Surfactant and Polyelectrolyte Gel Particles that Swell Reversibly

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Surfactant and Polyelectrolyte Gel Particles that Swell Reversibly Yakov Lapitsky, William J. Eskuchen, and Eric W. Kaler* Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed February 14, 2006. In Final Form: April 15, 2006 Mixing of oppositely charged surfactants and polyelectrolytes in aqueous solutions can lead to associative phase separation, where the concentrated phase is a viscous liquid, gel, or precipitate. In recent years, this phenomenon has been exploited to form gel-like particles, ranging from approximately 100 to 4000 µm in diameter, whose stability depends on equilibrium phase behavior. As the sample composition is varied, these particles either remain stable (in a two-phase mixture) or dissolve over time. Here, we present the formation of reversibly swelling gel particles from mixtures of N,N,N-trimethylammonium-derivatized hydroxyethyl cellulose (JR-400) and sodium dodecyl sulfate (SDS), whose swelling is controlled by the ambient solution conditions. The effects of cross-linking density and surfactant concentration are investigated by gravimetry and confocal microscopy. The resulting particles have a core/shell morphology and undergo reversible swelling/collapse transitions which, depending on the cross-link density, can be either gradual or abrupt with changing SDS concentration.

1. Introduction Oppositely charged surfactants and polyelectrolytes form aggregates over a broad range of conditions.1-3 Their attractive interactions can give rise to associative phase separation, where the sample can have the form of a viscous liquid, gel, or precipitate.2-8 In recent years, this phenomenon has been exploited to form surfactant/polyelectrolyte gel particles for controlled encapsulation and release by dropwise addition of an aqueous cellulose-based polyelectrolyte solution to a solution of oppositely charged surfactant.9-11 Because of the relatively high viscosity of the cellulose-based polyelectrolyte solution, mixing of the two solutions is not instantaneous. As a result, before the two solutions can mix, the surfactant diffuses into the polyelectrolyte drop to form a gel bead. The size of the resulting particle reflects the size of the parent drop and varies between 100 and 4000 µm. The stability of these particles depends on the phase behavior,11 so particles can remain stable under some solution conditions and irreversibly dissolve under others. The phase behavior (see Figure 1) and microstructure of such materials has been studied in detail, using mixtures of sodium dodecyl sulfate (SDS) and N,N,N-trimethylammonium-derivatized hydroxyethyl cellulose (JR-400, Amerchol, Inc.).1,12-16 Single-phase solutions form in both the polymer-rich and surfactant-rich portions of the phase map. The polymer-rich solu* To whom correspondence should be addressed. Phone: +1-302-8313553. Fax: +1-302-831-6751. E-mail: [email protected]. (1) Goddard, E. D. Interaction of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993; p 171. (2) Piculell, L.; Lindman, B. AdV. Colloid Interface Sci. 1992, 41, 149. (3) Thalberg, K.; Lindman, B. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993; p 203. (4) Ilekti, P.; Martin, T.; Cabane, B.; Piculell, L. J. Phys. Chem. B 1999, 103, 9831. (5) Marques, E. F.; Regev, O.; Khan, A.; Miguel, M. G.; Lindman, B. Macromolecules 1999, 32, 6626. (6) Swanson-Wethamuthu, M.; Dubin, P. L.; Almgren, M. J. Colloid Interface Sci. 1997, 186, 414. (7) Wang, Y. L.; Kimura, K.; Dubin, P. L.; Jaeger, W. Macromolecules 2000, 33, 3324. (8) Wang, Y. L.; Kimura, K.; Huang, Q.; Dubin, P. L.; Jaeger, W. Macromolecules 1999, 32, 7128. (9) Babak, V. G.; Merkovich, E. A.; Galbraikh, L. S.; Shtykova, E. V.; Rinaudo, M. MendeleeV Commun. 2000, 3, 94. (10) Ferres, M. R. J.; Serrabasa, P. E.; Liron, I. M.; Llorens, A. A. Procedure for preparing capsules and for encapsulation of substances, Patent No. ES2112150, Spain, 1998. (11) Lapitsky, Y.; Kaler, E. W. Colloids Surf., A 2004, 250, 179.

Figure 1. Phase map of the dilute SDS/JR-400/water mixture (redrawn from refs 13 and 15).

tions are viscous (intrinsic viscosity ≈ 100-1000 P),1,17 loosely associated networks with relaxation times of approximately 1-10 s.17 The surfactant-rich solutions, on the other hand, are less viscous than the surfactant-free solutions of the same polyelectrolyte concentration.1 When the SDS and JR-400 are mixed in more stoichiometric proportions, associative phase separation occurs. This two-phase envelope begins above the critical aggregation concentration (CAC) and follows the charge equivalence line (where the number of anionic SDS molecules is equal to the number of cationic sites on JR-400). When the SDS concentration is slightly above the charge equivalence line, a solid gel-like material forms.14-16 This material retains the shape that it assumes upon formation and allows the production of gel particles.11 Upon further surfactant addition, the resolubilization phase boundary is reached, at which point the surfactant is in substantial excess and the gel complex is redissolved into a single-phase solution of moderate viscosity.14-16 (12) Svensson, A.; Sjostrom, J.; Scheel, T.; Piculell, L. Colloids Surf., A 2003, 228, 91. (13) Sjostrom, J.; Piculell, L. Colloids Surf., A 2001, 183-185, 429. (14) Goldraich, M.; Schwartz, J. R.; Burns, J. L.; Talmon, Y. Colloids Surf., A 1997, 125, 231. (15) Goddard, E. D.; Hannan, R. B. J. Am. Oil Chem. Soc. 1977, 54, 561. (16) Goddard, E. D.; Hannan, R. B. J. Colloid Interface Sci. 1976, 55, 73. (17) Chronakis, I. S.; Alexandridis, P. Macromolecules 2001, 34, 5005.

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When the overall sample composition is inside the two-phase envelope on the phase map, stable gel particles form.11 In contrast, addition of polyelectrolyte drops to surfactant solutions such that the final surfactant concentration is above that of the twophase envelope produces unstable gel particles that dissolve with time as the solution equilibrates.11 The time for particle dissolution can be controlled by adjusting the surfactant concentration and can be varied from a few minutes to several days. While this behavior is desirable for some applications (e.g., where a burst release of the encapsulated material is needed), there are cases where it may also be advantageous to prevent (or reverse) this dissolution process. This can be done by covalently cross-linking the polyelectrolyte chains, which can be done by reacting the particles with divinyl sulfone (DVS) in the presence of sodium hydroxide (NaOH).13,18-20 The cross-linked gel networks do not dissolve but rather undergo swelling and collapse transitions that depend on the equilibrium surfactant/polyelectrolyte phase behavior.13,21-26 Because the extent of swelling is an equilibrium property, particle swelling (and solute permeability) can be reversibly controlled by varying the supernatant solution composition. As a result, the encapsulated material is trapped under ‘stable particle’ conditions (where the gel is in a collapsed conformation) and released under ‘unstable particle’ conditions (where the gel is swollen). Here, we demonstrate how to prevent surfactant/polyelectrolyte particle dissolution by cross-linking the JR-400 chains. This allows swelling under unstable (single-phase/resolubilization) solution conditions, which can be reversed upon returning to stable (twophase) solution conditions. The effects of cross-link density on equilibrium swelling, swelling and collapse kinetics, and particle stability are investigated by gravimetry, UV-vis spectrometry, and confocal microscopy. The particles exhibit reversible swelling and shrinking when they are cycled between the one-phase and two-phase sample compositions. As expected, this swelling/ collapse transition is greatest when the cross-link density is low, in which case the particle volume varies by up to a factor of 10, and is fastest in its approach to equilibrium when the cross-link density is high. 2. Experimental Section 2.1. Materials. All experiments were performed using Millipore Milli-Q deionized water (18.0-18.3 MΩ‚cm resistivity). SDS (MP Biomedicals, Inc.), sodium dodecyl benzyl sulfate (SDBS, TCI Organic Chemicals), divinyl sulfone (DVS, TCI America), and cationically modified hydroxyethyl cellulose (JR-400) with a nominal molecular weight of 500 0005 were used as received without further purification. The solution pH was adjusted with a 0.1 N sodium hydroxide (Acros Organics) stock solution. 2.2. Gel Particle Preparation. All solutions used in the particle preparation contained 10 mM NaOH. A 3 wt % (3.2 mM) JR-400 solution (1.33 g) was added dropwise to 2.67 g of 1.4 wt % (49 mM) SDS solution (drop diameter ≈ 2 mm) with a syringe through a 20-gauge syringe needle over a period of approximately 30 s. DVS (120 µL) was then immediately added to the mixture to initiate the cross-linking reaction. After gentle agitation, the reaction cocktail was left to react over time periods ranging from 2 to 480 min. To (18) Anbergen, U.; Oppermann, W. Polymer 1990, 31, 1854. (19) Rosen, O.; Piculell, L. Gels Networks 1997, 5, 185. (20) Rosen, O.; Sjostrom, J.; Piculell, L. Langmuir 1998, 14, 5795. (21) Piculell, L.; Sjostrom, J.; Lynch, I. Prog. Colloid Polym. Sci. 2003, 122, 103. (22) Isogai, N.; Gong, J. P.; Osada, Y. Macromolecules 1996, 29, 6803. (23) Khokhlov, A. R.; Kramarenko, E. Y.; Makhaeva, E. E.; Starodubtzev, S. G. Macromolecules 1992, 25, 4779. (24) Jeon, C. H.; Makhaeva, E. E.; Khokhlov, A. R. Macromol. Chem. Phys. 1998, 199, 2665. (25) P., N.; Hansson, P. J. Phys Chem. B 2005, 109, 23843. (26) Andersson, M.; Rasmark, P. J.; Elvingson, C.; Hansson, P. Langmuir 2005, 21, 3773.

Lapitsky et al. terminate the reaction, 10 mL of a 50:50 water/ethanol solution (also containing 10 mM NaOH) was added to the vessel and left for at least 2 h. 2.3. Gel Swelling Experiments. After terminating the reaction, the particles were filtered from the water/ethanol solution and added to SDS solutions of variable compositions, ranging between 0.05 and 0.50 wt % (5-6 particles per sample). The swelling was measured by periodically sequestering the particles from the supernatant solution and weighing them. These measurements were repeated until the particle mass reached a constant value. To measure the rate of surfactant release from collapsing gel particles, SDS was replaced with SDBS, whose concentration can be determined by UV measurements. Here, three freshly prepared gel particles were equilibrated in 0.5 wt % SDBS for at least 2 days. They were then transferred into 2 g of 0.05 wt % SDBS solution, and the SDBS concentration increase in the supernatant was monitored over time using a Perkin-Elmer Lambda 2 UV-vis spectrometer, operating at a wavelength of 273 nm. The molecular extinction coefficient of SDBS is 0.595 L/g‚cm. 2.4. Confocal Microscopy. Gel particles were placed into surfactant solutions containing 3.3 × 10-2 mg/mL 3,6-bis(dimethylamino)acridine (Acridine Orange, Fisher) fluorescent dye and left to equilibrate for at least 48 h to allow the Acridine Orange to absorb into the gel complex. The particles were then observed using a Zeiss 510 NLO multiphoton confocal microscope, operating with an excitation wavelength of 488 nm and a pass permission wavelength of 505 nm. Images were obtained by scanning the xy plane at the center of the gel particle.

3. Results and Discussion 3.1. Gel Particle Formation. Upon the dropwise addition of the JR-400 solution to the SDS solution, a thin gel shell forms around the polyelectrolyte drops. The surfactant concentration used in the particle formation process is slightly above the two-phase envelope (see Figure 1). However, the drop remains intact long enough for chemical cross-links to form between the JR-400 molecules, which ultimately prevents gel redissolution. When the particles are placed in the 50:50 ethanol/water quenching solution (which weakens surfactant/polyelectrolyte interactions and quickly dissolves un-cross-linked particles) the particles swell but remain intact, indicating that the JR-400 chains are successfully cross-linked. The resulting gel particles (see Figure 2) range from 1 to 5 mm in diameter, depending on solution SDS concentration and cross-link density. Their structure is composed of a densely crosslinked opaque shell and a clear, sparsely cross-linked gel core, which can be seen by cutting the particle in half. Similar core/ shell morphologies have been seen in polyelectrolyte gels with homogeneous cross-link densities that are in equilibrium with surfactant solutions. In this case, the polyelectrolyte is in excess and the shell thickness is determined by the sample stoichiometry.27,28 The case here is opposite to that: the surfactant is in excess, the cross-link density is nonhomogeneous, and the shell thickness is transport limited (i.e., not determined by equilibrium phase behavior). Instead, the core/shell morphology reflects the fact that the physical gelation that occurs when the surfactant diffuses into the JR-400 drop concentrates the polyelectrolyte by a factor of about 10.29 As a result, the physical gel forms an opaque, dense network on the outside of the drop, while the remaining polyelectrolyte in the core of the drop is relatively dilute and homogeneous. The cross-linking reaction fixes this structure and produces the core/shell morphology. The mass (27) Hansson, P.; Schneider, S.; Lindman, B. J. Phys. Chem. B 2002, 106, 9777. (28) Hansson, P.; Schneider, S.; Lindman, B. Prog. Colloid Polym. Sci. 2000, 115, 342. (29) Lapitsky, Y.; Kaler, E. W. Colloids Surf., A 2006, in press.

Particles that Swell ReVersibly

Figure 2. Cross-linked SDS/JR-400 gel particles (10 min reaction) in (a) 0.5 wt % SDS solution (swollen state) and (b) 0.05 wt % SDS solution (collapsed state).

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Figure 4. Fluorescent confocal micrographs showing cross-sections of the (a) outer shell, (b) interstitium, and (c) core of a sparsely cross-linked (2 min reaction) particle (4-5 mm in diameter) in 0.5 wt % SDS.

Figure 3. The schematic of the core/shell gel particle structure.

transfer limitation of the DVS penetration into the core of the drop may further contribute to the inhomogeneous cross-link density. A closer examination using fluorescence confocal microscopy reveals that the core/shell structure is composed of three layers (as drawn in Figure 3): a dense shell (Figure 4a), a sparse and porous interstitial layer (Figure 4b), and a homogeneous core (Figure 4c). The large pores in the interstitial layer make the layer weak and can cause the shell to flake off. 3.2. Reversible Swelling. When the particles are cycled between 0.05 (1.7 mM) and 0.5 wt % (17 mM) surfactant solutions, they swell reversibly by up to a factor of 10 (Figure 5) over a wide range of cross-link densities, which can be adjusted by varying the reaction times. For lower cross-link densities (e.g., those from 2 and 20 min reactions), the thin outer shell is damaged during the cycling process and, in the case of the particles crosslinked for 20 min, is eventually lost. The loss of the outer shell is reflected in the modest decrease in particle mass between the cycles.

Figure 5. Reversible swelling of SDS/JR-400 particles cross-linked in 0.01 M NaOH and 30 µL/g DVS solution for 2 ((), 20 (0), and 200 min (2). See text for description.

The damage to the particle shells can be linked to the interstitial layer structure (see Figure 4b), whose large pores make it weak and likely prone to mechanical failure. Most of this damage occurs during the collapse phase of the cycle. These observations are consistent with the reflectance confocal micrographs of the particle surface (Figure 6). For the sparsely cross-linked particle (2 min reaction), the surface morphology is rougher in the collapsed state (Figure 6a) than in the expanded state (Figure 6b). This suggests the presence of larger voids in the layer beneath the shell, which should cause the gel to break at lower stresses. The reduction in the void size upon swelling reflects a more homogeneous distribution of polyelectrolyte chains in the expanded state than in the collapsed state. The surface morphology

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Figure 6. Surface roughness of reversibly swelling surfactant/polyelectrolyte gel particles: (a) sparsely cross-linked (2 min reaction) in 0.05 wt % SDS, (b) sparsely cross-linked (2 min reaction) in 0.5 wt % SDS, (c) densely cross-linked (200 min reaction) in 0.05 wt % SDS, and (d) densely cross-linked (200 min reaction) in 0.5 wt % SDS.

of the highly cross-linked particles (200 min reaction), on the other hand, is smooth for both the collapsed (Figure 6c) and expanded (Figure 6d) states and is consistent with the higher resilience of these particle shells. These differences in surface properties also reflect the variability in shell thickness. Because the shell grows concurrently with the cross-linking reaction, increasing the reaction time allows a thicker shell to form. Surprisingly, while the very thin shells formed from the 2 min reaction are damaged upon cycling between solutions, shell damage is most dramatic in particles formed using intermediate (20 min) reaction times. This is probably due to differences in the flexibility of the shells. Because the sparse core undergoes a more dramatic swelling/collapse transition than the dense shell, the core and shell exert a tensile stress on the interstitial layer of the particle. While the thin shell (2 min reaction) is likely supple enough to conform to the volume transitions of the particle core, the thicker and stiffer shell formed in the 20 min reaction tears away from the core as the particle is cycled. The densely cross-linked particles formed from the 200 min reaction, on the other hand, have very thick shells and therefore are stable to the swelling/collapse cycle. The surface of these particles is smooth and unaffected by the sparse interstitial layer, which lies far beneath the dense shell.

3.3. Equilibrium Swelling. The extent of equilibrium swelling depends strongly on cross-linking reaction time. This dependence is most dramatic at low reaction times of less than about 50-100 min (Figure 7) in both 0.05 and 0.5 wt % SDS solutions. Despite scatter in the data (caused by batch variability), the equilibrium particle mass can clearly be varied by a factor of 2 or 3 by adjusting the cross-link density. The swelling/collapse transition is the greatest when the cross-link density is low where the particle mass varies by up to an order of magnitude between the two states. In the limit of high cross-linking density, particles in both the expanded and collapsed states have masses of approximately 10-15 mg, which is close to the particle mass during the crosslinking reaction. How the amount of equilibrium swelling depends on the SDS concentration can vary greatly depending on the cross-link density (Figure 8). The SDS/JR-400 phase diagram (Figure 1) suggests that a sharp swelling transition should occur at the redissolution phase boundary in the limit of low polyelectrolyte concentration, which is at the SDS concentration of about 3 mM (0.09 wt %).13,21 This transition occurs in the sparsely cross-linked (2 min reaction) particles (Figure 8), where the particle mass triples as the SDS concentration is raised from 0.08 to 0.10 wt %. Further addition of surfactant results in additional particle swelling (by a factor

Particles that Swell ReVersibly

Figure 7. Particle swelling in (4) 0.05 and (9) 0.5 wt % SDS as function of cross-linking reaction time. The dashed lines are guides to the eye.

Figure 8. Equilibrium particle swelling of sparsely cross-linked (2 min reaction) particles (() and densely cross-linked (200 min reaction) particles (0) as a function of supernatant SDS concentration. The dashed lines are guides to the eye.

of about 2), until the curve plateaus at an SDS concentration of about 0.3 wt %. In the highly cross-linked (200 min reaction) gels, no sharp transition is observed in the surfactant concentration range examined, but rather there is only a gradual and monotonic mass increase with increasing SDS concentration. This markedly different trend shows that varying the cross-link density enables the control of the abruptness of the swelling/collapse transition with respect to surfactant concentration. The smoother volume transitions with increasing cross-link density probably reflect the increased configurational constraints in the network.23,30,31 These may amplify the unfavorable electrostatic interactions,30 which resist charge inversion caused by the surfactant adsorption that accompanies the swelling transition,32 thereby reducing its abruptness. 3.4. Swelling and Collapse Kinetics. The swelling and collapse rates increase with the particle cross-link density. While the loosely cross-linked particles (2 min reaction) fail to attain their steady-state mass after 20-26 h of equilibration (Figure 5), densely cross-linked particles (200 min reaction) reach their equilibrium mass within 4 h. This is likely because fewer structural rearrangements need to be made to attain the equilibrium polyelectrolyte conformation when the gel is densely cross-linked. Another noteworthy feature of the swelling/collapse kinetics is that loosely cross-linked particles (2 min reaction), when added to 0.05 wt % surfactant, initially swell before collapsing. This effect occurs over the first 60-90 min after the particles are

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Figure 9. Solution SDBS concentration as a function of time illustrates the rate of surfactant release from swollen gel particles into the dilute (∼0.05 wt % SDBS) supernatant solution. The dotted line is a guide to the eye.

added to the 0.05 wt % solution and causes a sharp increase in the particle mass by about 20%. This could be an osmotic effect induced by the initially high SDS concentration difference between the surfactant-rich gel and dilute supernatant solution, similar to that observed in other systems.33-35 To test this hypothesis, the rate of surfactant transport out of the gel particles was measured and compared with the time scale of the swelling that precedes the collapse. For this experiment, SDS was replaced by the UV-active SDBS. The particles were swollen in 0.5 wt % SDBS and then transferred into 0.05 wt % SDBS. The release of excess surfactant from the particles was then monitored over time by measuring the supernatant SDBS concentration (see Figure 9). The SDBS concentration in the supernatant reaches a plateau at around 30 min, which is around the same time when the particle mass crests after its immersion into the 0.05 wt % SDS solution. This suggests that the particles begin shrinking when the SDBS equilibrates with the supernatant and supports the proposed hypothesis.

4. Conclusions Reversibly swelling surfactant/polyelectrolyte gel particles can be prepared by dropwise addition of polyelectrolyte solution to a solution of oppositely charged surfactant followed by a crosslinking reaction. These particles undergo swelling/collapse transitions wherein their mass changes by up to an order of magnitude. The equilibrium gel mass is governed by the mixture phase behavior and can be controlled by altering the solution composition and cross-link density. Altering the cross-link density enables adjustment of the magnitude of the transition and its abruptness with respect to changes in solution conditions. The equilibrium gel swelling/shrinking transitions are greatest in magnitude and most abrupt at low cross-link densities but reach equilibrium more rapidly for highly cross-linked particles. Acknowledgment. The authors gratefully acknowledge K. Czymmek for confocal microscopy support and B.E. Steuer for useful experimental suggestions. LA0604329 (30) Okuzaki, H.; Osada, Y. Macromolecules 1995, 28, 4554. (31) Starodubtzev, S. G. Vysokomol. Soedin. 1990, 31B, 925. (32) Ohbu, K.; Hiraishi, O.; Kashiwa, I. J. Am. Oil Chem. Soc. 1982, 59, 108. (33) Wood, J. M.; Attwood, D.; Collett, J. H. Int. J. Pharm. 1981, 7, 189. (34) Lee, P. I. J. Conrolled Release 1985, 2, 277. (35) Scranton, A. B.; Klier, J.; Peppas, N. A. Polymer 1990, 31, 1288.