Hydrophobically associating polymer adsorption

Li (2007) observed that the adsorption of a hydrophobically associating water-soluble polymer, AP-2, did not follow the Langmuir-type isotherm. Figure 5.37 shows that the adsorption increased to a maximum and then decreased as the polymer concentration was increased. The reason is probably that the hydrophobic polymer has an adsorption layer of multiple molecules on rock surfaces. When the polymer concentration is increased, the adsorption layer becomes thicker because of more adsorption. When the polymer concentration is further increased, the molecular interaction in the liquid is stronger than that between the adsorbed molecules and rock surfaces. Then the adsorbed molecules may leave the rock surfaces and redissolve into the liquid. Thus, the adsorption decreases. 

The associative mechanism of thickening has been variously described, but is generally thought to result from nonspecific hydrophobic association of water-insoluble groups in water-soluble polymers 34, 35). For associative ASTs, the terminal hydrophobes of the ethoxylated side chains are considered to be the primary interactive components. These hydrophobes can interact with each other via intermolecular association, and can also interact with hydrophobic particle surfaces when present. The specific interaction with dispersed-phase components such as latex particles has been shown to be one of surface adsorption (36). In essence, the associative component of thickening in dispersed-phase systems also has dual character resulting from the building of structure within the aqueous phase and interaction with particle surfaces. 

This equation means that, in polymer gels, hydrophobic interaction is inhibited and adsorption usually takes place noncooperatively. In order for the neighboring surfactant molecules to interact hydrophobically, the polymer chain must be flexible and little energy consumption should accompany the deformation of the polymer chains upon association of surfactant molecules. 

Figure 5.15(d) resembles a BET adsorption isotherm and represents a combination of two interactions indicated by the broken curves speciHc localized sorption at low concentrations, followed by clustering or aggregate formation at high concentrations (note that clustering is not associated with capillary condensation as it is in the BET case). Sorption of water by hydrophobic polymers such as cellulosic materials follows this behavior. 

In a systematic study on the adsorption of mixtures of associative thickeners of the polyurethane type and nonylphenol ethoxylates on a hydrophobic latex, it was demonstrated that polymers with hydrophobic side chains along the backbone could displace the nonionic surfactant provided that the concentration of polymer hydrophobe units was high enough. For associative thickeners with only terminal hydrophobic chains, it was found that the size of these chains is a decisive factor in competitive adsorption. In addition, the distance between the hydrophobic end-groups is of importance the shorter the distance, i.e. the higher the concentration of hydrophobic chains, then the more effective the polymer is in displacing the nonionic surfactant. 

In this chapter I present the current state of three aspects in physicochemistry of nanoparticles critical for understanding structure and properties of nanocomposites. This also relates to adsorption and chemisorption of macromolecules on nanoparticle surfaces from solutions the generation of interfaces phenomena of surface conductivity and specific interactions that depend on the chain origin and length, its conformation, the composition of copolymers and so on. In polyelectrolytes similarly charged with nanoparticles, hydrophobic polymers are inclined to associate ionic groups and form domains as microphases of ion regions. 

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. 

In the case of HM-polymers abundant evidence exists that the alkyl groups in the dissolved polymer can be preassociated. This means that centers will be provided for the adsorption/association of surfactant ions (and molecules). Their composition and size will change with surfactant concentration, as will their nature—intermolecular and/or intramolecular. Fine points in this case include the number, type, chain length, and distribution of hydrophobic groups and, again, the detailed structure of the base polymer. In fact, the number of possible variations on this structural theme is vast ... 

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