Soil removal nonpolar

Oil and grease are mostly long-chain hydrocarbons that are very nearly nonpolar. Our most common solvent is water, a polar substance that does not dissolve nonpolar substances. To use water to wash soiled fabrics, greasy dishes, or our bodies, we must enable the water to suspend and remove nonpolar substances. Soaps and detergents are emulsifying agents that accomplish this. Their function is controlled by the intermolec-ular interactions that result from their structures. 

Oily Soil Nonpolar soil has been found to be removed from hydrophobic substrates (e.g., polyester) more effectively by POE nonionics than by anionics (Fort, 1968 McGuire, 1975), and investigations of this type of soil removal have concentrated on the use of POE nonionics. POE nonionics have also been found (Rutkowski, 1971) to remove oily soils and prevent their redeposition at lower bath concentrations than anionics (i.e., nonionic surfactants are more efficient for these purposes than anionics). The greater efficiency of nonionics in soil removal is presumably due to their lower CMCs in the prevention of soil redeposition it is probably due to their greater surface coverage per molecule when adsorbed on substrate and soil. 

For commercial POE nonionics with different types of hydrophobic groups of approximately equivalent chain length and the same degree of oxyethylenation (9 mol EO), the order of decreasing nonpolar soil removal from polyester/ cotton was nonylphenol adduct > secondary Cn — C15 alcohol adduct > linear primary C12 — C15 alcohol adduct. This was the order of decreased effectiveness of equilibrium jow reduction and of reduced rate of yow reduction (Dillan, 1984). 

Nonionics have been shown also to be more effective than ionics in the removal of oily soil from relatively nonpolar substrates (polyester, nylon). On cotton, however, a relatively hydrophilic fiber, anionics can outperform nonionics in detergency, and both of these are superior to cationics (Fort, 1968). The effects here may be due to differences in the orientation of adsorption of the different types of surfactants on the different substrates. On nonpolar substrates and soils, POE nonionics are adsorbed (Chapter 2) from aqueous solution via dispersion forces or hydrophobic bonding with their hydrophobic POE groups oriented toward the adsorbent and their hydrophilic POE groups toward the bath. Adsorption of the surfactant in this fashion on the substrate lowers the substrate-bath interfacial tension jSB and facilitates soil removal (equation 10.3) adsorption in this fashion on both substrate and soil produces a steric barrier that inhibits soil redeposition. 

Comparable results were obtained in formulations containing sodium silicate as a builder together with (M-5% sodium tripolyphosphate, using 250 ppm hard water and a bath temperature of 49°C (Illman, 1971). A nonionic surfactant prepared by polyoxyethylenation of a C12-15 alcohol mixture with 9-11 mol of ethylene oxide generally showed similar detergency to an anionic prepared by sulfation of a Ci2 i5 alcohol mixture previously polyoxyethylenated with 3 mol of ethylene oxide at all percentages of sodium tripolyphosphate, and both were considerably superior to a linear tridecylbenzenesulfonate and a sulfated C16-I8 alcohol mixture. The nonionic was somewhat better than the sulfated POE alcohol for removing nonpolar fatty soil from Dacron-cotton permapress, and the reverse was true for the removal of polar soil from Dacron-cotton permapress and carbon soil from cotton, but similar results for the two surfactants were obtained for clay removal from both Dacron-cotton permapress and cotton, and polar and nonpolar fatty soil from cotton. 

Particulate soils arise from dust, dirt, soot, hydrocarbons, metal oxides and even from hair products based on materials such as silicas or aluminas from about 1pm to less than 0.1-pm particle size see Figure 5-3. The removal of particulate soil is not controlled by the hydrophilicity of the fiber surface. Particulate soil removal depends on the bonding of the particle to the surface, the location of the particle [14], and the size of the particle. Particle size is perhaps the most critical variable for the removal of particulates. As the particle size decreases, the area of contact with the fiber increases, making it more difficult to remove from the hair. At particle sizes of less than 0.1 pm, it is very difficult to remove material from hair surfaces by ordinary shampooing [15]. When the soil particle consists of nonpolar components, its adhesion depends mainly on Van der Waals forces (e.g., waxes or polymeric resins and dimethicone polymers and the molecular size and shape are critical to their removal). Unless very high molecular weights are involved, the removal of such soils is oftentimes easier than for cationic polymers where adhesive binding includes a combination of ionic and Van der Waals forces. 

Water-soluble soil (sodium chloride, sugar) appears to be removed by solubilization (Chapter 4, Section II) into free water in the interior of surfactant micelles in the solvent. Surfactant micelles in nonpolar solvents are formed with the polar heads oriented into the interior of the micelle. Water is added to the dry-cleaning solvent and is solubilized into the interior of these micelles. Some of this water in the interior is bound strongly to the polar heads of the surfactants in the interior of the micelle and some is essentially free water. Studies (Aebi, 1959) have shown that it is the free water that dissolves water-soluble soil rather than the bound water. In the absence of any free water in the solvent, water-soluble soil is not removed to any significant extent. The water-soluble soil appears to be removed from fibrous surfaces by a process involving hydration of the soil followed by solubilization (Monch, 1960 Rieker, 1973). 

Low bioavailability of hydrocarbon pollutants can limit the biodegradation by indigenous micro-communities in soils. CDs enhance desorption of the nonpolar contaminants from the solid surface and transfer them to the water-phase biofilms, where the hydrocarbon-degrading microbes work. Therefore, this special character of CDs and their derivatives can be used for enhanced removal of hydrocarbon contaminants from soil [92]. This is so-called CD-enhanced pump and treat technology for the removal of dense, non-aqueous phase liquid from the saturated soil. For example, HP- -CD solution was added into the source zone... 

Of particular practical importance is the primary mechanism of detergency for oily soils. In that case, the main role of the detergent solution is to displace or roll up the oily soil from the solid surface so that it can be more easily and completely removed from the surface by mechanical action. Obviously, for a nonpolar surface, such action must be limited by the above balance of adhesive and cohesive forces. Due to the complexity of the situation it is difficult to generalize as to how or even if, the addition of a surfactant will improve a given situation. (That point is discussed in the following section.) However, one can venture the following propositions ... 

In general, nonpolar (oily) soil is removed from hydrophobic substrates by polyethylene oxide nonionics better than by anionics. Also, nonionics are found to remove oily soils better than anionics at lower temperatures, due to lower CMC, and are generally better at preventing soil redeposition because of the greater surface area covered per molecule when adsorbed on substrate or soil. ... 

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