Polymer-surfactant interactions: modes of association and methods of analysis.
Polymers and surfactants are common ingredients in many products, including paints, inks, cosmetics, adhesives and detergent formulations. There is much that is known about the behavior of each of these ingredients separately, but the dynamic interactions between them in a completed formulation are only beginning to be understood. It is a fact that the efficacy of polymers can be affected by the presence of surfactants, and vice versa. The purpose of this article is twofold: to describe the types of associations that occur between polymers and surfactants in water-based formulations, and to discuss a few of the many suitable methods for measuring the degree of interactions between them.
A surfactant is a surface active agent, a material of fairly low molecular weight that is amphiphilic in nature. That is, it possesses both a hydrophilic and a hydrophobic portion (see Figure 1). There are three types of surfactants: nonionic (in which the headgroup is hydrophohic and the "tail" is hydrophilic, and generally ethoxylated), cationic (possessing positive charge in the headgroup), and anionic (possessing negative charge in the headgroup). For both cationic and anionic surfactants, the headgroup is hydrophilic and the "tail" is hydrophobic.
[FIGURE 1 OMITTED]
When incorporated into water, surfactants strive to reduce their surface energy by isolating their hydrophobic segments from the aqueous phase. There are three primary ways in which they can do this:
1. Adsorption at an interface (particle, liquid surface, air bubble).
2. Formation of micelles (self association).
3. Association with other components of the formulation (as in polymer/surfactant complexes, e.g.).
Micelles: Aggregates of Surfactant Molecules
The dual nature of surfactants leads to a struggle between the hydrophilic heads attempting to increase their contact with water and the hydrophobic tails trying to avoid it. This tension leads to a sort of compromise: an optimal surface area for the water-surfactant interface specific to each type of molecule. The shapes in which the molecules arrange themselves depend partly on the optimal surface area, as well as partly on the fluid volume of the hydrocarbon chains and the maximum length at which they can still be considered fluid. Although many structures can fit the geometry, one is usually best from a thermodynamic perspective.
At low concentrations of surfactant, the solution looks like any other: surfactant molecules distributed randomly throughout the water. When the concentration gets high enough, however, the molecules begin to arrange themselves in hollow spheres, rods and disks called micelles. The surface of a micelle is a layer of polar heads dissolved in the water, while the inner portion consists of hydrophobic tails screened from the water by the hydrophilic heads. Generally, the most favored (lowest energy) conformation of a micelle is spherical (see Figure 2).
[FIGURE 2 OMITTED]
The critical micelle concentration (CMC) is the concentration of surfactant in solution that must be reached for micelles to begin to form. Below this point, the surface tension of the solution will reach a minimum; once the CMC is reached, the surface tension will not change further. Therefore, the surface tension at the CMC is the lowest value that the system can achieve.
In addition to forming micelles, the surfactant can isolate its hydrophobic groups from the aqueous phase by associating them with the hydrophobic moieties on a polymer. In particular, hydrophobic polymers can interact strongly with nonionic surfactants. In general, anionic surfactants demonstrate a stronger interaction with nonionic polymers than do nonionic or cationic surfactants. The reason for a difference in cationic vs. anionic behavior may be due to the different hydration characteristics of the cationic and anionic headgroups.
Anionic surfactants have been shown to interact strongly with neutral synthetic polymers PVA (polyvinyl alcohol), PEO (polyethylene oxide), and PVP (polyvinyl pyrrolidone); hydrophobic attraction is expected to be the primary mode of interaction since no conspicuous electrostatic forces operate between them.
However, cationic surfactants do not show the same affinity for these polymers (nor do nonionics). They interact more strongly with more hydrophobic polymers such as PPO (polypropylene oxide) or PVA-Ac (polyvinyl acetate). (1)
Sprycha and Krishnan (2) showed that styrene/maleic anhydride copolymers with higher styrene content had a greater degree of interaction with Surfynol SE-F (a nonionic acetylenic diol surfactant). SMA 3000 contains a 3:1 molar ratio of styrene to maleic anhydride, whereas SMA 1000 is more hydrophilic, at 1:1 styrene-maleic anhydride (see Figure 3).
[FIGURE 3 OMITTED]
It was noted that all combinations of polymer and Surfynol SE-F caused a rise in surface tension (over that for Surfynol SE-F alone), due to adsorption of surfactant molecules along the polymer chain.
A half ester of SMA 1000, esterified with butyl cellosolve, was also evaluated in this experiment. It was found to have a stronger association with Surfynol SE-F than did SMA 1000 alone. The reason for this is that hydrophobic associations described above are operant, and, in addition, interactions between the ethoxylated portion of the surfactant and the ester functionality on the polymer exist.
Surfactants are often added to a product in order to affect its ability to wet a surface to which it is applied. In addition, surfactants are used in pigmented formulations to enhance wetting of the pigment surfaces during manufacture. This wetting of the pigment surfaces aids in the dispersion of such pigments.
Therefore, in a formulation where the polymer and surfactant have a substantial affinity for one another, an excess of surfactant would be needed in order to have a sufficient amount present at a substrate or pigment surface to modify surface tension thereupon.
Association of surfactant and polymer can also alter the rheological characteristics of the system. Adsorption of surfactant along a polymer chain can cause conformational changes of that polymer as well as three-dimensional networking. The degree to which the viscosity of the system is affected depends on how much the polymer conformation is changed due to surfactant adsorption.
Sprycha and Krishnan (3) found that addition of Surfynol SE-F to SMA 1000 and SMA 3000 caused minimal viscosity changes to solutions of these polymers. Conversely, addition of the surfactant to the half ester of SMA 1000 entailed a high degree of viscosity increase. One can conclude from this that conformational changes to the SMA 1000 and SMA 3000 were minimal, despite a high degree of polymer/surfactant interaction. It can also be concluded that there were substantial conformational changes to the half ester upon introduction of the surfactant. It is quite possible that the half ester became more extended when surfactant was adsorbed thereupon. Entanglement of polymer chains at that point would result in a viscosity rise.
Kevelam (4) describes a "polyelectrolyte effect" creating the viscosity rise for multiple ionic micelles bound to a nonionic polymer molecule. In this case, the charged groups repel one another and cause the polymer backbone to expand. This expansion causes the polymer molecules to sweep out a larger volume in solution.
Alternatively, adsorption of surfactant can cause contraction of polymer chains in some instances. Pereira et al (5) describe the effects of sodium dodecyl sulfate (SDS) on poly(N-isopropylacrylamide) [PNIPAM]. PNIPAM solubility decreases with temperature and will experience a slow transition from expanded coils to smaller globules upon heating to above 31[degrees]C. With a low amount of SDS, a similar transition is noted. Interestingly enough, however, at higher SDS concentrations, the coil-to-globule transition was sharp and first order in nature. It occurred above the 0 temperature * (worse than [theta] conditions for PNIPAM--meaning that PNIPAM was insoluble). At the highest SDS level, this transition proceeded gradually beginning at the [theta] temperature of the PNIPAM chains. Deep in the worse than 0 (i.e. insoluble) regime, a first order collapse ensued. Such an appearance of two collapse transitions, one gradual, and one first order, suggest that such a collapse is a complex process.
What has happened is that the temperature at which the chain collapses is moved further into the worse than 0 regime due to the adsorption of the surfactant. Eventually, however, the entropic losses of surfactant moving from the solution to the polymer chain and the worsening of the solvent quality in the medium cannot be offset by the adsorption energy--thus, the surfactant moves back into the solution phase and the polymer coils contract. Hence, the transition now becomes first order, with a discontinuous drop in polymer dimensions (see Figure 4).
[FIGURE 4 OMITTED]
The conclusions drawn from the above are that the association of surfactant with a polymer can result in abstraction of that surfactant such that it is not available to modify surface or interracial tensions. Additionally, adsorption of the surfactant onto a polymer chain can modify its dimensions such that it affects the rheology of the formulation or solubility of the polymer. These changes can be somewhat deleterious. However, there can be advantages to polymer-surfactant complex formations as well. One notable instance occurs when one is trying to improve the solubilization of a third species.
Surfactants are often used to solubilize and suspend materials in an aqueous medium, i.e. in dyeing operations or in emulsion polymerization. However, there are limits to the amount of material that a given surfactant can incorporate into its micelles. In certain cases where a polymer-surfactant complex is used, and the surfactant concentration in the complex is fairly high, the polymer/surfactant complex may show solubilization behavior greater than that of the surfactant alone and at concentrations below the CMC of the surfactant. Some examples (6) of nonionic homopolymer/anionic surfactant interactions that exhibit this behavior are:
(a) Sodium alkylsulfates containing 10-16 carbon atoms at concentrations below their CMC form complexes with serum albumin that solubilize oil-soluble azo dyes and isooctane. The moles of dye solubilized per mole of surfactant appear to increase with an increase in the chain length of the surfactant, the number of surfactant molecules adsorbed per mole of protein, and the concentration of the protein. Another example (7) utilizes Orange OT solubilized in mixtures of sodium alkylsulfates of similar alkyl chain lengths ([C.sub.10]-[C.sub.16]) and PVP. In this case, inclusion of the polymer allowed for solubilization of the dye at lower surfactant amounts than if the PVP had been excluded from the system (see Figure 5).
[FIGURE 5 OMITTED]
(b) Addition of polyoxyethylene glycols to solutions of sodium dodecyl sulfate and sodium p-octylbenzenesulfonate increased their solubilization power for Yellow OB. As the degree of polymerization of the glycol increased, the extent of solubilization of the dye increased. This effect is believed to be due to the formation of two types of complexes between the surfactant micelles and the glycol. Low molecular weight polyoxyethylene glycols (degree of polymerization < 10-15) are believed to form micelle-glycol complexes in which the glycol is adsorbed on the surface of the micelle in a manner similar to that of small polar compounds and the dye is located mainly in the inner core of the micelle. Higher molecular weight glycols are believed to form true polymer-surfactant complexes in which the glycol is in the form of a random coil bound to the surfactant with its hydrophilic groups oriented towards the aqueous phase. Here the dye is solubilized in the polyoxyethylene-rich region.
In general, the more hydrophobic the polymer, the greater the adsorption of surfactant onto it from water. Hydrophilic groups on the polymer can interact with water and weaken surfactant-polymer interactions. In considering the interaction of anionic surfactants and nonionic polymers, the degree of surfactant adsorption appears to follow the approximate order: polyvinylpyrrolidone ~ polypropylene glycol > polyvinylacerate > methylcellalose > polyethylene glycol > polyvinyl alcohol. Interestingly enough, PVP is fairly hydrophilic, yet acts as if it is more hydrophobic in its degree of interaction with anionic surfactants. This is potentially due to the fact that PVP becomes weakly protonated in aqueous solution (further borne out by the fact that cationic surfactants interact only very weakly or not at all with PVP).
Many studies have been done on complexation of anionic surfactants with nonionic homopolymers and hydrophobically-modified polymers, as the results of such interactions in the area of rheological properties and surface tension modification are quite measurable. However, though it appears that cationic surfactants and nonionic surfactants appear to have a much lower degree of association with the polymer as measured by these means, this is not necessarily the case. Ruckenstein (8) reports that though interaction between n-octylthioglucoside (OTG) and polypropylene oxide (PPO) was not evident by measurements of CMC (it remained constant), microcalorimetry studies detected interaction between the two species. Additional evidence for this was a change in turbidity of the PPO solution at the CMC of OTG, perturbed clouding behavior of PPO, and the reduced Krafft temperature of OTG.
Mechanism of Interaction
The classical behavior when incorporating surfactant into water begins with a gradual drop in surface tension as the surfactant concentration is increased. At this point, the surfactant exists in the solution as single molecules, or unimers, as they are sometimes termed.
At the CMC, the surface tension curve abruptly levels off, and no further lowering of the surface tension is experienced (see dotted line in Figure 6).
[FIGURE 6 OMITTED]
This point corresponds to the formation of micelles. When a surfactant molecule reaches its solubility limit and begins to self-associate in the form of micelles, there is no further allocation of surfactant to the air-water interface, and thus no additional surface tension reduction.
Incorporating a polymer into the solution changes the surface tension profile dramatically, howeve1: Initially, the surface tension of the polymer solution drops as surfactant is added, as before. Note the solid line in Figure 6; initially, it is co-linear with the dotted line depicting surfactant alone.
However, as the concentration of the surfactant increases, an early point of inflection is reached. This corresponds to the critical aggregation concentration, or CAC (denoted [T.sub.1]). At this point, the surfactant molecules begin to associate with the polymer in solution. Like micellization, this behavior allows the surfactant to isolate its hydrophobic portions from the aqueous medium. However, the CAC is always below the CMC.
As the surfactant concentration is raised further, the surface tension will remain fairly constant. This is due to the fact that the surfactant, in continuing to adsorb onto the polymer, is being abstracted from the aqueous phase and therefore is not present to modify the solution's surface tension (same case as that which occurs during micellization).
However, eventually, the surface tension of the medium again begins to gradually decrease. What has occurred at the point where this begins is that the polymer has become saturated by surfactant (at [T.sub.2]'). Continued addition of surfactant increases the concentration in the bulk aqueous phase, therefore: the surface tension is again modified. Figure 6 shows an idealized plateau for the region [T.sub.1]-[T.sub.2]'. However, a gradual drop to the value at concentration [T.sub.2] is most often observed (9); it is not a foregone conclusion that a surface tension plateau in the [T.sub.1]-[T.sub.2]' region exists.
As in the case for surfactant alone, eventually the concentration in the bulk reaches the CMC (at [T.sub.2]), and the curve abruptly flattens as micellization begins.
There are two accepted mechanisms of polymer-surfactant complex formation: one involves association of the surfactant to the polymer as unimers; the other involves micellization of the surfactant on or in the vicinity of the polymer chain. For polymers with hydrophobic groups, unimeric binding is dominant, whereas for hydrophilic polymers, the micelle formation scenario is more probable.
When micelles are formed along a hydrophilic polymer chain, they are assumed to be of uniform size whether polymer is present or not. The complex takes on a structure similar to that of a necklace (see Figure 7).
[FIGURE 7 OMITTED]
The driving force for interaction between the polymer and the surfactant micelles is to further stabilize the interface between the hydrophobic core of the micelle and the aqueous phase.
The total concentration of surfactant in the system is described by Nagarajan (10) as:
[[S.sub.T]] = [[S.sub.f]] + [[S.sub.b]] + m[[S.sub.m]]
where [[S.sub.f]] is the concentration of the free surfactant molecules, [[S.sub.b]] is the concentration of the surfactant molecules bound to the polymer, and [[S.sub.m]] is the concentration of the micellized surfactant molecules in the solution. The extent of this partitioning depends on the intrinsic binding constant, [K.sub.b], the micellization equilibrium constant, [K.sub.m], the critical or optimal micelle size, m, the size of the surfactant clusters bound at a polymer binding site, [lambda], and the number of binding sites per polymer molecule, n:
(11) [[S.sub.T]] = [[S.sub.f]] + [[P.sub.T]] n[lambda][K.sub.b][[[S.sub.f]].sup.[lambda]]/1 + [K.sub.b][[[S.sub.f]].sup.[lambda]] + m[K.sub.m][[[S.sub.f]].sup.m]
The value of [K.sub.b] is arbitrarily taken to be twice that of [K.sub.m] in recognition of the fact that surfactant/polymer binding occurs more readily than surfactant micellization (CAC occurs before CMC). This theory assumes only one type of micelle forms (spherical).
Classical polymer-surfactant interactions (nonionic homopolymer/anionic surfactant) as described above are very different from hydrophobically-modified polymer/single tailed surfactant associations, despite the fact that the hydrophobic effect plays a critical role in both cases. In the classical case of SDS and polyethylene oxide (PEO) association, micelles are bound to the polymer. To have a viscosifying effect, the surfactant concentration should exceed the CMC. However, the opposite is true when hydrophobically-modified polymers are involved; the viscosifying effect is most pronounced at low surfactant concentrations, where the surfactants are present as monomers. (12)
Hydrophobically modified polymers can self-associate (intra- or inter-molecularly) through entanglement of their alkyl side chains. Intramolecular associations cause the formation of compact globules, which causes a significant decrease in the viscosity of the solution. The flexibility of the backbone will affect how readily such hydrophobic domains form.
On the other hand, formation of intermolecular associations between hydrophobic side chains depends on the concentration of polymer in solution. These associations will increase the viscosity of the polymer solution. Additionally, an increasing number and/or length of the hydrophobic side chains will increase the maximum viscosity that can be achieved.
If the polymer concentration exceeds the critical overlap concentration, an addition of surfactant will increase viscosity. This is due to an increase in the number and the strength of the hydrophobic chain interactions resulting from adsorption of individual surfactant molecules or micelles onto interpolymeric hydrophobic domains (see Figure 8.)
[FIGURE 8 OMITTED]
A maximum is reached at about the CMC of the surfactant. Continued addition of surfactant causes the viscosity to drop (see Figure 9).
[FIGURE 9 OMITTED]
This can be explained by the fact that as the CMC is exceeded, each polymer hydrophobe becomes individually solubilized by a single micelle. As a consequence, the interpolymeric network is broken down.
Since in this case, hydrophobic forces predominate, the viscosity effect between these species will occur even if there are repulsive electrostatic forces between the polymer back. bone and surfactant.
Touhami (13) et al described the study of polymer-surfactant complexes as either "polymer-centered" or "surfactant-centered." The difference between the two vantage points is subtle, a sort of "chicken or egg?" approach. In the polymer-centered case, the description of forming complexes centers around the possession by the polymer of suitable binding sites for surfactant molecules. In the surfactant centered case, creation of polymer-surfactant complexes is seen as a consequence of or is related to surfactant micellization. Whether one looks at it from the vantage point of the polymer or of the surfactant, it is generally recognized that polymer-surfactant association results in surfactant micellization, though one that is in some way perturbed by the presence of the polymer.
For a system of PEG/SDS, it was concluded that increasing the level the polymer in solution increased the CMC of the surfactant, and the CAC was independent of the surfactant concentration (see Figure 10).
[FIGURE 10 OMITTED]
It was also noted that as the polymer concentration increased, the bound surfactant concentration increases (see Figure 11).
[FIGURE 11 OMITTED]
Other Measurements of Polymer/Surfactant Interactions
In addition to the methods referred to above (viscosity methods, dye solubilization experiments, surface tension experiments), there are a few other notable instrumental methods for elucidating the degree and nature of polymer-surfactant associations.
One popular method utilizes fluorescence emission, using pyrene as probe molecule. This is useful in case where the surfactant-polymer complex is sufficiently surface active itself. For these instances, surface tension measurements will not be adequate to assess the degree of association present.
The ratio of the intensities of first and third bands of pyrene may be used of as an index of the microscopic changes occurring in the polymer/surfactant complex. In the absence of the polymer, pyrene shows only one transition corresponding to the micellization process. In the presence of the polymer, the pyrene demonstrates two plateau regions, one corresponding to [T.sub.1], and one to [T.sub.2] (from surface tension studies). (14)
Pyrene is a nonpolar, polyaromatic molecule with low solubility in water. The fluorescence spectrum of pyrene possess a vibrational band structure which exhibits a strong sensitivity to the polarity of the environment. Relative intensities of the peaks at 372 and 385 nm undergo significant changes on going from solutions in nonpolar to solutions in polar solvents. [I.sub.1] increases with solvent polarity, while [I.sub.3] remains relatively unaffected. [I.sub.1]/[I.sub.3] increases from 0.64 to 1.45 on going from cyclohexane to ethanol; in water, [I.sub.1]/[I.sub.3] = 1.95.
Pyrene is also used to determine CMCs. Below the CMC of the surfactant, pyrene resides in water. As it diffuses into micelles; I1/I3 drops to 1.2.
The total fluorescence energy of pyrene in micelles is larger than in water (due to the fact that the lifetime of the excited state of pyrene is shorter in water than in hydrophobic environments).
The total fluorescence intensity provides information concerning the packing of the hydrophobic polymer chains in the domains. If the chains are closely packed, water is excluded from them and the fluorescence intensity will be high. Close packing of alkyl chains also increases the microviscosity of domains. This slows the diffusive motion of pyrene and dissolved oxygen in the domains (oxygen acts as a quencher for pyrene fluorescence). Therefore, quenching will be less efficient than in more 'open' (loosely packed) domains.
Another useful method for assessing polymer-surfactant interactions is photon correlation spectroscopy. Cosgrove (15) et al showed the effect of SDS on adsorption of PEO on silica. PEO adsorbs strongly on silica, whereas SDS does not. PCS measurements were used to determine the thickness of the adsorbed layer. SDS brings about a progressive thinning of the adsorbed PEO layer, reaching a limit at the CMC of the surfactant (see Figure 12).
[FIGURE 12 OMITTED]
Beyond the CMC, the thickness of the adsorbed layer increases again. This suggests that SDS micelles are bound by the attached PEO chains.
In summary, factors that play a part in the interaction of surfactant with polymers are:
1. Counterion effects/electrostatic effects.
2. Hydrophobicity around alkyl portion near ionic headgroups.
3. Size of headgroup.
4. Steric and polar factors such as hydrogen bonding or hydration.
5. Hydrophobicity of polymers.
6. Concentration of polymer and surfactant species present.
Interactions between polymers and surfactants in solution can be useful or detrimental to the characteristics of a formulation depending on its end use. Effects on rheology and surface tension are common.
Creating complexes that have the ability to solubilize colorants or other reagents to a greater degree than either the surfactant or the polymer alone is a synergistic benefit of these interactions.
* [theta] temperature--This is the lowest temperature (or highest in the case of PNIPAM which demonstrates insolubility with temperature) at which the polymer is soluble in the given solvent or system.
(1.) Saito and Anghel in Kwak, J. C. T. (ed), Polymer-Surfactant Systems. Surfactant Science Series, Vol. 77, Marcel Dekker, New York, 1998, Chapter 9.
(2.) Sprycha, R.; Krishnan, R., Interfacial Dynamics, Marcel Dekker, 1999.
(3.) ibid., p. 5.
(4.) Kevelam, J., thesis, Polymer-Surfactant Interactions: Aqueous Chemistry of Laundry Detergents, Rijksuniversiteit Groningen, 1998.
(5.) Pereira, G. G.; Williams, D. R. M.; Napper, D. H. Langmuir 1999, 15, 906-909.
(6.) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd Edition, Wiley-Interscience, New York, 1989.
(7.) Jonsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons, New York, 1998.
(8.) Ruckenstein, E., Langmuir 1999, 15, 8086-8089.
(9.) Goddard, E. D. Journal of Colloid and Interface Science, Elsevier Science, 2001.
(10.) Nagarajan, R., Polym. Prepr. 22:33, 1981.
(11.) Kwak, J. C. T. (ed), Polymer-Surfactant Systems. Surfactant Science Series, Vol. 77, Marcel Dekker, New York, 1998.
(12.) Kevelam, J., thesis, Polymer-Surfactant Interactions: Aqueous Chemistry of Laundry Detergents, Rijksuniversiteit Groningen, 1998, p. 31.
(13.) Touhami, Y.; Rana, D.; Neale, G. H.; Hornof, V., Colloid Polym. Sci. 2001, 279:297-300.
(14.) Kunjappu, J. T., American Ink Maker, March 1999.
(15.) Cosgrove, T., Mears, S. J., Obey, T., Thompson, L., and Wesley, R. D., Colloids Surf. 149, 329 (1999).
A longtime Ink World Editorial Advisory Board member, Lisa Hahn is president of Flexo Tech, and is the 2002 National Association of Printing Ink Manufacturers' (NAPIM) Technical Achievement Award recipient. She is a frequent presenter at the National Printing Ink Research Institute's (NPIRI) annual convention and during its summer courses. Ms. Hahn's Technical Tidbits column appears Ink World on a regular basis.
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