Encapsulation of 2,4-Dihydroxybenzophenone into Dodecylbenzenesulfonate Modified Layered Double Hydroxide for UV Absorption Properties

Abstract

New organic-inorganic composite of 2,4-dihydroxybenzophenone (BP-1) encapsulation into dodecylbenzenesulfonate (DBS) modified layered double hydroxide (LDH) was successfully prepared. The surface, structural, thermal and absorption properties of the BP-1/DBS-LDH nanohybrid was characterized by BET analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TG) and diffuse reflectance UV-Vis absorbance spectra (DRUV-vis). The interlayer configuration of composite and the adsorption mechanism of BP-1 on MgAl-DBS-LDH were discussed. It was suspected that DBS anions located in the form of monolayer arrangement with a $75^{\circ}$ anti parallel angle between dodecylbenzenesulfonate chain axis. The diffuse reflectance UV-Vis absorbance results revealed that the UV absorbing wavelength of BP-1/DBS-LDH evidently extends to about 400 nm, which shows that the BP-1/DBS-LDH has the potential application as a UV absorber.

Introduction

The use of UV screens is increasing because of growing several problems such as causing sunburn, acceleration of aging, cancer by ultraviolet radiation emitted from the sun.1-3 Organic UV filters such as benzophenone (BP) and their derivatives have been widely used in a variety of cosmetics because of their excellent UV ray absorption ability.4 However, in vivo and in vitro studies have shown that some benzophenone (BP) derivatives tend to be absorbed in the body through the skin and possess estrogenic effect. When a sunscreen lotion that contained 2-hydroxy-4-methoxybenzo-phenone (BP-3) was applied on human skin, 1-2% of the amount applied was absorbed into the bloodstream within 10 h.5-7 It has been suggested that these problems can be resolved by incorporation of organic materials in interlayer galleries of inorganic layered materials to avoid direct contact of organic molecules to the human skin.3,8 The layered double hydroxides (LDHs), also known as hydrotalcite-like compounds (HTLc) are a class of anionic clays whose structures are based on brucite-like layers. LDHs have the general formula [MII 1-xMIIIx(OH)2]x+(An−)x/n]·mH2O, where MII and MIII represent divalent and trivalent metal cations, respectively.9 Typically, layered double hydroxides (LDHs) have attracted significant attention due to their potential applications as catalysts, absorbents, controlled-release formulation.10-16

Over the past decades, great improvement has been advanced by intercalation of organic anions into the LDH interlayer to produce multifunctional materials such as UV absorbent. Despite many researches focused on intercalation of organic UV absorbents such as cinnamic acid, naphthalene sulfonic acid, benzoic acid and their derivatives into LDH,3,18-22 only few about benzophenone derivatives were reported.8,17,20,23 Moreover, most of the common benzophenone series UV absorber is hydrophobic, so it is difficult to intercalate these hydrophobic molecules into the LDH interlayer. Many studies have shown that surfactant modified clays can enhance their sorptive capacity for organic molecules, such as trichloroethylene, tetrachloroethylene, benzophenone, and phenol compounds, etc.24-27 When anionic surfactants are inserted between the LDH layers, the interlayer surface can be transformed from hydrophilic to hydrophobic, and this transformation can effectively enhance LDH materials’ affinity for hydrophobic compounds, making surfactant modified LDH alternative hydrophobic organic sorbents. So one promising route to the synthesis of hydrophobic UV absorber intercalated LDH is to absorb them in surfactant modified LDH.

In this work, we reported a novel layered nanohybrid by encapsulation of hydrophobic UV absorber, 2,4-dihydroxybenzophenone (BP-1), into dodecylbenzenesulfonate (DBS) modified layered double hydroxides and investigated the interlayer configurations, thermal stability and UV absorbance ability of the synthesized nanohybrids.

 

Experimental

Materials. Mg(NO3)2·6H2O, Al(NO3)3·9H2O, NaOH, 2,4-dihydroxybenzophenon (BP-1), Na2CO3, and sodium dodecylbenzenesulfonate (SDBS) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All reagents were analytical pure and used as raw materials without further purification. CO2-free deionized water was used throughout the experimental processes.

Synthesis of MgAl-DBS-LDH. The dodecylbenzenesulfonate (DBS) intercalated LDH was prepared by the standard aqueous coprecipitation method using N2 atmosphere and CO2-free water. A mixture solution of 50 mL 1 mol/L Mg(NO3)2·6H2O and 25 mL 1 mol/L Al(NO3)3·9H2O (Mg/Al molar ratio = 2:1) was slowly added to 500 mL 0.1 mol/L SDBS solution at room temperature with vigorously stirring under N2 atmosphere to prevent the formation of MgAl-CO3-LDH. The pH of the reaction mixture was adjusted 10.0 ± 0.2 by dropwise addition of 1 mol/L NaOH solution. Then the resulting slurry was crystallized at 70 °C for 48 h. The white precipitate was isolated by filtration, washed with hot decarbonated water several times and dried at 80 °C for 24 h.

Preparation of 2,4-Dihydroxybenzophenone (BP-1) Incorporated MgAl-DBS-LDH. BP-1was incorporated into MgAl-DBS-LDH as follows: MgAl-DBS-LDH was added to a methanol solution of BP-1 and the mixture was stirred for 3 days at 60 °C. The slurry was centrifuged with the solid material and washed three times using methanol and then dried at 60 °C.

Characterization. X-ray diffraction (XRD) patterns of the samples were collected using a Bruker D8 Advanced XRD diffractometer at Cu Kα radiation and a fixed power source (40 kV and 40 mA, λ= 1.5406 Å). The d-spacing of LDHs was calculated with Bragg equation: 2dsinθ = nλ. Fourier transform IR were recorded in the range 400-4000 cm−1 on a Nicolet NEXUS 470 IR spectrophotometer using KBr pellet technique. The specific surface area and pore-size distribution were evaluated using the nitrogen adsorption (Quantachrome Autosorb 1-C). Thermogravimetric analysis (TG) curves were obtained on a NETZSCH STA 449C instrument in the temperature range of 30-700 °C with a heating rate of 10 °C min-1 in air. A scanning electron microscope (FEI NOVA NanoSEM 450) was used to study the surface morphology of sample. Diffuse reflectance UV-Vis absorbance spectra were recorded on a Shimadzu UV-2550 spectrometer equipped with an integrating sphere attachment using BaSO4 as background.

Measurement of Loading Amount of BP-1 in MgAl-DBS-LDH. The loading amount of BP-1 in MgAl-DBS-LDH was measured by a Shimadzu UV-2550 model UV-vis spectroscopy using the following method28: 0.01 g of the BP-1/DBS-LDH and 1 mL of 6 mol/L HNO3 solution were added into a 50 mL volumetric flask, the balance was filled with ethanol. The concentration of BP-1 in the resulting solutions was determined by monitoring the absorbance at λmax = 404 nm with UV-vis spectroscopy to calculate the BP-1 value. The concentration was calculated by regression analysis according to the standard curve obtained from a series of standard solution of BP-1.29

 

Results and Discussion

X-ray Diffraction. XRD patterns of the MgAl-CO3-LDH, MgAl-DBS-LDH and BP-1 incorporated MgAl-DBSLDH (assigned to BP-1/DBS-LDH) are shown in Figure 1. The XRD pattern of the contrast MgAl-CO3-LDH (Figure 1(a)) exhibits typical characteristics of the LDH phase corresponding to the basal spacing of 0.75 nm, which agrees well with the literature data.20 After intercalation, the d-spacing of MgAl-DBS-LDH is expanded to 2.96 nm, which clearly demonstrating the successful intercalation of DBS into the LDH layer (Figure 1(b)). However, for BP-1 incorporated MgAl-DBS-LDH, there is decrease in d-spacing, but the well-ordered layered structure is maintained (Figure 1(c)). The phenomena of decrease in d-spacing after incorporating hydrophobic organic into DBS-LDH have been reported by Bruna et al., Nie and Hou.30,31 They all believe that the decrease could be due to the adsorption of organic which displaces the interlayer water molecules. In this work, comparing with MgAl-DBS-LDH, the d-spacing value of BP-1/DBS-LDH changes markedly from 2.96 to 2.72 nm. Therefore, it could be assumed that the decrease would be caused by the rearranged of DBS in the interlayer after BP-1 incorporated.

Figure 1.X-ray diffraction patterns of (a) MgAl-CO3-LDH, (b) MgAl-DBS-LDH and (c) BP-1 incorporated MgAl-DBS-LDH at 2θ range of 2-70°.

Based on the basal spacing d003 of 2.96 nm for MgAl-DBS-LDH observed by XRD, and subtracting the thickness of brucite layer (0.48 nm), the gallery height is calculated to be 2.48 nm, which is bigger than that of DBS anions (2.26 nm).32 So, as shown in Figure 2, it is suspected that DBS anions located in the form of monolayer arrangement by turning the hydrophobic group. As for BP-1/DBS-LDH, the basal spacing d003 was 2.72 nm, so the gallery height is 2.24 nm, which is slight shorter than the length of DBS anion. As shown in Figure S1, except for strength decrease there is no obvious new peaks in the ATR-IR spectrum of BP-1/DBS-LDH. Therefore as shown in Figure 2(b), it was suspected that DBS anions maybe locate in the form of monolayer arrangement with a 75° anti parallel angle between dodecylbenzenesulfonate chain axis.

Fourier Transform Infrared Spectroscopy. Figure 3 shows the FT-IR spectra of BP-1, MgAl-CO3-LDH, MgAl-DBS-LDH, and BP-1/DBS-LDH, respectively. In the spec-trum of the MgAl-CO3-LDH, shown in Figure 3(a), the absorption band around 3442, 1360 and 453 cm−1 can be ascribed to the characteristic peaks of the MgAl-CO3-LDH.20 As shown in Figure 3(c), the absorption bands at 2957, 2926 and 2855 cm−1 are due to the stretching vibration of C-H. The absorption bands at 1466 and 1375 cm−1 are the characteristic of bending vibration and CH3 stretching vibration, respectively. The FT-IR spectrum of MgAl-DBS-LDH shown in Figure 3(c) contains both the characteristic peaks of DBS at 1179, 1131, 1038, 1011 and 832 cm−1, which indicates that the DBS anions have been intercalated into the interlayer galleries of the LDH.26 Owing to the character absorption bands of BP-1 superimpose onto that of MgAl-DBS-LDH, the peaks of the BP-1/DBS-LDH are no shifted in position, but the CH scissoring at 1464 cm−1 becomes relatively weak and the C=C aromatic stretchings at 1411 and 1378 cm−1 become relatively broad (see Figure S2 and S3). These changes can be tentatively attributed to the strong sorbatesorbent interactions involving π-π interaction occurring between the phenyl rings of both BP-1 and MgAl-DBS-LDH.33

Figure 2.The possible interlayer arrangements of (a) MgAl-DBS-LDH and (b) BP-1/DBS-LDH.

Figure 3.FT-IR spectra of (a) BP-1, (b) MgAl-CO3-LDH, (c) MgAl-DBS-LDH, and (d) BP-1/DBS-LDH.

Figure 4.Adsorption-desorption isotherms of nitrogen gas for MgAl-DBS-LDH and BP-1/DBS-LDH.

Surface Property and BP-1 Loading Amount. The isotherms of nitrogen adsorption and desorption at 77 K for the MgAl-DBS-LDH and BP-1/DBS-LDH are plotted in Figure 4. The adsorption-desorption isotherms can be categorized as type IV,34 with hysteresis loops observed in the range of 0.40-0.99 P/P0, indicating the presence of mesopores (Figure 4).35 This type of hysteresis loops does not exhibit any limiting adsorption at high P/P0 region, which is commonly attributed to particle aggregates with slit-shaped pores. The general aspect of the isotherm for BP-1/DBS-LDH is not changed very much from that of MgAl-DBS-LDH. The BET surface areas of MgAl-DBS-LDH and BP-1/DBS-LDH are 17.3 and 12.2 m2g−1, respectively. Figure 5 shows plots of BJH desorption pore size distribution for MgAl-DBS-LDH and BP-1/DBS-LDH. The BJH desorption pore size distribution analysis of the two sample shows most pore diameters are around 4 nm (Figure 5), which are attributed to the interpaticle spaces.36 According to the result of UV-vis spectroscopy, the BP-1 loading amount in BP-1/DBS-LDH is 6.05 wt %.

Figure 5.Pore size distributions for MgAl-DBS-LDH and BP-1/DBS-LDH.

Figure 6.Scanning electron micrograph for (a) MgAl-CO3-LDH, (b) DBS-LDH and (c) BP-1/DBS-LDH.

Figure 6(a) shows the morphology of MgAl-CO3-LDH obtained by a scanning electron microscope. As can be seen, the MgAl-CO3-LDH particles are of typical plate-like shape with the lateral size of about 100 nm. After DBS intercalation, the DBS-LDH particles are agglomerated, which could be because of DBS absorption on the surface of intercalated LDH (Figure 6(b)). Moreover, BP-1/DBS-LDH particles also show agglomerated and the surfaces are diffuse and not as sharp as that of MgAl-CO3-LDH, which has been observed previously by others.25

Thermogravimetric Analysis. Figure 7 demonstrates TG curves of MgAl-CO3-LDH, MgAl-DBS-LDH, and BP-1/ DBS-LDH, respectively. As shown in Figure 7(a), the thermal decomposition of MgAl-CO3-LDH occurs in three steps corresponding to loss of adsorbed and interlayer water, dehydroxylation, and a combination dehydroxylation-decarbonation reaction, respectively. For MgAl-DBS-LDH (Figure 7(b)), thermal degradation takes place in several steps too.37 The first and second mass loss is observed from the ambient temperature to about 400 °C, which is correspond to the removal of interlayer water and dehydroxylation, respectively. The next mass losses at temperature around 400-700 °C are attributed to the combustion of the organic guest-DBS. Comparing with MgAl-DBS-LDH, the TG curve of BP-1/ DBS-LDH (Figure 7(c)) shows the same mass loss process, except the total mass loss percentage of slight higher, which also demonstrates that BP-1 is encapsulated in the interlayer of LDH.

Figure 7.TG curves of (a) MgAl-CO3-LDH, (b) MgAl-DBS-LDH, and (c) BP-1/DBS-LDH.

Figure 8.UV-Vis absorbance spectra of (a) BP-1, (b) BP-1/DBS-LDH, (c) MgAl-DBS-LDH, (d) MgAl-CO3-LDH.

UV Ray Absorption Ability Analysis. UV-Vis diffuse reflectance absorbance spectra are measured for MgAl-CO3-LDH, MgAl-DBS-LDH, BP-1, and BP-1/DBS-LDH, respectively (Figure 8). Similar to the result of ZnAl-NO3-LDH, the curve of MgAl-CO3-LDH (Figure 8(d)) shows UV absorption at 200-250 nm and 350 nm, which could be attributed to the existence of carbonate ions in the interlayer galleries. As shown in Figure 8(c), the UV absorbance curve indicated that MgAl-DBS-LDH has a strong UV absorption at about 230-280 nm. All of above-mentioned materials do not show obvious absorption property at wavelength more than 300 nm, except that BP-1 shows intensive UV absorption ability at the wavelength less than 400 nm. It can be seen that The UV absorbing wavelength of BP-1/DBS-LDH evidently extends to about 400 nm (Figure 8(b)). So with encapsulation of BP-1 into interlayer galleries of the MgAl-DBS-LDH, the composite shows excellent UV absorption ability, and the spectrum is red shifted (from 280 nm to about 400 nm). The result shows that the composite has potential applications as a UV absorber.

 

Conclusion

BP-1/DBS-LDH was successfully prepared by encapsulation BP-1 into SDBS modified LDH. After encapsulation of BP-1, the interlayer distance of the MgAl-DBS-LDH is 2.72 nm. It was suspected that DBS anions located in the form of monolayer arrangement with a 75° anti parallel angle between dodecylbenzenesulfonate chain axis. Comparing with MgAl-DBS-LDH, the UV absorbing wavelength of BP-1/DBS-LDH evidently extends to about 400 nm, which shows that the BP-1/DBS-LDH has the potential application as a UV absorber.