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Experimental and DFT Characterization of Halloysite Nanotubes Loaded with Salicylic Acid
Journal: The Journal of Physical Chemistry Manuscript ID jp-2016-06964n.R2
Manuscript Type: Article Date Submitted by the Author: n/a
Complete List of Authors: Spepi, Alessio; Universita degli Studi di Pisa, Dipartimento di Chimica e Chimica Industriale
Duce, Celia; Universita degli Studi di Pisa Dipartimento di Chimica e Chimica Industriale
Pedone, Alfonso; Università di Modena e Reggio Emilia, Modena, Italy, Department of chemistry
Presti, Davide; Università di Modena e Reggio Emilia, Chemical and Geological Sciences
Gonzalez Rivera, José; Universita degli Studi di Pisa Dipartimento di Chimica e Chimica Industriale
Ierardi, Vincenzo; Università di Genova,
Tine, Maria; Universita di Pisa, Dipartimento di Chimica e Chimica Industriale
1
Experimental and DFT Characterization of
Halloysite Nanotubes Loaded with Salicylic Acid
Alessio Spepi1, Celia Duce1*, Alfonso Pedone,2 Davide Presti,2 José-Gonzalez Rivera1, Vincenzo Ierardi3, Maria Rosaria Tiné1
1
Chemistry and Industrial Chemistry Department, University of Pisa, Via G. Moruzzi 13, I-56124 Pisa, Italy.; E-mail: celia.duce@unipi.it. Tel: +39 0502219311
2
Department of Chemical and Geological Sciences, University of Modena and Reggio-Emilia, via G. Campi 103, I-41125 Modena, Italy.
3
Nanomed Labs Physics Department, University of Genova, Largo R. Benzi 10, 16132, Genova, Italy 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58
Abstract
Halloysite nanotubes (HNTs) and salicylic acid (SA) are natural substances widely used in different fields. HNTs are very promising as nanocarriers because of their biocompatibility, atoxicity, anti-inflammatory properties and capacity to maintain the biological activity of immobilized enzymes. Due to its bactericidal and antiseptic properties, salicylic acid (SA) is used in pharmaceutical formulations, and as an additive for preserving foods and cosmetics. In this study, we set up a procedure for the loading of HNTs with SA for their possible application in active food packaging. Pristine HNTs were studied together with acidic etched HNTs with enlarged internal lumen, and various pH values for the loading solutions were tested in order to obtain the maximum loading. The HNTs - empty and loaded with SA - were characterized by TG-FTIR, FTIR SEM, STEM and nitrogen adsorption/desorption isotherms measurements. We obtained a maximum loading of 10.5% (w/w), using HNTs pretreated with H2SO4 2 M at 25°C for 48h and a solution of
sodium salicylate at pH 8. We also characterized the interaction of SA-HNTs at a molecular level by combining ATR-FTIR measurements and periodic density functional theory (DFT) calculations. We believe that the information on the SA-HNT complexes derived from our research should help to improve the current knowledge of SA-clay interactions. In addition, it should be of interest for environmental and earth sciences since SA is used to model natural organic matter (NOM) in both experimental and theoretical studies of NOM adsorption on different kinds of mineral surfaces.
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3
INTRODUCTION
Loading nanocarriers with active molecules is one of the most exploited ways to tailor some properties (e.g. solubility, rate of diffusion availability on the body, rate of release) of the loaded molecule to the specific needs of the material designer. Besides their low cost, high availability and “green” nature, halloysite nanotubes (HNTs) are very promising as nanocarriers. This is due to their biocompatibility, atoxicity, anti-inflammatory properties and capacity to maintain the biological activity of immobilized enzymes 1-3. They have been loaded with a wide range of molecules from antioxidants to antibiotics, anticancer and anti-inflammatory drugs1-5 and used in nano architecture for drug delivery in oral systems6 as tablets and capsule fillers7, in bone cements8, in active food packaging9, in antibacterial and anticorrosion protective coating10,11 as well as for bionanocomposites, membranes12 or films13.
HNTs have an external diameter of 40-70 nm, an inner lumen diameter of 10–15 nm, and a length of 1500±500 nm. They are formed by aluminosilicate layers with SiOH and AlOH groups on the external and internal surfaces respectively, producing a different chemistry of the lumen and outer surfaces. By changing the pH, the ionic strength, and the ionic media of the solution, the HNT charge can be changed and consequently a molecule can be lead inside or outside the HNT lumen14. Controlled acid or basic etching can also be used for selective aluminum oxide or silica etching and to enlarge the HNT lumen15,16.
Due to its bactericidal and antiseptic properties, salicylic acid (SA) is a natural compound used in a wide range of pharmaceutical formulations and as an additive for food17,18 and cosmetics19,20. With its carboxylic and phenolic functionality, salicylic acid is used to model natural organic matter (NOM) in both experimental and theoretical studies of NOM adsorption on different kinds of mineral surfaces, such as aluminum and iron oxides, hydroxides and clays21-26. The shifts of -COO -stretching C-OH bending and C-C of aromatic ring and the appearance of new absorption bands in the SA/mineral surface interfacial FTIR spectra of salicylate have been commonly used to characterize the salicylate-aluminum oxide and aluminum hydroxide surface complexes21,24.
In their work on the coordination of SA on aluminum and iron (III) oxides, Biber and Stumm21 hypothesized the presence of two sodium salicylate (NaSA)-δAl2O3 complexes: a monodentate
coordination of the carboxyl group assisted by the formation of a hydrogen bond associating the phenolic OH group with a deprotonated surface hydroxyl group and a bidentate coordination accounting for the decrease in the separation between antisymmetric and symmetric stretching absorption frequencies ∆ν (∆ν = νas – νs) of –COO- moiety compared to the free ligand.
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The ∆ν (∆ν = νas – νs) of –COO- moiety of salicylate in the metal complex and in solution are
often compared to gain insight into the complex that salicylate forms with a solid surface. If ∆ν in the complex is smaller than that of salicylate in solution, a bidentate mononuclear bond is postulated (the carboxylate forms two bonds with one metal atom). If the value of ∆ν is higher in the complex than that of salicylate in solution, a monodentate mononuclear bond is postulated (one oxygen of the carboxylate binds to one metal atom). If, on the other hand, the values of ∆ν are similar, a bridging bidentate complex is hypothesized (two oxygens of the carboxylate form bonds with two metal atoms)21,23,24.
Thus when studying the interaction between salicylate (here named SA-) and aluminum hydroxide, Guan et al.24 found that after the absorption onto aluminum, the band at 1578 cm-1 (asymmetric vibration of carboxylate moiety) shifts to a lower wavenumber of about 20 cm-1. This shift may be due to the asymmetric vibration of the bidentate or bridging complexes. As the ∆ν value of the complex is comparable to that of the free ligand value, the authors hypothesized that a bridging complex is more likely to be formed than a bidentate one23.
In their study on the interactions of salicylic acid and illite clay, Kubicki et al.25 found similar IR spectra to those of Biber and Stumm21, however their molecular orbital calculations indicating that the experimental frequencies correlated better with the presence of a bridging bidentate salicylate-octahedral Al3+ complex, do not totally rule out the presence of a small amount of monodentate complex, but do not support the presence of a bidentate complex.
In this study, we set up a procedure to load HNTs with SA for their possible application in active food packaging. Pristine HNTs were studied together with acidic etched HNTs with an enlarged internal lumen, and various pH values of the loading solutions were tested in order to obtain the maximum loading. The HNTs - empty and loaded with SA - were characterized by TG-FTIR, FTIR SEM, STEM and nitrogen adsorption/desorption isotherms measurements.
In addition, we investigated the SA-HNT interaction at a molecular level using ATR-FTIR results and periodic DFT calculations. The structures of the NaSA were optimized in vacuum, in water solution, and when chemisorbed onto the kaolinite slab.
EXPERIMENTAL
Materials. Pristine halloysite nanotubes, salicylic acid (SA) (99.9%), sodium salicylate (NaSA) (99.5%), H2SO4 (98 %), NaOH (≥ 97.0, pellets) and EtOH (≥ 99.8%) were purchased from Sigma
Aldrich and used without further purification.
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5
Thermogravimetry. A TA Instruments Thermobalance model Q5000IR equipped with an FTIR (Agilent Technologies) spectrophotometer Cary 640 model for Evolved Gas Analysis (EGA) was used. TG measurements were performed at a rate of 10 °C/min, from 40 °C to 900 °C under air flow (25 ml/min) using Pt crucibles. TG-FTIR measurements were performed at a rate of 20 °C/min, from 40 °C to 900 °C under nitrogen flow (70 ml/min), from 600 to 3000 cm-1 with a 4 cm
-1
width slit. A background spectrum was taken before each analysis in order to zero the signal in the gas cell and to eliminate the contribution due to the amount of ambient water and carbon dioxide. The amount of sample in each experiment varied between 4 and 8 mg. TG data were employed to estimate the yield of loading of NaSA into HNTs using the following equation:
% =
∙ 100 (1)
Where R HNT is the residual mass of empty HNTs, Rs is the residual mass of the NaSA loaded HNTs and Rp is the residual mass of pure NaSA, according to Odlyha et al.27
SEM, STEM and X-Ray elemental analysis. Pristine, acidic etched and SA loaded HNTs were characterized with an Ultra High Resolution Field Emission Scanning Electron Microscopy (UHR-FE-SEM), model CrossBeam® 1540XB by Zeiss equipped with a Scanning Transmission Electron Microscopy (STEM) detector. An elemental composition of the various halloysite samples was determined using X-ray Microanalysis. Electrons were accelerated at 15 kV for imaging purposes and at 10 kV for X-ray Microanalysis.
Nitrogen adsorption/desorption isotherms. Nitrogen adsorption/desorption isotherms were recorded at 77K using a surface area analyzer Beckman Coulter SA 3100. All the samples (pristine HNT, acid treated-HNT and HNT-loaded with salicylic acid) were outgassed for 90 min at 50°C under vacuum conditions (P = 1·10-3 mmHg). The isotherm was measured over the relative pressure range (PS/P0) from 0.01 to 0.991. The specific surface area (SBET) was calculated using the Brunauer-Emmett-Teller (BET) equation in the low relative pressure interval from 0.05 to 0.2; The Langmuir model and the t-plot method were used to obtain further information on the size of the monolayer and micropore volume at lower relative pressures (PS/P0 < 0.05). Total pore volume was determined at the relative adsorption pressure of 0.9814. The pore size distribution was calculated with the adsorption branch of the isotherm using the Barrett-Joyner-Halenda (BJH) method. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58
Fourier transform infrared spectroscopy. Infrared spectra were recorded using an FT-IR Agilent Technologies Spectrophotometer model Cary 640, equipped with an universal attenuated total reflectance accessory (ATRU). A few micrograms of sample powder were used with the following spectrometer parameters; resolution: 4 cm-1, spectral range: 600-4000 cm-1, number of scans: 128. Agilent spectrum software was used to process FTIR spectra.
Halloysite etching by sulfuric acid. The procedure followed for the HNT lumen etching was similar to that reported in the literature15. Two suspensions of halloysite were obtained by dispersing 5 g of halloysite in 500 ml of a 2M H2SO4 solution. The suspensions were magnetically
stirred for 48 hours, one on a hot plate at the controlled temperature of 50°C, the other at 25°C. In both cases, processed halloysite was washed five times with DI water, until the pH of the supernatant from the washing stage was in the range 6-7, similar to that of the pure halloysite suspension. The samples were dried in the oven at 70°C and characterized by TG, X-ray Microanalysis, SEM images and nitrogen adsorption/desorption isotherms
Halloysite loading with SA. The procedure followed for the HNT loading was similar to that reported in the literature1,2,10.
The suspensions of pristine or etched HNTs in water were prepared by adding 5 g of pristine HNTs to a concentrated solution of NaSA in water (1 g/ml) and the pH was adjusted at 6 and 8 with NaOH 0.1 M. All the suspensions were evacuated in a vacuum jar, using a vacuum pump, kept under vacuum for 3 h, and then cycled back to atmospheric pressure. This process was repeated three times in order to increase the loading efficiency. Finally, HNTs were separated from the solution by centrifugation, washed with water, dried in an oven at 70°C and characterized by TG-FTIR, ATIR and nitrogen adsorption/desorption isotherms.
COMPUTATIONAL DETAILS
The structure and vibrational properties of the NaSA molecule in vacuum and in solution, of the salicylate ion (SA-) with implicit + explicit water solvation, and of salicylate adsorbed onto a flat halloysite surface were investigated at the DFT level using the Gaussian0928 and CRYSTAL0929,30 suite of programs.
The B3LYP31 functional was used for all calculations. In Gaussian09 calculations the 6-31+G(d,p) Pople’s basis set was used. Unrestricted calculations (UB3LYP) were carried out for the salicylate ion, as it is an open-shell doublet configuration. Solvent effects were taken into account using the Conductor-like Polarizable Continuum (C-PCM) model32 and, in the case of SA-, adding three explicit molecules of water close to the –COO- and –OH moieties. Additional tests with more water molecules did not lead to changes in the structural and vibrational properties of the system.
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7 Further calculations on the SA- in vacuum, and with C-PCM implicit solvation are reported in ESI (see Figures S.1 and S.2).
All the solid-state models considered are neutral, closed-shell configurations. The triple-ζ pob_TZVP basis set33, devised to treat periodic systems, was employed for CRYSTAL09 calculations. The tolerance on the total electronic energy in the self-consistent procedure (SCF) was set to 10-6 a.u. (keyword TOLDEE 6), whereas the thresholds for truncating bielectronic exchange and Coulomb integrals (TOLINTEG) were set to 10-6 10-6 10-6 10-6 10-12 a.u. for a structural “pre-optimization” step.
To improve convergence stability, the Fock/Kohn-Sham matrix derivatives mixing between cycle N and cycle N-1 was set to 25% (keyword FMIXING 25). A negative level shifting of Fock/KS eigenvalues of 1.0 eV was applied to lock the system to a non-conducting state. A Pack-Monkhorst grid of 8 k-points in the Irreducible Brillouin Zone was defined, by setting a shrinking factor of 2.
Geometry optimizations were performed by setting the TOLDEG and TOLDEX parameters, which correspond to the tolerances on the root-mean-square gradients and displacements, to 3·10-5 a.u. and 1.2·10-4 a.u., TOLDEE was set at the default value (10-7 a.u.), and TOLINTEG thresholds increase at 10-7 10-7 10-7 10-7 10-14 a.u.
Harmonic vibrational frequencies and intensities were computed, for SA- adsorbed onto halloysite, through the Berry Phase approach implemented in CRYSTAL0934.
Finally, a comment on why the BSSE and dispersive interactions in the SA/kaolinite calculations is necessary. The BSSE correction is usually computed and appropriately identified for physisorption processes35 whereas, as shown later, experimental thermogravimetric data clearly indicate a chemical interaction between oxygen atoms (carboxyl- and/or hydroxyl groups) of the salicylate moiety with aluminum atoms of the halloysite surface. In order to study diverse (mono- and bi-dentate) models of the chemical interacting species, many oxygen atoms were removed from the halloysite surface and replaced by the oxygen of the carboxylate moiety of SA-. This made impossible to identify two fragments which were simultaneously closed-shell and neutral when isolated, as is required when applying the CounterPoise (CP) correction which is currently the only scheme available in CRYSTAL to correct the BSSE.
We were thus unable to account for BSSE corrections as well as computing binding energies of the adsorbates. Concerning the dispersive interactions, dispersive correction schemes are known in general to produce over binding (especially when both vdW and hydrogen bonds are present), whereas the BSSE correction yields the opposite effect. Since the use of a hybrid functional without BSSE and dispersion correction leads to ‘good’ geometries (i.e. to small structural deviations from a
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geometry computed using a hybrid functional + BSSE correction + dispersion correction) due to error cancellation, as reported in the literature36 for different systems, we adopted this approximation in this work.
RESULTS AND DISCUSSION
Our goal was to obtain an HNT loading with the highest SA content. We thus investigated the loading of pristine HNTs and of HNTs etched by treatment with 2M sulfuric acid at different temperatures for 48h. The latter treatment was chosen since, as reported in literature, it should enlarge the diameter of the HNT lumen and thus increases the tube loading capacity.
HALLOYSITE ETCHING BY SULFURIC ACID
SEM and SEM X-ray elemental analysis
Two different HNT samples were treated with 2M sulfuric acid for 48h at 25°C and 50°C, respectively. SEM X-ray elemental analysis shows that etching reduces the aluminum content of HNTs. The aluminum/silicon ratio decreases from 1, for pristine HNTs, to 0.8 for HNTs etched at 25°C, and to 0.2 for HNTs etched at 50°C. SEM images (see Figure 1) also reveal that when the etching temperature is increased, the tubes present broken points and the halloysite walls appear more friable and porous. HNT fragments and silica nanoparticles are also present, as reported in the literature4. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57
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Figure 1 : SEM images of pristine HNTs (A), for HNTs etched by a 2M H2SO4 solution for 48h at
25°C (B) and 50°C (C).
Nitrogen adsorption/desorption isotherms
The analysis of adsorption/desorption isotherms also indicates an adversely strong modification of HNTs after acidic etching at 50°C for 48h. Both pristine and acid-treated HNTs exhibit a type II isotherm, according to the IUPAC classification, typical of mesoporous solids (see Figure 2). However, while pristine HNTs and HNTs etched at 25°C exhibit an H4-type hysteresis loop (horizontal and parallel along a wide PS/P0 range), suggesting a slit-like pore morphology, HNTs etched at 50°C have an H3-type hysteresis loop (almost vertical and parallel for a narrower PS/P0 range), often associated with mesoporous aggregated samples such as plate-like particles.
HNTs etched at 25°C also show a similar pore distribution to pristine HNTs, with a slightly higher pore volume and BET surface area. HNTs etched at 50°C on the other hand, show a broader pore distribution and higher BET surface area, micropore area, and volume, suggesting the formation of micropores where aluminum atoms are displaced (see Figure 2(B) and Table 1). We therefore chose pristine HNTs and HNTs etched at 25°C, for loading with sodium salicylate.
Table 1: NaSA loading percentages and textural properties of pristine, 2M H2SO4 acidic etched and
SA loaded HNTs. 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58
Sample NaSA loading % 1 BET Area m2/g 2 Pore volume (Ads) ml/g Monolayer volume cm3/g 3 Micropore area m2/g 4 Micropore volume cm3/g Pristine HNTs - 70.9 0.1637 16.2996 10.196 0.00434 HNTs etched at 25°C - 113.6 0.2488 26.0882 10.326 0.00379 HNTs etched at 50°C - 227.5 0.6252 52.2617 31.812 0.01226 Pristine HNTs + NaSA pH=8 6.6 60.6 0.3624 13.9306 0 0 HNTs etched at 25°C + NaSA pH=8 10.5 24.7 0.3004 5.6858 0 0
1. Calculated with the Brunauer Emmet Teller equation in the linear interval of relative pressure from 0.05 to 0.2. 2. Total Pore volume, calculated at the relative pressure of 0.9814.
3. Micropore surface area, determined with the t-plot method. 4. Micropore volume, determined with the t-plot method.
Figure 2: Nitrogen adsorption/desorption isotherms (A) and pore size distribution (B), using the adsorption branch of the isotherm for pristine HNTs and acid etched at 25°C HNTs, loaded with SA at pH 8.
HALLOYSITE LOADING WITH SALICYLIC ACID
We used pristine HNT suspensions and a solution of NaSA in water (1 g/ml) at their natural pH, of around 6, reaching 3.7% loading. In order to then optimize the SA loading, we increased the pH of the SA solution to 8 in order to maximize the negative charge of salicylate, while remaining
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11 within the pH range of 4÷8.5 where the inner surface of HNTs is positively charged1,15. Under these conditions, we obtained an improvement in the loading of up to 6.6%. When the NaSA solution at pH 8 was used to load HNTs etched at 25°C, the percentage of loading reached 10.5%, which is almost the maximum that can be achieved with HNTs15.
STEM , TG and TG-FTIR
The loading efficiency was evaluated from TG data by using the values of residual mass at 800°C and the equation (1) reported in the experimental section. (see Table 1 in ESI). Table 1 reports the loading percentages with NaSA at pH 8 of pristine HNTs and HNTs etched at 25°C together with their textural properties. Figure 3 reports the scanning transmission electron microscopy (STEM) images of HNTs etched at 25°C, both empty (A) or loaded with NaSA at pH 8 (B). The figure 3 shows the differences clearly. While in panel (A) the empty lumen of the HNT is evident, it is not visible in the loaded HNT (B).
Figure 3: STEM images of HNTs etched at 25°C, empty (A) and NaSA loaded at pH=8 (B).
The comparison among the TG or DTG curves of pure NaSA, empty HNTs, and HNTs loaded with NaSA shows that when NaSA is loaded inside HNTs, its single degradation step at 139°C splits into two steps at higher temperatures (around 260°C and 400°C, see Figure 4), thus highlighting the strong interaction of NaSA with the HNT surface. The FT-IR spectra of the evolved gases reported in Figure 5A reveal that the step at 267 °C is due to the decarboxylation of the carboxylic moiety (signals at 2360, 2310 and 668 cm-1 due to CO2) while the FT-IR spectra
reported in Figure 5B reveals that the step at 397°C is related to the volatilization of the phenolic
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residue (stretching and bending OH at 3650 cm-1 and 1288 cm-1, stretching C-H at 3053 cm-1, stretching C=C at 1595 and 1500 cm-1, stretching C-O at 1185 cm-1 and bending C-H at 878, 757 and 696 cm-1 due to phenol vapour). The drastic modification of the NaSA thermal degradation, when NaSA is located inside HNTs, suggests a strong interaction of the salicylate moiety chemisorbed on the aluminum hydroxide surface.
Figure 4: DTG of NaSA, HNTs etched at 25°C, empty and loaded with NaSA at pH 8, recorded under air flow at 10°C/min.
Figure 5: FTIR spectrum of gases evolved at 267°C (A) and at 397 °C (B) by HNTs etched at 25°C loaded with NaSA at pH 8, under nitrogen flow.
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13 Nitrogen adsorption/desorption isotherms
Figure 6 shows the N2 adsorption/desorption isotherms for pristine HNTs and HNTs etched at
25°C both loaded with NaSA at pH 8. After SA loading, all materials keep the type II isotherm form. The SBET area was reduced from 70.9 to 60.63 m2/g for pristine HNTs, and from 113.6 to 24.7
m2/g for etched HNTs. The micropore area and micropore volume are also reduced (the values are beyond the sensitivity of the instrument (1·10-5 m3/g) (see Figure 6 and Table 1). This suggests that SA is initially uniformly loaded on micropores up to saturation. The salicylate inside the halloysite lumen then produces a partial pore blockage accounting for the BET area reduction and the corresponding reduction in the micropore area, micropore volume parameters, and mesoporosity of the material.
Figure 6: Nitrogen adsorption/desorption isotherms (A) and pore size distribution (B), using the adsorption branch of the isotherm for pristine HNTs and acid etched at 25°C HNTs, loaded with SA at pH = 8.
ATR-FTIR spectroscopy
We gained further insights into the SA-HNTs interactions by comparing the ATR-FTIR spectra of loaded HNTs and those acquired on a solid sample of NaSA obtained by evaporation from the 1M solution at pH 8 (Figure 7). Salicylate absorbs in a spectral region where HNTs do not give signals, thus the spectrum of SA inside HNTs is clearly recognizable (Figure 7).
It is well known that the coordination of salicylate with a metal ion changes the symmetry, bond strength, and angle of salicylate functional groups and consequently their corresponding normal modes of vibration22,23. Table 2 lists the vibrational frequencies corresponding to the distinguishable peaks in the spectra of sodium salicylate - both pure and inside HNTs - obtained in our study
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together with those reported in the literature for sodium salicylate pure23,26 and sodium salicylate interacting with planar aluminum oxide surfaces21,24,25. The table also reports the usual assignments proposed in the literature and for the SA in solution, the assignment provided by the DFT calculations carried out in this work and described in the following. Note that our ATR-FTIR spectra are those of NaSA absorbed on HNT inner surface, whereas the NaSA on planar aluminum surfaces are obtained indirectly by subtracting the supernatant and solid phase spectra from the salicylate/aluminum oxide or aluminum hydroxide suspension spectrum.
Figure 7: ATR-FTIR spectra of HNTs etched at 25°C loaded with NaSA and 1M NaSA solution at pH=8 in the range 1200 to 1750 cm-1.
Our spectra of pure NaSA at pH 8 are in good agreement with those reported in the literature. The spectra of NaSA inside HNTs show that all the vibrational frequencies shift with respect to those of pure NaSA and some new peaks arise. The bending mode (Ph-OH at 1336 cm-1) and the stretching band (Ph-O at 1255 cm-1) of the phenolic group shift by about 15 and 10 cm-1 respectively to a lower wavenumber than those of free salicylate anion. This suggests that the Ph-OH group is involved in the interaction between salicylate and aluminum with a weakening of the hydrogen bonding. The interaction with the halloysite also changes the π- electron density of the benzene ring, thus altering the absorption bands of its C-C vibrations (1592, 1487 and 1458 cm-1). In addition, unlike the spectra of NaSA on planar aluminum oxide/hydroxide surfaces, we found that the νas and νs of –COO- split into two components.
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15
Table 2: Vibrational frequencies of sodium salicylate pure and inside HNTs obtained in our study are listed together with those reported in the literature for sodium salicylate pure and sodium salicylate interacting with planar aluminum oxide surfaces.
Sample νs –COO
-νas –COO
-ν Ph-OH δ Ph-OH ν C-C ring ν C=O Ref.
NaSA pH=5.5 1387 a 1572 1253 1343 a 1623 / 1591/ 1488 / 1458 - 23 NaSA pH=5 1385 1576 1254 1341 1591 / 1589/ 1487 / 1458 - 24 NaSA pH=8 1387 a 1574 1255 1336 a 1620 / 1592/ 1487 / 1458 - This work NaSA vacuum, G09 1388 (1360)* + δ Ph-OH 1591 (1560) + δ Ph-OH 1295 (1269) 1350 (1323) + ring modes 1671(1638)+ δ Ph-OH / 1634 (1601) 1524 (1493) /1502 (1471) - This work NaSA water (PCM), G09 1368 (1341) + δ Ph-OH 1599 (1567) + δ Ph-OH 1281 (1255) 1342 (1315) 1668 (1634) /1630 (1597) 1520 (1490) /1490 (1460) - This work SA- water (PCM + explicit), G09 1399 (1371) + δ Ph-OH 1581 (1549) + δ H2O 1255 (1230) 1323 (1297)/ 1366 (1339) + ring modes 1675 (1642) 1649 (1616) 1628 (1595) + δ H2O / 1530 (1499) + δ Ph-OH / 1475 (1446) - This work NaSA-illite pH=3 1389 1553 1264 1335 1650 / 1603/ 1472 / 1457 - 25 NaSA-δAl2O3 pH=7 1381 1534 1259 1340 1604 / 1568/ 1469 / 1454 1708 21 NaSA-Al(OH)3 pH=7 1400 1557 1252 1333 1605 / 1578/ 1472 / 1468 1711 24 NaSA-HNTs pH=8 1398, 1377 1577, 1561 1244 1321 1650-1600b/ 1598/1484/ 1464 - This work a
Doublet produced by the coupling of the symmetric stretching mode of the carboxylic group and the bending mode
of the phenolic (Ph-O-H) group23.
b
Broad band in the range 1650-1600 cm-1.
*In parentheses, the vibrational frequencies corrected for anharmonic effects are reported.
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DFT calculations
Design of theoretical models
In order to shed light on the SA-HNT interactions and on the interpretation of the FTIR spectra, we optimized the structure of bare salicylate with the implicit + explicit solvent (see details in Section ‘Computational Details’), NaSA in vacuum and in water solution, and the SA- chemisorbed onto a kaolinite slab (halloysite), which had been studied in a previous work.37 Figure 8 shows the structures investigated. For the solid-state ones, all supercells have the same lattice constants ( a = 10.3099 Å, b = 10.3675 Å, c = 20.0000 Å, α,β = 90.0000°, γ = 119.6942°), which were fixed throughout the geometry optimizations. Six different SA-HNT chemisorbed configurations were considered: a) SA- in monodentate coordination with respect to both the carboxyl group and the deprotonated hydroxyl group, with oxygen atoms bound to different Al atoms (78 atoms per cell),
b) SA- in monodentate coordination with respect to both the carboxyl group and the deprotonated
hydroxyl group, both bound to the same Al atom (this configuration is similar to that called bidentate complex in Kubicki’s work25; 78 atoms per cell), c) SA- in monodentate coordination with respect to the carboxyl group with the hydroxyl group forming three intermolecular H-bonds with the halloysite slab, and the –C=O moiety forming three H-bonds (similar to the monodentate model studied by Kubicki et al.; 81 atoms per cell), d) SA- in bidentate coordination with respect to the -COO group, with carboxyl- oxygen atoms bound to the same Al atom (78 atoms per cell), e) SA- in bidentate coordination with respect to the -COO group, with the carboxyl- oxygen atoms bound to different Al atoms (similar to the bidentate bridging model studied by Kubicki et al.25; 81 atoms per cell), and f) a monodentate coordination with respect to the -COO group (79 atoms per cell), with an H-bond associating the phenolic OH group with a deprotonated surface hydroxyl group, which corresponds to the monodentate structure suggested by Biber et al.21 (see Fig. 2.10 of the original work)21. Note that to maintain a non-charged unit cell for model f), another H atom was removed from the kaolinite surface. All the solid-state models considered are thus neutral and closed-shell singlet configurations in their electronic ground state. Figure 8 shows the structural changes occurring after geometry optimizations; the blue sticks represent the initial geometry of SA, whereas the final optimized structure is represented with ball and sticks.
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17
Figure 8: (top) Optimized structure of sodium salicylate (-COO- oxygen atoms are labelled: O1 is
the oxygen near the hydroxyl moiety). (bottom) Optimized crystalline structures a) - f) of SA -chemisorbed onto a flat gibbsite layer; H-bonds related to the SA- are highlighted with green dashed lines. The input geometry of SA- is depicted in blue. Aluminum atoms involved in chemisorption are represented as spheres. The unit cell is shown in orange.
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Structural features
Table S.1 in the Supporting Information reports the structural parameters, which mainly reflect geometrical changes in the isolated NaSA and SA- species, as well the chemisorbed configuration of salicylate: angles and distances related to the –COOH and to the phenyl –OH moieties. Differences between NaSA molecules in vacuum and in water are very small, moreover the starting geometries are very close to the relaxed ones. Slight variations can be observed between the initial and final configuration of the O-C-O (O1-C-O2) angle (COO- group) of solvated NaSA, probably due to mild electrostatic effects on the partial double bond. Large variations (ca. 4°-5°) are shown by all the angular parameters of SA- after optimization: the presence of explicit water molecules and the consequent formation of H-bonds with the –COO- moiety lead to a rotation of the latter (see Fig. S.1 in ESI), whereas bond distances are comparable to other theoretical models.
Mild changes with respect to the starting structures can be observed for models a) and b), with the exception of the O-C-O angle (carboxyl group) and of the C-O-O (deprotonated OH group) angles which vary by about +5° and +/-3°, respectively, while maintaining the initial SA configuration. Large displacements are hampered by the two H-bonds on the carboxyl moiety.
The monodentate model c) undergoes a visible displacement during optimization, although SA -retains the same molecular structure: only intermolecular H-bonds are affected (only those of the relaxed structure are shown in Fig. 8 c)), whereas covalent bonds remain unchanged. This is the only case in which SA- almost ‘spreads’ over the gibbsite surface, holding five intermolecular H-bonds. Figure 8 c), in fact, highlights a marked rotation of the whole phenyl group around the C atom of the carboxyl moiety, a rotation of the OH group towards a bridging oxygen of the surface, from which three intermolecular H-bonds are formed, and a rotation of the O2 oxygen of the carboxyl group. Such changes are reflected by the measurements reported in Table S.1, especially the O1-C-O2 angle (119.81° to 121.96°), the C-O1 distance (1.349 Å to 1.345 Å), and the C-O-H angle (109.47° to 116.76°).
SA moiety in model d) experiences a noticeable rotation that leads to the formation of two intermolecular H-bonds. We recall from Figure 8 d) that the phenyl OH group donates a proton to one of the two bridging oxygens of the gibbsite surface.
The relaxation of the bidentate SA- model e) instead, produces a monodentate binding accompanied by three intermolecular H-bonds. Consequently, notable changes with respect to the starting geometry can be observed for both O1-C-O2 and C-O-H angles (110.74° to 122.74° and 115.82° to
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19 107.94°). These are related to the formation of an intermolecular H-bond and the simultaneous covalent bond-breaking of the carboxyl O1 atomic species with aluminum.
Interestingly, Table S.1 highlights that the relaxed model e) retains very similar structural features with respect to molecular NaSA. All the other models, conversely, show non-negligible structural variations, especially the angles related to COOH and OH groups. Model f), which corresponds to the monodentate absorption proposed by Biber et al.21 , has a structure after optimization that is not very different from the initial one, although Table S.1 reports visible differences on the O1-C-O2 and C-O-H/C-O-O angles. This is due to the displacement of a hydrogen atom, i.e. to the deprotonation of the phenyl OH moiety, in favor of a basic surface bridging oxygen. This configuration thus seems to be more stable than the protonated- SA- form proposed in the literature.
Vibrational features I: SA- and NaSA species
The computed vibrational spectra of NaSA in vacuum and in solution, of bare SA- with explicit water molecules (plus implicit solvation) and of SA- adsorbed onto the kaolinite slab were compared with the experimental counterparts in Figure 9. The computed frequencies were scaled by a factor of 0.9838 in order to empirically account for the non-harmonic effects which were not considered in our harmonic calculations.
Figure 9 a) reports the IR spectra of the NaSA molecule and SA,- whereas the numerical values of the related vibrational frequencies are included in Table 2. The salicylate ion is treated with both implicit and explicit solvation (three water molecules forming an intramolecular H-bond with the carboxyl and the hydroxyl moieties), calculations in vacuum and with implicit solvation only are reported in Figure S.2 of the ESI. Though explicit solvation is desirable to obtain quantitative insights into the vibrational properties of this system, Fig. S.2 shows that in the present case C-PCM quality implicit water is sufficient to obtain the main IR spectral features of SA-. This also justifies the use of C-PCM only, for the NaSA molecule.
The NaSA frequencies show very good agreement with the experimental ones, thus highlighting the accuracy of the calculations. The normal modes corresponding to the vibrational frequencies in the range 1200-1800 cm-1 are reported in Figures S.4 and S.5 of the ESI. The assignment corresponds to the one reported in Table 2 and in the literature, although the stretching and bending modes of the –COO- and Ph-OH moieties are always coupled with vibrational modes of the benzene ring and sometimes between themselves. Interestingly, Fig. 9 a) shows that both solvated SA- models cannot fully describe the experimental spectrum in the range of 1450-1500 cm-1, due to absence of a counter-ion. As an indirect effect, the latter (as in the case of NaSA and of the solid-state models)
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would enhance C-C ring vibrations, that for SA- are almost negligible in such range. Ring modes are also combined with bending modes of explicit water (see Table 2). A great difference between the model in vacuo and the two solvation models of SA- is also evident: neither the position nor the quality of experimental IR bands can be described. Conversely, the presence of sodium in the NaSA model means that the effect of the counter-ion is dominant over solvation: in this case, the vibrational features are notably less different between the model in vacuo and that treated with C-PCM water.
Vibrational features II: solid-state models
The situation becomes much more complicated when salicylate is chemisorbed onto the kaolinite slab, as inferred by examining the simulated spectra reported in Figure 9 b). Our calculations show that none of the five SA-HNT models alone can reproduce the experimental IR spectrum, but this spectrum seems to be due to SA- bound in several modes rather by just one. In fact, the peaks of all the spectra of the five models lie under the very broad peaks of the experimental spectra.
The eigenvectors of the normal modes corresponding to the vibrational frequencies ranging between 1200 and 1800 cm-1 are reported in Figures S.5-S.10 of the ESI, which report both harmonic and scaled values.
Fig. 9 shows that all peaks of our models in the range 1550-1700 cm-1 are compatible with the broad band of the experimental spectrum of SA in HNT. Models a) and e) seem to give a better description of such bands in terms of both the intensity and position of the main peak. The modes associated with such bands correspond mainly to the C-C and C-H ring modes, often mixed with the OH bending (models c), e) ) and the –C=O stretching (models b), d) and f)).
Except for model d) and f), the modes located in the middle region of frequencies (1550-1400 cm
-1
) are those that best correlate with the simulated and experimental spectra, though they are still associated with ring modes, which are always present. The other models always feature the two main peaks in this range, which are blue-shifted from configurations with two oxygens covalently bound to the gibbsite surface to one oxygen covalently bound. Fig S.5-S10 in the ESI suggest for models c) - e) visible mixed contributions between C-H modes and the phenyl OH bending. The non-deprotonated form of the phenyl OH moiety in the latter two models could be the cause of the blue-shift mentioned above.
The low-frequency range (1400-1200 cm-1) of the spectrum presents complex features. Even experimentally (SA in HNT) broad bands are present, and single mode contributions are difficult to identify. In fact, C-H ring modes are mixed with the symmetric and asymmetric C-O stretching of
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21 the carboxyl moiety. In the case of model a), the C-O stretching of the deprotonated phenyl OH group, whose O atom is covalently bound to the kaolinite surface, is also present in this region (see figure S.6 in the ESI, 1208 cm-1). Model f), in this region presents three bands (1236/1321/1357 cm
-1
) related to the ring modes and to the carboxyl C-O stretching (1321/1357 cm-1). These ring modes compare well with the position of the corresponding experimental broad bands. This spectrum provides more evidence of the possible presence of a monodentate configuration of deprotonated SA inside the HNT, partly confirming the model suggested by Biber et al.21
Figure 9: Simulated and experimental IR spectra of SA- (PCM + explicit water), NaSA (vacuum,
PCM) and of SA isolated (a) and adsorbed on the kaolinite slab (b). Simulated spectra were
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produced from the calculation of normal modes, scaled by a 0.98 factor, and convoluted with a Gaussian function with a half-width of 8 cm-1.
CONCLUSIONS
In this work we set up a procedure to load HNTs with SA. In order to reach the maximum loading, we used both pristine HNTs and acidic etched HNTs with an enlarged lumen and solutions of NaSA with different pH values. With a multi-technique approach (TG-FTIR, ATR-FTIR, SEM, STEM, nitrogen adsorption/desorption isotherm measurements), we fully characterized both the starting materials and the resulting SA/HNT products. ATR-FTIR measurements were used together with DFT calculations to shed light on the interaction between SA- and HNTs inside the HNT lumen.
The main results were:
• a mild acidic etching of pristine HNTs (2 M H2SO4 at 25°C for 48h) increases the HNT’s
lumen without changing the structural features;
• a maximum loading of 10.5% (w/w) was obtained using HNTs pretreated with 2 M H2SO4
at 25°C for 48h and a solution of sodium salicylate at pH 8;
• the salicylate inside the halloysite lumen produces a partial pore blockage and a reduction in the BET area, micropore area, micropore volume parameters, and mesoporosity of the material;
• when NaSA is loaded inside HNTs, its single thermal degradation step at 139°C splits into two steps related to decarboxylation at 267 °C and to the volatilization of phenolic residues at 397°C;
• the analysis of ATR-FTIR interfacial spectra highlights the different interaction of the salicylate with the inner curved surface of the nanotubes with respect to the NaSA/aluminum oxide-hydroxide planar surfaces. The Ph-OH group seems to be involved in the interaction between salicylate and aluminum, with a weakening of the hydrogen bond. On the other hand the splitting of the νas and νs of –COO- into two
components suggests the presence of different complexes SA/HNTs inside the nanotube lumen;
• DFT calculations show that none of the six considered SA-HNT models alone can reproduce the experimental IR spectrum. In fact, the experimental spectra seem to refer to an NaSA molecule in solution and adsorbed in HNTs with a large number different configurations. However, based on our calculations it seems that the salicylate prefers to
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23 adsorb in a monodentate and bridging mode rather than a bidentate mode. In fact, in the latter configuration, the geometry of the carboxylate moiety is much more strained and always tends to convert in a monodentate configuration. The vibrational spectra of the monodentate and bridging adsorbate models also compares better with the experimental one.
SA-loaded HNTs seem to be a promising system for the controlled release of SA. We are already planning to evaluate the kinetics of SA release in water. We believe that the information on the SA/HNT complexes presented in this paper improves the current knowledge of SA-clay interactions. This is likely to be of interest for environmental and earth sciences since SA is used to model natural organic matter (NOM) in both experimental and theoretical studies of NOM adsorption on different kinds of mineral surfaces.
ASSOCIATED CONTENT Supporting Information
Additional information on TG-DTG, nitrogen absorption/desorption, pore size distribution, FTIR data for pristine HNTs and HNTs etched at 25°C and 50°C, loaded with sodium salicylate, structural parameters of theoretical models a) – f), as well as calculated IR normal modes and related eigenvectors.
AUTHOR INFORMATION Corresponding Author
*E-mail: celia.duce@unipi.it. Tel: +39 0502219331
All authors contributed extensively to the work presented in this paper: C.D., A.P., and M.R.T. designed the research; A.S., J.G.R., and V.I. performed the experiments and characterizations; A.P and D.P. performed the DFT calculations; C.D, A.P. analyzed the data; C.D., A.P., and M.R.T. wrote the paper.
Notes
The authors declare no conflict of interest.
ACKNOWLEDGEMENTS
This work was supported by the project FIRB 2012 (No. RBFR12ETL5), funded by the Italian Ministry of University and Research, and by the project PRA_2016_46 funded by the University of
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Pisa. The authors would like to thank Dr. Jorge Tovar and the Center for Colloid and Surface Science in Florence, for the measurements of nitrogen adsorption/desorption isotherms
A. P. would like to thank the University of Modena and Reggio-Emilia for supporting this work by funding the internal project (Fondo di Ateneo per la Ricerca, FAR2014) entitled “Role of modular phyllosilicates for the capture and storage of CO2: an experimental and computational
investigation”.
ABBREVIATIONS
HNTs, halloysite nanotubes; SA, salicylic acid; NaSA, sodium salicylate; SA-, salicylate; TG thermogravimetry; DTG, differential thermogravimetry; SEM, Scanning electron microscopy; BET, Brunauer-Emmett-Teller theory; STEM, Scanning Transmission Electron Microscopy; ATR-FTIR, attenuated total reflectance-Fourier transformed infrared spectroscopy; NOM, natural organic matter; DFT, density functional theory; FTIR, Fourier Transform Infrared Spectroscopy; EtOH, ethyl alcohol.
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