Conformation, polymeric surfactants

The extremely low CMCs have been advantageous for several applications, since only traces of polymer are required to form micelles. High dilution effects, that are problematic in the case of classical surfactants, do not alter polymeric micelles. The surface activity at the air - water, of the amphiphilic block copolymer or polymeric surfactants must be different from the classical surfactants, because of their much lower diffusion coefficients and their much complex conformations. 

Diblock copolymers consisting of soluble and insoluble parts (Fig. 2b) act much as grafted chains once they are adsorbed on the surface. However, the thermodynamics of the initial solution, consisting primarily of micelles, and the conformation of the insoluble blocks on the surface affect the coverage in ways not well understood (e.g., Munch and Gast, 1988 Marques et al., 1988 Gast, 1989). Many dispersants or polymeric surfactants are synthesized in this way (Reiss et al, 1987). 

In addition to giving information about the shape and internal structure of colloidal aggregates, SANS studies can also be used profitably to determine the thickness and conformation of polymer layers adsorbed onto the surface of colloidal particles such as latex nanoparticles, and in some special cases, the surface of emulsion droplets. ° In such studies, the particles on which the polymer is adsorbed must generally be very accurately contrast matched to the solvent so as to allow information to be obtained only about the adsorbed layer. SANS studies have also been recently used in combination with differential scanning calorimetry and visual inspection of the solutions, to draw up a (simplified) partial phase diagram of the aggregation behavior of a polymeric surfactant in water.t ... 

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. 

The position of point A in the curve shown in Fig. 1.7 is characteristic for a sufficient purity of the surfactant and solvent. The point is located at a definite surface tension and concentration (CMC). As a rule, a minimum in the y(c) or y(log c) plot is evidence of traces of highly surface-active impurities which affect the results [147]. In chapters 2 to 4 the effect of lateral molecular interaction with increasing alkyl chain length, the effects of changes in orientation and conformation, essentially shown by polymeric surfactants such as proteins, may also lead to significant changes in the adsorption behaviour. 

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 )...

This chapter, will begin with a brief description of polymeric surfactants and their solution properties, followed by a description of the fundamental principles of using polymeric surfactants for stabilization of emulsions (as well as suspensions), starting with a section on the adsorption and conformation of these molecules at the interface. This is followed by a section on stabilization of dispersions by polymeric surfactants. Particular... 

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... 

Despite their potential importance, there are few spectroscopic studies of polymerizable surfactants. The only well-characterized polymerized surfactants are the polydiacetylenic fatty acids. The polymerization mechanism (1,4 addition) was only recently discovered as a result of studies on crystals of non-surfactant monomers which become brightly colored on polymerization [1], indicating the formation of polymer concurrent with a solid-solid phase transition. Several groups, particularly that of Ringsdorf, have pioneered the study of polydiacetylenic fatty acids [2-5]. A significant amount of spectroscopic work has been done on non-surfactant polydiacetylenes [6-9]. In addition, Lando and co-workers have used electron diffraction from mono- and bilayers of fatty acids to establish the conformation of the diacetylene backbone chains in this ordered surfactant system [10,11], and others have performed Raman spectroscopy on Langmuir-Blodgett films of similar surfactants [12]. This provides one with a powerful "basis set" of information from which to study other polydiacetylenic systems. 

The key to understanding how surfactants and polymers (to be referred to as polymeric surfactants) function as stabilisers is to know their adsorption and conformation at the solid/liquid interface. This is the objective of the present chapter, which is a survey of the general trends observed and some of the theoretical treatments. 

Understanding the adsorption and conformation of polymeric surfactants at interfaces is key to knowing how these molecules act as stabilizers. Most basic ideas on adsorption and conformation of polymers have been developed for the solid/liquid... 

A-B, A-B-A block and BAn graft type polymeric surfactants are used to stabilise emulsions and suspensions [18]. B is the anchor chain that adsorbs very strongly at the 0/W or S/L interface, whereas the A chains are the stabilising chains that provide steric stabilisation. These polymeric surfactants exhibit surface activity at the 0/W or S/L interface. The adsorption and conformation of these polymeric surfactant at the interface has been described in detail in reference 18. 

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