Article pubs.acs.org/Langmuir
Interaction of Poly(N‑isopropylacrylamide) with Sodium Dodecyl Sulfate below the Critical Aggregation Concentration Nobuo Uehara* and Minami Ogawa Graduate School of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan S Supporting Information *
ABSTRACT: Interaction between the thermoresponsive polymer poly(Nisopropylacrylamide) (P-NIP) and sodium dodecyl sulfate (SDS) both above and below its phase transition temperature was examined under dilute conditions. Above the lower critical solution temperature (LCST) of P-NIP (32 °C), 0.01 wt % P-NIP specifically interacted with 1.0 × 10−5 mol/L SDS to form a precipitate. However, when SDS was added at concentrations above or below 1.0 × 10−5 mol/ L, the P-NIP solution remained clear above the LCST. A fluorometric probe, Nphenyl-naphthalene, indicated that the hydrophobicity of the aggregates composed of P-NIP and SDS changed at an SDS concentration of 1.0 × 10−5 mol/L. Although the hydrophobicity of the precipitate was similar to that of P-NIP alone at less than 1.0 × 10−5 mol/L, it approached that of SDS homomicelles as the SDS concentration increased above 1.0 × 10−5 mol/L. Dynamic light scattering and turbidimetry studies showed no P-NIP phase transition above an SDS concentration of 1.0 × 10−5 mol/L, which is much lower than the reported critical association concentration (CAC) of SDS with P-NIP. This indicates that P-NIP interacted with SDS above the LSCT at much lower SDS concentration than the reported CAC.
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INTRODUCTION Thermoresponsive polymers are stimuli-responsive polymers that undergo reversible phase transitions in response to changes in temperature. The phase transition temperature of poly(Nisopropylacrylamide) (P-NIP), a representative thermoresponsive polymer, is 32 °C, which is known as the lower critical solution temperature (LCST).1,2 When linear P-NIP is hydrated, it adopts a random coil configuration below the LCST and becomes globular above the LCST because of shrinkage of polymer chains due to dehydration. This reversible phase transition of P-NIP means that its fundamental properties and its practical applications have been extensively studied.3 The phase transition of P-NIP in an aqueous solution is affected by the other species in solution. Water-miscible solvents elevate the LCST of P-NIP through solvation of the P-NIP chains,4−6 whereas inorganic salts depress the LCST through salting out.7−10 Surfactants also influence the phase transition of P-NIP through hydrophobic interactions and the formation of aggregates.11−13 The interaction of the common anionic surfactant, sodium dodecyl sulfate (SDS), with P-NIP has been intensively investigated.14−26 The hydrophobic interaction of SDS with P-NIP globules introduces negative charges to the conjugates, which increases the solubility of the globules and elevates the LCST of the polymer.21,24−26 When a solution containing SDS and P-NIP is heated above the LCST, the SDS molecules bound to the PNIP polymer globules are expelled by electrostatic repulsion.14 However, the interaction between SDS and P-NIP during the © 2014 American Chemical Society
phase transition and the molecular structure of the resulting aggregates are still not fully understood. For example, there are several different literature values of the critical aggregation concentration (CAC), which is the lowest SDS concentration at which it associates with P-NIP.1,21,24−26 Schild and Tirell reported a CAC value of 0.23 mg/mL,26 whereas Walters et al. reported it was 0.37 mg/L.21 Furthermore, Hsu et al. reported that CAC varied from 0.02 to 0.3 mg/L as a function of solution pH when a P-NIP copolymer with acrylic groups was used.27 The differences were caused by the intermolecular association of P-NIP chains and the turbidity of the solution resulting from the globules precipitating. To overcome this problem, single-chain globular particles of P-NIP have been prepared by using SDS micelles.28 The solution was heated to form P-NIP globules in the SDS micelles, and then SDS was removed by electrodialysis.29 However, there have been few studies of the interaction between P-NIP and SDS at low concentrations.29 Because the coil-to-globule transition of a single chain of P-NIP can be regarded as a model of the morphological change in nonionic biopolymers in biological samples, the interaction between P-NIP and SDS under dilute conditions should provide fundamental information about the behavior of biopolymers during treatment with SDS, such as in SDS-poly acrylamide gel electrophoresis. Received: March 19, 2014 Revised: May 2, 2014 Published: May 8, 2014 6367
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Figure 1. Fluorescence spectra of 2.28 × 10−7 mol/L PN in 0.01 wt % P-NIP solution as a function of temperature. Solution temperatures were set at 30, 32, 32.5, 33, and 34 + 2n (n = 0−8) °C. and DLS at various temperatures. Measurement wavelength for transmittance was set at 850 nm to avoid unintended interferences from PN.
In this paper, we studied the interaction of SDS with P-NIP at low concentrations below 0.01 wt %, where P-NIP did not precipitate above the LCST. The formation of P-NIP and SDS aggregates was examined with light scattering, turbidimetry, and fluorometry with a fluorometric probe molecule. The transmittance and dynamic light scattering (DLS) studies indicated that the critical interaction of both species occurred at an SDS concentration of 1.0 × 10−5 mol/L, which was much lower than the reported CAC.21,26 A fluorometric probe, N-phenylnaphthalene (PN)18,30 showed that the hydrophobicity of the conjugate changed at the concentration of SDS. Thus, SDS interacts with P-NIP at much lower concentrations than the reported CAC.
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RESULTS AND DISCUSSION Phase Transition Behavior of Dilute P-NIP. Although PNIP solutions with concentrations below 0.01 wt % remained almost clear, even when the solution was heated above the LCST, the fluorescent probe, PN, which responds to the hydrophobicity of its neighbor,30 indicated the formation of PNIP globules in solution even at low concentrations as shown in Figure 1. The change in the PN peak around 410 nm was also sharp above the LCST, indicating the formation of hydrophobic aggregates followed by the inclusion of PN. Figure 1 also shows another spectral band around 563 nm, which is not from the inherent fluorescence of PN and originated from incident light scattered by the globular aggregates because there was no band-pass filter mounted on the fluorometer. We consider that this peak is assigned to an anti-Stokes line of Raman scattering. The elastic scattering of incident light of 327 nm was observed at 654 nm as a twice wavelength, but it was eliminated in Figure 1. We used the peak at 563 nm to monitor the formation of P-NIP globules because its intensity is proportional to the P-NIP concentration. SDS surfactant micelles formed above the critical micelle concentration (CMC) produced no spectral bands around 563 nm. The temperature dependences of the signal intensities at 410 and 563 nm in the fluorescence spectra are summarized in Figure S1 (Supporting Information). Interaction of P-NIP with SDS. The interaction of P-NIP with SDS above the LCST of P-NIP (∼32 °C) causes the formation of aggregates above the CAC of SDS. The conventional model predicts that necklace-like aggregates are formed as a result of the interaction between P-NIP and SDS.14,27 The introduction of negative charges increases the solubility of the associated species and elevates the LCST.21,24−26 When the solution containing P-NIP and SDS was heated, P-NIP globules expelled the bound SDS to accommodate the negative charges. The behavior of the P-NIP and SDS aggregates under dilute conditions was different from that under conventional conditions. Ionic species, such as electrolytes and buffering agents not only facilitate the phase transition of P-NIP but also interact with SDS electrostatically. To investigate the interaction of P-NIP and SDS, we used a solution containing p-NIP and SDS without any buffering agents or electrolytes.
EXPERIMENTAL SECTION
Apparatus. Transmittance and fluorescence spectra of a P-NIP solution were measured with a V-650 spectrophotometer (JASCO, Japan) and an FP-6100 spectrofluorometer (JASCO, Japan), respectively. A Zetasizer Nano ZS DLS system (Malvern, UK) was used to determine the hydrodynamic radii of aggregates. Reagents. N-Isopropylacrylamide, PN, SDS, 3-mercaptopropionate, and potassium persulfate were obtained from Wako Pure Chemicals (Tokyo, Japan). Other reagents were of analytical reagent grade. N-Isopropylacrylamide was recrystallized twice with hexane before polymerization. P-NIP was synthesized by the radical polymerization reported by Cole et al.31 with slight modifications. The crude precipitate was recrystallized with methanol and diethyl ether to remove the unreacted monomers. The purified precipitate was dissolved in water and then freeze-dried. The polymer was obtained as a fluffy solid (average molecular weight, 1.0 × 104 by GPC with a polystyrene standard). A stock solution of 1.0 wt % P-NIP was prepared by dissolving P-NIP (0.10 g) in water (9.90 g). A working solution of 0.1 wt % P-NIP was prepared by diluting the stock solution 10-fold with water and allowing it to stand for at least 3 days before use. A surfactant solution of SDS was prepared by dissolving SDS in water to a concentration of 1.0 × 10−2 mol/L and diluted before use. A 1 × 10−3 mol/L methanolic PN stock solution was prepared by dissolving suitable amounts of PN in methanol. A 1.0 × 10−5 mol/L PN solution was prepared by diluting the stock solution with 0.1 wt % P-NIP solution. Preparation and Measurement of P-NIP Solutions Containing a Surfactant. Samples of P-NIP solutions containing SDS were prepared from appropriate amounts of 0.1 wt % P-NIP solution and SDS solution in a 10 mL volumetric flask followed by dilution to the mark. If necessary, a 1 × 10−5 mol/L of PN solution was added to the volumetric flask prior to dilution. The effect of the methanol in the sample solutions on the phase transition was negligible because the methanol concentration was less than 1 × 10−2 vol %. The sample solutions were subjected to transmittance measurements, fluorometry, 6368
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The effect of SDS concentration on the phase transition of PNIP was determined from the transmittance of the solution. Figure 2 shows the relationship between the transmittance of
Figure 3. Effect of SDS concentration on the fluorescence spectra of 2.28 × 10−7 mol/L PN in 0.05 wt % P-NIP solution at 40 °C. Excitation wavelength, 327 nm.
Figure 2. Transmittance of 0.03 wt % P-NIP solution containing SDS as a function of solution temperature. Transmittance was set at wavelength at 850 nm. SDS concentration: □, 0 mol/L; Δ, 1 × 10−6 mol/L; ■, 2 × 10−6 mol/L; ◆, 5 × 10−6 mol/L; ●, 1 × 10−5 mol/L; ○, 1 × 10−4 mol/L. Inset; Transmittance of 0.03 wt % P-NIP solution as a function of the SDS concentration of SDS at 30 °C (□) and 40 °C (○).
the 0.03 wt % P-NIP solution and SDS concentration. The transmittance of the P-NIP solution steeply decreased at 32 °C when 1 × 10−5 mol/L SDS was added. Adding SDS at a concentration more or less than 1 × 10−5 mol/L recovered the transmittance of the solution above the LCST. Thus, adding 1 × 10−5 mol/L of SDS created the specific conditions where the solubility of the P-NIP globules decreases. The inset in Figure 2 shows that the transmittance dipped around 1.0 × 10−5 mol/L SDS for a 0.03 wt % P-NIP solution. To examine the interaction between P-NIP and SDS further, the effect of the SDS concentration on the transmittance of PNIP solutions at 40 °C was investigated (data not shown) . The dip increased as the concentration of P-NIP increased for the same SDS concentration, suggesting that the concentration of SDS governed the association between SDS and P-NIP. This concentration was independent of the P-NIP concentration, which is consistent with the results published by Meewes and co-workers.22 Fluorometry with the fluorometric probe, PN, was also used to study effect of SDS on the association of the two components. The fluorescence of PN in the spectra increased as the hydrophobicity of the environment increased and a hypsochromic shift was observed. Figure 3 shows typical fluorescence spectra of PN in a 0.05 wt % solution of P-NIP and SDS. The spectral band around 410 nm was attributed to the fluorescence of PN in a hydrophilic environment. In addition to the inherent fluorescence, another spectral band was also observed around 563 nm in Figure 3, similar to that in Figure 1. The spectral band at 563 nm was caused by the scattering of incident light by the aggregates and it depended on the SDS concentration. Figure 4 shows the dependence of the intensities of both spectral bands on the SDS concentration. As the P-NIP concentration increased, the intensity of the peak at 563 nm reached a maximum at an SDS concentration of 1.0 × 10−5 mol/L. The maximum light scattering and minimum transmittance were observed at the same SDS concentration,
Figure 4. Signal intensities at 410 and 563 nm as a function of SDS concentration in P-NIP solution containing 2.28 × 10−7 mol/L PN at 40 °C. Excitation wavelength, 327 nm; transmittance wavelength, 850 nm.
which indicates that the amount of P-NIP aggregates reached a maximum at this concentration. Further increases in the SDS concentration above 1.0 × 10−5 mol/L reduced the signal intensity at 563 nm for every P-NIP concentration. The decrease in signal intensity is explained by the contribution of SDS in the aggregates, because SDS and its micelles solely do 6369
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The thermoresponsive change in the hydrodynamic radii of P-NIP was observed below an SDS concentration of 1.0 × 10−5 mol/L as a result of the phase transition. The hydrodynamic radii of P-NIP aggregates showed a sharp increase at the thermoresponsive temperature (32 °C), because globules were formed by the dehydration of the P-NIP chains. The hydrodynamic radii of the aggregates formed above the phase transition temperature increased with the SDS concentration up to 1.0 × 10−5 mol/L. This indicates that SDS molecules were taken into the P-NIP globules. Note that large variations of hydrodynamic diameters are observed around the phase transition temperature in Figure 6. They reflect various indefinite structures of the aggregates during the phase transition process. Contrarily, since globules formed above the phase transition temperature were relatively rigid, the variation of the hydrodynamic diameters of the globules was small. No obvious changes in the hydrodynamic radii of P-NIP around the phase transition temperature were observed at SDS concentrations greater than 1.0 × 10−5 mol/L. The hydrodynamic radii of P-NIP decreased slightly when the solution was heated above the phase transition temperature under these conditions. These results indicate that SDS concentrations greater than 1.0 × 10−5 mol/L suppressed not only the phase transition of P-NIP, but also variation of the hydrodynamic radii. This agrees with the experimental observation that anionic SDS molecules on the outside of the P-NIP globules repel each other and cause many smaller globules to form.14 Thus, our DLS results revealed sharp changes in the properties of P-NIP globules at an SDS concentration of 1.0 × 10−5 mol/ L. Figure 7 shows the change in the hydrodynamic radii of PNIP globules below and above the LCST and the transmittance of the solution. The critical SDS concentration is at 1.0 × 10−5 mol/L, and this divides the two regions where different P-NIP globules form (original data are summarized in Supporting Information Figure S2). A P-NIP solution containing less than 1.0 × 10−5 mol/L SDS exhibited phase transition changes, such as an increase in hydrodynamic radii and a decrease in transmittance, whereas the thermoresponsive changes in the PNIP solution were suppressed above the critical SDS concentration. This is consistent with the fluorometric studies, where the hydrophobicity of the aggregates below the concentration was similar to that of P-NIP globules on their own, although it reached that of SDS micelles as the SDS concentration exceeded the critical concentration of SDS. Thus, the critical concentration of SDS, 1.0 × 10−5 mol/L, is the point at which the behavior of the aggregates composed of P-NIP and SDS changes under dilute conditions, and it is much lower than the reported CAC.21,26 In conclusion, an SDS concentration of 1.0 × 10−5 mol/L was the critical concentration at which the interactions between SDS and P-NIP caused the aggregates to precipitate above the LCST. The thermoresponsive properties of P-NIP were suppressed at this SDS concentration, which is much lower than the reported CAC for the SDS and P-NIP system. Thus, SDS interacts with P-NIP at a much lower concentration than previously reported. These results provide new insights into the interactions of SDS with nonionic biopolymers such as polysaccharides and polypeptides under dilute conditions.
not produce any spectral bands at 563 nm. The SDS concentration slightly affects the intensity of the PN fluorescence at 410 nm. Both profiles indicate that SDS affected the light scattering properties of the P-NIP aggregates, although it slightly influences their hydrophobicity. We propose that SDS was included in P-NIP globules and increased the size of the aggregates at SDS concentrations of less than 1.0 × 10−5 mol/L. However, the size of the P-NIP aggregates decreased abruptly at concentrations greater than 1.0 × 10−5 mol/L because of the negative charges of the SDS in the aggregates. This is consistent with the SDS molecules being arranged on the outside of the aggregate.14,27 A slight increase in the fluorescence intensity at 5 × 10−3 mol/L for every P-NIP concentration was caused by the formation of SDS homomicelles because the SDS concentration was above the CMC and PN was included in the micelles. The shape of the spectral band for PN around 410 nm (Figure 3) shows that the fluorescence intensity at 400 nm decreased with the SDS concentration. Since the spectra of PN increase and shift hypsochromically as increase in hydrophobicity of its vicinity, the shape of the spectral band was evaluated ratiometrically to estimate the hydrophobicity of the aggregates. The ratio of the intensity at 410 to that at 400 nm increases when the hydrophobicity increases. Figure 5 shows
Figure 5. Intensity ratio of the fluorescence intensity at 410 nm to that at 400 nm as a function of SDS concentration. Data are taken from Figure 6.
the effect of SDS concentration on the ratio. The ratio of the intensities for 0.05% P-NIP solution was 1.05, whereas it was 1.50 for the 0.01 mol/L SDS solution, where SDS micelles were formed. The intensity ratio remained almost constant below an SDS concentration of 1.0 × 10−5 mol/L, and was almost identical to that of the P-NIP solution. However, it increased with the SDS concentration above 1.0 × 10−5 mol/L and approached the concentration of the SDS micelle solution. The profile of the ratio also indicates that the hydrophobicity of the aggregates was controlled by the SDS concentration in this concentration range. Thus, the main parameters that controlled the hydrophobicity of the aggregates changed at an SDS concentration of 1.0 × 10−5 mol/L. Dynamic Light Scattering Studies. The hydrodynamic radii of water-soluble compounds in solutions can be determined by DLS. The hydrodynamic radii of P-NIP globules in an SDS solution were estimated by DLS as a function of SDS concentration and solution temperature. Figure 6 shows that the addition of 1.0 × 10−5 mol/L SDS increased the hydrodynamic radii of the P-NIP globules above the phase transition temperature, which caused P-NIP to precipitate and decrease the transmittance. 6370
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Figure 6. Hydrodynamic radii of the aggregates as a function of solution temperature and SDS concentration determined by DLS at a P-NIP concentration of 0.005 wt %. Each datum is available in the Supporting Information (Figure S2).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Grant-in-aid for Science Research Japan B(23310054) and C(23510123) of Ministry of Education, Japan.
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Figure 7. (a) The transmittance and (b) the hydrodynamic radii of PNIP aggregates with SDS in 0.01 wt % P-NIP solution at 30 °C (□) and 40 °C (○) as a function of SDS concentration.
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ASSOCIATED CONTENT
S Supporting Information *
Effect of solution temperature on the maximum fluorescence wavelength of PN and signal intensities at 410 and 563 for the 0.01 wt % P-NIP solution containing 2.28 × 10−7 mol/L PN and change in histograms of DLS of PNIPAAm and PNIPAAm/SDS solutions. This material is available free of charge via the Internet at http://pubs.acs.org. 6371
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