Surface-active agents adsorption

These surface active agents have weaker intermoiecular attractive forces than the solvent, and therefore tend to concentrate in the surface at the expense of the water molecules. The accumulation of adsorbed surface active agent is related to the change in surface tension according to the Gibbs adsorption equation... 

The flux of surface-active agents from the surface into the bulk of the liquid may be controlled by the slower of the following processes 1) adsorption or desorption of surfactants at the surface or 2) diffusion of surfactants from the liquid bulk to the surface. Consequently, Levich evaluated the solution for a single drop... 

Adsorption is one of the primary components of surfactant effectiveness. Surfactants are adsorbed on interfaces of the aqueous solution. That may be another solution which is immiscible with water, a gas phase, or a solid surface. Solid surfaces in particular are significantly altered by the adsorption of surface-active agents. Hence this effect is used in many fields. 

The inhibition of Streptococcus mutans adherence to hydroxyapatite with combinations of alkyl phosphates and nonionic surfactants was tested. Seven alkyl phosphate derivatives and three nonionic surfactants were examined for their ability to inhibit the adherence of 3H-labeled cells of S. mutans to hydroxyapatite treated with buffer or parotid saliva. No compound by itself effectively hindered binding of bacteria to hydroxyapatite. A combination of certain of the alkyl phosphates, notably a disodium phosphate of 1-octadecanol, and nonionic surfactant at a 1 1 molar ratio gave a strong inhibition of S. mutans adherence. Treatment with this combination resulted in 98% reduction of adherence. Adsorption of the two types of surface-active agents alone and in combinations was studied using 14C-labeled agents. Electrophoretic measure-... 

Finally, one must realize that, whatever sophisticated electrodes and potentiometers one uses, their proper functioning may be disturbed by the adsorption of adventitious substances such as proteins or surface-active agents and by precipitation or film formation on the electrode surface, unless this is prevented by supersonic vibration or another cleaning procedure. 

Of special interest in liquid dispersions are the surface-active agents that tend to accumulate at air/ liquid, liquid/liquid, and/or solid/liquid interfaces. Surfactants can arrange themselves to form a coherent film surrounding the dispersed droplets (in emulsions) or suspended particles (in suspensions). This process is an oriented physical adsorption. Adsorption at the interface tends to increase with increasing thermodynamic activity of the surfactant in solution until a complete monolayer is formed at the interface or until the active sites are saturated with surfactant molecules. Also, a multilayer of adsorbed surfactant molecules may occur, resulting in more complex adsorption isotherms. 

DAF is used to remove suspended solids by decreasing their apparent density they then rise and float on the water surface. DAF is also used to remove soluble iron, VOCs, oils, and surface active agents by oxidation, air stripping, and surface adsorption. The flotation technology is becoming one of the most important technologies for groundwater decontamination, industrial effluent treatment, and water purification.58-6170... 

This is very similar to the flotation procedure described under basic physical treatments. In the case of foam fractionation, not only are the pollutants raised to the surface where they can be skimmed off, but a froth, like beer foam, is produced in which the pollutants become concentrated. The key to the process is the adsorption of the pollutants onto the surface-active agents that cause the froth to form. Sometimes a surfactant is added so that non-surface-active components can be removed. 

P. Somasundaran and K.P. Ananthapadmanabhan, "Physico Chemical Aspects of Adsorption on Surface Active Agents on Minerals", Croatica Chemica Acta, 1979, p. 67-86. 

If one adds an inorganic salt, such as NaCl, instead of detergent, then no foam is formed. Foam formation indicates that the surface-active agent adsorbs at the surface, and forms a TLF (consisting of two layers of amphiphile molecules and some water). This has led to many theoretical analyses of surfactant concentration (in the bulk phase) and surface tension (consequent on the presence of surfactant molecules at the surface). The thermodynamics of surface adsorption has been extensively described by the Gibbs adsorption theory (Chattoraj and Birdi, 1984). 

In addition to lowering surface tension, surface-active agents contribute to emulsion stability by oriented adsorption at the interface and by formation of a protective film around the droplets. Apparently, the first molecules of a surfactant introduced into a two-phase system act to form a monolayer additional surfactant molecules tend to associate with each other, forming micelles, which stabilize the system by hydrophilic-lipophilic arrangements. This behavior has been depicted by Stutz et al. ( ) and is shown in Figures 1-5. 

In any consideration of what is presently known regarding adsorption from aqueous solution it is necessary to distinguish between neutral species, simple ions, complex ions, surface active agents, polyelectrolytes, and charged and uncharged high polymers. Much of the data available in the literature relates to the adsorption of polymers and complex-... 

The adsorption of small ions (sulfate ions) at the surface of solutions of surface active agents (cationic, anionic, and non-anionic) was measured... 

There are differences in isotherm shape, and for DTAB the behavior is not amenable to a simple explanation. Of particular interest are plots of the amount adsorbed against the mean ionic activity of the surface active agent (including the counterion of the added electrolyte). In the case of DTAB all the data, including others at various salt concentrations up to 0.5M, lie on one line which, after an initial steep rise, is linear to the c.m.c. This indicates that for other than the initial strong adsorption at low concentrations (possibly because of specific interactions with the surface) the adsorption follows the law of mass action. For SDS a similar result is obtained except that positive deviations from the straight line occur below a — 4 X 10 3M for the cases (salt concentration < O.lAf) when there is a point of inflection in the isotherm. These deviations may reflect specific interactions of the DS" with the surface when the ions are adsorbed in parallel orientation. 

The adsorption behavior of AB- or ABA-type block copolymers in which block A is polyelectrolytic and block B hydrophobic is very interesting. As expected, these polymers serve as dispersants, micelle-forming agents and surface-active agents. 

Rosen, M.J. Adsorption of surface-active agents at interfaces the electrical double layer. In Surfactant and Interfacial Phenomena, 2nd ed., WUey New York, 1978, pp. 

Nearly all of the treatment processes in which fluids are injected into oil wells to increase or restore the levels of production make use of surface-active agents (surfactant) in some of their various applications, e.g., surface tension reduction, formation and stabilization of foam, anti-sludging, prevention of emulsification, and mobility control for gases or steam injection. The question that sometimes arises is whether the level of surfactant added to the injection fluids is sufficient to ensure that enough surfactant reaches the region of treatment. Some of the mechanisms which may reduce the surfactant concentration in the fluid are precipitation with other components of the fluid, thermally induced partition into the various coexisting phases in an oil-well treatment, and adsorption onto the reservoir walls or mineral... 

Kamegawa and Yoshida (612] studied the pH dependence of the adsorption of several surface-active agents by chemically modified carbons. The uptake of nonionic polyoxyethylene nonylphenyl ether (PO) was not affected by pH, but it deereased with increasing outgassing temperature of the carbon. The uptakes of both the cationic octadecyltrimethylammonium chloride (OT) and the anionic sodium dodecylbenzenesulfonate (DB) exhibited a shallow minimum at intermediate pHs. The authors concluded that the van der Waals force was dominant in the adsorption of PO, while electrostatic attraction and repulsion, respectively, contributed to the behavior of OT and DB. 

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