Soil, and substrates

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. 

Even the simplest detersive system is surprisingly complex and heterogeneous. It can nevertheless be conceptually resolved into simpler systems that are amenable to theoretical treatment and understanding. These simpler systems are represented by models for substrate-soHd soil and substrate-Hquid sod. In practice, many sod systems include soH—Hquid mixtures. However, removal of these systems can generally be analyzed in terms of the two simpler model systems. Although these two systems differ markedly in behavior and stmcture, and require separate treatment, there are certain overriding principles that apply to both. 

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. 

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,... 

Solid particles usually adhere to the hair surface through van der Waals or ionic forces [116-118], In water, the ease of removing these soils from a surface depends upon the relative affinities for each other of the water, soil, and substrate. These affinities are expressed as Wa, the work of adhesion, which is defined as the free energy change per unit area involved in removing an adhered solid particle from a surface (in this case the hair fiber) to which it is adhered. In water, Wa can be expressed as... 

Correlations between the chemical structure of the surfactant and its detergency are complicated by the differing soils and substrates to be cleaned, by the amount and nature of builders present, by the temperature and hardness of the water used in the bath, and by the different mechanisms by means of which soils are removed. Correlations are therefore valid only when many of these variables are specified and controlled. 

The general principle of surfactant action to remove soils from a surface is illustrated for oily soil in Fig. 3. When the soil is detached from the substrate, the surface between soil and substrate is destroyed, but two new surfaces are created—that between soil and bath and between substrate and bath. Detachment will be thermodynamically favorable when the sum of the interfacial tensions of the two new surfaces is less than that of the destroyed surface. A primary function of surfactants is to adsorb at the soil/bath and substrate/bath interfaces and reduce these interfacial tensions to make detachment more thermodynamically favorable. As surfactants adsorb and modify interfacial tensions, the contact angle (8 in Fig. 3) increases, and the contact area between soil and substrate decreases. As shown in Fig. 4, this results in the... 

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