Hydrophobically associating polymer surfactant effects

In the trimer series, the hydrophobic domains in the fluorescence data reflect isocyanurate associations. The hydrophobe associations responsible for effective thickening occur at higher concentrations. Probe studies similar to those conducted in oxyethylene-oxypropylene block polymers (12, 13) are warranted. The difference of importance to the rheology of dispersed systems is likely related to the cohesiveness of the surfactant hydrophobe interaction (21). 

The effect of monovalent and divalent salts on the solubility of these hydrophobically associating polymers (HAPs) is similar to that of ionic surfactants. An increase in salt content decreases solubility. With increasing salinity, the hydrocarbon chains are forced into closer proximity to the point where the subtle balance between hydrophobic associative forces and hydrophilic hydration forces breaks down, and phase separation results. Divalent cations have a larger effect on decreasing polymer solubility than do alkaline earth or monovalent cations. This is particularly so when the polymer contains anionic functionality such as acrylate or sulfonate. Another interesting phenomena occurs in mixed salts with certain polymer compositions when the ratio of divalent to monovalent cation is varied. A window of solubility is observed similar to that found with anionic surfactant solutions. 

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. 

Because the main driving force for surfactant self-association in polymer-surfactant mixed systems is the hydrophobic effect, the binding of surfactants to polyelectrolytes exhibits a similar dependence on the length of the alkyl chain as known for free micellization. Surfactants with longer hydrocarbon chains bind more strongly to polyions than those with shorter chains, and the binding starts a lower surfactant concentrations. In this context, a convenient parameter to characterize polyelectrolyte-surfactant systems is the critical aggregation concentration, cac, which is a counterpart of the well-known critical micellization concentration, cmc, but applies to solutions of surfactants in the presence of a polymer. It is defined as the... 

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. 

As surfactants will compete for hydrophobic sites in an aqueous solution (or interface), it can be expected that many properties of hydrophobically modified polymers will change, depending on the exact conditions of the solution. These can include increases, or decreases, in viscosity and such properties as foaming, emulsification, and wetting. For example, Danino et al. (94) report a rather complete study, using several techniques, of the polyamphiphile poly(disodium maleate-coalkylvinylether) and its mixtures with anionic or nonionic surfactants that have a disruptive effect on the association of this polymer. 

Figure 29 Effect on surfactant micelle morphology by addition of a hydrophobically modified associative polymer.

In formulations such as household cleaners or cosmetic and toiletry products, it is common for surfactants to be present in order to provide the required detergency effect. Consequently, the water phase has an assembly of micelles in addition to those which form from the groups in the polymer. In practise, the hydrophobes in the polymer become incorporated in the surfactant micelles originating from the components of the formulation which results in association or interactions greater than that which would arise from the polymer surfactant groups alone. 

Abstract This paper reviews possible phase diagrams of associating polymer solutions in which phase separation and molecular association interfere. Paying special attention on the structure and reorganisation of the network junctions, we study competition between phase separation and gelation. The molecular structure of associating micelles, or multiple cross-link junctions, in the networks is analyzed from the sol/gel transition lines. The effect of added surfactants on the formation of reversible gels in hydrophobically modified polymer solutions is also studied under the assumption of the existenee of a minimum multiplicity required for stable cross links. To describe... 

Consistent with the postulated hydrophobic association in aqueous media, addition of organic solvents such as DMSO, DMF and acetone at constant polymer concentration causes a drop in Brookfield viscosity. This behavior is illustrated in Fig. 7.8 for the addition of acetone. There is no effect until about 5% by volume and then there is a dramatic 3-30 fold decrease at an acetone concentration of 15%. Similar effects are observed for the addition of urea and ionic and nonionic surfactants (Fig. 7.9). In the latter case however, the viscosity vs additive profile is more complicated. There is a sharp initial decrease followed by an increase. The nature of this increase at higher surfactant concentration is unclear at present. The sharp decrease is consistent with association of the... 

The opposite effect on quenching by neutral molecules when small amounts of detergent are added to aqueous solutions of 3 could be attributed to several possible factors. First, the addition of detergent to the polymer may help in affording a more hydrophobic environment in its vicinity that may enhance the polymer-small molecule association. Also, since surfactants such as DTAB are well known to solubilize organic molecules in water, it appears reasonable that clusters of the detergent may combine with the neutral quenchers and function as chaperones to increase their effective solubility and thus enhance the complex formation with the polymer. 

Incorporation of long-chain hydrocarbon hydrophobes into a cellulose ether backbone leads to an interesting new class of polymeric surfactants. Their enhanced solution viscosity can be explained in terms of intermolecular associations via the hydrophobe moieties. Entropic forces cause the polymer hydrophobes to cluster to minimize the disruption of water structure. The same thermodynamic principles that are used to explain the micellization of surfactants can be applied to explain the solution behavior of HMHEC. HMHECs interact with surfactants that modify their solution viscosities. The chemical nature and the concentration of the surfactant dictate its effect on HMHEC solution behavior. The unique rheological properties of HMHEC can be exploited to meet industrial demands for specific formulations and applications. 

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