Detergency mechanisms

Of relevance is Young s equation, which relates the interfacial tension between the surfactant solution and oil (yow), oil and solid substrate (yos) and surfactant solution and solid substrate (yws) to the equilibrium contact angle 0  

Several factors have been studied with regard to their eflfect on the emulsification mechanism for the removal of mixtures of mineral oil and polar organic alcohols or acids from polyester. The rate of emulsification of mineral oil/oleic acid mixtures from polyester films was found to change as the oleic acid content was varied. Other factors such as electrolyte concentration and temperature were also found to have large effects on the rate of soil removal by this mechanism. 

Direct solubilization of oily soils into surfactant micelles can occur to a significant extent if a large excess of surfactant relative to oil is present and if the surfactant is above its critical micelle concentration (CMC). The solubilization of very small oil drops from polymer fibers has been visualized for a variety of nonpolar oils representative of liquid laimdry soils.  

Another mechanism of oily soil removal involves the formation of liquid crystalline phases at the detergent solution/oil interface. After formation the intermediate phase is broken off by agitation and emulsified into the aqueous solution, allowing fiesh contact of the remaining soil with the detergent solution. 

Soil solubilization rates are often enhanced when surfactant-rich phases, either isotropic or liquid crystalline in nature, are present in the washing solution. Such phases exist, for instance, when nonionic surfactants are above their cloud points. These phases can either solubilize oily soils directly or interact with soil to form intermediate surfactant-rich phases such as microemulsions containing large amounts of oil. Under favorable conditions, the intermediate phases can be emulsified into the washing bath. 

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Fig. 12-1. Polypropylene is widely used for housewares and appliances since it will withstand hot water, detergents, mechanical abuse and is low in cost. This injection-molded washing machine agitator is a typical heavy-duty application served well by polypropylene. (Courtesy Hercules, Inc.)...

Diesel fuels, like gasoline, are formulated with additives that affect the process of combustion, in this case to improve the cetane number. Diesel fuels also contain detergents for irijection systems as well as compounds for improving the fuel s low tempierature rheology. Finally, decreasing particulate emissions is a problem of increasing concern, but the mechanism of action to promote this effect is not clearly understood. 

Anionic Surfactants. PVP also interacts with anionic detergents, another class of large anions (108). This interaction has generated considerable interest because addition of PVP results in the formation of micelles at lower concentration than the critical micelle concentration (CMC) of the free surfactant the mechanism is described as a "necklace" of hemimicelles along the polymer chain, the hemimicelles being surrounded to some extent with PVP (109). The effective lowering of the CMC increases the surfactant s apparent activity at interfaces. PVP will increase foaming of anionic surfactants for this reason. 

Although it is impossible to Hst all the practical detersive systems that might be encountered, a large proportion fall in a small number of classes. This classification disregards surfactant stmcture and type of substrate (fibrous or hard surface) and is restricted to a consideration of the soil present on the substrate, the mechanical action employed, the bath ratio, and the detergent used. Some of the more commonly encountered detersive systems are classified on this basis in Table 1. 

System Soil Mechanical action Bath ratio Detergent... 

Detergency is mainly affected by the concentration and stmcture of surfactant, hardness and builders present, and the nature of the soil and substrate. Other important factors include wash temperature length of time of washing process mechanical action relative amounts of sod, substrate, and bath, generally expressed as the bath ratio, ie, the ratio of the bath weight to substrate weight and rinse conditions. 

Adsorption of bath components is a necessary and possibly the most important and fundamental detergency effect. Adsorption (qv) is the mechanism whereby the interfacial free energy values between the bath and the soHd components (sofld soil and substrate) of the system are lowered, thereby increasing the tendency of the bath to separate the soHd components from one another. Furthermore, the soHd components acquire electrical charges that tend to keep them separated, or acquire a layer of strongly solvated radicals that have the same effect. If it were possible to foUow the adsorption effects in a detersive system, in all their complex ramifications and interactions, the molecular picture of soil removal would be greatly clarified. 

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