Adsorption of polymeric surfactants

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

The adsorption of polymeric surfactants is more complex, since in this case the process is irreversible and produces a high-affinity isotherm with a steep rise in the adsorption value at low polymer concentrations (in this region most of the molecules are completely adsorbed). Subsequently, the adsorbed amount remains virtually constant, giving a plateau value that depends on the molecular weight, temperature and solvency of the medium for the chains (this topic was discussed in detail in Chapter 6). 

This chapter described the basis principles involved in stabilization of dispersions by polymeric surfactants. The first part described polymeric surfactants and their solution properties. The second part described the adsorption of polymeric surfactants and their conformation at the interface. The methods that can be applied to determine the adsorption and conformation of polymeric surfactants were briefly described. The third part dealt with the stabilization mechanism produced using polymeric surfactants. Two main repulsive forces were considered. The first arises from the unfavorable mixing of the chains on close approach of the particles or droplets, when these chains are in good solvent conditions. This is referred to as mixing or osmotic repulsion. The second force of repulsions... 

A plot of r versus C2 gives the adsorption isotherm. Two types of isotherms can be distinguished a Langmuir type for reversible adsorption of surfactants (Fig. 4.10) and a high affinity isotherm (Fig. 4.11) for irreversible adsorption of polymeric surfactants. 

After reviewing various earlier explanations for an adsorption maximum, Trogus, Schechter, and Wade [244] proposed perhaps the most satisfactory one so far (see also Ref. 243). Qualitatively, an adsorption maximum can occur if the surfactant consists of at least two species (which can be closely related) what is necessary is that species 2 (say) preferentially forms micelles (has a lower CMC) relative to species 1 and also adsorbs more strongly. The adsorbed state may also consist of aggregates or hemi-micelles, and even for a pure component the situation can be complex (see Section XI-6 for recent AFM evidence of surface micelle formation and [246] for polymeric surface micelles). Similar adsorption maxima found in adsorption of nonionic surfactants can be attributed to polydispersity in the surfactant chain lengths [247], Surface-active impuri-... 

A characteristic increase of 90 after the horizontal section is apparently more pronounced when the potassium oxalate K2C2O4 is used as the electron donor instead of the sulfide ions (Fig. 2.25). A qualitative similarity of the adsorption isotherms and the MO concentration dependence on the initial quantum yield indicates that the adsorbed dye molecules take part in the reaction. Note that all kinetic curves attain the same value of the stationary quantum yield ratio depends on the nature of polymeric surfactant used for stabilization of CdS colloid. With PAA, this ratio equals ca. 0.5, and 0.6 with PVS. 

Garcia-Delgado, R.A., Cotoruelo, L.M., and Rodriguez, J.J. (1992). Adsorption of anionic surfactant mixtures by polymeric resins. Sep. Sci. Technol., 27, 1065-76. Pendleton, P., Wu, S.H., and Badalyan, A. (2002). Activated carbon oxygen content influence on water and surfactant adsorption. J. Colloid Interface Sci., 246, 235-40. 

The behavior of the surfactant molecules in an emulsion polymerization is complex. The adsorption of the surfactant on the rapidly and continually growing surface of the monomer-swollen latex particles reduces their concentration in the aqueous phase, and also upsets the balance in equilibrium between the dissolved surfactant and the surfactant present in the inactivated micelles (those in which polymerization is not occurring), as shown in Figure 5.11. The point is quickly reached at which the surfactant concentration in the solution falls below its critical micelle concentration, CMC. When this occurs, the inactive micelles become unstable and disintegrate to restore the balance. In time all of the micelles disappear and the monomer droplets shrink in size. After a conversion of 10-20%... 

This is attributed to the failure of the large molecules to enter the pores of the solid. More complex isotherm shapes are encountered as in the case of the adsorption of alkyl surfactants on silica and alumina. For example, the adsorption isotherm of sodium dode-cylsulfonate on alumina consists of four regions depending on the dominant adsorption mechanism. Adsorption of polymeric reagents on minerals typically results in a pseudo-Langmuirian type isotherm as shown in Fig. 4.7 for the adsorption of polyacrylamide on Na-kaolinite (Hollander et al., 1981). 

Understanding the adsorption and conformation of polymeric surfactants at interfaces is key to understanding how these molecules act as stabilizers for suspensions and emulsions. Most basic theories on polymer adsorption and conformation have been developed for the solid/liquid interface (9). The same concepts may be applied for the liquid/liquid interface, with some modifications whereby some part of the molecule may reside within the oil phase, rather than simply staying at the interface. Such modifications do not alter the basic concepts, particularly when one deals with the stabilization by these molecules. 

Figure 16.1. Various conformations of polymeric surfactants adsorbed on a plane surface (a) random conformations of loops-trains-tails (homopolymer) (b) preferential adsorption of short blocks (c) chain lying flat on the surface (d) AB block copolymer with loop-train conformation of B and long tail of A (e) ABA block copolymer, as in (d) (f) BA graft with backbone B forming small loops and several tails of A ( teeth )...

In this overview, the first section will on general classification of polymeric surfactants. This is followed by a section on preparation of polymeric snrfactants, with particular reference to sugar-based molecules. This is followed by a discussion of their solntion properties. The next section will be devoted to the adsorption of polymeric snrfactants at the solid/liquid (S/L) interface, whereby a summary will be given to some of the theoretical treatments and the methods that... 

As will be discussed later, one of the main features of effective steric stabilization is strong adsorption (anchoring) of the chains to the interface. This anchoring is produced by the use of polymeric surfactants, the main topic of this chapter. These materials have attracted considerable attention in recent years for stabilization of many o/w and w/o emulsions. Apart from their effectiveness in prevention of flocculation and coalescence of the droplets, they are also expected to cause no skin irritation. The high molecular weight of the surfactants prevents their penetration through the skin and hence they do not cause any disruption of the stratum corneum. 

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