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Interaction of Sodium Dodecyl Sulfate with Methacrylate-PEG Comb Copolymers H. Middleton, R. J. English, and P. A. Williams* Centre for Water Soluble Polymers, North East Wales Institute, Plas Coch, Mold Road, Wrexham LL11 2AW, United Kingdom
G. Broze Colgate Palmolive, Research and Development Inc., Avenue du Parc Industriel, B-4041 Milmort, Herstal, Belgium Received December 1, 2004. In Final Form: March 8, 2005 A series of sodium methacrylate and poly(ethylene glycol) (PEG) comb copolymers (MAA/PEG) with approximate PEG chain lengths of 7, 11, and 22 ethylene oxide units were synthesized by free radical polymerization. Their weight-average molecular mass was found to be ∼66 000. A commercial sample of a PEG comb polymer with an acrylic backbone was also used in the studies (Sokalan HP 80). The interaction of the MAA/PEG comb polymers and pure sodium methacrylate (SPMA) with sodium dodecyl sulfate (SDS) was studied by ESR spectroscopy using 5-doxyl stearic acid (5-DSA) spin probe and by conductivity measurements. Surfactant aggregation in water occurred at SDS concentrations lower than the surfactant critical micelle concentration (cmc) and depended on the polymer concentration. The observations have been attributed to changes in the effective ionic strength of the systems due to the polymer itself, and it has been concluded that there is no interaction between the MAA/PEG comb copolymers or SPMA and SDS. This has been confirmed by the fact that the decrease in surfactant aggregation concentration is similar in magnitude to the decrease observed on adding NaCl when counterion ion condensation effects are taken into account. It is apparent that the electrostatic repulsions between the surfactant molecules and the methacrylate backbone of the MAA/PEG comb copolymers inhibit association of SDS with the PEG side chains.
Introduction Comb polymers with poly(ethylene glycol) (PEG) side chains are commonly used as dispersants in a wide range of industrial sectors such as surface coatings and inks. They adsorb onto organic and inorganic particulates such as dyes and pigments and prevent aggregation through steric repulsive forces.1 PEG comb polymers with a cationic backbone have been shown to sterically stabilize biological surfaces such as red blood cells and fibroblasts,2 and Owens and Gingell showed that PEG comb polymers can prevent cell adhesion onto glass substrates.3 Comb polymers are also used to modify surfaces to prevent protein or bacterial adhesion, for example, as coatings for medical devices such as catheters and cardiovascular implants.4,5 It is well established that sodium dodecyl sulfate (SDS) binds to poly(ethylene oxide) (PEO).6-10 Binding occurs at an SDS concentration, referred to as the critical aggregation concentration (cac), that is significantly less than the critical micelle concentration (cmc ) 8 mM), namely ∼4 * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Thetford, D. In Surfactants in polymers, coatings, inks and adhesives; Karsa, D. R., Ed.; Sheffield Academic Press: 2003; p 120. (2) Elbert, D. L.; Hubbell, J. A. Chem. Biol. 1998, 5 (3), 177-183. (3) Owens, N. F.; Gingell, D. J. Cell Sci. 1987, 87, 667-675. (4) Elbert, D. L.; Hubbell, J. A. Macromolecules 1997, 30, 69476956. (5) Desai, N. P.; Hossainy, S. F. A.; Hubbell, J. A. Biomaterials 1992, 13 (7), 417-420. (6) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (7) Shirahama, K. Colloid Polymer Sci. 1974, 252, 978-981. (8) Xia, J.; Dubin, P. L.; Kim, Y. J. Phys. Chem. 1992, 96, 6805. (9) Smitter, L. M.; Guedez, J. F.; Muller, A. J.; Saez, A. E. J. Colloid Interface Sci. 2001, 236, 343-353. (10) Cabane, B. J. Phys. Chem. 1977, 81, 1639.
Figure 1. Constitution of MAA/PEG comb polymers employed in the present study. In each case, the ratio of MAA:PEG segments in the polymer was kept constant at ∼63:37 by weight. Polymer 350MA, n ) 7; polymer 550MA, n ) 11; polymer S10W, n ) 22.
mM SDS in water and ∼1 mM in 0.1 M NaCl.7 The binding isotherms take on a typical sigmoidal shape, indicating that the process is highly cooperative. The surfactant aggregates are believed to resemble free SDS micelles but have a lower aggregation number.10 They are bound to the polymer chains, which more or less retain their overall dimensions, and thus the complex resembles a string of beads. For PEO with a molecular mass of 145 000, the number of surfactant aggregates per chain was determined to be 7 at an ionic strength of 0.05 M, increasing to 26 at 0.4 M.8 Dubin et al. found differences in the behavior of the surfactant with different cations and concluded that the cations simultaneously coordinate with the oxygens along the polymer chain while being electrostatically bound to the surfactant.11 (11) Dubin, P. L.; Gruber, J. H.; Xia, J.; Zhang, H. J. Colloid Interface Sci. 1992, 148 (1), 35-41.
10.1021/la040133o CCC: $30.25 © 2005 American Chemical Society Published on Web 04/30/2005
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Table 1. Reagents Used for Polymer Synthesis reagent
supplier
methacrylic acid bisomer MPEG 350MA (PEG chain length 7 units) bisomer MPEG 550MA (PEG chain length 11 units) bisomer MPEG S10W (PEG chain length 22 units) mercaptoacetic acid azo-bis-4-cyanovaleric acid
Aldrich Laporte Performance Chemicals Laporte Performance Chemicals Laporte Performance Chemicals Aldrich Acros Organics
Table 2. Characteristics of the Comb Copolymers polymer
no. of PEG chains per molecule
length of PEG chain
Mw (g mol-1)
Mn (g mol-1)
Mw/Mn (g mol-1)
350MA 550MA S10W Sokalan HP80
42 31 12 n/a
7 11 22 n/a
66 000 67 000 65 000 44 000
52 000 52 000 38 000 18 000
1.3 1.3 1.7 2.5
This paper reports on a study of the interaction of SDS with PEG comb copolymers with a methacrylate backbone with SDS using a nitroxide spin probe. Materials and Methods Materials: Synthesis and Characterization of MAA/PEG Comb Copolymers. Synthesis. The comb polymers were synthesized by free radical copolymerization of methoxyPEG methacrylate and methacrylic acid monomers. The idealized structure of the copolymers is illustrated in Figure 1. Details of the reagents used are given in Table 1. A 37 g sample of bisomer MPEG macromer, 63 g of methacrylic acid, 200 mL of solvent (50:50 EtOH/H2O), and 0.84 g of mercaptoacetic acid (chain transfer agent) dissolved in 30 mL of 50/50 EtOH/H2O were added to a round-bottomed flask, complete with agitator and condenser, and allowed to equilibrate to 60 °C in a water bath. The mixture was purged with nitrogen before the addition of azo-bis-4-cyanovaleric acid initiator (1 g in 9 g of 50/50 EtOH/H2O) and left overnight with constant agitation. The polymers were converted to the sodium salt form by adjustment of the pH to 9 using NaOH and dialyzed against distilled water using dialysis tubing with a molecular mass cutoff of 12-14 000 Da for 5 days; the water was changed regularly. The polymers were freeze-dried to give a solid product. Molecular Mass. The molecular mass distributions of the polymers were obtained by gel permeation chromatography using a Hemabio column connected to multiangle laser light scattering (Wyatt DAWN) and Optilab refractive index detectors. The eluent was 0.1 M NaCl and the flow rate was set at 1 mL/min. A 1% solution of each polymer (made up in 0.1 M NaCl) was filtered through a 0.45 µL filter and injected onto the column through a 50 µL injection loop. A value of 0.213 mL g-1 was used for the refractive index increment (dn/dc).17 The weight-average, Mw, and number-average, Mn, molecular mass values obtained are given in Table 2. Polymer Architecture. The mean number of oxyethylene units (n) in each of the methoxyPEG methacrylate macromers was determined using 1H NMR (Bruker NMR spectrometer running at 500 MHz for protons). Solutions of bisomer 350MA (n ) 7) and bisomer 550MA (n ) 11) were prepared in D2O (2%). Bisomer S10 (n ) 22), which was supplied as a solution in water (50%), was isolated by freeze-drying prior to reconstitution in D2O. The values of n for each monomer were determined by comparison of the peaks corresponding to the resonances of the methacrylate R-methyl protons (δ ) 0.95 ppm) [or terminal methyl group protons (δ ) 1.3 ppm)] and oxyethylene methylene protons (δ ) 3.55 ppm), respectively. The number of PEG side chains per polymer chain was also estimated using 1H NMR and molecular weight data. Solutions of the polymers (Na+ form, 2%) were prepared in D2O and their (12) Vrij, A.; Overbeek, J. Th. G. J. Colloid Sci. 1962, 17, 570. (13) Tanaka, R.; Williams, P. A.; Meadows, J.; Phillips, G. O. Carbohydr. Polym. 1990, 12, 443-459. (14) Zhao, F.; Rosen, M. J.; Yang, N. L. Colloids Surf. 1984, 11, 97. (15) Binana-Limbele, W.; Zana, R. Colloids Surf. 1986, 21, 483-494. (16) Methemitis, C.; Morcellet, M.; Sabbadin, J.; Francois, J. Eur. Polym J. 1986, 22, 619-627. (17) Stellner, K. L.; Scamehorn, J. F. Langmuir 1989, 5, 570.
proton spectra were recorded. The mole fractions of methacrylate and oxyethylene segments in the polymers was estimated by comparison of the areas of the peaks corresponding to the resonances of the backbone methacrylate R-methyl protons (δ ) 0.95 ppm) and side-chain oxyethylene methylene protons (δ ) 3.55 ppm), respectively. Knowledge of Mn values of the polymers enabled the approximate number of PEG side chains per backbone to be calculated. The 350MA, 550MA, and SW10 polymers were found to contain 42, 31, and 12 PEG side groups per chain, respectively. Sokalan HP80. In addition to the polymers synthesized above, work was also carried out on a commercial sample, namely Sokalan HP80 supplied by BASF (Germany). This is reported to be a PEG comb polymer with an acrylate backbone. The molecular mass was determined as discussed above and found to be 44 000 (Table 2). Poly(methacrylic acid). Poly(methacrylic acid) was obtained from Aldrich and solutions were adjusted to neutral pH using sodium hydroxide. Methods: Polymer-Surfactant Interaction. PEG Comb Polymers. The interaction of the MAA/PEG comb polymers with SDS was investigated using 5-doxyl stearic acid (5-DSA) (supplied by Aldrich) spin probe as described by Tanaka et al.13 The nitroxide spin probe gives rise to a three-line spectrum, and the shape and intensity of the lines are dependent on the probe mobility. In water, where the probe is highly mobile, the lines are narrowed due to isotropic tumbling; however, when the probe is incorporated into a surfactant micelle its mobility is restricted, resulting in line broadening due to anisotropic effects. The critical micelle concentration (cmc) of SDS was determined by measuring the ESR spectrum of 5-DSA (5 × 10-6 M) in the presence of increasing concentrations of SDS. Spectra were recorded on a Bruker ESP 300 ESR spectrometer using a flat quartz cell suitable for aqueous solutions. Measurements were then performed on polymer solutions containing varying SDS concentrations (1-100 mM). The rotational correlation time, τc, of the probe was calculated from the spectra using the following relationship:14
τc ) [6.08ω(ho/h+1)0.5 + (ho/h-1)0.5 - 2]
(1)
where ω corresponds to the line width of the central line, and h+1, ho, and h-1 are the heights of the peaks from low to high field, respectively. Sodium Polymethacrylate (SPMA). The interaction of SPMA with SDS was studied in the presence of varying concentrations of NaCl by conductivity measurements. SPMA solutions (100 cm3 each) at varying concentrations were transferred to a dry beaker and titrated against SDS solution (100 mM). The conductivity was measured after addition of 0.5 cm3 of SDS solution using the Jenway 4010 conductivity meter. The cmc was determined from the intercept of the lines for the lower and upper parts of the titration plot.15,16
Results and Discussion The ESR spectra for 5-DSA alone and in the presence of increasing concentrations of SDS are presented in
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Figure 4. ESR spectra obtained for 5-DSA (5 µM in water) in the presence of the MAA/PEG comb polymer 350MA (10 g L-1) and varying concentrations of SDS: (a) no surfactant present; (b) 3 mM SDS; (c) 4 mM SDS; (d) 5 mM SDS. Figure 2. ESR spectra for 5-DSA (5 µM in water) in the presence of increasing concentrations of SDS: (a) no surfactant present; (b) 1 mM SDS; (c) 7 mM SDS; (d) 8 mM SDS; (e) 9 mM SDS.
Figure 3. Influence of various PEG comb polymers on the critical micelle concentration of SDS in water measured by monitoring the rotational correlation time of the spin probe 5-DSA as a function of the concentration of surfactant. Polymer concentration ) 10 g L-1. Symbols: open squares, no polymer present; filled circles, HP80; triangles, 350MA; filled squares, 550MA; open circles, S10W. Dashed lines are intended as a guide to the eye.
Figure 2. It is noted that in the presence of low concentrations of SDS three sharp lines are obtained, indicating rapid isotropic tumbling. However, as the SDS concentration is increased, significant line broadening occurs owing to anisotropic effects in keeping with results published previously.13 This change occurs due to the fact that the probe molecules prefer to reside predominantly within the SDS micelles rather than the aqueous phase and that their mobility is consequently reduced. This is further illustrated in Figure 3, which plots τc as a function of SDS concentration. τc is seen to increase dramatically at SDS concentrations of ∼8 mM. This concentration corresponds to the cmc of the surfactant and is in accordance with values reported in the literature.17
The ESR spectra obtained for 5-DSA in the presence of 1% polymer (350MA) and varying SDS concentrations are shown in Figure 4. It is noted that the spectrum of the probe does not change in the presence of the MAA/PEG comb polymers alone without surfactant (compare spectra in Figure 2a and Figure 4a), indicating that there is no direct interaction between the probe and the polymer itself. At low SDS concentrations (Figure 4a,b) motionally narrowed spectra are observed. However, it is noted that the spectra show line broadening above a critical SDS concentration of 4 mM SDS (Figure 4c). The corresponding values of τc were calculated, and the results are given in Figure 3 together with values obtained for 550MA, SW10, and Sokalan HP80. τc was found to increase markedly at much lower concentrations than for SDS alone, namely ∼4 mM SDS for polymers 350MA, 550MA, and Sokalan HP80 and ∼3 mM for S10W. These values are similar to values reported for the critical aggregation concentration of SDS with PEO itself.6,7,18 Experiments were undertaken at varying polymer concentration, and the results are shown in Figures 5 and 6 for 350MA and S10W, respectively. The SDS concentrations at which τc increased sharply were found to change considerably with polymer concentration. At 0.1% polymer, the increase is close to the cmc, but at higher polymer concentrations the increase occurs at lower SDS concentrations. This is unexpected since, for PEO itself, several workers have shown that the interaction with SDS is only weakly dependent on polymer concentration.6,18-20 However, it has been shown that the cmc of ionic surfactants is reduced in the presence of noninteracting polyelectrolytes of the same charge due to changes in the ionic strength of the system.15,16 Values for the cmc of SDS in the presence of sodium polyacrylate and hydrolyzed polyacrylamide have been shown to be close to those (18) Schwuger, M. J. J. Colloid Interface Sci. 1973, 43, 491. (19) Arai, H.; Murata, M.; Shinoda, K. J. Colloid Interface Sci. 1971, 37, 22. (20) Lange, H. Kolloid Z. Z. Polym. 1971, 243, 101.
Interaction of SDS with MAA/PEG Comb Copolymers
Figure 5. Influence of variation in the concentration of MAA/ PEG comb polymer 350MA on the critical micelle concentration of SDS in water, measured by monitoring the rotational correlation time of the spin probe 5-DSA as a function of the concentration of surfactant. Symbols: triangles, 1.0 g L-1 350MA; circles, 5.0 g L-1 350MA; squares, 10 g L-1 350MA. Dashed lines are intended as a guide to the eye.
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Figure 8. Influence of variation in the concentration of MAA/ PEG comb polymer 350MA on the critical micelle concentration of SDS in 0.5 M NaCl(aq), measured by monitoring the rotational correlation time of the spin probe 5-DSA as a function of the concentration of surfactant. Symbols: open squares, no polymer present; triangles, 1.0 g L-1 350MA; filled squares, 10 g L-1 350MA. Dashed lines are intended as a guide to the eye.
effects. According to Manning,21 the fraction of counterions bound to a polymer chain can be calculated from the following relationship:
θ ) Z-1[1 - (Zξ)-1]
(2)
where Z is the valency of the counterions and ξ is the linear charge density parameter which is given by
ξ ) e2/orkTb Figure 6. Influence of variation in the concentration of MAA/ PEG comb polymer S10W on the critical micelle concentration of SDS in water, measured by monitoring the rotational correlation time of the spin probe 5-DSA as a function of the concentration of surfactant. Symbols: squares, 1.0 g L-1 S10W; circles, 10 g L-1 S10W. Dashed lines are intended as a guide to the eye.
Figure 7. Influence of sodium chloride, sodium poly(methacrylate), and PEG comb polymers on the critical micelle concentration of SDS in aqueous solution: squares, NaCl; triangles, sodium poly(methacrylate); circles, 350MA; inverted triangles, S10W. Dashed lines are intended as a guide to the eye.
obtained in the presence of NaCl when counterion condensation effects are taken into account. The cmc of SDS has been determined in the presence of SPMA and NaCl by conductivity measurements, and the results are shown in Figure 7 together with the values for the MAA/PEG comb copolymers. The values in the presence of the PEG comb polymers and SPMA are the same indicating, therefore, that there is no interaction between the SDS and the PEG chains of the comb polymers. cmc values are lower in the presence of NaCl compared to equivalent concentrations of polymer. However, this can be explained by counterion condensation
(3)
where e is the elementary charge, o is the permittivity in a vacuum, r is the relative permittivity, k is Boltzmann’s constant, T is absolute temperature, and b is the spacing between charged groups. For SPMA, b has a value of 0.25 nm and hence θ is 0.65. This means that two-thirds of the counterions are condensed on the polymer chains and only about one-third are free. If this effect is taken into account, the values for the cmc in the presence of the polymers are identical to those in NaCl, confirming that there is no interaction between the polymers and the SDS. The spin probe technique was used to determine the cmc of SDS in the presence of 0.5 M NaCl. The value of τc with increasing SDS concentration is shown in Figure 8 and is seen to increase sharply at an SDS concentration of 0.4 mM corresponding to micelle formation. This concentration is in agreement with values reported in the literature for the cmc of SDS obtained by other techniques17 and is attributed to screening of the repulsions between the sulfate headgroups by the electrolyte which promotes surfactant aggregation. The experiment was repeated in the presence of 0.1% and 1% 350MA polymer, and the values of τc obtained are also shown in Figure 8. The values for τc superimpose on the values for SDS alone and are not dependent on polymer concentration as was the case in the absence of electrolyte. This further supports the conclusion that the dependency on polymer concentration in the absence of added electrolyte is a consequence of the effective ionic strength due to the polymer itself. In the presence of 0.5 M NaCl the ionic strength due to the polymer is completely swamped by the electrolyte. Conclusions It has been shown that there is no interaction between methacrylate-PEG comb copolymers or SPMA and SDS (21) Manning, G. S. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 909922.
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surfactant. The decrease in the cmc of SDS observed in the presence of the polymers has been shown to be due to the effective increase in the ionic strength of the solution due to the polyelectrolyte counterions.
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Acknowledgment. The authors wish to thank Colgate Palmolive for providing financial support to undertake this work. LA040133O
