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Uptake and Release of Anionic Surfactant into and from Cationic Core-Shell Microgel Particles Melanie Bradley,*,† Brian Vincent,† and Gary Burnett‡ School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, and New Products DiVision (Oral Health Care), GSK, Weybridge KT13 0DE, United Kingdom ReceiVed May 29, 2007. In Final Form: June 21, 2007 Core-shell microgel particles, in the colloidal size range, have been prepared and characterized, where the core and the shell are both copolymers, based on N-isopropylacrylamide, but where the core and shell contain different pH-responsive groups having widely separated acid dissociation constants (pKa). The core contains vinylpyridine (VP), which has a pKa value of 4.92, and the shell contains 2-(dimethylamino)ethyl methacrylate (DMAEM), which has a pKa value of 8.4. The dispersion properties, and the uptake and release of an anionic surfactant, sodium dodecylbenzenesulfonate (SDBS), have been studied for both the core and the core-shell microgel particles as a function of pH changes. Both the core and the core-shell particles have been shown to swell as the pH decreases over the range from 7 to 3. However, despite the large differences in the pKa values of the VP and DMEAM groups, no distinct steps in the swelling ratio-pH curve for the core-shell particles were observed, and it is postulated that the boundary between the core and shell regions may be somewhat extended, rather than sharp. The uptake of the anionic surfactant SDBS has been shown to depend on two distinct attractive interactions between the surfactant molecules and the microgel particles: electrostatic and hydrophobic. A reasonable correlation between the minimum in the particle diameter, for both the core and the core-shell particles, and the point of charge neutralization, in the presence of SDBS, has been established.
Introduction Microgel particles are stimulus-responsive, cross-linked polymer particles. Poly(N-isopropylacrylamide) (PNIPAM) has been the most widely studied microgel system; it shows a temperaturedriven, swelling-deswelling volume phase transition (VPT) at 32 °C, just below body temperature.1 An independent pH response may be incorporated into PNIPAM microgel particles through copolymerization of N-isopropylacrylamide (NIPAM) with an acidic or basic functionalized monomer. Many studies have focused on temperature- and pH-responsive microgels since ionization affords more control over the VPT. Another way to control the responsive properties of a microgel is by changing the morphology of the particles through the preparation of coreshell microgel particles. A further level of control of the temperature and/or pH response may be obtained using microgel particles having a core-shell type of structure.2-9 For example, Lyon et al.2-5 have studied pH- and temperature-responsive core-shell microgel particles composed of NIPAM and NIPAM-co-acrylic acid (AAc) (with either as the core and the other as the shell). They have shown that the swelling properties of the core are affected by the shell. When the shell collapses first, it exerts an inward pressure on the core, but this only has a small effect. On the other hand, when * To whom correspondence
[email protected]. † University of Bristol. ‡ GSK.
should
be
addressed.
E-mail:
(1) Saunders, B.; Vincent, B. AdV. Colloid Interface Sci. 1999, 80, 1-25. (2) Jones, C. D.; Lyon, L. A. Macromolecules 2000, 33, 8301. (3) Gan, D.; Lyon, L. A. J. Am. Chem. Soc. 2001, 123, 8203. (4) Gan, D.; Lyon, L. A. J. Am. Chem. Soc. 2001, 123, 7511. (5) Jones, C. D.; Lyon, L. A. Macromolecules 2003, 36, 1988. (6) Berndt, I.; Richtering, W. Macromolecules 2003, 36, 8780. (7) Berndt, I.; Pedersen, J. S.; Richtering, W. J. Am. Chem. Soc. 2005, 127, 9372. (8) Berndt, I.; Popescu, C.; Wortmann, F.-J.; Richtering, W. Angew. Chem., Int. Ed. 2006, 45, 1081. (9) Berndt, I.; Pedersen, J. S.; Lindner, P.; Richtering, W. Prog. Colloid Polym. Sci. 2006, 133, 35.
the core collapses first, the swollen shell exerts a strong outward pressure on the core, leading to the appearance of a distinct third VPT, in addition to those expected for PNIPAM and PNIPAMco-AAc. The effect of the shell on modulating the overall particle behavior is dependent on the shell thickness in these temperature/ pH-responsive core-shell systems.10 Increasing shell thickness has also been shown to increase the VPT of the core in PNIPAM core/poly(N-isopropylmethacrylamide) (PNIPMAM) shell microgel particles.6-9 The VPT for NIPMAM is 44 °C, compared to 32 °C for the NIPAM core (VPT). Again, the temperature shift was interpreted in terms of a restraining elastic force developed in the shell, which offsets the thermodynamic driving force for shrinking the core. The VPT of the core was also shown to depend on the cross-link density of the shell. Most work on core-shell microgel particles has focused on the VPT of temperature-responsive particles. However, Plunkett et al.11 have prepared dual (temperature and pH) responsive coreshell gel particles containing a vinylpyridine (VP) core and a 2-(dimethylamino)ethyl methacrylate (DMAEM) shell. The core and the shell were fabricated in isolation using a masked photopolymerization procedure. The resultant particles were larger than colloidal microgels and showed two pH-dependent swelling transitions. In this study, as a follow-up to the work of Plunkett et al.,11 we have prepared core-shell microgel particles, in the colloidal size range, where the core and the shell are both copolymers, based on NIPAM, but where the core and shell contain different pH-responsive groups having widely separated acid dissociation constants (pKa). The core contains VP, which has a pKa value of 4.92,12 and the shell contains DMAEM, which has a pKa value of 8.4.13 The aim of this study was to compare the dispersion (10) Jones, C. D.; Lyon, L. A. Langmuir 2003, 19, 4544. (11) Plunkett, K. N.; Moore, J. S. Langmuir 2004, 20, 6535. (12) Reich, H. E.; Levine, R. J. Am. Chem. Soc. 1955, 77, 4913. (13) C¸ avus, S.; Gu¨rdag, G. Polym. Bull. 2007, 58, 235.
10.1021/la701571w CCC: $37.00 © 2007 American Chemical Society Published on Web 07/27/2007
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properties, and the uptake and release of an anionic surfactant, for the core and the core-shell microgel particles as a function of pH changes. The temperature response, per se, will not be reported here. Experimental Section Materials. NIPAM and N,N′-methylenebisacrylamide (BA) (both from Fisher) were recrystallized from hexane and ethanol, respectively. VP (Aldrich, 97%) and DMAEM (Aldrich) were filtered through basic alumina columns. The initiator 2,2′-azobis[2-(methylamidino)propane] dichloride (V50; Aldrich, 97%), cetylpyridinium chloride (CPC) surfactant (Aldrich), and sodium dodecylbenzenesulfonate (SDBS) surfactant were used as received. All solutions were prepared with Milli-Q water. Core-Shell Microgel Synthesis. Core-shell microgel particles were prepared by sequential, seeded-growth, free-radical precipitation polymerization reactions. Core (C) PVP-co-NIPAM particles were prepared by adding NIPAM (0.5 g), BA (0.08 g), and VP (0.1 g) to 95 mL of Milli-Q water in a 100 mL three-necked, round-bottom flask. This solution was purged with nitrogen and thermostated at 70 °C. The polymerization reaction was initiated through the addition of V50 initiator (0.04 g dissolved in 5 mL of Milli-Q water) and was left to proceed overnight with continuous stirring. The core-shell (C-S) particles were then prepared as follows. A 50 mL sample of the core particle dispersion was used as a seed for further growth. A small amount of CPC surfactant (0.02 g, less than the critical micelle concentration (cmc) in the total aqueous phase), dissolved in an additional 40 mL of water, was added to help stabilize the particles during the seeded-growth stage. The dispersion was then purged with nitrogen and thermostated at 70 °C. A monomer solution, prepared by adding NIPAM (0.25 g), BA (0.04 g), and DMAEM (0.05 g) to 5 mL of Milli-Q water, and an initiator solution (0.02 g of V50 in 5 mL of Milli-Q water) were fed into the seed microgel dispersion over a period of 3 h. The seeded polymerization reaction was left to proceed overnight with continuous stirring. The final core-shell particle dispersion was purified by replacing the supernatant with Milli-Q water, following centrifugation. This purification technique was repeated several times. Characterization of the Microgel Particles. The solids content was determined gravimetrically, and hence, the yields of the C and the C-S microgel preparations could be calculated. When required, the pH was adjusted by the addition of small quantities of (0.1 M) HCl or NaOH. The diffusion coefficients of the C and C-S microgel particles (at a particle concentration of 0.1 wt %) were determined by dynamic light scattering (DLS) using a Brookhaven Instruments Zeta Plus instrument, fitted with a 15 mW laser (λ ) 678 nm) and with the detector set at 90°. The Stokes-Einstein equation was then used to calculate the hydrodynamic diameter of the particles. The electrophoretic mobility values of the C and C-S microgel particles were determined using a Brookhaven Instruments ZetaPALS apparatus. The binding isotherms for SDBS to both the C and the C-S microgel particles were determined using the following (solution depletion) method. A known amount of surfactant solution was added to a given quantity of microgel dispersion and the system left to equilibrate for at least 7 days. The pH values of the microgel dispersion and surfactant solution were adjusted prior to mixing. The particles were then separated from solution by centrifugation at 10000 rpm for 20 min using a Sorvall RC 5b Plus centrifuge. The equilibrium surfactant concentration was then determined from a calibration graph of light absorbance (223 nm) versus surfactant concentration.
Results and Discussion Dispersion Properties of the Core and Core-Shell Microgel Particles. Plots of the swelling ratio as a function of pH for the C and C-S microgel particles are shown in Figure 1. At 25 °C both the NIPAM-based core and core-shell particles are relatively swollen already, compared to the collapsed state at temperatures
Figure 1. Swelling ratio as a function of pH at 25 °C: (O) PVPco-NIPAM core microgel particles; (b) PVP-co-NIPAM/DMAEMco-NIPAM core-shell microgel particles.
above the lower consolute solution temperature of PNIPAM (32 °C1,14). At 25 °C and pH 7 the hydrodynamic diameters for the C and C-S microgel particles, as determined by DLS, are 345 ( 7 nm (polydispersity of 2%) and 462 ( 23 nm (polydispersity of 5%). This indicates a shell thickness of ∼50 nm at 25 °C and pH 7. The pyridine groups in the core and the tertiary amine groups of the DMAEM units in the shell are both basic and become increasingly more protonated with decreasing pH, resulting in a buildup of positive charge within the microgel particles, causing them to swell further. The swelling transition, on lowering the pH, for pure PVP particles has been observed to be fairly “sharp”, occurring below pH 4.5,15 i.e., in the region of the pKa value of the VP groups. Here, as may be seen in Figure 1, for the PVPco-NIPAM core microgel particles, with decreasing pH, the swelling starts at pH 6 and increases slowly down to pH 4.5, after which it then increases more rapidly, in line with the results for pure PVP microgel particles referred to above. The greater “ease of swelling” between pH 4.5 and pH 6.5 observed here for the microgel particles containing a (weight) ratio of 5:1 of NIPAM to VP monomer, compared to pure VP, may well have to do with the more hydrophilic nature of NIPAM compared to VP. Hydrophobic particles do not swell readily in water, and a significant degree of charge buildup inside such particles may be required before they can begin to swell. For weak acids and bases half the potentially ionizable groups are ionized when the pH equals the pKa. Hence, for pure VP particles, which are fairly hydrophobic, swelling is only observed when the pH is actually around the actual pKa of the VP moieties (4.92). For the more hydrophilic (NIPAM-co-VP) core particles studied here, on the other hand, swelling will begin when the pH is a couple of units above the pKa value of VP. The pKa values of VP and DMAEM are separated by several pH units (see earlier), and if the swelling transition of the core and shell of the C-S microgel particles were independent, then it would be expected that a two-step swelling curve would be obtained with pH variation; however, as shown in Figure 1 this is not the case. Dual pH-responsive core-shell microgels containing a 4-VP core and DMAEM shell, where the core and shell are distinct, have been shown previously to have two-step swelling profiles.11 (14) Pelton, R. AdV. Colloid Interface Sci. 2000, 85, 1. (15) Loxley, A.; Vincent, B. Colloid Polym. Sci. 1997, 275, 1108.
Surfactant Uptake into and Release from a Microgel
Figure 2. Electrophoretic mobility as a function of pH at 25 °C: (O) PVP-co-NIPAM core microgel particles; (b) PVP-co-NIPAM/ DMAEM-co-NIPAM core-shell microgel particles.
Using reasoning similar to that outlined above in discussing the relationship between the onset of swelling and the pKa values for microgel particles, for the C-S microgel particles one might expect to see swelling begin at pH values significantly above the pKa value for DMAEM (8.4). This does not seem to occur; swelling only appears to start below a pH value of ∼6.5 (Figure 1), i.e., at a value similar to that for the core particles themselves. It should be remembered, however, that the shell thickness is only ∼25% of the total particle diameter, at pH 7, so small degrees of swelling of the shell around pH 7 ( 1 may be difficult to detect within the experimental error. Below pH ≈ 6.5, the C-S microgel particles do expand more rapidly with decreasing pH compared to the C microgel particles. At pH 3 the shell has expanded to a thickness of ∼140 nm, i.e., about 35% of the total core-shell diameter. An alternative explanation may also be proposed for the observations that, first, no swelling is observed (Figure 1) for the C-S microgel particles on lowering the pH, until a value of ∼6.0 is reached, and, second, that two distinct swelling transitions as such are not observed, as with the systems reported in ref 11. This has to do with the fact that, in preparing the C-S microgel particles, (some of) the second monomer mixture added in to grow the shells around the particle cores could have been imbibed by the core, and hence, further polymerization could have occurred within the cores (at least in their periphery regions), rather than a distinct, new shell forming around the cores. This is likely since the monomers are very soluble in the aqueous phase and the core particles are to some extent swollen with water. The VP and DMAEM groups would not then be as physically separated as intended, and a transition “zone” would exist, rather than a sharp boundary between the VP-containing cores and the DMAEM-containing shells. In that case the swelling of the composite particles would now be more “averaged” over the swelling transitions of the two monomer moieties, rather than exhibiting two distinct transitions. This would, therefore, be an alternative explanation for the pH-dependent swelling behavior observed for the C-S particles in Figure 1. The variation of the electrophoretic mobility with pH, for the C and C-S microgel particles, is presented in Figure 2. The first observation to note is that the maximum mobility value (1.4 × 10-8 m2 s-1 V-1) is somewhat lower than the values typically observed for hard latex particles (∼4 × 10-8 m2 s-1 V-1 16). This is because the microgel particles are already swollen with (16) Semmler, M.; Mann, E. K.; Ricka, J.; Borkovec, M. Langmuir 1998, 14, 5127.
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water molecules at 25 °C and pH 7. The mobilities of swollen microgel particles are known to be less than those for the corresponding deswollen particles. The trends observed in Figure 2 reflect quite closely those for the particle swelling itself as a function of (decreasing) pH, shown in Figure 1. For pure PNIPAM particles, where the only charge contribution comes from the initiator residues at (or close to) the particle surface, the mobility decreases as the microgel particles swell (say with decreasing temperature, below the lower consolute temperature for PNIPAM);17 this is simply because the surface charge density (i.e., the surface charge per unit area) decreases as the particle size increases. With the C and C-S particles studied here, the mobility increases as the microgel particles swell. This is because the swelling arises from an increase in the bulk charge density within the microgel particles with decreasing pH. For pure PVP microgel particles, the mobility does not increase significantly until the pH is pH 5 > pH 7. This simply reflects the fact that, as already stated, there are two contributions to the mechanism of absorption: electrostatic
(19) Kokufuta, E.; Nakaizumi, S.; Ito, S.; Tanaka, T. Macromolecules 1995, 28, 1704. (20) Linse, P.; Piculell, L.; Hansson, P. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; CRC Press: Boca Raton, FL, 1998; p 193.
Surfactant Uptake into and Release from a Microgel
attraction and hydrophobic attraction. The former contribution is highest at pH 3, intermediate at pH 5, and negligible at pH 7. The fact that, at all SDBS equilibrium concentrations, the absorbed amount at pH 7 is so much lower than at pH 5 or 3 would suggest that maybe it is easier for the micelle-like aggregates of the anionic surfactant molecules to form (and, indeed, maybe the aggregates are larger) around a positive charge group than through purely hydrophobic association with the hydrophobic moeities within the microgel particles. The fact that, at all three pH values, a plateau value has not been reached, either in the absorbed amount (Figure 4) or in the hydrodynamic diameter (Figure 3), implies that the microgel particles have not been saturated with SDBS molecules, even at an equilibrium SDBS concentration of ∼2 mM. Indeed, the absorbed amount appears to increase at a higher rate at the higher equilibrium SDBS concentrations studied. This could be because the more “opened up” the microgel particles become (i.e., the larger their diameter, Figure 3), the more easily the anionic surfactant molecules can penetrate into their interior. That would then leave open the question as to whether the reported absorbed amounts in Figure 4 are actually true equilibrium values. If one assumes that all the VP and DMAEM used in the preparation of the C and C-S microgel particles are present in the final particles, then one can readily calculate the maximum number of charge groups per unit mass of particles. From the absorption data at pH 3, presented in Figure 4, one can then determine the equilibrium concentration of SDBS for which the ratio of absorbed anionic SDBS molecules to cationic (VP + DMAEM) groups reaches unity (i.e., the point of charge neutralization). For the C microgel particles this occurs at 0.5 mM SDBS and for the C-S microgel particles at 0.4 mM SDBS. These two values are comparable to, but slightly higher than, the
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SDBS equilibrium concentrations where the hydrodynamic diameter of the corresponding microgel particles reaches a minimum in each case. This small discrepancy could simply imply that not all the VP and/or DMAEM monomer was actually incorporated during the particle synthesis. One could have analyzed, e.g., by IR or NMR spectroscopy, the actual monomer content of the particles, but this was not done in this work.
Conclusions Both NIPAM-co-VP copolymer microgel particles and NIPAMco-VP/NIPAM-co-DMEAM core-shell microgel particles have been shown to swell as the pH decreases over the range from 7 to 3. This is consistent with the buildup of cationic charge groups within the particles, due to protonation of the VP and DMEAM monomers. However, despite the large differences in the pKa values of the VP and DMEAM groups, no distinct steps in the swelling ratio-pH curve for the core-shell particles were observed, and it is postulated that the boundary between the core and shell regions may be somewhat extended, rather than sharp. The uptake of the anionic surfactant SDBS has been shown to depend on two distinct attractive interactions between the surfactant molecules and the microgel particles: electrostatic and hydrophobic. A reasonable correlation between the minimum in the particle diameter, for both the core and the core-shell particles, and the point of charge neutralization, in the presence of SDBS, has been established. Acknowledgment. M.B. and B.V. acknowledge GSK, New Products Research, Oral Healthcare R&D, Weybridge, U.K., for provision of research funds to enable this research to be carried out. LA701571W
