Surfactant-Hydrophobically Modified Polymer Interaction

Low charge density, hydrophobically modified polybetaines were shown to interact and comicellize with nonionic, anionic, cationic, and amphoteric surfactants [181-183] and many ionic organic dyes [264,265]. The association mechanism of hydrophobically modified polymers and surfactants in dependence on the concentration of interacting components can be modeled by two pathways (Scheme 21) [183]. 

In this chapter, recent developments in the design of hydrogels formed via hydro-phobic interactions are reviewed, with a special emphasis on the function of surfactant in their dynamic and mechanical properties. Micellar copolymerization is a simple technique for the synthesis of such hydrogels. A particular advantage of this technique is the blocky structure of the resulting hydrophobically modified polymers, significantly enhancing their associative properties. 

Wang Y, Han B, Yan H. Microcalorrmetry study of interaction between ionic surfactants and hydrophobically modified polymers in aqueous solutions. Langmuir 1997 13 3119-3123. 

For hydrophobically modified polymers [such as hydrophobically modified hy-droxyethyl cellulose or poly(ethylene oxide)] the interaction between the surfactant micelles and the hydrophobic chains on the polymer can result in the formation of cross-links, i.e. gel formation (Figure 15.4). However, at high surfactant concentrations, there will be more micelles that can interact with the individual polymer chains and the cross-links are broken. 

Fig. 15.4. Scheme of interaction between hydrophobically modified polymer chains and surfactant micelles. 

When So = 2, this model reduces to the variable multiplicity model in which junctions of arbitrary multiplicity can coexist at the probability determined by the thermodynamic balance. In the case of micro-crystalline junctions, for instance, it is natural to assume that a minimum number Sq greater than 2 of the crystalline chains is required for a junction formation. This is because, the surface energy terms will prevent small-k units from being stable, leading to the existence of the critical multiplicity for the nucleation of the crystallites. Similarly, a minimum aggregation number is required for the stability of micelles formed by hydrophobes on water-soluble polymers. As we will see later, surfactants added to the solution cause complex interaction with hydrophobically modified polymers due to the existence of this minimum multiplicity. 

Magny, B., Iliopoulos, I., Audebert, R. et al. (1992) Interactions between hydrophobically modified polymers and surfactants. Prog. Colloid Polym. Sci., 89, 118-121. 

A class of systems extensively investigated by means of PFG-NMR are colloids. They are usually hydrophobically modified water-soluble polymers, that is, polymers with a water-soluble skeleton bearing one or more hydrophobic units, which allow the self-assembling of the polymer in water solution and the interaction with surfactants.77... 

In this paper, the results on the Interactions of hydrophobe modified cationic polymer with surfactants Is presented and the results are compared with those for the unmodified polymer. 

Both of the types of polymer mentioned above can be modified by the incorporation of hydrophobic monomers onto the essentially hydrophilic acrylate backbone. The effect of this is to modify their characteristics by giving them so-called associative properties. These hydrophobes can interact or associate with other hydrophobes in the formulation (e.g., surfactants, oils, or hydrophobic particles) and thus build additional structures in the matrix [3-11]. These associative polymers are termed cross-polymers when they are based on carbomer-type chemistry [12] and hydrophobically modified alkali-soluble emulsions (HASEs) when based on ASE technology. 

Photophysical studies on a conformational transition of PMA induced by cationic surfactants have been reported (7). The stretched PMA chain at pH 8 collapses on addition of cationic surfactants that is, the hydrophobic interactions between the cationic surfactants that are bonded to the PMA chain lead to refolding of the polymer chain, and thus provide a hydrophobic site for fluorescence probes at pH 8. The cationic polyelectrolyte poly(4-vinylpyridine) quatemized with n-dodecyl bromide (8 i0) or hexadecyl bromide (11) are also examples of hydrophobically modified polyelectrolytes. 

Hydrophobically modified water-soluble polymers (HMWSPs) exhibit enhanced solution viscosity and unique rheological properties. These properties can be explained in terms of intermolecular associations via hydrophobes. This chapter describes the synthesis and solution properties of HMWSPs, Particularly discussed are the solution properties of hydrophobically modified hydroxyethytjcellulose (HMHEC) in aqueous and surfactant systems, HMHECs interact with surfactants and thus modify solution viscosities. The structure and the concentration of the surfactant dictate the solution behavior of HMHEC. The unique solution properties of HMHEC can be exploited to meet industrial demands for specific formulations or applications. 

Polymer-surfactant interactions are the basis for the rheological behavior of MHAPs. Other surfactant-polymer systems have previously been investigated. One example is the interaction of surfactants with polymers such as poly(ethylene oxide), which results in greater solution viscosities than with the polymer alone (e.g., ref. 25 and references therein). The interaction of surfactants or latexes with hydrophobically modified water-soluble polymers has also been shown to produce unique rheology (2, 5, 26, 27). In these systems, the latex particles or the surfactant micelles serve as reversible cross-link points with a hydrophobic region of a polymer molecule in dynamic association with a latex particle or surfactant micelle (27). 

These viscosity effects of the addition of surfactant to a solution of a hydrophobically modified water-soluble polymer are general, although the effect will be modified by other interactions such as electrostatic ones. As exemplified in Figure 20.11, addition of an oppositely charged surfactant to a solution of a hydrophobically... 

Hydrophobically modified polymCTs provide anoth means for polymers and surfactants to interact because the hydrocarbon chains of the polymers may be incorporated into micelles. Various types of behavior arc possible. At surfactant concmtrations only slightly above the CMC, chains from more than one polymer... 

Surfactant concentration (varied after polymerization) greatly affects the viscosity of associating polymer systems. Iliopoulos et al. studied the interactions between sodium dodecyl sulfate (SDS) and hydrophobically modified polyfsodium acrylate) with 1 or 3 mole percent of octadecyl side groups [85]. A viscosity maximum occurred at a surfactant concentration close to or lower than the critical micelle concentration (CMC). Viscosity increases of up to 5 orders of magnitude were observed. Glass et al. observed similar behavior with hydrophobically modified HEC polymers. [100] The low-shear viscosity of hydrophobically modified HEC showed a maximum at the CMC of sodium oleate. HEUR thickeners showed the same type of behavior with both anionic (SDS) and nonionic surfactants. At the critical micelle concentration, the micelles can effectively cross-link the associating polymer if more than one hydrophobe from different polymer chains is incorporated into a micelle. Above the CMC, the number of micelles per polymer-bound hydrophobe increases, and the micelles can no longer effectively cross-link the polymer. As a result, viscosity diminishes. 

While hydrophobically modified nonionic polymers are encountered far less frequently than their ionic counterparts, one can find several fluorescence studies of their interaction with surfactants. These include investigations of the HM-HEC/SDS system by Dualeh and Steiner (100) and by Sivadasan and Somasundaran (117) of the same polymer with added SDS and also the nonionic surfactant CnEOg. A comprehensive study. 

The categories of polyelectrolyte and hydrophobically modified (HM) polymers are very different from the above. In these the association with the surfactant is considered to be polymer directed in view of the strong adsorption sites they provide for the said surfactant. This is borne out by extremely low c.a.c. values frequently observed in these systems, which confirm the high affinity of the adsorption processes. Conceptually (at least) interaction mechanisms in these systems are more straightforward. 

Finally, by way of illustration, we mention a polymer type currently receiving much attention. The polymer is referred to as hydrophobically modified ethoxylated urethane (HEUR), the reaction product of a PEG and a diisocyanate, end-capped with a long chain alcohol or amine. HEURs are already recognized as having much potential as associative thickeners in coating formulations. Their structure suggests they will show pronounced interaction with surfactants in solution and this is indeed the case. Here we refer to two recent studies. The first by Hulden (95) included surface tension studies of the polymer... 

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