaces. It is frequently desirable to change the surface of a polymer. Nonpolar surfaces of plastics are characterized by static electricity buildup, non-wetting, poor adhesion, low printability, and poor dyeing. These disadvantages can be overcome by grafting polar vinyl monomers upon the surface by irradiation. A. S. Hoffman describes radiation grafting of polyelectrolytes upon nonpolar surfaces, and A. Chapiro and co-workers discuss radiation grafting of acrylic acid and vinyl pyridene upon Teflon films. 

On nonpolar surfaces, any structural or other factor that results in a decrease in yM (Chapter 5) decreases the contact angle and improves wetting. Thus, the addition of a water structure-breaking additive (/V-methylacetamide), which increases the surface tension of an aqueous solution of sodium dodecyl sulfate, causes an increase in its contact angle on polyethylene, whereas the addition of a (Water structure promoter (fructose, xylose), which decreases the surface tension of the surfactant solution, reduces its contact angle (Schwuger, 1971). 

Hydrophobic Possessing nonpolar surfaces that are not wetted by water literally, water hating. 

FIGURE 17.10. The critical surface tension of wetting, ctc, is usually found to be consistent for a homologous series of hydrocarbon liquids (curve 1), a series of alkyl halides (curve 2), and miscellaneous polar liquids (curve 3) on a nonpolar surface such as Teflon. Results on polar surfaces are less unequivocal, but still can be useful. 

When one considers the effects of low-molecular-weight surfactants on wetting, it is helpful to divide the subject into two regimes the effect on nonpolar surfaces and that on polar surfaces the reason is that, for nonpolar surfaces, it is often possible to assume that adsorption at the S2 interface will be negligible. If surfactant adsorption at the S2 interface is small (e.g., F2 0), as is usually observed for the situation where fluid 2 is air, Equations (17.41) and (17.42) indicate that... 

Reversed-phase chromatography The stationary phase here is nonpolar in most cases it is derivati-zed silica that carries Cig (i.e., C18H37) or Cg (i.e., CgHiy) groups. The mobile phase is polar, in most cases a mixture of water (or buffer solution) with methanol, acetonitrile, or tetrahydrofuran. Such an eluent cannot wet the surface of the stationary phase and the solutes are retained owing to an energy gain... 

Polyolefins, such as polyethylene and polypropylene, are nonpolar polymers. This means that any polar materials will not easily wet the surface of (adhere to) these polymers. One area where this has important implications is in the printing conversion process. Inks cannot absorb into most films because they are typically nonporous. The only way the ink will stay on the surface of the film is if it wets the polymer rather than puddle up. To promote the ink adhering to the surface, it becomes necessary to treat the film to make it more polar. 

Surfactants, which are also called surface active agents, reduce the surface tension of water, allowing it to "wet nonpolar substances such as grease and oil. Surfactants are able to do this because they have both a nonpolar end and a polar end. 

In alcohol, which wets both surfaces, particles do not adhere to each other. The hydrophilic areas are wetted by adsorption of the OH ends of alcohol molecules. The hydrophobic areas are wetted by the hydrocarbon groups of alcohol (or other molecules with polar and nonpolar segments). 

This equation is derived by Blake for the case of wetting of nonpolar surface by aqueous solutions of surfactants [37]. Equation (34) has been used to determine the adsorption of... 

Lower effective viscosity at a liquid/liquid interface has been correlated experimentally with spherical molecular shapes" and theoretically with high surface tension. " A local dip in density is also a feature of liquids that partially wet hydrophobic surfaces." These observations may explain the reduced effective viscosity experienced by the nonpolar solute at the water/CCl4 interface, but, this remains an open issue, as the X-ray reflectivity measurements... 

Even after organic modification of the clays, polypropylene does not wet the surface of clays because it is nonpolar. It is necessary to blend in a functionalized polymer such as maleated polypropylene (PP-g-MA) that wets the modified clay surface more readily and is also miscible with the bulk polymer. Okada and coworkers were the first to produce polypropylene layered silicate nanocomposites by melt compounding the modified elay with PP-g-MA and PP. The progress made since then in preparing and characterizing polypropylene layered silicate nanocomposites is reviewed in this chapter. We discuss advances in formulations, preparation methods and characterization then proceed to effects of the dispersion state (intercalated vs. exfoliated) and of silicate loading on crystallinity, mechanical performance and other properties, and end with a summary of progress to date with these composites. All the results presented in this chapter refer to isotactic polypropylene nanocomposites with layered or smectite clays. 

Surfaces that do not have strong surface chemical bonds that were broken tend to be nonpolar and are not readily wetted. Substances such as graphite and talc are examples that can be broken along weakly bonded layer planes without rupturing strong chemical bonds. These solids are naturally floatable. Also, polymeric particles possess... 

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