Removal of Liquid Soil

Removal of Liquid Soil Removal of liquid (oily) soil by aqueous baths is accomplished mainly by a roll-back or roll-up mechanism in which the contact angle that the liquid soil makes with the substrate is increased by adsorption of surfactant from the cleaning bath. 

When surfactants of the proper structure are present in the bath, they will adsorb at the substrate-bath (SB) and liquid soil-bath (OB) interfaces in such a fashion (i.e., with the hydrophilic group oriented toward the aqueous bath) as to reduce jSB and Job, with consequent reduction in the work to remove the soil from the substrate. Reduction in ySB will also cause a decrease in cos 0 and an increase in 0, resulting in the observed roll-back of the liquid soil. Many investigators of oily soil removal, 

FIGURE 10-1 Contact angle at the bath-liquid soil-substrate junction. 

FIGURE 10-2 Complete removal of oil droplets from substrate by hydraulic currents (arrows) when 9 remains constant at 90°. Reprinted with permission from A. M. Schwartz, in Surface and Colloid Science, Vol. 5, E. Matijevic (Ed.), Wiley, New York, 1972, p. 212. 

In high-speed spray cleaning, a critical factor is the dynamic surface tension reduction of the surfactant solution (Chapter 5, Section IV), rather than its 

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Detergency, or the power of a detergent product to remove soil, depends on the ability of surfactants to lower the interfacial tension between different phases. This can be explained for a typical case where removal of liquid soil is aided by surfactant adsorption onto the soil and substrate surfaces from the cleaning bath (Figure 2) using Young s equation,... 

The phenomena at the liquid/liquid interface are of outstanding importance for the removal of oily soil from the surface. The interfacial tension is one of the decisive parameters in the rolling-up process. This parameter vary considerably, de-... 

The removal of liquid oily soils from surfaces is generally understood in terms of three basic mechanisms the roll - back of droplets of oily soil, the surfaces of which are modified by the presence of an adsorbed layer of surfactant direct emulsification of macroscopic droplets of soil and the direct solubilization of the oily soil into surfactant micelles or other interfacial phases formed (1-3). 

The importance of a surfactant - rich phase, particularly a lamellar one, to detergency performance was noted for liquid soils such as C16 and mineral oil (3.6). Videomicroscopy experiments indicated that middle phase microemulsion formation for C12E04 and Cjg was enhanced at 30 °C, while at 18 °C, oil - in - water, and at 40 °C, water - in - oil microemulsions were found to form at the oil - bath interface (3.6). A strong temperature dependence of liquid soil removal by lamellar liquid crystals, attributed to viscosity effects, has been noted for surfactant - soil systems where a middle - phase microemulsion was not formed (10). 

Since the 1990s enzyme mixtures have been commonly used in heavy-duty liquids. Most products contain a minimum of a protease for removal of proteinaceous soils and an amylase to facilitate starchy food-based soil removal. Some products contain lipases for degrading fatty or oily soils and cellulases to improve fabric appearance by cleaving the pills or fuzz formed on cotton and synthetic blends. 

Oily soils containing amphiphilic species, such as fatty acids or fatty alcohols, can also be removed from substrates as a result of the formation of liquid crystal or mesomorphic phases between the amphiphile and a detergent. The liquid crystals are then broken up by subsequent osmotic penetration by water [140-142], Removal of solid soils by mesophase formation can be accelerated by increasing the temperature. This has been reported for stearyl alcohol [143] and for lauric, palmitic, and stearic acids [128, 129] and is likely due at least in part to the increased penetration of the soils at higher temperatures [128,129,143],... 

Removal of solid soils by penetration without liquid crystal formation has been reported for tripalmitin, octadecane, and tristearin [143-145]. In these cases penetration of detergents occurred at crack and dislocation sites of soils. 

On the other hand, if the surfactant has appreciable solubility in both liquids, then very different factors may determine the value of the interfacial tension. Although low liquid-liquid interfacial tension is important in promoting emulsification (Chapter 8) and in the removal of oily soil by detergents (Chapter 10), advances in our knowledge of the factors governing the reduction at that interface stem from the intense interest in enhanced oil recovery by use of surfactant solutions. 

Unlike the experiments carried out below the cloud point temperature, appreciable solubilisation of oil was observed in the time frame of the study, as indicated by upward movement of the oil-microemulsion interface. Similar phenomena were observed with both tetradecane and hexadecane as the oil phases. When the temperature of the system was raised to just below the PITs of the hydrocarbons with C12E5 (45°C for tetradecane and 50°C for hexadecane), two intermediate phases formed when the initial dispersion of Li drops in the water contacted the oil. One was the lamellar liquid crystalline phase La (probably containing some dispersed water). Above it was a middle-phase microemulsion. In contrast to the studies below the cloud point temperature, there was appreciable solubilisation of hydrocarbon into the two intermediate phases. A similar progression of phases was found at 35°C using n-decane as the hydrocarbon. At this temperature, which is near the PIT of the water/decane/C Es system, the existence of a two-phase dispersion of La and water below the middle-phase microemulsion was clearly evident. These results can be utilised to optimise surfactant systems in cleaners, and in particular to improve the removal of oily soils. The formation of microemulsions is also described in the context of the pre-treatment of oil-stained textiles with a mixture of water, surfactants and co-surfactants. 

Fundamental principles leading to the removal of oily soil from the solid substrate by the so-called roll-up mechanism, in which liquid oil is displaced from the surface by the washing solution in the form of dispersed tiny droplets [60], are essentially the same as those evoked in the attachment of air bubbles onto a mineral surface. 

Figure 12.13 Mechanisms of liquid soil removal emulsification (a) versus roll-up (b) mechanism...

Water-insoluble liquid soils are commonly known as oily soils. Naturally occurring oily soils include hydrocarbons, saturated or unsaturated fatty acids, esters of fatty acids, and alcohols. Natural oily soils found on textiles are mixtures of oily components. Frequently, oil soils contain dispersed solid particulate matter (e.g., used motor oil). The most important properties of oily soils are their viscosity [1,2], polarity [3,4], and solubility in detergent solutions or dry-cleaning solvents. The removal of oily soil by detergency is facilitated by a low viscosity at the wash temperature. The polarity of soil affects adhesion of the soil on fibers, interaction with... 

The cleaning process proceeds by one of three primary mechanisms solubilization, emulsification, and roll-up [229]. In solubilization the oily phase partitions into surfactant micelles that desorb from the solid surface and diffuse into the bulk. As mentioned above, there is a body of theoretical work on solubilization [146, 147] and numerous experimental studies by a variety of spectroscopic techniques [143-145,230]. Emulsification involves the formation and removal of an emulsion at the oil-water interface the removal step may involve hydrodynamic as well as surface chemical forces. Emulsion formation is covered in Chapter XIV. In roll-up the surfactant reduces the contact angle of the liquid soil or the surface free energy of a solid particle aiding its detachment and subsequent removal by hydrodynamic forces. Adam and Stevenson s beautiful photographs illustrate roll-up of lanoline on wood fibers [231]. In order to achieve roll-up, one requires the surface free energies for soil detachment illustrated in Fig. XIII-14 to obey... 

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