Detergency surfactant adsorption

Adsorption. Many studies have been made of the adsorption of soaps and synthetic surfactants on fibers in an attempt to relate detergency behavior to adsorption effects. Relatively fewer studies have been made of the adsorption of surfactants by soils (57). Plots of the adsorption of sodium soaps by a series of carbon blacks and charcoals show that the fatty acid and the alkaU are adsorbed independently, within limits, although the presence of excess aLkaU reduces the sorption of total fatty acids (58). No straightforward relationship was noted between detergency and adsorption. 

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 Gibbs equation relates the extent of adsorption at an interface (reversible equilibrium) to the change in interfacial tension qualitatively, Eq. (4.3) predicts that a substance which reduces the surface (interfacial) tension [(Sy/8 In aj) < 0] will be adsorbed at the surface (interface). Electrolytes have the tendency to increase (slightly) y, but most organic molecules, especially surface active substances (long chain fatty acids, detergents, surfactants) decrease the surface tension (Fig. 4.1). Amphi-pathic molecules (which contain hydrophobic and hydrophilic groups) become oriented at the interface. 

Surfactant adsorption on solids from aqueous solutions plays a major role in a number of interfacial processes such as enhanced oil recovery, flotation and detergency. The adsorption mechanism in these cases is dependent upon the properties of the solid, solvent as well as the surfactant. While considerable information is available on the effect of solid properties such as surface charge and solubility, solvent properties such as pH and ionic strength (1,2,3), the role of possible structural variations of the surfactant in determining adsorption is not yet fully understood. 

This brief review has attempted to discuss some of the important phenomena in which surfactant mixtures can be involved. Mechanistic aspects of surfactant interactions and some mathematical models to describe the processes have been outlined. The application of these principles to practical problems has been considered. For example, enhancement of solubilization or surface tension depression using mixtures has been discussed. However, in many cases, the various processes in which surfactants interact generally cannot be considered by themselves, because they occur simultaneously. The surfactant technologist can use this to advantage to accomplish certain objectives. For example, the enhancement of mixed micelle formation can lead to a reduced tendency for surfactant precipitation, reduced adsorption, and a reduced tendency for coacervate formation. The solution to a particular practical problem involving surfactants is rarely obvious because often the surfactants are involved in multiple steps in a process and optimization of a number of simultaneous properties may be involved. An example of this is detergency, where adsorption, solubilization, foaming, emulsion formation, and other phenomena are all important. In enhanced oil recovery. 

For optimum surfactant adsorption at solid/liquid interfaces, mixed micelles have more efficient packing, which in turn contributes to better detergency by lowering the CMC and interfacial tension. 

Muller M, Grosse I, Jacobasch HI, Sams P (1998) Surfactant adsorption and water desorption on thin cellulose films mtmitored by in-situ ATR FTTR spectroscopy. Tenside Surfactants Detergents 35(5) 354... 

Surfactant adsorption at solid surfaces is, in practice, exploited to facilitate detergency, control wetting and penetration of solutions, stabilize foams and emulsions, and collect minerals in flotation operations. From this list alone, it is clear that this field is of tremendous importance. Consequently, significant effort has been directed toward acquiring a better understanding of the adsorptive nature of different surfactant molecules at solid surfaces. ... 

The understanding of the interfacial behavior of aqueous surfactant solutions is a major issue in surface science both from a theoretical and from a technological point of view. On the one hand, the interpretation of several colloid phenomena requires detailed knowledge of the adsorption layer of the system [1] on the other hand, the performance of many commercial products and industrial technologies (e.g. detergents, pharmaceutical applications, food and mineral processing, oil recovery) [2] is based on the adsorption of surfactant molecules. This explains the widespread interest in surfactant adsorption studies and the fact that this phenomenon is still the subject of intensive experimental and theoretical investigation [3]. 

Figure 18 Illustration of configurations of three phases involved in detergency (solid-oil-wash-ing bath) and mineral flotation (solid-air-fiotation bath), both pertaining to wetting phenomena in the presence of surfactants, (a) Oily droplet and first 6i contact angle prior to surfactant adsorption and subsequent rollback when 9i > 90°. (b) Air bubble and relevant contact angle 62 that wUl be made greater than 90° upon surfactant adsorption, with resulting good adhesion of the air bubble to the surface.

The properties (e.g. cleaning and stabilizing capabilities) of surfactants depend on both solution properties (temperature, time, presence of salts and cosurfactants) and their own characteristics, especially CMC, the Krafft point and their chemistry. The surfactant chemistry and especially the balance between hydrophobic and hydrophilic parts is quantified using tools like the CPP or HLB (critical packing parameter, hydrophilic-lipophilic balance, respectively). For example, it is often observed that detergency increases with concentration especially up to CMC and is often best at CPP values around 1. We will meet the important concept of CPP again in Chapter 7 where we will see that surfactant adsorption on solid surfaces is connected to CPP. 

Long-term water-detergent (surfactant) interactions in systems where the water leak into the oil was stopped (or limited water presence occurred due to humidity condensation, which subsides when the oil becomes hot during the equipment exploitation) result in the formation of inverse micelles. It was also shown that the electrochemical electron-transfer reactions (either as direct redox charge transfer or through adsorption-mediated processes) for separated detergent and water are several orders of magnitude faster than the... 

The discussion above introduced some basic concepts related to the properties of fluid-fluid, and particularly liquid-vapor interfaces. The practical effects of surface tension lowering were not addressed because they generally appear in the context of phenomena such as emulsification, foaming, wetting, and detergency, to be discussed later. For further details on the subject of surface tension lowering and surfactant adsorption at fluid interfaces, the reader is referred to the works cited in the Bibliography. 

The adsorption of surface-active materials onto a solid surface from solution is an important process in many situations, including those in which we may want to remove unwanted materials from a system (detergency), change the wetting characteristics of a surface (waterproofing), control the triboelectric properties of a surface (static control), or stabilize a finely divided solid system in a liquid where stability may otherwise be absent (dispersion stabilization). In these and many other related applications of surfactants or amphiphilic materials, the ability of the surface-active molecule to situate itself at the solid-liquid interface and produce the desired effect is controlled by the chemical natures of the components of the system the solid, the surfactant, and the solvent. The following discussions summarize some of the factors related to chemical structures that significantly affect the mechanisms of surfactant adsorption and the orientation with which adsorption occurs. 

Thus, adding surfactants to minimize the oil-water and solid-water interfacial tensions causes removal to become spontaneous. On the other hand, a mere decrease in the surface tension of the water-air interface, as evidenced, say, by foam formation, is not a direct indication that the surfactant will function well as a detergent. The decrease in yow or ysw implies, through the Gibb s equation (see Section III-5) adsorption of detergent. 

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