Solid-liquid interface surfactant adsorption

Dispersion properties can be modified by adsorption of surfactants at the solid-liquid interface. Surfactant adsorption can alter the dispersion properties by changing the van der Waals attraction, electrostatic repulsion, and the steric forces between the particles as discussed earlier. The extent of the modification depends on the adsorption density (surface coverage), packing and orientation of molecules at the interface, and the nature of charges on the molecule. Therefore, it is important to first discuss the adsorption process itself in terms of the dominant mechanisms and possible orientations. 

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. 

The formation of multilayer structure can be carried out by several ways 1) adsorption on liquid/liquid interface - Langmuir-Blodgett films [5,6] 2) adsorption on solid/liquid interface - alternate adsorption of oppositely charged polyelectrolytes (PE) and surfactants on flat surfaces or spherical particles [7,8], To control the process of multilayer systems formation, it is necessary to under-... 

When dealing with a foam, gas—liquid interfaces will be present in addition to solid—liquid and liquid—liquid interfaces. Surfactant adsorption at the gas—liquid interface is obviously required for foam formation and therefore cannot be considered a mechanism of surfactant loss. Because gas is always the nonwetting fluid, the presence of a gas phase is not expected to affect contact between the solid and the aqueous phase and is not likely to affect adsorption of a water-soluble surfactant at the solid—liquid interface. Limited data comparing surfactant adsorption from a foam with adsorption from a bulk liquid during flow through a sand pack have indicated that this is, indeed, the case (34). If surfactant adsorption at the gas—liquid interface were to affect adsorption at the solid—liquid interface, the effect would likely be a reduction in adsorption on the solid because of a reduced surfactant concentration in the bulk aqueous phase. 

Effect of Pol3rmer. In recent years, as described previously, much attention, both experimental and theoretical, has been focused on surfactant-polymer interaction in solution. Less experimental work, however, is done on the interaction between polyelectrolyte and surfactant of similar charge at the solid-liquid interface. Static adsorption experiments from the chemical literature indicate that the polymer does not affect the adsorption of the surfactant onto solid material as long as the surfactant concentration is above the CMC, apparently owing to the availability of sufficient surface sites for adsorption of the surfactant molecules [40, 41],... 

For the solid-liquid system changes of the state of interface on formation of surfactant adsorption layers are of special importance with respect to application aspects. When a liquid is in contact with a solid and surfactant is added, the solid-liquid interface tension will be reduced by the formation of a new solid-liquid interface created by adsorption of surfactant. This influences the wetting as demonstrated by the change of the contact angle between the liquid and the solid surface. The equilibrium at the three-phase contact solid-liquid-air or oil is described by the Young equation ... 

Taking Simultaneous Micellizadon and Adsorption Phenomena into Consideration In the presence of an adsorbent in contact with the surfactant solution, monomers of each species will be adsorbed at the solid/ liquid interface until the dual monomer/micelle, monomer/adsorbed-phase equilibrium is reached. A simplified model for calculating these equilibria has been built for the pseudo-binary systems investigated, based on the RST theory and the following assumptions ... 

Solid/liquid (S/L) interface, surfactant adsorption at, 24 138-144 Solid-liquid encapsulation process, 16 444 Solid-liquid equilibria (SLE), 22 302 strategic separation schemes and, 22 310-311t... 

Surface wave, 17 422. See also S-wave Surfactant adsorption, 24 119, 133-144 at the air/liquid and liquid/liquid interfaces, 24 133-138 approaches for treating, 24 134 measurement of, 24 139 at the solid/liquid interface, 24 138-144 Surfactant blends, in oil displacement efficiency, 13 628-629 Surfactant-defoamers surface tension, <5 244t Surfactant-enhanced alkaline flooding,... 

Somasundaran, P., T. W. Healy, and D. W. Fuerstenau (1964), "Surfactant Adsorption at the Solid-Liquid Interface - Dependence", J. of Physical Chemistry 68, 3562-3566. 

H.S. Hanna and P. Somasundaran, "Physico-Chemical Aspects of Adsorption at Solid/Liquid Interfaces, Part II. Berea Sandstond/Mahogony Sulfonate System", in Improved Oil Recovery by Surfactants and Polymer Flooding, D.O. Shah and R.S. Schecter, eds.. Academic Press, 1977, p. 253-274. 

This paper describes a study of the dispersibility of Graphon (graphitized Spheron 6) in aqueous solutions of sodium dodecyl sulfate (SDS) an dodecyl trimethylammonium bromide (DTAB), and its relation to the adsorption behavior of the surfactants at the solid/liquid interface, with a view to determine the controlling process in the dispersibility of these systems. 

In the various sections of this chapter, I will briefly describe the major characteristics of FT-IR, and then relate the importance of these characteristics to physiochemical studies of colloids and interfaces. This book is divided into two major areas studies of "bulk" colloidal aggregates such as micelles, surfactant gels and bilayers and studies of interfacial phenomena such as surfactant and polymer adsorption at the solid-liquid interface. This review will follow the same organization. A separate overview chapter addresses the details of the study of interfaces via the attenuated total reflection (ATR) and grazing angle reflection techniques. 

Surfactants at Interfaces. Somewhat surprisingly, the successes described above in the in-situ studies of protein adsorption have not inspired extensive applications to the study of the adsorption of surfactants. The common materials used in the fabrication of IREs, thalliumbromoiodide, zinc selenide, germanium and silicon do, in fact, offer quite a range in adsorption substrate properties, and the potential of employing a thin layer of a substance as a modifier of the IRE surface which is presented to a surfactant solution has also been examined in the studies of proteins. Based on the appearance of the studies described below, and recent concerns about the kinetics of formation of self-assembled layers, (108) it seems likely that in-situ ATR studies of small molecules at solid - liquid interfaces ("wet" solids), will continue to expand in scope. 

The adsorption of surfactants at the liquid/air interface, which results in surface tension reduction, is important for many applications in industry such as wetting, spraying, impaction, and adhesion of droplets. Adsorption at the liquid/liquid interface is important in emulsification and subsequent stabilization of the emulsion. Adsorption at the solid/liquid interface is important in wetting phenomena, preparation of solid/liquid dispersions, and stabilization of suspensions. Below a brief description of the various adsorption phenomena is given. 

The adsorption of nonionic surfactants on polar and nonpolar surfaces also exhibits various features, depending on the nature of the surfactant and the substrate. Three types of isotherms may be distinguished, as illustrated in Fig. 7. These isotherms can be accounted for by the different surfactant orientations and their association at the solid/liquid interface as illustrated in Fig. 8. Again, bilayers, hemimicelles, and micelles can be identified on various substrates. 

Surfactants also aid the comminution of the particles by bead milling, whereby adsorption of the surfactant at the solid/liquid interface and in cracks facilitates their disruption into smaller units. 

J.S. Clunie, B.T. Ingram, Adsorption of non-ionic surfactants, in Adsorption from Solution at the Solid-Liquid interface (see sec. 2.10b), p. 105. 

Adsorption from Solution at the Solid/Liquid Interface, G.D. Parfitt, C.H. Rochester, Eds., Academic Press (1983). (Contains chapters on adsorption of smEill molecules (G.D. Parfitt and C.H. Rochester), adsorption from mixtures of miscible liquids (J.E. Lane) and adsorption of non-ionic surfactants (J.S. Clunle,... 

Some essential discoveries concerning the organization of the adsorbed layer derive from the various spectroscopic measurements [38-46]. Here considerable experimental evidence is consistent with the postulate that ionic surfactants form localized aggregates on the solid surface. Microscopic properties like polarity and viscosity as well as aggregation number of such adsorbate microstructures for different regions in the adsorption isotherm of the sodium dedecyl sulfate/water/alumina system were determined by fluorescence decay (FDS) and electron spin resonance (ESR) spectroscopic methods. Two types of molecular probes incorporated in the solid-liquid interface under in situ equilibrium conditions... 

Hanna, H.S., Somasundaran, P, 1977. Physico-chemical aspects of adsorption at solid/liquid interfaces, 11 Mahogany sulfonate/Berea sandstone, kaolinite. In Shah, D.O., Schechter, R.S. (Eds.), Improved Oil Recovery by Surfactant and Polymer Flooding. Academic Press, pp. 253-274. 

Adsorption of Surfactants at the Air/Liquid, Liquid/Liquid, and Solid/Liquid Interfaces... 

Adsorption of Polymeric Surfactants at the Solid/Liquid Interface... 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

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