Article pubs.acs.org/IECR
In Situ Encapsulation of Hydrogen Peroxide in a Silica Matrix in the Presence of Divalent Metal Ions (Mg2+ and Ca2+) Fulya Sudur Zalluhoglu,§ Ezgi Melis Dogan,§ Naigambi Patience Namusuubo,§ Nese Orbey,*,§ and Edwin Jahngen‡ §
Chemical Engineering Department and ‡Chemistry Department, University of Massachusetts Lowell, 1 University Avenue, Lowell, Massachusetts 01854 United States ABSTRACT: Encapsulating hydrogen peroxide (H2O2) in silica hydrogels is a simple, environmentally friendly, and cost-effective method that is easy to scale up. Sodium silicate is the most commonly used aqueous silicate in sol−gel chemistry. Previously, we studied the effects of Na+ and K+ ions in the starting silicate precursor on the structure of hydrogels and the stability of entrapped H2O2. In the present study, we present the results obtained when divalent ions, Mg2+ and Ca2+, were introduced in the sol. The use of divalent metal ions resulted in hydrogel structures that are different from those previously obtained. H2O2 stability increased with the addition of Mg2+ and Ca2+ ions and with decreasing pH. At low pH values, 93% of the peroxide was retained at the end of 10 days with Mg2+-containing hydrogels, compared to 91% retention with Ca2+-containing hydrogels, 87% retention with K+-containing hydrogels, and 68% retention with a unmodified sodium silicate precursor. The results show that the structure of the hydrogels can be changed using different types and amounts of metal ions to tailor the release of H2O2 for an intended application. 2(SiO−Na +) + CaCl 2 → 2SiO−Ca 2 + + 2NaCl
1. INTRODUCTION Encapsulating hydrogen peroxide (H2O2) in silica hydrogels and xerogels to improve its stability is currently the most promising method because of the formation of strong hydrogen bonds between the peroxide and the silica gel surface.1−6 Previous work done in our laboratories revealed the significant effect of the monovalent ions [sodium (Na+) and potassium (K+)] on the structure of the silica hydrogels.4−6 Therefore, in this study, we sought to study the effect of ions other than sodium and potassium on the structure of the hydrogels and hence the stability and release (%) of H2O2. To this end, magnesium (Mg2+) and calcium (Ca2+) ions were selected because they are both divalent, but their sizes are different from each other. To date, neither peroxide stability nor the effect of divalent ions on the silica hydrogel properties has been studied in detail. Sodium silicate, or “water glass”, is the most commonly used aqueous silicate in sol−gel chemistry. Silanol (SiOH) groups are formed by removing Na+ ions from sodium silicate with a cation-exchange resin.7−9 Magnesium and calcium silicate are not soluble in water; therefore, to prevent precipitation and ensure the formation of a uniform 3D network, we introduced Mg2+ and Ca2+ ions into the sol using magnesium and calcium chloride. The Na+ ions in the sol were partially replaced with these ions, as shown in eqs1 and 2.10 2(SiO−Na +) + MgCl2 → 2SiO−Mg 2 + + 2NaCl © 2017 American Chemical Society
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Two researchers have studied the effects of different metal ions on the silica gel formation and gel strength but did not use H2O2 in the formulations. The effect of these ions on the hydrogel structure has not been studied either. Hamouda and Amiri10 observed that the presence of Mg2+ or Ca2+ ions resulted in faster gelation as a result of the charge-screening effect, with Ca2+ ions enhancing the gel strength better than Mg2+ ions. Using tetraethylorthosilicate [TEOS, Si(OC2H5)4] as the starting precursor, Bansal11 studied the effects of nine different metal cations on the gelation time of silica gel. He also did not include H2O2 in his work. Bansal11 found that the gel time and activation energy for gel formation sharply increased in the presence of copper (Cu2+), aluminum (Al3+), lanthanum (La3+), or yttrium (Y3+) ions, which became incorporated in the gel network during polymerization. On the other hand, lithium (Li+), Na+, Mg2+, Ca2+, and strontium (Sr2+) ions bonded weakly to the silica gel and became entrapped without becoming part of the network. Although the gelation time was decreased, no appreciable effect on the activation energy was observed, and these ions were considered to be network modifiers. Received: Revised: Accepted: Published:
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January 20, 2017 February 14, 2017 February 14, 2017 February 14, 2017 DOI: 10.1021/acs.iecr.7b00278 Ind. Eng. Chem. Res. 2017, 56, 2607−2614
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Industrial & Engineering Chemistry Research
Figure 1. Spectroscopic characterization of the hydrogels and xerogels containing (a) Mg2+ and (b) Ca2+ ions.
Ca2+ to Na+ was 0.2:1. Once a homogeneous solution was obtained, 30 wt % H2O2 and 7 μL of phosphoric acid (H3PO4) were added. The pH values of the sols were controlled by adding different amounts (0−3.2 g) of an ion-exchange resin (Amberlite IR 120, hydrogen form). The mixture was stirred with an overhead mixer for 4 min, and the sol was separated from the resin. Sol was then transferred to a glass Petri dish (D = 100 mm and h = 15 mm) and placed in a BlueM gravity oven at 40 °C until gelation was observed. The sol−gel transition point is reached when a strong increase in the viscosity is observed, resulting in a “wet gel” or “hydrogel”. Upon drying by evaporation, the gel network contracts, expels liquid from the pores, and forms a xerogel. 2.3. Characterization. To study the formation of a silica network, attenuated-total-reflectance Fourier transform infrared (FTIR) spectroscopy (Thermo Scientific Nicolet 6700 FTIR spectrometer) was used with silica hydrogels and xerogels containing Mg2+ and Ca2+ ions. Spectra were obtained in the transmission mode from 600 to 4000 cm−1. The H2O2 content of the hydrogels was measured by potassium permanganate (KMnO4) titration immediately after gelation and remeasured daily to assess H2O2 retention in the hydrogel. Three repeated titrations were performed for each gel sample. Hydrogels gradually turned into xerogels during storage because of the loss of H2O2 and water. Scanning electron microscopy (SEM) and Brunauer− Emmett−Teller (BET) analyses were used to study the morphology and structure of the hydrogels. To prevent the collapse of the pores due to capillary pressure during drying, hydrogel samples were stored in 10 mL of ethanol (200 proof, anhydrous, >99.5%) overnight for solvent exchange and subsequently dried with supercritical carbon dioxide (Tousimis SAMDRI-795). The aerogels thus formed were degassed at room temperature (RT) under vacuum and analyzed using a Nitrogen Sorption Porosimeter Quantachrome Autosorb-3B instrument. The specific surface area was obtained by BET analysis. The pore-size distribution was calculated by the Barrett−Joyner−Halenda (BJH) method. The pore diameter was evaluated from the desorption isotherm. The morphology of the aerogels was visualized with a JEOL JSM 7401F scanning electron microscope. All samples were coated with gold by using a vacuum sputter coater and imaged with an accelerating voltage of 10−15.0 kV. For release (%) studies, equal amounts of hydrogel were placed in water in shaker flasks and placed on an orbital shaker (Chang Bioscience Inc., KJ201BD). Samples were agitated at RT and 100 rpm, and supernatants were removed at different time intervals ranging from 0 to 60 min. The amount of H2O2 released into water was determined by KMnO4 titration.
The synthesis of H2O2-containing xerogels using sodium silicate was first reported in 2007 by Ż egliński and colleagues,1 who tested the antibacterial activity of xerogels. Subsequently, Bednarz et al.2 entrapped H2O2 within silica xerogels, which they used as oxidants in organic synthesis. Wolanov et al.3 studied aluminate and alumina xerogels and compared their properties with those of silica xerogels synthesized using sodium silicate. More recently, Dogan et al.6 investigated the effects of the K+/Na+ ion ratio on the gelation time, hydrogel structure, stability, and release of H2O2. The results showed that the gelation time decreased and the short-term stability of H2O2 increased with the addition of K+ ions. They concluded that the properties of the hydrogels can be tailored by adjusting the amounts of K+ and Na+ ions and the pH of the sol for intended applications. The objective of the present study was to understand the effects of calcium and magnesium ions on the structure and morphology of the hydrogels and the stability and release of H2O2. The experimental procedure followed, and the results obtained are presented in the following sections. Currently, H2O2 is used in pulp and textile bleaching, tooth whitening, skin disinfection, and water treatment.12−15 Retarding the decomposition of H2O2 and controlling its release using different amounts of Na+, Mg2+, and Ca2+ ions would enhance the use of H2O2 in these applications and enable new applications that are currently not practical including injectable hydrogels for wound healing.16
2. EXPERIMENTAL PROCEDURE 2.1. Materials. Magnesium chloride (MgCl2; ACS reagent, >99%) and calcium chloride (CaCl2; ACS reagent, >99%) were purchased from J.T. Baker. A sodium silicate solution (reagent grade, 10.6% Na2O and 26.5% SiO2), Amberlite IR120 (hydrogen form), hydrogen peroxide (H2O2; containing inhibitor, 30 wt % in H2O, ACS reagent), orthophosphoric acid (85 wt % in H2O and 99.99% trace metals basis), potassium permanganate (ACS reagent >99%), and sulfuric acid (99.999%) were purchased from Sigma-Aldrich Chemical Co. All chemicals were used without further purification. 2.2. Hydrogel Synthesis. Hydrogels containing Mg2+ or Ca2+ ions along with Na+ ions were synthesized by using MgCl2, CaCl2, and sodium silicate. Silicates of magnesium and calcium have low solubility in water. Precipitation occurs when MgCl2 and CaCl2 solutions in water have concentrations of 1.5% by weight (wt %) and higher. Therefore, we used 1 wt % of these salts to prevent precipitation. Sodium silicate was added dropwise to solutions in an amount such that its weight fraction in the solution was 28% for all formulations. The resulting molar ratio of Mg2+ to Na+ and 2608
DOI: 10.1021/acs.iecr.7b00278 Ind. Eng. Chem. Res. 2017, 56, 2607−2614
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3. RESULTS AND DISCUSSION In this work, we partially replaced Na+ ions in the starting precursor by ions other than K+, to study the effects of the ion size and charge on the hydrogel properties. Mg2+ and Ca2+ ions were selected to replace Na+ ions because they are both divalent ions and Mg2+ ions are smaller than Na+ ions, whereas the sizes of the Ca2+ and Na+ ions are similar. Previous findings with only Na+ ions in the sol are included in all figures for ease of discussion. 3.1. Formation of a Silica Network and Gel Time. The bonds that formed in hydrogels and xerogels containing Mg2+ or Ca2+ are studied using FTIR spectroscopy (Figure 1). Spectra obtained for hydrogels and xerogels containing Mg2+ or Ca2+ ions were very similar to the results obtained in our previous study.4 Figure 1 shows the O−H stretching frequencies (3700−3000 cm−1 band) that were generated from H2O2, water, and SiOH groups. No stretching vibrations were observed beyond 3700 cm−1, indicating complete saturation of silica with hydrogen bonds.17 The 2900−2700 cm−1 band reflects the vibrations of H2O2.18 The 1400−1300 cm−1 band corresponds to the bending vibrations of H−O−O, reflecting the presence of water molecules in pores and of H2O2 bonded to SiOH groups.17 Asymmetric stretching vibrations of Si−O−Si (at 1100 cm−1) were observed for both hydrogel samples, indicating the presence of tetrahedral silicate units.19 Silica hydrogels and xerogels containing different ions had very similar spectra and SiOH bonding efficiencies. Therefore, either the hydrogel or xerogel form can be used to stabilize H2O2, depending on the application. Understanding the effects of other ions on the gel time is essential for large-scale hydrogel synthesis, in terms of obtaining reproducible results and choosing the optimal gel time for a particular application. The gel time generally depends on various factors, such as the size, charge, and coordination number of the ion, as well as the total ion concentration in the sol. Figure 2 shows the effects of the Mg2+ and Ca2+ ion contents on the gel time and pH of the sols. The results previously
and rapid aggregation of the particles was observed. When the pH of the sol was acidic, the charge on the silica particles changed from negative to positive, and the silica particles again repelled each other. Similar trends for the dependence of the gel time on the pH of the medium were observed with sodium silicate. For all pH values studied, the addition of Mg2+ and Ca2+ ions resulted in faster gelation compared to the previous results obtained in the presence of Na+ ions only. For example, the addition of Mg2+ and Ca2+ ions decreased the gel time to 8 and 35 min, compared to the 65 min that was observed previously with only Na+ ion in the sol. A decrease in the gelation time was also observed in our previous work with the addition of K+ ions to the starting precursor.6 This is an expected result because the total ion concentration is higher in sols with Mg2+, Ca2+, or K+ ions compared to sols with only Na+ ions. Inorganic salts at higher ion concentrations result in fewer electrostatic interactions between the silicate particles by reducing the dielectric constant of the aqueous medium.21 In addition, because of their divalent nature, Mg2+ and Ca2+ allow a more branched network to form in the sol, with sites available for hydrogen bonding between the SiOH groups and peroxide molecules. Sols with Mg2+ ions gelled faster compared to sols with Ca2+ ions (Figure 2). Sols of Ca2+ and Mg2+ had similar ion concentrations. Mg2+ has the smallest size (86 pm) compared to Ca2+ (114 pm) and Na+ (116 pm). However, the smaller size of Mg2+ ions allowed SiOH groups with H2O2 molecules to be brought closer together, such that they occupied less space in the sol compared to sols with Ca2+ or Na+. Although the sizes of Ca2+ and Na+ are very close to each other, faster gelation was observed with Ca2+ because of its divalent nature (explained above). 3.2. RT Stability of H2O2. To study the effects of the presence of Mg2+ and Ca2+ ions on the stability of entrapped H2O2, we replaced the cations (Mg2+, Ca2+, and Na+) with H+ ions by using different amounts of an ion-exchange resin and adjusting the pH, as explained in section 2.2. Before the ionexchange process, the pH of the sols containing Mg2+ or Ca2+ was 8.8. The amount of ion-exchange resin used determined the pH of the sol, which was 4.6 for sols containing Mg2+ ions at the highest amount of ion-exchange resin and 4.1 for sols containing Ca2+ ions. Stability data for sols containing Mg2+ for pH = 4.6 and 8.8 were obtained previously in our laboratories5 up to 50 days. These data are included in Figures 3a and 4a,c for comparison. For all pH values, the highest H2O2 stability was obtained when Mg2+ ions were present in the sol (Figure 3). After 70 days, 70% of the peroxide was retained at a pH of 4.6 when the hydrogels contained Mg2+ ions (Figure 3a), compared to 59% retention at a pH of 4.1 when the hydrogels contained Ca2+ (Figure 3b). We believe that this result occurred because Mg2+ is also a divalent ion, but has a smaller size compared to Ca2+. Therefore, Mg2+ caused a more branched network to form during the conversion from sol to hydrogel. When Ca2+ ions were present, the H2O2 stability was slightly lower than that of hydrogels containing Mg2+ ions. In the literature, the stability of H2O2 was studied with silica xerogels at lower initial peroxide concentrations. Retention values reported range between 50 and 71% for initial peroxide concentrations of 8 and 3.8 wt %, respectively.1,2 In our previous studies with a higher initial peroxide concentration of 20 wt %, 47% retention was obtained after 70 days of storage at RT with hydrogels containing K+
Figure 2. Relationship between the gel time and pH in the presence of Mg2+, Ca2+, K+, and Na+ ions.
obtained in the presence of Na+ and K+ ions are included for comparison.4,6 The relationship between the gel time and pH followed a similar trend for both cases; at high pH values (pH 6−8), the sol began to adsorb hydroxyl ions, resulting in longer gelation times and a negatively charged surface. The gel time started to decrease as the pH value increased (pH > 8). We believe that faster gelation is obtained because the stability of the negatively charged sol is disrupted due to the presence of the cations.20 Gelation was most rapid between pH values of 4 and 6, as the sol passed through the isoelectric point. At the isoelectric point, the sol did not carry a net electrical charge, 2609
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Figure 3. RT stability of hydrogels in the presence of (a) Mg2+and (b) Ca2+ ions.
containing only Na+.4 This figure also shows the short-term (first 10 days of storage) stability curves of hydrogels containing Mg2+, Ca2+, and Na+ ions. When the same amounts of ion-exchange resin were used for hydrogels containing Mg2+ or Ca2+ ions, the resulting pH values differed slightly because of the difference in the affinity of the ion-exchange resin to each ion. Better H2O2 stability was observed with hydrogels containing Mg2+ compared to the other two ions over the entire storage period for short- and long-term storage. The size of the Ca2+ ions is very close to that of the Na+ ions, and as seen in Figure 4a,b, in pH range of 7.5−8.8, no significant increase in the H2O2 stability was observed compared to our previous results obtained with hydrogels containing only Na+.4 We observed a significant increase in the short-term stability at pH values of 7.5 and 4 (Figure 4b,c). As the sol became more acidic (pH < 5), H2O2 retention values for hydrogels containing Mg2+ and Ca2+ ions became more similar to each other (Figure 4c). This is because the higher affinity of the ion-exchange resin to Mg2+ and Ca2+ compared to that to Na+ resulted in greater replacement of these ions with H+ ions, resulting in a higher residual amount of Na+ in the sol.22 At pH < 5, the highest amount of ion-exchange resin was added to the sol. For example, 93% of the peroxide was retained at the end of 10 days at a pH of 4.6 for Mg2+-containing hydrogels, compared to 91% retention with Ca2+-containing hydrogels at a pH of 4.1 and 68% retention with an unmodified sodium silicate precursor at a pH of 3.6. In the previous study,6 87% retention was observed in the presence of K+ ions at a pH of 4 at the end of 10 days. The presence of Mg2+ or Ca2+ ions resulted in better short- and long-time peroxide stability compared to our previous results. 3.3. Morphology and Structure of Hydrogels. The effects of the Mg2+ and Ca2+ contents on the hydrogel structure were studied by BET and SEM analyses. Figure 5 and Tables 1 and 2 report the effects of the Mg2+ and Ca2+ contents on the surface area and average pore diameter of the hydrogels. For all formulations, the surface area of the hydrogels increased with decreasing pH. The presence of Mg2+ and Ca2+ ions in the sol formulation resulted in a decrease of the surface area (Figure 5a). At high pH values, the surface area was 512 m2/g for Mg2+containing hydrogels and 565 m2/g for Ca2+-containing hydrogels. The rate of increase in the surface area was initially very high (at pH = 7.5−8.8) and then decreased and plateaued. At the lowest pH values, the surface area increased to 1247 m2/ g for Mg2+-containing hydrogels and 1191 m2/g for Ca2+containing hydrogels. This increase can be explained by the high dissolution rate of the silica, due to the presence of more ions at high pH.21,23 In this state, the silica dissolves and reprecipitates, forming large-diameter pores and causing a
Figure 4. Comparison of the H2O2 stability in hydrogels containing Mg2+, Ca2+, and Na+ ions at different pH values.
ions at a pH of 4.6 We believe that the better stability values obtained in the present work are because of an increase in the number of sites available for hydrogen bonding between the SiOH groups and peroxide molecules due to the divalent nature of the ions. Figure 4 shows the peroxide retention values of hydrogels containing Mg2+ or Ca2+ at different pH values, together with the results previously obtained in our laboratory for hydrogels 2610
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Figure 5. Structural properties of silica hydrogels containing Mg2+ and Ca2+ ions at different pH values: (a) surface area; (b) average pore diameter.
Table 1. BET Data for Hydrogels Containing Mg2+ Ions a b c d e f
pH
gel time (min)
SBET (m2/g)
average Dp (nm)
8.80 8.36 8.02 7.56 6.85 4.62
60 70 83 40 13 8
512 757 793 1102 1085 1247
15.9 5.6 4.9 3.8 4.3 4.3
and 38.2 nm, respectively, at high pH values. At low pH, small oligomers formed smaller-sized pores, due to the low dissolution rate of the silica.21 The smallest-sized pores were obtained in the presence of Mg2+ ions because of its small atomic size. However, compared to the previous studies,4−6 smaller average pore diameters were observed in the presence of Na+ and K+ ions (24.3 and 27.7 nm) than with hydrogels containing Ca2+ ions, although the size of the K+ ion (152 pm) is larger than those of the Ca2+ and Na+ ions. Nitrogen adsorption/desorption isotherms and BJH poresize distributions of the hydrogels are presented in Figures 6 and 7. Almost all Mg2+-containing hydrogels showed the type IV isotherm with a hysteresis loop, indicating mesoporous materials (Figure 6b−f). Hydrogels synthesized at the highest pH showed the type III isotherm, indicating the presence of macropores with type H3 hysteresis. Type H3 hysteresis is indicative of slit-shaped pores.25 As the pH decreased, the hysteresis loops transformed to H2, indicating the formation of ink-bottle-shaped pores with limited microporosity (Figure 6b−f).26 Hydrogels containing Ca2+ ions showed the type III isotherm at pH values of 8.8−7.7 (Figure 7a−d) and the type IV
Table 2. BET Data for Hydrogels Containing Ca2+ Ions a b c d e f
pH
gel time (min)
SBET (m2/g)
average Dp (nm)
8.80 8.40 8.20 7.72 7.35 4.09
70 180 240 120 52 35
565 574 678 847 837 1191
38.2 36.5 23.1 15.3 7.9 4.3
reduction in the surface area (Figure 5b).24 The average pore diameter decreased with decreasing pH of the sol. For sols containing Mg2+ and Ca2+ ions, the pore diameters were 15.9
Figure 6. Adsorption/desorption isotherms and pore-size distributions of hydrogels containing Mg2+ ions at different pH values. 2611
DOI: 10.1021/acs.iecr.7b00278 Ind. Eng. Chem. Res. 2017, 56, 2607−2614
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Figure 7. Adsorption/desorption isotherms and pore-size distributions of hydrogels containing Ca2+ ions at different pH values.
Figure 8. SEM images of hydrogels containing (a) Mg2+ and (b) Ca2+ ions at different pH values.
hydrogels containing Mg2+ or Ca2+ (Figures 6b−f and 7a−f). The pore diameter range shifted from 10−100 to 1−10 nm with decreasing pH. A more homogeneous pore-size distribution, with smaller pores and denser gel structure, was obtained at low pH values. We believe that this condition leads to a higher hydrogen-bonding affinity on the silica surface, which would improve H2O2 entrapment and stability at RT, as discussed in the previous section. Figure 8 shows the effects of the Mg2+ and Ca2+ ion contents on the morphology of the hydrogels, as revealed by SEM
isotherm at pH values of 7.4−4.1 (Figure 7e,f) with hysteresis loops. Type III and IV isotherms are indicative of macropores and mesapores, respectively. Hysteresis loops transformed from type H3 to H2 as the pH decreased. At the lowest pH range, type H4 hysteresis was observed, suggesting the presence of structures with narrow-slit pores, including pores in the micropore region.25 A bimodal pore-size distribution was observed in Mg2+containing hydrogels at the highest pH value (Figure 6a). At lower pH values, pore-size distributions were unimodal for 2612
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Figure 9. Percent of H2O2 released as a function of time from hydrogels containing (a) Mg2+ and (b) Ca2+ ions at different pH values.
analysis. Mg2+ and Ca2+ ions affected the formation of the silica network by influencing the hydrogen-bonding capacity and strength. A more compact and denser gel structure was obtained as the pH was decreased in both cases. The formation of smaller particles was observed with decreasing pH in hydrogels containing Mg2+ (Figure 8a). However, quantitative results could not be obtained from SEM. Therefore, we carried out nitrogen sorption experiments. The results were similar, and we were able to obtain more detailed and quantitative information from BET and BJH analyses. 3.4. Release Rate. Percent release is defined as the ratio of the amount of H2O2 released over time to the initial amount of H2O2. Ż egliński et al.1 studied the release profile of peroxide from xerogels and reported that the entire H2O2 content was released into water within 10 min. In the present work, we examined the effects of the Mg2+ and Ca2+ ion contents at different pH values on the percent release of H2O2 from silica hydrogels (Figure 9a,b). In all experiments, H2O2 was released via a two-step biphasic process, with an initial fast liberation from open pores near the bulk surface during the first 10 min, followed by a much slower release via diffusion of the remaining liquid from the matrix from 10 to 60 min. As discussed in the previous sections, the pH has a major effect on the structure of the silica hydrogels. For example, the average pore diameter changed from 15.9 to 4.3 nm in hydrogels containing Mg2+ as the pH decreased from 8.8 to 4.6. In hydrogels containing Ca2+, the average pore diameter decreased from 38.2 to 4.3 nm as the pH decreased to 4.1. Larger pores in the network cause a higher rate of H2O2 release. Because Ca2+-containing hydrogels had relatively large pores, more H2O2 was released at all pH values compared to Mg2+containing hydrogels. Specifically, 65% of the peroxide released from the hydrogels containing Mg2+ ions in 60 min compared to 74% release for Ca2+-containing hydrogels, 72% release for K+-containing hydrogels, and 70% release for Na+-containing hydrogels. The lowest percent release value was obtained for Mg-containing hydrogels because of its small pore diameters as discussed above. These results show that the use of divalent and monovalent metal ions allows us to obtain different hydrogel structures that can be used to tailor the release (%) of H2O2 to the specific application.
stability of the encapsulated H2O2 increased with the addition of Mg2+ and Ca2+ ions and with decreasing pH over the entire period. At the lowest pH values studied (pH = 4.6 and 4.1), 70% of the peroxide was retained with hydrogels containing Mg2+ ions, compared to 59% retention with hydrogels containing Ca2+ ions at the end of 70 days. At the highest pH value, the average pore diameter and surface area were 15.9 nm and 512 m2/g for Mg2+-containing hydrogels and 38.2 nm and 565 m2/g for Ca2+-containing hydrogels, respectively. As the pH values decreased in these sols, the gel structure became more compact, both pore diameters decreased to 4.3 nm, and the surface areas increased to 1247 m2/g for Mg2+-containing hydrogels and 1191 m2/g for Ca2+-containing hydrogels. The release of H2O2 decreased with decreasing pH for hydrogels containing Mg2+ or Ca2+. Encapsulating H2O2 in silica hydrogels is a simple, environmentally friendly, and cost-effective method that is easy to scale up. For intended applications, the hydrogel properties can be tailored to control the stability and release of H2O2 by adjusting the amounts of Mg2+, Ca2+, and Na+ ions in the sol.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Nese Orbey: 0000-0003-3973-5736 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the startup funds from University of MassachusettsLowell. Financial support from the Fulbright Foreign Student Program for E.M.D. is gratefully acknowledged. Financial support for N.P.N from University of Massachusetts−Lowell Louis Stokes Alliances for Minority Participation (LSAMP) Program is also gratefully acknowledged.
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REFERENCES
(1) Ż egliński, J.; Cabaj, A.; Strankowski, M.; Czerniak, J.; Haponiuk, J. T. Silica xerogel-hydrogen peroxide composites: Their Morphology, Stability, and Antimicrobial Activity. Colloids Surf., B 2007, 54, 165− 172. (2) Bednarz, S.; Rys, B.; Bogdal, D. Application of Hydrogen Peroxide Encapsulated in Silica Xerogels to Oxidation Reactions. Molecules 2012, 17, 8068−8078.
4. CONCLUSIONS The size and charge of ions in the sol have important effects on the hydrogel structure and, hence, the stability and percent release of entrapped H2O2. The gelation time decreased as MgCl2 and CaCl2 were added to the starting precursor. The 2613
DOI: 10.1021/acs.iecr.7b00278 Ind. Eng. Chem. Res. 2017, 56, 2607−2614
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DOI: 10.1021/acs.iecr.7b00278 Ind. Eng. Chem. Res. 2017, 56, 2607−2614
