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Interaction between Poly(acrylic acid) and an Ethoxylated Nonionic Surfactant I. D. Robb* and P. Stevenson Centre for Water Soluble Polymers, North East Wales Institute, Wrexham, Wales, U.K. LL11 2AW Received March 13, 2000. In Final Form: June 26, 2000 The interaction between uncharged polymer and nonionic surfactants is usually weak or nonexistent, except for the association between polyacids and ethoxylated nonionic surfactants. In this paper we have studied the interaction between poly(acrylic acid) and an ethoxylated nonyl phenol ether. Below pH 3 an insoluble complex forms, which redissolves on addition of excess surfactant. While the amount of surfactant required to form the maximum amount of precipitate or redissolve the complex is linearly dependent on polymer concentration, that required for the onset of precipitation is not. Despite the polydisperse nature of the ethoxylate chain, little fractionation of the surfactant after precipitation was observed. The nonionic surfactant was preferentially partitioned into water rather than dodecane, except where the polymer/ surfactant complex formed; in this case the complex dissolved mainly in the dodecane, forming a gel containing the oil. The origins of the effect of solution conditions and polymer characteristics on complex formation are discussed.
Introduction The interaction between polymers and surfactants is of importance in many colloidal systems and has been reviewed in several reports.1-3 Most attention has been paid to systems containing ionic surfactants with charged or uncharged polymers, as these usually exhibit the strongest interaction. Much less interest has been shown in association between nonionic surfactants and polymers. Association between polymers and surfactants usually occurs as a result of surfactant aggregate formation on the polymer being favored in comparison to that in solution. This often produces a critical concentration for the onset of association, except where4-6 the polymer has hydrophobic domains into which the surfactant can bind individually. Surfactant aggregation (in solution or on polymers) is promoted by hydrophobic interaction between the chains but opposed by repulsion between the hydrophilic headgroup. Repulsion between the headgroups of ionic surfactants is much larger than that between nonionic surfactants as shown by the much lower critical micelle concentrations (cmc’s) of nonionic surfactants compared to those of comparable alkyl chain length ionic surfactants. In addition neutron scattering studies7 at the air/water (a/w) interface show the water of hydration around the ionic headgroup extends a short distance (one or two carbon atoms) down the alkyl chain. Thus aggregate formation of ionic surfactants on hydrophobic polymers, is promoted both by hydrophobic interaction of the polymer chain with the carbon atoms near the headgroup of the ionic surfactant and by increasing the distance between * To whom correspondence should be addressed. (1) Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadamanabhan, K. P., Eds.; CRC Press: London, 1993. (2) Piculell, L.; Lindman, B. Adv. Colloid Interface Sci. 1992, 41, 149. (3) Robb I. D In Anionic Surfactants; Surfactant Science Series; Marcel Dekker: New York; Vol. 11, Chapter 3. (4) Thuresson, K.; Soderman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909. (5) Anthony, O.; Zana, R. Langmuir 1996, 12, 3590. (6) Thuresson, K.; Nystrom, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730. (7) Lu, J.; Thomas, R. K.; Penfold, J. Adv. Colloid Interface Sci. 2000, 84, 143.
ionic groups compared to a simple micelle. In contrast nonionic surfactants, having weaker repulsion between their headgroups, show much weaker association8,9 with most polymers. However association between ethoxylated nonionic surfactants (of the form CnEOm) and polyacids has been reported.10-16 In these systems, hydrogen bonding between the COOH and the oxygen of the ethylene oxide chain as well as hydrophobic interaction promotes surfactant aggregation on the polymer chain. Association between poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) at low pH has been observed15 and attributed to hydrogen bonding between the polymer and surfactant. Surface tension data10-12 show that below the onset of binding in the bulk of solution, the area per molecule of CnEOm surfactants at the air/water (a/w) interface is essentially unchanged by the addition of PAA. This contrasts with the change in area per molecule observed when nonionic polymers such as PEO are added to ionic surfactants. Here addition of the polymer causes the area per molecule of surfactant in the a/w interface to decrease, even at sufficiently low surfactant concentrations where no interaction or association takes place in the bulk of solution. This is probably a reflection of the fact that repulsion between nonionic headgroups is smaller than that between ionic groups. Thus although the polymer binds to the nonionic headgroup in the interface, there is little scope to decrease repulsion between these headgroups and hence alter their number density in the interface. The onset of binding of nonionic surfactants to PAA occurs at lower CnEOm concentrations12 with in(8) Brackman, J.; van Os, N. M.; Engberts J. B. F. N. Langmuir 1988, 4, 1266. (9) Winnik, F. M. Langmuir 1990, 6, 522 (10) Maloney, C.; Huber, K. J. Colloid Interface Sci. 1994, 164, 463. (11) Anghel, D. F.; Winnik, F. M.; Galatanu, N. Colloids Surf., A 1999, 149, 339. (12) Anghel, D. F.; Saito, S.; Baran, A.; Iovescu, A. Langmuir 1998, 14, 5342. (13) Saito, S.; Taniguchi, T. J. Am.Oil Chem. Soc. 1973, 50, 276. (14) Saito, S.; Taniguchi, T. J. Colloid Interface Sci. 1973, 44, 114. (15) Baranovsky, V.; Shenkov, S.; Rashkov, I.; Borisov, G. Eur. Polym. J. 1992, 28, 475. (16) Vasilescu, M.; Anghel, D. F.; Almgren, M.; Hansson, P.; Saito, S. Langmuir 1997, 13, 6951.
10.1021/la000386r CCC: $19.00 © 2000 American Chemical Society Published on Web 08/12/2000
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creasing n, is only slightly dependent11 on m, and is largely independent13 of temperature. At pH < 3.0, PAA can form a precipitate with CnEOm, redissolving on addition of excess surfactant. Redissolution occurred at lower surfactant concentration with increasing m, probably due to the better steric stability with longer EO chains. Aggregation numbers of the surfactant on PAA chains were found16 to be lower than those obtained for pure surfactant in free micelles. In this paper we report the interaction between PAA and ethoxylated nonionic surfactants and the consequent effect on the distribution of the surfactant between oils and water. Experimental Section To measure the distribution of the surfactant between oil and water, an ethoxylated nonyl phenol surfactant, Synperonic NP13, supplied by ICI Surfactants was used. Concentrations of surfactant in oil and water were measured using absorbance at 272 nm. Partitioning of the surfactant between the oils (octanol, hexane, and dodecane) with equal weights of water was performed, using UV absorbance to measure surfactant concentration and HPLC to measure polymer concentrations. The oils obtained from Aldrich were 99% pure and used as received. An analysis of the surfactant using 1H NMR showed the average number of carbon atoms in the alkyl chain to be 9.1 and the average number of ethoxylate units to be 12.7. The ethoxylate distribution was measured using evaporative light scattering detection (ELSD). In this technique, the surfactant is eluted down a column to separate the molecules by EO number, the eluate converted to a stream of fine droplets, and the solvent evaporated. Scattered laser light was then used to measure the amount of dried surfactant in each drop. The distribution of ethoxylate groups is shown in Figure 7. The cmc of the surfactant was measured by surface tension using a Kruss K8 DuNuoy tensiometer to be 0.006% (w/w) Several samples of PAA of different molecular masses ranging from 100 000 (supplied by Ciba Geigy), 175 000 (supplied by Aldrich), 160 000 (supplied by Ciba Geigy) to 220 000 Da (supplied by BDH) were used. The molecular mass distribution was measured by gel permecation chromatography (GPC), using a DAWN multiangle laser light scattering detection system with the polymers in a solution at pH 9 and 0.1 mol dm-3 NaCl. The concentration of polymer in any aqueous system was analyzed by raising the pH of the polymer/surfactant solutions to 8 and passing down the column of an HPLC apparatus containing a hydrophobically modified packing. This separated the polymer and surfactant and the polymer concentration was registered using a refractive index detector. Partitioning of the polymer and surfactant between oils and water was performed using equal weights of oil (octanol, hexane, and dodecane) and water.
Figure 1. Onset of precipitation (/) and redissolution boundary (O) of PAA-ENI surfactant complex as a function of temperature for systems containing 0.05% w/w PAA175K at pH 3 in the presence of varying concentrations of Synperonic NP13.
Figure 2. Ratio of Synperonic NP13/PAA in the insoluble complex as a function of total NP13 added: (b) using 0.2% w/w PAA220K at pH 2.5; (4) using 0.2% w/w PAA 175K at pH 3; (/) using 0.2% PAA 160K at pH 2.9.
Results Addition of the nonionic surfactant NP13 to poly(acrylic acid) at pH < 3.0 results initially in the formation of a precipitate, followed by its dissolution to form one phase. The temperature dependence of this phase separation is shown in Figure 1. The initial onset of precipitation (0.012-0.014%) occurs above the cmc of the pure surfactant (0.006%) and is essentially temperature independent whereas the redissolution boundary shows a marked temperature dependence. The precipitated polymer/surfactant complex was isolated from several systems and analyzed, the ratio of surfactant to polymer in the complexes being shown in Figure 2. The surfactant:polymer ratio does not remain constant, but increases with increasing amount of added surfactant. Experiments were performed to obtain the effect of pH and ionic strength on the onset of precipitation, maximum precipitation, and redissolution of the precipitate. The concentration of NP13 surfactant remaining in solution after removal of any polymer/surfactant precipitate is
Figure 3. Concentration of Synperonic NP13 in supernatant solutions at 25 °C after precipitating with 0.2% w/w PAA160K at pH 2.90 (O), pH 2.90 in the presence of 0.01M NaCl (4), pH 2.90 in the presence of 0.05 M NaCl (2), and pH 2.50 (b). The dashed line is for Synperonic NP13 in water with no PAA present.
shown in Figure 3. Increasing pH from 2.5 to 2.9 led to a higher surfactant concentration for the onset of precipitation, while addition of electrolyte to the system at pH 2.9 allowed precipitation to occur at a lower surfactant concentration. The total redissolution of the polymer/ surfactant complex was taken to be the concentration of surfactant at which the surfactant in solution became equal to the total in the system. The data in Figure 4 show that at pH 2.5 redissolution occurred at 0.6% surfactant, higher than the 0.45% at pH 2.9. However addition of NaCl (0.05 M) at pH 2.9 caused redissolution to occur at lower surfactant concentrations. The point of maximum precipitation was taken where the concentrations of
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Figure 4. Concentration of Synperonic NP13 in supernatant solutions at 25 °C after precipitating with 0.2% w/w PAA 160K at pH 2.90 (O), pH 2.90 in the presence of 0.05 M NaCl (2), and pH 2.50 (b). The dashed line is for Synperonic NP13 in water with no PAA present.
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Figure 7. Distribution of ethoxylate group of Synperonic NP13 in supernatant solution after precipitating various amounts with a constant PAA 160K concentration of 0.2% w/w at pH 2.50: (9) freeze-dried Synperonic NP13; (]) Synperonic NP13 that has not been freeze-dried; (2) distribution of EO groups after 0.45% w/w NP13 becomes complexed; (b) distribution of EO groups after 0.50% w/w NP13 becomes complexed; (4) distribution of EO groups after 0.25% w/w NP13 becomes complexed, (/) distribution of EO groups after 0.35% w/w NP13 becomes complexed.
Figure 5. Amount of PAA 175K (b) and Synperonic NP13 (4) remaining in aqueous solution after precipitating at pH 3.0.
Figure 8. Concentration of Synperonic NP13 in supernatant solutions at 25 °C after precipitating with 0.05% w/w PAA 175K at pH 3.0 (2) and 0.05% w/w PAA 100K at pH 3.0 (4). The dashed line is for Synperonic NP13 in water with no PAA present.
Figure 6. Concentration of Synperonic NP13 required to precipitate (0), redissolve (O), and cause maximum precipitation of PAA 175K (4) as a function of PAA 175K concentration. The dashed lines show the linearity associated with redissolution and maximum precipitation.
polymer and surfactant in solution were a minimum. The data in Figure 5 show that as surfactant was added to a constant polymer concentration (0.2%), the concentration of polymer and surfactant remaining in solution passed through a minimum at 0.33% surfactant. This corresponded to a surfactant:polymer ratio (w/w) of 1.5, after which the concentration of both polymer and surfactant free in solution increased. The concentration of surfactant required for (a) the onset on precipitation, (b) maximum precipitation, and (c) redissolution are plotted in Figure 6. As the surfactant is a commercial material with a distribution of both alkyl and ethoxylate chain lengths, it was possible that fractionation of the surfactant would
occur on precipitation. This was particularly so with regard to EO lengths which associate directly with the PAA and show the greatest spread of lengths. To study this, various amounts of NP13 surfactant were added to 0.2% PAA 160K at pH 2.5 and the ethoxylate distribution of the surfactant remaining in solution was measured by ELSD. The EO distributions of the pure surfactant and of that remaining in solution after precipitation of various amounts of polymer/surfactant complex are presented in Figure 7. They indicate that there was very little difference between the ethoxylate chain length distribution of the pure surfactant and that remaining in solution after precipitation with the polyacid, indicating little fractionation as a function of ethoxylate chain length had taken place. The dependence of phase separation on the molecular mass of the polymer is shown in Figure 8. There was a small effect of molecular mass on redissolution, more surfactant being required for higher molecular mass polymers. It is well-known that nonionic surfactants distribute between oils and water and are sometimes more soluble in polar oils than in water. The effect of added PAA on the distribution of the surfactant between equal weights of oil and water was measured for the oils octanol, hexane, and dodecane. For octanol/water mixtures, at total con-
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Figure 10. Gel phase formation after the aqueous PAA 175K (0.2% w/w)-surfactant solution, below pH 3, is mixed with equal weights of dodecane.
Figure 9. Concentration of Synperonic NP13 in aqueous solution after partitioning between dodecane and water in the presence of 0.05% w/w PAA175K at pH 3.0 (0) and absence of PAA (2). The dashed line is for Synperonic NP13 in pure water in the absence of PAA.
centrations up to 0.2% surfactant, about 91% of the surfactant partitioned into the oil. Addition of PAA at pH 3 increased the proportion of the surfactant in the octanol to 99% at 0.05% PAA. The distribution of mixtures of the polymer and surfactant between the more nonpolar oils of hexane or dodecane and water was measured, and the data for dodecane-water are shown in Figure 9. The data for hexane-water were the same within experimental error. In the absence of polymer the surfactant was preferentially distributed into the water. In the presence of polymer (0.05%) the surfactant remained preferentially in the water up to 0.01% surfactant, which is close to the concentration at which precipitation starts in pure water with that amount of polymer. Increasing concentrations of surfactant then distribute preferentially into the dodecane over the region where the precipitate formed in pure water. At high surfactant concentrations (0.15-0.2%) most of the surfactant had returned to the water. In the polymer/surfactant systems with either dodecane or hexane with water, no precipitate was observed in the water or oil phase. The concentrations of polymer and surfactant remaining in water after partitioning between dodecane and water for a constant polymer concentration of 0.2% are the same as those in Figure 5 within experimental error. They show that the polymer as well as the surfactant distribute between the dodecane and water. The maximum preference for the oil occurs at about 0.33% surfactant, corresponding closely to the point of maximum precipitation in pure water. With the dodecane/ water system containing polymer and surfactant, it was noticed after centifuging the whole system that the dodecane phase existed partly as a gel and partly as a solution. The volume of the gel passed through a maximum (at about 0.4% surfactant) with increasing surfactant content as shown in Figure 10. The gel was easily broken by stirring with a spatula. Discussion The well-known19 decrease in solubility and second virial coefficient of poly(ethylene oxide) chains with increasing temperature are the main cause of cloud points20 of nonionic surfactants and their decrease in cmc with increasing temperature. However the formation of a (17) Saito, S. J. Colloid Interface Sci. 1994, 165, 505. (18) Cole, M. L.; Whateley, T. L. J. Colloid Interface Sci. 1996, 180, 421. (19) Malcolm, G.; Rowlinson, G. Trans. Faraday Soc. 1957, 53, 421. (20) Clint, J. Surfactant Aggregation; Blackie: London, 1992; p 154.
precipitate between a polyacid and a nonionic surfactant is caused by both the hydrogen bonding between the ethoxylate chains and the carboxylic acid groups together with the hydrophobic association between the alkyl chains of the surfactant. While the sharp onset of binding indicates cooperative binding, the data in Figure 2 show that much of the initial contact of the ethoxylate headgroups is with the acid groups on the polymer chain rather than other ethoxylate chains as in free micelles. Thus there is little temperature dependence for the onset of precipitation. In contrast the redissolution of the precipitate depends on the ability of the ethoxylate chain to act as a steric stabilization agent for the polymer/ surfactant complex. This is temperature dependent, as shown by the existence of the cloud points of pure nonionic surfactants, making redissolution quite temperature dependent. As surfactant is added to the polyacid, the amount of complex precipitated passes through a maximum and eventually the system returns to one phase. The ratio of surfactant to polymer in the precipitate increases with increasing surfactant (Figure 2) showing that the complex has no fixed stoichiometry, there being only a small number of charged species in the precipitate to require any fixed ratio of constituents in the separated phase. Adsorption of the surfactant onto the polymer/surfactant precipitate accounts for the increased surfactant:polymer ratio. Precipitation of the polymer/surfactant complex would be expected to be strongly influenced by small numbers of charges on the polymer chain. On precipitation the counterion would be restricted to the small precipitated phase, resulting in a significant loss of entropy on precipitation. The effect of this is observed with the data in Figure 3 where small changes in pH from 2.9 to 2.5 significantly altered the onset of precipitation. At pH 2.9 about 0.5% of the COOH groups would be ionized,21 so that addition of electrolyte or lowering the pH to 2.5 facilitates phase separation, as expected. For similar reasons redissolution of the polymer/surfactant complex (Figure 4) is also facilitated at the higher pH of 2.9 compared to 2.5. The surfactant concentration at both the point of maximum precipitation and redissolution are linearly dependent (Figure 6) on polymer concentration, as would be expected if the polymer/surfactant complex is the same at these points for different polymer concentrations. However the surfactant concentration for onset of precipitation is not linearly dependent on polymer concentration, possibly for the following reason. It has been shown22,23 for the adsorption of molecules to substrates, (21) Katchalsky, A. J. Polym. Sci. 1957, 13, 69. (22) Hall, D. G. Trans Faraday Soc. 1971, 67, 2516. (23) Tanford, C. Physical Chemistry of Macromolecules; JohnWiley: New York, 1961; p 573.
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including the binding of species such as protons or surfactants to macromolecules, that δN ˜ /δµ2 ) [N ˜ 2 - (N ˜ )2]/ kT, where N is the number of surfactant molecules (species 2) per polymer chain, so that δN ˜ /δµ2 is the average number of surfactant molecules bound to the polymer as the surfactant concentration is increased. For δN ˜ /δµ2 to suddenly increase, there must be an uneven distribution of surfactant on the polymer chains; i.e., some polymer chains must have clusters of surfactant while other have very little. Thus since each polymer chain does not have the same number of surfactants in the early stages of binding, it is reasonable to expect that the surfactant concentration at the onset of precipitation does not vary linearly with polymer concentration. Complex formation between polymer and surfactant might be expected to be dependent on ethoxylate chain length and hence result in some fractionation of the surfactant. The fact that little fractionation (Figure 7) was observed may be the result of two competing factors which will tend to cancel, i.e., longer EO chain surfactants will have greater solubility in water but also have stronger interaction with the polymer. In addition the clustering of surfactants on the polymer chain tends to randomize the surfactant, also reducing any fractionation. It is reported that ethoxylated nonionic surfactants distribute preferentially24-26 into polar oils from water in agreement with the results here with octanol. However the surfactant was preferentially distributed into water compared to the nonpolar oils, dodecane and hexane, except in the presence of PAA. From the concentrations of polymer and surfactant remaining in the water, it is clear that the polymer/surfactant complex dissolves readily
into dodecane, producing a weak gel having a maximum volume close to the point of maximum precipitation in the aqueous phase. Gel formation is probably the result of association between COOH groups on different polymer chains, interrupted by their association with ethoxylate groups.
(24) Marquez, N.; Anton, R.; Graciaa, A.; Lachaise, J.; Salager, J.-L. Colloids Surf., A 1995, 100, 225. (25) Graciaa, A.; Lachaise, J.; Sayous J. G.; Grenier, P.; Yiv, S.; Schechter, R. S.; Wade, W. H. J. Colloid Interface Sci. 1983, 93, 474. (26) Salager, J.-L.; Marquez, N.; Anton, R. E.; Graciaa, A.; Lachaise, J. Langmuir 1995, 11, 37.
Acknowledgment. The authors thank Iain Fairweather for the ELSD analysis of the surfactant and Unilever for supporting P.S. during this work.
Conclusions The insoluble complex formed at low pH between poly(acrylic acid) and a nonyl phenyl ethoxylate surfactant redissolves in excess surfactant which acts as a steric stabilizing agent. Because of this, the insoluble complex does not have a fixed stoichiometry but has an increasing surfactant:polymer ratio approaching redissolution. The point of redissolution and maximum precipitation occurs at a constant surfactant:polymer ratio; however this ratio at onset of precipitation is dependent on the polymer concentration, possibly because of the nonuniform distribution of the surfactant on the polymer chains. Small amounts of charge on the polymer chain can have a significant retarding effect on precipitation of the complex. Although the surfactant used here had a wide range of ethoxylate chain lengths, little fractionation of the surfactant between the complex and solution was observed. In the presence of water and dodecane, the surfactant alone partitioned preferentially into the dodecane; addition of the polymer caused partitioning of the polymer and surfactant into the oil mainly where the insoluble complex formed in water alone. The polymer/surfactant complex was able to form a weak gel in the dodecane.
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