Adsorption detergent, 519 surfactant

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. 

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. 

The adsorption of surfactant mixtures on metal oxide surfaces (e.g., minerals) from aqueous solutions is an important process in such applications as enhanced oil recovery and detergency. Since surfactants used in real-world applications are almost always mixtures,... 

Adsorption of surfactants has developed into a domain on Its own. So far we have only introduced the non-ionic part (sec. 2.7d). Abundant applications are found in detergency, flotation, enhanced oil recovery, drug administration and other pharmaceutical purposes, paints, cosmetics, ceramic materials and the stabilization of suspensions in general. 

The adsorption of surfactants onto solid surfaces is important with respect to their detergent properties, their use as wetting agents in solid pharmaceutical dosage forms, and as stabilizers for suspension formulations. The mode of action of surfactants in each of these systems is discussed further below. 

Non-specific interactions between monolayer surfaces and solute molecules were studied by Whitesides and coworkers. They found that sodium dodecyl sulphate and other detergents reversibly adsorb from aqueous solutions on the surface of hexadecanethi-olate monolayer on the gold surface. Adsorption is well described by the Langmuir isotherm. Formation of full monolayers was observed at concentrations close to the critical micelle concentration for the surfactant. Monolayers presenting ethylene glycol groups at the interface, HS(CH2)ii(0CH2CH2)60H, resist adsorption of surfactants at these concentrations . 

The adsorption of surfactant molecules at an interface decreases the interfacial tension. The decrease of the water-air interfacial tension explains the foaming property. The addition of a surfactant into a biphasic liquid system renders emulsion formation possible by the decrease of the liquid-liquid interfacial tension. Wetting and detergency are two important... 

Some metallic detergents, such as sulfonates, in particular the overbased sulfonates, function as a rust inhibitor by forming a film through adsorption of surfactant molecules or neutralizing the acidic materials, preventing them from attacking the metal surfaces. Acidic materials are commonly produced by incomplete combustion of fuels or oil oxidation. 

Proteins are themselves surface-active compounds with an amphiphilic nature. The interfacial behavior of proteins is different from that of low-molecular-weight amphiphiles with a simple structure, namely, detergents, because proteins are highly complex polymers made up of a combination of 20 different amino acids (this point is described in detail in Chapter 3 of this book). Normally, proteins take on the folded compact structure, in which nonpolar amino acid residues are located in the interior and hydrophilic residues are exposed to molecular surfaces. Since hydrophobic interactions play dominant roles in the adsorption of surfactants to the air-water and oil-water interfaces, such a native structure of proteins should be modified to make fiiU use of the surface activity of proteins [1]. 

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]. 

Adsorption of surfactants on to solid surfaces is important in many of their applications, for example in detergency, when a dirt particle is surrounded by adsorbed surfactant molecules. It is also crucial to the solubilization of solid materials, for example latex and pigment particles in paints. 

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... 

Clearly, the extent and orientation of adsorption of surfactant molecules onto a solid substrate are of primary importance to their action in the detergency process. Therefore, any alterations in molecular structure that affect such adsorption... 

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. 

The analytical methods for a-sulfo fatty acid esters reported in the literature deal with the determination of the surfactants in different matrices like detergents or product mixtures from the fabrication. The methyl esters of a-sulfo fatty acids can be separated from a mixture of different surfactants together with sulfonated surfactants by adsorption on an anionic exchanger resin such as Dowex 1X2 or 1X8. Desorption from the exchanger resin is successful with sodium hydroxide (2%) in a 1 1 mixture of isopropanol and water [105]. 

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