A Brief Review Of Polymer/Surfactant Interaction
A Brief Review of Polymer/Surfactant InteractionBy: Robert Y. Lochhead and Lisa R. Huisinga, The Institute for Formulation Science,The University of Southern MississippiThis brief review of polymer-surfactant interaction opens by describing how polymersbehave in solution. Then we survey the literature on the interaction of nonionic polymerswith surfactants, and the interaction of polyelectrolytes with ionic surfactants of oppositecharge. After a brief discussion of polymer adsorption at interfaces, we consider theimplications of these interactions on the design of shampoo products.Polymers in Dilute and Semi-dilute SolutionPolymer-surfactant interaction in personal care compositions usually occurs in aqueousmedia. In order to understand the concepts of this type of polymer-surfactant interaction,it is first necessary to grasp how typical polymers behave in solution. The condition for apolymer molecule to dissolve is that the polymer-solvent interaction is greater than bothpolymer-polymer and solvent-solvent interactions. If this condition is achieved thepolymer will dissolve and, depending upon the concentration, a dilute solution or semidilute solution will be formed.A dissolved polymer can occupy many times the volume of the polymer moleculeitself—that is, a polymer swells when it is dissolved and the volume inside the swollenpolymer contains solvent. It is not unusual for a dissolved polymer to be swollen to athousand times its original size. In a dilute solution each dissolved polymer molecule willbe isolated. If the polymer concentration is increased, eventually there comes a pointwhen the entire space is filled with swollen polymer molecules and above thisconcentration the polymer can only occupy the solution if the molecules entangle andthread through each other’s domains.The concentration of the onset of entanglement is called the “critical overlapconcentration” (C*). Above the critical overlap concentration the system is in the semidilute regime. When polymers phase-separate from solution, they usually do so in thesemi-dilute or concentrated condition and therefore they are in an entangled state.Polymer scientists gain conceptual understanding of the process of separation byintroducing the concept of correlation length. The correlation length is known morecolloquially as the “blob size.” In dilute solution, the blob size is the size of the entirepolymer molecule and in semi-dilute solution the blob size becomes the distance betweenentanglement points (Figure 1).1
The blob size decreases as polymer concentration increases even in dilute solution. This adepicted in Figure 2 in which g(r) represents the blob size and the horizontal axisrepresents polymer concentration.Interaction of Nonionic Polymers with SurfactantsThe field of polymer surfactant interaction owes a great deal to Suji Saito, whose earlywork formed the basis of much of the formal research that has been conductedsubsequently. In 1952 he observed that the water-insoluble hydrophobic polymerpolyvinyl acetate completely dissolved in micellar sodium dodecyl sulfate solution.1 Thiswas intriguing because the polymer molecules were substantially too large to fit into themicelle and therefore the existing theories of solubilization could not explain thisphenomenon.Based upon simple viscosity measurements, Saito and Sata proposed a model of micellaraggregates along the polymer chain. This “pearls on a string model” is now well acceptedand has been validated by more sophisticated methods such as neutron scattering.12
In a 1957 publication, Saito extended this model to explain the sodium dodecyl sulfateinduced increase in the viscosities of aqueous solutions of the hydrophilic polymersmethyl cellulose and poly(N-vinylpyrrolidone).2,3 For these hydrophilic polymers, heexplained that ionic repulsion between the “micellar pearls” caused expansion of thepolymer chain, which in turn caused an increase of this viscosity. Today, we would referto this as an increase in the ionic persistence length of the molecule—or an increase in“blob size.”The model was further advanced by Jones in a study of polyethylene oxide interactionwith sodium dodecyl sulfate in aqueous solution.4 Jones noted that in the presence of thepolyethylene oxide the normal surface tension curve of the surfactant showed apremicellar breakpoint, T1, followed by a slow descent to meet the normal micelle curveat higher concentrations,T2, than the measured critical micelle concentration (CMC) ofthe surfactant. Jones described the T1 point as the lowest surfactant concentration atwhich interaction occurred between the surfactant and polymer and T2 as the surfactantconcentration at which both the polymer and the air-water interface became “saturated”with surfactant and normal micelles first appeared (Figure 3).Jones’ concepts and methods are still used today to probe polymer-surfactant interactions.A careful NMR study by Professor Nagarajan of Penn State University showed thatpolyethylene oxide decorated the outside of surfactant spherical micelles, penetratingdeeper than the micelle’s palisade layer and the polymer extended between manymicelles to form the “pearls on a string.”5Hydrophobically modified hydroxyethylcellulose is usually supplied as the hydrophilicpolysaccharide backbone with less than one mole percent hydrophobic modification. Theslight modification provides sufficient hydrophobic interaction between the chains toform a temporary network and to confer enhanced aqueous thickening properties on themolecule. It is interesting to note that the hydrophobically modified species phaseseparates from the unmodified species in aqueous solution. This is attributed to the factthat the hydrophobic associations form a network having a mesh size smaller than theunmodified polymer in solution;6 that is, upon hydrophobic modification the blob size ofthe polymer becomes smaller. This example demonstrates the fact that similar polymers3
with different blob sizes will not thermodynamically mix in solution.The network is not complete, however, because this polymer has a relatively stiffpolysaccharide backbone and a number of the hydrophobes on the backbone will besterically restricted from intermolecular hydrophobic association in aqueous solution. Theaddition of surfactant to solutions of this polymer, in the region of the CMC, causes adramatic increase in viscosity followed by an equally spectacular decrease in viscosity tolevels below that measured for the polymer solution in the absence of surfactant (Figure4).This behavior has been attributed to comicellization of the polymer hydrophobes withsurfactant hydrophobes.7 The comicellization is stoichiometric and when micelles firstform, they link hydrophobes that were previously isolated, and as a consequence a betternetwork of smaller blob size is formed and this results in an increase and the viscosity.As more surfactant micelles are introduced, a micelle concentration will be reached atwhich comicellization will not result in junction zones but rather in repulsion betweenpolymer chains as they become effectively polyions. The loss of network structure resultsin the observed dramatic loss in the viscosity at concentrations immediately above thecritical micelle concentration.Similar behavior is observed for hydrophobically modified alkali swellable acrylatethickeners, as exemplified by acrylates/steareth-20 methacrylates copolymer, but in thiscase the viscosity increase is less dramatic. On the contrary, completely differentbehavior has been observed for hydrophobically modified ethoxylated urethanethickeners. These are block copolymers having a poly(ethylene oxide) chain end-cappedwith hydrophobes, or they consist of hydrophobes grafted to a poly(ethylene oxide)chain.The flexibility of the polyethoxy chain allows these molecules to form micelles bythemselves at very low concentrations. A network structure is formed by some of thepolymers stretching from micelle to micelle. In this case, even small quantities of a lowmolecular weight surfactant comicellize with the polymer micelles and this results in4
immediate breakdown of the network structure and loss of the viscosity even at surfactantconcentrations well below the CMC (Figure 5).Increase in the surfactant concentration, introduction of cosurfactants such ascocamidopropyl betaine, or increase in the ionic strength of the solution causes anincrease in the micelle size. Spherical micelles become rod-like or they may even grow tobecome worm-like or branched micelles. These large micelles form exceptionally largejunction zones and stoichiometric comicellization with hydrophobically- modifiedhydrophilic polymers results in a large increase in viscosity that can be maintained over abroad surfactant concentration range (Figure 6).8In general, hydrophilic polymers will phase-separate from concentrated liquid crystalphases by a mechanism of depletion that results from osmotic competition between the5
components in such “crowded” situations. However, hydrophobically-modifiedhydrophilic polymers can be induced to interact with hexagonal liquid crystal phase andto penetrate the interlamellar layers of lamellar liquid crystal phase. The conditions forthis occurring are that the reduction in free energy due to mixing of the hydrophobesmore than compensates for the loss of conformational free energy of the chain when itchanges shape from solution state to the stretched conformation within the galleries oflamellar phase9-11 and the blob size within the gallery must be less than the width of thelamellar interlayer.12Interaction of Polyelectrolytes with Ionic Surfactants of Opposite ChargeSince the inception of conditioning shampoos in the 1970s, the concept of forming anddepositing complex coacervates has held the attention of conditioning shampooformulators. The interaction between a polyion and its counterions is described by atheory that was developed by Professor Gerald Manning at Rutgers University.13-16 Thistheory is based upon the concept that counterions in the presence of polyions can exist inone of two states; that is, either free in solution or condensed to the counterion. Manningasserted that if the polyion possessed an ionic charge above a certain critical chargedensity, then sufficient counterions would condense on the polyion chain to maintain thecharge density at its critical level.The significance of this is that the ultimate charge density of any polyion is limited to thiscritical value. Thus, Manning predicts that for sodium polyacrylate as the complete salt inpure water, about 65% of the sodium ions would condense on the chain and themaximum charge density that could be achieved for the polyacrylate ion wouldcorrespond to about 35% of the carboxylate groups. If the ionic charge of the counter ionsis increased, then a higher proportion of the counterions would condense. Thus, Manningpredicts that 82% of divalent counterions would condense on a polyacrylate chain and thehighest change density that the polyion could reach would correspond to only 18% of theacrylate groups.An increase in the ionic strength of the solution would also inevitably lead to a higherproportion of condensed ions. Decreased counterion solubility is also expected to lead toa greater proportion of condensed ions. Due to hydrophobic interaction, amphipathicsurfactant ions are necessarily less soluble in water than simple salt ions, such as chlorideor bromide. It would be expected, therefore, and it is observed in practice that surfactantions condense readily upon polyions and that these amphipathic ions readily ionexchange for the more soluble chloride, bromide and sulfate counterions associated withcationic polyions.Interaction of cationic polysaccharides with anionic surfactants forms the basis of themodern conditioning shampoo and the mechanism is well known. In the 1970s, Goddard,who continues to be the leader in field,17,18 showed that polyquaternium-10 and commonanionic surfactants formed coacervates that are one-phase systems at shampooconcentrations but they phase separate upon dilution during the shampooing process todeposit conditioning agents on the hair. Goddard’s explanation for the mechanism ispresented in Figure 7, which is a depiction of a binary polymer- surfactant phasediagram.’6
At low surfactant concentration, below the CMC, the anionic surfactants condense on thepolycation and the resulting ion-pair converts the cationic site into a hydrophobesubstituted site. Hydrophobic interaction within and between the modified polycationchains causes phase separation and this phase separation persists if the polycation:surfactant anion equivalent ratio is maintained at stoichiometric equivalence.It is notable that the surfactant-treated polycation displays a rapid increase in viscosityaround the surfactant CMC in similar fashion to hydrophobically-modifiedhydroxyethylcellulose and indeed for this system an elastic gel is formed.19, 20 Above theCMC comicellization with surfactant micelles results in a one-phase system.Fluorescence spectroscopy and 13C NMR techniques have shown the presence of hemimicelles along the polycation chain in the region of the phase separation and havedelineated crucial differences in that hemi-micelle structure depending upon the detailedstructure of the surfactant.21 It was also shown in this work that the addition of sodiumchloride moved the onset of the phase separation to higher surfactant concentrations, inaccordance with Manning theory, and resulted in “resolubilization” at lower surfactantconcentrations. This result is consistent with the salt-enhancing water structure, which inturn enhances the hydrophobic effect, and causes a lowering of the CMC.’Polymer Adsorption at InterfacesWhen a dissolved polymer adsorbs at an interface, if the interaction free energy betweenthe polymer and the interface is low, the polymer will absorb close to its solutionconfirmation. This type of interaction has been named “mushroom adsorption” becausepolymers with one anchor point appear to have a mushroom stem and a “button” made upof the cloud of polymer in its swollen conformation (Figure 8).7
It is generally accepted that most real polymers possess several anchor groups along thechain and these are adsorbed as trains where the interaction between polymer and surfaceis high, and as loops and tails where the interaction between the polymer and solvent ishigh (Figure 9).For example, this should be the case for adsorption of slightly charged polyquaternium11 to hair at pH values above the isoelectric point of the hair.If the interaction between the surface and the polymer is strong, the polymer adsorbs in aconformation that is flat and aligned with the surface. For example, this would be thecase with polyquatenium-6 and hair at high pH, where the polymer and the hair surfacewould carry opposite ionic charges.8
Recent Advances in Conditioning ShampoosThe interaction of polymer and surfactant bearing opposite ionic charges is utilized inconditioning shampoos and it results in the formation of a complex coacervate thatseparates upon dilution of the shampoo composition and during the rinsing stage ofshampooing. Complex coacervate formation depends upon a number of parameters suchas molecular weight, concentration, ionic strength of the solution, change density of theinteracting components, pH and temperature.22-24Confocal fluorescence scanning microscopy and scanning electron microscopy have beenused to show that deposition of the coacervate occurs preferentially at the cuticle edges25but measurement of the wetting force of single hair fibers reveals that the coating on thehair has relativity uniform surface free energy along the hair fiber.26Polyquaternium-10 is a watersoluble polymer that forms clear films and the improvementconferred upon hair appearance has been ascribed to such films.27 It has been reportedthat polyquaternium–10 of high charge density forms solid-like gels over a limitedconcentration range, whereas the low charge-density species form a liquid-like gel over amuch broader concentration range.26 In this context, it is interesting that the inclusion ofhigh molecular weight poly(ethylene oxide) reduces the particle size of the coacervate,produces higher foam volume and density, reduced combing forces, enhanced depositionand gives more uniform deposition on hair.28 An investigation of the mechanism ofpoly(ethylene oxide) synergism is warranted.Clear depositing systems have been claimed for lower molecular weight guarhydroxypropyltrimonium chloride and it would be interesting to investigate if this findingcorrelates with a smaller polymer blob size.29 The coacervate deposits on the hair and itcan co-deposit other beneficial agents such as silicone fluids, gums and resins. Suchconditioning shampoos should confer the wet hair attributes of softness and ease of wetcombing, and the dry-hair attributes of good cleansing efficacy, long-lasting smooth,moistened feel, manageability control, and no greasy feel. Particle sizes below 5 micronsare reported to deposit efficiently on hair because they are trapped within the coacervateupon dilution.30 It has been asserted that the polymer-surfactant coacervate alone deliversgood wet conditioning but does not give good dry feel.Recent patent applications have been directed towards insoluble particles other thansilicones. For example, PPG- 15 stearyl ether,31 condensates of adipic acid andpentaerythritol, polybutene and mineral oil32 have recently been revealed in the patentliterature as attempts to provide manageability control for dry hair, reducing interfiberfriction, providing a moisturized feel, while alleviating the “greasy feel” of conventionalcomplex coacervate-based conditioning shampoos.The opposite effect is targeted in coacervate-driven deposition of particles (titaniumdioxide, clay, pearlescent mica, or silica) to confer interfiber fiction in order to enhancestyleability of the hair.33 It is also seen in spherical particles (hollow silica, hollowpolymer spheres) for slip and conditioning attributes.33 In this case high molecular weight(100,000 to 3 million Daltons) cationic guar polymers are specified with a charge densityof less than 4-5 meq/g.Specific mixtures of cationic polymers have been claimed to deliver more uniformcoverage and thinner deposited films than conventional coacervate-based conditioningshampoos.34 This same source cites the use of mixtures of poly(acrylamide-co-9
acrylamidopropyl trimonium chloride), hydroxypropyl guar trimonium chloride, andsilicone quaternium-13.The influence of cationic polymer on surfactant self-associated structures is shown in arecent patent application that reveals that synthetic polymers such aspoly(methacrylamido propyltrimethylammonium chloride) (MAPTAC) cause phaseseparated lyotropic liquid crystals to form in shampoo compositions and that these liquidcrystalline coacenvates confer conditioning benefits on hair.35 Another recent patentapplication36 reveals that styling and gloss benefits can be conferred from rinse-offcompositions containing an anionic surfactant, a cationic polymer and an amphiphilic,branched block copolymer. An investigation of the fundamental physical mechanismsthat underpin this technology could lead cosmetic formulators to new and useful deliverysystems.Measuring and Characterizing Deposition from ShampoosThe multiple attribute consumer assessment study is an important hurdle to qualifyproducts for market. Common attributes that are tested by consumer study are cleansing,ease of wet and dry combing, hair softness, and lather amount and creaminess.34Secondary ion mass spectrometry can be used to detect the distribution of silicone on thehair. This technique is especially useful to assess whether the distribution is even orlocalized on, for example, cuticle edges or regions of weathering or damage. 34The thickness of silicon layers on hair ca
A Brief Review of Polymer/Surfactant Interaction By: Robert Y. Lochhead and Lisa R. Huisinga, The Institute for Formulation Science, The University of Southern Mississippi This brief review of polymer-surfactant interaction opens by describing how polymers behave in solution. Then we survey the literature on the interaction of nonionic polymers
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