Nonionic polymers polymer-surfactant interaction

When polymers are employed in personal care formulations, in many (if not most) cases they occur as cocomponents with surfactants. Recognizing a widespread tendency of such components to interact and affect each other s properties (sometimes in dramatic ways). Chapter 4 outlines a number of methods to investigate and analyze snch interaction for the main types of polymers—nonionic, ionic, hydrophobic, and proteinaceons. Chapter 5 presents an illustrative selection of polymer/surfactant interactions in applied systems that demonstrates how they can be selected to achieve beneficial performance effects. 

Nonionic surfactants are a special case. Because of the absence of electrical repulsive forces at the micellar periphery, aggregation takes place at a much lower concentration. For the same R group the c.m.c. of a nonionic surfactant can be two orders of magnitude lower than that of an ionic surfactant. Also, micellar shapes other than spherical, e.g., ellipsoids, are frequently encountered. All of the above factors, as will be seen, have a bearing on the subject of polymer/surfactant interaction. 

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. 

To describe and summarize conditions prevailing in homogeneous mixtures of an interacting (nonionic) polymer/ionic surfactant pair, where the concentration of both components changes, we utilize a phase diagram constmcted by Cabane and Duplessix (155) for the PEO/SDS system. (See Fig. 36.) In field I no complex formation occurs... 

Interaction of nonionic surfactants with most polymers is relatively weak and therefore signiflcant increases in viscosity resulting from polymer-surfactant interactions do not occur unless the polymer has been suitably hydrophically modified (see below). However, interaction of the EO groups of nonionic surfactants with polyacrylic acid-based polymers and copolymers is known and this can lead to substantial alterations in viscosity (91,92). This behavior is associated with the specific interaction of polyethers and polycarboxylic acids (92). 

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. 

Blankschtein and co-workers [65] have done pioneer work through theoretical modeling, aided by the computer, to predict the properties of mixed surfactant systems. Also, based on the necklace model proposed by Shirahama et al. [67,68], they have proposed a molecular thermodynamic theory of the com-plexation of nonionic polymers and surfactants in diluted aqueous solutions [66], Application of this method can help predict the interaction parameters for several nonionic polymer-surfactant mixtures. 

In this work a new interaction model is introduced to eliminate the mean interaction parameters and to analyse the possibility of complex formation between nonionic polymers and surfactants. 

Saito, S. (1987). Polymer-surfactant interactions. In Schick., M., ed. Nonionic Surfactants Physical Chemistry. 

PIC Piculell, L., Bergfeldt, K., and Gerdes, S., Segregation in aqueous mixtures of nonionic polymers and surfactant micelles. Effects of micelle size and surfactant headgroup/polymer interactions, J. Phys. Chem., 100, 3675, 1996. 

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. 

This behavior shows that the dimensions of these polymers are independent of pH, ionic strength (in the ranges studied) and presence or absence of Tergitol or polyethyleneoxide. This result is of considerable help in interpretation of GPC behavior since in the absence of polymer-glass substrate interactions, the molecular weight calibration curves (log MW vs. elution volume) should be independent of pH, ionic strength or the two nonionic surfactants investigated. 

Surface tension measurements indicated no bulk interaction between the anionic surfactant and the anionic or nonionic polymer. 

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. 

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]. 

Figure 13.2 shows the dynamic IFT for the two systems (1) 0.2% OP (nonionic) + 0.2% PS (petroleum sulfonate) +1.1% NaCl (without polymer), and (2) the same as (1) but with 0.1% 3530S polymer. From this figure, we can see that the IFTs for the two systems were almost the same. This figure demonstrates that there was not a strong interaction between the polymer and surfactants. However, polymer increases water viscosity to affect surfactant transport, so dynamic IFT was affected within a short time. Figure 13.2 shows that the dynamically stable IFT with addition of polymer was a little bit higher than that without polymer. 

Incorporation of surfactants in HPMC compressed matrices has been shown to cause retardation of drug release [110-112]. The first mechanism postulated was that anionic surfactants were capable of binding to nonionic polymers to increase the viscosity [110], More recently, drug-surfactant ionic interaction has been put forward instead, resulting in the formation of a complex with low solubility in water [111,112]. Note that such an effect can unintentionally occur upon putting some surfactant into the release medium in order to increase wettability. 

Fig. 2 (a-c) Physical polymer-network cross-linking provided by mixed micelles in hydrogels formed via hydrophobic interactions in surfactant solutions. Mixed micelles are formed by aggregation of hydrophobic blocks of per-se hydrophilic polymers and surfactant alkyl tails, (b) Nonionic polymer and ionic surfactant gel system at the state of preparation. For clarity, charges are not shown, (c) Ionic polymer and oppositely charged surfactant gel system after extraction of free micelles, (d) Structure of the hydrophobic monomers used in the micellar polymerization... 

The phase behaviour of PS systems is also affected by specific interactions between the two cosolutes, similar to hydrophobic interactions in the case of HM-polymers. This may enhance phase separation for nonionic systems but decrease it for ionics. For a mixture of oppositely charged polymer and surfactant,... 

Figure 20.34. Swelling of covalent gels is mainly controlled by electrostatic interactions. Therefore, the binding of an ionic surfactant (exemplified by sodium dodecyl sulfate) into a nonionic polymer gel (cross-linked ethylhydroxyethyl cellulose) leads a large increase in the gel volume. Gel swelling starts in the vicinity of the CAC, where also the swelling of an adsorbed polymer layer takes place. (Redrawn from O. Rosen and L. Piculell, Polym. Gels Networks, 5 (1997) 185)...

If the primary mechanism of ionic surfactant-nonionic polymer interaction is hydrophobic or dispersion-related, the adsorption of surfactant will almost... 

It should be noted that the interaction of nonionic polymers with cationic smfactants is much weaker than with anionic surfactants. The reason for this difference in behavior is not entirely clear but may stem from a small positive charge acquired by the EO chain in aqueous solution. 

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