Micelle polymer-surfactant interaction

Polymer/Surfactant Interactions. Interaction between polymers and surfactants was recently reviewed by Robb (11) and surfactant association with proteins by Steinhardt and Reynolds (12). Polymer/surfactant interactions are highly dependent on the chemical nature of the polymer and the surfactant. In general, surfactants tend to associate with uncharged polymers in aggregates rather than individual surfactant molecules interacting with the macro-molecule. The ability of surfactants to form micelles is thought to be an important factor in the role of surfactant behavior in interactions with polymers. Individual surfactant... 

The solubilization of the HMHEC in the surfactant was attributed to the interactions between surfactant micelles and polymer-bound hydrophobes. The effect of pH on polymer-surfactant solution viscosity was explained in terms of charge effects at the surface of the surfactant micelles. Steiner (13) proposed that at pH levels above or below the isoelectric point, the surfactant has a net charge on the head groups that causes repulsion within a single micelle. This repulsion leads to a relatively open micelle-aqueous phase interface through which polymer-bound hydrophobes can enter and experience stable polymer-surfactant interactions. These interactions anchor the polymer chains in an extended configuration. 

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). 

FIGURE 14.5. In some polymer-surfactant interactions there is evidence for the formation of micelle-like or hemimicelle aggregates of surfactant molecules along the polymer chain—something like a string of pearls. ... 

FIGURE 14.12. Polymer-surfactant interactions are important in many areas of polymer science and technology, especially emnlsion polymerization. In snch processes surfactants and micelles perform several dnties snch as emulsification of monomers (a), solnbihzation of growing oUgomeric free radical chains (b), and stabilization of growing and final polymer particles (c). 

The striking difference of behavior between SDS and TTAB as far as changes of CMC and 3 are concerned suggest that the polymer-surfactant interaction occurs at the level of the surfactant ionic head group and does not involve the surfactant alkyl chain. This is turn means that the penetration of the polymer in the micelle will be restricted to the head group region, with little if any at all, penetration of the polymer in the micelle hydrophobic interior. Thus, the polymer surfactant interaction can be looked at as an adsorption of the polymer chain on the micelle surface. ... 

An elegant innovation to aid the study of polymer/surfactant interaction was the introduction of a fluorescent label directly onto the polymer molecule by covalent bonding. [See reviews by Winnik (43,44).] This approach has been particularly useful in systems, such as combinations of nonionic polymers and nonionic surfactants, where interaction is weak. For example, pyrene-labeled hydroxypropylcellulose (HPC) gave evidence of association with weakly reactive OTG (n-octyl-P-D-thioglucopyranoside) but only at concentrations near its c.m.c. (119). Experiments with pyrene-labeled PNIPAM have been reported by Winnik et al. (120), who obtained evidence of noncooperative association of this polymer with anionic and cationic surfactants. A polymer that has been terminally labeled with pyrene groups is PEO (121) in mixtures with SDS at lower concentrations fluorescence data indicated the polymer chain cyclized. At higher concentration the pyrene groups were located in separate micelles. 

The raising of the cloud point of a nonionic surfactant by an anionic surfactant can be considered to be a special case of polymer/surfactant interaction. Here the polymer is a hydrophobically substituted species in which the hydrophilic moiety with repeating units (most often ethylene oxide) is an oligomer rather than a tme polymer. This phenomenon has been of much importance, and has long been known, to formulation chemists and involves the close association of the two species in mixed micelles—the complexes in this case. 

Fig. 3 Polymer-surfactant interaction is sketched. In the network of telechelic polymers (/ = 2), mixed micelles are formed at the junctions. Isolated micelles made up of pure surfactants also exist in the solution...

One way of getting information on polymer-surfactant interactions is to analyze the influence of the polymer on the micellization process of the surfactant. A convenient experimental route for monitoring that perturbation, as shown by Jones [12], is to follow the surface-tension properties of the surfactant in the presence and in the absence of a non-surface-active polymer. 

More specific polymer-surfactant interactions can be obtained if the polymer chain is modified to introduce more hydrophobic groups along the chain. Such modifications provide more opportunity for hydrophobic interactions between the surfactant tail and the polymer. In addition, they may result in increased intra- and interchain polymer interactions, producing more compact chain conformations in aqueous solution or multichain polymer aggregates or micelles (Figure 7.9). 

The ability of surfactants to form complexes with polymer chains may also affect the ultimate properties and stability of the resulting polymer, especially when the macromolecule exhibits some affinity for or reactivity with water. Perhaps the best documented case of the effect of surfactant on latex stability is that of polyvinyl acetate. The stability of PVAc latexes has been found to vary significantly depending on the surfactant employed in its preparation. It has also been found that PVAc could be dissolved in concentrated aqueous solutions of SDS and that it did not precipitate on dilution. The results suggest that, in this case at least, solubilization did not occur in the micelle, but that extensive adsorption of surfactant onto the polymer chain was required. They also indicate that a strong, stable PVAc-SDS complex is formed that produces a water-soluble structure that is essentially irreversible, imlike normal micelle formation. Cationic and nonionic surfactants had little or no solubilizing effect under identical conditions, indicating the specific nature of many, if not most, polymer-surfactant interactions. 

As with normal hydrocarbon-based surfactants, polymeric micelles have a core-shell structure in aqueous systems (Jones and Leroux, 1999). The shell is responsible for micelle stabilization and interactions with plasma proteins and cell membranes. It usually consists of chains of hydrophilic nonbiodegradable, biocompatible polymers such as PEO. The biodistribution of the carrier is mainly dictated by the nature of the hydrophilic shell (Yokoyama, 1998). PEO forms a dense brush around the micelle core preventing interaction between the micelle and proteins, for example, opsonins, which promote rapid circulatory clearance by the mononuclear phagocyte system (MPS) (Papisov, 1995). Other polymers such as pdty(sopropylacrylamide) (PNIPA) (Cammas etal., 1997 Chung etal., 1999) and poly(alkylacrylicacid) (Chen etal., 1995 Kwon and Kataoka, 1995 Kohorietal., 1998) can impart additional temperature or pH-sensitivity to the micelles, and may eventually be used to confer bioadhesive properties (Inoue et al., 1998). 

Recently, a new class of inhibitors (nonionic polymer surfactants) was identified as promising agents for drug formulations. These compounds are two- or three-block copolymers arranged in a linear ABA or AB structure. The A block is a hydrophilic polyethylene oxide) chain. The B block can be a hydrophobic lipid (in copolymers BRIJs, MYRJs, Tritons, Tweens, and Chremophor) or a poly(propylene oxide) chain (in copolymers Pluronics [BASF Corp., N.J., USA] and CRL-1606). Pluronic block copolymers with various numbers of hydrophilic EO (,n) and hydrophobic PO (in) units are characterized by distinct hydrophilic-lipophilic balance (HLB). Due to their amphiphilic character these copolymers display surfactant properties including ability to interact with hydrophobic surfaces and biological membranes. In aqueous solutions with concentrations above the CMC, these copolymers self-assemble into micelles. 

This chapter reviews the wide range of colloidal systems amenable to investigation by FT - IR spectroscopy. Molecular level information about die interactions of amphiphilic substances in aggregates such as micelles, bilayers, and gels can be obtained and related to the appearance and stability of the various phases exhibited. The interactions of polymers, surfactants and proteins with interfaces, which substantially modify the solid - liquid or liquid - air interface in many important industrial and natural processes, can also be monitored using FT - IR. 

Polymers Interact with surfactants and mlcroemulslons In diverse. Interesting and technologically Important wavs(1.21. The mechanisms that are responsible for the Interactions Include the usual panoply of forces Involved In the interaction of any two different molecules lon-lon, lon-dlpole, dlpole-dlpole, and van der Waals forces all modulated by the presence of solvent and/ or other species such as dissolved salts. All may play a role. The special factors Involved in surfactant/polymer and polymer-/mlcroemulslon Interactions that form the basis for their particular interest lies in their tendencies to form a variety of supermolecular clusters and conformations, which In tuim may lead to the existence of separate phases of coexisting species. Micelles may form In association with the polymer, polymer may precipitate or be solubilized, mlcroemulslon phase boundaries may change, and so on. 

---