Redox-Responsive Gel-Sol/Sol-Gel Transition in Poly(acrylic. acid) Aqueous Solution Containing Fe(III) Ions Switched by

Transcription

1 Supporting Information Journal of the American Chemical Society Redox-Responsive Gel-Sol/Sol-Gel Transition in Poly(acrylic acid) Aqueous Solution Containing Fe(III) Ions Switched by Light Fei Peng, Guangzhao Li, Xinxing Liu, Shuizhu Wu and Zhen Tong* Research Institute of Materials Science, South China University of Technology, Guangzhou , China S1

2 Experimental Materials: Poly(acrylic acid) (PAA, Alfa Aesar, 25 wt% aqueous solution) was used to prepare hydrogels with weight average molecular weight M w = and distribution M w /M n = 2.0, which were determined with a GPC (Waters 2410) at 35 C using 0.1 M aqueous solution of Na 2 SO 4 as the eluent (0.6 ml/min) and narrowly distributed poly(ethylene oxide) as the standard. 1,10-phenanthroline, citric acid monohydrate (C 6 O 6 H 8 H 2 O), and ferric chloride hexahydrate (FeCl 3 6H 2 O) were purchased from Tianjin No.1 Chemical Reagent Factory, China. All reagents were of analysis grade. Water used in all experiments was purified through deionization and filtration with a Millipore purification apparatus to the resistivity higher than 18.0 MΩ cm. Gel Preparation and Dissolution: Appropriate amounts of FeCl 3 6H 2 O and citric acid monohydrate were dissolved into pure water to form a Fe(III)-citrate aqueous solution and its ph was adjusted to desired value by adding NaOH solution (10 mol/l). The Fe(III) concentration in the final solution was fixed at 0.1 mol/l and the ratio of FeCl 3 6H 2 O to C 6 O 6 H 8 H 2 O was 1:2 in mole. Aqueous solution of PAA (25 wt%, 2 g) and Fe(III)-citrate aqueous solution (0.5 ml) at different ph were mixed in a small vial and stirred for 4 min. After being stood for a few minutes, the viscosity obviously increased and the solution gradually changed into a gel. The dissolution of the gel in a tightly sealed vial was conducted by simulated sunlight irradiation with a metal halide lamp at wavelength higher than 313 nm (CMH-250 Beijing, China) in a dark ark and the light intensity at the sample was 80 mw/cm 2 as detected by an optical energy indicator (SGN-I Tianjin, China). Rheology Measurements: The rheology properties of the PAA with Fe-citrate aqueous system before and after light irradiation were measured with a stress controlled rheometer AR-G2 (TA) using a parallel plate of 40 mm diameter at 25±0.1 C. The linear viscoelastic S2

3 region was determined by the strain sweep at 6.28 rad/s and then the frequency sweep was carried out at strain between 2 ~ 10 % to ensure the linear response. The zero shear viscosity η 0 for the PAA + Fe-citrate after irradiation was determined by extrapolating the viscosity data at shear rate of 0.1 ~ 100 s -1 to zero. The as-prepared gel and PAA aqueous solution were incubated at room temperature for 3 days prior to the rheology measurement. Other samples after irradiation were measured immediately. The results are represented in Figures S1 and S2. Fe(III) and Fe(II) Concentration Assay: The concentration of Fe(II) and Fe(III) ions in aqueous solutions was determined according to the procedure reported in literature to avoid the formation of Fe(III) oxyhydroxide precipitates. 1 The absorbance at 510 nm, at which there was a maximum for the Fe-phenanthroline system, was monitored with an UV-Vis spectrophotometer UV-3010 (Hitachi) at 25 C after equilibrated for 2 min to avoid the further oxidation of Fe(II). The absorbance at 510 nm for the Fe(III) solutions with 1,10-phenanthroline (0.06 g/l) in sodium acetate buffer (0.1 mol/l) was recorded before and after reduction by hydroxylamine hydrochloride (20.0 g/l). Linear calibration curves of absorbance against the concentrations of Fe(II) and Fe(III) ions were derived on the Fe(III) standard solutions of 0 ~ 40 µmol/l as shown in Figure S3. The concentration of ferric and ferrous ions in the aqueous solution of PAA was determined in the same way using the calibration curves. The solution was diluted with pure water, which was irradiated with ultrasonic for 15 min and bubbled with argon for 30 min prior to use to remove dissolved oxygen. S3

4 Fe(III) - OOC-C-OH hv 2Fe(II) + OOC-C-OH CH 3 2CO 2+ C=O 2Fe(III) C=O +H + + Fe(II) Fe(III) C-OH + CO 2 CH 3 Scheme S1. Photochemical reactions of Fe(III)-citrate complex in aqueous solution under the sunlight irradiation Dynamic modulus 10 3 PAA + Fe-citrate o C G', G" (Pa) ω / rad s -1 Figure S1. Storage modulus G (filled symbols) and loss modulus G (open symbols) at 25 o C for the PAA (20 wt%) + Fe-citrate aqueous systems as a function of angular frequency ω: as-prepared PAA + Fe(III) gel (G, G ), PAA + Fe(III) gel after irradiated for 12 min (G, G ), PAA + Fe(III) gel after irradiated for 12 min and oxidation for 5 days (G, G ), and 20 wt% of original PAA aqueous solution (G, G ). S4

5 G', G'' (Pa) PAA + Fe(III)-citrate 25 o C ph = 2 ph = ph = ω / rad s -1 Figure S2. Storage modulus G (filled symbols) and loss modulus G (open symbols) at 25 o C for the 20 wt% of PAA mol/l Fe(III)-0.04 mol/l citrate aqueous systems a function of angular frequency ω at ph 2, 3 and Calibration curves for Fe(III) and Fe(II) concentration assay Fe-phenanthroline 510 nm, 25 o C 40 A 1 x r = A2 x r = Fe / µmol/l 0 Figure S3. Calibration curve of absorbance vs. Fe ion concentration in aqueous solutions S5

6 containing 1,10-phenanthroline of 0.06 g/l in sodium acetate buffer of 0.1 mol/l. A1: absorbance for the solution of Fe(III) completely reduced to Fe(II) by hydroxylamine hydrochloride of 20.0 g/l; A 2 : absorbance for the solution of Fe(III) without reduction. r is the standard error. References (1) Viollier, E.; Inglett, W. P.; Hunter, K.; Roychoudhury, N. A; Cappellen Van, P. Appl. Geochem. 2000, 15, (2) Zhang, C.; Wang, L.; Wu, F.; Deng, N. Environ Sci & Pollut Res. 2006, 13, S6